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 MnO ₂ . While the capacitance properties of MnO ₂ , Mn ₂ O ₃ , P2-Na _0.5 MnO ₂ , and O3-NaMnO ₂ etc. are reported, there is no report yet on the capacitive performance of P2-Na _2/3 MnO ₂ , 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 _2/3 MnO ₂ by a hydrothermal method followed by annealing at a high temperature of about 900 °C for 12 h. For comparison, manganese oxide Mn ₂ O ₃ (without pre-sodiation) is synthesized by following the same method, but annealing at 400 °C. While P2-Na _2/3 MnO ₂ exhibits a high specific capacitance of 234 F g ^-1 , Mn ₂ O ₃ can deliver only 115 F g ^-1 when cycled at 0.4 A g ^-1 in an aqueous electrolyte of 1.0 M Na ₂ SO ₄ in a three-electrode cell. An asymmetric supercapacitor Na _2/3 MnO ₂ ‖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 _2/3 MnO ₂ and AC with an operational voltage of 2.0 V and possesses excellent cycling stability. This asymmetric Na _2/3 MnO ₂ ‖AC supercapacitor can be cost-effective considering the high abundance, low-cost and environmental friendliness of Mn-based oxides and aqueous Na ₂ SO ₄ electrolyte.
Competing Interests: There are no conflicts of interest to declare.
(This journal is © The Royal Society of Chemistry.)