| Nickel hydroxide is the cathode material of nickel-based batteries. The alkaline nickel-based batteries have been extensively used for their high security, high specific energy, high reliability and low cost, especially in the areas of electric vehicles, microelectronic devices and aerospace industry. The nickel hydroxide has two crystal phases, namely a-Ni(OH)2and β-Ni(OH)2, and respectively converted into γ-NiOOH and β-NiOOH when charging. The theoretical capacity of β-Ni(OH)2is289mAh/g, while α-Ni(OH)2is482mAh/g.In this paper, samples of nano a-Ni(OH)2containing SO42-, NO3-, Cl-and doped with Yb、Cu、Y、La are prepared by ultrasonic assisted precipitation and the impact of source/doping element/buffer on the structure of Ni(OH)2is studied. X-ray diffraction (XRD), infrared spectroscopy, transmission electron microscope (TEM), scanning electron microscope(SEM), high resolution transmission electron microscopy(HRTEM), laser particle size analyzer(PSD) are used to characterize the crystal structure, morphology and particle size distribution. Electrochemical properties of the synthesized samples were studied by Ni-MH battery performance test and cyclic voltammetry. The results indicate that preparation conditions of the nickel sources, doping elements, kinds of buffer solution and cation-anion had a greater impact on the phase of Ni(OH)2. Larger cationic radius was conducive to the formation of α-Ni(OH)2and insetting of anions. The proportion of a-Ni(OH)2samples doped with two elements is larger than one doped with single element. The XRD diffraction peaks of Yb-doped samples are anisotropic broadening, and the morphology of samples are polycrystalline. The synthesized products are mainly a-Ni(OH)2, whose average particle size is about50～200nm and decreases as the ratio increasing of Yb-doping. The positive electrode are prepared with the nano samples mixed with micron-level spherical nickel hydroxide at the ratio of8Wt.%. The specific capacity increases initially and then decreases with increasing Yb-doping ratio, the electrode containing1.44wt.%Yb shows the maximum discharge capacity of295.1mAh/g at0.2C, which is higher about60mAh/g than μm-scale spherical nickel electrode. Otherwise it has lower charging voltage, higher discharge voltage plateau and better cycle performance.Embedded anions are measured by Fourier transform infrared spectrum (FTIR) indicating the existing of SO42-and NO3-between the layers of the layered structure of a-Ni(OH)2.The peaks of association hydroxyl of samples are all broad and strong, while the peaks of free hydroxyl at about3642.6cm-1decrease gradually as the content of Yb-doped increase, showing that α-Ni(OH)2content increases successively.The crystal structure of the samples was determined by X-ray diffraction (XRD). The results indicate that the diffraction peaks shift to the right differently as the difference of valence state among SO42-mNO3-, Cl-and Y3+, Yb3+, Cu2+. In addition, ions with different valence states causing different interference on the crystal lattice of Ni(OH)2. Ions with larger radiu and higher valence state can more greatly interfere with the lattice and crystallinity is worse.In short, reaction conditions, the doped element, the buffer and preparation methods have an impact on the crystalline phase and the agglomeration of samples. Larger radius anions of nickel sources or buffer contribute to the formation of a-Ni(OH)2-Samples prepared with nickel sulphate as nickel source contain more α-Ni(OH)2than samples with nickel nitrate or nickel chloride as nickel source. Samples with Na2HPO4as buffer contain more α-Ni(OH)2than ones with Na2CO3as buffer. |
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