Analysis of ejecta ignition and velocity from 18650 Li-ion battery cells during thermal runaway

Lithium-ion batteries are increasingly deployed across a range of energy storage applications, heightening the need to understand failure mechanisms such as thermal runaway. This study presents a new experimental platform focusing on optical accessibility to quantify the transient ejection and ignition dynamics from a single 18650-format cell with an NMC-811 cathode. High-speed optical diagnostics, including luminosity imaging, spectral emission, photodetection, and sound measurements, were employed to characterise the spatiotemporal evolution of ejected fragments and flaming ignition. Light and sound data are temporally correlated, demonstrating that ignition is followed by flame propagation that occurs rapidly and intermittently, with significant temporal variability despite consistent thermal runaway initiation temperatures. A gaseous ignition mode is identified where solid fragments ejected from the battery coated with metallic lithium ignite pyrophorically, acting as an ignition kernel for the flammable gaseous effluent. Atomic emission of neutral lithium (Li I) at 611 nm and 671 nm is detected during ignition and flaming ignition and is hypothesised to form via lithium plating or thermal decomposition of LiC 6 during thermal runaway. Atomic emission of sodium and potassium is also found, all of which is superimposed on the grey body distribution from luminous ejecta. In addition, the high-speed images are cross-correlated to provide time-resolved velocity fields and their statistical distributions of ejected particles during the entire thermal runaway event. Axial velocities possess a log-normal distribution, with peak velocities as large as 35 m s −1 . This work sets the platform for subsequent studies employing high-speed optical diagnostics to deliver a quantitative database to enable modelling of Li-ion battery thermal runaway and to understand flame-induced heating of adjacent cells in a battery module. • High-speed optical diagnostics to measure the ejection and ignition dynamics • Emission of neutral lithium (Li I) at 671 nm during intermittent ignition events • Pyrophoric combustion of ejected fragments can ignite the gaseous effluent • Time-resolved ejecta velocity fields are extracted from high-speed images