Study On The Synthesis Of ZIFs-based Graphene Composites And Mechanism Of Lithium Storage

Driven by changes in energy technology and new technology industries,lithium-ion batteries are emerging as energy storage devices.Many researchs have devoted to devolope of high-energy density and power density,ultra-long cycle life,high safety factor,and low-cost lithium-ion batteries.Developing a high electrochemical performance electrode material is one of feasible approaches to enhance the performance of lithium-ion batteries.In this paper,graphene is used as raw material to construct zeolite imidazolate skeleton compounds(ZIFs)based graphene composites and investigates its electrochemical performance as anode materials in lithium-ion batteries.Further explore the transmission kinetics and capacitance quantitative analysis of composite materials to better understand the charge storage mechanisms.The main work of the paper is summarized as follows:(1)ZIF-8 nanoparticles were grown on the surface of graphene oxide(GO)in situ,and then ultra-high nitrogen-doped porous carbon graphene nanosheets(NPC@GNS)were prepared by one-step pyrolysis.The results show that the NPC@GNS composite can maintain a high specific capacity of 906.6 mAh/g after 100 cycles at a current density of 0.5 A/g.It can still provide an average specific capacity of 378.2 mAh/g after 1000 cycles at a high current density of 5 A/g.Excellent cycle stability and rate performance confirm the reliability of sufficient "point and plane" 2D sandwich structures.Some micropores and abundant mesoporous channels facilitate rapid transfer of Li+.The charge storage mechanism indicates that more than 41.30%of the charge storage is controlled by surface capacitance In short,the higher capacitance contribution rate of the total capacity and the unique structure of NPC@GNS can achieve excellent charge storage capacity at large current densities,and also exhibit fast charge and discharge performance.(2)Fe3O4 N-doped graphene nanosheets(Fe3O4@NGNS)composite was obtained by simple hydrothermal synthesis using Fe-ZIF precursor and GO as raw materials.Electrochemical tests show that the nitrogen content of Fe3O4@NGNS composites is 6.02%.And it can maintain a reversible specific capacity of 912.8 mAh/g after 100 cycling at a current density of 0.2 A/g.It can still provide a reversible specific capacity of 502.3 mAh/g after 200 cycles at a high current density of 2 A/g.The excellent cycle stability performance and rate performance of composite mainly due to that situ-synthesis strategy can effectively avoid the agglomeration of nano-spherical Fe3O4 and graphene can buffer the huge volume change of Fe3O4.The results of the charge storage mechanism research show that more than 33.87%of the charge storage comes from the surface capacitance effect of the active material The unique nano-spherical structure uniformly disperses and firmly anchored on the graphene network can enhancethe stability of composites.The special structure of composites is benifite to enhance the ion transmission,and finally exhibit fast charging and discharging characteristics.