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

Among polyanionic-based electrode materials developed for Na-ion batteries, one of the most promising families turns out to be Na 3 V 2 (PO 4 ) 2 F 3– y O y (0 ≤ y ≤ 2). Here, we present the influence of the oxygen substitution for fluorine on the structural and transport properties of Na 3 V 2 (PO 4 ) 2 F 3– y O y, 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 Na 3 V 2 (PO 4 ) 2 F 3– y O y, the higher is the electronic conductivity, in relation with the mixed valence state V 3+ /V 4+ within the bioctahedral units V 2 O 8 F 3– y O y, and the better are the electrochemical performances in Na-ion batteries.