The need for rechargeable batteries is growing fast, but at the same time there’s a global shortage of lithium, the key raw material. A joint research group from Helmholtz Centre (HZB) and Humboldt University of Berlin has therefore embarked on a project to bring sodium-ion battery technology on to the point where it could be a substitute for lithium-ion.

"In contrast to lithium-ion batteries, which are based on the storage of lithium ions in the positive and negative electrodes of the battery, we work with sodium ions, as they also occur in cheap table salt,” says Professor Philipp Adelhelm, who heads the jointly-founded research group Operando Battery Analysis, in a press release.

This is achieved by the “co-intercalation” of sodium Na+ ions. Intercalation is a method commonly used in battery technology whereby material properties are altered by inserting ions into layered electrode materials. During discharging, the Na+ ion moves from the anode (negative) and enters the cathode (positive) through the electrolyte. During charging, the opposite happens.

In this case, co-intercalation is where Na+ ions are inserted into the layers or lattice together with their “solvation shell” which, says Adelhelm, “makes it possible to realise completely new storage reactions.” Until now, this was only achievable at the anode (made of graphite), but Adelhelm's team has succeeded in extending the concept to the cathode made with titanium disulphide (TiS2).

"We are just starting to understand co-intercalation batteries," adds HZB’s Dr Katherine A. Mazzio. "The process could improve efficiency by enabling better performance at low temperatures. It could also be used to improve alternative cell designs, such as using multivalent ions instead of storing Li+ or Na+, which are particularly sensitive to the solvation shell.”

The project was funded by the European Research Council under the EU’s Horizon 2020 programme.