MnO2+delta (Manganese oxide) nanoflakes were synthesized for use as electrode material in electrochemical supercapacitors. The nanoflakes were produced via RF-magnetron sputtering with various excess oxygen contents (delta), and the electrochemical supercapacitive properties of the Mn2+delta nanoflakes were investigated as a function of delta with the use of a Na2SO4 electrolyte. The excess oxygen (delta) induces the MnO2+delta nanoflakes to form a thin open structure, and mu-Raman measurements revealed that the MnO2+delta nanoflakes formed a bimessite phase with a layered structure. X-ray photoelectron spectroscopy was used to obtain quantitative information on both the oxidation state and the chemical composition of the nanoflake electrodes. The crystallinity of the nanoflakes improved when the oxygen partial pressure increased during sputtering. At an optimal delta similar to 0.6, the electrochemical stability and the capacity retention significantly improved, and electrochemical impedance spectroscopy revealed that easy access of Na+ ions into the nanoflakes at an optimal delta value resulted in a low diffusion resistance, playing a key role in determining the improvement in the supercapacitor characteristics. (C) 2015 Elsevier Ltd. All rights reserved.