| Soils, the bottom of ocean and lake sediments contain large amounts of manganese nodules, of which manganese oxide minerals are the main component. The applications of manganese oxide in many fields are growing concerned. There is an important theoretical and practical significance for further investigating manganese oxide properties and promoting the development and resource use of manganese oxides. There are many types of manganese oxides, which can be divided into one-dimensional tunneled, two-dimensional layered and three-dimensional network structures according to spatial structure. Manganese oxides are resource-rich, low cost, environmentally friendly materials, which have important applications, such as in the field of lithium-ion secondary battery cathode materials, super capacitors, catalysts, adsorbents, magnetic materials and nuclear waste fixed materials.Up to now, people have realized that the Li-ion battery is a new generation of high specific capacity battery for high voltage, long-cycle life, low self-discharge and environment-friendly etc. Recently, more and more research has been focused on improving the performance of Li secondary battery. The investigation of negative and electrolytes have great progress, while the development of the Li-ion battery has been limited by its positive materials, which is one of the key components of the Li-ion battery system. On the other hand, it is known that the performance of manganese oxide cathode materials is greatly influenced by the synthesis methods. For example, solid state method, which is used in industry widely, wastes more power and time. At the same time, the materials synthesized using this method seriously agglomerate and its crystals and particles can not be well controlled, which leads to the degradation of materials’ performance. Therefore, more and more researchers focus on looking for new methods to prepare the cathode materials with excellent performance.Based on enhancing the discharge capacity and cyclic stability of manganese oxide materials and decreasing the production cost of Li-ion battery, this work study the synthetic methods of common manganese oxides (todorokite, cryptomelane and birnessite) and the modifiedmanganese oxides by doping, characterize the physical and chemical properties, and investigated the electrochemical performances of as-prepared materials for cathode of lithium ion battery using X-ray diffraction (XRD), Rietveld structure refinement, thermal gravimetric analysis (TGA), component analysis, specific surface area tests, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), galvanostatic charge/discharge and cyclic voltammetry, etc. The main results obtained are as follows:1. A series of cobalt-doped todorokites were synthesized by refluxing process at atmospheric pressure from the transformation of birnessites. Their unit cell parameters, water content, the average oxidation state of Mn (AOS), and crystal morphology (including grain size) were characterized, and the valence state of cobalt ions in all cobalt-doped samples was +3. Rietveld structure refinement showed that doped cobalt ions replaced the equivalent sites of Mn2 and Mn4, and the average bond lengths between metal ions and oxygen anions changed systematically. It was further shown that the cobalt doping improved the stability of the crystal structure and the voltage platform of discharge for todorokites with the combination of electrochemical analysis, and so the discharge capacity and cyclic stability of cobalt-doped todorokites were improved. Among them, doped todorokites with Co content of 10%-15% displayed better discharge performance. Tod-Co10% showed a high initial discharge capacity of 219 mAh/g and cycling capacity of 102 mAh/g after 100 cycles. The effect of doping cobalt on electrochemical performance of todorokite was further proved with cyclic voltammetry by using powder microelectrode.2. A newly arisen method——microwave heating method was used to prepare todorokite with different concentrations of cobalt doping. The differences with refluxing method were that,1) The intensity of diffraction peaks in all samples prepared by microwave heating was strong, and the peak width at half height was narrow, exhibiting that they all had high crystallinity; 2) The AOS of Mn increased slightly with increasing dopant amount; 3) Tod-Co20% had a minimum flake particles, a highest cobalt doping and a lowest initial discharge capacity, but a best cyclic stability and a high coulombic efficiency of charge/discharge. The same as refluxing method were that,1) The water content and the cell volume in todorokites increased by elevating cobalt amount; 2) Todorokite samples primarily consisted of trilling at 120°to each other with flake crystal particles, and with the increase of doped-cobalt content, the particle size decreased and aggregated into spherical secondary particles with high content of cobalt; 3) The specific discharge capacity and the cyclic stability of cobalt-doped todorokites were all better than those of undoped todorokite. 3. Three different methods (refluxing method, sol-gel method and high temperature calcination method) could be used to prepare pure cryptomelane. Because of different synthetic methods, the physical and chemical properties of the three samples were quite different. There were marked differences between Cry-60 and Cry-sol-gel in morphology, but the battery performance was similar with each other. The former had a better cyclic capacity. Because of its low water content, small cell volume and surface area, the larger crystal particles, and the irregular sizes and morphology, Cry-calcine synthesized by the calcination method showed the minimum cycle capacity. Cryptomelane synthesized by reflux method had a better battery performance. Owing to a moderate pretreatment temperature, specific surface area, water content and grain size, Cry-80 sample exhibited the best battery performance, and the tunneled structure of all cryptomelane electrodes synthesized by reflux method is still stable after 50 cycles of charge/discharge.4. By further doping cobalt ions, a series of cryptomelanes with different cobalt concentrations were prepared at reflux condition. The unit cell volume decreased with the increase of doped cobalt content, the valence state of cobalt ions are +3, but the water content increased gradually. The tunnel water content of cryptomelanes influenced the cycle capacity of the battery:more tunnel water were extracted for cryptomelane heat treated at 300℃, leading to the easy collapse of tunnel structure during charge/discharge, and all electrodes showed the poor discharge performance; Most of the tunnel water was not removed after sinter at 140℃, and played the role of supporting the tunnel which could elastically expand or shrink during the insertion/extraction of Li+, increase the cycle stability of electrodes materials. Doping cobalt ions further improved the cycle stability of the cryptomelane, but the excessive doped-content will reduce the capacity of inserting lithium. Therefore, Cry-Co5% electrode heat treated at 140℃displayed the best discharge performance, its initial discharge capacity was up to 254 mAh/g, and maintained at 138mAh/g after 50 cycles.5. A series of birnessites with different cobalt concentration were prepared by a one-step oxidation of the mixture of MnCl2 and CoCl2 using O2 at strong alkaline condition. The influences of the doped cobalt content on the unit cell parameters, crystal size, water content, surface area, AOS of Mn, valence state of Co, proportion of O with different chemical state and crystal morphology (including the particles size) were investigated. Combining TG and component analysis, the birnessite with different layered water content could be prepared by changing the temperature of heat treatment. The galvanostatic charge/discharge results were that the discharge capacity of undoped birnessite decreased rapidly, and the cycle stability of birnessites were all promoted by doping cobalt. Because a part of layered water was extracted by fit heat treatment (at 130℃for 4 h) to increase the insertion/extraction sites of Li+, the residual layered water supported the layer structure, and doping cobalt made the skeleton of MnO6 octahedra more firm, birnessite with doping cobalt 10% showed the best cycle stability, whose initial discharge capacity was as high as 224 mAh/g, and kept at 115 mAh/g after 100 cycles at 20 mA/g between 2.0 and 3.8 V (vs. Li/Li+). The discharge test with different rate showed that Bir-Co10% electrode had a better discharge capacity at high current density, because of its large surface area.6. A series of birnessites with different doped cobalt contents were firstly synthesized by controlling the added amount of Co(CH3COO)2 using sol-gel method with the raw materials of KMnO4, sucrose and Co(CH3COO)2. The XRD results showed that the main compositions of all samples were monoclinic structural birnessites, containing a few impurity of hausmannite, whose content increased a little with increasing dopant amount. Owing to the passive film at the surface of cobalt-doped birnessites synthesized by sol-gel method, their open circuit voltages were low, and the internal resistances of the batteries were large. The discharge capacity for all samples were small at first cycle, and up to the largest discharge capacity by the activation of charge at first cycle. The content of hausmannite in birnessites increased gradually with increasing dopant amount, resulting in the deterioration of the battery performance. Undoped birnessite heat-treated at 130℃for 4 h with fit layered water extracted and the lowest content of hausmannite showed the best cycle stability; the cyclic voltammogram exhibited that Bir-Co0% electrode still retained high electrochemical activation after 60 cycles of charge/discharge. The redox peaks of Bir-Co0% electrode had fine symmetry, lesser voltage difference, and few decay of current. |
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