In lithium-ion batteries, chemical additives are used as co-solvents to primary electrolytes to improve capacity and power retention. These additives facilitate the formation of a passivation layer, the solid electrolyte interphase (SEI), on the electrode surface. In this work, SEIs are formed in neat electrolyte and in electrolyte containing fluoroethylene carbonate and vinylene carbonate. The formed SEIs are then compared using a redox couple to probe their physical properties. For passivated samples, the impedance response of the redox couple shows the presence of multiple time constants, with processes at longer time scales corresponding to redox couple transport in the porous layer of the SEI. Samples passivated with additive-containing electrolyte versus neat electrolyte exhibit less redox-couple kinetic and mass-transport impedance. Simulations of the electrode-electrolyte interface indicate that compact and porous layer growth lead to slowed redox kinetics and mass transport. Model results suggest that SEI thicknesses are found to be at least an order of magnitude larger than expected compared to graphite electrodes. SEM cross sections of SEIs formed by neat and additive-containing electrolyte support the model findings. Experimentally measured formation charges, coupled with FTIR measurements of SEI composition, suggest that polymerization reactions are causing the unexpected film growth. (C) The Author(s) 2016. Published by ECS. All rights reserved.