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© 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Lithium–sulfur batteries (Li–S) have become a viable alternative to future energy storage devices. The electrochemical reaction based on lithium and sulfur promises an extraordinary theoretical energy density, which is far higher than current commercialized Li-ion batteries. However, the principal disadvantage impeding the success of Li–S batteries lies in the severe leakage and migration of soluble lithium polysulfide intermediates out of cathodes upon cycling. The loss of active sulfur species incurs significant capacity decay and poor battery lifespans. Considerable efforts have been devoted to developing various sulfur host materials that can effectively anchor lithium polysulfides. Herein, a comprehensive review is presented of recent advances in sulfur host materials. On the basis of the electrochemistry of Li–S batteries, the strategies for anchoring polysulfides are systematically categorized into physical confinement and chemical bonding. The structural merits of various sulfur host materials are highlighted, and the interaction mechanisms with sulfur species are discussed in detail, which provides valuable insights into the rational design and engineering of advanced sulfur host materials facilitating the commercialization of Li–S batteries. Future challenges and promising research prospects for sulfur host materials are proposed at the end of the review.

Challenges from the insulating S and Li2S2/Li2S (Li2S1–2) discharge products are restricting the development of the high-energy-density Li–S battery system. The deposition of insulating Li2S1–2 on the surfaces of S based cathodes (e.g., S and Li2S) limits the reaction kinetics, leading to inferior electrochemical performance. In this work, the impact of binders on the deposition of Li2S1–2 on S based cathodes is revealed, along with the interaction between polyvinylidene difluoride and Li2S/polysulfides. This interaction can obstruct the electrochemical reactions near the binder, leading to dense deposition of insulating Li2S1–2 that covers the cathode surface. Without such a binder, localized and uniform Li2S1–2 deposition throughout the whole cathode can be achieved, effectively avoiding surface blockage and significantly improving electrode utilization. A full battery constructed with a binder-free Li2S cathode delivers a gravimetric and volumetric energy density of 331.0 Wh kg−1 and 281.5 Wh L−1, under ultrahigh Li2S loading (16.2 mgLi2S cm−2) with lean electrolyte (2.0 µL mgLi2S−1), providing a facile but practical approach to the design of next-generation S-based batteries.

The lithium–sulfur (Li–S) battery is widely regarded as a promising energy storage device due to its low price and the high earth-abundance of the materials employed. However, the shuttle effect of lithium polysulfides (LiPSs) and sluggish redox conversion result in inefficient sulfur utilization, low power density, and rapid electrode deterioration. Herein, these challenges are addressed with two strategies 1) increasing LiPS conversion kinetics through catalysis, and 2) alleviating the shuttle effect by enhanced trapping and adsorption of LiPSs. These improvements are achieved by constructing double-shelled hollow nanocages decorated with a cobalt nitride catalyst. The N-doped hollow inner carbon shell not only serves as a physiochemical absorber for LiPSs, but also improves the electrical conductivity of the electrode; significantly suppressing shuttle effect. Cobalt nitride (Co4N) nanoparticles, embedded in nitrogen-doped carbon in the outer shell, catalyze the conversion of LiPSs, leading to decreased polarization and fast kinetics during cycling. Theoretical study of the Li intercalation energetics confirms the improved catalytic activity of the Co4N compared to metallic Co catalyst. Altogether, the electrode shows large reversible capacity (1242 mAh g−1 at 0.1 C), robust stability (capacity retention of 658 mAh g−1 at 5 C after 400 cycles), and superior cycling stability at high sulfur loading (4.5 mg cm−2). NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

To address the serious capacity fading in lithium–sulfur batteries, a 3D ferroconcrete-like aminated carbon nanotubes network with polyaniline coating as an effective sulfur host to contain polysulfide dissolution is presented here. In this composite, the cross-linked aminated carbon nanotubes framework provides a fast charge transport pathway and enhancement in the reaction kinetics of the active material to greatly improve the rate capability and sulfur utilization. The ethylenediamine moieties provide strong adhesion of polar discharge products to nonpolar carbon surfaces and thus efficiently prevent polysulfide dissolution to improve the cycle stability, confirmed by density functional theory calculations. The outside polyaniline layers structurally restrain polysulfides to prevent the shuttle effect and active material loss. Benefiting from these advantages, the synthesized composite exhibits a high initial capacity of 1215 mAh g−1 and a capacity of 975 mAh g−1 after 200 cycles at 0.2 C. Even after 200 cycles at 0.5 C, a capacity of 735 mAh g−1 can be maintained, among the best performance reported. The strategy in this work can shed some light on modifying nonpolar carbon surfaces via the amination process to chemically attach sulfur species for high-performance lithium–sulfur batteries.

As the lightest member of transition metal dichalcogenides, 2D titanium disulfide (2D TiS) nanosheets are attractive for energy storage and conversion. However, reliable and controllable synthesis of single- to few-layered TiS nanosheets is challenging due to the strong tendency of stacking and oxidation of ultrathin TiS nanosheets. This study reports for the first time the successful conversion of TiCT MXene to sandwich-like ultrathin TiS nanosheets confined by N, S co-doped porous carbon (TiS@NSC) via an in situ polydopamine-assisted sulfuration process. When used as a sulfur host in lithium–sulfur batteries, TiS@NSC shows both high trapping capability for lithium polysulfides (LiPSs), and remarkable electrocatalytic activity for LiPSs reduction and lithium sulfide oxidation. A freestanding sulfur cathode integrating TiS@NSC with cotton-derived carbon fibers delivers a high areal capacity of 5.9 mAh cm after 100 cycles at 0.1 C with a low electrolyte/sulfur ratio and a high sulfur loading of 7.7 mg cm, placing TiS@NSC one of the best LiPSs adsorbents and sulfur conversion catalysts reported to date. The developed nanospace-confined strategy will shed light on the rational design and structural engineering of metal sulfides based nanoarchitectures for diverse applications.

One of the most challenging problems in the development of lithium–sulfur batteries is polysulfide dissolution, which leads to cell overcharge and low columbic efficiency. Here, we propose the formation of a thin conformal Li-ion permeable oxide layer on the sulfur-carbon composite electrode surface by rapid plasma enhanced atomic layer deposition (PEALD) in order to prevent this dissolution, while preserving electrical connectivity within the individual electrode particles. PEALD synthesis offers a fast deposition rate combined with a low operating temperature, which allows sulfur evaporation during deposition to be avoided. After PEALD of a thin layer of aluminium oxide on the surface of electrode composed of large (ca. 10 μm in diameter) S-infiltrated activated carbon fibers (S-ACF), significantly enhanced cycle life is observed, with a capacity in excess of 600 mA·h·g−1 after 300 charge–discharge cycles. Scanning electron microscopy (SEM) shows a significant amount of redeposited lithium sulfides on the external surface of regular S-ACF electrodes. However, the PEALD alumina-coated electrodes show no lithium sulfide deposits on the fiber surface. Energy dispersive spectroscopy (EDS) studies of the electrodes’ chemical composition further confirms that PEALD alumina coatings dramatically reduce S dissolution from the cathodes by confining the polysulfides inside the alumina barrier.

CuS, CoS, and CuS/CoS onto fluorine-doped tin oxide glass substrates were deposited to function as counter electrodes for polysulfide redox reactions in CdS/CdSe quantum dot–sensitized solar cells (QDSSCs). Relative to a Pt electrode, the CuS, CoS, and CuS/CoS electrodes provide greater electrocatalytic activity, higher reflectivity, and lower charge-transfer resistance. Measurements of fill factor and short-current density reveal that the electrocatalytic activities, reflectivity, and internal resistance of counter electrodes play strong roles in determining the energy-conversion efficiency (η) of the QDSSCs. Because the CuS/CoS electrode has a smaller internal resistance and higher reflectivity relative to those of the CuS and CoS electrodes, it exhibits a higher fill factor and short-circuit current density. As a result, the QDSSC featuring a CuS/CoS electrode provides a higher value of η. Under illumination of one sun (100 mW cm−2), the QDSSCs incorporating Pt, CuS, CoS, and CuS/CoS counter electrodes provide values of η of 3.0 ± 0.1, 3.3 ± 0.3, 3.8 ± 0.2, and 4.1 ± 0.2%, respectively.

There is a critical need to evaluate lithium-sulfur (Li-S) batteries with practically relevant high sulfur loadings and minimal electrolyte. Under such conditions, the concentration of soluble polysulfide intermediates in the electrolyte drastically increases, which can alter the fundamental nature of the solution-mediated discharge and thereby the total sulfur utilization. In this work, we present an investigation into various high donor number (DN) electrolytes that allow for increased polysulfide dissolution, and demonstrate how this property may in fact be necessary for increasing sulfur utilization at low electrolyte and high loading conditions. The solvents dimethylacetamide, dimethyl sulfoxide, and 1-methylimidazole are holistically evaluated against dimethoxyethane as electrolyte co-solvents in Li-S cells, and they are used to investigate chemical and electrochemical properties of polysulfide species at both dilute and practically relevant conditions. The nature of speciation exhibited by lithium polysulfides is found to vary significantly between these concentrations, particularly in regards to the S(3)(•−) species. Furthermore, the extent of the instability in conventional electrolyte solvents and high DN solvents with both lithium metal and polysulfides is thoroughly investigated. These studies establish a basis for future efforts into rationally designing an optimal electrolyte for a lean electrolyte, high energy density Li-S battery.

Polysulfide dissolution into the electrolyte and poor electric conductivity of elemental sulfur are well-known origins for capacity fading in lithium–sulfur batteries. Various smart electrode designs have lately been introduced to avoid these fading mechanisms, most of which demonstrate significantly improved cycle life. Nevertheless, an in-depth understanding on the effect of sulfur microstructure and nanoscale electron transport near sulfur is currently lacking. In this study, the authors report an organized nanocomposite comprising linear sulfur chains and oleylamine-functionalized reduced graphene oxide (O-rGO) to achieve robust cycling performance (81.7% retention after 500 cycles) as well as to investigate the reaction mechanism in different regimes, i.e., S8 dissolution, polysulfide conversion, and Li2S formation. In the nanocomposite, linear sulfur chains terminate with 1,3-diisopropylbenzene are covalently linked to O-rGO. The comparison with control samples that do not contain either the capping of sulfur chains or O-rGO reveals the synergistic interplay between both treatments, simultaneously unveiling the distinct roles of confined sulfur nanodomains and their adjoining electron pathways in different reaction regimes.

A reversible room-temperature aluminum–sulfur (Al-S) battery is demonstrated with a strategically designed cathode structure and an ionic liquid electrolyte. Discharge–charge mechanism of the Al-S battery is proposed based on a sequence of electrochemical, microscopic, and spectroscopic analyses. The electrochemical process of the Al-S battery involves the formation of a series of polysulfides and sulfide. The high-order polysulfides (Sx2−, x ≥ 6) are soluble in the ionic liquid electrolyte. Electrochemical transitions between S62− and the insoluble low-order polysulfides or sulfide (Sx2−, 1 ≤ x < 6) are reversible. A single-wall carbon nanotube coating applied to the battery separator helps alleviate the diffusion of the polysulfide species and reduces the polarization behavior of the Al-S batteries.

Safety and the polysulfide shuttle reaction are two major challenges for liquid electrolyte lithium–sulfur (Li–S) batteries. Although use of solid-state electrolytes can overcome these two challenges, it also brings new challenges by increasing the interface resistance and stress/strain. In this work, the interface resistance and stress/strain of sulfur cathodes are significantly reduced by conformal coating ≈2 nm sulfur (S) onto reduced graphene oxide (rGO). An Li–S full cell consisting of an rGO@S-Li10GeP2S12-acetylene black (AB) composite cathode is evaluated. At 60 °C, the all-solid-state Li–S cell demonstrates a similar electrochemical performance as in liquid organic electrolyte, with high rate capacities of 1525.6, 1384.5, 1336.3, 903.2, 502.6, and 204.7 mA h g−1 at 0.05, 0.1, 0.5, 1.0, 2.0, and 5.0 C, respectively. It can maintain a high and reversible capacity of 830 mA h g−1 at 1.0 C for 750 cycles. The uniform distribution of the rGO@S nanocomposite in the Li10GeP2S12-AB matrix generates uniform volume changes during lithiation/delithiation, significantly reducing the stress/strain, thus extending the cycle life. Minimization of the stress/strain of solid cells is the key for a long cycle life of all-solid-state Li–S batteries.

Lithium–sulfur (Li–S) batteries have a much higher energy density than Li ion batteries and thus are considered as next generation batteries for electric vehicle applications. However, the problem of rapid capacity fading due to the shuttling of soluble polysulfides between electrodes remains the main obstacle for practical applications. Here, a thin and selective interlayer structure has been designed and produced to decrease the charge transfer resistance and mitigate the shuttling problem, simply by coating the surface of cathode with a thin film of functionalized boron nitride nanosheets/graphene. Due to this thin and ultralight interlayer, the specific capacity and cycling stability of the Li–S batteries with a cathode of sulfur-containing porous carbon nanotubes (≈60 wt% sulfur content) have been improved significantly with a life of over 1000 cycles, an initial specific capacity of 1100 mA h g−1 at 3 C, and a cycle decay as low as 0.0037% per cycle. This new interlayer provides a promising approach to significantly enhance the performance of Li–S batteries.

Freestanding cathode materials with sandwich-structured characteristic are synthesized for high-performance lithium–sulfur battery. Sulfur is impregnated in nitrogen-doped graphene and constructed as primary active material, which is further welded in the carbon nanotube/nanofibrillated cellulose (CNT/NFC) framework. Interconnected CNT/NFC layers on both sides of active layer are uniquely synthesized to entrap polysulfide species and supply efficient electron transport. The 3D composite network creates a hierarchical architecture with outstanding electrical and mechanical properties. Synergistic effects generated from physical and chemical interaction could effectively alleviate the dissolution and shuttling of the polysulfide ions. Theoretical calculations reveal the hydroxyl functionization exhibits a strong chemical binding with the discharge product (i.e., Li2S). Electrochemical measurements suggest that the rationally designed structure endows the electrode with high specific capacity and excellent rate performance. Specifically, the electrode with high areal sulfur loading of 8.1 mg cm−2 exhibits an areal capacity of ≈8 mA h cm−2 and an ultralow capacity fading of 0.067% per cycle over 1000 discharge/charge cycles at C/2 rate, while the average coulombic efficiency is around 97.3%, indicating good electrochemical reversibility. This novel and low-cost fabrication procedure is readily scalable and provides a promising avenue for potential industrial applications.

A comprehensive approach is reported to construct stable and high volumetric energy density lithium–sulfur batteries, by coupling a multifunctional and hierarchically structured sulfur composite with an in-situ cross-linked binder. Through a combination of first-principles calculations and experimental studies, it is demonstrated that a hybrid sulfur host composed by alternately stacking graphene and layered graphitic carbon nitride embraces high electronic conductivity as well as high polysulfide adsorptivity. It is further shown that the cross-linked elastomeric binder empowers the hierarchical sulfur composites—multi-microns in size—with the ability to form crack-free and compact high-loading electrodes using traditional slurry processing. Using this approach, electrodes with up to 14.9 mg cm−2 sulfur loading and an extremely low electrolyte/sulfur ratio as low as 3.5: 1 µL mg−1 are obtained. This study sheds light on the essential role of multifaceted cathode design and further on the challenges facing lithium metal anodes in building high volumetric energy density lithium–sulfur batteries.

Despite the 3–5 fold higher energy density than the conventional Li-ion cells at a lower cost, commercialization of Li–S batteries is hindered by the insulating nature of sulfur and the dissolution of intermediate polysulfides (Li2S X , 4 < X ≤ 8) into the electrolyte. The authors demonstrate here multichannel carbon nanofibers that are composed of parallel mesoporous channels connected with micropores as sulfur containment. In addition, hydroxyl functional groups are formed on the carbon surface through a chemical activation to enhance the interaction between sulfur and carbon. In the sulfur embedded composite, the mesoporous multichannel enhances the active material utilization and sulfur loading, while the micropores act as a reaction chamber for sulfur component and trap site for polysulfide with the assistance of the functional groups. This sulfur–carbon composite electrode with 2.2 mg cm−2 sulfur displays excellent performance with high rate capability (initial capacity of 1351 mA h g−1 at C/5 rate and 847 mA h g−1 at 5C rate), maintaining 920 mA h g−1 even after 300 cycles (a decay of 0.07% per cycle). Furthermore, a stable reversible capacity of as high as ≈1100 mA h g−1 is realized with a higher sulfur loading of 4.6 mg cm−2.

A NaSICON-type Li+-ion conductive membrane with a formula of Li1+ x Y x Zr2− x (PO4)3 (LYZP) (x = 0–0.15) has been explored as a solid-electrolyte/separator to suppress polysulfide-crossover in lithium-sulfur (Li-S) batteries. The LYZP membrane with a reasonable Li+-ion conductivity shows both favorable chemical compatibility with the lithium polysulfide species and exhibits good electrochemical stability under the operating conditions of the Li-S batteries. Through an integration of the LYZP solid electrolyte with the liquid electrolyte, the hybrid Li-S batteries show greatly enhanced cyclability in contrast to the conventional Li-S batteries with the porous polymer (e.g., Celgard) separator. At a rate of C/5, the hybrid Li ||LYZP|| Li2S6 batteries developed in this study (with a Li-metal anode, a liquid/LYZP hybrid electrolyte, and a dissolved lithium polysulfide cathode) delivers an initial discharge capacity of ≈1000 mA h g−1 (based on the active sulfur material) and retains ≈90% of the initial capacity after 150 cycles with a low capacity fade-rate of

The lithium-sulfur battery is a compelling energy storage system because its high theoretical energy density exceeds Li-ion batteries at much lower cost, but applications are thwarted by capacity decay caused by the polysulfide shuttle. Here, proof of concept and the critical metrics of a strategy to entrap polysulfides within the sulfur cathode by their reaction to form a surface-bound active redox mediator are demonstrated. It is shown through a combination of surface spectroscopy and cyclic voltammetry studies that only materials with redox potentials in a targeted window react with polysulfides to form active surface-bound polythionate species. These species are directly correlated to superior Li-S cell performance by electrochemical studies of high surface area oxide cathodes with redox potentials below, above, and within this window. Optimized Li-S cells yield a very low fade rate of 0.048% per cycle. The insight gained into the fundamental surface mechanism and its correlation to the stability of the electrochemical cell provides a bridge between mechanistic understanding and battery performance essential for the design of high performance Li-S cells.

Lithium-sulfur (Li-S) batteries are being considered as the next-generation high-energy-storage system due to their high theoretical energy density. However, the use of a lithium-metal anode poses serious safety concerns due to lithium dendrite formation, which causes short-circuiting, and possible explosions of the cell. One feasible way to address this issue is to pair a fully lithiated lithium sulfide (Li2S) cathode with lithium metal-free anodes. However, bulk Li2S particles face the challenges of having a large activation barrier during the initial charge, low active-material utilization, poor electrical conductivity, and fast capacity fade, preventing their practical utility. Here, the development of a self-supported, high capacity, long-life cathode material is presented for Li-S batteries by coating Li2S onto doped graphene aerogels via a simple liquid infiltration–evaporation coating method. The resultant cathodes are able to lower the initial charge voltage barrier and attain a high specific capacity, good rate capability, and excellent cycling stability. The improved performance can be attributed to the (i) cross-linked, porous graphene network enabling fast electron/ion transfer, (ii) coated Li2S on graphene with high utilization and a reduced energy barrier, and (iii) doped heteroatoms with a strong binding affinity toward Li2S/lithium polysulfides with reduced polysulfide dissolution based on first-principles calculations.

The sulfur cathode in traditional lithium–sulfur batteries suffers from poor cyclability due to polysulfide shuttling effect as well as large volume change during charge/discharge processes. Gum arabic (GA), a low cost, nontoxic, and sustainable natural polymer from Acacia senegal, is adopted as a binder for the sulfur cathode to address these issues. The excellent mechanical properties of GA endow the cathode with high binding strength and suitable ductility to buffer volume change, while the functional groups chemically and physically confine sulfur species within the cathode to inhibit the shuttling effect of polysulfides. Additionally, GA shifts the electrode fabrication process from the organic solvent process to an aqueous process, eliminates the use of toxic organic solvents, and achieves uniformly distributed electrode with lower impedance. A remarkable cycling performance, i.e., 841 mAh g−1 at low current rate of C/5, is achieved throughout 500 cycles due to the bifunctions of the GA binder.

Lithium–sulfur (Li–S) redox flow battery (RFB) is a promising candidate for high energy large-scale energy storage application due to good solubility of long-chain polysulfide species and low cost of sulfur. Here, the fundamental understanding and control of lithium polysulfide chemistry are studied to enable the development of liquid phase Li–S redox flow prototype cells. These differ significantly from conventional static Li–S batteries targeting for vehicle electrification. A high solubility of the different lithium polysulfides generated at different depths of discharge and states of charge is required for a flow battery in order to take full advantage of the multiple electron transitions. A new dimethyl sulfoxide based electrolyte is proposed for Li–S RFBs, which not only enables the high solubility of lithium polysulfide species, especially for the short-chain species, but also results in excellent cycling with a high Coulombic efficiency. The challenges and opportunities for the Li–S redox flow concept have also been discussed in depth.

The rapidly increasing demand for electrical and hybrid vehicles and stationary energy storage requires the development of “beyond Li-ion batteries” with high energy densities that exceed those of state-of-the-art Li-ion batteries. Li–S batteries, which have very high theoretical capacities and energy densities, are believed to be one of the most promising candidates. The sulfur-based electrochemical reaction requires novel electrolytes to replace the classical carbonate-based electrolyte systems inherited from Li-ion batteries because carbonates are incompatible with the intermediate polysulfides in Li–S batteries. In addition, the theoretical specific capacities and projected energy densities of Li–S batteries are difficult to achieve experimentally, mainly because of the electronically insulating nature of sulfur and lithium sulfide cathodes, and the shuttle effect; this is a serious issue associated with the dissolution and diffusion of soluble polysulfides in most potential electrolytes and causes rapid capacity fading. It is therefore highly desirable to explore, modify, and/or optimize electrolytes for Li–S batteries to address these issues and improve their capacities, cycling stabilities, rate performances, and energy densities. An overview of recent developments in electrolytes for Li–S batteries is provided with a focus on the chemistry of polysulfides in different electrolyte media, including polysulfide solubility and its relevance to battery performance.