High Defect Formation Energies Limit Ionic Conductivity in Na3Ga2Li3F12 and Na3Al2Li3F12

Lithium-conducting fluoride garnets exhibit ionic conductivities of ∼10–10 S cm–1, far below those of their oxide counterparts. Here, we investigate the origin of this poor transport in Na3Ga2Li3F12 and Na3Al2Li3F12 through a combination of structural characterization and first-principles calculations. Both compounds are highly ordered, with no significant antisite disorder, and exhibit intrinsic ionic conductivities of ∼2 × 10–11 S cm–1 at 100 °C. Nudged elastic band calculations reveal low Li-ion migration barriers (0.16 to 0.29 eV), indicating that the fluoride garnet framework does not inherently restrict Li-ion diffusion. However, the formation energies for lithium Frenkel and Schottky defect pairs are high (1.9 to 2.4 eV), indicating that the intrinsic concentrations of mobile Li-ion defects are negligibly small. These results establish that the low conductivity of stoichiometric fluoride garnets arises from high defect-formation energies, rather than high migration barriers.