Northwestern University researchers have developed a flexible, self-supporting 3D conducting graphenic scaffold incorporating Si nanoparticles that exhibits excellent rate performance and tolerance to structural deformation. The electrode material represents an attractive high power-high capacity anode material for Li-ion batteries. |
Imagine
a cellphone battery that stayed charged for more than a week and
recharged in just 15 minutes. That dream battery could be closer to
reality thanks to Northwestern University research.
A
team of engineers has created an electrode for lithium-ion batteries —
rechargeable batteries such as those found in cellphones and iPods —
that allows the batteries to hold a charge up to 10 times greater than
current technology. Batteries with the new electrode also can charge 10
times faster than current batteries.
The
researchers combined two chemical engineering approaches to address two
major battery limitations—energy capacity and charge rate—in one fell
swoop. In addition to better batteries for cellphones and iPods, the
technology could pave the way for more efficient, smaller batteries for
electric cars.
The technology could be seen in the marketplace in the next three to five years, the researchers said.
A paper describing the research is published by the journal Advanced Energy Materials.
“We
have found a way to extend a new lithium-ion battery’s charge life by
10 times,” said Harold H. Kung, lead author of the paper. “Even after
150 charges, which would be one year or more of operation, the battery
is still five times more effective than lithium-ion batteries on the
market today.”
Kung
is professor of chemical and biological engineering in the McCormick
School of Engineering and Applied Science. He also is a Dorothy Ann and
Clarence L. Ver Steeg Distinguished Research Fellow.
Lithium-ion
batteries charge through a chemical reaction in which lithium ions are
sent between two ends of the battery, the anode and the cathode. As
energy in the battery is used, the lithium ions travel from the anode,
through the electrolyte, and to the cathode; as the battery is
recharged, they travel in the reverse direction.
With
current technology, the performance of a lithium-ion battery is limited
in two ways. Its energy capacity—how long a battery can maintain its
charge—is limited by the charge density, or how many lithium ions can be
packed into the anode or cathode. Meanwhile, a battery’s charge
rate—the speed at which it recharges—is limited by another factor: the
speed at which the lithium ions can make their way from the electrolyte
into the anode.
In
current rechargeable batteries, the anode—made of layer upon layer of
carbon-based graphene sheets—can only accommodate one lithium atom for
every six carbon atoms. To increase energy capacity, scientists have
previously experimented with replacing the carbon with silicon, as
silicon can accommodate much more lithium: four lithium atoms for every
silicon atom. However, silicon expands and contracts dramatically in the
charging process, causing fragmentation and losing its charge capacity
rapidly.
Currently,
the speed of a battery’s charge rate is hindered by the shape of the
graphene sheets: they are extremely thin—just one carbon atom thick—but
by comparison, very long. During the charging process, a lithium ion
must travel all the way to the outer edges of the graphene sheet before
entering and coming to rest between the sheets. And because it takes so
long for lithium to travel to the middle of the graphene sheet, a sort
of ionic traffic jam occurs around the edges of the material.
Now,
Kung’s research team has combined two techniques to combat both these
problems. First, to stabilize the silicon in order to maintain maximum
charge capacity, they sandwiched clusters of silicon between the
graphene sheets. This allowed for a greater number of lithium atoms in
the electrode while utilizing the flexibility of graphene sheets to
accommodate the volume changes of silicon during use.
“Now
we almost have the best of both worlds,” Kung said. “We have much
higher energy density because of the silicon, and the sandwiching
reduces the capacity loss caused by the silicon expanding and
contracting. Even if the silicon clusters break up, the silicon won’t be
lost.”
Kung’s
team also used a chemical oxidation process to create miniscule holes
(10 to 20 nanometers) in the graphene sheets—termed “in-plane
defects”—so the lithium ions would have a “shortcut” into the anode and
be stored there by reaction with silicon. This reduced the time it takes
the battery to recharge by up to 10 times.
This
research was all focused on the anode; next, the researchers will begin
studying changes in the cathode that could further increase
effectiveness of the batteries. They also will look into developing an
electrolyte system that will allow the battery to automatically and
reversibly shut off at high temperatures—a safety mechanism that could
prove vital in electric car applications.
The Energy Frontier Research Center program of the U.S. Department of Energy, Basic Energy Sciences, supported the research.
In-Plane Vacancy-Enabled High-Power Si–Graphene Composite Electrode for Lithium-Ion Batteries
