Preparation And Properties Of Lithium-ion Battery Cathode Materials For Lithium Iron Phosphate

Olivine-structured LiFePO₄ is considered as a promising cathode material for lithium-ion batteries, due to its advantages such as environmentally friendliness, abundant raw materials, inexpensive price and high safety. However, it suffers from low conductivity and poor rate performance, which prevents it from commercial use. To improve the electrochemical performances of LiFePO4 material, the synthesis condition, metal doping, carbon coating, electrochemical behavior and kinetics of lithium insertion-extraction process of LiFePO₄ as cathode materials were studied in detail in this dissertation.The cathode material LiFePO4 was synthesized by carbothermal method, using LiH₂PO₄ and Fe₂O₃ as raw materials. The crystalline structure, morphology, carbon contents and electrochemical performances were investigated by X-ray diffraction, scanning electromicroscope, carbon/sulfur analyzer and electrochemical tests. The effects of synthesis time, temperature and carbon contents on electrochemical performances of LiFePO₄ material were discussed. In this experiment, the optimized synthesis condition was obtained, the ration of Li/Fe was 1, the amount of carbon in precursor compound was 12%, the material was synthesized at 750℃for 15h, under a flowing inert gas in the process.The cathode material LiFePO₄ synthesized by above synthesis condition. The results showed that the initial discharge capacity of LiFePO₄ was 141.8 mAh/g in 2.5～4.2V and at 0.1C rate and 125.SmAh/g at 1C rate, retaining 95.3% after 100 cycles.The structure and properties of LiFePO₄ doped by Mg²⁺ and Ti⁴⁺ were investigated. Doped LiFePO4 exhibited the initial discharge capacity of 146mAh/g and 150.8 mAh/g in 2.5～4.2V and at 0.1C rate, 130.5mAh/g and 133.7 mAh/g at 1C rate, as Mg²⁺ and Ti⁴⁺ doping content were 2mo1% and lmol% respectively; These indicated the doped material has high discharge capacity and good cycling property. Meanwhile electrochemical discharging behavior of LiFePO₄ material at high rates was investigated in this dissertation, which exhibited a satisfied result. The kinetics behaviors of olivine LiFePO₄ and modified samples were studied by means of cyclic voltammetry and electrochemical impedance spectroscopy measurements. The results indicated that only one oxidative and reductive electrochemical reaction occurred during the charge and discharge of LiFePO₄. The material doped by metal ion which possessed less electrochemical polarization and better reversibility in the lithium insertion-extraction process. Electrochemical impedance spectroscopy indicated that the doped material also possessed less charge transfer resistance. And then improvement of the electrochemical properties of LiFePO₄ was realized.