Controllable Synthesis And Electrochemical Performance Of Transition Metal Nitride(Carbide) Based Cathode For Lithium Sulfur Batteries

Lithium-Sulfur（Li-S）battery is considered as one of most promising next generation energy storage devices because of its high energy density(2560 Wh kg^-1)and abundant sources of sulfur raw materials.However,the practical application of Li-S battery is hindered by the shuttling effect of lithium polysulfides（Li PSs）and sluggish sulfur/lithium sulfide redox kinetics.Developing a sulfur host material with high conductivity and fast reaction kinetics to improve its electrochemical performance is of great significance.To solve the above problems,in this doctoral dissertation,based on the highly conductive transition metal nitride/carbide through rational structural design and composition manipulation,a series of transition metal nitride/carbide-based composites/heterostructures as sulfur host materials with high conductivity and high catalytic conversion activity have been prepared,which significantly improved the electrochemical performance of lithium-sulfur batteries.The main research contents and conclusions are summarized as following:（1）In view of transition metal nitride compounds with low specific surface area and open structure cannot effectively inhibit the dissolution and diffusion of polysulfides,we developed a novel strategy to simultaneously achieve large specific capacity,high rate capability,and long cycle life by utilizing mesoporous niobium nitride microspheres/N-doped graphene nanosheets（Nb N@NG）hybrids as multifunctional host materials for sulfur cathodes.The mesoporous Nb N microspheres chemically immobilized Li PSs via Nb-S chemical bonding and catalytically promote conversion of Li PSs into insoluble Li₂S resulting in enhanced redox reaction kinetics.The highly conductive nitrogen-doped graphene nanosheets can further inhibit the dissolution and loss of polysulfides through physical barrier,and form a three-dimensional conductive network with Nb N to promote electron transport and ion diffusion.Benefiting from the synergistic of Nb N and NG,the S/Nb N@NG cathode demonstrated a large capacity of 948 m Ah g^-1 at 1 C,high-rate capability of 739 m Ah g^-1 at 5 C,and excellent cycle stability with a capacity decay of 0.09%per cycle for over 400 cycles.（2）In view of the unclear mechanism of Li PSs adsorption and catalytic conversion by transition nitride materials（TMN）,taking vanadium nitride（VN）as a model,the changes of surface chemical states of VN obtained at different nitriding temperatures and the corresponding relationships with its chemically adsorption were investigated.It was proved that the residual oxygen content and surface chemical states in VN played a crucial role in enhancing its chemisorption and catalytic conversion kinetics.The effective immobilization of Li PSs could be achieved by the spontaneous redox reaction of VN to form the V-S chemical bond and the active intermediates.At the same time,the V-S chemical bond formed by self-vulcanization improved the intrinsic electrocatalytic activity of VN materials,and promoted the rapid transformation of Li PSs and the kinetics of redox reaction.Thanks to its strong chemical anchoring effect and enhanced intrinsic electrocatalytic activity,the VN prepared via low temperature nitriding exhibited high specific capacity,excellent rate performance and good cycling stability.（3）For single TMN compounds,the adsorption and catalytic capacity is limited and the bidirectional catalytic promotion towards sulfur reduction and lithium sulfide oxidation processes cannot be achieved simultaneously.Therefore,based on the design of bifunctional catalyst host materials,we constructed a two-dimensional Mo S₂-Mo₅N₆Mott-Schottky heterostructure bifunctional catalyst by in-situ topological chemical nitriding method.Mo S₂-Mo₅N₆ Mott-Schottky heterojunction can form an internal electric field at the heterogeneous interface to promote the effective separation of charge,drive the rapid transfer/diffusion of electrons/ions,and realize the bidirectional promotion of Li PSs conversion.By controlling the degree of nitridation,the structure and composition of the materials are regulated,thus balancing the anchoring ability（by Mo S₂）and catalytic activity（by Mo₅N₆）toward Li PSs,and synergistic boosting the adsorption-diffusion-conversion process of Li PSs.As a result,the Mo S₂-Mo₅N₆@C after three hours of nitriding shows a maximum specific capacity of 1117 m Ah g^-1 at0.2 C,an excellent rate performance of 799 m Ah g^-1 at 5 C and a good cycling performance of 240 cycles at 1 C with a low decay rate of 0.057%per cycle.（4）In order to solve the problems of complex preparation process,unstable interface structure and catalytic deactivation of transition metal carbides based heterostructures,we designed and synthesized a hierarchical Co/Mo2C heterostructure encapsulated in graphitic carbon matrix nanosheets arrays grown on carbon cloth（Co/Mo₂C@GC//CC）through an in-situ phase separation process as the highly efficient host material for Li-S battery.The Co/Mo₂C heterostructures with abundant heterointerfaces can catalyze and accelerate the transformation kinetics and redox reaction of liquid Li PSs,reduce the nucleation and decomposition barrier of Li₂S,and accelerate the nucleation growth and rapid uniform deposition of solid Li₂S.At the same time,the carbon nanotubes and graphitized carbon layer can be combined with the substrate carbon cloth to form a three-dimensional conductive network structure,which can promote electron transport and rapid diffusion of lithium ions,and also effectively keep the activity of the catalyst and improve the mechanical stability of the electrode structure.As a consequence,the Co/Mo₂C@GC//CC electrode with Li₂S₆ loading shows a large reversible specific capacity(1416 m Ah g^-1 at 0.2 C),superior rate performance(995 m Ah g^-1 at 0.2 C)and robust cycle stability(915 m Ah g^-1 after 250cycles at 0.5 C).