Transformation from microcrystalline LiMn1-xCrxO2 to 1D nanostructured β-Mn1-xCrxO2:: Promising electrode performance of β-MnO2-type nanowires

Cr-substituted beta-Mn1-xCrxO2 nanowires with nonaggregated morphology have been synthesized by the chemical oxidation of layered LiMn0.9Cr0.1O2 microcrystals under hydrothermal conditions at elevated temperature. According to powder X-ray diffraction analysis, a persulfate treatment at 200 degrees C gives rise to a unique transformation from a monoclinic-layered structure to a pyrolusite (beta-MnO2-) type structure. Electron microscopic analyses clearly demonstrated that the oxidized derivative consists of well-separated single-crystalline nanowires with a diameter of similar to 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 beta-MnO2-type nanowires show promising electrode performance for Li+ ion batteries, superior to those of pristine LiMn0.9Cr0.1O2, bulk beta-MnO2, and other structuretype manganese oxide nanowires including unsubstituted beta-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.