Modeling initiation and propagation of thermal runaway in pouch Li-ion battery cells: Effects of heating rate and state-of-charge

Abstract

The implementation of lithium-ion batteries (LIB) in battery energy storage systems (BESS) for utility-scale renewable energy generation has led to increased fire and explosion hazards. These hazards are attributable to the heat and flammable gases released during LIB thermal runaway (TR). TR can be triggered within an individual LIB cell and cause intra-cell TR propagation (TRP), the energy release from which causes TR in adjacent cells, leading to inter-cell TRP. Safety assessments are critical to preventing such incidents, and numerical modeling is a key tool in developing such assessments. This work presents an inverse modeling approach utilizing measurements of cell-level experiments of large-format 63Ah pouch cells used in BESS. Such an approach is a practical alternative to existing approaches that rely on a detailed thermochemical characterization of LIB internal components which are seldom available when dealing with commercial batteries encountered in BESS. Owing to the critical role played by gas and solid venting for external flaming and inter-cell TRP, we develop an approach to capture these quantities in the model. Two key parameters affecting TRP are the state-of-charge (SOC) and heating rate. The former provides some control of the system hazard, the latter affects TR behavior of LIB cells subject to different levels of preheating often encountered in BESS fires. We develop the capability to capture the effects of these parameters in the model. To determine model parameters, we rely on existing single-cell experimental data at different heating rates, and augment this with new experiments for the same type of cells at different SOCs. The modeling results recover the experimental TR and intra-cell TRP behavior quantitatively. Analysis reveals that, at lower heating rates, the temperature is greater at the time of the onset of TR, but the location is farther away from the heated cell surface; its location and the greater internal temperature then facilitate faster intra-cell TRP. When SOC is lower, the reduced internal heat release delays the onset of TR, and we find that its location moves away from the heated surface and intra-cell TRP is slower. Our findings provide physical insights to TR initiation and intra-cell propagation in support of large-scale TRP model development.