Engineers create a high performance all-solid-state battery with a pure-silicon anode
UC San Diego News Center
September
23, 2021 – Engineers created a new type of battery that weaves two promising
battery sub-fields into a single battery. The battery uses both a solid state
electrolyte and an all-silicon anode, making it a silicon all-solid-state
battery. The initial rounds of tests show that the new battery is safe, long
lasting, and energy dense. It holds promise for a wide range of applications
from grid storage to electric vehicles.
The battery technology is described in
the Sept. 24, 2021 issue of the journal Science. University of
California San Diego nanoengineers led the research, in collaboration with
researchers at LG Energy Solution.
Silicon anodes are famous for their
energy density, which is 10 times greater than the graphite anodes most often
used in today’s commercial lithium ion batteries. On the other hand, silicon
anodes are infamous for how they expand and contract as the battery charges and
discharges, and for how they degrade with liquid electrolytes. These challenges
have kept all-silicon anodes out of commercial lithium ion batteries despite
the tantalizing energy density. The new work published in Science
provides a promising path forward for all-silicon-anodes, thanks to the right
electrolyte.
"With this battery configuration,
we are opening a new territory for solid-state batteries using alloy anodes
such as silicon," said Darren H. S. Tan, the lead author on the paper. He
recently completed his chemical engineering PhD at the UC San Diego Jacobs
School of Engineering and co-founded a startup UNIGRID Battery that has
licensed this technology.
Next-generation, solid-state batteries
with high energy densities have always relied on metallic lithium as an anode.
But that places restrictions on battery charge rates and the need for elevated
temperature (usually 60 degrees Celsius or higher) during charging. The silicon
anode overcomes these limitations, allowing much faster charge rates at room to
low temperatures, while maintaining high energy densities.
The team demonstrated a laboratory scale
full cell that delivers 500 charge and discharge cycles with 80% capacity
retention at room temperature, which represents exciting progress for both the
silicon anode and solid state battery communities.
Silicon as an anode to replace graphite
Silicon anodes, of course, are not new.
For decades, scientists and battery manufacturers have looked to silicon as an
energy-dense material to mix into, or completely replace, conventional graphite
anodes in lithium-ion batteries. Theoretically, silicon offers approximately 10
times the storage capacity of graphite. In practice however, lithium-ion
batteries with silicon added to the anode to increase energy density typically
suffer from real-world performance issues: in particular, the number of times
the battery can be charged and discharged while maintaining performance is not
high enough.
Much of the problem is caused by the
interaction between silicon anodes and the liquid electrolytes they have been
paired with. The situation is complicated by large volume expansion of silicon
particles during charge and discharge. This results in severe capacity losses
over time.
“As battery researchers, it’s vital to
address the root problems in the system. For silicon anodes, we know that one
of the big issues is the liquid electrolyte interface instability," said
UC San Diego nanoengineering professor Shirley Meng, the corresponding author
on the Science paper, and director of the Institute for Materials
Discovery and Design at UC San Diego. “We needed a totally different
approach,” said Meng.
Indeed, the UC San Diego led team took a
different approach: they eliminated the carbon and the binders that went with
all-silicon anodes. In addition, the researchers used micro-silicon, which is
less processed and less expensive than nano-silicon that is more often used.
An all solid-state solution
In addition to removing all carbon and
binders from the anode, the team also removed the liquid electrolyte. Instead,
they used a sulfide-based solid electrolyte. Their experiments showed this
solid electrolyte is extremely stable in batteries with all-silicon anodes.
"This new work offers a promising
solution to the silicon anode problem, though there is more work to do,"
said professor Meng, "I see this project as a validation of our approach
to battery research here at UC San Diego. We pair the most rigorous theoretical
and experimental work with creativity and outside-the-box thinking. We also
know how to interact with industry partners while pursuing tough fundamental
challenges."
Past efforts to commercialize silicon
alloy anodes mainly focus on silicon-graphite composites, or on combining
nano-structured particles with polymeric binders. But they still struggle with
poor stability.
By swapping out the liquid electrolyte
for a solid electrolyte, and at the same time removing the carbon and binders
from the silicon anode, the researchers avoided a series of related challenges
that arise when anodes become soaked in the organic liquid electrolyte as the
battery functions.
At the same time, by eliminating the
carbon in the anode, the team significantly reduced the interfacial contact
(and unwanted side reactions) with the solid electrolyte, avoiding continuous
capacity loss that typically occurs with liquid-based electrolytes.
This two-part move allowed the
researchers to fully reap the benefits of low cost, high energy and
environmentally benign properties of silicon.
Impact & Spin-off Commercialization
“The solid-state silicon approach
overcomes many limitations in conventional batteries. It presents exciting
opportunities for us to meet market demands for higher volumetric energy,
lowered costs, and safer batteries especially for grid energy storage,” said
Darren H. S. Tan, the first author on the Science paper.
Sulfide-based solid electrolytes were
often believed to be highly unstable. However, this was based on traditional
thermodynamic interpretations used in liquid electrolyte systems, which did not
account for the excellent kinetic stability of solid electrolytes. The team saw
an opportunity to utilize this counterintuitive property to create a highly
stable anode.
Tan is the CEO and cofounder of a
startup, UNIGRID Battery, that has licensed the technology for these silicon
all solid-state batteries.
In parallel, related fundamental work
will continue at UC San Diego, including additional research collaboration with
LG Energy Solution.
“LG Energy Solution is delighted that
the latest research on battery technology with UC San Diego made it onto the
journal of Science, a meaningful acknowledgement,” said Myung-hwan Kim,
President and Chief Procurement Officer at LG Energy Solution. “With the latest
finding, LG Energy Solution is much closer to realizing all-solid-state battery
techniques, which would greatly diversify our battery product lineup.”
“As a leading battery manufacturer, LGES
will continue its effort to foster state-of-the-art techniques in leading
research of next-generation battery cells,” added Kim. LG Energy Solution said
it plans to further expand its solid-state battery research collaboration with
UC San Diego.
The study had been supported by LG
Energy Solution’s open innovation, a program that actively supports
battery-related research. LGES has been working with researchers around the
world to foster related techniques.
Title of Paper
“Carbon Free High Loading Silicon Anodes
Enabled by Sulfide Solid Electrolytes,” in the Sept. 24, 2021 issue of Science.
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