Effects of Mixed Phases on Electrical Conductivities for (CeF3)1–m (CaF2) m Fast-Fluoride-Ion-Conducting Solid Electrolytes

The realization of all-solid-state fluoride batteries (FBs) is critical for next-generation rechargeable batteries with high specifications, such as energy density, power capacity, lifespan, and reliability. However, no standard choice for solid electrolytes and electrodes in FBs has been established. In addition, the mechanism by which F– ions travel through a working device is not yet fully understood. In this study, we performed X-ray diffraction (XRD) and Raman spectroscopy measurements for Ca-doped CeF3 fluoride-ion conductors, (CeF3)1–m (CaF2) m . As a result, an amount of Ca substitution in CeF3, m, was limited to approximately 0.08, i.e., (CeF3)0.92(CaF2)0.08. Moreover, AC impedance measurements were performed, and we found that (CeF3)0.9(CaF2)0.1 exhibits the highest electrical conductivity. Neutron diffraction (ND) analysis was conducted to clarify the structural features. We found two phases on (CeF3)0.9(CaF2)0.1: the primary phase of Ce0.92Ca0.08F2.92 with a tysonite-type structure and a small amount of the secondary phase with a cubic structure. This study provides new insights into the further improvement of the electrical conductivity of (CeF3)0.9(CaF2)0.1, owing to the small amount of the secondary phase.