Designing High-performance Lithium Sulfur Batteries Based On Catalytic Effect

Owing to the advantages of high theoretical energy density,the lithium-sulfur(selenium)batteries have been regarded as the most promising next-generation energy storage systems with great potential.However,the shuttle effect of soluble intermediates as well as the sluggish redox kinetics of the active species render the lithium-sulfur(selenium)batteries with low capacity and short cycle life,greatly limiting their practical applications.Therefore,the key to achieve high-performance lithium-sulfur(selenium)batteries is to suppress the shuttle effect at source and further to improve the utilization of the active materials.Herein,from the point of designing advanced catalysts for enhancing the reaction dynamics of active species,the strategies for constructing high-performance lithium-sulfur(selenium)batteries are proposed in this thesis,which involve optimizing the reaction interface between active species and catalysts by precisely manipulating the structure,component and surface chemistry of catalysts.Superior adsorption ability towards active materials is the precondition for catalysts to fully exert their catalytic role.Based on this line,a ternary composite interlayer structure was constructed using Ti O₂,graphene oxide,MWCNTs as the basic components.The dynamic balance between the interception efficiency of active material and the transmission efficiency of electron and ion can be realized via accurately adjusting the proportion of each elementary component.By properly combining the structure and function of different composite nano-carbon materials and inorganic components,this work gives full play to the advantages of each component and synergically alleviates the shuttle effect of polyselenides.At the same time,this work also lays a good theoretical and experimental foundation for the design and construction of high-performance catalyst components.The structure,chemical composition and surface chemistry of titanium carbide can be precisely regulated by efficient and convenient hydrothermal oxidation strategy,and a new type of two-dimensional heterostructure of titanium carbide/titanium dioxide was in situ prepared.This unique heterostructure possesses the merits of high conductivity,high specific surface area,excellent polysulfide chemical anchoring and catalytic conversion ability,which enables the rapid“capture-transfer-conversion”process of polysulfides on its surface.Therefore,A thin and compact interlayer is fabricated by mixing such heterostructure together with highly conductive graphene,which effectively restrains the shuttling of polysulfides and improves the overall electrochemical performance of lithium-sulfur batteries.We further design a bi-directional electrocatalyst for lithium-sulfur by in situ nitridation of binary transition-metal oxides(TMOs)in NH₃with optimized conditions,in the process of which a heterostructure consisting of metal-alloy nanoparticles and metal nitrides grown together was synthesized.The heterogeneous structure of metal alloy and metal nitride can realize the"dynamic"adsorption and transformation process of active materials at different charging and discharging stages,and the smooth heterogeneous interface ensures the rapid migration of active substances during cycling,thus significantly enhancing the reaction kinetic process and utilization efficiency of the active species.Some nitrogen atoms can be successfully removed from the Nb₄N₅ nanofiber by high temperature hydrogen treatment and thus the conductive metallic compound material with abundant nitrogen-defects is constructed.As the functional catalyst in lithium-sulfur batteries,such material can significantly reduce the reactive energy barrier of the active species at the defect surface and directly convert them due to the high electrical conductivity,thus effectively avoiding the transfer resistance of active materials,electrons and ions among different interphase interfaces and achieving efficient catalytic ability towards active materials with the limited specific surface area.The strategy of inhibiting the shuttle effect at the source by enhancing the reaction kinetics of active materials is proposed in this thesis,which provides a good theoretical basis and solution for the construction of high-performance lithium-sulfur(selenium)battery.