Ultrasonic metal welding has been used widely to join battery cell terminals, or tabs (either Al or Cu), with bus bars (Cu) to form assembled battery packs in battery electric vehicles. However, the mechanism of ultrasonic welding for Al/Cu is still not well understood. In this chapter, the microstructures of the ultrasonic welds between three layers of lithium-ion battery tabs (either Al or Cu) and bus bars were studied. From the microstructure analysis, the weld formation mechanism and failure modes were investigated. It was found that the metal inter-mix or interlocking is the main weld formation mechanism among Al tabs,...