Since its commercialization in 1991 by Sony, Lithium-ion (Li-ion) has become the dominant rechargeable battery technology for consumer electronic devices and is poised to become commonplace for industrial and transportation use, as well. For electric vehicles, the Li-ion battery is a widely used energy source because the energy density (which extends an electric vehicle’s range) and power density (which increases discharge/charge efficiency) is typically doubled in contrast to previously used technologies, such as nickel metal hydride and lead acid.
Similar to those used in consumer electronics, Li-ion battery systems for electric vehicles require thousands of times more power, and they are expected to function under varied conditions for more than 10 years. To compete economically with dominating combustion engines, the battery system needs to continue to enhance the performance yet cut costs by half. Reaching these goals will require innovative and reliable manufacturing processes. Among them, lasers stand out as a unique and proven tool.
Categorized by shape, Li-ion battery cells are mainly found in cylindrical, prismatic and pouch shapes, as seen in Figure 1. Each shape has its advantages. For example, the prismatic shape allows OEMs to minimize the packaging size and accounts for the majority share of Li-ion batteries in the electric vehicle market.
Manufacturing prismatic Li-ion battery systems includes a complicated assembly process of multiple layers of varied metals, including aluminum (mainly 1000 and 3000 series), copper, nickel and nickel-plated copper in thicknesses from 0.03 mm to 2 mm. With the unique advantages of low heat input, high welding speed and great flexibility, lasers play an important role in prismatic Li-ion battery manufacturing processes.
In fact, lasers have been used in mass production of prismatic Li-ion batteries in applications including hermetic sealing of the battery housing, welding anode cathode terminals to the cell housing, welding terminals with bus bars and welding of the safety vent cap.
In the beginning of the production chain, shown in Figure 2, the aluminum or copper foil is coated and compressed to create the electrode foil. This foil is produced on a coil, which needs to be cut in sheets using a process called slitting or sheet cutting. The current technology for this production step is a mechanical cut, but cutting this kind of material creates additional costs because mechanical abrasions make it necessary to change tools frequently.
Next, the taps of the electrode foils are welded together using laser welding or ultrasonic welding. The technological challenge in this step is welding of thin foils and material mixes such as copper-copper, aluminum-aluminum or aluminum-copper joints. Here, battery manufacturers must achieve a very low electrical resistance in the joint for a high quality weld of the battery cell.
The electrode foils are then packed in the battery housing. For prismatic Li-ion batteries, the housing is normally made of an aluminum alloy. After packaging, the housing is closed and sealed.
Because the welding seam must be gas tight, there are stringent requirements for the welding process. As the electrical parts are already in the housing, only minimal heat can be tolerated, and only a few welding technologies are currently available that meet this requirement. Laser welding is one of the fastest and most reliable welding technologies, and thus, widely used in mass production of prismatic battery cells.
In the final step of the cell production, the battery housing is filled with electrolytes. After filling, the hole must be closed using a welding process with nearly the same requirements as the previous step. The cells are then assembled into battery modules and blocks. Also in this step, joining technologies are needed to connect the cells and modules.
Hermetic sealing of housings
Typical laser applications for prismatic Li-ion batteries begin with the hermetic sealing of the battery housing. This can be seen in Figure 3.
Most Li-ion battery manufacturers choose an aluminum alloy for the housing material given its light weight and low cost. The most common material used is the 1000 and 3000 series with thickness ranges from 0.6 mm to 1 mm. The housing is normally “deep drawn” to form a prismatic container, and the top plate is welded onto the container with a 3-kW disk laser frequently being the laser of choice.
There are typically two scenarios: welding from the top (butt joint) and welding from the side (overlap joint). Figure 4 shows actual welding in the two scenarios with a 3-kW disk laser.
The advantages of welding from the top include higher welding quality and automation convenience. There is potential risk, however, of welding spatter splashing into the battery cell. Also, the necessary part preparation accuracy isn’t sufficiently met by the deep-draw process. Welding from the top is forgiving of those factors, but it requires highly dynamic multi-axis machines as the laser power and welding speed must be perfectly matched, especially in the corners.
Anode cathode terminal
In addition to the housing, typical laser applications for prismatic Li-ion batteries include welding of the anode cathode terminal in a single cell.
Here, the positive terminal is made of aluminum and the negative terminal is made of copper. Depending on the mechanical designs of different manufacturers, the terminals have two types of joints: the butt joint and the fillet joint, as seen in Figure 5a and 5b. The butt joint is a more popular design as it can be welded quickly with scanner optics.
Due to the material thickness (up to 2 mm) and highly reflective nature of aluminum and copper, the welding of the terminals requires higher laser power. Fiber lasers have similar absorption rates compared to disk lasers, but fiber laser usage is greatly limited due to the potential risk of damage by back reflection.
Disk lasers, on the other hand, are not sensitive to back reflection due to the much lower power density on the internal optical components. Thus, disk lasers are better choices for welding terminals, especially for the copper negative terminals, which are more reflective. A 6-kW disk laser with PFO33 (programmable focusing optics) scanner optics have been proven to be the most productive and reliable solution in mass production of Li-ion battery terminals.
Safety vent cap
Safety protection is a fundamental function integrated into the Li-ion battery system, and the safety vent is one of the essential components. In the event of internal pressure and rapid temperature increases caused by environmental stress or over-charge, the safety vent will open to allow the gas to vent.
The safety vent cap is normally nickel or nickel-plated copper with a thickness of 0.03 mm to 0.5 mm, and it is welded onto the top plate of the housing. Minimum distortion is required, and a lamp-pumped Nd:YAG pulsed laser with scanner optics is ideal for this purpose.
Spatter-free welding can be achieved by adapting the pulse shape. And, the productivity is boosted with burst mode (rapid firing within a short period of time).
A copper bus bar is used to connect the terminals of single cells to produce the higher voltage that is required to power electric motors. A nut and bolt connection has its limits because the parts might loosen over time, and the oxidized surface has a relatively high resistance. Laser welding has been used for welding terminals with bus bars including welding dissimilar materials, such as aluminum and copper, as seen in Figure 6.
A 4-kW or 6-kW disk laser is used to generate nonporous and crack-free welding, which guarantees both mechanical strength and electrical conductivity.
When the terminals or bus bars are joined with a butt joint, or when a larger cross section is required due to the high current, the so-called laser wobbling welding or laser beam oscillation can be used. As shown in Figure 7, a secondary motion with linear or curvilinear (circular or ellipse) movement is superimposed to the feeding motion of the laser beam along the welding path. This welding technique will be able to bridge a bigger gap and create a larger cross section with better surface quality.
For the production of Li-ion batteries and so much more, laser welding is a popular joint process due to its high speed, small heat-affected zone and low occurrence of deformation. Among all available laser technologies, disk lasers, with their insensitivity to back reflection, are an ideal choice for welding highly reflective materials, such as aluminum and copper.
Using the disk laser for laser welding is an attractive joining solution as it is fast, precise and flexible and supports different joint types and materials and produces nonporous and crack-free welds. Combined with PFO scanner optics, the assembly automation process could be simplified, and the wobbling process could be used to even further improve the Li-ion battery’s mechanical and electrical performance.