High Li-Ion Conductivity in Pyrochlore-Type Solid Electrolyte Li_2–xLa_(1+x)/3M₂O₆F (M = Nb, Ta)

Abstract

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 Liion 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.

Doping effects of conductivity improvement in anti-perovskite Na3OBr solid electrolytes

“…[2][3][4][5][6][7][8][9][10] There are some studies reporting that Li-ion solid-state electrolytes have reached an ionic conductivity of 10 −2 S cm −1 , which is sufficient for most device applications. [11][12][13][14] However, to achieve the requirements mentioned above, a substitute cation for Li is strongly needed. Sodium (Na) is one of the alkali metals with similar chemical properties to Li, and is also low-cost and highly abundant.…”