background image
The Good News
2015 December/January
Pg 3 - The Sunshine Express
Quantum dots made from fool¡¯s gold
boost battery performance
November 11, 2015: If you add quantum dots - nano-
crystals 10,000 times smaller than the width of a human
hair - to a smartphone battery it will charge in 30 sec-
onds, but the effect only lasts for a few recharge cycles.
However, a group of researchers at Vanderbilt University
report, in the Nov. 11 issue of the journal ACS Nano, that
they have found a way to overcome this problem: Making
the quantum dots out of iron pyrite, commonly known as
fool¡¯s gold, can produce batteries that charge quickly and
work for dozens of cycles.
The research team headed by Assistant Professor of
Mechanical Engineering Cary Pint and led by graduate
student Anna Douglas became interested in iron pyrite
because it is one of the most abundant materials in the
earth¡¯s surface. It is produced in raw form as a byprod-
uct of coal production and is so cheap that it is used in
lithium batteries that are bought in the store and thrown
away after a single use.
Despite all their promise, researchers have had trouble
getting nanoparticles to improve battery performance.
¡°Researchers have demonstrated that nanoscale ma-
terials can significantly improve batteries, but there is
a limit,¡± Pint said. ¡°When the particles get very small,
generally meaning below 10 nanometers (40 to 50 atoms
wide), the nanoparticles begin to chemically react with
the electrolytes and so can only charge and discharge a
few times. So this size regime is forbidden In commercial
lithium-ion batteries.¡±
Aided by Douglas¡¯ expertise in synthesizing nanoparticles,
the team set out to explore this ¡®ultrasmall¡¯ regime.
materials is like pushing chocolate chips
into the cake and then pulling the intact
chips back out. With the interesting ma-
terials we¡¯re studying, you put chocolate
chips into vanilla cake and it changes into a
chocolate cake with vanilla chips.¡±
As a result, the rules that forbid the use
of ultrasmall nanoparticles in batteries no
longer apply. In fact, the scales are tipped
in favor of very small nanoparticles.
¡°Instead of just inserting lithium or sodium
ions in or out of the nanoparticles, storage
in iron pyrite requires the diffusion of iron
atoms as well. Unfortunately, iron diffuses
slowly, requiring that the size be smaller
than the iron diffusion length ¨C some-
thing that is only possible with ultrasmall
nanoparticles,¡± Douglas explained.
A key observation of the team¡¯s study
was that these ultrasmall nanoparticles
are equipped with dimensions that allow
the iron to move to the surface while the
sodium or lithium reacts with the sulfurs in
the iron pyrite. They demonstrated that this
isn¡¯t the case for larger particles, where the
inability of the iron to move through the
iron pyrite materials limits their storage
Research A Major Step
Pint believes that understanding of chemi-
cal storage mechanisms and how they
depend on nanoscale dimensions is critical
to enable the evolution of battery perfor-
mance at a pace that stands up to Moore¡¯s
law and can support the transition to elec-
tric vehicles.
¡°The batteries of tomorrow that can charge
in seconds and discharge in days will not
just use nanotechnology, they will benefit
from the development of new tools that will
allow us to design nanostructures that can
stand up to tens of thousands of cycles and
possess energy storage capacities rivaling
that of gasoline,¡± said Pint. ¡°Our research is
a major step in this direction.¡±
Coauthors of the paper with Pint and
Douglas include mechanical engineering
graduate students Rachel Carter and Adam
Cohn and interdisciplinary materials science
graduate students Keith Share and Landon
Oakes. The research was funded in part
by National Science Foundation grant EPS
1004083 and NSF¡¯s graduate research fel-
lowship program grant 1445197.
They did so by adding millions of iron pyrite quantum
dots of different sizes to standard lithium button bat-
teries like those that are used to power watches, auto-
mobile key remotes and LED flashlights. They got the
most bang for their buck when they added ultrasmall
nanocrystals that were about 4.5 nanometers in size.
These substantially improved both the batteries¡¯ cy-
cling and rate capabilities.
The researchers discovered that they got this re-
sult because iron pyrite has a unique way of chang-
ing form into an iron and a lithium-sulfur (or sodium
sulfur) compound to store energy. ¡°This is a different
mechanism from how commercial lithium-ion batter-
ies store charge, where lithium inserts into a material
during charging and is extracted while discharging, all
the while leaving the material that stores the lithium
mostly unchanged,¡± Douglas explained.
According to Pint, ¡°You can think of it like vanilla cake.
Storing lithium or sodium in conventional battery