The researchers' vision is of vehicles where a large part of the car-body or aeroplane-fuselage consists of structural lithium ion batteries. Multi-functional carbon fiber can work as battery electrodes and load-bearing material consecutively. Source: Yen Strandqvist, Chalmers University of Technology
Researchers at the Chalmers University of Technology in Sweden have discovered that carbon fibers can be used for more than just reinforcement in aircraft and cars. The fibers can operate like battery electrodes and can store energy indirectly.
Carbon fibers are a multifunctional material key player to weight reduction in planes and vehicles. In order to be powered by electricity, rather than fuel, planes need to be much lighter than they are. Weight is also an important factor in electric cars because the lighter a vehicle is, the more distance a car can get from a battery charge.
Chalmers researchers looked into how carbon fibers do more than just act as a lighter reinforcement material for planes and cars. They found that carbon fibers can also be used to store energy.
"A car body would then be not simply a load-bearing element, but also act as a battery," said Leif Asp, professor of material and computational mechanics at Chalmers University of Technology. "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 percent."
The microstructure of carbon fibers affects the fiber’s electrochemical properties. As a result, carbon fibers can operate as electrodes in lithium-ion batteries. Looking at the microstructure of carbon fibers, researchers determined that the fibers with small and poorly integrated crystals had good electrochemical properties with lower stiffness. Fibers with large and highly oriented crystals have greater stiffness but the electrochemical properties are too low to be used in batteries.
"We now know how multifunctional carbon fibers should be manufactured to attain a high energy storage capacity, while also ensuring sufficient stiffness," Asp said. "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 aviation, the thickness of carbon fiber may need to be increased to compensate for the reduced stiffness in batteries. The thickness also could increase the storage capacity of a battery.
"The key is to optimize vehicles at system level — based on the weight, strength, stiffness and electrochemical properties. 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. 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," added Asp.
The study was published in Multifunctional Materials.
