Effects of defect chemistry and kinetic behavior on electrochemical properties for hydrothermal synthesis of LiFePO4/C cathode materials

Lithium iron phosphate cathode materials for lithium ion batteries were synthesized successfully by hydrothermal synthesis method using LiOH·H2O and FeSO4·7H2O as resources of Li and Fe, respectively. The X-ray diffraction, the scanning electron microscopy, the surface areas, the Fourier transform infrared spectra and electrochemical properties of LiFePO4/C cathode materials prepared at different pH values were investigated. The solubility product principle is reasonable to illustrate the chemical composition of the final products in the hydrothermal process. A full Rietveld refinement of X-ray diffraction patterns and Fourier transform infrared spectra results of pure LiFePO4/C prepared at pH 8, 9, 10 supports that cation mixing is the intrinsic characteristic of LiFePO4/C synthesized by hydrothermal process. The lithium diffusion kinetics is affected by the interaction between the occupation of Fe on Li sites and the diffusion path length. In the applied pH range, there is an optimal pH value of 10 for the formation, morphology, and lithium diffusion kinetics, and hence the best electrochemical performances of LiFePO4/C. The mechanisms of nucleation and crystal growth, the defect chemistry and the kinetic behavior are widely discussed in the study, and are essential for optimizing the properties of LiFePO4.