Principle of electrode SEI film formation

The formation principle of electrode SEI film The formation process of SEI film can be divided into two situations: (1) the lithium electrode covered by the original passivation film; (2) the bare lithium surface has no original passivation film. The surface of the lithium electrode is usually covered with a layer of original passivation film. Spectroscopy method research shows that the original surface film of lithium has a double-layer structure. The outer layer is mainly composed of li-0h-li2c03, and the inner layer is mainly composed of Li20. Electrode SEI film formation principle During the contact process, the lithium electrode and the organic electrolyte, the lithium electrode on the surface of the important component of the original film, Li20 (inner layer), LiOH and Li2003 (outer layer), will react with electrophilic solvent molecules such as ether and alkane. Carbonic acid is partially soluble in electrolyte. Then the metal lithium of part or all of the surface film of the original component is replaced by the reaction product composition of the electrolyte. The interface membrane component near the surface of the metal lithium continues to react, and the interior of the SEI film near the lithium forms a layer of low oxidation state component, and the outer film The surface deposition is a dynamic process, forming some high oxidation state components, and at the same time, a porous structure appears due to the surface dissolution of some components. During the cycle, the dissolution of lithium will accelerate the rupture of the original film on the surface of the lithium, causing a violent reaction between the metal lithium and the electrolyte. Due to the strong chemical reaction activity of metallic lithium, bare lithium without an initial passivation film is usually prepared in situ on the surface of electrolyte lithium. When a newly prepared lithium electrode surface passivation layer will be exposed to the organic electrolyte, all components of the electrolyte and the lithium electrode will undergo a reduction reaction. When some products are reduced on the surface of the lithium electrode, they will hinder this process so that further reactions will pass In the case of electron transfer, it is more selective, so further reducing the electrolyte component will have a high selectivity, resulting in a multilayer structure on the surface of the lithium electrode formed by the SEI film. The structure and composition of the SEI film formed on the surface of the lithium electrode through the above process will undergo further changes in the subsequent storage process. These changes include the reduction of surface components near the lithium surface and the diffusion of water, resulting in the hydration of the SEI film components. This last process is almost impossible to prevent. Even if water contains only 105, it can also occur in the electrolyte. This is because the commonly used lithium electrode surface passivation components of the electrolyte are highly hygroscopic, and water can penetrate the SEI membrane from the electrolyte. The chemical composition leads to the degradation of the cycle performance of the lithium electrode. Water also diffuses to the surface and directly reacts with lithium. Regardless of the different electrolytes, the different reactions on the surface of the lithium electrode, the reaction solvent, the solubility of the lithium salt, impurities (h2o, H, etc.), and the reaction product of the electrolyte, its formation process is determined by the solvent The delicate balance between lithium salt and impurities determines the reduction process, and the initial formation of SEI film. The surface of the lithium electrode usually undergoes an aging process and gradually changes its structure and performance. The heterogeneous chemical structure of the SEI film leads to uneven lithium deposition. , And (or) is immediately and effectively passivated by SEI when depositing lithium, and the lithium cycle efficiency is high. At the same time, it can be seen that SEI membrane components are not easy to absorb water, such as Li2C03 and life. Better components to achieve effective passivation of active lithium electrodes, so the presence of Q02 or a small amount of high-frequency organic electrolyte can significantly improve lithium electrodes. Performance. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: Four essential components for lithium batteries