Researchers from the Department of Energy’s SLAC National Accelerator Laboratory (SLAC) and Stanford University have made the first clear, detailed images of the solid-electrolyte interphase (SEI) layer in the wet environment of a working Li-ion battery by using cryogenic electron microscopy (cryo-EM).

Although formation of SEI is believed to be inevitable, researchers hope to stabilize and control the growth of this layer in a way that maximizes the battery’s performance. But until now they have never had a clear picture of what the SEI looks like when it’s saturated with electrolyte, as it would be in a working battery. A paper on the work is published in Science.

Although liquid-solid interfaces are foundational in broad areas of science, characterizing this delicate interface remains inherently difficult because of shortcomings in existing tools to access liquid and solid phases simultaneously at the nanoscale. This leads to substantial gaps in our understanding of the structure and chemistry of key interfaces in battery systems. We adopt and modify a thin film vitrification method to preserve the sensitive yet critical interfaces in batteries at native liquid electrolyte environments to enable cryo–electron microscopy and spectroscopy. We report substantial swelling of the solid-electrolyte interphase (SEI) on lithium metal anode in various electrolytes. The swelling behavior is dependent on electrolyte chemistry and is highly correlated to battery performance. Higher degrees of SEI swelling tend to exhibit poor electrochemical cycling.

—Zhang et al. 2022

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

—Stanford professor Yi Cui, co-corresponding author

This is the latest in a series of groundbreaking results over the past five years that show cryo-EM, which was developed as a tool for biology, opens “thrilling opportunities” in energy research, the team wrote in a separate review of the field published in July in Accounts of Chemical Research.

Cryo-EM is a form of electron microscopy, which uses electrons rather than light. By flash-freezing their samples into a clear, glassy state, scientists can look at the cellular machines that carry out life’s functions in their natural state and at atomic resolution. Recent improvements in cryo-EM have transformed it into a highly sought method for revealing biological structure in unprecedented detail, and three scientists were 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 Stanford professor Wah Chiu to explore whether cryo-EM could be as useful a tool for studying energy-related materials as it was for studying living systems.

The researchers 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. However, to make those images they had to take the battery parts out of the electrolyte, so that the SEI dried into a shrunken state.

To capture the SEI in its wet native environment, the researchers adapted a thin-film vitrification method to freeze very thin films of the electrolyte liquid that contained tiny lithium metal wires, which offered a surface for corrosion and the formation of SEI.

To make atomic-scale images of this layer in its native environment, researchers inserted a metal grid into a working coin cell battery (left). When they removed it, thin films of electrolyte clung to tiny circular holes within the grid, held in place by surface tension, and SEI layers had formed on tiny lithium wires in those same holes. Researchers blotted away excess liquid (center) before plunging the grid into liquid nitrogen (right) to freeze the films into a glassy state for examination with cryo-EM. This yielded the first detailed images of the SEI layer in its natural swollen state. (Zewen Zhang/Stanford University)

First, they inserted a metal grid used for holding cryo-EM samples into a coin cell battery. When they removed it, thin films of electrolyte clung to tiny circular holes within the grid, held in place by surface tension just long enough to perform the remaining steps.

However, those films were still too thick for the electron beam to penetrate and produce sharp images. So Chiu suggested a fix: sopping up the excess liquid with blotter paper. The blotted grid was immediately plunged into liquid nitrogen to freeze the little films into a glassy state that perfectly preserved the SEI. All this took place in a closed system that protected the films from exposure to air.

In these wet environments, SEIs absorbed electrolyte and swelled to about twice their previous thickness.

Cryo-EM images of electrolyte clinging to holes in a sample grid show why it’s important to blot away excess electrolyte before freezing and imaging the samples. At 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 beneath. After blotting (bottom), the grid (left) and its tiny holes (right) can clearly be seen and probed with beams of electrons. SLAC and Stanford researchers used this method to make the first realistic cryo-EM images of a layer called SEI that forms on the surfaces of electrodes due to chemical reactions with the battery electrolyte. (Weijiang Zhou/Stanford University)

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 worst battery performance.

Right now that connection between SEI swelling behavior and performance applies to lithium metal anodes, but we think it should apply as a general rule to other metallic anodes, as well.

—Zewen Zhang, lead author

Yi Cui is director of Stanford’s Precourt Institute for Energy and an investigator with 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. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) and Stanford Nanofabrication Facility (SNF). The research was funded by the DOE Office of Science.

Resources

• Zewen Zhang et al. (2022) “Capturing the swelling of solid-electrolyte interphase in lithium metal batteries”, Science doi: 10.1126/science.abi8703

• Zewen Zhang et al. (2021) “Cryogenic Electron Microscopy for Energy Materials” Accounts of Chemical Research, July 2021 doi: 10.1021/acs.accounts.1c00183