The formation process of SEI film is an electrochemical reaction process. When the voltage reaches a certain value, a series of physical and chemical changes will occur on the surface of the negative electrode. In actual production, this step is mainly through battery formation to form a film.
Extended data
Lithium dendrite growth is one of the fundamental problems that affect the safety and stability of lithium ion batteries. During the cycle of lithium-ion battery, the growth of lithium dendrites will lead to the instability of the interface between the electrode and electrolyte, and destroy the solid electrolyte interface (SEI) film formed from it. During the growth of lithium dendrites, the electrolyte will be continuously consumed, leading to the irreversible deposition of metallic lithium, forming dead lithium, and the coulomb efficiency is low.
The formation of lithium dendrites may even pierce the diaphragm, leading to internal short circuit of lithium-ion battery, which in turn leads to thermal runaway of the battery and combustion explosion. The growth mechanism of lithium dendrites is still controversial in academic circles. Because lithium-ion batteries are afraid of water and oxygen, the commonly used technical means to characterize SEI are very limited.
Using various electron microscope techniques to solve this problem is very important for understanding the evolution process of lithium dendrite growth at nanometer scale. Conventional transmission electron microscope is easy to damage SEI and electrode structure due to the irradiation of high-energy particles.
Although frozen electron microscope can solve this problem, due to the limitation of use conditions, it is impossible to use normal temperature electrolyte in the experiment and to realize in-situ observation.
On the basis of in-situ electrochemical atomic force microscopy (EC-AFM), some scholars have deeply studied the film-forming mechanism of various negative materials of lithium-ion batteries. Taking advantage of the fact that the film-forming potential of SEI film is more accurate than the deposition potential of metallic lithium, two-step real-time in-situ observation experiments have been designed to study lithium dendrites.
Researchers can use EC-AFM to study the growth process of SEI films based on vinyl carbonate (EC) and fluoroethylene carbonate (FEC) in real time, and then observe the growth of in-situ lithium dendrites.
Through the analysis of Young's modulus, CV spectrum and EIS impedance spectrum of the SEI film formed by these two electrolytes, combined with XPS spectrum analysis, the researchers found that the SEI film formed by FEC electrolyte contains more LiF inorganic salts. Because LiF has better hardness and stability, SEI film has higher strength, which can effectively inhibit the growth of lithium dendrites.
reference data
Baidu Encyclopedia-Interface Features
Baidu Encyclopedia-fluoroethylene carbonate
Baidu encyclopedia-lithium dendrite