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Laser Cleaning Metal Improves Battery Pack Reliability and Increases Manufacturing Throughput

Have you ever tried putting tape on a dirty, grimy surface? Yes? Then you  know it doesn’t stick very well and that you generally need to clean the area first. The same is true when trying to bond or weld two metals together: the more surface contaminants present, the weaker the bond or weld and the less conductive the finished part will be.

This is especially important when manufacturing battery packs. A battery pack consists of two or more individual batteries which, depending on the device, are connected in series or parallel to provide energy to a device. The more energy required, the more batteries that are needed.

The electrical connection between battery cells is typically achieved by a metal strap – or tab – attached to each cell. Every connection is critical to the overall performance of the finished battery.

Why is Cleaning Needed?

Battery packs are typically assembled in two places: cells are often manufactured in one plant and transported to another plant to be assembled into a pack. The surface of the cells can acquire a variety of contaminants during this transportation. Furthermore, operators handling the battery cells are likely to leave fingerprints on the surface.

Dirt and fingerprints = potential contamination at the weld joint – these kinds of organic contaminants or the presence of corrosion inhibiting agents like thin film coatings can contribute to low tensile strength joints in welding processes leading to poor adhesion, cracks and porosity between the materials. Therefore, it’s a good idea to clean the joint interface before welding to help ensure defect-free joints, which in-turn result in stronger welds and improved battery performance.

How is Cleaning Achieved?

Traditional methods to remove contaminants and thin-film coatings include sand blasting, dry ice blasting and plasma cleaning. But these methods are costly, time-consuming, inflexible multi-step procedures that also pose risks to the operator and environment by dispersing fine particles that can be adsorbed into the lungs (particularly sandblasting). Laser cleaning, on the other hand, is a clean, fast process. Benefits of laser cleaning battery anodes and cathodes, include quality and reliability improvements of the joint strength, fewer damaged parts, and increased processing speed. Typically, this laser cleaning process is implemented just prior to the bonding process.

 How Does Laser Cleaning Work?

Laser cleaning – sometimes referred to as laser ablation or laser machining, are all terms used to describe the removal of the top layers of material by laser irradiation. The amount of material removed depends on the intensity, pulse length and wavelength of the laser, as well as the material itself. By scanning the laser beam across the surface several times, the contaminants are stripped away exposing the base material and leaving a clean surface.

Using a laser to clean paint from metal – 1/4 complete
Using a laser to clean paint from metal – 3/4 complete

Benefits of Laser Cleaning vs Traditional Thin Film Removal Techniques

Quality improvements

Laser cleaning of the battery anode and cathode before welding creates an extremely clean surface for joining helping to ensure quality, repeatable results. Traditional cleaning techniques may actually leave debris if not properly used.

Less part damage

Media blasting cleaning techniques use abrasives to rip away contaminants which are effective, but can cause micro denting on the surface of the part. By contrast, laser cleaning removes only surface materials without affecting the surface morphology of the substrate.

Increased speed and safety

Laser cleaning is a significantly faster process than traditional media blasting. Reports suggest that it can be up to 15 times faster. In addition, media blasting produces debris that may be harmful to breathe requiring additional equipment or cleaning steps.

Lower cost

The cost for operating a laser cleaning system, including electricity and consumables, is significantly less than for other cleaning methods. In fact, reports indicate that a laser ablation / cleaning system can pay for itself within a year.

Seeing is Believing

The figure below shows the anode and cathode of a Li-ion battery before and after laser cleaning. On the left, a battery received from a cell manufacturer. Note the significant amount of debris and darker appearance. On the right, the same battery after laser cleaning: cleaner, brighter. A pull test showed that the bond on the cleaned anode was significantly stronger than on the dirty anode.

Li-ion battery before (left) and after (right) laser cleaning

Here’s another example. On the left, an image of a battery can with a fingerprint (see arrow). On the right is the same surface after laser cleaning. The customer welded tab material to the surfaces with and without fingerprints and performed pull tests. Samples subject to laser ablation showed significant increase in weld strength with less variation.

Battery can with fingerprint (left) and after laser cleaning (right)