Study Of Synthesis Of Lifepo₄ By Hydrothermal Method And Its Property Modification

Lithium ion batteries are used in cellular phones, notebook computes, and digital cameras. They have also attracted considerable attention as potential power sources for electric and hybrid vehicles. The active material in the cathode is widely either LiCoO2 or LiMnO2 because of the high reversible capacities. However, LiCoO2 is toxic and causes collapse of construction for overcharge. LiMnO2 are limiting for the cycling and thermal stability.Lithium iron phosphate (LiFePO4) has been regarded as one of the most promising cathode materials, due to its lower cost, improved safety, highly reversible and repeatable property, this material was intended for Hybrid Electric Vehicle(HEV) and plug-in hybrid electric vehicles (PHEV). However, the low lithium-ion diffusion rate and low electronic conductivity has constricted its commercial applications. In recent years,Improvements in the electrode conductivity have been introduced by doping other atoms, carbon coating and reducing the particle size.LiFePO4 can be formed by different methods including co-precipitation method, solid-state and hydrothermal method.The LiFePO4 were synthesized by the hydrothermal method and synthesis process from starting reagents of LiOH,(NH4)2Fe(SO4)2·6H2O,H3PO4 and ascorbic aid. The basic parameters method can be knowed in the hydrothermal. For example:pH, the reaction temperature and the reaction time.Reducing agents were added in the synthesis process. For example:Hypop-hosphorous acid, critric acid and ascorbic acid. The results were found by the x-ray diffraction(XRD), scanning electron microscopy(SEM) and the electrochemical performance. The reducing agents prohibit the oxidation of Fe2+ to Fe3+. Generally, the surface of the LiFePO4 can obtain Fe3+, which reduce the electrochemical performance. In addition, the reducing agents can decompose carbon. The carbon coating significantly improves the electrochemical performance and reducing the particle size. However, organic acids (critric acid and ascorbic acid) are better than the hypophosphorous acid clearly. The surfactant agents were added in the synthesis process. For example: polyvinylpyrro-Lidone (K-15), polyethylene Glycol and glucose. The results were found no effect for crystal construction of LiFePO4 Firstly, the surfactant agents were coated the particles, then, Carbon was coated the surface of the particles in the high temperature and pressure.The electrochemical performance was improved. Through the experiments, polyvinylpyrro-Lidone (K-15)coating is better than polyethylene Glycol and glucose.