Commercial porous activated carbons have a high surface area and gravimetric capacitance but a low pack density. In this work, highly ordered graphitic carbons with different microcrystalline structures were prepared by the calcination and KOH-activation of petroleum coke. When the graphitic carbons were electrochemically activated at 4 V, specific capacitance was increased to 166 F/g from a very low initial value caused by the low surface area. The energy and power density reached 48.5 Wh/kg (32.8 Wh/L) and 6106 W/kg (4132.4 W/L), respectively, at 3.3 A/g. Electrochemical activation is believed to be a voltage-driven ion intercalation process, in which abundant ion-accessible sites are created and can be used for mixed ion adsorption/intercalation charge storage. The effects of solvents, applied voltage, and cation selection on the ion intercalation behavior were systematically studied using galvanostatic charge-discharge and cyclic voltammetry techniques. The results revealed that higher applied voltage, shorter chain length, and weaker solvent-ion interactions favor ion intercalation to the positive and negative electrodes, consequently leading to symmetrical capacitive responses and maximum cell performance. (C) 2014 The Electrochemical Society. All rights reserved.