R. Colin Johnson Courtesy of EE Times (08/14/2007 9:42 AM EST)

PORTLAND, Ore. — Rensselaer Polytechnic Institute researchers said they have developed a paper-thin battery by immersing a carpet of vertical nanotubes in an ionic liquid electrolyte. The result is a cellulose paper that stores electrical energy.

The RPI team produced a supercapacitor by placing a second nanotube electrode on the other side of the paper. They then added a lithium electrode atop the paper, creating what they claim is a paper-thin rechargable battery.

"The carbon nanotubes are embedded in the paper, and the electrolyte is soaked into the paper, so it really looks, feels and weighs about the same as paper," said RPI professor Robert Linhardt.

Nanocomposite paper uses carbon nanotubes (black section) to create ultra-thin energy storage batteries and supercapacitors.

The supercapcitor and rechargeable battery are the result of a year and half of collaborative research among three RPI labs. One lab was making carbon nanotube-based structures, which were adapted to serve as a battery electrode. By growing the nanotubes vertically on a sheet, liquid cellulose was poured between the "forest of nanotubes" to form the battery. Another lab added a lithium-based top electrode to create either a rechargable battery or a supercapacitor by adding a second nanotube electrode.

Ionic liquids first disolved the cellulose, turning it into a gel. The fluids also serve as the battery electrolyte, carrying ions from one one side of the paper battery to the other.

Each sheet of battery-paper generated about 2.4 volts with a power density of about 0.6 milliamps/cm². For higher voltages, paper can be stacked. For more current, the sheets can be expanded to larger areas. The battery-paper operates from minus 100 degrees up to 300 degrees Fahrenheit, and can deliver quick surges of current, the RPI researchers claim. It can also be rolled twisted or cut into many shapes.

So far, the RPI researchers have only cycled their paper batteries through 100 rechargings. But they claim no deterioration in performance has been detected after recharging. Next, they plan long-term testing of the batteries to determine the maximum number of rechargings, and to optimize the design for higher power densities.

Since the paper is biocompatible, the researchers will also experiment with powering medical implants with paper batteries that could use blood and sweat as liquid electrolytes.

Finally, the researchers want to perfect methods for printing batteries and supercapacitors using traditional roll-to-roll printing presses.