The cycling performance and SEI composition of Si nano-particle anodes in electrolytes containing 5-25 wt% fluoroethyelene carbonate (FEC) and 3-6 wt% vinylene carbonate (VC) has been investigated by a combination of by electrochemical cycling, electrochemical impedance spectroscopy, IR-ATR and XPS. The incorporation of FEC or VC changes the cycling performance, impedance, electrode morphology, and SEI structure of Si nano-particle electrodes. Cells cycled with standard carbonate electrolytes have poor capacity retention and the anode surface is primarily covered by lithium alkyl carbonates and lithium carbonate. The electrodes cycled in electrolyte containing 10-15 wt% FEC have the smallest impedance and best capacity retention. The reduction of electrolyte containing FEC forms a stable SEI consisting of poly(FEC), LiF, lithium carbonate and lithium alkyl carbonates. Reduction of electrolytes containing VC results in higher impedance and the generation of lithium carbonate, poly(VC) and traces of LiF, and lithium alkyl carbonates. (C) 2014 The Electrochemical Society. All rights reserved.