Tailoring Polyhedral Linkage in K1–xBaxM2F7+x (M = 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 K_1–xBa_xM₂F_7+x (M = Yb, Lu) as F^– conductors with tunable fluorine contents resulting in a polyhedral bond transition. The optimized composition K_0.9Ba_0.1Yb₂F_7.1 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 K_0.9Ba_0.1Lu₂F_7.1. 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.