Synthesis And Characterization Of Graphene Based Composites As Anode Materials For Lithium-ion Batteries

Rechargeable lithium-ion batteries have been widely used as power sources in different areas owing to their high energy density, high operating potential, long cycle life. The low specific capacity (about 330mAh/g) of commercialized graphite-based anode materials can not satisfy the requirements for high energy density of batteries, therefore it is urgent to develop new anode materials with larger capacity. Graphene has been considered as a potential candidate of anode material for lithium-ion battery because of its special properties, such as good mechanical property and electrical conductivity, which was relating to its special structure of single or few layers of graphite. In this thesis, graphene/Co(OH)2 and graphene/CoO composite were synthesized successfully and the performance of these composites as lithium-ion batteries anode materials were studied.Synthetic methods of graphene oxide and graphene nano sheets (GNS) were suggested first. Graphite oxide was obtained through oxidation of graphite using strong oxidant, and then GNS with few layered and compact structure were obtained through reduction of graphite oxide. Initial discharge capacity of GNS as anode material was 1472.3mAh/g at the current density of 200mA/g and initial Coulombic efficiency was 25.6%. After 50 cycles, the reversible capacity was 228.3mAh/g. These data showed the cycle performance of GNS as anode material for lithium-ion batteries was not satisfied.A GNS/Co(OH)2 composite as a high performance anode material was firstly prepared through a simultaneous hydrothermal method.. The key factors of reaction conditions were investigated and the optimized reaction conditions were: grapheme oxide density 0.375mg/ml, pH 10, reaction temperature 150℃and reaction time 5h.? The microscopic structure and surface morphology of as-prepared GNS and composites were analyzed by XRD and TEM.? Electrochemical properties of the material were measured by charge-discharge test and cyclic voltammetry(CV). Ex-situ X ray diffraction(ex-situ XRD) technique was applied to analyze the lithium insertion and extraction mechanism of GNS/Co/(OH)2 electrode. The result showed that initial discharge capacity of GNS/Co(OH)2 was 1519.8mAh/g at the current density of 200mA/g and initial Coulombic efficiency was 70.3%,the reversible capacity was remained as 87.7% after 50 cycles.? The structure, morphology and electrochemical performance of the obtained GNS/CoO composite through calcination of GNS/Co/(OH)2 were systematically investigated by X-ray diffraction (XRD), transmission electron microscope (TEM) and electrochemical measurements. The result showed that the cycling stability of GNS/CoO material was poor and further improvement was needed.In a conclusion, GNS/Co(OH)2 material showed a high capacity and excellent cycling stability, which was proved to be a promising anode material for lithium-ion batteries. Through further research of its mechanism and further improvement of its preparation technology, a new anode material for lithium-ion batteries with low price and higher capacity is going to be obtained.