Tailoring the Composition of a Mixed Anion Iron-Based
Fluoride Compound: Evidence for Anionic Vacancy and Electrochemical
Performance in Lithium Cells
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Abstract
Microwave-assisted synthesis allows
stabilizing Fe-based fluoride
compounds with hexagonal tungsten bronze (HTB) network. The determination
of the chemical composition, i.e., FeF<sub>2.2</sub>(OH)<sub>0.8</sub>·(H<sub>2</sub>O)<sub>0.33</sub>, revealed a significant deviation
from the pure fluoride composition, with a high content of OH groups
substituting fluoride ions. Rietveld refinement of the X-ray diffraction
data and Mössbauer spectroscopy showed that the partial OH/F
substitution impact on the structure (interatomic distances, angles,
and so on) and the local environment of iron (isomer shift and quadrupole
splitting distribution). The thermal behavior of the hydroxyfluoride
compound has been thoroughly investigated. From room temperature to
350 °C under Ar flow, the HTB-type structure remains stable without
any fluorine loss and only water departure. At <i>T</i> >
350 °C, the structure started to collapse with a partitioning
of anions leading to α-FeF<sub>3</sub> and α-Fe<sub>2</sub>O<sub>3</sub>. Within 200 °C ≤ <i>T</i> ≤
350 °C, the chemical composition can be tuned with different
contents of OH<sup>–</sup>/O<sup>2–</sup> and structural
water. By an adequate thermal treatment, it has been shown that anionic
vacancies formed by dehydroxylation reaction could be stabilized within
the HTB network yielding a compound containing three different anions,
i.e., FeF<sub>2.2</sub>(OH)<sub>0.8–<i>x</i></sub>O<sub><i>x</i>/2</sub>□<sub><i>x</i>/2</sub>. XRD Rietveld analysis, atomic pair distribution function, and Mössbauer
spectroscopy confirmed the formation of under-coordinated iron FeX<sub>5</sub>□<sub>1</sub> (X = O<sup>2–</sup>, F<sup>–</sup>, and OH<sup>–</sup>) atoms. Different compositions have been
prepared by thermal treatment at <i>T</i> ≤ 350 °C
and their electrochemical properties evaluated in lithium cell. Structural
water seems to block the diffusion of lithium within the hexagonal
cavities. Increasing the content of anionic vacancies significantly
improves the reversible capacity emphasizing a peculiar role on electrochemical
properties. Pair distribution functions obtained on lithiated and
delithiated samples indicated that the HTB network was maintained
(in the 2–4.2 V voltage range) during the intercalation processes