SOLID STATE SYNTHESIS OF Li0.33MnO2 AS POSITIVE ELECTRODE MATERIAL FOR HIGHLY STABLE 2V AQUEOUS HYBRID SUPERCAPACITORS

Many studies demonstrate the great potential of MOx, especially MnO₂, as electrode materials for high energy supercapacitors. However, monoclinic Li_XMnO₂, which has been traditionally used as cathode material for Li ion batteries, was barely studied as an electrode material in the field of supercapacitors, despite its improved stable structure and conductivity relative to pure MnO₂. 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 was synthesized through a thermal solid-state synthesis. Li_0.33MnO₂ electrodes exhibit wide operational potential window, between −1.25 and 1.25 V vs SCE, which enables them to serve as either negative or positive electrodes. This electrode material exhibits high specific capacity 140 mAh g⁻¹ at low current densities 0.1 A g⁻¹, and 76 mAh g⁻¹ at 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, 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 voltammetry and XRD.