This is more battery research and in this
case they are growing a forest of nano batteries in order to gain in energy density. No one really knows what the successful
protocol will ultimately be, so we must push forward on every good idea.
This looks a long ways from been optimized so
it is good to see early success in methodology.
This is something that could be commercialized into circuit boards and miniature
devices.
Anyway, the battery rush continues.
Better batteries from the bottom up
12/9/2010
CONTACT: Mike Williams
PHONE: 713-348-6728
EMAIL: mikewilliams@rice.edu
The batteries employ vertical arrays of nickel-tin
nanowires perfectly encased in PMMA, a widely used polymer best known as
Plexiglas. The Rice laboratory of Pulickel Ajayan 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 was reported this week in the online edition of
the journal Nano Letters.
"In a battery, you have two electrodes separated by a
thick barrier," said Ajayan, professor in mechanical engineering and
materials science and of chemistry. "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 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," Ajayan said.
"We wanted to figure out how the proposed 3-D designs
of batteries can be built from the nanoscale up," said 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."
The researchers, led by Gowda and postdoctoral researcher
Arava Leela Mohana Reddy, worked for more than a year to refine the process.
\
"To be fair, 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.
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 said.
They feel the PMMA coating will increase the number of
times a battery can be charged by stabilizing conditions between the nanowires
and liquid electrolyte, which tend to break down over time.
The team is also studying how cycling affects nanowires
that, like silicon electrodes, expand and contract as lithium ions come and go.
Electron microscope images of nanowires taken after many charge/discharge
cycles showed no breaks in the PMMA casing -- not even pinholes. This led the
researchers to believe the coating withstands the volume expansion in the
electrode, which could increase the batteries' lifespans.
Co-authors are Rice graduate student Xiaobo Zhan; former
Rice postdoctoral researcher Manikoth Shaijumon, now an assistant professor at
the Indian Institute of Science Education and Research, Thiruvananthapuram,
India; and former Rice research scientist Lijie Ci, now a senior research and
development manager at Samsung Cheil Industries.
The Hartley Family Foundation and Rice University
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