An active carbon (AC)/AC electrochemical capacitor, taking advantage of a high-concentrated lithium trifluoromethane sulfonate (LiTFS) or lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) aqueous electrolyte, was demonstrated with an extended operating voltage of 2.5 V, which is the largest value till now for aqueous carbon-based capacitors. The AC electrode is entirely capacitive in these two electrolytes and the stable potential window of the single AC electrode can reach −1.2 to 1.2 V versus the saturated calomel electrode (SCE). The performance of the AC-based capacitor is evaluated in two- and three-electrode cells using a combination of electrochemical impedance (EIS), cyclic voltammetry (CV), galvanostatic discharge-charge, and self-discharge (SD, i.e., leakage current) measurements. At 0.5-mA cm−2 charge-discharge rate, the AC/AC capacitor presents 5.5 wh kg−1 and 4.5 wh kg−1 energy density for 20 m LiTFS and LiTFSI electrolyte, respectively. The results suggest that a thorough utilization of such lithium salt aqueous electrolytes with widening electrochemical stable potential window will no doubt lead to further development of electrochemical capacitors toward superior performance.