Ion-Conduction Properties of LiCsF₂-Based Fluoride Materials

Solid electrolytes with high Li+ conductivity and excellent electrochemical stability are required for the realization of all-solid-state lithium-ion batteries. In this study, LiCsF2, which has been proposed to possess a wide electrochemical stability window, was fabricated and its ion-conduction properties were investigated. LiCsF2 and Mg2+–LiCsF2 were fabricated via ball milling. The dissolution of MgF2 in LiCsF2 via variation of the lattice parameters of LiCsF2 was suggested. The conductivity of LiCsF2 was of the order of 10−8 S/cm at room temperature, and the activation energy for ion conduction was estimated as 1.3 eV. Li deposition/dissolution currents were not clearly observed in the cyclic voltammetry (CV) curves of Mg2+–LiCsF2. The conductivity of Mg2+–LiCsF2 significantly increased upon increasing the relative humidity of the measurement atmosphere. Based on the voltage variation in the water vapor concentration cell, it was concluded that the major conduction carrier in Mg2+–LiCsF2 after exposure to moisture was H+.