Revealing the Unusual Mechanism of Mixed Cationic and Anionic Redox in Oxyfluorosulfide Cathode for All-Solid-State Fluoride-Ion Batteries

All-solid-state fluoride-ion batteries (FIBs) have been considered next-generation energy storage devices because of their high theoretical energy density. However, previously reported metal/metal fluoride active materials suffer from severe degradation due to large volume expansion; consequentially, fluoride-ion-intercalating active materials fail to deliver a high capacity. Here, oxyfluorosulfide Sr2F2Fe2OS2 (SFFOS) with a layer structure is reported as a topotactic fluoride-ion (de)intercalation host. This material delivers a high reversible capacity of more than 340 mAh g–1, which remains well maintained after 20 cycles. The intercalation reaction mechanism for this high and stable capacity is elucidated via X-ray diffraction and fine structure analysis. Wherein both Fe2+/Fe3+ redox and sulfide ion redox are involved in charge compensation during charge/discharge processes, where the sulfide ion redox contributes to the whole voltage range of −1.5 to 1.5 V and Fe2+/Fe3+ redox only contributes from the middle state of charge. Fluoride ions can not only insert into the Sr–S interstitial sites but also combine with Fe cations. Meanwhile, excessive fluoride-ion intercalation leads to the formation of S–S bonds in the SFFOS lattice. These results highlight the oxyfluorosulfides with layer structure as a new class of active materials for constructing high-performance FIBs.