Electrochemical scanning tunneling microscopy observation of highly oriented pyrolytic graphite surface reactions in an ethylene carbonate-based electrolyte solution

In order to elucidate surface reactions on graphite negative electrodes of secondary lithium ion batteries, topographical changes of the basal plane of a highly oriented pyrolytic graphite (HOPG) in 1 M LiClO4/ethylene carbonate-diethyl carbonate (1:1 by volume) were observed under polarization by electrochemical scanning tunneling microscopy. A step edge on the basal plane of HOPG was treated as a model of the edge plane of HOPG. When the sample potential was stepped to 1.1 V vs Li/Li+, two kinds of hill-like structure of ca. 10 Angstrom height appeared on the HOPG surface. The first hill was formed far apart from a step edge and was almost unchanged with time. The second hill was formed in the vicinity of the step and was spread out with time. The formation of the second hill caused the exfoliation of graphite layers. The observed height of the hills was comparable to the values of the increment of the interlayer spacing for ternary graphite intercalation compounds of alkali metal with solvent molecules prepared by a solution method. It was considered that the intercalation of solvated lithium ion is responsible for the formation of the hills and that this process corresponds to the initial stage of the solvent decomposition and subsequent film formation processes.