A facile, scalable and cost-efficient redox reaction is developed to prepare micro-powders of a quasi-crystallised, mesoporous birnessite-type manganese oxide, K2Mn4O9. In 1 M KOH electrolyte, the K2Mn4O9 powder shows a high specific capacitance of 754 F g(-1) at 1 A g(-1) (calculated with the net weight of K2Mn4O9 micro-powder only). Meanwhile, the electrode retains 91% of its initial capacitance after 5000 cycles at a high current density of 5 A g(-1). By simply adding carbon nanotubes (CNTs) into the reaction system, the specific capacitances of as-prepared K2Mn4O9/CNTs composites are further increased to 929 and 1055 F g(-1) at 1 A g(-1) in 1 and 6 M KOH electrolyte (corresponding to 69 and 77% of the theoretical capacitance of MnO2), or 600 and 674 F g(-1) at 5 A g(-1), respectively. Significantly, a maximum energy density of 62 Wh kg(-1) at a power density of 852 W kg(-1) could be achieved based on a K2Mn4O9/CNTsllactivated carbon asymmetric supercapacitor (ASC). At the same time, the ASC device exhibits a decent long cycle life with 85% specific capacitance retained after 1000 cycles, suggesting its wide application potential in low-cost high energy density storage systems. (C) 2015 Elsevier B.V. All rights reserved.