Preparation and Characterization of a Stable FeSO₄F‑Based Framework for Alkali Ion Insertion Electrodes

Polyanionic electrode materials offer an attractive combination of safety benefits and tunable redox potentials. Thus far, phosphate-based phases have drawn the most interest with a subsequent surge of activity focused on the newly discovered family of fluorosulfate phases. Here, we report the preparation of a new potassium-based fluorosulfate, KFeSO₄F, which, with removal of K, leads to a new polymorph of FeSO₄F crystallizing in the high-temperature structure of KTiOPO₄. This new phase which contains large, empty channels, is capable of reversibly inserting 0.9 Li⁺ per unit formula and can accommodate a wide variety of alkali ions including Li⁺, Na⁺, or K⁺. This finding not only expands the rich crystal chemistry of the fluorosulfate family but further suggests that a similar strategy can apply to other K-based polyanionic compounds in view of stabilizing new attractive host structures for insertion reactions.