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

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 MnO₂ in WIS electrolytes-based supercapacitors remains unclear. Herein, MnO₂ nanoflowers are successfully grown on mesoporous bowl-like carbon (MBC) particles to generate MnO₂/MBC composites, which not only increase electroactive sites and inhibit the pulverization of MnO₂ 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 MnO₂/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 MnO₂ + xLi⁺ + xe^-↔Li_xMnO₂ takes place during charging and discharging. Therefore, the asymmetric MnO₂/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.