Assessing Technologies for Dielectric Protection of Battery System Components
Governments and vehicle manufacturers around the globe are pledging to transition from internal combustion engines to electric vehicles (EV). As EV manufacturers continue to vie for more market share, they must alleviate certain consumer fears, one of which is range anxiety.
To reduce anxiety, EV manufacturers are increasing energy density and in turn, mileage. With the growth and development of battery pack technologies, a greater emphasis is placed on pack design optimization, with manufacturers seeking more compact solutions to improve energy density and lower costs.
Higher energy density requires powerful high-voltage batteries that operate up to 900 volts. Higher voltages necessitate greater precautions to prevent arcing between electrical components. Thus, with increased energy density comes a need for increased battery safety.
One major area of focus is to improve the electrical isolation performance of dielectric materials as well as their adhesion to battery/pack components and thermal interface materials, and their ease of application.
Dielectric protection is critical in the assembly of high-voltage battery packs. Dielectric materials are typically polymeric and require the following performance attributes:
- High dielectric strength
- Good adhesion to substrates
- Durable after exposure to chemical, thermal and mechanical forces
- Applied at thin film thickness (50-250 microns)
- Easily applied in a high-throughput manufacturing process
For example, the battery cell, cooling plate, and module walls
Electrical isolation of EV components is required at the cell, module and pack levels—for example, in battery cells, side plates, cooling plates, walls and bus bars.
Several types of dielectric materials exist on the market today. In a recent white paper developed by Parker Lord, four of these materials were compared based on several factors. They are:
- A PET film commercially used in electrical isolation applications.
- A heat-cured powder coating that was designed for electrical application with good edge coverage.
- LORD® JMC-700K, a Parker Lord heat-cured solvent-borne, epoxy-based coating.
- Sipiol® UV, a Parker Lord UV-cured and 100% solids acrylate-based coating.
Table 1: List of dielectric materials evaluated in this white paper
Testing the Dielectric Materials
For the study, five tests were completed:
- Hipot Test: High Potential (Hipot) equipment was used to measure the breakdown voltage of dielectric materials per ASTM D3755. The breakdown voltage is the minimum voltage where a material experiences electrical breakdown and becomes electrically conductive.
- Adhesion: This test looks at how well the coating stick to the aluminum. Crosshatch adhesion was conducted per ASTM D3359. A rating of 5 indicates excellent adhesion and a rating of zero indicates poor adhesion.
- Environmental Aging: Coated specimens for hipot and adhesion data were aged at 85°C / 85% RH for up to three weeks in an ESPEC environmental chamber. Another set of specimens were exposed to thermal cold shock cycling from -40°C to 100°C for up to 120 cycles using a Thermotron SE400 environmental chamber.
- Failure Mode: The failure mode was analyzed to determine the weakest point for the bonding assembly. A cohesive coating failure indicates that the weakest point is the cohesive strength of the coating.
- Thermal Conductivity: Thermal conductivity was measured according to ASTM D5470 using 1400 TIM tester. Aluminum disks (33 mm diameter, 3 mm thick) were coated with varying film thicknesses (50 to 200 micron) of dielectric material. The disks were compressed to 50 kPa pressure in the TIM tester.
The choice of dielectric materials for specific battery components depends on many factors. These include:
Cost and Productivity
In addition to the performance, the economics of a dielectric material solution is critical to the decision of which to implement for a manufacturer. While each dielectric material has its target application, each product’s application time, ease of automation, labor and energy costs, and the potential equipment investment required for a full-rate automotive application should be evaluated.
When considering cost and productivity, PET films are easy to apply to geometrically simple parts but challenging for complex parts. On the other hand, powder coating has the advantage that excess material can be collected and reused but can be difficult to achieve even coating thickness on complicated geometries. LORD JMC coatings use standard applications and large curing ovens while also not relying on static deposition. Meaning, it is sprayed where it is wanted and not sprayed where it is not wanted. Meanwhile, Sipiol UV has low upfront capital investment and can be robotically spray-applied, making it an efficient option.
Electrical isolation/ breakdown voltage
Electrical isolation is a primary requirement for safety and performance of EV battery components. Minimum breakdown voltage (BDV) is used to quantify a material’s electrical isolation performance and is measured via direct current (DC) hi-pot equipment in the air. Higher material breakdown voltage translates into improved electrical isolation and improved safety and performance of EV battery components.
Components near a battery cell typically require electrical isolation that meets a voltage withstand of ~3 kV, while power electronics and cooling plate components may require a voltage withstand of ~5 kV. Hence, dielectric materials with breakdown voltages greater than 6 kV can meet both requirements.
Of the four materials evaluated by Parker Lord, PET film had the highest dielectric strength (85-149 kV/mm) and provided good and consistent electrical isolation. However, it has lower adhesion strength and is typically limited to applications on flat surfaces without protrusions, valleys and edges. In next-generation EVs, OEMs are increasingly relying on the battery itself for structural support—meaning consistent, robust adhesion is vital.
Powder coating had the lowest dielectric strength and, therefore, requires a thicker film to meet the typical voltage withstand requirements.
Sipiol UV and LORD JMC coatings can both achieve high voltage withstand with a thickness of 90-120 microns and are recommended for battery components with sharper edges and radii.
Edge coverage
Often battery components will have edges and corners that require coverage for electrical isolation. The electrical isolation performance depends on film thickness. Thus, determining the coating film thickness on an edge is important for understanding the electrical isolation performance.
Four coatings were applied to two different substrates and analyzed for coverage and thickness. Of these coatings, Sipiol UV and powder coating were effective in uniformly coating sharp corners, but LORD JMC had difficulty with sharp corners and imperfections in the metal. Therefore, a higher thickness target may be needed for very sharp radii to achieve adequate electrical protection.
Figure 3: DC Breakdown voltage in air as a function of film thickness for PET, powder coating, LORD JMC coating, and Sipiol UV coating. The error bars are the standard deviation. The green dashed line indicates voltage withstand requirement of 3 kV and the blue dashed line denotes the high voltage withstand requirement of 5 kV.
Thermal conductivity
Thermal management is another critical requirement for the safety and performance of EVs. Poor thermal management may result in thermal runaway events, creating a hazardous fire condition. Also, poor thermal management reduces the electronic components’ efficiency because excessive heat increases electrical resistance and lowers the power and energy of the EV. It is, therefore, important to remove heat from battery and powertrain components efficiently.
The heat transfer from battery components to cooling medium requires materials with high thermal conductivity and low interfacial thermal impedance.
For EV applications requiring high electrical isolation and heat transfer, LORD JMC is the preferred product, followed by powder coating, Sipiol UV, and PET. It is important to consider both a material’s inherent thermal conductivity and the required film thickness for electrical isolation when assessing its overall heat transfer capabilities.
Adhesion and environment
Adhesion is also important to consider when evaluating dielectric materials. Once the dielectric material is applied, it must remain on the component surface to provide dielectric protection for the life of the product. In addition, good adhesion to the metal substrate and adhesive is important for effective heat transfer. The required adhesion strength varies depending on where it is used.
For example, EV battery components such as side plates or battery lids may need lower levels of adhesion strength because the dielectric material is not bonded structurally to other components. In this example, the dielectric material must provide adequate strength for handling light impact, abrasion, and the environment.
On the other hand,” higher adhesion strength is critical for EV manufacturers that are moving from “cell-to-module” to “cell-to-pack” or larger module design to lower costs and improve the battery pack’s energy density. In cell-to-pack designs, battery components are often structurally bonded together to provide structural strength that withstands stronger vibration, impact and the environment.
Of the materials tested, Sipiol UV had the highest adhesion strength, followed by powder coating and LORD JMC. While PET film had the lowest adhesion strength, plasma treatment could improve its performance. thermal shock cycling had minimal negative effects on adhesion for all materials, while exposure to 85°C / 85% RH condition had moderate reductions in adhesion strength. All dielectric materials tested have excellent crosshatch adhesion to aluminum before and after environmental exposure.
Conclusion
Continuous development of dielectric materials is ongoing, with new developments enabling safer, higher-density battery packs. Products are currently being developed to improve heat transfer, adhesion, corner coverage, ease of application, and cost.
Eric Dean, Global Business Development Manager, Parker Lord.
