Hybrid nanoribbons of vanadium oxide and graphene could jump-start the development of advanced lithium-ion (Li-ion) batteries capable of powering electric cars.
Researchers from Rice University, based in Houston, USA, claim the nanoribbons (consisting of a foam-like graphene lattice filled with vanadium oxide) can be used to make high-performance cathodes that capitalise on graphene’s excellent conductivity and vanadium’s ability to store lithium.
This combination imbues the cathode with both high energy and high power density. Depending on their intended purpose, energy storage devices normally have only one of these two different properties, offering either long operation time or rapid energy dispensing.
Li-ion batteries’ high energy density makes them perfect for smartphones, but slow ion and electron diffusion has previously hindered their use in applications such as electric vehicles that require a high power density. The Rice team claims that its nanoribbons could be used to improve the design of high power Li-ion batteries, as their structure allows fast diffusion of both lithium ions and electrons.
The method makes use of the basic properties of both materials and the fact that they consist of atomically thin layers, says Dr Robert Vajtai, Faculty Fellow at Rice’s Department of Mechanical Engineering and Materials Science. ‘One of the advantageous properties of these materials is electronic. Sometimes it is catalytic, but often it’s for energy storage devices because they need a very high surface area. These materials, being only a few atoms thick, naturally have the highest surface area that you can make.’
The thin nature of the nanoribbons provides a short solid-state diffusion length for the lithium, something that Vajtai claims improves on current cathode materials. He explains that the 10nm thickness combined with the small pore size (one micron or less), ‘means that all the lithium ions that you bring into the vanadium oxide only need to travel a very short distance inside the electrolyte, increasing the speed of the device.’
Although vanadium pentoxide has previously been used for lithium-based energy storage, its initial promise as an electrode material was limited by its low electrical conductivity. Vajtai says that this was because the material was being used in macroscopic crystal form, a structure that differs dramatically in thickness and porosity. ‘Even a millimetre is really large compared to our nanostructure [which measures 10nm across], so if you have one Li-ion, it needs to travel 100,000 times further to charge or discharge that layer’.
To unlock the power of the vanadium, the team suspended graphene oxide nanosheets together with powdered vanadium pentoxide in water and heated them in an autoclave. The process involved simultaneous hydrothermal synthesis and chemical reduction, with the vanadium pentoxide reduced to ribbons of vanadium dioxide, and the graphene oxide reduced to a graphene lattice. ‘The solvent dismounts the vanadium oxide and reduces the graphene oxide and they selforganise into a layered foam system, with graphene adding mechanical stability,’ says Vajtai.
Using this relatively straightforward hydrothermal process, the method would be easy to scale up, claims Vajtai. He adds that as the nanoribbons could be dispersed in a solvent, they could be suitable for use in components in paint-on polymer batteries previously demonstrated by the Rice lab. ‘It’s a big step to go from a silicon chip to get it to provide the same qualities when you grind it down and paint it out from a solvent,’ says Vajtai. ‘But this material is already finely structured in the solution, it is also a porous structure so when you paint it out you don’t need to re-optimise the system.’