Nano Sandwich Improves Battery Life

Friday, April 17, 2015 @ 04:04 PM gHale

A small sandwich may be the key to better cellphones and other rechargeable electronics. These “sandwiches” consist of nanosheets and they could improve rechargeable lithium-ion batteries.

The focus of the research is on the lithium cycling of molybdenum disulfide, or MoS2, sheets, which Gurpreet Singh, assistant professor of mechanical and nuclear engineering at Kansas State University describes as a “sandwich” of one molybdenum atom between two sulfur atoms.

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In his latest research, the team found silicon carbonitride-wrapped molybdenum disulfide sheets show improved stability as a battery electrode with little capacity fading.

Singh’s team observed that molybdenum disulfide sheets store more than twice as much lithium — or charge — than bulk molybdenum disulfide reported in previous studies. The researchers also found the high lithium capacity of these sheets does not last long and drops after five charging cycles.

“This kind of behavior is similar to a lithium-sulfur type of battery, which uses sulfur as one of its electrodes,” Singh said. “Sulfur is notoriously famous for forming intermediate polysulfides that dissolve in the organic electrolyte of the battery, which leads to capacity fading. We believe that the capacity drop observed in molybdenum disulfide sheets is also due to loss of sulfur into the electrolyte.”

To reduce the dissolution of sulfur-based products into the electrolyte, the researchers wrapped the molybdenum disulfide sheets with a few layers of a ceramic called silicon carbonitride, or SiCN. The ceramic is a high-temperature, glassy material prepared by heating liquid silicon-based polymers and has much higher chemical resistance toward the liquid electrolyte, Singh said.

“The silicon carbonitride-wrapped molybdenum disulfide 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,” Singh said.

After the reactions, the research team also dissembled and observed the cells under the electron microscope, which provided evidence the silicon carbonitride protected against mechanical and chemical degradation with liquid organic electrolyte.

Singh and his team now want to better understand how the molybdenum disulfide cells might behave in an everyday electronic device — such as a cellphone — that recharges hundreds of times. The researchers will continue to test the molybdenum disulfide cells during recharging cycles to have more data to analyze and to better understand how to improve rechargeable batteries.

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