Suppression of thermal runaway in lithium iron phosphate batteries by supercritical CO2 jets

Lithium iron phosphate (LFP) batteries are increasingly used in electric vehicles and energy storage systems. However, effective mitigation of their thermal hazards remains challenging. This work characterized the flow-field evolution and axial attenuation of temperature depression in supercritical CO2 (SCO2) jets and assessed their effectiveness in suppressing battery TR. SCO2 was injected at safety-valve opening to evaluate early intervention, whereas SCO2, liquid CO2 (L-CO2), and gaseous CO2 (G-CO2) were compared at the onset of severe TR. The SCO2 jet developed successively through rapidly expanding, mixing-transition, and gas diffusion regions, while its centerline temperature depression decayed exponentially with axial distance. With a 5mm nozzle at an injection pressure of 9Mpa, the temperature depression decreased from more than 75 °C at 2 cm to 11.8°C at 72cm. Under the tested conditions, SCO2 injection at safety-valve opening prevented the sharp temperature rise, secondary abrupt drop in expansion force, and pronounced voltage collapse associated with severe TR, indicating that early intervention inhibited further TR development. During severe TR, all three media extinguished the external flame within approximately 1 s but could not immediately terminate self-sustaining internal heat generation. Relative to the unsuppressed condition, SCO2, L-CO2, and G-CO2 reduced apparent heat accumulation by 62.75%, 53.46%, and 41.38%, respectively, and decreased the peak unheated-surface temperature by 84.95°C, 64.00°C, and 33.55°C, respectively. Finally, based on the jet impingement characteristics and battery heat-transfer processes, a coupled suppression mechanism involving SCO2 was revealed.