Synthesis And Characterization Of LiFePO₄

LiFePO₄is one of the most potential positive materials in the lithium ion battery. It hasbecome a focus of research materials inland and abroad due to safety，low raw materials cost,good thermal stability and good environmental compatibility etc. But lithium iron phosphatealso has its own weakness, such as lower electronic conductivity and low lithium ionmigration rate, which terribly affects its electrochemical performance and the applicationprocess in the lithium batteries. In this paper, LiFePO₄was obtained by gel process with theFePO₄precursor which was obtained by co-precipitation, the oxalate and citric acid whichwere used as carbon sources and lithium hydroxide as lithium source. The obtained LiFePO₄powder with spherical and the narrow distribution was of the small and uniform particle size.For preparing the FePO₄precursor, effects of the parameters on as-synthesized LiFePO₄were studied, such as the choice of the precipitator, the control of pH, adding speed ofammonia water, contents of phosphoric acid and aging time. The results showed that theoptimum process was that ammonia water chosen as the precipitator which was added at thespeed of1.4ml/sec, pH was adjusted to3.5, the molar ratio of phosphoric acid and ferrousphosphate was1:1, and the aging time was10minutes.For preparation LiFePO₄by Gel method, effects of the parameters on as-synthesizedLiFePO₄were studied, such as the water amount, the molar ratio of citric acid and oxalic acid,the choice of carbon sources, stirring time, the drying temperature, mixing method and addingorder of raw material. The results showed that adding water was250g/molar of precursor andthe molar ratio of oxalic acid and citric acid was1:0.67. The mixing method was that citricacid was first added into the water, and then added the oxalic acid, lithium hydroxide and theFePO₄precursor in order, finally the mixtures were stirred for15hours and dried at65-120oC. The addition of carbon sources made the obtained LiFePO₄have more uniform andsmaller with the increasing sucrose. Seen from the size dispersion of as-synthesized LiFePO₄,the result of the adding sucrose was better than PEG.During the heating treatment, the effects calcination temperature, heating curves and theholding time on the powder were studied. The results showed that particle size was bigger andthe surface was smoother with increasing temperature. At700oC., as-synthesized LiFePO₄began to appear the phenomenon of sintering. The faster heating rate is, the smaller grain sizeis, and the slower heating rate is, the better the dispersion of grains is. With increasingholding time, particles size became bigger and the shape became spherical.