First realistic portraits of a squishy layer that is key to battery performance

Cryo-EM snapshots of the solid electrolyte interphase, or SEI, reveal its natural swollen state and offer a new approach to lithium-metal battery design.

Lithium metal batteries can store far more charge in a given space than lithium-ion batteries can today, and the race is on to create them for electric cars, electronics and other power applications. new generation.

But one of the obstacles is a silent battle between two of the battery components. The electrolyte, the liquid between the two electrodes, corrodes the surface of the lithium metal anode, coating it with a thin layer of gunk known as the solid electrolyte interphase, or SEI.

Although SEI formation is considered inevitable, researchers want to stabilize and manage the growth of this layer to maximize battery performance. But they never got a clear picture of what the SEI looks like when saturated with electrolyte, as it would be in a working battery.

Now, researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have taken the first high-resolution images of this layer in its plump, spongy natural state. This breakthrough was made possible by cryogenic electron microscopy, or cryo-EM, a breakthrough technology that reveals details as small as atoms.

The results, they said, suggest the right electrolyte can minimize swelling and improve battery performance, giving scientists a potential new way to modify and improve battery design. They also offer researchers a new tool to study batteries in their daily work environment.

The team described their work in an article published in Science on January 6, 2022.

“No other technology can examine this interface between the electrode and the electrolyte with such high resolution,” said Zewen Zhang, a Stanford doctoral student who led the experiments with SLAC and Stanford professor Yi Cui. and Wah Chiu. “We wanted to prove that we could image the interface at these previously inaccessible scales and see the native, pristine state of these materials as they are found in batteries.”

Cui added, “We find that this swelling is almost universal. Its effects have not been widely appreciated by the battery research community before, but we have found it to have a significant impact on battery performance.

SEI SLAC coated lithium wire

This video shows a metallic lithium wire covered with a layer called SEI and saturated with the surrounding liquid electrolyte; the dotted lines represent the outer edges of this SEI layer. As the electrolyte is removed, the SEI dries out and shrinks (arrows) to about half its previous thickness. SLAC and Stanford researchers have used cryo-EM to create the first clear, detailed images of the SEI layer in the humid environment of a working battery. The results suggest new ways to improve the performance of next-generation batteries. Credit: Zewen Zhang/Stanford University

An “exciting” tool for energy research

This is the latest in a series of groundbreaking results over the past five years that show that cryo-EM, which was developed as a tool for biology, opens up “exciting opportunities” in energy research, has writes the team in a separate journal of the field published in July in Chemical research accounts.

Cryo-EM is a form of electron microscopy, which uses electrons rather than light to observe the world of toddlers. By rapidly freezing their samples in a clear, glassy state, scientists can observe the cellular machinery that performs life’s functions in their natural state and at atomic resolution. Recent improvements in cryo-EM have made it a highly sought-after method for revealing biological structure in unprecedented detail, and three scientists have been awarded the 2017 Nobel Prize in Chemistry for their pioneering contributions to its development.

Inspired by many success stories in biological cryo-EM, Cui teamed up with Chiu to explore whether cryo-EM could be as useful a tool for studying energy-related materials as it is for studying living systems. .

One of the first things they looked at was one of those pesky SEI layers on a battery electrode. They published the first atomic-scale images of this layer in 2017, along with images of finger-like growths of lithium wire that can puncture the barrier between the two halves of the battery and cause short circuits. or fires.

But to make these images, they had to remove the battery parts from the electrolyte, so the SEI dried into a shrunken state. What it looked like wet inside a working battery was anyone’s guess.

Next Generation SLAC Lithium Metal Batteries

In next-generation lithium-metal batteries, the liquid between the electrodes, called the electrolyte, corrodes the surfaces of the electrodes, forming a thin, spongy layer called SEI. To create atomic-scale images of this layer in its native environment, the researchers inserted a metal grid into a working button battery (left). When they pulled it out, thin films of electrolyte clung to tiny circular holes in the grid, held in place by surface tension, and layers of SEI had formed on tiny lithium wires in those same holes. Researchers removed excess liquid (center) before submerging the grid in liquid nitrogen (right) to freeze the films in a glassy state for examination with cryo-EM. This gave the first detailed images of the SEI layer in its natural inflated state. Credit: Zewen Zhang/Stanford University

Blotting paper to the rescue

To capture SEI in its soggy native environment, the researchers found a way to make and freeze very thin films of the electrolytic liquid containing tiny wires of metallic lithium, which provided a surface for corrosion and SEI formation.

First, they inserted a metal grid used to hold cryo-EM samples in a button cell. When they pulled it out, thin films of electrolyte clung to tiny circular holes inside the grid, held in place by surface tension just long enough to complete the remaining steps.

However, these films were still too thick for the electron beam to penetrate and produce sharp images. So Chiu suggested a solution: blot the excess liquid with blotting paper. The blotted grid was immediately immersed in liquid nitrogen to freeze the small films to a glassy state that perfectly preserved the SEI. All of this took place in a closed system that protected the films from exposure to air.

Cryo EM Images Electrolyte SLAC

Cryo-EM images of electrolyte clinging to holes in a sample grid show why it is important to remove excess electrolyte before freezing and imaging samples. At the top, excess electrolyte has frozen into a thick layer (right) and sometimes even formed crystals (left), blocking the microscope’s view of the tiny circular samples below. After the transfer (bottom), the grid (left) and its tiny holes (right) can be clearly seen and probed with electron beams. SLAC and Stanford researchers used this method to create the first realistic cryo-EM images of a layer called SEI that forms on the surface of electrodes due to chemical reactions with battery electrolyte. Credit: Weijiang Zhou/Stanford University

The results have been dramatic, Zhang said. In these humid environments, the SEIs absorbed electrolytes and swelled to about twice their previous thickness.

When the team repeated the process with half a dozen other electrolytes of varying chemical compositions, they found that some produced much thicker SEI layers than others – and that the layers that swelled the most were associated with the worse battery performance.

“At present, this link between SEI swelling behavior and performance applies to lithium metal anodes,” Zhang said, “but we believe it should generally apply to other anodes as well. metallic.”

The team also used the super-fine tip of an atomic force microscope (AFM) to probe the surfaces of the SEI layers and verify that they were more squishy in their wet, puffy state than in their dry state.

In the years since the 2017 paper revealed what cryo-EM can do for energetic materials, it has been used to zoom in on materials for solar cells and cage-like molecules called metallo-organic frameworks that can be used in fuel cells, catalysis and gas storage.

As for next steps, the researchers say they’d like to find a way to image these materials in 3D – and image them while they’re still inside a working battery, to the most realistic image to date.

Yi Cui is director of the Precourt Institute for Energy at Stanford and a research fellow at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC. Wah Chiu is co-director of the Stanford-SLAC Cryo-EM facilities, where the cryo-EM imaging work for this study took place. Some of this work was done at the Stanford Nano Shared Facilities (SNSF) and the Stanford Nanofabrication Facility (SNF). The research was funded by the DOE Office of Science.

References: “Capturing Solid Electrolyte Interphase Swelling in Lithium Metal Batteries” by Zewen Zhang, Yuzhang Li, Rong Xu, Weijiang Zhou, Yanbin Li, Solomon T. Oyakhire, Yecun Wu, Jinwei Xu, Hansen Wang, Zhiao Yu, David T. Boyle, William Huang, Yusheng Ye, Hao Chen, Jiayu Wan, Zhenan Bao, Wah Chiu and Yi Cui, January 6, 2022, Science.
DOI: 10.1126/science.abi8703

“Cryogenic Electron Microscopy for Energetic Materials” by Zewen Zhang, Yi Cui, Rafael Vila, Yanbin Li, Wenbo Zhang, Weijiang Zhou, Wah Chiu and Yi Cui, July 19, 2021, Chemical research accounts.
DOI: 10.1021/acs.accounts.1c00183

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