Temperature Dependence of Structural and Transport Properties for Na3V2(PO4)2F3 and Na3V2(PO4)2F2.5O0.5

Abstract

Among polyanionic-based electrode materials developed for Na-ion batteries, one of the most promising families turns out to be Na3V2(PO4)2F3-yOy (0 ≤ y ≤ 2). Here, we present the influence of the oxygen substitution for fluorine on the structural and transport properties of Na3V2(PO4)2F3-yOy, as a function of temperature, through the comparison of the two compositions y = 0 and y = 0.5. An order-disorder phase transition is observed, whatever the content in oxygen, in relation with changes in the ionic conductivity and thus with the mobility of the Na+ ions within the tunnels of the tridimensional structure. The richer the content in oxygen in Na3V2(PO4)2F3-yOy, the higher is the electronic conductivity, in relation with the mixed valence state V3+/V4+ within the bioctahedral units V2O8F3-yOy, and the better are the electrochemical performances in Na-ion batteries.