Exploratory Studies of the Carbon/Nonaqueous Electrolyte Interface by Electrochemical and In Situ Ellipsometry Measurements

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Abstract
An electrochemical cell was fabricated and used for in situ ellipsometry studies of the carbon/electrolyte interface during potential cycling in nonaqueous electrolytes. The initial studies were conducted on smooth carbon films obtained by pyrolysis of a positive photoresist on a wafer at . The potential of the carbon film in + ethylene carbonate‐dimethyl carbonate was scanned from open circuit to , and the ellipsometry parameters Δ and Ψ were recorded. Detectable changes in the ellipsometry parameters Δ and Φ occurred at potentials near , where a large cathodic current was observed. These changes were attributed to the formation of a solid electrolyte interface (SEI) layer. Subsequent potential scans between and showed no significant change in the ellipsometry parameters or the current‐potential profiles, indicative of an SEI layer which remained unchanged. X‐ray photoelectron spectroscopy measurements indicated that and were the major elements present in the SEI layer on the carbon surface. Preliminary analysis of the ellipsometry data using a single‐layer effective medium approximation model indicated that the SEI layer was thick. ©1998 The Electrochemical Society

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