Electrochemical capacitance properties of pre-sodiated manganese oxide for aqueous Na-ion supercapacitors

Mn-based oxides are widely investigated as electrode materials for electrochemical supercapacitors, because of their high specific capacitance in addition to the high abundance, low cost, and environmental friendliness of Mn. The pre-insertion of alkali metal ions is found to improve the capacitance properties of MnO₂. While the capacitance properties of MnO₂, Mn₂O₃, P2-Na_0.5MnO₂, and O3-NaMnO₂etc. are reported, there is no report yet on the capacitive performance of P2-Na_2/3MnO₂, which has already been studied as a potential positive electrode material for Na-ion batteries. In this work, we have synthesized sodiated manganese oxide, P2-Na_2/3MnO₂ by a hydrothermal method followed by annealing at a high temperature of about 900 °C for 12 h. For comparison, manganese oxide Mn₂O₃ (without pre-sodiation) is synthesized by following the same method, but annealing at 400 °C. While P2-Na_2/3MnO₂ exhibits a high specific capacitance of 234 F g^-1, Mn₂O₃ can deliver only 115 F g^-1 when cycled at 0.4 A g^-1 in an aqueous electrolyte of 1.0 M Na₂SO₄ in a three-electrode cell. An asymmetric supercapacitor Na_2/3MnO₂‖AC is assembled, which can exhibit a SC of 37.7 F g^-1 at 0.1 A g^-1 with an energy density of 20.9 W h kg^-1, based on the total weight of Na_2/3MnO₂ and AC with an operational voltage of 2.0 V and possesses excellent cycling stability. This asymmetric Na_2/3MnO₂‖AC supercapacitor can be cost-effective considering the high abundance, low-cost and environmental friendliness of Mn-based oxides and aqueous Na₂SO₄ electrolyte.