Synthesis, Characterization and Electrochemical Properties of a Fluoride-Substituted Spinel LiMn2O4 Cathode Material

Conventional high-temperature (800 °C) synthesis of fluoride-substituted samples from LiF has been found to encounter the volatilization of fluorine and to limit the fluorine content, as reported in our earlier paper. In the present study, a fluoride-substituted spinel LiMn2O4 material was synthesized by using a solid-state reaction method with (NH4)HF2 as the fluorine source. The structure and the electrochemical properties of the synthesized samples were characterized by using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), surface area analysis, Fourier transform infrared (FT-IR) spectroscopy, cyclic voltammetry (CV) and charge-discharge tests. As indicated by XRD, all samples showed a spinel cubic structure with space group Fd3m. In comparison with the pure LiMn2O4, fluoride substitution leads to an increase in the lattice parameter, as well as the crystal size, and a decrease in the oxidation state of manganese. From N2-sorption, LiMn2O3.85F0.15 was found to have the smallest specific surface area and the largest pore diameter compared to the LiMn2O4 and the LiMn2O3.95F0.05 samples. Additionally, the majority of the pores in all samples were found to be located in the region of mesopores. The results of the electrochemical study showed that a fluoride substitution into LiMn2O4 improved the initial discharge capacity of the materials. Among the synthesized samples, LiMn2O3.85F0.15 can be used as a starting material for producing better performing spinel-type cathode materials.