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

The fundamental electrical transport properties including ionic conductivity, dielectric constants, loss tangent, and relaxation time constants of Li-excess garnet-type cubic (space group la (3) over bard) Li(5+2)xLa(3)Ta(2-x)Y(x)O(12) (x = 0.25, 0.5 and 0.75) have been studied in the temperature range of similar to 50 to 50 degrees 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 delta) 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 similar to 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.