Effect of low pressure on thermal runaway behavior of lithium-ion batteries induced by internal short circuit with optical diagnostics

Thermal runaway (TR) of lithium-ion batteries (LIBs) triggered by internal short circuits (ISCs) poses severe safety risks and becomes even more critical under low-pressure conditions. This study investigates the influence of ambient pressure on ISC-induced TR behavior using commercial 18650-type LiNi1/3Co1/3Mn1/3O2 cells. Experiments are conducted in a constant-volume combustion chamber with simultaneous measurements of temperature, pressure, recoil force, and voltage, and Schlieren diagnostics are employed to visualize the TR evolution. The results show that reduced ambient pressure primarily affects the venting and deflagration stages. At the onset of safety valve opening, shock waves are consistently generated and their propagation velocity increases as the pressure decreases. During venting, lower pressure narrows the venting angle due to reduced air density and flow resistance. In the deflagration stage, ambient pressure shows a nonlinear effect on TR intensity, with maximum pressure and deflagration strength generally decreasing at lower pressures but showing an anomalous intensification at 0.6 bar where the maximum pressure variation reaches 3.77 bar. This abnormal behavior arises from the increase in the equivalence ratio of the combustible gas as pressure decreases, with the equivalence ratio approaching unity at 0.6 bar. Furthermore, oxygen limitation promotes incomplete reactions, leading to hydrocarbon and CO accumulation and reduced flame propagation velocity under extremely low pressures. These findings collectively provide critical insights into TR mechanisms under low ambient pressure conditions, supporting safety management for LIBs in aerospace and high-altitude energy storage applications.