Ultrasonic Welding: Green Manufacturing Technology for Battery Cell Production
In the dynamic world of electric vehicles, the efficient and high-quality production of battery cells in cylindrical, prismatic, or pouch form is crucial. Production methods that are not only fast and reliable but also environmentally friendly and compatible with future use of recycled materials are in demand. When it comes to creating perfectly conductive electrical connections between different materials in cost-effective mass production, ultrasonic joining processes are an excellent choice.
The automotive industry increasingly favours cylindrical battery cells. They offer high energy density, mechanical robustness, longevity, and can be produced and scaled relatively easily: aluminium and copper foils, separated by a separator layer, are rolled up in fully automated production lines, equipped with electrical contacts, and packaged in a cylindrical cell. A battery for an average electric car contains between 1,000 and 9,000 such cells, depending on its power, making a highly efficient production method indispensable to ensure quality, consistency, and the required high quantities. Telsonic’s Soniqtwist torsional ultrasonic welding process provides ideal conditions for welding the poles of cylindrical battery cells. The process has already proven itself in many different industrial areas, and the systems can be easily and cost-effectively fully integrated into various production plants (Figure 1). Examples include round and ring indexing machines as well as continuous mode or linear indexing systems.
Practical Advantages in Welding the “Jelly Roll”
Today’s cylindrical battery cells typically consist of a 12 µm thick aluminium foil and an 8 µm thick copper foil separated by a thin, laminated separator layer. This sandwich structure is then rolled up into the so-called “jelly roll,” placed in cylindrical housings, and then an aluminium collector is welded on. This is done by a high-frequency torsional welding movement, usually at a frequency of 20 kHz, which connects the jelly roll to the cathode (the + pole) via the aluminium disc or collector that links the foils. The rod-shaped sonotrode, with a diameter of 2-6 mm, penetrates 70-160 mm into the jelly roll (Figure 2) and welds the aluminium disc to the insulated bottom of the housing. After that, the cell is sealed with a lid and filled with electrolyte through an opening, which is then also sealed. After cell finishing, which includes forming, degassing, and maturation, the cell is charged. The finishing parameters vary depending on the cell manufacturer and significantly affect the cell’s performance.
Compared to laser technology, which also convinces in terms of precision but reaches its limits in integration into high-speed production environments, productivity, and costs, ultrasonic technology offers several advantages: the torsional ultrasonic welding process allows, depending on the production plant, a productivity of up to 400 ppm (parts per minute) with welding times under 300 ms. Thus, nothing stands in the way of efficient mass production.
Environmentally Friendly and Without Side Effects
Simultaneously, the process is tolerant to the materials used: battery manufacturers aim to produce as green batteries as possible with low CO2 footprints, fossil-free energy, and an innovative circular system. Topics such as battery recycling are of essential importance. Here, ultrasonic welding technology also convinces, as recycled materials can be processed. Laser technology, for example, is not suitable for the latter because they often contain substances that reflect the laser beam, negatively affecting the welding result. Unlike other fusion welding processes, the ultrasonic method, due to its low heat development—30 to 40% below the melting point—also minimizes the risk of pore formation, splashes, and hot cracks. Pores in the pole connections could, for example, lead to cracks and thus failures during later operation due to vibrations.
Ultrasonic welding systems also consume very little energy, as the systems operate at the resonance point with minimal energy expenditure; this saves a lot of energy compared to other joining processes. Ultrasonic welding does not require protective equipment, does not cause hazardous emissions or waste, and does not need additional materials such as flux or solder. These aspects align with the environmental goals of e-mobility, making torsional ultrasonic welding a forward-looking green technology in battery cell production for electric vehicles. It supports sustainability goals and enhances the company’s image.
Production Quality in Focus
As sustainability is closely linked to production quality and error prevention, Telsonic relies on a digital platform that enables seamless integration and control of ultrasonic components from the lab to series production. With TelsoAssist, the individual ultrasonic components are digitally connected into a complete system. The integration of sensors enables precise measurements of important parameters and correct process control. The software supports all phases of a project, from application development in the lab to integration, commissioning, production, and service and support.
Through consistently used software-supported documentation, machine-readable real-time data are available to monitor processes, recognize trends, ensure quality, and continuously optimize. Self-check functions of the tools used, an image-supported menu guide for the operator, simple visualization, maintenance counters for wear parts and equipment management, as well as barcode readers ensure that the right tools with validated parameters are used for each application, serving consistent error prevention. The perfectly coordinated solution of software and hardware, intuitive dashboards, and network-wide data exchange via various interfaces such as ProfiNet, EtherNet/IP, EtherCat, and POWERLINK offer transparency and efficiency in production and enable the development of AI functionalities.
Technical Box: Torsional Ultrasonic Welding
The torsional ultrasonic welding process Soniqtwist is a high-frequency friction welding process. The sonotrode performs alternating torsional movements around the longitudinal axis in one direction and the other. This is done in short times (between 0.1 and 0.4 s) with a frequency of 20 kHz and amplitudes up to 50 µm into the interface of the two parts. At the same time, the torsional movement of the sonotrode ensures that the surroundings of the welding zone are practically not affected by the ultrasound. Power levels up to 14.4 kW can be transferred with the process. Thus, high strengths can be achieved even with difficult material combinations and large welding areas. Pull-off strengths of several 100 N/mm2 are possible, depending on the component and requirement profile.
Axel Schneider, Head of Business Development Battery at TELSONIC Ultrasonics,
