A Fundamental Study On The Development Of Lithium Iron Phosphate Cathode

Lithium iron phosphate, LiFePO₄, has recently attracted significant interest because of its low hygroscopicity, low cost and environmentally friendly components. The aim of the present study were to focus on the preparation processes, the modification of materials, the structural characterization, the electrochemical properties, and the kinetics behaviors of the olivine lithium iron phosphate as cathode materials for rechargeable lithium batteries.Effects of the preparation conditions, such as precursor material, annealing temperature and time, atmosphere and grinding process et al., on the structure, particle size and electrochemical properties of LiFePO₄ were studied by using TG-DSC, XRD, SEM, LSD, CV, EIS and electrochemical charge-discharge tests to optimize LiFePO₄ production process.Attempts were made for the first time to use TG-DTA to study the solid state reaction processes of LiFePO₄. The activation energies of each reaction process, reaction orders and frequency factors were determined by making use Doyle-Ozawa method and Kissinger method. The method provides reliable predication for further scale-up and research of the process.The effects of different carbon source on the performance of LiFePO₄ were systematically investigated. The results demonstrate that pyrogenation and glycerin, added as conductive precursor before the formation of the crystalline phase, can optimize the performance of LiFePO₄ effectively. The carbon content was optimized, and the results show that the material obtained by adding 10 wt. % C have much higher discharge capacity. The material also displays a more stable cycle-life than the others.The effects of cation doping on the physicochemical structure and electrochemical performance of produced cathode were systematically investigated. at different doping position. The doping mechanism was discussed from the aspects of cation valence, cation radius and the crystal defect of LiFePO₄. And the criterion to choose the cations was proposed, that is the supervalent cations, which have the similar radius with Li+, have good doping effects.Attempts were made for the first time to improve the co-precipitation method by designing feed-in device to control the growth speed of the deposit, which could make deposit well-proportioned. The results indicated that the improved co-precipitationmethod was an effective route to improve the electrochemical of LiFePO₄/C material.The open circuit voltage and the differential capacitance plot for LiFePO₄ with the change of intercalation compositions have been measured. The lithium ion diffusion coefficients have been obtained by EIS method and the results indicate that its values are change with the change of lithium ion intercalation compositions.