Asymmetric supercapacitors have potential applications in renewable-energy technology owing to their remarkable electrochemical properties. A high-voltage asymmetric supercapacitor was developed based on a core–shell-structured MnO2@carbon sphere composite (MnO₂@CS) as the cathode, nitrogendopedactivated carbon as the anode and a neutral aqueous Na₂SO₄ solution as the electrolyte. MnO₂@CS was successfully fabricated by hydrothermally growing MnO₂ on the surface of carbon spheres. A nitrogencontaining benzoxazine resin was adopted as a precursor to produce in situ nitrogen-doped activated carbon.Such an aqueous electrolyte-based asymmetric supercapacitor can be cycled reversibly in the high-voltage region of 0–1.9 V and exhibits a superior energy density of 8 Wh kg^-1 at an ultrahigh power density of 9627 W kg^-1 owing to the matching of MnO₂@CS and porous nitrogen-doped activated carbon. Moreover,the asymmetric supercapacitor presents acceptable cycling performance, with 74.4% retention after 1000 cycles at 1 A g^-1, and a charge–discharge efficiency of the electrode of almost 100%.