Trajectory analysis of ejected particles during thermal runaway of large-format lithium-ion cells

Thermal runaway (TR) in lithium-ion batteries is accompanied by rapid gas release and particle ejection, both of which present significant safety hazards. This work investigates the particle dynamics of TR-driven outgassing in a large-format prismatic nickel-manganese-cobalt oxide cell with a LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathode through controlled heater-induced initiation and high-speed optical diagnostics. A series of four experiments were conducted under free-jet and obstructed-jet configurations, with synchronized voltage, surface temperature, and visual data acquisition. A dedicated image-processing pipeline based on OpenCV and TrackPy processed high-speed recordings at 35,200 frames per second to extract projected two-dimensional particle trajectories, velocities, and ejection angles from a single free-jet test. The results indicate time-dependent particle discharge behavior characterized by high initial velocities, broad in-plane ejection angles, subsequent directional shifts and a varying projected cone width. The event specific particle velocities were found to be within the range reported in optical studies for small-format cells. Furthermore, ejection angle analysis revealed a broad outgassing process with a total in-plane width exceeding 180°. The findings demonstrate the event-resolved application of the proposed methodology for quantitative analysis of visible TR ejecta within the image plane and provide new insights into the transient nature of particle-driven venting.