Preparation Of Iron Phosphate And Its Application

Layered LiNi0.5Co0.2Mn0.3O2 and spinel LiMn2O4 are promising cathode materials for next-generation Li-ion battery, due to their low toxicity, environmental-friendliness, low cost and relatively high energy density. However, the cycling stability of LiNi0.5Co0.2Mn0.3O2 deteriorates rapidly in the charge-discharge cycle process because of side reactions between LiNi0.5Co0.2Mn0.3O2 and electrolyte which leads to the structure change of the material surface and increase in polarization electrode reaction. For spinel LiMn2O4, the capacity decays rapidly especially at high temperature (60 ℃) in charge-discharge cycle process due to dissolution of Mn and Jahn-Teller effect.Surface coating is one of the effective means to improve the electrochemical properties of cathode materials. It can reduce side reactions between cathode materials and electrolyte by forming a coating layer in the cathode material surface which makes cathode structure more stable in the process of charge-discharge. In this paper, co-precipitation method is adopted to coat LiNi0.5Co0.2Mn0.3O2 and spinel LiMn2O4 with homemade nano FePO4 suspension to effectively improve cathode materials’ cycle performance.The best conditions to prepare pure FePO4 were obtained by using liquid phase method:pH=2.0, T=80 ℃, molar ratio of phosphorus and iron=1:1. Under this condition, nano FePO4 suspention is successfully prepared by using hydrothermal method at 80 ℃ for 10 h in sealed reaction kettle. Then, the effects of solid phase reaction temperature, sintering time and coating quantity on the structure and electrochemical performance of coated LiNi0.5Co0.2Mn0.3O2 by co-precipitation method were studied respectively.The optimal conditions are determined:solid phase reaction temperature 400 ℃, sintering time 12 h, coating quantity 2 wt%. The results shows that FePO4 nano coating layer is uniformly adhering to the suface of LiNi0.5Co0.2Mn0.3O2 and it is still α-NaFeO2 layered structure. The discharge capacity of pure LiNi0.5Co0.2Mn0.3O2 sample and that with 2 wt% FePO4 coating at 1 C rate after 100 cycling times are 109.6 and 120.6 mAh/g, respectively, and the corresponding capacity retention rate are 76.12%,85.84%,respectively. The influence of coating quantity on the electrochemical performance of LiMn2O4 was also explored by controlling solid phase reaction temperature 400 ℃ and sintering time 12 h. The results show that, the 2 wt% FePO4 coating LiMn2O4 has the best high temperature cycle performance among the coating quantities and the first discharge capacity changes little after coating and capacity retention rate increases from 74.01% to 87.82%. Cyclic Voltammetry and Electrochemical Impedance Spectroscopy testing results reveal that FePO4 coating can reduce the polarization in electrode reaction and suppress the increase in membrane impedance and charge transfer impedance to improve the cycling performance of cathode materials.