Probing NMC|graphite batteries with ultrasounds: in operando non-destructive through-thickness diagnostics and SoC estimation

Ultrasonic analysis is emerging as a promising non-destructive technique to monitor Li-ion batteries relying on (de)lithiation-induced variations in the mechanical properties of the active materials at the electrodes. Ultrasonic state of charge (SoC) estimations are usually based on shifts of a single peak of the time of flight (ToF) whose behavior is dominated by the contribution of graphite, while effects from the cathode have never been observed. Here, we retain ToF as a core metric but advance to multi-peak analysis. We demonstrate how pulse-echo tests provide live insights through the thickness of the battery, showing two sets of peaks, whose intensity is coherent with Li content at the anodes and cathodes, respectively. This finding has the potential to allow non-destructive through-thickness diagnostics for local state of health (SoH) evaluations in real time. We provide a demonstration of gas formation between two electrodes, as a first alert in case of battery failure. A frequency domain analysis reveals that the transmitted signal is made of three main waves and the spectral power of two of them is a linear indicator of the charge stored in the battery, even during constant voltage charging. • Pulse-echo ultrasound enables depth-resolved battery diagnostics. • Acoustic impedance variations distinguish anode and cathode dynamics. • Gas formation is detected as early failure alert. • Spectral power of transmitted signals quantitatively tracks the SoC.