Revisiting the Role of Polysulfides in Lithium-Sulfur Batteries

Optimized Cerium vanadate catalytic host with simple heterostructure engineering achieving regulated polysulfide deposition for high-performance Lithium-Sulfur batteries under harsh conditions

Meliorating the behavior deposition of lithium polysulfides (LiPS) is crucial for enhancing the electrochemical performance of sulfur cathodes, which could be implemented by the precise modulation on the catalytic host. Herein, heterostructure engineering is employed to tune up the catalytic capability of CeVO4, by introducing CeO2 through a simple adjustment in the addition sequence of reactants. The formed CeVO4/CeO2 heterostructure has been demonstrated to exhibit appropriate interaction strength with LiPS for accelerating the catalytic conversion process, as well as an engineered surface for inducing three dimensional (3D) Li2S deposition, thereby endowing the corresponding sulfur cathodes with excellent electrochemical performance under harsh conditions. The S/CeVO4/CeO2 cathode could deliver a high capacity of 36.5 mAh cm-2 (1278.9 mAh g-1) under high sulfur loading of 28.5 mg cm-2 and lean electrolyte dosage of 6 μL mg-1, and also function well even under a wide temperature range (-30 ∼ 60 °C) or matching with polymer solid electrolyte. This work offers a simple yet practical strategy to modulate the catalysts for sulfur cathodes with heterostructure design, thereby advancing the practical implementation of the lithium-sulfur battery.

Advanced Electrolyte Additives for Enhanced Homogeneous Sulfur Fixation in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are regarded as leading contenders for next-generation energy storage owing to their exceptional theoretical energy density. However, severe sulfur electrode depletion causes rapid capacity fading and compromised cycling stability. Electrolyte engineering effectively enables homogeneous sulfur fixation, improving battery performance. The study investigates the mechanisms behind these homogeneous reactions, focusing on sulfur fixation processes. Sulfur fixation is explored through multiple perspectives, including the inhibition of polysulfide shuttling, mitigation of electrode passivation, and the combined application of both strategies. Regarding polysulfide shuttling inhibition, three distinct mechanisms for sulfur fixation are identified: 1) chemisorption-based sulfur fixation, involving the formation of chemical bonds with polysulfides; 2) redox-mediated sulfur fixation, which accelerates the kinetics of sulfur species; and 3) hybrid sulfur fixation, which combines elements of both approaches. Furthermore, the review analyzes current methods for homogeneous sulfur fixation, focusing on electrolyte designs that enable homogeneous sulfur fixation under limited conditions. It provides insights to optimize electrolytes, advancing Li-S battery performance and commercialization.

Synergistic Promotion of Adsorption-Conversion for Lithium Polysulfides through Ion/Electron Coconductive Catalytic Triple-Phase Interface by the Geometric Torsion of Lithium Lanthanum Titanate Electrocatalyst

Rationally designing and constructing ionic/electronic coconductor electrocatalysts with adjustable active sites to enhance the redox kinetics of lithium-sulfur batteries (LSBs) in lean electrolyte conditions is a challenge. Herein, this study presents the promotion for lithium polysulfides (LiPSs) redox kinetics through the construction of ion/electron coconductive catalytic triple-phase interface using lithium lanthanum titanate/carbon (LLTO/C) nanofibers, which is due to manipulating the geometric torsion of BO6 octahedron in LLTO. Experiments and theoretical calculations demonstrate that geometric torsion changes the coordination environment of O-Ti-O in LLTO and causes oxygen vacancy and lattice distortion. This enhances the local electronic state density, ion migration rate, and high catalytic activity of LLTO, thereby resulting in a synergistic effect for good chemisorption and rapid catalytic conversion of LiPSs. Therefore, when the modified separator is used, LSBs with the electrocatalyst realize a high discharge capacity of 1024 mA h·g-1 at a high current rate of 2 C. Upon a high sulfur loading of 9 mg ·cm-2 and a lean electrolyte/sulfur ratio of 4 μL· mg-1, an acceptable areal capacity of 9.1 mA h· cm-2 is achieved. This work provides a new perspective for the rational design of ionic/electronic coconductor catalysts for LSBs.

Polysulfide chemistry in metal-sulfur batteries

Renowned for their high theoretical energy density and cost-effectiveness, metal-sulfur (M-S) batteries are pivotal in overcoming the current energy storage bottlenecks and accelerating the transition toward a cleaner society. Polysulfides (PSs) serve as essential intermediates in M-S batteries and bridge the electrochemical redox processes of sulfur, playing a decisive role in controlling the electrode behaviors and regulating the battery performances. Understanding PS chemistry across diverse battery environments is key to advancing M-S batteries. This review aims to provide a comprehensive overview of the PS chemistry in high-energy-density battery systems and outline future research directions. The compositions, properties, and characterization methods of PSs are introduced to facilitate a fundamental understanding of the PS chemistry in working batteries. Following this, a thorough examination of the chemical and electrochemical behaviors of PSs and their impacts on electrode performances is conducted to deepen the insights into the PS reactions in batteries. Building on this foundation, representative PS regulation strategies are discussed, focusing on molecular modification, solvation optimization, and interfacial regulation, to achieve superior M-S battery performances. Challenges of PSs in practical M-S batteries are finally analyzed, and perspectives on the future research trends of PS chemistry are presented.

Molecular Electrocatalysts in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries face challenges due to the sluggish reaction kinetics of sulfur species, which reduces sulfur utilization and thus lowers performance. Molecular electrocatalysts, with their clear and adequately exposed active sites, offer a reliable way to enhance reaction kinetics in lithium-sulfur batteries. This review elaborates on the reaction processes and mechanisms of molecular electrocatalysts, focusing on both the sulfur reduction reaction (SRR) and sulfur evolution reaction (SER) to explore their potential working principles. Additionally, we analyze the design strategies for novel catalysts aimed at inhibiting the diffusion of lithium polysulfides (LiPSs). This paper aims to design molecular electrocatalysts that facilitate the multiphase conversion of sulfur species, providing guidance for the commercialization of Li-S batteries.

Oxygen Dopants in MoS2 Catalysts as Dual Anchors for a Lithium Polysulfide Chain to Accelerate Conversion to Solid Li2S: A Strategy to Mitigate the Shuttle Effect in Li-S Batteries

The development of lithium-sulfur batteries (LSBs) is hindered by the solubility of polysulfide intermediates. Herein, we synthesized oxygen-doped MoS2 on a highly conductive CNT as a cathode material for LSBs. The spaced oxygen dopants on the catalyst surface enable Li polysulfide chains to adsorb parallel to the catalyst surface. This configuration restricts solubility and accelerates the conversion of soluble Li polysulfides to insoluble Li2S. Therefore, the Mo(S-O)2/CNT cathode demonstrates an impressive discharge capacity (1410.4 mAh g-1 at 0.2 C and 880.3 mAh g-1 at 2 C) along with exceptional cycle stability, retaining 62.7% capacity after 100 cycles at 0.2 C and showing a low decay rate of 0.094% per cycle over 400 cycles at 2 C in LSBs. This work will inspire further research on the deanchoring design of Li2S from negatively charged polar dopants in catalysts as well as studies on the synchronization of catalysis with the anchoring-deanchoring process.

Biomass-derived carbon dots: synthesis, modification and application in batteries

Biomass-derived carbon dots (BCDs) have attracted considerable attention for their promising attributes, including low toxicity, excellent solubility, biocompatibility, and eco-friendliness. Their rich surface chemistry and impressive photoluminescent properties have sparked widespread research interest, particularly in areas such as sensing and biomedicine. However, the potential applications of BCDs in the energy sector, especially in electrochemical energy storage batteries, have received scant review focus. This article systematically consolidates the selection of carbon sources, synthesis methods, modification strategies, and the corresponding characterization techniques for BCDs. Application strategies in energy storage batteries are explored, with the underlying connection between the role of BCDs in batteries and their structural properties being analyzed, providing comprehensive insights from synthesis and characterization to application. Furthermore, a preliminary discussion is initiated on the current limitations in material regulation and design within research, and potential avenues for enhancement are proposed.

Halide as Catholyte in Composite Cathode to Enhance Cycling Stability of All-Solid-State Lithium-Sulfur Batteries

All-solid-state lithium-sulfur batteries (ASSLSBs) using inorganic solid-state electrolytes can effectively alleviate the polysulfide shuttle effect in liquid electrolytes and improve the energy density. However, the electrochemical window of sulfide-based catholytes in composite cathodes is relatively narrow, which makes the evaluation of electrochemical performance of sulfur cathodes in ASSLSBs complicated. The decomposition of the sulfide catholytes increases the interfacial resistance, thus reducing the battery cycle life. To overcome these challenges, Li3YCl5I has been developed with a wide window of electrochemical stability for a catholyte suitable for a composite cathode. Its ionic conductivity is as high as 1.67 × 10-3 S cm-1, which is conducive to the rapid transport of lithium ions. The ASSLSB based on the Li3YCl5I catholyte exhibits a discharge specific capacity of 1084.05 mAh g-1 at 45 °C. Additionally, it maintains 81.5% capacity after 100 cycles, significantly exceeding the retention rate of 54.5% for a battery using Li6PS5Cl.

Investigation of Lithium Polysulfide Reduction Associated with the Potential-Limiting Step in Lithium-Sulfur Batteries

The electrochemical reduction of sulfur has the capability to deliver high energy density in lithium-sulfur batteries; however, its effectiveness is lessened by the requirement of excessive electrolyte for high performance. The potential-limiting step of this reaction was previously proposed to be the reduction of the final soluble intermediate Li2S4 to solid Li2S2/Li2S. In this work, this step was further investigated. It was discovered that nominal Li2S4 solutions with concentrations relevant to battery operation disproportionate into Li2S6 in the solution phase and Li2S in the solid phase. Li2S6 is the more likely reactant for the observed electrochemical reduction, which manifests two discernible steps. The first reduction step occurs at 2.2 V vs Li/Li+. Its specific capacity is almost invariable with scan rate, but its relative contribution to the total reduction capacity decreases with reactant concentration. The product is a semisoluble polysulfide such as Li2S4 or Li2S3, which can be further reduced in the second step or oxidized at 2.3 V. The second reduction step, with Li2S as the product, occurs at 2.1 V and is electrochemically more irreversible than the first reduction. Its specific capacity declines significantly with increased scan rate but is less impacted by concentration.

A high-power lithium-sulfur battery combining a catholyte and a composite cathode

A lithium battery benefits from the conversion reaction of a soluble polysulfide and a sulfur-based electrode. The battery with S-content from 4.5 to 6.5 mg cm-2 achieves exceptional energy and power, from 700 W h kgS-1 and 100 W kgS-1 at 0.1C to 100 W h kgS-1 and 35 kW kgS-1 at 50C over 500 cycles.

Interplay between Two Structure Types in the Incommensurately Modulated A14+2/3Ta8Se46+2/3 Compounds (A = Rb, Cs)

Incommensurately modulated crystals are a rare class of materials that are notoriously difficult to characterize properly. We have synthesized two new incommensurately modulated compounds, Rb14+2/3Ta8Se46+2/3 and Cs14+2/3Ta8Se46+2/3, based on the M2Q11 (M = Nb, Ta; Q = S, Se) unit using high-temperature solid-state synthesis. Using superspace crystallography in combination with second harmonic generation measurements, we confirmed both materials to be noncentrosymmetric, falling into the superspace group P1(αβγ)0, while the basic cell suggests C2/c. These materials can be structurally understood as ordered combinations of two known structure types, A6Ta4Se22 and A12Ta6Se35 (A = K, Rb, Cs). While both modulated compounds share structural similarities with the aforementioned known phases, they represent novel structures rather than a literal combination of the two phases, such as a composite. Additionally, both Rb14+2/3Ta8Se46+2/3 and Cs14+2/3Ta8Se46+2/3 were optically and thermally characterized, revealing identical band gaps of 1.63 eV and congruent melting points at 434 and 417 °C, respectively.

Scalable Li-Ion Battery with Metal/Metal Oxide Sulfur Cathode and Lithiated Silicon Oxide/Carbon Anode

A Li-ion battery combines a cathode benefitting from Sn and MnO2 with high sulfur content, and a lithiated anode including fumed silica, few layer graphene (FLG) and amorphous carbon. This battery is considered a scalable version of the system based on lithium-sulfur (Li-S) conversion, since it exploits at the anode the Li-ion electrochemistry instead of Li-metal stripping/deposition. Sn and MnO2 are used as cathode additives to improve the electrochemical process, increase sulfur utilization, while mitigating the polysulfides loss typical of Li-S devices. The cathode demonstrates in half-cell a maximum capacity of ~1170 mAh gS -1, rate performance extended over 1 C, and retention of 250 cycles. The anode undergoes Li-(de)alloying with silicon, Li-(de)insertion into amorphous carbon, and Li-(de)intercalation through FLG, with capacity of 500 mAh g-1 in half-cell, completely retained over 400 cycles. The full-cells are assembled by combining a sulfur cathode with active material loading up to 3 mg cm-2 and lithiated version of the anode, achieved either using an electrochemical pathway or a chemical one. The cells deliver at C/5 initial capacity higher than 1000 mAh gS -1, retained for over ~40 % upon 400 cycles. The battery is considered a promising energy storage system for possible scaling-up in pouch or cylindrical cells.

A Promising Approach to Ultra-Flexible 1 Ah Lithium-Sulfur Batteries Using Oxygen-Functionalized Single-Walled Carbon Nanotubes

Lithium-sulfur (Li-S) batteries represent a promising solution for achieving high energy densities exceeding 500 Wh kg-1, leveraging cathode materials with theoretical energy densities up to 2600 Wh kg-1. These batteries are also cost-effective, abundant, and environment-friendly. In this study, an innovative approach is proposed utilizing highly oxidized single-walled carbon nanotubes (Ox-SWCNTs) as a conductive fibrous scaffold and functional interlayer in sulfur cathodes and separators, respectively, to demonstrate large-area and ultra-flexible Li-S batteries with enhanced energy density. The free-standing sulfur cathodes in the Li-S cells exhibit high energy density maintaining 806 mAh g-1 even after 100 charge-discharge cycles. Additionally, oxygen-containing functional groups on the SWCNTs significantly improve electrochemical performance by promoting the adsorption of lithium polysulfides. Employing Ox-SWCNTs in both cathodes and interlayers, the study achieves high-capacity Li-S pouch cells that consistently deliver a capacity of 1.06 Ah and a high energy density of 909 mAh g-1 over 50 charge-discharge cycles. This strategy not only significantly enhances the electrochemical performance of Li-S batteries but also maintains excellent mechanical flexibility under severe deformation, positioning this Ox-SWCNT-based architecture as a viable, light-weight, and ultra-flexible energy storage solution suitable for commercializing rechargeable Li-S batteries.

Sulfurization of transition metal inorganic electrocatalysts in Li-S batteries

Lithium-sulfur (Li-S) batteries have garnered significant attention for their exceptional energy density, positioning them as a promising solution for next-generation energy storage. A critical factor in their performance is the use of transition metal inorganic compound electrocatalysts, prized for their distinctive catalytic properties. Recently, increasing interest has focused on the sulfurization of these catalysts in polysulfide-rich environments, a process that holds great potential for enhancing their efficiency. This review analyzes the sulfurization reactions of various transition metal compounds in Li-S batteries and their profound impact on electrochemical performance. By elucidating the sulfurization process with the assistance of advanced characterization techniques, we aim to reveal the true active sites and intrinsic catalytic pathways of sulfur redox electrocatalysts, offering new insights into the design of advanced catalysts for more efficient lithium polysulfide conversion. These findings are expected to accelerate the development of high-performance Li-S battery technologies.

Demineralization of a Carbon Material Derived from Palm Kernel Shells as a Process for the Enhancement of the Electrochemical Performance of Lithium-Sulfur Cells

Carbon materials derived from biomass have been widely used in Li-S batteries; however, the mineral matter present in the biomass could impact the properties of the carbons and affect the electrochemical performance. In this study, the removal of mineral matter from palm kernel shells is reported to identify the effect of minerals on the physicochemical properties of the derived activated carbon and correlate them to the electrochemical performance in Li-S batteries. The content of minerals such as silicon, iron, and potassium was decreased by acid washing. The textural and conductive properties of activated carbon were increased by the absence of minerals. Electrochemical results reveal that the demineralized sample used as a sulfur host can increase the capacity for high charge and discharge rates by 23%. Hence, the removal of mineral matter in the biomass is an important step to consider for the application of activated carbons as sulfur hosts in Li-S batteries.

Organic Thiolate as Multifunctional Salt for Rechargeable Lithium-Sulfur Batteries

The practical application of lithium-sulfur (Li-S) batteries is hindered by the severe shuttle effect of soluble polysulfide intermediates and the unstable lithium anode interface. Conventional lithium salts (e.g., LiPF6, LiTFSI) just serve as conducting salts to provide necessary free lithium cations for internal ion transport, lacking full utilization of the anions. Herein, lithium 4-fluorobenzenethiolate (F-PhSLi) as a multifunctional salt for rechargeable Li-S batteries, which is able to chemically react with sulfur to alter the redox pathway of sulfur cathode, accelerate the sulfur redox kinetics, and inhibit the shuttle effect of polysulfides is reported. Meanwhile, due to the redox activity of F-PhSLi, the reactive electrolyte can offer additional capacity. In addition, it also can construct a stable LiF-rich solid electrolyte interface layer on the lithium metal anode. Such reactive electrolyte endows Li-S batteries with ultrahigh discharge specific capacity, improved sulfur utilization, long-term storage ability, enhanced rate capability, and outstanding low-temperature performance. This work presents a new solution for developing high performance Li-S batteries.

Entrapment and Reactivation of Polysulfides in Conductive Amphiphilic Covalent Organic Frameworks Enabling Superior Capacity and Stability of Lithium-Sulfur Batteries

Inhibiting the shuttle of polysulfides is of great significance for promoting the practical application of lithium-sulfur batteries (LSBs). Here, an imine-linked covalent organic framework@carbon nanotube (COF@CNT) interlayer composed of triazine and boroxine rings is constructed between the sulfur cathode and the separator for polysulfides reception and reutilization. The introduction of CNT imparts the conductor characteristic to the interlayer attributed to electron tunneling in thin COF shell, and creates a hierarchical porous architecture for accommodating polysulfides. The uniform distribution of amphiphilic adsorption sites in COF microporous structure not only enables efficient entrapment of polysulfides while allowing the penetration of Li+ ions, but also provides a stable electrocatalytic channel for bidirectional conversion of active sulfur to achieve the substantially improved capacity and stability. The interlayer-incorporated LSBs deliver an ultrahigh capacity of 1446 mA g-1 at 0.1C and an ultralow capacity decay rate of 0.019% at 1C over 1500 cycles. Even at an electrolyte/sulfur ratio of 6 µL mg-1, an outstanding capacity of 995 mAh g-1 and capacity retention of 74.1% over 200 cycles at 0.2C are obtained. This work offers a compelling polysulfides entrapment and reactivation strategy for stimulating the study on ultra-stable LSBs.

Inhibiting Shuttle Effect and Dendrite Growth in Sodium-Sulfur Batteries Enabled by Applying External Acoustic Field

The room-temperature sodium-sulfur (RT Na-S) battery is a promising alternative to traditional lithium-ion batteries owing to its abundant material availability and high specific energy density. However, the sodium polysulfide shuttle effect and dendritic growth pose significant challenges to their practical applications. In this study, we apply diverse disciplinary backgrounds to introduce a novel method to stimulate polarized BaTiO3 (BTO) nanoparticles on the separator. This approach generates more charges due to the piezoelectric effect under stronger driving forces produced by applying a controllable acoustic field at the outer edge of the cell. The acoustically stimulated BTO attracts more polysulfides, thus reducing the shuttling effect from the cathode to the anode and ultimately enhancing the battery performance. Meanwhile, the acoustic waves create additional streaming flows, improving the uniformity of the sodium ion dispersion, enhancing the sodium ion transport and reducing the possibility of sodium dendrite development. We believe that this work offers a new strategy for the development of high-performance Na-S batteries.

Design of Coatings for Sulfur-Based Cathode Materials in Lithium-Sulfur Batteries: A review

Lithium-sulfur batteries (LSBs) are considered next-generation energy storage and conversion solutions owing to their high theoretical specific capacity and the high abundance/low-cost of sulfur-based cathode materials. However, LSBs still encounter significant challenges, including the low conductivities of sulfur-based materials, severe volumetric expansion of sulfur during the discharge process, and the persistent "shuttle effect" of polysulfides. In recent years, a tremendous amount of research has been conducted to address the above challenges by developing coating and compositing materials and corresponding fabrication strategies for sulfur-based cathode materials. In this study, the surface coating, compositing materials, and fabrication methodologies of LSB cathodes are comprehensively reviewed in terms of advanced materials, structure/component characterization, functional mechanisms, and performance validation. Some technical challenges are analyzed in detail, and possible future research directions are proposed to overcome the challenges toward practical applications of lithium-sulfur batteries.

Enhanced lithium host performance of multi-walled carbon nanotubes through acidic functionalization for lithium-sulfur batteries

Carbon nanotubes (CNTs) have been explored as a potential cathode material for lithium-sulfur (Li-S) batteries owing to their unique structure. However, traditional CNTs exhibit poor dispersion properties when preparing electrodes. The non-uniform distribution of the conductive agents hinders the formation of enough sites for sulfur loading, which results in the aggregation of sulfur/Li2S and severe polarization. In this study, we propose the acidic functionalization of CNTs in the cathode structure as a practical solution for mitigating the poor dispersion and polysulfide shuttling in lithium-sulfur batteries. Multiwalled CNTs were functionalized by oxidation through acidic treatment using sulfuric, nitric, and mixed acids. The cathode prepared with a mixture of sulfuric and nitric acids showed a coulombic efficiency of 99 % after 100 cycles, with a discharge capacity of 743 mAh g-1. These findings demonstrate the effectiveness of the acidic functionalization of CNTs as a promising approach for enhancing the electrochemical performance and commercial viability of lithium-sulfur batteries.

Sulfonic Acid-Functionalized Graphdiyne for Effective Li-S Battery Separators

Lithium-sulfur (Li-S) batteries enable a promising high-energy-storage system while facing practical challenges regarding lithium dendrites and lithium polysulfides (LiPSs) shuttling. Herein, a fascinating SO3H-functionalized graphdiyne (SOGDY) was developed by grafting SO3H onto GDY to modify the separator in Li-S batteries. It realizes structure-retained material transformation, that is, SOGDY retains the crystalline all-carbon network and uniform subnanopores from the initial GDY. The abundant SO3H and uniform pores create a rapid Li+ transport relay station, benefit rapid Li+ transport and even lithium deposition, and prevent lithium dendrite growth. The spatial obstruction and strong polar adsorption sites from SO3H effectively inhibit LiPS shuttling. Additionally, SOGDY establishes a fast electron-transfer pathway to facilitate the LiPS conversion. The SOGDY/PP separator exhibited steady cycling at 1 mA cm-2 over 3500 h in the Li∥Li symmetric battery and achieved outstanding low-temperature and high-rate performance in the Li-S battery with a high initial specific capacity of 804.5 mA h g-1 and a final capacity of 504.9 mA h g-1 after 500 cycles at 3 C and -10 °C. This work demonstrates that introducing a stable all-carbon network and uniform functionalized nanopores is an effective strategy to modify the Li-S battery separator.

Simple Framework for Simultaneous Analysis of Both Electrodes in Stoichiometric Lithium-Sulfur Batteries

A battery is composed of two electrodes that depend on and interact with each other. However, galvanostatic charging-discharging measurement, the most widely used method for battery evaluation, cannot simultaneously reflect performance metrics [capacity, Coulombic efficiency (CE), and cycling stability] of both electrodes because the result is generally governed by the lower-capacity electrode of the cell, namely the limiting reagent of the battery reaction. In studying stoichiometric Li-S cells operating under application-relevant high-mass-loading and lean-electrolyte conditions, we take advantage of the two-stage discharging behavior of sulfur to construct a simple framework that allows us to analyze both electrodes simultaneously. The cell capacity and its decay are anode performance descriptors, whereas the first plateau capacity and cell CE are cathode performance descriptors. Our analysis within this frame identifies Li stripping/plating and polysulfide shuttling to be the limiting factors for the cycling performance of the stoichiometric Li-S cell. Using our newly developed framework, we examine various previously reported strategies to mitigate these bottleneck problems and find modifying the separator with a reduced graphene oxide layer to be an effective means, which improves the capacity retention rate of the cell to 99.7% per cycle.

Advanced Separator Materials for Enhanced Electrochemical Performance of Lithium-Sulfur Batteries: Progress and Prospects

Lithium-sulfur (Li-S) batteries are promising energy storage devices owing to their high theoretical specific capacity and energy density. However, several challenges, including volume expansion, slow reaction kinetics, polysulfide shuttle effect and lithium dendrite formation, hinder their commercialization. Separators are a key component of Li-S batteries. Traditional separators, made of polypropylene and polyethylene, have certain limitations that should be addressed. Therefore, this review discusses the basic properties and mechanisms of Li-S battery separators, focuses on preparing different functionalized separators to mitigate the shuttle effect of polysulfides. This review also introduces future research trends, emphasizing the potential of separator functionalization in advancing the Li-S battery technology.

Designer Anions for Better Rechargeable Lithium Batteries and Beyond

Non-aqueous electrolytes, generally consisting of metal salts and solvating media, are indispensable elements for building rechargeable batteries. As the major sources of ionic charges, the intrinsic characters of salt anions are of particular importance in determining the fundamental properties of bulk electrolyte, as well as the features of the resulting electrode-electrolyte interphases/interfaces. To cope with the increasing demand for better rechargeable batteries requested by emerging application domains, the structural design and modifications of salt anions are highly desired. Here, salt anions for lithium and other monovalent (e.g., sodium and potassium) and multivalent (e.g., magnesium, calcium, zinc, and aluminum) rechargeable batteries are outlined. Fundamental considerations on the design of salt anions are provided, particularly involving specific requirements imposed by different cell chemistries. Historical evolution and possible synthetic methodologies for metal salts with representative salt anions are reviewed. Recent advances in tailoring the anionic structures for rechargeable batteries are scrutinized, and due attention is paid to the paradigm shift from liquid to solid electrolytes, from intercalation to conversion/alloying-type electrodes, from lithium to other kinds of rechargeable batteries. The remaining challenges and key research directions in the development of robust salt anions are also discussed.

NiS/NiCo2O4 Cooperative Interfaces Enable Fast Sulfur Redox Kinetics for Lithium-Sulfur Battery

Due to their high energy density and cost-effectiveness, lithium-sulfur batteries (LSBs) are considered highly promising for the next generation of energy storage technologies. However, the soluble lithium-polysulfides (LiPSs) notorious for causing the shuttle effect and the sluggish redox kinetics have hindered their practical commercialization. To tackle these challenges, a heterostructural catalyst featuring NiS-NiCo2O4 interfaces is developed, which serves as an interlayer for LSBs. These interfacial sites leverage the advantages of polar NiCo2O4 and conductive NiS, enabling smooth Li+ diffusion, rapid electron transport, and effective immobilization of LiPSs. This synergistic approach promotes the conversion of sulfur species, resulting in a high discharge capacity of 526 mAh g-1 at 3 C for cells with the NiS-NiCo2O4 interlayer. Additionally, remarkable cycling stability is achievable with an areal sulfur loading of ≈5.0 mg cm-2. It is believed that this research paves the way for practical applications of LSBs.

Boron dopant- and nitrogen defect-decorated C3N5 porous nanostructure as an efficient sulfur host for lithium-sulfur batteries

Active site implantation and morphology manipulation are efficient protocols for boosting the electrochemical performance of carbon nitrides. As a promising sulfur host for lithium-sulfur batteries (LSBs), in this study, C3N5 porous nanostructure incorporated with both boron (B) atoms and nitrogen (N) defects was constructed (denoted as ND-B-C3N5) using a two-step strategy, i.e., pyrolysis of the mixture of 3-amino-1,2, 4-triazole and boric acid to obtain B-doped C3N5 porous nanostructure and then KOH etching under hydrothermal condition to generate N defects. The doped B atoms in the C3N5 porous nanostructure are in the form of B-N bonds and grafted B-O bonds. N defects are primarily created at the CN-C positions of the triazine unit, leaving behind some N vacancies and cyano groups. Benefiting from the involvement of B dopants and N defects, the optimized ND-B-C3N5-12 sample exhibits ameliorative conductivity, mass transport, lithium polysulfides (LiPSs) adsorption ability, diffusion of Li+ ions, Li2S deposition capacity, sulfur redox polarization, and a reversible solid-solid sulfur redox process. Consequently, the ND-B-C3N5-12/S cathode delivers accelerated redox performance of polysulfides for LSBs, revealing capacities of 1091 ± 44 and 753 ± 20 mAh/g at 0.2C for the initial and 300th cycles, respectively. The ND-B-C3N5-12/S cathode is also endowed with desired sulfur redox activity and stability at 2C for 1000 cycles, holding an initial discharging capacity of 788 ± 24 mAh/g and a low decay rate of 0.05 % per cycle.

Integrating Lithium Sulfide as a Single Ionic Conductor Interphase for Stable All-Solid-State Lithium-Sulfur Batteries

As a very prospective solid-state electrolyte, Li10GeP2S12 (LGPS) exhibits high ionic conductivity comparable to liquid electrolytes. However, severe self-decomposition and Li dendrite propagation of LGPS will be triggered due to the thermodynamic incompatibility with Li metal anode. Herein, by adopting a facile chemical vapor deposition method, an artificial solid electrolyte interphase composed of Li2S is proposed as a single ionic conductor to promote the interface stability of LGPS toward Li. The good electronic insulation coupled with ionic conduction property of Li2S effectively blocks electron transfer from Li to LGPS while enabling smooth passage of Li ions. Meanwhile, the generated Li2S layer remains good interface compatibility with LGPS, which is verified by the stable Li-plating/stripping operation for over 500 h at 0.15 mA cm-2. Consequently, the all-solid-state Li-S batteries (ASSLSBs) with a Li2S layer demonstrate superb capacity retention of 90.8% at 0.2 mA cm-2 after 100 cycles. Even at the harsh condition of 90 °C, the cell can deliver a high reversible capacity of 1318.8 mAh g-1 with decent capacity retention of 88.6% after 100 cycles. This approach offers a new insight for interface modification between LGPS and Li and the realization of ASSLSBs with stable cycle life.

Quasi-Solid-State Electrolyte Induced by Metallic MoS2 for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are a promising high-energy-density technology for next-generation energy storage but suffer from an inadequate lifespan. The poor cycle life of Li-S batteries stems from their commonly adopted catholyte-mediated operating mechanism, where the shuttling of dissolved polysulfides results in active material loss on the sulfur cathode and surface corrosion on the lithium anode. Here, we report in situ formation of a quasi-solid-state electrolyte (QSSE) on the metallic 1T phase molybdenum disulfide (MoS2) host that extends the lifetime of Li-S batteries. We find that the metallic 1T phase MoS2 host is able to initiate the ring-opening polymerization of 1,3-dioxolane (DOL), forming an integrated QSSE inside batteries. Nuclear magnetic resonance analysis reveals that the QSSE consists of ∼13% liquid DOL in a solid polymer matrix. The QSSE efficiently mediates sulfur redox reactions through dissolution-conversion chemistry while simultaneously suppressing polysulfide shuttling. Therefore, while ensuring high sulfur utilization, it avoids degradation of both electrodes, as well as the concomitant electrolyte consumption, leading to enhanced cycling stability. Under a practical lean electrolyte condition (electrolyte-to-sulfur ratio = 2 μL mg-1), Li-S pouch cell batteries with the QSSE demonstrate a capacity retention of 80.7% after 200 cycles, much superior to conventional liquid electrolyte cells that fail within 70 cycles. The QSSE also enables Li-S pouch cell batteries to operate across a wider temperature range (5 to 45 °C), together with improved safety under mechanical damage.

Effective Bidirectional Mott-Schottky Catalysts Derived from Spent LiFePO4 Cathodes for Robust Lithium-Sulfur Batteries

It is deemed as a tough yet profound project to comprehensively cope with a range of detrimental problems of lithium-sulfur batteries (LSBs), mainly pertaining to the shuttle effect of lithium polysulfides (LiPSs) and sluggish sulfur conversion. Herein, a Co2P-Fe2P@N-doped carbon (Co2P-Fe2P@NC) Mott-Schottky catalyst is introduced to enable bidirectionally stimulated sulfur conversion. This catalyst is prepared by simple carbothermal reduction of spent LiFePO4 cathode and LiCoO2. The experimental and theoretical calculation results indicate that thanks to unique surface/interface properties derived from the Mott-Schottky effect, full anchoring of LiPSs, mediated Li2S nucleation/dissolution, and bidirectionally expedited "solid⇌liquid⇌solid" kinetics can be harvested. Consequently, the S/Co2P-Fe2P@NC manifests high reversible capacity (1569.9 mAh g-1), superb rate response (808.9 mAh g-1 at 3C), and stable cycling (a low decay rate of 0.06% within 600 cycles at 3C). Moreover, desirable capacity (5.35 mAh cm-2) and cycle stability are still available under high sulfur loadings (4-5 mg cm-2) and lean electrolyte (8 µL mg-1) conditions. Furthermore, the as-proposed universal synthetic route can be extended to the preparation of other catalysts such as Mn2P-Fe2P@NC from spent LiFePO4 and MnO2. This work unlocks the potential of carbothermal reduction phosphating to synthesize bidirectional catalysts for robust LSBs.

Kinetic Evaluation on Lithium Polysulfide in Weakly Solvating Electrolyte toward Practical Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are highly considered as next-generation energy storage techniques. Weakly solvating electrolyte with low lithium polysulfide (LiPS) solvating power promises Li anode protection and improved cycling stability. However, the cathodic LiPS kinetics is inevitably deteriorated, resulting in severe cathodic polarization and limited energy density. Herein, the LiPS kinetic degradation mechanism in weakly solvating electrolytes is disclosed to construct high-energy-density Li-S batteries. Activation polarization instead of concentration or ohmic polarization is identified as the dominant kinetic limitation, which originates from higher charge-transfer activation energy and a changed rate-determining step. To solve the kinetic issue, a titanium nitride (TiN) electrocatalyst is introduced and corresponding Li-S batteries exhibit reduced polarization, prolonged cycling lifespan, and high actual energy density of 381 Wh kg-1 in 2.5 Ah-level pouch cells. This work clarifies the LiPS reaction mechanism in protective weakly solvating electrolytes and highlights the electrocatalytic regulation strategy toward high-energy-density and long-cycling Li-S batteries.

Advanced Polymers in Cathodes and Electrolytes for Lithium-Sulfur Batteries: Progress and Prospects

Lithium-sulfur (Li-S) batteries, which store energy through reversible redox reactions with multiple electron transfers, are seen as one of the promising energy storage systems of the future due to their outstanding advantages. However, the shuttle effect, volume expansion, low conductivity of sulfur cathodes, and uncontrollable dendrite phenomenon of the lithium anodes have hindered the further application of Li-S batteries. In order to solve the problems and clarify the electrochemical reaction mechanism, various types of materials, such as metal compounds and carbon materials, are used in Li-S batteries. Polymers, as a class of inexpensive, lightweight, and electrochemically stable materials, enable the construction of low-cost, high-specific capacity Li-S batteries. Moreover, polymers can be multifunctionalized by obtaining rich structures through molecular design, allowing them to be applied not only in cathodes, but also in binders and solid-state electrolytes to optimize electrochemical performance from multiple perspectives. The most widely used areas related to polymer applications in Li-S batteries, including cathodes and electrolytes, are selected for a comprehensive overview, and the relevant mechanisms of polymer action in different components are discussed. Finally, the prospects for the practical application of polymers in Li-S batteries are presented in terms of advanced characterization and mechanistic analysis.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

Undercoordination Chemistry of Sulfur Electrocatalyst in Lithium-Sulfur Batteries

Undercoordination chemistry is an effective strategy to modulate the geometry-governed electronic structure and thereby regulate the activity of sulfur electrocatalysts. Efficient sulfur electrocatalysis is requisite to overcome the sluggish kinetics in lithium-sulfur (Li-S) batteries aroused by multi-electron transfer and multi-phase conversions. Recent advances unveil the great promise of undercoordination chemistry in facilitating and stabilizing sulfur electrochemistry, yet a related review with systematicness and perspectives is still missing. Herein, it is carefully combed through the recent progress of undercoordination chemistry in sulfur electrocatalysis. The typical material structures and operational strategies are elaborated, while the underlying working mechanism is also detailly introduced and generalized into polysulfide adsorption behaviors, polysulfide conversion kinetics, electron/ion transport, and dynamic reconstruction. Moreover, perspectives on the future development of undercoordination chemistry are further proposed.

Improving Rate Performance of Encapsulating Lithium-Polysulfide Electrolytes for Practical Lithium-Sulfur Batteries

The cycle life of high-energy-density lithium-sulfur (Li-S) batteries is severely plagued by the incessant parasitic reactions between Li metal anodes and reactive Li polysulfides (LiPSs). Encapsulating Li-polysulfide electrolyte (EPSE) emerges as an effective electrolyte design to mitigate the parasitic reactions kinetically. Nevertheless, the rate performance of Li-S batteries with EPSE is synchronously suppressed. Herein, the sacrifice in rate performance by EPSE is circumvented while mitigating parasitic reactions by employing hexyl methyl ether (HME) as a co-solvent. The specific capacity of Li-S batteries with HME-based EPSE is nearly not decreased at 0.1 C compared with conventional ether electrolytes. With an ultrathin Li metal anode (50 μm) and a high-areal-loading sulfur cathode (4.4 mgS  cm-2 ), a longer cycle life of 113 cycles was achieved in HME-based EPSE compared with that of 65 cycles in conventional ether electrolytes at 0.1 C. Furthermore, both high energy density of 387 Wh kg-1 and stable cycle life of 27 cycles were achieved in a Li-S pouch cell (2.7 Ah). This work inspires the feasibility of regulating the solvation structure of LiPSs in EPSE for Li-S batteries with balanced performance.

Kinetic Promoters for Sulfur Cathodes in Lithium-Sulfur Batteries

ConspectusLithium-sulfur (Li-S) batteries have attracted worldwide attention as promising next-generation rechargeable batteries due to their high theoretical energy density of 2600 Wh kg-1. The actual energy density of Li-S batteries at the pouch cell level has significantly exceeded that of state-of-the-art Li-ion batteries. However, the overall performances of Li-S batteries under practical working conditions are limited by the sluggish conversion kinetics of the sulfur cathodes. To overcome the above challenge, various kinetic promotion strategies have been proposed to accelerate the multiphase and multi-electron cathodic redox reactions between sulfur, lithium polysulfides (LiPSs), and lithium sulfide. Nowadays, kinetic promoters have been massively employed in sulfur cathodes to achieve Li-S batteries with high energy densities, high rates, and long lifespans. A comprehensive and timely summary of cutting-edge kinetic promoters for sulfur cathodes is of great essence to afford an in-depth understanding of the unique Li-S electrochemistry.In this Account, we outline the recent efforts on the design of sulfur cathode kinetic promoters for advanced Li-S batteries. The latest progress is discussed in detail regarding heterogeneous, homogeneous, and semi-immobilized kinetic promoters. Heterogeneous promoters, representatively known as electrocatalysts, function mainly by reducing the energy barriers of the interfacial electrochemical reactions. The working mechanism, activity regulation strategies, and reconstitution/deactivation processes of the heterogeneous promoters are reviewed to provide guiding principles for rational design. In comparison, homogeneous promoters are able to fully contact with the reaction interfaces and regulate the electron/ion-inaccessible reactants in working Li-S batteries. Redox mediators and redox comediators are typical homogeneous promoters. The former establishes extra chemical reaction pathways to circumvent the originally sluggish steps and boost the overall kinetics, while the latter fundamentally modifies the LiPS molecules to enhance their redox kinetics. For semi-immobilized promoters, the active units are generally anchored on the cathode substrate through flexible chains with mobile characteristics. Such a design endows the promoter with both heterogeneous and homogeneous characteristics to comprehensively regulate the multiphase sulfur redox reactions involving both mobile and immobile reactants.Overall, this Account summarizes the fundamental electrochemistry, design principles, and practical promotion effects of the various kinetic promoters used for the sulfur cathodes in Li-S batteries. We believe that this Account will provide an in-depth and cutting-edge understanding of the unique sulfur electrochemistry, thereby providing guidance for further development of high-performance Li-S batteries and analogous rechargeable battery systems.

A solid-state electrolyte for electrochemical lithium-sulfur cells

Post-lithium-ion batteries are designed to achieve high energy density and high safety by modifying their active material and cell configuration. In terms of the active material, lithium-sulfur batteries have the highest charge-storage capacity and high active-material utilization because of the use of a conversion-type sulfur cathode, which involves conversion between solid-state sulfur, liquid-state polysulfides, and solid-state sulfides. In terms of the configuration, solid-state batteries ensure high safety by using a solid-state electrolyte in between the two electrodes. Herein, we use a lithium lanthanum titanate (LLTO) solid-state electrolyte in the lithium-sulfur cell with a polysulfide catholyte electrode. The LLTO, which replaces the conventional liquid electrolyte, is a solid-state electrolyte that offers smooth lithium-ion diffusion and prevents the loss of polysulfides, while the highly active polysulfide electrode, which replaces the solid-state sulfur cathode, improves the reaction kinetics and the active-material utilization. The material and electrochemical analyses confirm the stabilized electrodes exhibit long-lasting lithium stripping/plating stability and limited polysulfide diffusion. Moreover, the morphologically and electrochemically smooth interface between the solid-state electrolyte and catholyte enables fast charge transfer in the cell, which demonstrates a high charge-storage capacity of 1429 mA h g-1, high rate performance, and high electrochemical efficiency.

Sodium-Selenium Batteries with Outstanding Rate Capability by Introducing Cubic Mn2 O3 Electrocatalyst

With their high volumetric capacity and electronic conductivity, sodium-selenium (Na-Se) batteries have attracted attention for advanced battery systems. However, the irreversible deposition of sodium selenide (Na2 Se) results in rapid capacity degradation and poor Coulombic efficiency. To address these issues, cubic α-Mn2 O3 is introduced herein as an electrocatalyst to effectively catalyze Na2 Se conversion and improve the utilization of active materials. The results show that the addition of 10 wt% Mn2 O3 in the selenium/Ketjen black (Se/KB) composite enhances the conversion from Na2 Se to Se by lowering activation energy barrier and leads to fast sodium-ion kinetics and low internal resistance. Consequently, the Mn2 O3 -based composite delivers a high specific capacity of 635 mAh ⋅ g-1 at 675 mA ⋅ g-1 after 250 cycles as well as excellent cycling stability for 800 cycles with a high specific capacity of 317 mAh ⋅ g-1 even at the high current density of 3375 mA ⋅ g-1 . Due to the cubic Mn2 O3 electrocatalyst, the performance of the composites is superior to existing state-of-the-art Na-Se batteries reported in the literature.

Metal Nanoclusters as a Superior Polysulfides Immobilizer toward Highly Stable Lithium-Sulfur Batteries

Due to their high designability, unique geometric and electronic structures, and surface coordination chemistry, atomically precise metal nanoclusters are an emerging class of functional nanomaterials at the forefront of materials research. However, the current research on metal nanoclusters is mainly fundamental, and their practical applications are still uncharted. The surface binding properties and redox activity of Au24 Pt(PET)18 (PET: phenylethanethiolate, SCH2 CH2 Ph) nanoclusters are herein harnessed as an high-efficiency electrocatalyst for the anchoring and rapid conversion of lithium polysulfides in lithium-sulfur batteries (LSBs). Au24 Pt(PET)18 @G composites are prepared by using the large specific surface area, high porosity, and conductive network of graphene (G) for the construction of battery separator that can inhibit polysulfide shuttle and accelerate electrochemical kinetics. Resultantly, the LSB using a Au24 Pt(PET)18 @G-based separator presents a high reversible specific capacity of 1535.4 mA h g-1 for the first cycle at 0.2 A g-1 and a rate capability of 887 mA h g-1 at 5 A g-1 . After 1000 cycles at 5 A g-1 , the capacity is 558.5 mA h g-1 . This study is a significant step toward the application of metal nanoclusters as optimal electrocatalysts for LSBs and other sustainable energy storage systems.

Accelerating S↔Li2 S Reactions in Li-S Batteries through Activation of S/Li2 S with a Bifunctional Semiquinone Catalyst

The reaction rate bottleneck during interconversion between insulating S8 (S) and Li2 S fundamentally leads to incomplete conversion and restricted lifespan of Li-S battery, especially under high S loading and lean electrolyte conditions. Herein, we demonstrate a new catalytic chemistry: soluble semiquinone, 2-tertbutyl-semianthraquinone lithium (Li+ TBAQ⋅- ), as both e- /Li+ donor and acceptor for simultaneous S reduction and Li2 S oxidation. The efficient activation of S and Li2 S by Li+ TBAQ⋅- in the initial discharging/charging state maximizes the amount of soluble lithium polysulfide, thereby substantially improve the rate of solid-liquid-solid reaction by promoting long-range electron transfer. With in situ Raman spectra and theoretical calculations, we reveal that the activation of S/Li2 S is the rate-limiting step for effective S utilization under high S loading and low E/S ratio. Beyond that, the S activation ratio is firstly proposed as an accurate indicator to quantitatively evaluate the reaction rate. As a result, the Li-S batteries with Li+ TBAQ⋅- deliver superior cycling performance and over 5 times higher S utilization ratio at high S loading of 7.0 mg cm-2 and a current rate of 1 C compared to those without Li+ TBAQ⋅- . We hope this study contributes to the fundamental understanding of S redox chemical and inspires the design of efficient catalysis for advanced Li-S batteries.

A Catalytic Electrolyte Additive Modulating Molecular Orbital Energy Levels of Lithium Polysulfides for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have ultrahigh theoretical specific capacity, but the practical application is hindered by the severe shuttle effect and the sluggish redox kinetics of the intermediate lithium polysulfides (LiPSs). Effectively enhancing the conversion kinetics of LiPSs is essential for addressing these issues. Herein, the redox kinetics of LiPSs are effectively improved by introducing 6-azauracil (6-AU) molecules to the organic electrolyte to modulate the molecular orbital energy level of LiPSs. The 6-AU as a soluble catalyst can form complexes with LiPSs via Li-O bonds. These complexes are liable to transform because of the elevated HOMO and the reduced LUMO energy levels as compared to the dissociative LiPSs, resulting in small energy gaps (Egap) and exhibiting stronger redox activity. Benefiting from the rapid conversion kinetics, the shuttling effect of LiPSs is alleviated to a great extent, so that sulfur utilization is improved and the lithium electrode is protected. In addition, the introduction of 6-AU modulates the deposition behavior of Li2S and eases the coverage of the cathode surface by the insulating Li2S layer. The Li-S battery containing 6-AU provides superior capacity retention of 853 mAh g-1 after 150 cycles at 0.2 C and shows remarkable high-rate performance and retains a specific discharge capacity of 855 mAh g-1 at 5 C. This study accelerates the kinetics of Li-S batteries by tuning the HOMO and LUMO energy levels of LiPSs, which opens an avenue for designing functional electrolyte additives.

Toward Sustainable Li-S Battery Using Scalable Cathode and Safe Glyme-Based Electrolyte

The search for safe electrolytes to promote the application of lithium-sulfur (Li-S) batteries may be supported by the investigation of viscous glyme solvents. Hence, electrolytes using nonflammable tetraethylene glycol dimethyl ether added by lowly viscous 1,3-dioxolane (DOL) are herein thoroughly investigated for sustainable Li-S cells. The electrolytes are characterized by low flammability, a thermal stability of ∼200 °C, ionic conductivity exceeding 10-3 S cm-1 at 25 °C, a Li+ transference number of ∼0.5, electrochemical stability window from 0 to ∼4.4 V vs Li+/Li, and a Li stripping-deposition overpotential of ∼0.02 V. The progressive increase of the DOL content from 5 to 15 wt % raises the activation energy for Li+ motion, lowers the transference number, slightly limits the anodic stability, and decreases the Li/electrolyte resistance. The electrolytes are used in Li-S cells with a composite consisting of sulfur and multiwalled carbon nanotubes mixed in the 90:10 weight ratio, exploiting an optimized current collector. The cathode is preliminarily studied in terms of structure, thermal behavior, and morphology and exploited in a cell using standard electrolyte. This cell performs over 200 cycles, with sulfur loading increased to 5.2 mg cm-2 and the electrolyte/sulfur (E/S) ratio decreased to 6 μL mg-1. The above sulfur cathode and the glyme-based electrolytes are subsequently combined in safe Li-S batteries, which exhibit cycle life and delivered capacity relevantly influenced by the DOL content within the studied concentration range.

Dually Sulphophilic Chromium Boride Nanocatalyst Boosting Sulfur Conversion Kinetics Toward High-Performance Lithium-Sulfur Batteries

The sluggish kinetics of sulfur conversions have long been hindering the implementation of fast and efficient sulfur electrochemistry in lithium-sulfur (Li-S) batteries. In this regard, herein the unique chromium boride (CrB) is developed via a well-confined mild-temperature thermal reaction to serve as an advanced sulfur electrocatalyst. Its interstitial-alloy nature features excellent conductivity, while the nano-lamination architecture affords abundant active sites for host-guest interactions. More importantly, the CrB nanocatalyst demonstrates a dual sulphophilicity with simultaneous Cr─S and B─S bondage for establishing strong interactions with the intermediate polysulfides. As a result, significant stabilization and promotion of sulfur redox behavior can be achieved, enabling an excellent Li-S cell cyclability with a minimum capacity fading rate of 0.0176% per cycle over 2000 cycles and a favorable rate capability up to 7 C. Additionally, a high areal capacity of 5.2 mAh cm-2 , and decent cycling and rate performances are still attainable under high sulfur loading and low electrolyte dosage. This work offers a facile approach and instructive insights into metal boride sulfur electrocatalyst, holding a good promise for pursuing high-efficiency sulfur electrochemistry and high-performance Li-S batteries.

Single-atom site catalysis in Li-S batteries

With their high theoretical energy density, Li-S batteries are regarded as the ideal battery system for next generation electrochemical energy storage. In the last 15 years, Li-S batteries have made outstanding academic progress. Recently, research studies have placed more emphasis on their practical application aspects, which puts forward strict requirements for the loading of S cathodes and the amount of electrolytes. To meet the above requirements, electrode catalysis design is of crucial significance. Among all the catalysts, single-atom site catalysts (SASCs) are considered to be ideal catalyst materials for the commercialization of Li-S batteries due to their high activity and highest utilization of catalytic sites. This perspective introduces the kinetic mechanism of S cathodes, the basic concept and synthesis strategy of SASCs, and then systematically summarizes the research progress of SASCs for S cathodes and, the related functional interlayers/separators in recent years. Finally, the opportunities and challenges of SASCs in Li-S batteries are summarized and prospected.

New Class of High-Energy, High-Power Capacitive Devices Enabled by Stabilized Lithium Metal Anodes

Lithium-ion capacitors (LIC) combine the energy storage mechanisms of lithium-ion batteries and electric double layer capacitors (EDLC) and are supposed to promise the best of both worlds: high energy and power density combined with a long life. However, the lack of lithium cation sources in the carbon cathode demands the cumbersome step of prelithiation of the graphite anode, mainly by using sacrificial lithium metal, hindering the mass adoption of LICs. Here, in a conceptually new class of devices termed lithium metal capacitors (LMC), we replace the graphite anode with a lithium metal anode stabilized by a complex yet stable solid-electrolyte interface (SEI). Via a specialized formation process, the well-explored synergetic reaction between the LiNO3 additive and controlled amounts of polysulfides in an ether-based electrolyte stabilizes the SEI on the lithium metal electrode. Optimized devices at the coin cell level deliver 55 mAh g-1 at a fast 30C discharge rate and maintain 95% capacity after 8000 cycles. At the pouch-cell level, energy densities of 13 Wh kg-1 are readily achieved, indicating the transferability of the technology to practical scales. The LMC, a new class of capacitive device, eliminates the prelithiation process of the conventional LIC, allowing practical production at scale and offering exciting avenues for exploring versatile cathode chemistries on account of using a lithium metal anode.

Electrolytes with moderate lithium polysulfide solubility for high-performance long-calendar-life lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries with high energy density and low cost are promising for next-generation energy storage. However, their cycling stability is plagued by the high solubility of lithium polysulfide (LiPS) intermediates, causing fast capacity decay and severe self-discharge. Exploring electrolytes with low LiPS solubility has shown promising results toward addressing these challenges. However, here, we report that electrolytes with moderate LiPS solubility are more effective for simultaneously limiting the shuttling effect and achieving good Li-S reaction kinetics. We explored a range of solubility from 37 to 1,100 mM (based on S atom, [S]) and found that a moderate solubility from 50 to 200 mM [S] performed the best. Using a series of electrolyte solvents with various degrees of fluorination, we formulated the Single-Solvent, Single-Salt, Standard Salt concentration with Moderate LiPSs solubility Electrolytes (termed S6MILE) for Li-S batteries. Among the designed electrolytes, Li-S cells using fluorinated-1,2-diethoxyethane S6MILE (F4DEE-S6MILE) showed the highest capacity of 1,160 mAh g-1 at 0.05 C at room temperature. At 60 °C, fluorinated-1,4-dimethoxybutane S6MILE (F4DMB-S6MILE) gave the highest capacity of 1,526 mAh g-1 at 0.05 C and an average CE of 99.89% for 150 cycles at 0.2 C under lean electrolyte conditions. This is a fivefold increase in cycle life compared with other conventional ether-based electrolytes. Moreover, we observed a long calendar aging life, with a capacity increase/recovery of 4.3% after resting for 30 d using F4DMB-S6MILE. Furthermore, the correlation between LiPS solubility, degree of fluorination of the electrolyte solvent, and battery performance was systematically investigated.

Enhancing Electron Conductivity and Electron Density of Single Atom Based Core-Shell Nanoboxes for High Redox Activity in Lithium Sulfur Batteries

Herein, an integrated structure of single Fe atom doped core-shell carbon nanoboxes wrapped by self-growing carbon nanotubes (CNTs) is designed. Within the nanoboxes, the single Fe atom doped hollow cores are bonded to the shells via the carbon needles, which act as the highways for the electron transport between cores and shells. Moreover, the single Fe atom doped nanobox shells is further wrapped and connected by self-growing carbon nanotubes. Simultaneously, the needles and carbon nanotubes act as the highways for electron transport, which can improve the overall electron conductivity and electron density within the nanoboxes. Finite element analysis verifies the unique structure including both internal and external connections realize the integration of active sites in nano scale, and results in significant increase in electron transfer and the catalytic performance of Fe-N4 sites in both Li2 Sn lithiation and Li2 S delithiation. The Li-S batteries with the double-shelled single atom catalyst delivered the specific capacity of 702.2 mAh g-1 after 550 cycles at 1.0 C. The regional structure design and evaluation method provide a new strategy for the further development of single atom catalysts for more electrochemical processes.

Spherical Sulfur-Infiltrated Carbon Cathode with a Tunable Poly(3,4-ethylenedioxythiophene) Layer for Lithium-Sulfur Batteries

Li-S batteries have received significant attention owing to their high energy density, nontoxicity, low cost, and eco-friendliness. However, the dissolution of lithium polysulfide during the charge/discharge process and its extremely low electron conductivity hinder practical applications of Li-S batteries. Herein, we report a sulfur-infiltrated carbon cathode material with a spherical morphology and conductive polymer coating. The material was produced via a facile polymerization process that forms a robust nanostructured layer and physically prevents the dissolution of lithium polysulfide. The thin double layer composed of carbon and poly(3,4-ethylenedioxythiophene) provides sufficient space for sulfur storage and effectively prevents the elution of polysulfide during continuous cycling, thereby playing an essential role in increasing the sulfur utilization rate and significantly improving the electrochemical performance of the battery. Sulfur-infiltrated hollow carbon spheres with a conductive polymer layer demonstrate a stable cycle life and reduced internal resistance. The as-fabricated battery demonstrated an excellent capacity of 970 mA h g^-1 at 0.5 C and a stable cycle performance, exhibiting ∼78% of the initial discharge capacity after 50 cycles. This study provides a promising approach to significantly improve the electrochemical performance of Li-S batteries and render them as valuable and safe energy devices for large-scale energy storage systems.

A Rational Design of a CoS2-CoSe2 Heterostructure for the Catalytic Conversion of Polysulfides in Lithium-Sulfur Batteries

Lithium-sulfur batteries are anticipated to be the next generation of energy storage devices because of their high theoretical specific capacity. However, the polysulfide shuttle effect of lithium-sulfur batteries restricts their commercial application. The fundamental reason for this is the sluggish reaction kinetics between polysulfide and lithium sulfide, which causes soluble polysulfide to dissolve into the electrolyte, leading to a shuttle effect and a difficult conversion reaction. Catalytic conversion is considered to be a promising strategy to alleviate the shuttle effect. In this paper, a CoS₂-CoSe₂ heterostructure with high conductivity and catalytic performance was prepared by in situ sulfurization of CoSe₂ nanoribbon. By optimizing the coordination environment and electronic structure of Co, a highly efficient CoS₂-CoSe₂ catalyst was obtained, to promote the conversion of lithium polysulfides to lithium sulfide. By using the modified separator with CoS₂-CoSe₂ and graphene, the battery exhibited excellent rate and cycle performance. The capacity remained at 721 mAh g^-1 after 350 cycles, at a current density of 0.5 C. This work provides an effective strategy to enhance the catalytic performance of two-dimensional transition-metal selenides by heterostructure engineering.

Multiscale Structural Engineering of Sulfur/Carbon Cathodes Enables High Performance All-Solid-State LiS Batteries

Constructing all-solid-state lithium-sulfur batteries (ASSLSBs) cathodes with efficient charge transport and mechanical flexibility is challenging but critical for the practical applications of ASSLSBs. Herein, a multiscale structural engineering of sulfur/carbon composites is reported, where ultrasmall sulfur nanocrystals are homogeneously anchored on the two sides of graphene layers with strong SC bonds (denoted as S@EG) in chunky expanded graphite particles via vapor deposition method. After mixing with Li_9.54 Si_1.74 P_1.44 S_11.7 Cl_0.3 (LSPSCL) solid electrolytes (SEs), the fabricated S@EG-LSPSCL cathode with interconnected "Bacon and cheese sandwich" feature can simultaneously enhance electrochemical reactivity, charge transport, and chemomechanical stability due to the synergistic atomic, nanoscopic and microscopic structural engineering. The assembled InLi/LSPSCL/S@EG-LSPSCL ASSLSBs demonstrate ultralong cycling stability over 2400 cycles with 100% capacity retention at 1 C, and a record-high areal capacity of 14.0 mAh cm^-2 at a record-breaking sulfur loading of 8.9 mg cm^-2 at room temperature as well as high capacities with capacity retentions of ≈100% after 600 cycles at 0 and 60 °C. Multiscale structural engineered sulfur/carbon cathode has great potential to enable high-performance ASSLSBs for energy storage applications.

Steering Bidirectional Sulfur Redox via Geometric/Electronic Mediator Comodulation for Li-S Batteries

Mediator design has stimulated ever-increasing attention to help tackle a surge of detrimental caveats in Li-S realms, mainly pertaining to rampant polysulfide shuttling and sluggish redox kinetics. Nevertheless, universal designing philosophy, despite being highly sought-after, remains still elusive to date. Herein, we present a generic and simple material strategy to allow the target fabrication of advanced mediator toward boosted sulfur electrochemistry. This trick is done by the geometric/electronic comodulation of a prototype VN mediator, where the interplay of its triple-phase interface, favorable catalytic activity, and facile ion diffusivity is conducive to steering bidirectional sulfur redox kinetics. In laboratory tests, the thus-derived Li-S cells manifest impressive cyclic performances with a capacity decay rate of 0.07% per cycle over 500 cycles at 1.0 C. Moreover, under a sulfur loading of 5.0 mg cm^-2, the cell could sustain a durable areal capacity of 4.63 mAh cm^-2. Our work is anticipated to lay a theory-to-application foundation for rationalizing the design and modulation of reliable polysulfide mediators in working Li-S batteries.