LAUSR

We here present the synthesis of a new material, Na3(VO)Fe(PO4)2F2, by sol-gel method. Its atomic and electronic structural description are determined by a combination of several diffraction and spectroscopy techniques, such as: synchrotron X-ray powder diffraction, 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 are localized on the same crystallographic position, their local environment can be differentiated by the use of 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 vs. Na+/Na, whereas the reduction of Fe3+ into Fe2+ has been observed at ~ 1.5 V vs. 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 conditions 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 de-sodiation mechanism occurring upon charging is investigated by operando synchrotron X-ray diffraction and operando synchrotron X-ray absorption at V K-edge.