While the first generation of their devices used the nickel-coated viruses for the electrodes, work published earlier this year investigated the feasibility of structuring electrodes with the active material deposited on top of each nickel-coated nanorod, forming a core/shell nanocomposite where every TMV particle contains a conductive metal core and an active material shell. In collaboration with Chunsheng Wang, a professor in the Department of Chemical and Biomolecular Engineering, and his Ph.D. student Xilin Chen, the researchers have developed several techniques to form nanocomposites of silicon and titanium dioxide on the metalized TMV template. This architecture both stabilizes the fragile, active material coating and provides it with a direct connection to the battery electrode.
In the third and final step, Chen and Gerasopoulos assemble these electrodes into the experimental high-capacity lithium-ion batteries. Their capacity can be several times higher than that of bulk materials and in the case of silicon, higher than that of current commercial batteries.
"Virus-enabled nanorod structures are tailor-made for increasing the amount of energy batteries can store. They confer an order of magnitude increase in surface area, stabilize the assembled materials and increase conductivity, resulting in up to a10-fold increase in the energy capacity over a standard lithium ion battery," Wang said.
A bonus: since the TMV binds metal directly onto the conductive surface as the structures are formed, no other binding or conducting agents are needed as in the traditional ink-casting technologies that are used for electrode fabrication.
"Our method is unique in that it involves direct fabrication of the electrode onto the current collector; this makes the battery's power higher, and its cycle life longer," said Wang.
The use of the TMV virus in fabricating batteries can be scaled up to meet industrial production needs. "The process is simple, inexpensive, and renewable," Culver adds. "On average, one acre of tobacco can produce approximately 2,100 pounds of leaf tissue, yielding approximately one pound of TMV per pound of infected leaves," he explains.
At the same time, very tiny microbatteries can be produced using this technology. "Our electrode synthesis technique, the high surface area of the TMV and the capability to pattern these materials using processes compatible with microfabrication enable the development of such miniaturized batteries," Gerasopoulos adds.
While the focus of this research team has long been on energy storage, the structural versatility of the TMV template allows its use in a variety of exciting applications. "This combination of bottom-up biological self-assembly and top-down manufacturing is not limited to battery development only," Ghodssi said. "One of our lab's ongoing projects is aiming at the development of explosive detection sensors using versions of the TMV that bind TNT selectively, increasing the sensitivity of the sensor. In parallel, we are collaborating with our colleagues at Drexel and MIT to construct surfaces that resemble the structure of plant leaves. These biomimetic structures can be used for basic scientific studies as well as the development of novel water-repellent surfaces and micro/nano scale heat pipes."
Funding for the research comes from the National Science Foundation, the Department of Energy Office of Basic Energy Sciences, the Maryland Technology Development Corporation, and the Laboratory for Physical Sciences at the University of Maryland. James Culver's work is conducted in collaboration with Purdue University professor Michael Harris.
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