Porphyrin‐Derived Graphene‐Based Nanosheets Enabling Strong Polysulfide Chemisorption and Rapid Kinetics in Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries hold great promise as a next‐generation battery system because of their extremely high theoretical energy density and low cost. However, ready lithium polysulfide (LiPS) diffusion and sluggish redox kinetics hamper their cyclability and rate capability. Herein, porphyrin‐derived graphene‐based nanosheets (PNG) are proposed for Li–S batteries, which are achieved by pyrolyzing a conformal and thin layer of 2D porphyrin organic framework on graphene to form carbon nanosheets with a spatially engineered nitrogen‐dopant‐enriched skin and a highly conductive skeleton. The atomic skin is decorated with fully exposed lithiophilic sites to afford strong chemisorption to LiPSs and improve electrolyte wettability, while graphene substrate provides speedy electron transport to facilitate redox kinetics of sulfur species. The use of PNG as a lightweight interlayer enables efficient operation of Li–S batteries in terms of superb cycle stability (cyclic decay rate of 0.099% during 300 cycles at 0.5 C), good rate capability (988 mAh g^?1 at 2.0 C), and impressive sulfur loading (areal capacity of 8.81 mAh cm^?2 at a sulfur loading of 8.9 mg cm^?2). The distinct interfacial strategy is expected to apply to other conversion reaction batteries relying on dissolution–precipitation mechanisms and requiring interfacial charge‐ and mass‐transport‐mediation concurrently.     

Solvent‐Mediated Li2S Electrodeposition: A Critical Manipulator in Lithium–Sulfur Batteries

Controlling electrochemical deposition of lithium sulfide (Li₂S) is a major challenge in lithium–sulfur batteries as premature Li₂S passivation leads to low sulfur utilization and low rate capability. In this work, the solvent's roles in controlling solid Li₂S deposition are revealed, and quantitative solvent‐mediated Li₂S growth models as guides to solvent selection are developed. It is shown that Li₂S electrodeposition is controlled by electrode kinetics, Li₂S solubility, and the diffusion of polysulfide/Li₂S, which is dictated by solvent's donicity, polarity, and viscosity, respectively. These solvent‐controlled properties are essential factors pertaining to the sulfur utilization, energy efficiency and reversibility of lithium–sulfur batteries. It is further demonstrated that the solvent selection criteria developed in this study are effective in guiding the search for new and more effective electrolytes, providing effective screening and design criteria for computational and experimental electrolyte development for lithium–sulfur batteries.     

Catalytic Polysulfide Conversion and Physiochemical Confinement for Lithium–Sulfur 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).     

Sulfiphilic Few‐Layered MoSe2 Nanoflakes Decorated rGO as a Highly Efficient Sulfur Host for Lithium‐Sulfur Batteries

Lithium‐sulfur batteries (LSBs) have been regarded as a competitive candidate for next‐generation electrochemical energy‐storage technologies due to their merits in energy density. The sluggish redox kinetics of the electrochemistry and the high solubility of polysulfides during cycling result in insufficient sulfur utilization, severe polarization, and poor cyclic stability. Herein, sulfiphilic few‐layered MoSe₂ nanoflakes decorated rGO (MoSe₂@rGO) hybrid has been synthesized through a facile hydrothermal method and for the first time, is used as a conceptually new‐style sulfur host for LSBs. Specifically, MoSe₂@rGO not only strongly interacts with polysulfides but also dynamically strengthens polysulfide redox reactions. The polarization problem is effectively alleviated by relying on the sulfiphilic MoSe₂. Moreover, MoSe₂@rGO is demonstrated to be beneficial for the fast nucleation and uniform deposition of Li₂S, contributing to the high discharge capacity and good cyclic stability. A high initial capacity of 1608 mAh g^?1 at 0.1 C, a slow decay rate of 0.042% per loop at 0.25 C, and a high reversible capacity of 870 mAh g^?1 with areal sulfur loading of 4.2 mg cm^?2 at 0.3 C are obtained. The concept of introducing sulfiphilic transition‐metal selenides into the LSBs system can stimulate engineering of novel architectures with enhanced properties for various energy‐storage devices.     

Highly Stable Lithium–Sulfur Batteries Based on Laponite Nanosheet‐Coated Celgard Separators

Lithium–sulfur (Li–S) batteries are of great interest due to their high theoretical energy density. However, one of the key issues hindering their real world applications is polysulfide shuttle, which results in severe capacity decay and self‐discharge. Here, a laponite nanosheets/carbon black coated Celgard (LNS/CB‐Celgard) separator to inhibit polysulfide shuttle and to enhance the Li⁺ conductivity simultaneously is reported. The polysulfide shuttle is efficiently inhibited through strong interactions between the O active sites of the LNS and polysulfides by forming the Li···O and O? S bonds. Moreover, the separator features high Li⁺ conductivity, fast Li⁺ diffusion, excellent electrolyte wettability, and high thermal stability. Consequently, the Li–S batteries with the LNS/CB‐Celgard separator and the pure S cathode show a high initial reversible capacity of 1387 mA h g^?1 at 0.1 C, high rate performance, superior cycling stability (with a capacity decay rate of 0.06% cycle^?1 at 0.2 C and 0.028% cycle^?1 at 1.0 C over 500 cycles), and ultralow self‐discharge. The separator could also enhance the performance of other batteries such as the LiFePO₄/separator/Li battery. This work sheds a new light on the design and preparation of novel separators for highly stable Li–S batteries via a “green” and cost‐effective approach.     

Highly Solvating Electrolytes for Lithium–Sulfur Batteries

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, an investigation into various high donor number (DN) electrolytes that allow for increased polysulfide dissolution is presented, and the way in which this property may in fact be necessary for increasing sulfur utilization at low electrolyte and high loading conditions is demonstrated. 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 with regard to the S₃^?? 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.     

Flexible and High‐Loading Lithium–Sulfur Batteries Enabled by Integrated Three‐In‐One Fibrous Membranes

Lithium–sulfur batteries are appealing as high‐energy storage systems and hold great application prospects in wearable and portable electronics. However, severe shuttle effects, low sulfur conductivity, and especially poor electrode mechanical flexibility restrict sulfur utilization and loading for practical applications. Herein, high‐flux, flexible, electrospun fibrous membranes are developed, which succeed in integrating three functional units (cathode, interlayer, and separator) into an efficient composite. This structure helps to eliminate negative interface effects, and effectively drives synergistic boosts to polysulfide confinement, electron transfer, and lithium‐ion diffusion. It delivers a high initial capacity of 1501 mA h g^?1 and a discharge capacity of 933 mA h g^?1 after 400 cycles, with slow capacity attenuation (0.069% per cycle). Even under high sulfur loading (13.2 mg cm^?2, electrolyte/sulfur ratio = 6 mL g^?1) or in an alternative folded state, this three‐in‐one membrane still exhibits high areal capacity (11.4 mA h cm^?2) and exceptional application performance (powering an array of over 30 light‐emitting diodes (LEDs)), highlighting its huge potential in high‐energy flexible devices.     

Designing Safe Electrolyte Systems for a High‐Stability Lithium–Sulfur Battery

Safety, nontoxicity, and durability directly determine the applicability of the essential characteristics of the lithium (Li)‐ion battery. Particularly, for the lithium–sulfur battery, due to the low ignition temperature of sulfur, metal lithium as the anode material, and the use of flammable organic electrolytes, addressing security problems is of increased difficulty. In the past few years, two basic electrolyte systems are studied extensively to solve the notorious safety issues. One system is the conventional organic liquid electrolyte, and the other is the inorganic solid‐state or quasi‐solid‐state composite electrolyte. Here, the recent development of engineered liquid electrolytes and design considerations for solid electrolytes in tackling these safety issues are reviewed to ensure the safety of electrolyte systems between sulfur cathode materials and the lithium‐metal anode. Specifically, strategies for designing and modifying liquid electrolytes including introducing gas evolution, flame, aqueous, and dendrite‐free electrolytes are proposed. Moreover, the considerations involving a high‐performance Li⁺ conductor, air‐stable Li⁺ conductors, and stable interface performance between the sulfur cathode and the lithium anode for developing all‐solid‐state electrolytes are discussed. In the end, an outlook for future directions to offer reliable electrolyte systems is presented for the development of commercially viable lithium–sulfur batteries.     

Fluorine‐Free Noble Salt Anion for High‐Performance All‐Solid‐State Lithium–Sulfur Batteries

Amongst post‐Li‐ion battery technologies, lithium–sulfur (Li–S) batteries have captured an immense interest as one of the most appealing devices from both the industrial and academia sectors. The replacement of conventional liquid electrolytes with solid polymer electrolytes (SPEs) enables not only a safer use of Li metal (Li°) anodes but also a flexible design in the shape of Li–S batteries. However, the practical implementation of SPEs‐based all‐solid‐state Li–S batteries (ASSLSBs) is largely hindered by the shuttling effect of the polysulfide intermediates and the formation of dendritic Li° during the battery operation. Herein, a fluorine‐free noble salt anion, tricyanomethanide [C(CN)₃^?, TCM^?], is proposed as a Li‐ion conducting salt for ASSLSBs. Compared to the widely used perfluorinated anions {e.g., bis(trifluoromethanesulfonyl)imide anion, [N(SO₂CF₃)₂)]^?, TFSI^?}, the LiTCM‐based electrolytes show decent ionic conductivity, good thermal stability, and sufficient anodic stability suiting the cell chemistry of ASSLSBs. In particular, the fluorine‐free solid electrolyte interphase layer originating from the decomposition of LiTCM exhibits a good mechanical integrity and Li‐ion conductivity, which allows the LiTCM‐based Li–S cells to be cycled with good rate capability and Coulombic efficiency. The LiTCM‐based electrolytes are believed to be the most promising candidates for building cost‐effective and high energy density ASSLSBs in the near future.     

In Situ Assembly of 2D Conductive Vanadium Disulfide with Graphene as a High‐Sulfur‐Loading Host for Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are deemed to be one of the most promising energy storage technologies because of their high energy density, low cost, and environmental benignancy. However, existing drawbacks including the shuttling of intermediate polysulfides, the insulating nature of sulfur, and the considerable volume change of sulfur cathode would otherwise result in the capacity fading and unstable cycling. To overcome these challenges, herein an in situ assembly route is presented to fabricate VS₂/reduced graphene oxide nanosheets (G–VS₂) as a sulfur host. Benefiting from the 2D conductive and polar VS₂ interlayered within a graphene framework, the obtained G–VS₂ hybrids can effectively suppress the polysulfide shuttling, facilitate the charge transport, and cushion the volume expansion throughout the synergistic effect of structural confinement and chemical anchoring. With these advantageous features, the obtained sulfur cathode (G–VS₂/S) can deliver an outstanding rate capability (≈950 and 800 mAh g^?1 at 1 and 2 C, respectively) and an impressive cycling stability at high rates (retaining ≈532 mAh g^?1 after 300 cycles at 5 C). More significantly, it enables superior cycling performance of high‐sulfur‐loading cathodes (achieving an areal capacity of 5.1 mAh cm^?2 at 0.2 C with a sulfur loading of 5 mg cm^?2) even at high current densities.     

High‐Performance and Low‐Temperature Lithium–Sulfur Batteries: Synergism of Thermodynamic and Kinetic Regulation

The intrinsic polysulfides shuttle, resulting from not only concentration‐gradient diffusion but also slow conversion kinetics of polysulfides, bears the primary responsibility for the poor capacity and cycle stability of lithium–sulfur batteries (LSBs). Here, it is first presented that enriched edge sites derived from vertical standing and ultrathin 2D layered metal selenides (2DLMS) can simultaneously achieve the thermodynamic and kinetic regulation for polysulfides diffusion, which is systematically elucidated through theoretical calculation, electrochemical characterization, and spectroscopic/microscopic analysis. When employed to fabricate compact coating layer of separator, an ultrahigh capacity of 1338.7 mA h g^?1 is delivered after 100 cycles at 0.2 C, which is the best among the reports. Over 1000 cycles, the cell still maintains the capacity of 546.8 mA h g^?1 at 0.5 C. Moreover, the cell exhibits outstanding capacities of 1106.2 and 865.7 mA h g^?1 after 100 cycles at stern temperature of 0 and ?25 °C. The superior low‐temperature performance is appealing for extended practical application of LSBs. Especially, in view of the economy, the 2DLMS is recycled as an anode of lithium‐ion and sodium‐ion batteries after finishing the test of LSBs. The low‐cost and scalable 2DLMS with enriched egde sites open a new avenue for the perfect regulation of the sulfur electrode.     

A Metal Organic Framework Derived Solid Electrolyte for Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) are currently considered as promising candidates for next‐generation energy storage technologies. However, their practical application is hindered by the critical issue of the polysulfide‐shuttle. Herein, a metal organic framework (MOF)‐derived solid electrolyte is presented to address it. The MOF solid electrolyte is developed based on a Universitetet i Oslo (UIO) structure. By grafting a lithium sulfonate (‐SO₃Li) group to the UIO ligand, both the ionic conductivity and the polysulfide‐suppression capability of the resulting ‐SO₃Li grafted UIO (UIOSLi) solid electrolyte are greatly improved. After integrating a Li‐based ionic liquid (Li‐IL), lithium bis(trifluoromethanesulfonyl)imide in 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide, the resulting Li‐IL/UIOSLi solid electrolyte exhibits an ionic conductivity of 3.3 × 10^?4 S cm^?1 at room temperature. Based on its unique structure, the Li‐IL/UIOSLi solid electrolyte effectively restrains the polysulfide shuttle and suppresses lithium dendritic growth. Lithium–sulfur cells with the Li‐IL/UIOSLi solid electrolyte and a Li₂S₆ catholyte show stable cycling performance that preserves 84% of the initial capacity after 250 cycles with a capacity‐fade rate of 0.06% per cycle.     

Capture and Catalytic Conversion of Polysulfides by In Situ Built TiO2‐MXene Heterostructures for Lithium–Sulfur Batteries

The detrimental shuttle effect in lithium–sulfur batteries mainly results from the mobility of soluble polysulfide intermediates and their sluggish conversion kinetics. Herein, presented is a multifunctional catalyst with the merits of strong polysulfides adsorption ability, superior polysulfides conversion activity, high specific surface area, and electron conductivity by in situ crafting of the TiO₂‐MXene (Ti₃C₂T_x) heterostructures. The uniformly distributed TiO₂ on MXene sheets act as capturing centers to immobilize polysulfides, the hetero‐interface ensures rapid diffusion of anchored polysulfides from TiO₂ to MXene, and the oxygen‐terminated MXene surface is endowed with high catalytic activity toward polysulfide conversion. The improved lithium–sulfur batteries deliver 800 mAh g^?1 at 2 C and an ultralow capacity decay of 0.028% per cycle over 1000 cycles at 2 C. Even with a high sulfur loading of 5.1 mg cm^?2, the capacity retention of 93% after 200 cycles is still maintained. This work sheds new insights into the design of high‐performance catalysts with manipulated chemical components and tailored surface chemistry to regulate polysulfides in Li–S batteries.     

A Comprehensive Approach toward Stable Lithium–Sulfur Batteries with High Volumetric Energy Density

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.