The high cost and limited range of electric vehicles can make them a tough sell, and their costliest and most limiting component are their batteries.
But batteries also open up new design possibilities because they can be shaped in more ways than gasoline tanks and because they can be made of load-bearing materials. If their chemistries can be made safer, batteries could replace conventional door panels and other body parts, potentially making a vehicle significantly lighter, more spacious, and cheaper. This could go some way toward helping electric cars compete with gas-powered ones.
Tesla Motors and Volvo have demonstrated early versions of the general approach by building battery packs that can replace some of the structural material in a conventional car. Dozens of other research groups and companies are taking further steps to make batteries that replace existing body parts, such as body panels and frames.
The ability to use batteries as structural materials is currently limited by the use of flammable electrolytes, but researchers are developing safer chemistries that could be used more widely. The approach also raises several practical questions: can the energy-storing body panels be engineered so that even if they’re dented, the car will still work? And how expensive will bodywork be? However, automakers could turn to the approach under pressure to sell more electric vehicles and hybrids to meet stringent future fuel economy standards.
Batteries are the single most expensive item in electric cars, so making them cheaper would make electric vehicles cheaper too. But even without significant breakthroughs, new battery designs could make a car lighter.
One example is the way Tesla has designed the battery for the Model S. The metal casing that protects the battery also serves to make the car frame more rigid, reducing the overall amount of metal needed (see “How Tesla Is Driving Electric Car Innovation”).
This month, Volvo demonstrated another approach using lithium-ion batteries, which are made of thin films of material that are rolled or folded up to form a battery cell. Researchers at the Lulea University of Technology in Sweden in collaboration with Volvo sandwiched these films between sheets of carbon-fiber composite. The resulting structure was used to replace plastic body parts and a small conventional battery on a hybrid version of the Volvo S80. (The car is a “stop-start” hybrid that uses a battery to make it possible to turn off the engine whenever the car isn’t moving.)
The U.S. Department of Energy’s Advanced Research Projects Agency for Energy is spending $37 million on projects seeking to use batteries as structural materials. (The program is called RANGE, which stands for Robust, Affordable, Next-Generation Energy Storage Systems). In two ARPA-E projects, researchers are figuring out ways to design battery packs to absorb energy in a crash to replace materials now used to protect passengers. For example, rather than packaging battery cells into a solid block, the cells could be allowed to move past each other in an accident, dissipating energy as they do.
Most of the approaches being explored so far still use conventional battery cells—the parts of the pack that actually store energy. If safer battery cells can be made, then this would provide even more flexibility in how a car can be designed. You wouldn’t need to enclose them in protective cases or regulate their temperature to prevent battery fires.
“When you’re not obsessed with protecting batteries, you can be a lot more creative. You’re not limited to the architecture of conventional cars,” says Ping Liu, who manages and helped conceive of ARPA-E’s RANGE project.
To this end, several researchers are developing new chemistries that don’t use flammable electrodes, so the batteries could be safely used as door panels. They’re considering replacing volatile electrolytes with less-flammable polymers, water-based materials, and ceramics (see “Solid-State Batteries”). Once they have a safer electrolyte, the researchers will look for ways to use the battery electrodes in a cell to bear loads.
Volvo has an experimental version of this approach that uses carbon fibers in composite materials to store and conduct electricity but also to strengthen the composites. The device was formed in the shape of a trunk lid. But it could only produce enough electricity to light up some LEDs, so it couldn’t replace the battery in an electric car or a hybrid. A newer version being developed at Imperial College in London replaces the epoxy that ordinarily holds together carbon fibers in a composite with a blend of stiff materials and ionic liquids that can conduct charged molecules. This forms a type of supercapacitor that could store enough energy to be used in place of a battery in a stop-start hybrid.
For electric cars and hybrids with larger batteries, supercapacitors don’t store enough energy. So to provide enough driving range, some researchers are developing lithium-ion batteries that use carbon fibers for one electrode, but use conventional lithium-ion materials for the opposite one. Others have developed a nonvolatile polymer electrolyte to replace conventional, flammable ones. The resulting material will make it possible to “do two jobs with one thing,” says Leif Asp, a professor at Lulea University. Several ARPA-E projects are taking this kind of approach.
These new electrolytes and load-bearing battery cells are likely more than a decade away from being useful in cars, however. It will be difficult to ensure that the battery stores large amounts of energy and can also be strong enough as a structural component.
Asp says the first applications could be in portable electronics, where load-bearing batteries could replace conventional plastic cases. But if car components can one day be made out of such materials, then batteries could finally go from a limiting factor to a selling point.
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