The first realistic portraits of Squishy Layer are the key to battery performance

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

Lithium-metal batteries can store much more charge in a given space than lithium-ion batteries today, and the race to build them for next-generation electric vehicles, electronics and other applications continues.

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

Although SEI formation is considered 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 looks like when it is saturated with electrolyte, as it would be in a working battery.

Now researchers from the National Laboratory of SLAC Accelerators of the Department of Energy and Stanford University have made the first images of this layer in high resolution in a natural plump state. This progress has been made possible by cryogenic electron microscopy, or cryo-EM, revolutionary technology that reveals small, like atoms, details.

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

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

“There are no other technologies that could look at this interface between electrode and electrolyte with such a high resolution,” said Zwen Zhang, a Stanford graduate student who led experiments with SLAC and Stanford professors Yi Cui and Wa Zhu. “We wanted to prove that we can display the interface at these previously inaccessible scales and see the original state of these materials as they are in the batteries.”

Cui added, “We believe this tumor is almost ubiquitous. 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 metal lithium 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 compresses (arrows) to about half of its previous thickness. SLAC and Stanford researchers used cryo-EM to make the first clear, detailed images of the SEI layer in the humid environment of an active battery. The results offer new ways to improve the performance of next-generation batteries. Credit: Tseven 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, 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 instead of light to observe the world of very small ones. By freezing their samples into a transparent, glassy state, scientists can look at cellular machines that perform vital functions in their natural state and with atomic separation. Recent improvements in cryo-EM have made it a highly sought-after method for unprecedented detailed detection of biological structure, and three scientists have been awarded the 2017 Nobel Prize in Chemistry for their innovative contribution to its development.

Inspired by the 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 annoying layers of SEI on the battery electrode. They published the first images of this layer on an atomic scale in 2017, as well as images of finger growths of lithium wire that can break through the barrier between the two halves of the battery and cause a short circuit or fire.

But to make these images, they had to remove the battery parts from the electrolyte to allow the SEI to dry in a shrunken state. How it looked wet inside a working battery, one could only guess.

In next-generation lithium-metal batteries, the fluid between the electrodes, called the electrolyte, corrodes the surfaces of the electrodes, forming a thin shaky layer called SEI. To make images of this layer on an atomic scale in its native environment, the researchers inserted a metal mesh into a working coin battery (left). When they removed it, thin films of electrolyte stuck to the tiny round holes in the grid, which were held in place by surface tension, and in these same holes on the small lithium wires formed layers of SEI. The researchers removed the excess liquid (center) before immersing the mesh in liquid nitrogen (right) to freeze the films in a glassy state for study using cryo-EM. This gave the first detailed images of the SEI layer in its natural swollen state. Credit: Tseven Zhang / Stanford University

Wetting paper comes to the rescue

To capture SEI in its raw native environment, researchers have devised a way to make and freeze very thin films of electrolyte fluid that contained tiny metallic lithium wires that created a surface for corrosion and SEI formation.

First, they inserted a metal mesh, used to hold cryo-EM samples, into a coin battery. When they removed it, thin films of electrolyte stuck to the tiny round holes in the grid that were held in place by the 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 suggested a fix: moisten the excess liquid with soaking paper. The coated mesh was immediately immersed in liquid nitrogen to freeze the small films in a glassy state that was perfectly preserved by SEI. All this took place in a closed system that protected the films from the effects of air.

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

The results were dramatic, Zhang said. Under these humid conditions SEI absorbs electrolytes and swells about twice the previous thickness.

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

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

The team also used an ultrathin atomic force microscope (AFM) tip to examine the surfaces of the SEI layers and make sure they were more shaky in the wet, swollen state than in the dry.

Years after the 2017 document showed what cryo-EM can do for energy materials, it has been used to increase materials for solar cells and cell-like molecules called organometallic skeletons that can be used in fuel cells, catalyst and gas storage .

As for the next steps, the researchers say they would like to find a way to detect these materials in 3D – and display them while they are still in the working battery, to get the most realistic picture.

And Cui is the director of the Stanford Precourt Institute of Energy and a researcher at the Stanford Institute of Materials and Energy Sciences (SIMES) at SLAC. Wu Chiu is one of the directors of Stanford-SLAC Cryo-EM Facilities, where work on cryo-EM drawing for this study was conducted. Part of this work was performed at Stanford Nano Shared Facilities (SNSF) and Stanford Nanofabrication Facility (SNF). The study is funded by the US Department of Science.

References: “Capturing Swelling of Solid Electrolyte Interphase in Lithium Metal Batteries” Zwen Zhang, Yuzhang Li, Rong Xu, Weijiang Zhou, Yanbin Li, Solomon T. Oyahire, Yetsun Wu, Jinwei Xu, Chiw Dei Yuen Wen Huang, Yusheng Ye, Hao Chen, Jiayu Wang, Zhenan Bao, Wah Qiu and Yi Cui, January 6, 2022, Science.
DOI: 10.1126 / science.abi8703

“Cryogenic Electron Microscopy for Energy Materials” Zwen Zhang, Yi Cui, Raphael Villa, Yanbin Li, Wenbo Zhang, Weijiang Zhou, Wa Zhu and Yi Cui, July 19, 2021, Chemical research accounts.
DOI: 10.1021 / acs.accounts.1c00183