Transformation from microcrystalline LiMn_1-xCr_xO ₂ to ID nanostructured β-Mn_1-xCr_xO ₂: Promising electrode performance of β-MnO₂-type nanowires

Cr-substituted β-Mn_1-xCr_xO₂ nanowires with nonaggregated morphology have been synthesized by the chemical oxidation of layered LiMn_0.9Cr_0.1O₂ microcrystals under hydrothermal conditions at elevated temperature. According to powder X-ray diffraction analysis, a persulfate treatment at 200 °C gives rise to a unique transformation from a monoclinic-layered structure to a pyrolusite-(β-MnO₂-) type structure. Electron microscopic analyses clearly demonstrated that the oxidized derivative consists of well-separated single-crystalline nanowires with a diameter of ∼30 nm and a length of several micrometers. Mn K-edge and Cr K-edge X-ray absorption spectroscopy and chemical analysis provide straightforward evidence for the substitution of chromium ions for the manganese sites of the pyrolusite-structured manganese oxide nanowires. Of special importance is that the present Cr-substituted β-MnO₂-type nanowires show promising electrode performance for Li⁺ ion batteries, superior to those of pristine LiMn_0.9Cr_0.1O₂, bulk β-MnO ₂, and other structure-type manganese oxide nanowires including unsubstituted β-MnO₂ nanowires. On the basis of the present experimental findings, we are able to conclude that the persulfate treatment under hydrothermal conditions provides a powerful method not only to prepare cation-substituted manganese oxide nanowires but also to improve the electrode performance of microcrystalline metal oxides through nanostructure fabrication.