Structure, Lithium-Ion Conductivity Coupled with Second-Order Jahn–Teller Effect, and Electrochemical Stability of Sr-Based Perovskite-Type Solid Electrolytes

We synthesized polycrystalline perovskite-type Li-ion-conducting oxides (general formula: ABO₃), Sr_0.5–xLi_0.3+2xTi_0.3Ta_0.7O₃ (x = 0.030–0.100), and assessed their crystal structure, microstructure, ionic conductivity, and electrochemical stability. Based on first-principles calculations, local structure changes accompanied by Li-ion diffusion were discussed. It was found that the average structure of Sr_0.5–xLi_0.3+2xTi_0.3Ta_0.7O₃ (x = 0.030–0.100) is a cubic perovskite-type one, and at x = 0.042, i.e., Sr_0.458Li_0.384Ti_0.3Ta_0.7O₃, the highest bulk ionic conductivity and the total ionic conductivity at 300 K were observed to be 1.87 × 10^–3 and 1.05 × 10^–3 S cm^–1, respectively, which are greater than those of La_2/3–xLi_3xTiO₃(LLTO). The first-principles calculations suggested that BO₆ octahedra are distorted, and the Li-ion diffusion is assisted by the dynamic distortion of BO₆ octahedra coupled with the second-order Jahn–Teller effect. The reduction potential of Sr_0.458Li_0.384Ti_0.3Ta_0.7O₃ was 1.6–1.7 V vs Li/Li⁺, which is comparable to that of LLTO. A cell using a Sr_0.458Li_0.384Ti_0.3Ta_0.7O₃ pellet with a deposited thin film LiCoO₂ cathode on one side was successfully operated as a secondary battery at room temperature, indicating that the compound can be applied as a solid electrolyte for Li-ion batteries.