Atomistic insights into fluoride ion conduction in Ba_1-xSn_xF₂ from ¹⁹F PFG-NMR studies

Currently, a variety of solid F^- conductors are being considered as potential electrolytes for the development of all-solid-state fluoride-ion batteries. Among those, PbSnF₄ type solid F^- conductors are promising due to their high ionic conductivity at ambient temperatures. Here we have developed a PbSnF₄-type solid electrolyte Ba_1-xSn_xF₂ (x = 0.54) (BSF), which exhibits high ionic conductivity of approximately 9 × 10^-3 Scm^-1 at 298 K. In this study, we investigated the origin of its high conductivity and the underlying ion conduction mechanism using nuclear magnetic resonance (NMR) spectroscopy. Using ¹⁹F static and magic angle spinning (MAS)-NMR analyses it is revealed that the fluoride ions residing in Sn-rich environment of BSF are highly mobile, and they primarily contribute to the ionic conductivity. Also, the diffusion coefficient (D_NMR) measured by pulsed field gradient (PFG)-NMR technique is estimated to be 6 × 10^-12 m² s^-1 at 298 K which is higher than the diffusion coefficient of some well-known lithium ion conducting solid electrolytes. These findings indicate that by using NMR spectroscopic techniques atomistic insights into the ion conduction mechanism and the dynamics could be evaluated successfully in solid F^- conductors.