Clarifying fluoride-ion conduction of La0.9Ba0.1F2.9 solid electrolyte

Development of fluoride-ion conductors is currently one of the main bottlenecks in realizing fluoride-ion batteries, yet many details regarding the fluoride-ion conduction mechanism hitherto remain unknown. It is well-known that La0.9Ba0.1F2.9 (LBF) has succeeded in significantly improving the ionic conductivity by introducing vacancies into the fluoride-ion sites by substituting part of La in LaF3 (P3¯c1) with Ba. Herein, a conventional solid-state electrolyte LBF (not P3¯c1 as in LaF3 but P63/mmc) was synthesized by traditional solid-state synthesis. An introduction of fluoride vacancies significantly enhanced the fluoride-ion conductivity by approximately two orders of magnitude, compared to that of LaF3, to approximately 10−6 S cm−1 at room temperature. Rietveld analysis of the temperature-dependent synchrotron x-ray diffraction patterns revealed the fluorine site-splitting from F2 along the c axis into two fluorine sites: F2 and F3. The increase in metastable fluorine sites shortens the F–F distance in the crystal structure, which could play the crucial role of fluoride-ion conduction altogether with the fluoride-ion vacancies. This work presents a proposed fluoride-ion conduction mechanism and comprehensive structural analysis, which is expected to play a key role in the understanding of the high fluoride-ion conductivity of this material.