Local impedance spectroscopic and microstructural analyses of Al-in-diffused Li₇La₃Zr₂O₁₂

A cylindrical Li₇La₃Zr₂O₁₂ specimen in contact with an Al₂O₃ crucible is sintered and the effect of Al diffusion on the microstructure, phase composition, and Li-ion conductivity of Li₇La₃Zr₂O₁₂ is investigated. The decrease in the Li-ion conductivity and Al concentration with increasing distance from the contact area with the Al₂O₃ crucible is confirmed using spatially resolved local-impedance spectroscopy and compositional analysis. The region close to the Al₂O₃ crucible shows abnormally large grains (size > 0.7 mm) within a matrix of fine grains (size: ∼10 μm), abundant intergranular liquid, the Li ₇La₃Zr₂O₁₂ phase mostly in cubic form, and a high Li-ion conductivity (8.5 × 10^-5 S cm ^-1). In contrast, the region far from the Al₂O₃ crucible exhibits uniform fine grains (size: ∼10 μm), scarce intergranular liquid, the tetragonal form of the Li₇La ₃Zr₂O₁₂ phase, and a low Li-ion conductivity (1.1 × 10^-6 S cm^-1). The incorporation of Al into the Li₇La₃Zr₂O₁₂ lattice and the resulting increase in the number of Li-ion vacancies is suggested to be the main reason for the increase in the Li-ion conductivity. The origins of the abnormal grain growth induced by the diffusion of Al are discussed in relation to the atomic surface structures of Li₇La₃Zr₂O ₁₂ and the presence of intergranular liquid phase.