Towards Battery Like Energy Density of MnO ₂ Electrodes in Water‐in‐Salt Electrolyte Environments

Enhancing the energy density of supercapacitors (SCs) is essential for advancing their capabilities, broadening their range of applications, and making them more competitive with other energy storage technologies like batteries. Although aqueous SCs are emerging as a desirable technology, their energy density is severely restricted by the low thermodynamic stability window of aqueous electrolytes. Achieving a high‐energy supercapacitor that leverages the benefits of an aqueous electrolyte is both highly desirable and challenging. In this study, we address this challenge by utilizing a water‐in‐salt 17 m NaClO ₄ (WiS) electrolyte that allows an operational window of 2.87 V, coupled with engineered potassium‐incorporated manganese dioxide (K‐MnO ₂ ) electrode. Our research demonstrates that the insertion of potassium ions (K ⁺ ) into manganese dioxide (MnO ₂ ) significantly enhances its pseudocapacitive behavior via the Mn ³⁺ /Mn ⁴⁺ redox mechanism. This enhancement, facilitated by the wide potential range provided by the electrolyte, results in a high areal capacitance of 1051 mF/cm ² (294.3 F/g). Furthermore, the asymmetric supercapacitor, featuring an activated carbon negative electrode and K‐MnO ₂ positive electrode, operates at 2.3 V, achieving an energy density of 59.5 Wh/kg. This study underscores the critical role of well‐defined electrochemical interfaces in enhancing the energy density of SCs. It also offers a powerful approach to utilizing WiS electrolytes, enabling wide‐window operation of electrodes in aqueous SCs to achieve battery‐like energy densities.