In another bid to create the much-hoped-for 10X batteries, researchers at North Carolina State University are rolling their own.
As noted in many articles, lithium batteries infused with silicon have a bad habit of swelling and contracting as they charge and discharge, pulverizing the silicon eventually. Depending on the surrounding materials, the destruction can take place fairly quickly, leading to reduced cycle life for the battery.
North Carolina scientists are fighting to extend battery life, though, with what they call “A Novel Nano-architecture for Flexible Lithium Ion Battery Electrodes,” part of the title of their paper in the journal Advanced Materials.
Many battery electrodes are some form of graphite composite, and the impetus to wrap these anodes or cathodes in silicon has strong motivation. “Putting silicon into batteries can produce a huge increase in capacity—10 times greater,” Dr. Philip Bradford, assistant professor of textile engineering, chemistry and science at NC State says. “But adding silicon can also create 10 times the problems.”
Growing aligned carbon on a substrate can make a true carbon nanotube forest, with almost any type of substrate as the forest floor. NCSU researchers toppled this forest, aligned the “logs” and coated that sheet with silicon.
A sheet of aligned and coated carbon nanotubes was rolled onto a cylinder, the basis for making powerful, long-lasting electrodes.
According to the University, “When the silicon-coated carbon nanotubes were aligned in one direction like a layer of drinking straws laid end to end, the structure allowed for controlled expansion so that the silicon is less prone to pulverization, said Xiangwu Zhang, associate professor of textile engineering, chemistry and science at NC State.”
He explains, “There’s a huge demand for batteries for cell phones and electric vehicles, which need higher energy capacity for longer driving distances between charges. We believe this carbon nanotube scaffolding potentially has the ability to change the industry, although technical aspects will have to be worked out. The manufacturing process we’re using is scalable and could work well in commercial production.”
The research team’s paper was published August 1. Authors included Kun Fu, Ozkan Yildiz, Hardik Bhanushali, Yongxin Wang, Kelly Stano, Leigang Xue, Ziangwu Zhang, and Philip D. Bradford.
The research was supported by the Donors to the American Chemical Society Petroleum Research Fund.
Vertically grown aligned carbon nanotube (CNT) forests are drawn into aligned CNT sheets and coated with silicon layers.
The paper’s abstract details the manufacturing process for these electrodes. One ends up with a sandwich structure somewhat similar to those used in aircraft, and which could combine the best elements of structural strength and electrical storage capabilities.
“In the pursuit of high performance lithium ion batteries (LIBs), significant effort has been expended to explore high performance cathode and anode materials. Silicon has the greatest lithium storage capacity per unit mass, and is therefore one of the most promising potential candidates to replace graphite as the anode material in future generations of batteries. The main challenge in utilizing silicon comes from the structural failure induced by its large volume change (>300%) during electrochemical cycling, leading to capacity loss. New designs, in which silicon and carbon can act in a mutually beneficial way, so that silicon can fully contribute to the capacity while maintaining cyclic stability, are needed. With this in mind, this communication describes novel, binder-free, thin sheet anodes for LIBs using aligned carbon nanotube (CNT) based silicon films which were processed in a way that is conducive to future commercial production. The horizontal super-aligned CNT sheets provided high surface area and a porous structure to facilitate both the uniform chemical vapor deposition of silicon during fabrication and the electrochemical kinetics between the silicon and the electrolyte during use. The CNT-based silicon composite sheets had both high specific energy capacity and stable cycle performance. This work also revealed an interesting new mechanism of deformation for silicon coated CNT structures after electrochemical cycling. A spring-like deformation behavior of the aligned CNTs helped to explain the electrochemical stability of the crystalline silicon coatings. These findings will guide future work to optimize this unique nano-architecture for further increases in energy density and stability. This aligned CNT scaffold design may be extended to other anode and cathode materials utilized in thin and flexible LIBs.”