Ultrasonic inspection of lithium-ion pouch cells subjected to localized thermal abuse

Safety concerns associated with thermal runaway (TR) in lithium-ion cells have limited their adoption in high-power applications such as battery electric vehicles or energy storage systems. In practice, cells are more likely to encounter localized heating than total volume heating, so detecting and preventing TR in this thermal loading environment remains critically important. In this work, we employ ultrasonic inspection (US) to probe NMC lithium-ion cells subjected to localized, thermal loading to illustrate the utility of ultrasonic methods for TR detection. The method employs US Gaussian pulses with two different center frequencies propagating along two different paths to gain additional knowledge of the internal state of a mechanically-confined, thermally-loaded pouch cell during localized heating between 200∘C and 450∘C in different iterations of the test. The time of flight shift (TOFS) and signal amplitude (SA) are used to indicate thermal loading. For a given frequency-path pair the TOFS and SA trends under different levels of thermal loading are consistent. Damage indicators are developed using the US characteristics to give a time-before-TR warning. We demonstrate that through the use of multiple frequency-path pairs and ultrasonic metrics, US measurements can detect an impending battery failure up to twenty-five minutes in advance of TR.