Carbon-composite batteries Making the bodywork Multi-function materials for building and powering electric cars

Apr 26th 2010

From The Economist online
COMPARED with other electric vehicles, the Tesla Roadster is what you might call a hot rod. With an impressive range of 380km (236 miles) between charges, the two-seater sports car is capable of going from zero to 96kph in just 3.9 seconds. But as is so often the case, such performance comes at a cost. For a vehicle that weighs around 1,200kg (2,645 pounds) a whopping 450kg of that is taken up by the car’s batteries.

Although new battery technologies are emerging, their weight and size is likely to remain a drag on the development of electric and hybrid cars, forcing manufacturers to come up with new and inventive ways to shed weight and free up space. One solution which researchers are exploring is to build cars using a hybrid material: a carbon composite that is also capable of storing electrical energy. That way, car designers could combine structural form with electrical function.
Carbon composites are extremely strong and light. They are already used in products ranging from tennis racquets to aircraft wings. Some supercars are built with the material, but it is generally too expensive for mass-produced vehicles. With the additional ability to store energy, however, carbon composites could become a lot more attractive for the automotive industry, says Emile Greenhalgh of Imperial College, London, who is leading the research as part of a broader EU project called STORAGE into incorporating different battery materials into the bodywork of a car. The work’s academic and industrial partners include the British Ministry of Defence, which is interested in extending the range of robotic drones, and Volvo.
Like traditional composites, Dr Greenhalgh’s material consists of woven sheets of carbon fibres which are made rigid using a cured resin. To enable this material to store electrical energy, two layers of woven fibres are made into a sandwich, separated by a thin layer of a glass-based insulating material. The resin within the carbon layers is laced with lithium ions, so that each layer acts like an electrode, causing the positively charged lithium ions to collect in one layer when a voltage is applied, and a current to flow when the sandwich is placed in a circuit. All this is encapsulated within further layers to ensure that it is electrically isolated.
Strictly speaking the composite behaves not like a battery but more like a capacitor, or rather a supercapacitor, says Dr Greenhalgh. Batteries are good at storing large amounts of charge but slow at delivering it; for capacitors the reverse is the case. Supercapacitors have a big internal surface-area that allows a large amount of energy to be delivered rapidly and are used in some electric cars to provide a short burst of power for rapid acceleration and in hybrids to recover energy during braking.
To get similar characteristics from composites, the carbon fibres are first chemically treated with an alkali which creates lots of tiny pits on their surface. This massively increases their surface area and hence the charge they can hold, but without impairing the physical strength of the material.
Another challenge lies in resolving the two conflicting requirements of the resin. “You want it very rigid and stiff, but from an electrical point of view you also want it to allow ions to flow through the material,” says Dr Greenhalgh. With traditional resins you either get one or the other. Dr Greenhalgh’s solution was to use a polymer gel-based resin that combines two networks of cross-linking structures, one that holds the material together and the other providing a conduit for charged particles.
The result is a material that has an energy density of about 0.005 watt-hours per kilogram. Not much, admittedly, when compared to the 128 watt-hours per kilogram of a lithium-ion battery, like that found in many electric cars. But this is just the beginning, says Dr Greenhalgh. Increasing the operating voltage will boost the energy density of the composite significantly. And by covering the fibres with carbon nanotubes, at least 20 watt-hours per kilogram is expected by the end of next year.
Using a single material for two functions has great potential for carmakers, says Per-Ivar Sellergren, a senior engineer at Volvo’s Material Laboratory in Gothenburg. If the electrical storage of composites could be boosted close to that of existing lithium-ion batteries, then it would take only the roof, the bonnet and the boot lid to power an electric vehicle for 130km, he says.
Multi-function composites could also reduce the amount of wiring in vehicles, says Dr Greenhalgh, allowing the rear lights, for example, to be powered by the material used to build the boot. Some instruments, like satnavs, could be powered by energy stored in their casings. And compared with most rechargeable batteries, supercapacitors tend to have a longer working life. In time that too would become a more obvious benefit. Most drivers of electric cars have not had their vehicles long enough to have had to pay thousands of dollars to replace their worn-out battery packs.