Study on Microstructure Characterization and Correlation Mechanism of Transport Properties in the BaSnF ₄ Solid Electrolyte

Fluoride ion batteries (FIBs) are considered a promising high-energy-density alternative to conventional lithium-ion batteries. Tetragonal BaSnF 4 exhibits high fluoride-ion conductivity; however, its detailed conduction mechanism, particularly regarding the occurrence of the cubic phase in the tetragonal phase, remains to be further clarified. In this work, a sample predominantly composed of t-phase BaSnF 4 with a trace amount of c-phase, prepared by ball milling followed by sintering, achieved a high room-temperature conductivity of 2.71 × 10 −4 S·cm −1 . This enhanced conductivity is attributed to fluoride vacancies generated at the heterointerface between the cubic and the tetragonal phase. Solid-state 19 F nuclear magnetic resonance (NMR) reveals the exchange of fluoride ions between the two phases, suggesting that fluoride ions can migrate across the phase boundary. These findings demonstrate that the common existence of a cubic phase in tetragonal BaSnF 4 can be exploited through controllable preparation parameters to enhance ionic conductivity.