Experimental Visualization of Interstitialcy Diffusion Pathways in Fast-Fluoride-Ion-Conducting Solid Electrolyte Ba0.6La0.4F2.4

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.