Effects of Sr ²⁺ Doping on Phase Stability and Lithium-Ion Conductivity in Pyrochlore-type Oxyfluoride Solid Electrolytes

Solid electrolytes are expected to improve safety compared to liquid electrolytes. Among them, oxide-based solid electrolytes are attractive because of their higher chemical stability than sulfide- and chloride-based counterparts. In particular, pyrochlore-type materials such as Li 1.25 La 0.58 Nb 2 O 6 F are promising lithium-ion conductors because they exhibit high ionic conductivity (>10 mS cm –1 ). However, there are currently no effective strategies for further enhancing their ionic conductivities. This study investigated Sr 2+ doping as a potential approach for improving the ionic conductivity. Structural analysis confirmed the successful substitution of Sr into the oxyfluoride pyrochlore structure of Li 1.25 La 0.58 Nb 2 O 6 F within a specific composition range, resulting in lattice expansion. However, the ionic conductivities of the Sr-doped samples decreased to approximately 0.17–6.3 mS cm –1 at 25 °C. This may be related to the incorporation of Sr 2+, which could potentially fill lithium vacancies and hinder lithium-ion migration. Although Sr doping did not yield the expected improvement in ionic conductivity, this work provides critical insights into structural constraints and compositional optimization, which serve as guidelines for the design of pyrochlore-type oxyfluoride solid electrolytes with high ionic conductivity.