Résumé
Vanadium phosphate positive electrode materials attract great interest in the field of Metal-ion batteries due to their ability to exchange several electrons per transition metal. These multi-electron reactions combined with the high voltage of corresponding redox couples will allow the achievement of high energy density at the positive electrode level in Alkali-ion batteries. During this talk, we will discuss about new attractive phases and mechanisms identified in different systems as shortly highlighted hereafter:- Safety is also of high importance to ensure large-scale development of a Na-ion battery technology. We have shown the good thermal stability of the polyanionic materials Na3V2-yVy(PO4)2F3-yOy and revealed that, whatever the state of charge and the oxygen content (1 ≤ x ≤ 3 and y = 0, 0.07 and 0.12), the thermal degradation leads, quite unexpectedly, to the formation of crystalline Na3V2(PO4)2F3 in addition to an amorphous phase.- A new class of NASICON phases NaxV2(PO4)3 has been obtained. Its synthesis as well as structural and physico-chemical properties will be described, considering also that on the contrary to typical Na3V2(PO4)3 a slopping voltage profile is obtained with two domains at 3.46 and 4.16 V vs. Na+/Na and an extremely small polarization and great capacity retention.- New potassium and vanadium oxyfluoride phosphates of the KTiOPO4 structure type and with a chemical composition KVPO4F1-yOy (0 ≤ y ≤ 1) have been synthesized. In particular, the compound KVPO4F5O0.5 appears promising and has been the subject of an in-depth study of the redox mechanism occurring during the de-intercalation of potassium. It was possible to demonstrate and rationalize the preferential oxidation of the VO4F2 site of cis configuration before the trans configuration in KVPO4F, and then to propose an analogous redox mechanism for partially substituted phases.