Bild: Chalmers University of Technology

Big breakthrough for “massless” energy storage systems

Researchers at Chalmers University of Technology have produced a structural battery that works ten times better than any previous version. It contains carbon fibers that serve simultaneously as an electrode, conductor and load-bearing material. Their latest research breakthrough paves the way for essentially “massless” energy storage in vehicles and other technologies.

The batteries in today’s electric cars make up a large part of the vehicle’s weight without performing a load-bearing function. A structural battery, on the other hand, is a battery that works both as a power source and as part of the structure – for example, in a car body. This is called “massless” energy storage because the weight of the battery essentially disappears when it becomes part of the supporting 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 is based on years of research, including earlier discoveries with certain types of carbon fibres. These are not only stiff and strong, but also have a good ability to chemically store electrical energy. This work was named one of the top ten scientific breakthroughs of 2018 by Physics World.

The first attempt to make a structural battery was made back in 2007, but it has so far proved difficult to produce batteries with both good electrical and mechanical properties.

But now development has taken a real step forward: researchers at Chalmers, in collaboration with the Royal Institute of Technology in Stockholm (KTH), have unveiled a structural battery whose properties in terms of electrical energy storage, stiffness and strength far exceed anything that has gone before. Their multifunctional performance is ten times higher than that of previous structural battery prototypes.

The battery has an energy density of 24 Wh/kg, which is about 20 percent capacity compared to similar lithium-ion batteries currently available. However, because the weight can be greatly reduced, less energy is needed to power an electric car, for example, and the lower energy density also leads to greater safety. And with a stiffness of 25 GPa, the structural battery can actually compete with many other common building materials.

“Previous attempts to make structural batteries have resulted in cells that have either good mechanical properties or good electrical properties. But here we have succeeded in using carbon fibres to design a structural battery that has both competitive energy storage capacity and stiffness,” explains Leif Asp, professor at Chalmers and leader of the project.

Superlight electric bikes and consumer electronics could soon be a reality

The new battery has a negative electrode made of carbon fiber and a positive electrode made of aluminum foil coated with lithium iron phosphate. They are separated by a glass fibre fabric in an electrolyte matrix. Although the researchers have managed to create a structural battery that is ten times better than any previous, they did not choose the materials to break records – rather, they wanted to study and understand the effects of the material architecture and the thickness of the separator.

A new project is now underway, funded by the Swedish Space Agency, to increase the performance of the structural battery even further. The aluminum foil is replaced by carbon fiber as the supporting material in the positive electrode, increasing both stiffness and energy density. The glass-fibre separator is replaced by an ultra-thin variant that enables a significantly greater effect – and also faster charging cycles. The new project is expected to be completed within two years.

Leif Asp, who is also leading this project, estimates that such a battery could achieve an energy density of 75 Wh/kg and a stiffness of 75 GPa. This would make the battery about as strong as aluminum, but comparatively much lighter in weight.

“The next-generation structural battery has fantastic potential. If you look at consumer technology, it could well be possible within a few years to produce smartphones, laptops or electric bicycles that weigh half as much as they do today and are much more compact,” says Leif Asp.

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

“We’re really only limited by our imaginations here. We have received a lot of attention from many different companies in connection with the publication of our scientific articles in this field. There is understandably a lot of interest in these lightweight, multifunctional materials,” says Leif Asp.

Read the article in the journal Advanced Energy & Sustainability Research: A Structural Battery and its Multifunctional Performance