Synthesis Of Cathode Material LiFePO₄ Of Lithium Ion Batteries By Sol-Gel Method

LiFePO₄ is considered as a promising cathode material for lithium-ion batteries due to its merits such as low cost, environmental friendliness and good electrochemical capability, etc. However, the electron conductivity of LiFePO₄ is just 10^-10～10^-3 cm²/s. The capacity of LiFePO₄ is low at high discharge rate. On the base of a systematical on the research and the development of LiFePO₄ as a cathode material of lithium ion battery, a Sol-Gel method is adopted to prepare LiFePO₄ by using glycol as solvent and LiOH, FeC₂O₄ and NH₄H₂PO₄ as reactants in this paper in the aims of developing an available technique to prepare LiFePO₄ with high properties. The effects of the fabrication parameters including sintering temperatures, pre-sintering temperature of the gel precursor, sintering atmospheres and surface active reagent addition on the microstructure and electrochemical properties of LiFePO₄ were investigated. The high rate capacity of LiFePO₄ is increased by optimizing the fabrication technique.The microstructure of LiFePO₄ prepared by different fabrication parameters was characterized by XRD, SEM and TEM, etc. Cyclic Voltammogram (CV) and Electrochemical Impendance Spectroscopy (EIS) were used to study the electrochemical property of LiFePO₄ cathode and the charge and discharge capacity were also measured.The results show that Ethylene Glycol decomposed in-situ forming a network carbon film covering the surface of LiFePO₄ particles during the sintering process, and effectively improved the electronic contact of the LiFePO₄ particles thus improved the electrochemical property of LiFePO₄ cathode. Sintering temperatures, pre-sintering temperature of the gel precursor, sintering atmospheres and surface active reagent addition affected the crystallization and electrochemical properties of LiFePO₄.In the case of a pre-sintering temperature of 100℃ and a sintering atmosphere of 5%H₂+N₂ were used, only phase of LiFePO₄ was detected in the synthesized products by XRD when the temperature ranging from 600 to 700℃. When the sintering temperature was increased to 750℃, impurities of Li₃PO₄, FeP and Fe₂P formed in the products. The LiFePO₄ particles sintered at 700℃ was homogeneous and spherical, which display the largest discharge capacity among the samples sintered in different temperature. The discharge capacity reached 141 mAh/g at the rate of 1C, where 1C is 170 mAh/g, and the capacity almost preserved after 200 cycles.On the fabrication condition of sintering temperature of 700℃ and sintering atmosphere of 5%H₂+N₂ were used, the samples pre-sintered at 90℃ and 110℃ displayed high phase purity of LiFePO₄ of small particle size and uniform distribution. As the temperature increasesto 130℃ and 150℃, impurities of Li₃PO₄, Fe₂P and FeP formed in the products, causing a decrease of the capability of LiFePO₄. The sample pre-sintered at 110℃ exhibits the largest capacity.On the fabrication condition of sintering temperature of 700℃ and pre-sintering temperature of 110℃ were used, little amount of hydrogen (5%) favors the formation of a uniform distributed small size of LiFePO₄ of high purity. When the content of hydrogen was increased to 15%, impurity phases formed in the products and the distribution of the LiFePO₄ size was uniform, causing a decrease of the discharge capacity. The LiFePO₄ sintered in 5%H₂+N₂ demonstrated a higher capacity than that either sintered in pure N₂ or in atmosphere with higher H₂ content.The addition of surface active reagent of soybean lecithin led to the formation of impurity of Fe₂P, causing a decrease of the capacity of the electrode. Moreover, with increasing the addition of the soybean lecithin, the particle size of LiFePO₄ increased and became uniform, which also reduced the capacity of LiFePO₄.