Lithium-ion batteries are a cornerstone of our technology due to their energy storage capabilities. But lithium itself is not that great when one considers the environmental and monetary costs of its production. A better alternative would be sodium, which is abundant on Earth but making sodium batteries is not without its difficulties. However, one of those might have been sorted.

Researchers from Chalmers University of Technology have been able to build high-performance electrode materials for sodium batteries with a special type of graphene. The battery shows energy capacity very close to what can be found in standard lithium batteries. The breakthrough is published in Science Advances.

In lithium batteries, the anode is made of graphite, the material in a pencil lead, which is layer upon layer of graphene. Lithium ions are small enough to move in and out of graphene layers, but sodium is much chunkier so it can't use the same trick. And that’s where the Janus graphene comes in. This version of graphene can be organized in layers that allow sodium ions to flow.

“We have added a molecule spacer on one side of the graphene layer. 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,” lead author Dr. Jinhua Sun said in a statement.

The capacity of a sodium battery using standard graphite is 35 milliampere hours per gram, which is less than one-tenth of what you get from a standard lithium battery. The new approach with the Jannus graphene gets to 322 milliampere hours per gram. Still less than lithium, but much closer. And they are fully reversible and have high cycling stability.

“It was really exciting when we observed the sodium-ion intercalation with such high capacity. 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,” added Professor Aleksandar Matic at the Department of Physics at Chalmers.

The name of this graphene comes from the Roman god, Janus. Graphene is made of carbon organized in a single one-atom-thick layer. Janus is known for having two faces so here it symbolizes the fact that this layer looks different on one side due to the molecule spacer. Janus is also the god of doors, and the team liked the idea that the material will open the door to new technology.

“Our Janus material is still far from industrial applications, but the new results show that we can engineer the ultrathin graphene sheets – and the tiny space in between them – for high-capacity energy storage. We are very happy to present a concept with cost-efficient, abundant and sustainable metals,” concluded Vincenzo Palermo, Affiliated Professor at the Department of Industrial and Materials Science at Chalmers.