Thermal runaway behavior analysis during overheating for commercial LiFePO4 batteries under various state of charges

It is anticipated that safety may still act as a restraint in the search for acceptable compromise with the overall performance and cost of Li-ion batteries of the future. Motivated by this, the thermal runaway behaviors of 4 types of LiFePO4 batteries under various state of charges were studied, including electrical, thermal, and jet. To gain a better insight into the vent mechanism, the components, and contents of the main thermal runaway gases were analyzed qualitatively and quantitatively. Given reliability and timeliness, a gas-based early warning method was established according to the venting behavior. Results indicate that the overheating process may be divided into 3 stages: preheating, venting, and thermal equilibrium/thermal runaway, and the maximum values of temperature, pressure, and mass loss exhibit an uptrend with the increase of state of charge. It is noteworthy that the main vent-gas are H2, CO, CO2, and alkanes, and the amount of gases increases obviously with the increase of state of charge. The high proportion of H2, CO2, and alkanes makes the gases flammable and potentially toxic. Furthermore, the gas-based approach can provide early warning signals several minutes before thermal runaway and the warning time decreases with the increase of state of charge. In short, this work provides new insights into the thermal runaway behavior and early safety warning of Li-ion batteries.