Designing Realizable and Scalable Techniques for Practical Lithium

Perspective Cite This: J. Phys. Chem. Lett. 2018, 9, 1398−1414

pubs.acs.org/JPCL

Designing Realizable and Scalable Techniques for Practical Lithium Sulfur Batteries: A Perspective Yusheng Ye,† Feng Wu,†,‡ Sainan Xu,† Wei Qu,† Li Li,†,‡ and Renjie Chen*,†,‡ †

Beijing Key Laboratory of Environmental Science and Engineering, School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China ABSTRACT: To progress from the coin lithium sulfur (Li−S) cell to practical applications, it would be necessary to investigate industrially scalable methods to produce high-quality and large quantities of Li−S configurations. In this Perspective, we focused on the feasibility of scalable production of high-quality and large quantities of cathode composite, the construction of highly safe and highly stable electrolyte, and durable lithium metal anode. The results presented here suggest that the construction of highly secondary microstructures from nanoparticles is the key solution to achieve scalable cathode composite. Developing unconventional electrolyte solvent is a meaningful approach to develop high safety Li−S batteries. The high performance and high stability of lithium metal anode will enlighten the practical application of Li−S batteries. This Perspective presents outlooks for the key scalable techniques of realizable Li−S cell in the near future and provides promising strategies to accomplish long-cycle-life, high-energy-density Li−S batteries.

D

confidence to get involved in Li−S batteries because today’s pouch Li−S batteries suffer from many bottlenecks, such as poor cycling life, poor rate performance, and safety issues.4 Understanding their working mechanism would help us find more appropriate solutions to solve these problems. Sulfur is a relatively variable element with a wide variety of oxidation states. The multielectron Li−S chemistry holds complicated multistep redox reactions between lithium and sulfur based on a solid− liquid−solid transition

eveloping renewable and sustainable energy technologies is critical to address increasing global energy consumption and climate change concerns. Since the use of graphite as anode, lithium-ion batteries (LIBs) have achieved great success in replacing the highly polluting and nonrenewable fossil fuels.1 Large demand for portable electronics, electric vehicles, and microelectronic devices also promotes the gradual flourishing of LIBs. However, the inherent limitations of the Li-ion reaction mechanism that makes this type of batteries cannot meet the pursuit of lightweight and high-energy-density energy storage devices. The goal of increasing the share of electric vehicles on the roads, however, calls for energy-storage devices that embrace lower cost and higher energy density as well as longer cycling life. Without end, scientists and engineers never stop seeking higher and better substitutes to make a better choice. Lithium metal batteries, such as lithium−sulfur (Li−S) batteries are satisfying candidates because of their attractively high energy densities (2600 W h kg−1).2,3 Coupled to high-capacity sulfur as positive electrode and lithium metal as negative electrode, Li−S batteries would be more efficient and cost-effective. In addition, earth-abundance, low mass, and environmentally friendly impact are also flash points that make it possible for them to achieve the goal of low-cost and longer mileage/endurance electric vehicles. However, even after more than 10 years as new favorites of scientists, there is still a thing against large-scale production of Li−S batteries. Industrial circles still do not have enough

S80 + 4e− → S24 − S04 + 4e− → 2S2 − + S22 −

Advanced spectroscopy and electrochemical investigation have offered important fundamental insights into Li−S chemistry. The process of Li−S batteries can be summarized as a 2-plateau charge/discharge voltage profile in ether-based electrolytes. During the discharging process, S8 first opens its ring and is lithiated to form long-chain lithium polysulfide intermediates (Li2Sx, 4 ≤ x ≤ 8). This is corresponding to the first discharging voltage plateau (∼2.3 V) with 25% theoretical capacity of sulfur (418 mAh g−1). Long-chain lithium polysulfide intermediates are generally soluble in liquid-ether-based organic electrolyte. Subsequently, long-chain polysulfides further lithiated to insoluble short-chain lithium sulfides, which correspond to the second voltage plateau (∼2.1 V) with 75% theoretical capacity (1255 mAh g−1). Soluble polysulfide intermediates continuously shuttle and diffuse through the separator, reaching and reacting

Even after more than 10 years as new favorites of scientists, there is still a thing against large-scale production of Li−S batteries. © 2018 American Chemical Society

Received: November 29, 2017 Accepted: February 26, 2018 Published: February 26, 2018 1398

DOI: 10.1021/acs.jpclett.7b03165 J. Phys. Chem. Lett. 2018, 9, 1398−1414

The Journal of Physical Chemistry Letters

Perspective

with lithium metal anode to form an unstable solid electrolyte interface (SEI). As a result, some of them are consumed or remaining dissolved in electrolyte to a certain extent. All of these sulfur species do not reprecipitate back to cathode at the end of discharge, causing severe capacity loss and decay. Actually, effective techniques and materials in experimental coin cells usually show disillusionary results for large-scale production and their use in practical Li−S cells.5 We emphasize that the design of realizable and scalable techniques for cathode, electrolyte, and lithium metal anode may be insurmountable processes for the practicality of Li−S batteries. As shown in Figure 1, the industrial process of Li−S batteries needs to be

Although some good results have been achieved at the laboratory level, it is time to consider their feasibility to meet the urgent demands for large-scale production. ratio of electrolyte to sulfur is about 3, the sulfur content of all cathodes (except aluminum foil) should be higher than 70 wt %. In a word, the sulfur content of cathode composite should be higher than 80 wt % normally. At this level, some sulfur will cover the conductive host surface. Yang and coworkers have proposed a series of multicore−shell sulfur composite with high sulfur content up to 91 wt %.10−12 Zhang and coworkers have also prepared a cauliflower-like hierarchical porous C/S cathode with 75 wt % sulfur content, which is thought can facilitate the lithium-ion and electrons transport.13 It is clear that at both of these two end points of sulfur content range, sulfur almost completely covered the carbon hosts. Here, the sulfur species dissolution is essential but also has negative impacts on Li−S battery performance. After charge and cycle, the dissolved polysulfides break the original shape and reprecipitate on the surface of the electrode; this effect alters the morphology of the cathode in each cycle. Sulf ur Loading Approach. Different sulfur loading approaches greatly affect sulfur morphology in sulfur-based composite. There are five main sulfur loading approaches: sulfur melting method, sulfur dissolution and recrystallization, sulfur vapor infusion, physical mixing method, and solution reaction method. Table 1 compares the advantages and disadvantages of different Table 1. Advantages and Disadvantages of Different Sulfur Loading Approaches

Figure 1. Key techniques and their integration for industrial Li−S batteries.

given to the integration of a wide range of batch-ready configurations. Among them, cathode, elctrolyte, and anode are the most important three challenges. It should be pointed out that the optimized sulfur secondary particles could help to maintain structural integrity and inhibit volume shrinkage during slurry coating and drying. Liquid-electrolyte-based and solid-statebased Li−S batteries are two mainstreams near-practical Li−S batteries. On the basis of the advanced anode developments, safety hazards and lithium dendrite formation can be crippling. The integration of these techniques can make more improvements for practical Li−S batteries. Cathode Is Still Unsatisfactory. To overcome the electronic limitation of elemental sulfur and lithium sulfides and the dissolution of lithium polysulfides in the electrolyte, nanoscale approaches have been germinated to develop efficient hosts for sulfur cathode.6 A power strategy is to load sulfur in conductive host materials to improve cathode conductivity as well as confine lithium polysulfide species within the host. A variety of carbons, metal oxides, and conductive polymers are thus dedicated to extract the entire capacity of sulfur cathodes as close as possible.7,8 Although some good results have been achieved at the laboratory level, it is time to consider their feasibility to meet the urgent demands for large-scale production. To compare with state-of-art lithium ion batteries, a typical areal-specific capacity of sulfur cathode higher than 4 mAh cm−2 would be more competitive.9 Sion Power proposed a Li−S prototype cell with 350 Wh kg−1, which maintained at 250 Wh kg−1 after 80 cycles. Taking an average of 300 Wh kg−1 and assuming the average practical capacity of sulfur cathode is 1000 mAh g−1 and the mass

advantages sulfur melting

large-scale

disadvantages agglomeration, large follow-up work, low cathode porosity energy consuming

sulfur vapor wide range of infusion applications sulfur dissolution− accurate control, toxic, large environmental impact, recrystallization energy-preserving human hazards physical mixing large-scale, simple, fast highly depends on the character of conductive hosts solution reaction large-scale, economic hydrogen sulfide exhaust gas treatment

sulfur loading approaches. High-temperature melting method usually operates at 155 °C, where the viscosity of sulfur is lowest. After sulfur melted and incorporated with the conductive host, the composite suffered from a solidification step. Agitation and coiling rate are two key parameters that are worthy of attention by this method. Moreover, an agglomeration phenomenon appears frequently, especially at high sulfur content. Much follow-up work needs to be carried out to get high-quality cathode composites, such as ball milling and sieving. However, the low melting temperature of sulfur puts forward more stringent conditions in the ball-milling process. Furthermore, the obtained sulfur cathodes by this way have poor porosity. In our view, the porosity of sulfur cathode is very important because it affects the electrolyte infiltration. Only for the elemental sulfur converted to lithium polysulfide can the capacity be released more completely in liquid-electrolyte-based Li−S batteries. Sulfur vapor infusion is an impressive method by vaporizing sulfur at elevated temperature in vacuum and then intercalating it into the voids of different scale host materials. 1399



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energy-preserving method to prepare sulfur-based mixture. However, because of the different chemical interaction between sulfur and host, this method is highly dependent on the character of conductive hosts. By comparison, the solution reaction method is a low-environment-impact and economic method to prepare sulfur-based composites. The only concern is the hydrogen sulfide tail gas treatment. Today there are many solutions to deal with hydrogen sulfide, which ensures the operability and feasibility of this method. We can easily control the sulfur content by simply adjusting the amount and concentration and adding speed of reactant. More importantly, the obtained cathode morphology is fluffy and porous, which is a benefit for electrolyte infiltration. To meet the natural abundance and low environmental impact of Li−S cells, complex synthesis methods, contaminated processes, and expensive materials should be avoided in their design to facilitate their large-scale production and ensure that they have a price advantage over conventional LIBs. Although various host materials and flexible polymer coating layers have been intensively investigated to trap the polysulfides within the cathode area, it would be really very different when these sulfur composites were engaged in pouch Li−S batteries.14 One reason is that the fabrication of high sulfur loading cathode without sacrificing specific capacity is a necessary step.15−20 An important breakthrough in such sulfur cathode development was reported by Nazar and coworkers. They prepared a highly scalable graphene− sulfur composite (GSC) with a sulfur loading content of 87 wt % (Figure 2).21 In this report, micron-sized sulfur particles were enveloped by reduced graphene oxide, which not only can form a highly conductive network around the sulfur particles but also can efficiently trap the polysulfides through favorable

Extensive conductive materials can be used as hosts for sulfur insertion by regulating temperature. Essentially, the uptake of elemental sulfur can be efficiently controlled. The downside is that this method is an energy-consuming process. Sulfur dissolution−recrystallization method is another approach to load sulfur into conductive host. This method can be achieved at room temperature and ambient pressure and does not need any complex procedures. Sulfur has poor solubility in conventional organic solvents. Table 2 compares the solubility of sulfur Table 2. Solubility of Sulfur in Some Organic Solvents

solvent

solubility (g/100 g saturated solution)

carbon disulfide acetone benzene toluene trichloroethylene tetrachloroethylene dichloroethylene tetrachloroethane

29.50 2.65 2.07 2.02 1.63 1.53 1.28 1.23

solvent

solubility (g/100 g saturated solution)

chloroform pentachloroethane vinyl chloride carbon tetrachloride anhydrous ether anhydrous ethanol anhydrous methanol

1.22 1.2 0.84 0.83 0.28 0.05 0.03

in some organic solvents. Carbon disulfide has good solubility of sulfur, which is an order or several orders of magnitude higher than other solvents. Unfortunately, it is highly toxic and may cause large human hazards. Even some relatively low-sulfursolubility solvents with lower degrees of toxicity also have potential risks to human health. Physical mixing method is a simple,

Figure 2. (a) One-pot synthesis step for the production of GSC. Reproduced with permission from ref 21. Copyright 2012 Royal Society of Chemistry. (b) Fabrication of monodisperse polymer-encapsulated S nanospheres. Reproduced with permission from ref 22. Copyright 2013 Proceedings of the National Academy of Sciences of the United States of America. (c) Schematic illustration and SEM images of micro-nano-structured CS@CTAB/FC composite fabrication. Reproduced with permission from ref 11. Copyright 2017 Royal Society of Chemistry. (d) Core−shell graphene-encapsulated sulfur (GES) composites as superior cathode materials for lithium−sulfur batteries. Reproduced with permission from ref 23. Copyright 2013 Royal Society of Chemistry. 1400



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Building stable secondary particles is thus of interest to a variety of structural designs because of their distinct physiochemical properties. Taking into account the poor electronic conductivity of sulfur, it is acceptable that decreasing the size of the nanoscale can boost the performance of sulfur-based cathode. Accordingly, one of the biggest challenges associated with the use of highcapacity sulfur cathode is how to increase the structural stability of sulfur electrodes. The organized integration of nanoparticles into large secondary structures is a facile and straightforward strategy that takes care of both nanoparticles’ and large secondary particles’ advantages. The pinhole-like void space between each nanoparticles allows the sulfur to expand without changing the whole secondary particle size. The obtained secondary particles are nanoparticle-dependent, which guarantees the fast electron and ions kinetics process. Besides, large and stable secondary particle sizes are attractive for industrial applications due to their good flow and handling properties. Low volumetric capacity is another challenge of Li−S batteries that is rarely considered in research papers. Because of spaceefficient packing inside the secondary particles, integrated secondary particles have higher tap density than that of primary loose nanosized particles. It is a key parameter to fabricate thick electrode with high mass loading. The chemical reaction between oligomer resin precursor and porogen through an emulsion polymerization is studied to synthesize carbon-nanotube-interpenetrated mesoporous N-doped carbon spheres (MNCS/CNT) (Figure 3a).25 The MNCS/ CNT-sulfur composite can effectively trap soluble polysulfides by its nitrogen sites and mesopores, mitigating the loss of active material. Moreover, the high tap density (0.95 g cm−3) and the large particle size of the microsized MNCS/CNT-sulfur composite allow us to make fairly compact electrodes with >5 mg S cm−2 high sulfur loadings. A secondary cauliflower-like structured C/S composite with enormous hybrid macro-, meso-, and micropores was developed as Li−S cathode (Figure 3b). With 75% high sulfur content and 6−14 mg cm−2 high sulfur loading, this kind of cathode could be close-packed on the aluminum foil. The increased pore volume of reconfigured secondary particles facilitates the electrolyte infiltration and Li-ion transport across the whole cathode.13 To diminish the impact of volume changes during discharge, Byon and coworkers present a bipyramidal shaped sulfur cathode based on 3D multiwalled CNT frameworks. Conducting tubes interwoven in the sulfur particles provides a uniform conducting pathway for high electrical conductivity. 35 vol % sulfur-free region can release the mechanical stress during volume expansion and maintain the cathode from being damaged.26 Integrating carbon nanoparticles into microsized secondary microparticles was developed to address thick sulfur cathode issues.9 Slurry based on this composite is easy to be casted into cathodes with practically usable mass loadings. Inspired by this way and to further increase the interaction electrical connection of nanoparticles, self-assembly methods were proposed to integrate scattered nanocomposites into large secondary particles and to construct integrated thick sulfur cathodes.27 Besides, to reach large-scale synthesis, one-step spray drying approach was used to fabricate wool-ball-like nanocomposite of carbon/sulfur microspheres (SD-C/S).28 Spray drying is a facile and effective approach that has been widely applied in spherical shape granulating in industry. Taking advantage of this approach, the large-scale synthesis of C/S nanocomposite for practical Li−S batteries becomes possible. Novel Cathode Techniques. Although the structure and materials design of sulfur cathodes get some success in capacity retention

hydrophilic−hydrophilic interactions. More importantly, they proposed an essential synthesis method by combining a mixture of conductive host and soluble polysulfide (Na2S∼2.4), followed by aiding HCl oxidant. In this way, the overall gravimetric capacity of Li−S pouch batteries based on this cathode is expected to be greatly increased. Polymeric films are also investigated to avoid the transportation of Li2Sx moieties to the anode side and to maintain the stability of cathode. On the basis of the solution reaction method, Yang and coworkers proposed a series of multicore−shell structured composite cathode materials with polyaniline polymer networks.10,11 The inner conductive polyaniline that grafted to the acetylene black particle can hold sulfur particles, while the outer polyaniline coating can relieve the volume expansion of the cathode and hold sulfur particles, both of which can restrain the diffusion of lithium polysulfides. Another solution reaction synthesis method based on sodium persulfate as sulfur source was reported to fabricate monodisperse polymer (polyvinylpyrrolidone)-encapsulated hollow sulfur nanospheres. Similarly, this approach is suitable for room-temperature operation and scalable production.22 Uniform core−shell structure graphene-encapsulated sulfur (GES) composites based on a solution-chemical reaction−deposition method was also synthesized.23 The synergistic effects of highly conductive graphene and core−shell structures are thought to facilitate good transport of electrons, preserve fast transport of lithium ions, and alleviate the polysulfide shuttle problem. Graphene here can also greatly improve the rate performance of Li−S batteries. The high sulfur content of 83.3 wt % substantially increases the overall gravimetric specific capacity of cathode with respect to the total mass of pouch Li−S batteries. Structural Integrity. As mentioned before, high sulfur loading is very important for designing high-energy-density Li−S batteries. Slurry coating and constructing 3D current collector sulfur-based cathode are the two main methods to achieve high sulfur loading cathode. 3D carbon foam, graphene foam, and carbon fiber cloth are common 3D current collector sulfur-based cathodes.24 Market penetration of the coin cell of 3D current collector sulfur-based cathode from laboratory research to industrialization requires us to consider several issues: (1) The current output effect of largescale 3D current collector is limited so far. (2) The relative high uptake of liquid electrolyte by 3D current collectors is usually larger than 2D coating cathode, which significantly affects the energy density of Li−S batteries. (3) The low tap density of 3D current collector further decreases the volumetric capacity of Li−S capacity. (4) Complex technical process increases the cost of Li−S batteries. Actually, most of high areal loading sulfur electrodes based on 3D current collector are free-standing and aluminum-foil-free. Such aluminum-foil-free and “free-standing” electrodes have high implementation costs to be applicable for industrial production. From a practical point of view, the conventional coating method is more feasible for constructing highsulfur loading cathode. For slurry coating method, increasing sulfur loading means increasing electrode thickness. Attempt to do so usually results in limited kinetics for both lithium ions and electrons transportation. Moreover, thick sulfur electrodes often suffer from cracking and peeling-off problems. Structural failure caused by particle fracture has long been considered an essential reason for poor cycling performance and fast capacity fading. Large volumetric expansion of sulfur particles upon lithiation easily results in electrode structural damage, which further aggravates the dissolution of intermediate polysulfides. More badly, this kind of structural damage issues is often irreversible in most cases. 1401



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Figure 3. Secondary particles discussed here: (a) Carbon-nanotube-interpenetrated mesoporous nitrogen-doped carbon spheres. Reproduced with permission from ref 25. Copyright 2015 WILEY-VCH. (b) Cauliflower-like cauliflower-like hierarchical porous particles. Reproduced with permission from ref 13. Copyright 2015 Nature Publishing Group. (c) 3D multiwalled carbon nanotube frameworks. Reproduced with permission from ref 26. Copyright 2014 WILEY-VCH. (d) Integrated Ketjen Black microparticles. (e) Woolball-like carbon microspheres. Panels d and e are reproduced with permission from refs 9 and 28. Copyright 2015 WILEY-VCH.

will lead to an increasing electrolyte usage, which, in turn, decreases the energy density of practical Li−S batteries. Moreover, the role of the introduced interlayer as a current collector is often limited in pouch batteries. After designing high sulfur loading cathode materials, the last, but not least, method is cathode technique of undamaged thick electrode, which can be easily enlarged. One major reason for thick electrode cracks is the large volume shrinkage when removing solvent from the slurry by solvent-based methods widely used in the present stage. In addition to this, there are unwished side effects and chemical incompatibility added to the system during the drying process of cathode sheets, and sulfur evaporation and morphology change also make the efforts in vain. By the way, the removal of solvent increases the fabrication time and reduces production efficiency and may bring the new contamination to the environment, such as the low-volatile NMP. Moreover, the complete removal of solvent during cathode fabrication is a big challenge. Thus the development of new cathode techniques is an important branch in the process of industrialization. Kaskel and coworkers presented a solvent-free, simple pressing/thermal treatment method for the forming process of flexible freestanding Li−S cathode foils starting from melt infiltrated carbon−sulfur nanocomposite and water-free PTFE binder.34 The mechanical stability was ensured through adhesion forces provided by the 3D PTFE fiber network and structural support provided by carbon nanotube (CNT). A loose, open particle structure allowing the penetration of electrolyte shown from SEM indicated that porosity is controllable using this technology. Utilizing an optimized DUT-19 carbide-derived carbon with cubic ordered mesopores, a stable capacity of >740 mAh g−1 as well as a high discharge−charge efficiency of >96% was achieved over 160 cycles at a current load of 167 mA g−1 (C/10). This pressing/thermal treatment method is especially advantageous for large-scale experiment

and long-term cycling performance, it should be noted that most of these cathodes are under low sulfur loading (usually 2.0 mg cm−2) and sulfur fraction of 60% (90% of Li-ion batteries), so it is difficult for these electrodes to attain the real energydensity potential of Li−S batteries. Once thick electrodes are prepared by traditional blade-casting method, they will fracture and show poor rate performance with low sulfur utilization because of the high carbon content and bad electrolyte immersion through the thick cathodes. From a battery configuration perspective, permselective membranes serve as a separator coating or interlayer and play an upper-current-collecting role that appears to suppress shuttle effect. However, plate lugs are always welding at Al foil in the real pouch batteries. Current cathode fabrication approaches usually suffer from large ohmic polarization, whose collecting effect could be considered minimal. Thereupon, Manthiram and coworker reported pure sulfur electrode with ultrahigh areal loading of 13.9 mg cm−2 covering carbon paper as an upper current collector by plain blade-casting method and achieved the highest areal capacity of 19.2 mAh cm−2 of sulfur cathode, which is four times higher than that of available Li-ion battery electrode.29 More importantly, no complex host material or nanosized sulfur resynthesis or time-wasting process is required in this cathode technique, and it may open new avenues for the revolution of high-energy-density and industry-adopted Li−S batteries. Similarly, a high-flux graphene oxide membrane was directly coated on sulfur cathode surface by shear alignment (Figure 4).30 The shear-aligned graphene oxide membrane is thought to maintain good kinetics of the chargetransfer processes in Li−S batteries. This method is easy to operate, which may help to bring this technique closer to commercial process. Notably, setting interlayers between cathode and separator is a common method to enhance cycle performance in laboratory coin cell, which is thought to trap polysulfides and act as an upper current collector.31−33 However, the use of interlayer 1402



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Figure 4. New cathode techniques discussed here: (a) Shear alignment method. Reproduced with permission from ref 30. Copyright 2016 American Chemical Society. (a) Solvent-free method. Reproduced with permission from ref 34. Copyright 2013 Royal Society of Chemistry. (c) Phase inversion method. (d) 3D printing method. Panels c and d are reproduced with permission from refs 35 and 36. Copyright 2016 WILEY-VCH.

Figure 5. Reaction mechanism of polysulfides in carbonate electrolyte. Reproduced with permission from ref 37. Copyright 2013 American Chemical Society.

Yang and cowokers for the first time put this technique into Li−S batteries by printing sulfur copolymer-graphene architectures with well-designed periodic microlattices as cathode with the ink composed of S particles, 1,3-diisopropenylbenzene (DIB), and condensed GO dispersion.36 Moreover, reduced GO can provide high electrical conductivity for insulating sulfur to improve the utilization of active materials. With superadded diffusion-inhibiting effect for the soluble intermediate polysulfides by strong covalent S−C bonds in the sulfur copolymer and facilitating effect for electrolyte into sulfur by periodic microlattices, these 3D printing materials exhibit high reversible capacity of 812.8 mAh g−1 and good cycle performance as cathode for Li−S batteries. Such a hot 3D printing technique can be widely tried to construct special architectures in this general way. What is more, it can promote the process of industrial automation in various energy-storage fields. Electrolyte: Solvent Is the Key; Solid Electrolyte Is Ultimate. The low complexity of using liquid electrolytes makes liquid electrolytebased Li−S batteries be either foreseen or a reality. Carbonates are a good choice in lithium ion batteries with low electrolyte/active materials ratios. However, the situation completely changes in Li−S batteries. Only the sulfur is strongly confined in a special porous host material, or sulfur is covalently immobilized on polymeric composites, carbonates can be used with a different reaction mechanism, such as sulfur trapped in microporous carbons and polyacrylonitrile−sulfur cathodes. Thus these electrolytes are unsuitable for typical Li−S batteries. Because the highly electronegative oxygen atoms of carbonates show more attractive electron attraction than the single-bonded carbon atoms connected to them (Figure 5), the single-bonded carbon atoms are

and material optimization because of the reproducibility and quickness. Li and coworkers developed a phase inversion method by simply immersing current collector coated with electrode slurry into a water coagulation bath.35 In this way, they have constructed a porous sulfur electrode with interconnected polymer skeleton. It is interesting to find that no obvious electrode deformation was observed during the phase inversion process. More importantly, this method is environmentally friendly and energy-saving for large-scale practical application. Extrusion-type 3D printing used in wide applications is a rapid and accurate object construction technique based on a digital model file adhering ink materials such as liquid metals and polymers layer by layer. It shows more advantages when the object structure is more complex because material addition and structure formation can be finished in just one step, which is the most exciting point of 3D printing technique. During this direct ink writing process, suitable inks with high viscosity and shear-thinning features are essential to the success of the well-defined print products, which means the viscosity of the ink decreases with the increase in shear stress through a nozzle, and it can recover the initial high viscosity after extrusion to guarantee 3D structure retention. Coincidentally, Wallace and coworkers demonstrated gel-like behaviors of highly concentrated graphene oxide (GO) suspensions, so it is possible to print it into filaments and build 3D networks, but only a few works about GO-based microsupercapacitors or lithium-ion microbatteries with extrusion-type 3D printing technique have been carried out, and suitable inks and favorable architectures with high electrochemical performances are still in their infancy. 1403



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partially positive and act as electrophiles.37 It is possible for polysulfides to attack these carbon atoms on the carbonate molecules. Up to now, LiN(SO2CF3)2 (LiTFSI) lithium salt in mixtures of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) has been the most studied liquid electrolyte in Li−S batteries. DME is a polar solvent with a relatively high dielectric constant and low viscosity. This solvent is a good for polysulfides dissolution and ensures complete redox reaction. With DME solvent, sulfur can be easily reduced to insoluble products (Li2S2 or Li2S) when passing through dissolved polysulfide intermediates. DOL is a nonpolar solvent and is thought to generate a protective SEI layer on the surface of Li anode by cleavage of cyclic structures. Even though a series of electrolyte-containing DOL/DME mixtures have been found, these mixtures still need to be further optimized to achieve a trade-off among several key parameters such as viscosity, ion conductivity, electrochemical windows, and safety.38 Highly flammable organic electrolytes for Li−S batteries are still identified as one of the serious safety issues. Undoubtedly, LiNO3 is a significant breakthrough as an additive by forming a more stable and thicker SEI layer. The oxidizing nature of LiNO3 in ether-based electrolytes was found to effectively prevent the reaction based on shuttling mechanism between lithium polysulfides and metallic lithium. Unfortunately, flammable organic electrolytes still have huge security risks. Besides, LiNO3 would gradually exhaust during cycling. Moreover, it may cause explosive hazards with the cathode composition like gun powder (KNO3, sulfur, and carbon). Considering these factors, the LiNO3 additives should not be heavily relied upon in the future. Developing LiNO3-free electrolyte is a considerable method that can shed light on the further development of Li−S batteries.39 In addition, it is also possible to change the solvents to prevent polysulfide shuttling and increase the safety of pouch Li−S batteries.

LiNO3 additives should not be heavily relied upon in the future.

Figure 6. (a) Images of chemically made “Li2S8” dissolved in different solvents with different dielectric constant. Reproduced with permission from ref 40. Copyright 2014 American Chemical Society. (b) Photo presents the solubility of a series of mixtures of Li2Sx (1 ⩽ x ⩽ 8) with and without P2S5 in TEGDME. Reproduced with permission from ref 45. Copyright 2015 American Chemical Society. (c) Polysulfide solubility test of 1.0 M chemically made “Li2S8” in fluorinated electrolyte solvents. Panels b and c are reproduced with permission from refs 44 and 46. Copyright 2015 American Chemical Society.

Unconventional Ether Electrolyte. In all configurations of Li−S batteries, sulfur, carbon-based conductive, electrolyte, and lithium metal are all combustible, making them very unstable during operation. Among them, liquid electrolytes, such as carbonates or ethers, not only have relatively low flash points and low boiling points but also are highly volatile and inflammable. Li−S batteries based on these organic liquid electrolytes would suffer from large serious safety issues. Organic solvents with high dielectric constants can facilitate the dissolution of Li salts and increase the number of charge carriers. Figure 6a shows the chemically made “Li2S8” dissolved in different organic solvents with dielectric constants.40 Solvents with high dielectric constant such as DMSO (46.540) exhibit stronger ability to stabilize charged species compared with solvents with low dielectric constant such as DME (7.075). The rates of these charged species disproportionation reactions affect the rate of Li−S batteries. The reaction rate of low dielectric solvents is higher than that of high dielectric solvents. Thus electrolyte solvent with acceptable dielectric constant accompanied by suitable catalyst that can rapidly eliminate the accumulation of sulfur species in cells would be a possible strategy to design safe and high-performance Li−S batteries in future researches. For example, P2S5 was introduced as an efficient additive for Li−S electrolyte, which was proven to promote the dissolution of Li2S and eliminate the shuttling of polysulfides by passivating metallic lithium.41,42 P2S5 forms a complex with Li2Sx and facilitates the

dissolution of lithium sulfide/polysulfides in ether-based electrolytes (Figure 6b). The addition of redox mediators such as V2O5, LiI, or CoS2 also has a similar capability to promote the oxidation of Li2S to more soluble species.43,44 A long-chain ether-based organic solvent, tetraethylene glycol dimethyl ether (TEGDME), was investigated as an alternative electrolyte solvent due to its low dielectric constant (7.79), low viscosity, and low vapor pressure properties.45 Fluorinated ethers also have superior properties, making them promising cosolvents for Li−S electrolyte. 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE)46,47 and 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane48 with low melting points, low viscosities, low flammability, and poor coordination abilities have been studied as cosolvents to mitigate polysulfide dissolution and promote conversion kinetics from long-chain lithium polysulfides to short-chain lithium sulfides. More Safe Electrolyte. Except for the common used ether electrolytes mentioned above, sulfone-based electrolytes are more promising solvents when concerning the safety issue because 1404



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liquid electrolytes dominate in the field of Li/S batteries at present and novel liquid-based electrolytes have just emerged as candidates, it is unnegligible to use solid-state electrolyte for pouch Li−S batteries. The unique properties of solid electrolytes render some of these types of electrolyte valuable to create safe Li−S batteries with high performance. Polymer electrolyte and inorganic solid electrolytes are the two main trends of solid electrolyte. Among them, the closest step that moves liquid electrolytes toward the solid state is polymer electrolytes. Polymer electrolyte holds many advantages, such as favorable mechanical properties, easy fabrication into thin films, stable interface with Li metal, and high modulus to prevent lithium dendrite formation. Polystyrene-b-poly(ethylene oxide) copolymer electrolyte is expected to dissolve alkali salts and retain mechanical rigidity simultaneously.57 By adding polysulfides, the morphologies and phase behaviors of the copolymer electrolyte were changed. This study demonstrated the possibility of using block copolymer electrolytes for Li−S batteries. Moreover, poly(vinylidenefluoride-cohexafluoropropylene) (PVDF-HFP) also can provide excellent chemical stability and mechanical integrity for processing freestanding films with high ionic conductivity.58 Inorganic solid electrolytes can physically block the dissolution and diffusion of polysulfides, suppress the formation of Li dendrites, and prevent leakage, volatilization, and flammability of electrolytes. Moreover, only lithium ion migration in the electrolyte interior enables nearly united lithium ion transference numbers. Sulfide glasses as inorganic solid electrolytes show high lithium ion conductivities (higher than ∼10−4 S cm−1) at room temperature. Nanoporous β-Li3PS4 has been prepared as a solidstate electrolyte with high ionic conductivity at room temperature (1.6 × 10−4 S cm−1). Besides, this solid-state electrolyte shows superior chemical stability when matching with lithium metal anode and has a wide electrochemical window of 5 V.59 With Li3PS4+5 as the cathode and nanoporous β-Li3PS4 as inorganic solid electrolyte, such assembled Li−S batteries have an impressive cyclability.60 As shown in Figure 7a, a high initial discharge capacity of 1272 mAh g−1 with 100% Coulombic efficiency was obtained at room temperature. Even after 300 cycles, the cathode showed a high capacity of 700 mAh g−1. Thio-LISICONs are found as lithium superionic conductors with conductivities greater than 10−3 S cm−1 at room temperature and used as solid electrolytes for Li−S batteries.61 The all-solidstate Li−S battery showed a reversible capacity of 900 mAh g−1 at a current density of 0.013 mA cm−2. Without polysulfides dissolution, only one discharge plateau was observed. Interface compatibility between electrolyte and electrode (including cathode and anode) is a virtual criterion to assess the feasibility of solidstate electrolyte. Highly deformable LiBH4 can facilitate tight interface formation between cathode and LiBH4 electrolyte.62 The high reducing ability of LiBH4 also allows the use of a lithium metal as anode to enhance the energy density of Li−S batteries. Moreover, this solid-state electrolyte has a high lithium ionic conductivity, >2 × 10−3 S cm−1 above 117 °C. The highly deformable nature of LiBH4 enables a tight interface to be formed between the active materials and electrolyte particles. Even at a discharge rate of 0.5 C (corresponding to 2.5 mA cm−2), all-solid-state Li−S batteries retained discharge capacities of 630 mAh g−1. Employing solid-state electrolytes as separators to potentially impede the diffusion of soluble species is currently recognized as one of the most promising way to consider the high performance of Li−S batteries. Under this examination, the chemical compatibility between solid-state electrolyte material and polysulfide

they are less volatile and have low-toxicity properties. Sulfonebased electrolytes usually have relatively high viscosities and high melting temperatures. Thus sulfone-based electrolytes are rarely used because they show lower power performance than ethers. Tetramethylene sulfone (TMS) and ethyl methyl sulfone (EMS) are the two most intensively investigated sulfones in Li-S batteries. EMS has a lower viscosity than TMS and thus has a higher ionic conductivity for Li−S electrolytes. LiTFSI dissolved in EMS as electrolyte provides both high ionic conductivity and wide liquid range. TMS single solvent containing 1 M LiTFSI also has been directly used as electrolyte for assessing the formation of Li2S and detecting the real-time morphology during battery operation. In a word, because of its low donor ability, TMS has a much lower solubility to polysulfides than DME, thus suppressing the redox shuttle reaction. When sulfone was used alone, the cell showed a poor cycle life and large polarization. Combining the high donor number and low viscosity of ethers with the higher dielectric constant and oxidation potential of sulfones is another feasible approach to design safe and high-performance Li−S batteries. Thus, TMS is explored as a cosolvent in binary solvent mixtures for Li−S batteries. DME and THF have been studied as cosolvents with sulfone. However, both of them showed unsatisfied cycling performance. Li−S batteries based on sulfone/DOL mixed electrolyte cosolvents showed high initial discharge capacities.49 EMS is suggested to be used when combined with a less viscous cosolvent with a low melting point TEGDME.50 The viscosity and solvation ability of the solvents would affect the reactions of dissociation of the S82− and disproportionation of lithium polysulfides in the electrolyte solutions. Ionic liquids (ILs) have been identified as alternative electrolyte solvents because they have many attractive properties, such as high thermal stabilities, low flammabilities, negligible volatilities, and wide electrochemical potential windows. IL electrolytes with low donor ability has also been found to limit the dissolution or diffusion of polysulfide moieties for Li−S secondary batteries. ILs’ solvation behavior is another important property for Li−S battery electrolytes. It is worth mentioning that the presence of LiTFSI in ILs can further reduce the polysulfide solubility, which might be attributed to the anions [TFSI]− serving as the solvent forms Li[TFSI]2 complexes. Thus [TFSI]−-based ILs have relatively low viscosities, high conductivities, and compatibilities with the lithium metal anode.51 Even so, low Coulombic efficiency is still a big obstacle that Li−S batteries confront. Slow Li+ transport in highly viscous IL-based electrolytes can be relieved by increase in temperature. Unfortunately, high-temperature operation would further cause rapid capacity decay and lower Coulombic efficiency. High temperature may increase the solubility of Li2S and lead to serious side reactions. In our view, it might be possible to use ILs to form stable SEI films, but much research must be involved. Sulfones (methyl isopropyl sulfone) mixed with viscous ILs are studied to alleviate polysulfide dissolution and shuttle phenomena by virtue of the low-coordination ability of ILs.52 The presence of sulfones here is expected to reduce the viscosity and improve the ionic conductivity of mixed ILs-based electrolyte. Solid-State Li−S Electrolyte. Solid electrolyte is an ultimate way to solve the shuttle problem for Li−S batteries.53−56 Even though

Solid electrolyte is an ultimate way to solve the shuttle problem for Li−S batteries. 1405



Perspective

Figure 7. (a) Chemical reactions of lithium polysulfidophosphates (LPSPs) and the cycling performance of Li3PS4+5 as the cathode for all-solid-state Li−S batteries at the rate of C/10. Reproduced with permission from ref 60. Copyright 2013 WILEY-VCH. (b) Crystal structures of Li1+xTi2−xAlx(PO4)3 (LATP) and the cycle performance of obtained solid-state Li−S cell based on LATP solid-state electrolyte. Reproduced with permission from ref 63. Copyright 2016 American Chemical Society.

efficiently separate electrodes and prevent lithium dendrite penetration. The porous solid-state framework layer can locally confine different cathode materials and accommodate their volume change. Results showed that the integrated sulfur cathode loading can reach >7 mg cm−2, higher than a close to practical level of sulfur loading. The proposed hybrid Li−S battery based on this bilayer solid-state electrolyte exhibited a high initial Coulombic efficiency (>99.8%) and high average Coulombic efficiency (>99%) for subsequent cycles (Figure 8b). Because solid-state electrolytes have a large possibility of creating all-solid-state cells with controlled lower polysulfide solubility and enhanced lithium metal anode stability, we strongly believe that industrial manufacturing will promote all-solid batteries as an ultimate solution.

solution is the most important key parameter to be addressed. Manthiram and coworkers have proposed a comprehensive approach by assessing the durability of Li1+xTi2−xAlx(PO4)3 in contact with polysulfide solution (Figure 7b).63 It was thought that Li1+xTi2−xAlx(PO4)3 gets lithiated in contact with lithium polysulfide solution and Li2CO3 layers are formed on the particle surface, which effectively blocks the interfacial lithium-ion transport between the liquid electrolyte and solid-state electrolytes. The obtained solid-state Li−S cell exhibited an initial discharge capacity of 978 mAh g−1 at C/10 rate and enabled a long-cycling-life Li−S battery. Although solid-state electrolytes show good promising value and have certain beneficial properties in future application, it should be noted that most inorganic solid-state cells usually have to be operated at high temperature. Moreover, there are still many difficulties in forming effective solid-electrolyte/solidelectrode interfaces for electrochemical reactions. These weaknesses strongly limit the practical application of solid-state Li−S batteries. Because the diffusion rate of lithium ions through a solid electrolyte is very slow, it is more difficult to match them with thick cathode. High sulfur loading means thick electrode to some extent, resulting in limited kinetics for both lithium ions and electrons. The above-mentioned problem intrinsically reduces the power capabilities of Li−S batteries, let alone their operation at low temperatures. The cathode material prepared by mechanochemical treatment on the mixture of inorganic solid-state electrolyte, sulfur, and conductive carbon is a good way to solve ion conductivity in thick cathode because the ionic conductivity of the sulfur cathode is of paramount importance in the cycling of these Li−S batteries. By increasing the contact between active material and solid electrolyte, it is possible to reduce their contact resistance and provide more ion transport channels. A big breakthrough was designed by Hu and coworkers.64 They reported a 3D solid-state electrolyte framework with a bilayer dense-porous structure, as shown in Figure 8a. The thin dense and rigid solidstate layer at the bottom has a high elastic modulus, which can

Because solid-state electrolytes have a large possibility of creating all-solid-state cells with controlled lower polysulfide solubility and enhanced lithium metal anode stability, we strongly believe that industrial manufacturing will promote all-solid batteries as an ultimate solution. It should also be emphasized that the batteries are inherently safer due to their lack of leakage risk and due to the thermally stable electrolytes of the solid-state electrolyte; both are important reasons for the development of all-solid-state Li−S batteries. Toward Diverse and High-Performance Lithium Metal Anode. Except for cathode and electrolyte views, lithium metal anode is another rigorous obstacle for high-energy-density Li−S batteries to enter the commercial market. Although the highest theoretical capacity (3860 mAh g−1) and the most electronegative potential (−3.04 V versus standard hydrogen electrode) are very prominent 1406



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Figure 8. Schematic of the 3D bilayer garnet solid electrolyte and its corresponding hybrid solid-state bilayer Li−S battery. Voltage profile and cycling performance of the hybrid bilayer Li−S cell with a high sulfur mass loading of ∼7.5 mg cm−2 at 0.2 mA cm−2. Reproduction with permission from ref 64. Copyright 2017 Royal Society of Chemistry.

electrostatic shielding layers and finally inhibiting lithium dendrite.65,66 Although these researches occupied a large part of the content, their performances are usually not as good as expected on the large scale because the naturally generated SEI film is still too weak to completely resist the dendrite and the continuous consumption of electrolyte components is still a fundamental problem. As a consequence, there comes a round-bout way explained as “host” designs for lithium metal including hollow carbon spheres, graphene, and polymer nanofibers. These materials are demonstrated to build robust interfacial layers, which could successfully accommodate lithium volumetric expansion without breaking and hinder dendrite growth without reducing lithium-ion conductivity simultaneously. The promising approach of stretchable and compressible artificial SEI interlayer has been moved toward reality by some groups. Cui and coworkers presented a novel coating of silica@poly(methyl methacrylate) (SiO2@PMMA) core−shell nanospheres as an interfacial layer on lithium metal anode with nanoscaled pores, high flexibility, and good mechanical and thermal stability (Figure 9).67 The SiO2 core mainly resisted the growth of lithium dendrite by its high modulus of 68 GPa, while PMMA adhered SiO2 nanospheres together to form the artificial SEI flexible membrane and protected SiO2 from reacting with lithium metal above all. Owing to the synergistic effect between SiO2 and PMMA, the nanopores formed among the packed SiO2 nanospheres were smaller than lithium nucleation size to suppress dendrite and larger than lithium-ion transport diameter to ensure lithium-ion transmission through this artificial SEI. This solution, in principle, improved the Coulombic efficiency of lithium metal anode and contributed a new idea of artificial SEI layer for lithium metal anode protection. On the basis of the perspective of the last example, highmodulus host materials show promise in mechanically suppressing volume expansion and dendrites of lithium metal negative electrode. From counter but conventional understanding, extremely soft polymer coating to improve lithium metal anode

Lithium metal anode is another rigorous obstacle for high-energydensity Li−S batteries to enter the commercial market. advantages of the lithium metal anode, there are concomitant fatal flaws decreasing its use in popularity. (i) Lithium undergoes infinite volume expansion during lithium stripping/plating process due to its “hostless” electrode property, leading to mechanical instability. (ii) The highly reactive lithium metal anode can react easily with liquid electrolyte and soluble intermediate long-chain lithium polysulfides to form an unstable SEI layer that could not totally resist the large volume change during cycling. (iii) The unstable SEI layer will be introduced to the formation of cracks and pits as the cycle of charge and discharge increases, which exposes fresh Li to lower impedance and makes for enhanced flux of lithium-ion at these points. (iv) After uneven Li deposition across the breakage of the SEI layer, Li dendrites with high surface area come into being and grow, facilitated by SEI defects; the dendritic problem is the core crux and has become a headache for researchers all over the world. (v) Lithium dendrites causing dead lithium accumulation and increased impedance not only increase electrolyte decomposition and large loss of active materials sulfur, resulting in low Coulombic efficiency and poor cycle life, but also penetrate the separator after occupying limited free space in the battery, eventually leading to a short-circuit, hot runaway, and dangerous fire. Significant research efforts have been made to address these issues; here we discuss several effective strategies in recent research progress. Stretchable and Compressible Artif icial SEI Layer. Numerous studies focused on anode surface modification to control lithium plating and improve the stability and uniformity of SEI directly from the optimization of electrolyte formulations, such as solvents, salts, and additives of lithium halides, or by introducing 1407



Perspective

Figure 9. Artificial interfacial layer anode: (a) a core−shell silica@poly(methyl methacrylate) (SiO2@PMMA) coating and (b) a soft and flowable polymer coating. Panel a is reproduced with permission from ref 67. Copyright 2017 American Chemical Society. Panel b is reproduced with permission from ref 68. Copyright 2016 American Chemical Society.

of lithium metal electrode. The nasty volume expansion means anode volume change from thorough discharged state to charged state, and this could cause internal pressurization and even bulge in the implemented battery. So, in recent years, much more neoteric ideas have been explored, and research emphasis has been moved onto anode host constructing and current collectors. However, many of the host designs are accompanied by resistance increase and active sulfur loss, and the complex host building-up procedure also increases the cost of the practical application. In such a context, Lu and coworkers presented a facile and low-cost strategy with only one step of copper mesh embedding into lithium metal through mechanical pressure to fabricate 3D porous Cu current collector/Li-metal (3D Cu/Li) composite electrode (Figure 10).69 This 3D Cu provided a cage to confine lithium volume change by accommodating Li into its 3D porous structure, resulting in smooth lithium plating, favoring the utilization of anode lithium metal materials and ensuring the anode’s Coulombic efficiency. Compared with flat Cu current collector, the 3D porous composite structure improved the specific area of lithium metal anode by leaps and bounds, promoted the electrochemical reaction kinetics, and reduced the interfacial resistance. From a macro explanation, the dendrites and the thickness of lithium anode can be controlled at a stationary level, which could guarantee the separator

electrodeposit morphology can also be explained. A more straightforward strategy has been realized to achieve flat and dense Li plating by coating highly viscoelastic and intrinsically flowing polymer directly across the lithium metal surface to be pinhole-free during repeated charging and discharging.68 The simulation measurement indicated that the self-healing polymer repaired the pinholes or cracks and avoided the hot spot creation in SEI, homogenizing lithium ion flux and weakening ionic-conductivity excessive increase at these SEI defects. Under the effect of this adding structure, more strain on the interfacial stability of lithium metal electrode was overcome, and dendritefree lithium deposition was observed at current density up to 5 mA cm−2. Comparatively speaking, this result is more attractive than the performance of the electrolyte-additive-modification method, which usually is only discussed at relatively low current densities (0.1 to 0.5 mA cm−2). These two works highlight the significance of stretchable and compressible artificial SEI layer for suppressing lithium dendrite and provide more potential for lithium metal application. Constructing Anode Host. Plenty of research work focuses on interface stability by making efforts on the comprehensive properties of SEI to prevent lithium dendrites from shooting out from anode surface, but this train of thought could not effectively govern hostlessness that is regarded as another fatal weakness 1408



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Figure 10. Lithium anode based on different hosts: (a) Copper mesh. Reproduced with permission from ref 69. Copyright 2017 WILEY-VCH. (b) Graphene oxide. Reprinted with permission from ref 70. Copyright 2016 Nature Publishing Group. (c) Copper nanowire. Reproduced with permission from ref 71. Copyright 2015 American Physical Society. (d) Mesoporous AlF3 framework. Reprinted with permission from ref 72. Copyright 2017 AAAS.

stabilizing the interface mechanically and electrochemically. The composite anode exhibits good flexibility,

Designing Realizable and Scalable Techniques for Practical Lithium

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