Engineering a TiNb2O7-Based Electrocatalyst on a Flexible Self-Supporting Sulfur Cathode for Promoting Li-S Battery Performance

Bidirectional enhancement of Li2S redox reaction by NiSe2/CoSe2-rGO heterostructured bi-functional catalysts

The high activity barriers of Li2S nucleation and deposition limit the redox reaction kinetics of lithium polysulfides (LiPSs), meanwhile, the significant shuttle effect of LiPSs hampers the advancement of Li-S batteries (LSBs). In this work, a NiSe2/CoSe2-rGO (NiSe2/CoSe2-G) sulfur host with bifunctional catalytic activity was prepared through a hard template method. Electrochemical experiment results confirm that the combination of NiSe2 and CoSe2 not only facilitates the bidirectional catalytic function during charge and discharge processes, but also increases the active sites toward LiPSs adsorption. Simultaneously, the highly conductive rGO network enhances the electronic conductivity of NiSe2/CoSe2-G/S and provides convenience for loading NiSe2/CoSe2 catalysts. Benefitting from the exceptional catalytic-adsorption capability of NiSe2/CoSe2 and the presence of rGO, the NiSe2/CoSe2-G/S electrode exhibits excellent electrochemical properties. At 1C, it demonstrates a low capacity attenuation of 0.087 % per cycle during 500 cycles. The electrode can maintain a discharge capacity of 927 mAh/g at a sulfur loading of 3.3 mg cm-2. The bidirectional catalytic activity of NiSe2/CoSe2-G offers a prospective approach to expedite the redox reactions of active S, meanwhile, this work also offers an ideal approach for designing efficient S hosts for LSBs.

Selective Reduction of Multivariate Metal-Organic Frameworks for Advanced Electrocatalytic Cathodes in High Areal Capacity and Long-Life Lithium-Sulfur Batteries

Lithium-sulfur batteries hold great promise as next-generation high-energy-density batteries. However, their performance has been limited by the low cycling stability and sulfur utilization. Herein, we demonstrate that a selective reduction of the multivariate metal-organic framework, MTV-MOF-74 (Co, Ni, Fe), transforms the framework into a porous carbon decorated with bimetallic CoNi alloy and Fe3O4 nanoparticles capable of entrapping soluble lithium polysulfides while synergistically facilitating their rapid conversion into Li2S. Electrochemical studies on coin cells containing 89 wt % sulfur loading revealed a reversible capacity of 1439.8 mA h g-1 at 0.05 C and prolonged cycling stability for 1000 cycles at 1 C/1060.2 mA h g-1 with a decay rate of 0.018% per cycle. At a high areal sulfur loading of 6.9 mg cm-2 and lean electrolyte/sulfur ratio (4.5 μL:1.0 mg), the battery based on the 89S@CoNiFe3O4/PC cathode provides a high areal capacity of 6.7 mA h cm-2. The battery exhibits an outstanding power density of 849 W kg-1 at 5 C and delivers a specific energy of 216 W h kg-1 at 2 C, corresponding to a specific power of 433 W kg-1. Density functional theory shows that the observed results are due to the strong interaction between the CoNi alloy and Fe3O4, facilitated by charge transfer between the polysulfides and the substrate.

Proton-Induced Defect-Rich Vanadium Oxides as Reversible Polysulfide Conversion Sites for High-Performance Lithium Sulfur Batteries

Lithium-sulfur (Li-S) batteries have attracted attention due to their high theoretical energy density, natural abundance, and low cost. However, the diffusion of polysulfides decreases the utilization and further degrades the battery's life. We have successfully fabricated a defect-rich layered sodium vanadium oxide with proton doping (HNVO) nanobelt and used it as the functional interface layer on the separator in Li-S batteries. Benefiting from the abundant defects of NVO and the catalytic activity of metal vanadium in the electrochemical process, the shuttle of polysulfides was greatly decreased by reversible chemical adsorption. Moreover, the extra graphene layer contributes to accelerating the charge carrier at high current densities. Therefore, a Li-S battery with G@HNVO delivers a high capacity of 1494.8 mAh g^-1 at 0.2 C and a superior cycling stability over 700 cycles at 1 C. This work provides an effective strategy for designing the electrode/separator interface layer to achieve high-performance Li-S batteries.

d-p Hybridization-Induced "Trapping-Coupling-Conversion" Enables High-Efficiency Nb Single-Atom Catalysis for Li-S Batteries

Single-atom catalysts have been paid more attention to improving sluggish reaction kinetics and anchoring polysulfide for lithium-sulfur (Li-S) batteries. It has been demonstrated that d-block single-atom elements in the fourth period can chemically interact with the local environment, leading to effective adsorption and catalytic activity toward lithium polysulfides. Enlightened by theoretical screening, for the first time, we design novel single-atom Nb catalysts toward improved sulfur immobilization and catalyzation. Calculations reveal that Nb-N₄ active moiety possesses abundant unfilled antibonding orbitals, which promotes d-p hybridization and enhances anchoring capability toward lithium polysulfides via a "trapping-coupling-conversion" mechanism. The Nb-SAs@NC cell exhibits a high capacity retention of over 85% after 1000 cycles, a superior rate performance of 740 mA h g^-1 at 7 C, and a competitive areal capacity of 5.2 mAh cm^-2 (5.6 mg cm^-2). Our work provides a new perspective to extend cathodes enabling high-energy-density Li-S batteries.

Catalytic Effects of Electrodes and Electrolytes in Metal-Sulfur Batteries: Progress and Prospective

Metal-sulfur (M-S) batteries are promising energy-storage devices due to their advantages such as large energy density and the low cost of the raw materials. However, M-S batteries suffer from many drawbacks. Endowing the electrodes and electrolytes with the proper catalytic activity is crucial to improve the electrochemical properties of M-S batteries. With regard to the S cathodes, advanced electrode materials with enhanced electrocatalytic effects can capture polysulfides and accelerate electrochemical conversion and, as for the metal anodes, the proper electrode materials can provide active sites for metal deposition to reduce the deposition potential barrier and control the electroplating or stripping process. Moreover, an advanced electrolyte with desirable design can catalyze electrochemical reactions on the cathode and anode in high-performance M-S batteries. In this review, recent progress pertaining to the design of advanced electrode materials and electrolytes with the proper catalytic effects is summarized. The current progress of S cathodes and metal anodes in different types of M-S batteries are discussed and future development directions are described. The objective is to provide a comprehensive review on the current state-of-the-art S cathodes and metal anodes in M-S batteries and research guidance for future development of this important class of batteries.

Combined physical confinement and chemical adsorption on co-doped hollow TiO2 for long-term cycle lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have long been expected to be promising high-energy-density secondary batteries because of their high theoretical specific capacity and element abundances. Yet, their poor cyclability and low rate-capacity strongly limited their practical application. Herein, a nitrogen and sulfur dual doped hollow TiO₂ sphere is designed and synthesized for the sulfur host. The dual doped hollow TiO₂ can enhance the adsorption ability of soluble lithium polysulfides, which effectively promote the conversion reaction of lithium polysulfides from high-order to low-order in Li-S batteries. What is more, the hollow spherical TiO₂ host provides a deposition space for lithium polysulfides and blocks polysulfide migration from the cathode to the electrolyte. Both theoretical calculations and experimental studies confirmed that the electrochemical properties of the sulfur electrode are significantly improved by the dual doped hollow TiO₂ sphere. The typical as-prepared dual doped hollow TiO₂ cathode coated sulfur has a capacity of 1258 mA h g^-1 for the first discharge and a capacity decay as low as 0.0648% per cycle during 500 cycles with a sulfur loading of 3.8 mg cm^-2.