Stress-Strain Monitoring of Li-Ion Battery Through Single-Frequency Ultrasonic Testing

The safety, performance, and lifespan of lithium-ion (Li-ion) batteries heavily depend on understanding their internal chemo-mechanical state. Conventional battery management systems, which depend on terminal electrical measurements, offer limited insight into the dynamic stress and strain that develop during operation. This paper introduces a non-destructive, in-situ method for monitoring the internal stress state of a commercial Li-ion pouch cell using single-frequency ultrasonic testing. Initially, a relation between stress-strain buildup within the battery and the amplitude of the transmitted ultrasonic signal is established. Then, a 120 kHz ultrasonic signal is transmitted through the cell in a through-transmission setup, and the amplitude of the received signal is analyzed as the main indicator for stress-strain and degradation monitoring. Experimental results over four charge-discharge cycles show a strong correlation between signal amplitudes and the State of Charge, with amplitude generally increasing during charging due to intercalation-induced compressive stress. The technique demonstrates high sensitivity, capturing non-monotonic behaviors attributed to electrode phase transitions. Additionally, a gradual, cycle-by-cycle decrease in overall signal amplitude is observed, providing a direct measure for tracking cumulative mechanical degradation and State of Health. These results demonstrate that ultrasonic signal amplitude is a comprehensive, versatile feature capable of offering real-time insights into the complex acousto-mechanical behavior of Li-ion batteries, offering a promising enhancement for next-generation battery diagnostics.