Application Of Co₉S_8-x-MoS_2-xHeterojunction With Rich Defects In Lithium-Sulphur Battery As Cathode Host Materials

In order to realize the energy revolution of"carbon peak"and"carbon neutral"and the appeal of environmental protection,lithium sulfur battery is one of the most promising energy-storage ideas at present.However,due to the shuttle effect and slow redox kinetics of intermediate polysulfides,its commercial application has been hindered.In order to solve the above problem and improve the electrochemical performance of lithium sulfur batteries,two sulfur host materials combining the advantages of carbon material and metal sulfide have been synthesized due to the heterojunction strategy and defect-engineering design.Besides,the correlation between the defects on the heterojunction materials and reaction kinetics in Li-S batteries is studied.The main contents in this research include:(1)The structure design of Co₉S_8-x-MoS_2-x-r GO heterojunction as sulfur host material and the catalysis mechanism of defect on the heterostructure.Co₉S_8-x-MoS_2-x-r GO materials with different defect concentrations are prepared by simple hydrothermal method and high temperature heat treatment.Then,the existence of sulfur vacancies and heterostructure is confirmed by XRD,XPS and HRTEM.The initial specific capacity can reach 1043 m Ah g^-1 at the current density of 0.5 C for the lithium sulfur battery with the highest defect concentration host materials.After 300cycles,the capacity decay rate of each cycle is only 0.123%.Electrochemical tests show that the defect sites of the heterojunction materials can significantly improve the electrical conductivity of the host materials and ensure the electron transport for the sulfur electrode.Theoretical calculation and polysulfide adsorption tests show that the overall chemisorption effect of heterojunction materials with defect sites is enhanced,which inhibits the shuttle effect and improves the cycle stability of the battery.Finally,the symmetric cell data and the corresponding electrochemical data show that the defects improve the surface active adsorption sites,promote the surface adsorption-catalysis conversion process of high-order polysulfides,and produce the active intermediate S₃^*-,which acts as a homogeneous catalytic mediator and accelerates the deposition and dissociation kinetics process of polysulfides.(2)The structure design and study on electrochemical properties of Co₉S_8-x-MoS_2-x-Co NC heterostructure.Co₉S_8-x-MoS_2-x heterostructure is successfully grown on the Co NC materials derived from ZIF-67 through simple hydrothermal and heat treatment methods,which is used as the host material for lithium-sulfur batteries.In this host material,Co₉S_8-x-MoS_2-x has a good catalytic ability,and Co NC has a good chemisorption effect.The coupling of both realizes the synergy of chemisorption and catalytic effect.The MoS₂ nanosheets coated on the Co NC nanocube improve the pore size of materials from micropore to mesopore,which provides more space for sulfur and accelerates the conduction of lithium ions and the diffusion of electrolyte.Thanks to the unique structure of the composite,the S@Co₉S_8-x-MoS_2-x-Co NC electrode exhibits superior cycle stability and rate capability.At a high current density of 2 C,the initial discharge specific capacity of S@Co₉S_8-x-MoS_2-x-Co NC can reach 659 m Ah g^-1.With the sulfur loading of 3.04 mg cm^-2,the area capacity can reach 2.36 m A h cm^-2 at the current density of 0.1 C.