Multidimensional signal evolution during nail-penetration-induced thermal runaway in lithium-ion batteries with different states of charge: An optical investigation

Thermal runaway (TR) remains a critical safety challenge for lithium-ion batteries, necessitating diagnostic techniques to unravel its dynamic evolution for early detection and mitigation. Compared to well-studied thermal and electrical abuse scenarios, this work establishes a novel framework to investigate the evolution of multidimensional signals of 18650-type ▪ cells with varied states of charge (SOCs) during nail-penetration-induced TR. Experiments are conducted in a constant volume combustion chamber equipped with a motor-driven nail penetration system and a Schlieren optical setup. Multidimensional signals, including voltage, recoil force, pressure, temperature, optical imaging, and venting gas components, are measured. By optical diagnostics, key gas venting and combustion parameters, such as venting velocity, venting angle, and flame propagation speed, are quantified. The results reveal that, across all tested SOCs, the recoil force serves as the most reliable indicator for early TR warning at mechanical abuse conditions. Shock waves are experimentally visualized near safety valves, attributed to high gas venting velocities in cells with SOCs from 50% to 120%. Jet fire phenomena are observed in cells with SOCs above 70%, featuring flame propagation speeds an order of magnitude higher than those at 50% SOC condition. Moreover, higher SOC levels lead to increased gas generation, with an elevated proportion of flammable components such as H2 and CH4, lowering its flammability limit and intensifying combustion hazard. Finally, a radar chart-based risk evaluation method is proposed to quantitatively assess TR hazards. These findings collectively provide new insights into TR dynamics, early warning, and risk assessment under mechanical abuse conditions.