Electrochemical AFM Observation of the HOPG Edge Plane in Ethylene Carbonate-Based Electrolytes Containing Film-Forming Additives

Abstract

The effects of film-forming additives in electrolyte solutions on the formation of a surface film on edge plane pyrolytic graphite electrodes in lithium ion batteries were investigated by electrochemical atomic force microscopy (AFM). Surface films were formed at around 1.2 V through reductive decomposition of the additives including vinyl ethylene carbonate. The thickness of the surface films, ca. 20 nm, was less than half of that for an electrolyte with no additives, and remained unchanged after the second potential cycle. Most of the surface films could be scraped off by repeated scanning of the surface in contact-mode AFM. However, a very thin surface layer, which was in close contact with the edge plane, remained even after repeated scans by AFM. No precipitate was observed on the scraped part in the subsequent potential cycle, indicating that the remaining thin surface layer could act effectively as a passivating film for the edge plane graphite electrode. Such a passivating surface layer was found to be formed uniformly over the entire surface of the edge plane. These results clearly indicate that surface films with high passivation ability can be effectively formed on the edge plane during the initial cycle by the presence of film-forming additives in electrolytes.