October 5, 2011 — Stanford researchers, led by Yi Cui, an associate professor of materials science and engineering, fabricated electrode materials that significantly increase lithium-ion battery energy storage capacity. Sulfer-coated hollow carbon nanofibers, and an electrolyte additive, improved the battery cathode. In previous research (2007), Cui’s group fabricated battery anodes with silicon nanowires. Cui envisions silicon nanowire anodes and sulfur-coated carbon cathodes combined in next-generation batteries.
Sulfur offers "10x higher charge storage capacity," Cui explains, "with about half the voltage of the existing battery. Higher charge capacity and lower voltage results in a lithium-ion battery with 4-5x the energy storage capability of today’s Li-ion products.
Prior attempts at incorporating sulfur — which is low cost and nontoxic — have failed to produce commercially viable products. Lithium-sulfur batteries fail quickly when cycling through charging. Sulfer has conventionally coated open carbon structures, exposed to the battery’s electrolyte. Intermediate reaction products, lithium polysulfides, dissolve into the electrolyte solution and reduce energy density.
Cui’s team developed a cathode fabrication that they expect will avoid these issues. "On the one side we don’t want a large surface area contacting the sulfur and the electrolyte," said graduate student Wesley Guangyuan Zheng, "on the other hand we want a large surface area for electrical and ionic conductivities." In this work, sulfur coats the inside of a hollow carbon nanofiber, protected from the outside. This was achieved with a commercially available filer technology.
The nearly closed cathode design prevents polysulfides from significantly leaking out into the electrolyte solution. The length of a hollow nanofiber is about 300x its diameter, containing polysulfides thanks to the long, skinny shape.
Graduate student Yuan Yang then included an electrolyte additive that enhances the battery’s charge and energy efficiency, known as the coulombic efficiency. "Without the additive you put 100 electrons into the battery and you get 85 out. With the additive, you get 99 out," Cui said.
Judy J. Cha of the Stanford Department of Materials Science and Engineering and Seung Sae Hong of the Stanford Department of Applied Physics also contributed to this research.
The results were published online Sept. 14 in the journal Nano Letters: Access "Hollow Carbon Nanofiber-Encapsulated Sulfur Cathodes for High Specific Capacity Rechargeable Lithium Batteries," Nano Letters, Sept. 14, 2011, at http://pubs.acs.org/doi/abs/10.1021/nl2027684.
Courtesy of Sarah Jane Keller, science-writing intern at Stanford News Service. Learn more at www.stanford.edu.