Ion-Conduction Properties of LiCsF₂-Based Fluoride Materials

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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⁺.

Conductivity of Mg²⁺–LiCsF₂ under relative humidities of 40% (circle), 60% (square) and, 100% (triangle).