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(PO 4 ) 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, 57 Fe Mössbauer, and 31 P solid-state nuclear magnetic resonance spectroscopy. The crystal structure of this newly obtained phase is similar to that of Na 3 (VO) 2 (PO 4 ) 2 F, with a random distribution of Fe 3+ ions over vanadium sites. Even though Fe 3+ and V 4+ ions situate on the same crystallographic position, their local environment can be studied separately using 57 Fe Mössbauer and X-ray absorption spectroscopy at Fe and V K edges, respectively. The Fe 3+ ion resides in a symmetric octahedral environment, while the octahedral site of V 4+ is greatly distorted due to the presence of the vanadyl bond. No electrochemical activity of the Fe 4+ /Fe 3+ redox couple is detected, at least up to 5 V, whereas the reduction of Fe 3+ o Fe 2+ has been observed at ∼1.5 V versus Na + /Na through the insertion of 0.5 Na + into Na 3 (VO)Fe(PO 4 ) 2 F 2 . Comparing to Na 3 (VO) 2 (PO 4 ) 2 F, the electrochemical profile of Na 3 (VO)Fe(PO 4 ) 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 Fe 3+ 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.