Lithium Lanthanum Titanates: A Review

To date, the highest bulk lithium ion-conducting solid electrolyte is the perovskite (ABO₃)-type lithium lanthanum titanate (LLT) Li _3xLa_(2/3)-x□_(1/3)-2xTiO₃ (0 < x < 0.16) and its related structure materials. The x ≈ 0.1 member exhibits conductivity of 1 × 10^-3 S/cm at room temperature with an activation energy of 0.40 eV. The conductivity is comparable to that of commonly used polymer/liquid electrolytes. The ionic conductivity of LLT mainly depends on the size of the A-site ion cation (e.g., La or rare earth, alkali or alkaline earth), lithium and vacancy concentration, and the nature of the B-O bond. For example, replacement of La by other rare earth elements with smaller ionic radii than that of La decreases the lithium ion conductivity, while partial substitution of La by Sr (larger ionic radii than that of La) slightly increases the lithium ion conductivity. The high lithium ion conductivity of LLT is considered to be due to the large concentration of A-site vacancies, and the motion of lithium by a vacancy mechanism through the wide square planar bottleneck between the A sites. It is considered that BO₆/TiO ₆ octahedra tilting facilitate the lithium ion mobility in the perovskite structure. The actual mechanism of lithium ion conduction is not yet clearly understood. In this paper, we review the structural properties, electrical conductivity, and electrochemical characterization of LLT and its related materials.