The Na-ion battery technology appears as a reliable, sustainable and environmentally friendly alternative to the Li-ion one, especially for stationary energy storage. As for the Li-ion technology, safety aspect is of high importance to ensure large-scale development. In this work, we studied the thermal stability and decomposition mechanisms of carbon-coated Na3V2(PO4)2F3 and two fluorine-rich phases belonging to the solid-solution Na3V3+2-yV4+y(PO4)2F3-yOy (y = 0.07 and y = 0.12), that family of compounds being often considered among the most promising positive electrode materials for Na-ion batteries. This study shows the good thermal stability of these polyanionic materials and reveals that a low O2- for F- substitution has a very limited effect on the thermal stability of fully re-intercalated materials recovered in the discharged state of the battery, whereas it has a beneficial impact for highly de-intercalated ones, obtained by in-depth charges. Furthermore, whatever the state of charge and the oxygen content in NaxV2(PO4)2F3-yOy (1<=x<=3 and y = 0, 0.07 and 0.12), the thermal degradation leads, quite unexpectedly, to the formation of crystalline Na3V3+2(PO4)2F3 in addition to an amorphous phase. The fluorination of the partially oxygen for fluorine substituted material was clearly demonstrated by X-ray diffraction (XRD) and solid state nuclear magnetic resonance spectroscopy (NMR) on materials recovered after differential scanning calorimetry (DSC) analyses. The formation of a fully sodiated crystalline phase from the thermal degradation of the material obtained in charged states of the battery, with or without presence of electrolyte, was never reported before.
Thermal stability of Na3-xV2(PO4)2F3-yOy: influence of F- for O2- substitution and degradation mechanisms
13 March 2023, Version 1
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