Study On Synthesis And Lithium Storage Performance Of Transition Metal Sulfides-Based Composites

With the progress of our society and the rapid development of science and technology,the use of mobile phones,digital cameras,tablets and other equipments is increasing,energy consumption and demand is also increasing.At the same time,the continuous depletion of non-renewable energy such as coal and petroleum makes the energy and environment problems imminent,and the development and use of new clean and renewable energy become the key to solve the problem.Therefore,the requirements for energy storage equipment and devices are constantly increasing.Among many new energy storage devices,lithium-ion batteries?LIBs?have attracted much attention due to their advantages of long cycle life,high energy density and environmental friendliness.However,the low theoretical capacity of commercial graphite anodes(372 mAh g^-1)will greatly limit the application of LIBs in the future.Developing new negative materials has become the key research direction of LIBs.Recently,transition metal sulfides?TMSs?have become a potential negative material for LIBs due to the high theoretical capacity,environmental friendliness and good safety performance.However,as a negative material of LIBs,the low conductivity and the serious volume effect of TMSs during the cycle make low structure stability,which lead to poor electrochemical performance and cycle stability for the battery,and greatly limit its practical applications.In order to solve these two main problems,we modified the TMSs negative electrode material,which effectively improved the lithium storage capacity and cycle stability of the batteries.The main modification research contents are as follows:?1?Co_1-xS-rGO composite was prepared by a simple one-step solvothermal reaction and freeze-drying process.Graphene oxide?GO?was firstly prepared by modified Hummer's method.Then GO was added as raw material and base material into the solution containing cobalt nitrate hexahydrate and thiourea.During the solvothermal reaction process,GO was reduced to rGO and Co_1-xS nanoparticles grew uniformly on the surface of rGO.The results show that the presence of rGO greatly hinders the agglomeration of Co_1-xS nanoparticles,effectively improves the electrical conductivity and structural stability of the composite materials,and LIBs show good electrochemical properties.At the current density of 200 mA g^-1,the discharge capacity of Co_1-xS-rGO composite can reach 643 mAh g^-1 after 100 cycles.At the high current density of 1000mA g^-1,the discharge capacity of the composite remains at 625 mAh g^-1 after 200cycles,which was close to the theoretical specific capacity of Co_1-xS(683 mAh g^-1).?2?Hexagonal flower-like Co_1-xS/MoS₂ composite was successfully prepared by two-step hydrothermal reaction.Flower-like Co_1-xS base material was fabricated by changing the reaction temperature,time and solvent.Then,a layer of MoS₂ in-situ grew on the surface of pure Co_1-xS by hydrothermal reaction,and the flower-like Co_1-xS/MoS₂ composite was successfully fabricated.Characterization and performance tests show that Co_1-xS/MoS₂ composite have better structural stability and reaction kinetics than the pure Co_1-xS and MoS₂.The composite exhibits excellent capacity,rate performance and long cycle stability.The initial discharge specific capacity of Co_1-xS/MoS₂ composite can reach 1698 mAh g^-1 at 200 mA g^-1,and the remained specific capacity is 1033 mAh g^-1 after 100 cycles.Even at the current density of 1000mA g^-1,the capacity of the first cycle is 887 mAh g^-1,and remains 715 mAh g^-1 after500 cycles.