Experimental Visualization of Interstitialcy Diffusion Pathways in Fast-Fluoride-Ion-Conducting Solid Electrolyte Ba_0.6La_0.4F_2.4

All-solid-state fluoride shuttle batteries (FSBs) have the potential to become the next generation of rechargeable batteries. However, there are gaps in the fundamentals of developing all-solid-state FSBs. For example, the mechanism by which the F– ions travel through a working device is not yet fully understood. In this work, we use a cutting-edge neutron diffractometer and a suite of analysis programs to perform Rietveld refinements. We employ the maximum entropy method to experimentally determine the F– ion diffusion pathways in the superior solid electrolyte with a fluorite-type structure, namely, Ba0.6La0.4F2.4. We show that the excessive F– ions, located at the specific interstitial sites, migrate to the neighboring F– ion sites based on the interstitialcy diffusion mechanism at the operating temperature for all-solid-state FSBs. Understanding the diffusion mechanism of F– ions plays a key role in the development of solid electrolytes for all-solid-state FSBs, particularly for those that can operate at room temperature.