The Preparation Of Carbon Group Oxides/Graphene Composites And Electrochemical Performance

As an effective electrochemical energy storage device, the lithium-ion batteries?LIBs? have attracted great interests in recent years, with higher energy density, longer cycling life and batter safety. The key to improve the electrochemical performance and optimize battery performance of the Lithium-ion batteries is developing new anode active materials. Currently, the theoretical capacity of graphite commonly used anode material in commercial LIBs is 372 mAhg^-1. Carbon group elements, such as Sn and Si are important members of anode family, which have been widely investigated due to their high theoretical Li-ion storage capacities (>600 mAhg^-1). Si-based and Sn-based LIBs anodes offer specific capacity an order of magnitude beyond that of conventional graphite. However,the formation of stable anodes is a challenge because of significant volume changes occurring during their electrochemical alloying and dealloying with Li.Therefore it is meaningful to search an effective way to ease the volume change and improve the conductivity to make the electrochemical performance of the anode materials better.Based on the special structure, excellent physical and chemical properties of graphene, this article attempts to dope carbon group oxides such as SnO₂ or SiO₂ material on the basis of graphene, composites was prepared, and their structure,morphology and electrochemical performance have been tested. Specific content including:1. The preparation of graphene by oxidation - reduction method and its structure.The graphite was obtained by the modified Hummers method with the natural graphite as raw materials, and the graphene was prepared by reduction of graphite oxide under hydrothermal condition with the hydrazine hydrate as reducing agent. And the successful preparation of graphene can be determined by means of characterization electron microscopy and field emission electron microscopy.2. The preparation of SiO₂/graphene composite and electrochemical performanceThe SiO₂/graphene composite was obtained by hydrothermal reaction under the alkaline condition of graphite oxide and tetraethyl orthosilicate as raw materials.Amorphous SiO₂ is attached to the surface of graphene with a spherical structure having an average diameter of about 100 nm, Electrochemical tests showed that the SiO₂ /graphene composite exhibited the first charge and discharge capacity are respectively 129.5mAhg^-1 and 244.1mAhg^-1,and remain at 73.6 mAhg^-1 for 50th cycles at a current density of 200mAg^-1.3. The preparation of the porous SnO₂ nanospheres/graphene composite and electrochemical performanceThe porous SnO₂ nanospheres/graphene composite was obtained by hydrothermal reaction under the acid condition of graphite oxide and SnCl₂·2H2O as raw materials.By changing the experimental parameters, it can be found that with the increase of the content of graphite oxide, the increase of hydrothermal reaction temperature, the increase of hydrothermal reaction time and the increase of concentrated hydrochloric acid content will promote the growth of SnO₂ nanospheres in the composites, with the diameter of SnO₂ nanospheres increased, while the crystallinity of SnO₂ nanospheres will also be affected. The resulting composite had a specific surface area of 59.50 m2/g,an average pore size of 3.188 nm and a porosity of 0.086 cm3g^-1. Electrochemical tests showed that the porous SnO₂ nanospheres/graphene composite exhibited the first charge and discharge capacity are respectively 1520.76 mAhg^-1 and 952.2 mAhg^-1. After 50 cycles, the discharge capacity of the composite structure is still 683.1 mAhg^-1,the charge capacity is also 658.26 mAhg^-1,and the cycle performance has been improved.