The effect of fluoroethylene carbonate (FEC) additive has been studied in the formation of solid electrolyte interphase (SEI) over Si-based anode using in-situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). SEI species were observed in the first lithiation cycle at an onset potential of 1.4 V in electrolyte containing 2 wt% vinylene carbonate (VC) + 10 wt% FEC and at 1.1 V in electrolyte without FEC additive. With blended VC and FEC, high carbon containing species including poly (FEC), poly (VC), and polycarbonates were identified, while poly (VC) and polycarbonates formed in the absence of FEC. The FEC additive also led to a higher content of organic phosphorous fluorides as compared to the electrolyte containing no FEC. Electrochemical analyses indicated that the combination of 2 wt% VC and 10 wt% FEC resulted in lower impedances and improved the stability of the Si-electrode through cycling as compared to that without FEC. DRIFTS provided evidence that similar SEI species formed after the initiation in the first cycle, and this formation was recorded for five cycles. Finally, possible reactions related to the reductive decomposition of FEC and VC are discussed. (c) The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.