Characterizing the structure of lithium metal batteries using local ultrasonic resonance spectroscopy (LURS)

Nondestructive characterization of battery structures is important as both a research tool and as a means for developing reliable prognostics for batteries in service. Local Ultrasonic Resonance Spectroscopy (LURS) is a technique that measures spatially localized through-thickness vibrational resonances in layered materials. In battery cells, LURS measurements can reveal layer spacing and changes in mechanical properties. This study examines changes in structure that occur from fabrication to end of life for batteries cycled under different conditions as a demonstration of the capabilities of the LURS approach. Lithium metal pouch cell batteries were studied in both single- and multi-layer form factors. The cells were electrically cycled under constant current conditions at charge rates ranging from 0.2 C to 2 C, where 1 C is the charge rate (C-rate) required to fully charge a battery in one hour. In addition to varying charge rates, cells were also cycled under different temperatures and loading conditions, leading to a wide variety of electrode structures at end of life. LURS scans were conducted at various points in the battery lifetime to examine how damage developed. Multiple processing methods are shown, which help to reveal information about the internal resonance in each case and the ways in which resonance changes due to cell aging and spatial variation in the layered structure. Scan data in some instances showed evidence of manufacturing defects such as foreign object debris (FOD) on the electrode surface. In other cases, scan data showed spatial variation in degradation of the lithium anode surface that was dependent on the charge rate, loading scenario, or cycling temperature.