Lithium-sulfur batteries: progress and prospects.

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Lithium–Sulfur Batteries: Progress and Prospects Arumugam Manthiram,* Sheng-Heng Chung, and Chenxi Zu Accordingly, new battery chemistries are being intensively explored. Recently, lithium–air (Li–air) and lithium–sulfur (Li–S) batteries with theoretical energy densities of, respectively, 3500 W h kg−1 and 2500 W h kg−1, have attracted much attention. However, Li–air batteries are met with insurmountable challenges due to several intrinsic problems and low practical energy density.[2] Compared with Li–air batteries, the challenges of Li–S batteries are less severe, so Li–S batteries are believed to be more feasible for practical utility.[3] The high energy density of Li–S batteries originates from the sulfur cathode having a high theoretical capacity of 1672 mA h g−1 and the lithium-metal anode having a high capacity of 3860 mA h g−1. Also, sulfur is abundant and environmentally benign. During discharge, Li+ ions are produced at the lithium-metal anode and move through the electrolyte to the sulfur cathode, while the electrons flow through the external circuit, producing Li2S as the final discharge product at the cathode (Figure 1). Despite the advantages of high energy density and low cost, the practical utility of Li–S batteries is challenging to realize. The main obstacles that prevent the commercialization of Li–S batteries are the low electrochemical utilization of sulfur and fast capacity fading. First, both sulfur and the discharge product Li2S are electronically and ionically insulating. Therefore, incorporation of sulfur into a conductive matrix (e.g., carbon, polymer, or metal) is usually required, which reduces the energy density. Second, unlike insertion-compound cathodes, sulfur undergoes a series of complicated compositional and structural changes with the formation of soluble polysulfide intermediates, resulting in poor mechanical stability and severe capacity fading.[4] Third, the dissolved polysulfide species shuttle between the cathode and anode, which results in active material loss from the cathode, a low Coulombic efficiency, and passivation of the lithium-metal surface with insoluble products, e.g., Li2S/Li2S2. Fourth, the lithium-metal anode degrades due to surface passivation and an unstable solid-electrolyte interphase (SEI) that is formed with the organic electrolyte, prohibiting the long-term cycling stability of Li–S batteries. In order to address the aforementioned problems with Li–S batteries, many efforts and improvements have been made in the past decade.[4a,5] The improvements include: i) novel cell component materials and structures (e.g., the cathode, binders, the electrolyte, and the anode); ii) mechanistic understanding of the Li–S redox chemistries; and iii) innovation

Development of advanced energy-storage systems for portable devices, electric vehicles, and grid storage must fulfill several requirements: low-cost, long life, acceptable safety, high energy, high power, and environmental benignity. With these requirements, lithium–sulfur (Li–S) batteries promise great potential to be the next-generation high-energy system. However, the practicality of Li–S technology is hindered by technical obstacles, such as short shelf and cycle life and low sulfur content/loading, arising from the shuttling of polysulfide intermediates between the cathode and anode and the poor electronic conductivity of S and the discharge product Li2S. Much progress has been made during the past five years to circumvent these problems by employing sulfur– carbon or sulfur–polymer composite cathodes, novel cell configurations, and lithium-metal anode stabilization. This Progress Report highlights recent developments with special attention toward innovation in sulfur-encapsulation techniques, development of novel materials, and cell-component design. The scientific understanding and engineering concerns are discussed at the end in every developmental stage. The critical research directions needed and the remaining challenges to be addressed are summarized in the Conclusion.

1. Introduction Availability of energy at an affordable cost without adverse environmental consequences is one of the major challenges to modern society. The increasing consumption and limited availability of fossil fuels, along with the environmental impact caused by burning fossil fuels, have prompted the development of alternate, sustainable, clean energy technologies. Renewable energy sources, such as solar and wind, are appealing in this regard, but the effective utilization of these intermittent energy sources requires efficient and economical electrical energy storage (EES) systems. Rechargeable batteries are the most viable option for EES. Among the various rechargeable battery systems, lithium-ion batteries offer the highest energy density. However, the energy density of conventional lithiumion batteries with insertion-compound cathodes (e.g., LiCoO2, LiMn2O4, and LiFePO4) and anodes (graphite) is limited to fulfill the demands of electric vehicles and smart grids.[1]

Prof. A. Manthiram, S.-H. Chung, C. Zu Materials Science and Engineering Program & Texas Materials Institute The University of Texas at Austin Austin, TX 78712, USA E-mail: [email protected]

DOI: 10.1002/adma.201405115

Adv. Mater. 2015, DOI: 10.1002/adma.201405115

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Arumugam Manthiram is the Cockrell Family Regents Chair in Engineering and the Director of the Materials Science and Engineering Program and the Texas Materials Institute at the University of Texas at Austin (UT-Austin). His research interests are in the area of materials for rechargeable batteries, fuel cells, and solar cells, including novel synthesis approaches for nanomaterials. See http://www.me.utexas.edu/~manthiram/ for further details. Figure 1. Schematic and voltage profiles of a Li–S cell. Reproduced with permission.[6] Copyright 2012, Nature Publishing Group.

in cell configuration. In this Progress Report, we present the advances made regarding Li–S batteries in the past decade, with a focus on aspects that boost the sulfur utilization, cycle stability, and power density of Li–S batteries. Specifically, innovations in cathode structures and novel cell configurations are presented, e.g., use of sulfur–carbon and sulfur–polymer nanocomposites, porous polysulfide reservoirs, porous current collectors, binders, free-standing composite electrodes, interlayers between the cathode and the separator, surface-coated separators, polysulfide catholytes, sandwiched cathode structures, lithium-metal protection, and Li2S activation. Based on the progress made with these strategies, future directions for further improvements and practical utility of Li–S batteries are presented.

Sheng-Heng Chung is currently a Ph.D. student in the Materials Science and Engineering Graduate Program at UT-Austin. He obtained his B.S. (2006) in Resources Engineering from National Cheng Kung University and his M.S. (2008) in Materials Science and Engineering from National Tsing Hua University in Taiwan. His research is focused on process modification and cell-configuration design for batteries. Chenxi Zu is currently a Ph.D. student in the Materials Science and Engineering Graduate Program at UT-Austin. She obtained her B.Eng. (2011) in Materials Science and Engineering from Beihang University in China. Her research interests are high-energy-density Li–S batteries, lithium anodes, and surface and interface studies.

2. Lithium–Sulfur Cells with a Sulfur Cathode The Li–S battery, fabricated with a sulfur cathode, is the most promising high-energy-density system.[6] The conventional cathode contains pristine sulfur mixed with conductive carbon and binder.[7] However, with a pure sulfur cathode prepared with sulfur–carbon binder mixtures, it is difficult to effectively utilize and stabilize the active material. The clusters or agglomerates of insulating sulfur particles form inactive cores within the sulfur–carbon binder mixtures, which limits the redox reaction of the cathode. The dissolved polysulfides that form at intermediate discharge/charge states lead to low discharge/ charge efficiency and loss of active material. The end discharge products (Li2S2/Li2S mixture) that are formed by the reduction of the dissolved polysulfides redeposit on the surface of the electrodes, form inactive agglomerates/zones, and block ion and electron transport in the electrode. In addition, the structure of sulfur changes by 80 vol%, involving the phase transitions of sulfur–polysulfide–sulfide, which breaks down the interparticle contact among the insulating sulfur, the conductive carbon, the binder, and the current collector. This leads to rapid capacity fading during cycling.[8] To solve these scientific challenges, modification of the physical/chemical properties and the morphology of sulfur is

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the first step in the development of Li–S technology. Pristine sulfur in the cathode mixtures is replaced by various sulfur– porous-carbon nanocomposites or sulfur–conductive-polymer nanocomposites to increase the cathode conductivity and suppress polysulfide migration.[9] However, the addition of extra conductive additives may reduce the sulfur content in the composite cathode and increase the complexity for widespread use. Therefore, a balance between sulfur loading and battery performance is required. Moreover, since Li–S cells involve a conversion reaction, compared with conventional insertion-compound cathodes, modification in the cell components along with the design of novel cathode configurations could be a promising strategy for improving the performance of Li–S batteries.[4d,10]




PROGRESS REPORT Figure 2. SEM observation, schematic model, and cell performances of various sulfur-based nanocomposites: a) porous hollow carbon@sulfur nanocomposite, b) sulfur–polyaniline nanotube, and c) poly(ethylene glycol) (PEG)-wrapped graphene–sulfur nanocomposite. a) Reproduced with permission.[14] Copyright 2011, Wiley-VCH. b) Reproduced with permission.[31] Copyright 2012, Wiley-VCH. c) Reproduced with permission.[33] Copyright 2011, American Chemical Society.

2.1. Composite Cathodes A high-performance Li–S cell relies heavily on the optimization within the cathode configuration. A practical cathode design should include high cathode conductivity, outstanding polysulfide-trapping capability, and a robust electrode structure. The most promising approach is the encapsulation of sulfur within conductive additives to form a sulfur-based composite cathode. Porous carbon substrates and conductive polymers are essential to enable the redox accessibility of the insulating sulfur (conductivity of 5 × 10−30 S cm−1). The high surface area and porosity of porous carbon materials, as well as the chemical gradient created by polymer coatings could satisfy the critical requirements of good electronic and ionic conductivities, as well as retention of polysulfides within sulfur-based composite cathodes. Moreover, a porous carbon substrate or a soft polymer can buffer huge volume changes of the sequestered active material.[4b,5c] Surface functionalization techniques further modify the morphologies of the nanocomposites to limit the diffusion of polysulfide out of the nanocomposites or trap the migrating polysulfides.[11]

2.1.1. Sulfur–Carbon Nanocomposites Conductive carbon acts well as an effective electronic conductor to enhance the utilization of the insulating sulfur. Its loose clusters serve as a porous framework to contain the redox products.[12] However, poor links between the active material and the carbon matrix, as well as the unstable architecture of the carbon clusters result in rapid capacity fading and low efficiency during the initial several cycles. This results in unstable and poor


cyclability with a cycle life of less than 50 cycles. Recent progress on sulfur–carbon composite cathodes has minimized these problems. Sulfur–carbon nanocomposites benefit from their hierarchical micro-/mesoporous structural design, satisfying the criteria for encapsulating sulfur into porous substrates.[4a,4c,4d,11a] Ji et al. first presented a sulfur–mesoporous-carbon composite synthesized by a melt-diffusion process.[13] The CMK-3 ordered mesoporous carbon, synthesized by using the SBA-15 silica template, has a high conductivity, uniform and narrow mesopores (3 nm), a large pore volume (2.1 cm3 g−1), and an interconnected porous structure. As a result, at 155 °C, liquid sulfur with the lowest viscosity achieved excellent active material encapsulation in the mesoporous space and hence, for the first time, exhibited high reversible capacity with good efficiency in Li–S batteries.[4b,13] Jayaprakash et al. utilized a sulfurvaporization route to infuse gaseous sulfur into porous hollow carbon with a mesoporous shell, as shown in Figure 2a.[14] The use of mesoporous hollow carbon capsules can encapsulate and sequester up to 70 wt% active material in their interior and porous shell. As a reference, this sulfur content refers to the content of active material in the nanocomposite. The carbon protection shell minimizes polysulfide dissolution and the shuttle effect. The utilization of sulfur vapor leads to molecular-level contact between the insulating active material and the conductive carbon shell. Moreover, the mesoporous shell allows access of electrolyte and preserves fast Li+-ion transport. Therefore, this scalable procedure produces efficient uptake of elemental sulfur with effective ion and electron transport for achieving outstanding cyclability. These porous hollow carbon@sulfur nanocomposites, for the first time, provided a long cycle life of 100 cycles and a high reversible capacity approaching 1000 mA h g−1 at a high cycling rate.



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Zhang et al. reported double-shelled hollow carbon spheres synthesized by way of hollow SnO2 spheres as the template.[15] However, the larger core and thicker shell may decrease the redox accessibility of the encapsulated sulfur as compared with the single-shelled hollow carbon spheres reported by Jayaprakash et al.[14] Recently, Peng et al. reported a hollow graphene nanoshell for sulfur encapsulation, which might be a promising strategy to solve the concern of the non-conductive sulfur core.[16] The resulting hollow graphene nanoshells had a diameter of 10–30 nm with a high pore volume of 1.98 cm3 g−1 for excellent sealing of the active material in the conductive carbon shell. The small diameter of the graphene nanoshell limits the particle size of the encapsulated sulfur well and, therefore, suppresses the formation of the non-conductive core. As a result, a high sulfur utilization of 91% was achieved at a C/10 rate. It is interesting to find a reversible discharge capacity of 419 mA h g−1 and stable Coulombic efficiency of 95% without the addition of lithium nitrate after 1000 cycles at a high 1C rate. The corresponding capacity fading rate was only 0.06% per cycle. The reported cycle life and capacity fading rate were, respectively, longer and lower than the values of other Li–S batteries based on graphene in lithium-nitrate-free electrolytes. In addition to heat-treatment procedures, a chemical-synthesis approach is another favorable process for synthesizing sulfur–carbon composites and creating a strong binding between the sulfur and the carbon substrate. Our group reported a carbon-wrapped sulfur nanocomposite synthesized in aqueous solution at room temperature, which is a facile and nontoxic manufacturing process.[17] The resulting core–shell structured sulfur–carbon nanocomposites encapsulated the precipitated sulfur at the interspaces between the clusters of carbon nanoparticles. The strong chemical bonding between the sulfur nucleates and the dispersed carbon allowed this approach to be applicable to any carbon substrates. However, the formation of the sulfur core in the core–shell-structured sulfur–carbon nanocomposites may cause an inactive and non-conductive core in the agglomerates of active material.[17] A similar problem can also be found with the core–shell-structured carbon–sulfur composite prepared by a sulfur deposition method. In a case reported by Wang et al., the non-conductive sulfur shell on the nanocomposite may limit the rate capability and utilization of the active material.[18] Hence, both nanocomposites showed a limited enhancement in high-rate cycle performance. The reversible discharge capacity of the former and the latter after 50 cycles were, respectively, 697 mA h g−1 at a C/4 rate and 336 mA h g−1 at a C/2 rate. To prevent sulfur aggregation, Ji et al. reported an appealing method by using graphene oxide as a sulfur immobilizer.[19] Graphene oxide has strong reactive functional groups on its surface to bond the nanosized sulfur particles (tens of nanometers).[20] The nanocomposites were prepared by a two-step chemical deposition and thermal treatment process. The resulting graphene oxide–sulfur nanocomposites had a thin and uniform sulfur coating on the conductive graphene oxide sheet to avoid the aggregation of large sulfur particles. The nanocomposite exhibited good electrochemical reversibility and capacity stability with a discharge capacity of >950 mA h g−1 4


for 50 cycles. Inspired by this, our group prepared hydroxylated graphene–sulfur nanocomposites that achieved uniform sulfur distribution on graphene and overcame the problem of non-conductive sulfur aggregation in the chemical preparation stage.[21] This approach attaches the amorphous sulfur onto hydroxylated graphene nanosheets by chemical bonding . The added hydroxyl functional groups limit the growth of bulk crystalline sulfur during composite preparation and inhibit polysulfide dissolution into the liquid electrolyte during cell cycling. However, the nanoscale sulfur coating on the carbon substrates may incur the concern of a low sulfur content in the composite cathode. On the other hand, although other solution-based synthesis processes also offer unique nanocomposites with high performance, the use of the toxic CS2 solution or the production of the toxic H2S byproduct may nullify the advantages of the environmentally friendly sulfur.[22] In general, the common merit of these chemical-synthesis processes is the use of an aqueous solution at room temperature to create strong chemical bonds between the sulfur and the carbon substrate. Overall, the advances in the development of carbon substrates (porous carbon,[13–15,23] carbon fibers (CNFs),[22c,24] carbon nanotubes (CNTs),[25] graphene,[19,21,25c,26] and even carbide-derived carbon[27] and active-material encapsulation techniques have boosted cell performance, with an increase in discharge capacity from less than 500 mA h g−1 to above 1500 mA h g−1 and an enhancement in cycle life from 20 to ca. 1000 cycles.[13,23c,25d,28] Although these sulfur–carbon nanocomposites have shown progress in cell performance, a high specific capacity (>1200 mA h g−1) and an excellent cycle stability during long cycle life (>80% capacity retention over 100 cycles) could not be realized simultaneously with any single composite material.[5a] To develop high-capacity sulfur–carbon composite cathodes, porous carbon hosts with tunable micro-/mesopores are suggested to be combined with a suitable sulfur encapsulation process to achieve outstanding sulfur immobilization and ion/ electrolyte transference. The combination of a highly conductive carbon network with a porous carbon nanostructure is a novel method for a composited conductive/porous carbon host to effectively utilize the immobilized sulfur for Li–S batteries. In a conductive/porous carbon host, the conductive carbon materials (CNFs, CNTs), and graphene) effectively raise the redox capability of the encapsulated active material that are protected by the nanoporous carbon substrate.[29a−d] Moreover, heteroatom doping can chemically improve the reactivity and facilitate chemical sulfur-adsorption to the chemically stable carbon host.[23g,29e−i] To achieve practical applications, an increase in total accessible pore volume will be essential to enhance sulfur loading and the sulfur content. After impregnating sulfur into open pores, voids are necessary to ensure good electrolyte impregnation and fast lithium-ion transference to achieve an outstanding high-rate cell performance. On the other hand, these extra voids can also buffer volume changes of the active material. The graphitization level and the mechanical strength of the carbon hosts dominate the improvements in, respectively, cathode conductivity and the integrity of the nanocomposite structure.[4a,4d,22a]




Polymers are another type of frequently used additive in rechargeable Li–S batteries, especially the conductive polymer coating on sulfur particles. Conductive polymers can be tailored or used to modify the surface of cathodes to facilitate ion and charge transport.[11b] The corresponding synthesis strategies aim at confining sulfur and its redox products in nanocomposites with controlled morphology. Core–shell structures, in which the insulating sulfur is the core and the conductive polymers are the shell, offer shorter pathways for ion and electron transport and more freedom for compositional change. Wang et al. introduced the first sulfur–conductive polymer nanocomposites synthesized with polyacrylonitrile (PAN) and sublimed sulfur at ca. 300 °C.[11c] Following this, extensive efforts have targeted the synthesis of sulfur–conductive-polymer nanocomposites with various conductive polymers and structures to enhance the core– shell structure.[4a,d] Our group has reported several core– shell structured sulfur–polypyrrole nanocomposites with various microstructure and morphologies as composite cathodes.[30] The polypyrrole coating functions as a stable interface between the liquid electrolyte and the polysulfide species, allowing the accessibility of ions and charge, but sequestering the diffusion of the active material. This demonstrates that sulfur–conductive-polymer nanocomposites containing dispersed sulfur and conductive nanoparticles could become a viable approach to chemically overcome some of the persistent problems associated with rechargeable lithium–sulfur batteries.[30b] In addition to the core–shell structure, conductive polymers have recently been used to mimic the structure of CNTs and CNFs. Xiao et al. reported a self-assembled polyaniline nanotube for enhancing sulfur encapsulation, as shown in Figure 2b.[31] Electropositive groups on the sulfur–polyaniline nanocomposites attract polysulfides through electrostatic forces, reducing the loss of active material during cell cycling and proving long-term cyclability (cycle life of 500 cycles). This progress implies that additional functionalities could be introduced into conductive polymers to further improve the performance. Thus, our group has investigated the effect of a mixed ionic/electronic conductor (MIEC) in sulfur–polypyrrole nanocomposites for Li–S batteries.[32] The MIEC of polypyrrole was synthesized with poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA). The resulting sulfur–MIEC nanocomposites reduced the overpotential and electrochemical impedance. The excellent electrochemical stability obtained at various cycling rates was attributed to the MIEC that facilitates ion and electron transfer and captures polysulfides within the cathode region. Although conductive or functional polymers have stronger chemical polysulfide-trapping capability than the chemically inert carbon, the relatively low electrical conductivity of the polymer additives (as compared with carbon) may limit the progress on both raising the active material utilization and reutilizing the trapped active material.[10a] In general, sulfur– polymer nanocomposites can achieve outstanding cycle stability, but still may not help to increase the practical specific capacity.


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2.1.3. Functional-Polymer-Supported Sulfur–Carbon Nanocomposites

2.1.2. Sulfur–Polymer Nanocomposites

Recent advancements in nanoscience and nanotechnology have offered exciting opportunities for the development of a mixed soft-polymer coating with conductive/porous carbon substrates for sulfur-based nanocomposites. Various polymer-coated sulfur–carbon nanocomposites that can effectively encapsulate the redox products and maintain a robust but porous electrode structure have been reported. These advantages are essential for improving the electrochemical performance.[4b,11a,22a] As a pioneer in high-performance composite cathodes, Ji et al. first demonstrated the enhanced discharge capacity and stability of a polyethylene glycol (PEG)-coated sulfur–CMK-3 nanocomposite.[13] These PEG-coated sulfur–carbon nanocomposites physically contain the active material in their ordered CMK-3 mesoporous carbon substrates and further chemically retard polysulfide diffusion by creating a chemical gradient in the nanocomposites. In addition to improving the electrochemical reversibility, the soft-polymer coating can tolerate huge volume changes from the trapped active material. Figure 2c depicts a PEG-wrapped graphene–sulfur nanocomposite reported by Wang et al.[33] The synthesized sub-micrometer sulfur particles are coated with PEG surfactants and graphene sheets, which function as a chemical and physical protection coating for trapping the polysulfide intermediates. The soft PEG coating further accommodates the volume change of the wrapped sulfur particles during cell cycling. The conductive graphene coating offers the encapsulated sulfur particles a robust and electrically conductive shell. Therefore, PEG/graphene–sulfur nanocomposites show high and stable discharge capacities up to 600 mA h g−1 over more than 100 cycles. This depicts a significant improvement in the cyclability of polymercoated sulfur–carbon nanocomposites from 20 cycles (at a C/10 rate) to 140 cycles (at a C/2 rate). Wu et al. presented a polyaniline-coated sulfur–multiwall CNT (MWCNT) nanocomposite.[34] The polyaniline that was synthesized by a rapid in situ chemical oxidation polymerization covered the sulfur–carbon mixture to form a gel-like cathode composite. The resulting composite possesses a conductive MWCNT network to improve the cathode conductivity. The porous polyaniline shell reduces the Li+-ion transfer pathway and prevents the dissolution of the active material. Thus, the synergistic effect between the porous carbon substrates and the functional polymer coatings greatly advances the cycle performance and extends the long-term cyclability of the composite cathodes from less than 50 cycles to 500 cycles. In view of the results of the intensive research in this area, we may conclude that the encapsulation of sulfur within a nanoporous substrate is a good approach to make advanced Li–S batteries. Based on the physical adsorption/absorption or chemical anchoring/trapping capability, these nanocomposites in different contexts suppress the polysulfide dissolution and diffusion issue. It should also be emphasized that the integration of conductive and porous substrates with a composited nanospace, heteroatom doping, and functional surface coatings shows even better electrochemical performance than the use of these individual approaches. However, these approaches may still be ineffective in stopping polysulfides that carry negative



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charges from constantly migrating toward the lithium anode during cell discharge. This may result in fast capacity fading during the initial several cycles, which usually happens in composite cathodes. Moreover, although conductive additives improve the capacity and cyclability, these electrochemically inert materials reduce the gravimetric energy density and limit the sulfur content and loading in Li–S batteries. In many cases, the total sulfur content in the cathode region is not more than 65 wt% by weight and the sulfur loading is not higher than 2 mg cm−2 of the cathode.[35]

2.1.4. Smaller Sulfur Molecule Encapsulation The developments regarding microporous carbon synthesis and sulfur-encapsulation techniques have opened another useful research direction to limit the rapid capacity fading by avoiding the formation of soluble polysulfide intermediates (Li2Sn, n = 4–8). The concept is different from the conventional methods that mainly depend on chemical sulfur–carbon/ polymer bonding or physical polysulfide-absorption/adsorption capacity of the porous hosts. By using smaller sulfur molecules (S2−4), the aim is to confine them in the confined space of a conductive microporous carbon matrix as the starting active material. As a result, this approach may theoretically eliminate the formation of soluble polysulfides and improve the close contact between S2−4 and the conductive carbon host.[4a,10a] Zhang et al. prepared sulfur–carbon spherical composites and encapsulated sulfur into the micropores of carbon.[36a] An electrochemical stability up to 500 cycles and a superior highrate performance were obtained, although the sulfur content was only 42 wt%. Xin et al. encapsulated smaller sulfur molecules into a core/porous-sheath structure comprising a composited CNT@MPC matrix. The small sulfur molecules were shown to be loaded into the microporous carbon, which had a critical pore size of 0.5 nm. The encapsulated metastable small sulfur molecules (S2–4) were tightly held in the confined nanospace of the conductive microporous carbon matrix. This limited the formation of cyclo-S8 molecules with the dimensions of 0.7 nm. The resulting small-sulfur-molecule–microporouscarbon-coated CNT (CNT@MPC) nanocomposites avoid the unfavorable transformation between cyclo-S8 and S42−. Electrochemical analyses provide convincing evidence that the small-sulfur-molecule–carbon nanocomposites show a single discharge plateau at 1.9 V (the lower-discharge plateau) and a single reduction peak in the cyclic voltammetry curves. This provides evidence that the discharge/charge process effectively avoids polysulfide formation and hence limits the loss of active material. As a result, a high reversible capacity of 1149 mA h g−1 was obtained after 200 cycles. Xin et al. continuously enhanced this concept by using a hollow porous carbon substrate to encapsulate the smaller sulfur molecules.[37] However, they pointed out a challenge with this method. The previously used microporous carbon may not have sufficient pore volume to host more than 50 wt% of the smaller sulfur molecules in the resulting nanocomposites. Thus, hollow micro-/mesoporous carbons are used as the new substrate. The hollow porous carbon has a microporous carbon shell to accommodate and sequester chain-like smaller sulfur

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molecules, which can prevent the loss of active material. In addition, the hollow carbon has a hierarchical porous structure to facilitate electron and Li+-ion transport, which leads to better and faster electrochemical reaction as compared with the group’s previous work.[10a,36b] Excellent long-term cyclability with a high reversible capacity of 730 mA h g−1 after 600 cycles at a C/10 rate and with a limited self-discharge for 30 days were achieved. Ye et al. further improved the application of smaller sulfur molecules by using a carbon host with a hierarchical micro-/mesoporous structure.[23c] The hierarchical carbon matrix not only has abundant micropores to store chain-like sulfur molecules in the composite, but also has mesopores to ensure fast Li+-ion transport. The limited active material loss and fast electrochemical kinetics show a long lifespan of 800 cycles with a reversible capacity of 600 mA h g−1 at a 1C rate. A series of research concludes that the concept of using chain-like small sulfur molecules is a promising approach to improve cell performance. The small sulfur molecules are confined within the limited nanospace of the micropores. The limited nanospace and the strong interaction between the active material and carbon substrate avoid the formation of unfavorable soluble polysulfides and subsequent polysulfide dissolution and diffusion. Close contact with the conductive carbon host further improves the activity of the active material. Thus, an effective cooperation between the smaller sulfur molecule and the microporous carbon may overcome the severe polysulfide diffusion problem in conventional Li–S cells. The use of micro-/mesoporous carbon has been shown to facilitate electron and Li+-ion transference, as well as increase the loading of smaller sulfur molecules to above 50 wt%. As a reference, this sulfur content refers to the content of active material in the nanocomposite. We may conclude that this new research concept may be suitably applied to different microporous substrates (e.g., carbon, polymers, or oxides) with hierarchically tunable mesopores for developing practical Li–S batteries.

2.1.5. Inorganic-Compound–Sulfur Composites As an alternative to conductive carbon and polymers, inorganic compounds such as TiO2 or TinO2n−1 have been used as inherently polar substrates to adsorb the sulfur species. Seh et al. developed sulfur–TiO2 yolk–shell nanoarchitectures with an internal void space to accommodate the volume expansion of sulfur.[38a] The capacity degradation was as small as 0.033% per cycle for 1000 cycles. In addition, the conductive Magnéli phase Ti4O7 was revealed to be an effective matrix for binding/ interacting with sulfur species, resulting in better accommodation of Li2S, a higher reversible capacity, and improved cycling performances.[38b,c]

2.2. Porous Polysulfide Reservoirs The application of porous substrates in composite cathodes to contain soluble polysulfides leads to another new cell configuration. A new containment strategy is to embed porous additives directly in the cathode matrix as an internal polysulfide reservoir. This promising concept was first reported by




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Ji et al., as shown in Figure 3a,b.[39] An ordered mesoporous silica substrate (SBA-15) is embedded within the sulfur–carbon composite cathode for trapping and then storing the polysulfides formed during cycling. The polysulfide-trapping capability of the porous additive is through surface absorption. The suppressed polysulfide diffusion issue further limits the redeposition of the insulating Li2S/Li2S2 mixtures onto the surface of the cathodes, which mitigates the rapid capacity fading and short cycle life. An appealing merit of embedding the polysulfide reservoir within the cathode is that a small fraction of the porous substrates (10 wt% in the cathode) can greatly immobilize the migrating polysulfides within the composite cathode. The same research group further investigated the understanding of the polysulfide-trapping mechanism. Evers et al. presented the use of a porous TiO2 additive (less than 4 wt% in the cathode) as a polysulfide reservoir in composite cathodes, which further increased the capacity retention and extended the cycle life to 200 cycles.[40] They demonstrated that the soluble polysulfide species are majorly absorbed within the nanoporous additives and minorly trapped by surface binding. Recent advances in porous polysulfide reservoirs include extended cycle life and limited capacity fading. Wei et al. used a pig-bone-based hierarchical porous carbon (named BHPC) that was derived from pig bones as another appealing polysulfide reservoir.[41] After heat and KOH alkali treatments, the BHPC that was subsequently activated at 850 °C achieved a high surface area of 2156 m2 g−1 and a large pore volume of 2.26 cm3 g−1. The highly porous BHPC possesses interconnected pores with a broad pore-size distribution, containing macropores, mesopores, and micropores. The

meso-/macropores facilitate the transport of Li+ ions and the micropores help retain the capacity during cell cycling. As a result, a small amount of BHPC additive (6 wt% in the cathode) improves the cycle stability and the utilization of sulfur during cycling. Our group utilized a carbonized-eggshell membrane (CEM) as a polysulfide absorbent with a pure sulfur cathode, as shown in Figure 3c.[42] The CEM polysulfide absorbent derived from a natural eggshell membrane has a unique porous structure with a high porosity and surface area for accommodating the soluble polysulfides. The suppressed polysulfide diffusion mitigates the rapid capacity loss and the redeposition of the insulating Li2S/Li2S2. Thus, the porous CEM provides good long-term cyclability (150 cycles) with a high capacity retention rate of 85% and a low capacity fading rate of 0.10% per cycle for Li–S cells. This reconfirms the progress on the development of the porous polysulfide reservoir initiated by Nazar and co-workers.[39,40] The intensive research provides convincing evidence that this new cell configuration facilitates good cyclability. Such an enhanced battery performance is achieved with less than 10 wt% additive in the cathode, which satisfies the criteria of high-performance and high sulfur content/loading. Future work may involve the investigation and enhancement of the interaction between porous polysulfide reservoirs (metal oxides or carbon substrates) and polysulfides to determine whether the polysulfide absorption is due to the porous architecture or via a physical/chemical effect. In addition, the characteristics of the pore size, surface area, and trapping capability of the substrates on the battery performance can be studied in detail to achieve future long-term electrochemical cycling.

Figure 3. a) Schematic model of the polysulfide reservoir. Cell performances of various polysulfide reservoirs: b) SBA-15 polysulfide reservoir and c) CEM polysulfide reservoir. a,b) Reproduced with permission.[39] Copyright 2011, Macmillan Publishers Limited. c) Reproduced with permission.[42] Copyright 2014, American Chemical Society.




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Figure 4. a) Schematic model of the porous current collector. b,c) Cell performances of various porous current collectors: porous H-030 carbon paper (b) and 3D aluminum foam/carbon nanotube scaffold (c). b) Reproduced with permission.[48a] Copyright 2014, Elsevier. c) Reproduced with permission.[51] Copyright 2014, Elsevier.

2.3. Porous Current Collectors The conventional current collector used in Li–S battery research is a 2D aluminum foil, which is just a flat supporter in the cathode. Moreover, the aluminum foil may encounter oxidation and corrosion during cycling, causing the sulfur to lose electrical contact with the current collector and increasing the internal resistance of the battery.[43] Therefore, appropriate, alternative current collectors are of great interest for long-term cycle stability and high energy density. A new cell configuration design that uses a 3D conductive/porous-metal current collector has demonstrated improvements with regard to cell cyclability for various rechargeable battery systems.[44] In the Li–S battery scenario, the conductive/porous matrix not only works as an inner conductive framework to guarantee fast electron transport, but also as an active material container to stabilize the active material mixtures in its conductive skeleton. This design enables superior electrochemical stability of the sulfur cathodes with a capacity retention rate of 92% after 50 cycles.[45] Ballauff and co-workers conducted quantitative analysis on the capacity fading of Li–S batteries with different cell configurations.[46] According to the capacity-fading model, they concluded that porous-metal current collectors have positive effects on the long-term cycle stability of batteries. The improved cycle stability may result from the stronger interaction between the metal substrates and sulfur species.[47] However, the weight of the metal-foam substrate may incur concerns regarding reducing the low energy density. Therefore, lightweight porous carbon substrates are more promising current collectors for increasing the accessible reactive area and providing a stable morphology during cycling.[48] A nanocellular carbon consisting of carbon-nanofoam plates and interwoven

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carbon fibers was used as a bifunctional current collector with a readily prepared pure sulfur cathode.[49] The nanofoam plates that were tightly attached onto the fibrous network served as reservoirs to store the active material and localize the dissolved polysulfides. The coalescing carbon fibers functioned as an embedded conductive network to improve the redox reaction. As a result, the bifunctional conductive/porous current collectors offer high discharge capacity and good cycle stability. Another example of progress regarding the development of porous current collectors is the use of a commercial H-030 carbon paper as the substrate with a pure sulfur cathode, as shown in Figure 4a,b.[48a] The highly porous H-030 carbon paper has a high porosity of 80% and low density of 0.4 g cm−3. The abundant nanoporous space allows the pure sulfur cathode to achieve a high sulfur loading (2.3 mg cm−2) and sulfur content (80 wt%). The close connection between the active material and the conductive/porous current collector leads to a good rate capability with higher discharge capacity and longer cyclability, as compared with both the aluminum foil current collector and our previous porous current collector research.[45,49] The advantage of embedding various conductive/porous substrates in sulfur cathodes is notable, which allows us to conclude that the porous current collector is a promising cell configuration for Li–S batteries. Nevertheless, we would like to emphasize that porous carbon substrates offer better cycle performance than metal substrates. The difference between these selected substrates is dominated by their pore sizes and morphologies. We find that porous metal substrates have large pores several tens of micrometers in size. After these large pores hold the active material, inactive cores may form agglomerates of active material. On the other hand, porous carbon substrates have smaller pores less than 10 µm in size and also have





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a micro-/mesoporous structure, which decreases the formation of large non-conductive agglomeration in their nanospace. Also, the nanospace of carbon substrates can swell the liquid electrolyte, which is a benefit for stabilizing the electrochemical reaction within the porous cathode.[48a,49] Zhang et al. investigated the effect of a nickel foam and a carbon-fiber cloth as the porous current collector in sulfur– polypyrrole composite cathodes.[50] Although the carbon substrate has lower electrical conductivity than the metal one, the cell with the carbon-fiber cloth has more stable electrochemical performance compared with that of nickel foam. The reported advantages of carbon fibers result from its chemical, structural, and morphological characteristics, which improves the affinity toward sulfur–polypyrrole nanocomposites and therefore enhances stable charge-transfer conditions over that of the metal counterpart. This result reconfirms the observations and findings in our current collector research. Cheng et al. designed a 3D aluminum-foam/CNT scaffold that integrates the advantages of both the metal substrate and the CNT network, as shown in Figure 4c.[51a] The combination of sulfur–CNT mixtures and aluminum foam not only builds long- and short-range electron pathways, but also creates a vast void space in the sulfur cathode. Thus, the composited metalfoam/CNT scaffolds possess conductive pathways for facilitating efficient electron transport and an abundant porosity for approaching a high sulfur loading of 7.0 mg cm−2. The high-loading sulfur cathode delivers a high initial discharge capacity of 860 mA h g−1. However, the cell could only cycle for 15 stable cycles. Moreover, although the reported maximum permitted sulfur loading in the cathodes approaches 12.5 mg cm−2, the initial discharge capacity drops down to 3.12 mA h cm−2, which is equal to less than 300 mA h g−1. This implies that more efforts are needed to optimize the cyclability and feasibility of sulfur cathode with such a high active material loading. Zhou et al. utilized a graphene foam electrode to host a high sulfur loading of 3.3–10.1 mg cm−2, successfully realizing a high areal capacity of 13.4 mA h cm−2.[51b] The interconnected pores of the graphene-foam current collector not only store a high amount of the active material, but also prevent the loss of the encapsulated active material during cell cycling. As a result, the cell employing the S–graphene-foam electrode displayed a low capacity fading rate of 0.07% per cycle for 1000 cycles. The bifunctional current collectors embedded within the pure sulfur cathode have shown that they can contain the active material and improve the electrolyte absorption of the resulting cathode. This ensures close contact between the insulating active material and the conductive matrix. The porous current collector inherently has a high mechanical strength to ensure the complete electrode structure is retained during cycling. As such, a stable and fast electrochemical redox reaction is guaranteed. In addition, the use of a highly porous substrate further allows the sulfur loading in the porous carbon current collector to approach 2–10 mg cm−2, which implies a promising gravimetric energy density. However, the volumetric energy density of the Li–S cell is strongly related to the thickness of the porous carbon current collector. Furthermore, the use of a rigid and fragile substrate may limit the flexibility of the cell, which should be avoided in future developments.

2.4. Binders In addition to the current collector, it may be necessary to customize other cell components for Li–S batteries. Conventional polytetrafluoroethylene (PTFE) or polyvinylidene difluoride (PVdF) binders are used to link sulfur particles or sulfur-based nanocomposites with the conductive carbon and the current collector. Although they are chemically stable during cycling, conventional binders can neither effectively tolerate the huge volume changes occurring during cycling nor suppress the polysulfide dissolution and migration. Thus, alternative binders that can create robust electrode architecture and possess polysulfide-retention capability have been considered for Li–S batteries.[4a,11b]

2.4.1. Alternative Binders Shim et al. first investigated sulfur cathodes with a poly(ethylene oxide) (PEO) binder with different mixing processes, ball milling, and mechanical stirring methods.[7] The study indicated that the preparation methods affect the morphology of the PEO binder and the porosity in the sulfur cathodes, which influences the cycle performance. A cationic polyelectrolyte binder, poly(acrylamide-co-diallyldimethylammonium chloride) (AMAC), was used with high-loading sulfur electrodes (sulfur content of 80 wt%), as reported by Zhang.[52] AMAC is insoluble in organic solvents. Therefore, these electrodes with the AMAC binder retained a porous and complete structure during cycling and hence exhibited better cyclability than that with the PEO binder. Zhang further points out a problem of using the AMAC single-binder. The AMAC dissociates chloride ions and then creates galvanic cells between sulfur particles and the aluminum metal in the process of aqueous slurry coating. This leads to the pitting corrosion of the metal current collector. The solution is to introduce a dual-binder approach with poly(acrylonitrile–methyl methacrylate) (ANMMA) as the second binder to bind the cathode mixtures to the current collector, which has been proven to be effective in eliminating the corrosion issue. Sun et al. used a natural gelatin polymer as the binder in cathode preparation.[53] A gelatin binder has a high adhesion ability, ensuring the structure stability of the sulfur cathode, as well as a good dispersion ability, mitigating the aggregation of the active material during cathode preparation and cell cycling. Other alternative binders that can enhance the adhesion among cathode mixtures, or suppress the agglomeration of cathode material mixtures, have also been reported.[53,54] A good binder in cathode preparation should not only be a high-adhesion agent but also be a strong dispersion medium. This is beneficial for having a complete electrode structure and favors the uniform distribution between the insulating sulfur particles and the conductive carbon additives, which ensures a good electrical contact and results in high sulfur utilization. Although these newly developed binders seem to have better battery performance than the conventional binder, stable long-term cyclability cannot be found with them. This implies that good polysulfide-retention capability and long-term



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capability. This is because Pluronic F-127 is an amphiphilic copolymer that contains a hydrophobic poly(propylene oxide) (PPO) middle block and two hydrophilic PEO end blocks. The F-127copolymer uses the hydrophobic PPO block to adhere to the hydrophobic sulfur in nanocomposites and utilizes the hydrophilic PEO blocks to create a chemical gradient, limiting the severe diffusion of polysulfides out of the cathode. As a result, the Pluronic copolymer not only maintains a uniform electrode structure during cycling, but also reduces the dissolution of polysulfides, and further limits the formation of a dense Li2S inactivation layer on the cathode.[13] In addition, PEO can facilitate Li+-ion transference by an electrostatic coordination of Li ions with ether oxygen atoms.[58] Therefore, cathodes using a 10 wt% Pluronic F-127 copolymer achieve an excellent electrochemical stability at high 1C and 2C rates and even allow the rate capability to approach a 4C rate. The progress of binder development has extended the cycle life to approach 100 cycles and improve the rate capability to a 4C rate. This emphasizes that the binder accounting for about 5–20% by weight in the cathode material mixtures should be well designed for Li–S batteries. In addition to guaranteeing a complete structure of the electrodes and a uniform distribution of the cathode material mixtures, we may conclude that the functional binder in the future should be low cost and have a low electrical resistance, as well as have a strong chemical polysulfide-trapping capability to ensure a long cell cycle life.

2.5. Free-Standing Sulfur-Based Composite Electrodes Figure 5. Schematic model and cell performance of the Pluronic F-127 block copolymer. Reproduced with permission.[57] Copyright 2012, American Chemical Society.

electrochemical cycling may be an essential factor in the binder design.

2.4.2. Functional Binders Recently investigated functional binders include PEO, PEG, and F-127 block copolymer. Lacey et al. first investigated the role of PEO and PEG in cathodes.[55] PEO and related derivatives of PEG were used as binders, coatings, or electrolyte additives in Li–S cells to study the cell performance. This research clearly demonstrated that a common and similar performance improvement can be obtained with all three approaches. The enhanced electrochemical reversibility during cell discharge and the suppressed cathode passivation at the end of discharge result from the polyether chains, which can modify the solvent system at the interface. The PEO binder or the surface cathode coatings are found to dissolve or swell in the liquid electrolyte. The ether-modified solvent interface delays the precipitation of insoluble discharge products and hence affords an improvement in the reutilization of the active material.[56] Our group used the Pluronic F-127 block copolymer in a sulfur–microporous carbon composite cathode preparation, as shown in Figure 5.[57] The Pluronic F-127 partially replaces PVdF binder in the electrode to provide a polysulfide-retention

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In addition to modifying the conventional cell components, another new approach is to remove inactive materials from the cell by directly employing a sulfur-based nanocomposite as a free-standing composite cathode. The sulfur-based composite electrode is a novel binder-free and current collector-free electrode design. The direct application of the composite electrode not only eliminates the bulk resistance from the added binder but also decreases the net weight of the electrode. As a reference, the weight of the binder is about 5–20% by the weight in the cathode material mixture. The weight of the conventional aluminum foil current collector accounts for about 15–20% of a battery, by weight.[43a] Progress on free-standing composite cathode is focused on the development of highly conductive substrates with a light weight and a high porosity. The most essential requirement is that the applied conductive/ porous substrate must have either a free-standing shape or a self-weaving characteristic, which is important to guarantee its normal function as an electrode. Moreover, the flexible and robust substrate should retain its complete structure after impregnating the active material and during cell cycling.

2.5.1. Templated Assembly of Composite Electrodes Elazari et al. presented the first promising free-standing composite electrode, which is a sulfur-impregnated activated carbon-fiber cloth (Figure 6a).[59] The activated carbon fiber cloth disks (Kynol 2000) have a high surface area of 2000 m2 g−1 and a total pore volume of 1 cm3 g−1. The carbon disks are




PROGRESS REPORT Figure 6. SEM observation or schematic model and cell performance of various free-standing sulfur-based composite electrodes: a) sulfur-impregnated activated carbon fiber cloth, scale bar = 10 µm, b) S@C NW electrode, and c) self-weaving sulfur–carbon composite cathode. a) Reproduced with permission.[59] Copyright 2011, Wiley-VCH. b) Reproduced with permission.[62] Copyright 2013, Wiley-VCH. c) Reproduced with permission.[65] Copyright 2012, Royal Society of Chemistry.

overlaid with sulfur for pre-impregnation at 150 °C and then heated to 155 °C for 10–15 h to impregnate the microporous nanospace with melting sulfur. After the active-material-encapsulation process, the activated carbon fiber tightly absorbs the dispersed sulfur into its micropores (60 wt% in this free-standing sandwiched cathode with a high sulfur loading of 5.5 mg cm−2. Accordingly, we are likely to realize high-performance Li–S batteries possessing high practical gravimetric energy densities through a simple, low-cost method.

4. Lithium-Metal Anodes Despite the high capacity of the metallic lithium anode of ca. 3860 mA h g−1, practical utility of the lithium-metal anode is hampered by its poor cycling efficiency and safety hazards.

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The poor cycling efficiency and safety concerns are due to dendritic redeposited lithium and unevenly distributed electrolyte decomposition products on the lithium-metal surface. Furthermore, corrosion of metallic lithium becomes more severe in Li–S batteries. In addition to lithium-dendrite formation and pulverization, which are common problems associated with lithium-metal anodes in organic electrolytes, dissolved intermediate polysulfide species in Li–S batteries can migrate to the lithium-metal anode and be reduced, leading to a passivation by insoluble species, e.g., Li2S2/Li2S, on the surface of the lithium-metal anode, deteriorating the activity of the metallic lithium.[113]

4.1. Passivation Layer on the Lithium-Metal Surface Recently, lithium-metal protection has attracted much attention. Among various lithium-metal protection strategies for Li–S batteries, most efforts are focused on building a stable passivation layer on the lithium-metal surface. For example, Mikhaylik et al. disclosed that an additive with N–O bonds, e.g., LiNO3, can suppress polysulfide shuttling and improve the reversible capacity of Li–S batteries.[114] In addition, it was further explained by Aurbach et al. that LiNO3 reacts with polysulfide species and lithium metal in the Li–S cell, forming effective passivation on the lithium-metal surface and blocking the direct contact between the lithium metal and the polysulfide species.[115] In this way, polysulfide shuttling is controlled. However, it is worth noting that satisfactory passivation by LiNO3 alone is challenging to achieve and LiNO3 is consumed gradually with cycling. Eventually, “polysulfide shuttling” could be prominent. Furthermore, LiNO3 adversely decomposes at the cathode below 1.6 V, negatively impacting the cathode surface chemistry and deteriorating battery performance. Recently, Xiong et al. reported that polysulfide species play a critical part in forming a stable passivation layer on the lithium-metal surface.[116] Two sublayers together form the self-limiting barrier to suppress further chemical reactions, where the top layer consists of oxidized products of polysulfides and the bottom layer is composed of the reduced products of polysulfides and LiNO3. The improved lithium-surface chemistry due to the formation of a self-limiting barrier for chemical reactions in the presence of polysulfide species was also reported by Demir-Cakan et al.[108] However, the mechanism of the self-limiting effect is not fully understood. In order to construct a reliable passivation layer on the lithium-metal surface, our group proposed using MX (where M is, for example, Cu, Fe, Ni, Co etc., and X represents the anions that control the release of M in the non-aqueous system) to initiate lithium-surface passivation layer formation. MX can interact with the lithium metal and polysulfide species, changing the physical/chemical properties of the lithium-metal surface. Our preliminary results show that copper acetate can be used as an electrolyte additive to initiate the formation of a protective passivation layer on the surface of the lithiummetal anode.[117] To demonstrate this concept, the CNF-paper/ dissolved-polysulfide cell was used. The CNF paper has macrosized interspaces and allows direct contact between the corrosive polysulfide species and the lithium-metal anode; therefore,




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Figure 10. a) Electrochemical performances of the Li/polysulfide batteries cycling without copper acetate (control cell) and with copper acetate (experimental cell) at a rate of C/5. b) SEM images showing the lithium-metal surface of the control cell after the first charge and after the 100th charge, and of the experimental cell after the first charge and after the 100th charge. The scale bars in (b) represent 100 µm. Reproduced with permission.[117] Copyright 2014, American Chemical Society.

cycle. XPS analysis reveals that the lithium surface in the experimental cell has much less polysulfide adhesion after the first charge, compared with the lithium surface in the control cell. The experimental cell has a new passivation film composed of Li2S, Li2S2/CuS/Cu2S species, lithium salts, and electrolyte decomposition products (denoted as LixSOy).[118] LixSOy did not accumulate with cell operation, indicating a stabilized lithiummetal surface and that fewer parasitic reactions had occurred in the experimental cell. It is noted that the intrinsic physical/ chemical properties of the passivation layer initiator MX can influence homogeneities and the electrochemical characteristics of the passivation layer formed in situ. Advanced characterization techniques are needed for a better understanding of the formation of the passivation layer and the lithium deposition. Alternatively, researchers have used cesium or rubidium ions with an effective reduction potential below the standard reduction potential of Li+ ion to build the electrostatic shield around the initial dendrite tips.[119] These additive cations are positively charged and force further deposition of lithium to the adjacent regions of the protrusion, thus preventing dendrite growth. Compared with other strategies, these additive cations are not consumed during cell operation. Further experimental efforts are needed to apply this strategy in Li–S batteries. On the other hand, a Li+-ion conducting solid electrolyte layer can be built on the lithium-metal surface to protect the lithium metal and suppress polysulfide shuttling in Li–S batteries. For example, phosphorous pentasulfide (P2S5) has been used to initiate a Li3PS4 passivation layer on lithium metal.[167] Li3PS4, with a high lithium-ion conductivity, has been reported to block direct contact between the lithium metal and the polysulfides.[120] Moreover, a Li3N protection layer has also been fabricated on the surface of a lithium-metal anode by in situ surface treatment. A capacity of 773 mA h (g sulfur)−1 was obtained using a sulfur–Ketjen Black composite cathode after 500 cycles.[121]

4.2. Novel Anode Configurations and Li-Alloying Materials the effectiveness of copper acetate in initiating a robust passivation film could be evaluated. Figure 10a presents the cycling performance of the CNF-paper/dissolved-polysulfide cells. In contrast to the control cell without copper acetate, which shows a sudden capacity drop around 100 cycles, the experimental cell with copper acetate and the new passivation film exhibits superior cycling stability. The capacity drop in the control cell correlates to degradation of the lithium-metal anode, further evidence of which is provided by scanning electron microscopy (SEM). As indicated by the SEM data in Figure 10b, the lithium-metal surface in the control cell was characterized by bulk insoluble species, e.g., Li2S/Li2S2 precipitates, along with unevenly deposited mossy lithium after the first charge. Relatively, the experimental cell has a smoother lithium surface, which is covered by a passivation film with spherical features. Furthermore, the lithium surface is highly rough in the control cell after the 100th charge. This lithium-metal surface of low quality explains the cell failure after 100 cycles. In contrast, uniform lithium deposition was identified in the experimental cell, which was believed to be attributed to the passivation layer formation in the first


In addition to building a robust passivation, alternative methods include innovation in the electrode configuration, and using alternative Li/M (M = Si, Sn, C, etc.) alloys. For example, Ji et al. constructed SiO2- or SiC-decorated, anisotropic, spatially heterogeneous carbon-fiber-paper current collectors.[122] It was shown that this current collector enables dendrite-free lithium deposition at a high current density of 4 mA cm−2. In terms of using Li-alloying materials, Yan et al. integrated a prelithiated Si/C microsphere anode, a S/C composite cathode, and an ionic liquid (n-methyl-nallylpyrrolidinium bis(trifluoromethanesulfonyl)imide) electrolyte to fabricate Li–S batteries.[123] A high initial discharge capacity of 1457 mA h g−1 was obtained with a sulfur content of 32 wt% in the cathode. Similarly, Li/Sn alloys were used by Hassoun and Scrosati to couple with a Li2S/C nanocomposite cathode and a gel-type electrolyte.[124] A reversible capacity of ca. 800 mA h g (Li2S)−1 was obtained for 30 cycles. Despite the advantages of being a safer anode option, Li-alloying materials usually suffer from a huge volume change, resulting in poor capacity retention.



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In general, it is challenging to eliminate the insulating passivation on the lithium-metal surface in Li–S batteries or prevent the dendrite-formation/lithium-metal-pulverization problems caused by the organic electrolyte. Advances in the electrolyte system are highly demanded. Solid–electrolyte systems may be a possible solution for the long-term cycling of Li–S batteries; however, the low ionic conductivity of the solid electrolyte and its compatibility with the lithium metal and the operating voltage need to addressed.

5. Lithium–Sulfur Cells with a Li2S Cathode As mentioned previously, lithium-metal degradation remains a persistent problem in Li–S batteries, resulting in safety concerns in the practical utility of Li–S batteries. Li2S cathode material with a theoretical capacity of ca. 1166 mA h g−1 coupled with a high-capacity, lithium-free anode (e.g., Si, Sn, or carbon-based anodes) has, therefore, attracted much attention. However, challenges are associated with Li2S utilization. For example, Li2S has intrinsically low electronic and ionic conductivities. Moreover, it is sensitive to moisture and oxygen, which limits the manufacturing routes. Recently, many efforts have been made to promote the utility of Li2S cathode.

be achieved for 150 cycles. The nature of the activation was assumed to be due to dramatically enhanced electronic transfer from the active material to the electrode assisted by RM. In addition, our group proposed using an additive to change the surface environment of the Li2S bulk particle and enhance its electrochemical activity. It has been reported by Lin et al. that Li2S reacts with P2S5 in the ether-based electrolyte;[120] therefore, P2S5 can be used to suppress Li2S passivation on the lithium-metal surface, forming Li3PS4 with a fast Li+-ion conductivity. Inspired by this work, our group systematically studied the interaction between P2S5 and Li2S in ether-based electrolyte and used hybrid electrolyte with an ether component and P2S5 to realize the efficient use of Li2S bulk cathode materials at room temperature.[128] The interaction between P2S5 and Li2S led to reduced cell resistance and lowered the initial charging voltage plateau to 65% and a sulfur loading of >2 mg cm−2 in the whole cathode.

novel chemical synthesis approaches for Li2S particles with high electrochemical activity, good control over morphology, and a high degree of lithiation will promote the utility of Li2S. Finally, the successful coupling of Li2S and lithium-free anode needs further investigation. In summary, there have been significant advances in lithium–sulfur batteries in recent years. Although some challenges still exist, we would like to emphasize that the wide range of materials and cell configurations presented have the potential to achieve impressive success in the coming years. Therefore, the development of active-material containers, functional binders, porous current collectors, surface-coated separators, and anode materials, combined with customized cell configurations might promote the feasibility of practical lithium– sulfur batteries for future energy-storage applications.

Acknowledgements This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0005397.

6.2. Lithium–Sulfur Cells with Polysulfide Catholytes Li/polysulfide batteries exhibit superior sulfur utilization and reversibility, attributed to the intimate contact between the semi-liquid cathode material and the conductive matrix. However, increasing the concentration of free polysulfide molecules in the electrolyte requires advanced strategies to control the polysulfide shuttling and protect the lithium-metal anode. Moreover, new characterization techniques should be applied to study the SEI layer formed on the lithium-metal surface in Li/polysulfide batteries. It is possible to assemble the Li/ polysulfide cells into flow-batteries; similarities or differences between the Li/polysulfide flow-batteries and static batteries can be compared.

6.3. Lithium–Sulfur Cells with a Stabilized Anode Lithium-metal degradation due to side reactions with the organic electrolyte and polysulfides introduces safety concerns for the practical utility of Li–S batteries. In order to promote the use of lithium metal, satisfactory passivation between lithium metal and the liquid electrolyte is one critical factor that needs to be controlled. In addition, alternative Li/M (M = Si, Sn, C, etc.) alloy anodes could be a feasible approach for enabling a safer Li–S battery. Solid electrolytes may provide a solution to suppress lithium surface passivation and dendrite formation; however, incompatibility between the solid electrolyte and the lithium-metal anode and the surface/interface resistance should be considered.

6.4. Lithium–Sulfur Cells with a Li2S Cathode Mechanistic investigations regarding the insulating nature of Li2S and its activation are needed. Specifically, efficient electrochemical systems or chemical strategies that enable the use of micrometer-sized Li2S will be of interest. In addition, 24


Received: November 7, 2014 Revised: December 10, 2014 Published online:

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Antisensibility: progress and prospects.

RNA capping: progress and prospects.

SCN8A encephalopathy: Research progress and prospects.

Hypomyelinating leukodystrophies: translational research progress and prospects.

Global mental health: policy, progress, and prospects.

Laser ignited engines: progress, challenges and prospects.

Orphan products: origins, progress, and prospects.

Uterus transplantation: current progress and future prospects.

Progress and prospects in ergot alkaloid research.

Type 1 diabetes--progress and prospects.

Progress and prospects for blood-stage malaria vaccines.

Progress and prospects for engineered T cell therapies.

DNA-informed breeding of rosaceous crops: promises, progress and prospects.

High cancer drug prices 4 years later-Progress and prospects.

Computer Vision Malaria Diagnostic Systems-Progress and Prospects.

Progress and prospects of early detection in lung cancer.

Noninvasive Imaging of Nanomedicines and Nanotheranostics: Principles, Progress, and Prospects.

Dietary fibre, diabetes, and hyperlipidaemia. Progress and prospects.

Integrated health and social care in England--Progress and prospects.

Progress and prospects for L2-based human papillomavirus vaccines.

High drug-loading nanomedicines: progress, current status, and prospects.

Noncoding RNAs in gastric cancer: Research progress and prospects.

Medical school hotline: Homelessness in Hawai'i: challenges, progress, and prospects.

Progress and Prospects of Anti-HBV Gene Therapy Development.