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

A perovskite oxyfluoride SrFeO₂F_x can accomodate excess F^– beyond x = 1 in the structure with oxygen redox (O–O bond formation). The Sr-substituted CaFeO₂F_x (Ca_0.8Sr_0.2FeO₂F _x ) provides an extremely large capacity of 580 mAh g⁻¹ (2595 mAh cm⁻³). This research will provide new design principles for the development of all-solid-state fluoride-ion battery cathode materials and the development of new mixed-anion compounds.

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 SrFeO₂F _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 SrFeO₂F _x with Fe²⁺/Fe³⁺ redox and oxygen redox. Furthermore, Sr-substituted CaFeO₂F_x (Ca_0.8Sr_0.2FeO₂F _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.