Study On Metal-doped LiFePO₄ As Cathode Material For Rechargeable Lithium Batteries

Recently, olivine-type LiFePO₄ has received much attention as lithium insertion compounds for cathode material of rechargeable lithium batteries. Compared with other cathode materials, LiFePO₄ possesses many advantages such as suitable discharge plateau, a high theoretical charge capacity, excellent electrochemical stability, abundant raw materials, low cost and environmental friendliness. In this paper, the lithium-ion battery's development and common cathode materials were reviewed firstly, then the structure and electrochemical performance of lithium iron phosphate were discussed detaildely. We studied the LiFePO₄ both in theory and in experiment according to the research focus and its defects as cathode materials of lithium ion batteries.Based on first-principles, the electronic structures and diffusion energy barriers for pure and doped LiFePO₄ using density functional theory (DFT) were investigated. The result shows that LiFePO₄ is an indirect bandgap semiconductor whose band gap is about 0.562eV, which is close to different results obtained by other groups. Band gaps of LiFePO₄ doped with metal ions are narrower than that of pure LiFePO₄. Valence bands of energy band diagrams broaden more or less for LiFePO₄ doping in Li sites; but they split for LiFePO₄ doping in Fe sites. The average Li-O nuclear separations in LiFePO₄ doping in Li sites are larger than them in pure LiFePO₄. These indicate that doped LiFePO₄ have wider lithium ion pathways which is benefit to the lithium ion transport. The average Li-O nuclear separations lengthen at different extent for LiFePO₄ doping in Fe sites, except for the LiNi0.25Fe0.75PO₄. Diffusion energy barriers for Li ions along the diffusion pathway are calculated in pure and metal-doped materials. Results show that diffusion energy barriers decrease in these dopants. Hence, though LiFePO₄ possesses one-dimensional lithium ion diffusion pathway, the cations doping in Li sites wouldn't block lithium ion motion.Experimentally, we synthesize LiFePO₄ under microwave irradiation, solid phase reaction with microwave irradiation and iron phosphate reduced by Fe powder. The X-ray diffraction results of LiFePO₄ indicate that samples having pure single phase and complete crystal structures may be obtained using above methods. Especially, the samples, synthesized by solid phase reaction with microwave irradiation, possess smooth surface and purity peaks in XRD patterns. So this method can remove shortcomings of microwave irradiation and solid phase reaction and its application is with a good prospect. That iron phosphate is reduced by Fe powder to lithium iron phosphate may reduce cost and obtain obvious economic benefits.