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 graph ite negative electrodes of rechargeable lithium-ion batteries, topogra phical changes of the basal plane of a highly oriented pyrolytic graph ite 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 similar to 1 nm height appeared on the surface of highly oriented pyrolytic gr aphite at 0.95 V versus Li/Li+, and then changed at 0.75 V to irregula r shaped blister-like features with a maximum height of similar to 20 nm. In 1 M LiClO4/EC + dimethoxyethane, hemispherical blisters of simi lar to 20 nm height appeared at 0.90 V. These morphology changes, hill and blister formation, were attributed to the inercalation of solvate d Li+ ions into graphite interlayers and to the accumulation of its de composed products, respectively. On the other hand, only rapid exfolia tion and rupturing of graphite layers were observed in 1 M LiClO4/prop ylene carbonate (PC), which was considered to be responsible for cease less solvent decomposition when graphite electrodes are charged in PC- based solutions. From the observed topographical changes, it was concl uded that the intercalation of solvated Li+ ions is a necessary step f or stable surface film formation on graphite. (C) 1997 Elsevier Scienc e S.A.