Professor Leif Asp with a bobbin of carbon fiber yarn. The electrodes in a structural lithium ion battery consist of carbon fiber yarn arranged in a grid in a polymer. Every length of yarn consists of 24,000 individual carbon fibers.
Photo courtesy of Johan Bidell, Chalmers University of Technology

Discovering that carbon fibers can work as battery electrodes, storing energy directly, opens new opportunities for structural batteries, where the carbon fiber becomes part of the energy system. The use of this type of multifunctional material can contribute to a significant weight-reduction in the aircraft and vehicles of the future – a key challenge for electrification, say researchers at Chalmers University of Technology in Sweden.

Passenger aircraft need to be much lighter than they are today to be powered by electricity. A reduction in weight is also very important for electric vehicles (EVs) to extend the driving distance per battery charge.

Leif Asp, professor of material and computational mechanics at Chalmers, conducts research into the ability of carbon fibers to perform more tasks than simply to act as a reinforcing material. They can store energy, for example.

“A car body would then be not simply a load-bearing element, but also act as a battery,” Asp says. “It will also be possible to use the carbon fiber for other purposes such as harvesting kinetic energy, for sensors, or for conductors of both energy and data. If all these functions were part of a car or aircraft body, this could reduce the weight by up to 50%.”

Asp headed up a multidisciplinary group of researchers who recently published a study on how the microstructure of carbon fibers affects their electrochemical properties – their ability to operate as electrodes in a lithium-ion battery.

The researchers work with structural lithium ion batteries (see image below) where the negative electrodes are made of carbon fiber and the positive electrodes are made of cathode-coated carbon fiber.

Researchers studied the microstructure of different types of commercially available carbon fibers and discovered that carbon fibers with small, poorly oriented crystals have good electrochemical properties but lower stiffness. Carbon fibers with large, highly oriented crystals have greater stiffness, but the electrochemical properties are too low for use in structural batteries.

“We now know how multifunctional carbon fibers should be manufactured to attain a high energy storage capacity, while also ensuring sufficient stiffness,” Asp says. “A slight reduction in stiffness is not a problem for many applications such as cars. The market is currently dominated by expensive carbon fiber composites whose stiffness is tailored to aircraft use. There is therefore some potential here for carbon fiber manufacturers to extend their utilization.”

In the study, the types of carbon fiber with good electrochemical properties had a slightly higher stiffness than steel, whereas the types whose electrochemical properties were poor were slightly more than twice as rigid as steel.

Illustration credit: Yen Strandqvist, Charlmers University
of Technology

The researchers are collaborating with automotive and aviation companies. Asp explains that for the aviation industry, it may be necessary to increase the thickness of carbon fiber composites to compensate for the reduced stiffness of structural batteries. This would, in turn, also increase their energy storage capacity.

“The key is to optimize vehicles at system level – based on the weight, strength, stiffness, and electrochemical properties,” Asp explains. “That is something of a new way of thinking for the automotive sector, which is more used to optimizing individual components. Structural batteries may perhaps not become as efficient as traditional batteries, but since they have a structural load-bearing capability, very large gains can be made at system level.”

He continues, “In addition, the lower energy density of structural batteries would make them safer than standard batteries, especially as they would also not contain any volatile substances.”

Chalmers University of Technology