Ultrasound Propagation in Lithium‐Ion Battery Cell Materials: Basis for Developing Monitoring and Imaging Methods

State‐of‐the‐art methods to monitor the degree of wetting of lithium‐ion batteries during production, which are applicable at an industrial scale, are not capable of determining the spatial electrolyte distribution in the porous network of electrodes and separator. Ultrasound can bridge this gap as it has previously been proven to be a suitable method to monitor or evaluate the infiltration of porous structures, for example, epoxy resin in carbon fiber composite. Herein, the acoustic properties of different battery cell materials are evaluated over a wide frequency range in both the dry and wet states, for the first time. Furthermore, a collection of input parameters, from the experimental setup and the evaluation routine to the acoustic parameters of the battery materials themselves, are provided. These parameters are needed for the adaptation of ultrasonic nondestructive testing to the monitoring of the wetting process of a lithium‐ion battery. Most importantly, this work demonstrates that a large change in the sound velocity (up to 100%) is observed when the porous structure of a lithium‐ion battery is filled with electrolyte and thus can serve as an appropriate evaluation figure of merit.