Iron-based Compound/Carbon Composites For Lithium-ion Batteries

Lithium-ion batteries have attracted extensive interest due to high energy densities and excellent performance. With the rapid developments of portable electronics and electric vehicles, lithium-ion batteries with low-cost, high performance, and high safety are in great demand. Traditional carbon anode materials can not meet the requirements of the next generation lithium-ion battery due to a low capacity of372mAh-g-1. Consequently, developments of novel anode materials with higher capacity, high stability, high safety and low cost are particularly urgent.Because of their high theoretical capacity, low cost and environment friendly advantages, iron oxides Fe2O3, Fe3O4as anode materials of the lithium-ion battery are becoming the focus of attention. However, these materials are currently with some problems such as low rate capability and short cycle life. Composites of iron oxides and carbon with designed nanostructures are expected to enhance the electrochemical performance. In general, crystal structure, morphology and nanosize of composites have significant influences on their electrochemical properties. Therefore in this thesis, several iron-based compounds and their carbon composites were synthesized, characterized and electrochemically evaluated.1. Nano-plate LiFePO4as cathode materials was synthesized by hydrothermal method through adding ascorbic acid to inhibit particle’s growth. The reaction temperature and time of hydrothermal treatments on LiFePO4nanoparticles’ morphology and electrochemical performance were examined. The optimum hydrothermal conditions of180℃,10h were found to achieve the best electrochemical performance.2. Fe2O3nanoparticles as anode materials for lithium-ion batteries were prepared by the hydrothermal method. Different morphologies such as pyramid, porous spheres were achieved through the addition of surfactants such as EDTA. But the Fe2O3nanoparticles demonstated limited cycle life. The cycle performance was found to be sensitive to the ratio of acetylene black added in the anode. To improve the cycle life of Fe2O3anode materials, it is important to synthesize composites of oxides with carbon.3. The nanocomposites of Fe3O4and mesoporous carbon were synthesized by heating iron-based xerogels in N2. Varying the amounts of iron compounds, gelation agents and solvents affected the morphology of xerogels and resulting Fe3O4/C composites. The effects of heat treatment conditions on morphology, specific surface area and electrochemical properties are also examined. The results showed that the best electrochemical performance was achieved on the600℃,4h heat treated P2-2X sample, which is attributed to the favorable nanostructure in which Fe3O4nanoparticles dispersed homogeneously in the mesoporous carbon matrix.