Electrochemical Characterization of SEI-Type Passivating Films Using Redox Shuttles

Although redox shuttles have been demonstrated to provide overcharge protection for up to 200 cycles, the existence of the solid-electrolyte-interphase (SEI), a passivating film on the negative electrode, seems to be inherently incompatible with a successful shuttle reaction. In this work, the kinetics of ferrocene is measured in the presence and absence of passivating films using rotating-disk-electrode voltammetry. The steady-state current-voltage curve is described by a simple model of Butler-Volmer kinetics and a through-film limiting current. The presence of a passivating film decreases both the limiting current and the effective rate constant. Both parameters decrease with increased passivation time; a decreasing porosity is a possible explanation for the latter observation. The characterization method developed in this work allows direct measurements of the effect of passivating films, thus contributing to understanding of passivation phenomena in nonaqueous electrolytes.