Researchers at Rice University believe a hybrid material they have developed combining vanadium oxide (VO2) and graphene could revitalize the use of lithium-ion (Li-ion) batteries for powering all-electric vehicles.
While Li-ion batteries for hybrid vehicles have enabled that car segment to grow rapidly over the years, the all-electric vehicle has languished as a niche market. This is in large part because Li-ion batteries just don’t have the charge life or short recharging capabilities for them to make sense for most people’s driving habits. The demise of companies that have developed nanomaterials for Li-ion batteries in all-electric vehicles, like A123 Systems and Ener1, underscores just how difficult it has been to get Li-ion batteries to perform at levels necessary to make electric vehicles to take a stronger foothold in the market.
To address this shortcoming, Pulickel Ajayan, professor of engineering at Rice, and his team turned to the well-characterized use of VO2 for cathodes because of their high energy and power density. While vanadium pentoxide has been used in Li-ion batteries, oxides have not been so readily adopted because they have a low electrical conductivity that translates into slow charge and discharge rates.
Ajayan and his team overcame this problem by essentially baking graphene into the VO2, a process that imparted graphene’s high electrical conductivity into the ribbon-like hybrid material that makes up the cathodes. The graphene is able to pass its conductivity to the hybrid material even though the VO2 accounts for 84 percent of the cathode’s overall weight.
The challenge for the researchers was finding the right method for "baking" the graphene into the VO2. In a process described in the journal Nano Letters, the researchers suspended graphene oxide nanosheets along with vanadium pentoxide in water and then heated the suspension for hours in an autoclave. The result was that the vanadium pentoxide had been reduced into vanadium oxide and had taken the form of crystallized ribbons, and the graphene oxide had been reduced to graphene. When characterized, the VO2 ribbons had a web-like coating of graphene and were about 10 nanometers thick, 600 nanometers wide, and tens of micrometers in length.
"These ribbons were the building blocks of the three-dimensional architecture," said Shubin Yang, lead author of the research, in a press release. "This unique structure was favorable for the ultrafast diffusion of both lithium ions and electrons during charge and discharge processes. It was the key to the achievement of excellent electrochemical performance."
As far as performance, the cathodes are capable of holding 204 milliamp hours of energy per gram and remained stable after 200 cycles even at high temperatures (75 degrees Celsius).
"We think this is real progress in the development of cathode materials for high-power lithium-ion batteries," Ajayan said in the press release. "This is the direction battery research is going, not only for something with high energy density but also high power density. It’s somewhere between a battery and a supercapacitor."
Image: Rice University/Ajayan Group