Revealing the Unusual Mechanism of Mixed Cationic and Anionic Redox in Oxyfluorosulfide Cathode for All-Solid-State Fluoride-Ion Batteries
Zulai Cao, Kentaro Yamamoto, Toshiyuki Matsunaga, Toshiki Watanabe, Mukesh Kumar, Neha Thakur, Ryogo Ohashi, Shintaro Tachibana, Hidenori Miki, Kazuto Ide, Hideki Iba, Hisao Kiuchi, Yoshihisa Harada, Yuki Orikasa, Yoshiharu Uchimoto
Abstract
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 Sr 2 F 2 Fe 2 OS 2 (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 Fe 2+ /Fe 3+ 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 Fe 2+ /Fe 3+ 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.