Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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G.Yan, M.Sathiya, G.Rousse, Q.Jacquet, M.Deschamps, R.David, B.Mirvaux, J.W.Freeland, J-M.Tarascon, 'Higher energy and safer sodium ion batteries via an in situ electrochemically made disordered new Na3V2(PO4)2F3 material', Nat. Commun. (in press)

The growing need to store an increasing amount of renewable energy in a sustainable way has rekindled interest for Na-ion battery technology, owing to the abundancy of Na . Presently, Na-ion batteries based on Na3V2(PO4)2F3/C are the subject of intense research focused on improving the energy density by harnessing the 3rd sodium, which has so far been reported to be electrochemically inaccessible. In this study, by using proper electrochemical oxidizing conditions, we were able to trigger the activity of the 3rd sodium via the formation of a disordered NaxV2(PO4)2F3 phase of tetragonal symmetry (I4/mmm space group). This phase can reversibly uptake 3 Na+ per formula unit over the 1 to 4.8 V voltage range, with the last one being re-inserted at 1.6 V vs Na+/Na0. We tracked the Na-driven structural/charge compensation mechanism associated to the new phase and found, by combined X-ray diffraction and X-ray absorption measurements, that it remains disordered on cycling while its average vanadium oxidation state varies from 3 to 4.5. Full Na-ion cells based on such a newly in situ made NaxV2(PO4)2F3 as positive electrode and carbon as negative electrode show a 10-20 % increase in the overall energy density, while offering other practical and beneficial aspects, such as providing an over-discharge protection and easier monitoring for the state of charge of NVPF batteries. Overall, this study offers unprecedented fundamental-applied insight towards the development of a highly performing Na3V2(PO4)2F3/C Na-ion system.