Electrochemical surface analysis of LiMn2O4 thin-film electrodes in LiPF6/propylene carbonate at room and elevated temperatures

Abstract

Degradation of LiMn2O4 in LiPF6-based electrolyte solution is complicated due to the influence of PF6− anion. Decomposition of PF6− anion accelerates both of dissolution of manganese ion and surface-film formation. In this study, surface states of LiMn2O4 thin-film electrodes in LiPF6/propylene carbonate (PC) derived from the surface-film formation were investigated using redox reaction of ferrocene and spectroscopic analyses. The spectroscopic analyses suggested that properties of the surface film depended the operation temperature (30 °C and 55 °C); a thinner surface film composed of LiF and PC decomposition products formed on LiMn2O4 at 30 °C and a thicker surface film was formed at 55 °C. The redox reaction of ferrocene clearly showed that LiMn2O4 was completely passivated at 30 °C, while it was partially passivated at 55 °C, indicating the surface film formed at 55 °C was not compact and LiMn2O4 was exposed to the electrolyte solution. It was one of the causes of the rapid degradation of LiMn2O4 at elevated temperatures in LiPF6-based electrolyte solution.