Study On Lithium-Ion Diffusion Kinetics Of LiFePO₄

As the cathode material of lithium ion battery, the olivine-structured LiFePO₄ is the most promising cathode material to replace LiCoO₂ due to its rich raw materials source, low-cost, non-toxicity, environmental friendliness, good thermal stability and high safety. But the low electrical conductivity and lithium ion diffusion coefficient hinder its commercial development. In recent years, many researchers have improved the electrical conductivity and lithium ion diffusion coefficient through doping, surface coating, particle size reduction and other methods. However, few studies were made on microscopic electronic structure, leading to the lack of appropriate theoretical support.Starting from the theoretical aspect, the crystal models of undoped and doped samples with doping V, Mo, Mn on iron site and doping F on oxygen site were simulated through CASTEP module of Materials studio software. Based on the first-principle of the Density Functional Theory(DFT) and ultra-soft pseudo potential plane-wave method, the electronic structures such as energy band structure, density of states of undoped and doped samples were caculated, and changes rules before and after doping were summarized. Starting from the experiment aspect, the conclusions from the theory were verified by the electrochemical properties.Calculations showed that LiFePO₄ is a direct-gap ion-conductor, in which the Li-ion is relatively free. The doped samples maintained single olivine structure, with a decreased total energy. The unit cell volume of doped samples all increased except for the F-doped one. Fermi level was significantly increased after F-doping, and the conduction band crossed the Fermi level, so F-doping could increase the conductivity. The electrochemical results showed that the initial discharge capacity of LiFePO_3.96F_0.04/C was 152.2mAh/g at 0.2C rate, higher than the undoped one and the other three doped ones. From cyclic voltammetry curves, it was found that the F-doped sample exhibited minimum battery polarization and better reversibility and cycleability. The lithium ion diffusion coefficients of the undoped sample and samples doped with V, Mo, Mn, F were 1.73×10^-11, 5.26×10^-11, 0.81×10^-10, 9.77×10^-11, 1.52×10^-9cm²/s, respectively, leading to the better performance of the F-doped sample.