Manganese Oxide Micron Synthesis And Characterization

In this dissertation,α-MnO₂ nanorods,α-MnO₂ spheres, and spinel LiMn₂O₄ microspheres have been synthesized, respectively. Their electrochemical properties have been characterized, accordingly. The details are presented as follows.Firstly, single-crystalα-MnO₂ nanorods were prepared by simple hydrothermal decomposition of KMnO₄ under acidic conditions. SEM and TEM images indicate that the as-synthesizedα-MnO₂ products are composed of nanorods with diameters 30-70 nm and have a length up to2.6μm. The growth process of the MnO₂ nanorods was studied by analyzing phase structures and morphologies of the products obtained at different reaction stages. An "agglomeration-dissolution/recrystallization-anisotropic growth-Ostwald ripening" process was proposed to explain the formation ofα-MnO₂ nanorods. Meantime, the effects of various experimental parameters on the phases and morphologies of the final products were investigated in detail. Electrochemical studies show that the MnO₂ nanorods prepared for 5 and 8 h exhibit ideal capacitive behaviors with initial discharge capacities of 84.9 and 75.3 F/g, respectively. In addition, the as-preparedα-MnO₂ nanorods display fine discharge characteristics in laboratory-made Zn-MnO₂ cells. The discharge capacities of theα-MnO₂ nanorods are 278 and 235 mAh/g, respectively, at current rates of 62.5 and 375 mA/g, accordingly.Secondly,α-MnO₂ spheres were synthesized by a mild wet chemical method. SEM and TEM images indicate that the resultant products consist of spherical particles with diameters ranging from 0.3 to 3μm. It is found that the mild reaction condition favors the formation ofα-MnO₂ spheres and the morphologies of the MnO₂ products vary with the increase of reaction temperatures. The formation of theα-MnO₂ spheres was discussed based on the experimental results. Electrochemical studies show that theα-MnO₂ nanorods obtained at room temperature exhibit fine electrochemical properties in laboratory-made Li-ion cells. The initial discharge capacity was 206.2 mAh/g and the discharge capacity retained 154.7 mAh/g over 15 cycles. In addition, it is seen that the products prepared at 70℃show excellent discharge characteristics in laboratory-made Zn-MnO₂ cells with discharge capacities of 280 and 235 mAh/g, respectively, at current rates of 50 and 500 mA/g, accordingly.Thirdly, spinel LiMn₂O₄ products were synthesized by solvothermal and high temperature solid-state reaction, respectively, by using the pre-obtainedα-MnO₂ spheres as precursors. SEM images indicate that the resulting products prepared by solvothermal reaction exhibit irregular shapes while the products obtained by solid-state reaction are microspheres having diameters of 1-10μm. Electrochemical studies show that the LiMn₂O₄ products prepared by solid-state reaction exhibit better electrochemical properties that that prepared by solvothermal method.