Studies On The Activation Process Of Nickel Hydroxide Electrode

More and more national governments around the world have been realizing the importance of the development of electric vehicle due to the increasingly serious problems of environmental pollution and energy consumption. Among the electric vehicles, the hybrid electric vehicle is the most mature one at the present time. As the most mature electrochemical power sources for hybrid electric vehicle, Ni/MH power batteries have good comprehensive electrochemical performance. However, with the fast development of Ni/MH power batteries industrialization, the specific energy, the specific power and cycle life of Ni/MH power batteries should be further promoted and the production cost of the batteries should be reduced. The settlement of these issues strongly depends upon basic research of applicable technology of Ni/MH power batteries. The study of the activation process of Ni/MH power batteries positive electrode belongs to this basic research. Focusing on the activation process of Ni(OH)2 electrode, some innovative works referring to the activation behavior of electrode material Ni(OH)2, the study of Ni(OH)2 containing Ni(Ⅲ), differences in the effects of Co and CoO additions on the performance of Ni(OH)2 electrode, the performance of Ni/MH power batteries after long time storage have been done in this thesis. The main progress of this thesis is summarized as follows:1. The activation process of Ni(OH)2 was studied by a single particle microelectrode method with an improved apparatus. The color change of the single particle could be seen during the charging and discharging process. The result indicates that the Ni(Ⅲ) formed in the charging process cannot be completely reduced under nonnal reduction conditions. The study of the activation process manifests that CV is an effective way to activate the Ni(OH)2 particle. The variation of charge efficiency during the activation process means that in the initial stage the major reaction has faster activation speed than that of the side reaction (oxygen evolution), and then the activation speed of these two reactions become the same. The competition between the major and side reactions is an important characteristic of the activation process. The analysis of normalized output rate reveals that the activation process does not improve the electrochemical reaction rate of the particle. A model to describe the activation process of a single particle is proposed from an overall perspective. The intrinsic characteristic of the activation process is that a layer of active mass with better electric conductivity is formed starting from the surface and is finally dispersed in the whole particle.2. Partially oxidized Ni(OH)2 was prepared by oxidizing Ni(OH)2 with strong oxidant in an alkaline medium. The oxidation degree of Ni(OH)2 can be increased by raising reaction temperature and increasing reaction time. Elevated temperature can expedite proceeding of the oxidation reaction. The surface composition and morphology of Ni(OH)2 particles are changed by strong oxidant. Compared with the original Ni(OH)2, the content of cobalt and zinc on the surface layer of Ni(OH)2 particle containing 5% Ni(Ⅲ) increases. The tiny particles on the surface of Ni(OH)2 containing 10% Ni(Ⅲ) have loose contact with each other. The surface of Ni(OH)2 containing 20% Ni(Ⅲ) has been severely damaged by strong oxidant. Because Ni(Ⅲ) existed in the surface layer of Ni(OH)2 can improve the electric conductivity of the material, this has resulted in a faster activation of the Ni(OH)2 containing Ni(Ⅲ) than the original Ni(OH)2 in the initial activation process. This result has further confirmed the intrinsic nature of the activation of Ni(OH)2 that has been revealed in the chapter 2. Because the particle surface of Ni(OH)2 containing 10% Ni(Ⅲ) and 20% Ni(Ⅲ) has been excessively oxidized, the cycle performance of both these two materials become worse. The Ni(OH)2 containing 5% Ni(Ⅲ) has good comprehensive electrochemical performance.3. The electrochemical behavior of Co and CoO as additives in the positive electrode of Ni/MH power batteries in an alkaline medium during first charging at 50°C was analyzed. The structural and textural evolutions of Co and CoO during charging were studied, and an oxidation mechanism was proposed. Moreover, the effects of the two additives in the positive electrode on the performance of Ni/MH power batteries were investigated. The experimental results reveal that different oxidation products can be obtained according to the type of starting material. When the starting material is CoO, only Co3O4 is formed. A CoOOH phase is present together with a Co3O4 phase when Co is used as a starting material. The oxidation mechanism of Co and CoO reveals that two factors, the solubility of cobalt and kinetics of the reaction that consumes CoOOH, can significantly influence the amounts of Co3O4 and the remaining CoOOH. According to the electrochemical performance of the CoO-added and Co-added batteries and the cobalt oxidation behavior, the highly compact CoOOH phase, which works well in connecting the nickel foam and Ni(OH)? particles, enhances the high rate charge and discharge performance of the Ni/MH power battery. The Co3O4 phase, which works well in connecting Ni(OH)2 particles with each other, increases the utilization of Ni(OH)2, consequently improving the capacitive performance of the Ni/MH power battery. Therefore, both additives are necessary to create a power battery.4. CoO-added battery and Co-added battery which have stored for two years were chosen as research objects. The performance of the two batteries was studied by analyzing the physical properties and electrochemical performance of positive electrodes as well as the physical properties of electrolyte and separator. The electrochemical performance of the two positive electrodes still remains unchanged comparing with that of the two positive electrodes before storage. This result manifests that the cobalt conductive networks which fomied in the first charging at 50°C have good electric conductivity and stability after the long time storage. The Ni(OH)2 particles in the CoO-added positive electrode stack compactly, whereas the Ni(OH)2 particles in the Co-added positive electrode stack loosely. The content of cobalt in the surface of CoO-added positive electrode is larger than the content of cobalt in the surface of Co-added positive electrode. And the content of cobalt in the surface of Ni(OH)2 particles is still larger in the case of CoO-added positive electrode than that in the case of Co-added positive electrode. The concentration of Al、Ce、La、Mn elements which dissolved from the negative electrode is very low. This has little effect on the performance of the two batteries. Because the oxygen evolution has significant effect on the positive electrode of Co-added battery, this results in the penetration of large amount of Ni(OH)2 particles into the separator. However, in the case of CoO-added battery there is only a small amount of Ni(OH)2 particles that have inserted into the separator.