Graphene-coated ribbons of vanadium oxide, seen in a scanning electron microscope image, show promise as electrode for lithium-ion batteries, according to researchers at Rice University. (Credit: Ajayan Group/Rice University)
Last week I told you about some fascinating work coming out of the University of Stuttgart on extremely flexible vanadium pentoxide paper like material. Researchers are interested in the material for battery electrode and supercapacitor applications.
Researchers at Rice University also are looking at vanadium oxide for lithium battery electrode applications and recently reported on VO2-graphene hybrid ribbons for cathodes in an article in Nano Letters published by the American Chemical Society (subscription required).
The work comes out of Pulickel Ajayan’s group. Ajayan is a professor in the Mechanical Engineering and Materials Science Department and in the Chemistry Department and is known for his creative thinking about batteries. Last summer, for example, we told you about his work on paintable batteries.
According to the paper’s abstract, although lithium-ion batteries have high energy density, their full potential in applications is not yet realized because “they lack suitable electrodes capable of rapid charging and discharging to enable a high power density critical for broad applications.” In a press release, Ajayan says that vanadium oxide has long interested the battery research community and that vanadium pentoxide has been used in some Li-ion batteries. However, he points out that oxides generally charge and discharge slowly because their electrical conductivities are low.
Ajayan’s group addressed the slow charge-discharge problem by “baking” high-conductivity graphene on VO2 ribbons. The graphene forms a web like coating on the ribbons and serves as a “speedy conduit for electrons and channels for ions.”
The team reports promising results. Half-cell tests show that the cathodes fully charge and discharge in 20 seconds and retain 90 percent of their initial charge capacity even after 1,000 cycles. The team says their best cathode samples were up to 84 weight percent “lithium-slurping” VO2 and held 204 milliamp hours of energy per gram. They also appear to be highly stable. The press release reports the “capacity for lithium storage remained stable after 200 cycles,” even at high temperatures regimes above 75°C, where the effectiveness of other cathode materials tends to attenuate. (I’m not sure how or whether the insulator-to-metal phase transition that VO2 undergoes at 67°C is a factor. I’ll update this post when I find out.)
The ribbons are made in a simple-sounding hydrothermal process, but in the press release Subin Yang, lead author of the paper, admits, “One challenge to production was controlling the conditions for the co-synthesis of VO2 ribbons with graphene.” They make the hybrid ribbons by heating a water suspension of graphene oxide nanosheets and vanadium pentoxide powders for hours in an autoclave. The V2O5 reduces completely to VO2 and crystallizes into ribbon like structures that are 10 nanometers thick, up to 600 nanometers wide, and tens of micrometers long. Meanwhile, the graphene oxide reduces into graphene and forms a web like coating on the ribbons.
Ajayan thinks this hybrid material could be used in the paintable batteries his team is working on, too.
Full details are in the paper, “Bottom-up approach toward single-crystalline VO2-graphene ribbons as cathodes for ultrafast lithium storage,” Shubin Yang, Yongji Gong, Zheng Liu, Liang Zhan, Daniel P. Hashim, Lulu Ma, Robert Vajtai, and Pulickel M. Ajayan, Nano Letters, DOI: 10.1021/nl400001u.