More than four years ago, James Tour at Rice University developed a method by which cylindrical carbon nanotubes could be unzipped to form graphene nanoribbons (GNR). About 18 months after making that discovery, Tour described his work here on the pages of IEEE Spectrum.
Today, Tour and his colleagues have found an application for their GNR material that could increase the storage capacity of lithium ion (Li-ion) batteries.
The research, which is described in the journal ACS Nano ("Graphene Nanoribbon and Nanostructured SnO2 Composite Anodes for Lithium Ion Batteries"), has developed a method by which the GNR can be combined with tin oxide in a way that gives it greater storage capacity than the theoretical maximum of tin oxide alone. The prototype device that the Rice team developed still managed to maintain a storage capacity more than twice that of traditional graphite after 50 charge-discharge cycles.
While the unzipping of carbon nanotubes into GNR made the cover of Nature back in 2009--and created suspensions with well-defined distributions of graphene platelets--there was some doubt as to whether the process was scalable. However, according to the press release, Tour and his team can produce GNR in bulk quantities.
With a bulk manufacturing process in place, Tour mixed the GNR with tin oxide particles about 10 nanometers wide into a slurry. After using a cellulose gum binder and water, the mixture of GNR material and tin oxide was applied to a current collector and placed into a button-style Li-ion battery.
In the lab tests, the prototype battery had an initial charge capacity of more than 1520 milliamp hours per gram (mAh/g). After repeated charge-discharge cycles that number began to plateau at about 825 mAh/g.
For many years, researchers have been attempting to use silicon as a replacement for the traditional graphite used in Li-ion anodes. However, despite silicon’s vastly superior storage capacity, these anodes would become brittle from repeated charge-discharge cycles and quickly crack becoming useless. As a result, nanostructured silicon has been the focus of much recent research for replacing graphite. Just two months ago, Lansing, MI-based XG Sciences, Inc. announced commercial intentions for a nanostructured silicon based on graphene.
We’ll have to check back to see where that technology has developed, but in the meantime Tour and his team believe the GNR/tin oxide hybrid material they have developed could address the brittleness issue.
“Graphene nanoribbons make a terrific framework that keeps the tin oxide nanoparticles dispersed and keeps them from fragmenting during cycling,” said Tour in a press release. “Since the tin oxide particles are only a few nanometers in size and permitted to remain that way by being dispersed on GNR surfaces, the volume changes in the nanoparticles are not dramatic. GNRs also provide a lightweight, conductive framework, with their high aspect ratios and extreme thinness.”
I am all in favor of improving the ubiquitous Li-ion battery. The required recharging of my smart phone in the middle of the day is an indication of how much these improvements are needed for mobile devices. Li-ion batteries may have an ongoing role in hybrid vehicles. But maybe it’s time to put to rest the idea that they are a viable solution for powering all-electric vehicles.
Illustration: Jian Lin/Tour Group
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.