Heavy batteries are one of the downsides of hybrid and electric vehicles. In the Tesla roadster, for example, they account for a third of the car's weight. But a new material, patented by Imperial College London, could change all that by doing double duty as both a structural and a functional element. In automobiles, this would mean strong and lightweight body panels that could also store and release electrical energy. The stuff could even work in mobile phones and laptop computers, allowing the outer casings to also function as the batteries.
Material research is taking place as part of a multimillion-dollar, three-year project funded by the European Union, involving researchers at Imperial and several European companies, including Volvo Car Corp. Dr. Emile Greenhalgh of Imperial's Aeronautics Dept. is coordinating the new project, announced in February. One of the goals is to develop the composite material for use in a car's wheel well. Volvo hopes to then fit this wheel well system into prototype cars for test runs: It could reduce the number of traditional batteries required to power the electric motor. Use of the composite material will reduce the weight of the existing system (conventional wheel well plus conventional battery) by at least 15%.
One of Volvo's engineers, Per-Ivar Sellergren, based in Sweden, has been working on the idea of merging batteries with body panels for more than 20 years, an idea Volvo patented in 1995. The Swedish scientist, who remains heavily involved in the research project, believes the multifunctional body panels could completely replace today's battery technology within a decade.
The composite material under development, made of carbon fibers and a polymer resin, will store and release energy much faster than traditional batteries can, say researchers. At Imperial, the research focus is on developing supercapacitors, which offer a rapid charge, but today carry only 10% of the energy of a conventional battery. Within the larger research project, other companies are focusing on composite batteries, which they believe could have superior energy storage compared with traditional batteries.
The project is now in its first stage, with scientists concentrating on boosting the amount of energy the composite can store. Today that figure is about 0.005 W-hr/kg. In comparison, modern lithium-ion batteries offer closer to 150 W-hr/kg in energy density. The Imperial team hopes to boost the new material's energy density by growing carbon nanotubes on the carbon fiber surface, thereby increasing the material's surface area and its ability to hold more energy. Dr. Greenhalgh hopes to hit at least 20 W-hr/kg by late 2011. He says the new supercapacitors won't yet replace the battery packs in hybrid cars, but can make these packs smaller, lighter weight, and less expensive.
“I anticipate the material will provide benefits within the next three years, as this is the timescale for it to be used as a demonstrator in Volvo's car,” says Greenhalgh. “I then anticipate it will go into volume production with ACG, the industrial partner working on scale-up, after about five years. Finally, I expect that the material will be in widespread use in about 10 years, though this is only an estimate.”
Continue to next page
Safety concerns are another issue and will require further testing before consumer use. So far, the idea is to protect passengers from electric shock by sandwiching the new material between layers of traditional composites, according to the research team.
“There is a concern that these materials could potentially behave energetically during an impact or penetration condition,” notes Greenhalgh. “However, work done on similar types of materials by QinetiQ and Army Research Labs in the U.S. suggests these concerns are unfounded. As demonstrated by Formula One, composite materials have an excellent safety record during impact or collisions.”
Imperial College London, www.imperial.ac.uk
Volvo Car Corp., www.volvocars.com
How to make a carbon-composite battery
A cured resin is applied to woven sheets of carbon fibers to make the material rigid and stiff. Fibers are also treated with an alkali to create widespread surface pitting, increasing the surface area and thereby the capacity to hold more electricity. To facilitate electrical energy storage, two layers of woven fibers surround a thin layer of insulating material, made of glass fibers, forming a sandwich. The resin also includes lithium ions, causing each layer to act like an electrode where the positively charged ions pool in one layer when voltage is applied, and with current flowing when the material is put in a circuit. Additional layers enclose the material to guarantee electrical isolation.