Coating systems Optical quality control for Li-ion battery coatings

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For the production of accumulators within the scope of automotive electrification, manufacturers need pilot plants where coating variants and processing technologies can be tested under conditions close to production. A conversation with a Swiss engineering company that operates in this field worldwide.

The automotive industry expects explosive growth in the coming decades with the transition from combustion to electric drives. The key to this is the further development of batteries, whose performance and costs have not yet met customer expectations. Manufacturers need pilot plants to test coating variants and processing technologies under production-like conditions. A Swiss engineering firm is active worldwide in this field.

"We have evolved from a manufacturer of textile dyeing machines to a globally active engineering partner for sophisticated coating technologies," reports Clemens Disch, Sales & Marketing Manager of Mathis AG in Oberhasli, Switzerland. With around 60 employees, the family-run company develops and implements state-of-the-art coating systems for high-tech industries such as the automotive sector, the chemical industry, the manufacturing of Li-ion batteries, and solar cell producers. As a partner to their customers, Mathis specialists help create truly functional plant technologies for new ideas and technological approaches. With such pilot plants, customers can test the practical feasibility of new approaches. This allows thorough examination of the feasibility of the plant concept, ensuring that large-scale plants built later with high capital expenditure deliver the desired results. For such projects, Mathis relies on a tightly-knit network of proven supply and development partners from the surrounding area.

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Lithium accumulators for the automotive sector

"Although the principle has been known for some time, lithium-ion batteries remain one of the hottest development topics for automotive manufacturers," adds Nico Brand from the technical engineering department. The everyday usability of currently available solutions regarding range, charging speed, cycle stability, and cost cannot convince many customers. To make progress, numerous possible variants must be investigated and optimized. In principle, such a battery is made up of cells, each consisting of an anode and cathode with an intermediate separator foil. The anode is a copper foil coated with a graphite-containing anode material. The cathode consists of aluminum with a coating containing ions of transition metals such as cobalt, nickel, manganese, or iron. The separator foil is permeable to the lithium ions of the electrolyte, thus enabling charge transport. The coatings are applied as a pasty mass in one or multiple layers, either simultaneously or sequentially. They contain partly highly toxic solvents, requiring elaborate drying and exhaust air cleaning processes. The application is carried out by various methods such as slot die coating or doctor blades in thicknesses between 10 to 350 micrometers. To produce efficiently, as wide bands as possible are often processed industrially, on which separate strips are applied. Sometimes, the strips are also executed in an interrupted manner, creating separate "patches." Depending on the equipment, the coating is initially applied on only one side or simultaneously on both the top and bottom sides. In single-sided processes, the band is then coated on the opposite side in a second pass. For the production of the actual batteries, the finished bands are cut into appropriately sized pieces, with no offset allowed between the top and bottom sides.

Fast CIS special sensor technology for position control

"One of the critical points in manufacturing such coating systems is thus the precise adherence to tight position tolerances between the layers on the top and bottom sides of the foils," explains Clemens Disch. The reference is always the band edge on the right in the running direction. Relative to this, the coating must maintain a defined distance with a position tolerance of ± 0.2 millimeters. The CCD camera systems used so far were not reliable; the current width of 760 millimeters (approx. 29,9 inches) already required the use of several cameras side by side. Instead, the choice fell on a so-called Contact Image Sensor (CIS). These are "line cameras," as also used in copying systems. However, much higher accuracy and speed are required for coating. The CIS sensor now used is 800 millimeters wide and consists of nearly 19,000 linearly arranged monochrome photocells with fixed-focus optics. The individual photocells have a spacing of 0.042 millimeters. Also integrated are two rows of LEDs, each in front of and behind the row of photocells. A significant difference from the simple CIS sensors in copiers is the scan speed, as the band running through at up to 10 meters (approx. 32,8 feet) per minute must be captured with a sampling rate of almost four MHz.

Another important aspect in selecting the sensor is the ability to handle the most challenging contrast ratios, from metallic shiny and reflective to matte black absorbing. Therefore, in this case, an additional setup with a diagonally arranged light bar was required. Two such systems are used per plant, one for the top and one for the bottom of the band. The upper sensor measures both the band edge position and the width of the coating(s), while the lower sensor only monitors the band position.

Swiss support for high-tech sensor from the Far East

"A suitable sensor is found in the product range of a Chinese manufacturer," Brand recalls. A decisive prerequisite for the choice was – in addition to fulfilling the required hardware properties – the comprehensive engineering support from its Swiss representative, Compar AG in nearby Freienbach. Compar is a system integrator for machine vision and robotics with decades of experience and has developed a software platform called Visionexpert for this purpose. Visionexpert is a flexible tool for solving various tasks in the field of image capture and optical defect detection. Thanks to its ease of use and numerous interfaces, it is ideal for the efficient development of customer-specific solutions.

After making contact in mid-2022, an initial order was placed in August. This was followed by several months of development cooperation, which proceeded to the mutual satisfaction of both parties. Compar set up the CIS sensors fully for the task, so no calibration is required later. In addition, the software was developed for optimal lighting adjustment, image evaluation, and translation into control signals for correcting the band position on the rollers.

Mathis was able to secure an initial order for a pilot coating system equipped with CIS sensors in January 2023, with the internal approval of the functionally assembled line occurring in May 2023. Customer approval followed after a successfully passed Factory Acceptance Test (FAT) in December 2023. The CIS sensor has been functioning smoothly since the first installation. Meanwhile, Mathis is already equipping a second line with this sensor technology, and four more projects are already in the pipeline.

Further projects planned

"Compar has decades of experience in developing camera-based systems for automation and quality control and has proven to be a competent and responsive partner," says Disch. The CIS system initially remained at Compar for two months after delivery to work out the optimal settings for different films and coating variants. This particularly involved the lighting, for which an additional inclined light bar was installed. Subsequently, the system was mounted on the pilot plant. Here, Mathis' software department integrated the signals from the CIS control into the PLC of the band system. The collaboration worked very well right from the start. Compar proved to be a development partner on an equal footing, always responding quickly to questions or problems. Even after the system was installed in the band system, Compar employees were often on-site within less than an hour if needed. They also had good experiences with online support and collaboration by phone. Compar has proven to be a supply partner with whom they look forward to engaging in further projects in the future, according to C. Disch.