Abstract: As a new type of clean energy, lithium-ion battery has been used in various fields for its high energy density and extended cycle life, but in some conditions such as overheating, overcharging and short circuit, the lithium-ion battery would get into thermal runaway or even thermal explosion. To study the thermal explosion behaviors of the lithium-ion battery, a series of thermal explosion experiments of 18650 lithium-ion battery under different stages of charge (SOCs) in hermetic space was carried out using extend volume accelerating rate calorimeter (EV-ARC) and a stainless steel pressure canister(the volume of this vessel is 292mL). In these experiments, the lithium-ion battery was overheated until it got into thermal explosion and the stage SOC of sample cells was 0, 25%, 35%, 50%, 65%, 75%, 85% and 100%, respectively. By the experiment trails, we could get some vital thermal hazard characters of batteries, such as the thermal explosion initial temperature, the maximum temperature, the maximum pressure, temperature rise rate, pressure rise rate, et al. The experimental results showed that there was no thermal explosion at 0% SOC, while it occurred under other conditions. When the battery got into thermal explosion, the maximum surface temperature of the battery and the maximum canister internal pressure increase with the increase of SOC. The thermal energy released from the battery under different SOCs during thermal explosion was calculated using the initial temperature and the maximum temperature of the battery. When the battery was fully charged, the explosion equivalent value was 5.45 g TNT, which is about 1.5 times higher than that of 25% SOC, and the maximum canister internal pressure was 40.69 bar at 100% SOC. In hermetic space, the thermal explosion hazards of 18650 lithium-ion batteries increase with the increases SOC.

Key words: hermetic space, lithium-ion batteries, thermal explosion, EV-ARC