Study on the Multi-Stage Evolution of Thermal Runaway and the Flammability Threshold of Gas Generation in Lithium Iron Phosphate Batteries Based on SOC Gradient

Lithium batteries are widely used in fields such as engineering micro-machines, robotics, and transportation. However, safety issues caused by battery thermal runaway limit their further promotion. This study used a sealed heating pressure chamber (SHPC) to perform “heat-wait-seek (HWS)” stepwise heating on a 50 Ah lithium iron phosphate (LiFePO4) battery to trigger thermal runaway. It was found that the state of charge (SOC) has a significant impact on the safety of the battery. There was no significant correlation between the valve opening temperature (T1) and the temperature at which the battery’s thermal runaway rapidly self-heats (T2) and SOC. However, as SOC increased, the maximum temperature (T3) of the battery’s thermal runaway increased, reaching up to 357.4 °C. The mass loss rate due to thermal runaway increased with SOC. The critical point of the battery’s safety valve was essentially independent of SOC and was mainly influenced by temperature. After thermal runaway, the mixed gas was passed through a gas chromatograph (GC) to detect its composition. When the SOC was below 50%, the total gas production from thermal runaway increased slowly (0.68–0.90 mol). Above 50% SOC, the total gas production from the battery increased sharply (at 75% SOC, 1.17504 mol; at 100% SOC, 2.33047 mol). Among these gases, the amount of H2 increased sharply with SOC (from 0.01 mol at 0% SOC to 0.93 mol at 100% SOC), while the amount of CO2 remained almost constant. Considering the inerting effect of CO2 in the gas produced during thermal runaway of LiFePO4 batteries, the lower flammability limit of the mixed gas increased as SOC decreased (from 6.91% at 100% SOC to 55.43% at 0% SOC). The risk of explosion during thermal runaway of high SOC batteries significantly increased. Notably, within the SOC range of 25% to 100%, the flammable range remained stable at 34–43%, but at 0% SOC, it sharply dropped to 0.5%. Therefore, batteries that are deeply discharged have higher safety.