Ultra-thin ribbons made of vanadium oxide coated with graphene can provide a superior material for lithium-ion battery cathodes, according to new research at Rice University in Houston. The team led by Rice materials scientist P. M. Ajayan published its findings online in a recent issue of the journal Nano Letters (paid subscription required).
Graphene is a form of carbon, one atom thick, structured with its atoms in a hexangonal lattice like a chicken wire. The material is very light, strong, chemically stable, and can conduct both heat and electricity, with applications in fields such as electronics, energy, and health care. Vandium oxide (VO2) is inexpensive and has the ability to increase the storage capacity of batteries, but has low electrical conductivity and is slow to charge and discharge.
The Rice researchers devised a process for coating vanadium oxide with graphene, thus combining the electrical conductivity of graphene with the storage properties of vanadium oxide. As explained by lead author Shubin Yang, the process suspends raw materials graphene oxide in sheets and powdered vanadium pentoxide in water, then heating it for hours. This process reduces the vanadium pentoxide to crystallized VO2, while the graphene oxide was reduced to pure graphene.
The result was ribbons about 10 nanometers thick and up to 600 nanometers wide; 1 nanometer equals 1 billionth of a meter. “These ribbons were the building blocks of the three-dimensional architecture,” says Yang. “This unique structure was favorable for the ultrafast diffusion of both lithium ions and electrons during charge and discharge processes.”
Lab tests of the ribbons built into cathodes showed their lithium storage capacity remains stable after 200 cycles, even at temperatures exceeding 167 degrees F, where other cathode materials decay, even at lower charge-discharge rates. The researchers found the test cathodes fully charge and discharge in 20 seconds and retain more than 90 percent of their initial capacity after more than 1,000 cycles.
“We think this is real progress in the development of cathode materials for high-power lithium-ion batteries,” says Ajayan. “This is the direction battery research is going, not only for something with high energy density but also high power density.”
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