Aqueous carbon-based supercapacitors are now reaching the energy density limits set by electric double layer mechanism and water decomposition voltage window. Here, we construct a high-energy-density carbon-based supercapacitor with a new strategy of asymmetric electrolyte design by adding K-3[Fe(CN)(6)] and 2,6-dihydroxyanthraquinone (2,6-DHAQ) as positive and negative redox additives in 2 M KOH solution respectively. Modified activated carbon felt (ACF) serves as low-cost free-standing electrodes with no binders and conducive additives, porous structure of which facilitates excellent electrolyte permeability and fast ion transport. And it shows great potential for large-scale energy storage due to unique flexible and scalable properties. The corresponding supercapacitor integrating the advantages of redox activity of K-3[Fe(CN)(6)] in positive potential and 2,6-DHAQ in negative potential achieves higher ion utilization, extended operation window to 2 V, high cell capacitance of 79 F g(-1) and excellent cycling performance with 84% capacitance retention after 5000 cycles. Most importantly, it delivers high energy density of 39.1 Wh kg(-1) maintaining superior power density, much higher than those of previously reported supercapacitors with symmetric aqueous redox electrolytes. The asymmetric redox additives in aqueous electrolytes for ACF-based supercapacitors may represent a new approach to high-performance energy storage.