Synthesis And Modification Of Nano LiFePO₄ As Cathode Materials

Olivine-structured orthophosphate LiFePO₄ has become a highly promising cathode material for lithium ion batteries due to its high operating voltage (³.5V vs. Li/Li+) and large theoretical gravimetric capacity (¹70mAh/g), as well as its low cost, non-toxicity and safety advantages. However, pure LiFePO₄ shows low reversible capacity and poor charge-discharge characteristics at high current density because of its poor electronic conductivity. Some methods such as reducing particle size, adding conductive additive, and cationic doping were taken to improve the electrochemical performances of LiFePO₄, and some progresses have been achieved so far.In this thesis, pure LiFePO₄ was successfully prepared by hydrothermal reaction. The synthesis conditions were systematically studied. Under the optimum condition, the as-prepared LiFePO₄ was modified. The thermal stability of LiFePO₄ was analysed by TG-DTA; the structure and morphology were characterized by XRD, SEM. Electrochemical performance was investigated by galvanosatic charge-discharge cycling test, CV and EIS.Experimental results show that LiFePO₄ can be successfully synthesized by hydrothermal reaction. Hydrothermally reacted at 180℃for 18h, the obtained LiFePO₄ shows the lowest cationic disorder, good crystallinity, optimal particle size and the best electrochemical performance. The initial discharge capacity is 142 mAh/g. Adding reducing agent, such as ascorbic acid, in the starting materials can effectively prevent the formation of impurities. Heating mode will also impact on the product's performance.Carbon coating can effectively enhance the electrochemical performance for LiFePO₄. LiFePO₄ coated with 3 wt.% carbon exhibits the best performance. The discharge capacity is 142 mAh/g at 0.2C, and the capacity fade is only 0.7% over 50 cycles. Carbon coating methods can also influence the performance of LiFePO₄. The results indicate that LiFePO₄ grinding with carbon source can reach the best performance. Silver coating is also a way to improve the electrochemical performance for LiFePO₄. LiFePO₄ coated by 1% silver has the best performance, which shows a discharge capacity of 133 mAh/g. After 25 cycles, the capacity fade is 19.5%.Mn²⁺ doping has some influences on the electrochemical performance of the olivine cathode. In particular, the carbon-free LiFe0.95Mn0.05PO₄ exhibits a specific capacity of 132 mAh/g at 0.2 C, which is 10% higher than that of the pure LiFePO₄.