In this paper we present a comprehensive structural, chemical and electrochemical characterization of monoclinic Li0.33MnO2 as a positive electrode material for aqueous high-voltage hybrid supercapacitors. The monoclinic Li0.33MnO2, which is traditionally used as cathode material for lithium ion batteries, was synthesized through a simple thermal solid-state synthesis. The monoclinic Li0.33MnO2 electrode exhibits a wide operational potential window ranging between −1.25 and 1.25V vs SCE, which enables it to serve as either a negative or a positive electrode. In addition, this electrode material exhibits a high specific capacity of 140mAhg−1 at a low current density of 0.1Ag−1, and 76mAhg−1 at high current density of 1Ag−1 in this range of potentials. Hybrid supercapacitors composed of Li0.33MnO2 positive electrode and activated carbon (AC) negative electrode were fabricated. They exhibit outstanding electrochemical performance in terms of operational potential window, cycleability, and energy and power density. The Li0.33MnO2/AC hybrid capacitor has an energy density of 13.5 Wh kg−1 at power density of 100Wkg−1, which is twice than that of MnO2/AC and AC/AC supercapacitors, and an energy density of 7 Wh kg−1 at 1000Wkg−1, which is seven times higher than that of AC/AC capacitors at this power density. Furthermore, this hybrid capacitor presents an excellent cycle life with 80% specific capacitance retention after 12,000 cycles to 2V. The electrochemical charge storage mechanism of the monoclinic Li0.33MnO2 was investigated by cyclic voltammetry and X-ray diffraction.