Solid state synthesis of Li_0.33MnO₂ as positive electrode material for highly stable 2V aqueous hybrid supercapacitors:

In this paper we present a comprehensive structural, chemical and electrochemical characterization of monoclinic Li_0.33MnO₂ as a positive electrode material for aqueous high-voltage hybrid supercapacitors. The monoclinic Li_0.33MnO₂, which is traditionally used as cathode material for lithium ion batteries, was synthesized through a simple thermal solid-state synthesis. The monoclinic Li_0.33MnO₂ electrode exhibits a wide operational potential window ranging between −1.25 and 1.25 V 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 140 mAh g⁻¹ at a low current density of 0.1 A g⁻¹, and 76 mAh g⁻¹ at high current density of 1 A g⁻¹ in this range of potentials. Hybrid supercapacitors composed of Li_0.33MnO₂ 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 Li_0.33MnO₂/AC hybrid capacitor has an energy density of 13.5 Wh kg⁻¹ at power density of 100 W kg⁻¹, which is twice than that of MnO₂/AC and AC/AC supercapacitors, and an energy density of 7 Wh kg⁻¹ at 1000 W kg⁻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 2 V. The electrochemical charge storage mechanism of the monoclinic Li_0.33MnO₂ was investigated by cyclic voltammetry and X-ray diffraction.