La_1−xSrxF_3−x: A Solid-State Electrolyte for Fluoride Ion Battery with High Ionic Conductivity and Wide Electrochemical Potential Window

Abstract

Fluoride ion conductors are developed for use as solid-state electrolytes in fluoride ion batteries which are one of promising candidates for next-generation storage batteries. Ba-doped LaF3 (La0.9Ba0.1F2.9: LBF) is mainly used as a solid-state electrolyte in fluoride ion batteries. However, room temperature conductivity of LBF is considerably low, on the order of 10-6 S cm-1 and it is still unclear the optimal elements to be doped to LaF3. In this study, we have explored La0.9SrxBa0.1-xF2.9 system (x = 0, 0.01, 0.025, 0.05, 0.1), in which Ba in LBF is substituted for Sr and investigated the composition dependence of ionic conductivity. We elucidate that the higher concentration of Sr without Ba can significantly improve the ionic conductivity, and the maximum ionic conductivity of La0.9Sr0.1F2.9 is 1.5 × 10-5 S cm-1 at room temperature, which is one order of magnitude larger than that of LBF. The higher ionic conductivity of LSF is due to the larger grain size and higher sintering density of LSF compared to LBF, which results in lower grain boundary resistance. The LSF total ionic conductivity of 10-4 S cm-1 can be achieved at 350 K, which significantly lowers operating temperature of fluoride ion batteries down to 350 K.