On flammability limits of battery vent gas: Role of diffusion, radiation and chemical kinetics

An experimental and computational investigation has been conducted on the determination of the flammability limits of the multi-component NCA (Nickel, Cobalt, and aluminum) battery vent gas (BVG) blended with an inert gas (CO₂, H₂O, and N₂), and on the understanding of associated limit phenomena in general. The simulation results based on detailed chemical kinetics and transport properties agree well with the experimental data from low to high inert gas ratios for both lean and rich limits. In addition, when the inert gas ratio is very high, an absolute limit state exists, at which the lean and rich limits merge, and beyond which flame propagation is not possible. The combustible regime increases in the order of CO₂, H₂O and N₂. The effects of each inert gas on the flame temperature, laminar flame speed, flame broadening, diffusion and radiation are identified and investigated. Furthermore, a linear growth law of the normalized sensitivity with the dominant chain termination reaction rate to the chain branching reaction rate was found near the lean flammability limit. Based on first principle concept of competition between the dominant chain branching and termination reactions, the criterion of a unified flammability exponent is extended to the current multi-component mixture system. This outcome yields useful insight into the fundamental processes of flammability limits, and scrutinizes the heat loss and chain termination effects of flammability limits. Results of this study provide guidance for hazard mitigation and fire suppression for the thermal runaway of lithium-ion batteries.