Electrochemistry of highly ordered pyrolytic graphite surface film formation observed by atomic force microscopy

In situ electrochemical atomic force microscopy (ECAFM) was used to study 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 experiments confirmed that distinct reduction reactions occur when the potential of a fresh HOPG electrode is swept from 2.4 to 0.01 V vs. Li/Li+ in a Li/HOPG cell. The reactions appear to be irreversible, since no evidence of corresponding oxidation reactions was observed. ECAFM results, when combined with slow-scan cyclic voltammetry data, suggest that different electrochemical reactions are occurring in these two electrolytes. The deposits were found to be several hundreds of nanometers thick. 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 HOPG electrodes in lithium-ion batteries.