Abstract
Lithium-ion batteries (LIBs) are widely used in electric vehicles and energy storage systems to combat environmental pollution and instability of renewable energy. However, the fire accidents caused by LIB thermal runaway (TR) also attracted many attentions. The ignition of the gaseous products ejected during LIB TR by hot particles is one of the critical fire origins, of which the current understanding is still limited to qualitative observation rather than quantitative analysis. In this study, a series of experimental and numerical investigations have been conducted to investigate the spot ignition process and critical ignition temperature (Tcr) of the gaseous products during LIB TR. In the ignition tests of free-fall sphere, the flame propagates with double or multiple flame fronts due to the moving ignition source in combustion vessel. The model-predicted Tcr agrees well with experimental measurements. The relative magnitude between the sphere temperature (Tsphere) and Tcr determines the ignition position along vessel. Tcr is insensitive to the shape of ignition sources providing their dimensions are similar. For the LIB TR scenario with small particles, Tcr of one d=2 mm particle is 1350 K, which is rational to attain as evidenced by the cell’s internal temperature measurements and electrochemical property. The numerical predictions further reveal that the Tcr of multiple small particles (d=0.2mm) is higher than that of a single large one (d=2mm) although their total diameters or surface areas are the same, indicating that the small particles have higher specific surface area and faster heat dissipation. These findings shed new light on the ignition mechanism of LIB with quantitative information about the ignition threshold to inform the development of fire prevention strategies.
Novelty and significance statement TR induced LIB fires in energy storage system and electric vehicles pose significant hazards to life and property. The ignition of TR gases by hot particles is considered as one of the most important fire origin mechanisms, of which the understanding is still limited to experimental observation rather than quantitative prediction. This study pioneeringly investigated the particle-ignition process and thresholds of LIB TR gases using a combination of purposely designed experiments and numerical modelling approach. The results revealed the effects of particle size, shape and fuel compositions on the propensity to ignition. The experimental findings and validated modelling approach provided new insight about the ignition mechanism of LIB TR products in a quantitative manner for first time, which is a big step forward versus previous qualitative observations, shedding light on the fire origin and prevention of LIB systems.