Research On The Metallic Ions Doping Modification Of LiFePO₄ Cathode Materials

Using as the positive electrode active material in lithium ion battery (LIB), lithium iron phosphate has the advantages of abundant raw materials resources, cheap price, good heat stability and friendly environment. But the electronic conductivity and the chemical diffusion coefficient of Li+ are both too low. These are large obstacles to restrain its application. One of the effective methods to improve its electrochemical performance is doping metallic ions into LiFePO₄. The influences of radius, valency, doping amount and doping position of the doping ions on the electrochemical performance have been studied systematically.To investigate the influences of doping, LiFePO₄ was synthesized with solid state synthesis at first. The XRD determinations show that LiFePO₄/ FePO₄ two phases coexist and transform to each other in the whole charge-discharge process of LiFePO₄. Using electrochemical impedance method the deintercalation/ intercalation of Li+ in the charge-discharge process of LiFePO₄ was investigated. The experimental result show that chemical diffusion coefficient of Li+ increases rapidly at the beginning of charge– deintercalation then the tendency of increasing decreases; At the beginning of discharge– intercalation, the charge of chemical diffusion coefficient is not large, but decreases rapidly at the end of discharge.To investigate the influences of doping ions on the electrochemical performances. Y³⁺ was selected to dop LiFePO₄ in the Li-site and the Fe-site. The experimental results show that the specific discharge capacity of LiFePO₄ increases with increasing the doping content. In the condition of same doping content, the electrochemical performance of doping Y³⁺ in the Fe-site is better than that in the Li-site. The miscellaneous phase appears when the doping content is too much, and the electrochemical performance becomes worse.On the basis of the above experiments the influences of four kinds of ions in lanthanide series: La³⁺, Nd³⁺, Ga³⁺ and Er³⁺, which have different radii and same valency, have been investigated. The experimental results show that the smaller the radius of doping ion, the better the electrochemical performance of LiFePO₄. The influences of four kinds of metallic ions Ga³⁺, Ti4+, Ta5+ and Mo6+, whose radii are closely but the valencies are different, have been investigated. The experimental results show that the best electrochemical performance occurs during doping Ti⁴⁺.