Roles of lithium ions and La/Li-site vacancies in sinterability and total ionic conduction properties of polycrystalline Li3xLa2/3−xTiO3 solid electrolytes (0.21 ≤ 3x ≤ 0.50)

The Li0.33La0.55TiO3 solid electrolyte has a maximum grain ionic conductivity of 1.13 × 10−3 S cm−1 among the Li3xLa2/3−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. Li0.21La0.60TiO3 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 Li0.33La0.55TiO3 in the Li3xLa2/3−xTiO3 system studied, in spite of its lower grain ionic conductivity.