Modification Of LiFePO₄ For Cathode Material Of Lithium Ion Battery

This paper demonstrates the modification of LiFePO4 by carbon doping, Mg2+ doping and mesoporous compositing with carbon aerogels to improve its electronic conductivity and Li+ diffusion coefficient, and solves the problems of the low specia capacity in applying and the poor electrochemical performance in high current charge/discharge of LiFePO4.LiFePO4/C composite was prepared via liquid phase method by using PEG as carbon source. Then the structure, morphology and electrochemical performance of the composite were studied by changing the quantity of carbon doping. The results show that carbon doping can reduce the size of the particles, shorten the distance of charge migration and improve the electronic conductivity, which was six orders of magnitude higher than not doped. The results also indicate that the best doping quantity of carbon is 9.40%(wt%), under this condition, the initial discharge capacity of the LiFePO4/C composites was up to 156.2mAh/g,121.4mAh/g and 85.4mAh/g at 0.1C,1C and 5C.LiFe1-xMgxPO4/C composite was prepared using Mg(NO3)2·6H2O as Mg2+ source to partially replace Fe. The results show that it is favor for the Li+ diffusion ability by Mg2+ doping and the best doping quantity of Mg2+ is 10%(mol%), under this condition, the initial discharge capacity of the LiFe1-xMgxPO4/C composites was up to 158.1.mAh/g,131.6mAh/g and 101.OmAh/g at 0.1C,1C and 5C.The carbon aerogels were oxided in ammonia solutions heating by microwave. It is found that the treatment can increase the amount of hydrophilic functional proups on CAs, which is of advantage to ameliorate its dispersion in water-based solvent. In addition, the pore structure is modified and the degree of graphitization is improved, which are closely related to the electrical conductivity of CAs. The results reveal that CAs treated for 10second has better dispersion in aqueous solution and the highest conductivity,58%higher than the CAs without treatment.The C-LiFePO4/CA-10 mesoporous composite was synthesized by using carbon aerogels as hard template and PEG as reducing agent. The results indicate that the C-LiFePO4/CA-10 composite exhibits high conductivity and behaves excellent electrochemical properties. Its discharge capacities under 0.1C were all above 164.8mAh/g, and its initial discharge capacities under 1C and 5 C were 130.6mAh/g and 103.5mAh/g respectively. After 30cycles, its capacities maintained at 98.36%,97.55% and 97.70% respectively, which shows excellent cycle stability.