High-performance aqueous asymmetric supercapacitors based on K + and Na + co-preinserted δ-MnO 2 nanocrystals

Layed δ-MnO 2 nanocrystals are fabricated through a facile precipitation reaction at ambient conditions with Na 2 SO 3 as reductant. The high content of co-preinserted K + and Na + and water content in δ-MnO 2 are responsible for the enhanced electrochemical performance. Specifically, δ-MnO 2 delivers a high specific capacitance of 303.7 F g −1 at 0.5 A g −1 , together with steady cycle property. Moreover, an asymmetric supercapacitor (ASC) is fabricated with δ-MnO 2 as the positive electrode and reduced graphene oxide (RGO) as the negative electrode. The device displays a large energy density of 53.2 Wh kg −1 at 250 W kg −1 with a potential window of 0–2.0 V in aqueous electrolyte, and 91.1% capacitance retention after 5000 cycles. These results can well demonstrate the potential application of layed δ-MnO 2 nanocrystals as the active electrode material in energy storage systems. Graphical abstract Layed δ-MnO 2 nanocrystals are fabricated through a simple precipitation reaction at ambient condition using a new reductant Na 2 SO 3 . The high content of co-preinserted K + and Na + ions and water in δ-MnO 2 are responsible for the enhanced electrochemical performance. δ-MnO 2 //RGO asymmetric supercapacitor exhibits a high energy density in aqueous electrolyte.