Ultrasound Spectroscopy of Lithium-Ion Battery Cells

This thesis explores ultrasound (US) as a non-destructive diagnostic tool for lithium-ion batteries across their life cycle, from manufacturing to aging. US is highly sensitive to mechanical stiffness and structural changes during lithiation and delithiation. It detects effects such as electrode wetting, solid elecrolye interphase (SEI) formation, lithium plating, and other aging processes. Unlike conventional electrochemical methods, US directly captures mechanical and electrolyte-related changes that often remain hidden in electrical data. The study systematically examines the influence of state of charge (SoC), state of health (SoH), temperature, cell thickness, and frequency on ultrasonic signals. A dedicated setup enabled simultaneous operando measurement of thickness and US observables, allowing changes in the US signals caused by thickness variations to be separated from intrinsic material effects. An ultrasound spectroscopy method was developed to analyze frequency-dependent resonances. The first resonance frequency proved sensitive to SoC, offering a robust new parameter for SoC estimation. At resonance, signal amplitude increased more than fivefold from fully discharged to fully charged, while remaining largely temperature-independent. A transfer matrix model theoretically confirmed the phenomenologically observed resonances and their SoC dependence, revealing localized pressure maxima of standing waves in specific components near resonance edges. This suggests applications such as targeted high-intensity US for battery healing or material separation in recycling. Measurements on aged cells showed that US is sensitive to degradation. Correlating US spectroscopy measurements with post-mortem analyses linked resonance shifts to lithium plating, SEI growth, and cathode cracking. Finally, US monitoring of wetting and formation improved understanding of gas and electrolyte dynamics, with potential to optimize manufacturing and shorten wetting procedures.