Monitoring the Crystal Structure and the Electrochemical Properties of Na3(VO)2(PO4)2F through Fe3+ Substitution

We here present the synthesis of a new material, Na3(VO)­Fe­(PO4)2F2, by the sol–gel method. Its atomic and electronic structural descriptions are determined by a combination of several diffraction and spectroscopy techniques such as synchrotron X-ray powder diffraction and synchrotron X-ray absorpt...

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Veröffentlicht in:ACS applied materials & interfaces 2019-10, Vol.11 (42), p.38808-38818
Hauptverfasser: Nguyen, Long H. B, Olchowka, Jacob, Belin, Stéphanie, Sanz Camacho, Paula, Duttine, Mathieu, Iadecola, Antonella, Fauth, François, Carlier, Dany, Masquelier, Christian, Croguennec, Laurence
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Sprache:eng
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Zusammenfassung:We here present the synthesis of a new material, Na3(VO)­Fe­(PO4)2F2, by the sol–gel method. Its atomic and electronic structural descriptions are determined by a combination of several diffraction and spectroscopy techniques such as synchrotron X-ray powder diffraction and synchrotron X-ray absorption spectroscopy at V and Fe K edges, 57Fe Mössbauer, and 31P solid-state nuclear magnetic resonance spectroscopy. The crystal structure of this newly obtained phase is similar to that of Na3(VO)2(PO4)2F, with a random distribution of Fe3+ ions over vanadium sites. Even though Fe3+ and V4+ ions situate on the same crystallographic position, their local environment can be studied separately using 57Fe Mössbauer and X-ray absorption spectroscopy at Fe and V K edges, respectively. The Fe3+ ion resides in a symmetric octahedral environment, while the octahedral site of V4+ is greatly distorted due to the presence of the vanadyl bond. No electrochemical activity of the Fe4+/Fe3+ redox couple is detected, at least up to 5 V, whereas the reduction of Fe3+ to Fe2+ has been observed at ∼1.5 V versus Na+/Na through the insertion of 0.5 Na+ into Na3(VO)­Fe­(PO4)2F2. Comparing to Na3(VO)2(PO4)2F, the electrochemical profile of Na3(VO)­Fe­(PO4)2F2 in the same cycling condition shows a smaller polarization which could be due to a slight improvement in Na+ diffusion process thanks to the presence of Fe3+ in the framework. Furthermore, the desodiation mechanism occurring upon charging is investigated by operando synchrotron X-ray diffraction and operando synchrotron X-ray absorption at V K edge.
ISSN:1944-8244
1944-8252
DOI:10.1021/acsami.9b14249