33 E-Mobility Engineering | September/October 2025 Battery pack materials | Tech focus Steel Steel has been fighting back against aluminium. With the same crash safety performance and roughly the same weight, the high silicon steel design for a 70 kWh battery is up to 50% cheaper to fabricate compared with an aluminium alternative. This was the finding of analysis based on a scenario of 200,000 vehicles/year over a period of seven years. It included material and manufacturing costs, tooling investments for parts manufacture, and body-in-white production including leak testing and corrosion protection. The melting point of a 0.8 mm sheet of steel is 1410 C. In fire tests, the temperature of the steel battery housing cover barely exceeds 1000 C, even after 20 minutes, demonstrating the impressive safety reserves of steel. By contrast, a 1.1 mm thick sheet of aluminium reaches its melting point of 610 C after just 15 seconds in a fire test; after 30 seconds the material collapses. High silicon content allows thinner material with the same strength to be used to make slim and thin-walled crash structures, saving packaging space and freeing up room for large batteries. Battery housing with multichamber profiles made from highstrength steels can support very high loads in side-on collisions and prevent contact being made between the housing parts and battery modules. Life cycle assessments show that steel is more sustainable, with up to two thirds lower greenhouse gas emissions in the production of a steel battery housing compared with those associated with an aluminium design. Aluminium However, aluminium producers point out that recycling aluminium uses only 5% of the energy required for primary aluminium production and reduces CO2 emissions by more than 90%. Modern manufacturing techniques, such as hot stamping and adhesive bonding, enable significant weight reductions with aluminium without substantial cost increases. As less material is required to achieve the same structural integrity, the reduction in material usage can offset the per-unit cost of aluminium compared with that of steel, leading to overall cost savings. Additionally, lighter vehicles benefit from secondary weight savings. For instance, replacing 400 kg of steel with 240 kg of aluminium results in a primary weight reduction of 160 kg, which can lead to an additional 40–60 kg reduction in other components, such as brakes and suspension systems. Beyond production scrap, aluminium can be recovered from end-of-life vehicle recycling initiatives. Through scrap sorting and segregation, specific aluminium alloys can be captured from decommissioned vehicles to feed back into the production process. A lighter aluminium battery pack means a car can use smaller parts, including brakes, suspension parts, batteries and motors, yet achieve the same acceleration, performance and range with lower emissions. Aluminium also absorbs more energy that steel per kilogram. Therefore, a lightweight aluminium battery pack can be safer for the occupant than a comparable steel-based vehicle. When kinetic energy is absorbed in a carbon footprint without impacting performance. However, the lower stiffness than that of steel means reinforcement might be needed for crash loads, both fatigue and crack propagation can be a concern if not designed carefully, and the material is more expensive than steel for equivalent strength. Steel enclosures have high strength and excellent crash protection with reasonable cost-effectiveness for structural parts, and they are easy to weld and form into rigid enclosures. A high silicon content reduces the carbon footprint and allows thinner structures to compensate for the fact that steel is heavier than aluminium. Steel also has lower thermal conductivity, which reduces passive heat spreading, but it is susceptible to corrosion unless treated or coated. Composite materials, particularly thermoplastics, can have a very high strength-to-weight ratio, are corrosionproof and can be moulded into complex shapes with excellent fatigue resistance. But they also have poor thermal conductivity unless hybridised with conductive inserts. However, despite these issues and irrespective of their high cost and more complex manufacturing process, composites have been used in high-end designs and can now be used in mid- and even lowend designs. A battery enclosure using Selectrify high silicon steel (Image courtesy of ThyssenKrupp)
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