Synthesis And Performance Studies Of LiFePO₄ And Doped LiFePO₄ Cathode Materials For Li-ion Batteries

Lithium iron phosphate with an ordered olivine structure has many excellences in contrast to commercial Li-ion cathode material at present such as good charge-discharge character, high reversible capacity and good cyclic stability, which is a promising cathode material due to iron's abundant resources, non-toxicity, environmental benign. Based on the reviews for research progress of LiFePO₄ cathode material, the structure, working principle, preparation methods and existy mostly problem were further introduced. Solid-reaction was used to synthesize lithium iron phosphate. The crystalline structure, morphology of particle and electrochemical performance were investigated by x-ray diffraction, scanning electron microscopy and constant current charge-discharge experiments. The effects on the physical characteristics and electrochemical performances of sample by different experimental conditions were investigated systematically.With FeC₂O₄ · 2H₂O, Li₂CO₃, NH₄H₂PO₄ which were activated by ball-milling as raw materials, LiFePO₄ was prepared by sintered at high temperature under the protection of the inert gases. Characterized by X- Ray diffraction, the sample was found with an order olivine structure, no impurity and single phase. The sample was made into simulated batteries to be tested on the electrochemical capability, which showed fine electrochemical performance. It's indicated by the orthogonal experiments that temperature was the most influential one among many elements, and then the reaction time, rate of temperature ascending and mole ratio of / Li⁺ /Fe²⁺.The second specific capacity of the charge-discharge with the rate of 0.1C at room temperature of the samples synthesized under the optimized conditions reached 145.7mAh/g, 85.7% of the theoretical capacity, with almost no descension after circulating for 10 times. But in the high-rate test the capacity decayed fast and polarized seriously.The effect of the Mn²⁺ doping was studied, which showed that Fe and Mn could form a solid solution with order olivine structure. And the LiFe_0.8Mn_0.2PO₄ has the best electrochemical performance. LiFe_0.8Mn_0.2PO₄ synthesized under the optimum conditions displays a good cycle performance and high reversible capacity at room temperature, which first specificcharge-discharge capacity is 136.5/131.7mAh/g and has 126.7/126.2mAh/g after 50 cycles and 119.6/118.8mAh/g after 100 cycles at 0.5C rate.Compared LiFePO4 with LiFeo.gMno.2PO4, both synthesized under the same conditions, the latter got the better performance, .The average specific capacity of LiFePO4 reduced from 142mAh/g to 120nAh/g when the discharge rate ascended form 0.1C rate to 0.5C rate while that of the LiFeo.8Mno.2PO4 reduced from 135mAh/g to 130mAh/g. The result showed that the doping of Mn2+ decreased the polarization and increased the diffusive ability of Li+. The AC impedance of both materials were tested and the diffusion modulus was calculated, of which the LiFePO4 was 2.371 X 10"16cm2/s and that of LiFeo.sMno.2PO4 was 4.411 X 10"14cm2/s.