Contacting of metallized polymer foils as current collectors with ultrasonic welding for Li-Ion batteries

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Aluminum-polymer current collectors have the potential to increase the safety of Li-ion batteries by suppressing the extension of internal short circuits. Hence, the cell heats up only locally and for a limited time, which prevents a thermal runaway. However, in pouch cells the polymer layer electrically isolates the different plies from each other. This makes contacting particularly challenging, as electrical conduction paths must occur for the top ply to the tab through the insulating polymer.
In this poster, metal-to-ultrasonic welding for such aluminum-polymer current collectors was investigated. Ultrasonic welding is an established process for contacting current collectors because it brings little heat to the weld zone and can join many plies together. Hereby, the knurling of the horn often penetrates the plies. This plus a potential displacement of the polymer could enable the generation of conductive pathways in the aluminum-polymer foils. Joints were made with two different aluminum tabs (20 µm and 100 µm thickness), as well as with a varying number of plies (one to 12 layers). T-peel tests and 4-point-probe measurements were performed to characterize the joints.

The peel resistance is strongly dependent on the number of plies clamped in the testing machine. This and the fracture surface indicate that for joint strength, the crack sensitivity of the foil was more critical than the adhesion strength. So, the principle share of failure was due to substrate fracture.
In the 4-point-probe measurements, the electrical resistance increased for particularly thin and thick joints. The number of plies was not decisive here, but the total thickness of the tab and the plies together. The electrical resistance was in the range of 6 to 14 mΩ for a weld area of 11×7 mm² and thus powers of ten above those of purely metallic welds. It follows that other contacting approaches are necessary for such current collectors to reduce the energy loss at high current flows.

Summarized, the results show no correlation between peel strength and electrical resistance. Nevertheless, a mechanical joint could be created with metal ultrasonic welding. In this context a two-step joining process with a fixation step and one for electrical contacting might be more promising.

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