The researchers genetically engineered the viruses to self-assemble nanoscale battery films by creating billions of random variations, then using the survival-of-the-fittest principle to select those that best performed desired tasks.

Last year, the team demonstrated the ability to use micro-contact printing techniques to fabricate flexible battery films from anode materials that were self-assembled by viruses. The demonstration used a traditional cathode material.

Genetically-engineered viruses can now also self-assemble nanoscale cathode material, providing the final component necessary for commercialization.

"We have used genetic engineering to grow a cathode material--nanowires of lithium ion phosphate plus silver, which then pickup a single carbon nanotube at their tip to increase their conductivity," said Belcher.

MIT's previously demonstrated self-assembly of the anode material using a different virus that coats itself with cobalt oxide and gold to form a nanowire. The new virus harnesses a similar method to coat itself with iron phosphate and silver, then uses molecular recognition to pick up nanotubes on their ends for more efficient electron transport.

"We first tried to engineer the material without the nanotubes, but its conductivity was not good enough. So we found a virus that would attach to nanotubes by virtue of molecular recognition," said Belcher. "That was the hardest part, since only two out of a billion viruses--each with a different genetic code and selected by the survival-of-the-fittest principle--picked up a nanotube on its tip."

The resulting material--which can be mass produced in solution, then dried into a powder--consists of about 5 percent carbon nanotubes. In demonstrations, batteries stamped out using the micro-contact printing method were able to be recharged hundreds of times with no detectable drop in performance.

The MIT researchers next hope to refine their lithium-based material formulations to provide even better performance by adding metals to their lithium phosphate mix, including lithium magnenese phosphate or lithium nickel phosphate.

Funding for the MIT research was provided by the U.S. Army Research Office Institute and the National Science Foundation through its Materials Research Science and Engineering Centers program.