Study On Hydrothermal Synthesis And Electrochemical Performance Of Carbon-coated Lifepo₄

Olivine-structured LiFePO₄, as a promising cathode material for lithium-ion battery, is considered as the most competitive material for rechargeable lithium-ion battery in the future due to its merits such as low cost, good electrochemical performance and security, etc. However, the low conductivity of LiFePO₄ and poor lithium diffusion limit the high rate capacity in lithium cells. Hydrothermal methods were used to prepare LiFePO₄ and the mechanism of the reaction were explored. The research on when the carbon precursor should be added was carried out. The effects of heat treatment, selection of different carbon resources were disscussed in the heating process.LiFePO₄ was synthesized by hydrothermal method using FeSO₄, H₃PO₄, LiOH as raw materials. Citric acid, as the complexing agent, and ascorbic acid as the reducing agent supply a stable environment to Fe in the process of reaction. Little LiFePO₄ crystal nucleus and lots of amorphous Fe₃(PO₄)₂ and Li₃PO₄ were produced when prepared the mixed fluid. Fe²⁺ Li⁺ PO₄^3- enter in the crystal lattice to synthesized regular LiFePO₄ crystal in the earlier 4 hours. Some lattice variance were bring in in the next synthetic time. The optimal systhesis process was using water as solvent, FeSO₄ : H₃PO₄ : LiOH=1: 1:3(mol) as raw materials, to hydrothermal react at 180°C for 6h. The concentration of the liquid compound was 0.3mol/l. The sample was prism-like particles, and average size was 0.6μm, tap density was 0.97g/ml.Glucose as carbon sources was added into the compound before hydrothermal. The particle size was smaller to be 0.3μm, and the crystal structure was more regular. The sample displayed better electrochemical performance after heat treatment. The specific charge-discharge capacity respectively were 127. 1mAh/g, 102.6 mAh/g at the first cycle at 0.1C. After 10 cycles, the sample keep 93.14% of the capacity.Microwave method and atmosphere method were designed in the heat treatment process. The efficiency of microwave heat treatment is high which can decompose organic carbon such as glucose in few minutes so as not to increase the particle size. The shortage of microwave method was it can't decompose the carbon completely. The atmosphere heat treatment method which was capable of decomposing the organic carbon completely, however, will increase the particle size, and this method has a low efficiency. Samples with glucose as carbon source, heated by these two methods, have a better performance than that with urea as carbon source. LiFePO₄-C samples synthesized through hydrothermal method with glucose as carbon sources were tested followed by microwave heat treatment, and the samples displayed charge-discharge specific capacity of 122.7mAh/g and 104.5mAh/g and had no reduction basiclly.