Improving the production of battery cells

According to a multitude of press releases that cross our desk, battery manufacturing is a hot market now with substantive growth, led by accelerated demand from the electric vehicle (EV) industry. One, for example, cites General Motors’ plans to sell one million EVs by 2025, and another states that Chinese automakers sold 1.21 million vehicles in 2020, projecting growth to almost 2 million this year. That’s tantalizing to battery makers and more so to companies that supply battery welding equipment (especially laser-based), which has the laser industry drooling as it forecasts significant growth for this technology that had been stagnant in the mid-teens for many years.

Battery welding is not a new laser application. In one of my trips to China in the late 1980s, I visited a laser welding system builder in Wuhan that was supplying solid-state laser systems to join battery pairs, a booming job-shop industry then. But it wasn’t until EVs caught on that laser battery welding started to take off. And over the ensuing years, thanks in no small part to Elon Musk’s success with the electric Tesla, that e-mobility became a buzzword in the laser welding community. As an aside, I had the pleasure of driving a prototype laser-welded lithium battery-powered Tesla Roadster at the LASER World of PHOTONICS show in Munich in spring 2011.

That said, in a recent press release, the Fraunhofer Institute for Laser Technology (Fraunhofer ILT; Aachen, Germany) is working towards setting up high-performance production centers for battery cells for the automotive industry, and their scientists are investigating how laser technology can be used to economically contact and join dissimilar materials so that battery cells can be manufactured efficiently and reliably, and if they can be interconnected to form modules and packs in order to dependably cover the upcoming large demand for storage capacity.

Efficiency boost for lithium-ion batteries

One such is the BMBF project “HoLiB” with which lithium-ion batteries can be manufactured much more productively than before. Fraunhofer ILT is developing and qualifying a laser process that can be used to connect anodes and cathodes to the contacts: the arrester tabs. Because the anodes are made of copper, the cathodes of aluminum, and the arrester tabs of both materials, the Aachen researchers decided to test three different beam sources: a blue diode laser (wavelength: 450 nm), a green disk laser (515 nm), and an infrared fiber laser (1070 nm) are being used. Johanna Helm, a research associate at Fraunhofer ILT, explains, “The test of the three beam sources has already shown that the film stack can be welded through with process reliability. We are currently verifying the process windows and carrying out welding tests on the arrester tabs.”

A soliton suggested was using a turntable with several stations on which 20 anodes and cathodes are stacked by a stacking wheel at 0.1-second intervals so that a stack is ready within two seconds. When a stack is at a station on the turntable, the turntable continues to rotate rapidly so that the rotating stacking wheel can deposit further anodes and cathodes on the next free space. In parallel, the laser-based contacting process for the first deposited stack can start without any loss of time.

Nanosecond laser pulses protect heat-sensitive components

In the AiF project MikroPuls, scientists are examining how to bond battery cells more efficiently and Fraunhofer ILT is developing processes for joining copper, aluminum, and steel with an infrared fiber laser pulsed in the nanosecond range. These are demanding processes because the thin electrical contacts are thermally sensitive and may not be heated too much. A balance is important here: If too little welding energy is applied, the connection lacks mechanical stability; if too much energy is applied, the batteries’ mode of operation is impaired or their service life is shortened. Elie Haddad, research associate at Fraunhofer ILT, says, “This is where the fast MikroPuls process can play out its advantages, which can even be used to generate copper welds at a maximum average power of 200 W while introducing little energy into the components.”

Reliable laser welding of dissimilar materials

The dissimilar joints between copper and aluminum, for example, also pose a particular challenge because intermetallic phases form quickly, deteriorating the quality of the weld seam, leading to high contact resistances that result in either high losses due to heat or brittle joints that can no longer withstand mechanical forces. An important role is played by selectively identifying the optimum parameters with which users can also reliably generate dissimilar joints that have a consistent welding depth and high weld quality.

Tests with copper-aluminum joints on pouch cells and copper-steel joints on cylindrical cells showed that micro-pulse joining can achieve just as good joints as continuous-wave (CW) welding, with significantly lower energy consumption, higher repeatability, and fewer intermetallic phases. The only disadvantage is that the welding process generally takes longer, so there are still parameters that need to be improved.

A system is in operation that integrates both a CW fiber laser and a nanosecond pulsed fiber laser. The beam sources can be controlled individually. The system can not only join, but also remove material—for example, to structure surfaces.

The following institutes are working on the project “HoLiB – High throughput processes for the production of lithium ion batteries” funded by the German Federal Ministry of Education and Research (BMBF), (duration: Oct. 1, 2019 – Sept. 30, 2022):

TU Braunschweig, Institute of Machine Tools and Production Technology IWF (Coordinator)
TU Braunschweig, Institute of Joining and Welding Technology (IFS)
TU Berlin, Institute for Machine Tools and Factory Management (IWF) and
Fraunhofer ILT

For further information, see www.prozell-cluster.de/en/projects/holib.

The following companies are involved in the project-accompanying committee for “MikroPuls – Fine contacting of thermally sensitive materials in electrical engineering by means of short laser pulses,” which is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) as well as by the AiF Arbeitsgemeinschaft industrieller Forschungsvereinigungen "Otto von Guericke" e.V. (German Federation of Industrial Research Associations "Otto von Guericke") and the DVS Deutscher Verband für Schweißen und verwandte Verfahren e. V. (German Welding Society) (duration: Oct. 1, 2019 – Sept. 30, 2021):