Exploring the reactivity of Na3V2(PO4)3/C and hard carbon electrodes in sodium-ion batteries at various charge states

The interest in post-lithium batteries as an alternative to lithium-ion batteries boosted recently due to their substantial abundance, low cost, inherent safety, and sustainability. In recent years, the crucial need for the improvement of battery safety has been emphasized and safety remains a critical barrier for post-lithium technology. Therefore, the thermal stability and reaction enthalpies of electrochemically de-sodiated sodium vanadium phosphate (Na3V2(PO4)3/C) positive electrode and commercial coconut-shell derived hard carbon (HC) at various states of charge (SOCs) were systematically investigated. This study employed the 3D Tian-Calvet calorimeter (C80) and thermogravimetric analysis coupled with mass spectrometry (TGA-MS), to gain comprehensive insights into the thermodynamic aspects of these materials. Thermal stability of electrode materials at distinct sodiation / de-sodiation states draws great attention in cell design and is one of the reasons for the strong state of charge (SOC) dependence of the thermal runaway phenomenon, which represents the most critical safety issue for batteries. This combined experimental approach provides a comprehensive understanding of thermal stability and associated reactions in both, sodium vanadium phosphate (NVP) and hard carbon (HC) electrodes. NVP/C reacts with the electrolyte between 150 and 300°C, releasing ∼400 J/g heat, although it thermally decomposed beyond 150°C. The sodiated HC initiates decomposition at 100°C, releasing ∼750 J/g heat in two steps in a reaction to the electrolyte. These data can facilitate optimizing the design of thermal management systems according to the cell's thermal performance.