The effectiveness of ethylene sulfite (ES) and/or vinylene carbonate (VC) as electrolyte additives in Li(Ni1/3Mn1/3Co1/3)O-2 (NMC)/graphite pouch cells was studied using in situ measurements of gas evolution, ultra high precision coulometry (UHPC), automated storage experiments and electrochemical impedance spectroscopy (EIS). Cells containing only ES produced large amounts of gas during both formation and cycling. UHPC cycling and storage results showed that cells containing only ES had lower coulombic efficiency, larger charge end point capacity slippage rate and larger voltage drop during storage than control cells. Moreover, the impedance of cells containing only ES increased after cycling. These results show that ES is not a competitive additive when used on its own. When ES is combined with VC, there are several benefits to cell performance. Cells with 2% VC + ES (1% or 2%) produced virtually no gas during formation and cycling and showed similar performance in UHPC testing to cells containing only 2% VC. The most interesting characteristic is that the use of both ES and VC greatly reduced (by about 50%) the cell impedance after cycling. EIS studies of positive and negative electrode symmetric cells showed that the introduction of ES (1% or 2%) to 2% VC-containing electrolyte led to a significant decrease in impedance at the negative electrode. (C) 2014 The Electrochemical Society. All rights reserved.