Uncovering Sodiated HC dominated thermal runaway mechanism of NFPP/HC pouch battery

Sodium-ion batteries (SIBs) are considered a promising technology for large-scale energy storage systems (LSESS) because of their rich resources and outstanding electrochemical performance. However, the safety of SIBs is rarely discussed, and the thermal stability is critical to the application of the battery, especially for LSESS. In this study, the thermal runaway mechanism of Na3Fe2(PO4)(P2O7)||hard carbon (NFPP/HC) pouch batteries dominated by heat generation from the sodiated anode has been uncovered. The heat generation analysis based on battery and material levels shows that the exothermic reaction between HC and the electrolyte begins to occur at 100 °C (the exothermic reaction between NFPP and the electrolyte is near 230 °C), and the reaction between the anode and electrolyte releases a large amount of heat, while NFPP materials exhibit less and milder exothermic behavior. Meanwhile, the melting temperature of the separator is extremely close to the triggering temperature of thermal runaway. Therefore, the exothermic reaction between HC and the electrolyte can cause the separator to melt, thus triggering thermal runaway of the SIBs. More seriously, when sodium plating occurs, the safety of the battery will further deteriorate. Considering the characteristic of great heat generation in the early stage of thermal runaway of SIBs, the ceramic-coated separators with higher thermal stability and higher wettability are applied to SIBs, which significantly improve battery safety. This study reveals the mechanism of thermal runaway in SIBs (NFPP/HC), which is expected to provide guidance for the research of safer SIBs.