Mechanics of Ultrasonic Wave Propagation in Cycling Li-Ion Batteries & Applications for SOC Determination

There has been recent interest in applying ultrasonic methods to ‘image’ a variety of battery geometries and chemistries. We demonstrate some results of applying MHz ultrasonics to help accurately determine lithium-ion state of charge (SOC) and state of health (SOH). The propagation of elastic waves through a material is directly related to its material properties. As the electrochemical and mechanical properties of a material are inherently coupled, incorporating some simple ultrasonic metrics into SOC determination allows for significantly improved accuracy over traditional direct-measurement SOC methods. We propose a simple physical model to explain the changes in ultrasonic signals with respect to the electrochemical changes.