First realistic portraits of the Squishy layer, which is the key to battery performance

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

Lithium-metal batteries can store much more charge in a space than lithium-ion batteries can today, and the race is to create them for the next generation of electric cars, electronics and other applications.

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

Although SEI formation is thought to be inevitable, researchers want to stabilize and control the growth of this layer to maximize battery performance. But they never had a clear idea of ​​what SEI looked like when it was saturated with electrolyte, as it would be in a working battery.

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

The results, they said, suggest that the right electrolyte can minimize swelling and improve battery performance – giving scientists a potential new way to adjust and improve battery design. They also give researchers a new tool for studying batteries in their everyday work environments.

The team describes its work in a document published in science on January 6, 2022

“There are no other technologies that can look at this interface between an electrode and an electrolyte with such a high resolution,” said Zeven Zhang, a Stanford PhD student who led the experiments with SLAC and Stanford professors Yi Cui and Wah Chiu. “We wanted to prove that we can portray the interface on this hitherto inaccessible scale and see the pristine, natural state of these materials, as they are in batteries.

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

This video shows a lithium metal wire coated 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 and shrinks (arrows) to about half its previous thickness. Researchers from SLAC and Stanford used cryo-EM to make the first clear, detailed images of the SEI layer in a humid environment with a working battery. The results offer 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 revolutionary results over the past five years that show that cryo-EM, developed as a tool for biology, opens up “exciting opportunities” in energy research, the team wrote in a separate review of the field published in July. Chemical research accounts.

Cryo-EM is a form of electron microscopy that uses electrons, not light, to observe the world of the very young. By freezing their samples in a flash, in a transparent, glassy state, scientists can examine cellular machines that perform the functions of life in their natural state and with atomic resolution. Recent improvements in cryo-EM have made it a highly sought-after method for discovering biological structure with 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, Qui has partnered with Chiu to explore whether cryo-EM can 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 annoying SEI layers on the battery electrode. They published the first atomic-scale images of this layer in 2017, along with images of finger-like lithium wire growths that can break 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 that the SEI could dry in a shrunken state. How it looked wet inside a working battery, everyone can guess.

In next-generation lithium-metal batteries, the fluid between the electrodes, called the electrolyte, corrodes the surfaces of the electrodes to form a thin, dense layer called SEI. To make atomic scale images of this layer in its native environment, the researchers inserted a metal grid into a working coin battery (left). When removed, thin films of electrolyte adhered to small round holes in the grid held in place by surface tension, and SEI layers formed on small lithium wires in these same holes. The researchers wiped off the excess liquid (center) before immersing the grid in liquid nitrogen (right) to freeze the films in a glass state for cryo-EM testing. This led to the first detailed images of the SEI layer in its natural swollen state. Credit: Zewen Zhang / Stanford University

Absorbent paper help

To capture SEI in its humid natural environment, researchers have devised a way to make and freeze very thin films of electrolyte fluid that contains small lithium metal wires that offer a surface for corrosion and SEI formation.

First, they inserted a metal grid used to hold cryo-EM samples in a coin cell battery. When it was removed, thin films of electrolyte clung to small round holes in the grid, held in place by surface tension long enough to perform the remaining steps.

However, these films were still too thick for the electron beam to penetrate and create sharp images. So Chiu proposed a solution: soak up the excess liquid with absorbent paper. The touched grid is immediately immersed in liquid nitrogen to freeze the small films in a glassy state, which perfectly preserves the SEI. All this happens in a closed system that protects the films from exposure to air.

Cryo-EM images of electrolyte adhering to holes in the sample grid show why it is important to remove excess electrolyte before freezing and imaging samples. On top, the excess electrolyte froze in a thick layer (right) and sometimes even formed crystals (left), blocking the microscope’s view of the small round samples below. After absorption (below), the lattice (left) and its small holes (right) can be clearly seen and examined with electron beams. Researchers at SLAC and Stanford used this method to make the first realistic cryo-EM images of a layer called SEI, which forms on electrode surfaces due to chemical reactions with the battery electrolyte. Credit: Weijiang Zhou / Stanford University

The results were dramatic, Zhang said. In these wet environments, SEIs absorb electrolytes and swell to about twice their previous thickness.

When the team repeated the process with half a dozen other electrolytes with different 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 worst battery performance.

“Currently, this link between SEI’s swelling behavior and performance applies to lithium metal anodes,” Zhang said, “but we think it should be applied as a general rule to other metal anodes as well.”

The team also used the super-fine tip of an atomic force microscope (AFM) to study the surfaces of the SEI layers and see if they were rougher in the wet, swollen state than in the dry state.

In the years since the 2017 document revealed what cryo-EM can do for energy materials, it has been used to increase solar cell materials and cell-like molecules called metal-organic frames that can be used in fuel cells, catalysis and gas storage.

As for the next steps, the researchers say they would like to find a way to depict these materials in 3D – and depict them while still in battery life for the most realistic picture to date.

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

References: “Capture of solid electrolyte intermediate 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 energy 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