| As one of the transition metal oxides, manganese oxides have been widely applied in the fields of catalysis, ion exchange, molecular adsorption, biological sensor, high density magnetic recording media, lithium ion battery, and supercapacitor due to their outstanding advantage including abundant raw materials, low cost, low toxicity, and environmentally friendly characteristics. Manganese oxides have attracted more and more attention owing to their diversity of fundamental structure, which affects their physical-chemistry properties. Especially in the field of green chemical power sources, manganese oxides can be used as negative electrode material of lithium-ion batteries, also it also can be applied to super capacitor because of polyvalent form. Manganese oxides can be performed as precursor to synthesize orthorhombic LiMnO2(o-LiMnO2) and LiMn2O4, which is one of the widely uesed cathode materials, and the former shows great potential applications in lithium-ion battery.Mn2O3of low valence state is mainly synthesized by precursors such as MnO2, MnOOH, MnCO3, Mn-NTA by calcination at400～600℃. In this paper, a-Mn2O3was first hydrothermally synthesized, and then pyrolyzed to a-Mn2O3at350℃. The electrochemical reaction mechanism of a-Mn2O3used as electrode materials in lithium battery and supercapacitor was investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and the XRD characterization of intermediate product to study of. At present, o-LiMnO2was fabricated through a two-step process:precursor was first synthesized, and then calcined at high temperature in argon gas. In this work, the o-LiMnO2was synthesized by one-step hydrothermal process. The experimental results demonstrated that unit cell parameters would affect the electrochemical properties of o-LiMnO2. Al doped o-LiMnO2was further hydrothermally synthesized using the similar reaction process. The influence of the Al doping on crystal cell parameters and electrochemical properties of o-LiMnO2was studied by XRD structure refinement and infrared spectra, cyclic voltammetry, and AC impedance. In order to improve the electrochemical performance of o-LiMnO2, a adopted hydrothermal method was conducted to synthesize o-LiMnO2/CNT, and the lithium battery performance was improved to a certain degree. The main topics and results of the research are summarized as fellows:.1) A series of a-Mn2O3microstructures with different morphologies were successfully prepared from the transformation of MnF2using heat treatment at350℃for12h. MnF2precursors were fabricated using hydrothermal reactions of manganese acetate and hydrofluoric acid solution in ethanol, water, and glycerol, respectively. The molar ratio of HF/Mn(CH3COO)2and solvents affected the morphologies of MnF2and the corresponding α-Mn2O3. When HF/Mn(CH3COO)2molar ratio was2:1in ethanol to synthesize MnF2precursor, microspheric α-Mn2O3was obtained (Mn2O3-2a). As HF/Mn(CH3COO)2molar ratio increased to12:1, polyhedral a-Mn2O3was formed. The viscosity influenced the morphologies of MnF2and corresponding Mn2O3possibly due to the steric effect. When water of lower viscosity used as solvent in hydrothermal reaction, the particle size of a-Mn2O3became bigger and irregular (Mn2O3-2w). When glycerol of higher viscosity was used instead, smallest particles of a-Mn2O3were obtained (Mn2O3-2g). Mn2O3-2a exhibited excellent initial discharge capacity for lithium anode materials, and specific surface area have no obvious influence on intercalation lithium, capacity. The formation of LiAl alloy did much contribution to the initial discharge capacity. Spherical a-Mn2O3exhibited excellent lithium storage capacity of2899mAh g-1at first cycle and265mAh g-1after15cycles.2) Mn2O3was transformed into burserite and exhibited acceptable capacitive properties after activation of many cycles. Large surface area and temperature accelerated the formation of burserite and showed high specific capacitance. Mn2O3-2g had the highest specific capacitance of202F g-1and kept steady after400cycles of charge/discharge activation. The various microstructures of α-Mn2O3as-prepared had obvious influence on the microstructures of burserite. At the same time, the particle size of Mn2O3affected the transformation to burserite, which had significant influence on properties and reaction mechanism of capacitor. This work facilitates the preparation and application of a-Mn2O3with different microstructures and understanding of influencing factors and reaction mechanism for their electrochemical lithium storage and supercapacitive properties.3) It is very difucult to synthesize the metastable o-LiMnO2by conventional method. In this work, pure-phased and aluminium-doped o-LiMnO2cathode materials with high discharge capacity and good cyclic stability were prepared by a simple one-step hydrothermal treatment of MnCl2, ethylene diamine tetraacetic acid tetrasodium salt (EDTA), LiOH, AlC13and NaCIO solutions at180℃for24h. The component and doping content were affected by hydrothermal temperature and the amount of LiOH, NaCIO and AICl3in reaction system. The addition of EDTA and NaClO facilitated the formation of pure-phased and aluminium-doped o-LiMnO2, respectively. Layered o-LiMnO2was transformed into spinel LiMn2O4in air at high temperature of300-600℃. When heat-treatment temperature was increased from300to600℃, cell parameter a was decreased from0.8178run (M300) to0.8125nm (M600).So the unit cell volume of M600changed a lot in phase transition in3.3-2.4V platform, leading to electrochemical performance degradation. Remarkably attenuation occurred at4.3-3.6V platform for the original o-LiMnO2. Al doping would change the ionic radius and unit cell parameters of o-LiMnO2, and it was found that a reduced, b increased, and c changed disorderly. It should be pointed out that Al/Mn molar ration in o-LiMnO2phase was only0.34(M5),0.58(M10),0.91(M15), and1.22(M20) when Al/Mn molar ratio in reaction system was controlled as0.05:1,0.10:1,0.15:1, and0.20:1, respectively. And M10showed the best electrochemcal performance for lihtium storage. The appropriate amount of Al-doping in M10show the best unit cell stability, which lead to litter reaction resistance and better performance. These results suggested that the unit cell parameters a and b values would affect the reaction resistance, while the lattice parameter c value would affect the migration rate of lithium ions.4) This work successfully synthesized o-LiMnO2/CNT by the hydrothermal method. During the synthesis of single phase o-LiMnO2/CNT, the added mount of LiOH and NaCIO in the reaction system influenced by-COOH and CNT. Electrochemical performance of the o-LiMn02with loading raw CNT had been improved successfully. However, the electrochemical performance would be descended by loading carboxylated CNT, likely due to that the group of-COOH would affect the combination of CNT and o-LiMnO2, and then affected the electrochemical performance. |
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