RICE (US) — Faster-charging three-dimensional microbatteries may soon be able to power new generations of remote sensors, display screens, smart cards, flexible electronics, and biomedical devices.
The batteries employ vertical arrays of nickel-tin nanowires perfectly encased in PMMA, a widely used polymer best known as Plexiglas.
Researchers found a way to reliably coat single nanowires with a smooth layer of a PMMA-based gel electrolyte that insulates the wires from the counter electrode while allowing ions to pass through.
The work is reported online in the journal Nano Letters.
“In a battery, you have two electrodes separated by a thick barrier,” says Pulickel Ajayan, professor in mechanical engineering and materials science and chemistry at Rice University.
“The challenge is to bring everything into close proximity so this electrochemistry becomes much more efficient.”
Ajayan believes he’s done that by growing forests of coated nanowires—millions of them on a fingernail-sized chip—for scalable microdevices 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,” he explains.
“We wanted to figure out how the proposed 3-D designs of batteries can be built from the nanoscale up,” says Sanketh Gowda, a graduate student in Ajayan’s lab.
“By increasing the height of the nanowires, we can increase the amount of energy stored while keeping the lithium ion diffusion distance constant.”
“To be fair, the 3-D concept has been around for a while,” says postdoctoral researcher Arava Leela Mohana Reddy. “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.”
The same approach is being tested on nanowire systems with higher capacities.
The process builds upon the lab’s previous research to build coaxial nanowire cables that was reported in Nano Letters last year.
In the new work, the researchers grew 10-micron-long nanowires via electrodeposition in the pores of an anodized alumina template. They then widened the pores with a simple chemical etching technique and drop-coated PMMA onto the array to give the nanowires an even casing from top to bottom. A chemical wash removed the template.
They have built one-centimeter square microbatteries that hold more energy and that charge faster than planar batteries of the same electrode length.
“By going to 3-D, we’re able to deliver more energy in the same footprint,” Gowda says.
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