Two-faced graphene offers sodium-ion battery a tenfold boost in capacity
Whether it's forming special foams that act as thermal switches, mixing with ceramics to form ultra-tough electrolytes or being used to cage delicate silicon particles, graphene is already shaping the future of battery technology in some interesting ways. Scientists in Sweden have now deployed a novel form of the wonder material in a sustainable sodium battery, and shown how it can be used to bring about a tenfold increase in capacity.
Scientists searching for new and improved battery designs are drawn towards widely abundant sodium as a replacement for expensive and difficult-to-source lithium. These sodium-ion batteries would function much like today's lithium-ion batteries, generating power by shuttling ions between a pair of electrodes in a liquid electrolyte, but as it stands their performance isn't quite up to scratch.
Part of the reason for this is because of the larger size of sodium ions, as compared to lithium ions, so they don't gel so well with the graphite electrodes that consist of stacked layers of graphene. Normally, ions would freely move in and out of the graphite electrode as the battery is cycled in a process known as intercalation, but the bulkier sodium ions are unable to be efficiently stored within the structure. This significantly hampers the sodium ion battery's performance and affords it a capacity of around 35 mAh/g, a tenth of what is offered by lithium-ion chemistry.
In search of a solution to this problem, scientists from Chalmers University of Technology turned to a novel form graphene with peculiar properties. Named after a Roman god famed for having two faces, the team's Janus graphene features molecules on only one side that act as both spacers and active interaction sites for sodium ions.
We've seen this thinking applied to so-called Janus particles before, allowing for spheres that both attract and repel water, for example. In this case, the molecules found only on one face of the graphene material facilitate electrostatic interactions between the stacked sheets while also creating more space between them, which the team found brought about great gains in capacity.
“We have added a molecule spacer on one side of the graphene layer," explains team member Jinhua Sun. "When the layers are stacked together, the molecule creates larger space between graphene sheets and provides an interaction point, which leads to a significantly higher capacity."
By using their novel Janus graphene instead of graphite, the scientists achieved a capacity of 332 mAh/g in their experimental sodium battery, which is around 10 times higher than conventional designs and approaching the capacity for lithium in graphite.
“It was really exciting when we observed the sodium-ion intercalation with such high capacity," says study author Professor Aleksandar Matic. "The research is still at an early stage, but the results are very promising. This shows that it’s possible to design graphene layers in an ordered structure that suits sodium-ions, making it comparable to graphite."
The research was published in the journal Science Advances.