Self-adhesive dielectric insulation for High Voltage Energy Storage Components
Range anxiety, long charging times and concerns regarding a perceived lack of charging infrastructure is often mentioned in the media as among the barriers to wider consumer adoption of Electric Vehicles (EVs) The response from the automotive industry has been to increase the energy density of the EV battery packs and to enable faster charging and greater range. Amongst the solutions has been the introduction of 800 Volt (V) systems, which allow faster charging than the more traditional 400 V systems. However, the increase in battery energy density and faster charging is also a challenge to the electrical safety of these vehicles.
With voltages and currents in excess of what consumers are used to and also to protect those engaged in servicing and maintaining EVs, electrical safety is also a prime concern. The safety standard ISO 26262, an international standard specifically tailored for the automotive industry, classifies EVs having high-voltage systems, necessitating specific safety measures. The standard gives guidelines and requirements for achieving functional safety and encompasses the entire vehicle development lifecycle, from design through production, to the operation, maintenance, and end of life.
Included in the safety considerations for high voltage systems are adequate insulation and isolation solutions to minimize the risk of electrical shock and short circuits. Insulating materials such as high dielectric strength polymers are used to separate conductive parts and prevent unintended electrical contact. These are often applied as film solutions to insulate conductive EV battery pack components.
Applications for Insulating films
Generally, there are three application scenarios for battery insulation films, starting with cell wrapping, insulating end and side plates in stacks and modules, and protecting metal components in the pack.
Variations of the cell-to-pack (CTP) design are becoming more popular as manufacturers strive to increase the energy density in the pack. The CTP design gives up the module layout by integrating batteries into groups and directly integrating the cells in the battery pack. This technological innovation brings higher requirements for the insulation performance of the battery pack and other components such as busbars and fluid cooling plates.
Insulation is usually enabled by two solutions – coatings (powder or liquid systems) and polymer film laminating, and the mainstream film covering processes include cold lamination of pressure-sensitive films or hot pressing of heat activated adhesives. Pressure-sensitive adhesives offer the advantage of cheaper and faster processing, while coatings and heat-laminated films can offer higher bond strengths.
Performance Requirements of Battery Insulation Films
With performance needs such as fast charging, companies along the EV industry value chain have put forward many performance requirements for battery insulation film, often featuring one or more of the following seven attributes – dielectric strength, thermal resistance, flame retardancy, adhesive strength, durability and manufacturability.
Dielectric strength: The bulk dielectric strength, often expressed as the breakdown voltage, is the most common key parameter. The bulk dielectric strength of insulation film materials is usually around 3.7 kV AC or 4.2 kV DC (IEC 60243-1 & 2). This can be delivered by films even as thin as 23 microns, although 50-micron films are easier to handle. For higher-performance systems, dielectric strength as high as 5 kV or even 10 kV can also be required.
Polymer films have an advantage over coatings in that they have a uniform thickness across the entire surface to be protected, not impacted by surface tension effects on edges and shoulders that could cause film reduction. In compensation, coatings often have to be applied as thicker layers to guarantee the minimum thickness along edges, and where this impacts the heat flow, may require thermally conductive additives that add cost and complication
Electrical surface resistance such as a tracking index (e.g., IEC 60112) can also be satisfied by these films; polyolefins have the best tracking ranking followed by polyesters. Unfortunately, films like polyimide that have the highest heat resistance also have the lowest tracking performance, and surface coatings may be required to bridge any weakness. Balancing the area of insulation with the tracking index can help determine the most efficient solution, e.g., a polymer film with a lower tracking index can be compensated for by enlarging the insulating surface.
Thermal resistance: Components that are part of the battery thermal management system also require the insulation to allow the heat to pass through. Often this is specified as the thermal conductivity, but the “k” value is not a good indicator to calculate how much energy will pass through the insulating layer. Factors such as the thickness of the layer and the wetting of the material interfaces are key parameters, and therefore the thermal resistance (units being Kelvins per watt (K/w)) is a better indicator of the temperatures reached at the cooling plate surface.
Flame retardancy: Combustion and even the explosion of cells in EV batteries have always been a serious concern for consumers, battery manufacturers and automotive OEMs. More and more industry players have begun to further prioritize the flame-retardant properties of materials. Flame retardant polymer films and adhesives are available that help the components they are insulating to reach the flammability standards as described in UL 94, commonly applied to define the burning behavior of pack materials.
Adhesive strength: Self-adhesive insulating films are based on similar adhesives used for many applications in automotive engineering and have a track record of performing under typical operating conditions such as vibrations and temperature changes. Crash-resistant pack designs however increasingly call for higher shear resistance and more durable adhesive strength of film materials. Whatever the performance needs, or the substrate material, there is an optimum adhesive that can secure the insulating film.
Durability: Although EVs, with their increasing connectivity and use of leading-edge Human-Machine Interfaces, are being described as smartphones on wheels, actually they also require much greater longevity, and usually have a product life cycle of 8-10 years. Such an “ultra-long durability” poses challenges to the wear and environmental resistance of such insulating materials. Polymer films and coatings have proven their durability in many automotive applications.
Manufacturability: To achieve insulation between the cooling plate and the inside of the battery pack, a large area of insulation film may need to be bonded. Both coatings and film laminations can be automated, and there are many equipment suppliers with experience in this area. Applying self-adhesive film insulation to pack components can be optimized for any production volume, from prototyping to series production where short cycle times are necessary.
Self-adhesive Insulation Film Solutions
Through continuous technological innovation and a basis in the science of pressure-sensitive adhesives, Avery Dennison develops solutions for suppliers in the EV industry value chain to help them better compete in this high-growth market and effectively respond to challenges related to electrical safety.
The range starts with cell wrapping solutions, where our PET laminates offer a high level of puncture resistance to protect cells from damage as well as the electrical insulation. The range continues with thin, self-adhesive single and double-coated films for insulating cooling plates and ribbons as well as other conductive pack components.
Some applications such as cell connect systems and busbars sometimes call for added flame retardancy, and the company has developed special coatings and adhesives that help modules meet the UL® 94 performance criteria. The company’s coating solutions not only meet flame-retardant requirements but also withstand the 1,000-hour aging test that is twice the Chinese national standard, delivering on the “ultra-long durability” of EVs.
For adhesive strength, Avery Dennison has developed various types of adhesive solutions. For example: pressure-sensitive adhesives with a shear strength of around 2 MPa, UV-activated adhesives with a shear strength of more than 5 MPa, and heat-activated adhesives with a shear strength of more than 7 MPa. At the same time, through special treatment, the company has successfully improved the wetting and outgassing effect of its adhesives, delivering bubble-free bonding even on complex surfaces such as sheet metal parts to ensure the manufacturability and aesthetics of relevant processes and components.
Conclusion
Thin, self-adhesive polymer films can deliver the electrical safety required from high voltage energy storage devices such as EV batteries. They are easy to apply, scalable from small runs and prototyping to high volume production and can be applied manually or on automated lines.
With more than 100 years of developing pressure-sensitive adhesives, coating and laminating a wide variety of films and foils, Avery Dennison have developed superior solutions for electrical insulation. Our products can be found in automobiles across the world, including in safety-critical applications like airbags, brake systems, and durable warning labels. This competence in materials science gives us the ability to support EV battery designers and engineers in finding the optimal solution.
Andrew Christie Business Development Manager Automotive Market
