With the rapid development of portable electronic equipment and electric vehicles, people in addition to the pursuit of lithium battery capacity, charge and discharge faster, the more concerned about is how to protect the safety of lithium batteries because of the lithium battery explosion from time to time And other events, inevitably make people nervous. How to solve the problem of lithium battery safety premise is that scientists as far as possible in-depth and comprehensive understanding of the causes of lithium battery explosion.
The current scientific explanation is that the surface of the electrode lithium deposition will form a 'dendrites' (dendrites), and it will continue to grow, resulting in internal battery short circuit caused by battery failure or may cause a fire, but how to understand and research from the atomic structure level, And then to find a solution to the problem, in the past the lack of effective technical means.
This month has just won the 2017 Nobel Prize for Chemical Technology (cryo-EM) technology, to provide a strong technical support. Stanford University, the US Department of Energy directly under the SLAC National Accelerator Laboratory Professor Cui Yi, 1997 Year's Nobel Prize winner Zhu Diwen et al.'s research team captured the image of the first atomic-grade lithium metal dendrites by cryo-EM. The results were published on October 27th local time In the international academic journal "Science" on.
The above image shows that each lithium metal dendrite is a long strip, and the formation of perfect hexahedral crystal, and previously observed by electron microscopy is only irregular shape of the crystal.Cui Yi said, 'research results are very Excited, but also for the relevant research to open a new situation!
Frozen electron microscopy, as the name suggests is the use of cryopreservation, at low temperatures using transmission electron microscopy (Transmission Electron Microscope, referred to as TEM) to observe the microscopic technology of the sample.Freeze electron microscopy is an important structural biology research method is to obtain biological macromolecules Structure of the important means.
Because the image is the key to understanding the mechanism, scientific breakthroughs are often based on the use of the naked eye to obtain the visual conception of the target. For a long time, it is believed that TEM is not suitable for observing biomolecules because powerful electron beams can destroy biological materials. Electron microscopy, so that researchers can freeze the biological molecules, unprecedented observation and analysis of the movement process, these representations for the understanding of life chemistry and the development of pharmacology have a decisive impact.For this reason, frozen electron microscopy will also promise this year Bell chemistry will be incorporated into the arm.
In the TEM image at room temperature, the dendrites of lithium are corroded by exposure to the air, and the electron beam also melts a large number of holes above. Right: The image under cryo-EM, the frozen environment preserves its original Of the state, indicating that it has a clear interface of the crystal nanowires.
For lithium and other materials, it is not possible to use a projection electron microscope to view the results of the dendritic atomic level. Similar to the biomaterial, when the TEM is used at room temperature, the edge of the dendrites will be curled or even melted by electron beam impingement. Yan Na Li, a doctoral student at Stanford University, said that the preparation of the transmission electron microscope sample was carried out in the air, but the lithium metal would soon be corroded in the air. 'Whenever we tried to observe the metal with a high power electron microscope When lithium is in, the electrons 'drill holes' in the dendrites and even melt it completely.
Yanbin Li, a Stanford doctoral student who participated in the study, said, 'It's like cutting a tree with a magnifying glass in the sun, but if you can cool the leaves, the problem will be solved: you focus your light on the leaves, The same will be lost, the leaves will not be destroyed. This is what we can use frozen electronic microscope can achieve the effect of the use of battery materials on the imaging, the difference is very obvious.
Therefore, the freeze electron microscope not only makes biochemistry into a new era, but also for scientists for the first time at the atomic level to see the complete structure of lithium dendrites.The researchers also found that in the carbonate-based electrolyte in the dendrites along a Grow in specific directions for single crystal nanowires, some of which will be knotted during 'growth', but their crystal structures remain intact.
Another researcher at Stanford University, who participated in the study, said that the solid electrolyte interface (SEI) was also seen, and that different SEI nanostructures formed in different electrolytes were also revealed because when the battery was charged and discharged , The same coating will be formed on the metal electrode, so control its generation and stability for the efficient use of the battery is essential.
Using cryo-EM, scientists can observe how electrons emerge from the atoms in the dendrites, revealing the position of a single atom (left). Scientists can even measure the distance between atoms (top right) and the atomic spacing Indicating that they are lithium atoms (bottom right).
The press release released by the SLAC shows that under the microscope, researchers use different techniques to observe how electrons emerge from dendritic atoms to reveal the location of individual atoms in the crystal and its solid electrolyte interfacial coating. When adding chemicals that are commonly used to improve battery performance, the atomic structure of the solid electrolyte interfacial coating becomes more orderly, and this will help explain why the additive will work.
'We are very excited, this is the first time we can get such a detailed dendritic image, but also for the first time we see the solid electrolyte interface layer of nano-structure.' Yanbin Li said, 'This tool can help us understand the different Of the electrolytes are what kind of effect, and why some of the electrolyte is better than the other.
The data from these experiments can be used to further understand the mechanism of battery failure.Although this work is to lithium metal as an example to prove the practicality of cryo-EM, but this method may also be extended to involve light-sensitive Materials such as silicon carbide or sulfur. The research team also said that they plan to focus on more understanding of the chemical properties and structure of the solid electrolyte interface layer.
