(Source: PNNL official website)
According to foreign media reports, scientists of the Northwest Pacific National Laboratory (PNNL) in the United States installed a specially designed lithium-ion battery into a secondary ion mass spectrometer. During the operation of the battery, the formation of SEI was observed at the molecular level.
To a large extent, the first few hours of the first use of lithium-ion batteries determine its performance. Because during this time, a group of molecules will self-assemble to form the internal component of the battery, the solid electrolyte interphase membrane (SEI), and will have an impact on the battery in the next few years. The key role of the SEI membrane is to allow some particles to pass through. A further understanding of SEI membranes is a key step in the manufacture of more viable, longer-lasting and safer lithium-ion batteries. However, the formation of SEI membranes has always been a mystery. Researchers have tried a variety of techniques and have been able to witness its formation process at the molecular level.
SEI is a very thin film material that does not exist when manufacturing batteries. Only when the battery is charged for the first time will the molecules gather and form this structure through an electrochemical reaction. This structure is like a channel, so that lithium ions can flow back and forth between the positive and negative electrodes. Most importantly, the SEI membrane forces electrons to bypass, keeping the battery running and making energy storage possible. It is precisely because of SEI that lithium-ion batteries can power mobile phones, laptop computers and electric vehicles. Scientists use various raw materials and want to make the best SEI membrane. However, I don't understand the process of SEI, just like a chef only has ingredients but not cooking. Researchers from the Northwest Pacific National Laboratory (PNNL) and the US Army Research Laboratory of the US Department of Energy set out to study how SEI membranes are produced.
The researchers used PNNL's patented technology to penetrate the high-energy ion beam into the SEI film that was just formed during the operation of the battery, sent some materials into the air, and captured it for analysis, while relying on surface tension to help control the liquid electrolyte. Then, the SEI material was analyzed using a mass spectrometer. This patented technology is called "liquid SIMS" (liquid SIMS). Through this technology, the research team was able to observe the formation of SEI unprecedentedly and solve the problems that occur when the lithium ion battery works. Zihua Zhu, head of the PNNL research team, said: "On the solid foundation of this technology, we can scientifically understand the molecular activities in this complex structure. Through these findings, people are expected to adjust the chemical composition of the electrolyte and the electrode to produce Better battery. "
The PNNL team teamed up with Kang Xu, an expert in the US Army Research Laboratory, to solve the problem together. It is confirmed that SEI is composed of two layers. The research team further understood the chemical composition of each layer and determined the chemical process that produced this structure in the battery. They found that the layer structure close to the negative electrode was thin and dense. This layer repels electrons but allows lithium ions to pass through. The outer layer is thicker, just beside the electrolyte, which can adjust the interaction between the liquid and the rest of the SEI membrane. In contrast, the inner layer is slightly harder and the outer layer is slightly thicker.
The results of this study help to understand the role of lithium fluoride in the electrolyte of lithium-ion batteries. Researchers have shown that the more lithium fluoride in the SEI membrane, the better the battery performance. The research team showed how lithium fluoride can be part of the inner layer of SEI to help add more fluorine to the structure. Wang: "With this technology, you can understand not only the existence of molecules, but also their structure."
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