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.