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nanotimes News in Brief Energy Storage //

Building Energy Storage Device on a Single Nanowire © Text: Mike Williams / Rice

11-09 :: September 2011

T

he world at large runs on lithium ion batteries. New research at Rice University shows that tiny worlds may soon do the same.

The Rice lab of Prof. Pulickel Ajayan has packed an entire lithium ion energy storage device into a single nanowire. The researchers believe their crea- tion is as small as such devices can possibly get, and could be valuable as a rechargeable power source for new generations of nanoelectronics.

In their paper, researchers described testing two versions of their battery/supercapacitor hybrid. The first is a sandwich with nickel/tin anode, polyethyle- ne oxide (PEO) electrolyte and polyaniline cathode layers; it was built as proof that lithium ions would move efficiently through the anode to the electrolyte and then to the supercapacitor-like cathode, which stores the ions in bulk and gives the device the ability to charge and discharge quickly.

The second packs the same capabilities into a single nanowire. The researchers built centimeter-scale arrays containing thousands of nanowire devices, each about 150nm wide. Ajayan‘s team has been inching toward single-nanowire devices for years. The researchers first reported the creation of three- dimensional nanobatteries last December. In that

project, they encased vertical arrays of nickel-tin nanowires in PMMA, a widely used polymer best known as Plexiglas, which served as an electrolyte and insulator.

They grew the nanowires via electrodeposition in an anodized alumina template atop a copper substrate. They widened the template‘s pores with a simple chemical etching technique that created a gap bet- ween the wires and the alumina, and then drop-coa- ted PMMA to encase the wires in a smooth, consi- stent sheath. A chemical wash removed the template and left a forest of electrolyte-encased nanowires.

In that battery, the encased nickel-tin was the anode, but the cathode had to be attached on the outside.

The new process tucks the cathode inside the nano- wires, said Ajayan, a professor of mechanical engi- neering and materials science. In this feat of nano- engineering, the researchers used PEO as the gel-like electrolyte that stores lithium ions and also serves as an electrical insulator between nanowires in an array.

After much trial and error, they settled on an easily synthesized polymer known as polyaniline (PANI) as their cathode. Drop-coating the widened alumina pores with PEO coats the insides, encases the anodes