The batteries in today’s electric cars constitute a large part of the vehicles’ weight, without fulfilling any load-bearing function. A structural battery, on the other hand, works as both a power source and as part of the structure; for example, in a car body. This is termed “mass-less” energy storage because the battery’s weight vanishes when it becomes part of the load-bearing structure. Calculations show that this type of multifunctional battery could greatly reduce the weight of an electric vehicle.
Researchers developed a structural battery that 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 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 battery has an energy density of 24 Wh/kg, meaning approximately 20 percent capacity compared to comparable lithium-ion batteries currently available. But since the weight of the vehicles can be greatly reduced, less energy will be required to drive an electric car, for example, and lower energy density also results in increased safety. And with a stiffness of 25 GPa, the structural battery can compete with other commonly used construction materials.
In further work, the performance of the structural battery will be increased. 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 that will give a much greater effect as well as faster charging cycles.