Oxygen Vacancy-Induced Directional Ordering of Li-Ion Pathways for Enhanced Ion-Conducting Solid Electrolytes

Defects in perovskite oxide solid electrolytes (SEs) impact Li-ion conductivity. However, the role of oxygen vacancies (V_o) in transport behavior has been less explored. Herein, our study elucidates the microscopic origin of the role of V_o in enhancing the total ionic conductivity of a prototype lithium lanthanum titanate while maintaining its insulating properties. Scanning transmission electron microscopy and theoretical calculations reveal that the presence of V_o significantly lowers the activation energy of Li-ion migration. The V_o is revealed to be preferentially aligned parallel to c-planes and causes modulated lattice expansion in an alternating manner, resulting in easy directional Li-ion transport. The effect of V_o-assisted Li-ion transport is optimized through the hierarchical rearrangement of structural features at multiple length scales close to the direction of the V_o arrays. Our results offer novel insights into the microscopic origins of superior ion conductivity facilitated by V_o, contributing to the design of high-performance SEs.