With the rapid development of portable electronic devices and electric vehicles, people are not only pursuing larger capacity and faster charging and discharging speeds for lithium batteries, but also increasingly concerned about ensuring their safety. The occasional lithium battery explosions and other incidents inevitably cause anxiety. Solving the lithium battery safety problem requires scientists to gain a thorough and comprehensive understanding of the causes of lithium battery explosions.
The current scientific explanation is that lithium deposition on the electrode surface forms dendrites, which continue to grow, causing internal short circuits that can lead to battery malfunctions or even fires. However, in the past, there was a lack of effective technical means to understand and study this problem at the atomic structure level and find solutions.
The cryo-electron microscopy (cryo-EM) technique, which just won the 2017 Nobel Prize in Chemistry this month, provided strong technical support for this. A research team led by Professor Yi Cui of Stanford University and the SLAC National Accelerator Laboratory (affiliated with the U.S. Department of Energy), along with Steven Chu, the 1997 Nobel Laureate in Physics, captured the first image of atomic-level lithium metal dendrites using cryo-electron microscopy (cryo-EM). The research results were published in the international academic journal *Science* on October 27th.
Each lithium metal dendrite is a long, perfectly formed, six-sided crystal. Previously, only irregularly shaped crystals were observed under an electron microscope. Cui Yi stated, "The research results are very exciting and have opened up a completely new field for related research!"
Cryo-electron microscopy, as the name suggests, is a microscopic technique that uses cryo-fixation to observe samples at low temperatures using a transmission electron microscope (TEM). Cryo-electron microscopy is an important method in structural biology research and a crucial means of obtaining the structures of biological macromolecules.
Because images are key to understanding mechanisms, scientific breakthroughs often rely on successfully acquiring visual images of targets with the naked eye. For a long time, TEM was considered unsuitable for observing biomolecules because the powerful electron beams would damage biological materials. However, the advent of cryo-electron microscopy has allowed researchers to "freeze" biomolecules, observing and analyzing their movement processes like never before. These characterizations have a decisive impact on the understanding of biochemistry and the development of pharmacology. For this reason, cryo-electron microscopy is also slated for this year's Nobel Prize in Chemistry.
Left image: In a TEM image at room temperature, lithium dendrites are corroded due to exposure to air, and the electron beam melts numerous pores into them; Right image: Cryo-EM image, the freezing environment preserves its original state, showing crystalline nanowires with well-defined interfaces.
The same applies to materials like lithium; transmission electron microscopy (TEM) cannot be used to examine dendrites at the atomic level. Similar to biological materials, when using TEM at room temperature, the dendrite edges curl or even melt due to electron beam impacts. Yanbin Li, a Stanford University doctoral student involved in this work, explained, "The TEM samples were prepared in air, but lithium metal corrodes very quickly in air." He added, "Every time we try to observe metallic lithium with a high-powered electron microscope, electrons 'drill holes' in the dendrites, even completely melting them."
Yanbin Li, a Stanford University doctoral student involved in the study, said, "It's like looking at a leaf with a magnifying glass in the sunlight. But if you can cool the leaf, the problem is solved: you focus the light on the leaf, the heat dissipates, and the leaf isn't damaged. That's what we can achieve with cryo-electron microscopy, and the difference is very obvious when applied to imaging battery materials."
Therefore, cryo-electron microscopy not only ushered in a new era for biochemistry, but also allowed scientists to observe the complete structure of lithium dendrites at the atomic level for the first time. Researchers also discovered that dendrites in carbonate-based electrolytes grow into single-crystal nanowires along a specific direction. Some of these nanowires may become knotted during the "growth" process, but their crystal structure remains intact.
Yu Zhangli, a Stanford University doctoral student who also participated in the research, stated that a solid electrolyte interphase (SEI) membrane was observed, and different SEI nanostructures were revealed to form in different electrolytes. Because the same coating forms on the metal electrodes during battery charging and discharging, controlling its formation and stability is crucial for the efficient use of the battery.
Using cryo-EM, scientists were able to observe how electrons are ejected from atoms within dendrites, thus revealing the position of individual atoms (left image). Scientists were even able to measure the distances between atoms (top right image), and the interatomic spacing precisely indicates that they are lithium atoms (bottom right image).
A press release from SLAC reveals that researchers used various techniques under a microscope to observe how electrons are ejected from the atoms of dendrites, revealing the location of individual atoms within the crystal and its solid electrolyte interphase (SEI) coating. When they added chemicals typically used to improve battery performance, the atomic structure of the SEI coating became more ordered, which helps explain why the additives work.
“We are very excited. This is the first time we have been able to obtain such detailed images of dendrites, and the first time we have seen the nanostructure of a solid electrolyte interfacial film,” said Yanbin Li. “This tool can help us understand what different electrolytes do and why some electrolytes are more effective than others.”
The data observed in these experiments can provide a deeper understanding of battery failure mechanisms. While this work uses lithium metal as an example to demonstrate the practicality of cryo-EM, this approach could potentially be extended to other studies involving beam-sensitive materials such as silicon lithium or sulfur. The research team also stated that they plan to focus on further understanding the chemical properties and structure of the solid electrolyte interface film.
