High Li-Ion Conductivity in Pyrochlore-Type Solid Electrolyte Li 2– x La (1+ x )/3 M 2 O 6 F (M = Nb, Ta)

All-solid-state Li-ion batteries featuring both a high energy density and safety are desirable. Sulfide-based solid electrolytes with high conductivities have been actively studied. However, such electrolytes easily react with moisture in air to generate toxic H 2 S. Therefore, non-sulfide-based solid electrolytes with high ionic conductivity are needed. In this study, we discovered high ionic conductivity in pyrochlore-type oxyfluoride Li 2– x La (1+ x )/3 M 2 O 6 F (M = Nb, Ta), which was stable in air. Li 1.25 La 0.58 Nb 2 O 6 F exhibited a bulk ionic conductivity of 7.0 mS cm –1 and a total ionic conductivity of 3.9 mS cm –1 at room temperature (∼298 K), which are higher than those of any previously reported oxide solid electrolytes. The conduction path of pyrochlore-type structure covers the F ions located in the tunnels created by MO 6 octahedra. The conduction mechanism is the sequential movement of Li ions while changing bonds with F ions. Li ions move to the nearest Li position always passing through metastable positions. Immobile La 3+ bonded to the F ion inhibits the Li-ion conduction by blocking the conduction path and vanishing the surrounding metastable positions. We not only successfully synthesized a Li-ion conductor with high conductivity and stability in air but also pioneered a new class of superionic conductors with a pyrochlore-type oxyfluoride.