Using laser irradiation to initiate and control the behaviour of battery thermal runaway

Laser irradiation has been identified by researchers as an innovative, highly repeatable, and non-invasive method of initiating localised internal short circuit (ISC) induced thermal runaway (TR) in lithium-ion batteries. However, for laser irradiation to become a deterministic and impactful standardised trigger, effective in assessing the resilience of safety critical systems to the effects of TR, the impact of specific laser irradiation properties including laser power, operation mode and heating location on the behaviour of thermal runaway must be better understood. In this study, for the first time, laser irradiation is comprehensively evaluated. Focussing on the failure of single 21700 cylindrical cells; the effect of several laser properties including output power, operation mode and heating location are evaluated against the target cells propensity to engage in thermal runaway. The severity and behaviour of the thermal event and the failure modes exhibited are systematically evaluated. Post-mortem analysis including non-destructive X-ray computed tomography (XCT) and physical cell inspection are deployed to visualise the internal structure of cells and gain further insights into the nature of the underpinning reasons for failure. The results indicate that high power irradiation (>100 W) perforates the cell casing prior to ignition, forming a molten keyhole and internal short circuit (ISC) in the outer electrode layers which propagates through the laser perforation as a sustained spark. Two TR behaviours are observed for medium power irradiation (>50 W) with 70 W continuous irradiation initiating ISC in the outer jelly roll layers adjacent to the laser spot, reducing the tensile strength of the cell casing which is eventually exceeded by localised gas pressure resulting in rupture and fire burst. Gradual heating in 50 W tests results in delocalised hotspot generation and nominal failure through engineered vents. The use of XCT provides new insight into the laser heating method which is compared with traditional TR trigger methods. Finally, a tuning exercise is conducted to assess the repeatability of laser induced TR. The exercise identifies 65 W CW irradiation as an optimised setting to reproduce sidewall rupture, achieving the desired failure in 75 % of tests. • The effects of specific laser test parameters on thermal runaway behaviour and cell rupture were investigated. • Temperature, residual mass, fire behaviour and voltage change were characterised. • Likelihood of thermal runway and rupture with respect to laser parameters evaluated • X-ray computed tomography of internal structure of cells after laser induced thermal runaway