Combining Effects Of Oxide On The Electrochemical Performance Of LiFePO₄ Cathode Materials

The low conductivity of LiFePO₄ was improved by the method of metal ion doping and metal oxide coating. The intrinsic conductivity of LiFePO₄ can be increased by metal ion doping, which can optimize material performances. The surface conductivity of LiFePO₄ and the uniformity of the carbon coating can be improved by the method of metal oxide coating, which is considered as an effective means. Therefore, it has theoretical and practical value when the two effects of metal oxide coating and metal oxide doping on LiFePO₄ are being combined.The modified samples of LiFePO₄ were prepared by carbon thermal reduction method in this paper. The microstructures, morphologies and electrochemical performances of these composites were characterized by galvanostatic charge-discharge test system, XRD, SEM, TEM, and the testing of conductivity. The effects of metal cation doping, carbon and metal oxide co-coating and the combining effects of metal oxide coating and doping were studied.MgO, Mo₂O₃ and V2O5 were selected as doped metal oxides and the performance of the doped samples was shown as follows: the morphology and particle size of material were affected little by doping and a high crystallinity of olivine LiFePO₄ structure was synthesized. Charge-discharge tests showed that doping enhance the rate capability effectively, especially at the high-rate. The discharge capacity of Li0.99Mg0.01FePO₄/C is about 120mAh/g at 5C rate, which is higher than the other doped samples. The attenuation ratio of Li0.99Mg0.01FePO₄/C, which is higher than the un-improved sample of 16%, is 19% at 3C rate after 200 charge and discharge cycles.The electrochemical properties of coated samples showed that the discharge capacity of coated samples at different rates were higher than the unimproved sample and the capacity fading of all coated samples were less than 10mAh/g at 3C rate after 200 cycles, exhibited excellent cycling performance. The result shows that the modified metal oxide coating can improve the rate capacity and cycle performance of materials. Al₂O₃ coated sample shows the best cycling performance.Li0.99Mg0.01FePO₄/Al₂O₃/C (LMFP/Al/C) was prepared by carbon thermal reduction and co-precipitation method. The rate capability and cycling performance of LiFePO₄ materials can be improved by the composite effects of carbon and Al₂O₃ coating and Mg2+ doping. The discharge capacity of sample at different rates (0.2, 1, 2, 3 and 5C) is about 150, 138, 125, 118 and 110mAh/g respectively, which is higher than the other modified materials. The initial capacity of LMFP/Al/C at 3C rate is as high as 129mAh/g, with almost no attenuation after 200 cycles.