High Defect Formation Energies Limit Ionic Conductivity in Na3Ga2Li3F12 and Na3Al2Li3F12
Michael J. Brady, Hauke Glück, Jessica L. Andrews, Erica Truong, Alexander G. Squires, Ifeoluwa P. Oyekunle, Rachel C. Monger, Wolfgang G. Zeier, Yan-Yan Hu, David O. Scanlon, Benjamin J. Morgan, Brent C. MelotAbstract
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.