Microbatteries with Nanowire Hearts
Source: Product Design & Development (12/14/10)
"Nanostructured Li-ion battery charges faster, lasts longer than those used now."
The batteries employ vertical arrays of nickel-tin nanowires perfectly encased in PMMA, a widely used polymer best known as Plexiglas. Rice found a way to reliably coat single nanowires with a smooth layer of a PMMA-based gel electrolyte that insulates them from the counter electrode while allowing ions to pass through.
"In a battery, you have two electrodes separated by a thick barrier," said Mechanical Engineering, Materials Science and Chemistry Professor Ajayan. "The challenge is to bring everything into close proximity so this electrochemistry becomes much more efficient."
Ajayan and his team feel they've done that by growing forests of coated nanowires—millions of them on a fingernail-sized chip—for scalable micro devices with greater surface area than conventional thin-film batteries. "You can't simply scale the thickness of a thin-film battery, because the lithium-ion kinetics would become sluggish," Ajayan said.
"We wanted to figure out how the proposed 3-D battery designs can be built from the nanoscale up," said graduate student Sanketh Gowda. "By increasing the height of the nanowires, we can increase the amount of energy stored while keeping the lithium-ion diffusion distance constant."
The researchers, led by Gowda and postdoctoral researcher Arava Leela Mohana Reddy, worked for more than a year to refine the process.". . .The 3-D concept has been around for a while," Reddy said. "The breakthrough here is the ability to put a conformal coat of PMMA on a nanowire over long distances. Even a small break in the coating would destroy it." He said the same approach is being tested on nanowire systems with higher capacities.