| Manganese oxide (MnO2) has been investigated widely because of its applications in batteries, separation and catalysis, and used as cathodes for batteries for long time due to its good economy and low toxicity. However, the specific capacity of MnO2 in practical Li/MnO2 batteries is lower compared to its theoretical value, and its cyclic performance is not good for application in rechargeable batteries. Their electrochemical activity depends on many factors, but mainly on the crystal structure of manganese dioxide. Therefore, the synthesis of the manganese dioxide with good crystal structure is of great importance.The traditional method for the synthesis of manganese dioxide include redox precipitation, hydrothermal, low-temperature solid state method and sol-gel method, etc. However, the size of manganese dioxide synthesized by these methods is generally nano size-based. By means of high pressure technology, many new substances or substances with some special morphology and crystal structure can be made up. The flux, which can reduce the reaction temperature, is more conducive to the synthesis of a large single crystal. Therefore, this paper investigated the synthesis of the layered manganese oxide and its structure characterization and electrochemical properties. The manganese dioxide synthesized by high pressure methods, has large size and good electrochemical performance.MnO2 was used as the starting material, KOH as the flux, and the reaction is under the conditions of 100 MPa,400℃and 24 h. In this paper, it is the first synthesis of a novel layered compound, K0.33MnO1.62··0.4H2O. The crystal structure, morphology and electrochemical properties have been studied deeply by means of powder X-ray diffraction (XRD), energy dispersive spectrometer (EDS), thermogravimetric (TG), scanning electron microscopy (SEM), and infrared spectroscopy (IR), ect. The product is flaky crystal, its thickness could reach about 3-4μm, and the largest diameter was 40-50μm Meanwhile, it also has good thermal stability, which is stable up to 700℃. And it shows that, it has good electrochemical performance, which offers a new way for preparation of the batteries of high capacity and good performance.In addition, we also discussed the influences of the reaction time, reaction temperature, pressure and reactant ratio on the composition and morphology of K0.33MnO1.62·0.4H2O. With increasing reaction time,δ-MnO2 was first generated, and then a small amount of Mn3O4 would appear. When a certain time is reached, they all converted into the target product, and the size of the product also increases. With rising reaction temperature, the K0.33MnO1.62·0.4H2O gradually transformed intoδ-MnO2, and a small amount of Mn3O4 was found, and the size of the product would be reduced. The starting materials could not be reacted with each other at the low ratio, and they gradually reacted with increasing the ratio. They would be all transformed into the product when the ratio reaches 1:20. The crystal size of the product increases with increasing of the ratio of reactants. K0.33MnO1.62·0.4H2O can only be synthesized by high pressure, and the crystal size of product synthesized by high pressure was significantly larger than that synthesized under the room pressure.We successfully synthesizedδ-MnO2 by high pressure flux, and characterized its structure and properties. Compared with the conventional synthetic methods, the single crystal ofδ-MnO2 synthesized by high pressure flux has a larger size. The corresponding X-ray diffraction results show that the crystallinity ofδ-MnO2 is good, and it is with monoclinic, space group C2/m, and the distance between the layers is about 0.72 nm. Its scanning electron microscopy shows that,δ-MnO2 was homogeneous single crystal with circular sheet, the thickness was about 2-3μm, the maximum diameter was 30-40μm. The TG curve shows that it has good thermal stability, and can be stable up to 800℃. By means of the infrared spectroscopy, we preliminarily analyzed the local structure information ofδ-MoO2. |
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