AMERICAN CERAMIC SOCIETY

ceramics in energy
Simple nanoscale processes improve electrode performance in Li-ion
batteries and supercapacitors
Stanford University associate professor
of materials science and engineering
Yi Cui knows his way inside and out of
a carbon nanotube, and he’s using his
knowledge of that terrain to design new
electrodes for lithium-ion batteries and
ultracapacitors. Two papers published
in recent weeks in Nano Letters describe
the details.
The first paper describes an approach
to cathode design for Li-ion batteries
that incorporates sulfur. Sulfur is an
attractive cathode material because of
its high storage capacity at relatively
low voltage. It also is inexpensive,
abundant and nontoxic. According to
a Stanford news release, batteries with
sulfur cathodes can store four to five
times as much energy as existing Li-ion
batteries.
There have been previous cathode
designs that coated sulfur onto porous
carbon structures. However, they fail
quickly during the charge–recharge
cycle because intermediate lithium
polysulfide compounds are in contact
with the electrolyte solution and dissolve
into it. Cui’s graduate student,
Wesley Guangyuan Zheng, describes
the problem they tackled, saying, “[W]
e don’t want a large surface area contacting
the sulfur and the electrolyte,
and on the other hand we want a large
surface area for electrical and ionic
conductivities.”
The Cui team separated the sulfur
from the electrolyte by simply moving
it inside the cathode. Adapting a
commercially available water filtration
process, they coated the interior of
CNTs with sulfur. The new design prevents
polysulfides from leaking into the
electrolyte solution, while still allowing
easy transport of Li ions through the
CNT wall. Tests showed a high specific
capacity after 150 charge–discharge
cycles. In the same paper, they reported
improved coulumbic efficiency gained
by adding LiNO 3 to the electrolyte.
American Ceramic Society Bulletin, Vol. 90, No. 9 | www.ceramics.org
Cui’s second paper also investigates
electrode efficiency, in this case
one made with manganese dioxide, a
promising material for supercapacitors
(also called ultracapacitors) because of
its high theoretical specific capacity,
low cost and nontoxicity. However,
although MnO 2 is blessed with a high
charge storage capacity, it has low electrical
and ionic conductivity, so getting
the charge in or out quickly is a barrier.
Scanning electron microscope photo of
hollow carbon nanofiber-encapsulated
sulfur tubes, at the heart of a new battery
design.
To improve the conductivity of the
electrode at its surface, two conductive
coatings were investigated: carbon
nanotubes and a conductive polymer.
Coatings were applied by dipping a
graphene-MnO 2 nanostructured composite
electrode into a solution of the
coatings.
Both coatings increased electrode
conductivity, and therefore capacitance.
The specific capacitance of the
CNT-coated electrode increased by 25
percent and that of the polymer-coated
electrode increased by 45 percent. The
paper also reports that the coated electrodes,
which the authors describe as
ternary composites, delivered superior
cycling performance, retaining more
than 95 percent of their capacitance
after more than 3000 cycles.
In a Technology Review story, it was
pointed out that the energy density of
(Credit: Wesley Guangyuan Zheng; Stanford Univ.)
the electrode has yet to be reported.
Visit www.stanford.edu/group/
cui_group n
Optimizing YBCO superconductor
magnet design
Twenty-five years after their
discovery, high-temperature super- conductors
are starting to be used in applications.
Since the beginning, much
attention has focused on the ytrrium
barium copper oxide family of superconductors,
and they are starting to be
used in some systems. But, basic physics
still presents a few obstacles.
One of them is the issue of “quenching,”
which is the sudden loss of the
superconductive property. Quenching
can be catastrophic because the sudden
release of stored electricity can destroy
systems. Quench protection is designed
into most superconducting magnet
systems, but quench onset must be
detected early enough for the protection
mechanism to react effectively.
A difficulty yet to overcome with
YBCO materials is that the “normalzone
propagation velocity” is slow.
The implications of this are that the
property changes that signal the start of
quench, such as voltage and temperature,
are too small to detect.
Complicating the obstacles provided
by physics alone, is the challenge facing
system designers to develop products
based on the properties of existing
materials. Device configuration can
influence the quench properties, but to
what extent has not been clear.
A recent paper by Wan Kan Chan
and Justin Schwartz at North Carolina
State University introduces a way of
looking at how device geometry and
material selection affect the properties
of a YBCO superconductor. In the news
release, Schwartz says, “This approach
moves us closer to the ideal of having
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