Room-Temperature Electrochemical Fluoride (De)insertion into CsMnFeF₆

Abstract

We report on the reversible, electrochemical (de)fluorination of CsMnFeF 6 at room temperature using a liquid electrolyte. CsMnFeF 6 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 CsMnFeF 6 for multiple cycles. Ex situ X-ray absorption spectroscopy confirmed that both Mn 2+ and Fe 3+ 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 CsMnFeF 6 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 CsMnFeF 6 from the pyrochlore structure into an orthorhombic polytype that continues cycling fluoride ions reversibly.

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

“…Cycles six and seven are shown in orange and blue, respectively. Reproduced with permission from ref . Copyright 2022 American Chemical Society.…”

“…Likewise, CsMnFeF 6 was investigated as a cathode material for room-temperature FIBs by Jessica and co-workers in 2022 . CsMnFeF 6 in a defect pyrochlore structure with suitable particle size and phase purity was obtained through a mechanochemical method.…”

“…Likewise, CsMnFeF 6 was investigated as a cathode material for room-temperature FIBs by Jessica and co-workers in 2022. 594 CsMnFeF 6 in a defect pyrochlore structure with suitable particle size and phase purity was obtained through a mechanochemical method. The presence of Frenkel defects was required to endow the F − ion's diffusion through the network, so the defect pyrochlore CsMn 2+ Fe 3+ F 6 (Figure 25d) with the disorder in the MnFeF 6 substructure and ordered anionic vacancies became a suitable fluoride intercalation host, allowing fast F − -ion transport at room temperature.…”

Composite anode for fluoride-ion batteries using alloy formation and phase separation in charge and discharge processes

“…32)), therefore, they are the most promising materials at present. However, using these materials faces another problem: most electrolytes, whether solid [33][34][35] or liquid, 6,9,22,[36][37][38][39][40] are at risk of decomposition because their cathodic limits are located almost the same or higher than the RPs of La/LaF 3 and Ce/CeF 3 . This severely limits the number of available electrolytes.…”

Construction and Interfacial Modification of a β-PbSnF4 Electrolyte with High Intrinsic Ionic Conductivity for a Room-Temperature Fluoride-Ion Battery

“…Christe et al claimed that tetrahydrofuran (THF) can stabilize free fluoride in quaternary ammonium salts, which provides guidance for the development of LE for FIBs . Inspired by the meaningful discovery, Melot and coworkers , used a solution of 1 M tetra- n -butylammonium fluoride (TBAF) in THF as an LE to construct liquid FIBs. They found that the electrodes (CsMnFeF 6 and ReO 3 ) can realize reversible electrochemical (de)­fluorination behavior.…”