A high-voltage hybrid electrochemical capacitor (HEC) which uses nongraphitizable carbon for a negative electrode and activated carbon for a positive electrode has been constructed with lithium salt for the electrolyte. Under high-voltage conditions, large capacity loss during charge-discharge cycling was observed when the positive electrode was highly polarized at a potential over 4 V vs Li/Li+. This capacity loss was induced by the increase of the interfacial resistance at the negative electrode. A large amount of lithium fluoride (LiF) was observed on the surface of the negative electrode after charge-discharge cycling. It is considered that the LiF was generated by a reaction of hydrofluoric acid (HF), generated by the decomposition of hexafluorophosphate anion (PF6-), with lithium alkyl carbonate (semicarbonate, ROCOOLi) or lithium carbonate (Li2CO3) on the surface of the negative electrode. The generation of LiF and the capacity loss were suppressed by providing lithium metal between the positive electrode and the negative electrode. It was thus confirmed that the suppression of LiF generation, caused by high polarization of activated carbon positive electrode, can realize a high-voltage HEC with good cycleability. (c) 2007 The Electrochemical Society.