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Nano 'sandwich' makes a better battery electrode

17 April 2015

Research team finds silicon carbonitride-wrapped molybdenum disulphide sheets make more stable electrodes for rechargeable lithium-ion batteries.

Image courtesy of the researchers/Kansas State University

Gurpreet Singh, an assistant professor of mechanical and nuclear engineering at Kansas State University, and his research team observed that molybdenum disulphide sheets store more than twice as much lithium — or charge — than bulk molybdenum disulphide reported in previous studies. The researchers also found that the high lithium capacity of these sheets does not last long and drops after five charging cycles.

"This kind of behaviour is similar to a lithium-sulphur type of battery, which uses sulphur as one of its electrodes," says Singh. "sulphur is notoriously famous for forming intermediate polysulphides that dissolve in the organic electrolyte of the battery, which leads to capacity fading. We believe that the capacity drop observed in molybdenum disulphide sheets is also due to loss of sulphur into the electrolyte."

To reduce the dissolution of sulphur-based products into the electrolyte, the researchers wrapped the molybdenum disulphide sheets with a few layers of silicon carbonitride (SiCN). This ceramic is a high-temperature, glassy material prepared by heating liquid silicon-based polymers and has much higher chemical resistance toward the liquid electrolyte.

"The silicon carbonitride-wrapped molybdenum disulphide sheets show stable cycling of lithium-ions irrespective of whether the battery electrode is on copper foil-traditional method or as a self-supporting flexible paper as in bendable batteries," says Singh.

After the reactions, the research team also dissembled and observed the cells under the electron microscope, which provided evidence that the silicon carbonitride protected against mechanical and chemical degradation with liquid organic electrolyte. The team now intends to test the molybdenum disulphide cells during multiple recharging cycles.

In other research, the team has shown that when silicon carbonitride and boron nitride nanosheets are combined, they have high temperature stability and improved electrical conductivity. Additionally, these silicon carbonitride/boron nitride nanosheets make better battery electrodes.

"This was quite surprising because both silicon carbonitride and boron nitride are insulators and have little reversible capacity for lithium-ions," says Singh. "Further analysis showed that the electrical conductivity improved because of the formation of a percolation network of carbon atoms known as 'free carbon' that is present in the silicon carbonitride ceramic phase. This occurs only when boron nitride sheets are added to silicon carbonitride precursor in its liquid polymeric phase before curing is achieved."

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