Glass coated cathode promises lithium-sulfur battery performance gains

Lithium-sulfur batteries have shown to be capable of producing up to 10 times more energy than conventional batteries, which makes them ideal for applications in energy-demanding electric vehicles. But commercializing sulfur batteries has proved difficult to achieve with one of the main problems being the tendency for lithium and sulfur reaction products, called lithium polysulfides, to dissolve in the batterys electrolyte and travel to the opposite electrode permanently. The result causes the batterys capacity to decrease over its lifetime.

The scientises have been investigating a strategy to prevent the 'polysulfide shuttling' phenomenon by creating nano-sized sulfur particles, and coating them in silica (SiO2), otherwise known as glass.

The work is in a reported paper entitled 'SiO2 Coated Sulfur Particles as a Cathode Material for Lithium-Sulfur Batteries' and published online in the journal Nanoscale.

The researchers have been working on designing a cathode material in which silica cages 'trap' polysulfides having a thin shell of silica, and the particles polysulfide products now face a trapping barrier a glass cage. The team used an organic precursor to construct the trapping barrier.

Our biggest challenge was to optimize the process to deposit SiO2 not too thick, not too thin, about the thickness of a virus, said Mihri Ozkan.

Graduate students Brennan Campbell, Jeffrey Bell, Hamed Hosseini Bay, Zachary Favors, and Robert Ionescu found that silica-caged sulfur particles provided higher battery performance, but felt further improvement was necessary because of the challenge with the breakage of the SiO2 shell.

We have decided to incorporate mildly reduced graphene oxide (mrGO), a close relative of graphene, as a conductive additive in cathode material design, to provide mechanical stability to the glass caged structures, said Cengiz Ozkan.

The new generation cathode provided more improvement than the first design, since the team engineered both a polysulfide-trapping barrier and a flexible graphene oxide blanket that harnesses the sulfur and silica together during cycling.

The design of the core-shell structure essentially builds in the functionality of polysulfide surface-adsorption from the silica shell, even if the shell breaks, explained Brennan Campbell. Incorporation of mrGO serves the system well in holding the polysulfide traps in place. Sulfur is similar to oxygen in its reactivity and energy yet still comes with physical challenges, and our new cathode design allows sulfur to expand and contract, and be harnessed.