Room-Temperature Electrochemical Fluoride (De)insertion into CsMnFeF6

• Published in: ACS energy letters 2022-07, Vol.7 (7), p.2340-2348
• Main Authors: Andrews, Jessica L., McClure, Eric T., Jew, Kenneth K., Preefer, Molleigh B., Irshad, Ahamed, Lertola, Matthew J., Robertson, Daniel D., Salamat, Charlene Z., Brady, Michael J., Piper, Louis F. J., Tolbert, Sarah H., Nelson Weker, Johanna, Chmelka, Bradley F., Dunn, Bruce S., Narayan, Sri R., West, William C., Melot, Brent C.
• Format: Article
• Language: English
• ISSN: 2380-8195; 2380-8195
• DOI: 10.1021/acsenergylett.2c01324

We report on the reversible, electrochemical (de)­fluorination of CsMnFeF6 at room temperature using a liquid electrolyte. CsMnFeF6 was synthesized via three methods (hydrothermal, ceramic, and mechanochemical), each of which yields 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 mechanochemical CsMnFeF6 for multiple cycles. Ex situ X-ray absorption spectroscopy confirmed that both Mn2+ and Fe3+ are redox active. The cell impedance decreases after one cycle, suggesting that the formation of fluoride vacancies in early cycles generates mixed-valent Fe and enhances the material’s conductivity. Ex situ synchrotron diffraction revealed subtle expansion and contraction of the CsMnFeF6 cubic lattice on insertion and removal, respectively, during the first two cycles. New reflections intensify in the ex situ diffraction patterns from cycle 3, corresponding to a topotactic transformation of CsMnFeF6 from the pyrochlore structure into an orthorhombic polytype that continues cycling fluoride ions reversibly.