Research Progress toward the Practical Applications of LithiumâSulfur

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Research Progress toward the Practical Applications of Lithium− Sulfur Batteries Joshua Lochala,† Dianying Liu,† Bingbin Wu, Cynthia Robinson, and Jie Xiao* Department of Chemistry & Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, United States ABSTRACT: The renaissance of Li−S battery technology is evidenced by the intensive R&D efforts in recent years. Although the theoretical capacity and energy of a Li−S battery is theoretically very high, the projected usable energy is expected to be no more than twice that of state-of-the-art Li-ion batteries, or 500 Wh/kg. The recent “sulfur fever” has certainly gathered new knowledge on sulfur chemistry and electrochemistry, electrolytes, lithium metal, and their interactions in this “new” system; however, a real advance toward a practical Li−S battery is still missing. One of the main reasons behind this is the sensitivity of Li−S batteries to the experimental testing parameters. Sophisticated nanostructures are usually employed, while the practicality of these nanomaterials for batteries is rarely discussed. The sulfur electrode, usually engineered in a thin-film configuration, further poses uncertainties in the knowledge transfer from the lab to industry. This review article briefly overviews the recent research progress on Li−S batteries, followed by a discussion of the Li−S battery system from the authors’ own understandings collected from their past few years of research. The critical findings, the unresolved issues, and the scientific gap between lab research and industrial application are discussed. The future work in Li−S battery research is also explored to propel relevant research efforts toward industrial applications. KEYWORDS: Li−S batteries, electrolyte, thick electrode fabrication, pouch cell design, energy storage

1. INTRODUCTION The Li−S battery system was first explored more than 30 years ago, when organic polar aprotic solutions were identified to enable the reversible electrochemical reactions of polysulfides.1,2 Starting from the mid-2000s, after almost two decades, Li−S batteries have regained a position of paramount research interest because of the urgent need to develop high-energy and low-cost new energy storage technologies for vehicle electrification and grid applications.3The Li−S battery has been demonstrated by its use to power the Zephyr high-altitude pseudosatellite aircraft, which flew for over 14 days.4 The Li−S batteries supplied by Sion Power Company have a high energy of 350 Wh/kg after minimization of the total pack weight. A current benchmark of Li-ion batteries is the Panasonic NCR18650B, which has a gravimetric energy density of ca. 240 Wh/kg. The high energy of the Li−S battery comes from the electrochemical couple, i.e., S and Li.5 The S cathode undergoes multiple electron transfer reactions during cycling and thus has a high theoretical capacity of ca. 1680 mA·h/g. The lithium anode has the lowest electrochemical potential and is the lightest metal on the periodic table, translating to its high gravimetric energy.6 Although the average operating voltage of S in the cell (ca. 2.0 V) is lower than that of state-of-the-art Liion batteries (LIBs), the high theoretical capacity of S compensates for the voltage deficiency.7 The Li−S battery has been considered one of the most promising next-generation battery technologies.8 However, critical challenges exist before market penetration of Li−S batteries can be achieved. Low Coulombic efficiency and prodigious battery capacity loss are © 2017 American Chemical Society

rooted in the dissolution of polysulfides in the electrolyte (Figure 1).9 The Li-metal anode itself has tremendous

Figure 1. Schematic illustration of the polysulfide shuttle effect in a Li−S battery using liquid electrolyte. The dissolved long-chain polysulfides produced at the cathode side migrate to the anode side and get reduced. Some of the slightly reduced polysulfides move back to the cathode, forming the well-documented “shuttle” reactions. Some of the reduced polysulfides on the anode side permanently reside on Li metal in the form of Li2S or Li2S2. Reprinted with permission from ref 9. Copyright 2014 Elsevier.

problems in terms of dendrite formation, pulverization, and solid electrolyte interphase (SEI) buildup (Figure 1), all of which further worsen the cycling stability and safety of this “beyond Li-ion” battery technology. Cathode Issues. The majority of the published Li−S work is related to the S cathode.10 Most approaches try to confine the soluble species to the cathode side. Carbon is one of the Received: May 4, 2017 Accepted: June 15, 2017 Published: June 15, 2017 24407

DOI: 10.1021/acsami.7b06208 ACS Appl. Mater. Interfaces 2017, 9, 24407−24421

Spotlight on Applications

ACS Applied Materials & Interfaces

Figure 2. (a) Schematic diagram of sulfur (yellow) confined in the interconnected pore structure of mesoporous carbon CMK-3. Reprinted with permission from ref 3. Copyright 2009 Nature Publishing Group. (b) Schematic of the synthetic process of yolk−shell S/TiO2 nanostructures. Reprinted with permission from ref 17. Copyright 2013 Nature Publishing Group. (c) Illustration of MOF structures. Two different sizes of pores are represented by dark-yellow and blue spheres. Reprinted from ref 18. Copyright 2014 American Chemical Society. (d) Schematic illustration of the construction and cycling of the PANI/S composite. Reprinted with permission from ref 19. Copyright 2012 John Wiley & Sons, Inc. (e) Schematic model of the bifunctional interlayer architecture. Reprinted from ref 21. Copyright 2014 American Chemical Society. (f) Schematic of MOF@GO separators in lithium−sulfur batteries. The MOF@GO separator acts as an ionic sieve toward the soluble polysulfides. The enlarged image illustrates the MOF pore size (approximately 9 Å). Reprinted with permission from ref 23. Copyright 2016 Nature Publishing Group.

most popular materials used to build a scaffold for sulfur because of the flexibility of the carbon structures and their high electronic conductivity, which improves the utilization rate of insulating sulfur.3,11 Linda Nazar’s group demonstrated that using mesoporous carbon (Figure 2a) can slow the diffusion of dissolved polysulfides.3 Graphene-wrapped sulfur can also enhance the cycling ability of Li−S cells.12−14 Similarly, carbon nanotubes and to some extent zipped graphene seem to effectively immobilize polysulfides, especially after decoration with functional groups.15 In addition to carbon hosts, various metal oxides (Figure 2b) have been investigated for S confinement.16,17 It was found that surface adsorption and diffusion need to be balanced for nonconductive oxides to maximize the performance of Li−S cells. A more intricate form of metal oxide, e.g., a metal−organic framework (MOF), shows positive effects in attracting polysulfide anions, probably through Lewis acid−base reactions (Figure 2c).18 Polymers, such as polyaniline (Figure 2d)19 and polypyrrole20 with mixed ionic and electronic conductivities, are reported to form both physical and chemical bonds with polysulfides, which accordingly improve the cycling stability of Li−S batteries. At first glance, all of these reported approaches seem to work well for Li−S batteries; however, the real differences among these numerous host materials are not clear. Most work deals with very thin (300 Wh/kg) is needed, the loading must be further increased, which will be discussed in the last section. Unfortunately, most of the published data resides far below the discussed S loading level.69 Challenges exist when fabricating thick sulfur electrodes using nanocomposites. Nanoparticles facilitate Li+ transport by shortening the diffusion path and also alleviate the volume expansion of electrode

those cells containing organosulfides, suggesting that they are different than the traditional Li−S batteries. Roles of S 3 •− Radicals in the Electrochemical Reactions. The production of S3•− radicals is related to the disproportionation or dissociation reaction of S62− anions. Therefore, the radicals can be used to index the existence of Li2S6 (S62−). The concentration of the radicals demonstrates periodic changes (but never goes to zero) during repeated cycling (Figure 7c),57 which suggests that S62− exists throughout the whole cycle. S3•− radicals were also detected in the chemically synthesized Li2Sx series by ex situ EPR spectroscopy, indicating that a chemical equilibrium instead of a stoichiometric Li2Sx is produced after mixing of S and Li2S. Combined with the in situ EPR results, this suggests that a temporal chemical equilibrium among different polysulfides, instead of step-by-step chain cleavage of polysulfides, exists at each different potential. That is, chemical and electrochemical reactions occur concurrently during cycling (Figure 7d). The proposed reaction mechanism helps to explain why the voltage profiles for discharge and charge are different no matter how low the current density is. The residual S3•− radicals at the end of the discharge facilitate the conversation of Li2S/Li2S2 to long-chain polysulfides in the next charge process, leading to a different reaction pathway during charging (Figure 7d). When the process is started directly from commercial Li2S, the overpotential during the first charge is usually much higher because of a lack of sulfur radicals. Once radicals are generated, 24414




between energy and power needs to be considered instead of achieving the highest energy. The detailed sulfur loading and coating weight on each electrode therefore must be carefully designed depending on the required power/energy ratio. For thick S cathodes, external pressure is applied to the fresh electrodes to adjust the thickness and porosity of the electrodes. This step helps to enhance the adhesion between S/C particles and the current collector, improving the cycling stability. Pressing the electrode also reduces its porosity, resulting in reduced electrolyte intake; however, electrolyte wetting also becomes challenging. Accordingly, capacity is sacrificed after electrode pressing. Vacuum does not work for DOL/DME-based electrolytes, which are highly volatile. Carbon nanotubes and graphene are found to facilitate fast electrolyte penetration56 into the interior electrode, probably because of the hydrophilic functional groups on their surfaces. S electrodes with different microstructures will have different responses to the external pressure. A balanced performance among the thickness, electrolyte intake, initial capacity, reversible capacity, and cycling stability needs to be identified (Figure 8c−e). More work needs to be conducted from the electrolyte/additive point of view to facilitate the wetting of thick S electrodes. Again, the challenge of uniform distribution of a minimum amount of electrolyte is only observable in thick electrodes, highlighting the importance of investigating and understanding the electrodes at relevant scales. Transition of the Dominant Influencing Factor in Li−S Cells during Cycling. The charge flows through the cathode and anode in any battery are always equal. When a thin S cathode is used, the total number of electrons participating in the electrochemical reaction is very limited. Accordingly, the counter electrode, i.e., the Li metal, undergoes very shallow cycling at the same time. If more S is loaded on the cathode, the participation of the Li anode is also increased during each stripping/deposition process. After the same number of cycles, more Li will be consumed irreversibly in a cell coupled with thick S electrodes than in one with thin cathodes. The end result is the rapid increase in cell impedance, which terminates the operation of batteries consisting of high-S-loading electrodes. That is, the thicker the S cathode is, the faster is the observed capacity degradation. For the same S cathodes, the rate or current density also impacts the morphology of Li formed on the anode side. A higher current density means that less time is required to deposit the same amount of Li metal on the anode. Higher current density promotes the formation of smaller Li grains with higher surface area.75 The more expansive Li metal interphase with the electrolyte will then irreversibly generate more SEI components. The buildup of these components at the Li grain surfaces prevents the compact (dense) fusion of the Li metal grains together, resulting in increased porosity (Figure 9a,b), and restricts the metallic Li− Li contacts at the Li metal grain boundaries. It is not surprising to see deep Li corrosion after limited cycling when a Li-metal electrode is coupled with a thick S cathode (Figure 9c,d).56 If soluble polysulfides exist, the “contamination”/passivation of the Li anode gets even worse, as proved by X-ray photoelectron spectroscopy (Figure 9d). A general Li−S failure mechanism can be described as follows. In the beginning, capacity decay is mainly related to the loss of dissolved polysulfide. Then a temporary balance of S diffused in and out is reached, as reflected by the relatively stable cycling after the first 10−20 cycles. When this stable cycling starts and how long it lasts depend on the cathode

materials that undergo conversion reactions. However, it is very difficult to coat high-surface-area nanoparticles uniformly onto a traditional current collector without the formation of pinholes and cracks. The thicker the electrode coating is, the easier it is for the nanoparticles to come off.70 This is particularly true if the binder content is less than 10% in order to minimize the weight of nonactive materials within the cell. Although relatively thick electrodes may still be punched from poorly coated electrodes, that is only useful for publication purposes. Large-area uniform coatings on the current collector need to be emphasized for large-format cell fabrication. The low tap density of the nanomaterials also substantially sacrifices the volumetric energy density of the batteries, limiting the practical application of Li−S batteries for electrical vehicles. The key challenge is how to increase the particle size while still taking advantage of the unique properties derived from nanostructures. One example is to integrate nanosized primary porous carbon into secondary micron-sized particles (Figure 8a).56,69 S is then infiltrated into these interconnected pores and homogeneously distributed within the primary particles. The as-prepared S/C particles have improved tap density and easily form a uniform coating with adjustable thickness, which becomes adaptable by industry. Other reported works on fabricating thick S cathodes are based on a similar idea of synthesizing secondary particles, although different approaches are used.71−74 A closer inspection of the thick S electrodes indicates that the areal capacity does not increase linearly with S loading (Figure 8b). Instead, a peak areal capacity is seen at a S loading of about 3.5 mg/cm2 for the electrodes composed of integrated Ketjenblack carbon particles.56 This is the case because the electronic conductivity of the sulfur cathode as a whole decreases when the electrode thickness increases. Additionally, when immersed in the electrolyte, the thick S electrode cannot easily be fully wetted by the electrolyte. Thus, the ionic conductivity of the whole S cathode also decreases with increased S coating on the electrode. When the electrode becomes too thick and dense, the utilization rate/capacity of sulfur decreases rapidly because of this decreased ionic and electronic conductivity. The areal capacity (mA·h/cm2) is the product of the S loading (mg/cm2) and the usable capacity of S (mA·h/g). The former increases while the latter decreases as the electrode is thickened. At a certain limit, e.g., a sulfur loading of 3.5 mg/cm2, a balance point between the sulfur loading and sulfur utilization rate is reached, as reflected by the maximum areal capacity in Figure 8b. This maximum areal capacity and its corresponding S loading largely depend on the porosity and tortuosity of the S/C composite electrode. Through appropriate modifications, the peak value can be further pushed to a higher deliverable energy/capacity. For example, specific areal capacities of 9 mA·h/cm2 or even higher have recently been reported for S cathodes.28 For electrodes with high S loadings, a comparison of the electrode thickness with that of state-of-the-art cathodes for Li-ion batteries is necessary since the thickness is directly related to the volume of the final cell. As a reference, the thickness of a traditional LiNi1/3Mn1/3Co1/3O2 cathode with an areal capacity of 4 mA·h/ cm2 is approximately 90−100 μm. If the high-S-content cathode is accompanied by an unusually high thickness, volumetric energy density will be largely sacrificed. In other words, the volumetric S loading (g/cm3) is a more direct parameter in determining the battery energy. It should be pointed out that for traction batteries, an appropriate balance 24415




battery, prelithiated graphite is used as the anode while the cathode remains sulfur. In general, graphite only works in the presence of EC, which forms a protective SEI layer to avoid continuous cointercalation of solvent molecules, which would exfoliate the graphite layers. Unfortunately, EC is incompatible with polysulfides and the sulfur radicals generated during cycling of Li−S batteries.54 Inspired by Yamada’s work,80 concentrated LiTFSI/DOL was prepared and found to enable reversible cycling of graphite without EC.78 The voltage profiles in Figure 10a are consistent with standard discharge−charge curves, indicating that both S and Li are stable in the concentrated electrolyte. After 100 cycles (Figure 10b), a capacity retention of 81.3% was achieved at 0.5C with a high Coulombic efficiency of above 97%. It should be noted that there was no LiNO3 additive in the concentrated LiTFSi/DOL electrolyte, indicating that the shuttle reaction was largely reduced as a result of the removal of Li metal. Reduced solubility of polysulfides in the concentrated electrolyte should also help. A key question in this work is how the graphite lattice is protected by the ether-based electrolyte. If graphite is cycled in regular 1 M LiTFSI/DOL, it cannot reversibly react with Li+ ions.78 Although DOL can be polymerized if a Lewis acid exists in the contaminated electrolyte,81 the polymerized DOL cannot effectively protect the graphite lattice, as reflected by the poor cycling. The surfaces of graphite (harvested from Li/graphite half cells) cycled in the concentrated electrolytes were surprisingly clean (Figure 10c). In contrast, graphite cycled in 1 M LiTFSI/DOL experienced severe exfoliation (Figure 10d). On the basis of the pioneering work of Yamada80 and Wang and Xu82 on concentrated electrolytes, a fundamentally new SEI formation mechanism has been proposed for concentrated electrolytes. It is hypothesized that a reversible protecting layer, precipitated from partially solvated salts, can be induced by an electric field on the electrode surface when it is in contact with a highly concentrated electrolyteregardless of its specific composition. More detailed information can be found in a recent publication.83 A few drawbacks of the Li-ion S battery should be addressed. The prelithiation step of graphite is not cost-effective and brings more risks in terms of controlling the Li content in the cell. When either lithium powder or an ultrathin lithium film is used, homogeneous and full prelithiation of graphite is still very challenging. If the graphite is overlithiated, dendrites may readily form before cell operation, increasing the safety concerns. If Li is deficient, the cycling ability will be poor. The cell balance between the S cathode and the graphite anode needs to be carefully optimized. Using a Li2S cathode may help address these issues, but the activation and long-term cycling stability of Li2S seem to be challenging as well.84 The energy of Li-ion S batteries is also less than that of Li−S cells because of the difference in anodes. A 255 Wh/kg Li-ion S battery was projected in a pouch cell with ca. 3 A·h capacity.77 The main advantage of a Li-ion S battery may come from its cost, which may enable its deployment in large-scale energy storage.

Figure 9. (a, b) Cross-sectional SEM images of the Li anodes harvested from a Li/lithium nickel cobalt aluminum oxide cell after 100 cycles at charge rates of (a) 0.2C and (b) 0.5C. The faster the deposition of Li during charge is, the more Li transits from dense to porous structures. (c) surface and (d) cross-sectional views of a Li anode harvested from Li−S cells after 100 cycles. The existence of dissolved polysulfides accelerates the passivation of Li metal and increases the cell impedance quickly. Reprinted with permission from ref 56. Copyright 2015 John Wiley & Sons, Inc. (e) Long-term cycling of a Li−S cell with a thick S cathode. After about 180 cycles, a sudden capacity degradation due to the impedance buildup on the Li anode side is seen. When a thin-film S electrode is used, this sudden capacity drop is largely delayed.

structure, electrolyte, and testing conditions. Meanwhile, the SEI quickly accumulates on the anode side, and the electrolyte is drained during the process of Li pulverization upon cycling. A sudden capacity drop after about 100 cycles is always seen (Figure 9e),76 suggesting that the impedance increase on the Li anode side becomes the principal factor that terminates the cell. The reported thousands of stable cycles of rechargeable Li metal batteries most often reflect the fact that only a very small portion of Li participates in the stripping/deposition during each cycle, which can be traced to the very thin S cathode design. The progressive buildup of cell impedance on Li metal is slow in the cells containing low-S-content cathodes where much less Li is consumed during each cycle. Li-Ion S Batteries. The root cause of Li metal failure is the highly reactive surface of Li and its pulverization during cycling. Similar to the case of Si,77 continuous exposure of new surfaces to the electrolyte leads to quick accumulation of the SEI by irreversible consumption of the electrolyte. To mitigate the volume change of the anode, graphite, an intercalation compound, has been explored to temporarily replace Li metal and construct a so-called “Li-ion S battery”.78,79 In a Li-ion S

2. DESIGN OF HIGH-ENERGY Li−S BATTERIES The button cell is the most common testing vehicle for lab research. However, it is hard to guarantee that everything will still remain the same on scale-up of the cells using different dimensions. This is especially true when Li metal is used. The surface of fresh Li is usually covered by a combination of different salts (e.g., Li2O, Li2CO3, Li3N) depending on the storage environment. As a result, even on the same Li metal, 24416




Figure 10. (a) Charge/discharge curves of a lithiated graphite/S full cell in 5 M LiTFSI/DOL electrolyte without LiNO3 and (b) the corresponding cycling stability and Coulombic efficiency. (c, d) TEM images of graphite after being cycled in a Li/graphite half-cell consisting of (c) 5 M and (d) 1 M LiTFSI/DOL electrolyte for five cycles and (insets) the corresponding electron diffraction patterns. Reproduced with permission from ref 78. Copyright 2015 Royal Society of Chemistry.

Table 1. Li−S Cell Parameters for 350 and 400 Wh/kg Designs

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Figure 11. Cell designs for Li−S batteries: (a) 350 Wh/kg; (b) 400 Wh/kg. The slight increase of 50 Wh/kg in going from (a) to (b) requires a largely increased S coating on the cathodes, which incurs many risk consequences that limit the battery’s performance.

In order to further increase the cell energy from 350 to 400 Wh/kg, without increasing the cell dimensions, the coating weight of S on each side needs to be ca. 6.7 mg/cm2 × 80% × 80% = 4.3 mg/cm2 (Figure 11 and Table 1), so the total S loading must be 8.6 mg/cm2 on the double-side-coated electrode. Accordingly, the thickness of each side of the electrode must increase from 38 to 56 μm when the energy density is increased by 50 Wh/kg. The cell balance, i.e., the ratio of the anode and cathode capacities, is reduced from 1.7 to 1.5 to provide more room to accommodate increased sulfur content. About 11 layers of S electrodes are now needed without changing the thickness of the pouch cell. Although the total amount of electrolyte in the cell does not change much, the electrolyte/capacity ratio is reduced from 2.0 to 1.8 g·A−1· h−1 as a result of the increased S content. The relatively lower electrolyte content in the 400 Wh/kg design is expected to affect the long-term cycling stability of the Li−S batteries. The projected capacity and energy for this aggressive design are 2980 mA·h and 401 Wh/kg, respectively. Although the energy increase is only 50 Wh/kg in going from Figure 11a to Figure 11b, the largely thickened cathode raises many issues such as the electrolyte wetting, the mechanical durability between thick electrodes and current collectors, the decrease in cycling stability due to irreversible electrolyte consumption, and so on. A few considerations need to be discussed concerning Table 1. Cell swelling is expected for Li−S batteries since both S and Li suffer large volume changes during cycling; therefore, at least 20% of the volume has been reserved in the cell design to accommodate this cell swelling. Once swelled, the cell begins to lose electronic/ionic conductivity at various locations within the cell, which can spell disaster for the battery operation. The removal of the copper foil from the design (Table 1) in order to use Li metal as both the anode and the current collector raises some concern in regard to the loss of electronic contact given the randomized location of the pulverized Li particles that occur.85 That is, the excess amount of Li that is designed as the current collector may also directly participate in the electrochemical reactions. During long-term cycling, the Li metal will gradually become the “killing factor” for Li−S batteries. The risk in the 400 Wh/kg design will be higher because of the decreased cell balance, i.e., the smaller excess amount of Li metal on the anode side. Without success in stabilizing the Li

the surface properties will vary significantly at different locations since the distribution of the lithium salts is not uniform on the surface. This will lead to differences in SEI components formed later on the Li. The distribution of current density is also disrupted, which further expounds the differences of the SEI properties formed at different locations of Li metal. When the size of the Li metal is increased, for example in pouch cells, more inconsistencies will occur on this large-area Li metal. In this regard, methods that can homogenize the Li surface properties or produce a uniform artificial SEI would enhance the consistency of the Li metal anode and thus improve the reproducibility of Li−S cells. Table 1 shows the cell designs for 350 and 400 Wh/kg Li−S batteries. All of the cell components have been considered, but only the most related parameters are included in Table 1 to simplify the discussion. The calculation is based on a Z-typestacking cell phone battery with dimensions of 3.3 mm thickness, 39 mm width, and 96 mm length. If it is assumed that the S/C composite contains 80% S and the composite cathode occupies 80% of the weight of the whole electrode (excluding the Al substrate), the S content in the cathode would be 80% × 80% = 64%. The copper current collector is removed from the Li metal anode in the current design and will be discussed later. In order to deliver the 350 Wh/kg energy goal, the coating weight of S needs to be at least 4.5 mg/cm2 × 80% × 80% = 3 mg/cm2 for each side. Since both sides of the Al current collector are coated, the loading of S must be 3 mg/ cm2 × 2 = 6 mg/cm2. The average voltage used in the calculation is 1.9 V at a rate of 0.1C, and the capacity is 1000 mA·h/g. At least 15 layers of double-side-coated cathodes are needed in this 300 Wh/kg design. The amount of electrolyte in the pouch cell is 5.46 g, which translates to an electrolyte/ capacity ratio of 2 g·A−1·h−1. For comparison, standard Li-ion batteries with the same dimensions have an electrolyte/capacity ratio of 1.5 g·A−1·h−1 for mobile device batteries. For EV design, this electrolyte/capacity ratio needs to be increased to 4.5 g·A−1·h−1 in order to prolong the cycling of traction batteries. The space reservation to accommodate cell volume expansion is 20% in the 350 Wh/kg design, since both S and Li will undergo volume expansion during cycling. The projected capacity from this design is about 2729 mA·h for the 350 Wh/ kg cell design. 24418




summarizes the experiences or lessons learned from the authors’ own research. Hopefully, it will provide some useful information and new insights for researchers in the same field and inspire new solutions to address the real challenges in this exciting battery technology.

metal anode, it is hard to achieve Li−S battery technology with both high energy and sufficient cycling stability.

3. CONCLUDING REMARKS Although very attractive, Li−S battery technology needs to overcome many roadblocks before its market penetration can be achieved. Since Li−S batteries belongs to the family of rechargeable Li metal batteries, the most critical challenges are to extend the reversible cycling of Li metal and remove the safety concerns related to Li metal. S cathode research starts from thin films but eventually needs to step into the thick electrode configuration with reasonable S content for practical use. It is helpful to always compare the S cathode with a stateof-the-art high-energy cathode in terms of both gravimetric and volumetric capacity/energy before further tailoring the electrode properties. The feasibility of industrial adoption of the proposed S cathode fabrication and modification needs careful consideration as well. A single approach or recipe will never work since the dominant failure mechanism of Li−S cells changes upon cycling; a battery is a system, not just an individual material or electrode. The key still exists in the electrolytes and their interfaces with both the Li metal and the S cathode. Dual electrolytes may be useful in the different sides of Li−S batteries if an appropriate separation membrane can be identified. More research is urgently needed in the following areas, including but not limited to: (1) high-tap-density S/C composites with decent electrochemical performance; (2) electrolytes/additives that can enhance the uniform wetting of thick S cathodes; (3) approaches or additives that effectively adjust the cathode porosity/tortuosity to minimize the electrolyte intake without sacrificing cell performance; (4) binders that further confine polysulfides and help wetting; (5) methods that favor the uniform redistribution of polysulfides during repeated cycling; (6) modified separators to prevent “shuttle reactions” and reduce self-discharge of Li−S batteries; (7) fabrication of a thin but dense Li metal anode with consistent surface properties; (8) approaches that mitigate and eventually eliminate the continuous SEI accumulation within Li; (9) methods that smooth out the detrimental dendritic Li that may short the cells; (10) engineering approaches to maintain the intimate contact of all components inside Li−S cells. It should be noted that items (8) and (9) are two different failure mechanisms of Li metal that are always entangled during cycling. The Li-metal anode is known to be very challenging because of its high reactivity. Future work associated with the Li-metal anode needs more effort to be pursued in the area of electrolytes, additives, and separators, which should function corporately. A standard testing protocol may also be needed for an effective evaluation of different approaches used to protect the Li metal anode. Because of the limited knowledge of the authors, there are many important points that may have been missed. It was also impossible for the authors to cover all of the excellent work reported in literature in this short review article. Different understandings or opinions will also exist for the conclusions drawn in this work. This review article, however, mainly

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jie Xiao: 0000-0002-5520-5439 Author Contributions †

J.L. and D.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Arkansas Research Alliance for financial support. J.X. also thanks her colleagues, Drs. Dongping Lv and Jianming Zheng at Pacific Northwest National Laboratory, for their helpful suggestions and discussions.

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