EV Battery Production: Exploring Joint Technologies

Joints are important electrical and mechanical connections in producing electric vehicle (EV) batteries. They link individual battery cells to make a full battery pack. However, the process of making joints has evolved over the years due to various technologies. This first part of the multipart FAQ will discuss ultrasonic welding and laser welding, two commonly used technologies for creating joints for efficient EV battery production.

Ultrasonic welding — for high-strength, dissimilar materials

Ultrasonic welding has become an important way to join parts in producing EV batteries, especially pouch cells. This advanced method uses high-frequency ultrasonic vibrations of 20 kHz or more to make solid-state links under pressure. This allows materials to join together without melting, which is a significant benefit when making batteries.

Ultrasonic vibrations perpendicular to the pressing force make the process work when thin pieces are joined. These high-frequency vibrations cause frictional heat, which softens the metal and makes it easier to bend plastically. This causes materials to connect at the atomic level, creating strong bonds at temperatures that are usually only a third to a half of the melting point of the materials. The ultrasonic welding process is illustrated in Figure 1.

Figure 1. An illustration of ultrasonic welding to join two battery cells. (Image: Batteries, MDPI)

The technology is very flexible; it can join materials as thin as 5 µm and work with more than 100 layers of groups. Manufacturers use a 40 kHz frequency with a low amplitude for smaller battery parts like tabs and foils. Bigger parts like busbars and cables need a 20 kHz frequency with a higher amplitude.

Advantages of ultrasonic welding

The technique’s main benefit is that it reduces intermetallic compounds and defects like porosity and hot cracking, which could otherwise hurt the performance and stability of the battery. Ultrasonic welding is a fast process, capable of joining components at rates up to 400 parts per minute. This high speed allows for high-volume production, crucial for meeting the growing demand for EV batteries.

The process requires relatively low energy input, making it an energy-efficient joining method. This aligns with the sustainability goals of EV production. Ultrasonic welds can join dissimilar materials commonly used in battery production, like aluminum and copper. This versatility is essential for connecting various battery components.

The solid-state nature of the process ensures minimal heat input to the battery, reducing the risk of damaging sensitive components. This is important for preserving battery performance and safety. Ultrasonic welding is a flexible process that can be adapted to different battery designs and production layouts as well as easily integrated into automated production lines.

Disadvantages of ultrasonic welding

But EV makers have to think about some limits. Ultrasonic welding is mostly useful for pouch cells because shocks and pressure can damage cylindrical or prismatic cells. The process requires careful planning of how to access the anvil and sonotrode in the battery design (Figure 2), and when the sonotrode sticks sometimes, it can put battery parts at risk.

Figure 2. A schematic drawing of ultrasonic welding of a lid foil onto a microcavity when the sonotorde and anvil get closer. (Image: ResearchGate)

Laser welding for non-contact, high-speed process

Laser welding is a non-contact process using a focused laser beam to melt and fuse materials. It is a promising alternative to the traditional ultrasonic welding method for joining current collectors to tabs in EV Li-ion pouch cells. It is possible to work quickly with laser welding, create custom weld patterns, and weld any joint shape while joining dissimilar metals. Three types of joints can be used when laser welding busbars to battery cell terminals: the lap joint, the fillet joint, and the laser spot welding, as illustrated in Figure 3.

Figure 3. Three types of laser joining battery connections — fillet joint (left), lap joint (middle), and multiple spot welds (right). (Image: Batteries, MDPI)

Advantages of laser welding

Laser welding uses a highly focused laser beam, which gives the welder exact control over the process. Because of this accuracy, the welds are narrow and deep, which reduces the area affected by heat and the chance of damaging the delicate battery parts.

Due to its thin welds and fast welding speed, laser welding does not generate much heat in the assembly during the welding process. This is necessary for welding battery tabs because the chemicals inside are very heat-sensitive.

Due to the small cross-section of the laser beam and its energy concentration, laser welding allows for high welding speeds. This method can successfully join a variety of material combinations, including aluminum to aluminum, aluminum to steel, copper to steel, and copper to aluminum. This versatility is beneficial for connecting battery tabs and terminals, often made from different metals.

Disadvantages of laser welding

Brittle intermetallic compounds can form in the fusion zone when laser welding dissimilar metals, such as aluminum and copper. These compounds, like Al₃Cu₂ and Al₂Cu₃ (Figure 4), can weaken the joint and make it more prone to cracking. Their formation is more pronounced when welding copper foils to aluminum tabs.

Figure 4. Formation of intermetallic compounds such as Al₃Cu₂ and Al₂Cu₃ during laser welding. (Image: Elsevier)

Laser-welded joints generally exhibit higher electrical contact resistance and elevated operating temperatures than ultrasonic joints. The higher electrical resistance leads to increased energy loss through Joule heating and can negatively impact battery efficiency and lifespan.

Summary

The use of a particular EV battery joining technology depends on many factors. Ultrasonic welding is useful for dissimilar materials where the temperature while joining is less than the melting point of the metals used. As EV makers focus more on design aspects of batteries to fit them into EVs, this technology serves the need.

Laser welding is good for non-contact welding processes and can create highly custom welds. However, it has a high upfront cost — the higher contact resistance results in a higher power loss, especially during high current transfer.

The next FAQ will focus on resistance spot welding and wire bonding.