Boosting the Energy Density of Bowl‐Like MnO2@Carbon Through Lithium‐Intercalation in a High‐Voltage Asymmetric Supercapacitor with “Water‐In‐Salt” Electrolyte

Abstract Highly concentrated “‘water‐in‐salt”’ (WIS) electrolytes are promising for high‐performance energy storage devices due to their wide electrochemical stability window. However, the energy storage mechanism of MnO 2 in WIS electrolytes‐based supercapacitors remains unclear. Herein, MnO 2 nanoflowers are successfully grown on mesoporous bowl‐like carbon (MBC) particles to generate MnO 2 /MBC composites, which not only increase electroactive sites and inhibit the pulverization of MnO 2 particles during the fast charging/discharging processes, but also facilitate the electron transfer and ion diffusion within the whole electrode, resulting in significant enhancement of the electrochemical performance. An asymmetric supercapacitor, assembled with MnO 2 /MBC and activated carbon (AC) and using 21 m LiTFSI solution as the WIS electrolyte, delivers an ultrahigh energy density of 70.2 Wh kg −1 at 700 W kg −1 , and still retains 24.8 Wh kg −1 when the power density is increased to 28 kW kg −1 . The ex situ XRD, Raman, and XPS measurements reveal that a reversible reaction of MnO 2 + xLi + + xe − ↔ Li x MnO 2 takes place during charging and discharging. Therefore, the asymmetric MnO 2 /MBC//AC supercapacitor with LiTFSI electrolyte is actually a lithium‐ion hybrid supercapacitor, which can greatly boost the energy density of the assembled device and expand the voltage window.