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Silica-coated sulfur nanoparticles with mildly reduced graphene oxide as Li-S battery cathode

One of the main obstacles to the commercialization of high-energy density lithium-sulfur batteries is the tendency for lithium polysulfides—the lithium and sulfur reaction products—to dissolve in the battery’s electrolyte and travel to the opposite electrode permanently. This causes the battery’s capacity to decrease over its lifetime.

To prevent this polysulfide shuttle, researchers in the Bourns College of Engineering at the University of California, Riverside have fabricated SiO2-coated sulfur particles (SCSPs) for cathode material. With the addition of mildly reduced graphene oxide (mrGO) to the material, SCSPs maintain more than 700 mAh g−1 after the 50th cycle. A paper on their work is published in the RSC journal Nanoscale.

The silica shell functions as a trapping barrier for the polysulfides; 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.

—Mihri Ozkan, co-corresponding author

Ozkan-glass
A schematic illustration of the process to synthesize silica-coated sulfur particles. Click to enlarge.

Graduate students Brennan Campbell, Jeffrey Bell, Hamed Hosseini Bay, Zachary Favors, and Robert Ionescu in Cengiz Ozkan’s and Mihri Ozkan’s research groups (earlier post) found that silica-caged sulfur particles provided a substantially 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.

—Cengiz Ozkan, co-corresponding author

The resulting cathode material combines 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. 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.

— Brennan Campbell

The work was funded by the Winston Chung Global Energy Center at UC Riverside.

Resources

  • Brennan Campbell, Jeffrey Bell, Hamed Hosseini Bay, Zachary Favors, Robert Ionescu, Cengiz S. Ozkan and Mihrimah Ozkan (2015) “SiO2-coated sulfur particles with mildly reduced graphene oxide as a cathode material for lithium–sulfur batteries” Nanoscale doi: 10.1039/C4NR07663J

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