ELECTROCHEMISTRY OF HIGHLY ORDERED PYROLYTIC-GRAPHITE SURFACE-FILM FORMATION OBSERVED BY ATOMIC-FORCE MICROSCOPY

In situ electrochemical atomic force microscopy (ECAFM) was used to st udy the surface films that form on highly ordered pyrolytic graphite ( HOPG) electrodes during cathodic polarization in 1 M LiClO4 EC:DMC (1: 1) and 1 M LiPF6 EC:DMC (1:1) electrolytes. Cyclic voltammetry experim ents confirmed that distinct reduction reactions occur when the potent ial of a fresh HOPG electrode is swept from 2.4 to 0.01 V vs. Li/Li+ i n a Li/HOPG cell. The reactions appear to be irreversible, since no ev idence of corresponding oxidation reactions was observed. ECAFM result s, when combined with slow-scan cyclic voltammetry data, suggest that different electrochemical reactions are occurring in these two electro lytes. The deposits were found to be several hundreds of nanometers th ick. Auger analysis confirmed the presence of elements consistent with electrolyte reduction products. These observations have implications for understanding the formation of a solid electrolyte interphase on H OPG electrodes in lithium-ion batteries.