Direct Visualization of Nucleation and Growth Processes of Solid Electrolyte Interphase Film Using in Situ Atomic Force Microscopy

An understanding of the formation mechanism of solid electrolyte interphase (SEI) film at the nanoscale is paramount because it is one of the key issues at interfaces in lithium-ion batteries (LIBs). Herein, we explored the nucleation, growth, and formation of SEI film on highly oriented pyrolytic graphite (HOPG) substrate in ionic liquid-based electrolytes 1-butyl-1-methyl-pyrrolidinium bis(fluorosulfonyl)imide ([BMP]+[FSI]-) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMP]+[TFSI]-) by in situ atomic force microscopy (AFM) and found that the types of anions have significant influence on the structure of the formed SEI. In [BMP]+[FSI]- containing LiFSI, a compact and thin SEI film prefers to grow in the plane of HOPG substrate, while a rough and loose film tends to form in [BMP]+[TFSI]- containing LiTFSI. On the basis of in situ AFM observations, the relationship between the SEI structure and the electrochemical performance was clarified.