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, Na-3(VO)Fe(PO4)(2)F-2, 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, Fe-57 Mossbauer, and P-31 solid-state nuclear magnetic resonance spectroscopy. The crystal structure of this newly obtained phase is similar to that of Na-3(VO)(2)(PO4)(2)F, 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 Fe-57 Mossbauer 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 similar to 1.5 V versus Na+/Na through the insertion of 0.5 Na+ into Na-3(VO)Fe(PO4)(2)F-2. Comparing to Na-3(VO)(2)(PO4)(2)F, the electrochemical profile of Na-3(VO)Fe(PO4)(2)F-2 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.