Electrochemical Oxidative Fluorination of an Oxide Perovskite

Abstract

We report the successful electrochemical intercalation of F-ions into a densely packed perovskite oxide from a liquid electrolyte at room temperature. Using galvanostatic oxidation and electrochemical impedance spectroscopy coupled with operando X-ray diffraction, we show that roughly 0.5 equivalents of F-ions can be inserted onto the vacant A-site of the perovskite ReO3. Density functional theory calculations indicate that the intercalated phase is thermodynamically unfavorable compared to other less densely packed polymorphs of ReO3F. Pairing X-ray spectroscopy, neutron total scattering measurements, and magic-angle spinning 19F NMR confirms a rapid decomposition of the product on removal from the cell but nevertheless, these results clearly demonstrate that small anions like fluoride can be intercalated into solids as readily as alkali cations at room temperature, which opens new opportunities to electrochemically fluorinate many new materials. File list (2)download file view on ChemRxiv FxReO3.pdf (12.07 MiB) download file view on ChemRxiv supporting_information.pdf (5.70 MiB)

Towards the high energy density batteries via fluoride ions shuttling in liquid electrolytes: A review

“…Thus far, some of these challenges have been addressed individually; a thorough design however yet to be developed. For instance, David et al demonstrated remarkably high fluoride salt solvation of 2 M within an ether‐based solvent, which was chemically stable against the decomposition as well 22 ; Yamamoto et al reported a fluorohydrogenate ionic liquid electrolyte reaching ionic conductivity as high as 100 mS/cm 24 ; dual‐ion incorporation F − chemistry, 25,26 F − based flow battery, 27 and F − intercalation/deintercalation design 28 have also been developed which will be further discussed in detail accordingly. Nowroozi et al presented a general review regarding the recent progress of the electrolyte, cathode, and anode materials of FIBs focusing partly on liquid electrolytes and particularly on the development of the solid‐state electrolytes 29 .…”

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

“…Building off of these advances, our group recently reported on the electrochemical fluorination of an oxide host, ReO 3 , from a liquid fluoride electrolyte, tetra-n-butylammonium fluoride (TBAF) dissolved in THF, at room temperature. 19 Although clear evidence for fluoride incorporation was obtained through a combination of electrochemical cycling, 19 F NMR techniques, and operando X-ray diffraction studies, reversible cycling of fluoride at room temperature was not achievable due to the instability of fluorinated ReO 3 . 19 Following this work, we began to explore the defect pyrochlore CsMnFeF 6 (Fig.…”

“…19 Although clear evidence for fluoride incorporation was obtained through a combination of electrochemical cycling, 19 F NMR techniques, and operando X-ray diffraction studies, reversible cycling of fluoride at room temperature was not achievable due to the instability of fluorinated ReO 3 . 19 Following this work, we began to explore the defect pyrochlore CsMnFeF 6 (Fig. 1).…”

“…TBAF was chosen based on prior work that showed a high fluoride conductivity at room temperature over a relatively wide electrochemical window. 19 Loose powder working electrodes, prepared by ball milling conductive carbon and CsMnFeF 6 , were utilized.…”

“…While better reversibility was achieved, the higher voltage feature still could not be resolved and opening the window further lead to irreversible reactions that may correspond to fluorination of the carbon additive or attack of the electrolyte. 16,19,29,30 The local bonding environments of the Fe atoms within CsMnFeF 6 were analyzed directly using zerofield 57 Fe Mössbauer spectroscopy, which is sensitive to the types of coordinating ligands and symmetry of Fe atoms based on the absorption of gamma rays by Fe nuclei in solid materials. Mössbauer spectra were acquired at room temperature for the three CsMnFeF 6 materials, a representative example of which is shown in Figure 5 for the mechanochemical material; the spectra for the other materials are shown in Figures S1b and S2b.…”