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Nobel Prize application Atomic cryogenic microscopy reveals the mystery of lithium battery explosion Jan 22, 2018

With the rapid development of portable electronic devices and electric vehicles, people are more concerned with how to ensure the safety of lithium batteries in addition to pursuing a larger capacity of lithium batteries and faster charging and discharging. Because from time to time lithium battery explosion and other incidents, must not let people nervous.


How to solve the safety problem of lithium battery is based on the premise that scientists should understand the cause of explosion of lithium battery as far as possible and comprehensively. At present, the scientific explanation is that lithium deposition on the electrode surface forms "dendrites," and that it will continue to grow, causing a short circuit within the battery that could cause the battery to malfunction or could cause a fire. But how to understand and research from the atomic structure level and then find a solution to the problem, in the past the lack of effective technical means.


This month has just won the 2017 Nobel Prize in Chemistry cryo-EM (cryo-EM) technology, to provide a strong technical support. Stanley Cui, a professor at SLAC's National Accelerator Laboratory directly under the US Department of Energy, and a research team led by Steven Chu, a Nobel laureate in physics, captured the first atomic level by cryo-EM Image of lithium metal dendrite. The research results were published in the international academic journal "Science" on October 27 local time.


The above image shows that each lithium metal dendrite is a long, perfectly shaped hexahedron. Previously observed by electron microscopy only irregular shape of the crystal. Cui Yi said, "The research results are very exciting, but also for the relevant research has opened a whole new situation!"


Cryo-electron microscopy, as its name implies, is a microscopic technique for observing samples at low temperatures using a transmission electron microscope (Transmission Electron Microscope, TEM for short) using cryosecure. Cryo-electron microscopy is an important structural biology research method and an important means of obtaining the structure of biological macromolecules.


Because the image is the key to understanding the mechanism, scientific breakthroughs are often based on using the naked eye to successfully target their visual constellations. It has long been assumed that TEMs are not suitable for viewing biomolecules because powerful electron beams can destroy biological materials. However, the development of cryo-EM allows researchers to "freeze" biomolecules and observe and analyze the movement process unprecedentedly, all of which have a decisive influence on biochemical understanding and pharmacology. Because of this, frozen electron microscope will also be included in this year's Nobel Chemistry.


Left: In the TEM image at room temperature, dendrites of lithium are corroded by exposure to the air and the electron beam also melts out a large number of holes on the top; right: images under cryo-EM, the frozen environment preserves the original State, indicating that it has a clear interface of crystalline nanowires.

The same is true for materials such as lithium, which can not be observed using projection electron microscopy for atomic-level dendrites. Similar to biological materials, dendritic edges curl and even melt by electron beam impact when using TEM at room temperature. Yanbin Li, a doctoral student at Stanford University who participated in the work, said: "The preparation of TEM samples is carried out in the air, but the lithium metal will soon be corroded in the air." "Whenever we try to observe with a high power electron microscope When lithium metal, the electron will 'drill holes' in the dendrites and even melt it completely. "


Yanbin Li, a Ph.D. student at Stanford University who participated in the study, said: "It's like using a magnifying glass to look at the leaves in the sun, but if you can cool the leaves, the problem will be solved: you focus the light on the leaves, heat The same will be lost, the leaves will not be damaged.This is what we can achieve with a cryo-electron microscope effect, the use of battery material imaging, the difference is very obvious. "


Therefore, the use of cryo-electron microscopy not only allows biochemistry to enter a new era, but it also enables scientists to see the complete structure of lithium dendrites at the atomic level for the first time. The researchers also found that dendrites in carbonate-based electrolytes grew into monocrystalline nanowires in a particular direction. Some of them will be knotted during "growth", but their crystal structure is still intact.


Another Ph.D. student at Stanford University, Yuzhang Li, who participated in the study, said that solid electrolyte interface films (SEIs) can be seen as well as revealing different SEI nanostructures formed in different electrolytes. Because the same coating is also formed on the metal electrode when the battery is charged and discharged, controlling its generation and stabilization is crucial to the efficient use of the battery.


Using cryo-EM, scientists can observe how electrons are ejected from atoms in the dendrite, revealing the location of individual atoms (left). Scientists can even measure the distance between atoms (top right), and the atomic spacing just shows that they are lithium atoms (bottom right).

A press release from SLAC shows that under the microscope, researchers used different techniques to observe the way electrons pop up from dendritic atoms, revealing the location of individual atoms in the crystal and its solid electrolyte interface film coating. When they add chemicals that are commonly used to improve battery performance, the atomic structure of the solid electrolyte interface film coating becomes more orderly, and this will help explain why the additives play a part.


"We are excited that this is the first time we have been able to obtain such an elaborate dendritic image and the first time we have seen nanostructures in solid electrolyte interface films." Yanbin Li said, "This tool can help us understand different Of electrolytes, and why some electrolytes work better than others. "


The relevant data observed from these experiments can provide a better understanding of the mechanism of battery failure. Although this work is based on lithium metal as an example to demonstrate the usefulness of cryo-EM, this approach may also extend to other studies involving beam-sensitive materials such as lithiated silicon or sulfur. The team also said they plan to focus more on the chemical properties and structure of the solid electrolyte interface film.