In the world of electric vehicle (EV) battery manufacturing, laser welding is a cornerstone technology that offers unparalleled precision, reliability, and efficiency in joining battery cells and busbars. Laser welding enables the production of consistently high-quality batteries and battery assemblies and allows for more complex, higher performance EV battery designs. However, taking full advantage of the undeniable benefits of laser welding requires some key considerations from tooling to quality assurance (QA) before battery production can begin.
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There are two basic approaches to the design of the tooling that presses busbars or collector plates to battery cell terminals: weld masks or single cell clamps. The choice between these clamping methods significantly impacts production efficiency and adaptability.
Welding masks promise speed and efficiency by clamping the busbar to several cells at once – the downside is that dimensional tolerances must necessarily be tighter to ensure sufficient busbar-to-cell contact across a wide area. Conversely, single cell clamping accommodates greater variations in cell alignment or geometry, simplifying production and reducing costs. However, this flexibility comes at the cost of speed. Laser welding offers very high busbar-to-cell welding speeds, sometimes exceeding a dozen cells per second, but slower clamping methods can limit welding rates.
Laser welding speeds can exceed a dozen cells per second for cylindrical cell connections
Laser welding is an exceptionally precise process and cell positioning must be consistent and precise to ensure uniform welds. Variations in cell alignment within the battery pack can lead to misaligned welds or inadequate weld penetration that compromise structural integrity. Tighter cell holder designs often reduce battery placement variation but may risk over-compression and cell damage during installation. Optimizing battery pack design with alignment guides and minimizing gaps enhances weld accessibility and quality.
Designing an effective busbar or current collector plate involves more than optimizing electrical performance. Considerations like thickness, which dictates stiffness and flexibility, impact both tooling and the optimal laser parameters. Thicker busbars, often used for prismatic cells, are efficient at carrying currents but are harder to bend for contact with cell terminals. Additionally, thicker materials can increase laser penetration time.
Busbar material is a particularly critical consideration when designing EV battery busbars. Copper has long been a popular choice for both busbars and other EV battery features due to its excellent conductivity. However, aluminum has continued to grow in popularity as a replacement for copper busbars as it offers good electrical performance while reducing battery pack weight. Aluminum busbars often weigh as little as half as much as copper busbars.
Fortunately, lasers designed for EV battery welding maintain high welding speeds and excellent weld quality for a wide variety of busbar designs and materials. Battery welding lasers often offer highly focused beams with excellent beam quality that achieve weld penetration quickly without creating a large heat-affected zone.
Each year, EV battery manufacturers make millions, or even billions, of busbar-to-cell welds, making efficient automation critically important. There are many factors that drive battery design but one illustrative example is terminal placement on cylindrical cells.
Cylindrical cells can be designed with both the positive and negative terminal on the top or the positive terminal on the top and the negative on the bottom. Choosing between these designs dictates production speed and complexity. The traditional top/bottom design enables simplified busbar design but necessitates an additional battery handling step to flip the assembly for a second welding pass. The top/top design, which has become more common with the introduction of style cells, allows for faster production cycles with reduced battery handling but demands both precise weld placement within tight tolerances and more complex busbar design.
Regardless of battery design or battery welding requirements, laser welding lends itself well to automation. An effective EV battery laser welding system fits the stage of production, from R&D to full-scale manufacturing, and addresses tooling and battery handling requirements.
When incoming batteries have consistent surface quality and tolerances, laser welding is a highly stable and repeatable process. However, if dimensional or positional properties vary unexpectedly, the result may be a faulty weld. Faulty welds result in expensive rework or scrap and, in the worst-case scenario, catastrophic failure in the final product. It is therefore necessary to accurately, and efficiently, measure and test every busbar-to-terminal weld.
Real-time weld measurement technology directly measures weld geometry as the weld is made.
Destructive testing produces accurate results but is expensive and incapable of measuring every weld. Methods like photodiode are performed in-process for each weld but can only take indirect measurements and provide imperfect results. EV battery manufacturers have increasingly turned to real-time weld measurement. Real-time weld measurement directly measures critical factors like weld depth as the weld is made, returning highly accurate data comparable to destructive testing. Additionally, trends in weld measurement data can detect process drift, helping battery manufacturers prevent future unacceptable welds.
As part of my technical consulting work, I’ve had the opportunity to work on every aspect of EV battery manufacturing and design.
The battery is the most expensive part in an electric car, so a reliable manufacturing process is important to prevent costly defects. Electric vehicle batteries are also in high demand, which puts pressure on manufacturers to maximize production without compromising quality. As a result, robot automation is almost everywhere during battery manufacturing.
The production process of electric batteries includes many steps. Before going over each step, let’s review the structure of battery cells.
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The most important component in an EV battery is the cell. Cells are where the energy of the battery is stored. While different chemistries exist, lithium-ion batteries are the most common on the market because they offer the best compromise between energy density and cost.
Three types of EV battery cells are manufactured: cylindrical cells, prismatic cells, and pouch cells. Cylindrical cells are by far the most popular, but prismatic cells are also important.
All types of cells include the following components, which are assembled in similar ways.
A coating called the slurry is applied on current collectors to help produce and conduct electricity. The slurry is a homogeneous paste that is mixed under vacuum to make sure that there are no air bubbles or moisture in it.
Here’s what it contains:
The slurry does not contain the same materials for the anode and the cathode. In lithium-ion batteries for example, it contains lithium-metal oxide for the cathode and graphite for the anode.
Before mixing, the properties of raw materials used in the slurry are measured and validated before mixing.
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Once the slurry is ready, electrode sheets are coated with it in a machine called the coater. Here are the different operations performed during this process:
The electrode sheets are cut so that they have the exact shape needed. The sheets not only need to fit into the casing, but they need to meet specific form factors.
Laser cutting is the technology typically used for high-volume production. In smaller production lines or R&D labs, die cutters are also used.
Laser marking is used to permanently identify the anode and cathode sheets with 2D codes. This will be used to:
The electrode and separator sheets form what is known as a stack. The stack is a series of sheets that are alternated (anode, separator, cathode, separator, etc.) The stack will later be inserted in the cell casing, whether it’s a cylindrical case, and prismatic case, or a pouch.
Different methods are used to create these stacks:
A metal strip is welded to the anode stack of copper foils, and another to the cathode stack of aluminum foils. Both ultrasonic bonding and laser welding can be used to make these connections. These metal strips (or tabs) are then connected to the cell terminals and will later be joined to a busbar to create an electrical circuit.
Electrolyte is poured into the casing and is absorbed by the separator sheets. The choice of the electrolyte chemistry is important for the battery’s performance. For example, additives can be used to affect factors like viscosity and improve conductivity.
After the electrolyte is poured and absorbed, air bubbles are removed in a vacuum chamber. This step is called degassing, and it ensures that the electrolyte is uniformly distributed.
The casing is then sealed hermetically using methods such as crimping, laser welding, ultrasonic bonding, or heat sealing.
Forming involves the initial charging and testing of battery cells. During this step, cells are connected and undergo multiple charge and discharge cycles (with resting in between) that help set the cells’ electrochemical properties.
The final step of cell manufacturing (before module and pack assembly) is cell inspection.
After forming, cells undergo final testing to verify their electrical properties like capacity, voltage, and internal resistance. Mechanical properties are also validated with vision cameras. Cells that do not meet requirements are rejected.
Cells with similar properties are grouped together. They will be assembled in the same modules to ensure the high performance of the module as a whole.
During the assembly of battery modules and packs, sealants and adhesives are applied using a dispenser. Adhesives are used to join components together while providing additional properties like thermal conductivity, electrical conductivity, or insulation.
They are applied on battery housings, cells casings, and other components like cooling tubes. The curing time is controlled with active methods like UV light curing to prevent slowdowns.
To maximize the strength of the adhesive bonds, laser surface preparation is used before adhesives and sealants are applied. This process removes all contaminants and can modify the surface roughness if needed.
When the battery pack is assembled, other components that are essential for the good functioning of the battery need to be integrated.
Laserax provides laser solutions for EV battery manufacturing. You can learn more about our solutions here. Or, contact us if you want to discuss one of the following:
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