Scientists from the Massachusetts Institute of Technology have harnessed the construction talents of tiny viruses to build ultra-small "nanowire" structures for use in very thin lithium-ion batteries. Such batteries could power various electronics as well as vehicles and, since they are very small, can enable longer battery lives compared to existing accumulators.
The goal of the work, led by MIT Professors Angela Belcher, Paula Hammond and Yet-Ming Chiang, is to create batteries that cram as much electrical energy into as small or lightweight a package as possible. The batteries they hope to build could range from the size of a grain of rice up to the size of existing hearing aid batteries. By manipulating a few genes inside these viruses, the team was able to coax the organisms to grow and self-assemble into a functional electronic device.
Batteries consist of two opposite electrodes – an anode and cathode – separated by an electrolyte. In the current work, the MIT team used an intricate assembly process to create the anode. Specifically, they manipulated the genes in a laboratory strain of a common virus, making the microbes collect exotic materials – cobalt oxide and gold. Because these viruses are negatively charged, they can be layered between oppositely charged polymers to form thin, flexible sheets. As a result, the scientists got a dense, virus-loaded film that serves as an anode.
In their research, the MIT team altered the virus's genes so they make protein coats that collect molecules of cobalt oxide, plus gold. The viruses then align themselves on the polymer surface to form ultrathin wires. Each virus, and thus the wire, is only 6nm in diameter, and 880nm in length.
“We can make them in larger diameters, but they are all 880nm in length, which matches the length of the individual virus particles. Once we've altered the genes of the virus to grow the electrode material, we can easily clone millions of identical copies of the virus to use in assembling our batteries,” said Ms. Belcher.
For the metal oxide the researchers chose cobalt oxide because it has very good specific capacity, which will produce batteries with high energy density, meaning it can store two or three times more energy for its size and weight compared to previously used battery electrode materials. Adding the gold further increased the wires' energy density, according to the researchers.
Equally important, the reactions needed to create nanowires occur at normal room temperatures and pressures, so there is no need for expensive pressure-cooking technology to get the job done.
The work is important also because energy density is a vital quality in batteries. A lack of energy density – meaning the amount of charge a battery of a given size can usefully carry – is what has hampered development of electric cars, since existing batteries are generally too heavy and too weak to compete with gasoline as an energy source. Still, battery technology is gradually being improved and may someday even become competitive as the price of oil escalates.
“The nanoscale materials we've made supply two to three times the electrical energy for their mass or volume, compared to previous materials,” the team reported.
The researchers' work was spurred by growing evidence that 'nanostructured' materials can improve the electrochemical properties of lithium-ion batteries, compared to more conventional batteries based on older technologies.
But to create new battery materials, Ms. Belcher noted, special control is needed so just the right amounts of the exotic materials end up exactly where they belong. Cobalt oxide has reportedly shown excellent electrochemical cycling properties, and is thus under consideration as an electrode for advanced lithium-ion batteries.
This work was funded by the Army Research Office Institute of Collaborative Biotechnologies, the Institute of Soldier Nanotechnologies and the David and Lucille Packard Foundation.