Preparation, Structure And Electrochemical Performances Of Al-substituted α-Ni(OH)₂ By Homogeneous Precipitation

It has become an important subject to prepare stable α-Ni(OH)₂ with the good electrochemical performances because the traditional active material β-Ni(OH)₂ has little improving space. In this thesis, α-Ni(OH)₂ samples with different additives which is stable in alkaline solution and has excellent electrochemical performance were prepared by homogeneous precipitation method in organic solution with urea as precipitator.In the third chapter, the effects of several conditions on homogeneous precipitation were explored in detail. The optimized technology for preparing Al-substituted α-Ni(OH)₂ was as follows: adding 0.7mol/L [Ni²⁺] solution and 4.5mol/L urea solution in 30% alcohol-water system to vessel with 1% TX-100, reacting for five hours at 90℃±2℃ with stirring, and controlling PH value and adding rate at 0.5 mL/min. The experimental results showed that a special flaky α-Ni(OH)₂ with thickness of 10-20 nm and a discharge capacity of 320mAh/g can be obtained under the optimized conditions.The electrochemical performances tests showed that the Al-substituted α-Ni(OH)₂ has the lower charging potential and the higher discharge potential than that of β-Ni(OH)₂. The results of cyclic voltammetry also showed that the as-synthesized α-Ni(OH)₂ has the better electrochemical reversibility. Al-substituted α-Ni(OH)₂ samples had higher diffusion coefficient of proton and lower charge transfer resistance than β-Ni(OH)₂ due to the large proton defects and interlayer distance.In the fifth chapter, Al-Zn co-substituted α-Ni(OH)₂ was prepared by homogeneous precipitation method. The experimental results showed that the as-prepared Al-Zn co-substituted α-Ni(OH)₂ had the special flaky surface microstructure, the flake thickness is about 5-10 nm. The pure α-Ni(OH)₂ phase can't be obtained only by the substitution of Zn at 20%(mol). With the substitution of Ni by Al and Zn element, the charge efficiency, crystallite size, cyclic stability and electrochemical reversibility were improved compared with that of Al-substituted α-Ni(OH)₂, however, the discharge potential, discharge capability and electrochemical activation were worsen. Electrochemical impedance spectroscopy(EIS) investigation showed that Al-Zn co-substituted α-Ni(OH)₂ and Al-substituted α-Ni(OH)₂ have the same impedance equivalent circuits.In the sixth chapter, Al-Co co-substituted α-Ni(OH)₂ was prepared by homogeneous precipitation. The results of electrochemical tests showed that the charge efficiency performance increased and the discharge potential and discharge capability tended to decreased for the Al-Co co-substituted α-Ni(OH)₂ compared with the Al-substituted α-Ni(OH)₂ with CoO additve. The results of cyclic voltammetry and proton diffusion coefficients showed that a suitable Al-Co co-substitution for Ni element in α-Ni(OH)₂ can improve the electrochemical reversibility.