In what sounds like a solution in search of a problem, a few years ago researchers at Rice University figured out a way to create graphene ribbons by “unzipping” carbon nanotubes. Well, now it looks like a problem has been found, and it’s a big one with huge implications for the solar and wind power markets, to say nothing of electric vehicles and portable electronics. In the latest twist, the Rice team has applied carbon nanotube-derived graphene nanoribbons to energy storage, and the result is a promising new platform for creating more durable, lightweight and efficient lithium-ion batteries.
How To Make A Graphene Nanoribbon
Before graphene (aka “the strongest material in the world”) came along, carbon nanotubes were the next big thing in advanced electronics, and Rice University was among the institutions looking for efficient, low cost ways to manufacture them in bulk.
In 2009 a research team at the university came up with a kind of low cost “carbon nanotube stew” based on common fluid-based industrial processes. That same year, another team developed a low cost method for “unzipping” carbon nanotubes — basically, slicing them straight down the middle — to create graphene nanoribbons.
The process involved splitting open a carbon nanotube by exposing it to sulfuric acid and potassium permanganate at room temperature. The result is graphene, a sheet of graphite only one atom thick with powerful conductive properties and a strength estimated at 200 times that of steel.
The graphene sheets resulting from this process are water-soluble and can be painted onto a surface using standard fabrication methods.
Applying Graphene Nanoribbons to Lithium-Ion Batteries
Despite its great promise for next-generation electronics and energy storage, graphene faces an enormous stumbling block in terms of cost-effective, commercial scale manufacturing, so the combination of low cost carbon nanotube production with low cost graphene production has given Rice a running start on the next step, commercial production of a competitively priced, next-generation lithium-ion battery.
For the latest experiments, the Rice team created graphene nanoribbons from carbon nanotubes using a sodium/potassium solution. They used that to make a “slurry” composed of graphene nanoribbons along with nanoscale particles of tin oxide and a bit of water, all bound together with cellulose gum, a common food additive.
As reported in the current issue of the American Chemical Society’s ACS Nano journal, when the researchers applied the slurry to the anode of small “button-style” lithium-ion batteries the results were promising.
After 50 discharge cycles, the batteries retained far more capacity — more than double — than Li-ion batteries using standard graphite anodes.
Mike Williams of Rice University runs down the numbers:
“Lab tests showed initial charge capacities of more than 1,520 milliamp hours per gram (mAh/g). Over repeated charge-discharge cycles, the material settled into a solid 825 mAh/g.”
That could be just the beginning. According to Williams, lead researcher Jian Lin is confident that the new battery could handle “many more” cycles without a significant loss of capacity.
Part of the reason for improved durability is the increased flexibility that graphene nanoribbons lend to the anode. Conventional Li-ion batteries use silicon and other materials that break down and lose efficiency. With a graphene nanoribbon platform the tin oxide particles maintain a consistent size, rather than expanding and contracting.
The Future Of Energy Storage
We’ve noted before that sulfur-lithium technology is emerging as a promising alternative to conventional lithium-ion batteries, but as the Rice project demonstrates there is still vast room for improvement without sulfur. Don’t look for that “affordable” Tesla to make the $20,000 range any time soon (last we heard, $40,000 was the mark), but a significant drop in the cost of advanced batteries will make a huge difference in the up-front cost of owning an electric vehicle.
The development of advanced energy storage also has vast implications for national security as well as domestic well-being. As hammered home with great regularity by the Department of the Navy among others, the global petroleum market is an increasingly volatile and risky place to bank on the future. Intermittent sources like wind and solar are promising, domestically sourced alternatives, but mainstreaming them depends on pushing energy storage into utility-scale territory.
That explains why both the Office of Naval Research and the Air Force Office of Scientific Research provided support for the new Rice project along with Sandia National Laboratory and Boeing.
The Navy and Air Force research offices are also behind a plethora of funding for other graphene projects, including a Navy-funded graphene nanoribbon project aimed at exploiting the terahertz band.