Compared with batteries, electrochemical capacitors have the potential for providing higher power density and good cyclability. Pseudocapacitance results from either superficial or multi electron-transfer faradaic reactions with fast charge/discharge properties. An increasing number of metal oxides exhibiting pseudocapacitive behaviors have been discovered in recent years. We have previously discovered a new nanocrystalline oxide material having the formula of MnFe₂O₄that have so far been the only oxide electrode showing pseudocapacitance in both aqueous and organic Li-ion electrolytes. The discovery of this material has opened up the possibility of setting up a high voltage asymmetric supercapacitors of substantially enhanced energy density over the state-of-the-art EDLC organic supercapacitors.

    In this study, MnFe₂O₄ spinel oxide electrodes was synthesized and characterized for supercapacitor applications using organic Li-ion electrolyte. The change in structure, valence state and local atomic arrangement upon charging/discharging are investigated with in operando X-ray analyses. The operation voltage window is carefully determined in view of leakage current for the upper voltage limit. The lower voltage window is selected to maximize the energy density of the spinel oxide electrode. For the purpose of increasing the energy density of cell up for the demand of electronic devices, asymmetric capacitor is assembled using the spinel oxide as positive electrode and pre-lithiated silicon coated graphite (SGF) as negative electrode. It is shown that the asymmetric capacitor has greatly increased the energy density by asymmetric mass matching between anode and cathode and by extending the overall operation voltage window.