On the chain-branching explosion limits of large-format LiFePO₄ battery vent gas in energy storage

Abstract

Thermal runaway of large-format lithium iron phosphate (LFP) batteries releases combustible vent gases that may trigger explosions in battery energy storage systems. However, the temperature- and pressure-dependent explosion limits and underlying chemical mechanisms remain insufficiently understood. Using a 628 Ah LFP battery subjected to external heating, this work experimentally characterizes the vent gas composition at different states of charge (SOC) and computationally investigates the explosion limit behavior of vent gas–air mixtures with detailed chemical kinetics. Although H₂ is the most abundant combustible species, the minor component C₂H₄ plays an important role in both the second and third explosion limits, whereas C₂H₆ exerts a secondary influence mainly in the intermediate-pressure second limit region. Increasing SOC shifts the second explosion limit region toward lower temperatures, whereas the third explosion limit region at 100% SOC shifts toward higher temperatures and pressures than those at lower SOCs. The explosion limit curve exhibits a crossover point near 1 bar and rotates counterclockwise with increasing equivalence ratio, owing to the competing effects of oxygen availability and fuel enrichment. Sensitivity and reaction path analyses further reveal a pressure-dependent transition in the dominant explosion chemistry. Low-pressure explosion behavior is governed by H₂–O₂ chain-branching reactions, whereas elevated pressures promote a HO₂–H₂O₂ reaction regime in which H₂O₂ decomposition and hydrocarbon oxidation control radical production. Results of this study are expected to advance the understanding of pressure-dependent explosion chemistry in LFP battery vent gases and provide guidance for explosion hazard assessment and safety design of energy storage systems.