Structural battery could enable 'massless' energy storage

Swedish researchers have produced a structural battery that is claimed to perform 10 times better than all previous versions.

Containing carbon fibre that serves simultaneously as an electrode, conductor and load-bearing material, the battery is said to pave the way for essentially ‘massless’ energy storage in vehicles and other technology. It has been described in the journal Advanced Energy & Sustainability Research.

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, is one that works as both a power source and as part of the structure — for example, in a car body. This is termed ‘massless’ energy storage, because in essence 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.

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 fibre. In addition to being stiff and strong, they also have a good ability to store electrical energy chemically. But while the first attempt to make a structural battery was made as early as 2007, it has so far proven difficult to manufacture batteries with both good electrical and mechanical properties.

Now Chalmers researchers, in collaboration with KTH Royal Institute of Technology, have presented a structural battery with properties claimed to far exceed anything yet seen in terms of electrical energy storage, stiffness and strength — including multifunctional performance that is 10 times higher than previous structural battery prototypes.

The structural battery uses carbon fibre as a negative electrode and a lithium iron phosphate-coated aluminium foil as the positive electrode. The carbon fibre acts as a host for the lithium and thus stores the energy. Since the carbon fibre also conducts electrons, the need for copper and silver conductors is also avoided — reducing the weight even further. Both the carbon fibre and the aluminium foil contribute to the mechanical properties of the structural battery. The two electrode materials are kept separated by a fibreglass 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 fibres and other parts.

The battery has an energy density of 24 Wh/kg, meaning approximately 20% 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 many other commonly used construction materials.

“Previous attempts to make structural batteries have resulted in cells with either good mechanical properties or good electrical properties,” said Chalmers Professor Leif Asp, leader of the project. “But here, using carbon fibre, we have succeeded in designing a structural battery with both competitive energy storage capacity and rigidity.”

Despite their success, the researchers did not choose the battery materials to try and break records — rather, they wanted to investigate and understand the effects of material architecture and separator thickness. Now 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 aluminium foil will be replaced with carbon fibre as a load-bearing material in the positive electrode, providing both increased stiffness and energy density. The fibreglass separator will be replaced with an ultrathin 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.

Prof Asp, who is leading this project too, 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 aluminium, but with a comparatively much lower weight.

“The next-generation structural battery has fantastic potential,” Prof Asp said. “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.”

And in the longer term, it is absolutely conceivable that electric cars, electric planes and satellites will be designed with and powered by structural batteries.

“We are really only limited by our imaginations here,” Prof Asp said. “We have received a lot of attention from many different types of companies in connection with the publication of our scientific articles in the field. There is understandably a great amount of interest in these lightweight, multifunctional materials.”

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