Understanding the combustion characteristics and establishing a safety evaluation technique based on the overcharged thermal runaway of lithium-ion batteries

As an abusive method to trigger thermal runaway (TR), the overcharge method has been widely used to evaluate the safety status of lithium-ion batteries (LIBs). In this study, the combustion characteristics of LIBs were systematically studied by overcharging batteries at different charging rates (C-rates). The results indicated that a high C-rate could cause jet flames with temperatures as high as 1350 °C at a relatively low state of charge. Furthermore, certain maximum heat release rate (HRR) and mass loss rate (MLR) values of 192 kW and 1785 g/s were obtained at 1C, respectively. X-ray photoelectron spectroscopy results for sulfur showed that the valence of sulfur was related to the peak HRR. Moreover, a novel safety evaluation technique denoted as the relative overcharge safety status (OCSS) was presented to highlight the safety status of the overcharging battery. We found that the OCSS at different C-rates presented a linear relationship with the dimensionless time and obtained the safety threshold, safety boundary, and safety level to easily evaluate the TR risk. Therefore, the OCSS method may provide timely, real-time evidence for battery management systems when batteries are overcharged.