Origin of Improved Electrochemical Activity of β-MnO2 Nanorods: Effect of the Mn Valence in the Precursor on the Crystal Structure and Electrode Activity of Manganates

1D nanorods/nanowires of manganese oxides with different crystal structures and morphologies were prepared and characterized to understand the influence of the Mn valence in the solid-state precursor oil the electrochemical activity of these nanomaterials and to elucidate the mechanism responsible for the excellent activity of beta-MnO2 nanorods as well. According to powder X-ray diffraction analyses, treating manganese oxide precursors that have an oxidation state of <=+3 with persulfate ions under hydrothermal conditions yields manganese oxides with the beta-MnO2 structure. In contrast, the use of a LiMn2O4 precursor with a higher Mn valence leads to the formation of the alpha-MnO2-structured manganese oxide. Electron microscopic studies clearly show a 1D nanorod-type morphology for the beta-MnO2 material, whereas a 1D nanowire-type morphology with a higher aspect ratio is observed for the alpha-MnO2 material. The diameter of the beta-MnO2 nanorods decreases as the Mn valence in the precursors becomes smaller. According to electrochemical measurements, the formation of nanorods dramatically improves the electrode performance of the beta-MnO2 phase. This compares with a relatively weak performance enhancement for the alpha- and delta-MnO2 phases upon the nanowire formation. The Optimum electrode property results from the smaller beta-MnO2 nanorods prepared with the MnO precursor. Li-7 magic angle spinning nuclear magnetic resonance spectroscopy clearly demonstrates that Li+ ions in the lithiated beta-MnO2 phase are adsorbed mainly on the sample Surface. On the basis of this finding, we attribute the improved electrode performance of the beta-MnO2 nanorods to their expanded surface area.