Temperature Dependence of Structural and Transport Properties for Na₃V₂(PO₄)₂F₃ and Na₃V₂(PO₄)₂F_2.5O_0.5

Abstract

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 orderdisorder 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 the electronic conductivity, in relation with the mixed valence state V 3+ /V 4+ within the bi-octahedral units V 2 O 8 F 3y O y , and the better the electrochemical performances in Na-ion batteries.

Ag3V2(PO4)2F3, a new compound obtained by Ag+/Na+ ion exchange into the Na3V2(PO4)2F3 framework

“…A much higher energy barrier is calculated for Na hops between neighboring rings, of the order of 0.6 eV and above 43 . These hops are required for macroscopic diffusion; in fact, recently the activation energy for Na conduction in Na3V2(PO4)2F3 was measured by Impedance Spectroscopy and reported to be 680 meV (or even greater) 44,45 , in good agreement with the calculations.…”

LiVPO4F1–yOy Tavorite-Type Compositions: Influence of the Concentration of Vanadyl-Type Defects on the Structure and Electrochemical Performance

“…This was recently observed in the mixed valence phase Na3V2(PO4)2F2.5O0.5 for which a significant improvement of the electronic conductivity has been evidenced compared to that of the pure V 3+ phase (i.e. Na3V2(PO4)2F3) 36…”

“…This work is part of a series of studies conducted in our groups and dealing with vanadyltype defects in vanadium fluoride phosphate materials (LiVPO4F 7,8,17 and Na3V2(PO4)2F3 27,36 ). The combination of advanced characterizations by diffraction and spectroscopy allowed an in-depth understanding of the LiVPO4F1-yOy phases, from their long range average structure to their local one.…”

Vanadium‐Based Nanomaterials: A Promising Family for Emerging Metal‐Ion Batteries

“…Bianchini et al revealed the detailed phase diagram during Na extraction and structural distortion using high angular and intensity resolution synchrotron radiation diffraction. Broux et al investigated the influence of the oxygen substitution for fluorine on the structural and transport properties of Na 3 V 2 (PO 4 ) 2 F 3 as a function of temperature. Yu group investigated the intrinsic ionic and electronic conductivities, as well as the chemical diffusion coefficient of Na + of Na 3 V 2 (PO 4 ) 2 F 3 .…”