Lithium coordination disorder controlling ionic conductivity in mixed-halide borate glasses

Abstract

Understanding the structure–property relationships in glasses remains challenging owing to their structural complexity, especially in multicomponent practical glasses. In this work, neutron diffraction with isotopic substitution was employed to investigate lithium coordination environments in halide (chloride and/or bromide)-doped borate glasses, which exhibit variations in lithium-ion conductivity associated with anion mixing. Lithium-specific neutron pair distribution function analysis reveals that the conductivity enhancement in oxide–halide mixed glasses originates from the breaking of one short Li–O bond and the formation of two longer Li–halogen bonds. The formation of LiO₃Cl₂ and LiO₃Br₂ units is observed in chloride- and bromide-doped glasses, respectively, whereas a glass containing equal amounts of chloride and bromide exhibits LiO₃ClBr units in addition to LiO₃Cl₂ and LiO₃Br₂ units. These results suggest that the coexistence of multiple lithium–oxygen–halogen polyhedral units hinders lithium-ion migration, leading to reduced ionic conductivity in halide–halide mixed glasses. This work provides new insights into the structure–property relationships in lithium-ion conducting glasses through lithium-specific structural analysis.