Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithium–sulfur battery

Transition Metal Phosphides: The Rising Star of Lithium-Sulfur Battery Cathode Host

Lithium-sulfur batteries (LSBs) with ultra-high energy density (2600 W h kg-1 ) and readily available raw materials are emerging as a potential alternative device with low cost for lithium-ion batteries. However, the insulation of sulfur and the unavoidable shuttle effect leads to slow reaction kinetics of LSBs, which in turn cause various roadblocks including poor rate capability, inferior cycling stability, and low coulombic efficiency. The most effective way to solve the issues mentioned above is to rationally design and control the synthesis of the cathode host for LSBs. Transition metal phosphides (TMPs) with good electrical conductivity and dual adsorption-conversion capabilities for polysulfide (PS) are regarded as promising cathode hosts for new-generation LSBs. In this review, the main obstacles to commercializing the LSBs and the development processes of their cathode host are first elaborated. Then, the sulfur fixation principles, and synthesis methods of the TMPs are briefly summarized and the recent progress of TMPs in LSBs is reviewed in detail. Finally, a perspective on the future research directions of LSBs is provided.

Interfacial Engineering of Ru Nanocluster-Modified TiO2 Nanotube-Assisted Regulation of Lithium Polysulfide Reactions

The inhibition of lithium polysulfide (LiPS) diffusion and the acceleration of reaction kinetics are two major challenges for the practical application of lithium-sulfur (Li-S) batteries. Herein, through an interface engineering strategy, a multifunctional sulfur host based on Ru nanocluster-modified TiO2 nanotubes (TiO2-Ru) was designed. The TiO2-Ru interface field effect, combined with the hollow nanotube structure and the strong chemical action of TiO2, enhanced the LiPS trapping ability and inhibited the "shuttle effect". Furthermore, the high catalytic activity of Ru nanoclusters reduced the energy barrier of multistep LiPS reactions, thus speeding up the electrode kinetics. As a result, the TiO2-Ru-based composite sulfur cathode delivered excellent electrochemical performance, including an extremely low capacity loss of ∼0.015% per cycle and an increased areal capacity of ∼6.1 mAh cm-2 at 4.8 mg cm-2. This work contributes to a better sulfur cathode design from insights into morphology and phase interface engineering.

Mo2C-Loaded Porous Carbon Nanosheets as a Multifunctional Separator Coating for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries have emerged as one of the promising next-generation energy storage devices. However, the dissolution and shuttling of polysulfides in the electrolyte leads to a rapid decrease in capacity, severe self-discharge, and poor high-temperature performance. Here, we demonstrate the design and preparation of a Mo₂C nanoparticle-embedded carbon nanosheet matrix material (Mo₂C/C) and its application in lithium-sulfur battery separator modification. As a polar catalyst, Mo₂C/C can effectively adsorb and promote the reversible conversion of lithium polysulfides, suppress the shuttle effect, and improve the electrochemical performance of the battery. The lithium-sulfur battery with the Mo₂C/C =-modified separator showed a good rate of performance with high specific capacities of 1470 and 799 mAh g^-1 at 0.1 and 2 C, respectively. In addition, the long-cycle performance of only 0.09% decay per cycle for 400 cycles and the stable cycling under high sulfur loading indicate that the Mo₂C/C-modified separator holds great promise for the development of high-energy-density lithium-sulfur batteries.

Synthesis of the Ni2P-Co Mott-Schottky Junction as an Electrocatalyst to Boost Sulfur Conversion Kinetics and Application in Separator Modification in Li-S Batteries

To overcome the shuttling effect and sluggish conversion kinetics of polysulfides, a large number of catalysts have been designed for lithium-sulfur (Li-S) batteries. Herein, a Mott-Schottky junction catalyst composed of Co nanoparticles and Ni₂P was designed to improve polysulfide kinetics. Our investigations reveal the rearrangement of charges at the Schottky junction interface and the construction of the built-in electric field are crucial for lowering the activation energy of the dissolved Li₂S_n reduction and Li₂S nucleation reaction. Furthermore, a series of experimental and electrochemical tests were performed to demonstrate that the Schottky catalytic effect enhanced the synergistic catalytic effect. With a Ni₂P-Co@CNT catalyst, the battery exhibits an initial specific capacity of 874 mAh g^-1 at a rate of 4.0 C, and the decay rate per cycle is 0.049% in 700 cycles. Meanwhile, the battery shows 0.118% decay rate per cycle at 0.5 C in 100 cycles at a high sulfur loading of 10 mg cm^-2. The Schottky heterojunction structure proposed here has been shown to have a good catalytic effect on the reduction of Li₂S_n and nucleation of Li₂S, which provides a profound guidance for efficient and rational catalyst design.

MoP Quantum Dot-Modified N,P-Carbon Nanotubes as a Multifunctional Separator Coating for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have the advantages of high energy density and low cost and are considered promising next-generation energy storage systems, but the shuttle effect and slow sulfur redox kinetics severely limit their practical applications. Herein, MoP quantum dot-modified N,P-doped hollow PPy substrates are adopted as separator modification coatings for LSBs. The MoP quantum dots exhibit excellent chemisorption and catalytic conversion capabilities for polysulfides, while the N,P-doped PPy substrates can provide flexible channels for Li⁺/electron transport and act as a physical barrier to suppress the shuttle effect. As a result, LSBs assembled with modified separators exhibit excellent rate capability (739 mAh/g at 3 C) and cycle performance (600 mAh/g at 1 C after 600 cycles, 0.052% decay per cycle). Moreover, even under a high sulfur loading of 3.68 mg/cm², areal capacities of 3.58 and 2.92 mAh/cm² for the 1st cycle and 110th cycle are achieved. In addition, according to density functional theory calculations, MoP quantum dots have large adsorption energy for S₈ and Li₂S_n, which further confirms the possibility of lowering the initial nucleation energy barrier of Li₂S and helps to improve the kinetics of the subsequent Li₂S reaction. This study proposes a novel method for using transition-metal phosphides as catalysts in high-performance LSBs.

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.

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.

Cobalt-Based Double Catalytic Sites on Mesoporous Carbon as Reversible Polysulfide Catalysts for Fast-Kinetic Li-S Batteries

Li-S batteries are considered to be the most promising next-generation advanced energy-storage systems. However, the sluggish reaction kinetics and the "shuttle effect" of lithium polysulfides (LiPSs) severely limit their battery performances. To overcome the complex and multiphase sulfur redox chemistry of LiPSs, in this study, we propose a new type of cobalt-based double catalytic sites (DCSs) codoped mesoporous carbon to immobilize and reversibly catalyze the LiPS intermediates in the cycling process, thus eliminating the shuttle effect and improving the charge-discharge kinetics. The theoretical calculation shows that the well-designed DCS configuration endows LiPSs with both strong and weak binding capabilities, which will facilitate the synergistic and reversible catalytic conversion. Furthermore, the experimental results also confirm that the DCS structure shows significantly enhanced catalytic kinetics than the single catalytic sites. The Li-S battery equipped with the DCS structure displays an extremely high discharge capacity of 918 mA h g^-1 at a current density of 0.2 C and can reach a capacity of 867 mA h g^-1 after 200 cycles with an ultralow capacity attenuation rate of 0.028% for each cycle. This study opens new avenues to address the catalytic requirements both in discharging and charging processes.

Cerium Oxysulfide with O-Ce-S Bindings for Efficient Adsorption and Conversion of Lithium Polysulfide in Li-S Batteries

Understanding of adsorption and kinetic conversion of polysulfide lithium (LiPSs) in Li-S batteries is quite crucial for the design of efficient effective sulfur carriers. Herein, based on the possible interactions with LiPSs, Ce₂O₂S with unique O-Ce-S bindings is proposed to be used as a promising carrier additive and a 2D Ce₂O₂S/C composite is synthesized via a one-facile NaCl-template method and subsequent sulfuration under 700 °C. The 2D Ce₂O₂S/C exhibits a stronger adsorption capability than CeO₂/C through the adsorption test for Li₂S₆. Combined with XPS and DFT results, the superiority is mainly originated from the formation of S-S and Li-S bonds between LiPSs and the lattice S on the surface of Ce₂O₂S. The 2D Ce₂O₂S/C composite also exhibits a better catalytic ability than CeO₂ according to the change of the free energies of the polysulfides during the discharge process, which coincides with the lower oxidation potential for Li₂S₂/Li₂S transition by cyclic voltammetry. Resultantly, the cathodes using the Ce₂O₂S/C composite as a carrier manifest an enhanced rate and cycling performances. Hence, our work paves a phenomenon wherein Ce₂O₂S with O-Ce-S bindings is more beneficial to improve the cycling stability of Li-S batteries than CeO₂ containing single Ce-O bonds, which may be also suitable for other kinds of metallic sulfur oxide compounds.

Multifunctional Metal Phosphides as Superior Host Materials for Advanced Lithium-Sulfur Batteries

For the past few years, a new generation of energy storage systems with large theoretical specific capacity has been urgently needed because of the rapid development of society. Lithium-sulfur (Li-S) batteries are regarded as one of the most promising candidates for novel battery systems, since their resurgence at the end of the 20th century Li-S batteries have attracted ever more attention, attributed to their notably high theoretical energy density of 2600 W h kg^-1 , which is almost five times larger than that of commercial lithium-ion batteries (LIBs). One of the determining factors in Li-S batteries is how to design/prepare the sulfur cathode. For the sulfur host, the major technical challenge is avoiding the shuttling effect that is caused by soluble polysulfides during the reaction. In past decades, though the sulfur cathode has developed greatly, there are still some enormous challenges to be conquered, such as low utilization of S, rapid decay of capacity, and poor cycle life. This article spotlights the recent progress and foremost findings in improving the performance of Li-S batteries by employing multifunctional metal phosphides as host materials. The current state of development of the sulfur electrode of Li-S batteries is summarized by emphasizing the relationship between the essential properties of metal phosphide-based hybrid nanomaterials, the chemical reaction with lithium polysulfides and the latter's influence on electrochemical performance. Finally, trends in the development and practical application of Li-S batteries are also pointed out.

Metal-Organic-Framework-Derived Nanostructures as Multifaceted Electrodes in Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) are considered up-and-coming future-generation energy storage systems because of their prominent theoretical energy density. However, the practical applications of MSBs are still hampered by several critical challenges, i.e., the shuttle effects, sluggish redox kinetics, and low conductivity of sulfur species. Recently, benefiting from the high surface area, regulated networks, molecular/atomic-level reactive sites, the metal-organic frameworks (MOFs)-derived nanostructures have emerged as efficient and durable multifaceted electrodes in MSBs. Herein, a timely review is presented on recent advancements in designing MOF-derived electrodes, including fabricating strategies, composition management, topography control, and electrochemical performance assessment. Particularly, the inherent charge transfer, intrinsic polysulfide immobilization, and catalytic conversion on designing and engineering of MOF nanostructures for efficient MSBs are systematically discussed. In the end, the essence of how MOFs' nanostructures influence their electrochemical properties in MSBs and conclude the future tendencies regarding the construction of MOF-derived electrodes in MSBs is exposed. It is believed that this progress review will provide significant experimental/theoretical guidance in designing and understanding the MOF-derived nanostructures as multifaceted electrodes, thus offering promising orientations for the future development of fast-kinetic and robust MSBs in broad energy fields.

Recent Advances in Molybdenum-Based Materials for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries as power supply systems possessing a theoretical energy density of as high as 2600 Wh kg^-1 are considered promising alternatives toward the currently used lithium-ion batteries (LIBs). However, the insulation characteristic and huge volume change of sulfur, the generation of dissolvable lithium polysulfides (LiPSs) during charge/discharge, and the uncontrollable dendrite formation of Li metal anodes render Li-S batteries serious cycling issues with rapid capacity decay. To address these challenges, extensive efforts are devoted to designing cathode/anode hosts and/or modifying separators by incorporating functional materials with the features of improved conductivity, lithiophilic, physical/chemical capture ability toward LiPSs, and/or efficient catalytic conversion of LiPSs. Among all candidates, molybdenum-based (Mo-based) materials are highly preferred for their tunable crystal structure, adjustable composition, variable valence of Mo centers, and strong interactions with soluble LiPSs. Herein, the latest advances in design and application of Mo-based materials for Li-S batteries are comprehensively reviewed, covering molybdenum oxides, molybdenum dichalcogenides, molybdenum nitrides, molybdenum carbides, molybdenum phosphides, and molybdenum metal. In the end, the existing challenges in this research field are elaborately discussed.

Bifunctional Atomically Dispersed Mo-N2/C Nanosheets Boost Lithium Sulfide Deposition/Decomposition for Stable Lithium-Sulfur Batteries

The sluggish kinetics of lithium polysulfides (LiPS) transformation is recognized as the main obstacle against the practical applications of the lithium-sulfur (Li-S) battery. Inspired by molybdoenzymes in biological catalysis with stable Mo-S bonds, porous Mo-N-C nanosheets with atomically dispersed Mo-N₂/C sites are developed as a S cathode to boost the LiPS adsorption and conversion for Li-S batteries. Thanks to its high intrinsic activity and the Mo-N₂/C coordination structure, the rate capability and cycling stability of S/Mo-N-C are greatly improved compared with S/N-C due to the accelerated kinetics and suppressed shuttle effect. The S/Mo-N-C delivers a high reversible capacity of 743.9 mAh g^-1 at 5 C rate and an extremely low capacity decay rate of 0.018% per cycle after 550 cycles at 2 C rate, outperforming most of the reported cathode materials. Density functional theory calculations suggest that the Mo-N₂/C sites can bifunctionally lower the activation energy for Li₂S₄ to Li₂S conversion and the decomposition barrier of Li₂S, accounting for its inherently high activity toward LiPS transformation.

Lithium-Sulfur Batteries: Advances and Trends

A review with 132 references. Societal and regulatory pressures are pushing industry towards more sustainable energy sources, such as solar and wind power, while the growing popularity of portable cordless electronic devices continues. These trends necessitate the ability to store large amounts of power efficiently in rechargeable batteries that should also be affordable and long-lasting. Lithium-sulfur (Li-S) batteries have recently gained renewed interest for their potential low cost and high energy density, potentially over 2600 Wh kg−1. The current review will detail the most recent advances in early 2020. The focus will be on reports published since the last review on Li-S batteries. This review is meant to be helpful for beginners as well as useful for those doing research in the field, and will delineate some of the cutting-edge adaptations of many avenues that are being pursued to improve the performance and safety of Li-S batteries.

Accelerated polysulfide conversion on hierarchical porous vanadium-nitrogen-carbon for advanced lithium-sulfur batteries

With high theoretical specific density, low cost, and non-toxicity, Li-S batteries are regarded as a promising candidate for next-generation energy storage systems. However, the shuttling of soluble Li polysulfides (LiPSs) results in self-discharge and rapid capacity degradation. Herein, nitrogen-doped hierarchical porous carbon with embedded highly dispersed vanadium (v)-N_x sites (V-N-C) is developed as a high-performance Li-S battery cathode for the first time. The metal-organic polymer supramolecule structure formed by the electrostatic/hydrogen bond interaction of chitosan-VO₃^- strongly stabilizes V to generate a high density of V-N_x/C sites. During the discharge/charge process, the unique V-N_x/C active sites can serve as efficient catalysts to accelerate the redox kinetics of LiPSs, while the hierarchical porous carbon structure of V-N-C benefits the diffusion/transfer of Li⁺/e^- and suppresses the shuttling of LiPSs. As a result, the S/V-N-C composite delivers a high specific capacity of 1111.2 mA h g^-1 at 0.5C and maintains 573.6 mA h g^-1 at 5C with a low capacity decay rate of 0.087% per cycle (over 500 cycles at 1C). The rate performance of the developed V-N-C cathode in Li-S batteries is superior to that of most of the reported M-N-C and carbon material/metal compound composite electrodes.