Room Temperature Electrochemical Fluoride (De)Insertion into the Defect Pyrochlore CsMnFeF6

We report on the reversible, electrochemical (de)fluorination of the defect fluoride pyrochlore CsMnFeF6 at room temperature using a liquid electrolyte. CsMnFeF6 was synthesized via three different methods (hydrothermal, ceramic, and mechanochemical), each of which yield products of varying particle size and phase purity. Using galvanostatic cycling, we found that after three oxidative/ reductive cycles, approximately one fluoride ion can be reversibly inserted and removed from mechanochemically synthesized CsMnFeF6 for multiple cycles. Ex-situ X-ray absorption spectroscopy confirmed that both the Mn2+ and Fe3+ in this composition are redox active during cycling. Electrochemical impedance spectroscopy and ex-situ synchrotron powder diffraction were utilized to investigate the delayed onset of significant fluoride (de)insertion. We observed decreased impedance after one full cycle and subtle expansion and contraction of the CsMnFeF6 cubic lattice on oxidation (insertion) and reduction (removal), respectively, over the first two cycles. Our results suggest the formation of fluoride vacancies in early cycles generates mixed-valent Fe that enhances the conductivity and improves the reversibility in later cycles.