Inserting molecular spacers between the graphene layers in this novel anode material creates sufficient space for sodium ions (green). Image: Yen Strandqvist/Chalmers University of Technology.In the search for sustainable energy storage, researchers at Chalmers University of Technology in Sweden have come up with a novel concept for fabricating high-performance electrode materials. This involves using a novel type of graphene to store one of the world's most common and cheap metal ions – sodium. The results show that the energy capacity of this novel electrode material can match those used in today’s lithium-ion batteries.
Even though lithium ions work well for energy storage, lithium is an expensive metal, and there are concerns regarding long-term supply and environmental issues. Sodium, on the other hand, is an abundant, low-cost metal, and a main ingredient in seawater (and kitchen salt). This makes sodium-ion batteries an interesting and sustainable alternative technology, reducing our need for critical raw materials. However, one major challenge is increasing the energy capacity of sodium-ion batteries.
At the current level of performance, sodium-ion batteries cannot compete with lithium-ion cells. One limiting factor is the graphite anode, composed of stacked layers of graphene, used in today’s lithium-ion batteries.
Lithium ions can intercalate in graphite, which means they move in and out of the graphene layers as the battery is charged and discharged. Sodium ions are larger than lithium ions and interact differently, and so cannot be efficiently stored in the graphite structure. But the Chalmers researchers have come up with a novel solution to this problem.
“We have added a molecule spacer on one side of the graphene layer,” explains Jinhua Sun, a researcher in the Department of Industrial and Materials Science at Chalmers and first author of a paper on this work in Science Advances. “When the layers are stacked together, the molecule creates a larger space between the graphene sheets and provides an interaction point, which leads to a significantly higher capacity.”
Typically, the capacity of sodium intercalation in standard graphite is about 35 milliampere hours per gram, which is less than one tenth of the capacity for lithium-ion intercalation in graphite. In contrast, the specific capacity for sodium ions in the novel graphene-based anode is 332 milliampere hours per gram – approaching the value for lithium ions in graphite. The results also showed full reversibility and high cycling stability.
“It was really exciting when we observed the sodium-ion intercalation with such high capacity,” says Aleksandar Matic, a professor in the Department of Physics at Chalmers. “The research is still at an early stage, but the results are very promising. This shows that it’s possible to design graphene layers in an ordered structure that suits sodium ions, making it comparable to graphite.”
The graphene layers used in the anode have asymmetric chemical functionalization on opposite faces and are therefore called Janus graphene, after the two-faced ancient Roman God Janus. As a consequence, the molecules between the stacked graphene layers are connected by a covalent bond to the lower graphene sheet and interact through electrostatic interactions with the upper graphene sheet. These molecules act as both spacers and active interaction sites for the sodium ions.
“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. We are very happy to present a concept with cost-efficient, abundant and sustainable metals,” says Vincenzo Palermo, a professor in the Department of Industrial and Materials Science at Chalmers.
This story is adapted from material from Chalmers University of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.