Lithium-ion battery electrodes bound together by a new highly
conductive material have a much greater storage capacity—a development
that could eventually increase the range of electric cars and the life
of smart-phone batteries without increasing their cost. Unlike many
high-capacity electrodes developed over the last few years, these can be
made using the equipment already found in today's battery factories.
Battery binder: This microscopy image shows a silicon electrode
before charging (left) and after 32 cycles. A new binder keeps the
particles close together
The key is a stretchy, highly conductive polymer binder that can be
used to hold together silicon, tin, and other materials that can store a
lot of energy but that are ordinarily unstable. Researchers at the
Lawrence Berkeley National Laboratory painstakingly engineered this new
polymer binder and used it to make a silicon anode for a rechargeable
lithium-ion battery with a storage capacity 30 percent greater than
those on the market today. It's also more stable over time than
previously developed electrodes.
When a lithium-ion battery is charged, lithium ions are taken up by
one of the electrodes, called the anode. The more lithium the anode can
hold, the more energy the battery can store. Silicon is one of the most
promising anode materials: it can store 10 times more lithium than
graphite, which is used to make the anodes in the lithium-ion batteries
on the market today. "Graphite soaks up lithium like a sponge, holding
its shape, but silicon is more like a balloon," says Gao Liu, a researcher at the Berkeley Lab's Environmental Energy Technologies Division.
However, because the silicon anodes swell and shrink, changing in
volume by three or four times as they're charged and discharged, the
capacity of the battery fades over time. "After a few rounds of charge
and discharge, pretty soon the silicon particles are not in touch with
each other," which means the anode can't conduct electricity, says Liu.
One approach to the problem is to structure these anodes in a totally
different way, for example growing shaggy arrays of silicon nanowires
that can bend, swell, and move around as lithium enters and exits. This
approach is being commercialized by Amprius, a startup in Palo Alto,
California. But growing nanowires requires new processes that aren't
normally used in battery manufacturing.
Today's anodes are made by painting a solvent-based slurry of
graphite particles held together with a binder, a simple process that
keeps costs low. The Berkeley researchers believe the key to making new
battery materials like silicon work is to stick with this manufacturing
process. That meant coming up with a rubbery binder that would stick to
silicon particles, remain highly conductive in the harsh environment of
the anode, and stretch and contract as the anode swells and deflates.
Most work on advanced batteries has focused on the active materials, but "we have pushed these materials to the limit," says Yury Gogotsi, professor of materials science and engineering at Drexel University. "Now what's limiting us are the binders."
Reading through papers on silicon battery binders, Liu noticed that
researchers were making "fatal mistakes"—choosing polymers that lose
their conductivity in the kinds of conditions found in an anode, for
example. He worked with theoretical chemists to come up with a list of
polymers with the right electrical properties for the job. Once they
found one, they altered it to make it much stickier. Once they developed
and characterized this new material, they were able to make silicon
anodes using conventional processes, and test them in batteries.
The Berkeley group's anodes have been tested in over 650 charging
cycles. They maintain a storage capacity of 1,400 milliamp hours per
gram—much greater than the 300 or so stored by conventional anodes. Full
batteries incorporating the anodes store about 30 percent more total
energy than a commercial lithium-ion battery. Typically, battery
capacity increases by about 5 percent a year, Liu notes. He says they've
tested the binder in other battery anodes, including those made of tin,
that have similar potential and problems, and that it should work for
any such materials.
The storage capacity of these batteries is nearly as good as those made from pure silicon nanowires with no binders, says Yi Cui,
professor of materials science and engineering at Stanford and one of
the founders of Amprius. That's impressive, he says, considering that
the binder doesn't store any lithium.
Liu's group is now collaborating with researchers at 3M on the anode
research. 3M is scaling up production of silicon-based battery
materials designed to not expand quite so much during charging, says
Kevin Eberman, who is developing battery materials products at 3M Electronics
in St. Paul, Minnesota. But to make them work, a good binder is key.
The company is providing the Berkeley group with materials to test. Liu
says the Berkeley group has patented the binders, and is in talks with a
few companies about ways to commercialize them.
By Katherine Bourzac
From Technology Review
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