To determine the passivation mechanism of the Solid-Electrolyte-Interphase (SEI), glassy carbon was held at potentials from 0.1 to 0.9 V vs. lithium for different lengths of time. The resulting SEI was characterized electrochemically, using steady-state and transient ferrocene kinetics. Experiments were interpreted with macroscopic models for film formation and through-film ferrocenium reduction. Formation experiments demonstrate that growth is limited by transport of a charged species, but that electron migration through the SEI cannot be the limiting process. Ferrocene experiments show that both through-film transport and kinetics decrease with more passivation time. Comparison with models suggests that a decreasing porosity is a more likely explanation than either an increasing thickness or a decreased area of active sites. For the same amount of formation charge, the SEI formed at lower potential passivates the electrode more effectively. At long times and low potentials, the SEI is unstable. A passivation mechanism in which soluble intermediates of electrolyte reduction diffuse away from the electrode is proposed.
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