Ultra‐High‐Capacity of Earth‐Abundant Cathodes Enabled by Excess Fluoride‐Ion Insertion/Extraction

Abstract All‐solid‐state fluoride‐ion batteries (FIBs) are highly prospective candidates for next‐generation batteries due to their superior energy density and safety to lithium‐ion batteries (LIBs). Nevertheless, the full potential of FIBs remains unrealized, as practical cathode materials remain elusive. Here, a perovskite oxyfluoride SrFeO 2 F x is reported to exhibit topotactic F − (de)intercalation and provide a large capacity of 350 mAh g −1 (1843 mAh cm −3 ) with negligibly small volume expansion (≈0.5%). The large capacity is attributed to excess F − (de)intercalation of ≈2.3 mol in SrFeO 2 F x with Fe 2+ /Fe 3+ redox and oxygen redox. Furthermore, Sr‐substituted CaFeO 2 F x (Ca 0.8 Sr 0.2 FeO 2 F x ) has the highest capacity of 580 mAh g −1 (2595 mAh cm −3 ) among cathode materials using the topotactic ion (de)intercalation, including LIBs. For the first time, the possibility is presented that the utilization of ultimate anion redox associated with the ion (de)intercalation to achieve specific capacities surpassing current active materials by over twofold.