Ignition of lithium-ion battery vent gases: Combined experimental and numerical investigation

The demand for Lithium-ion batteries (LiBs) is increasing amid the global transition towards sustainable energy solutions. However, these cells present an inherent risk of thermal runaway (TR), which releases flammable gases and incandescent particles. Upon venting, these gases may ignite when exposed to air, generating a significant amount of heat, thus increasing the likelihood of TR propagation across battery modules containing hundreds or thousands of cells. Despite significant progress in TR characterization, the exact ignition mechanism of the vented gases remains unclear; both particle-assisted ignition and auto-ignition (AI) are hypothesized, yet lack direct evidence in the literature. In this study, the role of incandescent particles in enhancing ignition is confirmed through high-speed imaging of TR venting events. Additionally, the images capture ignition kernels forming in the absence of particles, providing strong evidence that AI also occurs. A comparison between the order of magnitude of numerically estimated AI delay times and experimentally observed values demonstrates that AI requires temperatures higher than the commonly reported cell wall temperature ( ∼ 950 K). Measurements using two-color pyrometry (2-CP) reveal that the cell cap reaches temperatures of ∼ 1200 K—well within the range necessary for AI in the gas phase—confirming AI and indicating that cell wall temperatures underpredict actual gas temperatures. The insights gained in this study are particularly valuable for vent gas combustion modeling, as they clarify ignition mechanisms and provide improved boundary conditions for CFD models, aiding in the design of safer battery modules. • Incandescent particles contribute to Li-ion cell vent gases ignition. • Auto-ignition in the gas phase occurs in the later stages of thermal runaway. • Jet flames formed via auto-ignition anchor to the cell in the final phase. • Vent gas temperatures are significantly underestimated by cell wall measurements.