Sodium pre-doped manganese oxide (denoted as SMO) is prepared via a simple, warm-water synthesis method for the application of asymmetric supercapacitors. The pre-doping quantity of sodium significantly affects the crystallite size and morphology of resultant SMO nanoparticles (NPs), leading to the variation in capacitive performances. The crystalline structure and surface morphology of SMO NPs are examined by X-ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopic (TEM) analyses. Among 4 samples, the SMO NPs prepared in the 35-mL precursor solution containing 15 mmol Na2SO4 (denoted as SMO-15) shows the highest specific capacitance (similar to 153.7 F g(-1)) as well as acceptable capacitance retention. An aqueous asymmetric supercapacitor with a cell voltage of 2.4 V is demonstrated using activated carbon as the negative electrode and SMO-15 as the positive electrode. The specific energy and power of this device (based on total mass of active materials) measured at 20 A g(-1) are equal to 22.72 Wh kg(-1) and 24.41 kW kg(-1), respectively. (C) 2015 The Electrochemical Society. All rights reserved.