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Ramping up lithium-sulfur batteries
Chemists at the Nanosystems Initiative Munich Clus-
ter at Ludwig Maximilian University of Munich (LMU) and
at the University of Waterloo, Ontario, synthesized a new
material that could lead the way to state-of-the-art lithium-
sulfur batteries. They produced a novel type of nanofiber featuring
a highly ordered and porous structure that gives it an extraordinarily
high surface-to-volume ratio.
“The high surface-to-volume ratio and high pore volume are im-
portant because they allow sulfur to bind to the electrode in a finely di-
vided manner, with relatively high loading. This enhances the efficiency
of the electrochemical processes that occur in the course of charge-dis-
charge cycles. And the rates of the key reactions at the sulfur electrode-
electrolyte interface, which involve both electrons and ions, are highly
dependent on the total surface area available,” explains chemistry professor Thomas Bein.
To synthesize the carbon fibers, chemists prepared a porous, tubular silica template,
starting from commercially available, but
nonporous fibers. This template is then
filled with a special mixture of carbon, sil-
icon dioxide, and surfactants, which is
heated at 900°C. Finally, the template and
the SiO2 are removed by etching. During
this process, the carbon nanotubes—and
pore size—shrink more than they would
without the confining template, and the
fibers themselves become more stable.
.
Shooting nanoribbons
The Rice University lab of materials
scientist Pulickel Ajayan, Houston, discovered that nanotubes that hit a target end-first
generally turn into ragged clumps of atoms. But nanotubes that happen to broadside the
target unzip into handy ribbons, which can be used in composite materials for strength as
well as in applications that take advantage of their desirable electrical properties.
Researchers fired pellets of randomly oriented, multiwalled carbon nanotubes from a
light gas gun built by the lab of materials scientist Enrique Barrera with funding from
NASA. Pellets impacted an aluminum target in a vacuum chamber at about 15,000 mph.
When the resulting carbon rubble was inspected, nanotubes that smashed into the target
end-first or at a sharp angle simply deformed into crumpled tubes. But ones that hit length-
wise actually split into ribbons with ragged edges.
According to Ajayan, the process eliminates the need to clean chemical residues from
nanoribbons produced through current techniques. “One-step, chemical-free, clean, and
high-quality graphene nanoribbons can
be produced using our method. They are
potential candidates for next-generation
electronic materials,” he says. For more in-
formation: Pulickel Ajayan, 713.348.5904,
, .
Molecular simulations and electron
microscope images show what happens to a
carbon nanotube when the end of it strikes a
target directly at about 15,000 mph—they
split into useful nanoribbons. Courtesy of
Ajayan Group/Rice University.
ADVANCED MATERIALS & PROCESSES • SEPTEMBER 2014 16
NANOTECHNOLOGY
briefs
The University of Manchester,
UK, will host Graphene Week 2015
next June. Isolated at Manchester
in 2004, graphene has captured
the attention of scientists
worldwide with its potential to
revolutionize the materials world
due to its incredible properties. As
the world’s first 2D material, it is
ultra-light, yet immensely tough; it
is 200 times stronger than steel,
but is incredibly flexible; and it is
fire retardant yet retains heat. The
university is also building a $28.3
million National Graphene Institute
(NGI), set to open in spring 2015.
.
A project at IBM, Armonk, N.Y.,
aims to build transistors with
carbon nanotubes, which are
expected to replace silicon
transistors around 2020. According
to the semiconductor industry’s
roadmap, transistors will then
require features as small as 5 nm
to keep up with the continuous
shrinking of computer chips. The
company recently made chips with
10,000 nanotube transistors and is
now working on a transistor design
that could be built on silicon
wafers used today with minimal
changes to existing design and
manufacturing methods.
Simulations suggest that these
new microprocessors could be five
times as fast as silicon ones using
the same amount of power.
.
Novel nanofibers
hold promise for
advanced
lithium-sulfur
batteries.
Courtesy of
LMU.
BNNT LLC, Newport News, Va., has made its
new boron nitride nanotubes (BNNTs) avail-
able. BNNTs are as strong as their more fa-
mous cousin, carbon nanotubes, but are
superior in many ways. For example, they
have a much higher resistance to heat, high
voltage, and neutron radiation. Unlike most
nanotube products, which occur in powder
form, the new BNNTs are cotton-like in ap-
pearance. At the molecular scale, they are
thin—about 3-5 nm in diameter—as well as
highly crystalline, few-walled, and feature
aspect ratios approaching one million.
.