Lithium-sulfur batteries could deliver five times the energy of a lithium-ion battery of the same weight. However, side reactions

alter the sulfur and carbon cathode, introducing efficiency and capacity loss that challenge Li-S battery development. Now researchers have placed a nanostructured brush-like interlayer atop the cathode to capture the molecular products of the side reactions, appreciably slowing the capacity loss that typically happens during successive charging cycles. The work is published in Advanced Functional Materials.

During a Li-S battery charging cycle, lithium ions leave the anode and nestle into the sulfur in the cathode. Long chains of lithium and sulfur form at the cathode and dissolve in the organic liquid electrolyte, irreversibly removing active material from the electrode and causing the battery to lose capacity over time. The dissolved polysulfides can also travel between the electrodes, performing an additional redox reaction that lowers the Coulombic efficiency of the cell. Finally, dissolved polysulfides can form short chain polysulfides at the Li metal anode, which then deposit on the surface of the anode.

To physically trap the polysulfides at the cathode, researchers have added porous materials such as ceramics, graphene derivatives, or metal oxide nanoparticles to the electrode material. Teng Zhao and R. Vasant Kumar at the University of Cambridge, Renjie Chen at the Beijing Institute of Technology, and their colleagues wanted to build a cathode using a nanostructured material that chemically bonds the polysulfides, as well as physically traps them.

Inspired by villi, the nutrient-gathering fingers on the membrane of intestinal cells, the researchers synthesized zinc oxide nanowires on a nickel foam mat. The oxygen atoms in the nanowires bind to lithium in the polysulfides. This binding also keeps the polysulfides electrically connected to the cathode, says Paul Coxon, at the University of Cambridge and one of the researchers involved with the study.

The researchers placed this modified foam mat atop a sulfur-doped cathode made from multi-walled carbon nanotubes, using it as an interlayer between the cathode and the cell separator in a coin cell. After 500 charging cycles, the cell retained 81% of its capacity. When the researchers examined the interlayer using x-ray photoelectron spectroscopy, they found four different sulfur-containing species there.

The researchers also synthesized the nanowires on a cheaper, flexible, and lighter carbon nanofiber mat instead of the nickel foam, and again they built a coin cell using this nanostructured interlayer. After fully discharging its capacity within one hour for 200 charging cycles, the cell’s capacity was 776 mA h g^-1, averaging 0.05% capacity loss per cycle.

Jie Xiao, at the University of Arkansas, likes that the high sulfur loading of the cathode in these cells, 3 mg cm^-2, is closer to realistic conditions than many Li-S batteries being tested, which she says could make this approach practical for commercial Li-S batteries.

Read the abstract in Advanced Functional Materials.