The genetically engineered battery wrap is fabricated by dipping a scaffold into three beakers--the first containing the polyelectrolyte, the second containing the genetically engineered virus and the third containing a solution of cobalt ions.
The first dip deposits a film of polyelectrolyte on the scaffold that can range from microns down to 100 nanometers thick. The second is the virus dip, which deposits a single layer of the 6 x 880-nm M13 bacteriophage (the viruses are negatively charged, which enables them to array themselves along the positively charged patterns on the scaffold).
The third dip then enables the genetically engineered virus to pull cobalt-oxide and gold ions out of the solution and surround itself with that material. After the third dip, the scaffold is dried and the battery wrap peeled off. All steps are performed at normal room temperatures and pressures.
"We made a cobalt-oxide anode first, and discovered it had excellent properties," said Belcher. "Then we increased its current density by genetically engineering a virus that could grow two different materials at the same time [cobalt-oxide and gold] and found the combination made an even better electrode. The viruses do their work by pulling the cobalt-oxide and gold ions out of the solutions into which the polyelectrolyte is dipped."
The viruses are genetically altered by manually tweaking individual genes, then cloning millions of copies from the hand-tailored original, enabling different versions of M13 to be prescribed for collecting different molecules--here, cobalt-oxide and gold.
Belcher said it only takes about 30 minutes for the viruses to assemble the inorganic molecules in a monolayer above, below and adjacent to themselves. After that, the polyelectrolyte is dried out, and the 6-nm-diameter viruses dehydrate, becoming harmlessly entombed inside a sealed compartment of inorganic cobalt and gold.
Currently, the materials scientists are demonstrating their battery wrap by wrapping it around a conventional lithium cathode, but the group is already genetically engineering a new variation of M13 that will be able to grow the cathode in a monolayer on the other side of the film.
"Potentially, when we grow a lithium layer on the other side of the polyelectrolyte for the other cathode, we could use this material to make batteries as thin as 100 nm," said Belcher.
So far, the scientists have demonstrated the ability to stack sheets of batteries atop each other in a comb structure that can be wired in parallel to increase current-carrying capabilities. In addition, they are wiring groups of combs in series to raise the voltage output and to recharge. Currently, their highest voltage battery is a 3-V version.
"Our next step will be to experiment with growing different electrode materials plus self-assemble an entire battery, including both the cobalt anode and the lithium cathode," said Belcher.
Belcher is MIT's Germeshausen professor of materials science and engineering and biological engineering; Chiang is Kyocera professor of materials science and engineering; and Hammond is Mark A. Hyman professor of chemical engineering. The professors led a team of five additional researchers comprising materials science and engineering graduate students Ki Tae Nam, Dong-Wan Kim, Chung-Yi Chiang and Nonglak Meethong, plus postdoctoral associate Pil Yoo.
Belcher predicted that the final packaged battery wrap could store two or three times more energy for its size and weight than conventional batteries today. In addition, its conformability should make possible much thinner battery-powered portable devices.
Funding for the research was provided by the Army Research Office Institute of Collaborative Biotechnologies, the Institute of Soldier Nanotechnologies, and the David and Lucille Packard Foundation.