Study Of Lithium Iron Phosphate Prepared By Aqueous Precipitation

Owing to its steady crystal structure, high specific capacity, good discharge platform and long cycle life, Lithium iron phosphate is considered as one of these most promising cathode materials for lithium-ion battery. But there are three defects limiting Lithium iron phosphate to be the key materials of power battery. Firstly, the poor conductivity causes the specific capacity and rate capability of the material to be displayed difficultly. Secondly, the tap density of lithium iron phosphate is not high enough, which largely affects its volume energy density. Finally, there is high energy consumption and large exhaust emissions in the current production. Aiming to solve these problems, some solutions have been proposed. Iron phosphate has been prepared using trivalent iron compound instead of bivalent iron compound, focusing on the research on controlling the particle size and morphology of precursor and these process parameters of LiFePO4 prepared by CRT (carbothermal reduction). Based on the above, we doped titanium dioxide into LiFePO4 to enhance its bulk conductivity. The work focuses on the following four parts aiming to solve the above three defects.(1) Spherical iron phosphate was prepared by precipitation. These factors such as iron sources, water temperature and PH were studied in the precipitation. Under these conditions, ferric chloride and phosphoric acid as raw materials, above 90℃, PH = 0.9, the spherical particle of iron phosphate in the range of 1 ~ 2μm can be prepared.(2) Controlled-morphology growth of iron phosphate assisted by surfactant. On the basis of Zeta potential, small molecular surfactant and polymer surfactant were introduced. The data show that the sample modified by small molecular SDS (sodium dodecyl sulfate) surfactant was surface smooth and spherical-likely particles, which is favor for the high material tap density; while the polymer surfactant PVP (polyvinyl pyrrolidine) modified iron phosphate was particles with lamellar and agglomeration.(3) Lithium iron phosphate was obtained by CTR. The ingredients, dispersing agents, reducing agents, sintering process conditions and other major parameters influencing on structure and properties were studied in the process of CTR. The Li:Fe= 1.02:1, alcohol medium as dispersing agents , glucose as reducing agents and sintering at 750℃for 12 hours were the optimization conditions. Studies have shown that, with acetylene black as carbon source, at 0.1C, the initial capacity of lithium iron phosphate is up to 152 mAh / g. However, at 0.2C, the specific capacity of the sample has fell to 125 mAh/g after 50 cycles, while in the case of glucose, the specific capacity still maintained 130 mAh/g. (4) Lithium iron phosphate composite was prepared using TiO2 as dopant. Based on the CTR, with glucose as carbon source, titanium dioxide as the dopant, the effects of titanium on the crystal structure of lithium iron phosphate were carefully investigated. Doped lithium iron phosphate improved the initial capacity and rate capacity. The first charge capacity and discharge capacity of the sample reached 157.9 mAh /g and 155.2 mAh / g. at 1C rate, and its specific capacity is still 130 mAh / g. after 100 cycles. The results of CV (cyclic voltammetry) and EIS (electrochemical impedance spectroscopy) showed that both the polarization and resistance of doped LiFePO4 composite has been obviously reduced.