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Synthesis and Modification of Nano LiFePO4 as Cathode Materials

Author: PengXingYuan
Tutor: HuangYunHui
School: Huazhong University of Science and Technology
Course: Materials Science
Keywords: Lithium ion battery Cathode material Olivine LiFePO4 Carbon coating Silver coating Mn2+ doping
CLC: TM912
Type: Master's thesis
Year: 2011
Downloads: 75
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Abstract


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.

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CLC: > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Battery
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