The vast majority of vehicles on the road today are powered by traditional fuels, but make no mistake, electric vehicles (EVs) are making serious inroads. In 2016, 777,497 EVs were sold globally according to Forbes, up by 41% compared to 2015. Slowly but surely, personal transportation is becoming more reliant on electricity. On top of that, the diverse and growing array of infotainment and navigation electronics offered in modern vehicles means that today’s vehicles have unprecedented electricity needs.
As a result, EV batteries have to provide more power, more cycles and a longer lifetime if EVs are to ever truly flourish. Lithium-ion (Li-ion) battery packs remain the go-to power source for the EV industry due to their impressive power density and charging efficiency. However, these batteries have relatively short operating lives and degrade quickly with age, issues that are exacerbated by challenging automotive environments.
EVs powered by Li-ion technologies haven’t quite caught up to traditional vehicles in terms of range or power density, meaning that an EV needs to be recharged more often than a gasoline vehicle needs to be refueled. Each recharge cycle marginally reduces the overall capacitance of the battery, shortening the battery lifespan.
Additionally, recharging causes an internal chemistry change that manifests as a slight expansion of the physical dimensions of the battery cells, which can cause delamination of the internal battery cells and components or even battery pack deformation. This can prevent effective thermal management, further endangering the battery lifespan, and in worse case scenarios leading to thermal runaway.
Battery research indicates that optimal battery lifespan occurs when a moderate amount of pressure is applied to ensure electrical and thermal connections while the battery ‘breathes’ during its discharge and recharge cycles. In large battery packs with many cells, this breathing can be considerable. However, there are some surprising material technologies that help prevent delamination and deformation in pouch-type, actively-cooled, Li-ion battery packs, thereby keeping more EVs and hybrids on the road every day.
Dielectric foams can accommodate the dimensional changes and variances of the battery cells, but deliver enough pressure to the cell package to prevent misshaping and disconnections. The foam has a spring-like characteristic, but is in fact better than a spring. The more a spring deflects, the higher the potential return energy. But foams can be engineered to deliver the same, consistent return energy across a wide range of compression amounts, a property known as compression force deflection (CFD). Springs are also thermally and electrically conductive and can create hard spots in the battery.
Foam cushioning in the battery also has an impressive compression set—the ability of a material to resist permanent deformation under compressive loads. The performance of specially engineered polyurethane- and silicone-based foams will outlast the lifespan of the battery, which isn’t true for other potential materials solutions such as other elastomers. Another advantage is foam’s remarkable operational temperature range, much larger than most other rubbers.
Foam materials are reliable even under the stresses of the harsh automotive environment. They have excellent high and low temperature resistance. They are also thermally insulative, encouraging heat to be exhausted to the heat sink and not transferred to neighboring battery cells. This insulative property isn’t reduced as the foam compresses. This is significant as excess heat is the foremost threat to batteries and electronics. The dielectric nature of the foam averts arcing between cells as well.
Protecting the battery components is a supreme concern and foam materials offer important provisions.
Foam compression pads reduce the severity of vibration and shock on the battery components, which is important for any sensitive system in automotive applications. Cushioning also provides a quieter ride for vehicle occupants as it reduces the potential sources of rattle in the battery. By sealing the gaps between cells and other components, specially-engineered foams prevent the ingress of contaminants such as moisture and debris. Li-ion batteries that overheat can go into thermal runaway, a rare but serious event where the batteries combust. So fire resistance is another important trait for compressive battery pads, which is provided by the specially engineered silicone foams.
The complicated requirements of compression pads leave these specially engineered microcellular polyurethane and silicone as the optimal materials. High performance polyurethane foams are effective in temperatures between -40°C - 121°C and are easy to cut to size. Saint-Gobain Performance Plastics’ Foams and Tapes division offers specially-engineered polyurethane compression pads in their products Norseal® PF45 and Norseal PF47. For applications that require a larger temperature range, high-performance silicone foams are the better choice, with an operational temperature range of -51°C - 204°C. These Silicone foams also help mitigate the threats of thermal runaway, as they have low smoke generation and low flame spread qualities with a UL94 rating of V-0. For these needs, Saint-Gobain supplies Norseal F12 and Norseal F20.
All of the Norseal products deliver consistent, reliable pressure (CFD) to the battery components throughout a wide range of compressive forces. Saint-Gobain has a variety of products with different CFD values, making compressive pads suitable for diverse applications. Compressive pad materials can also be supplied with or without an adhesive layer.
Li-ion batteries are vital to EVs and will remain so for the foreseeable future. To expedite EV growth with current technologies, every possible gain in battery efficiency needs to be implemented. Manufacturers are thinking creatively about how to achieve longer battery lives and Saint-Gobain’s experience and engineering will be key in these innovations. Saint-Gobain is already working on the next generation of compression pad materials for tomorrow’s EV batteries.
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