Tailoring Polyhedral Linkage in K_1–<i>x</i>Ba_<i>x</i><i>M</i>₂F_7+<i>x</i> (<i>M</i> = Yb, Lu) for Enhanced Fluoride-Ion Conductivity

The development of high-performance fluoride-ion conductors is crucial for advancing all-solid-state fluoride-ion batteries, yet effective conductors combining high ionic conductivity and reduction resistance remain limited to fluorite and tysonite structures. Here, we report K1–xBaxM2F7+x (M = Yb, Lu) as F– conductors with tunable fluorine contents resulting in a polyhedral bond transition. The optimized composition K0.9Ba0.1Yb2F7.1 achieves an F– conductivity of 2.0 × 10–6 S cm–1 at 298 K, surpassing typical fluorite-type conductors based on Group II–III elements. Furthermore, a high reduction resistance with a reduction potential of −2.58 V vs Pb/PbF2 was attained for K0.9Ba0.1Lu2F7.1. Structural analyses reveal that interstitial fluorine modifies the pentagonal bipyramid chain linkages from corner- to edge-sharing. Molecular dynamics simulations reveal that concerted migration occurs through a pathway comprising edge-sharing interstitial, corner-sharing equatorial, and apical sites aligned along the c-axis with a low energy barrier of 0.30–0.35 eV. This study introduces fluoride-ion conductors composed of electropositive cations with tunable fluorine content across a wide composition range (0.025 ≤ x ≤ 1), offering a balanced combination of moderate ionic conductivity and high reduction resistance.