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Lithium-ion has been the dominant battery technology since the 1990s, when it made its consumer debut with the Sony Walkman. Now, the technology underpins the modern world – it’s in laptop batteries, cars and phones. But lithium is expensive, and there are concerns over the damage its extraction may cause to the environment in certain areas. For years, scientists have been looking for alternatives to lithium to use in batteries, anticipating price spikes and shortages as the move to electric vehicles increases demand for the silvery metal – which is only found in an easily extractable form in a few areas of the planet.
Sodium is one candidate – it’s abundant and cheap, but at present sodium-ion batteries can’t compete with lithium-ion cells. One limiting factor has been the graphite used in the anode – sodium ions are bigger than lithium ones and can’t move in and out of the graphite layers of the anode as easily. But researchers at Chalmers University of Technology, Sweden, have come up with a promising solution.
“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 researcher Jinhua Sun at the Department of Industrial and Materials Science at Chalmers and first author of a new paper, published in Science Advances.
The new form of graphene improves the capacity for sodium intercalation by nearly tenfold, bringing it closer to that of lithium, with full reversibility and stability. “It was really exciting when we observed the sodium-ion intercalation with such high capacity. 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,” says Professor Aleksandar Matic at the Department of Physics at Chalmers.
The novel graphene is called Janus graphene, after the Roman god Janus – the two-faced deity associated with new beginnings, doors and gates, and the first steps of a journey.
It takes its name from the fact that it’s asymmetric, with different chemical functionalisation on opposite sides. It has a unique artificial nanostructure – on the upper face of each graphene sheet is a molecule that acts as a spacer, and an active interaction site for 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, Affiliated Professor at the Department of Industrial and Materials Science at Chalmers.
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