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 on the electrochemical activity of these nanomaterials and to elucidate the mechanism responsible for the excellent activity of β-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 β-MnO₂ structure. In contrast, the use of a LiMn₂O₄ precursor with a higher Mn valence leads to the formation of the α-MnO₂-structured manganese oxide. Electron microscopic studies clearly show a 1D nanorod-type morphology for the β-MnO₂ material, whereas a 1D nanowire-type morphology with a higher aspect ratio is observed for the α-MnO₂ material. The diameter of the β-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 β-MnO₂ phase. This compares with a relatively weak performance enhancement for the α- and δ-MnO₂ phases upon the nanowire formation. The optimum electrode property results from the smaller β-MnO₂ nanorods prepared with the MnO precursor. ⁷Li magic angle spinning nuclear magnetic resonance spectroscopy clearly demonstrates that Li⁺ ions in the lithiated β-MnO₂ phase are adsorbed mainly on the sample surface. On the basis of this finding, we attribute the improved electrode performance of the β-MnO₂ nanorods to their expanded surface area.