Silicon anodes muscle in on battery technology
Scientists show exactly how promising approach to better batteries breaks
down

Date:
October 5, 2021
Source:
DOE/Pacific Northwest National Laboratory
Summary:
One effort toward better batteries for electric vehicles is hitting
overdrive, thanks to new findings about silicon anodes.



FULL STORY ========================================================================== Silicon is a staple of the digital revolution, shunting loads of signals
on a device that's likely just inches from your eyes at this very moment.


==========================================================================
Now, that same plentiful, cheap material is becoming a serious candidate
for a big role in the burgeoning battery business. It's especially
attractive because it's able to hold 10 times as much energy in an
important part of a battery, the anode, than widely used graphite.

But not so fast. While silicon has a swell reputation among scientists,
the material itself swells when it's part of a battery. It swells so
much that the anode flakes and cracks, causing the battery to lose its
ability to hold a charge and ultimately to fail.

Now scientists have witnessed the process for the first time, an important
step toward making silicon a viable choice that could improve the cost, performance and charging speed of batteries for electric vehicles as
well as cell phones, laptops, smart watches and other gadgets.

"Many people have imagined what might be happening but no one had actually demonstrated it before," said Chongmin Wang, a scientist at the Department
of Energy's Pacific Northwest National Laboratory. Wang is a corresponding author of the paper recently published in Nature Nanotechnology.

Of silicon anodes, peanut butter cups and packed airline passengers
Lithium ions are the energy currency in a lithium-ion battery,
traveling back and forth between two electrodes through liquid called electrolyte. When lithium ions enter an anode made of silicon, they
muscle their way into the orderly structure, pushing the silicon atoms
askew, like a stout airline passenger squeezing into the middle seat on
a packed flight. This "lithium squeeze" makes the anode swell to three
or four times its original size.



==========================================================================
When the lithium ions depart, things don't return to normal. Empty
spaces known as vacancies remain. Displaced silicon atoms fill in many,
but not all, of the vacancies, like passengers quickly taking back the
empty space when the middle passenger heads for the restroom. But the
lithium ions return, pushing their way in again. The process repeats as
the lithium ions scoot back and forth between the anode and cathode, and
the empty spaces in the silicon anode merge to form voids or gaps. These
gaps translate to battery failure.

Scientists have known about the process for years, but they hadn't
before witnessed precisely how it results in battery failure. Some have attributed the failure to the loss of silicon and lithium. Others have
blamed the thickening of a key component known as the solid-electrolyte interphase or SEI. The SEI is a delicate structure at the edge of the
anode that is an important gateway between the anode and the liquid electrolyte.

In its experiments, the team watched as the vacancies left by lithium
ions in the silicon anode evolved into larger and larger gaps. Then
they watched as the liquid electrolyte flowed into the gaps like tiny
rivulets along a shoreline, infiltrating the silicon. This inflow allowed
the SEI to develop in areas within the silicon where it shouldn't be,
a molecular invader in a part of the battery where it doesn't belong.

That created dead zones, destroying the ability of the silicon to store
lithium and ruining the anode.

Think of a peanut butter cup in pristine shape: The chocolate outside is distinct from the soft peanut butter inside. But if you hold it in your
hand too long with too tight a grip, the outer shell softens and mixes
with the soft chocolate inside. You're left with a single disordered
mass whose structure is changed irreversibly. You no longer have a true
peanut butter cup. Likewise, after the electrolyte and the SEI infiltrate
the silicon, scientists no longer have a workable anode.



==========================================================================
The team witnessed this process begin immediately after just one battery
cycle.

After 36 cycles, the battery's ability to hold a charge had fallen dramatically. After 100 cycles, the anode was ruined.

Exploring the promise of silicon anodes Scientists are working on ways
to protect the silicon from the electrolyte.

Several groups, including scientists at PNNL, are developing coatings
designed to act as gatekeepers, allowing lithium ions to go into and
out of the anode while stopping other components of the electrolyte.

Scientists from several institutions pooled their expertise to do
the work.

Scientists at Los Alamos National Laboratory created the silicon nanowires
used in the study. PNNL scientists worked together with counterparts
at Thermo Fisher Scientific to modify a cryogenic transmission electron microscope to reduce the damage from the electrons used for imaging. And
Penn State University scientists developed an algorithm to simulate the molecular action between the liquid and the silicon.

Altogether, the team used electrons to make ultra-high-resolution images
of the process and then reconstructed the images in 3D, similar to how physicians create a 3D image of a patient's limb or organ.

"This work offers a clear roadmap for developing silicon as the anode
for a high-capacity battery," said Wang.

At PNNL, the work is part of a broad research program exploring silicon
anodes, including original materials like coatings, new ways to make
the devices, and a new electrolyte that increases battery life.

In addition to Wang, other PNNL authors of the paper include Yang
He, Yaobin Xu, Haiping Jia, Ran Yi, Miao Song, Xiaolin Li (also a
corresponding author) and Ji-Guang (Jason) Zhang.

========================================================================== Story Source: Materials provided by
DOE/Pacific_Northwest_National_Laboratory. Original written by Tom
Rickey. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yang He, Lin Jiang, Tianwu Chen, Yaobin Xu, Haiping Jia, Ran Yi,
Dingchuan Xue, Miao Song, Arda Genc, Cedric Bouchet-Marquis,
Lee Pullan, Ted Tessner, Jinkyoung Yoo, Xiaolin Li, Ji-Guang
Zhang, Sulin Zhang, Chongmin Wang. Progressive growth of the
solid-electrolyte interphase towards the Si anode interior
causes capacity fading. Nature Nanotechnology, 2021; DOI:
10.1038/s41565-021-00947-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211005101915.htm

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