New composites made of polymer and ceramic might help solve major energy-storage problems for electric and hybrid vehicles, researchers say.
Polymers are ideal for helping electric and hybrid vehicles store energy in capacitive devices because of their light weight, ability to withstand strong electric fields, mechanical flexibility, and scalability to mass production. However, existing two-dimensional films of a polymer called BOPP cannot stand up to the high temperatures under the hood of a car without considerable additional cooling equipment. This adds to the weight and expense of the vehicles.
Previous research looked into 2D films of composites comprising ceramic nanoparticles mixed with polymer. Though this combination yielded higher operating temperatures and charge-discharge efficiencies, there was a significant drawback: a relatively low dielectric constant, or ability to withstand electric fields without breakdown. This kept energy densities unacceptably low.
Now scientists at Penn State University have developed a sandwich-like composite dubbed SSN-x that features separate layers of polymer-ceramic pairings. These can operate at high temperatures while also maintaining a high dielectric constant. The researchers detailed their findings in the 22 August online edition of the journal Proceedings of the National Academy of Sciences.
The outer layers of the SSN-x sandwich are a composite made from boron nitride nanosheets in a c-BCB polymer matrix. These serve as excellent electrical insulators that block charge injection from the capacitive device’s electrodes. This freed the scientists to adjust the recipe for the central layer, made from a composite of barium titanate nanoparticles and c-BCB, to have a high dielectric constant. The result was a big boost in energy density and power density.
“Before, people normally just looked at two-dimensional films and varied their material compositions to tune their properties,” says Qing Wang, a materials scientist at Penn State and lead researcher of the study. “Now we look at three-dimensional structures, which provide a much larger regime for us to tune [the materials'] compositions and optimize their properties.”
In experiments, the researchers showed that the composite could operate at 150 °C—a temperature that makes it well suited for electric vehicle operations. It withstood 24 hours of continuous operation over more than 30,000 cycles of discharging and recharging with no signs of degradation. And at these high temperatures, SSN-x outperformed state-of-the-art polymers in terms of energy density, power density, how efficiently they could be discharged and recharged, and how many discharge-recharge cycles they could undergo before they started to break down.
“We'd now like to see if this material can be produced at a reasonable cost at large scales,” Wang says. “We'd also like to work with electrical engineers and system designers to see how this material can be efficiently integrated into hybrid and electric vehicles.”
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.