Thermal stability of Na3-xV2(PO4)2F3-yOy: influence of F- for O2- substitution and degradation mechanisms

Abstract

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.

Increasing Tap Density of Carbon-Coated Na3V2(PO4)2F3 via Mechanical Grinding: Good or Bad Idea?

“…Then, the impact of the ball milling on the thermal stability of carbon-coated Na 3 V 2 (PO 4 ) 2 F 3 was investigated by DSC analyses of materials recovered at various states of charge and in contact with the electrolyte (Figure c). Before cycling, DSC curves for both NVPF@C and NVPF@C-90 are characterized by rather similar profiles with an exothermic signal peaking at ∼280 °C, in perfect agreement with the results reported by Pablos et al for carbon-coated NVPF . The enthalpy of the heat generated is slightly higher for the ball-milled material which may suggest a higher reactivity between the electrolyte salt and solvents, the polymer binder, and the active material upon heating.…”

“…Before cycling, DSC curves for both NVPF@C and NVPF@C-90 are characterized by rather similar profiles with an exothermic signal peaking at ∼280 °C, in perfect agreement with the results reported by Pablos et al for carbon-coated NVPF. 40 The enthalpy of the heat generated is slightly higher for the ball-milled material which may suggest a higher reactivity between the electrolyte salt and solvents, the polymer binder, and the active material upon heating. These results can be explained by a more oxidized carbon coating, as observed by XPS or by smaller particles and slightly larger specific surface areas.…”

Recent Advances on F-Doped Layered Transition Metal Oxides for Sodium Ion Batteries

“…Turning to large scale applications, there have been two schools favoring either layered or polyanionic compounds such as NaFePO 4 [77]. Polyanionic cathode materials have been industrialized on a small scale, but polyanionic compounds inherently suffer from low stacking density [78], and it is not a perfect choice.…”