Synthesis of Fast Fluoride-Ion-Conductive Fluorite-Type Ba_1–<i>x</i>Sb<i>_x</i>F_2+<i>x</i> (0.1 ≤ <i>x</i> ≤ 0.4): A Potential Solid Electrolyte for Fluoride-Ion Batteries

Abstract

Toward the development of high-performance solid electrolytes for fluoride-ion batteries, fluorite-type nanostructured solid solutions of Ba_{1- x}Sb _xF_{2+ 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 Ba_{1- x}Sb _xF_{2+ 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 Ba_0.7Sb_0.3F_2.3 and Ba_0.6Sb_0.4F_2.4 compositions at 160 °C, respectively. In comparison to isostructural Ba_0.3La_0.7F_2.3, the ionic conductivity of Ba_0.7Sb_0.3F_2.3 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, Ba_0.7Sb_0.3F_2.3 was tested as a cathode material against Ce and Zn anode using La_0.9Ba_0.1F_2.9 as the electrolyte. Ba_0.3Sb_0.7F_2.3/La_0.9Ba_0.1F_2.9/Ce cell showed high discharge and charge capacities of 301 and 170 mA h g^-1, respectively, in the first cycle at 150 °C.