Janus Graphene: Asymmetric, Two-Faced Material Could Turn Sodium-Ion Batteries Into Sustainable Energy Storage

A new nanomaterial, called the Janus Graphene, could further improve sodium-ion batteries as a potentially sustainable energy storage solution.

Scientists from Chalmers University of Technology in Sweden present their novel idea in creating high-performance electrode materials for sodium-ion batteries. Using a specially-engineered form of graphene, one that is asymmetric and has two uneven faces, it can store metal ions with capacities that match modern lithium-ion batteries.

Details about the use of the so-called Janus Graphene in improving sodium-ion batteries are described in the Science Advances report "Real-Time Imaging of Na Reversible Intercalation in "Janus" Graphene Stacks for Battery Applications," published August 25.

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Finding a Cost-Effective Alternative for Lithium-Ion Batteries

Today, lithium-ion batteries are among the best energy storage solutions available. However, lithium itself is an expensive metal, raising concerns about long-term supply and the environmental issues that come with sourcing the material. On the other hand, sodium used in sodium-ion batteries is relatively cheap, owing to the reactive metal's abundance - being the sixth most abundant element on Earth, according to PubChem. Furthermore, sodium is the main component of seawater and salt. These advantages have made sodium-ion batteries an interesting alternative, especially in terms of cost and sustainability. The remaining challenge for utilizing sodium, however, is in improving its energy capacity.

Currently, lithium-ion batteries far outclass their sodium-ion counterparts. One of the promising areas of improvement for the energy storage solution is in the use of graphite, which is composed of stacked layers of graphene used as the anode material in lithium-ion batteries. Ions intercalate, or pass in between layers, of graphene and make energy storage possible. However, sodium ions are significantly larger than lithium ions, making them react differently in relation to a graphite anode.

The new technology works around this problem by adding a "molecule spacer" on one side of graphene. As the layers stack together, the spacers also cumulatively create a larger space and provide an interaction point, improving the battery's capacity, according to Jinhua Sun, first author of the paper and a researcher at Chalmers' Department of Industrial and Materials Science.

After applying the novel solution, researchers were able to increase the capacity of sodium intercalation from 35 milliampere hours per gram (mAh/g) in the conventional graphite to 332 mAh/g with the novel graphene - an increase of about ten times.

Made Possible With the Janus Graphene

The novel graphene that could significantly improve sodium-ion batteries has an asymmetric chemical function on each of its faces. It is named the Janus graphene, after the two-faced Roman god Janus - the god of beginnings and commonly associated with passageways. Researchers find the name additionally fitting, as the Janus graphene could open new doors for the future of sodium-ion batteries as sustainable energy storage solutions.

Vincenzo Palermo, Affiliated Professor at the Department of Industrial and Materials Science at Chalmers who initiated the Janus graphene project, explains in a Chalmers news release that their 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."