Flammability characteristics of the battery vent gas: A case of NCA and LFP lithium-ion batteries during external heating abuse

•Flammability characteristics of the battery vent gas were studied.•The UFL and LFL curves of the vent gas form a peninsula shape as SOC decreases.•A fire safety threshold for commercial 18,650 LIBs is proposed in this paper.•CO2 dilution can change the battery vent gas from flammable to non-flammable. The purpose of this research was to evaluate the safety of lithium-ion batteries (LIBs) from the perspective of the flammability characteristics of the battery vent gas (BVG). The BVG released by commercial 18650 LIBs with Lix(Ni0.80Co0.15Al0.05)O2 (NCA) and LixFePO4 (LFP) cathodes during external heating abuse was used to study its flammability characteristics at diferent state of charge (SOC). Flammability characteristics included the upper flammability limit (UFL), lower flammability limit (LFL), flammability range (RF), flammability hazard index (HF), LIB fire safety threshold (SOCF), and diluent ratio threshold (DRF). Effects of external carbon dioxide (CO2) on the flammability characteristics of the BVG were also studied. The results show that the UFL and LFL curves of the BVG form a peninsula shape for both types of batteries as the SOC decreases, with RF being essentially unchanged first and then decreased dramatically. For the LFP battery, the LFL of the BVG is higher, and the RF is lower, resulting in obviously lower HF than those of the NCA battery at the same SOC. Using CO2 dilution can significantly reduce the BVG flammability and extend the SOCF. At the same SOC, the DRF is obviously lower for LFP LIBs than the NCA type.