Co4N Nanosheet Assembled Mesoporous Sphere as a Matrix for Ultrahigh Sulfur Content Lithium-Sulfur Batteries

A hierarchically porous CoS2-NiS2 heterojunction for high-performance lithium sulfur batteries

The commercial development of lithium-sulfur batteries (LSBs) has been significantly hindered by the "shuttle effect" and the slow redox kinetics of lithium polysulfides (LiPSs). To solve these problems, researchers have designed a variety of catalysts for the conversion of LiPSs. However, the multi-step lithium polysulfide conversion process is difficult to be effectively solved by a single-function catalyst. In this study, a novel hollow nanocage composite CoS2/NiS2 with heterogeneous interfaces (CoS2-NiS2) is designed to dual modify the cathode host and separator. These CoS2-NiS2 heterojunctions can enhance the trapping ability of LiPSs and improve the wettability, thereby mitigating the shuttling of LiPSs. Besides, the CoS2-NiS2 heterogeneous interface exhibits a strong internal electric field, which effectively enhances the electronic conductivity of the composites. Moreover, this strong electric field at the CoS2-NiS2 heterojunctions offers excellent catalytic ability for sulfur reduction so that can remarkably promote the conversion of LiPSs. Due to these multi-functional characteristics, the CoS2-NiS2 nano-composite demonstrates excellent adsorption and catalytic properties. The as-obtained battery achieves a high initial specific capacity of 1037.4 mAh g-1 and a low decay rate of 0.047 % per-cycle. This study offers a comprehensive understanding of interfacial catalytic mechanisms and their impact on sulfur reaction kinetics, particularly for binary metallic-compounds heterojunctions.

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.

Vertical Graphene Growth on LDH Nanosheets and Carbon Cloth Nanofibers with NiCo Nanoparticles as a Freestanding Host for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries have been recognized as one of the excellent candidates for next-generation energy storage batteries because of their high energy density and low cost and low pollution. However, lithium-sulfur batteries have been challenged by low conductivity, low sulfur utilization, poor cycle life, and the shuttle effect of polysulfides. To address these problems, we report here an independent mixed sulfur host. First, NiCoAl-layered double hydroxide (LDH) nanosheets were uniformly grown on carbon cloth (CC) by a hydrothermal method. Then, vertical graphene (VG) was uniformly vertically grown on the composite structures to form VG@LDH/CC by a plasma enhanced chemical vapor deposition (PECVD) method. Graphene and LDH nanosheets forming a three-dimensional mesh structure can effectively physically block lithium polysulfides, store singlet sulfur, and improve the conductivity of the cathode. In addition, during the growth of graphene, the Ni and Co ions in the LDH nanosheets are reduced to NiCo nanoparticles, which can enhance the chemical adsorption of polysulfides, thus effectively mitigating the "shuttle effect" and improving the electrical conductivity of the material. The lithium sulfur batteries with derived sulfur anodes (VG@LDH/CC-S) exhibited excellent electrochemical properties, including excellent rate performance (780.8 mAh g-1 at 3C) and impressive cycling stability (capacity decay of about 0.0755% per cycle after 750 cycles at 0.5C).

Metal-N Coordination in Lithium-Sulfur Batteries: Inhibiting Catalyst Passivation

Lithium-sulfur (Li-S) batteries exhibit great potential as the next-generation energy storage techniques. Application of catalyst is widely adopted to accelerate the redox kinetics of polysulfide conversion reactions and improve battery performance. Although significant attention has been devoted to seeking new catalysts, the problem of catalyst passivation remains underexplored. Herein, we find that metal-N coordination has a previously overlooked role in preventing the catalyst passivation. In the case of nickel, the Ni catalyst reacts with S8 to produce NiSx compounds on the surface, leading to catalyst passivation and slow the kinetics of LiPSs conversion. In contrast, when Ni is coordinated with N (typically Ni-N4), S8 remains stable on the surface. The Ni-N4 exhibits excellent resistance to passivation and rapid kinetics of LiPSs conversion. Consequently, the sulfur cathode with Ni-N4 exhibits a high rate capability of 604.11 mAh g-1 at 3 C and maintains a low capacity decay rate of 0.046 % per cycle over 1000 cycles at 2 C. Furthermore, preventing S passivation in M-N coordination applies not only to Ni-N4 but also to various coordination numbers and transition metals. This study reveals a new aspect of metal-N coordination in inhibiting catalyst passivation, improving our understanding of catalysts in Li-S batteries.

CeO2/Co heterostructure encapsulated in hollow necklace-like carbon fiber as an advanced host material for high-performance lithium-sulfur batteries

The shuttle effect of lithium polysulfides (LiPSs) and the sluggish reaction kinetics of LiPSs conversion pose serious challenges to the commercial feasibility of lithium-sulfur (Li-S) batteries. To address these obstacles, herein, we construct CeO2/Co heterostructures in hollow necklace-like carbon fibers (CeO2/Co-CNFs) as the cathode host material for Li-S batteries. The specific surface area of fibers is significantly enhanced by using a template, thereby promoting the utilization efficiency of sulfur. Meanwhile, CeO2/Co-CNFs show strong conductivity, effective adsorption to LiPSs, and robust catalytic activity for LiPSs conversion. As a result, the Li-S battery with CeO2/Co-CNFs displays 961 mAh g-1 at 0.2 C, with an 86 % capacity retention rate after 100 cycles. At 2.0 C current density, the composite cathode maintains an initial discharge capacity of 782 mAh g-1, with a mere 0.044 % capacity loss per cycle. Furthermore, in situations with limited electrolytes, high sulfur loading, and high areal mass loading, the composite cathode can provide a high areal capacity of 6.2 mg cm-2 over 100 cycles. This work provides a useful approach for investigating high-performance Li-S battery cathodes.

Regulating Electron Filling and Orbital Occupancy of Anti-Bonding States of Transition Metal Nitride Heterojunction for High Areal Capacity Lithium-Sulfur Full Batteries

The commercialization of lithium-sulfur (Li-S) battery is seriously hindered by the shuttle behavior of lithium (Li) polysulfide, slow conversion kinetics, and Li dendrite growth. Herein, a novel hierarchical p-type iron nitride and n-type vanadium nitride (p-Fe2N/n-VN) heterostructure with optimal electronic structure, confined in vesicle-like N-doped nanofibers (p-Fe2N/n-VN⊂PNCF), is meticulously constructed to work as "one stone two birds" dual-functional hosts for both the sulfur cathode and Li anode. As demonstrated, the d-band center of high-spin Fe atom captures more electrons from V atom to realize more π* and moderate σ* bond electron filling and orbital occupation; thus, allowing moderate adsorption intensity for polysulfides and more effective d-p orbital hybridization to improve reaction kinetics. Meanwhile, this unique structure can dynamically balance the deposition and transport of Li on the anode; thereby, more effectively inhibiting Li dendrite growth and promoting the formation of a uniform solid electrolyte interface. The as-assembled Li-S full batteries exhibit the conspicuous capacities and ultralong cycling lifespan over 2000 cycles at 5.0 C. Even at a higher S loading (20 mg cm-2) and lean electrolyte (2.5 µL mg-1), the full cells can still achieve an ultrahigh areal capacity of 16.1 mAh cm-2 after 500 cycles at 0.1 C.

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.

Reinforcing the Adsorption and Conversion of Polysulfides in Li-S Battery by Incorporating Molybdenum into MnS/MnO Nanorods

The Sabatier principle suggests that an excessive adsorption of lithium polysulfides (LiPSs) by metal compounds may hinder their conversion in the absence of a conversion module. Therefore, it is imperative to establish a synergetic effect mechanism between "strong adsorption" and "rapid conversion" for LiPSs. To achieve this coexistence, a molybdenum-doped MnS/MnO@C porous structure is designed as a multifunctional coating on the polypropylene (PP) separator. The incorporation of MnS/MnO@C enhances the adsorption capacity towards LiPSs, while molybdenum facilitates subsequent conversion. Benefiting from the synergistic effect of each component and its large specific surface area, the cell with Mo-doped MnS/MnO@C coating achieves smooth adsorption-diffusion-conversion processes and exhibits an appreciable rate performance with outstanding cycling stability. Even when sulfur loading increases to 9.68 mg cm-2 , the modified battery delivers an excellent initial areal capacity of 11.69 mAh cm-2 and maintains 6.97 mAh cm-2 after 50 cycles at 0.1 C. This study presents a promising approach to simultaneously accomplish "strong adsorption" and "rapid conversion" of polysulfides, offering novel perspectives for devising dual-functional modified separators.

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.

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.

Single-Atomic Dispersion of Fe and Co Supported on Reduced Graphene Oxide for High-Performance Lithium-Sulfur Batteries

High theoretical energy density and low cost make lithium-sulfur (LSB) batteries a promising system for next-generation energy storage. LSB performance largely depends on efficient reversible conversion of elemental sulfur to Li2S. Here, well-designed sulfur host materials including Fe or Co single atoms embedded on N-doped reduced graphene oxide (MNC/G with M = Fe or Co) are proposed to tackle the LSB challenges and enhance the electrochemical performance. Using a combination of Mössbauer spectroscopy and high-resolution scanning electron microscopy, the atomic dispersion of Co and Fe was revealed up to relatively high mass loadings. After optimization of the electrolyte/sulfur (E/S) ratio, FeNC/G shows the most promising cycle performance combining a constant high discharge capacity at low E/S values with the lowest polarization. In particular, the material FeNC/G@S with a high sulfur loading (9.4 mg cm-2) delivers a high area capacity of 7.7 mAh cm-2 under lean electrolyte conditions (6 mL g-1).

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.

Recent Progress for Concurrent Realization of Shuttle-Inhibition and Dendrite-Free Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have become one of the most promising new-generation energy storage systems owing to their ultrahigh energy density (2600 Wh kg-1 ), cost-effectiveness, and environmental friendliness. Nevertheless, their practical applications are seriously impeded by the shuttle effect of soluble lithium polysulfides (LiPSs), and the uncontrolled dendrite growth of metallic Li, which result in rapid capacity fading and battery safety problems. A systematic and comprehensive review of the cooperative combination effect and tackling the fundamental problems in terms of cathode and anode synchronously is still lacking. Herein, for the first time, the strategies for inhibiting shuttle behavior and dendrite-free Li-S batteries simultaneously are summarized and classified into three parts, including "two-in-one" S-cathode and Li-anode host materials toward Li-S full cell, "two birds with one stone" modified functional separators, and tailoring electrolyte for stabilizing sulfur and lithium electrodes. This review also emphasizes the fundamental Li-S chemistry mechanism and catalyst principles for improving electrochemical performance; advanced characterization technologies to monitor real-time LiPS evolution are also discussed in detail. The problems, perspectives, and challenges with respect to inhibiting the shuttle effect and dendrite growth issues as well as the practical application of Li-S batteries are also proposed.

Boosting Lean Electrolyte Lithium-Sulfur Battery Performance with Transition Metals: A Comprehensive Review

Lithium-sulfur (Li-S) batteries have received widespread attention, and lean electrolyte Li-S batteries have attracted additional interest because of their higher energy densities. This review systematically analyzes the effect of the electrolyte-to-sulfur (E/S) ratios on battery energy density and the challenges for sulfur reduction reactions (SRR) under lean electrolyte conditions. Accordingly, we review the use of various polar transition metal sulfur hosts as corresponding solutions to facilitate SRR kinetics at low E/S ratios (< 10 µL mg-1), and the strengths and limitations of different transition metal compounds are presented and discussed from a fundamental perspective. Subsequently, three promising strategies for sulfur hosts that act as anchors and catalysts are proposed to boost lean electrolyte Li-S battery performance. Finally, an outlook is provided to guide future research on high energy density Li-S batteries.

Towards Practical Application of Li-S Battery with High Sulfur Loading and Lean Electrolyte: Will Carbon-Based Hosts Win This Race?

As the need for high-energy-density batteries continues to grow, lithium-sulfur (Li-S) batteries have become a highly promising next-generation energy solution due to their low cost and exceptional energy density compared to commercially available Li-ion batteries. Research into carbon-based sulfur hosts for Li-S batteries has been ongoing for over two decades, leading to a significant number of publications and patents. However, the commercialization of Li-S batteries has yet to be realized. This can be attributed, in part, to the instability of the Li metal anode. However, even when considering just the cathode side, there is still no consensus on whether carbon-based hosts will prove to be the best sulfur hosts for the industrialization of Li-S batteries. Recently, there has been controversy surrounding the use of carbon-based materials as the ideal sulfur hosts for practical applications of Li-S batteries under high sulfur loading and lean electrolyte conditions. To address this question, it is important to review the results of research into carbon-based hosts, assess their strengths and weaknesses, and provide a clear perspective. This review systematically evaluates the merits and mechanisms of various strategies for developing carbon-based host materials for high sulfur loading and lean electrolyte conditions. The review covers structural design and functional optimization strategies in detail, providing a comprehensive understanding of the development of sulfur hosts. The review also describes the use of efficient machine learning methods for investigating Li-S batteries. Finally, the outlook section lists and discusses current trends, challenges, and uncertainties surrounding carbon-based hosts, and concludes by presenting our standpoint and perspective on the subject.

Binary Sulfiphilic Nickel Boride on Boron-Doped Graphene with Beneficial Interfacial Charge for Accelerated Li-S Dynamics

The "shuttle effect" and slow conversion kinetics of lithium polysulfides (LiPSs) are stumbling block for high-energy-density lithium-sulfur batteries (LSBs), which can be effectively evaded by advanced catalytic materials. Transition metal borides possess binary LiPSs interactions sites, aggrandizing the density of chemical anchoring sites. Herein, a novel core-shelled heterostructure consisting of nickel boride nanoparticles on boron-doped graphene (Ni₃ B/BG), is synthesized through a graphene spontaneously couple derived spatially confined strategy. The integration of Li₂ S precipitation/dissociation experiments and density functional theory computations demonstrate that the favorable interfacial charge state between Ni₃ B and BG provides smooth electron/charge transport channel, which promotes the charge transfer between Li₂ S₄ -Ni₃ B/BG and Li₂ S-Ni₃ B/BG systems. Benefitting from these, the facilitated solid-liquid conversion kinetics of LiPSs and reduced energy barrier of Li₂ S decomposition are achieved. Consequently, the LSBs employed the Ni₃ B/BG modified PP separator deliver conspicuously improved electrochemical performances with excellent cycling stability (decay of 0.07% per cycle for 600 cycles at 2 C) and remarkable rate capability of 650 mAh g^-1 at 10 C. This study provides a facile strategy for transition metal borides and reveals the effect of heterostructure on catalytic and adsorption activity for LiPSs, offering a new viewpoint to apply boride in LSBs.

Anchoring and Catalytic Effects of rGO Supported VS2 Nanosheets Enable High-Performance Li-Organosulfur Battery

As a high-energy-density cathode material, organosulfur has great potential for lithium batteries. However, their practical application is plagued by electronic/ionic insulation and sluggish redox kinetics. Hence, our strategy is to design a self-weaving, freestanding host material by introducing reduced graphene oxide-supported VS₂ nanosheets (VS₂ -rGO) and carbon nanotubes (CNTs) for lithium-phenyl tetrasulfide (Li-PTS) batteries. Unique host materials not only provide physicochemical confinement of active materials to boost the utilization but also catalyze the conversion of active materials to accelerate redox kinetics. Therefore, Li-PTS cell based on the 3D VS₂ -rGO-CNTs (VSGC) host material shows excellent cyclability, with a slow capacity decay rate of 0.08% per cycle over 500 cycles at 0.5 C, and a high areal capacity of 3.1 mAh cm^-2 with the PTS loading of 7.2 mg cm^-2 . More importantly, the potential for practical applications is highlighted by the flexible pouch cell with a high areal capacity (4.1 mAh cm^-2 ) and a low electrolyte/PTS ratio (3.5 µL mg^-1 ). This work sheds light on elevating the electrochemical performance of Li-organosulfur batteries through the effective catalytic and adsorbed host material.

Coordinating Interface Polymerization with Micelle Mediated Assembly Towards Two-Dimensional Mesoporous Carbon/CoNi for Advanced Lithium-Sulfur Battery

Lithium-sulfur battery has attracted significant attention by virtues of their high theoretical energy density, natural abundance, and environmental friendliness. However, the notorious shuttle effect of polysulfides intermediates severely hinders its practical application. Herein, a novel 2D mesoporous N-doped carbon nanosheet with confined bimetallic CoNi nanoparticles sandwiched graphene (mNC-CoNi@rGO) is successfully fabricated through a coordinating interface polymerization and micelle mediated co-assembly strategy. mNC-CoNi@rGO serves as a robust host material that endows lithium-sulfur batteries with a high reversible capacity of 1115 mAh g^-1 at 0.2 C after 100 cycles, superior rate capability, and excellent cycling stability with 679.2 mAh g^-1 capacity retention over 700 cycles at 1 C. With sulfur contents of up to 5.0 mg cm^-2 , the area capacity remains to be 5.1 mAh cm^-2 after 100 cycles at 0.2 C. The remarkable performance is further resolved via a series of experimental characterizations combined with density functional theory calculations. These results reveal that the ordered mesoporous N-doped carbon-encapsulated graphene framework acts as the ion/electron transport highway with excellent electrical conductivity, while bimetallic CoNi nanoparticles enhance the polysulfides adsorption and catalytic conversion that simultaneously accelerate the multiphase sulfur/polysulfides/sulfides conversion and inhibit the polysulfides shuttle.

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).

Tailoring Lithium Fluoride Interface for Dendrite-Free Lithium Anode to Prolong the Cyclic Stability of Lithium-Sulfur Pouch Cells

Lithium-sulfur (Li-S) cells have been regarded as attractive alternatives to achieve higher energy densities because of their theoretical specific energy far beyond the lithium-ion cells. However, the achieved results of Li-S cells are exaggerating the cycle performance in their pouch formats because the considerable works are based on the coin cells where flood electrolyte and endless Li supply ensure the Li metal with nature structure features, resulting in a negligible effect on cycle performance caused by the Li dendrites and electrolyte dissipation during cycles. Herein, we demonstrate a strategy to enable the Li metal with lithium fluoride (LiF)-rich solid electrolyte interface via integrating a reinforced interface (RI) embedded with nano-LiF particles on the surface of the Li metal anode. The RI interface enables the solvent molecules of the electrolyte to gain fewer electrons from Li anode, resulting in a lower leakage current of assembled RI||Li-S cell (~ 0 μA) than pristine Li anode (~ 1.15 µA). Moreover, these results show that suppressing lithium dendrite growth is more urgent than inhibiting the shuttle effect of polysulfides in the pouch cell format. As a result, the RI layer-engineered Li metal bears witness to the cyclic stability of Li anode over 800 h, thus achieving stable cycles of Ah-scale Li-S pouch cell with an energy density of 410 Wh/kg at a current of 200 mA per cell. Our study demonstrates that the suppression of lithium dendrites by the RI could be a promising method to prolong the cycle number of Li-S pouch cells.

Ultrathin Two-Dimensional Fe-Co Bimetallic Oxide Nanosheets for Separator Modification of Lithium-Sulfur Batteries

The shuttle effect is understood to be the most significant issue that needs to be solved to improve the performance of lithium-sulfur batteries. In this study, ultrathin two-dimensional Fe-Co bimetallic oxide nanosheets were prepared using graphene as a template, which could rapidly catalyze the conversion of polysulfides and inhibit the shuttle effect. Additionally, such ultrathin nanostructures based on graphene provided sufficient active sites and fast diffusion pathways for lithium ions. Taking into account the aforementioned benefits, the ultrathin two-dimensional Fe-Co bimetallic oxide nanosheets modified separator assembled lithium-sulfur batteries delivered an incredible capacity of 1044.2 mAh g^-1 at 1 C and retained an excellent reversible capacity of 859.4 mAh g^-1 after 100 cycles. Even under high loading, it still achieved high area capacity and good cycle stability (92.6% capacity retention).

Understanding the Catalytic Kinetics of Polysulfide Redox Reactions on Transition Metal Compounds in Li-S Batteries

Because of their high energy density, low cost, and environmental friendliness, lithium-sulfur (Li-S) batteries are one of the potential candidates for the next-generation energy-storage devices. However, they have been troubled by sluggish reaction kinetics for the insoluble Li₂S product and capacity degradation because of the severe shuttle effect of polysulfides. These problems have been overcome by introducing transition metal compounds (TMCs) as catalysts into the interlayer of modified separator or sulfur host. This review first introduces the mechanism of sulfur redox reactions. The methods for studying TMC catalysts in Li-S batteries are provided. Then, the recent advances of TMCs (such as metal oxides, metal sulfides, metal selenides, metal nitrides, metal phosphides, metal carbides, metal borides, and heterostructures) as catalysts and some helpful design and modulation strategies in Li-S batteries are highlighted and summarized. At last, future opportunities toward TMC catalysts in Li-S batteries are presented.

Carbon Coated Metal-Based Composite Electrode Materials for Lithium Sulfur Batteries: A Review

Lithium-sulfur battery is one of the most promising secondary battery systems due to their high energy density and low material cost. During the past decade, great progress has been achieved in promoting the performances of Li-S batteries by addressing the challenges at the laboratory-level model systems. With growing attention paid to the application of Li-S batteries, new challenges at practical cell scales emerge as the bottleneck. However, challenges remain for the commercialization of lithium-sulfur batteries. The current review mainly focused on metal-based catalysts decorated-carbon materials for enhanced lithium sulfur battery performance. Firstly, the synthesis methods of various carbon-sulfur composites are discussed, as well as the influence of different material structures on the electrochemical performance. Secondly, a variety of catalysts, including metal atoms, metal oxides, sulfides, phosphides, nitrides, and carbide-decorated carbon nanomaterials, are systematically introduced to determine how lithium can be enhanced by suppressing polysulfides and promoting redox conversion reactions. Also, analyzed the multi-step electrochemical reaction mechanism of the battery during the charging and discharging process, and provide a feasible path for the practical application of high energy density lithium-sulfur batteries.

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.

Copper Zinc Sulfide (CuZnS) Quantum Dot-Decorated (NiCo)–S/Conductive Carbon Matrix as the Cathode for Li–S Batteries

Sulfur composites consisting of electrochemical reactive catalysts/conductive materials are investigated for use in lithium–sulfur (Li–S) batteries (LSBs). In this paper, we report the synthesis, physicochemical and electrochemical properties of CuZnS quantum dots (CZSQDs) decorated with nickel–cobalt–sulfide ((NiCo)–S)) mixed with reduced graphene oxide (rGO)/oxidized carbon nanotube (oxdCNT) (rGO/oxdCNT) ((NiCo)–S@rGO/oxdCNT) composites. These composites are for the purpose of being the sulfur host cathode in Li–S batteries. The as-prepared composites showed a porous structure with the CZSQDs being uniformly found on the surface of the rGO/oxdCNT, which had a specific surface area of 26.54 m²/g. Electrochemical studies indicated that the (NiCo)–S@rGO/oxdCNT cells forming the cathode exhibited a maximum capacity of 1154.96 mAhg⁻¹ with the initial discharge at 0.1 C. The smaller size of the CZSQDs (~10 nm) had a positive effect on the CZSQDs@(NiCo)–S@rGO/oxdCNT composites in that they had a higher initial discharge capacity of 1344.18 mAhg⁻¹ at 0.1 C with the Coulombic efficiency being maintained at almost 97.62% during cycling. This latter property is approximately 1.16 times more compared to the absence of the Cu–Zn–S QD loading. This study shows that the CuZnS quantum dots decorated with a (NiCo)–S@rGO/oxdCNT supporting matrix-based sulfur cathode have the potential to improve the performance of future lithium–sulfur batteries.

Ni-CeO2 Heterostructures in Li-S Batteries: A Balancing Act between Adsorption and Catalytic Conversion of Polysulfide

Lithium-sulfur (Li-S) batteries have attracted considerable attention over the last two decades because of a high energy density and low cost. However, the wide application of Li-S batteries has been severely impeded due to the poor electrical conductivity of S, shuttling effect of soluble lithium polysulfides (LiPSs), and sluggish redox kinetics of S species, especially under high S loading. To address all these issues, a Ni-CeO₂ heterostructure-doped carbon nanofiber (Ni-CeO₂ -CNF) is developed as an S host that combines the strong adsorption with the high catalytic activity and the good electrical conductivity, where the LiPSs anchored on the heterostructure surface can directly gain electrons from the current collector and realize a fast conversion between S₈ and Li₂ S. Therefore, Li-S batteries with S@Ni-CeO₂ -CNF cathodes exhibit superior long-term cycling stability, with a capacity decay of 0.046% per cycle over 1000 cycles, even at 2 C. Noteworthy, under a sulfur loading up to 6 mg cm^-2 , a high reversible areal capacity of 5.3 mAh cm^-2 can be achieved after 50 cycles at 0.1 C. The heterostructure-modified S cathode effectively reconciles the thermodynamic and kinetic characteristics of LiPSs for adsorption and conversion, furthering the development of high-performance Li-S batteries.

Understanding the interactions between lithium polysulfides and anchoring materials in advanced lithium-sulfur batteries using density functional theory

Lithium-sulfur batteries (LSBs) are promising energy storage devices because of their high theoretical capacity and energy density. However, the "shuttle" effect in lithium polysulfides (LiPSs) is an unresolved issue that can hinder their practical commercial application. Research on LSBs has focused on finding appropriate materials that suppress this effect by efficiently anchoring the LiPSs intermediates. Quantum chemical computations are a useful tool for understanding the mechanistic details of chemical interaction involving LiPSs, and they can also offer strategies for the rational design of LiPSs anchoring materials. In this perspective, we highlight computational and theoretical work performed on this topic. This includes elucidating and characterizing the adsorption mechanisms, and the dominant types of interactions, and summarizing the binding energies of LiPSs on anchoring materials. We also give examples and discuss the potential of descriptors and machine learning approaches to predict the adsorption strength and reactivity of materials. We believe that both approaches will become indispensable in modelling future LSBs.

Recent Advances and Strategies toward Polysulfides Shuttle Inhibition for High-Performance Li-S Batteries

Lithium-sulfur (Li-S) batteries are regarded as the most promising next-generation energy storage systems due to their high energy density and cost-effectiveness. However, their practical applications are seriously hindered by several inevitable drawbacks, especially the shuttle effects of soluble lithium polysulfides (LiPSs) which lead to rapid capacity decay and short cycling lifespan. This review specifically concentrates on the shuttle path of LiPSs and their interaction with the corresponding cell components along the moving way, systematically retrospect the recent advances and strategies toward polysulfides diffusion suppression. Overall, the strategies for the shuttle effect inhibition can be classified into four parts, including capturing the LiPSs in the sulfur cathode, reducing the dissolution in electrolytes, blocking the shuttle channels by functional separators, and preventing the chemical reaction between LiPSs and Li metal anode. Herein, the fundamental aspect of Li-S batteries is introduced first to give an in-deep understanding of the generation and shuttle effect of LiPSs. Then, the corresponding strategies toward LiPSs shuttle inhibition along the diffusion path are discussed step by step. Finally, general conclusions and perspectives for future research on shuttle issues and practical application of Li-S batteries are proposed.

Multifunctional integrated VN/V2O5 heterostructure sulfur hosts for advanced lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries show great potential in future electric transportation and large-scale grid storage applications because of their attractive theoretical energy density (2600 W h kg^-1) and relatively abundant sulfur reserves. However, the rapid capacity decay and unsatisfactory sulfur loading caused by the lithium polysulphide (LiPS) dissolution and low electrical conductivity of sulfur are the most urgent issues plaguing its practical applications. Herein, we report a multifunctional nanoporous (NP) VN/V₂O₅ binary host that can efficiently resolve the above conflicts by the synergy between the functions of two materials. The inner V₂O₅ facilitates rapid trapping of numerous LiPSs while the outer porous VN with abundant NP channels offers high conductivity and mild chemisorption, thereby improving the localization and catalytic conversion ability of LiPSs. Accordingly, the designed cathodes with 1.87 mg cm^-2 sulfur content achieve an acceptable areal specific capacity (2.72 mA h cm^-2), excellent rate capability (963 mA h g^-1 at 5.0C), and cycling stability. Remarkably, the cathodes with ultrahigh sulfur loading of 9.02 mg cm^-2 deliver a satisfactory areal specific capacity (12.12 mA h cm^-2) and still maintain excellent durability.

CoNiO2 /Co4 N Heterostructure Nanowires Assisted Polysulfide Reaction Kinetics for Improved Lithium-Sulfur Batteries

The "shuttle effect" of soluble polysulfides and slow reaction kinetics hinder the practical application of Li-S batteries. Transition metal oxides are promising mediators to alleviate these problems, but the poor electrical conductivity limits their further development. Herein, the homogeneous CoNiO2 /Co4 N nanowires have been fabricated and employed as additive of graphene based sulfur cathode. Through optimizing the nitriding degree, the continuous heterostructure interface can be obtained, accompanied by effective adjustment of energy band structure. By combining the strong adsorptive and catalytic properties of CoNiO2 and electrical conductivity of Co4 N, the in situ formed CoNiO2 /Co4 N heterostructure reveals a synergistic enhancement effect. Theoretical calculation and experimental design show that it can not only significantly inhibit "shuttle effect" through chemisorption and catalytic conversion of polysulfides, but also improve the transport rate of ions and electrons. Thus, the graphene composite sulfur cathode supported by these CoNiO2 /Co4 N nanowires exhibits improved sulfur species reaction kinetics. The corresponding cell provides a high rate capacity of 688 mAh g-1 at 4 C with an ultralow decaying rate of ≈0.07% per cycle over 600 cycles. The design of heterostructure nanowires and graphene composite structure provides an advanced strategy for the rapid capture-diffusion-conversion process of polysulfides.

Co4 N-Decorated 3D Wood-Derived Carbon Host Enables Enhanced Cathodic Electrocatalysis and Homogeneous Lithium Deposition for Lithium-Sulfur Full Cells

The sluggish kinetics of sulfur conversion in the cathode and the nonuniform deposition of lithium metal at the anode result in severe capacity decay and poor cycle life for lithium-sulfur (Li-S) batteries. Resolving these deficiencies is the most direct route toward achieving practical cells of this chemistry. Herein, a vertically aligned wood-derived carbon plate decorated with Co₄ N nanoparticles host (Co₄ N/WCP) is proposed that can serve as a host for both the sulfur cathode and the metallic lithium anode. This Co₄ N/WCP electrode host drastically enhances the reaction kinetics in the sulfur cathode and homogenizes the electric field at the anode for the uniform lithium plating. Density functional theory calculations confirm the experimental observations that Co₄ N/WCP provides a lower energy barrier for the polysulfide redox reaction in the cathode and a low adsorption energy for lithium deposition at the anode. Employing the Co₄ N/WCP host at both electrodes in a S@Co₄ N/WCP||Li@Co₄ N/WCP full cell delivers a specific capacity of 807.9 mAh g^-1 after 500 cycles at a 1 C rate. Additional experiments are performed with high areal sulfur loading of 4 mg cm^-2 to demonstrate the viability of this strategy for producing practical Li-S cells.

Customized Structure Design and Functional Mechanism Analysis of Carbon Spheres for Advanced Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are attracting much attention due to their high theoretical energy density and are considered to be the predominant competitors for next-generation energy storage systems. The practical commercial application of LSBs is mainly hindered by the severe "shuttle effect" of the lithium polysulfides (LiPSs) and the serious damage of lithium dendrites. Various carbon materials with different characteristics have played an important role in overcoming the above-mentioned problems. Carbon spheres (CSs) are extensively explored to enhance the performance of LSBs owing to their superior structures. The review presents the state-of-the-art advances of CSs for advanced high-energy LSBs, including their preparation strategies and applications in inhibiting the "shuttle effect" of the LiPSs and protecting lithium anodes. The unique restriction effect of CSs on LiPSs is explained from three working mechanisms: physical confinement, chemical interaction, and catalytic conversion. From the perspective of interfacial engineering and 3D structure designing, the protective effect of CSs on the lithium anode is also analyzed. Not only does this review article contain a summary of CSs in LSBs, but also future directions and prospects are discussed. The systematic discussions and suggested directions can enlighten thoughts in the reasonable design of CSs for LSBs in near future.

Single-dispersed polyoxometalate clusters embedded on multilayer graphene as a bifunctional electrocatalyst for efficient Li-S batteries

The redox reactions occurring in the Li-S battery positive electrode conceal various and critical electrocatalytic processes, which strongly influence the performances of this electrochemical energy storage system. Here, we report the development of a single-dispersed molecular cluster catalyst composite comprising of a polyoxometalate framework ([Co₄(PW₉O₃₄)₂]¹⁰⁻) and multilayer reduced graphene oxide. Due to the interfacial charge transfer and exposure of unsaturated cobalt sites, the composite demonstrates efficient polysulfides adsorption and reduced activation energy for polysulfides conversion, thus serving as a bifunctional electrocatalyst. When tested in full Li-S coin cell configuration, the composite allows for a long-term Li-S battery cycling with a capacity fading of 0.015% per cycle after 1000 cycles at 2 C (i.e., 3.36 A g⁻¹). An areal capacity of 4.55 mAh cm⁻² is also achieved with a sulfur loading of 5.6 mg cm⁻² and E/S ratio of 4.5 μL mg⁻¹. Moreover, Li-S single-electrode pouch cells tested with the bifunctional electrocatalyst demonstrate a specific capacity of about 800 mAh g⁻¹ at a sulfur loading of 3.6 mg cm⁻² for 100 cycles at 0.2 C (i.e., 336 mA g⁻¹) with E/S ratio of 5 μL mg⁻¹.

Efficient electrochemical energy storage in Li-S batteries is hindered by sluggish sulfur redox reactions. Here, the authors propose a polyoxometalate/multilayer graphene composite as a bifunctional electrocatalyst for battery performance improvement.

Highly catalytically active CoSe2 supported on nitrogen-doped three dimensional porous carbon as a cathode for high-stability lithium-sulfur battery

Lithium-sulfur batteries, promising secondary energy storage devices, were mainly limited by its unsatisfactory cyclability owing to inefficient reversible conversion of sulfur and lithium sulfide on the cathode during the discharge/charging process. In this study, nitrogen-doped three-dimensional porous carbon material loaded with CoSe 2 nanoparticles (CoSe 2 -PNC) is developed as a cathode for lithium-sulfur battery application. A combination of CoSe 2 and nitrogen-doped porous carbon can efficiently improve the cathode activity and its conductivity, resulting in enhanced redox kinetics of the charge/discharge process. The obtained electrode exhibits a high discharge specific capacity of 1139.6 mAh g -1 at a current density of 0.2 C. After 100 cycles, its capacity remained at 865.7 mAh g -1 corresponding to a capacity retention of 75.97%. In a long-term cycling test, a discharge specific capacity of 546.7 mAh g -1 was observed after 300 cycles performed at a current density of 1 C.

Polysulfide Catalytic Materials for Fast-Kinetic Metal-Sulfur Batteries: Principles and Active Centers

Benefiting from the merits of low cost, ultrahigh-energy densities, and environmentally friendliness, metal-sulfur batteries (M-S batteries) have drawn massive attention recently. However, their practical utilization is impeded by the shuttle effect and slow redox process of polysulfide. To solve these problems, enormous creative approaches have been employed to engineer new electrocatalytic materials to relieve the shuttle effect and promote the catalytic kinetics of polysulfides. In this review, recent advances on designing principles and active centers for polysulfide catalytic materials are systematically summarized. At first, the currently reported chemistries and mechanisms for the catalytic conversion of polysulfides are presented in detail. Subsequently, the rational design of polysulfide catalytic materials from catalytic polymers and frameworks to active sites loaded carbons for polysulfide catalysis to accelerate the reaction kinetics is comprehensively discussed. Current breakthroughs are highlighted and directions to guide future primary challenges, perspectives, and innovations are identified. Computational methods serve an ever-increasing part in pushing forward the active center design. In summary, a cutting-edge understanding to engineer different polysulfide catalysts is provided, and both experimental and theoretical guidance for optimizing future M-S batteries and many related battery systems are offered.

Organosulfur polymer-based cathode materials for rechargeable batteries

Organosulfur polymer cathode materials have shown promising electrochemical performances in rechargeable batteries. This review covers recent developments of the polymer cathodes and the remaining challenges and future prospects are discussed.

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.

Combined enhanced redox kinetics and physiochemical confinement in three-dimensionally ordered macro/mesoporous TiN for highly stable lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries with tremendous energy density possess great promise for the next-generation energy storage devices. Even though, the shuttle effect and sluggish redox kinetics of lithium polysulfides (LiPSs) seriously restrict practical applications of Li-S batteries. Herein, a three-dimensionally ordered macro/mesoporous TiN (3DOM TiN) nanostructure is established via using poly (methyl methacrylate) PMMA spheres as template. The interconnected macro/mesoporous channels are constructed to effectively alleviate the stacking of composite materials and render a large portion of inherent active sites exposed on the surface region. Moreover, TiN exhibits high electrical conductivity, which efficiently enhances charge-transfer kinetics and guarantees the favorable electrochemical performance of sulfur cathode. More importantly, the as-prepared 3DOM TiN suppresses the shuttle effect and improves the redox kinetics significantly due to strong affinity toward LiPSs. Attributed to these unique features, the S/3DOM TiN electrode achieves an ultrahigh initial discharge capacity of 1187 mAh g^-1at 0.2 C, and stable cycling performance of 552 mAh g^-1over 500 cycles at 1 C. Meanwhile, the discharge capacity retention of 701 mAh g^-1(3.5 mAh cm^-2) can be endowed for the S/3DOM TiN electrode under high sulfur loading of 5 mg cm^-2after 100 cycles at 0.1 C. Therefore, the 3DOM TiN nanostructure electrocatalyst provides a promising path for developing practically useable Li-S batteries.

Efficient capture and conversion of polysulfides by zinc protoporphyrin framework-embedded triple-layer nanofiber separator for advanced Li-S batteries

The practical application of Lithium-sulfur (Li-S) batteries is significantly inhibited by (i) the notable 'shuttle effect' of lithium polysulfides (LiPS), (ii) the corrosion of the lithium interface, and (iii) the sluggish redox reaction kinetics. The functional separator in the Li-S battery has the potential to provide the perfect solution to these problems. Herein a triple-layer multifunctional PVDF-based nanofiber separator, which contains GoTiN/PVDF layer on the top and bottom and ZnTPP/PVDF layer on the middle, is designed. The polarity and porous structure of this multifunctional separator can greatly improve the wettability of electrolytes and enhance the transportation of Li⁺. With the zinc-based porphyrin framework (ZnTPP) structure, this separator has a strong chemisorption and LiPS conversion ability, which greatly prevent the 'shuttle effect'. Consequently, the designed multilayer separator showed excellent electrochemical performance. As a result, the cell with GoTiN@ZnTPP@GoTiN nanofiber membrane displayed an initial discharge capacity of 1180 mAh/g with a benign capacity retention of 65.9% at 0.5C and high coulombic efficiency of more than 98.5% after 100 cycles. Even at 2C, it can still release a capacity of 798 mAh/g. Moreover, the remarkable capacity of 591 mAh/g could be achieved with a high sulfur load of 5.76 mg/cm² under a current density of 0.1C. Based on these merits, this novel and scalable multifunctional separator is a promising candidate to replace the conventional PP separator for advanced Li-S batteries to deal with various challenges.

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.

Embedding Cobalt Atom Clusters in CNT-Wired MoS2 Tube-in-Tube Nanostructures with Enhanced Sulfur Immobilization and Catalyzation for Li-S Batteries

Lithium-sulfur batteries are one of the most promising next-generation energy storage systems. The efficient interconversion between sulfur/lithium polysulfides and lithium sulfide is a performance-determining factor for lithium-sulfur batteries. Herein, a novel strategy to synthesize a unique tube-in-tube CNT-wired sulfur-deficient MoS₂ nanostructure embedding cobalt atom clusters as an efficient polysulfide regulator is successfully conducted in Li-S batteries. It is confirmed that encapsulating MWCNTs into hollow porous sulfur-deficient MoS₂ nanotubes embedded with metal cobalt clusters not only can accelerate electron transport and confine the dissolution of lithium polysulfide by physical/chemical adsorption, but also can catalyze the kinetics of polysulfide redox reactions. Based on DFT calculations, in situ spectroscopic techniques, and various electrochemical studies, catalytic effects of CNT/MoS₂ -Co nanocomposite in Li-S battery are deeply investigated for the first time. The CNT/MoS₂ -Co composite cathode exhibits a very remarkable rate capability (641 mAh g^-1 at 5.0 C) and excellent cycling stability (capacity decay rate of 0.050% per cycle at 5.0 C) even at high sulfur mass loading of 3.6 mg cm^-2 . More crucially, CNT/MoS₂ -Co tube-in-tube nanostructures present a superior specific capacity of 650 mAh g^-1 in a Li-S pouch cell at 0.2 C (4.0 mg cm^-2 ).