Thermal Stability of Na3-x V2(PO4)2F3-y O y : Influence of F- for O2- Substitution and Degradation Mechanisms

The Na-ion battery technology appears as a reliable,sustainable,and environmentally friendly alternative to the Li-ion one, especiallyfor stationary energy storage. As for the Li-ion technology, the safetyaspect is of high importance to ensure large-scale development. Inthis work, we studied the thermal stability and decomposition mechanismsof carbon-coated Na3V2(PO4)(2)F-3 and two fluorine-rich phases belonging to the solid-solutionNa(3)V(3+) V-2-y (4+) ( y )(PO4)(2)F3-y O y (y = 0.07 and y = 0.12), thatfamily of compounds being often considered among the most promisingpositive electrode materials for Na-ion batteries. This study showsthe good thermal stability of these polyanionic materials and revealsthat a low O2- for F- substitutionhas a very limited effect on the thermal stability of fully reintercalatedmaterials recovered in the discharged state of the battery, whereasit has a beneficial impact for highly deintercalated ones, obtainedby in-depth charges. Furthermore, whatever the state of charge andthe oxygen content in Na x V2(PO4)(2)F3-y O y (1 & LE; x & LE;3 and y = 0, 0.07 and 0.12), the thermal degradationleads, quite unexpectedly, to the formation of crystalline Na3V3+ (2)(PO4)(2)F-3 in addition to an amorphous phase. The fluorination of thepartial oxygen for fluorine substituted material was clearly demonstratedby X-ray diffraction (XRD) and solid state nuclear magnetic resonancespectroscopy (NMR) on materials recovered after differential scanningcalorimetry (DSC) analyses. The formation of a fully sodiated crystallinephase from the thermal degradation of the material obtained in chargedstates of the battery, with or without the presence of electrolyte,was never reported before.

Automated summary

Na-ion battery technology is a reliable and sustainable alternative to Li-ion technology for stationary energy storage. This study investigates the thermal stability and decomposition mechanisms of positive electrode materials for Na-ion batteries and reveals unexpected findings, such as the formation of crystalline phases and the fluorination of materials during thermal degradation. These findings contribute to a better understanding of the behavior and properties of Na-ion batteries.