Epitaxial growth and lithium ion conductivity of lithium-oxide garnet for an all solid-state battery electrolyte

Abstract

Epitaxial thin films of Al-doped Li₇La₃Zr₂O₁₂ (LLZO) with a cubic garnet-type structure were successfully synthesized using pulsed laser deposition to investigate the lithium ion conduction in grains. Two orientations of the films were obtained depending on the Gd₃Ga₅O₁₂ (GGG) substrate orientation, LLZO(001)/GGG(001) and LLZO(111)/GGG(111). The ionic conductivities in the grains of the (001) and (111) films were 2.5 × 10⁻⁶ and 1.0 × 10⁻⁵ S cm⁻¹ at 298 K, respectively, which were lower than those of polycrystalline LLZO of over 10⁻⁴ S cm⁻¹. X-ray reflectometry and inductively coupled plasma mass spectrometry revealed a large amount of Al³⁺ of over 0.6 moles substituted for Li⁺. These results indicate that the Al³⁺ substitution in the LLZO lattice decreases the number of movable lithium ions and blocks the three-dimensional lithium migration pathway. The lattice mismatch between the film and the substrate induced the lattice distortion of the LLZO, resulting in different conductivities between the (001) and (111) films. The epitaxial-film model system directly clarified a substantial impact of the Al substitution and the lattice distortion on the lithium ion conductivity in the LLZO.