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), Sr0.5–x Li0.3+2x Ti0.3Ta0.7O3 (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 Sr0.5–x Li0.3+2x Ti0.3Ta0.7O3 (x = 0.030–0.100) is a cubic perovskite-type one, and at x = 0.042, i.e., Sr0.458Li0.384Ti0.3Ta0.7O3, 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 La2/3–x Li3x TiO3(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 Sr0.458Li0.384Ti0.3Ta0.7O3 was 1.6–1.7 V vs Li/Li+, which is comparable to that of LLTO. A cell using a Sr0.458Li0.384Ti0.3Ta0.7O3 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.