Sandwich‐like Catalyst–Carbon–Catalyst Trilayer Structure as a Compact 2D Host for Highly Stable Lithium–Sulfur Batteries

Relay-Type Catalysis by a Dual-Metal Single-Atom System in a Waste Biomass Derivative Host for High-Rate and Durable Li-S Batteries

An environmental-friendly and sustainable carbon-based host is one of the most competitive strategies for achieving high loading and practicality of Li-S batteries. However, the polysulfide conversion reaction kinetics is still limited by the nonuniform or monofunctional catalyst configuration in the carbon host. In this work, we propose a catalysis mode based on "relay-type" co-operation by adjacent dual-metal single atoms for high-rate and durable Li-S batteries. A discarded sericin fabric-derived porous N-doped carbon with a stacked schistose structure is prepared as the high-loading sulfur (84 wt %) host by a facile ionothermal method, which further enables the uniform anchoring of Fe/Co dual-metal single atoms. This multifunctional host enables superior lithiophilic-sulfiphilic and electrocatalytic capabilities contributed by the "relay-type" single-atom modulation effects on different conversion stages of liquid polysulfides and solid Li2S2/Li2S, leading to the suppression of the "shuttle effect", alleviation of nucleation and decomposition barriers of Li2Sx, and acceleration of polysulfide conversion kinetics. The corresponding Li-S batteries exhibit a high specific capacity of 1399.0 mA h g-1, high-rate performance up to 10 C, and excellent cycling stability over 1000 cycles. They can also endure the high sulfur loading of 8.5 mg cm-2 and the lean electrolyte condition and yield an areal capacity as high as 8.6 mA h cm-2. This work evidentially demonstrates the potential of waste biomass reutilization coupled with the design of a single-atom system for practical Li-S batteries with high energy density.

Engineering Oxygen Vacancies in In2O3 with Enhanced Polysulfides Immobilization and Selective Catalytic Capability

Lithium-sulfur (Li-S) battery is identified as an ideal candidate for next-generation energy storage systems in consideration of its high theoretical energy density and abundant sulfur resources. However, the shuttling behavior of soluble polysulfides (LiPSs) and their sluggish reaction kinetics severely limit the practical application of the current Li-S battery. In this work, a series of In2O3 nanocubes with different oxygen vacancy concentrations are designed and prepared via a facile self-template method. The introduced oxygen vacancy on In2O3 can effectively rearrange the charge distribution and enhance sulfiphilic property. Moreover, the In2O3 with high oxygen vacancy concentration (H-In2O3) can slightly slow down the solid-liquid conversion process and significantly accelerate the liquid-solid conversion process, thus reducing the accumulation of LiPSs in electrolyte and inhibiting the shuttle effect. Contributed by the unique selective catalytic capability, the prepared H-In2O3 exhibits excellent electrochemical performance when used as sulfur host. For instance, a high reversible capacity of 609 mAh g-1 is obtained with only 0.044% capacity decay per cycle over 1000 cycles at 1.0 C. This work presents a typical example for designing advanced sulfur hosts, which is crucial for the commercialization of Li-S battery.

Surface-Phosphided Metal Oxide Microspheres as Catalytic Host of Sulfur to Enhance the Performance of Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are one of the most promising high-energy density secondary batteries due to their high theoretical energy density of 2600 Wh kg-1. However, the sluggish kinetics and severe "shuttle effect" of polysulfides are the well-known barriers that hinder their practical applications. A carefully designed catalytic host of sulfur may be an effective strategy that not only accelerates the conversion of polysulfides but also limit their dissolution to mitigate the "shuttle effect." Herein, in situ surface-phosphided Ni0.96Co0.03Mn0.01O (p-NCMO) oxide microspheres are prepared via gas-phase phosphidation as a catalytic host of sulfur. The as-prepared unique heterostructured microspheres, with enriched surface-coated metal phosphide, exhibit superior synergistic effect of catalytic conversion and absorption of the otherwise soluble intermediate polysulfides. Correspondingly, the sulfur cathode exhibits excellent electrochemical performance, including a high initial discharge capacity (1162 mAh gs-1 at 0.1C), long cycling stability (491 mAh gs-1 after 1000 cycles at 1C), and excellent rate performance (565 mAh gs-1 at 5C). Importantly, the newly prepared sulfur cathode shows a high areal capacity of 4.0 mAh cm-2 and long cycle stability under harsh conditions (high sulfur loading of 5.3 mg cm-2 and lean electrolyte/sulfur ratio of 5.8 μL mg-1). This work proposes an effective strategy to develop the catalytic hosts of sulfur for achieving high-performance Li-S batteries via surface phosphidation.

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.

Interfacial "Double-Terminal Binding Sites" Catalysts Synergistically Boosting the Electrocatalytic Li2S Redox for Durable Lithium-Sulfur Batteries

Catalytic conversion of polysulfides emerges as a promising approach to improve the kinetics and mitigate polysulfide shuttling in lithium-sulfur (Li-S) batteries, especially under conditions of high sulfur loading and lean electrolyte. Herein, we present a separator architecture that incorporates double-terminal binding (DTB) sites within a nitrogen-doped carbon framework, consisting of polar Co0.85Se and Co clusters (Co/Co0.85Se@NC), to enhance the durability of Li-S batteries. The uniformly dispersed clusters of polar Co0.85Se and Co offer abundant active sites for lithium polysulfides (LiPSs), enabling efficient LiPS conversion while also serving as anchors through a combination of chemical interactions. Density functional theory calculations, along with in situ Raman and X-ray diffraction characterizations, reveal that the DTB effect strengthens the binding energy to polysulfides and lowers the energy barriers of polysulfide redox reactions. Li-S batteries utilizing the Co/Co0.85Se@NC-modified separator demonstrate exceptional cycling stability (0.042% per cycle over 1000 cycles at 2 C) and rate capability (849 mAh g-1 at 3 C), as well as deliver an impressive areal capacity of 10.0 mAh cm-2 even in challenging conditions with a high sulfur loading (10.7 mg cm-2) and lean electrolyte environments (5.8 μL mg-1). The DTB site strategy offers valuable insights into the development of high-performance Li-S batteries.

Dual Design on Hierarchically Hollow MoTe2/C with Ion/Electron Channel Engineering for High-Performance Sodium Storage

Transition-metal tellurides have been investigated as novel anode materials for application in sodium-ion batteries (SIBs) due to their rich active sites and unique and controllable layered nanostructures. However, the weak structural strength and inferior intercalation/deintercalation kinetics inhibit the development of transition-metal tellurides. In this work, MoTe2/C composites with two different hollow nanostructures are designed and prepared. By adjustment of the precursor structure, MoTe2/C-2 exhibits superior sodium-storage performance because of its uniquely hollow nanostructure with self-assembled 2D flexible nanosheets grown on the external surface. MoTe2/C-2 delivers a higher specific capacity (276 mAh g-1 at 0.1 A g-1 after 300 cycles), much more than MoTe2/C-1 (201 mAh g-1 at 0.1 A g-1 after 300 cycles), and exhibits a long-time cycling performance (131 mAh g-1 at 1 A g-1 after 2000 cycles). The excellent sodium-storage performance derived from the rational structure design is beneficial for shortening the ion paths, facilitating the sodiation/desodiation process, and reinforcing the intrinsic structural stability, thus boosting the reaction kinetics and prolonging the cycling life. Meanwhile, the assembled full-cell maintains 101 mAh g-1 at 0.1 A g-1 after 50 cycles and lights an electric watch. The findings provide several new views for preparation of more transition-metal tellurides with multi-ion/electron migration channel engineering.

Room-temperature reversible F-ion batteries based on sulfone electrolytes with a mild anion acceptor additive

Rechargeable fluoride ion batteries (FIBs) as an emerging anion shuttle system are attracting much attention due to their potential advantages in terms of energy density, cost and safety. A liquid electrolyte system enables the FIB operation at low or room temperature due to its higher ionic conductivity than that of a solid F-ion electrolyte. However, the insolubility of fluoride salts in aprotic solvents limits the development of liquid F-ion electrolytes. Although the boron-based anion acceptors (AAs) can facilitate the dissolution of F-ion salts, they are prone to lead to a tough desolvation process for F- due to strong Lewis acidity and therefore an inferior electrochemical performance. Here, a new non-boron AA (6-thioguanine) with moderate Lewis acidity is proposed to dissolve F- in the sulfone solvent. The ionic conductivity of the corresponding electrolytes reaches a level of mS cm-1 at room temperature. A model FIB coin cell is successfully operated with high conversion reaction reversibility based on the coupled defluorination/fluorination mechanism of electrodes, enabling a low overpotential of 0.36 V and a reversible capacity of 126 mA h g-1 after 40 cycles.

2D nanocomposite materials for HER electrocatalysts - a review

Hydrogen energy has the potential to be a cost-effective and strong technology for brighter development. Hydrogen fuel production by water electrolyzers has attracted attention. 2D nanocomposites with distinctive properties have been extensively explored for various applications from hydrogen evolution reactions to improving the efficiency of water electrolyzer, which is the most eco-friendly, and high-performance for hydrogen production. Recently, typical 2D nanocomposites such as Metal-Free 2D, TMDs, Mxene, LDH, organic composites, and Heterostructure have recently been thoroughly researched for use in the HER. We discuss effective ways for increasing the HER efficiency of 2D catalysts in this paper, And the unique advantages and mechanisms for specific applications are highlighted. Several essential regulating strategies for developing 2D nanocomposite-based HER electrocatalysts are included such as interface engineering, defect engineering, heteroatom doping, strain & phase engineering, and hybridizing which improve HER kinetics, the electrical conductivity, accessibility to catalytic active sites, and reaction energy barrier can be optimized. Finally, the future prospects for 2D nanocomposites in HER are discussed, as well as a thorough overview of a variety of methodologies for designing 2D nanocomposites as HER electrocatalysts with excellent catalytic performance. We expect that this review will provide a thorough overview of 2D nanocatalysts for hydrogen production.

Boosting Electrochemical Nitrogen Fixation via Regulating Surface Electronic Structure by CeO2 Hybridization

Electrocatalytic nitrogen reduction reaction (NRR) paves a sustainable way to produce NH3 but suffering from the relatively low NH3 yield and poor selectivity. High-performance NRR catalysts and a deep insight into the structure-performance relationship are higher desired. Herein, a molten-salt approach is developed to synthesize tiny CeO2 nanoparticles anchored by ultra-thin MoN nanosheets as advanced catalysts for NRR. Specifically, a considerably high NH3 yield rate of 27.5 µg h-1  mg-1 with 17.2% Faradaic efficiency (FE) can be achieved at -0.3 V vs (RHE) under ambient conditions. Experimental and density functional theory (DFT) calculations further point out that the incorporation of MoN with CeO2 can promotes the enlargement of the electron deficient area of nitrogen vacancy site. The enlarged electron deficient area contributes to the accommodation of lone pair electrons of N2 , which dramatically improves the N2 adsorption/activation and the key intermediates (*NNH and *NH3 ) generation, thus boosting the NRR performance.

Metal nitrides for seawater electrolysis

Electrocatalytic high-throughput seawater electrolysis for hydrogen production is a promising green energy technology that offers possibilities for environmental and energy sustainability. However, large-scale application is limited by the complex composition of seawater, high concentration of Cl- leading to competing reaction, and severe corrosion of electrode materials. In recent years, extensive research has been conducted to address these challenges. Metal nitrides (MNs) with excellent chemical stability and catalytic properties have emerged as ideal electrocatalyst candidates. This review presents the electrode reactions and basic parameters of the seawater splitting process, and summarizes the types and selection principles of conductive substrates with critical analysis of the design principles for seawater electrocatalysts. The focus is on discussing the properties, synthesis, and design strategies of MN-based electrocatalysts. Finally, we provide an outlook for the future development of MNs in the high-throughput seawater electrolysis field and highlight key issues that require further research and optimization.

Palygorskite-Derived Ternary Fluoride with 2D Ion Transport Channels for Ampere Hour-Scale Li-S Pouch Cell with High Energy Density

Although various excellent electrocatalysts/adsorbents have made notable progress as sulfur cathode hosts on the lithium-sulfur (Li-S) coin-cell level, high energy density (WG ) of the practical Li-S pouch cells is still limited by inefficient Li-ion transport in the thick sulfur cathode under low electrolyte/sulfur (E/S) and negative/positive (N/P) ratios, which aggravates the shuttle effect and sluggish redox kinetics. Here a new ternary fluoride MgAlF5 ·2H2 O with ultrafast ion conduction-strong polysulfides capture integration is developed. MgAlF5 ·2H2 O has an inverse Weberite-type crystal framework, in which the corner-sharing [AlF6 ]-[MgF4 (H2 O)2 ] octahedra units extend to form two-dimensional Li-ion transport channels along the [100] and [010] directions, respectively. Applied as the cathode sulfur host, the MgAlF5 ·2H2 O lithiated by LiTFSI (lithium salt in Li-S electrolyte) acts as a fast ionic conductor to ensure efficient Li-ion transport to accelerate the redox kinetics under high S loadings and low E/S and N/P. Meanwhile, the strong polar MgAlF5 ·2H2 O captures polysulfides by chemisorption to suppress the shuttle effect. Therefore, a 1.97 A h-level Li-S pouch cell achieves a high WG of 386 Wh kg-1 . This work develops a new-type ionic conductor, and provides unique insights and new hosts for designing practical Li-S pouch cells.

Co/Mon Invigorated Bilateral Kinetics Modulation for Advanced Lithium-Sulfur Batteries

Sluggish sulfur redox kinetics and Li-dendrite growth are the main bottlenecks for lithium-sulfur (Li-S) batteries. Separator modification serves as a dual-purpose approach to address both of these challenges. In this study, the Co/MoN composite is rationally designed and applied as the modifier to modulate the electrochemical kinetics on both sides of the sulfur cathode and lithium anode. Benefiting from its adsorption-catalysis function, the decorated separators (Co/MoN@PP) not only effectively inhibit polysulfides (LiPSs) shuttle and accelerate their electrochemical conversion but also boost Li+ flux, realizing uniform Li plating/stripping. The accelerated LiPSs conversion kinetics and excellent sulfur redox reversibility triggered by Co/MoN modified separators are evidenced by performance, in-situ Raman detection and theoretical calculations. The batteries with Co/MoN@PP achieve a high initial discharge capacity of 1570 mAh g-1 at 0.2 C with a low decay rate of 0.39%, uniform Li+ transportation at 1 mA cm-2 over 800 h. Moreover, the areal capacity of 4.62 mAh cm-2 is achieved under high mass loadings of 4.92 mg cm-2 . This study provides a feasible strategy for the rational utilization of the synergistic effect of composite with multifunctional microdomains to solve the problems of Li anode and S cathode toward long-cycling Li-S batteries.

GO-CoNiP New Composite Material Modified Separator for Long Cycle Lithium-Sulfur Batteries

Lithium-sulfur batteries with high capacity are considered the most promising candidates for next-generation energy storage systems. Mitigating the shuttle reaction and promoting catalytic conversion within the battery are major challenges in the development of high-performance lithium-sulfur batteries. To solve these problems, a novel composite material GO-CoNiP is synthesized in this study. The material has excellent conductivity and abundant active sites to adsorb polysulfides and improve reaction kinetics within the battery. The initial capacity of the GO-CoNiP separator battery at 1 C is 889.4 mAh g-1 , and the single-cycle decay is 0.063% after 1000 cycles. In the 4 C high-rate test, the single-cycle decay is only 0.068% after 400 cycles. The initial capacity is as high as 828.2 mAh g-1 under high sulfur loading (7.3 mg cm-2 ). In addition, high and low-temperature performance tests are performed on the GO-CoNiP separator battery. The first cycle discharge reaches 810.9 mAh g-1 at a low temperature of 0 °C, and the first cycle discharge reaches 1064.8 mAh g-1 at a high temperature of 60 °C, and both can run stably for 120 cycles. In addition, in situ Raman tests are conducted to explain the adsorption of polysulfides by GO-CoNiP from a deeper level.

Breaking Barriers to High-Practical Li-S Batteries with Isotropic Binary Sulfiphilic Electrocatalyst: Creating a Virtuous Cycle for Favorable Polysulfides Redox Environments

Investigations into lithium-sulfur batteries (LSBs) has focused primarily on the initial conversion of lithium polysulfides (LiPSs) to Li2 S2 . However, the subsequent solid-solid reaction from Li2 S2 to Li2 S and the Li2 S decomposition process should be equally prioritized. Creating a virtuous cycle by balancing all three chemical reaction processes is crucial for realizing practical LSBs. Herein, amorphous Ni3 B in synergy with carbon nanotubes (aNi3 B@CNTs) is proposed to implement the consecutive catalysis of S8(solid) → LiPSs(liquid) → Li2 S(solid) →LiPSs(liquid) . Systematic theoretical simulations and experimental analyses reveal that aNi3 B@CNTs with an isotropic structure and abundant active sites can ensure rapid LiPSs adsorption-catalysis as well as uniform Li2 S precipitation. The uniform Li2 S deposition in synergy with catalysis of aNi3 B enables instant/complete oxidation of Li2 S to LiPSs. The produced LiPSs are again rapidly and uniformly adsorbed for the next sulfur evolution process, thus creating a virtuous cycle for sulfur species conversion. Accordingly, the aNi3 B@CNTs-based cell presents remarkable rate capability, long-term cycle life, and superior cyclic stability, even under high sulfur loading and extreme temperature environments. This study proposes the significance of creating a virtuous cycle for sulfur species conversion to realize practical LSBs.

Furnishing Continuous Efficient Bidirectional Polysulfide Conversion for Long-Life and High-Loading Lithium-Sulfur Batteries via the Built-In Electric Field

Most catalysts cannot accelerate uninterrupted conversion of polysulfides, resulting in poor long-cycle and high-loading performance of lithium-sulfur (Li-S) batteries. Herein, rich p-n junction CoS2 /ZnS heterostructures embedded on N-doped carbon nanosheets are fabricated by ion-etching and vulcanization as a continuous and efficient bidirectional catalyst. The p-n junction built-in electric field in the CoS2 /ZnS heterostructure not only accelerates the transformation of lithium polysulfides (LiPSs), but also promotes the diffusion and decomposition for Li2 S the from CoS2 to ZnS avoiding the aggregation of lithium sulfide (Li2 S). Meanwhile, the heterostructure possesses a strong chemisorption ability to anchor LiPSs and superior affinity to induce homogeneous Li deposition. The assembled cell with a CoS2 /ZnS@PP separator delivers a cycling stability with a capacity decay of 0.058% per cycle at 1.0 C after 1000 cycles, and a decent areal capacity of 8.97 mA h cm-2 at an ultrahigh sulfur mass loading of 6 mg cm-2 . This work reveals that the catalyst continuously and efficiently converts polysulfides via abundant built-in electric fields to promote Li-S chemistry.

Sheet-on-sheet ZnIn2S4@RGO-modified separators with abundant sulfur vacancies for high-performance Li-S batteries

A novel sheet-on-sheet architecture with abundant sulfur vacancies (Vs) is designed by in situ growth of flake-like ZnIn2S4 on the reduced graphene oxide (Vs-ZIS@RGO) surface, which serves as a functional layer on the separators for high-performance lithium-sulfur batteries (LSBs). Benefiting from the sheet-on-sheet architecture, the separators exhibit rapid ionic/electronic transfer, which is capable of supporting fast redox reactions. The vertically ordered ZnIn2S4 shortens the diffusion pathways of lithium-ions and the irregularly curved nanosheets expose more active sites to effectively anchor lithium polysulfides (LiPSs). More importantly, the introduction of Vs adjusts the surface or interface electronic structure of ZnIn2S4, enhancing the chemical affinity to LiPSs while accelerating conversion reaction kinetics of LiPSs. As expected, the batteries with Vs-ZIS@RGO modified separators exhibit an initial discharge capacity of 1067 mA h g-1 at 0.5C. Even at 1C, the excellent long cycle stability (710 mA h g-1 over 500 cycles) with an ultra-low decay rate of 0.055% per cycle is also attained. This work proposes a strategy of designing the sheet-on-sheet structure with rich sulfur vacancies, which provides a new perspective to rationally devise durable and efficient LSBs.

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.

Interface Engineering Toward Expedited Li2 S Deposition in Lithium-Sulfur Batteries: A Critical Review

Lithium-sulfur batteries (LSBs) with superior energy density are among the most promising candidates of next-generation energy storage techniques. As the key step contributing to 75% of the overall capacity, Li₂ S deposition remains a formidable challenge for LSBs applications because of its sluggish kinetics. The severe kinetic issue originates from the huge interfacial impedances, indicative of the interface-dominated nature of Li₂ S deposition. Accordingly, increasing efforts have been devoted to interface engineering for efficient Li₂ S deposition, which has attained inspiring success to date. However, a systematic overview and in-depth understanding of this critical field are still absent. In this review, the principles of interface-controlled Li₂ S precipitation are presented, clarifying the pivotal roles of electrolyte-substrate and electrolyte-Li₂ S interfaces in regulating Li₂ S depositing behavior. For the optimization of the electrolyte-substrate interface, efforts on the design of substrates including metal compounds, functionalized carbons, and organic compounds are systematically summarized. Regarding the regulation of electrolyte-Li₂ S interface, the progress of applying polysulfides catholytes, redox mediators, and high-donicity/polarity electrolytes is overviewed in detail. Finally, the challenges and possible solutions aiming at optimizing Li₂ S deposition are given for further development of practical LSBs. This review would inspire more insightful works and, more importantly, may enlighten other electrochemical areas concerning heterogeneous deposition processes.

Crystal phase engineering of nanoflower-like hollow MoSe2boosting polysulfide conversion for lithium-sulfur batteries

The battery performance of sulfur cathode has obviously depended on the redox reaction kinetics of polysulfides upon cycling. Herein, an effective strategy was proposed to achieve the conversion from 2H (semiconductor phase) to 1T (metal phase) in hollow nano-flowered molybdenum selenide sphere (HFSMS) through crystal phase engineering. The HFSMS with different phase ratio was realized by regulating the proportion of reducing agents. Specifically, the 1T phase content can reach up to 60.8%, and then subsequently decreased to 59.1% with the further increase of the reducing agent. The as-prepared HFSMS with the 1T phase content of 60.8% showed a smallest Tafel slopes (49.99 and 79.65 mV/dec in reduction and oxidation process, respectively), fastest response time and highest response current (520 s, 0.459 mA in Li₂S deposition test), which further exhibited excellent catalytic activity and faster reaction kinetics. This result was verified by electrochemical performance, which manifested as stable cycle life with only 0.112% capacity decay per cycle. It was found that the hollow structure can ensures a rich sulfur storage space, and effectually buffer the volume changes of the active substance. More importantly, the improved performance is attributed to the introduction of the 1T phase, which significantly improves the catalytic activity of MoSe₂with promoting the polysulfide conversion.

Isolated Fe-Co heteronuclear diatomic sites as efficient bifunctional catalysts for high-performance lithium-sulfur batteries

The slow redox kinetics of polysulfides and the difficulties in decomposition of Li₂S during the charge and discharge processes are two serious obstacles to the practical application of lithium-sulfur batteries. Herein, we construct the Fe-Co diatomic catalytic materials supported by hollow carbon spheres to achieve high-efficiency catalysis for the conversion of polysulfides and the decomposition of Li₂S simultaneously. The Fe atom center is beneficial to accelerate the discharge reaction process, and the Co atom center is favorable for charging process. Theoretical calculations combined with experiments reveal that this excellent bifunctional catalytic activity originates from the diatomic synergy between Fe and Co atom. As a result, the assembled cells exhibit the high rate performance (the discharge specific capacity achieves 688 mAh g^-1 at 5 C) and the excellent cycle stability (the capacity decay rate is 0.018% for 1000 cycles at 1 C).

Dynamic Liquid Metal Catalysts for Boosted Lithium Polysulfides Redox Reaction

Designing efficient electrocatalysts with high electroconductivity, strong chemisorption, and superior catalytical efficiency to realize rapid kinetics of the lithium polysulfides (LiPSs) conversion process is crucial for practical lithium-sulfur (Li-S) battery applications. Unfortunately, most current electrocatalysts cannot maintain long-term stability due to the possible failure of catalytic sites. Herein, a novel dynamic electrocatalytic strategy with the liquid metal (i.e., gallium-tin, EGaSn) to facilitate LiPSs redox reaction is reported. The combined theoretical simulations and microstructure experiment analysis reveal that Sn atoms dynamically distributed in the liquid Ga matrix act as the main active catalytic center. Meanwhile, Ga provides a uniquely dynamic environment to maintain the long-term integrity of the catalytic system. With the participation of EGaSn, a tailor-made 2 Ah Li-S pouch cell with a specific energy density of 307.7 Wh kg^-1 is realized. This work opens up new opportunities for liquid-phase binary alloys as electrocatalysts for high-specific-energy Li-S batteries.

Advanced cathodic free-standing interlayers for lithium-sulfur batteries: understanding, fabrication, and modification

In the past decades, lithium-sulfur batteries (LSBs) have demonstrated huge practical potential due to their ultrahigh theoretical specific capacity, low price, and environmental friendliness. However, LSBs are still faced with the problems of volumetric expansion, slow reaction kinetics, and short working life due to the shuttling of polysulfides. The introduction of a free-standing interlayer is a good way to solve the problems because of the physical confinement, chemical entrapment, and conversion. This review summarizes the common fabrication methods of free-standing interlayers, including the power-originated and film-originated methods. The modification of the as-prepared free-standing interlayers is also accomplished into physical treatment, atomic doping, and compound introduction. Finally, we conclude and compare the different fabrication methods of free-standing interlayers and their modifications and put forward the outlook of the high-performance free-standing interlayers.

Poly(ionic liquid) Nanovesicle-Templated Carbon Nanocapsules Functionalized with Uniform Iron Nitride Nanoparticles as Catalytic Sulfur Host for Li-S Batteries

Poly(ionic liquid)s (PIL) are common precursors for heteroatom-doped carbon materials. Despite a relatively higher carbonization yield, the PIL-to-carbon conversion process faces challenges in preserving morphological and structural motifs on the nanoscale. Assisted by a thin polydopamine coating route and ion exchange, imidazolium-based PIL nanovesicles were successfully applied in morphology-maintaining carbonization to prepare carbon composite nanocapsules. Extending this strategy further to their composites, we demonstrate the synthesis of carbon composite nanocapsules functionalized with iron nitride nanoparticles of an ultrafine, uniform size of 3-5 nm (termed "Fe_xN@C"). Due to its unique nanostructure, the sulfur-loaded Fe_xN@C electrode was tested to efficiently mitigate the notorious shuttle effect of lithium polysulfides (LiPSs) in Li-S batteries. The cavity of the carbon nanocapsules was spotted to better the loading content of sulfur. The well-dispersed iron nitride nanoparticles effectively catalyze the conversion of LiPSs to Li₂S, owing to their high electronic conductivity and strong binding power to LiPSs. Benefiting from this well-crafted composite nanostructure, the constructed Fe_xN@C/S cathode demonstrated a fairly high discharge capacity of 1085 mAh g^-1 at 0.5 C initially, and a remaining value of 930 mAh g^-1 after 200 cycles. In addition, it exhibits an excellent rate capability with a high initial discharge capacity of 889.8 mAh g^-1 at 2 C. This facile PIL-to-nanocarbon synthetic approach is applicable for the exquisite design of complex hybrid carbon nanostructures with potential use in electrochemical energy storage and conversion.

Hollow Ni3Se4 with High Tap Density as a Carbon-Free Sulfur Immobilizer to Realize High Volumetric and Gravimetric Capacity for Lithium-Sulfur Batteries

Despite that the practical gravimetric energy density of lithium sulfur batteries has exceeded that of the traditional lithium-ion battery, the volumetric energy density still pales due to the low density of carbonaceous materials. Herein, hollow polar nickel selenide (Ni₃Se₄) with various architectures was designed and employed as a carbon-free sulfur immobilizer. Among them, hollow sea urchins like Ni₃Se₄ with high porosity (0.39 cm³ g^-1) and large specific surface area (82.7 m² g^-1) exhibit abundant adsorptive and electrocatalytic sites, which pledge excellent electrochemical performances of the Li-S battery. Correspondingly, the Ni₃Se₄-based sulfur electrode presents excellent rate endurability (581 mAh g^-1_-composite at 2.0 C) and superior cycle stability (ultralow fading rate of 0.042% per cycle during the 1000 cycles at 1.0 C). More importantly, thanks to the higher tap density (Ni₃Se₄/S: 1.57 g cm^-3 vs super P/S: 0.7 g cm^-3), the volumetric specific capacity of Ni₃Se₄-based cathodes is as high as 1699 mAh cm^-3_-composite at 0.1 C, which is almost 2.8 times that of the carbonaceous electrode. Hence, rational transition metal selenide architecture design with synergistic function of good conductivity, well-defined catalyst and adsorption, as well as high tap density provide a promising route toward high gravimetric and volumetric energy density of Li-S batteries.

Expediting Sulfur Reduction/Evolution Reactions with Integrated Electrocatalytic Network: A Comprehensive Kinetic Map

Electrocatalysts are considered the most promising candidates in ameliorating the slow kinetics of Li-S batteries (LSBs), however, the issue of insufficient catalytic capability remains to be addressed. Herein, we report an integrated catalytic network comprising graphitic carbon-encapsulated/bridged ultrafine NiCoP embedded in N, P-codoped carbon (GC-uNiCoP@NPC) as a highly competent catalyst for sulfur-based species conversions. By profiling the evolution map of Li-S chemistry via operando kinetic analyses, GC-uNiCoP@NPC is demonstrated to possess versatile yet efficient catalytic activity for sulfur reduction/evolution reactions, especially the rate-determining heterogeneous phase transitions. As a result, GC-uNiCoP@NPC enables high capacity and stable cycling of sulfur cathode under high areal loading and lean electrolyte. Moreover, pouch cells assembled under practical conditions present promising performance with a specific energy of 302 Wh kg^-1. This work not only conceptually expands the catalyst design for LSBs but also provides a comprehensive insight into the catalyst performance for Li-S chemistry.

Novel functional separator with self-assembled MnO2 layer via a simple and fast method in lithium-sulfur battery

Modifying separator with metal oxides has been considered as a strong strategy to inhibit the shuttling of soluble polysulfide in the lithium-sulfur battery (Li-S battery). Manganesedioxide (MnO₂), one kind of transition metal oxide, is widely applied to decorate the PP (Polypropylene) separator. However, the fabrication by physical coating is always multistep and complicated. Here, we design a simple and fast method to chemically decorate separator. Based on the oxidizing property of acidic KMnO₄ solution, the PP separator was oxidized and an ultrathin self-assembled MnO₂ layer was directly constructed on one side of separator, by immersing in acidic KMnO₄ solution for only 1 h. The self-assembled MnO₂ layer has the synergistic effect of adsorption and catalytic conversion on polysulfides, which can effectively inhibit the shuttle effect. It can also help battery to maintain excellent electrochemical kinetics in the electrochemical cycle and maintain the effective recycling of active substances. As a result, the shuttling of polysulfide is greatly prohibited by this novel functional separator, and cycling stability is outstandingly improved, with a low-capacity decaying of 0.058% after 500 cycles at 0.5C. The rapid and simple modification method proposed in this study has a certain reference value for the future large-scale application of lithium-sulfur battery.

Synthesis of atomic form nickel co-catalysts on TiO2 for improved photocatalysis by RAFT technique

Using the high incompatibility between the neutral stabilizer body and the block polyelectrolyte to drive phase separation during polymerization-induced self-assembly, a modular approach to systematically adjust ionic monomer/polymer solubility was...

Enhanced Catalysis of LIS3· Radical-to-Polysulfide Interconversion via Increased Sulfur Vacancies in Lithium–Sulfur Batteries

The practical application of lithium–sulfur (Li–S) batteries is seriously hindered by severe lithium polysulfide (LiPS) shuttling and sluggish electrochemical conversions. Herein, the Co₉S₈/MoS₂ heterojunction as a model cathode host material is employed to discuss the performance improvement strategy and elucidate the catalytic mechanism. The introduction of sulfur vacancies can harmonize the chemisorption of the heterojunction component. Also, sulfur vacancies induce the generation of radicals, which participate in a liquidus disproportionated reaction to reduce the accumulation of liquid LiPSs. To assess the conversion efficiency from liquid LiPSs to solid Li₂S, a new descriptor calculated from basic cyclic voltammetry curves, nucleation transformation ratio, is proposed.

Sulfur vacancies can harmonize chemisorption and generate amounts of radicals to boost the conversions of LiPSs.

Tessellated N-doped carbon/CoSe2 as trap-catalyst sulfur hosts for room-temperature sodium-sulfur batteries

The construction of highly conductive structure with excellent adsorption-catalytic properties to accelerate electron transfer and suppress polysulfides shuttle is considered as an effective strategy to achieve well-behaved sodium-sulfur batteries. Herein,...

Sulfur Compensation: A Promising Strategy against Capacity Decay in Li-S Batteries

Drastic capacity decay as a result of active sulfur loss caused by the severe shuttle effect of dissolved polysulfides is the main obstacle in the commercial application of Li-S batteries. Various methods have been developed to suppress the active sulfur loss, but the results are far from ideal. Herein, we propose a facile sulfur compensation strategy to improve the cyclic stability of Li-S batteries. The strategy is to compensate sulfur to the cathode by chemical reactions between additional sulfur and lithium polysulfides diffusing away from the cathode. The compensatory sulfur can effectively mitigate the loss of active sulfur in the cathode side caused by the shuttle effect and thus maintain the high capacity of the battery during charging and discharging for long life cycle assessments. Using this strategy, the specific capacity of the assembled Li-S batteries was maintained at >700 mA h g^-1 for more than 500 cycles at 1 C and >1000 mA h g^-1 for ∼100 cycles at 0.1 C, while the capacity of control batteries rapidly decreased to <200 mA h g^-1 under the same conditions.

Lithium-Sulfur Battery Cathode Design: Tailoring Metal-Based Nanostructures for Robust Polysulfide Adsorption and Catalytic Conversion

Lithium-sulfur (Li-S) batteries have a high specific energy capacity and density of 1675 mAh g^-1 and 2670 Wh kg^-1 , respectively, rendering them among the most promising successors for lithium-ion batteries. However, there are myriads of obstacles in the practical application and commercialization of Li-S batteries, including the low conductivity of sulfur and its discharge products (Li₂ S/Li₂ S₂ ), volume expansion of sulfur electrode, and the polysulfide shuttle effect. Hence, immense attention has been devoted to rectifying these issues, of which the application of metal-based compounds (i.e., transition metal, metal phosphides, sulfides, oxides, carbides, nitrides, phosphosulfides, MXenes, hydroxides, and metal-organic frameworks) as sulfur hosts is profiled as a fascinating strategy to hinder the polysulfide shuttle effect stemming from the polar-polar interactions between the metal compounds and polysulfides. This review encompasses the fundamental electrochemical principles of Li-S batteries and insights into the interactions between the metal-based compounds and the polysulfides, with emphasis on the intimate structure-activity relationship corroborated with theoretical calculations. Additionally, the integration of conductive carbon-based materials to ameliorate the existing adsorptive abilities of the metal-based compound is systematically discussed. Lastly, the challenges and prospects toward the smart design of catalysts for the future development of practical Li-S batteries are presented.

Two-dimensional materials towards separator functionalization in advanced Li-S batteries

Functional separators have played important roles in improving the electrochemical performance of lithium-sulfur (Li-S) batteries by addressing the key issues of both the sulfur cathode and lithium anode. Compared with other materials that are used for separator functionalization, two-dimensional (2D) materials with atomic layer thickness and infinite lateral dimensions feature several advantages of ultra-thin laminate structure, remarkable physical properties and tunable surface chemistry, which show potential applications in separator functionalization towards addressing the issues of both the shuttle effect and formation of Li dendrites in Li-S batteries. In this review, the unique advantages of 2D materials for separator functionalization in Li-S batteries and their common construction methods are introduced. Then, recent progress and advances in the construction of 2D materials functional separators are summarized in detail towards inhibiting the shuttle effect of polysulfides and suppressing Li dendrite growth in Li-S batteries. Finally, some opportunities and challenges of 2D materials for constructing high-performance functional separators are proposed. We anticipate that this review will provide new insights into separator functionalization for developing advanced Li-S batteries.

Boron nitride nanosheets wrapped by reduced graphene oxide for promoting polysulfides adsorption in lithium-sulfur batteries

The polysulfides shuttling and slow redox kinetics of sulfur-based cathodes have severely hindered the commercialization of lithium-sulfur (Li-S) batteries. Herein, distinctive three-dimensional microspheres composed of boron nitride (BN) nanosheets and reduced graphene oxide (rGO) were applied to act as efficient sulfur cathode hosts for the first time using in a spray-drying process. Using this construction, the robust microsphere structure could shorten ion diffusion pathways and supply sufficient spaces to alleviate the volumetric expansion of sulfur during lithiation. Besides, the synergistic effect between BN and rGO significantly enhanced polysulfides adsorption capability and accelerated their conversion, verified by the density functional theory (DFT) calculations and adsorption experiments. Consequently, the S-BN@rGO cathode could manifest the high initial capacity (1137 mAh g^-1 at 0.2 C) and remarkable cycling/stability performance (572 mAh g^-1 at 1 C after 500 cycles). These results shed light on a design concept of high-performance sulfur cathode host materials.

A Fe3N/carbon composite electrocatalyst for effective polysulfides regulation in room-temperature Na-S batteries

The practical application of room-temperature Na-S batteries is hindered by the low sulfur utilization, inadequate rate capability and poor cycling performance. To circumvent these issues, here, we propose an electrocatalyst composite material comprising of N-doped nanocarbon and Fe3N. The multilayered porous network of the carbon accommodates large amounts of sulfur, decreases the detrimental effect of volume expansion, and stabilizes the electrodes structure during cycling. Experimental and theoretical results testify the Fe3N affinity to sodium polysulfides via Na-N and Fe-S bonds, leading to strong adsorption and fast dissociation of sodium polysulfides. With a sulfur content of 85 wt.%, the positive electrode tested at room-temperature in non-aqueous Na metal coin cell configuration delivers a reversible capacity of about 1165 mA h g-1 at 167.5 mA g-1, satisfactory rate capability and stable capacity of about 696 mA h g-1 for 2800 cycles at 8375 mA g-1.

All-in-One Sulfur Host: Smart Controls of Architecture and Composition for Accelerated Liquid-Solid Redox Conversion in Lithium-Sulfur Batteries

The development of Li-S batteries (LSBs) is largely impeded by sluggish redox kinetics and notorious polysulfide shuttling. Herein, hierarchical MoC@Ni-NCNT arrays are reported as a multifunctional sulfur host in Li-S batteries, which comprised a flexible carbon fiber cloth substrate decorated with vertical MoC porous nanorods rooted by interconnected nitrogen-doped carbon nanotubes (NCNTs). In the designed host, the inner MoC porous backbone (composed of nanoparticles) along with the in situ-grafted interwoven NCNT shell can greatly maximize the host-guest interactive surface for homogeneous sulfur dispersion, thus realizing decent high-sulfur-loading performance. Ni nanoparticles, encapsulated within NCNTs in the outer shell, act as strong chemical-anchoring centers effectively trap-escaped polysulfides and propel the bidirectional sulfur transformation kinetics. In merit of sufficient adsorption and catalytic sites, the cell configured with the MoC@Ni-NCNT cathode delivers not only high capacity (1421 mA h g^-1 at 0.1 C) but also superior rate performance and ultralong lifespan. The cell can still achieve a superb areal capacity of 6.1 mA h cm² under an increased sulfur loading up to 6 mg cm^-2. This work could open a new avenue for the construction of a multifunctional cathode for high-performance LSBs.