Evaluation of fundamental transport properties of Li-excess garnet-type Li5+2xLa3Ta2−xYxO12 (x = 0.25, 0.5 and 0.75) electrolytes using AC impedance and dielectric spectroscopy

Abstract

The fundamental electrical transport properties including ionic conductivity, dielectric constants, loss tangent, and relaxation time constants of Li-excess garnet-type cubic (space group Iad) Li_5+2xLa₃Ta_2−xY_xO₁₂ (x = 0.25, 0.5 and 0.75) have been studied in the temperature range of −50 to 50 °C using electrochemical AC impedance spectroscopy. A correlation has been established between the excess Li content and the Li⁺ ion migration pathways. The loss tangent (tan δ) for all samples exhibits a relaxation peak corresponding to the dielectric loss because of dipolar rotations due to Li⁺ migration. Comparing the modulus analysis of Li-excess garnets with fluorite-type oxygen ion conductors, we propose the local migration of Li⁺ ions between octahedral sites around the “immobile” Li⁺ ions in tetrahedral (24d) sites. In the samples with x = 0.25 and 0.5, Li⁺ ions seem to jump from one octahedral (96h) site to another bypassing the tetrahedral (24d) site between them (path A), both in local and long-range order migration processes, with activation energies of ∼0.69 and 0.54 eV, respectively. For the x = 0.75 member, Li⁺ ions exhibit mainly long-range order migration, with an activation energy of 0.34 eV, where the Li hopping between two octahedral sites occurs through the edge which is shared between the two LiO₆ octahedra and a LiO₄ tetrahedron (path B). The present AC impedance analysis is consistent with the ab initio theoretical analysis of Li-excess garnets that showed two conduction paths (A and B) for Li ion conduction with different activation energies.