Preparation And Electrochemical Performance Of Transition Metal Sulfides And Oxides/Carbon Composites Cathode For Lithium-sulfur Battery

Lithium-sulfur battery is one of the most promising secondary batteries in the next generation due to the high theoretical specific capacity(1675 mAhg^-1).However,the conductivity of sulfur is poor and the kinetics of discharge process is sluggish,moreover,the intermediate lithium polysulfides(LiPSs)are easily dissolved and diffused in the electrolyte,resulting in capacity decay and poor cycling stability,which greatly restricts the development of lithium-sulfur batteries.Carbon material has excellent electrical conductivity,while polar materials such as transition metal sulfides/oxides can effectively adsorb Li PSs and prevent them from being lost in the electrolyte.Combining the advantages of polar material and carbon with reasonable structure design,the composites can accelerate the kinetic process of Li PSs conversion and use as an effective sulfur host to improve the cycle stability and long-life for lithium-sulfur battery.The rGO/1T'-ReS2 nanosheets composites were prepared as sulfur host via a hydrothermal method.The effects of the raw materials ratio and surfactant addition on the morphology of rGO/1T'-ReS2 nanosheets were studied.The effect of the composite structure of Re S2 grown vertically on rGO on the acceleration of Li PSs conversion was investigated.The first principle calculation and electrochemical analysis were carried on to reveal the mechanism of the adsorption and catalytic effect of ReS2 on LiPSs.The results show that ReS2 is stable in 1T'phase with good electrical conductivity,the active sites at its edge and base planes can both achieve efficient adsorption of LiPSs and accelerate the kinetics of redox reactions.Therefore,the composite exhibits good cycle stability and rate performance when used as a host for sulfur.At 1 C,the initial capacity of high sulfur surface loading(2.2 mgcm^-2)electrode can reach 601 mAhg^-1,and the capacity remain 73.3%after 400 cycles.In addition,CeO2-x@C hollow spheres composites were prepared as sulfur host using hydrothermal synthesis.The effects of raw material ratio and reaction conditions on the morphology of CeO2-x@C hollow spheres were studied.The effect of the composite structure and oxygen vacancies on the acceleration of Li PSs conversion was investigated.The mechanism of effectively adsorption of Li PSs and acceleration of electron transport and ion diffusion was revealed.The results show that CeO2 can effectively adsorb LiPSs.Through reasonable design of surface engineering,CeO2 is rich in oxygen vacancies,which changes the electronic state and accelerates the conversion reaction of Li PSs.Moreover,the carbon layer acts as a physical barrier to encapsulate LiPSs and improve the overall conductivity,the hollow sphere can inhibit the volume expansion.Therefore,the S cathode shows good cycling stability and rate performance.The reversible discharge specific capacity of the CeO2-x@C-S electrode at current densities of 0.1,0.2,0.5,1 and 2 C are 934,784,671,650 and 592 mAhg^-1,respectively.The initial capacity at high current density(1 C)is 700 mAhg^-1,and the capacity remain 68.6%after 400 cycles.