Optical diagnostics of the venting spray and combustion behaviour during Li-ion battery thermal runaway induced by ramp heating

Abstract

• Thermal Runaway assessed by means of Natural Luminosity and Schlieren. • LCO, NMC and LFP cathodes are compared at different states of charge. • Similar venting penetration at different state of charge. • Combustion is observed in LCO and NMC cathodes. • LFP chemistry represents the safest operation among chemistries. Lithium-ion cells are one of the solutions with the most significant potential for energy storage in future transportation. However, a controlled ambient temperature is crucial for its application due to thermal safety. Thermal runaway characterisation of three types of commercially available cylindrical cells, based on cathode composition of LiCoO 2 (LCO), LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC 622), and LiFePO 4 (LFP), is performed in the current investigation. The cells were preconditioned to achieve a state of charge (SOC) of 25%, 50%, and 100%; this allows for studying all the SOC operating windows used in electric vehicles. A continuous flow vessel is used as a testbed to monitor the cells going through ramp heating with clear visualisation. Schlieren and Natural Luminosity technique are applied with high-speed cameras. The cell surface temperature and the chamber temperature are measured with thermocouples. The chamber is filled with air that works as a heat transfer medium and chamber cleaner. The main novel of this work is high-quality images during both safety venting and combustion, which characterise the thermal runaway process in detail. High-speed cameras allow recording with more than 6,000 fps in both phenomena. The results show that the average temperature where the cells start the safety venting was found at 193°C. The exothermic onset was, on average, 238°C. The maximum temperature achieved in the cell surface was average for the nine experiments, 567°C. The high-speed camera recording shows that the LCO initiated the flame by a hot spot in the vent cap while the NMC ejected hot solid particles that burned the flammable gas. The LFP did not show any flame propagation for the three SOC tested.