The Fabrication, Structure And Properties Of LiFePO₄/C Cathode Materials For Lithium Ion Batteries

LiFePO4 has attracted wise attention due to its low cost, non-toxicity, flat voltage profile and excellent cycling stability. However, the poor electronic conductivity of this olivine-type material limits its high rate performance and thus prevent it being applied in the high power source for HEV or EV application, which are expected nowadays. A sol mechanical activation method is introduced in this thesis in order to optimize the morphology of LiFePO4/C materials and the content of Fe2P, and hence to improve high rate capability. The effects of synthesis conditions including the different solvents of the sol and mechanical activation time, pre-treatment of the dried gel, calcination temperature and time on the structure and electrochemical properties of LiFePO4/C materials are investigated by X-ray diffraction, scanning electron microscope, element analysis and electrochemical testing of galvanostatic charge-discharge, cyclic voltammetry, electrochemical impedance spectra etc.The results show that the sol mechanical activation plays an important role in reducing the particle size and its distribution and increasing the Fe2P content. The discharge capacities at different rates are effectively improved. Compared with distilled water, solvent of ethanol results in smaller LiFePO4 particles and possess better high rate capability. With the mechanical activation time increasing from 2 to 8 h, the high rate capability increases first and then decreases. The sample prepared with 4-hour of sol mechanical activation pocesses highest rate capability due to its moderate Fe2P content and small particle size.High temperature calcination is necessary to produce moderate amount of Fe2P but it commonly increases the particle size. Moreover, low temperature calcination leads to poor crystallinity of LiFePO4, which undermines high rate capability. The combination of low temperature calcination of 600℃and high temperature calcination of 700℃is used to optimize both the morphology and the Fe2P content of LiFePO4/C materials. The LiFePO4 material prepared with a calcination of 600℃for 20 h followed by a calcination of 700℃for 2-4 h shows the best rate capability. Its specific capaities at 0.1C,1C,5C,10C and 20C are 155mAh/g,140mAh/g, 120mAh/g, 110mAh/g,106mAh/g and 100mAh/g, respectively.