New breakthrough in sodium batteries: Double-sided graphene increases capacity by more than 10 times

Whether it’s made into a special foam that acts as a heat switch or mixed with ceramics to form a super-strong electrolyte, graphene is shaping the future of battery technology in some interesting ways. Now, scientists from Sweden have applied this wonder material to a sustainable sodium battery, increasing the capacity by more than ten times over traditional sodium batteries.

In their quest to improve and create battery designs, scientists have widely looked to Earth-abundant sodium as an excellent alternative to today's lithium-ion batteries. These sodium-ion batteries function much like today's lithium-ion batteries, generating electricity by shuttling ions between a pair of electrodes in a liquid electrolyte, but their performance has so far been less than satisfactory.

Part of the problem is that sodium ions are larger than lithium ions, so they don't integrate well with graphite electrodes, which are made up of stacked layers of graphene. Normally, as the battery cycles, ions will move freely in and out of the graphite electrode, a process called intercalation, but the larger sodium ions can't be stored effectively within the structure. This greatly hinders the performance of sodium-ion batteries, giving them a capacity of about 35 mAh/g, one-tenth that of lithium-ion chemistry.

To find a solution to this problem, scientists at Chalmers University of Technology turned to a new type of graphene with special properties. Named after a Roman god known for having two faces, the team's Janus graphene features molecules with only one side that act as both a spacer and an active interaction site for sodium ions.

We’ve seen this kind of thinking applied before to so-called Janus particles, for example, allowing a sphere to both attract and repel water. In this case, molecules found on only one face of the graphene material promoted electrostatic interactions between the stacked sheets while also creating more space between them, which the team found led to huge gains in capacity.

"We added a molecular spacer on one side of the graphene layers," explained team member Jinhua Sun. "When the layers are stacked together, the molecule creates more space between the graphene sheets and provides an interaction point, which leads to a significant increase in capacity."

By using their new Janus graphene instead of graphite, the scientists achieved a capacity of 332 mAh/g in their experimental sodium battery, which is about 10 times higher than conventional designs and close to the capacity of lithium in graphite.