The Sulfur Fixation Mechanism And Electrochemical Properties Of Carbon Complex Cobalt-based Sulfide

This thesis aims to the preparation of carbon composite cobalt-based sulfides by simultaneous and stepwise methods,and the synergistic effect of carbon materials and cobalt sulfides was investigated to study the inhibitory effect on the dissolution of polysulfide ions.Specifically,we synthesized the nitrogen-doped carbon polyhedral material with uniform distribution of CoS₂ by simultaneous sulfuration method,and explored its optimization effect on performance of lithium-sulfur half-cell and full-cell.S/CoS₂/rGO three-dimensional composite was prepared by step-by-step method,which effectively inhibiting the shuttle of polysulfide.The hollow double-shell Co9Sg@CNTs material was synthesized by step-by-step method.By adjusting the content of carbon materials,the effect of carbon composites in lithium-sulfur batteries was studied.The main contents can be summarized as follows:1,A simultaneous sulfuration method was developed to prepare a porous nitrogen-doped carbon polyhedral structure with uniform distribution of S/CoS₂.The cobalt-embedded nitrogen-doped carbon polyhedral material obtained by the pyrolysis of the metal organic framework ZIF-67 is compounded with sulfur at a high temperature,and the sulfur is fused into the porous carbon polyhedron while Co is sulfided into cobalt sulfide to synthesize a nitrogen-doped carbon polyhedron structure with uniform distribution of S/CoS₂.Thanks to the excellent conductivity of nitrogen-doped carbon polyhedra,the strong interaction of CoS₂ and nitrogen-doped carbon with polysulfide ions,the composite exhibits the good cycling stability.After 250 cycles,capacity was maintained at 702 mA h g^-1 at a current density of 0.5 C.The electrochemical performance of the full cell with S/CoS₂ uniformly distributed nitrogen-doped carbon polyhedral composite structure as the cathode and LixGe?LixSiGe?as the anode was also tested.2,A stepwise method was used to synthesize a three-dimensional graphene-loaded CoS₂ composite material by hydrothermal synthesis,and then the material was loaded with sulfur to obtain a S/CoS₂/rGO composite material.Graphene forms a three-dimensional conductive mesh to promote the rapid transport of electrons and ions.The physical limitation of graphene and the strong chemical interaction between CoS₂ and polysulfide ions inhibit the dissolution of polysulfide ions and promote cycling stability.A capacity of 811 mA h g^-1 was maintained at the current density of 1 C.3,A carbon nanotube-coated Co₉S₈ multi-shell hollow nanosphere structure?Co₉S₈@CNT?was designed and synthesized as a host for sulfur by the step-by-step preparation methods.The cobalt-glyceride complex precursor material is first sulfuration,and the double-shell hollow structure is formed via the Kirkendall effect;then the carbon nanotubes are in-situ catalytically grown and coated surface of double-shell Co₉S₈ hollow nanospheres accompanied with acetylene pyrolysis;finally,sulfur is filled by high temperature melting method.The structure effectively utilizes the synergy effect of the various components to improve the electrochemical performance of the lithium-sulfur battery.On the one hand,the carbon nanotubes interconnected to form a network structure as a conductive layer promotes the rapid transport of electrons and ions,and restricts the diffusion of polysulfide ions in the electrolyte on the other hand.The strong interaction between polar Co₉S₈ and polar polysulfide ions effectively inhibits the dissolution of polysulfi.de ions,while Co₉S₈ promotes the conversion of polysulfide ions.The multi-layered polar material composite design plays a double guarantee for suppressing the "shuttle effect" of polysulfide ions,and is beneficial to the rapid transmission of electrons and ions during electrochemical cycling.Theoretical calculations further confirm the strong interaction between Co₉S₈ and polysulfide ions,which is beneficial to the conversion of polysulfide ions.As a cathode material for lithium-sulfur batteries,Co₉S₈@CNT/S composites have a capacity of 1415 mA h g'1 at a current density of 0.2 C.Even at a high current density of 10 C,the capacity remains at 676.7.mA h g^-1.