Abstract: Asymmetric supercapacitors (ASCs) are promising candidates for high-power output applications, but their theoretical capacity remains largely unrealized due to the low capacitance of carbon negative electrodes. Traditional strategies for enhancing carbon capacity via structural optimization often compromise tap density, electrical conductivity, and rate performance. Herein, we address this bottleneck by incorporating 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl (4OT) as a redox mediator into the electrolyte, constructing ASCs with well-matched capacitance and potential windows between the two electrodes. Benefiting from the rapid redox kinetics of 4OT’s radical chemistry, the carbon electrode capacity can be flexibly tuned by adjusting the 4OT concentration without sacrificing rate performance. The Ni3S2/CoNi2S4 positive electrode exhibits a remarkable capacity, attributed to its superior conductivity, abundant active sites, and enhanced electrochemical activity. Notably, the addition of 50 mM 4OT effectively balances both the capacity and potential window of the two electrodes, enabling the ASC to achieve a maximum energy density of 55 Wh kg-1, which significantly surpasses previously reported values. This work demonstrates the great potential of rational redox mediator design for realizing capacity equilibrium and boosting the energy density of high-performance ASCs.