Synthesis of Fast Fluoride-Ion-Conductive Fluorite-Type Ba_1–xSb_xF_2+x (0.1 ≤ x ≤ 0.4): A Potential Solid Electrolyte for Fluoride-Ion Batteries

Toward the development of high-performance solid electrolytes for fluoride-ion batteries, fluorite-type nanostructured solid solutions of Ba1–xSbxF2+x (x ≤ 0.4) were synthesized by high-energy ball-milling method. Substitution of divalent Ba2+ by trivalent Sb3+ leads to an increase in interstitial fluoride-ion concentration, which enhances the ionic conductivity of the Ba1–xSbxF2+x (0.1 ≤ x ≤ 0.4) system. Total ionic conductivities of 4.4 × 10–4 and 3.9 × 10–4 S cm–1 were obtained for Ba0.7Sb0.3F2.3 and Ba0.6Sb0.4F2.4 compositions at 160 °C, respectively. In comparison to isostructural Ba0.3La0.7F2.3, the ionic conductivity of Ba0.7Sb0.3F2.3 is significantly higher, which is attributed to the presence of an electron lone pair on Sb3+. Introduction of such lone pairs seems to increase fluoride-ion mobility in solid solutions. In addition, Ba0.7Sb0.3F2.3 was tested as a cathode material against Ce and Zn anode using La0.9Ba0.1F2.9 as the electrolyte. Ba0.3Sb0.7F2.3/La0.9Ba0.1F2.9/Ce cell showed high discharge and charge capacities of 301 and 170 mA h g–1, respectively, in the first cycle at 150 °C.