Site-Selective Fluorination for Decoupling Oxidative Stability and Ionic Transport of Oxychloride Solid Electrolytes

Fluorination in solid electrolytes is widely used to enhance oxidative stability owing to fluorine's high electronegativity, yet strong Li–F interactions hinder Li-ion transport. Here we show that this anodic stability–ionic conductivity trade-off can be largely decoupled through lattice-level anion design in the highly Li-ion-conductive LiTaOCl4. By distinguishing oxygen's framework-linking role from chloride's transport-defining role, we targeted fluorination at framework-linking O-sites to minimize perturbation of the Li-ion conduction network. The fluorinated oxychlorides retain ~10 mS cm–1-class ionic conductivity while raising the oxidation onset from 4.08 to 4.27 V versus Li+/Li, improving capacity retention as catholytes cycled to 4.5 V. First-principles calculations reveal that fluorination stabilizes oxidation-prone O- and Cl-derived electronic states through an inductive effect, while transport-network topology and correlated lattice dynamics preserve fast Li-ion transport. These results establish site-selective anion occupation as a design principle for decoupling oxidative stabilization from fast ion transport in mixed-anion solid electrolytes.