Model Studies on the Solid Electrolyte Interphase Formation on Graphite Electrodes in Ethylene Carbonate and Dimethyl Carbonate: Highly Oriented Pyrolytic Graphite

Aiming at a deeper understanding of the solid electrolyte interphase (SEI) formation on carbon anodes in lithium-ion batteries, we performed a combined electrochemical and spectroscopic model study using structurally well-defined graphite model electrodes (highly oriented pyrolytic graphite, HOPG) and simplified model electrolytes (ethylene carbonate (EC)+1 M LiPF6 or dimethyl carbonate (DMC)+1 M LiPF6). In cyclic voltammetry measurements, we find initial activation of the reductive electrolyte decomposition at faster scan rates (1 or 10 mV s(-1)), whereas this is not the case at a slower scan rate (0.1 mV s(-1)). This activation effect, which is more pronounced for DMC, is explained by an increase in the HOPG surface area, presumably by electrode exfoliation; it is not observed on surface-defect-rich samples. XPS analysis shows that, regardless of the solvent and the scan rate, the SEI is mainly composed of LiF and only small amounts of solvent and other salt decomposition products.