Fluoride-Ion Conduction and Wide Electrochemical Stability Window for Ca10– x (HPO4) x (PO4)6– x F2– x (0 ≤ x ≤ 0.6)

F–-conducting apatites have unique structural features and thus have emerged as promising electrolytes for next-generation all-solid-state fluoride-ion batteries (FIBs). Structural and morphological analyses indicate that OH– in Ca10–x(HPO4)x(PO4)6–x(OH)2–x (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5, and 0.6) are exchanged with F– via chemical fluorination at 150 °C using XeF2. We investigate the F– conductivity of Ca10–x(HPO4)x(PO4)6–xF2–x synthesized with various x values. AC impedance spectroscopy reveals a large increase in ionic conductivity, reaching 5 × 10–5 S cm–1 at 80 °C, with a low activation energy of 0.15 eV for x = 0.3. The material is chemically stable at least up to 1200 °C. Due to the robust crystalline framework containing Ca, the electrochemical stability window (ESW) expands in the active direction, and an extremely high ESW of over 5 V is achieved. Although further increasing the ionic conductivity of this material remains a challenge, its excellent ESW is a unique advantage over existing solid electrolytes in next-generation FIBs, in which safety is a priority during high-voltage operation and in harsh environments.