As one of the important reactions in the chemical industry, synthetic ammonia not only produces a large number of indispensable raw materials, but also brings a series of energy consumption and pollution problems. The lithium medial conductive chemical synthesis of ammonia (LiMEAS) is a different way and provides a new idea of clean synthesis. Recently, Professor Li Yuzhang's team at the University of California, Los Angeles published an article in Nature Energy to further clarify the chemical reaction that occurs during the reaction. "This work demonstrates the importance of the lithium metal SEI interface and provides new insights and discoveries for our understanding of the LiMEAS process," the team said. ”
Web screenshot | References[1]
The following is the interpretation of the paper written by Yuan Xintong, the first author of the paper of Li Yuzhang's team, for the husk.
Ammonia, as a chemical with an annual output of more than 175 million tons, is undoubtedly an important way to use nitrogen resources in chemical synthesis, and it is also a key energy carrier in the industrial decarbonization process. However, the Haber–Bosch process, which has been used by the ammonia industry, relies heavily on hydrogen produced by high temperature and pressure and methane steam reforming, which is not only energy-intensive, but also very dependent on non-renewable fossil fuels. Therefore, in order to promote the sustainable development of the chemical industry and make full use of the thermodynamic reaction between lithium metal and nitrogen, lithium-mediated conductive chemical synthesis of ammonia (LiMEAS) has become one of the most promising methods.
In recent years, the LiMEAS method has made great progress, but its mechanism is still unclear, and the role of lithium metal and its surface passivation layer solid electrolyte interface (SEI) in the reaction process is still controversial. First, is the process thermochemically or electrocatalytically driven, i.e. will lithium metal be consumed in the process? Secondly, because the Fermi level of lithium metal is higher than the lowest unoccupied molecular orbital of existing electrolytes, it is easy to be reduced, so it is also necessary to confirm whether SEI participates in the reaction in this process.
At present, there are four mechanisms in the academic community:
- Thermochemical nitrogen fixation and protonation processes consume electrodeposited lithium, resulting in lithium ammonia and ethanol (Figure 1a);
- Lithium metal, lithium nitride and lithium hydride are used as electrocatalysts, and nitrogen is adsorbed, protonated and reduced on its surface to form ammonia (Figure 1b);
- The relative transfer rates of lithium ions, nitrogen, and protons in SEI determine selectivity, i.e., an unbalanced diffusion rate will result in excess lithium metal or lithium nitride or severe hydrogen evolution reactions (Figure 1c);
- The nature and concentration of the proton donor determine the transport nature of the SEI (Figure 1d).
Explanation of the mechanism of lithium-mediated ammonia synthesis | Photo courtesy of the team
In order to reveal the reaction mechanism of LiMEAS, in order to further study and apply the method, Professor Li Yuzhang's team used cryo-EM to analyze the whole process of the reaction. Cryo-EM results showed that the proton donor ethanol, by attacking amorphous organic components in the SEI, disrupts the SEI structure (Figure 2).
Ethanol's destruction of amorphous SEI can take many forms. From the perspective of physical structure, the hydrogen produced by the reaction of ethanol with lithium metal destroys the mechanical stability of SEI. From the perspective of SEI structure, the reaction between ethanol and lithium metal may produce advantageous SEI components with poor passivation and strong nitrogen permeability. It is also possible to chemically react directly with SEI components to produce a more permeable mesophase.
Electron microscopy results | Photo courtesy of the team
Combining the results of cryo-EM and X-ray photoelectron spectroscopy (XPS), proton donor ethanol appears to be the main driver of LiMEAS. In the absence of the introduction of a proton donor, the decomposition products of fluoroborate ions (BF4-) and tetrahydrofuran (THF) constitute the passivated SEI, while nitrogen and electrolytes cannot react with lithium because they cannot penetrate into and pass through the SEI. At the same time, lithium ions can diffuse in this SEI, resulting in the continuous formation of lithium metal dendrites (Figure 3a), which is not conducive to subsequent reactions. With the addition of ethanol, the amorphous SEI will be dominated by ethanol decomposition products, and nitrogen and electrolyte can penetrate into this interface, thereby continuously consuming the deposited lithium metal to form ammonia (Figure 3b), driving the reaction to continue to occur.
The role of SEI in LiMEAS | Photo courtesy of the team
The analysis results show that for the LiMEAS system, SEI with poor passivation can effectively synthesize ammonia, so it is necessary to introduce some proton donors to promote the reaction. However, if there is an excessive reaction between the lithium metal and the proton donor, a large amount of hydrogen gas is produced, resulting in irreversible loss of lithium metal. Therefore, in the subsequent design and improvement of the reaction, it is necessary not only to select the appropriate combination of lithium salt and solvent, but also to have a proton donor with high stability, so as to achieve SEI activation while avoiding the loss of lithium metal, and improve the reaction safety and production efficiency.
bibliography
[1] Steinberg, K., Yuan, X., Klein, C.K. et al. Imaging of nitrogen fixation at lithium solid electrolyte interphases via cryo-electron microscopy. Nat Energy (2022). https://doi.org/10.1038/s41560-022-01177-5
Author: Yuan Xintong
Translation: Jin Xiaoming
Typesetting: Yin Ningliu
Compilation source: Courtesy of the team
Research team
(Co)Corresponding Author Yuzhang Li: Assistant Professor of Chemical Engineering, UCLA. 2009-2013 University of California, Berkeley, Ph.D., 2013-2018 Stanford University (supervisor: Prof Cui), 2018-2020 Stanford University postdoctoral fellow (co-supervisor: Prof. Bob Sinclair & Professor Yi Cui), joined UCLA in July 2020. As the first author or corresponding author, he has published dozens of papers in internationally renowned academic journals such as Science, Nature Energy, Joule, Chem, Matter, etc., and has been selected as Clarivate Highly Cited Researchers for many times, with a total of more than 12,000 citations and an h-index of 41. He has been selected for the DOE Early Career award, the NSF Career award, the Forbes 30 Under 30 in Science list and many other honors and awards.
Corresponding author Karthish Manthiram, professor at Caltech.
(Co-)First author Katherine Steinberg: PhD student in MIT's Department of Chemical Engineering.
(Co-)First Author Xintong Yuan: PhD student in the Department of Chemical Engineering, UCLA.
Dissertation information
Published in the journal Nature Energy
Published: December 22, 2022
论文标题 Imaging of nitrogen fixation at lithium solid electrolyte interphases via cryo-electron microscopy
(DOI:https://doi.org/10.1038/s41560-022-01177-5)
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