Currently, a major constraint in employing supercapacitors as a solitary energy storage device in applications like electric vehicles is their low energy density. In aqueous asymmetric supercapacitors, the energy density is limited by the voltage window, which is governed by the electrode’s work functions. Here, the preinsertion of different metal cations such as Li + , Na + , and K + ions into manganese dioxide (MnO 2 ) to tune the electrode’s work function is demonstrated. Sodium-doped MnO 2 (NaMnO 2 ) exhibited a lower work function than Li + and K + preinserted electrodes. This lowering of the work function leads to a higher voltage window. The work function tuned NaMnO 2 is coupled with a high-work-function negative electrode material, molybdenum oxide (MoO 2 ), to fabricate a 2.5 V aqueous asymmetric supercapacitor. The supercapacitor delivered a maximum energy density of 78 Wh kg –1 , a power density of 4.6 kW kg –1 , and capacitance retention of 98.6% after 5000 cycles. This work brings new insights into the engineering of electrode’s work function for developing high-voltage and high-energy supercapacitors.