“Representation of a molybdenum disulfide (MoS2) sheet covered with SiCN ceramic”
“Representation of a molybdenum disulfide (MoS2) sheet covered with SiCN ceramic”

Researchers from Kansas State University have shown how miniature ‘sandwiches’ comprised of nanosheets could help improve common rechargeable lithium-ion batteries used in cellphones and other rechargeable electronics.

The team was exploring the lithium cycling of molybdenum disulfide (MoS2) sheets, where one molybdenum atom is sandwiched between two sulfur atoms. In their study, published in Scientific Reports [David et al. Sci. Rep. (2015) DOI: 10.1038/srep09792], silicon carbonitride-wrapped molybdenum disulfide sheets demonstrated improved stability as a battery electrode with little capacity fading, and able to store over double as much lithium (or charge) than bulk molybdenum disulfide shown in other studies.

Sulfur is well known for forming intermediate polysulfides that dissolve in the organic electrolyte of the battery, leading to capacity fading. This study demonstrated that the capacity drop in the molybdenum disulfide sheets could also be due to loss of sulfur into the electrolyte. To reduce this dissolution, wrapping the sheets in a few layers of the high-temperature ceramic silicon carbonitride, which is produced by heating liquid silicon-based polymers, offers much higher chemical resistance toward the liquid electrolyte. Once the reactions had taken place, the cells showed that the silicon carbonitride protected against mechanical and chemical degradation with liquid organic electrolyte.

The team had previously demonstrated that exfoliated sheets of tungsten disulfide (which has a similar structure to molybdenum disulfide) can store more than twice the amount of charge (or lithium) capacity than their bulk crystals. However, such large lithium capacity is short-lived as it starts to react irreversibly with the organic electrolyte identified as the main reason for capacity fading

As team leader Gurpreet 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.” The self-standing paper electrode is therefore useful for lightweight batteries, especially as the glass-like coating of the silicon carbonitride is known for its high chemical and thermal stability. It allows diffusion of lithium ions through it to reversibly react with molybdenum disulfide, but protects the reaction of molybdenum disulfide with the organic electrolyte during successive cycling.

There are interesting potential applications for the work in a range of innovative consumer electronics, and the team now hope to test the stability of the electrode material to improve our understanding of how the molybdenum disulfide cells would act in electronic devices that are recharged hundreds of times.