Acrylic acid nitrile (AAN) is introduced as a novel example out of the large class of vinylene groups containing film-forming additives for lithium-ion batteries. The electrochemical behaviour, especially the electrolyte additive reduction and the associated film formation in the presence of this compound is investigated with the in situ methods of Fourier transform infrared (FT-IR) spectroscopy and electrochemical quartz crystal micro balance (EQCMB). The results clearly point at a solid electrolyte interphase (SEI) formation mechanism, which proceeds via the cathodically induced polymerization of AAN. We suggest that the electro-polymerisation of vinylene groups is a main electrolyte reduction mechanism for a vinylene group containing electrolyte additives. The outstanding filming properties of vinylene compounds such as AAN allow the use of graphitic carbon anodes in PC-based electrolytes even when only 1 % of the additive is present in the electrolyte. (C) 2003 Elsevier Science B.V. All rights reserved.