Engineering news
This could open up new opportunities for structural batteries, where energy storage is integrated into the body of a vehicle, allowing for significant weight reductions in electric cars or aircraft.
"A car body would then be not simply a load-bearing element, but also act as a battery," explains Leif Asp, a professor of material and computational mechanics at Chalmers University of Technology, which carried out the work. “It will also be possible to use the carbon fibre 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 and colleagues have been exploring how the microstructure of carbon fibres affects their electrochemical properties. They discovered that carbon fibres with small and poorly oriented crystals have good electrochemical properties but a lower stiffness in relative terms. If you compare this with carbon fibres that have large, highly oriented crystals, they have greater stiffness, but the electrochemical properties are too low for use in structural batteries.
"We now know how multifunctional carbon fibres 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-fibre composites whose stiffness is tailored to aircraft use. There is therefore some potential here for carbon-fibre manufacturers to extend their utilisation."
The types of carbon fibre with good electrochemical properties had a slightly higher stiffness than steel, while those with poorer electrochemical properties were twice as rigid.
The researchers are now collaborating with the automotive and aerospace industries, working on optimising carbon fibre to find the right balance between stiffness and electrochemical properties that could allow it to be used as a battery. In the aviation industry, for instance, it might be necessary to double the thickness of carbon-fibre parts to compensate for reduced stiffness.
"The key is to optimise vehicles at system level – based on the weight, strength, stiffness and electrochemical properties,” says Asp. That is something of a new way of thinking for the automotive sector, which is more used to optimising 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."