Cell-to-cell variability in Li-ion battery thermal runaway: Experimental testing, statistical analysis, and kinetic modeling

As one of the most promising energy storage mediums, Lithium-ion batteries (LIBs) have attracted extensive research interest. A major challenge associated with LIB application is thermal runaway, which can be triggered under abused conditions and impose direct threats to lives and properties. Here, we select Li-ion batteries with lithium cobalt oxide cathode and graphite anode (18650, Samsung), with relatively simple chemistry, to revisit thermal runaway. The experiment is conducted using an accelerating rate calorimeter (EV+ ARC, Thermal Hazard Technology) following the heat-wait-seek strategy, and is repeated for 9 similar new cells with 100 % state of charge (SOC). Key features such as onset temperature of self-heating, the onset temperature of thermal runaway, maximum heat release rate, thermal runaway delay time, and maximum temperature are measured experimentally. Although the cells are all brand new and have been initiated similarly, obvious cell-to-cell variability has been observed in the measured exothermic onset temperature, delay time, and mass losses. It is also shown that the widely used four-step thermal runway model cannot quantitatively capture the activation energy from the heat release rate. To improve the kinetic modeling and accommodate the cell-to-cell variability, statistical analysis is conducted to process the experimental results. Mean and standard deviation of the frequency factor and activation energy has been acquired to determine the lower and upper bound of the kinetic modeling using a one-step global chemistry. The measured and simulated thermal runaway delay time has reached a reasonable agreement, with the uncertainty of the kinetic model considered. The role of reactant consumption during the heating process is also discussed. The identified cell-to-cell variability should not only be considered for cell-level safety evaluation and modeling, but also in the thermal runaway propagation of battery modules and packs.