b LRCS, Université de Picardie Jules Verne, F-80039 Amiens Cedex 1, France
c ILL, Institut Laue Langevin, F-38000 Grenoble, France
d CEMHTI, Université d’Orléans, F-45071 Orléans, France
e CELLS - ALBA Synchrotron, Cerdanyola del Vallès, E-08290 Barcelona, Spain
Contact: Laurence.Croguennec@icmcb.cnrs.fr
Vanadium-rich fluorinated-phosphates are attractive positive electrode materials for Li-ion and Na-ion batteries due to their high capacity, rate capability and long-term cycling stability.1,2
We already reported on a complex phase diagram as a function of the charge state for Na3V2(PO4)2F3.3 From structural determination based on high resolution X-ray powder synchrotron data and bond valence sum analysis we proposed two vanadium environments in NaV2(PO4)2F3, V3+ and V5+, instead of a single one (i.e. V4+). We will report on the operando investigation of the redox processes involved during sodium deintercalation and on the charge compensation mechanism on the V site, from X-ray absorption near edge structure measurements collected at the V k-edge. We will compare that mechanism to those observed for other vanadium-rich phosphates such as LiVPO4(F, O, OH).
We will also show how challenging is the control of oxygen over fluorine stoichiometry in these fluorinated phosphates. Existence of characteristic lithium defect environments has been for instance recently revealed using solid-state 7Li nuclear magnetic resonance in well-crystallized Tavorite LiVPO4F,4 despite they were not detected by high resolution X-ray and neutron diffraction as well as scanning transmission electron microscopy. Several studies performed on Na3V2(PO4)2F3 have revealed significant discrepancies in its structural description as well as in the electrochemical properties.5 Unit cell volumes ranging between 871 and 878 Å3 were for instance reported, whereas the phase diagram observed upon cycling was described either as complex with a series of two phase reactions or as a solid solution. Subtle differences in compositions exist. We will especially discuss in details the effect of a partial substitution of oxygen for fluorine and thus of a mixed valence state for vanadium (V3+,4+) on the structure and electrochemical properties of these materials.
Acknowledgements
CEA-Liten (Grenoble, France) and especially Loïc Simonin are acknowledged for their collaboration. This research is performed in the frame of the French network RS2E (http://www.energie-rs2e.com) and of the European network ALISTORE-ERI (http://www.alistore.eu). This project is partly funded by the French National Research Agency ANR (Descartes project SODIUM and Progelec project HIPOLITE) and by the H2020 European Program (Project NAIADES).
References
[1] Huang et al. J. Power Sources189, 748-751 (2009)
[2] Ponrouch et al. Energy & Environmental Science6(8), 2361-2369 (2013)
[3] Bianchini et al. Chem. Mater. 27(8), 3009 (2015)
[4] Messinger et al. Chem. Mater.27(15) 5212 (2015)
[5] Bianchini et al. Chem. Mater. 26(14), 4238 (2014)