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Fluoride-ion conductors have attracted great attention as solid electrolytes for all-solid-state fluoride-ion batteries with high energy densities surpassing those of conventional lithium-ion batteries. Well-known examples include fluorite-type PbSnF4 with intrinsic fluoride vacancies and tysonite-type La0.9Ba0.1F2.9 (LBF) with extrinsic fluoride vacancies introduced by aliovalent substitution. In contrast to the conventional dynamics of fluoride ions through fluoride vacancies, an isolated anion sublattice could provide a unique means for interstitial fluoride-ion diffusion because of its rotational flexibility. In this study, we employed Tl4.5SnF8.5, which contains fluoride ions located between isolated SnF6 octahedra, and investigated the relationship between the cell volume and the fluoride-ion conductivity upon varying the ionic radius of the tin site substituent with a fixed carrier amount. Tl4.5Sn0.9Y0.1F8.4 exhibited the maximum fluoride-ion conductivity and minimum activation energy. Ball milling of this material led to high room-temperature fluoride-ion conductivity comparable to that of LBF. Neural-network potential molecular dynamics was also used to elucidate the fluoride-ion diffusion mechanism. The fluoride ions of the SnF6 octahedra were found to undergo rotational motion, which mediated fluoride-ion diffusion and hopping of the interstitial fluoride ions. These dynamics of the isolated anion sublattice provide a new design strategy for fluoride-ion conductors to complement the previous approach based on the introduction of fluoride vacancies.
Takami et al. (Wed,) studied this question.