Anion Substitution at Apical Sites of Ruddlesden–Popper-type Cathodes toward High Power Density for All-Solid-State Fluoride-Ion Batteries

All-solid-state fluoride-ion batteries (FIBs) that use fluoride ions as carrier ions offer a new horizon for next-generation energy storage devices owing to their high specific capacities. Materials that utilize topochemical insertion and desorption reactions of fluoride ions have been proposed as cathodes for FIBs; among them, Ruddlesden–Popper-type perovskite-related compounds are promising cathode materials owing to reversible fluoride-ion (de)intercalations with low volume expansion compared to conversion-type cathode materials. Although it is essential to improve the power density of the compounds for practical application, the relationship between the structure and power density is still not clearly understood. In this study, we synthesized chemically fluorinated Ruddlesden–Popper compounds, LaSrMnO4 and apical-site-substituted oxyfluoride Sr2MnO3F, and examined the correlations between their structures and electrochemical properties; Sr2MnO3F showed better power density. Open-circuit voltage measurements, X-ray absorption spectroscopy, and synchrotron X-ray diffraction revealed that electrochemical F– insertion into LaSrMnO4 proceeds via a two-phase reaction with relatively high volume expansion, whereas that into Sr2MnO3F proceeds via a solid-solution reaction with relatively low volume expansion. The substitution of oxygen in the apical sites with fluorine suppressed phase transitions with large volume changes, resulting in improved power density.