TechFront New Developments in Manufacturing and Technology Nanotubes Hold Promise for Extending Battery Life R
esearchers at Rice University (Houston) have invented a new way to boost the efficiency of the ubiquitous lithium-ion (li-ion) battery by using ribbons of gra- phene that start as carbon nanotubes.
Proof-of-concept anodes—the part of the battery that stores lithium ions—built with graphene nanoribbons (GNRs) and tin oxide showed an initial capacity better than the theo- retical capacity of tin oxide alone, according to Rice chemist James Tour. After 50 charge-discharge cycles, the test units retained a capacity that was still more than double that of the graphite currently used for li-ion battery anodes. The research appeared in the June issue of the American Chemical Society journal ACS Nano.
Better batteries are greatly desired by everyone who car- ries a cell phone or computer or drives an electric car. The Rice team sees the potential for GNRs to contribute to their development. The technology was licensed just last November by AZ Electronic Materials (Branchburg, NJ), a producer of high-quality, high-purity specialty chemicals to the semicon- ductor and flat-panel display industries. Both Professor Tour and Ralph Dammel, AZ Electronic Materials’ chief technology officer, estimate that commercialization of the technology is about five years out. Tour and his colleagues developed a method for unzip- ping nanotubes into GNRs, revealed in a 2009 cover story in Nature. Since then, the researchers have figured out how to make graphene nanoribbons in bulk and are moving toward commercial applications. An area ripe for improvement is the humble battery, and in an increasingly mobile world, battery capacity is becoming a bottleneck that generally limits devices to less than a day’s worth of use.
In the new experiments, the Rice lab mixed graphene nanoribbons and tin oxide particles about 10-nm wide in a slurry with a cellulose gum binder and a bit of water, spread it on a current collector and encased it in a button- style battery. GNRs are a single atom thick and thousands of times longer than they are wide. The GNRs not only separate and support the tin oxide but also help deliver lithium ions to the nanoparticles.
Lab tests showed initial charge capacities of more than 1520-milliamp hours per gram (mAh/g). Over repeated charge-discharge cycles, the material settled into a solid 825 mAh/g. “It took about two months to go through 50 cycles,” said lead author Jian Lin, a postdoctoral researcher at Rice, who believes the material could handle many more without losing significant capacity.
A scanning electron microscope (SEM) image of unzipped graphene nanoribbons (GNR). Rice University researchers expect the GNRs to improve future lithium-ion batteries.
GNRs could also help overcome a prime difficulty with li-ion battery development. Lithium ions tend to expand the material they inhabit, and the material contracts when they’re pulled away. Over time, materials like silicon, which shows extraordinary capacity for lithium, break down and lose their ability to store ions. Other labs at Rice have made break- throughs that help solve the expansion problem by breaking treated silicon into a powder, achieving great capacity and many cycles.
GNRs take a different approach by giving batteries a degree of flexibility, Tour said. “Graphene nanoribbons make a terrific framework that keeps the tin oxide nanoparticles dispersed and keeps them from fragmenting during cy-
August 2013 |
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Image courtesy Rice University
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