Structural batteries have been under consideration for electric vehicles for some years. For example, Lord Paul Drayson’s record-breaking electric racing cars incorporated prototypes of these components. Put simply, structural batteries are energy storage devices which form part of the overall structure of the vehicle: this could be the bodywork or chassis of an electric car, or part of the fuselage or skin or the underlying supporting framework of an aircraft. In theory, performing "double duty" as both part of the structure and the energy storage capability of the vehicle is a method of reducing weight. However, previous prototypes of structural battery systems have been found wanting in both capacities.
Researchers from Chalmers University of Technology in Sweden have now found a new way of transforming composite panels into batteries. Led by Leif Asp, a professor of material and computational mechanics, the team has been researching how carbon fibres in composites could be used as components of lithium ion batteries formed out of the panelwork itself. "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 per cent," Asp claimed.
Using carbon as a component in lithium ion batteries is not in itself new; it is an established practice to use carbon – generally in its graphite form or a variant of it – as the positive electrode while lithium metal is the negative. Lithium ions migrate across the battery electrolyte and embed themselves within the carbon atomic structure: a process known as intercalation.
The Chalmers team's research has focused around determining what is the optimum structure for carbon fibres based on the polymer polyacrylonitride (PAN) to both store energy by intercalating lithium and act as reinforcement for a polymer composite. PAN-based fibres have much higher electrochemical capacity than fibre is based on pitch, they found.
In the journal Multifunctional Materials, they describe how fibres with small and poorly oriented crystals have good electrochemical properties but low stiffness, while fibres with large and well oriented crystals are very stiff but have electrochemical properties too poor to act as structural battery components.
In general, "good" battery fibres are slightly stiffer than steel, while "poor" battery fibres are 10 times stiffer. " 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," Asp said.
If aircraft manufacturers want to use this property of carbon, they may have to change their philosophy on how the element is used, the team said. They may need to increase the thickness of the carbon fibres in their composites to compensate for the reduction in stiffness, which would also improve the energy storage capability.
"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,” said Asp. “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."