Layered approach leads to composite paper electrode

Composite paper that can be used as a negative electrode in sodium-ion batteries has been developed at Kansas State University. 

Gurpreet Singh, assistant professor of mechanical and nuclear engineering, and his student researchers are said to be the first to demonstrate that a composite paper, made of interleaved molybdenum disulphide and graphene nanosheets, can be an active material to efficiently store sodium atoms and a flexible current collector.

Singh said in a statement: ‘Most negative electrodes for sodium-ion batteries use materials that undergo an ‘alloying’ reaction with sodium.

‘These materials can swell as much as 400 to 500 per cent as the battery is charged and discharged, which may result in mechanical damage and loss of electrical contact with the current collector.

‘Molybdenum disulphide, the major constituent of the paper electrode, offers a new kind of chemistry with sodium ions, which is a combination of intercalation and a conversion-type reaction.

‘The paper electrode offers stable charge capacity of 230 mAh.g-1, with respect to total electrode weight. Further, the interleaved and porous structure of the paper electrode offers smooth channels for sodium to diffuse in and out as the cell is charged and discharged quickly. This design also eliminates the polymeric binders and copper current collector foil used in a traditional battery electrode.’

The research appears in ACS-NANO in an article entitled MoS2/graphene composite paper for sodium-ion battery electrodes.

For the last two years the researchers have been developing new methods for quick and cost-effective synthesis of atomically thin two-dimensional materials — graphene, molybdenum and tungsten disulphide — in gram quantities, particularly for rechargeable battery applications.

For the latest research, the engineers created a large-area composite paper that consisted of acid-treated layered molybdenum disulphide and chemically modified graphene in an interleaved structured.

According to the university, the research marks the first time that such a flexible paper electrode was used in a sodium-ion battery as an anode that operates at room temperature.

Most commercial sodium-sulphur batteries operate close to 300 degrees Celsius, Singh said.

Singh said the research is important for two reasons. Synthesis of large quantities of single or few-layer-thick 2D materials is crucial to understanding the true commercial potential of materials such as transition metal dichalcogenides (TMD) and graphene.

Secondly, it furthers fundamental understanding of how sodium is stored in a layered material through mechanisms other than the conventional intercalation and alloying reaction.

In addition, using graphene as the flexible support and current collector is crucial for eliminating the copper foil and making lighter and bendable rechargeable batteries. In contrast to lithium, sodium supplies are essentially unlimited and the batteries are expected to be a lot cheaper.

The researchers are now working to commercialise the technology.