Graphene boosts sodium-ion battery 10x

Researchers from Chalmers University of Technology in Sweden have recently devised a method of using a novel form of the wonder material graphene in sodium-ion batteries, increasing their sustainability and capability, tenfold.

Much like today's lithium-ion, sodium-ion batteries function by generating power by shuttling ions between a pair of electrodes in a liquid electrolyte, yet their performance has always left something to be desired.

Part of their limitation is due to the larger size of sodium ions. Compared to lithium ions, sodium ions do not gel well enough with graphite electrodes, the most widely used material in battery anodes.

Considered to be a perfect material for anodes – the negative side of a battery – graphite has dominated the market since the birth of lithium-ion batteries, benefitting from its incomparable balance of relatively low-cost, vast abundance, high energy density, power density, and long cycle life.

As graphene is just a single layer of graphite, Chalmers scientists have found that a novel form of the 2D material, dubbed Janus graphene after the Roman god with two faces, features molecules on only one side that acts as both spacers and active interaction sites for sodium ions.

In a typical lithium-ion battery, ions 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 this structure. This significantly obstructs their performance, resulting in a capacity of around 35 milliampere hours per gram, only about a tenth of the 350 mAh/g capacity offered by lithium-ion chemistry.

"We have added a molecule spacer on one side of the graphene layer," explained Jinhua Sun, a research member at Chalmers. "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 the novel Janus graphene instead of graphite, the scientists were able to achieve a capacity of roughly 332 mAh/g in their experimental sodium battery, increasing the capability nearly tenfold and approaching the capacity of its lithium counterpart.

"It was really exciting when we observed the sodium-ion intercalation with such high capacity," said Professor Aleksander Matic, author of the study paper published in "Science Advances. "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 (lithium)."