Effect of heat treatment on the structure and electrochemical performance of FePO4 coated spinel LiMn2O4

The surface of LiMn2O4 was coated with a nano-layer of FePO4 by the co-precipitation method followed by heat treatment at different temperatures in air. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that the FePO4 coatings remained in the amorphous phase from 300 °C to 400 °C, whereas crystalline FePO4 was obtained above 500 °C. In addition, the diffusion of Fe ions into the LiMn2O4 lattice was promoted at higher temperatures. The results of the cycle capacity test showed that 3 wt% FePO4-coated LiMn2O4 heat treated at 400 °C exhibited optimum cyclability, with cycle retention of 90.3% after 100 cycles at elevated temperature (55 °C). The electrochemical impedance spectroscopy (EIS) results indicated that 3 wt%-400 °C FePO4-coated LiMn2O4 showed the lowest charge transfer resistance (Rct) value and the highest diffusion coefficient of the lithium ion of 9.85E−10 cm2 s−1. These results indicated that heat treatment at the proper temperature could maintain FePO4 in the amorphous phase, thereby facilitating Li ion transport and prevent the detachment of FePO4 and LiMn2O4, which was caused by the diffusion of Fe ions into LiMn2O4 lattice.