Solid electrolyte interphase (SEI) forming electrolyte additives are able to improve the performance of lithium ion and lithium metal batteries. In this work, the electrochemical performance of graphite and lithium metal, when using 1 M LiTESI (lithium bis(trifluoro-methanesulfonyl)imide) in tetraethylene glycol dimethyl ether (TEGDME) with and without the addition of different amounts of fluoroethylene carbonate (FEC), is compared. It is shown that 1 M LiTESI in TEGDME without additive is not able to form an effective SEI on graphite and that this electrolyte is also continuously decomposing on lithium metal. By the addition of >2 wt% FEC, an effective SEI is formed on both, lithium metal and graphite, enabling good cycling stability. Furthermore, 1 M LiTESI in TEGDME with FEC as additive is a suitable electrolyte for lithium iron phosphate (LFP) based lithium ion batteries. (C) 2015 The Electrochemical Society. All rights reserved.