O 2− /F − Anion Co‐Migration Enabled Regulation of Grain Boundary Structure for Lower Impedance in Li 0.32 La 0.56 Fe 0.01 Ti 0.94 Sb 0.06 O 3 Solid Electrolyte

Microstructural characterization of Li 0.32 La 0.56 Fe 0.01 Ti 0.94 Sb 0.06 O 3 (LLFeTSO) using AC‐TEM revealed that Fe 3+ /Sb 3+ ion co‐doping induces the migration of lattice oxygen to grain boundaries (GBs), enhancing GB conductivity. However, the atomic arrangement at the GBs remains highly ordered and lacks sufficient cation vacancy carriers. In this study, we synthesized Li 0.32 La 0.56 Fe 0.01 Ti 0.94 Sb 0.06 O 3− x F x solid electrolytes by introducing the F − anion as a heterovalent substituent for the O 2− anion. This substitution is highly likely to enable the introduction of charge compensation at the GBs through O 2− /F − anion co‐migration, which effectively generates cation vacancies. Analysis of integrated differential phase contrast (iDPC)‐STEM revealed that microscale lattice distortion at the GBs triggers significant anion loss, resulting in positively charged space‐charge regions that impede Li + migration. By optimizing the F − content, the sample with x = 0.08 achieved a total ionic conductivity of 9.0 × 10 −2 mS cm −1 at 25°C, representing a 53.8% improvement relative to the undoped LLFeTSO.