Garnet-Type Lithium Metal Fluorides: A Potential Solid Electrolyte for Solid-State Batteries

Abstract

In this work, we introduced a garnet-type lithium metal fluoride, Li3Na3M2F12 (M = Al, Sc, In), as solid-state lithium-ion conductors for the first time. The mechanically milled Li3Na3M2F12 compounds crystallized in a cubic garnet-like structure (Ia3̅d, No. 230). The ionic conductivities of Li3Na3Al2F12, Li3Na3Sc2F12, and Li3Na3In2F12 are 1.7 × 10–6, 8.2 × 10–6, and 2.4 × 10–6 S/cm at 300 °C and 1.2 × 10–10, 2.6 × 10–9, and 1.8 × 10–10 S/cm at 100 °C, respectively. Even though these fluoride garnets’ conductivity is less, it is still better than those of the oxide analogues Li3Ln3Te2O12 (Ln = Er, Gd, Tb, Nd). Moreover, we explored why garnet-type Li3Na3M2F12 has low ionic conductivity and presented strategies for further improving conductivities.