New Composites Target EV Applications

No longer a low-volume play, the latest composite materials offer EV developers new options for lightweighting, thermal management and structures.

Mid- and high-volume production of composites is transitioning from the high level of hand work traditionally required to increasingly automated processes. (TRB)

Lightweighting solutions that are financially viable, reliable and sustainable for conventionally powered vehicles are arguably even more important for electric vehicles (EVs). Range anxiety remains a major concern of many consumers and must be addressed if EVs are to have a significant impact on the industry and the environment. With current battery chemistries, extending the range of EVs requires more cells to be added, increasing curb weight.

EV structures (Audi eTron shown) offer many opportunities for composite applications. (Audi)

For example, the compact Chevrolet Bolt EV’s 65-kWh battery pack weighs nearly half a ton, which is almost one-third of the car’s curb weight. More extreme is the 2022 GMC Hummer EV full-size pickup. Its ~200-kWh pack, engineered to deliver over 350 miles of driving range, is estimated to weigh over 2,000 lb. (907 kg). The truck’s curb weight is over 9,000 lb (4082 kg).

The EV battery mass/vehicle range reality is forcing engineers to look for mass savings in other areas. Material selection plays a vital role in EV design, and the right material can be used to reduce the overall weight while still maintaining strength and integrity. Intelligent structural core designs used in combination with lightweight materials provide the strength and rigidity necessary to allow OEMs to move away from traditional steel components.

Composites have been somewhat overlooked due to high manufacturing costs and slow cycle times. But recent advances have streamlined the manufacturing process to make high-volume composites a realistic alternative for EVs. Thermoset and thermoplastic composites have the potential to offset the weight of large battery packs and electric propulsion systems. They can also aid in battery thermal management, providing high voltage insulation and allowing for more intelligent designs to further reduce parasitic weight. They also give opportunities to reduce noise, vibration and harshness (NVH) in many applications.

Mass and heat challenges

For EV battery pack applications, composites give engineers greater design freedom to improve packaging and thermal management, as well as reducing mass. (TRB)

The introduction of larger battery systems brings an unavoidable vehicle-mass increase. Battery weight, and how it is distributed in the vehicle, can affect performance parameters such as fuel consumption, acceleration dynamics and handling. EVs tend to house their batteries low, ideally beneath the floor and evenly distributed longitudinally and transversely. However, unlike traditional vehicles with internal combustion engines and fuel tanks, the weight of the battery in an EV is never diminished – if a bus starts its day with a 1-ton battery pack, it will end its day with a 1-ton pack. This may offer some benefits in terms of vehicle dynamics, but it is a challenge for vehicle range.

Increased weight isn’t the only issue. Lithium-ion (Li-ion) batteries are leading the way in EV development, but there are a number of ‘stress factors’ that significantly impact the performance and capacity of these batteries over time, many of which are exacerbated by increasing battery capacity or the number of cells. Extreme temperatures, which can either be a result of environmental conditions or localized heat exchange during battery operation, have a profound effect on charge/discharge rate and battery performance.

For example, charging in very low temperatures causes metallic lithium to form spikes around the anode, slowing the current. Temperature extremes also lead to the expansion and contraction of metallic components such as the battery box or chassis mounting points – but not the cells themselves. This can lead to stress fractures and failures over time.

Battery thermal management systems (BTMS) are engineered to maintain battery-cell temperatures within a fixed range – ideally between 20°C and 40°C – avoiding excessive fluctuations and maintaining an even cell-to-cell temperature. As EVs are fitted with more cells and larger battery systems, the greater the rate and increase in temperature, making thermoregulation even more important. However, the more complex the BTMS, the more weight it is likely to add to the battery.

Composite benefits

With battery weight inevitable, volumetric weight-saving strategies need to focus on parts of the vehicle that offer the best value, such as battery casings or the chassis. It’s here that lightweight materials, such as aluminum and composites, provide advantages over traditional steel components. For example, BMW used a lightweight, carbon fiber composite for the body of the i3 EV, to compensate for its 450-lb (204-kg) Li-ion battery pack.

These weight-saving strategies are being applied to internal vehicle components such as battery enclosures. While aluminum, for example, offers significant weight savings versus steel and uses similar production processes, composites offer even greater weight savings. They can be used to manufacture components up to 40% lighter than even aluminum with the same strength.

These crucial weight savings can be achieved because composite materials are volumetrically lighter, while allowing for intelligent design and geometries to save additional weight. For example, with pressed or stamped metal parts, the thickness of the sheet is determined by the requirements of the highest stress point and uniform throughout, adding to the unnecessary parasitic waste in low-stress areas. In contrast, advanced composite manufacturing processes allow the thickness of material to be tailored to meet the load deltas between low- to high-stress zones. Components can be molded to exactly match the requirements of the application.

The very nature of composites – a resin with embedded fibers – allows manufacturers to select materials and constructions based on the qualities that they need. For example, glass fiber has a degree of spring deformation and higher true failure strength than aluminum, so that it can resist continual impacts and variable loading. These properties have allowed it to be successfully used to manufacture antenna covers for the underneath of trains, which are frequently subjected to stone strikes.

Alternatively, both composite sheets and aluminum can be formed over a honeycomb core to increase rigidity and resist deformation without significantly increasing weight. This benefits vehicles used on rough terrain or construction sites. For even further protection against abrasion, polyurethane coatings and aramids can be added to composites, ideal for components such as skid plates.

Selection of materials for EV battery pack structures needs to consider more than just weight. When using conductive materials, such as metals, there inevitably needs to be an air gap or additional non-structural, non-conductive layer to avoid the risk of arcing, leading to volume inefficiency. Glass fiber is non-conductive, so it can sit tightly against the battery and does not require a spark gap for electrical isolation. Composites are, therefore, a space-saving option as well as a lightweight solution.

They also offer benefits in terms of thermal management. Using composite materials for battery cases gives manufacturers the freedom to choose whether they need a more thermally isolating structure (such as glass fiber), or a highly conductive material (for example carbon fiber), as well as allowing thermal interface materials – pads, adhesives and gels – to be used to optimize heat transfer between the battery and cooling device without adding significant weight or volume.

High volume production

Composites are not new and are commonly used by low volume industries, including aviation, as a weight-saving solution for structural applications and interior components. However, they have been largely overlooked by the auto industry for their costly and labor-intensive manufacturing process until recently. TRB Lightweight Structures has transformed the manufacture of composites from a largely manual process taking several hours to a fully automated press that takes a matter of minutes. Traditional slow-setting resins have been replaced with snap-cure resins and pre-preg materials. Parts are produced robotically, with a cycle time of less than 10 minutes.

Current vehicle manufacturing pipelines and vehicle designs have, for economic reasons, been based around metal pressing and stamping, so an immediate switch to composite production is unlikely. However, adopting a multi-material, combined approach using metals and composites, where appropriate, can serve near-term to help balance out the limitations of existing solutions. This approach brings benefits to efficiency, volumetric weight and cost, and has been recognized by the EU as an affordable solution to high-volume lightweighting for future electric vehicles.

Selecting the right materials for EV vehicles is not an easy feat, with each material presenting different potential benefits in terms of cost, ease of manufacturing or weight. Crucially, composites can also offer solutions for improved thermal management, saving space and increasing impact resistance. With the introduction of fast-paced manufacturing, the stigma that composites are only suited to low volume, high price applications is ending. Both mid- and high-volume production of composites is now entirely achievable with automated and efficient production lines, making composites an extremely valuable option for future EV design that can change the mobility industry.

Composites engineer Andrew Dugmore is VP New Business Development, TRB Lightweight Structures. The company’s new, highly automated 40,000-sq.ft. composites manufacturing center in Richmond, Kentucky is a joint venture with Toyota Tsusho America.