Interphases in Lithium-Sulfur Batteries: Towards Deployable Devices

Aug 3, 2018 - Qing Zhao , Jingxu (Kent) Zheng , and Lynden A. Archer. ACS Energy Lett. , Just Accepted Manuscript. DOI: 10.1021/acsenergylett.8b01001...
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Interphases in Lithium−Sulfur Batteries: Toward Deployable Devices with Competitive Energy Density and Stability Qing Zhao,† Jingxu Zheng,‡ and Lynden Archer*,†,‡ Robert Frederick Smith School of Chemical and Biomolecular Engineering and ‡Department of Materials Science & Engineering, Cornell University, Ithaca, New York 14853, United States

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ABSTRACT: Lithium sulfur batteries have been studied intensively because of their potential to create high specific energies that meet accepted targets for mobile and stationary storage. Substantial efforts have been devoted to improving the electron/ ion conductivity of the sulfur cathode, suppressing dissolution of polysulfide, and limiting parasitic reactions of polysulfide with lithium. Less consideration has addressed the interface stability of lithium, which is critical for long-term stability of cells that utilize high sulfur loadings and minimal electrolyte volume to maximize overall specific energy. Recent emphasis on all-solid-state batteries underscores the important role well-formed interphases must play in achieving high material utilization at practical rates. We suggest that advances toward practical Li−S cells will come from strategies for creating durable interphases on lithium that limit active material and electrolyte loss and that provide greater flexibility in electrolyte design.

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A typical Li−S battery (Figure 1a) is composed of a lithium anode, a separator soaked with electrolyte, and a sulfur/carbonbased composite cathode loaded onto an Al metal current collector. Notwithstanding the many impressive characteristics of Li−S electrochemical cells, a commercially viable Li−S battery does not exist on large scale today because the solution thermodynamics of sulfur and its reduction compounds are complex.11−13 This issue is particularly problematic in Li−S cells because any soluble sulfur polymorphs formed at the cathode are able to diffuse to the reactive Li anode to undergo chemical reduction in a parasitic process, which depletes the active mass of the cathode and anode over time. A large body of research has focused on creating substrates for sulfur cathodes in order to kinetically trap polysulfide species generated in the cathode to limit parasitic losses of active material.14−19 Significant efforts have also been given to developing strategies for sequestering sulfur and polysulfides in hosts that are able to accommodate the volume changes associated with the electrochemical conversion reactions at the Li−S battery cathode.6 As a consequence of these efforts and nearly a decade’s worth of development, the utilization of sulfur in the Li−S battery cathode is approaching 100% and Li−S cells with cycle life approaching thousands of cycles have already been reported.20,21 An electrolyte-to-sulfur ratio close to 4 μL/ mg is thought to be the lowest to guarantee sufficient solubility of LiPS to enable such high utilization of sulfur species in the

echargeable batteries that utilize metallic anodes are considered to be promising candidates for nextgeneration energy storage/conversion devices, especially when the metal anode is coupled with high-capacity conversion cathodes (sulfur, O2, CO2, etc.).1−4 Among these couples, rechargeable lithium−sulfur (Li−S) batteries that combine metallic lithium as anode and elemental sulfur as cathode offer the highest specific energy (2600 Wh/kg) of any solid-state battery. In addition to the intrinsically low cost and high earth abundance of the cathode material, the Li−S battery is an attractive candidate for next-generation storage for several additional reasons. First, Li−S batteries can be manufactured in similar configurations and form factors as commercial Li-ion batteries.5−8 Second, the electrode reactions are spontaneous, and their kinetics are relatively fast; Li−S cells typically do not require intervention with a catalyst or thermal source to achieve good discharge−charge rates. Finally, recent literature contributions have shown that Li−S cells exhibit high abuse tolerance.9,10 This is remarkable considering the high reactivity and low melting point of the Li metal anode, which makes it especially prone to thermal runaway.

Practical Li−S cells with high specific energies and long cycle stability require the cooperation of each component, especially when using low anodeto-sulfur and electrolyte-to-sulfur ratios. © 2018 American Chemical Society

Received: June 14, 2018 Accepted: August 3, 2018 Published: August 3, 2018 2104

DOI: 10.1021/acsenergylett.8b01001 ACS Energy Lett. 2018, 3, 2104−2113

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Figure 1. Illustration of configuration and energy density of Li−S batteries. (a) Configuration of Li−S batteries. The vital points related with energy density as well as long cycle stability have been labeled. (b) Energy density of Li−S batteries calculated by thickness of Li or electrolyteto-sulfur ratio at different sulfur content. The curves in blue are the relationship between thickness of Li and energy density, in which the electrolyte-to-sulfur ratio is constant at 4 μL/mg. The curves in red are the relationship between electrolyte-to-sulfur and energy density, in which the capacity ratio of lithium-to-sulfur is set constant at 2.

cathode.22 High areal loadings of sulfur (>50 mg/cm2) have likewise been reported, although these results are mostly achieved with thick current collectors (>200 um).23 The continuous advances made over the past decade have accelerated interest in commercializing Li−S batteries.24 This interest has in turn put a spotlight on several practices in the research literature, including the method of reporting the specific energy of Li−S cells in terms of mass of sulfur in the cathode. It is understood, however, that in a practical battery the mass of additional components must be taken into account to fully understand the competitive advantages of the Li−S cell relative to state-of-the-art Li-ion battery technology. These include the mass and thickness of lithium anode (Li-to-sulfur ratio), the amount of electrolyte used in the cell (electrolyte-tosulfur ratio), and the areal and volume loading to sulfur in the cathode. These effects can be captured more concretely in terms of the practical specific energy of a Li−S battery, which can be computed using the expression

of-the-art LiBs at all sulfur mass loadings in the Li−S battery cathode. It should be noted that for limited or thin lithium, the Li metal anode needs to be coated on copper or other lightweight metal current collector, in which the energy density can be further decreased. Meanwhile, sulfur utilization close to 100% (1675 mAh/g) is nearly impossible to achieve for a thick electrode. Therefore, we also add a line for 20 mg/cm2 sulfur cathode (thick blue lines), in which we estimate the capacity of 1000 mAh/g and the energy density are largely reduced to 60%. It is noted, however, that when a more commonly used and thicker commercial lithium (500 μm) is used as the anode, sulfur loadings above 5 mg/cm2 are required to achieve specific energy competitive with that of LIBs. Taking these observations one step further, we compute the practical specific energy of the Li− S cell in which 100% excess of Li is present in the anode and the electrolyte-to-sulfur ratio varies for the same sulfur mass loadings as before (see red curves of Figure 1b). It is apparent from the figure that irrespective of the sulfur mass loading, the specific energy increases as a strong nonlinear function of decreasing electrolyte-to-sulfur ratio, implying that in order to achieve the high specific energies (>500 Wh/kg) promised by the Li−S battery chemistry, in practical cells, electrolyte-tosulfur ratio values below 3 and a sulfur loading of at least 5 mg(s)/cm2 are required. Therefore, in addition to contemporary efforts to increase the sulfur loading in Li−S cells, efforts to reduce the amount of electrolyte in the cell are also a priority for future work.

1675 Ah/kg × 2.15 V × mass(s) mg/cm 2 (mLi + mEle + mcathode + mseparator + mcurrent collector ) (mg/cm 2)

Here, we have neglected the mass of any packaging material, which can produce an additional lowering of the cell-level energy density, by as much as 10% for pouch cells. Assuming the composition of sulfur in the cathode is 70 wt % of cathode, the areal density of cathode current collector (Al foil) is 4 mg/cm2, the electrolyte is 1 M LiTFSI in DOL/DME with a density of 1.25 mg/ul, and the mass of the separator is about 1.1 mg/cm2 (celgard 2320), the specific energy of the cells is easily computed (see Figure 1b). The figure also includes practical specific energies (green shaded area) achievable with state-of-the-art Liion cells. The blue curves report the effect of Li anode thickness on specific energy in Li−S cells in which the electrolyte-to-sulfur ratio is fixed at 4 μL/mg for varying sulfur mass loading at the cathode (e.g., 8 μL/cm2 for 2 mg(s)/cm2; 20 μL/cm2 for 5 mg(s)/cm2; 40 μL/cm2 for 10 mg(s)/cm2; 80 μL/cm2 for 20 mg(s)/cm2). The results show that the practical specific energy of the Li−S cell increases with increasing sulfur loading, and when a thin (100 μm) Li anode is used, the overall specific energy is close to or higher than 400 Wh/kg, exceeding values accessible in state-

Interphases that simultaneously protect Li from parasitic reactions, are able to flex and stretch to accommodate reversible volume change, and enable fast interfacial transport of Li are a requirement for progress. Beyond their impact on the practical specific energy of a Li−S battery, proper accounting of the mass of all components in the cell introduces additional, quite serious challenges associated with performance. In particular, the commonly used electrolyte composed of DOL (1,3-dioxolane)/DME (1,2-dimethoxy2105

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Figure 2. Typical methods of protecting Li anode in Li−S batteries, electrolyte additives, and corresponding electron micrographs. (a) An organic/inorganic interface generated by using SiCl4 contained ester-based electrolyte. Reproduced with permission from ref 34. Copyright 2018 Wiley-VCH. (b) Hybrid interface formed by poly(sulfur-random-1,3-diisopropenylbenzene) added electrolyte. Reproduced from ref 36. Copyright 2018 American Chemical Society. Artificial layer and corresponding electron micrographs (c) scheme of fabricating MoS2-coated Li anode and (d) SEM image of cross-sectional view. Reproduced with permission from ref 37. Copyright 2018 Nature Publishing Group. Separator modification and corresponding electron micrographs. (e) Optical photographs and SEM cross-section view of G@PC/PP separators. Reproduced with permission from ref 38. Copyright 2018 Elsevier Inc. (f) Schematic illustrating working principle of cross-linked PEGDMA membranes using dangling sulfonate groups to regulate ion transport (upper panel) and corresponding SEM image. Reproduced from ref 41. Copyright 2016 American Chemical Society.

similar to that of black gun powder.32 These observations imply that despite the widespread use of LiNO3 as a potent electrolyte additive for achieving high Coulombic efficiency and stable cycling in small, research-scale cells, use of this additive in larger format, practical Li−S batteries is potentially problematic. Thus, in order to accommodate the thin-Li anode, high sulfur loading cathodes, and low sulfur-to-electrolyte ratios required for high specific energies, stable artificial solid electrolyte interphase (SEI) designs are needed.33 There are typically three approaches for creating such interphases to stabilize a lithium anode: (i) use of electrolyte additives as sacrificial agents to react with Li and electrolyte components to form a self-limited coating on the Li anode in situ;34−36 (ii) depositing a coating layer on metallic lithium exsitu;37 and (iii) coating the Li-facing side of the separator.38−42 A preferable SEI on the metallic Li anode should contain both inorganic salts that enable fast Li-ion transport and Li-ion conducting organic molecules (or polymers) that can flex to buffer volume changes at the anode during repeated Li stripping and plating cycles.43,44 A recent study by the authors showed that a small amount of SiCl4 can react with previous poor SEI and form Si-interlinked OOCOR molecules with inorganic LiCl salts, which exhibit fast kinetics in Li−S batteries and long cycle stability (1000 cycles) (Figure 2a).34 This improvement is caused by the elasticity of a Si-interlinked oligomer that can accommodate the volume change of Li stripping−plating, and the low diffusion barrier of LiCl enables fast Li transport. Wang’s group also found that an aromatic-based organosulfide and Li salts can be formed on the surface of metal Li using poly(sulfurrandom-1,3-diisopropenylbenzene) additive, which enable the high Coulombic efficiency (CE) of 99.1% after 420 cycles and long cycle life of 1000 cycles in Li−S batteries (Figure 2b).36 Both works propose that a hybrid SEI is favorable for achieving high CEs in Li−S batteries. Recently, Cha et al. reported that two-dimensional (2D) MoS2 also provide an artificial protective layer on the surface of

ethane)-LiTFSI (LiN(CF3SO2)2) + LiNO3 is considered a requirement for stable Li−S battery operation because this electrolyte displays good solubility of lithium polysulfides (LiPS) generated at the cathode, which is advantageous for ensuring fast reaction kinetics of sulfur-based intermediate products. This same characteristic unfortunately leads to loss of active material in the cathode and to degradation of the Li anode as a consequence of parasitic reactions between dissolved LiPS and the Li metal anode. Inevitable capacity fading due to parasitic reactions between dissolved LiPS species and the Li anode can to an extent be masked in the small cells typically used in research publications, which often employ large excess of Li. In larger cells that meet any of the criteria defined in the last section as requirements for high practical specific energies, particularly those in which the anode/cathode capacity approaches unity, these reactions produce more rapid fading.25 A stable interface between the lithium anode and electrolyte is known to play a crucial role in inhibiting parasitic reactions between electrolyte components and rough or mossy deposition of Li at low current densities.26 Thus, considering the importance of high sulfur loading and low electrolyte-to-sulfur ratios in achieving high practical specific energies, the quality and stability of interfacial phases formed on the Li anode are as important as the more extensively studied capacity of cathode hosts to bind LiPS, particularly in cells that utilize thin lithium foil to achieve an anode-to-cathode ratio near unity. The combination of LiNO3 and polysulfides (either generated from discharge of the Li−S cell or produced by sacrificial additives introduced to the electrolyte) has been investigated for its ability to form a stable SEI, which can protect the Li surface.27−30 However, the most detailed studies indicate that the NO3− group is gradually exhausted during battery cycling and generates gases, including N2O and N2, which are detrimental to long-term stability of a closed electrochemical cell.31 The reaction is also reported to yield products that accumulate in the cathode to create a material with composition 2106

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Figure 3. (a) Energy density of Li−S batteries calculated by the capacity ratio of Li-alloy anode and sulfur. (b) Illustration (upper panel) and SEM image (lower panel) of air-stable LixSi nanoparticles anode that is encapsulated by graphene sheets (foil). (c) Corresponding discharge− charge curves of foil−S and Li−S batteries. Panels b and c are reproduced with permission from ref 52. Copyright 2017 Nature Publishing Group.

Li metal to improve the performance of Li−S cells.37 Specifically, these authors found that sputter coating of a thin (∼10 nm), 2D MoS2 layer on the surface of Li metal can be lithiated to form an artificial SEI that is effective in stabilizing the metal against parasitic reactions with dissolved LiPS, enabling stable cycling of Li−S batteries for over 1200 cycles with a high CE of 98% and 84% capacity retention (Figure 2c,d). Analysis by density functional theory (DFT) calculations reveals that the diffusion of Li+ in the SEI is accelerated by a surface migration pathway that prevents mossy or dendritic deposition during recharge. Although the mentioned high energy density (589 Wh kg−1) in this work is based only on the mass of the cathode (sulfur and CNTs), it suggests that 2D materials with semiconductor to metallic transport characteristics are promising SEI candidates for enabling Li−S cells with long-term cycling stability. Functional separators have been reported to play a similar role in Li−S batteries.45 Pei et al. coated 2D N-doped porous carbon nano sheets on commercial polypropylene separators (G@PC/ PP), which can be used to trap polysulfide because of the physical absorption caused by porous structures and chemical absorption caused by chemical absorption of N doping (Figure 2e).38 This approach is attractive because the coatings are thin (0.9 um) and enable the use of high loading of sulfur cathodes (12 mg/cm2) with stable cycling performance. Ion-selective membranes that facilitate preferential transport of cations have also been investigated for their ability to enable stable cycling of metal batteries.46 Ma et al. reported a UV-cross-linked polyethylene glycol-based separator that incorporates dangling sulfonate groups covalently tethered to the network forms a highly conductive membrane that selectively facilitates Li+ transport while preventing migration of polysulfide due to electrostatic interactions between SO32− and Sx2− and small pores generated from cross-linking (Figure 2f).41 The applied Li−S batteries demonstrate high efficiency over 98% even without the addition of LiNO3. Another advantage is high cation ion transference number (tLi+) of ion-selective membrane (>0.9), which can largely reduce the side-reaction on the anode side. Considering the rigorous conditions of handing metallic Li during cell assembly, especially the thin Li anodes required to achieve high specific energy, application of Li alloys has also been considered as a possible choice for the anode in Li−S batteries. Replacing the Li anode with a Li alloy also provides a mechanism for eliminating parasitic reactions with dissolved LiPS and electrolyte components, enabling smaller sulfur-to-

electrolyte ratios. The effectiveness of this approach can be evaluated using the formula below to compute the specific energy of a Li-alloy/sulfur cell: 1675 Ah/kg × voltage × mass(s) mg/cm 2 (manode + mEle + mcathode + mseparator + mcurrent collector ) (mg/cm 2)

The calculation of cathode and electrolyte is the same as Figure 1b with fixing sulfur loading as 10 mg/cm2. For the anode, the current collector is assumed to be a 9 μm Cu with areal density of 8 mg/cm2. The anodes of LiC6, Li4.4Sn, and Li4.4Si are designed to take up 90 wt %, 70 wt %, and 70 wt % of the anode, respectively. The mass of the anode is determined on the basis of the capacity of sulfur in the battery cathode. The working voltage is defined as 2.0 V (LiC6−S),47 1.5 V (Li4.4Sn−S),48,49 or 1.75 V (Li4.4Si−S)50−52 for different battery systems. Other hybrid anodes, such as Li−Bi, Li−hard carbon, and Li−Al, are also possible choices. As displayed in Figure 3a, the specific energy of a Li-alloy−S cell based on LiC6 or Li4.4Sn as anode shows no obvious superiority in comparison to commercial Li-ion batteries because of the low specific capacity (LiC6−S) or low discharge potential (Li4.4Sn−S). An alloy anode composed of lithiated Si, on the other hand, is potentially quite promising because of its high capacity and low potential (vs Li+/Li), enabling cells with specific energies as high as 500 Wh/kg. In a recent demonstration of this concept, Cui’s group reported an air-stable Li-alloy/graphene foil anode, in which the Li-alloy especially LixSi is encapsulated by graphene sheets (Figure 3b).52 As displayed in Figure 3c, the foil−S battery shows high utilization of sulfur cathode at the lower operation voltage at about 1.75 V.

Solid-state electrolytes that provide interface stability and high ionic conductivity are still needed. Solubility of LiPS at the cathode is both a requirement and a problem in the Li−S cell. It is required for achieving practical charge-transfer kinetics and high levels of cathode utilization in sulfur and its electronically insulating reduction products. It is problematic, on the other hand, because (a) it provides a mechanism for irreversible loss of the active material in the cathode; (b) it facilitates transport of soluble, reactive sulfur compounds to the anode which react with Li and provide a mechanism for irreversible loss of the active anode material; and (c) dissolved LiPS slows ion transport at both electrodesit 2107

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Figure 4. General design strategies of solid-state (SS) electrolyte. Designing polymer-based SS electrolyte with high ionic conductivity. (a) Schematic diagram of preparation of HNT-modified LiTFSI−PEO-based electrolyte with enhanced ionic conductivity. (b) Ionic conductivity comparison of LiTFSI−PEO and LITFSI−PEO−HNT. Reproduced with permission from ref 53. Copyright 2017 Elsevier Ltd. Designing inorganic compound-based SS electrolyte with ideal interface. (c) Schematic diagram of Li10GeP2S12 solid-state electrolyte-based lithium− sulfur battery. Corresponding (d) cyclic voltammogram and (e) discharge−charge profiles (0.05 C) of all-solid-state cell at 60 °C. Reproduced with permission from ref 69. Copyright 2017 Wiley-VCH.

illonite clays,59 corn starch,60 and SiO261 are all effective in enhancing room-temperature ionic conductivity of PEO. Meanwhile, some fillers such as LiN3 can form a thin and compact passivation layer with high conductivity in all-solidstate Li−S batteries.62 In some cases, the filler particles have also been reported to facilitate ion pair dissociation and to increase the lithium ion transference number (tLi+). For example, Miller’s group found that the combination of PEO and halloysite nanotubes (HNTs) that has typical dimensions of 10−50 nm can greatly increase the conductivity (1.11 × 10−4 S cm at 25 °C).53 It was described that the opposite surface chemistry of HNT takes critical roles in improving the conductivity. The outer silica surface with negative charge will absorb Li ions, while the inter aluminum surface with positive charge will adsorb TFSI− ions, which together dissociated LiTFSI. Meanwhile, the EO units of PEO with lone pairs also absorb Li ions, which make the polymer organized on the outer HNT surface. The channels formed by HNT, LiTFSI, and PEO can significantly shorten the path of Li-ion transfer and provide fast Li-ion transport (Figure 4a,b). The applied Li−S batteries with the LiTFSI−PEO−HNT electrolyte maintain the capacity retention of 87% after 100 cycles with outstanding CE close to 100% at each cycle. The chemical diversity of inorganic solid electrolytes under consideration for Li−S batteries is far larger than that of their organic polymer analogs. Li3PS4,63 Li1.5PS3.3(60Li2S-40P2S5),64 LiBH4−LiCl,65 Li2S−P2S5,66 and Al3+/Nb5+ codoped cubic Li7La3Zr2O12 (LLZO)67 have all been applied in Li−S batteries. Some inorganic solid electrolyte such as Li 6 PS 5 Br, 68 Li10GeP2S12,69 and Li7P2.9Mn0.1S10.7I0.370 display high ionic conductivity even at room temperature, which is comparable to conductivity values associated with liquid electrolytes. In spite of this advantage, the inorganic solid electrolytes are often limited by their high interface resistance, which reduces the overall electrode reaction kinetics. The application of LiIn68 and LiAl71 as the anode appear to provide a route toward lower interface resistance between the battery anode and a solid-state electrolyte, but analogous strategies for enabling fast interface transport and high sulfur utilization at the cathode are still lacking. An additional challenge is that many of the most attractive inorganic SSEs are chemically unstable in contact with

increases the viscosity of the electrolyte, producing a diffusion boundary layer near the cathode and a passivating layer at the anode. Considering these conflicting attributes of the electrolyte in a Li−S cell, it is unsurprising that Li−S cells designed according to the criteria enumerated above to achieve high specific energy rarely meet the cycle life requirements for a practical battery. Li−S cells in which a solid-state electrolyte (SSE) serves to separate the cell into “cathode” and “anode” compartments are the first step to a Li−S battery design that could resolve these conflicting demands. A full solid-state Li−S in which a SSE serves as both the separator and substrate for cation transport between the electrodes would be another alternative. Two types of solid-state electrolytes have been evaluated in Li−S batteries: solid polymer electrolytes and solid inorganic electrolytes. The polymer-based electrolytes usually combine Li salts (LiTFSI,53 LiCF3SO3,54 Li[N(SO2F)2]55) and polymers (like PEO, polysiloxane56) that promote ion pair dissociation by coordinating with the Li+ cation in the salt. These electrolytes are attractive for their ease of synthesis, low mass densities, compatibility with large-scale roll-to-roll manufacturing, flexibility, and stable mechanical properties at the moderate charging potentials in a Li−S cell. They are problematic, however, because the most attractive polymers are semicrystalline materials, with ion transport principally occurring in the amorphous phase. This means that except in very rare situations (see discussion about filled polymers below) where the amorphous phase percolates throughout the electrolyte, extremely low ionic conductivity can be expected in roomtemperature battery operations. A LiTFSI/Poly(ethylene oxide) (PEO)-based polymer electrolyte has, for example, been reported to exhibit an ionic conductivity of 6.35 × 10−7 S/cm at 25 ○C.53 Introduction of nanosized fillers that interact strongly with the polymer and/or salt provide mechanisms for frustrating crystallization of PEO and for ensuring that the amorphous domains percolate to macroscopic length scales to enable higher conductivity at room temperature. There is a large body of work focused on dispersion of nanofillers in PEO and its impact on ion transport.57 These studies show that nanofillers based on ZrO2,54,58 montmor2108

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efforts needed to encapsulate Li2S. Recently, Tan and coworkers reported a combustion reaction between lithium and CS2 vapor to prepare Li2S nanocrystals wrapped by graphene (Figure 5c,d).74 This simple procedure enables both in situ combination with conducive carbon but also high Li2S ratio (88%). As a testament to the effectiveness of the approach, electrochemical analysis of the materials showed that even at a loading of 10 mg/cm2, specific capacity values close to 1000 mA h/g are achieved for the first few cycles. In addition to a high sulfur/carbon ratio in the cathode, it is already clear that a high areal loading of sulfur is needed to achieve Li−S cells with practical specific energies. A threedimensional (3D) interconnected cathode in which sulfur is dispersed on and in a substrate that also serves as a current collector is an effective method to achieve high sulfur loading without compromising electrode kinetics (Figure 5e,f).75 Such 3D interconnected current collectors have been reported by the authors to uptake more than 2000% electrolyte, which is a dramatic improvement relative to the 35% electrolyte uptake in a conventional planar, 2D, case in which the active cathode material is coated on an Al current collector. It means that even at high sulfur loadings, electrode components that facilitate electronic and ionic transport coexist through the electrode. A consequence is that cathodes with high sulfur loading of 21.2 mg/cm2 on a 0.8 cm2 current collector (electrode density 1.74 g cm−3) can be achieved. With 60 μL of electrolyte (electrolyte-tosulfur ratio of approximately 3.5 μL/mg), the 3D cathodes deliver an initial areal capacity over 23 mAh cm−2 and maintain about 15 mAh cm−2 after 200 cycles. The polysulfide additives can be used to heal the high-loading sulfur electrodes that enable long cycling life.76 Strategies for augmenting sulfur loading in the cathode using polysulfide (Li2S6, Li2S8) additives in liquid electrolytes is also a choice to achieve high sulfur loading and high specific energies in Li−S batteries. An additional benefit is that the interphases formed when the dissolved polysulfides react with the Li anode have been reported to be highly effective in passivating the Li anode against continuous parasitic attack by other electrolyte components. As with approaches that utilize a high sulfur loading, a 3D current collector design is needed to preserve good electrochemical access to the dissolved LiPS. Further modifications including coating functional groups on carbon to increase its affinity with sulfur can be employed to increase the stability of the electrochemical processes at such cathodes in extended cycling studies. Manthiram’s group, for example, reported that a coaxial graphene-coated cotton carbon (CGCC) collector is a particularly effective catholyte host for Li−S batteries (Figure 5g).23 The authors showed that a cathode created by adding 60 μL of 1.5 M Li2S6 (containing 17.28 mg of S) catholyte on CGCC to obtain a nominal sulfur loading of 57.6 mg/cm2 (0.3 cm2) produces Li−S cells with high electrolyte-tosulfur ratio (4.2 μL/mg) and maintain areal capacity of about 20 mAh/cm2 after cycling (Figure 5h). They further found that TiS2 addition is effective as a conductive polysulfide adsorbent in Li polysulfides batteries.77 Most of the reported high loading sulfur or polysulfide loading are based on porous current collectors such as carbon fiber (carbon nanotube, graphene) materials, which would be very problematic in commercial cell assembly, particularly for tab connection and thermal management. Meanwhile, the amount of catholyte is still needed to be elevated to further evaluate the specific capacity Perspective and Outlook. A decade’s worth of research has underscored the promise of Li−S batteries for storing large

the Li anode. To solve this problem, Yao et al. report a new structure of solid-state Li−S batteries (Figure 4c).69 At the cathode side, the reduced graphene oxide (rGO)@sulfur nanocomposite is mixed with Li10GeP2S10-acetylene in order to maintain the high ion conductivity and reduce the stress and strain through the discharge−charge process. At the anode side, a 75% Li2S−24% P2S5−1% P2O5 layer was pressed on the surface of the metal Li anode to avoid the side reaction of Li10GeP2S10. The results in Figure 4d,e demonstrate that the all-solid-state Li−S batteries exhibit one reduction peak and one oxidation peak, respectively. In addition, the cell can operate with high capacity and CE (almost 100%) at low current of 0.05 C. The electrolyte layer applied in this work is quite thick (1000 um), relative to typical separator (about 25 um for Celgard). The higher mass density of the electrolytes in reality places quite strong constraints on thickness (sulfur-to-electrolyte ratio), which are rarely considered in evaluating the promise of fully solid-state Li−S batteries. Further efforts are clearly needed to make thinner organic solid-state electrolytes with a better interface. Additionally, the all-solid-state electrolytes in Li−S batteries usually display only one discharge plateau, which has been attributed to a one step solid-state conversion (from S to Li2S2 or Li2S). This holds promise for a Li−S enabled by SSE that can essentially stop the generation of polysulfide, and maintain high CE with long cycling performance. Unfortunately, the conversion kinetics in solid-state sulfur cathodes are slow, meaning that either low current density or elevated temperature operation are required to achieve full sulfur utilization and high specific energy.

Li−S cells that combine high sulfur loading in the cathode and a polysulfide-based catholyte with low electrolyte-to-sulfur ratios provide a promising route to practical batteries with high specific energies. There are numerous studies in the literature in which stable cycling of Li−S batteries is achieved using simple nanomaterial design strategies for achieving good ion and electron transport in the cathode. The effectiveness of traditional approaches for infiltrating sulfur into the cathode substrate, such as meltinfusion, is largely dependent on the accessible pore volume of porous substrate, with the inevitable consequence that low sulfur loadings are achieved. These limitations can to an extent be avoided using other approaches, including wet chemistry synthesis,72 incorporating the sulfur in the form of a polysulfide catholyte20,23 and electrodeposition,73 to achieve high sulfur loading without compromising the nanostructure of the cathode and its beneficial effects on transport. Figure 5a,b reports results from a wet chemistry synthesis that enables the sulfur loading up to 90 wt %. The authors dissolved sulfur in N-methylpyrrolidine (NMP) and also dispersed ketjen black (KB), carbon nanotube (CNT), and pristine graphene (PG) in NMP to form a colloidal dispersion.72 Addition of water to the dispersion precipitates sulfur in nanoparticulate morphologies on the surface of the carbon particles. In addition to sulfur composites, the Li2S cathodes can also be synthesized with high sulfur content. Because the reaction from Li2S to S is accompanied by a volume reduction, a highly loaded Li2S cathode can in principle accommodate the volume change at the electrode without any 2109

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Figure 5. High active materials ratio and loading in Li−S batteries. High sulfur ratio. (a) Schematic illustration of preparing KB-CNT-PG/S composite through wet chemical synthesis procedure. (b) Corresponding thermogravimetric analysis of composite. Reproduced from ref 72. Copyright 2016 American Chemical Society. High Li2S ratio. (c) Schematic illustration of preparing Li2S@graphene capsules by the reaction of Li and CS2. (d) Corresponding TEM images. Reproduced with permission from ref 75. Copyright 2017 Nature Publishing Group. High sulfur loading. (e) Photograph of prepared hollow carbon fiber foam (HCFF). (f) Corresponding SEM image with sulfur loading of 16.5 mg cm−2. Reproduced with permission from ref 76. Copyright 2016 Wiley-VCH. (g, h) High polysulfide loading. (g) Schematic illustration of preparing polysulfide/coaxial−graphene-coated cotton-carbon composite and (h) corresponding cycling performance. Reproduced with permission from ref 23. Copyright 2018 Elsevier Inc.

quantities of electric energy on the basis of the S cathode mass. As a result, Li−S batteries are considered the most promising next-generation energy storage and conversion technology, particularly for aerial vehicles. In order to be competitive with state-of-the-art lithium-ion battery technology in any of these applications, multiple advances in the design of the electrodes and electrolyte are required to minimize the mass of enabling materials. For Li−S cells that exhibit stable long-term cycling and high active material utilization, but which use thinner Li anodes, high active material loading in the cathode and low sulfur-to-electrolyte mass ratios are required for further progress. On the basis of the analysis presented earlier in the Perspective, we believe that a useful near-term target for research is Li−S cells composed of thin anodes in which the Li excess is 100% or lower. As the Li metal anode is chemically reactive toward the most widely used electrolyte cosolvent (DOL), this choice places stricter demands on the compactness of Li deposits formed during battery recharge and the quality and stability of interphases formed on the Li anode. This in turn means that complementary efforts to understand the origins of, and to design materials to prevent, mossy deposition of Li are crucial for progress. Because the overarching goal to achieve practical Li−S batteries also requires high specific energy, this places strict limits on the mass and chemistry of any artificial interfacial phases used to facilitate stable and uniform lithium stripping− deposition in Li−S cells. Thus, although cells in which the conventional Li anode is replaced with a Li-alloy anode can reduce the usage of metal Li, these types of anodes introduce new problems associated with their lower overall specific capacity. Efforts to introduce electrostatic shields composed of particulate, polymer, and interlinked coatings that selectively transport cations to the Li electrode are particularly promising when these strategies are coupled with state-of-the-art deposition techniques that achieve high-quality, defect-free coatings. Application of such coatings either directly as artificial interphases on the anode or as interlayers on the cathode seems

promising for large-scale application combining polymer and coating technology. While initial studies along these lines suggest that application of thin coatings on the separator is more straightforward, transfer of coatings formed on inert substrates onto Li has recently been demonstrated in a scalable process compatible with roll-to-roll manufacturing. A Li−S cell in which a polymer or inorganic solid-state electrolyte is employed offers multiple paths for research that could lead to Li−S batteries that are intrinsically safe. The chemical stability and ease with which ions transport across the interfaces such electrodes form presents significant challenges that are fertile grounds for research. It is understood, however, that the strict mass limitations on all components, except the active sulfur material in the cathode, required to achieve practical Li−S cells place demands on the chemistry and density of SSEs that are well beyond those typically considered in the literature at this point. For now though, the low charge potential, cost, mechanical properties, compatibility with large scale manufacturing, and inherently low mass density offered by SSEs based on organic polymers present the most straightforward path toward practical, all-solid-state Li−S batteries. Regarding the sulfur cathode, maintaining the high utilization and kinetics after enhancing the sulfur (or polysufides catholyte) loading and lowering the electrolyte-to-sulfur ratio (lowered to 2 μL/mg) are significant for increasing the entire energy density. The deployable devices with competitive energy density and stability can be further expected after well integrating each part of Li−S batteries.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lynden Archer: 0000-0001-9032-2772 2110

DOI: 10.1021/acsenergylett.8b01001 ACS Energy Lett. 2018, 3, 2104−2113

ACS Energy Letters

Perspective

Notes

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The authors declare the following competing financial interest(s): Professor Archer is a Founder and member of the Board of Directors of NOHMs Technologies. NOHMs is a technology company located in Rochester, New York seeking to commercialize electrolytes for lithium-ion batteries and electrodes for lithium-sulfur batteries. Biographies Qing Zhao received his B.S. (2012) and Ph.D. (2017) degrees from Nankai University. Currently, he is a postdoc in Lynden Archer’s group at Cornell University. His research interest focuses on the preparation of micro/nanostructured hybrid materials and their application in various battery fields. Jingxu Zheng received his B.S. degree from Shanghai Jiao Tong University in 2017. He is currently a Ph.D. student in Lynden Archer’s group at Cornell University. His current research interest focuses on rechargeable metal lithium batteries. Lynden A. Archer is the James A. Friend Family Distinguished Professor of Engineering and Director of the Energy Systems Institute at Cornell University. His research interests center on transport properties in bulk liquids and at electrochemical interfaces.

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ACKNOWLEDGMENTS We are grateful to the Department of Energy Basic Energy Sciences Program, through Award DE-SC0016082, for financial support.

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