Lifepo₄/c Composite Cathode Materials Prepared By Liquid-phase Method

Olivine LiFePO4 appears as an interesting positive electrode material for Li-ion batteries because of its low toxicity, good thermal stability, long cycle life and low cost. The rheological phase method and sol-gel method have the advantages of mixed well of the materials, small particle size which are good for the charge-discharge cycles. Ferric iron as the source to prepare LiFePO4 has the advantages of low cost and simple process, so in this study we chose ferric iron as Fe sources, with the rheological phase method and sol-gel to prepare pure phase and metal ions doped LiFePO4/C material, and test the cycling capability and rate performance at room temperature. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to character the structural and microscopy characteristics. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were used to character the cyclic reversibility and resistance of the materials.LiFePO4/C materials was prepared using sol-gel method and had been studied the influence of different complexing agents, calcining temperature, the lithium salt and content, sintering temperature, sintering time on the material properties had been studied. LiFePO4/C materials was prepared with precursors of ferric nitrate, ammonium dihydrogen phosphate, lithium, sucrose, citric acid, and the discharge capacity could reach to 122mAh/g at 0.2C, but Mg2+ and Al3+ doped materials using sol-gel method were not ideal.LiFePO4/C cathode material was prepared using rheological phase with the precursors of iron nitrate, ammonium dihydrogen phosphate, lithium hydroxide, and had been studied of different metal ions doping on the material properties. The results showed that after sintering 4h calcined at 350℃for the first process, and then sintering 18h calcined at 650℃, pure phase of lithium iron phosphate was prepared and its discharge capacity could reach to 123mAh/g at 0.2C. Doping Zr4+, Al3+, Mg2+ of LiFePO4/C material performance comparison showed that the material doped 2% Mg2+ showed the best performance, and the 0.2C,0.5C,1C discharge capacity reached to 140mAh/g,120mAh/g,112mAh/g, and the basic capacity after 50 cycles without degradation.LiFePO4/C cathode material was prepared using rheological phase with the precursors of iron nitrate, ammonium dihydrogen phosphate, lithium nitrate, sucrose. 0.2C discharge capacity of the LiFePO4/C cathode material could reach to 140mAh/g, 0.5C to 133mAh/g, 1C is 130mAh/g. Doping different content of Mg2+ of LiFePO4/C material performance comparison showed that the material doped 2% Mg2+ showed the best performance, and the 0.2C discharge capacity reached to 143mAh/g. At the same time high-rate performance of the material has been improved. When 2% doped samples charged at 1C rate, discharged at 1C,2C,5C,10C rate after 10 cycles, the discharge capacity could reached to 128mAh/g,109mAh/g,94mAh/g,84mAh/g.Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were used to character the cyclic reversibility and resistance of the materials using different methods.