Monitoring the crystal structure and the electrochemical properties of Na3(VO)2(PO4)2F through Fe3+ substitution

Long H. B. Nguyen (1, 2, 3) , Jacob Olchowka (1, 3, 4) , Stéphanie Belin (5) , Paula Sanz Camacho (1) , Mathieu Duttine (1) , Antonella Iadecola (3) , François Fauth (6) , Dany Carlier (1, 3, 4) , Christian Masquelier (2, 3, 4) , Laurence Croguennec (1, 3, 4)

1 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux
2 LRCS - Laboratoire réactivité et chimie des solides - UMR CNRS 7314
3 RS2E - Réseau sur le stockage électrochimique de l'énergie
4 ALISTORE-ERI - Advanced Lithium Energy Storage Systems - ALISTORE-ERI
5 SSOLEIL - Synchrotron SOLEIL
6 ALBA Synchrotron light source [Barcelone]

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.

Mots clés: Na+ diffusivity; solid-state NMR; synchrotron X-ray diffraction; Na3V2(PO4)2F3; Na3(VO)2(PO4)2F; Fe3+ substitution; sodium-ion batteries; sol−gel synthesis; Fe 3+ -substitution; sol-gel synthesis; Na + diffusivity

Domaines: Matériaux

Long H. B. Nguyen, Jacob Olchowka, Stéphanie Belin, Paula Sanz Camacho, Mathieu Duttine, et al.. Monitoring the crystal structure and the electrochemical properties of Na3(VO)2(PO4)2F through Fe3+ substitution. ACS Applied Materials & Interfaces, 2019, 11 (42), pp.38808-38818. ⟨10.1021/acsami.9b14249⟩. ⟨hal-02318063⟩