Effect of heat treatment on the structure and electrochemical performance of FePO_4 coated spinel LiMn_2O_4

摘要

The surface of LiMn_2O_4 was coated with a nano-layer of FePO_4 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 FePO_4 coatings remained in the amorphous phase from 300℃ to 400℃, whereas crystalline FePO_4 was obtained above 500℃. In addition, the diffusion of Fe ions into the LiMn_2O_4 lattice was promoted at higher temperatures. The results of the cycle capacity test showed that 3 wt% FePO_4-coated LiMn_2O_4 heat treated at 400℃ exhibited optimum cyclability, with cycle retention of 90.3% after 100 cycles at elevated temperature (55℃). The electrochemical impedance spectroscopy (EIS) results indicated that 3 wt%-400℃ FePO_4-coated LiMn_2O_4 showed the lowest charge transfer resistance (R_(ct)) value and the highest diffusion coefficient of the lithium ion of 9.85E-10 cm~2 s~(-1). These results indicated that heat treatment at the proper temperature could maintain FePO_4 in the amorphous phase, thereby facilitating Li ion transport and prevent the detachment of FePO_4 and LiMn_2O_4, which was caused by the diffusion of Fe ions into LiMn_2O_4 lattice.