Fluoride solid electrolytes

Abstract

Relations between the structure and electrical properties of fluoride systems with different structures are presented. Physical properties of fluorite-structured (MF2-RF3, MF2-AF, MF2-M′F2, M = Ba, Pb, R = La-Lu, Sc, Y, A = Li, Na, K, Rb, M′ = Ba, Cd, Mg), orthorhombic (RF3, R = Tb-Er, Y), tysonite-structured (RF3-MF2, R = La-Nd, M = Sr), and monoclinic (BaR2F8, R = Ho-Yb, Y) fluoride single crystals or ceramics (ErF3, MF2-RF3, M = Ca, Ba, R = La, Gd, Tb, Y), glasses (ZBLAN, PIBAL) and eutectic composites (LiF-RF3, R = La-Gd, Y, PbF2-RF3, R = Ho, Yb, Y, Sc, PbF2-AF, A = Li, Na, PbF2-MgF2) are compared. Anisotropy of electrical properties is explained. Models of aggregation of defects into clusters are proposed. In fluoritestructured crystals, the highest ionic conductivity was found for PbF2: 7 mol % ScF3 (at 500 K, σ500 = 0.13 S/cm). In tysonite-structured crystals, the highest ionic conductivity was found for LaF 3: 3 mol % SrF2 (σ500 = 2.4 × 10−2 S/cm). Different types of coordination polyhedrons and their different linking in orthorhombic and tysonite structures explain a large difference between conductivities in both structures.