Investigating the Welding Limit of Copper Foils to a Tab for Battery Applications with a Green Laser

The lithium-ion battery cells in electric vehicles are created in part by welding many layers of electrode foils to a tab to create the cell lead. While this process has conventionally involved an ultrasonic welding process, there is a desire to replace this process with laser welding to address certain limitations and remove a process step. The literature surrounding laser welding of multilayer thin foils to tabs is limited. This research aims to address this gap by developing a process window for green laser welding of multilayer foil-to-tab copper welds. Multiple copper foil layers, from 30 layers to 90 layers, were joined to a copper tab using a green laser with varying power and speed. The welds were analyzed based on micrographs, x-ray, failure load, and fracture mode. Process windows were developed for the various stack-ups. It was observed that very low heat input results in a narrow weld width and hence interfacial fracture under lap-shear loading. With suitable power and speed parameters, a process window exists where the welds have a minimum interface width greater than 180 µm, leading to fracture at the foil-weld fusion boundary. It was observed that excessive weld energy results in a weld with high porosity. Foil cut-through associated with excessive surface variation occurred under high speed and high-power conditions. As the number of foil layers increased, the process window is reduced due to the increasing number of defects present.