Effect of lithium phosphate on the structural and electrochemical performance of nanocrystalline LiFePO4 cathode material with iron defects

The influence of excessive lithium on the crystal structure, morphology, and electrochemical properties of Fe-deficient LixFePO4 (x = 1.00, 1.02, 1.04, 1.05) cathode material was investigated using co-precipitation and the carbon thermal method. The X-ray diffraction pattern and Rietveld refinement results show that excessive Li+ combines with the phosphate group to form Li3PO4 rather than occupying the Fe site to form antisite pairs. Benefitting from the intrinsic ion conductivity of lithium phosphate, the lithium diffusion coefficient of LixFePO4 increases. However, the charge transfer impedance (Rct) also increases with increasing Li content due to the presence of Li3PO4, a nonconductive phase that can block the electric transfer channel. The biphasic LixFePO4 (x = 1.02) compound shows the best performance. This work suggests an effective and easy way to improve the electrochemical performance by adding excessive Li, which leads to the formation of Li3PO4.