Ultrasound Spectroscopy of Lithium-Ion Batteries: Insights into Layering Distances

Improving diagnostics of lithium-ion batteries (LIBs), and in particular the measurement and prediction of the remaining capacity, can lead to a longer lifetime and safer use. One promising approach to LIB diagnostics is the use of ultrasound. Ultrasound signals can measure stiffness changes of the active materials occurring during cycling and aging and can be used to predict state-of-charge (SoC) and state-of-health (SoH) of the cells [1,2]. Traditionally, ultrasound signals have been analyzed in the time domain by studying the time of flight. However, a novel approach is to study the signals in the frequency domain. The aim of this study was to investigate the potential of ultrasound spectroscopy to provide insights into the layering distances, sound velocity of the layer constituents, and homogeneity of layer thickness of LIBs. A key consideration in analyzing ultrasound signals is the interference effect resulting from reflections at different layer interfaces in the battery. The most relevant reflections occur at the copper and aluminum current collectors, so one unit of (Copper-Anode-Separator-Cathode-Aluminum) can be considered as the basic resonator. In transmission, the first destructive interference (Antiresonance) occurs when the width of the resonator is a quarter of the wavelength, which results in a dip in the frequency response. The frequency of the dip can be found by sweeping the measurement frequency (i.e., ultrasound spectroscopy) and can providing insights into the resonator width and therefore layering distances. This work presents a study of a commercial LIB (nickel-rich NCM/Graphite) with a nominal capacity of 12 Ah from Kokam (SLPB065070180, Kokam Co., Ltd.) using ultrasound spectroscopy. The results exhibited an antiresonance at 2.26 MHz in the commercial LIB (Figure 1). This frequency and sound velocity (measured to be 1640m/s) were then used to calculate the wavelength at 726 µm, and the resonator width was calculated to be 726 µm/4 = 181 µm. This result is in good agreement with the measured width of one unit from post mortem analysis at ~187 µm. The changes in the antiresonance were then studied over the course of 1000 cycles. References [1] A.G. Hsieh, S. Bhadra, B.J. Hertzberg, P.J. Gjeltema, A. Goy, J.W. Fleischer, D.A. Steingart, Electrochemical-acoustic time of flight: in operando correlation of physical dynamics with battery charge and health, Energy Environ. Sci. 8 (2015) 1569–1577. https://doi.org/10.1039/C5EE00111K. [2