Ultrasonic assessment of aging in lithium metal pouch cells

Abstract

Lithium metal anodes are prone to non-uniform lithium plating that can lead to dendrite formation, internal short circuits, and catastrophic ignition. This type of structural defect is not captured by battery management systems, prompting the need to advance nondestructive evaluation (NDE) techniques that are suitable for periodic monitoring of the internal structure. An ultrasonic inspection technique is evaluated here as a means of identifying flaws and irregular lithium plating that can be a precursor to dendrite formation and, ultimately, battery failure.Lithium metal pouch cell batteries were aged through electrochemical cycling at a range of moderate constant-current charging and discharging rates. They were inspected intermittently using a tailored local ultrasonic resonance spectroscopy (LURS) approach that measures the local through-thickness resonances of the battery as an indicator of the underlying structure. LURS results showed clear structural resonances in cells in the range of 3–7 MHz, which shifted significantly to lower frequencies as batteries aged. In addition to point measurements, scanning LURS measurements were implemented to map resonance frequencies over the electrode surface. These resonance maps revealed spatial variations in structure that developed during aging and showed better contrast to structural variation than achieved using conventional amplitude-based c-scans of the specimens.NDE results from pouch cell measurements were compared to a destructive physical analysis of the electrode structures using scanning electron microscopy (SEM). The SEM results showed differences in the end-of-life lithium anode structure at different charge rates. The results of this study show the capability of LURS as a means for identifying changes in battery structures during aging and establish resonance trends that may be applied in future work for prognostic modeling of batteries in service.