Rechargeable Lithium–Sulfur Batteries - Chemical ... - ACS Publications

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Rechargeable Lithium−Sulfur Batteries Arumugam Manthiram,* Yongzhu Fu, Sheng-Heng Chung, Chenxi Zu, and Yu-Sheng Su Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

6.3. Chemical Synthesis of Li2S Cathode Materials 6.4. Solid-State Batteries with Li2S Cathodes 7. Characterization Techniques and Mechanistic Understanding 7.1. In Situ Characterization 7.2. Polysulfide Dissolution 7.3. Formation of the Passivation Layer 7.4. Kinetics 8. Electrolytes and Separators 8.1. Liquid Electrolytes 8.2. Carbonate-Based Electrolytes 8.3. Polymer/Solid-State Electrolytes 8.4. Separators 9. Anodes 9.1. Lithium Metal Anode 9.2. Silicon Anode 9.3. Carbon Anode 10. Binders 11. Cell Configurations 11.1. Interlayers 11.1.1. Polysulfide-Interception Mechanism 11.1.2. Constructional Materials for Interlayers 11.2. Porous Current Collectors 11.2.1. Sulfur Impregnation Strategy 11.2.2. Constructional Materials for Porous Current Collectors 11.3. Sandwiched Electrodes 11.4. Dissolved Polysulfide Catholytes 12. Voltage Window 12.1. Upper Voltage Plateau 12.2. Lower Voltage Plateau 13. Conclusions and Future Directions 13.1. Appropriate Dispersion of Active Sulfur 13.2. Efficient Absorbing Materials 13.3. Flexible Conductive Matrix 13.4. Stable Electrolyte Systems 13.5. Safe Anode Materials 13.6. Li2S Cathode 13.7. Novel Cell Configurations 13.8. Smart Recharge Settings Associated Content Supporting Information Author Information

CONTENTS 1. 2. 3. 4.

Introduction Principles of Lithium−Sulfur Batteries Historical Development Technical Challenges 4.1. Shuttle Mechanism 4.2. Self-Discharge 5. Sulfur Cathode 5.1. Conventional Sulfur Composite Electrodes 5.2. Sulfur−Porous Carbon Composite Materials 5.2.1. Carbon Materials 5.2.2. Sulfur−Carbon Composites 5.2.3. Synthesis Methods 5.3. Sulfur−Graphene Composite Materials 5.3.1. Graphene and Graphene Oxide 5.3.2. Sulfur−Graphene/Graphene Oxide Composites 5.4. Binder-Free Sulfur−Carbon Composite Electrodes 5.5. Sulfur−Polymer Composite Materials 5.5.1. Sulfur−Polyacrylonitrile Composites 5.5.2. Sulfur−Polypyrrole Composites 5.5.3. Sulfur−Polyaniline Composites 5.5.4. Other Sulfur-Conductive Polymer Composites 5.6. Sulfur−Metal Oxide/Chalcogenide Composite Materials 5.6.1. Metal Oxide Additives/Composites 5.6.2. Metal Oxide Coatings 5.6.3. Intercalation Compounds/Chalcogenide Composites 6. Lithium Sulfide Cathodes 6.1. Activation of Microsized Li2S Particles 6.2. Li2S−Carbon Composites © 2014 American Chemical Society

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Special Issue: 2014 Batteries Received: February 3, 2014 Published: July 15, 2014 11751

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Chemical Reviews Corresponding Author Notes Biographies Acknowledgments References

Review

insertion-oxide cathodes. However, the conversion reaction is accompanied by a volume change of about 80%. In addition, sulfur and lithium sulfide are both insulators, which necessitates the incorporation of conductive additives (e.g., carbon) into the electrodes. Moreover, the intermediate lithium polysulfides formed during the conversion reaction are soluble in the liquid electrolytes currently used. All of these create tremendous challenges in developing reversible, stable, and efficient sulfur cathodes. However, significant progress has been made with rechargeable Li−S batteries in recent years by developing novel nanocomposites, efficient electrolytes, and novel cell configurations. Also, an in-depth fundamental understanding of the Li−S cells has been gained by employing advanced characterization methodologies. Although the Li−S batteries are believed to be one of the most promising next-generation high-energydensity rechargeable battery chemistries, the cycle life and efficiency need to be improved for them to be employed in practical applications. In this review, we first present the principles, history, and technical challenges of Li−S batteries and then focus on sulfur and lithium sulfide composite cathode materials. Significant attention is paid to the characterization techniques used and the mechanistic understanding gained. We then cover the electrolyte, separators, anodes, and binders used in Li−S batteries. Finally, we focus on novel cell configurations including carbon interlayers between the sulfur cathode and the separator. Overall, in this review we describe the advances in Li−S batteries and provide guidance for future development in the field.

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1. INTRODUCTION The demand for energy increases steadily with time due to population and economic growth and advances in lifestyle. As energy usage increases, concerns about environmental pollution associated with the use of fossil fuel are becoming serious. To mitigate these issues and reduce our dependence on fossil fuel, alternative energy technologies based on renewable sources need to be developed and adopted, e.g., solar and wind energies. However, solar and wind energies are intermittent; therefore, it is critical for efficient and economical storage of electricity produced by renewable sources to be competitive.1 Rechargeable batteries are one of the most viable options for electrical energy storage (EES). Rechargeable battery systems, such as lead−acid, nickel−cadmium, nickel metal hydride, and lithium ion batteries, have serviced humankind for over a century with their use in a variety of applications, e.g., portable electronic devices and automobiles. As the functionalities of the portable electronics become more sophisticated and the demand for electric vehicles and storage of electricity from renewable sources increases, advanced rechargeable batteries need to be developed. Cost, energy, power, cycle life, safety, and environmental compatibility are some of the most important parameters to be considered. Li ion batteries have become prominent over the past two decades, particularly for portable electronics, as they offer much higher energy density than other rechargeable systems. The current Li ion technology is based on insertion-compound anode and cathode materials, which limit their charge-storage capacity and energy density. A further increase in energy density needs to be achieved through an increase in the chargestorage capacity of the anode and cathode materials or an increase in the cell voltage or both. However, the limited electrochemical-stability window of the currently available liquid electrolytes makes it difficult to increase the cathode operating voltage beyond ∼4.3 V.2 Also, the capacities of the insertion-oxide cathodes have reached a limit of ∼250 mA h g−1. On the other hand, the capacity of the graphite anode is also limited to ∼370 mA h g−1. Therefore, alternative cathode and anode materials that offer higher capacities need to be developed. To overcome the charge-storage limitations of insertion-compound electrodes, materials that undergo conversion reactions while accommodating more ions and electrons are becoming a promising option, but are highly challenging. With this perspective, for example, Li−O2 and Li− S batteries with high energy are being intensively pursued.3,4 However, the huge volume change accompanying the electrochemical reactions and maintenance of an electrochemically favorable electrode structure during long-term cycling are some of the challenges. Sulfur, one of the most abundant elements in earth’s crust, offers a high theoretical capacity of 1672 mA h g−1, which is an order of magnitude higher than those of the transition-metal oxide cathodes. The high capacity is based on the conversion reaction of sulfur to form lithium sulfide (Li2S) by reversibly incorporating two electrons per sulfur atom compared to one or less than one electron per transition-metal ion in the

2. PRINCIPLES OF LITHIUM−SULFUR BATTERIES A Li−S cell is an electrochemical storage device through which electrical energy can be stored in sulfur electrodes. A schematic of the components in a single Li−S cell and its operation (charge and discharge) is shown in Figure 1. A conventional

Figure 1. Schematic diagram of a Li−S cell with its charge/discharge operations.

Li−S cell consists of a lithium metal anode, an organic electrolyte, and a sulfur composite cathode. Because sulfur is in the charged state, the cell operation starts with discharge. During the discharge reaction, lithium metal is oxidized at the negative electrode to produce lithium ions and electrons. The lithium ions produced move to the positive electrode through the electrolyte internally while the electrons travel to the positive electrode through the external electrical circuit, and thereby an electrical current is generated. Sulfur is reduced to produce lithium sulfide by accepting the lithium ions and electrons at the positive electrode. The reactions occurring during discharge are given below, and the backward reactions will occur during charge. 11752

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solvents propylene carbonate, γ-butyrolactone, dimethylformamide, and dimethyl sulfoxide. The subsequent developments were focused mostly on primary Li−S cells.8 An important development during this period was the identification of the electrolyte solvent for Li−S cells from saturated aliphatic amines,6 to propylene carbonate,7 to a mixture of tetrahydrofuran (THF)−toluene,8c and finally to mixtures of dioxolane-based electrolytes, which are being widely used nowadays.8d,e The liquid electrolyte has a significant impact on the cell performance since the intermediate charged/discharged products lithium polysulfides are soluble in it. For example, more than 95% sulfur utilization was achieved at room temperature with LiClO4 dissolved in a mixture of THF− toluene by Yamin et al.,8c but with a low discharge current density (10 μA cm−2). In contrast, dioxolane-rich electrolytes have an order of magnitude higher ionic conductivity than the THF−toluene electrolyte.8d However, sulfur utilization in dioxolane-rich electrolytes was only 50% even at a very low discharge rate in primary Li−S cells due to the partial discharge product Li2S2. Further work by Peled et al.8e demonstrated rechargeable Li−S batteries with dioxolane-rich electrolytes. A notable work by Rauh and Abraham et al.8b demonstrated a cell with a configuration of Li/∼5 M S in Li2Sn, THF, 1 M LiAsF6/ C. Almost 100% of the theoretical capacity was achieved at 50 °C and 1.0 mA cm−2, and 75% cathode utilization was achieved at ∼4 mA cm−2. Cycling efficiencies started at about 95% at 25 °C but tended to deteriorate after 10−20 cycles. Starting in 2000, more efforts were focused on rechargeable Li−S batteries as evidenced by the exponentially growing number of publications. Efforts have been focused on developing more conductive sulfur−carbon composites and solid electrolytes, fabricating efficient electrodes and cell configurations, and understanding the degradation mechanism and limiting factors for long cycle life Li−S batteries. Mikhaylik9 illustrated the status of the Li−S battery technology in his Electrochemical Society (ECS) meeting presentation in 2010, as shown in the spider chart in Figure 3. In comparison to

negative electrode: anodic reaction (oxidation, loss of electrons) 2Li → 2Li+ + 2e−

(1)

positive electrode: cathodic reaction (reduction, gaining electrons) S + 2Li+ + 2e− → Li 2S

(2)

overall cell reaction (discharge) 2Li + S → Li 2S

(3)

The theoretical capacities of lithium and sulfur are 3.861 and 1.672 A h g−1, respectively, which leads to a theoretical cell capacity of 1.167 A h g−1 for the Li−S cell. The discharge reaction has an average cell voltage of 2.15 V. Hence, the theoretical gravimetric energy density for a Li−S cell is 2.51 W h g−1.3 Sulfur atoms show a strong tendency to catenation, forming long homoatomic chains or homocyclic rings of various sizes.4a Octasulfur (cyclo-S8), crystallizing at 25 °C as orthorhombic αS8, is the most stable allotrope at room temperature. During an ideal discharge process, cyclo-S8 is reduced and the ring opens, resulting in the formation of high-order lithium polysulfides Li2Sx (6 < x ≤ 8). As the discharge continues, lower order lithium polysulfides Li2Sx (2 < x ≤ 6) are formed with the incorporation of additional lithium. There are two discharge plateaus at 2.3 and 2.1 V with ether-based liquid electrolytes, which represent the conversions of S8 to Li2S4 and Li2S4 to Li2S, respectively. At the end of discharge, Li2S is formed, as shown in Figure 2.4b During the following charge, Li2S is converted to

Figure 2. Voltage profiles of a Li−S cell. Reprinted with permission from ref 4b. Copyright 2012 Macmillan Publishers Ltd.

S8 via the formation of the intermediate lithium polysulfides, resulting in a reversible cycle.5 However, the two charge voltage plateaus are normally overlapped with each other.

3. HISTORICAL DEVELOPMENT Sulfur was used as a positive electrode material in electric dry cells and storage batteries by Herbet and Ulam in 1962.6 The electrolyte was identified to be alkaline perchlorate, iodide, bromide, or chlorate dissolved in a primary, secondary, or tertiary saturated aliphatic amine, preferably selected from among propylic, butylic, and amylic amines. Later on, Rao7 specifically patented high-energy-density metal−sulfur batteries with organic electrolytes and presented the theoretical energy densities of metal−sulfur cells in 1966. The open-circuit voltage of the cell was observed to be between 2.35 and 2.5 V, which was slightly lower than the calculated 2.52 V. The electrolytes contemplated in the patent consist of one or more of the

Figure 3. Year 2009 status of the Li−S technology. Reprinted with permission from ref 9. Copyright 2010 Y. V. Mikhaylik.

the specifications of Li−S batteries set by the U.S. Advanced Battery Consortium (USABC), Li−S batteries meet the requirements of specific energy, specific power, power density, and low-temperature performance. However, the energy density, rate capability, and recharge time are barely met, and the cycle life and high-temperature performance of current Li− S batteries are far below the minimum requirements, which limit the Li−S technology from wide commercial applications. 11753

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have low Coulombic efficiency. The high-order polysulfides migrate toward the anode, react with the lithium metal, reduce to low-order polysulfides, migrate back to the cathode, form high-order polysulfides again, and so on.11 Severe shuttle behavior can lead to an infinite recharge and poor charge efficiency. The polysulfide shuttle mechanism has been particularly investigated in recent years. The shuttle equation below was first derived by Mikhaylik and Akridge5a to evaluate the degree of shuttle behavior, which includes the impact from the charge current and polysulfide diffusivity. The charge shuttle factor ( f C) is derived as

Significant improvements have been made since then, including a remarkable increase in the cycle life of Li−S batteries in some work presented in this review.

4. TECHNICAL CHALLENGES There are many challenges facing the Li−S technology, either with the materials or with the system. First of all, the high resistance of sulfur (∼10−30 S cm−1) and the intermediate products (i.e., lithium polysulfides Li2Sx) formed during cycling along with their structural and morphological changes are formidable challenges, resulting in unstable electrochemical contact within sulfur electrodes. In addition, the dissolved polysulfides shuttle between the anode and cathode during cycling, reacting with both the lithium metal anode and the sulfur cathode.5a Moreover, the electrochemical conversion of sulfur to lithium sulfide involves structural and morphological changes as well as repetitive dissolution and deposition of reactive species, which tend to passivate both the electrodes, leading to a significant increase in impedance. These issues result in a low utilization of the active material, poor cycle life, and low system efficiency. Thus, the conventional Li−S battery electrode shown in Figure 1 cannot meet all the requirements of Li−S batteries for practical applications. As a result, the following issues need to be overcome to make Li−S batteries feasible: The shuttle mechanism resulting from the dissolution of the active cathode material has to be eliminated to avoid low Coulombic efficiency and self-discharge behavior. Also, the amount of carbon additives needs to be reduced as much as possible since too much carbon could reduce the volumetric energy density of Li−S cells. Full utilization of the sulfur cathode to achieve the full capacity of sulfur (1.672 A h g−1) is another challenge because the lower plateau redox reaction is quite sluggish compared to the upper plateau redox reaction due to slow solid-state diffusion when low-order polysulfides are reduced to Li2S.113c In addition, some persistent issues such as dendrite formation and surface passivation remain to be solved with the lithium metal anode. Other alternative anode materials might be an option to replace lithium metal due to safety concerns.

fC =

k Squp[Stotal ] IC

(4)

where IC, kS, qup, and [Stotal] represent, respectively, the charge current, shuttle constant (heterogeneous reaction constant), specific capacity of sulfur contributed by the upper plateau, and total sulfur concentration. The value of qup is fixed and equal to 419 mA h g−1, which is a quarter of the full theoretical capacity of Li−S batteries (0.5 electron per sulfur atom).5a,12 The simulated charge profiles can be seen in Figure 5. There is no shuttle effect when f C approaches zero, which means the

Figure 5. Simulated charge plateaus with different charge shuttle factors f C. Reprinted with permission from ref 5a. Copyright 2010 The Electrochemical Society.

4.1. Shuttle Mechanism

system has an infinitely large current density, an infinitely small shuttle constant, or an infinitely small sulfur concentration. When f C > 1, the charge curves become horizontal without a sharp voltage rise, offering overcharge protection. However, prolonged shuttle reactions might cause severe corrosion of the lithium anode and result in a short cycle life.5a,11,13 In addition, the extension of the leveled upper plateau during charging is not brought into play for the polysulfide oxidation, but for the extra energy consumed by the polysulfide migration, leading to poor charge efficiency. Equation 4 assumes that the reaction rate is proportional to the active material concentration, so the high-order polysulfide concentration in the upper plateau can be presented by eq 5 in consideration of the impact from the shuttle constant and current density:

The intermediate redox species formed, i.e., the high-order lithium polysulfides Li2Sx (6 < x ≤ 8), are highly soluble in the liquid electrolyte with glyme-based and dioxolane solvents.5a,10 These various polysulfide anions can freely migrate between the cathode and anode, leading to a so-called polysulfide shuttle phenomenon, as illustrated in Figure 4. Fundamentally, the shuttle behavior is the main reason causing Li−S batteries to

I d[SH] = C − k S[SH] dt qup

(5)

where [SH] represents the concentration of high-order polysulfides. The equation can be simplified by assuming a slow charge reaction (kStC ≫ 1) and considering that the upper plateau capacity (Qup) depends on the concentration of high-

Figure 4. Illustration of the shuttle mechanism occurring in a Li−S cell. 11754

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d ln Q up

order polysulfides and the specific capacity. The reciprocal shuttle constant can be given by the differential equation as dQ up dIC

1 = kS

The shuttle constant kS can be obtained when small currents are applied by measuring the charge capacity from the upper plateau. As a result, the lower the constant the battery system possesses, the slighter the effect.

dt R

= −kS

(7)

This equation demonstrates that the self-discharge behavior still closely correlates with the shuttle constant, which means that both the shuttle effect and the self-discharge effect originate from the dissolution of the active material in the Li−S battery system. Consequently, design of electrode architectures and novel cell configurations are needed to avoid these unwanted side reactions. A lot of effort has been devoted to reducing the shuttle effect and improving the retention of active material within the sulfur electrode. Some approaches are focused on developing sulfur composites with favorable nanostructures and properties to improve the discharge capacity, cyclability, and Coulombic efficiency.3,15 Other approaches being pursued include novel cell configurations with trapping interlayers, Li/dissolved polysulfide cells, and efficient electrolytes.16 The following sections give some representative examples of these approaches.

(6)

charge output shuttle shuttle

4.2. Self-Discharge

Low self-discharge is another criterion to judge the practicality of energy-storage devices. Unfortunately, Li−S batteries have strong self-discharge behavior, like nickel−cadmium or traditional nickel−metal hydride batteries. Since the dissolution of polysulfides is inevitable in the Li−S cell while employing some nonaqueous electrolytes, sulfur/high-order polysulfides after charging continue to slowly dissolve in the electrolyte even in the resting state. When batteries are resting, the self-discharge occurs because the active material gradually dissolves and migrates to the anode due to the concentration gradient and then reacts with lithium metal followed by conversion into high-order polysulfides, resulting in a decrease in the opencircuit voltage and discharge capacity.5a,14 Figure 6 exhibits the initial discharge voltage plateaus of pristine sulfur cathodes with different resting times. With a

5. SULFUR CATHODE 5.1. Conventional Sulfur Composite Electrodes

After the prototype Li−S cell was designed about 30 years ago, the insulating active material and the polysulfide shuttle effect became major chronic technical problems impeding the Li−S technology.8b,c The solution to the problem of high cathode resistance is straightforward: an appropriate electrical conductor (conductive carbon/polymer additives) needs to be added and well-dispersed in the active material to ensure smooth electron transport between the electrical conductor and the active material. About 10 years ago, conductive carbons and conductive polymers were added to sulfur cathodes to form (i) S−C composites (see section 5.2) and (ii) sulfur-conductive polymer composites (see section 5.5). The conductive carbon was added to S−C composites to enhance the conductivity and active material utilization of the sulfur cathode. For example, the carbon black that is very frequently used for preparing the active material mixture paste has high electrical conductivity to decrease the cathode resistance.17 The active carbon has a high surface area and abundant micropores for absorbing the active material, which is essential for limiting polysulfide dissolution.18 On the other hand, the sulfur-conductive polymer composites started with the application of polyacrylonitrile (PAN) and showed a high initial discharge capacity of 850 mA h g−1.19a As a result, various types of conductive/porous carbon materials and conductive polymers have been incorporated into sulfur during the past decade.17−19 Among these conductive additives, porous/conductive carbon has received considerable attention due to its porous structure and higher electrical conductivity compared to that of polymers, which are essential criteria for simultaneously accommodating the active material and enhancing the cathode conductivity. The cathode conductivity is increased by two morphological routes: (i) formation of a conductive carbon network, e.g., carbon nanoparticle clusters, and (ii) intimate connection between the conductive framework and the insulating sulfur. Moreover, the engineered porous carbon, mesoporous carbon, and macroporous network not only promote retention of sulfur but also enhance the charge and electrolyte transport in the composites. So far, various carbon materials and synthesis routes that are dedicated to optimizing

Figure 6. Typical discharge plateaus of pristine sulfur cathodes with different resting times.

short resting time (30 min), the cell displays a typical twoplateau discharge profile. With a longer resting time of over a week, the upper discharge plateau disappears and only the lower discharge plateau can be observed. The open-circuit voltage also decreases with increasing resting time. This demonstrates that, with a conventional cell configuration, sulfur dissolves in the electrolyte during cell resting, resulting in a loss of the upper discharge plateau. Additionally, if the dissolved polysulfide species diffuse out of the cathode region, they will not be reutilizable, leading to permanent capacity fading. The level of self-discharge behavior can be determined by the shuttle equation (eq 5) as well and vice versa.5a Thus, Mikhaylik et al. devised a mathematical way of expressing the relationship between the upper plateau capacity, resting time, and shuttle constant as 11755

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Figure 7. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S− microporous carbon composite. Reprinted with permission from ref 22. Copyright 2010 The Royal Society of Chemistry

composites influence the cycling performance of the resultant composite cathodes. Therefore, we present below several carbon materials that are frequently used in S−PC composites and their electrochemical performances. We also summarize the synthesis methods and manufacturing parameters as well as the sulfur content in the cathode in Table S1 (Supporting Information). 5.2.1. Carbon Materials. The categories of porous carbons are classified by their pore size and major morphology. According to the pore size (D), the porous structure of these carbons has three major types, and each type of porous carbon possesses unique morphological advantages: (i) the micropore (D < 2 nm) has been demonstrated as the ideal container for accommodating and immobilizing the active material;22 (ii) the mesopore (2 nm < D < 50 nm) can enhance sulfur encapsulation as the designed pore size is small or can improve lithium ion and electrolyte transport as well as raise the tolerance toward high sulfur loading as the pore size is large;20,23 (iii) the macropore (D > 50 nm) is usually derived from an interwoven network of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) and is able to ensure excellent electrolyte immersion or suppress polysulfide migration due to its high electrolyte absorbability.24 In addition, coalescing CNT or CNF networks can further be decorated with various micro/ meso/macropores to from composited hierarchical porous structures via delicate synthesis methods, which may lead to improved cycling performance.25 According to the above categories, the frequently used carbon materials in S−PC composites include microporous carbon,22,26 mesoporous carbon,23a,27 hierarchical porous carbon,20,23b,28 carbon black,29 hollow carbon spheres,30 CNTs,24,25c,31 CNFs,25a,b,32 and graphene (see section 5.3 for a detailed discussion).33 These porous carbons possess at least one of the following specific functions for improving the cycling performance: (i) containing/immobilizing the active material, (ii) constraining/trapping the dissolved polysulfides, (iii) accelerating the charge transport, and (iv) absorbing/ channeling the liquid electrolyte. 5.2.2. Sulfur−Carbon Composites. Microporous Carbon. Figure 7 shows the microporous carbon and the sulfur− microporous carbon composite. The effect of the micropores on the electrochemical stability was evidenced by the microporous carbon sphere encapsulating the melting sulfur via a two-step heat treatment process.22 Zhang et al.22 designed the microporous carbon spheres to constrain the electrochemical active material and reaction inside their narrow micropores by their strong adsorption. As a result, sulfur− microporous carbon cathodes showed a long cycle life for 500 cycles with the remaining capacity around 650 mA h g−1.

the composite configuration have provided significant improvements in the cycling performance of Li−S cells. 5.2. Sulfur−Porous Carbon Composite Materials

The first sulfur−porous carbon (PC) composites were the sulfur−carbon nanocomposites presented by Wang et al.18 The applied active carbon serves as an electrical conductor for increasing the cathode conductivity and also as a storage container for the active material in its micropores. This concept made sulfur cathodes exhibit better cyclability compared to the pure sulfur cathode. After several years of efforts on S−PC composites, a sulfur−mesoporous carbon composite that displayed a high discharge capacity and stable cyclability was reported by Ji et al.20 in 2009. A high specific capacity of 1320 mA h g−1, approaching 80% sulfur utilization, and stable cyclability were accomplished by impregnating sulfur into a highly ordered nanosized mesoporous carbon (MPC). The hierarchical MPC not only satisfies the criteria listed in section 5.1 but also provides pathways for fast Li ion transport within the mesoporous carbon substrates. The intimate contact among the charges, active material, and liquid electrolyte with the conductive substrate results in high sulfur utilization. Furthermore, the ordered mesoporous space confines the active material within the composites and suppresses the free migration of polysulfide species, leading to highly reversible cyclability. Following this concept, numerous and various porous carbon substrates have been developed within the past 4 years and defined as quick cures for the two chronic problems described above. However, quick cures usually cause side effects as they address the major issues. Zhang et al.10 pointed out the side effects associated with the S−C composite cathode: several excellent cycling performances published are due to a very low S to C ratio in the cathode. The reasonable sulfur content (65%) and sulfur loading (2 mg cm−2) suggested for the sulfur cathode should be the fundamental tenet for Li−S cell development.10,16b,21 On the other hand, several engineering-oriented studies indicate that the initial morphology of sulfur may not enhance the capacity.10,12,21 The reason is that, during cell discharge, the originally solid sulfur is converted into soluble polysulfides and then the polysulfide species freely migrate to other preferred locations during subsequent discharge processes. The morphology of the rearranged sulfur may be influenced by the surrounding conductive carbon additive and not decided by its original morphology. As a result, it is reasonable to expect that the morphology of the active material in the S−PC composites is dominated by the carbon host and the connection between them. Therefore, it is instructive to look at how synthesis routes and manufacturing parameters of S−PC 11756

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Figure 8. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S− mesoporous carbon composite. Reprinted with permission from ref 23a. Copyright 2011 The Royal Society of Chemistry.

Figure 9. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S− hierarchical porous carbon composite. Reprinted with permission from ref 20. Copyright 2009 Macmillan Publishers Ltd.

MPC composites with an initial capacity as high as 1390 mA h g−1 and a capacity of 840 mA h g−1 after 100 cycles. Engineered Hierarchical Porous Carbon. Engineered hierarchical porous carbon includes bimodal micro/mesoporous carbon28a,c,d or a micro/mesopore-decorated porous carbon framework.20,28b,e,f The first significant achievement was contributed by the CMK-3 ordered MPC (Figure 9) reported by Ji et al.20 The high sulfur utilization in this system results from the complete redox reaction, which is stabilized within the nanosized electrochemical reaction chamber of the MPCs. Therefore, the MPC works not only as the electronic conduit but also as the active material stockroom. This success encourages the development of numerous hierarchical porous carbon materials with micro/meso/macropores for improving cycling performance by different physical/ chemical properties.20,23b,28 In the hierarchical porous carbon, the micropore is designed for encapsulating/immobilizing the active material, which improves the electrochemical activity and also suppresses the loss of active material. The macroporous network serves as the electrolyte pathway to ensure good electrolyte immersion. The mesopore usually serves as an essential supporter for reinforcing the physical properties of the micropore or the macroporous network. Generally, a mesopore with a smaller pore size can work with the micropore to accommodate the active material and trap the dissolved polysulfides, suppressing the severe capacity fade.20,23b,28c−e On the other hand, a mesopore with a larger pore size can facilitate charge transport and cooperate with the macroporous network to ensure electrolyte penetration, resulting in high utilization of sulfur.28a−c,f As a result, the major accomplishment of the hierarchical porous carbon is the high cooperation possibility among the “functional micro/meso/macroporous carbons”. Carbon Black. Conventionally, carbon black is only used as an “extra” conductive additive in the cathode. However, the low

Recently, microporous carbon has facilitated the use of smaller sulfur molecules as a new starting active material.26 The metastable S2−4 active material that is synthesized within the confined narrow space of the microporous carbon avoids the transition of the highly soluble polysulfides and, therefore, shows only the lower discharge plateau at 1.9 V and a single reduction peak in the cyclic voltammograms. This roundabout discharge/charge process limits the loss of the active material and thereby accomplishes a high reversible capacity of 1149 mA h g−1 after 200 cycles.26b Yin et al.26d have investigated the effect of chainlike sulfur molecules on cycling performance. The chainlike sulfur molecules have a strong interaction with the conductive carbon substrate and are confined within the micropores, avoiding the formation of unfavorable soluble polysulfides. Thus, an effective cooperation of the metastable S2−4 active material and the microporous carbon may overcome the severe polysulfide diffusion problem in conventional lithium−sulfur cells. Mesoporous Carbon. The mesoporous carbon in Figure 8 has been identified as an ordered sulfur encapsulation substrate.23a,27 After systematically analyzing a series of mesoporous carbon materials with tunable pore sizes and pore volumes, Li et al.23a indicated that mesoporous carbon with a larger pore size can tolerate a higher maximum sulfur loading and still display good cell performance under full sulfurfilling conditions, which stimulates hopes of improving the currently limited sulfur content/loading in S−PC composite cathodes. On the other hand, under partial sulfur-filling conditions and surface modification, the encapsulated sulfur has an optimized electrical contact with the mesoporous substrate, which can limit the dissolution/diffusion of polysulfides and ensure a steady supply of lithium ions.23a For example, an MPC that possesses a pore size of 22 nm and has 50% sulfur filling in the mesoporous spaces (the theoretical maximum sulfur loading in a composite is 84.7%) provides S− 11757

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Figure 10. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S− carbon black composite. Reprinted with permission from ref 29c. Copyright 2012 Elsevier.

Figure 11. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S− hollow carbon sphere composite. Reprinted with permission from ref 30a. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 12. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S−CNT composite. Reprinted with permission from ref 24b. Copyright 2012 The Royal Society of Chemistry.

nm.26e,30a,b Jayaprakash et al.30a reported porous hollow carbon−sulfur composites with excellent cycling performance: high capacity retention of 91% after 100 cycles. In the composite, the large interior void space of the porous hollow carbon allows the starting active sulfur and polysulfides to be stabilized within the conductive carbon sphere. Furthermore, the mesoporous shell ensures a stable supply of lithium ions and electrolyte. Hollow carbon spheres recently synthesized by a hydrothermal method have a small diameter of 80 nm.30c The small hollow carbon spheres may suppress the formation of inactive sulfur cores as filling sulfur inside the carbon spheres and enhance electron transport. Carbon Nanotubes. The CNTs function as an interwoven conductive network in the sulfur-based composites,24b,25c,31 as shown in Figure 12. In Han et al.’s work, a S−multiwall carbon nanotube (MWCNT) composite cathode that has 20 wt % MWCNTs, 20 wt % conductive carbon, 50 wt % sulfur, and 10 wt % poly(vinylidene difluoride) (PVdF) manifested a higher discharge capacity and longer cycle life than the regular sulfur cathode containing 0 wt % MWCNTs, 40 wt % conductive carbon, 50 wt % sulfur, and 10 wt % PVdF. This is because the MWCNTs have a high surface area for absorbing polysulfides and can form a three-dimensional (3D) conductive network

cost and high electrical conductivity of carbon black offer advantages for sulfur cathodes. After carbon black is coated on the sulfur particle or sulfur is distributed in the carbon black clusters, the sulfur−carbon black composites can improve the sulfur utilization, as shown in Figure 10.18,29 Our group reported a sulfur−carbon black composite cathode synthesized by a facile in situ sulfur deposition route to exhibit a high first discharge capacity.29c The improved sulfur utilization was attributed to the increased contact area between the conductive carbon black clusters and the insulating sulfur as well as to the good sulfur distribution in the carbon network structure. The composite showed good cyclability with a reversible discharge capacity of 777 mA h g−1 after 50 cycles. The improved cyclability is due to the carbon black matrix, which plays a protective role as an adsorbent agent to keep the soluble polysulfides within the electrode structure. This phenomenon can be observed with many other materials as reported in the literature, i.e., conductive carbon materials with intrinsic nanopores18 and porous carbon black clusters.29a−c Hollow Carbon Spheres. Figure 11 shows the hollow carbon spheres and their sulfur-based composites.30a The hollow carbon spheres, which are synthesized by a hard template route, possess a large interior void space with a diameter of 200−500 nm and a mesoporous shell with a thickness of 30−50 11758

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Figure 13. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S−CNF composite. Reprinted with permission from ref 25b. Copyright 2011 The Royal Society of Chemistry

Figure 14. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative S− graphene composite. Reprinted from ref 33c. Copyright 2011 American Chemical Society.

structure to enhance electron transport.31a Although the first study reported limited improvement in cell performance (initial discharge capacity ∼500 mA h g−1), which may result from the immature sulfur encapsulation technique, it showed that a simple mechanical mixing of the long/thin MWCNTs with the sulfur improves the capacity retention. After combination of the advanced sulfur encapsulation technique with CNTs, several cycle-stable sulfur cathodes have demonstrated that the CNT network is a promising host material. Guo et al.25c developed sulfur-impregnated disordered carbon nanotubes in a vacuum environment. After sulfur vapor is infused into the narrow pore channel in a carbon shell, the carbon substrate can avoid severe polysulfide dissolution. Our group reported a self-weaving sulfur−MWCNT composite cathode24b,c having a coalescing MWCNT network as the electrical and ionic pathway for facilitating a fast charge/ discharge process. Moreover, the sulfur−MWCNT composite has good electrolyte absorption ability, which confines the soluble polysulfides within the electrode and avoids the irreversible loss of the active materials during cycling. Carbon Nanofibers. The CNFs shown in Figure 13 have a nanosized shape similar to that of the nanotubes and, therefore, several morphological advantages similar to those of CNTs but without a hollow space in the middle.25a,b,32 The CNFs are also well-known for their good electrical conductivity and high structural strength. The high conductivity ensures the CNFs to be a suitable carbon conductor. Moreover, the CNF additives can form a network-like structure for suppressing the S/Li2S agglomeration that usually covers the sulfur cathode and produces inactivation areas.32 Rao et al.32b reported a nanocarbon/sulfur composite that has a CNF cluster as the 3D conductive network with nanoscale sulfur well dispersed in the CNF substrate, which enhances cathode conductivity and suppresses active material agglomeration. The advanced sulfur encapsulation techniques allow researchers to impregnate the porous CNFs or the hollow anodic aluminum oxide (AAO) template CNFs with the active

materials, achieving a uniform sulfur dispersion and localization within the porous structures or facilitating sulfur coating onto the inner surface of hollow CNFs for promoting rapid lithium ion transport.25a,b Ji et al.25b developed porous CNF−S composites, which exhibit a high initial discharge capacity and enhanced rate capability. The enhanced electrochemical performance is due to the high electrical conductivity of the porous CNFs that immobilize the active sulfur on their porous structure during cell operation. Zheng et al.25a reported hollow CNF-encapsulated sulfur cathodes based on the AAO template, which showed a high initial discharge capacity of 1560 mA h g−1 and a capacity approaching 730 mA h g−1 after 150 cycles. However, an obvious capacity fade (400−500 mA h g−1) appeared in the initial 50 cycles. Graphene. Graphene is a novel 2D carbon monolayer that can be extracted from graphite and is regarded as an ultralight, thin, and hard material with high conductivity. With these unique physical properties, graphene has risen abruptly in the materials field in recent years.34 The flexible characteristics of graphene render it suitable to be the sulfur carrier in Li−S batteries.33 The graphene in the composites coating or wrapping the sulfur can suppress the loss of active sulfur species due to dissolution during cycling, as shown in Figure 14. The detailed physical characteristics and cell performance of sulfur−graphene composites are discussed in section 5.3. 5.2.3. Synthesis Methods. After the importance of various carbon materials and their sulfur-based composites is understood, it is instructive to look at how various synthesis routes influence the cell performance since a suitable sulfur encapsulation will definitively improve the utilization of the active material and limit the loss of the active material.35 Moreover, the feasibility and the cost associated with various synthesis methods need to be sketched in detail for future developments in the Li−S battery area. According to the incorporation of sulfur and carbon hosts, the synthesis methods are classified into three major categories: (i) mechanical mixing, (ii) heat treatment, and (iii) solution-based synthesis. 11759

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single-step heat treatment at 155 °C. At this heat treatment temperature, the melting sulfur has the lowest viscosity and can easily diffuse into the porous carbon substrates. After the sulfur is imbibed into the interconnected porous spaces, the sulfur is well absorbed into the composite, which suggests that no severe sulfur covers the outside of the composite and the second step of heat treatment is unnecessary. The melting diffusion method optimizes the morphology of the S−PC composites well and has become the most frequently used heat treatment process.23a,25a,b,26,28a,c,e,f,30a−c,31b,33b Jayaprakash et al. utilized the sulfur vaporizing method to synthesize S−porous hollow carbon composites. The encapsulating sulfur accomplishes molecular contacts with the carbon host, providing an enhanced cycle stability with 91% capacity retention after 100 cycles. Solution-Based Synthesis: CS2, Na2S, and Water-Based Solutions. The solution-based synthesis route starts from preparing a sulfur-based solution and then mixing the carbon host with the dissolved sulfur to achieve a close connection. By going through a heterogeneous nucleation, the precipitated sulfur is strongly held by or absorbed into the carbon substrate. Currently, solution-based synthesis routes are supported by CS2, Na2S, and water-based solutions. Liang et al.23b utilized CS2 as the matrix solution to channel the dissolved sulfur into the hierarchical micro/mesopores of the activated MPC and to fill the micro/mesopores with the sulfur-containing solution, resulting in an excellent sulfur encapsulation within the porous carbon and thereby achieving a high initial discharge capacity of 1584 mA h g−1. The improved electrochemical performance indicates that the CS2based solution may be an attractive method for preparing the S−PC composites.23a,25c However, the application of the toxic CS2 may cancel the advantage from the environmentally friendly sulfur. The Na2S solution method starts with Na2S solution preparation by adding Na2S to distilled water and then follows the chemical deposition method in an aqueous solution: Sx2− + 2H+ → (x − 1)S↓ + H2S. The sulfur nucleates on the dispersed carbon substrates and accomplishes a strong incorporation into the porous carbon host. The molecular sulfur incorporated into the conductive carbon eliminates the high cathode resistance and provides a high active material utilization of above 70%.25b,32b However, the toxicity concern still exists with the byproduct H2S. In comparison with the two methods described above, a water-based solution synthesis route may be safer, more attractive, and more environmentally benign. The water-based solution involves the precipitation of elemental sulfur on the carbon nanoparticles29b or at the interspaces between carbon nanoparticle clusters29c in aqueous solution at room temperature. Both the core−shell carbon−sulfur composites and the carbon-wrapped sulfur network structure ensure that the resultant sulfur contacts the carbon black closely due to the heterogeneous nucleation of sulfur: (i) core−shell composites, SO2 + 2S2− + 4H+ → 3S↓ + 2H2O, and (ii) carbon-wrapped composites, S2O32− + 2H+ → S↓ + H2SO3. The chemical precipitation process leads to a high sulfur utilization of 74% for the core−shell composites and 67% for the carbon-wrapped sulfur composites. Most importantly, the water-based synthesis route avoids use of toxic raw materials and generation of toxic products. The heat treatment and solution-based syntheses have received considerable attention in recent years because they provide the best sulfur encapsulation conditions for S−PC

Mechanical Mixing Method: Magnetic Stirring and BallMilling. The mechanical mixing route that mixes materials by magnetic stirring is widely applied in almost all kinds of pure sulfur and sulfur-based cathode preparation.17,28b,29d,e,31a,36 The conventional pure sulfur cathode17 is prepared by direct mechanical mixing of elemental sulfur, conductive carbon, and a binder and may have poor contact between the insulating sulfur and the electrical conductor (the carbon black and the current collector). The nonconductive sulfur agglomerations may block the electron pathway, resulting in limited active material utilization. As a result, the initial discharge capacity of conventional cathodes may be as low as 300−500 mA h g−1.17,31a,36a,b Although the mechanical mixing process has several disadvantages and has turned from a major synthesis route into a minor process for preparing the active material mixture slurry, it is still a necessary process for cathode preparation. Most S−PC composites still need to be well mixed with extra conductive carbons to ensure their normal function and high performance. The typical mixture in the whole cathode may include S−PC composites, conductive carbons (5−20 wt %), and binders. On the other hand, some reports describe simply using mechanical mixing to prepare a pure sulfur cathode (without using S−PC composites) but achieve excellent cycling performance. In these cases, improvements may arise from the application of an adsorbent carbon agent (6 wt % in the cathode)28b or alternative porous current collectors.29d,e,36c The ball-milling method is a branch of the mechanical mixing route which uses attrition ball-milling. This process imparts high energy to the mixtures and facilitates better connection between sulfur and the conductive carbon host compared to magnetic stirring.23b,31e,32a Chen et al.31e synthesized S− MWCNT composites by ball-milling for 6 h. The synthesized composites form an interwoven/porous MWCNT structure and approach a high initial discharge capacity of 1580 mA h g−1. Heat Treatment Method: Two-Step Heat Treatment, Sulfur Melting Diffusion, and Sulfur Vaporization. The heat treatment method aims to melt or vaporize the solid sulfur and then impregnate carbon matrixes with the low-viscosity melted sulfur. The derived synthesis routes include the two-step heat treatment (sulfur melting and then vaporizing),18,22,24a,27,29a sulfur melting diffusion method,20,23a,25a,b,26,28a,c,e,f,30b,c,31b,33b and sulfur vaporizing method.30a,33c During heat treatment, the melting or vaporizing sulfur can diffuse into the nanosized porous substrates. These advanced processes effectively encapsulate the sulfur into the narrow porous spaces of the carbon host, resulting in a high initial discharge capacity and stable cyclability. As a result, the heat treatment route has dominated the preparation processes for S−PC composites. Wang et al.18 prepared S−C composites by the two-step heat treatment process. The first step of heat treatment at 200 °C is to melt the elemental sulfur and to allow the melting sulfur to diffuse into the porous carbon substrates. The second step is to vaporize and remove the sulfur that covers the outside surface of the S−C composites. The two-step heat treatment ensures a strong sulfur−carbon bonding and no obvious sulfur coverage on the surface of the composites. Following this concept, various two-step heat treatments have been derived to obtain higher active material utilization by modifying the following three parameters: heat treatment temperature, dwell time, and porous carbon substrates.22,24a,27,29a In 2009, the sulfur melting diffusion method for synthesizing S−MPC composites was reported by Ji et al.20 and involves a 11760

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electrode which exhibited a higher initial discharge capacity and lower electrochemical impedance than the pure sulfur electrode. However, severe capacity fading occurred in the first cycle with a low sulfur content of 17.6 wt % in the electrode. Improved cycling performance with a higher sulfur content was achieved by dissolving sulfur in an organic solvent initially, e.g., CS2, followed by melting to lower the viscosity of sulfur and improve the sulfur distribution.42 Through this method, Cao et al.42 synthesized a functionalized graphene sheet−sulfur (FGSS) nanocomposite with a sandwich-type architecture. The synthesized composite was further coated with a cationic exchange Nafion polymer film that mitigated the migration of polysulfide anions. Improved capacity retention as high as 84.3% over 100 cycles was obtained with a high sulfur content of 57 wt % in the electrode.42 Herein, it is worth mentioning that polymers were usually integrated into the sulfur−graphene composite to enhance the flexibility of the composite and assist the trapping of polysulfides. Furthermore, graphene was first decorated by carbon black nanoparticles and coated onto the surface of polyethylene glycol (PEG)-wrapped submicrometer sulfur particles. The PEG/graphene composite coating accommodated the volume expansion of the active material during discharge and prevented the polysulfide from diffusing out of the cathode, resulting in stable capacities of ∼600 mA h g−1 over more than 100 cycles.33c Similarly, polyacrylonitrile43 and polypyrrole44 have been incorporated into sulfur−graphene composites. On the other hand, to produce a sulfur−graphene composite cathode suitable for large-scale production and industrial application, solution-based one-pot reaction syntheses have been developed. As mentioned in section 5.2.3, the solution-based one-pot reaction synthesis is based on the redox reaction of sulfur-containing compounds and heterogeneous nucleation of sulfur on the surface of the conductive matrix.33d,45 Following this strategy, Evers et al.33d synthesized a graphene-wrapped sulfur composite with a high sulfur content of ∼87 wt % (∼78 wt % in the electrode). However, the reversible capacity dropped to 550 mA h g−1 after 50 cycles at a 0.2C rate, indicating that further improvement of this method is required. There seems to be a trade-off between the simple synthesis strategies and the superior performance of the graphene−sulfur composite, and this calls for innovation in materials synthesis. In addition, the interaction between sulfur and the functionalized graphene could be used to improve the performance of sulfur−graphene composite cathodes. Sulfur was anchored by a GO network through the C−S bond, resulting in enhanced cycling performance of Li−S batteries.33b The GO−S nanocomposite cathodes exhibited a superior reversible capacity of ∼1000 mA h g−1 and a rate capability of up to 2C in an ionic-liquid-based electrolyte.33b Nevertheless, the functionalities on the GO network could only anchor sulfur that is intimately in contact with the graphene substrate, resulting in dissolution of outer layer sulfur species.46 To address this problem, Song et al.46 developed this technology by protecting the GO−S nanocomposites with a cetyltrimethylammonium bromide (CTAB) coating layer and replacing the conventional PVdF binder with an elastomeric styrene− butadiene rubber (SBR)/carboxymethyl cellulose (CMC) binder. Cycling performance up to 1500 cycles with a low decay rate of 0.039% per cycle was demonstrated, indicating the feasibility of employing sulfur−graphene composite cathodes for practical Li−S batteries.

composite syntheses. On the other hand, mechanical mixing has mainly turned to serve as a support process: mixing the synthesized S−PC composites well with extra conductive carbon and binder for further improving the cathode conductivity. This synergy effect functions well and has made several significant improvements. However, there are some disadvantages: (i) misleading calculation of the sulfur content and (ii) low sulfur content/loading. First, the sulfur content value presented should pertain to the sulfur content in the whole cathode and not simply in the composite alone. For example, the real sulfur content is only 49 wt % in a cathode that is prepared by mixing 70 wt % S−C composite (S:C ratio in composite, 70:30), 15 wt % conductive carbon, and 15 wt % binder. Second, according to the statistical table included in the Supporting Information, several S−PC composite cathodes cannot reach a sulfur content of even 60 wt % and seem to exhibit excellent cell performance because of low sulfur loading in the cathode.10 5.3. Sulfur−Graphene Composite Materials

5.3.1. Graphene and Graphene Oxide. Graphene refers to a single-atom-thick two-dimensional (2D) carbon material with a honeycomb lattice. It could be the basic building block of 0D fullerenes, 1D nanotubes, and 3D graphite.34 There are principally three representative methods to produce “pristine graphene”: epitaxial growth method, micromechanical exfoliation method, and solvent-assisted exfoliation method.37 Epitaxial graphene is grown on epitaxially matched surfaces, and the method often involves chemical vapor deposition (CVD) processes.37 The micromechanical exfoliation method can be used to generate electrically isolated graphene of high quality suitable for fundamental studies, but large-area production has yet to be achieved.37 Moreover, exfoliation of graphite could be achieved by exposing graphite powder to organic solvents such as dimethylformamide (DMF) or Nmethyl-2-pyrrolidone (NMP) and applying high-intensity ultrasound, but the yield is low.37 In contrast to graphene, graphene oxide (GO) is a compound of carbon, oxygen, and hydrogen in variable ratios and is rich in epoxide, carbonyl, hydroxyl, phenol, and organosulfate functional groups, whose structure and properties depend on the particular synthesis method and degree of oxidation.38 In general, GO can be synthesized through the Brodie,39 Staudenmaier,40 and Hummers38a methods, all of which involve oxidation of graphite powder to various levels.37 Specifically, a combination of potassium chlorate (KClO3) and nitric acid (HNO3) is used in the Brodie and Staudenmaier methods, while the Hummers method involves oxidizing graphite by potassium permanganate (KMnO4) and sulfuric acid (H2SO4).37 The synthesized GO is hydrophilic and easily dispersed in water, making it suitable for the synthesis of sulfur−graphene composites. In general, graphene-based materials are less expensive to produce than some other carbon nanomaterials such as carbon nanotubes, which is advantageous when they are applied as raw materials for the synthesis of graphene-based composite materials.34 5.3.2. Sulfur−Graphene/Graphene Oxide Composites. With exceptional electronic conductivity and high mechanical strength and flexibility, graphene-based materials have been successfully integrated into the sulfur cathode to enhance its electrochemical reactivity and stability. To synthesize a sulfur− graphene composite, melting methods have been commonly used. Wang et al.41 reported a sulfur−graphene composite 11761

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Table 1. Overview of Some Selected Sulfur−Graphene Composite Electrodesa material

S content

method

electrolyte (binder)

sulfur−graphene nanosheets

22/18

heat treatment

LiTFSI in PEGDME 500 (PVdF)

Nafion-coated FGSS

72/57

heat treatment

LiTFSI in DOL + DME (PVdF)

sulfur−polyacrylonitrile−graphene composite sulfur−graphene oxide composite

47/38

heat treatment

LiPF6 in EC + DMC (PTFE)

66/46

heat treatment

LiTFSI in PYR14TFSI + PEGDME (−)

CTAB-coated sulfur−graphene oxide composite graphene/PEG-wrapped sulfur

80/56

heat treatment

70/56

graphene-enveloped sulfur

87/78

sulfur−hydroxylated graphene composite

50/40

solution-based method solution-based method solution-based method

LiTFSI in PYR14TFSI + DOL + DME (SBR/CMC) DOL + DME (−)

a

LiTFSI in DOL + TEGDME (−) LiCF3SO3 in DOL + DME (PVdF)

performance 600 mA h g−1 (40th cycle, 0.03C) 960 mA h g−1 (100th cycle, 0.1C) 1200 mA h g−1 (50th cycle, 0.1C) 954 mA h g−1 (50th cycle, 0.1C) 740 mA h g−1 (1500th cycle, 0.02C) 550 mA h g−1 (140th cycle, 0.5C) 550 mA h g−1 (50th cycle, 0.2C) 1021 mA h g−1 (100th cycle, 0.5C)

ref 41 42 43a 33b 46 33c 33d 33a

The sulfur content is presented as S concentration in the composite/S concentration in the electrode (wt %).

Figure 15. Schematic models, SEM observation (the SEM image of the carbon host is also shown), and cell performance of a representative binderfree sulfur−carbon composite electrode: (a) S−ACF composites and (b) S−VACNT composites. Part a reprinted with permission from ref 52. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Part b reprinted with permission from ref 53. Copyright 2011 The Royal Society of Chemistry.

Huang et al.47a used a vapor treatment approach to introduce amorphous sulfur homogeneously into mesoporous freestanding graphene papers and obtained a high discharge capacity of 1393 mA h g−1. Moreover, Jin et al.47b reported a flexible graphene sheet−sulfur paper electrode fabricated through an in situ redox reaction of sulfur-containing compounds and a vacuum-filtration procedure. The graphene framework functioned as both a conductive network and a supporting carrier for sulfur nanoparticles, leading to a high capacity retention of 83% and an energy density of 804 W h kg−1 after 100 cycles. The binder- and current-collector-free approach is environmentally friendly and beneficial for increasing the practical energy densities of Li−S batteries, which will be discussed in detail in the following section.

Furthermore, to examine the interaction between the sulfur species and a certain type of functional group on the graphene substrate, our group selectively decorated the pristine graphene with hydroxyl groups and achieved hydroxyl-group-induced heterogeneous nucleation of bonded sulfur at room temperature.33a Superior reversible capacities of 1021, 955, and 647 mA h g−1 were retained, respectively, at C/2, 1C, and 2C rates after 100 cycles.33a The stable high-rate performance was attributed to (i) the homogeneously distributed hydroxyl groups, which aided the formation of an amorphous sulfur layer attached to graphene and retention of polysulfides, (ii) small sulfur particles as a result of the enhanced interaction between hydroxyl groups and sulfur species, providing short pathways for ion and electron transport, and (iii) the flexible graphene-based substrate, which allowed complete electrolyte penetration and absorption of the strain from cycling-induced volume changes. Table 1 summarizes some selected major achievements in the sulfur−graphene composite cathodes. Recently, fibrous graphene has been fabricated into binderfree self-standing electrodes for direct use in Li−S batteries.47

5.4. Binder-Free Sulfur−Carbon Composite Electrodes

Binders, which are usually polymeric materials (e.g., poly(tetrafluoroethylene) (PTFE) or PVdF), are widely used in conventional Li ion and other batteries in which they only act as a binding additive to hold the active material and carbon additives together without contributing any capacity. They are 11762

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electrode with a PEG barrier to suppress the shuttle of polysulfides. The electrode with the PEG barrier showed a higher initial discharge capacity than that without the PEG barrier. In addition, the electrode showed high cycling stability with a degradation of 0.38% per cycle over 100 cycles without LiNO3 in the electrolyte. Our group developed a green process to fabricate binder-free sulfur−MWCNT electrodes (Figure 12).24b Sulfur was deposited onto MWCNTs by the water-based solution method (see section 5.2). The curly MWCNTs have a self-intertwining behavior, which helps in forming a free-standing composite electrode.57 The sulfur loading was up to 63 wt % in the electrode. The highly conductive MWCNTs improved the active material utilization at high rates. The absorption ability of the cathode framework localized the electrolyte and suppressed the migration of soluble polysulfides. The tortuous interspaces in the network of MWCNTs provide a stable accommodation of sulfur for rapid redox reactions and prevent sulfur/Li2S from forming extensive agglomerates during cycling, leading to good cyclability. Jin et al.58 developed a two-step process for fabricating free-standing S−CNT composite electrodes. A slightly oxidized CNT film was prepared first by vacuum filtration, and then sulfur was coated onto the surface of the CNTs by a heating process. The authors believe the oxidized CNTs contain oxygenic groups which can form strong covalent bonds between sulfur and CNTs.

effective and interrupt the electrochemical processes in these batteries less prominently. However, in Li−S batteries, binders could play a critical role in the cell performance. As the structure and morphology change upon cycling, binders cannot hold all the active materials, especially the soluble polysulfides. In addition, they could become “dead” sites for the electrochemical reactions between lithium and sulfur which can deteriorate the cell performance further. Therefore, functional or smart binders become more important in Li−S batteries than in Li ion batteries, which will be discussed in later sections. In this section, binder-free sulfur electrodes are discussed. The binder-free electrode structure has shown unique advantages in some electrodes in Li ion batteries, such as Co3O4/graphene films,48 TiO2 anodes,49 and silicon anodes.50 Some of these materials, like sulfur, also undergo significant compositional and structural changes upon cycling in Li ion batteries. Without a binder, electrodes should be able to maintain the supportive and integrated structures while ensuring efficient electron and ion access to the active materials. Carbon is the most effective additive in electrodes to provide electron pathways. Carbon can also be fabricated into various morphologies, e.g., spheres, particles, fibers, tubes, etc., which makes binder-free sulfur−carbon composite electrodes possible. In addition, N-methyl-2-pyrrolidone (NMP) is a solvent generally used in fabricating Li ion battery electrodes to disperse the polymer binder and active materials. However, NMP is harmful to human health and the environment.51 Without a binder, such solvents can be eliminated, facilitating “green” fabrication processes with these binder-free electrodes. Moreover, the active material loading and electrode conductivity can also be increased without a binder. Elazari et al.52 have utilized microporous activated carbon fibers as a binder-free substrate to impregnate sulfur, as shown in Figure 15a. The carbon fiber cloth provides sufficient mechanical strength and electron conduction pathways. In addition, sulfur can diffuse into the voids (≤2 nm) in the carbon fibers. The sulfur content, however, is relatively low (ca. 33 wt %), which is due to the large dense fibers used. An accessible capacity of 1250 mA h g−1 and a reversible discharge capacity higher than 800 mA h g−1 after 80 cycles were obtained. The authors suggested that the maximum accessible capacity is limited by the electrochemical kinetics. Dörfler et al.53 utilized vertically aligned carbon nanotubes (VACNTs) grown on a metal foil as a substrate to hold sulfur (Figure 15b) with a high sulfur content of up to 70 wt %. The VACNT was synthesized by a chemical vapor deposition process. The CNT films on the metal foil had a thickness of up to 100−200 μm and a density of about 0.06−0.13 g cm−3. The calculated free volume in the CNT films was about 94 vol %, which can hold high sulfur loadings. In addition, CNTs have high electronic conductivity and a high retention of the integrity of the conductive network because of the high aspect ratio. These favorable properties led to a discharge capacity of over 1300 mA h g−1 (800 mA h g−1 based on the mass of the electrode) with the binder-free sulfur−CNT electrode. A similar approach was developed by Hagen et al.,54 showing an even higher sulfur loading of 90 wt % with reasonable capacities and cyclability. Zhou et al.55 used a sulfate-containing AAO template to directly grow aligned S−CNTs without metal substrates. Sulfur contents of 23 and 50 wt % in S−CNTs have been demonstrated with discharge capacities of, respectively, 712 and 520 mA h g−1 over 100 cycles. Huang et al.56 modified the aligned S−CNT electrode by covering one end of the

5.5. Sulfur−Polymer Composite Materials

Polymers are playing a more significant role than binders in Li− S batteries, especially functional polymers with favorable properties such as conductive polymers (e.g., polypyrrole, polyaniline, poly(3,4-(ethylenedioxy)thiophene) (PEDOT), etc.). Since polymers can be synthesized by different methods and sulfur can be synthesized at low temperatures, a variety of sulfur−polymer hybrid materials have been developed with the target of reducing the dissolution and shuttle effect of polysulfides, improving electronic conductivity, and controlling the morphology of sulfur−polymer composite electrodes. A few strategies exploring polymers as a conductive matrix and/or barrier for blocking polysulfides are presented in the sections below. 5.5.1. Sulfur−Polyacrylonitrile Composites. The first sulfur−conductive polymer hybrid material was developed by Wang et al.19a They used elemental sulfur as a dehydrogenating reagent coheated with PAN at 280−300 °C. The −CN groups in PAN form heterocycles, while the main chain of the polymer becomes like that of polyacetylene, which is a conductive polymer. The composite is in the form of nanoparticles with an average diameter of 200 nm. Extra sulfur (53.4 wt %) was stabilized in the composite, which is electrochemically active. However, the discharge voltage profile of the composite consists of a voltage slope at ca. 2.1−1.8 V without the clear upper discharge voltage (2.3 V) and lower discharge voltage (2.1 V) plateaus that sulfur electrodes usually display. The composite material showed a capacity of 850 mA h g−1 in the first cycle, indicating almost all the sulfur was reduced to Li2S. The cell showed decreasing cyclability with a remaining capacity of 600 mA h g−1 after 50 cycles in cells with a geltype polymer electrolyte. Yu et al.59 further characterized chemically and physically such sulfur−PAN composite materials, which were prepared at different temperatures, and examined their cycling performance. What they found was that the carbon matrix and sulfur react with each other, forming 11763

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−C−S−S−C− bonds besides the dehydrogenation and cyclization of PAN when a mixture of sulfur and PAN are heated. The sample heated at 450 °C showed a stable capacity of ca. 480 mA h g−1 over 240 cycles. Fanous et al.60 advanced the understanding of the structure and fading mechanism of the sulfur−PAN composites in Li−S batteries. After removal of any remaining elemental sulfur in the composites via an extraction with toluene, the analysis results suggested that sulfur is exclusively covalently bound to carbon and not to nitrogen, as shown in Figure 16. An ether-based

500 nm, and the authors claimed that the sulfur particles were uniformly coated with the PPy nanoparticles. The discharge voltage profile showed the two typical plateaus of sulfur electrodes. The capacities of the S−PPy composite were higher than those of bare sulfur electrodes, which was attributed to the presence of PPy nanoparticles. Our group63 studied S−PPy composites that were synthesized by a two-step process. Sulfur particles were formed and mixed with presynthesized PPy nanoparticles by an oxidation method. It was found that the presence of PPy nanoparticles prohibits the agglomeration of sulfur during synthesis, resulting in small sulfur particles. In addition, the PPy nanoparticles in the composite electrode improved the electrochemical contact and rate capability as well as the porosity of the electrodes. Liang et al.64 and Sun et al.65 studied S−PPy composites with different morphologies, e.g., granular, tubular, and nanowire, in Li−S batteries. Our group has developed a strategy for fabricating core− shell-structured S−PPy composites with the sulfur particles as the core and a layer of PPy nanoparticles as the shell.66 Sulfur particles could be formed into bipyramidal or spherical shapes depending on the surfactant used by an in situ deposition method. A layer of PPy nanoparticles could be self-assembled onto the surface of sulfur particles due to the similar hydrophobic properties of sulfur and PPy. The coating of PPy on the sulfur particles can significantly improve the discharge capacities and cycling performance of sulfur while decreasing the charge transfer resistance. However, the sulfur particles are relatively large, resulting in low utilization of sulfur. To further improve the performance of S−PPy composites, a mixed ionic−electronic conductor (MIEC) consisting of conductive PPy and an ionic conducting polymer has been developed.67 The ionic conducting component in the MIEC can help transport lithium ions, retard polysulfide dissolution, and maintain a robust but porous electrode structure, improving the cycling performance. A variety of other approaches have also been developed to utilize PPy in Li−S batteries. For example, Qiu et al.68 synthesized poly(pyrrole-co-aniline) copolymer nanofibers with cetyltrimethylammonium chloride as a template, and they were used as a conductive additive in sulfur composite cathodes. Liang et al.69 synthesized a core−shell structure of a PPy− MWCNT composite by an in situ polymerization of Py in the presence of MWCNT. Sulfur was introduced by heating the sulfur powder and the PPy−MWCNT composite at 155 °C. The S−PPy−MWCNT composite with 25 wt % PPy showed a high discharge capacity of 726 mA h g−1 after 100 cycles. Zhang et al.70 prepared a S−PPy composite by a simple one-step ballmilling process without heat treatment. The electron microscopic analysis showed that a uniform sulfur coating was formed on the surface of PPy, and a first discharge capacity of 1320 mA h g−1 was obtained. Wang et al.71 prepared a ternary composite consisting of a PPy-coated S−CNT composite synthesized by an in situ polymerization method. They found that the PPy coating significantly improves the performance of the binary composites (S−CNT and S−PPy). 5.5.3. Sulfur−Polyaniline Composites. Polyaniline (PANI) is another conductive polymer that has been widely used in making S−PANI composites. PANI can be used as either a conductive coating of sulfur materials or a conductive matrix after pyrolization. Wu et al.72 used an in situ polymerization method to coat S−MWCNT composites with a layer of PANI. The sulfur-coated MWCNT composite was prepared by ball-milling and thermal treatment. The initial

Figure 16. Proposed chemical structure of a sulfur−PAN composite. Reprinted from ref 60. Copyright 2011 American Chemical Society.

electrolyte which can dissolve polysulfides resulted in a decrease in capacity upon cycling. With carbonate-based electrolytes, a stable cycle life was obtained. In addition, the polymer backbone, which consists of a conjugated π-system, affects the initial capacities obtained. Wang et al.61 found that elemental sulfur finely dispersed in the PAN-derived network is beneficial for high capacity and cycle stability, so the optimized synthesis temperature is 350 °C. At this temperature, the synthesized sulfur−PAN composite showed a reversible capacity of about 795 mA h g−1 and capacity retention of 98.1% after 50 cycles based on the second discharge capacity. In addition, the PAN-derived conjugated backbone contributes a lithium storage capacity of about 156 mA h g−1, which is equivalent to 0.29 Li atom per acrylonitrile-derived C3N unit. Yin et al.43a developed a PAN−graphene composite as a precursor to prepare S−PAN−graphene composites for highrate Li−S batteries. PAN−graphene composites were synthesized by an in situ polymerization method, and the S−PAN− graphene composite was prepared by pyrolyzing a mixture of sulfur powder and PAN−graphene composite at 300 °C. It was found that PAN nanoparticles less than 100 nm in size were anchored onto the surface of graphene and this structure was maintained after the S−PAN−graphene composite was formed. The composite with only 4 wt % graphene exhibited a capacity of ca. 1500 mA h g−1 in the first cycle and a capacity of ca. 800 mA h g−1 at a high rate of 6C. The good battery performance was attributed to the presence of graphene and the 3D structure of PAN−graphene, which acts as an electronically conductive matrix. 5.5.2. Sulfur−Polypyrrole Composites. Wang et al.62 synthesized sulfur−polypyrrole (PPy) composites first for Li−S batteries. The composite was synthesized by a chemical polymerization method. Sulfur powder was dispersed in a solution of sodium p-toluenesulfonate as a dopant, 4styrenesulfonic sodium salt as a surfactant, and pyrrole. FeCl3 was used as an oxidant to polymerize pyrrole to form the S− PPy composite. The PPy particles were in the range of 200− 11764

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discharge capacity was 1334.4 mA h g−1, and the capacity was 932.4 mA h g−1 after 80 cycles. Similarly, Li et al.73 coated a S− C composite with a nanolayer of PANI via a two-step process. The electrochemical performance of the composite was improved with high-rate performances, which was believed to be due to a synergistic effect from both the conductive carbon black in the matrix and the PANI coating. A discharge capacity of 636 mA h g−1 was obtained at a rate of 10C, and the discharge capacity retention was >60% over 200 cycles. Duan et al.74 introduced a layer-by-layer assembly technique to fabricate PANI-coated sulfur particles. The positively charged poly(allylamine hydrochloride) and negatively charged poly(styrenesulfonate sodium salt) were first alternatively adsorbed onto the surface of sulfur particles, and then a layer of PANI was formed on the outer shell of the polymer-coated sulfur by an in situ polymerization method. The composite was characterized, but no cell performance was presented. Recently, Zhou et al.75 reported novel S−PANI core−shell and yolk−shell nanocomposites, the yolk−shell nanocomposite being prepared through a heat treatment of the core−shell composite, as shown in Figure 17. The yolk−shell structure

Figure 18. Schematic illustration of the construction and discharge/ charge process of the S−PANI nanotubes. Reprinted with permission from ref 76. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

polysulfides, which leads to superior cycle stability and rate capability. A discharge capacity of 432 mA h g−1 was obtained at a 1C rate over 500 cycles. A follow-up study from the same group presented an in-depth characterization of the S−PANI composite.77 It confirmed that part of the sulfur was grafted onto PANI during heating and the PANI backbones were connected with disulfide bonds, forming a cross-linked structure. The performance of the S−PANI composite can be further improved by reducing the particle size. 5.5.4. Other Sulfur-Conductive Polymer Composites. Other conductive polymers, e.g., polythiophene (PTh) and PEDOT, have also been used to obtain sulfur−polymer composites. For example, Wu et al.78 coated sulfur particles with a layer of PTh by an in situ polymerization method. An initial discharge capacity of over 1100 mA h g−1 and a capacity of >800 mA h g−1 over 50 cycles were obtained. To reduce the sulfur particle size, Chen et al.79 used a membrane-assisted precipitation technique to prepare ultrafine sulfur nanoparticles with diameters in the range of 10−20 nm. The sulfur nanoparticles were coated with PEDOT to form a core− shell-structured S−PEDOT nanocomposite. An initial discharge capacity of 1117 mA h g−1 and a capacity of 930 mA h g−1 over 50 cycles were obtained. Yang et al.80 applied a PEDOT/poly(styrenesulfonate) (PSS)-based conductive polymer as a coating layer on a CMK-3 mesoporous carbon−sulfur composite, as shown in Figure 19. The polymer layer is not only rigid and stable but also ionically and electronically

Figure 17. (Top) Two-step synthesis route for the S−PANI nanocomposite with a yolk−shell structure, with the yellow sphere representing sulfur, the dark green shell representing PANI, and the black shell representing vulcanized PANI. (Bottom) SEM image showing the retained morphology of the S−PANI composite with the yolk−shell structure after long-term cycling. Reprinted from ref 75. Copyright 2013 American Chemical Society.

provides an internal void space to accommodate the volumetric expansion of sulfur during lithiation, thereby preserving the structural integrity and enhancing the cycling performance. Stabilized capacities of 765 mA h g−1 at a C/5 rate and 628 mA h g−1 at a C/2 rate were obtained. This study provides important insights and a novel methodology to develop sulfurconductive polymer nanocomposites with favorable structures and high performance. In addition to the core−shell structure with PANI, some other novel S−PANI composites have been developed. For example, Xiao et al.76 utilized self-assembled PANI nanotubes for sulfur encapsulation, as shown in Figure 18. The PANI nanotubes were treated at 280 °C with sulfur, which resulted in a partial reaction of sulfur with the polymer to form a 3D, crosslinked, structurally stable S−PANI composite in which the PANI backbones are interconnected with inter- and/or intrachain disulfide bonds. Such a structure provides an ideal confinement for the active materials, e.g., sulfur and

Figure 19. Schematic of a PEDOT/PSS-coated CMK-3−sulfur composite. Reprinted from ref 80. Copyright 2011 American Chemical Society. 11765

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for effectively addressing the polysulfide diffusion issue and thereby improving the cycle stability. Most importantly, these obvious improvements result from a small dosage of absorbing additives from 3.6 wt % (TiO2) to 15 wt % (Mg0.6Ni0.4O) in the whole cathode material. 5.6.2. Metal Oxide Coatings. Another attractive idea is S− metal oxide core−shell composites. Seh et al.88 designed a TiO2−sulfur yolk−shell composite that exhibited prolonged cyclability over 1000 cycles, showing that metal oxide shells might replace a portion of carbon to trap polysulfides, although the sulfur loading (0.4−0.6 mg cm−2) needs to be increased for practical applications. The idea of the yolk−shell structure is to avoid fracture of the TiO2 spheres during the volume expansion of the active material, which could lead to serious leakage of polysulfides. Extra void or pore space remaining in the cathode structure is desirable not only to retain the dissolved polysulfides but also to cushion the volume change during the subsequent charge/discharge processes. 5.6.3. Intercalation Compounds/Chalcogenide Composites. Different from the carbon additives and the metal oxide additives described above, intercalation compounds could participate in the discharge/charge reaction in a voltage range similar to that of sulfur, functioning as a secondary active material in the hybrid cathode. For example, TiS2 is an attractive secondary active material because it can discharge/ charge in the same voltage range (1.5−3.0 V) as sulfur.89 Moreover, TiS2 is a semiconductor with a band gap of about 1.0 eV and becomes conductive on insertion of lithium. The relatively conductive TiS2, in comparison with sulfur, may enhance sulfur utilization in hybrid cathode systems, leading to a better overall specific capacity.90 Various sulfur−TiS2 composites have been investigated by our group. According to our experimental data, TiS2 can cycle normally in the glymebased electrolyte just like that in the carbonate electrolytes, ensuring good compatibility with sulfur. Other possibilities (with the redox potential in parentheses) are VO2 (B) (2.0−3.0 V),91 TiO2 (B) (1.0−2.5 V),92 Li4Ti5O12 (1.2−2.0 V),93 and Li2C6O6 (1.5−2.5 V),94 which all have an operating voltage window that is compatible with that of sulfur.35

conductive, which can effectively block the dissolution of polysulfides and provide pathways for ions and electrons. The initial discharge capacity was ∼10% higher than that of the bare counterpart, and a capacity of >600 mA h g−1 was obtained over 150 cycles. Recently, Li et al.81 reported a novel polymer-encapsulated hollow sulfur nanosphere cathode. The polymer can be nonconductive poly(vinylpyrrolidone) (PVP) or conductive PEDOT, which can minimize the polysulfide dissolution. The empty space in the sulfur nanosphere allows sulfur to expand inward instead of outward upon lithiation. The nanosized sulfur spheres facilitate electron and ion transport. All of these features of the sulfur−polymer nanocomposites ensured excellent performance (1000 cycles at a C/2 rate with a capacity decay of 0.046% per cycle) and high rate capability with the PEDOT coating (reversible capacities of 849 and 610 mA h g−1 at, respectively, 2C and 4C rates). A follow-up study from the same group compared different conductive polymers, e.g., PPy, PANI, and PEDOT, as coating materials on sulfur nanospheres. It was found that the capability of these three polymers in improving the long-term cycle stability and highrate performance of the composite cathode decreased in the order PEDOT > PPy > PANI. 5.6. Sulfur−Metal Oxide/Chalcogenide Composite Materials

Not only conductive carbons/polymers but also other materials could be applied in the composite synthesis with sulfur. The alternative additive may serve as an absorbing agent for trapping the soluble polysulfides or may function as a supporting active material for generating extra capacity. To function as the absorbing agent, the alternative material must have a redox potential not overlapping with that of sulfur (1.5− 2.8 V vs Li+/Li0) to prevent unwanted electrochemical reactions and structural change during cell cycling. The density and the amount of the alternative materials should not be too large; otherwise, the energy density of the whole battery would deteriorate. To work as the secondary active material in a hybrid cathode, the alternative material must operate well under a voltage window similar to that of the primary active material, sulfur. The alternative materials should also have good compatibility toward the same electrolyte system as the primary sulfur active material. 5.6.1. Metal Oxide Additives/Composites. In 2001, Gorkovenko et al.82 patented the application of vanadium oxides, silicates, aluminum oxides, and transition-metal chalcogenides to sulfur cathodes for suppressing polysulfide migration and diffusion. However, the large particle size of these absorbing agents limits the absorbing ability, especially the electron transport properties, leading to limited improvement. Considering the size effect, Song et al.83 added nanosized manganese nickel oxide particles (30−50 nm) into sulfur cathodes as an absorbing material, which led to good cell performance with a high capacity retention of 85% for 50 cycles. In their analysis, they determined that a suitable absorbing agent may need the following requirements: small particle size, porous structure, and high surface area. Various nanosized metal oxides that fulfill these criteria and their performances are summarized in Table S2 in the Supporting Information. The representative metal oxides involve manganese nickel oxide,83,84 γ-alumina,85 silica,86 and titania.87 These nanosized additives have a polysulfide adsorbing effect

6. LITHIUM SULFIDE CATHODES Li2S is the end discharge product of sulfur. Li2S has a theoretical capacity of 1166 mA h g−1, and it is more desirable as the cathode material than sulfur because it allows the use of lithium-free anodes such as silicon, tin, or metal oxides. However, there are a number of challenges with utilizing Li2S, e.g., low electronic and ionic conductivities, sensitivity to moisture and oxygen, and limited synthesis routes. A number of efforts have been made recently to understand the electrochemical behavior of Li2S and to develop Li2S cathodes. 6.1. Activation of Microsized Li2S Particles

Li2S is considered to be electrochemically inactive because of some challenges. In a Li2S electrode, which is fabricated by a slurry casting method, the size of the Li2S particles is usually in the micrometer-scale range. Yang et al.95 found a potential barrier of ∼1 V at the beginning of the first charge of microsized Li2S. When a higher cutoff voltage is applied in the first charge to overcome this barrier, Li2S becomes active and the barrier does not appear again in the following cycles, as shown in Figure 20a. On the basis of the experimental observations, a model was proposed by the authors and is summarized in Figure 20b. In steps 1 and 2, it is a single-phase 11766

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between particles, which can generate an internal localized heat, improving the contacts between particles. Recently, Jeong et al.99 developed carbon-coated Li2S composites which were prepared with either a dry coating process (mechanical milling of Li2S and sucrose) or a wet coating process (PAN dissolved in NMP). These composites showed a reasonable cycle life over 50 cycles. 6.3. Chemical Synthesis of Li2S Cathode Materials

A number of chemical synthesis approaches have been developed to prepare Li2S−C composite materials or electrodes. Yang et al.100 developed a nanostructured Li2S−C composite by chemically lithiating a sulfur−CMK-3 composite with n-butyllithium. A sulfur−CMK-3 electrode was first prepared, and then a solution of n-butyllithium in hexane was drop-coated onto the electrode. After a drying process, a Li2S− CMK-3 composite electrode was obtained. More than 90% of the sulfur in the composite electrode was converted to Li2S based on the mass measurements. The initial discharge capacity was 573 mA h g−1 (based on the mass of Li2S), and the capacity was stabilized between 300 and 400 mA h g−1 after five cycles. The Li2S−CMK-3 composite cathode was further demonstrated in a full cell consisting of a silicon anode with a short cycle life of 20 cycles. The use of n-butyllithium to lithiate sulfur could increase the cost, and this approach may not be easy to scale up. Zheng et al.101 developed a Li2S−microporous carbon (MC) composite by a scalable in situ lithiation method. First, a sulfur−MC composite electrode was fabricated, and then the electrode was lithiated by spraying stabilized lithium metal powder (SLMP), followed by compression. The Li2S− MC electrode provided a stable capacity of 650 mA h g−1 (based on the mass of Li2S) over 900 cycles in half-cells. When coupled with a lithium-free graphite electrode, the Li2S−MC electrode showed a stable capacity of around 600 mA h g−1 over 150 cycles. This route could be close to practical application to produce Li2S−carbon composite materials. Recently, Archer’s group102 has reported two novel routes toward Li2S−C composite cathodes. The first approach is to utilize the strong interaction between lithium ions in Li2S and electron-donating groups in carbon-precursor polymers such as PAN.102a Li2S may function as a cross-linking agent between PAN polymers in the mixture of Li2S and PAN. After carbonization at elevated temperatures in an inert atmosphere, a Li2S−C composite can be obtained. After the first charge, which showed a high overpotential, the composite behaved exactly like Li2S. A stabilized discharge capacity of over 500 mA h g−1 (based on the mass of Li2S) was obtained over 20 cycles. The second approach is illustrated in Figure 21.102b Li2S is formed via the reduction of sulfates by carbon, which has long been used in the Leblanc process. First, resorcinol and formaldehyde react under acidic conditions to form a crosslinked resorcinol−formaldehyde (RF) gel. The lithium ions in Li2SO4 molecules could be linked with oxygen atoms in the RF gel. After carbonization at 900 °C, the Li2S−C composite was formed. The composite showed behavior similar to that of Li2S, and performance over 40 cycles was demonstrated. To further explore the synthesis methods for making Li2S electrodes, our group recently reported an in situ formed Li2S cathode in a lithiated graphite electrode,103 as illustrated in Figure 22. This approach uses polysulfide Li2S6 as a precursor and a prelithiated carbon paper (graphite) electrode. The porous carbon paper electrode acts as a lithium donor and a matrix for holding the polysulfide solution. As the potential of

Figure 20. (a) Voltage profiles of a pristine Li2S electrode during the initial three cycles showing the high cutoff voltage in the first charge and (b) a model for the initial charging of Li2S. Reprinted from ref 95. Copyright 2012 American Chemical Society.

reaction. Li2S is partially delithiated, forming Li2−xS, which has the same crystal structure as Li2S. The charge transfer of this process is slow, resulting in a large charge-transfer resistance (large potential barrier). In step 3, polysulfides are formed. The coexistence of Li2S and polysulfides makes the charge transfer easier than that in the previous steps. Once all the Li2S is converted to polysulfides at the end of charge (step 4), charge transfer among polysulfides is very easy. After the first charge, polysulfide nuclei exist in the electrolyte, which significantly improves the charge transfer in the following cycles. This study provided a practical approach to utilize Li2S as a cathode material, and this approach has been used in other work with microsized Li2S particles.96 For example, our group recently developed a Li2S−MWCNT sandwiched electrode which consists of pristine Li2S powder between two layers of MWCNT paper electrodes.96b The large Li2S particles in the sandwiched electrode required a high cutoff voltage in the first charge to activate Li2S. An initial discharge capacity of 838 mA h g−1 (based on the mass of Li2S) and a capacity of 680 mA h g−1 over 100 cycles have been obtained at a rate of C/10. 6.2. Li2S−Carbon Composites

To enhance the electronic conductivity of Li2S, a usual approach is to develop Li2S−C composites. A simple way to prepare a Li2S−carbon composite is ball-milling of pristine Li2S powder and carbon under an inert-gas atmosphere.96a,97 Cai et al.96a prepared a Li2S−C composite by high-energy ball-milling which showed an initial discharge capacity of 1144 mA h g−1 (based on the mass of Li2S) and a capacity of 411 mA h g−1 over 50 cycles at a rate of C/10. Spark-plasma sintering (SPS) has also been used in making Li2S−C composites.98 In this process, Li2S and carbon powder were mixed and loaded into a graphite die and then pressed uniaxially. A pulsed dc current was simultaneously applied to generate a spark discharge 11767

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Hassoun et al.97 first reported a cell with a gel-type polymer membrane as the electrolyte which was formed by trapping a carbonate-based solution saturated with Li2S in a poly(ethylene oxide)/lithium trifluoromethanesulfonate (PEO/LiCF3SO3) polymer matrix. With a Li metal anode, the cell operating at 60 °C showed a reversible capacity of over 1000 mA h g−1 (based on the mass of Li2S). With a Sn/C anode, the cell operating at 25 °C showed a reversible capacity of over 800 mA h g−1 over 30 cycles at a rate of C/20. A follow-up study from the same group showed a solid-state battery with a hot-pressed membrane of PEO/LiCF3SO3 complex containing nanosized ZrO2 particles and Li2S as the electrolyte.16c With a Li2S−C composite cathode, the half-cell could be reversibly cycled over 50 cycles at different rates. Takeuchi et al.98b reported an allsolid-state battery with a Li3PO4−Li2S−SiS2 solid-state electrolyte and a Li2S−C composite cathode which was prepared by the SPS process. The cell showed an initial discharge capacity of 920 mA h g−1 (based on the mass of Li2S) and a capacity of >700 mA h g−1 over 10 cycles. Nagao et al.104 utilized Li2S− P2S5 as a solid-state electrolyte with a conductivity of over 10−3 S cm−1 in a solid-state battery with Li2S−C composite cathodes. The cell showed a reversible capacity of >600 mA h g−1 (based on the mass of Li2S) over 10 cycles at 25 °C with high rate capability (up to 3.5C).

Figure 21. Synthesis scheme of a Li2S−C composite. Reprinted with permission from ref 102b. Copyright 2013 The Royal Society of Chemistry.

7. CHARACTERIZATION TECHNIQUES AND MECHANISTIC UNDERSTANDING The Li−S battery system is superior because of its high theoretical energy density. However, the practical energy density obtained with Li−S batteries is usually much lower than the theoretical value. This could result from the ineffective control of the complicated chemistry of the sulfur redox process, involving both compositional and structural changes of the electrodes. To better utilize the advantages of the Li−S battery system, a comprehensive understanding of the redox reaction is demanded. Advanced characterization techniques are introduced nowadays, and new reaction systems are designed for Li−S batteries. Parameters that influence the reaction kinetics could also be controlled to alleviate the limitations of the Li−S battery system.

Figure 22. Schematic showing the chemical reduction reaction of one Li2S6 molecule by lithium to form six Li2S molecules involving the diffusion/driving of lithium out of the graphene layers in graphite. Li2S6 represents the average molecular formula of polysulfides in the liquid electrolyte, and 3.56 Å is the spacing of (002) planes in the lithiated graphite. Reprinted from ref 103. Copyright 2013 American Chemical Society.

lithium stored in the lithiated graphite is close to that of metallic lithium, one polysulfide Li2S6 molecule would react with six lithium atoms in the lithiated graphite to form Li2S. This reduction reaction occurs over 6 days under the experimental conditions, and donation of lithium atoms is evidenced by the up-shifts of the (002) peaks in the X-ray diffraction (XRD) pattern of the lithiated graphite. The formed Li2S electrode exhibited a potential of 2.12 V vs Li/Li+ and a low overpotential in the first charge. On the basis of X-ray photoelectron spectroscopy (XPS) analysis, about 48.4% of the polysulfide was converted to Li2S. The electrode exhibited a stable capacity of about 800 mA h g−1 over 50 cycles at a rate of C/10. A lithium-rich sulfur cathode, i.e., Li2S, is a promising approach to advancing the Li−S technology and avoiding the safety concerns of a lithium metal anode. However, challenges exist if Li2S is used directly in the cathode because of its intrinsic low conductivity and reactivity. The lithiation approaches developed hold the promise of providing alternative ways to utilize Li2S in Li−S batteries. Novel, easily scalable chemical synthesis approaches are still needed to provide better control over morphology and the degree of lithiation.

7.1. In Situ Characterization

One of the most challenging issues with Li−S batteries is development of in situ probes to understand the working mechanism of the redox reactions. For example, opinions on the formation of Li2S and reappearance of sulfur, based on the results from ex situ measurements, are debatable.105 Some groups detected the formation of crystalline Li2S at the end of discharge,14c,106 while others observed Li2S formation in the middle of the lower discharge voltage plateau.107 On the other hand, S8 was reported to exist at the end of charge,107b,108 whereas other groups claimed that polysulfides would never be converted back to S8.5b,11,107a,109 To avoid the artifacts from post-treatments and understand the real working mechanism, in situ characterization methodologies are demanded. Nelson et al. and Cañas et al. used in situ XRD to study the structural changes in Li−S batteries.110 Both groups recorded the formation of crystalline S8 at the end of charge. However, Li2S was not detected at the end of discharge by Nelson et al.,110a whereas Cañas et al.110b observed the formation of Li2S at 60% discharge in the lower discharge voltage plateau. It was reasoned that Li2S was not detected in some previous work because (i) the in situ cell was not airtight, (ii) the discharge

6.4. Solid-State Batteries with Li2S Cathodes

Besides the liquid electrolyte used with most Li2S cathodes, solid-state electrolytes have also been studied. For example, 11768

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short-chain Li2S, which deteriorates the discharge capacity of the lower discharge voltage plateau.114a Nevertheless, polysulfide dissolution into the bulk electrolyte could negatively impact the cycle stability of Li−S batteries due to the parasitic reactions between polysulfides and the lithium metal anode.5a To suppress polysulfide dissolution into the bulk electrolyte and its migration and contact with the lithium metal anode, porous conductive substrates have been used to confine polysulfides within the cathode through both physical and chemical interactions.115 However, complete blocking of polysulfides is not possible since the dissolution of polysulfide is thermodynamically favorable in organic electrolytes. Recently, concentrated electrolytes have been reported to effectively alleviate polysulfide dissolution into the bulk electrolyte through a common ion effect. Shin et al.116 showed that a higher lithium salt (lithium bis((trifluoromethyl)sulfonyl)imide (LiTFSI)) concentration of up to 5 M was helpful in decreasing both the dissolution and diffusion of polysulfides. Furthermore, Suo et al.117 proposed a “solvent-insalt” (SIS) electrolyte in which the lithium salts had ultrahigh concentrations up to 7 M and held a dominant position in the lithium ion transport system. It was demonstrated that SIS electrolytes not only inhibited the dissolution of lithium polysulfides but also effectively protected the lithium metal anode against the formation of lithium dendrites.117 On the other hand, Park et al.118a and Ueno et al.118b revealed that polysulfide dissolution was highly sensitive to the anionic structure of the ionic liquids (ILs) in IL-based electrolytes. Various binary mixtures of ILs and lithium salts were studied as electrolytes for Li−S batteries, and the solvent effect of ILs was found to be related to its donor ability (competition of the IL’s anion against the polysulfides to interact with the lithium ions).118 For example, polysulfides were easily dissolved in the strongly basic N-butyl-Nmethylpyrrolidinium trifluoromethanesulfonate ([P14][OTf]) electrolyte with high donor ability, whereas polysulfide dissolution was hampered in the ILs having bulky fluorosulfonyl amide-type anions owing to the weak donor ability of these anions to the lithium ions of lithium polysulfides.118 In addition to organic and IL-based electrolytes, polymer electrolytes16c,36a,119 and inorganic solid electrolytes120 have been studied to offer physical and kinetic barriers to polysulfide dissolution into the bulk electrolyte. For example, Hassoun and Scrosati16c reported the synthesis of a polymer electrolyte membrane consisting of a PEO−LiCF3SO3 complex with finely dispersed nanosized ZrO2 particles and Li2S, which was believed to enhance the ionic conductivity and also prevent sulfide dissolution from the cathode. Reproducible cycles with Coulombic efficiency approaching 100% were obtained, indicating that polysulfide dissolution and shuttling were controlled.16c Furthermore, an all-solid-state configuration with an inorganic solid electrolyte was reported to realize close solid−solid contact, prohibiting the dissolution of polysulfides.120d Nagao et al.120d studied an all-solid-state battery with a sulfur−CMK-3 composite cathode and thioLISICON solid electrolyte and obtained stable cycling up to 30 cycles with high reversibility, indicating the feasibility of applying an inorganic solid electrolyte to address the polysulfide dissolution issue. More detailed discussion regarding the electrolytes used in Li−S batteries and their influence on the performance of Li−S batteries will be presented in sections 8.1−8.3.

capacity was low, or (iii) the penetration depth of the X-ray was not high enough.110b To elucidate the aforementioned contention, Waluś et al.105 also conducted in situ XRD characterization with high resolution. Their synchrotron-based results indicated the formation of crystalline Li2S at the beginning of the lower discharge voltage plateau and its conversion to crystalline S8 during the following charge.105 Additionally, it was found that sulfur recrystallized into another allotrope: monoclinic β-sulfur.105 To obtain a more comprehensive overview of the redox mechanism, other complementary in situ characterizations are necessary. In addition to in situ XRD, in situ X-ray absorption spectroscopy (XAS) is another important in operando characterization technique for probing Li−S batteries. Gao et al.111 studied seven liquid electrolytes and claimed that solvents played a key role in the electrochemical performance of Li−S batteries while lithium salts had no significant effect. In situ sulfur K-edge XAS spectra of sulfur electrodes in different electrolytes at the initial, discharged, and charged states were analyzed. It was found that ether-based solvents with low viscosity resulted in more complete polysulfide reduction as reflected by the more complete conversion of the S peak to a Li2S peak in the XAS spectra, whereas carbonate-based solvents could react with the reduced sulfur species as revealed by the possible presence of peaks of thioether and sulfonium functionalities in the XAS spectra, indicating instability of the cell.111 However, the cells were not studied under real-time in operando conditions, and the polysulfide spectral features were not verified or quantified.112 To obtain more accurate information regarding the electrochemical redox processes of Li−S batteries, Cuisinier et al.112 conducted in situ XAS characterizations of a cathode that had sulfur imbibed into the spherical carbon shells with tailored porosity, avoiding the overwhelming distortion of the XAS spectra as a result of bulk particles. To gain insight into the mechanism of sulfur dissolution and Li2S deposition, in situ XAS spectra were interpreted by the linear combination of some selected polysulfide species (α-S8, S62−, S42−, and S2−). Delay in Li2S precipitation was identified upon reduction owing to supersaturation of S2−, which was confirmed by the in situ XAS results analyzed from the linear combination simulation.112 The delayed Li2S formation could be a possible explanation for the failure to identify Li2S in the previous in operando XRD/tomography studies.110a On the other hand, oxidation of Li2S proceeded straightforwardly to polysulfides.112 The different responses in the XAS spectra corresponding to different steps in the reduction/oxidation evidenced the potential hysteresis existing in the discharge/charge process. 7.2. Polysulfide Dissolution

As mentioned previously, the intermediate lithium polysulfides generated during the operation of Li−S batteries are soluble in common organic electrolytes.8b,c,113 To understand the solvent effect on the polysulfide dissolution and cycling characteristics of Li−S batteries, Barchasz et al.114a compared solvents with different solvation abilities toward polysulfides. It was concluded that Li−S batteries require the use of highsolvation-ability solvents to increase sulfur utilization, such as tetraethylene glycol dimethyl ether (TEGDME)114b and 2ethoxyethyl ether (EEE). This is because solvents exhibiting low polysulfide solvation ability, e.g., 1,2-dimethoxyethane (DME), diethylene glycol dibutyl ether (DEGDBE), or 1,3dioxolane (DOL), could lead to a passivation of the cathode by 11769

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7.3. Formation of the Passivation Layer

reached the highest value for an electrolyte containing 0.4 M LiNO3, possibly due to the protective passivation layer formed with the modified electrolyte compositions. Recently, it was revealed that the polysulfide is one essential component to keep the stability of the passivation layer.126 The stable passivation layer in Li−S batteries was assumed to consist of two sublayers, the top layer composed of oxidized products of polysulfides and the bottom layer composed of the reduced products of polysulfides and LiNO3,126 consistent with the earlier work by Aurbach et al.123 In general, the efficacy of forming an enhanced passivation layer in the presence of LiNO3 has been confirmed by previous work. However, it is worth mentioning that LiNO3 is progressively consumed with the development of new lithium dendrites and the formation of a new passivation layer during Li−S cell operation, which limits its ability to stabilize the lithium surface for long-term cycling or in a polysulfide-rich environment. Additionally, LiNO3 could be irreversibly reduced on the cathode at potentials lower than 1.6 V, which negatively impacts the cycle stability of Li−S batteries.127 Therefore, further improvements to stabilize the lithium surface passivation layer are demanded. An alternative to LiNO3, lithium bisoxalatoborate (LiBOB), was identified to modify the passivation layer on the lithium surface in Li−S batteries.128a Higher discharge capacity and smoother lithium surface morphology were obtained in the presence of LiBOB in the electrolyte.128a Finally, phosphorus pentasulfide (P2S5) disclosed by Lin et al.128b as an electrolyte additive was found to passivate lithium surface, alleviating the polysulfide shuttling. The passivation layer that formed on lithium surface consisting mainly of Li3PS4 was reported to have fast lithium-ion conduction while blocking the direct contact between lithium metal and polysulfides. The long-term cycle stability of this passivation layer is still under investigation. In contrast to the passivation layer on the metallic lithium anode, the passivation layer that forms on the cathode surface in Li−S batteries is usually comprised of Li2S bulk particles, and it negatively influences the cycling performance of Li−S batteries.36b,114a,129 The formation of the Li2S passivation layer on the cathode occurs at the lower discharge voltage plateau36b and could decrease sulfur utilization by blocking lithium ion transport toward the inside of the cathode.36b The formation of this Li2S passivation layer is influenced by the electrolyte solvent selected and the discharge rate.36b,114a,129 Electrolytes with low polysulfide-solvation ability and high-rate discharge would result in more severe Li2S passivation on the cathode surface, leading to the breakdown of the conductive matrix, formation of irreversible Li2S, and deterioration of the cycle stability.36b Further discussion regarding the formation and properties of the Li2S passivation layer is available in sections 7.2 and 7.4.

The high energy density of Li−S batteries originates from the use of a high-capacity sulfur cathode and a lithium metal anode. Although metallic lithium has a large theoretical capacity of 3860 mA h g−1, it suffers from drawbacks such as dendrite formation and poor cycling efficiency as an anode for rechargeable lithium batteries because of the unstable passivation layer (solid electrolyte interphase, SEI) resulting from parasitic reactions between organic electrolytes and the lithium metal. The chemical composition and microstructure of the passivation layer on the lithium surface are greatly influenced by the electrolyte, which is comprised of the solvent and lithium salts.121 For example, DOL is one of the most commonly used electrolyte solvents in Li−S batteries.122 During the discharge/charge electrochemical process, DOL is reduced to several ROLi (R refers to alkyl) species and oligomers with −OLi edge groups, enhancing the flexibility of the passivation layer on the lithium surface, which assists the passivation layer’s ability to accommodate lithium morphological changes upon cycling.121a,123 Moreover, fluoroethylene carbonate (FEC) solvent was identified to help the formation of a robust protective layer by UV-curing polymerization.121c The protective layer on the lithium anode surface formed in the FEC-based electrolyte consists of LiF along with LixPFy and LixPOFy, which effectively mitigates the overcharging related to polysulfide shuttling in Li−S batteries.121c Recently, ILs have been considered as a promising new class of electrolyte solvents for Li−S batteries. An IL-enhanced passivation layer on the lithium surface is found to exhibit a smoother morphology and less complicated surface chemistry compared to that formed with the conventional organic electrolytes.84 Lithium metal was reported to be protected from the continuous attack by polysulfides with an N-methyl-N-butylpyrrolidinium bis((trifluoromethyl)sulfonyl)imide (Py14TFSI)-modified passivation layer, leading to improved Coulombic efficiency and cycle stability.84 In terms of lithium salts, lithium trifluoromethanesulfonate (LiCF3SO3) and LiTFSI used in Li−S batteries have high dissociation constants, high oxidation and temperature stability, nontoxicity, and insensitivity to moisture.124 Unfortunately, they seem to have no obvious positive effect on the passivation layer that forms on the lithium metal surface.124 In addition to electrolyte solvents and lithium salts, additives have been reported to effectively modify the physical and chemical properties of the passivation layer on the lithium surface.124 Recently, Mikhaylik125 patented an additive that includes N−O bonds, e.g., LiNO3. It was found that LiNO3 as an electrolyte additive could prevent polysulfide shuttling, leading to higher reversible capacities of the Li−S batteries.125 Aurbach et al.123 rigorously studied the effect of the presence of LiNO3 on the Li surface through electrochemical and spectroscopic analyses. With the combined use of Fourier transform infrared (FTIR) spectroscopy and XPS, LiNO3 was identified to suppress polysulfide shuttling because of its direct reduction by lithium to LixNOy species and its oxidation of sulfur species to LixSOy moieties, passivating the lithium anode surface and alleviating parasitic reactions between lithium and sulfur species.123 A more stable passivation layer on the lithium surface could thereby be expected. Moreover, the passivation layer that forms in the presence of LiNO3 is beneficial to the lithium cycling efficiency. Liang et al.64b studied the function of LiNO3 on lithium deposition/dissolution and concluded that the efficiency increased with the concentration of LiNO3 and

7.4. Kinetics

Yamin et al.113b studied the redox mechanism of polysulfides with a glassy carbon electrode in THF by cyclic voltammetry (CV), revealing one anodic peak and up to three cathodic peaks. The anodic peak was reported to result from the oxidation of polysulfides to elemental sulfur, in which the charge-transfer stage was preceded by a slow chemical reaction. The cathodic peaks were ascribed to reduction of high-order polysulfides to Li2S6 in a diffusion-controlled reaction, reduction of Li2S6 to Li2S5 through a slow disproportionation reaction, and reduction of Li2S5 to Li2S2 or Li2S. The kinetics of the reduction from S8 to Li2S4 or Li2S5 corresponding to the 11770

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upper discharge voltage plateau was believed to be fast.20,36b,113 Our group113c experimentally demonstrated that the long-chain polysulfide redox couple was able to deliver nearly full theoretical capacity at a rate as high as 5C with a carbon interlayer inserted between the cathode and separator, which shows that the high-rate application of Li−S batteries is not hindered by reactions in the upper discharge voltage plateau. In contrast, the reaction kinetics of the conversion from Li2S4 or Li2S5 to Li2S2 or Li2S corresponding to the lower discharge voltage plateau was believed to be slow, attributed to the extra energy required for nucleation of the solid phase and sluggishness of solid-state diffusion to convert Li2S2 to Li2S.20,113a A systematic investigation on the rate capability and cycling characteristics of Li−S batteries was conducted by Cheon et al.,36b and it was concluded that the lower discharge voltage plateau where solid Li2S formed is highly sensitive to the discharge rate. The combined use of electrochemical characterization and morphology observation of the S−C composite cathode suggested that the thick Li2S layer formed at the cathode surface was responsible for the diminution of the lower discharge voltage plateau at a high discharge rate.36b It was further reasoned that the diffusional resistance of polysulfides and lithium ions increased at high rates, leading to more localized Li2S passivation on the cathode surface.36b The localized Li2S passivation and its associated stress buildup caused destruction of the carbon matrix and the formation of irreversible Li2S, so the reversible capacity decreased. The slow reaction kinetics of the low-order polysulfides and their blockage of the cathode could be the major factor causing low rate capability with Li−S batteries. To improve the kinetics of Li−S batteries, major strategies include decreasing the sulfur particle size and confining sulfur into highly conductive substrates with high surface areas. Zhang et al.22 encapsulated sulfur into microporous carbon and obtained an enhanced rate capability, owing to the highly dispersed elemental sulfur inside the narrow micropores of carbon spheres with good electrical conductivity. In addition, the micropores could trap elemental sulfur and subsequent lithium polysulfides during cycling, avoiding the formation of a thicker Li2S insulating layer on the composite surface. Similarly, Xin et al.26b synthesized metastable small sulfur molecules of S2−4 in the confined space of a conductive microporous carbon matrix and achieved a favorable high-rate discharge capacity of 800 mA h g−1 at a 5C rate. Superior high rate capability found in this work was ascribed to the high-electronic-conductive core of the carbon nanotube and the microporous carbon on the surface that provided unperturbed lithium ion transport, forming an efficient mixed-conducting 3D network.26b It is worth mentioning that integrating sulfur with graphene or graphene oxide is another important strategy to enhance the reaction kinetics of Li−S batteries. As mentioned previously, with the high surface area, superior electronic conductivity, high mechanical strength, and flexibility, graphene-based material is a suitable substrate to confine polysulfides and accommodate the large volume expansion during lithiation, avoiding the formation of irreversible Li2S. The graphene-based material could be further fabricated as a free-standing electrode and used in advanced cell configurations.130 More detailed discussion regarding the use of sulfur−graphene/graphene oxide composites to improve Li−S batteries was presented in section 5.3. On the other hand, it was found that enhanced rate capability could be achieved by using functional polymers to modify the cathode architecture and retain a uniform nanocomposite

structure. Our group replaced the conventional PVdF binder with Pluronic F-127 block copolymer and stabilized the S− black pearl nanocomposite, preventing the formation of a dense passivation layer of Li2S on the cathode.131 Stable cycling with capability up to a 4C rate was obtained due to the uniformly dispersed active material within the electrodes in the presence of Pluronic F-127. Similarly, polymers (e.g., polyaniline,73,132 polyacrylonitrile133) were applied directly as a coating layer to stabilize the cathode. The enhanced rate capability could be due to the increased electronic conductivity of the composite cathode and the superior electrolyte absorption ability of the polymer coating, leading to a reduced lithium ion diffusion distance.

8. ELECTROLYTES AND SEPARATORS Electrolytes act as the ion transport pathway between the anode and cathode. Liquid electrolytes are widely used in batteries because of their high ionic conductivity. In Li−S batteries, the electrolyte is critical because the intermediate polysulfides could dissolve in the liquid electrolyte and shuttle between the cathode and anode. The solubility of polysulfides in a liquid electrolyte definitely affects the battery performance. In addition, the passivation caused by the reduction or oxidation of the electrolyte on the electrodes, which was discussed in section 7.3, is also related to the composition of the electrolyte, particularly the additives. A solid-state electrolyte could be better than a liquid electrolyte in terms of reducing the dissolution and shuttle of polysulfides. However, the low ionic conductivity and interfacial instability associated with most solid-state electrolytes lead to more issues when they are used in Li−S batteries. Separators used in Li ion batteries are porous polymer films, e.g., expanded polypropylene. They physically separate the anode and cathode, but allow ion movements in the liquid electrolyte they absorb. In Li−S batteries, functional separators that can be ion selective become more promising in reducing the shuttle of polysulfides. In this section, different electrolytes and functional separators are discussed. 8.1. Liquid Electrolytes

The selection of liquid electrolyte solvents for Li−S batteries starts with aliphatic amines, carbonates, and then ethers. DOL was first demonstrated by Peled et al.8d in 1989 to be an appropriate solvent in a liquid electrolyte for Li−S batteries. Presently, liquid electrolytes mainly containing DOL and other solvents such as DME and lithium salts such as LiCF3SO3 or LiTFSI are widely used in Li−S batteries. A variety of other solvents, including ionic liquids and short-chain PEO-based polymers, have also been studied over the past several years; however, it is difficult to compare them because of the complexity of the Li−S systems and different sulfur electrodes prepared in different laboratories. Only a few representative studies on liquid electrolytes are discussed here. Chang et al.134 studied a mixed electrolyte of TEGDME and DOL in Li−S batteries. TEGDME can readily solvate the lithium salt LiCF3SO3, and DOL can reduce the viscosity of the electrolyte. What they found is that shorter polysulfides are favorably formed in DOL-based electrolytes in the upper discharge voltage plateau while the lower discharge voltage plateau is dependent on the TEGDME:DOL ratio (30:70 is the best) and sulfur utilization in the lower discharge voltage plateau is related to the viscosity and conductivity of the electrolyte. An electrolyte with high viscosity can cause 11771

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terms of capacity, efficiency, and cycle stability was obtained. In addition, Zheng et al.84 found that the ionic liquid Py14TFSI can improve SEI layer formation on the lithium metal surface in Li−S batteries, which can suppress the corrosion of the lithium metal anode by polysulfides and thereby improve the cycling efficiency and stability. Park et al.141 studied the ionic liquid N,N-diethyl-N-methylN-(2-methoxyethyl)ammonium bis((trifluoromethyl)sulfonyl)amide ([DEME][TFSA]) as an electrolyte solvent in Li−S batteries. In comparison with the TEGDME solvent, [DEME][TFSA] showed superior performance in terms of cycle stability and efficiency. The authors claimed that [DEME][TFSA] has a low donor ability, which can significantly suppress the dissolution of lithium polysulfides, because of the weak Lewis basicity of the [TFSA]− anion. The follow-up study by Park et al.118a showed that the solubility of lithium polysulfides in ILs is strongly dependent on the anionic structure. The stronger the donor ability of an IL, the higher the solubility of lithium polysulfides. In addition, the battery performance is also influenced by side reactions of anions, such as BF4− and [FSA]−, with polysulfides and mass transport in viscous ILs. TFSI-based ILs show high capacities and cycling efficiencies. Another study from the same group118b reported the anionic effects on solvated IL electrolytes in Li−S batteries. Li salts (LiX) with different anions (X = N(SO2C2F5)2 (BETI), N(SO2CF3)2 (TFSA), CF3SO3 (OTf), BF4, and NO3) and glymes were prepared as electrolytes. It was found that the dissolution of lithium polysulfides in solvated IL electrolytes Li[BETI] and Li[TFSA] in glymes as shown in Figure 23 was

passivation on the surface of sulfur electrodes because of the low polysulfide diffusion. Ryu et al.14c investigated the selfdischarge behavior of Li−S batteries with the TEGDME electrolyte. What they found is the self-discharge of the cells depends on the current collectors. With stainless steel current collectors, the highest self-discharge rate is 59% per month. With aluminum current collectors, the self-discharge rate is 34% during the initial 80 days but only 36% after 360 days of storage. Choi et al.106b,135 found that the addition of a small amount of toluene in TEGDME has a remarkable effect of increasing the initial discharge capacity and stabilizing the cycling performance, which is attributed to the reduced lithium metal interfacial resistance. Shin et al.136 found the addition of TEGDME to N-methyl-N-butylpyrrolidinium bis((trifluoromethyl)sulfonyl)imide (PYR14TFSI)-based electrolyte can significantly enhance the ionic conductivity, particularly at low temperatures. In addition, the mixed electrolyte has good compatibility with lithium metal. Recently, Barchasz et al.137 revisited TEGDME/DOL binary electrolytes in Li−S batteries. By studying the electrochemical response of various solvent ratios, lithium salt concentrations, and use of additives, it was concluded that the solvation ability of the electrolyte is the key factor in high electrochemical performance. High DOL contents result in insufficient solvation ability and DOL polymerization. Kolosnitsyn et al.138 studied sulfolane as a solvent in a liquid electrolyte for Li−S batteries. When sulfolane was used alone, the cell showed a poor cycle life. The voltage profile presented an obvious degradation in the upper and lower discharge voltage plateaus. When sulfolane was used with ethers, e.g., DME, DOL, and THF, the cycling performances of these cells were still poor. However, the cell with the mixed electrolyte containing DOL showed the highest initial discharge capacity. When sulfolane was used with linear ethers, e.g., glyme, diglyme, and tetraglyme, the authors correlated the changes in the cycling performance to the viscosity and solvation ability of the solvents, which affect the reactions of dissociation of the primary dianion S82− and disproportionation of lithium polysulfides in the electrolyte solutions. Kim et al.139 studied the mixed electrolytes of DME/DOL with and without imidazolium salts (1-ethyl-3-methylimidazolium bis((perfluoroethyl)sulfonyl)imide (EMIBeti) and 1butyl-3-methylimidazolium hexafluorophosphate (BMIPF6)) in Li−S batteries. It was found that small amounts (5 or 10 vol %) of the imidazolium salts added to the electrolytes can enhance the discharge capacity, rate capability, and lowtemperature performance, which may be related to the enhancement of the electrochemical reaction of polysulfides by the imidazolium cation and improved stability of the surface morphology of the lithium metal anode. Xiong et al.128a studied LiBOB as an additive in an electrolyte of LiTFSI in DME/ DOL. The cells with LiBOB in the electrolyte showed improvements in discharge capacities and cycling performance, which were attributed to the improvement of the lithium metal anode in the presence of LiBOB. ILs are promising electrolyte solvents for Li−S batteries because of their nonflammability, good chemical stability, and high ionic conductivity. In Li−S batteries, ILs show interesting properties and performance. Wang et al.140 used an ionic liquid of N-methyl-N-propylpiperidinium bis((trifluoromethyl)sulfonyl)imide (PP13-TFSI), a viscous solvent, to counteract the high solubility and diffusion rate of lithium polysulfides in DME. With optimized ratios, superior battery performance in

Figure 23. Schematic illustrating the anionic effects on solvated ionic liquids in suppressing the dissolution of polysulfides in Li−S batteries. Reprinted from ref 118b. Copyright 2013 American Chemical Society.

significantly suppressed, leading to stable cycling performance. However, NO3− anions can be irreversibly reduced at the cathode, and BF4 anions can form unexpected byproducts with polysulfide anions. Li[OTf] in glyme is not a good electrolyte for Li−S batteries. Suo et al.117 reported a new class of nonaqueous liquid solvent-in-salt electrolytes for Li−S batteries. The authors chose the electrolyte containing LiTFSI in DME/DOL and 11772

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Figure 24. Proposed nucleophilic reaction mechanism of EC and DMC with polysulfides. Reprinted with permission from ref 142. Copyright 2013 Elsevier.

reversible capacity of 400 mA h g−1 over 20 cycles, and the utilization of electrochemically active sulfur was about 90%. Zhang et al.22,29a used 1 M LiPF6 in PC/EC/DEC (1:4:5, v/v/ v) with sublimed sulfur in acetylene black or carbon spheres. The voltage profile showed only the lower discharge voltage plateau at 1.8 V instead of the two voltage plateaus at 2.3 and 2.1 V. The authors attributed this potential hysteresis to sulfur embedded in the narrow pores of carbon materials. The sulfur in the carbon spheres showed a stable cycle life of 500 cycles. Xin et al.26b demonstrated that metastable small sulfur molecules of S2−4 in a microporous carbon matrix can work with carbonate electrolytes, showing similar voltage profiles and a long cycle life. Wang et al.143 demonstrated that sulfur in a microporous−mesoporous carbon material can also work with carbonate electrolytes. These results confirm that small sulfur confined in a microporous carbon matrix can avoid the irreversible reaction of polysulfides with carbonates in Li−S batteries. Recently, Lin et al.144 introduced a flame-retardant additive (dimethyl methylphosphonate (DMMP)) to carbonate-based electrolytes (LiPF6 in EC/EMC) for Li−S batteries. With 7−11 wt % DMMP in the electrolyte, the cells showed good cyclability over 50 cycles.

studied the physicochemical properties of this electrolyte with different ratios of salt to solvent. The lithium ion transference number in 7 M electrolyte was 0.73, which is much higher than that of traditional salt ion−solvent electrolytes (0.2−0.4). However, the ionic conductivity of this electrolyte was 0.814 mS cm−1 at room temperature, which is superior to that of an all-solid-state dry polymer or most of the inorganic electrolytes, although the viscosity was as high as 72 cP. Interestingly, the Tg of this electrolyte is −77.3 °C, which is much lower than that of typical “polymer-in-salt” systems (Tg > −10 °C) due to the flexible hinge in the S−N−S bond of TFSI−. In Li−S batteries, such an electrolyte can inhibit the dissolution of lithium polysulfides and protect the lithium metal anode against the formation of lithium dendrites, which leads to excellent battery performance. 8.2. Carbonate-Based Electrolytes

Carbonate-based electrolytes, such as LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC), are widely used in Li ion batteries. It is desirable to use such electrolytes so that graphite and other non lithium metal anodes can be used in Li−S batteries. However, carbonate-based electrolytes may be inappropriate for use in Li−S batteries because the intermediate polysulfides could react with the carbonate solvent, resulting in irreversible loss of active materials. Gao et al.111 studied different electrolytes, including one with EC/DEC solvent, in Li−S batteries by in situ sulfur K-edge XAS. On the basis of the different XAS results with the EC/DEC electrolyte, the authors proposed that the nucleophilic sulfide anions may chemically react with carbonates to form methylated thiolate and thioether. Similarly, Barchasz et al.114a also found instability of carbonate-based electrolytes in Li−S batteries. Yim et al.142 established the reaction mechanism of the carbonates with polysulfides by studying the carbonate decomposition. A plausible mechanism for the carbonate decomposition is suggested, as shown in Figure 24. The polysulfides can react with carbonates via nucleophilic addition or substitution to form thiocarbonates and other small molecules. Although the unsuccessful use of carbonate-based electrolytes in Li−S batteries has been demonstrated by several groups, a few studies show that carbonate-based electrolytes can be used in cells with electrodes containing short-chain sulfur species. Wang et al.18a studied a gel electrolyte membrane containing a liquid electrolyte of 1 M LiPF6 in PC/EC/DEC (1:4:5, v/v/v/) with a sulfur−carbon nanocomposite. The voltage profile showed a single discharge voltage plateau after the first cycle. The sulfur composite electrode exhibited a

8.3. Polymer/Solid-State Electrolytes

Polymer and solid-state electrolytes are more favorable than liquid electrolytes in terms of their capability of reducing the solubility of polysulfides and blocking the shuttle of polysulfides in Li−S batteries. In addition, they could protect the lithium metal anode and minimize dendrite formation, which is beneficial for improving the safety and cycle life of Li−S batteries. However, polymer and solid-state electrolytes usually have low ionic conductivity because of the high viscosity of polymers and high energy barrier for lithium ion transport in solid-state electrolytes. Short-chain polymers such as TEGDME and polyethylene glycol dimethyl ether (PEGDME) have been widely used in Li−S batteries because of their liquidlike property, which allowd good lithium ion transport. For example, Marmorstein et al.119a compared three polymer electrolytes, including PEO, poly(ethylene−methylene oxide) (PEMO), and TEGDME. It was found that the cell with TEGDME showed a much lower capacity fade rate than the cell with the other two polymers. Kim et al.145 studied electrolytes containing LiCF3SO3 in TEGDME with a sulfur−mesoporous hard carbon spherule composite cathode. The cell exhibited high capacity with excellent retention during cycling. In addition, the cell performed well at low temperatures, delivering a capacity of 500 mA h g−1 at 0 °C over 170 cycles. Shim et al.17 11773

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lithiated Nafion membranes in Li−S batteries. Nafion membranes in hydrogen form are used in proton exchange membrane fuel cells because of their high ionic conductivity and good chemical stability. Once protons in Nafion are replaced by lithium ions, it becomes a lithium ion conductor. It was found that the lithium ion conductivity of the lithiated Nafion membranes is 2.1 × 10−5 S cm−1 at room temperature and the Li ion transference number is 0.986, indicating high lithium ion selectivity. The cell with the lithiated Nafion membrane exhibited better capacity retention and higher Coulombic efficiency than the cells with liquid electrolytes. A recent work by Jin et al.150 demonstrated another perfluorinated polymer membrane with lithium sulfonyl dicyanomethide functional groups which showed a lithium ion conductivity on the order of 10−4 S cm−1 and a close-to-unity lithium ion transference number. The cell with this membrane showed a high initial discharge capacity of over 1100 mA h g−1 and a stable capacity of over 800 mA h g−1 over 100 cycles with high Coulombic efficiency.

compared three solvents with different molecular weights, i.e., triglyme, PEGDME 250, and PEGDME 500. The cells with PEGDME 250 and 500 showed very reversible charging and discharging behaviors with high Coulombic efficiency, while the cell with the triglyme solvent showed low efficiency because of the shuttle of polysulfides, indicating that polymers with high molecular weights can indeed reduce the shuttle effect in Li−S batteries. Long-chain PEO polymers have been widely used as a major component in polymer electrolytes in Li−S batteries. For example, Shin et al.146 studied the electrochemical properties and interfacial stability of (PEO)10LiCF3SO3 composite polymer electrolytes with titanium oxide additive. The addition of titanium oxide increased the ionic conductivity of the composite electrolyte and improved the interfacial stability, thereby improving the electrochemical performance. Hassoun et al.16c,97 developed a solid-state Li−S battery employing a PEO-based gel-type polymer membrane containing a PEO/ LiCF3SO3 polymer matrix with finely dispersed nanosized zirconia and a Li2S−C composite cathode. A long cycle life and high Coulombic efficiency were obtained at elevated temperatures. Liang et al.147 studied a PEO18/Li(CF3SO2)2N polymer electrolyte containing 10 wt % SiO2 with a sulfur−mesoporous carbon sphere composite in Li−S batteries. A discharge capacity of over 800 mA h g−1 was obtained over 25 cycles at 70 °C. Highly conductive glass-ceramic electrolytes based on thioLISICON have also been evaluated in Li−S batteries. For example, Hayashi et al.120b have developed an all-solid-state Li− S cell with Li2S−P2S5 electrolytes which retained a capacity of over 650 mA h g−1 for 20 cycles at room temperature. Hakari et al.120e from the same group studied Li2S−P2S5 glasses and acetylene−black composite electrode materials in all-solid-state batteries, which showed the largest charged capacity of 240 mA h g−1 (normalized by the weight of Li2S−P2S5). The authors120a also studied such electrolytes with Li2S−Cu composite cathodes and an In metal anode. Cu is involved in the cathode reaction by forming LixCuS. Nagao et al.104,120f also studied Li2S−P2S5 electrolytes with Li2S and sulfur composite cathodes. Kobayashi et al.120c tested another thio-LISICON glass-ceramic electrolyte with a composition of Li3.25Ge0.25P0.75S4 in Li−S batteries that showed a few reversible cycles. Yersak et al.120g studied FeS−S composite cathodes within all-solid-state batteries with a 77.5Li2S:22.5P2S5 glass electrolyte and found Li 2 S in the glass electrolyte can be involved in the electrochemical processes and contribute to the capacity. Recently, Lin et al.148 reported a novel class of sulfur-rich cathode materials, lithium poly(sulfidophosphate)s, which exhibited a high Li ion conductivity of 10−5−10−6 S cm−1. They demonstrated this cathode material with the solid-state Li3PS4 electrolyte to display a reversible capacity of over 1200 mA h g−1 at C/10 over 300 cycles at 60 °C. The lithium superionic conductor Li10GeP2S12 with an ionic conductivity of 12 mS cm−1 at room temperature demonstrated by Kamaya et al.120h could be a promising solid electrolyte for Li−S batteries.

9. ANODES The anode is an essential part of the Li−S battery system because the stability of the anode determines the long-term cycle stability of Li−S batteries. Metallic lithium may in principle be the ultimate anode for Li ion batteries and the primarily used anode in Li−S batteries due to its low potential and high capacity, leading to high energy density. However, metallic lithium is unstable in contact with organic electrolytes, which hampers the safety of rechargeable batteries based on metallic lithium. The status and limitations of metallic lithium anodes as well as alternative anodes are analyzed in the sections below. 9.1. Lithium Metal Anode

As mentioned previously, although metallic lithium has a high theoretical capacity of ∼3860 mA h g−1, it suffers from drawbacks such as dendrite formation and low lithium cycling efficiency, which negatively influence the cycling stability and safety of rechargeable lithium batteries.124 The dendrite formation and low lithium cycling efficiency were believed to result from the instability of the passivation layer (SEI layer) on the metallic lithium anode.124,151 The unstable SEI cannot accommodate the shape and volume changes of the lithium electrode during cycling, leading to nonuniform lithium deposition and dissolution, resulting in lithium dendrite formation.124 Moreover, the breakdown of the SEI results in the exposure of the fresh lithium surface to the electrolyte and parasitic reactions to form a new SEI layer, which decrease the lithium cycling efficiency.124 In contrast to lithium ion batteries, Li−S batteries involve the intermediate polysulfides dissolved in an organic electrolyte, causing more severe parasitic reactions on the lithium metal surface. Consequently, conservation of a stable passivation layer is more difficult, and degradation of the metallic lithium anode is even more serious in Li−S batteries.5a,123,152 Excess lithium is, therefore, required to couple with the sulfur cathode, which decreases the practical energy density of Li−S batteries. It is reasonable to comment that the success of Li−S batteries requires a reliable lithium metal anode. The reliability of the lithium metal anode depends significantly on the stability of its passivation layer, which could be improved by changing the electrolyte solvents and introducing additives as was previously described in section 7.3. In addition, innovations

8.4. Separators

Expanded polypropylene separators are widely used in Li−S batteries. Such separators have the limited ability to block the shuttle of polysulfides since they are not functionalized. Some efforts have been taken to evaluate other membranes with functional groups in them with the goal of reducing the transport of polysulfides. For example, Jin et al.149 applied 11774

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With a prelithiated columnar structured α-Si film electrode, they were able to achieve a reversible capacity of >380 mA h g−1 after 60 cycles, although the performance is still far from the requirement for practical applications.158 To enhance the electronic conductivity of the Si-based materials and suppress the volume expansion, the lithiated Si could be confined into a carbon matrix.159 Hassoun et al.159a disclosed a metal-free, S/C−Li/Si/C full cell in which sulfur was trapped in highly porous hard carbon spherules and coupled with a lithiated Si/C nanocomposite anode and a glycol-based electrolyte. A reversible capacity of ∼300 mA h g−1 was obtained over 100 cycles. Improved performance was further anticipated if problems such as mass balancing between the electrodes or polysulfide shuttling were addressed. Recently, more promising results have been obtained by Yan et al.159b by replacing the conventional organic electrolyte with an ionic liquid, N-methyl-N-allylpyrrolidinium bis((trifluoromethyl)sulfonyl)imide (RTIL P1A3TFSI) and employing a prelithiated Si/C microsphere anode and a S/C composite cathode to achieve a high initial discharge capacity of 1457 mA h g−1 and a capacity of ∼670 mA h g−1 after 50 cycles. However, the sulfur content in the cathode was only 32 wt %. Electrochemical performances of the full cell with higher sulfur loadings and charge/discharge rates need to be evaluated. Similar to Li/Si alloys, Li/Sn alloys with a capacity of ∼993 mA h g−1 are appealing as an anode for Li−S batteries.122 Hassoun et al.97 demonstrated this concept with a Sn/C anode, a Li2S/C nanocomposite cathode, and a gel-type electrolyte. A reversible capacity of ∼800 mA h g−1 (based on the mass of Li2S) was obtained over 30 cycles.97 Furthermore, Kim et al.160a synthesized a thin layer of Al on the lithium anode surface and noticed better cycle stability with the Li/Al anode than similar cells assembled with bare Li. It was concluded that the Li/Al alloy layer was efficient in protecting the lithium anode and mitigating the reaction between dissolved polysulfides and the lithium anode. Li/B alloy was also identified to be benificial in terms of restraining dendrite formation, reducing interfacial impedance of the electrode, and improving the cycle performance to a certain degree in Li−S batteries.160b Despite the high capacities of Li-alloying materials, the huge volume changes associated with the use of Li-alloying materials result in poor capacity retention, which hampers their practical utility.124

in the electrode and cell configuration could be feasible approaches for realizing safe lithium metal anodes for Li−S batteries.124 For example, Ji et al.151b reported an anisotropic, spatially heterogeneous carbon-fiber paper current collector with SiO2 or SiC decoration. The rationally designed current collectors were characterized to be dendrite-free by ex situ scanning electron microscopy (SEM) observation after a deep lithium deposition of 28.8 C cm−2 at a high current density of 4 mA cm−2. Furthermore, graphene sheets could be prepared as 3D current collectors and used to increase the anode surface area, thereby reducing the effective current density and suppressing lithium dendrite development.151c Despite the advances in lithium metal protection in previous studies, complete control of the stability of the passivation layer and the roughness of the lithium metal surface during cycling, especially in Li−S batteries, has not been achieved, which hinders the commercialization of Li−S batteries. 9.2. Silicon Anode

Silicon with its high room temperature theoretical capacity of ∼3579 mA h g−1 (Li15Si4) is a promising alternative for metallic lithium as an anode for Li−S batteries.153 The interest in Sibased anodes dates back to the 1990s.153a,b Nevertheless, application of Si-based anodes in rechargeable batteries was hampered by the severe capacity fade due to the mechanical failure of the active material caused by the huge volume changes occurring during cycling.154 The huge volume change of >300% could result in stresses exceeding the breaking stress of Si, leading to cracking of the Si particles and loss of interparticle contact and active Si for further reaction with lithium.154,155 Recently, this problem has been alleviated and stable cycling has been achieved by employing nanosized Si, involving controlled volume changes and a shorter lithium ion diffusion length.156 However, it is worth mentioning that the theretical gravimetric energy density of the Li15Si4−S couple is approximately 50% lower than that of the Li−S couple (assuming an average voltage of 1.8 V for the Li15Si4−S couple reaction).157−159 For using Si-based anodes in Li−S batteries, it is a prerequisite that lithium be introduced into the cathode or anode before assembly of the cell. One possible design is to couple the Si-based anode with a Li2S cathode. Yang et al.100 prepared a Si nanowire anode and a Li2S−mesoporous carbon composite cathode and obtained an initial discharge energy density of 630 W h kg−1 (based on the mass of the active electrode materials), suggesting the feasibility of a Li2S/C−Si cell configuration. The nanostructured design is essential for both the electrodes to enhance the electronic conductivity and alleviate the large volume changes. However, severe capacity degradation occurred within 20 cycles.100 An alternative design is to couple a prelithiated Si electrode with a sulfur cathode, which mitigated the aforementioned technical problems associated with metallic lithium anode.124 Liu et al.157 demonstrated a rapid prelithiation of Si nanowires, coupled it with a sulfur−mesoporous carbon cathode, and achieved a proof-of-concept S/C−Li/Si full cell. Nevertheless, the lithium cycling efficiency was poor, weakening its practical advantages compared to a metallic lithium anode.157 The poor electronic conductivity of this cycled Si electrode with an irregular blend of porous prelithiated Si nanowires and intact nanowires could be one major reason for the inferior cycling stability of the full cell. Elazari et al.158 also reported a prelithiated silicon anode in combination with a sulfur cathode.

9.3. Carbon Anode

Although graphite materials with the layered structure were reported as the host lattice for lithium ion intercalation/ deintercalation as early as 1976,161 they have not been successfully applied in Li−S batteries. One major factor preventing the use of graphite in Li−S batteries is the incompatibility of the supporting electrolyte systems. Similar to the use of a metallic lithium anode, the use of graphite materials as the anode in rechargeable lithium batteries requires a choice of electrolyte solutions that could lead to surface stabilization of the graphite particles, since the forces between graphene planes in graphite are relatively weak.162 A cointercalation of solvent molecules together with lithium ions between the graphene planes could result in a delamination of the graphene sheets and failure of the graphite electrode.162 This failure mechanism was observed for graphite electrodes processed in ether-based electrolytes, which are the most commonly used electrolytes in Li−S batteries.162 Recently, promising results have been obtained by Brückner et al.152 with the use of carbon-based anodes in Li−S batteries. 11775

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co-diallyldimethylammonium chloride) (AMAC), as the binder in high-loading sulfur electrodes. AMAC is a hydrophilic polymer but is not soluble in organic electrolytes, retaining a stable void structure in sulfur electrodes upon cycling. A highsulfur-loading (80 wt %) electrode with the AMAC binder exhibited better cyclability than that with the PEO binder. Wang et al.172 chemically converted β-cyclodextrin to carbonyl-β-cyclodextrin (C-β-CD) as a water-soluble binder for sulfur electrodes. The cell with the C-β-CD binder and carbonate-based electrolytes exhibited a high initial discharge capacity of 1543 mA h g−1 and a reversible capacity of 1456 mA h g−1 over 50 cycles. The high performance is related to the fact that C-β-CD can tightly wrap the sulfur composite and suppress its aggregation during cycling. Compared to other binders such as PVdF and PTFE, C-β-CD offers the advantages of high performance, low cost, and environmental benignity. Recently, Seh et al.173 selected PVP as a good binder for Li2S cathodes based on ab initio simulations. The results show that the CO groups in PVP have high binding energies of 1.14 and 1.30 eV with Li2S and Li−S• species, respectively, which are much higher than those with −F groups in PVdF, as shown in Figure 25. As a result, the Li2S electrode with PVP binder did

It was found that the hard carbon anode was stable in etherbased electrolytes, making it possible to construct S−Li/C full cells. Cycling stability was obtained for more than 550 cycles with capacity fading as slow as 0.08% per cycle.152 In contrast to graphite materials, the nongraphitized hard carbon has a wider interlayer spacing and cross-links between the domains of the graphene planes, leading to good cycling stability in ether-based electrolytes.

10. BINDERS The binder is critical for fabricating battery electrodes. However, conventional binders such as PTFE or PVdF are not effective for sulfur cathodes because of the dissolution of polysulfides as discussed before. In addition to the development of binder-free electrodes discussed in section 5.4, alternative binders have also been considered for Li−S batteries. This section summarizes the efforts on binders. PEO is one of the earliest alternative binders that has been studied in Li−S batteries. Cheon et al.163 studied sulfur cathodes with different PEO binder contents prepared by two different methods, ball-milling and mechanical stirring. The preparation methods affect the morphology of PEO and porosity in the sulfur cathodes, which influences the cycling performance. In addition, roll-pressing improves the integrity of sulfur cathodes containing a PEO binder, leading to a longer cycle life. Lacey et al.164 found PEO- or PEG-modified cells show improved electrochemical reversibility and suppressed passivation of the cathode at the end of discharge. The reason is that PEO can dissolve in the liquid electrolyte, modifying the electrolyte similarly to PEGDME solvent. Other polymers have also been studied as binders with sulfur cathodes. For example, Sun et al.165 first used gelatin, a natural polymer, as a binder in sulfur cathodes. They compared sulfur cathodes with gelatin and PEO binders and found that gelatin can not only improve the adhesion but also enhance the dispersion in sulfur cathodes, showing better performance than the electrodes with a PEO binder. Later they166 studied sulfur cathodes with a gelatin binder treated by a freeze-drying method, which resulted in a porous electrode structure. Higher initial and reversible capacities have been obtained with these electrodes compared to the normal electrodes with a compact structure. Huang et al.167 compared gelatin and PEO binders in sulfur electrodes. It was found that gelatin could enhance the redox reversibility of sulfur cathodes, resulting in better electrochemical performance than with the PEO binder. Wang et al.107b found the gelatin binder can maintain a porous sulfur electrode structure even after prolonged cycles. A capacity of 1235 mA h g−1 was achieved in the first discharge, and a reversible capacity of 626 mA h g−1 was retained after 50 cycles. Wang et al.168 also found that sulfur electrodes with gelatin binder show high rate capability. In addition, the pH of the gelatin solution was found to affect the battery cycling performance. Zhang et al.169 found the electrode prepared with a pH 10 gelatin solution showed higher discharge capacities and a more stable cycle life than those prepared with pH 5.0 and 8.0 gelatin solutions. He et al.170 utilized a water-soluble binder consisting of SBR and sodium CMC in sulfur electrodes. The SBR−CMC binder can not only act as an adhesion agent but also facilitate the dispersion of active materials, maintaining a uniform and electrochemically favorable structure in the sulfur electrode and leading to better cycling performance than that with the PVdF binder. Zhang171 at the U.S. Army Research Laboratory introduced a cationic electrolyte, poly(acrylamide-

Figure 25. (a, b) Ab initio simulations showing the most stable configuration and calculated binding energies of Li2S and Li−S• species with (a) PVP and (b) PVdF binders. Optical microscopy and digital camera images (inset) showing the electrode slurry of Li2S electrodes with (c) PVP and (d) PVdF binders in NMP solvent. Reprinted with permission from ref 173. Copyright 2013 The Royal Society of Chemistry.

not show large aggregates that are usually observed in the electrode with the PVdF binder and exhibited a stable cycle life over 500 cycles.

11. CELL CONFIGURATIONS As the Li−S batteries owe their high capacity to the occurrence of a chemical conversion reaction rather than an insertion reaction unlike in conventional Li ion batteries, the adaptability and the compatibility of the conventional cell configuration need to be reconsidered. The cell configuration modification has been determined as a crucial factor in improving the performance of Li−S batteries, especially the improvements in the sulfur content/loading and active material utilization. Several attractive novel configurations for the cathode/cell have exhibited significant enhancement in the electrochemical performance of pure sulfur cathodes or dissolved polysulfide 11776

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Figure 26. (a) Schematic model of the bifunctional interlayer architecture. SEM observation and elemental S mapping of various bifunctional interlayers: (b) microporous carbon, (c) MWCNTs, (d) metal foam, and (e) biomass. Part b reprinted with permission from ref 174a. Copyright 2012. Nature Publishing Group. Part c reprinted with permission from ref 16a. Copyright 2012 The Royal Society of Chemistry. Part d reprinted with permission from ref 176. Copyright 2013 Springer Science and Business Media. Part e reprinted with permission from ref 177a. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

26c),96b,175 (iii) porous metal foam (Figure 26d),176 and (iv) porous biomaterials (Figure 26e).177 However, attention should be paid to not impacting the energy density due to the additional weight associated with the introduction of an interlayer.10 11.1.1. Polysulfide-Interception Mechanism. The bifunctional interlayers listed above all fulfill a fundamental criterion: a layered porous architecture. The multilayer network structure is necessary for intercepting the migrating polysulfides, transporting the lithium ions, and channeling the liquid electrolyte. As a result, the active material and the electrochemical reaction can be stabilized within the cathode region of the cell. The polysulfide-interception mechanism is primarily contributed by the microporous structure and the layered porous network. First, the microporous structure in the interlayer functions as the absorption site for immobilizing the active material and limiting the soluble polysulfide shuttling into the electrolyte.174,175a,177 Second, the layered porous structure that is composed of interwoven tubes/fibers serves as the host and as a “fishnet” for accommodating and retaining the dissolved polysulfide species by its high electrolyte absorptivity.16a,177 Moreover, the interlayer must have high electrical conductivity for achieving a high discharge capacity and, more importantly, for reactivating the trapped polysulfides during subsequent cycles. After capturing the migrating polysulfides, the conductive interlayer matrix should freely transport the electrons into the trapped polysulfides and reactivate them, making them available to be reutilized even during long-term cycling. With the interception, absorption, and reutilization processes, the bifunctional interlayer system provides the Li−S

catholyte systems. The use of interlayers, porous current collectors, and sandwiched structures is presented below. Some of the physical/chemical parameters and cycling performances are presented in Tables S3−S5 in the Supporting Information. 11.1. Interlayers

Since commercial polymeric separators are perfect permeable membranes for liquid electrolytes in a battery, they work well to prevent an internal short circuit and permit lithium ion diffusion. However, even with a good cathode system, the Li−S battery might still have some polysulfides migrating from the cathode to the anode, and the polymeric separator cannot stop the free polysulfide migration. To tackle this challenge, our group introduced the concept of inserting a bifunctional interlayer between the separator and the cathode as a polysulfide-diffusion inhibitor.16a,d The bifunctional interlayer has evidenced that it can effectively capture/ localize the migrating polysulfides within the cathode region of the cell, resulting in long-term cycle stability with high sulfur utilization, as shown in Figure 26a. Moreover, the bifunctional interlayer offers additional electron pathways covering the top surface of the cathode, enhancing the conductivity of pure sulfur cathodes and assisting the reutilization of the trapped active material. Therefore, the bifunctional interlayer can be regarded as another kind of separator which aims to block the penetration of soluble polysulfides, resulting in less shuttle effect and better capacity retention. This concept has now blossomed into a series of novel interlayers based on various materials (Table S3 in the Supporting Information): (i) micro/mesoporous carbon (Figure 26b),113c,174 (ii) interwoven CNFs/CNTs (Figure 11777

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Figure 27. (a) Schematic model of the porous current collector architecture. SEM observation and cycling performance of the bifunctional interlayers with various preparation processes: (b) dissolved polysulfide catholyte, (c) thermal sulfur impregnation, and (d) active material paste absorption. Part b reprinted with permission from ref 180. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Part c reprinted with permission from ref 52. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Part d reprinted with permission from ref 29d. Copyright 2013 The Royal Society of Chemistry.

smaller than 2 nm, micropores and small mesopores may have better polysulfide interception compared to macropores and large mesopores.174,178 However, the large mesopores are necessary for assisting fast lithium ion transport and cooperating with macropores to form interconnected electrolyte pathways.113c,177 Third, the high electrolyte-absorption ability is important for stabilizing the electrolyte containing the dissolved polysulfides. Moreover, the excellent electrolyte absorption suggests that there are plenty of interspaces for accommodating the active material. Fourth, the conductivity of the interlayer influences sulfur utilization and reutilization, which controls the reversible capacity during long-term cycling. Therefore, the bifunctional interlayers should have a continuous carbon cluster framework113c,130,174 or interwoven fiber network16a,96b,175−177 as the electron pathway. Fifth, the structural stability of the interlayer requires both physical and chemical stabilities. The structural stability requires high mechanical strength, ensuring that the interlayer will not break during cell assembly or during a long-term discharge/ charge process. Moreover, a flexible porous network may channel off the volume changes from the active material during the charge/discharge process. The chemical structural stability requires that the interlayer must avoid being corroded or ruptured during cell cycling.

cells long-term cycle stability with a high sulfur content with regular sulfur cathodes (i.e., no need to employ sulfur−carbon composite cathodes). The high sulfur content in the cathode is allowable with these cell configurations because the highly conductive bifunctional interlayer works as the upper current collector to reduce the cathode resistance.16a Our group has made further progress by developing a bifunctional separator that integrates the interlayer with the separator,16g,h which is specially designed for Li−S cells to allow lithium ion flow but hinder polysulfide migration. 11.1.2. Constructional Materials for Interlayers. Although all the representative bifunctional interlayers described above show good cell performance, the cyclability of the cells is still influenced by the raw material used for the fabrication of the interlayers and its physical properties: (i) layered porous architecture, (ii) pore size, (iii) electrolyte absorptivity, (iv) electrical conductivity, and (v) structural stability. First, the multilayer porous network is a necessary factor for intercepting the migrating polysulfides. Our group has analyzed the thickness effect by designing a multilayer interlayer module with a controllable thickness and a tunable number of layers. The results demonstrate that a thicker interlayer provides better interception ability and more stable cyclability than a thinner interlayer. Thus, by taking a balance between cell performance and the weight of the interlayer, a suitable thickness of the multilayer porous network can be accomplished.175b Second, considering that the soluble polysulfides have chain lengths

11.2. Porous Current Collectors

In most cases, a 2D aluminum foil is used as the current collector for all types of lithium batteries. However, the structure and morphology of sulfur change a lot during cycling 11778

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Figure 28. (a) Schematic model of the compound-sandwiched electrode. Cell configuration and cycling performance of various derived Li−S cells: (b) Li2S cathode, (c) dissolved polysulfide catholyte, and (d) S cathode. Part b reprinted with permission from ref 96b. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Part c reprinted with permission from ref 177a. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Part d reprinted with permission from ref 130. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

11.2.1. Sulfur Impregnation Strategy. The PCC is a cathode configuration modification that encapsulates the active material into its porous spaces. There are three major categories of sulfur impregnation processes: (i) dissolved polysulfide catholyte (Figure 27b; see section 11.4 for a detailed discussion),16b,21,177a,179 (ii) thermal sulfur impregnation (Figure 27c),25a,c,26c,52 and (iii) active material paste absorption (Figure 27d).29d,e,36c Dissolved Polysulfide Catholyte. The combination of PCC with the liquid-phase polysulfide catholyte allows cells to have a high discharge capacity with a high sulfur loading of >2 mg cm−2, which is hard to simultaneously achieve with cells having the conventional cathode. When the catholyte is added to the PCC, the PCC functions as the active material reservoir that absorbs the electrolyte containing sulfur in its porous spaces, realizing excellent active material encapsulation and uniform sulfur coverage on the wall of the active material container.180 The excellent catholyte immersion and penetration in the PCC can improve the connection among the active material, charges, electrolyte, and conductive substrate. Moreover, the amount of active material in the catholyte that is absorbed in the PCC is more than the amount of active material in the S−PC composite cathode that is surface coated on a flat current collector.

because of its repeated dissolution and precipitation. Therefore, a 3D porous current collector (PCC) may offer additional benefits: (i) serving as the container for holding the sulfur and (ii) functioning as the absorbing points for confining soluble polysulfides. As a result, the PCC is able to stabilize the active material, liquid electrolyte, and electrochemical reaction within the cathode region of the cell. The modification of the necessary component that is already inside the cell should support low active material loss and suppress polysulfide shuttling as shown in Figure 27a. Moreover, the 3D conductive network improves the contact between the active material and the current collector as well, decreasing the cathode resistance. During cell operation, the 3D PCCs behave as the inner electron pathway, facilitating fast electron transport and reactivation of the inactivated areas. Thus, use of PCCs could be a viable and facile approach to improve the cycling performance of Li−S batteries. For example, metal foam,36c,179 porous carbons,16b,21,29d,e,52,96b,130,179,180 AAO templates,25a,c,26c and porous biomaterials177a have been studied to function as the 3D PCC (Table S4, Supporting Information) and offer better cyclability with a higher sulfur loading compared to those with the conventional 2D flat Al current collectors. 11779

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Thermal Sulfur Impregnation. Elazari et al.52 synthesized a sulfur-impregnated activated carbon fiber cloth by a two-step heat treatment process to encapsulate the melting sulfur into the carbon substrate. This approach also offered a significant improvement in the cycle stability: a high capacity retention approaching 95% after 80 cycles with a high discharge capacity above 800 mA h g−1 and a reasonable sulfur loading of around 1.6 mg cm−2.52 Following this concept, several AAO hard template-based approaches have been developed. Although AAO templates offer Li−S cells with high sulfur utilization approaching 90% and a long cycle life over 1000 cycles, the sulfur loading in the cathode declines back to as low as 1.0 mg cm−2.25a,c,26c Active Material Paste Absorption. Active material paste absorption is a facile and cost/time-effective process for preparing high-performance porous sulfur cathodes. Our group designed this method by modifying the conventional slurry casting method.29d,e,36c The paste-absorption method immerses the PCC into the active material paste, allowing the PCC to absorb the paste by the capillary forces and forcing the paste to penetrate into the porous substrate. The resultant porous cathode achieves uniform sulfur coverage on the surface of the conductive matrix, resulting in enhanced cathode conductivity and high discharge capacities. The excellent sulfur encapsulation in the porous spaces allows the absorbed active material to occupy the electrochemically favorable positions during subsequent cycles and leads to stable long-term cyclability for over 100 cycles. Moreover, the PCC can function as the active container to achieve a high sulfur loading of 80% and 2.3 mg cm−2.29e 11.2.2. Constructional Materials for Porous Current Collectors. For promoting the application of PCCs, design and development of promising substrates are critical. First, the PCC must have high electrical conductivity to realize fast election transport and thereby improve the utilization of the active material. Second, the PCC must have abundant porous spaces for accommodating the active material and the polysulfides in the cathode region during the charge/discharge process. According to these two criteria, metal foam,36c,179 porous carbon,16b,21,29d,e,52,96b,130,179,180 porous biomass,177a and polymer substrates are suitable PCC candidates and are introduced here. The AAO template method is not described here because of its complex manufacturing procedures and low sulfur loading. Different from the interlayer, PCC may need a micro/meso/ macroporous structure.29d,e,36c The micropores in the PCC aim to encapsulate and absorb the active material during cathode preparation or during cell cycling. The small mesopores assist micropores in enhancing sulfur encapsulation. The large mesopores not only accelerate the charge accessibility but also cooperate with the macroporous structure to absorb the electrolyte containing the dissolved polysulfides into the cathode.29d,e,36c,177a The macroporous structure is suggested to be contributed by the interwoven fiber architecture, which can further provide electron pathways inside the cathode. As a result, the electrochemical materials/reaction can be stabilized within the cathode region, thereby limiting the loss of active material.

advantages of the bifunctional interlayer and the PPC as shown in Figure 28a, was pursued by our group.96b This newly developed cell configuration showed excellent electrochemical performance with several derived Li−S systems (Table S5 in the Supporting Information): (i) Li2S cathode, (ii) polysulfide catholyte, and (iii) regular sulfur cathode. Our group developed carbon-sandwiched electrodes (Figure 28b) that involve the Li2S active material powder within two layers of self-weaving MWCNT electrodes.96b The upper MWCNT electrode works as the bifunctional interlayer, and the lower MWCNT electrode serves as the PCC. This unique sandwiched electrode architecture enhanced ion/electron transport and localized the cycled products within the sandwiched electrodes, facilitating a high capacity and excellent cyclability at high rates. Following this, our group has further developed a carbonized eggshell membrane reservoir for polysulfide catholyte cells (Figure 28c), including two layers of free-standing biomass carbon film and the dissolved polysulfides in between.177a The carbonized biomaterial localizes the dissolved polysulfides within the sandwiched electrode region, accomplishing a high discharge capacity, longterm cycle stability, and high sulfur loading of 3.2 mg cm−2. Zhou et al.130 also designed a graphene−sulfur sandwich structure with regular sulfur cathodes which effectively reduces the internal resistance and stores/reuses well the active material in the compound electrodes (Figure 28d). 11.4. Dissolved Polysulfide Catholytes

In 1979, Rauh et al.8b demonstrated the first lithium−dissolved polysulfide cell, showing a high utilization of sulfur (1.83 electron per sulfur) in the initial discharge because of the soluble polysulfides with high reactivity compared to solid sulfur particles. Polysulfides can be dissolved in THF with high concentrations (up to 10 M). However, the cyclability was not satisfactory. Zhang et al.16b modified the dissolved polysulfide electrolyte with LiNO3 to promote Li anode passivation and improve the cycling performance with a good capacity retention over 70 cycles. However, the average discharge capacity was only around 500 mA h g−1. Xu et al.181 utilized a DME electrolyte containing predissolved lithium polysulfides and LiNO3 as the lithium salt to suppress the migration of polysulfides into the electrolyte. A high capacity of 1450 mA h g−1 and almost 100% Coulombic efficiency with excellent cyclability were achieved. Chen et al.182 also found that the polysulfide-containing electrolytes can improve the cycling performance of Li−S batteries. Our group180 demonstrated a high initial capacity of 1600 mA h g−1 and a capacity of over 1400 mA h g−1 over 50 cycles at a C/10 rate with lithium− dissolved polysulfide cells with a binder-free MWCNT paper current collector. The nanoscaled MWCNT electrode structure significantly improved the utilization of active material in the polysulfide electrolyte, leading to a highly reversible system. Dissolved polysulfides can also be used in catholytes circulating in a half-flow-mode redox flow battery, as proposed by our group.35,113c To maintain the flow mode, the charge and discharge voltage should be controlled within the dissolved polysulfide region (Li2Sx, x ≥ 4) to avoid clogging in the electrode (see section 12.1).

12. VOLTAGE WINDOW In most cases, Li−S batteries are operated within a voltage window of 1.5−3.0 V for utilizing the capacity generated by the complete chemical reaction between the elemental sulfur and

11.3. Sandwiched Electrodes

In 2013, a compound-sandwiched electrode architecture that includes a bifunctional interlayer on the top, the active material in the middle, and a PCC at the bottom, which integrates the 11780

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renaissance if stabilized lithium metal anode and advanced carbon cathode current collectors are developed. With these great achievements and efforts, the rechargeable Li−S batteries have become the most promising high-energy-density system for next-generation electrical energy storage. Despite the considerable improvements achieved during the past few decades, it will still be a long time before the Li−S chemistry can be successfully employed in practical systems. Realizing high capacities with long-term cycle stability, while retaining a high sulfur loading/content essential for high practical energy density, is challenging. Therefore, future efforts should be directed toward basic science research and approaches that can lead to viable high-performance practical Li−S batteries. The efforts for developing high-performance Li−S batteries can be generalized as follows.

its end product, lithium sulfide. Currently, the cutoff voltage may shift from 1.5 to 1.8 V to avoid the irreversible capacity loss from the LiNO3 additive that is often used in electrolytes to passivate the metallic lithium anode and achieve high discharge/charge efficiency.10,127 In both voltage windows, two discharge plateaus for the reduction reaction and two charge plateaus for the oxidation reaction can be seen at moderate C rates in ether-based electrolytes. However, the kinetics of the upper and the lower discharge plateaus are quite different.20 The upper plateau (2.4−2.0 V) involves the transformation from sulfur (S8) to long-chain polysulfides (Li2S4), and this step is very fast in kinetics. The lower plateau, which is slow in kinetics, involves the electrochemical reaction to generate solid Li2S from the Li2S4 produced at the end of the upper plateau. 12.1. Upper Voltage Plateau

When utilizing only the upper plateau that has the voltage range from 2.4 to 2.0 V in the half-flow-mode redox flow battery, our group reported that the cell can achieve a very high rate capability (6.3 A g−1) and retain stable cyclability.35,113c In 2013, Yang et al.183 demonstrated this concept with a 5 M Li2S8 catholyte possessing energy densities of 97 W h kg−1 and 108 W h L−1. The half-flow-mode Li−S cell without the ionselective membrane that inherited the same strategy and used Li2S8 as a catholyte in the cell delivered a steady energy for more than 2000 cycles, which may become a promising system for large-scale storage of renewable energies.

13.1. Appropriate Dispersion of Active Sulfur

Although the solid-phase sulfur transforms into soluble polysulfides during cycling and changes its initial morphology, the utilization of the active material will be low if sulfur has insufficient contact with conductive agents. Poor connection between the insulating sulfur and the electrical conductor can result in inactive regions in the cathode, leading to low cathode conductivity and low capacity. 13.2. Efficient Absorbing Materials

Polysulfide dissolution and diffusion could lead to a series of severe shuttle issues. Novel conductive and porous materials need to be designed for effectively absorbing and accommodating the active material. Moreover, further improvement could be expected with well-balanced additive:sulfur ratios.

12.2. Lower Voltage Plateau 174b

Our group conceived a recharge approach by controlling the charge capacity for utilizing the lower plateau at ∼2 V. Thus, the redox reaction can be confined within the solid-state region, leading to highly reversible capacity output for extended cycles. Cyclability with an extremely low capacity degradation rate of 0.002% per cycle up to 500 cycles was obtained. Additionally, high Coulombic efficiency could be realized even without the addition of LiNO3 to the electrolyte.

13.3. Flexible Conductive Matrix

Li−S batteries suffer from large volume and structural changes during the sulfur redox reactions. Flexible and robust conductive matrixes are essential for accommodating the Li2S/Li2S2 deposition during discharge and suppressing the formation of irreversible Li2S to avoid deterioration of cycle stability caused by a breaking down of the conductive substrate through stresses induced by the large volume changes.

13. CONCLUSIONS AND FUTURE DIRECTIONS In this review, we have provided a comprehensive account of the developments in Li−S batteries, including major historical progress, associated technical obstacles, and component/ material developments. We also focused on the latest advent of cell configurations, which hold promise for a leap in Li−S battery research. Moreover, advanced characterization techniques that provide a better understanding of the mechanisms involved in the Li−S chemistry were introduced. The application of nanostructured composites, facilitating rapid ion/electron transport while trapping the polysulfide intermediates, have been intensively pursued with Li−S batteries. Sulfur−carbon nanocomposites, which are one dominant type of composite cathode in Li−S batteries, have shown promising performances with high capacity, enhanced rate capability, and long-term cycle stability. In addition, sulfur−polymer composites have offered the advantage of a versatile design of nanocomposite cathodes. Electrolytes also play a critical role in solving the problems with Li−S batteries. For example, the LiNO3 additive enhances the Coulombic efficiency of Li−S batteries and the stability of the lithium metal anode. In addition, novel cell configurations, e.g., carbon interlayer or porous current collector configurations, significantly improve the cycling performance of Li−S batteries even when coupled with a conventional pure sulfur electrode. Furthermore, Li−dissolved polysulfide cells could regain

13.4. Stable Electrolyte Systems

The current liquid electrolyte is far from meeting the demands of the practical utility of Li−S batteries because of the side reactions among polysulfides, electrolyte solvents, and lithium metal. A more reliable electrolyte is preferred to have a controlled dissolution of the polysulfides and to be compatible with the lithium metal anode. Development of suitable polymer or inorganic solid electrolytes may be a promising strategy. 13.5. Safe Anode Materials

A lithium metal anode in liquid electrolytes poses safety concerns due to lithium dendrite development during cycling. To address this problem, use of a protected lithium metal anode or alternative Li/M (M = Si, Sn, C, etc.) alloys is a feasible approach for building a safer Li−S battery. 13.6. Li2S Cathode

Another approach to facilitate the use of a metallic lithium-free anode is to employ Li2S rather than S as the cathode and couple it with high-capacity anodes such as silicon, tin, or metal oxides. This strategy, however, will need to overcome the initial activation barrier associated with micrometer-sized Li2S particles, possibly through the development alternative 11781

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Biographies

chemical synthesis approaches with good control over morphology and the degree of lithiation. 13.7. Novel Cell Configurations

Given the fact that Li−S batteries involve conversion reactions, and not insertion reactions, unlike those in the conventional Li ion batteries, innovations in cell configurations should be considered. Strategies involving interlayers, porous current collectors, and sandwiched electrodes could be attractive solutions to store the active material and confine the migrating polysulfides. Moreover, these novel cell configurations can be coupled with a pure sulfur cathode with high sulfur loadings. However, caution should be exercised that the weight/volume of these alternative porous/conductive substrates does not sacrifice the overall energy density. Coating the separator with a thin layer of carbon on the cathode side could potentially alleviate the weight concern.16g,h

Arumugam Manthiram is the Joe C. Walter Chair in Engineering and the Director of the Materials Science and Engineering Program and Texas Materials Institute at the University of Texas at Austin (UT-

13.8. Smart Recharge Settings

Austin). His research interests are in the area of materials for

Utilizing only the lower voltage plateau at ∼2 V has been proved effective in improving the cycle life of Li−S batteries. The only concern is that the utilization of the lower plateau must be high (>1000 mA h g−1); otherwise, the advantages of the high energy density of the sulfur cathodes will be weakened. Overall, the commercial viability of Li−S batteries could be enhanced with further scientific and technical advances. Development of advanced materials and characterization methods together with a smart engineering design will have a significant impact in the Li−S battery area. In addition, a stabilized lithium metal anode needs to be developed to couple with future long-cycle-life sulfur cathodes. With these improvements, rechargeable Li−S batteries could be the most promising high-energy-storage system for supporting a sustainable and mobile society.

rechargeable batteries, fuel cells, and solar cells, including novel synthesis approaches for nanomaterials. He has authored 550 publications, including more than 470 journal papers. See www.me. utexas.edu/∼manthiram for further details.

ASSOCIATED CONTENT S Supporting Information *

Table S1, parameters of representative S−PC composites, Table S2, parameters of representative metal oxide absorbing agents, Table S3, parameters of representative bifunctional interlayers, Table S4, parameters of representative porous current collectors, and Table S5, parameters of representative compound-sandwiched electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

Yongzhu Fu is currently an assistant professor at Indiana UniversityPurdue University Indianapolis (IUPUI). He obtained his B.E. (2000) and M.S. (2003) in chemical engineering from Tsinghua University and the Dalian Institute of Chemical Physics, respectively, in China, and his Ph.D. (2007) in materials science and engineering from UTAustin. He was a chemist postdoctoral fellow at Lawrence Berkeley

AUTHOR INFORMATION Corresponding Author

National Laboratory, research scientist at Lynntech, Inc., and research

*E-mail: [email protected]. Phone: 512-471-1791. Fax: 512-471-7681.

associate at UT-Austin before joining IUPUI in 2014. His research is

Notes

focused on new materials for electrochemical energy conversion and

The authors declare no competing financial interest.

storage. 11782

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research interests are in high-energy-density batteries, green chemistry, and sustainable technology.

ACKNOWLEDGMENTS Financial support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-SC0005397, Seven One Ltd., and Welch Foundation Grant F-1254 is gratefully acknowledged. REFERENCES (1) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Chem. Rev. 2011, 111, 3577. (2) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587. (3) (a) Skotheim, T. A. High Capacity Cathodes for Secondary Cells. U.S. Patent 5,460,905, Oct 24, 1995. (b) Skotheim, T. A. High Capacity Cathodes for Secondary Cells. U.S. Patent 5,462,566, Oct 31, 1995. (c) Peramunage, D.; Licht, S. Science 1993, 261, 1029. (d) Chu, M.-Y. Rechargeable Positive Electrodes. U.S. Patent 5,686,201, Nov 11, 1997. (e) Mikhaylik, Y. V.; Skotheim, T. A.; Trofimov, B. A. Lithium Batteries. U.S. Patent 6,936,382, Aug 30, 2005. (4) (a) Dahl, C.; Prange, A.; Steudel, R. Metabolism of Natural Polymeric Sulfur Compounds. Biopolymers Online; Wiley-VCH: Weinheim, Germany, 2005; p 35. (b) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Nat. Mater. 2012, 11, 19. (5) (a) Mikhaylik, Y. V.; Akridge, J. R. J. Electrochem. Soc. 2004, 151, A1969. (b) Akridge, J. R.; Mikhaylik, Y. V.; White, N. Solid State Ionics 2004, 175, 243. (c) Mikhaylik, Y. V. Methods of Charging Lithium Sulfur Cells. U.S. Patent 7,019,494, March 28, 2006. (d) Mikhaylik, Y. V. Methods of Charging Lithium Sulfur Cells. U.S. Patent 7,646,171, Jan 12, 2010. (6) Danuta, H.; Juliusz, U. Electric Dry Cells and Storage Batteries. U. S. Patent 3,043,896, July 10, 1962. (7) Rao, M. L. B. Organic Electrolyte Cells. U.S. Patent 3,413,154, Nov 26, 1966. (8) (a) Nole, D. A.; Moss, V. Battery Employing Lithium-Sulphur Electrodes with Non-Aqueous Electrolyte. U. S. Patent 3,532,543, Oct 6, 1970. (b) Rauh, R. D.; Abraham, K. M.; Pearson, G. F.; Surprenant, J. K.; Brummer, S. B. J. Electrochem. Soc. 1979, 126, 523. (c) Yamin, H.; Peled, E. J. Power Sources 1983, 9, 281. (d) Peled, E.; Sternberg, Y.; Gorenshtein, A.; Lavi, Y. J. Electrochem. Soc. 1989, 136, 1621. (e) Peled, E.; Gorenshtein, A.; Segal, M.; Sternberg, Y. J. Power Sources 1989, 26, 269. (9) Mikhaylik, Y. V. 216th Electrochemical Society Meeting Presentation, Vienna, Austria, Oct 4−9, 2010. (10) Zhang, S. S. J. Power Sources 2013, 231, 153. (11) Cheon, S. E.; Ko, K. S.; Cho, J. H.; Kim, S. W.; Chin, E. Y.; Kim, H. T. J. Electrochem. Soc. 2003, 150, A796. (12) Barchasz, C.; Molton, F.; Duboc, C.; Lepretre, J. C.; Patoux, S.; Alloin, F. Anal. Chem. 2012, 84, 3973. (13) Lee, Y. M.; Choi, N. S.; Park, J. H.; Park, J. K. J. Power Sources 2003, 119, 964. (14) (a) Mikhaylik, Y. V. Storage Life Enhancement in LithiumSulfur Batteries. U.S. Patent 6,436,583, Aug 20, 2002. (b) Ryu, H. S.; Ahn, H. J.; Kim, K. W.; Ahn, J. H.; Lee, J. Y.; Cairns, E. J. J. Power Sources 2005, 140, 365. (c) Ryu, H. S.; Ahn, H. J.; Kim, K. W.; Ahn, J. H.; Cho, K. K.; Nam, T. H. Electrochim. Acta 2006, 52, 1563. (15) Yang, Y.; Zheng, G.; Cui, Y. Chem. Soc. Rev. 2013, 42, 3018. (16) (a) Su, Y.-S.; Manthiram, A. Chem. Commun. 2012, 48, 8817. (b) Zhang, S. S.; Read, J. A. J. Power Sources 2012, 200, 77. (c) Hassoun, J.; Scrosati, B. Adv. Mater. 2010, 22, 5198. (d) Gorkovenko, A.; Skotheim, T. A.; Xu, Z.-S. Cathodes Comprising Electroactive Sulfur Materials and Secondary Batteries Using Same. U.S. Patent 6,878,488, April 12, 2005. (e) Boguslavsky, L. I.; Gavilov, A. B.; Mikhaylik, Y. V.; Skotheim, T. A. Composition Useful in Electrolytes of Secondary Battery Cells. U.S. Patent, 5,962,171, Oct 5, 1999. (f) Manthiram, A.; Su, Y.-S. Porous Carbon Interlayer for

Sheng-Heng Chung is currently a Ph.D. candidate in the Materials Science and Engineering Graduate Program at UT-Austin. He obtained his B.S. (2006) in resources engineering from the National Cheng Kung University and M.S. (2008) in materials science and engineering from the National Tsing Hua University in Taiwan. He was a research assistant in the Safety and Health Technology Center in Taiwan before joining UT-Austin. His research is focused on advanced cell configurations for Li−S batteries.

Chenxi Zu is currently a Ph.D candidate in the Materials Science and Engineering Graduate Program at UT-Austin. She obtained her B.E. (2011) in materials science and engineering from the Beijing University of Aeronautics and Astronautics (Beihang University) in China. Her research is focused on high-energy-density Li−S batteries, surface/interface studies, and metallic anode protection.

Yu-Sheng Su is currently working at Intel as a process engineer. He obtained his B.S. (2005) and M.S. (2007) in materials science and engineering from the National Tsing Hua University and his Ph.D. (2013) in materials science and engineering from UT-Austin. His 11783

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