Researchers at Rensselaer Polytechnic Institute developed an entirely new type of nanomaterial that could enable the next generation of high-power rechargeable lithium (Li)-ion batteries for electric automobiles, laptop computers, mobile phones, and other devices. The material, called a “nanoscoop” because it resembles a cone with a scoop of ice cream on top, is shown in the above scanning electron microscope image. Nanoscoops can withstand extremely high rates of charge and discharge that would cause today’s Li-ion batteries to rapidly deteriorate and fail. Credit: Rensselaer Polytechnic Institute |
An
entirely new type of nanomaterial developed at Rensselaer Polytechnic Institute
could enable the next generation of high-power rechargeable lithium (Li)-ion
batteries for electric automobiles, as well as batteries for laptop computers,
mobile phones, and other portable devices.
The
new material, dubbed a “nanoscoop” because its shape resembles a cone with a
scoop of ice cream on top, can withstand extremely high rates of charge and
discharge that would cause conventional electrodes used in today’s Li-ion
batteries to rapidly deteriorate and fail. The nanoscoop’s success lies in its
unique material composition, structure, and size.
The
Rensselaer research team, led by Professor Nikhil
Koratkar, demonstrated how a nanoscoop electrode could be charged and
discharged at a rate 40 to 60 times faster than conventional battery anodes,
while maintaining a comparable energy density. This stellar performance, which
was achieved over 100 continuous charge/discharge cycles, has the team
confident that their new technology holds significant potential for the design
and realization of high-power, high-capacity Li-ion rechargeable batteries.
“Charging
my laptop or cell phone in a few minutes, rather than an hour, sounds pretty
good to me,” said Koratkar, a professor in the Department of Mechanical,
Aerospace, and Nuclear Engineering at Rensselaer.
“By using our nanoscoops as the anode architecture for Li-ion rechargeable
batteries, this is a very real prospect. Moreover, this technology could
potentially be ramped up to suit the demanding needs of batteries for electric
automobiles.”
Batteries
for all-electric vehicles must deliver high power densities in addition to high
energy densities, Koatkar said. These vehicles today use supercapacitors to
perform power-intensive functions, such as starting the vehicle and rapid
acceleration, in conjunction with conventional batteries that deliver high
energy density for normal cruise driving and other operations. Koratkar said
the invention of nanoscoops may enable these two separate systems to be
combined into a single, more efficient battery unit.
Results
of the study were detailed in the paper “Functionally Strain-Graded Nanoscoops
for High Power Li-Ion Battery Anodes,” published in Nano Letters.
The
anode structure of a Li-ion battery physically grows and shrinks as the battery
charges or discharges. When charging, the addition of Li ions increases the
volume of the anode, while discharging has the opposite effect. These volume
changes result in a buildup of stress in the anode. Too great a stress that
builds up too quickly, as in the case of a battery charging or discharging at
high speeds, can cause the battery to fail prematurely. This is why most
batteries in today’s portable electronic devices like cell phones and laptops
charge very slowly—the slow charge rate is intentional and designed to protect
the battery from stress-induced damage.
The
Rensselaer team’s nanoscoop, however, was
engineered to withstand this buildup of stress. Made from a carbon (C) nanorod
base topped with a thin layer of nanoscale aluminum (Al) and a “scoop” of
nanoscale silicon (Si), the structures are flexible and able to quickly accept
and discharge Li ions at extremely fast rates without sustaining significant
damage. The segmented structure of the nanoscoop allows the strain to be
gradually transferred from the C base to the Al layer, and finally to the Si
scoop. This natural strain gradation provides for a less abrupt transition in
stress across the material interfaces, leading to improved structural integrity
of the electrode.
The
nanoscale size of the scoop is also vital since nanostructures are less prone
to cracking than bulk materials, according to Koratkar.
“Due
to their nanoscale size, our nanoscoops can soak and release Li at high rates
far more effectively than the macroscale anodes used in today’s Li-ion
batteries,” he said. “This means our nanoscoop may be the solution to a
critical problem facing auto companies and other battery manufacturers—how can
you increase the power density of a battery while still keeping the energy density
high?”
A
limitation of the nanoscoop architecture is the relatively low total mass of
the electrode, Koratkar said. To solve this, the team’s next steps are to try
growing longer scoops with greater mass, or develop a method for stacking
layers of nanoscoops on top of each other. Another possibility the team is
exploring includes growing the nanoscoops on large flexible substrates that can
be rolled or shaped to fit along the contours or chassis of the automobile.
Along
with Koratkar, authors on the paper are Toh-Ming Lu, the R.P. Baker
Distinguished Professor of Physics and associate director of the Center for
Integrated Electronics at Rensselaer; and Rahul Krishnan, a graduate student in
the Department of Materials Science and Engineering at Rensselaer.
This study was supported by the National
Science Foundation (NSF) and the New York State Energy Research and Development
Authority (NYSERDA).
