An ultrathin battery/supercapacitor hybrid contains thousands of nanowires, each of which is a fully functional battery. The Rice University lab of Pulickel Ajayan developed the device. Photo: Jeff Fitlow/Rice University
The 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 Professor Pulickel Ajayan has packed an entire lithium ion
energy storage device into a single nanowire, as reported in Nano Letters. The
researchers believe their creation is as small as such devices can possibly
get, and could be valuable as a rechargeable power source for new generations
In their paper, researchers described testing two versions of their
battery/supercapacitor hybrid. The first is a sandwich with nickel/tin anode,
polyethylene 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 150 nm wide.
Ajayan’s team has been inching toward single-nanowire devices for years. The
reported the creation of 3D nanobatteries last December (2010). 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 between the wires and the
alumina, and then drop-coated PMMA to encase the wires in a smooth, consistent
sheath. A chemical wash removed the template and left a forest of electrolyte-encased
A schematic shows nanoscale battery/supercapacitor devices in an array, as constructed at Rice University. The devices show promise for powering nanoscale electronics and as a research tool for understanding electrochemical phenomenon at the nanoscale. Image: Ajayan Lab/Rice University
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 nanowires, says Ajayan, a
professor of mechanical engineering and materials science. In this feat of
nanoengineering, 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 and leaves tubes at the
top into which PANI cathodes could also be drop-coated. An aluminum current
collector placed on top of the array completes the circuit.
“The idea here is to fabricate nanowire energy storage devices with
ultrathin separation between the electrodes,” says Arava Leela Mohana
Reddy, a research scientist at Rice and coauthor of the paper. “This
affects the electrochemical behavior of the device. Our devices could be a very
useful tool to probe nanoscale phenomenon.”
The team’s experimental batteries are about 50 microns tall, Reddy says.
Theoretically, the nanowire energy storage devices can be as long and wide as
the templates allow, which makes them scalable.
The nanowire devices show good capacity; the researchers are fine-tuning the
materials to increase their ability to repeatedly charge and discharge, which
now drops off after a about 20 cycles.
“There’s a lot to be done to optimize the devices in terms of
performance,” says the paper’s lead author, Sanketh Gowda, a chemical
engineering graduate student at Rice. “Optimization of the polymer
separator and its thickness and an exploration of different electrode systems
could lead to improvements.”