Battery coatings | Insight 49 then a fire protection layer. Developing multi-functional coatings is a challenge, particularly if two or more of the required functions seem incompatible. Solutions to this kind of problem have been found in material design, Dr Knecht at Henkel argues, along with selection of filler packages that can, for example, enable thermal conductivity while maintaining dielectric protection. “A lot of functions the industry thinks are incompatible just haven’t been figured out yet, excluding those few prohibited by the laws of physics,” argues Parker Lord’s Dean. “We have extensive expertise in all the primary chemistries: acrylic, silicone, urethane, and epoxy, and we make materials that flow like water or stretch like rubber bands,” he says. “The all-in-one coating system is on everyone’s wish list, but we have to come to terms with the fact that not all desired functions can be combined,” says PPG’s Collison. “This is why in standard automotive painting you have a primer, a base coat and a clear coat working cohesively together. We have to think about the engineered battery pack in the same way.” Munro adds that this is not a new set of challenges. “We’re constantly seeing a desire to be able to do things that feel diametrically opposed,” he adds. “We realise that we can’t break the laws of physics, but very often we find that we can bend them, and that there are typically exceptions to the rules.” Munro explains that most coatings are composite materials with, from an engineering point of view, similar benefits to those embodied in bulk materials but scaled down. “We’re very good at engineering at a nano scale, up through meso scale and micro scale in different materials, and that allows us to bend the rules pretty significantly.” Engineering a surface material that is both flexible and thermally conductive is one of these rule bending exercises. “By developing a composite approach on a microscopic scale, we’ve been able to use vibrations to make heat move while we’re actually dampening other vibration, he reports.” Further, both time and external stimulus can be taken into account when creating a material to meet conflicting requirements. “If we take a step back and think about it, the material is not always expected to do opposing things at the same time,” Munro says. “Some of the coatings that we’re applying can switch their thermal characteristics so that under most conditions they do a great job of moving heat, but if the temperature exceeds a threshold it will activate and become a thermal barrier instead.” “With the industry, there has not been much consolidation at this point in terms of design approach,” adds Munro. “And so all those trade-offs are going to be specific to individual designs and customer preferences. We are very aware that there is not necessarily a single solution that will fit everybody’s needs,” he reflects, “so we can work to understand what our customers are really trying to achieve and help guide them to the best approach.” Application issues Several methods are used for applying coatings to battery components, and all have their pros and cons. With dielectrics, Henkel’s Dr Knecht explains, the typical method is spray coating, which is good for laying down homogeneous material thickness on 3D shapes. The downsides stem from overspray, which leads to relatively high waste – unless the material can be recovered for reuse – and might require masking. Spraying is the standard method of applying fire protection coatings, he continues, as it is compatible with many materials, in addition to suiting 3D application well. The main alternative is flat stream coating, which produces less waste as it eliminates overspray and makes the whole process more efficient. When applying conductive materials, roll-to-roll (RTR) gravure coating (also referred to as roll coating) is the preferred technology. This is because it is a fast process that produces very uniform results, compared with alternatives such as slot-die coating. RTR gravure coating is a continuous process used for applying thin coatings onto flexible substrates, such as films and foils. The process is based on three rolls: the gravure roll has a pattern of cells or wells engraved onto its surface, and works with a smooth backing roll, plus a roll of substrate material between them. The gravure roll is partially immersed in the coating material and the excess wiped off, only leaving the material in the engraved cells. As the roll of E-Mobility Engineering | January/February 2024 BMW’s prismatic cells are spray coated in two layers by a specially developed machine, and the coating is then UV cured. A fully automated three-stage quality control process then checks the thickness and surface quality in preparation for a final high-voltage test (Image courtesy of BMW)
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