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). Design and charge‐storage mechanism of the present aqueous sodium ion capacitor. [ABSTRACT FROM AUTHOR]