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For more than a decade, scientists have
tried to improve lithium-based batteries by replacing the graphite in one
terminal with silicon, which can store 10 times more charge. But after just a
few charge/discharge cycles, the silicon structure would crack and crumble,
rendering the battery useless.
Now a team led by materials scientist Yi
Cui of Stanford University and SLAC National Accelerator
Laboratory has found a solution: a cleverly designed double-walled
nanostructure that lasts more than 6,000 cycles, far more than needed by
electric vehicles or mobile electronics.
“This is a very exciting development toward
our goal of creating smaller, lighter and longer-lasting batteries than are
available today,” Cui said. The results were published in Nature Nanotechnology.
Lithium-ion batteries are widely used to
power devices from electric vehicles to portable electronics because they can
store a relatively large amount of energy in a relatively lightweight package.
The battery works by controlling the flow of lithium ions through a fluid
electrolyte between its two terminals, called the anode and cathode.
The promise—and peril—of using silicon as
the anode in these batteries comes from the way the lithium ions bond with the
anode during the charging cycle. Up to four lithium ions bind to each of the
atoms in a silicon anode—compared to just one for every six carbon atoms in
today’s graphite anode—which allows it to store much more charge.
However, it also swells the anode to as
much as four times its initial volume. What’s more, some of the electrolyte
reacts with the silicon, coating it and inhibiting further charging. When
lithium flows out of the anode during discharge, the anode shrinks back to its
original size and the coating cracks, exposing fresh silicon to the
electrolyte.
Within just a few cycles, the strain of
expansion and contraction, combined with the electrolyte attack, destroys the
anode through a process called “decrepitation.”
Over the past five years, Cui’s group has
progressively improved the durability of silicon anodes by making them out of
nanowires and then hollow silicon nanoparticles. His latest design consists of
a double-walled silicon nanotube coated with a thin layer of silicon oxide, a
very tough ceramic material.
This strong outer layer keeps the outside
wall of the nanotube from expanding, so it stays intact. Instead, the silicon
swells harmlessly into the hollow interior, which is also too small for
electrolyte molecules to enter. After the first charging cycle, it operates for
more than 6,000 cycles with 85% capacity remaining.
Cui said future research is aimed at
simplifying the process for making the double-wall silicon nanotubes. Others in
his group are developing new high-performance cathodes to combine with the new
anode to form a battery with five times the performance of today’s lithium-ion
technology.
In 2008,
Cui founded a company, Amprius, which licensed rights to Stanford’s patents for
his silicon nanowire anode technology. Its near-term goal is to produce a
battery with double the energy density of today’s lithium-ion batteries.
