Researchers from Chalmers University of Technology, in collaboration with KTH Royal Institute of Technology in Stockholm, have produced a structural battery that performs ten times better than all previous versions. It contains carbon fiber that serves simultaneously as an electrode, conductor, and load-bearing material.

The new structural battery, which is described in an open-access paper in the journal Advanced Energy & Sustainability Research, features an energy density of 24 Wh kg⁻¹, an elastic modulus of 25 GPa, and tensile strength exceeding 300 MPa.

The structural battery uses carbon fiber as a negative electrode, and a lithium iron phosphate-coated aluminum foil as the positive electrode. The carbon fiber acts as a host for the lithium and thus stores the energy. Since the carbon fiber also conducts electrons, the need for copper and silver conductors is also avoided—reducing the weight even further. Both the carbon fiber and the aluminum foil contribute to the mechanical properties of the structural battery.

The two electrode materials are kept separated by a fiberglass fabric in a structural electrolyte matrix. The task of the electrolyte is to transport the lithium ions between the two electrodes of the battery, but also to transfer mechanical loads between carbon fibers and other parts.

The cell consists of a carbon fiber electrode and a lithium iron phosphate electrode separated by a fiberglass fabric, all impregnated with a structural battery electrolyte for combined mechanical and electrical function. Image: Marcus Folino

The development of structural batteries at Chalmers University of Technology has proceeded through many years of research, including previous discoveries involving certain types of carbon fiber. In addition to being stiff and strong, they also have a good ability to store electrical energy chemically. This earlier work was named by Physics World as one of 2018’s ten biggest scientific breakthroughs.

Structural battery composite fabrication, showing the steps: battery component manufacture, pouch‐cell manufacture, and curing of the SBE. Asp et al.

The first attempt to make a structural battery was made as early as 2007, but it has so far proven difficult to manufacture batteries with both good electrical and mechanical properties.

The battery has an energy density of 24 Wh/kg—approximately 20% capacity compared to comparable lithium-ion batteries currently available. But since the weight of the vehicles could be greatly reduced with the incorporation of the structural battery, less energy would be required to drive an electric car. Further, lower energy density also results in increased safety. With a stiffness of 25 GPa, the structural battery can really compete with many other commonly used construction materials.

A new project, financed by the Swedish National Space Agency, is underway, where the performance of the structural battery will be increased yet further. The aluminum foil will be replaced with carbon fiber as a load-bearing material in the positive electrode, providing both increased stiffness and energy density. The fiberglass separator will be replaced with an ultra-thin variant, which will give a much greater effect—as well as faster charging cycles. The new project is expected to be completed within two years.

Leif Asp, Professor at Chalmers and leader of the current project, is leading this new project too. He estimates that such a battery could reach an energy density of 75 Wh/kg and a stiffness of 75 GPa. This would make the battery about as strong as aluminum, but with a comparatively much lower weight.

The next generation structural battery has fantastic potential. If you look at consumer technology, it could be quite possible within a few years to manufacture smartphones, laptops or electric bicycles that weigh half as much as today and are much more compact.

—Leif Asp

In the longer term, it is conceivable that electric cars, electric planes and satellites could be designed with and powered by structural batteries.

The current project is run in collaboration between Chalmers University of Technology and KTH Royal Institute of Technology, Sweden’s two largest technical universities. The battery electrolyte has been developed at KTH. The project involves researchers from five different disciplines: material mechanics, materials engineering, lightweight structures, applied electrochemistry and fiber and polymer technology. Funding has come from the European Commission’s research program Clean Sky II, as well as the US Air Force.

Resources

• Asp, L.E., Bouton, K., Carlstedt, D., Duan, S., Harnden, R., Johannisson, W., Johansen, M., Johansson, M.K.G., Lindbergh, G., Liu, F., Peuvot, K., Schneider, L.M., Xu, J. and Zenkert, D. (2021), “A Structural Battery and its Multifunctional Performance.” Adv. Energy Sustainability Res., 2: 2000093 doi: 10.1002/aesr.202000093