The effects of different concentrations of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) (0%, 0.5%, 1%, 2%, 4%, and 6% by weight) in LiCoO2/graphite pouch cells in 1M LiPF6 ethylene carbonate (EC) : ethyl methyl carbonate (EMC) (3:7 w:w) electrolyte have been studied using high precision coulometry and electrochemical impedance spectroscopy measurements (EIS) on symmetric cells. High precision coulometry showed that when cells were formed at 40 degrees C, VC improves the cycling behavior by reducing the rate of the parasitic reactions at the positive electrode and improving the coulombic efficiency, whereas VEC does not affect the cycling behavior (compared to control). EIS measurements on symmetric cells showed that while VC affects the positive electrode for all concentrations studied and the negative electrode at concentrations higher than 1%, VEC does not affect the negative electrode for all concentrations studied and affects the positive electrode at concentrations higher than 2%. (C) 2013 The Electrochemical Society. All rights reserved.