Electrochemical AFM Study of Surface Films Formed on the HOPG Edge Plane in Propylene Carbonate-Based Electrolytes

Abstract

The effects of film-forming additives in electrolytes on the formation of a surface film on an edge plane pyrolytic graphite electrode in lithium-ion batteries were investigated in propylene carbonate (PC)-based solutions by electrochemical atomic force microscopy (AFM). A film-forming additive-derived surface film is formed when PC is used as a solvent because PC does not participate in the formation of a surface film. A surface film forms at around 1.2 V through the reductive decomposition of additives, such as vinylene carbonate. The precipitate layer can be removed by repeated scanning over 100 times in contact-mode AFM. The number of AFM scans required to remove the precipitate layer formed in PC-based electrolytes was much higher than that needed for ethylene carbonate-based electrolytes containing additives, even though the former was thinner by about 10 nm. These results indicate that a thin, dense and solid precipitate layer should form in PC-based electrolytes with additives. A very thin surface layer remains on the edge plane after repeated scans by AFM, and is in close contact with it. This remaining surface layer still functions as a protective film in subsequent potential cycles. The thermal stability of the surface film at an elevated temperature was also investigated.