Fast lithium-ion diffusion in pyrochlore-type oxyfluoride Li1.25La0.58Nb2O6F

Fast lithium-ion conductors with oxide frameworks are key materials for high performance solid-state rechargeable batteries. This study reveals fast Li + ion diffusion in the recently discovered pyrochlore-type lithium lanthanum niobium oxyfluoride, Li 2– x La (1+ x )/3 □ (2 x –1)/3 Nb 2 O 6 F (□ = vacancy), using pulsed-field gradient nuclear magnetic resonance (NMR) and impedance measurements. These analyses confirm that fast Li + ion diffusion is the origin of the high ionic conductivity. Moreover, 7 Li and 19 F NMR data suggest that local disorder at the Li + ion sites facilitate fast diffusion. Chemical shifts of the 19 F NMR can be explained by the number of La, Li and vacancies around fluorine. The Arrhenius plot exhibits a slight bending at approximately 200 K. The thermal expansion coefficient also changes from negative to positive at 200 K. These results suggest that Li + ions in pyrochlore-type oxyfluorides undergo an order–disorder phase transition. The insights provided by this study into the mechanism of fast Li + ion diffusion in pyrochlore-type oxyfluorides pave the way for fabricating solid electrolytes with improved performance over conventional solid electrolytes. • Pyrochlore-type oxyfluoride shows fast Li + ion diffusion. • Diffusion coefficient of Li + ions is clarified by PFG-NMR. • Ionic conductivity exhibits a bending at 200 K. • Thermal expansion coefficient changes from negative to positive at 200 K. • Order–disorder phase transition in pyrochlore-type oxyfluorides is suggested.