Janus graphene could boost capacity in sodium-ion batteries

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Researchers at Chalmers University of Technology in Sweden have presented a new concept to fabricate high-performance electrode materials for sodium batteries. The results show that the capacity could match lithium-ion batteries.

The ions intercalate in the graphite, which means that they can move in and out of the graphene layers and be stored for energy usage. Sodium ions are larger than lithium ions and cannot be efficiently stored in the graphite structure. The Chalmers researchers have come up with a novel way to solve this.   

“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,” says Jinhua Sun, researcher at Chalmers.

The capacity of sodium intercalation in standard graphite is typically about 35 milliampere hours per gram (mAh/g). This is less than one tenth of the capacity for lithium-ion intercalation in graphite. With the novel graphene, the specific capacity for sodium ions is 332 mAh/g, approaching the value for lithium in graphite. The results also showed full reversibility and high cycling stability.

The novel graphene has asymmetric chemical functionalization on opposite faces, and is therefore called Janus graphene, after the two-faced ancient Roman God Janus. The upper face of each 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 have uniform pore size, controllable functionalization density, and few edges.

The article “Real-time imaging of Na+ reversible intercalation in “Janus” graphene stacks for battery applications” summarizing the research was published in Science Advances.

Source: Chalmers Institute of Technology

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