Ultrasound can also be applied for defect detection in electrodes, particularly for metal contaminants. Metal contaminants have been demonstrated to act as sites for internal shorting and significantly reduce long-term cyclability. Furthermore, internal shorting is linked to thermal runaway, which can lead to combustion and other safety hazards. Thus, de- tection of contaminants is critical in the electrode and cell fabrication processes. Figure shows the acoustic images and associated optical images of aluminum shavings on top of the electrode (a,b) and embedded shavings (c,d). When the metal contaminants are on top of the electrode, optical images provide better detection from contrast and resolution. On the other hand, the acoustic image cannot resolve the small metal contaminants which is likely due to feature broadening as discussed prior. However, it is observed that the range of acoustic density increases when contaminants are present in the electrode from between 0.13-0.16 g/cm 3 in Figure (no contaminants) to 0.20-0.22 g/cm 3 as shown in Figure . This increase in the acoustic density is likely due to the metal particles having a higher density than the porous electrode. Scattering may have also affected the measurement due to protrusions of the metal contaminants out of the electrode surface. This shift of acoustic density range within the electrode can then be used to indicate the presence of contaminants for quality control by comparing to a standard acoustic density range from an electrode with no contaminants.