Multimodal contactless ultrasonic testing for Li-ion pouch cells

The present work demonstrates the application of a multimodal non-contact ultrasonic test technique as diagnostic tool for lithium-ion pouch cells. Different propagation modes of elastic waves (shear and longitudinal), different propagation directions (from normal to the cell plane to in the cell plane), and different wave analysis techniques (including time and frequency domain analysis, and propagation and resonant techniques) are applied. Air-coupled ultrasonic waves with frequencies ranging from 0.15 to 1.4 MHz were used in a through transmission configuration with varying incident angles from 0 to 60°. As practical example, degradation in lithium-ion pouch was investigated. For this purpose, pristine and two types of altered cells were used. Alterations were induced by two deliberate degradation processes (deep discharge and calendar ageing). The information provided by the proposed multimodal ultrasonic technique is analyzed in combination with structural and electroanalytical data, and the a priori knowledge about the induced alterations to provide a first insight into the capabilities of the proposed technique. While the deep-discharge process introduces minor modifications in the ultrasonic response, the calendar ageing process produces a severe change, most likely, the result of a local and randomly distributed degradation of the mechanical continuity between the different layers (linked to electrodes modification, solid electrolyte interface modification, loss of compactness, appearance of cracks, gas generation, delaminations, etc.). These technologies can find applications in the research of new batteries, for in-line quality test in fabrication and for second-life battery sorting. • Pouch cells are tested using air-coupled ultrasound (0.2–1.4 MHz). • Multimodality is the result of combining propagation and resonant techniques • Multimodality is the result of combining different propagation modes and directions • Changes in thickness, elastic moduli, and interfaces degradation, can be detected. • The non-contact nature allows fast, robust and accurate testing.