The use of double-sided metallized polymer foils as current collectors can significantly improve the safety of Li-Ion battery cells by completely avoiding thermal runaway. The approach is as follows: In the event of an internal short circuit in the cell, a greatly increased temperature occurs. This in turn melts the current collector locally, causing it to withdraw from the zone of internal short circuit, this prevents further chain reactions. However, a particular challenge is the cell production with these new materials, especially the contacting of pouch cells. There, several layers must be joined together in such a way that all current collectors have electrical contact. The metal ultrasonic welding process is often used for this purpose because it is fast, reproducible and suitable for thin metal foils like copper or aluminum. Since polymers generally have a very high electrical resistance, the contacting of polymer foils metallized on both sides poses special challenges. In this work, aluminum coated polymer foils in a total thickness of approx. 11 µm were used. Different parameter variations in the ultrasonic welding process in terms of energy, amplitude and joining force were investigated for this foil. The used layer number is between four and twelve. In addition, the effect of top and intermediate layers of pure aluminum foil was evaluated. For the joints, both the mechanical strength by peel tests and the electrical resistance in conductivity measurements were determined for this investigation.
The results showed that with a low number of layers, additional aluminum foil is necessary to achieve sufficient mechanical strength. This is not necessary for a 12-layer stack-up. In each case, substrate fracture was achieved. The conductivity measurements resulted in resistances in the milli ohms range.