A biological template ramps up electrode performance and scales down size.
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.
Using viruses to assemble inorganic materials has several advantages, says Daniel Morse, professor of molecular genetics and biochemistry at the University of California, Santa Barbara. First, the placement of the proteins, and the cobalt and gold that bind to them, is precise. The virus can also reproduce quickly, providing plenty of starting material, suggesting that this is manufacturing technique that could quickly scale up. And this assembly method does not require the costly processes now used to make battery materials.
“You could do this at the industrial level really quickly,” says Brent Iverson, professor of organic chemistry and biochemistry at the University of Texas at Austin. “I can’t imagine a way to template or scaffold nanoparticles any cheaper.”
Yet-Ming Chiang, materials science and engineering professor at MIT and one of Belcher’s collaborators, says that, while small batteries designed for specific applications could be made using this process within a couple of years, much work remains to be done. For example, cobalt oxide might not be the best material, so the researchers will be engineering viruses to bind to other materials.
One of the ways they have done this in the past is using a process called “directed evolution.” They combine collections of viruses with millions of random variations in a vial containing a piece of the material they want the virus to bind to. Some of the viruses happen to have proteins that bind to the material. Isolating these viruses is a simple process of washing off the piece of material –only those viruses bound to the material remain. These can then be allowed to reproduce. After a few rounds of binding and washing, only viruses with the highest affinity for the material remain.
The researchers also want to make viruses that assemble the negative electrode as well. They would then grow the positive and negative electrodes on opposite sides of a self-assembling polymer electrolyte developed by Paula Hammond*, another major contributor to the project. This would create self-assembled batteries, not just electrodes. Another goal is to make “interdigitated” batteries in which negative and positive electrode materials alternate, like the tines of two combs pushed together – this could pack in more energy and lead to batteries that deliver that energy in more powerful bursts.
And batteries could be just the beginning. Since the viruses have different proteins at different locations – one protein in the center and others at the ends – the researchers can create viruses that bind to one material in the middle and different materials on the ends. Already, Belcher’s group has produced viruses that coat themselves with semiconductors and then attach themselves at the ends to gold electrodes, which could lead to working transistors.
“If you can make batteries that truly are effective this way, it’s just mind-boggling what the applications could be,” Iverson says.
*Correction: The virus-battery work was the result of a collaboration between researchers at MIT. The original article mentions Angela Belcher and Yet-Ming Chiang. An important part of this work was the development of a self-assembling polymer electrolyte by Paula Hammond, MIT chemical engineering professor.
Home page image courtesy of Angela Belcher, MIT.
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