Thermal Safety of Na3V2(PO4)2F3/HC Sodium-Ion Batteries: Influence of Electrolyte Formulation and Aging

• NaFSI and NaODFB increase the onset exothermic reaction temperature • Less severe thermal runaway for aged batteries compared to pristine • ARC self-heating rates remained below 10°C/min Sodium-ion batteries are emerging as supplementary technology to lithium-ion batteries. This study focuses on investigating the thermal stability of Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) || hard carbon (HC) sodium-ion cells through the lens of Solid Electrolyte Interphase. Differential Scanning Calorimetry demonstrated a clear increase in SEI stability formed with improved electrolyte formulations (NaFSI and additives). At cell level, Accelerating Rate Calorimetric tests were performed on pristine versus aged batteries. Ramp heating analysis revealed consistent thermal behavior, with SEI degradation initiating near 155°C and thermal runaway (TR) triggering around 210°C in both pristine and aged batteries. However, Heat Wait Search analysis on aged cells exhibited less severe TR compared to pristine batteries. The higher onset temperature observed in aged batteries was attributed to enhanced thermal stability of the re-formed SEI, while the reduced thermal severity was proven to be unlinked to the lower energy density.