Roles of lithium ions and La/Li-site vacancies in sinterability and total ionic conduction properties of polycrystalline Li_3xLa_2/3-xTiO₃ solid electrolytes (0.21 ≤ 3x ≤ 0.50)

The Li_0.33La_0.55TiO₃ solid electrolyte has a maximum grain ionic conductivity of 1.13 × 10^-3 S cm^-1 among the Li_3xLa_2/3-xTiO₃ oxides (0.21 ≤ 3x ≤ 0.50), but the total ionic conductivity of its polycrystalline phase is not the highest. Owing to the grain-boundary resistances controlling the total resistances of bulk samples, an excellent solid electrolyte is mainly characterized by the grain-boundary resistances. With regard to the role of lithium ions, the substitution of La³⁺ ions by the Li⁺ ions weakens the strength of inter-ionic forces, leading to the decrease in the sintering temperature. The presence of La³⁺/Li⁺-site vacancies promotes the densification and grain growth and further results in rapid decreases in porosity and grain-boundary resistances. Li_0.21La_0.60TiO₃ with a larger amount of La³⁺/Li⁺-site vacancies can therefore exhibit the highest total ionic conductivity through rapidly decreasing its grain-boundary resistances by changing its microstructure, and it becomes a better polycrystalline solid electrolyte than Li_0.33La_0.55TiO₃ in the Li_3xLa_2/3-xTiO₃ system studied, in spite of its lower grain ionic conductivity.