High Performance Aqueous Sodium-Ion Capacitors Enabled by Pseudocapacitance of Layered MnO2

Aqueous sodium-ion capacitors (ASICs) are becoming increasingly important due to the remarkable advantages of aqueous electrolyte about the excellent ionic conductivity, non-flammability and low cost compared with organic systems. But, low capacitance of the electric double-layer capacitive material and narrow potential window of aqueous electrolyte both have negative effects on the enhancement of energy density. Therefore, we employ typical pseudocapacitive material, layered MnO2/CNTs composite as cathode to fabricate sodium ion capacitor. It needs to be emphasized that the electrochemical process involves two kinds of energy storage mechanisms, such as the reversible Na+ adsorption/desorption onto the surface of each layer and fast Na+ (de)intercalation into the 2D interlayer space. Thus, the composite delivers a high specific capacitance (322.5 F g(-1) at 0.5 A g(-1)) and an excellent cycle stability (5000 cycles with capacitance retention of approximately 90 %). By means of the synergistic effects of the layered MnO2/CNTs cathode, sodium-ion water-in-salt electrolyte (NaWiSE) and polyimide organic anode, the as-assembled ASIC achieves a high energy density of 78.5 Wh kg(-1), accompanied by high power density of 11000 W kg(-1) and excellent cycle performance (even 77 % capacity retention after 10000 cycles).