This composite image shows a silicon-carbon nanofiber electrode before (left) and after (right) being charged with lithium ions. Image: Pacific Northwest National Laboratory |
A study that examines a new type of silicon-carbon
nanocomposite electrode reveals details of how they function and how repeated
use could wear them down. The study also provides clues to why this material
performs better than silicon alone. With an electrical capacity five times
higher than conventional lithium battery electrodes, silicon-carbon
nanocomposite electrodes could lead to longer-lasting, cheaper rechargeable
batteries for electric vehicles.
Published online in Nano
Letters, the study includes videos of the electrodes being charged at
nanometer-scale resolution. Watching them in use can help researchers
understand the strengths and weaknesses of the material.
“The electrodes expand as they get charged, and that
shortens the lifespan of the battery,” said lead researcher Chongmin Wang
at the United States Department of Energy’s Pacific Northwest National
Laboratory. “We want to learn how to improve their lifespan, because
silicon-carbon nanofiber electrodes have great potential for rechargeable
batteries.
Plus and minus
Silicon has both advantages and disadvantages for use as a battery material. It
has a high capacity for energy storage, so it can take on a hefty charge.
Silicon’s problem, though, is that it swells up when charged, expanding up to
three times its discharged size. If silicon electrodes are packed tightly into
a battery, this expansion can cause the batteries to burst. Some researchers
are exploring nano-sized electrodes that perform better in such tight confines.
A multi-institution group led by PNNL’s Wang decided to
test nano-sized electrodes consisting of carbon nanofibers coated with silicon.
The carbon’s high conductivity, which lets electricity flow, nicely complements
silicon’s high capacity, which stores it.
Researchers at DOE’s Oak Ridge National Laboratory in Oak Ridge, Tenn.; Applied
Sciences Inc. in Cedarville, Ohio;
and General Motors
Global R&D
Center in Warren, Mich.,
created carbon nanofibers with a thin layer of silicon wrapped around. They
provided the electrodes to the team at PNNL to probe their behavior while
functioning.
First, Wang and colleagues tested how much lithium the
electrodes could hold and how long they lasted by putting them in a small
testing battery called a half-cell. After 100 charge-discharge cycles, the
electrodes still maintained a very good capacity of about 1000 milliAmp-hours per
gram of material, five to 10 times the capacity of conventional electrodes in
lithium ion batteries.
Although they performed well, the team suspected that the
expansion and contraction of the silicon could be a problem for the battery’s
longevity, since stretching tends to wear things out. To determine how well the
electrodes weather the repeated stretching, Wang popped a specially designed,
tiny battery into a transmission electron microscope, which can view objects
nanometers wide, in DOE’s EMSL, the Environmental Molecular Sciences Laboratory
on the PNNL campus.
They zoomed in on the tiny battery’s electrode using a
new microscrope that was funded by the Recovery Act. This microscope allowed
the team to study the electrode in use, and they took images and video while
the tiny battery was being charged and discharged.
Not crystal glass
Previous work has shown that charging causes lithium ions to flow into the
silicon. In this study, the lithium ions flowed into the silicon layer along
the length of the carbon nanofiber at a rate of about 130 nm/sec. This is about
60 times faster than silicon alone, suggesting that the underlying carbon
improves silicon’s charging speed.
As expected, the silicon layer swelled up about 300% as
the lithium entered. However, the combination of the carbon support and the
silicon’s unstructured quality allowed it to swell evenly. This compares
favorably to silicon alone, which swells unevenly, causing imperfections.
In addition to swelling, lithium is known to cause other
changes to the silicon. The combination of lithium and silicon initially form
an unstructured, glassy layer. Then, when the lithium to silicon ratio hits 15
to 4, the glassy layer quickly crystallizes, as previous work by other
researchers has shown.
Wang and colleagues examined the crystallization process
in the microscope to better understand it. In the microscope video, they could see the
crystallization advance as the lithium filled in the silicon and reached the 15
to 4 ratio.
They found that this crystallization is different from
the classic way that many substances crystallize, which builds from a starting
point. Rather, the lithium and silicon layer snapped into a crystal all at once
when the ratio hit precisely 15 to 4. Computational analyses of this
crystallization verified its snappy nature, a type of crystallization known as
congruent phase transition.
But the crystallization wasn’t permanent. Upon
discharging, the team found that the crystal layer became glassy again, as the
concentration of lithium dropped on its way out of the silicon.
To determine if repeated use left its mark on the
electrode, the team charged and discharged the tiny battery 4 times. Comparing
the same region of the electrode between the first and fourth charging, the
team saw the surface become rough, similar to a road with potholes.
The surface changes were likely due to lithium ions
leaving a bit of damage in their wake upon discharging, said Wang. “We can
see the electrode’s surface go from smooth to rough as we charge and discharge
it. We think as it cycles, small defects occur, and the defects
accumulate.”
But the fact that the silicon layer is very thin makes it
more durable than thicker silicon. In thick silicon, the holes that lithium
ions leave behind can come together to form large cavities. “In the
current design, because the silicon is so thin, you don’t get bigger cavities,
just like little gas bubbles in shallow water come up to the surface. If the
water is deep, the bubbles come together and form bigger bubbles.”
In future work, researchers hope to explore the thickness
of the silicon layer and how well it bonds with the underlying carbon to
optimize the performance and lifetime of the electrodes.
