The Synthesis And Lithium Storage Properties Of Transition Metal Oxide/Graphene Composites

In order to build a clean,low-carbon,safe and efficient energy system,as a new generation of energy storage devices,lithium-ion batteries have received extensive attention from all over the world.Transition metal oxides are considered to be one of the most promising cathode materials in the future because of their excellent lithium storage properties.Compared with single-component transition metal oxides,multi-component transition metal oxides show excellent electrochemical performance when used as the anode of lithium-ion batteries due to their complex chemical components and more active sites.By attaching transition metal oxides to the surface of graphene,the advantages of large specific surface area of graphene and high capacity of transition metal oxides can be exploited simultaneously,thus improving the transport rate of lithium ions.Based on the above design ideas,two kinds of transition metal oxide/graphene composites with different heterostructures were designed and constructed using graphene as the substrate.The main work of this thesis is briefed below:(1)Using graphene oxide(CGO)as the substrate,CuO/Cu2O/reduced graphene oxide(CuO/Cu2O/CRGO)was prepared by self-assembly in situ growth method.To investigate its electrochemical properties as anode materials for lithium ion batteries.The test results show that the CuO/Cu2O/CRGO composite material can maintain the high specific capacity of 797.2 mAh/g at the current density of 0.1 A/g for 100 cycles.After 200 long cycles at a high current density of 1 A/g,the composite can still provide an average specific capacity of 586 mAh/g.Combined with the analysis of the lithium storage performance of the composite,more than 54%of the capacitance contribution is caused by the surface capacitance effect of the active material,which provides the rapid charge-discharge characteristics.The excellent electrochemical performance of the electrode is mainly attributed to the synergistic effect between the multiple components.The heterojunction CuO/Cu2O attaches to the surface of CRGO,shorten the transport path between lithium ions and electrons,and reduces the agglomeration problem of heterojunction.Moreover,the introduction of CRGO enhances the electrical conductivity of the electrode.The three-dimensional network structure provides CuO/Cu2O with a reaction attachment point and a large specific surface area,which increases the contact area between the electrolyte and the electrode surface,and optimizes the ion and electron transport at the interface.(2)Preparation of porous layered CuO/Co3O4/reduced graphene oxide composite(CUO/Co3O4/CRGO)by carbonate assisted hydrothermal method.The electrochemical test results show that:Compared with CuO/CO3O4 electrode material,the electrochemical performance of CuO/Co3O4/CRGO composite electrode is significantly improved.When the current density is 0.1A/g,the initial discharge specific capacity can reach 1345.8 mAh/g,and the high reversible capacity can still maintain 894.6 mAh/g after 100 cycles.When the current density increases to 1 A/g for 400 cycles,the reversible capacity of the composite electrode is up to 618.7 mAh/g.The synergistic effect between heterostructure CuO/CO3O4 and CRGO was realized,which take advantage of multiple components.The three-dimensional porous layered structure can provide an effective diffusion channel and shorten the electron transport path.CRGO can inhibit the volume expansion of CuO/CO3O4 to a certain extent,improve the structural stability of the active substance in the process of deintercalation of lithium,and exhibit rapid charge-discharge characteristics.The lithium storage mechanism of CuO/Co3O4/CRGO composites shows that more than 61%of the capacitance contribution by the surface capacitance effect of active materials,showing excellent cycling and rate performance.