Synthesis And Electrochemical Performance Of Graphene/Transition Metal Oxide Composites

Nowadays,the commercial anode materials in domestic market are artificial graphite carbon,which are prepared from petroleum coke and needle coke through graphitization.Natural graphite is abundant in our country which can be used as anode material to reduce the cost of anode materials for lithium ion batteries significantly.And natural graphite can be exploited for producing graphene.The composites of graphene and transition metal oxides show not only good conductivity and stability,but also high specific capacity.In this paper,natural graphite is used as a raw material for preparing modified graphite and composite of graphene and transition metal oxides.The structure and electrochemical performances of modified natural graphite,graphene,graphene/ZnO composite and graphene/Mn₃O₄ composites are studied,and the lithium storage mechanisms are explained.To overcome the large first irreversible capacity and poor cycling performance of natural graphite,doping and oxidative modification are studied.Orthogonal experiment method is used to determine the optimum conditions for doping.The best doping amount of 1%,heat treatment time of 2.5 h,and heat treatment temperature of 650 oC are determined.The reversible capacity of the lithium doped graphite is 333.2 m Ah g-1,and the Coulomb efficiency reaches up to 92.7%.After50 cycles,the capacity retention is 347.5 mAh g-1.XRD,XPS and Raman analysis are employed to compare the structure and composition of natural graphite before and after modification.Moreover,the performance of natural graphite with different modification methods are studied.Because of the changes of interlayer spacing and the proportion of 3R crystalline phase of doped graphite,the microstructure of natural graphite is more beneficial for the insertion of lithium ions,thus improving the reversible capacity,coulomb efficiency and cycling performance.For modified graphite oxide,the original structural defects and active sites are peroxided.The oxygen-containing functional groups increase the interlayer spacing and passivate the surface.The consumption of lithium ions is reduced during the first charge and discharge cycle,and a stable SEI film is formed.Therefore,the diffusion of lithium ions is facilitated,and the electrochemical performance are finally enhanced.Graphene nanosheets are prepared via optimized redox method.XRD and Raman analysis reveals the structural changes from natural graphite to graphene.Rough surface,wrinkles,overlapping and microporous holes in graphene sheets are observed with SEM and TEM.HRTEM and AFM results show that the as-prepared graphene sheets are multilayer.The initial discharge capacity of the as-obtainedgraphene is 2059.2 m Ah g-1,and the reversible capacity is 848.3 m Ah g-1,the capacity retention rate could reach 69.5% after 30 cycles.The reversible lithium intercalation capacity is much higher than the theoretical one.The reason is that the as-prepared graphene has lots of lithium storage sites,including sites on the edge of the carbon layers,inside the interface,defects,vacancies and overlapped slices.In order to investigate electrochemical performance of the compounds of graphene oxide and transition metal oxides,ZnO with alloy-type lithium storage capability and Mn_xO_y with conversion-type lithium storage capability are selected.The influence of the composite structure on electrochemical properties is discussed.The GN/ZnO composite is synthesized via a simple liquid method.ZnO and graphene form three-dimensional structure through 0D-2D?zero dimensional-two dimensional?connection.The initial discharge capacity of GN/ZnO composite is1155.3 mAh g-1,and the reversible capacity reach 650.8 mAh g-1.After 30 cycles,the capacity retention rate is 60.1%.On the opposite,the capacity retention of ZnO is below 40% after 10 cycles.Graphene actes as a conductive network to increase the conductivity of ZnO,and a supporting flexible framework to sustain the volume change during the formation of alloy by lithiation reaction.The composite can remain the original structure after charging and discharging,and the sharply decreasing capacity of ZnO due to the structure collapse is thus largely avoided.Moreover,the 0D-2D stable structure of the composite can reduce the charge transfer resistance,and the synergistic effect of graphene and ZnO nanostructures improved the specific capacity and cycling stability of composite materials.Mica-like nanosheets Mn₃O₄ and Mn₃O₄-NS/GNS composite are prepared by the liquid precipitation method.The as-obtained Mn₃O₄ nanosheets is irregular.Mn₃O₄ nanosheets are embedded on the surface of the graphene forming a stable multi-layer 2D-2D overlapping structure,accompanied with nanoholes between the stacked nanosheets.The first reversible capacity of Mn₃O₄-NS/GNS composite is1003.0 mAh g-1.After 50 cycles,the remaining capacity is 1158.5 m Ah g-1.Even at a high current density of 1000 mA g-1,the capacity of 637.4 m Ah g-1 can be obtained after multiple cycles.In contrast with the composite,the capacity of uncomplexed Mn₃O₄ phase is only 90.5 mAh g-1 at the same current density.The mechanisum for the enhanced performance of the composite material is the formation of a stable 2D-2D structure,which provids additional lithium storage space,alleviates the stress of volume expansion during charge and discharge,and reduces the charge transfer resistance.This stabilized structure reduces the restacking of graphene nanosheets and improves the electron transfer and Li+diffusion rate.Therefore,the cycling and rate performances of composites are significantly enhanced.Compared with GN/MnO₂ connected by 0D-2D structure,this 2D-2D structure is beneficial to inhibit the volume effect of transition metaloxides during charge and discharge.This type of structure can be extended to other transition metal oxide lithium storage systems.