Researchers at the Department of Energy (DoE) have observed for the first time a swelling process that occurs when silicon is used as an anode in a battery, an important step forward to making the material viable for future devices, they said.
A team at Pacific Northwest National Laboratory (PNNL) and other collaborators observed the process, which causes the anode to flake and crack. That, in turn, causes the battery to lose its ability to hold a charge and ultimately to fail.
“Many people have imagined what might be happening but no one had actually demonstrated it before,” said Chongmin Wang, a scientist at PNNL who worked on the research.
Now that researchers have, they can find out how to solve the problem so silicon can be used to replace graphite in future lithium batteries for EVs. This paves the way for devices that cost less but have higher performance and charge faster, he said.
Indeed, silicon—which is already used widely in microprocessors and photovoltaics—is a promising battery material because it can hold 10 times as much energy in an anode than graphite. Solving the process of swelling could help scientists make good on this promise.
How It Was Done
The swelling of a silicon anode in a battery occurs because lithium ions penetrate the anode in such a way that it displaces the silicon atoms, creating a “lithium squeeze” that causes the anode to swell up to three or four times its original size.
When the lithium ions depart the anode during the storage process, empty spaces known as vacancies remain, in which some silicon atoms enter. However, lithium ions also continue to push their way in, ultimately leading to battery failure.
While scientists have known about this process for years, they had never witnessed why it creates failure in the battery before, and so didn’t attribute the cause correctly. The PNNL team managed to discover both why and when the process occurs in a lithium battery with a silicon anode.
In their experiments, the team used electrons to make ultra-high-resolution images of the anode swelling and then reconstructed the images in 3D, similar to how physicians create a 3D image of a patient’s limb or organ. In addition to researchers from PNNL, scientists from Los Alamos National Laboratory, Thermo Fisher Scientific, and Penn State University also worked on the project.
Problem and Solution
Based on researchers’ observations, the swelling process begins immediately after just one battery cycle. It causes the battery’s ability to hold a charge to fall significantly after 36 cycles; after 100 cycles, the anode was ruined.
By observing the process through experiments, the team watched as the vacancies left by lithium ions in the silicon anode evolved into larger and larger gaps.
This allowed the liquid electrolyte to flow into the gaps, infiltrating the silicon and causing silicon to reside in places where it shouldn’t be. This created dead zones that destroyed the ability of the silicon to store lithium and ruining the anode.
Researchers published a paper on their work in the journal Nature Nanotechnology.
Now that they know where the problem lies, scientists are working on ways to protect the silicon from the electrolyte to create viable silicon anodes. “This work offers a clear roadmap for developing silicon as the anode for a high-capacity battery,” Wang said.
One way to do this is through coatings, which several groups, including scientists at PNNL, are developing. The coatings would act as a type of security gate that allows lithium ions to go into and out of the anode while stopping other components of the electrolyte from doing the same.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.