Optimized carbon-coated Na₃V₂(PO₄)₂F₃ showed exceptional rate and electrochemical cycling capabilities, more than 4000 times at 1 C rate, as demonstrated by performance of hard carbon//Na₃V₂(PO₄)₂F₃ 18650 prototypes of 75 Wh kg⁻¹.¹ These attractive results, among others, attract a renewed interest in the field of Na-ion and Li-ion batteries for vanadium fluoride phosphates. The control of their electrochemical performance requires the careful tuning of the oxygen and thus vanadyle-type defects’ concentration in these materials. Indeed, the competition between the ionic V³⁺−F bond and the covalent V⁴⁺=O bond has a major effect on the structure of the pristine materials, and thus on the phase diagram and redox mechanisms involved upon their cycling in batteries.

It will be illustrated for series of LiVPO₄F_1-yO_y ^2,3 and Na₃V_2‑xM_x(PO₄)₂F_3-yO_y ^{1, 4-6} phases combining mainly Synchrotron X-ray diffraction and spectroscopy studies. Original, probably firstly unexpected, crystal chemistry and redox activity will be discussed.

Acknowledgements:

The authors thank the European Union’s Horizon 2020 Research and Innovation Program under grant agreement No 646433-NAIADES, the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01, HIPOLITE Progelec Project ANR-12-PRGE-0005-02 and SODIUM Descartes Project ANR-13-RESC-0001-02), and the French RS2E and European ALISTORE-ERI networks for funding.

References

1. T. Broux, F. Fauth, N. Hall, Y. Chatillon, M. Bianchini, T. Bamine, J.-B. Leriche, E. Suard, D. Carlier, Y. Reynier, L. Simonin, C. Masquelier, and L. Croguennec - Small Methods 2018, 1800215-1800226
2. E. Boivin, R. David, J.-N. Chotard, T. Bamine, A. Iadecola, L. Bourgeois, E. Suard, F. Fauth, D. Carlier, C. Masquelier, and L. Croguennec - Chemistry of Materials 2018, 30 (16), 5682-5693
3. E. Boivin et al., submitted
4. T. Broux, T. Bamine, F. Fauth, L. Simonelli, W. Olszewski, C. Marini, M. Ménétrier, D. Carlier, C. Masquelier, and L. Croguennec - Chemistry of Materials 2016, 28(21), 7683-7692
5. T. Broux, T. Bamine, L. Simonelli, L. Stievano, F. Fauth, M. Ménétrier, D. Carlier, C. Masquelier, and L. Croguennec - Journal of Physical Chemistry C 2017, 121(8), 4103-4111
6. H. B. L. Nguyen et al., submitted