Redox stoichiometric ratio mediated fast (4 h) hydrothermal synthesis of Na0.44MnO2 nanowires for high pseudocapacitive Na plus ion supercapacitors

Key issue for Na-ion supercapacitors is facile preparation of electrode active materials with high electrochemical performance. In this paper, 4 h preparation of Na0.44MnO2 nanowires (2.34 g/50 mL) were achieved by the redox hydrothermal reactions of KMnO4 and MnSO4. The fast synthesis is due to the stoichiometric ratio of raw materials (MKMnO4/MMnSO4 = 14/31): (1) fast generation of Na-birnessite from the redox reaction of KMnO4 and MnSO4, and (2) fast generation of Na0.44MnO2 from redox hydrothermal reaction of Na-birnessite and Mn2+. Na0.44MnO2 nanowire electrodes had a pseudocapacitive Na+ ion storage mechanism, a high specific capacity (263.6F/g at 0.1 A/g) and a good capacity retention (almost 100 %) after 5000 cycles at 10 A/g.