A technology for preventing capacity loss during charge-discharge cycling of a high-voltage hybrid electrochemical capacitor is presented. During high-voltage cycling of the capacitor, which combined the negative electrode of a lithium-ion battery with the positive electrode of an electric double-layer capacitor, extreme capacity loss occurred through a decline in the capacity of the negative electrode when the positive electrode was highly polarized (at over 4 V vs Li/Li+). A method of preventing this capacity loss by adding a lithium metal to the cell is shown here. (c) 2007 The Electrochemical Society.