The high energy density of rechargeable lithium-ion batteries has prompted an increasing number of car manufacturers to use them as energy storage systems in electric vehicles. Vehicle battery packs consist of multiple cells, which have to be interconnected via electrical contacts. Cells in flexible casings known as pouch cells are frequently used in such applications. They have strips of copper and aluminum respectively leading from the electrode layers and extending from the envelope to form positive and negative terminals.

To build a battery pack, the terminals of the individual cells have to be connected in series by linking the anode of one cell to the cathode of the next cell, and so on. This is normally done using screw connections, because aluminum and copper are dissimilar metals and thus difficult to bond reliably. Conventional welding and soldering techniques are not a practical alternative because the flexible cells risk being damaged by the heat.

Method

The use of a bonding method to connect the terminals, instead of screw connections, offers the advantage of a much lower electrical resistance and hence increased battery efficiency. Laser welding is well suited as a joining method because it allows the process to be automated and can be performed with a relatively low energy input if the process parameters are set appropriately. As part of the Fraunhofer “System Research for Electromobility” project, laser welding is therefore being qualified as a joining technique for the described task.

Result

In experimental tests, a process parameter window was identified within which it is possible to produce reproducible welded connections between the terminals with an adequate current carrying capacity. Current research is focused on qualifying the long-term stability of the connections.

Applications

Rechargeable lithium-ion batteries are at present the most promising energy-storage technology for electromobility applications. The availability of efficient energy-storage systems that can be manufactured at low cost is a vital enabling factor for this fast-growing branch of industry. Laser welding can help manufacturers to produce the high-performance batteries needed for mobile applications more cost-effectively.