Researchers at Chalmers University of Technology have pushed the performance of electrode material for sodium batteries so it matches lithium-ion batteries.
Using a novel graphene, the team at the Swedish institute reported the specific capacity for sodium ions was 332 milliampere-hours-per-gram— almost ten times that of the capacity of sodium intercalation in standard graphite.
The article “Real-time imaging of Na+ reversible intercalation in “Janus” graphene stacks for battery applications” was published in the journal Science Advances.
Sodium ions are larger than lithium ions and interact differently, which means they cannot be efficiently stored in the graphite structure— unlike lithium-ion cells where the graphite anode allows better ion intercalation.
Chalmers’ research uses Janus graphene (named after the two-faced ancient Roman God Janus) due to its asymmetric chemical functionalisation on opposite faces of the graphene.
The upper face of each Janus graphene sheet has a molecule that acts as both spacer and active interaction site for the sodium ions.
Each molecule, in between two stacked graphene sheets, is connected by a covalent bond to the lower graphene sheet and interacts through electrostatic interactions with the upper graphene sheet.
The graphene layers also have uniform pore size, controllable functionalisation density, and few edges.
Jinhua Sun, from the Department of Industrial and Materials Science at Chalmers and first author of the scientific paper, said by adding the molecule spacer when the layers were stacked together, the molecule creates a larger space between graphene sheets and provides an interaction point— which leads to a significantly higher capacity.
Vincenzo Palermo, affiliated professor at the Department of Industrial and Materials Science at Chalmers, said: “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.”