The role of pressure-sensitive adhesives in electric vehicle batteries
Electric Vehicle (EV) fire safety has been the subject of recent media reports comparing it with the safety record of internal combustion-engined vehicles.
In fact, making vehicles safe is of paramount importance for every automotive engineer, and this holds true for electric propulsion systems and their batteries.
What follows reports on how pressure-sensitive adhesives (PSA) help the EV sector make electric vehicles a safe option for consumers.
First, a bit of technical background: EVs powered by lithium-ion technologies rely on a host of materials and components in the battery pack to deliver the safety, performance and reliability demanded by consumers. These protect the valuable cells from impact, temperature extremes, fire propagation and electrical arcing.
Challenges for EVB engineers
Global standards are in place that define the minimum safety characteristics for Electric Vehicle Batteries (EVB). One well-known example is the requirement for occupants to have at least five minutes to exit a vehicle after the onset of a thermal runaway event. Such standards rely on tests that involve physical and thermal shock, flame exposure, impact and vibration, as well as electric overload tests that are generally performed on the battery pack. The challenge for engineers is meeting the battery pack requirements at a module, component, or even material-layer level.
As automotive and battery OEMs and their suppliers work on different solutions to meet pack performance targets, the EVB has become a complex system. Whether the design is traditional, with modules in a pack or as a cell-to-pack design, the cells, their connecting busbars, the battery management system and all the thermal management, flame propagation and electrical insulation solutions call for the assembly of many different components and materials.
A flexible and reliable bonding method is needed for this complex assembly, and this is where PSAs play a growing role. PSA tapes are an alternative to traditional mechanical fasteners such as clips, staples, pins, screws and even liquid adhesives, matching and sometimes exceeding them in strength and durability. The tapes also bring additional benefits such as sealing, surface protection, shielding, vibration absorption and the lowering of stress loads. Market estimates indicate that a typical EVB pack contains between 3 and 5 square meters of PSA tape.
"Pressure-sensitive adhesives are an effective solution for assembling and securing these materials within the modules and packs. They also contribute directly to battery performance."
- Andrew Christie
PSA tape constructions are modular. Adhesive formulation, carrier, facestock and liner selection are selected based on the materials to be bonded, the application method and the operating conditions (figure 1). This makes them an excellent option for bonding the various materials used in the battery module and pack construction, ranging from foams to fibrous materials, metals, ceramics and plastics. The construction is also related to the type of application. Transfer tapes consisting of adhesive only will bond materials, while double-coated tape will have additional features such as strength, dimensional stability, sealing and dielectric strength. A single-coated tape can protect surfaces from moisture or chemicals, wear and tear, or electromagnetic influences. PSA specialists can recommend the optimum construction for specific applications.
PSA solutions already play an important role in the automotive industry, demonstrating their endurance in applications including bonding badges, body side moldings, aerials and sensors to vehicle exteriors under all conditions. They are used in the interior to bond decorative and functional components; in safety systems, like airbags and sensors, and in powertrain modules. PSA solutions meeting global automotive industry needs are in virtually every vehicle that rolls off a production line.
For laminating and bonding material layers
Single Coated Tape
For protecting surfaces from external influences
Double Coated Tape
Brings added functions to material layer bonding
Figure 1: Some typical constructions of PSA solutions in EV battery applications.
Applications for PSAs in EVBs
PSA solutions contribute in different ways to various EVB bonding challenges (see figure 1). Let's take a look at some common applications.
Thermal propagation (runaway protection)
Materials with low thermal conductivity and often high heat resistance are commonly applied to reduce heat transfer and flame propagation between cells and modules. Micas, ceramic papers and aerogels are candidates for situations where heat propagation from an unstable cell or module needs to be controlled.
PSA tapes can be easily laminated to mica and ceramic papers for quick and reliable attachment to cells or frames. PSA films also help to protect delicate surfaces. When applied to exposed faces or even when encapsulating fragile ceramic materials, they prevent linting and dust generation in the battery enclosure.
By adding flame retardant properties to these PSA solutions, engineered laminates can meet flammability requirements such as those in the UL® 94 plastics flammability standard.
Pouch and prismatic cells expand and contract in varying degrees during the charge and discharge cycle. This expansion is controlled by placing elastic materials with a given compression set within the cell stack. Polyurethane foams are a popular option for this task, and PSA is ideal for attaching the layers. The porous materials’ low dielectric strength can be offset by applying PSA tapes featuring a filmic carrier or facestock.
PSA tapes can also address today’s trend towards multi-functional solutions that combine the compression set with thermal resistance. As an example, PSA tapes unite inorganic porous materials like ceramic papers with heat-resistant layers like mica to have an engineered compressible pad. Unlike wet adhesives, PSA tapes will not penetrate porous materials and degrade their elasticity.
Figure 2: Typical PSA applications in an EVB pack
Heater film bonding
PSA tapes are often a preferred option for mounting heater films to a module or pack enclosure for extended battery life in cold temperatures. Not only is this a simpler alternative to mechanical fasteners, but their visco-elastic nature also provides the additional benefit of accommodating expansion rate differences.
PSA tapes’ filmic carriers and facestocks offer the dielectric strength needed to meet EVB breakdown voltage requirements. On both steel and aluminum, the tapes can insulate components in the pack ranging from thermal exchanger plates to reinforcement ribs and enclosures.
Thermal interface materials
Heat spreaders are a pouch cell solution for directing heat flow away from the cell stack. Lighter-weight alternatives to copper or aluminum like graphite sheets are gaining favor for this application, but the mechanical attachment used for the metal foils may not be appropriate with these more delicate materials. PSA tapes can attach graphite sheets to mounting frames to meet both performance and productivity needs.
Where other thermal interface sheet materials are applied, transfer tapes can improve adhesion even under load and impact conditions. Plus, they can minimize layer detachment due to vibration.
Sometimes the question arises of whether thermally conductive PSA solutions are required. These specialized adhesives play a critical role in electronic engineering, where heat generated in small spaces, such as in chips and LEDs, needs to be removed. However, in EVB designs, standard PSA tapes can be considered because heat flow is usually several orders of magnitude higher given the much larger surface areas.
Flexible busbars offer weight-reduction and simplify manufacturing, but need to be secured reliably to perform during the battery’s planned operating life. Double-coated PSA tapes can secure these under demanding conditions and offer additional electrical insulation.
Single-coated film PSA tapes are used for wrapping prismatic and cylindrical cells, providing physical and insulative protection as well as a surface for labeling and identification.
Pack seal and gasketing
Thermal insulating or intumescent layers can be used between the battery pack and the vehicle floor to prevent the propagation of heat and flames. PSA tapes, already in use to secure heat shields, are also suitable to secure these protective layers.
PSA tapes also offer a viable option for assembling and securing foam or elastomeric pack seals prior to lid or connector attachment. There are appropriate PSA solutions for polyurethane, elastomeric or silicon seals. PSA tapes can also be designed for easier seal opening to facilitate maintenance and recycling.
PSA is a flexible solution for bonding and assembling the many different organic and inorganic materials that are part of a safe, durable and high-performance battery pack.
They contribute to better product design by simplifying the use of dissimilar materials, joining irregular shapes, saving space and contributing to light-weighting. Their insulating features add to electrical safety. Adding flame retardance features to PSA tapes facilitates composites and materials to meet UL® 94 V-0 and other flame-retardant requirements. Prototyping is quick, easy and can be completed without expensive tooling and equipment.
In the manufacturing process, PSA tapes lend themselves equally to manual or automated applications. Plus, they bond quickly, and the absence of a post-application cure saves time and relieves capacity restraints. They require no further processing steps, such as surface grinding, polishing or cleaning.
To take full advantage of PSA solutions’ flexibility and performance, bring your PSA expert on board early in the design stage. Early involvement allows adhesive engineers to fully examine opportunities for streamlining manufacturing processes, enhancing product performance and accommodating engineering changes as the project evolves.