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–xLa_(1+x)/3□_(2x–1)/3Nb₂O₆F (□ = 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, ⁷Li and ¹⁹F NMR data suggest that local disorder at the Li⁺ ion sites facilitate fast diffusion. Chemical shifts of the ¹⁹F 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.