Friday, April 07, 2006

Virus-Assembled Batteries

A biological template ramps up electrode performance and scales down size.

By Kevin Bullis

More than half the weight and size of today's batteries comes from supporting materials that contribute nothing to storing energy. Now researchers have demonstrated that genetically engineered viruses can assemble active battery materials into a compact, regular structure, to make an ultra-thin, transparent battery electrode that stores nearly three times as much energy as those in today's lithium-ion batteries. It is the first step toward high-capacity, self-assembling batteries.

Applications could include high-energy batteries laminated invisibly to flat screens in cell phones and laptops or conformed to fit hearing aids. The same assembly technique could also lead to more effective catalysts and solar panels, according to the MIT researchers who developed the technology, by making it possible to finely control the positions of inorganic materials.

"Most of it was done through genetic manipulation -- giving an organism that wouldn't normally make battery electrodes the information to make a battery electrode, and to assemble it into a device," says Angela Belcher, a researcher on the project and an MIT professor of materials science and engineering and biological engineering. "My dream is to have a DNA sequence that codes for the synthesis of materials, and then out of a beaker to pull out a device. And I think this is a big step along that path."

The researchers, in work reported online this week in Science, used M13 viruses to make the positive electrode of a lithium-ion battery, which they tested with a conventional negative electrode. The virus is made of proteins, most of which coil to form a long, thin cylinder. By adding sequences of nucleotides to the virus' DNA, the researchers directed these proteins to form with an additional amino acid that binds to cobalt ions. The viruses with these new proteins then coat themselves with cobalt ions in a solution, which eventually leads, after reactions with water, to cobalt oxide, an advanced battery material with much higher storage capacity than the carbon-based materials now used in lithium-ion batteries.

To make an electrode, the researchers first dip a polymer electrolyte into a solution of engineered viruses. The viruses assemble into a uniform coating on the electrolyte. This coated electrolyte is then dipped into a solution containing battery materials. The viruses arrange these materials into an ordered crystal structure good for high-density batteries.

[Click here for an illustration of the battery-forming process.]

These electrodes proved to have twice the capacity of carbon-based ones. To improve this further, the researchers again turned to genetic engineering. While keeping the genetic code for the cobalt assembly, they added an additional strand of DNA that produces virus proteins that bind to gold. The viruses then assembled as nanowires composed of both cobalt oxide and gold particles -- and the resulting electrodes stored 30 percent more energy.