Résumé
Polyanionic materials have been intensively studied as promising active materials for positive electrodes in Na-ion batteries thanks to their excellent stability upon cycling and the fast ionic mobility in their structural framework. Among them, Na3V2(PO4)2F3 and Na3(VO)2(PO4)2F are two of the most promising ones due to their high voltages for Na+-ion extraction and their high energy densities: 500 mWh g−1 and 495 mWh g−1, respectively. Here, we study the formation mechanism as well as the stability of these phases in aqueous media and the possible use of a washing step in water in order to remove undesirable impurities formed during the synthesis. Furthermore, the origin of the extra capacity observed at the high voltage region for Na3V2(PO4)2F3 and Na3V2(PO4)2F1.5O1.5 was studied by operando X-ray absorption spectroscopy.