We report on overcharge protection characteristics of N-ethyl-2-pyrrolidone (NEP) as electrolyte additive. The electrochemical stability of this organic compound was investigated on a platinum working electrode using linear sweep voltammetry. We investigated the overcharge protection effect on a lithium iron phosphate (LFP), a lithium nickel manganese cobalt oxide (NMC) and on a LiMn2O4 (LMO) cathode. Via electrochemical quartz crystal microbalance (EQCMB) measurements the effects of NEP during overcharge were studied and revealed that NEP acts not like a redox shuttle, but is forming a protecting polymeric film. The oxidation products on the cathode were studied by analyzing the surface of the lithium iron phosphate (LFP) and LiMn2O4 (LMO) cathode using XPS spectroscopy. (C) 2014 The Electrochemical Society. All rights reserved.