Electrochemical scanning tunneling microscopy analysis of the surface reactions on graphite basal plane in ethylene carbonate-based solvents and propylene carbonate

In order to elucidate the mechanism of surface film formation on graphite negative electrodes of rechargeable lithium-ion batteries, topographical changes of the basal plane of a highly oriented pyrolytic graphite were observed in a few electrolyte solutions under polarization by electrochemical scanning tunneling microscopy. In 1 M LiClO4/ethylene carbonate (EC) + diethyl carbonate, a hill-like structure of not, vert, similar 1 nm height appeared on the surface of highly oriented pyrolytic graphite at 0.95 V versus Li/Li+, and then changed at 0.75 V to irregular shaped blister-like features with a maximum height of not, vert, similar 20 nm. In 1 M LiClO4/EC + dimethoxyethane, hemispherical blisters of not, vert, similar 20 nm height appeared at 0.90 V. These morphology changes, hill and blister formation, were attributed to the intercalation of solvated Li− ions into graphite interlayers and to the accumulation of its decomposed products, respectively. On the other hand, only rapid exfoliation and rupturing of graphite layers were observed in 1 M LiClO4/propylene carbonate (PC), which was considered to be responsible for ceaseless solvent decomposition when graphite electrodes are charged in PC-based solutions. From the observed topographical changes, it was concluded that the intercalation of solvated Li+ ions is a necessary step for stable surface film formation on graphite.