Room-Temperature Electrochemical Fluoride (De)Insertion into CsMnFeF₆

Understanding the structural changes induced by cation intercalation has proven crucial to augmenting the performance of Li-ion batteries. However, little work has been done to understand the mechanism of anionic intercalation and its effects on host structures. The possibility of anions serving as charge carriers for electrochemical energy storage provides an alternative frontier to explore a variety of new rechargeable battery materials. Systems that leverage fluoride intercalation have recently been reported, but the solid electrolytes utilized in these cells suffer from low room temperature fluoride conductivity, thereby requiring high operating temperatures and limiting practical use. Recent advancements in liquid electrolytes include salt and solvent combinations that exhibit significant fluoride ion shuttle at room temperature and relatively large electrochemical windows. We will present on the reversible, electrochemical (de)fluorination of CsMnFeF₆ at room temperature using a liquid electrolyte. CsMnFeF₆ can be synthesized via three methods (hydrothermal, ceramic, and mechanochemical), each of which yield products in a defect pyrochlore structure with varying particle sizes and phase purities. After three galvanostatic cycles, approximately one fluoride ion can be reversibly (de)inserted into mechanochemically synthesized CsMnFeF₆ for multiple cycles. Ex-situ X-ray absorption spectroscopy confirms that both Mn²⁺ and Fe³⁺ are redox active. The cell impedance decreases after one cycle, suggesting the formation of fluoride vacancies in early cycles generates mixed-valent Fe and enhances the material’s conductivity. Ex-situ synchrotron diffraction shows subtle expansion and contraction of the CsMnFeF₆ cubic lattice on insertion and removal, respectively, during the first two cycles. New reflections intensify in the cycle 3 ex-situ diffraction, corresponding to the formation of a new phase from the topotactic transformation of CsMnFeF₆ in the pyrochlore structure into an orthorhombic polytype that continues cycle reversibly cycle fluoride ions for up to 10 cycles.