A few months back, the Air Force Office of Scientific Research (AFOSR) was proud to publish an article regarding a research accomplishment by Dr. Jim Tour and his research team at Rice University. AFOSR, along with other funding agencies, supported Dr. Tour's research effort to make graphene suitable for a variety of organic chemistry applications—especially the promise of advanced chemical sensors, nanoscale electronic circuits and metamaterials.
Four years ago, Tour's research team demonstrated that they could chemically unzip cylindrical shaped carbon nanotubes into soluble graphene nanoribbons (GNR) without compromising the electronic properties of the graphitic structure. A recent paper by the Tour team, published in IEEE Spectrum and partially funded by AFOSR, showed that GNR can significantly increase the storage capacity of lithium ion (Li-ion) by combining graphene nanoribbons with tin oxide.
By producing GNR in bulk, a necessary requirement for making this a viable process, the Tour team mixes GNR and 10 nanometer wide particles of tin oxide to create a slurry. By adding a cellulose gum binding agent and water, the mixture is then applied to a capacitor, which is then fitted to a button-style lithium-ion battery.
In the Tour lab tests, the prototype battery had an initial charge capacity of more than 1,520 milliamp hours per gram (mAh/g). After repeated charge-discharge cycles that number began to plateau at about 825 mAh/g, but after 50 discharge cycles, the batteries retained far more capacity—more than double—that of Li-ion batteries that employ standard graphite anodes.
The critical key that makes the increase in battery capacity possible is the significant improvement in flexibility that graphene nanoribbons lend to the anode. By comparison, conventional Li-ion batteries with graphite anodes break down and lose efficiency because of their inability to flex, as they expand and contract, with repeated charge and discharge cycles; over time the graphite cracks and the battery cannot charge. Conversely, anodes with a graphene nanoribbon platform allow the tin oxide particles to maintain a consistent size, rather than expanding and contracting, and thus eliminating the brittleness and cracking associated with a graphite-based anode.
This breakthrough may very well lead to the next generation of the lithium-ion battery—a promising new platform for creating more durable, lightweight and efficient lithium-ion power.