Review—From Nano Size Effect to In Situ Wrapping: Rational Design of Cathode Structure for High Performance Lithium − Sulfur Batteries

Rechargeable Li-S batteries have become attractive for next-generation energy storage technology due to their high speciﬁc energy, low cost, and materials earth abundance. However, the Li-S technology has not been successfully commercialized because of several outstanding challenges including fast capacity fading, low Coulombic efﬁciency, and limited rate capability. Over the past few years, considerableeffortshavebeendevotedtosolvingthechallengesassociatedwiththesulfurcathode.Inthisminireview,wesummarizeourrecentadvancesinLi-Scathodes,suchastheunderstandingofthenano-sizeeffectinsulfurmaterials,thedesignofpolymerfunctionaladditives,andtherealizationofanovelinsituwrappingapproachforLi-Scathodes.ThecurrentLi-StechnologycallsforarationaldesignoftheLi-ScathodethatcanleadtoexcellentoverallperformanceofLi-Sbatteries.©TheAuthor(s)2017.PublishedbyECS.ThisisanopenaccessarticledistributedunderthetermsoftheCreativeCommonsAttribution4.0License(CCBY,http://creativecommons.org/licenses/by/4.0/),whichpermitsunrestrictedreuseoftheworkinanymedium,providedtheoriginalworkisproperlycited.[DOI:10.1149/2.0081801jes]Allrightsreserved.

Sulfur is a promising cathode material, which is highly earth abundant and with a theoretical specific capacity as high as 1672 mAh g −1 .
When the sulfur cathode couples with a lithium metal anode to assemble rechargeable lithium-sulfur (Li-S) batteries, their theoretical specific energy density can reach 2600 Wh kg −1 , which is 3-5 folds higher than those of state-of-the-art Li-ion batteries. [1][2][3] Because of the high energy density and low cost, rechargeable Li-S batteries are promising for applications not only in consumer electronics but also in energy storage and electric vehicles. [4][5][6] Although the research on Li-S batteries has been ongoing for over three decades, two major challenges still remain in this field: one is the low specific capacity due to high electrical resistivity of elementary S and the solid reduction products ((Li 2 S 2 and Li 2 S)) in the cathodes; the other is the fast capacity fading owing to the shuttle effect, i.e. polysulfide intermediates formed during discharge/charge cycles dissolve in the electrolyte, diffuse to the anode, react with Li metal, and form insoluble Li 2 S 2 and Li 2 S at the anodic region, leading to the loss of lithium metal and sulfur cathodic materials. 7,8 In the past few years, researches have mainly focused on designing nanostructured sulfur host materials to address challenges related to poor electrical conductivity of S/Li 2 S 2 /Li 2 S. [9][10][11][12][13][14][15][16][17][18][19] A quantum leap for the nanostructure approach was the utilization of ordered mesoporous carbon CMK-3 to trap sulfur, as reported by Nazar et al. in 2009. 20 The conductive mesoporous carbon framework precisely constrains the growth of sulfur within its channels and thus enables fast electronic and ionic transport. Following this work, Cui et al. designed a series of sulfur nanostructures with complicated host materials demonstrating excellent electrochemical performances. 9 Various nanostructured host materials, including but not limited to carbon/sulfur, 11,14,16,17,21 polymer/sulfur, [22][23][24] and metal oxide/sulfur composites 12,25,26 have been investigated. These nanostructures provide new merits and opportunities: (1) in nanostructured sulfur cathodes the distances over which electrons and ions must diffuse are dramatically decreased; and (2) the nanostructured hosts also have large specific surface areas, which can provide abundant electron and ion transport paths. As a result, sulfur cathode with well-designed nanostructures exhibited both improved discharge capacity and enhanced rate performances. 9 Nevertheless, it should be noted that the nanostructure strategy also has its disadvantages, such as the poor tap density of the active material, low thermodynamic stability and serious agglomeration during electrochemical cycling. 27,28 Those drawbacks limit the volumetric energy density of the resulted batteries and also lead to high irreversible capacity and poor cycle life. 28 Recently, a promising strategy incorporating nano/micro hierarchical structures rather than nanometer-scaled structural features only has been developed to address the drawbacks mentioned above. [29][30][31] Through elegant design of hierarchical sulfur cathodes with nanocarbon networks, Jung et. al demonstrated that Li-S batteries with ultrafast charge/discharge rates and long-life can be fabricated and scaled up. 32 The sulfur hosts with appropriate nano/micro porous structures are now widely desired for the development of high performance sulfur cathodes. In addition to material developments, novel in situ and ex situ characterization techniques have also made great progresses, which contribute significantly to the in-depth understanding of nanostructured sulfur cathodes and will further guide the rational design of sulfur cathodes. [33][34][35][36][37][38][39][40][41][42][43] To address the issue of fast capacity fade, physical confinement is frequently employed in the design of host materials for sulfur cathodes. By coating with external physical barrier or physical absorption layer on the host materials, the solvated polysulfides are expected to be confined within the cathode, and thus the shuttle effect can be significantly suppressed. [44][45][46][47][48][49][50] However, in general, the polysulfides still tend to disengage from the cathode because both the sulfur host materials and the physical wrapping layers are usually "sulfiphobic". 51,52 In some recent studies, researchers have come to realize that an ideal confinement barrier should have a "sulfiphilic" surface and also be conductive for the absorbed polysulfides. 53 Thus, the attention on designing host materials for sulfur cathodes has extended from physical confinement to surface chemistry. [54][55][56][57][58] That is, the confinement barrier should not only trap the solvated polysulfides but also provide subsequent surface redox centers for the adsorbate. Aurbach et al. reported a groundbreaking work in which redox mediators were introduced to enable the application of Li 2 S cathodes for Li-S batteries. 59 Polysulfide mediators are reversible redox couples that can undergo recycled electrochemical reaction at the electrode surface. The addition of metallocenes as functional additives in electrolyte allows for enhanced utilization of Li 2 S cathodes. Nazar and Cui et al. further demonstrated improved cycle life and discharge capacity of sulfur cathodes by employing similar surface chemistry based strategies. 25,60 As subsequently reported by various other groups, the use of metal oxides, 2D metal carbides/carbonitrides, or doped carbon materials as sulfur hosts or functional additives can also help to suppresses the polysulfide shuttle and improve the polysulfide redox kinetics, often simultaneously. 21,[60][61][62][63][64][65][66][67][68][69][70][71] The downside of adding these inactive materials is the reduction in energy density of the final product the Li-S battery, and the requirement of high specific surface area of the these additives also brings the challenge of materials dispersibility in long and repeated cycles. Although there are still many obstacles to overcome, development of functional additives based on special surface chemistry will be beneficial for Li-S cathodes in the near future.
As there are already many excellent reviews of Li-S batteries available, 7,63,72,73 this mini review just highlights our recent work on sulfur cathodes that aimed at enhancing charge transfer and suppressing polysulfide shuttle. We demonstrated that the bottleneck of slow charge transfer kinetics could be improved by incorporating nanostructured materials or functional additives. In order to obtain Li-S batteries with excellent overall performance, we then developed a rational design strategy that could simultaneously improve the energy density and power density of the batteries. Furthermore, we introduced an in situ wrapping approach that aims at overcoming the conflict between allowing for electrolyte infiltration and suppressing polysulfide shuttle. Towards the end of this review, we will briefly highlight two future directions to advance the Li-S battery technology.

The Nano Size Effect in Li-S Cathodes
Control of S nanoparticle size.-Inspired by the commercialization of low-conductivity LiFePO 4 material for lithium ion batteries (LIBs) cathodes via the synthesis of particles that are hundreds of nanometers in size and coated with conducting carbon, 74,75 one of our earlier works compared the electrochemical properties of sulfur particles with different, i.e., nanometer vs. micrometer sizes. 76 A novel membrane-assisted precipitation process was developed for sulfur nanoparticle synthesis (Fig. 1a). Microfiltration hollow fiber membranes with an average pore size of 100 nm were used to disperse a S/CS 2 solution into a large volume of ethanol. The S/CS 2 solution would form micro-droplets when it passed through the pore of the membrane. Because sulfur is insoluble in ethanol, the sulfur solute in micro-droplets precipitated into nanoparticles immediately as the droplets passed the membrane. As the nucleation process of sulfur particles is restricted within each individual micro-droplets, the particle size of the resulted sulfur nanoparticles is thus mainly determined by the membrane pore size and the fluid dynamics across the membrane. In order to prevent the primary sulfur particles from aggregation, a surfactant (poly-vinylpyrimidine) was used as wrapping ligand to control further growth of the sulfur nanoparticles. This approach is quite unique compared to previous reports of sulfur particles synthesized via the reduction of Na 2 S 2 O 3 or other salts, which involves multi-stepped reduction of sulfide species and intricate nucleation/growth processes of sulfur particles. 77,78 The membraneassisted precipitation process reported by us involves only physical precipitation but no chemical reaction. Ultrafine sulfur nanoparticles (Nano-S) with diameter in the range of 10-20 nm were obtained successfully using this membrane-assisted precipitation method (Fig. 1b,  1c). The sulfur nanoparticles were then polymerized with poly (3,4ethylenedioxythiophene) to form S@PEDOT core@shell structures, with the aim of trapping polysulfides dissolution during cycling (Fig.  1d). Commercial powder sulfur (CP-S) with particle size in the range of 1-50 μm was used as a control material for comparison.
Impressively, the initial galvanostatic discharge/charge profiles showed that the nanosized sulfur (including Nano-S and Nano-S@PEDOT) exhibit high initial capacity of 1028 and 1117 mAh g −1 , respectively, while the CP-S only had 708 mAh g −1 (Fig. 1e). Also, the charge-transfer resistance of the cell with Nano-S cathode was much smaller than that with CP-S cathode (Fig. 1f), suggesting that the use of nanosized sulfur particles have improved the electrochemical kinetic characteristics in the Li-S cathode. Different from the common nanostructure strategies that focused on synthesis of sulfur host materials, 9 this work aimed at direct investigation of the nanosize effect. It has been demonstrated that the nanosized S particles facilitate charge transfer and significantly improve the utilization of the sulfur materials.
Different from the intercalation/deintercalation chemistry in LIBs, the operation of Li-S batteries is based on multi-stepped redox reactions between Li and S involving a solid-solution-solid transition. 7 The influence of the nano-size effect on the charge transfer kinetics in such a complicated battery system needs to be thoroughly investigated and better understood.
One of the key prerequisites of the detailed mechanistic study on the nano-size effect is the size-controlled synthesis of sulfur materials. Several methods have been reported to control the S particle size in recent years, including the design of carbon-sulfur composites, polymersulfur composites and hollow micro-/nanostructures etc. 9,31,77,79 These methods are fairly effective in controlling the size of the S particle products. In the meantime, we have developed an unique sulfur-amine chemistry-based method with high conversion efficiency. 80 Sulfuramine chemistry was first investigated in the 1960s to explain why the solubility of sulfur in amine solvents was much higher than that predicted by theoretical considerations. 81 Previous studies have revealed that sulfur mainly exists as polysulfide ions and sulfur radicals in amine solutions. 82,83 Since that octatomic S 8 molecules could be quantitatively recovered from sulfur-amine precursor solutions upon addition of diluted acid, 80 sulfur nanoparticles (S NPs) on a reduced graphene oxide (rGO) matrix were prepared for Li-S battery cathode applications (Fig. 2). 84 Compared to the membrane-assisted precipitation technique and most other methods reported up-to-date, the sulfur-amine chemistry-based method exhibits more flexible control and better mono-dispersity in the size of the S particle products.
Enhanced performance of nanosized sulfur.-The controlled synthesis of sulfur particles lays a solid base for subsequent studies on size-dependent electrochemical performances of the sulfur cathode material. 84 Smaller S NPs shows better sulfur utilization efficiency and rate performance. As shown in Fig. 3a, S NPs with smaller size exhibit greater initial galvanostatic discharge/charge capacities under 0.25 C, indicating better utilization of sulfur. The better S utilization and higher specific capacity of smaller S NPs mainly originates from the larger specific surface area of smaller S NPs that facilitates electron transfer and Li + diffusion. Furthermore, the charge transfer resistance, R ct , obtained from fitting the EIS spectra of freshly assembled cells at open circuit potential exhibited a monotonic decrease as the nanoparticle diameter decreases (Fig. 3b). The monotonic decrease of R ct could be attributed to the larger specific surface area and improved electrical contact between the smaller sulfur nanoparticles and conductive hosts. The EIS results corroborated that the nano-size of sulfur particles can improve electrochemical kinetic characteristics of sulfur, and the smaller S NPs, the faster kinetics.
The smaller S NPs also exhibit better rate performances, as shown in Fig. 3c. The synergistic effect of particle size and discharging rate was noticed as the following: 84 in large sized sulfur particles under high discharging current density, the high Li + flux, which is necessary for charge neutralization, could result in an insulating Li 2 S 2 and Li 2 S blocking layer at the sulfur particle surface and thus leads to low discharging capacity; smaller sulfur particle size and lower discharging rate allow for smooth Li + diffusion into the interior of sulfur particle, thus delaying the formation of insulating Li 2 S 2 and Li 2 S and improving the sulfur utilization (Fig. 3d-3i). As a result, the smaller sulfur nanoparticles exhibited higher reversible specific capacity at high rate discharge.

Push the limit of discharging/charging capacity.-Importantly,
we demonstrated that the theoretical discharging behavior can be experimentally realized in ultra-small sulfur nanoparticles. When the sulfur particle size was decreased down to 5 nm, the specific capacity reached the theoretical specific capacity of 1672 mAh g −1 at 0.1 C (Fig. 4a). The discharge curve also matches very well with theoretical prediction. The high-voltage plateau that corresponding to the solid-liquid two-phase reduction from elemental sulfur to polysulfides (at ∼2.35 V) exhibited a capacity of 418 mAh g −1 . The low-voltage plateau (at ∼2.14 V) together with the subsequent tailing slope corresponding to the liquid-solid two-phase reduction from the dissolved polysulfides to insoluble Li 2 S 2 /Li 2 S exhibited a capacity of 1254 mAh g −1 (Fig. 4a).
Detailed characterization of the discharged product of the 5 nm S NP cell at DOD of 100% showed no XRD peaks that corresponding to elemental sulfur (Fig. 4b). And X-ray photoelectron spectra (XPS) indicated two peaks at binding energy of 159.8 and 160.8 eV (at DOD of 100%), which are characteristic to Li 2 S (Fig. 4c). These results suggested that the theoretical limit of specific capacity and the theoretical discharge/charge behavior of Li-S battery can indeed be achieved with the ultra-small S NPs. In-depth understanding of the limiting factors in reaching the theoretical performance of elemental sulfur would guide further rational design of sulfur cathodes. But, on the other hand, we should note that the realization of theoretical discharge capacity does not necessarily lead to high energy density of the assembled cells. The energy density of a battery is determined by multiple factors including specific discharge capacity, discharge voltage, sulfur loading of the electrode, the amount of electrolyte etc. 85 Systematic engineering is crucial in obtaining high energy density cells from a high specific capacity material.

Polymer-Based Functional Additives to Promote Polysulfide Redox in Li-S Cathodes
Although nanostructure design is currently a highly popular strategy for promoting charge transfer of Li-S cathodes, it can only promote the initial solid-state discharge process of the Li-S cathodes. Once the polysulfide intermediates were generated, they tended to dissolve and diffuse into liquid electrolyte. 7 Subsequent reduction of polysulfides to form final Li 2 S 2 and Li 2 S product was thus rendered more difficult because of the repulsion between the polar polysulfides and the generally nonpolar conductive surface. 51,52 The limited redox efficiency of the polysulfides would thus lead to low sulfur utilization and slow electrochemical kinetics. 2,7 Therefore, development of nanostructure design alone is insufficient and kinetically unfavorable to promote charge transfer, especially for the polysulfide redox in the sulfur cathode.
As mentioned above, incorporation of functional additives into the sulfur cathodes would provide extra charge-transport paths for polysulfide redox, making it a promising complementary approach for promoting charge transfer in the sulfur cathodes. 25,59,60 Compared to the inorganic additives as reported in other studies, we demonstrated that polymer-based additives can also exhibit remarkable enhanced redox efficiency of the polysulfides. Poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA), which is a stable free radical polymer reported to be potential organic electrode materials, 86 can behave as an efficient additive for high performance Li-S batteries once it was activated via in situ electrochemical oxidation (Fig. 5a). 87 Three sets of independent experiments were performed to evaluate the role of the PTMA additive: (1) PTMA was in situ activated to PTMA + by charging to 4.0 V, and then cycled in the typical Li-S electrochemical window of 1.5-2.8 V in the subsequent cycles (PTMA + /S battery). (2) PTMA was cycled between 1.5-2.8 V without activation to PTMA + (PTMA/S battery). (3) PTMA was replaced with a  traditional PVDF binder, and the battery was also charged to 4.0 V for "activation" before subsequent cycled at 1.5-2.8 V (PVDF/S battery).
Interestingly, the initial discharge capacity of the PTMA + /S cell was as high as 1254 mAh/g, while that of the PTMA/S cell is only 692 mAh/g (Fig. 5b). Similarly, the PTMA + /S cell also exhibited improved rate performances compared to the controlled cells (Fig.  5c). Overall, the PTMA + /S cell presented significant improvements in both discharge capacity and rate performance. More detailed studies demonstrated that the activated PTMA + not only displayed strong binding affinity to polysulfides but also provided PTMA + -assisted additional redox sites. In addition to the usual redox path, where sulfur materials and polysulfides were directly in contact with the conductive network, the PTMA + were expected to become extra transport sites where the PTMA + assist the polysulfide redox (Fig. 6). Thus, it helped to improve the kinetics of the cathode reaction, resulting in improved discharge capacity and better rate performance. Compared to the inorganic-based additives as reported elsewhere, the polymer-based additive may also function as a binder. The multifunctional binder/additive may improve electrochemical properties of the final batteries without compromising the energy density. There have been several excellent reviews covering functional additives in Li-S batteries in recent years. [88][89][90] As reported in many previous studies, host materials with "sulfiphilic" surfaces can bind to polysulfide intermediates via strong surface-sulfur interactions, resulting in a significant suppression of the shuttle effect. However, in the case of functional binder PTMA + , it not only displays strong binding affinity to polysulfides but also provides additional catalytic activity and improves the kinetics of the cathode reaction, which is an advancement based on previous concepts. Considering the rich possibility in functional polymer design and synthesis, further advances toward high-performance Li-S battery with polymer-based multi-functional additives can be expected.

Rational Design of Li-S Cathode for Improved Overall Performance
Along with the intensive researches in the past decade, Li-S batteries have exhibited rapid improvements in energy density. Nevertheless, there exists a trade-off between energy density and power density, due to the inherent insulating nature of S and Li 2 S 2 /Li 2 S and hence poor electrode kinetics. Commercialization of the Li-S batteries is still yet to be realized because of relatively poor overall electrochemical performances. The current Li-S technology calls for rational design of Li-S cathode structures that can lead to excellent overall performance of Li-S batteries.
To improve energy and power densities simultaneously, sufficient redox sites inside the electrode are necessary for energy density, and integrated electron/ion pathways are essential for electrode kinetics. We thus designed a novel freestanding Li-S cathode, which consists of a composite film assembled from sulfur nanoparticles, rGO, and a multifunctional polymer poly(anthraquinonyl sulfide) (PAQS). 91 The thin film cathode was prepared using a simple self-assembled technique: the PAQS polymer solution was mixed with S nanoparticle on an rGO dispersion and then vacuum filtered to assemble a freestanding film with a layered and porous structure (Fig. 7a-7c). The major innovations of the cathode design include: (1) the sulfur nanoparticles provided a high initial specific capacity, and the layered and porous rGO structure provided electron/ion transport paths and also restricted polysulfide shuttling. (2) PAQS was not only a highly efficient sulfide trapping agent but also an excellent Li + conductor, which benefited the battery reaction kinetics at a high charge/discharge rate. As a result, the freestanding Li-S cathode showed an initial discharge capacity of 1255 mAh g −1 and displayed a reversible capacity of 615 mAh g −1 when discharged at a high rate of 8 C (Fig. 7d, 7e). And more impressively, it exhibited a capacity decay rate as low as 0.046% per cycles over 1200 cycles (Fig. 7f). 91

In Situ Wrapping Approach to Build Li-S Cathode with Better Cycle Life
While highly stable Li-S cathodes with lifetimes of more than 1000 cycles have been reported, 91 they are still not comparable to the current commercial LIBs. Even with design of sulfur cathodes incorporating diffusion barrier and sulfide trapping additives, a continuous capacity-fading mainly due to the polysulfide shuttle still remains a major challenge. Cathode material coating or wrapping, which typically involves the preparation of sulfur composite particles with a wrapping layer before assembling into coin cells for testing, has been widely investigated. 76,92 Such a pre-battery-assembly wrapping approach presents a dilemma: if the pre-assembly wrapping layer were designed to be perfectly compact and tight (so as to completely block the polysulfide diffusion), electrolyte would not be able to infiltrate into the cathodic composite. The resulted battery would then exhibit very poor electrochemical performance (Fig. 8b). Conversely, if the pre-assembly wrapping layer were designed to be imperfect with cracks or pores, which may allow for electrolyte penetration into the cathodic composite, then the solvated polysulfides would also leak out of the wrapping layer via the cracks or pores, resulting in improved, but nevertheless still decaying performance (Fig. 8c). The latter scenario essentially represents most cathode wrapping work reported up-to-date. 76,92,93 In our latest work, we have demonstrated a novel in situ wrapping approach that overcome the conflict between allowing for electrolyte infiltration and blocking the polysulfide diffusion. 94 Carbon/sulfur composite particles were first imperfectly coated with a wrapping layer, and then assembled into coin cells. The imperfect wrapping layer allows for infiltration of adequate electrolyte into the C/S particles. Importantly, a special functional additive was added in the electrolyte, aiming at reacting with the initially imperfect wrapping layer to form a second wrapping layer after the battery was assembled (Fid. 8d). This second, post-assembly and in situ formed wrapping layer was designed to close the pores of the imperfect coating layer.
To construct the multi-shell structure, a CMK-3/S composite was first prepared according to a literature procedure via the melt-diffusion method. 20 Then a polyacrylonitrile (PAN) layer was grown on CMK-3/S via free radical polymerization of acrylonitrile. The PAN layer was sulfurized through the reaction of CMK-3/S@PAN with sulfur to obtain a CMK-3/S@sulfurized PAN (CMK-3/S@PANS), 19 and the resulted CMK-3/S@PANS was assembled into coin cells as the cathode material. Triphenylphosphine (TPP) was added into the electrolyte, which was expected to form a TPS coating layer on top of the PANS layer (CMK-3/S@PANS@TPS) while allowing for electrolyte infiltration into the interior of the composite particles (Fig.  9a). 95 Transmission electron microscopy (TEM) was used to analyze the generation of the wrapping layer: the original CMK-3/S particles displayed a clean surface; a coating layer with a thickness of ∼40 nm or less was found on the surface of CMK-3/S@PANS particles; after soaking in TPP-added electrolyte, an additional coating layer as thin as ∼15 nm was observed in the CMK-3/S@PANS@TPS sample (Fig.  9b-9d). Our study demonstrated that the CMK-3/S@PANS@TPS cell exhibited remarkably enhanced cycling stability. In a glass cell test, no obvious change of color could be observed in the CMK-3/S@PANS@TPS cell, indicating highly effective blocking of the shuttling effect by the TPS layer ( Fig. 9e-9g). Moreover, the in situ wrapped C/S cathode exhibited an initial specific discharge capacity of 1246 mAh g −1 (0.25C), with a Coulombic efficiency of 98.2% over 100 cycles. Importantly (Fig. 9h, 9i), it exhibited a capacity decay rate as low as 0.030% and 0.034% per cycle over 1000 cycles at 1C and 2C, respectively (Fig. 9j).
This in situ wrapping strategy provides conceptually new opportunities for Li-S cathode materials development. Considering versatile in situ wrapping reaction types including photochemical or electrochemical methods and almost unlimited possibilities in molecular design of the in situ wrapping reagents, the stability of Li-S cathodes can certainly be further optimized. Importantly, this approach is in principle applicable to any C/S composites, and thus is compatible with scaling up attempts in the engineering of large capacity pouch cells.

Conclusions
Beyond summarizing our studies on sulfur cathodes, here we briefly highlight two future directions for the advancement of Li-S batteries as the following: (1) The cycling stability of the Li anodes has become a limiting bottleneck in the Li-S technology. 96,97 Uncontrollable interface reaction and low Coulombic efficiency during Li deposition/stripping are the major challenges in Li-S anodes. 98,99 In spite of these issues to be addressed, Li metal still poises to be the anode material of choice in rechargeable Li-S batteries in the near future. Commercially viable additives or nanostructure design for the Li anodes would be essential for their practical applications, 100-103 but the approaches must be efficient in both weight and volume so that they do not compromise the energy density of the whole battery.
(2) All-solid-state batteries composed of solid cathode, anode and electrolyte are promising to be the next breakthrough for Li-S batteries. 104 The replacement of liquid electrolytes by solid-state electrolyte was expected to enable a high-efficiency Li-S chemistry, eliminate the polysulfide shuttle, and provide an intrinsically safe battery design. 105,106 Advanced solid-state electrolytes with high ionic conductivity, including various glass-ceramic solid electrolytes and polymer-based electrolyte, have been demonstrated in recent years. [106][107][108] Nevertheless, the integration of the solid-state electrolytes into Li-S batteries is still a challenge mainly due to the high impedance at different electrode/electrolyte interfaces throughout the entire battery. 109 More recently, there have been efforts to investigate the interfacial behaviors between Li-ion conductors and cathode/anode in detail, with a growing number of in situ and ex situ analytical tools applied to studying the Li + transfer at interfaces. 110,111 All-solid-state Li-S batteries could be a highly promising alternative to the conventional liquid electrolyte Li-S batteries.
In summary, the prospectus of Li-S batteries has been improved remarkably over the past years. More breakthroughs are still needed in both enhancing the overall performance and understanding the working mechanisms of both traditional liquid and recent all-solidstate Li-S battery systems.