| Lithium-sulfur(Li-S)battery is a kind of rechargeable batteries with lithium as negative electrode and sulfur as positive electrode.It has a high theoretical specific capacity of 1675 mA·h/g,which is 3-5 times higher than traditional cathode materials based on transition metal oxides or phosphates,making Li-S batteries have great research and development potential in the field of new generation renewable energy.However,the non-conductivity of the sulfur positive electrode and the "shuttle effect" during the process of charging and discharging make the achievable energy density much lower than the theoretical value.Doped graphene is widely used in lithium-sulfur batteries due to its excellent electrical conductivity and strong adsorption to polysulfides,which effectively improves the utilization of sulfur in active substances and alleviates the shuttle effect.In this paper,the prepared graphene was modified and applied to the positive electrode of lithium-sulfur batteries,and its electrochemical performance was tested.The main research contents are as follows:First,graphite oxide(GO)was prepared by Hummer’s method,and nitrogen-doped graphene(NG)was prepared by hydrothermal method,and nitrogen-doped graphene/sulfur(NG/S)composite material was prepared by heat treatment.The microstructure and morphology of the prepared NG were characterized by XRD,SEM and TEM,respectively.The composite is amorphous carbon structure with a light sheet structure.The material has a specific surface area of 260.2 m2/g and NG/S displayed a first discharge specific capacity of 1252 mA·h/g.After 50 cycles of charge and discharge,it still maintains 568 mA·h/g,presenting a good cycle stability.Because of the nitrogen atom incorporation changing the polarity of the graphene material,the lone electron pair on the nitrogen atom strongly adsorbs the polysulfide in the process of charging and discharging of the lithium-sulfur batteries,which can effectively alleviate the"shuttle effect" and improve the cycling stability performance of Li-S batteries.Second,graphite oxide(GO)was prepared by Hummer’s method,and nitrogen-doped graphene was prepared by microwave method.The effect of microwave temperature on the morphology and electrochemical properties of prepared NG materials was explored.Then nitrogen-doped graphene/sulfur composite material was prepared by heat treatment.The composite material has a specific discharge capacity of 1172 mA·h/g at a rate of 0.1 C.After 100 cycles,the specific capacity of the discharge is stable at 423 mA·h/g,showing a good cycle stability.Third,nitrogen-doped graphene(MNG)was prepared by microwave treatment from graphite oxide(GO)impregnated by ammonium carbonate,and microwave nitrogen-doped graphene/sulfur(MNG/S)was obtained by in-situ chemical deposition as cathode composite of lithium sulfur battery.The microstructure and morphology of the prepared MNG and MNG/S were characterized by FT-IR,XRD,SEM,TEM and BET,respectively.The electrochurnical properties of the composites were investigated by constant current charge-discharge test and AC impedance test.As cathode materials of lithium-sulfur batteries,the MNG/S exhibited a high specific capacity of 1373 mA·h/g at 0.1 C,754 mA·h/g at 0.2 C,607 mA·h/g at 0.5 C,and 511 mA·h/g at 1 C rate. |
