The Preparation Of Bimetallic Sulfide Core-shell Microspheres And Their Performance In Lithium-sulfur Batteries

Lithium-sulfur(Li-S)battery is considered to be one of the most promising next-generation energy-storage devices because of its high energy density(2567 Wh/kg),attractive theoretical specific capacity(1675 mAh/g),low cost and natural abundance of sulfur.Consequently,the Li-S battery is hoped to meet the requirements of electric vehicles and large-scale electricity storage in the near future.In the past decade,Li-S batteries have been extensively investigated by many researchers.However,the shuttle effect of polysulfides and relatively slow reaction kinetics of sulfur cathode severely hinder the commercial applications of the Li-S battery.To overcome these issues,we have designed and synthesized a novel sulfur host material,which can suppress the shuttle effect of polysulfides and improve the redox kinetics of sulfur species.The details are as follows:Firstly,a cobalt-based bimetallic sulfide with a core-shell hollow microsphere structure was designed and prepared,and it was applied as a sulfur host for lithium-sulfur battery.The bimetal sulfides of NiCo2S4 and CoxVyS,core-shell hollow spheres was synthesized through a facile anion exchange method.The NiCo2S4 and Co,VyS,core-shell hollow spheres were physically characterized by SEM?TEM?XPS and XRD.Then,the S/NiCo2S4 and S/CoxVySz were prepared with the method of immersiom and diffusion,and physically characterized by TGA and SEM.The electrical characterization were tested by constant current charge-discharge method,CV and EIS.(1)The as-synthesized NiCo2S4 core-shell hollow spheres can not only provide sufficient space for loading sulur but also mitigate volumetric variation of sulfur during cycling.The adsorption experiments indicate that the NiCo2S4 host has a stronger entrapment ability towards lithium polysulfides than Super P due to the stronger chemical interaction.Furthermore,an electrochemical kinetic study reveals that the NiCo2S4 could serve as an electrocatalyst,significantly accelerating the kinetics of polysulfide redox reaction.S/NiCo2S4-n(n is 60,70 and 80)composites with different carrying capacity of sulfur were prepared and tested with normal method,S/NiCo2S4-60,S/NiCo2S4-70 and S/NiCo2S4-80 composites own capacities of 446.2,378.8 and 237.4 mAh/g and have capacity decay rate per cycle of 0.063%.0.074%and 1.04%.(2)The as-synthesized CoxVySz core-shell hollow spheres has a stronger entrapment ability towards lithium polysulfides than CoV-precursor due to the stronger chemical interaction.Furthermore,the EIS was conducted and the S/CoxVySz cathode exhibits much lower charge-transfer resistance compared to that of S/CoV-precursor cathode.The experimental results show that the sulfurization increases the reaction kinetics of the electrode and enhances the adsorption capacity of the polysulfides.The S/CoxVySz composite(sulfur content of 70 wt%)as the cathode for Li-S batteries exhibited remarkable cycle stability up to 500 cycles at 0.5 C with a low capacity decay rate of 0.063%per cycle.The research of this paper is of great significance for the development of bimetallic sulfide with hollow structure as the sulfur host material.