Theoretical Analysis of the Fire Boundaries of Lithium-Ion Battery Eruption Gases Caused by Thermal Runaway According to the Thermal Ignition Theory

According to the thermal ignition theory, the fire triangle (i.e. combustible, oxidizer, and ignition source) of lithium-ion battery (LIB) eruption gases, which are caused by thermal runaway, is key to causing LIB fires. The three fire boundaries corresponding to the fire triangle are the minimum battery eruption gases (BEG) concentration (cBEG, ignition), minimum oxygen concentration (cO2, ignition) and lowest temperature required for ignition. The purpose of this paper is to theoretically clarify the three fire boundaries of BEG. The BEG identification results of 29 thermal runaway tests in an inert atmosphere in open literature were summarized, and a BEG time-sequence diagram was drawn. According to Le Chatelier's mixing rule, the physical properties of gases and the thermal ignition theory, the three fire boundaries were theoretically analyzed for batteries with four types of cathode materials (i.e. LixCoO2 (LCO), LixFePO4 (LFP), LixNiyCozAl1-y-zO2 (NCA), and LixNiyCozMn1-y-zO2 (NMC)) at different state of charge (SOC) (0%~143%). The results showed that both of cBEG, ignition and cO2, ignition had different trends for different types of batteries with the increase in the SOC values at discharged states, but they were almost unchanged in the full and overcharged states. For the four types of batteries, from low to high, cBEG, ignition was NMC < LCO < NCA < LFP, and cO2, ignition was NCA < LFP < NMC < LCO. By controlling the SOC and/or selecting a reasonable battery type, cBEG, ignition and/or cO2, ignition could be changed, thereby changing the probability of battery fire. Batteries were prone to forced-ignition with forced-ignition sources regardless of the BEG temperature. When the eruption gases of a battery are at the eruption temperature without forced-ignition sources, the battery is prone to auto-ignition. However, the battery is hard to auto-ignited when the BEG temperature is cooled below the minimum self-ignition point (about 260 ℃) of the BEG components. The BEG ignition mode can be controlled by changing its temperature and ignition sources.These results can be used to guide battery pack design, thermal management system design and fire safety protection.