| In this paper, firstly, it aimed at and started from the long-term debates relative to the composition and structure of calcium zincate synthesized in advance which has been used in Zinc negative materials for Zinc-Nickel batteries, it successfully prepared one-up regular and big particles of tetragonal and hexagonal calcium zincate single-crystal, which provides the study on the crystal calcium zincate with the directly substance supporting and helps to analyze the constitution and structure of it. Secondly, this thesis briefly generalized the development of the secondary Zinc-Nickel batteries, and analyzed the problems which has been restricting the marketization and commercialization of the secondary Zinc-Nickel batteries so far and solutions about it. In order to improve the problems of shape change and dendrites formation which they significantly affect the cycling life of secondary zinc anodic electrode, it combined the nanocrystallization of the additives of the zinc negative electrode and the surface modification technique of the electrode materials and synthesized the materials of nanosized-MgSn(OH)6 surface modified zinc oxide and MgSn(OH)6 surface modified calcium zincate, and it systematically studies their electrochemical properties during the performance of the Zinc-Nickel batteries. The main conclusions of this paper are as follows:1. In this paper, it used the materials of CaCO3 and ZnO and successfully prepared the big particles of calcium zincate single-crystal with precipitation transformation method by isothermal in the reasonable concentration of KOH as base solution for the first time. Under 27℃constant temperature, tetragonal calcium zincate single-crystal were made in KOH base solution whose concentration were range from 7.5wt.% to 15wt.%, which they have regular crystal shape and surface smooth and the biggest one has the side length of about 2 mm and its longest diagonal exceeds 3 mm and its thickness is 0.5 mm or so. Under 40℃constant temperature, hexagonal calcium zincate single-crystal were synthesized in KOH base solution whose concentration were range from 22.5wt.% to 25wt.%, which they have regular crystal shape and smooth surface and the biggest one has the side length of about 1 mm and its thickness is 0.2 mm or so. This experimental evidence sufficiently indicates the fact that the Ksp of calcium zincate is less than the Ksp of calcium carbonate, which this is the thermodynamical criterion that CaCO3 can be successfully used to prepare calcium zincate single-crystal as calcium resource. Trial discussion about the factors of crystal nucleation and growth was made in the paper and maintains that the concentration of reactant ion and the temperature of reaction system are potentially the two most key factors in ones which influence the crystal nucleation and growth; this is because the environmental temperature and the concentration of reactant ion together influence the every crystal plane of predominant orientation of the growth rates in the formation of crystal nucleation and growth of crystal cell to a large extent, which conduce to changes of crystal appearance and to make tetragonal and hexagonal calcium zincate single-crystal which they show two shapes in appearance and they were made from CaCO3 and ZnO with precipitation transformation method by isothermal (27℃and 40℃) in the proper concentration of KOH as base solution Therefore, the concentration of reactant ion and the temperature of reaction system may be in need of controlling two key factors. Based on the data analysis and discussion of the tetragonal and hexagonal calcium zincate single-crystal characterized by optical microscope, XRD, TG-DSC, infrared spectroscopy and X-ray single crystal diffraction, the conclusions are drawn as follows:(1)The crystal structure of tetragonal and hexagonal calcium zincate single-crystal is identical and the crystal arrangement means of various atoms in space, atomic arrangement of law, means and interaction of the bonding between various atoms and the relevant crystal parameters are also exactly the same; the molecular expression of tetragonal and hexagonal calcium zincate Single-crystal is the same form of Ca[Zn(OH)3]2·2H2O.(2) The paper also referred to the necessary connection of structure and properties of material, and then carried out the tentative deep exploration on the structure and properties of calcium zincate single-crystal, and drew the following conclusions:Perfect crystal shape of either regular tetragonal or hexagonal calcium zincate (regardless of crystal size) should always have the same chemical expression of Ca[Zn(OH)3]2·2H2O as its form; and incomplete crystal shape of calcium zincate prepared in different conditions, their chemical expression may be the form of Ca[Zn(OH)3]2*nH2O(n=1～2). Perfect crystal of calcium zincate were prepared in alkaline solution, whose form of ion reaction is as follows:Ca2 ++2ZnCOH)3-+2H2O=Ca[Zn(OH)3)]·2H2O; Moreover, with the changes of concentration of alkali solution, reactant ions and the change in ambient temperature conditions, crystal nucleation and growth process of the formation of chemical bonds and crystal plane orientation and the growth rate advantage will be significant impacted, at the same time the number of crystal water of crystallization will also be affected, manifested as Ca[Zn(OH)3]2·nH2O(n=1-2). Thus, environmental temperature, the concentration of reactant ions, the number of crystal water of crystallization and other conditions will have a decisive cooperative effect on the integrity of calcium zincate crystal growth and crystal morphology of perfection; therefore, the samples of calcium zincate synthesized indifferent conditions have the different appearance of morphology and particle size and their physical and chemical properties of the individual differences will be shown.(3) This paper also started from the structure of calcium zincate single-crystal and took up reasonable explanations about its property of thermal performance; and the relations between the crystalline characteristics and performance of the electrochemical reaction of calcium zincate as the application of zinc anode material in nickel-zinc batteries were also discussed.2.This paper successfully synthesized the materials of nanosized-MgSn(OH)6 surface modified zinc oxide and MgSn(OH)6 surface modified calcium zincate with direct precipitation method, and systematically studied the electrochemical properties and the performance of the two surface modified zinc anode active materials and the following conclusions are drawn:(1) With the utilization of nanosized-MgSn(OH)6 surface modified ZnO as zinc anode active materials, the zinc electrode shows a smaller capacity fading rate and a better cycling stability; zinc electrode shows lower charge voltage, higher discharge voltage and higher midpoint discharge voltage than the pure ZnO electrode. The polarization of modified zinc oxide electrode decreases, their efficiency of charge current become high and their charge and discharge reversibility get stable, and the utilization of the zinc anodic active material was significantly improved and increased; and the corrosion resistance of zinc electrode in alkaline electrolyte can be significantly enhanced and the stability of zinc electrode in the alkali electrolyte can be also significantly enhanced; the cyclic reversibility of zinc electrode is increased, and there is a process of repeated activation in circulating zinc electrode during charge and discharge; after repeated activation of the zinc electrode, its charge transfer resistance decreased, the electrochemical activity of electrode materials can be improved; The higher the content of MgSn(OH)6 modification on the surface of ZnO is, then the higher the range of the cycle stability of the zinc electrode and the utilization of the zinc anodic active materials will be increased; when the modification content of MgSn(OH)6 reached 13wt.% and above, the decay rate of electrode capacity was very small, showing good cycle stability. The reason is, firstly, mainly because of MgSn(OH)6 modified on the surface of ZnO, which the MgSn(OH)6 on the surface of ZnO microencapsulated ZnO particles with the protective effect of zinc oxide materials and can effectively prevent ZnO from directly contacting with potassium hydroxide electrolyte, and reduce the dissolution of ZnO in the electrolyte, and the modifier can lock the electrode active materials with the role of zinc oxide, and zinc oxide is better to keep on the zinc electrode. Secondly, the main reason is that some MgSn(OH)6 modified on the surface of ZnO was reduced to metallic tin in the charge-discharge process; and the tin simple substance has always been wrapped the surface of zinc oxide particles, which the tin not only can enhance the conductivity of zinc electrode, and increase current efficiency, promote the uniform electrodeposition of zinc on the zinc electrode, the tin but also has a high overpotential of hydrogen evolution, and can inhibit the formation of hydrogen and increase the stability of the zinc electrode, so that the deformation of zinc electrode and the growth of zinc dendrite were effectively inhibited and he cycling stability of zinc electrode can be effectively maintained and the capacity of zinc electrode can play well. Therefore, nanosized-MgSn(OH)6 surface modified zinc oxide is a feasibility of a new means to effectively improve the electrochemical stability of the active materials of zinc anodic electrode. (2) The paper also roughly studied the performance of the samples of MgSn(OH)6-modified ZnO after calcination (referred to as calcined zinc oxide). The results indicate that nanosized-MgSn(OH)6 modified on the surface of zinc oxide generated MgSnO3 after calcination at 450℃which coated the surface of the particles of zinc oxide. The calcined zinc oxide showed better electrochemical performance than that of the pure zinc oxide and modified zinc oxide. This is because the MgSnO3 generated from nanosized-MgSn(OH)6 modified on the surface of zinc oxide after calcination maybe has a porous nanostructure in the material. And the porous channel of MgSnO3 may be more beneficial to be infiltrated by electrolyte and to maintain the electrolyte. The performance of MgSnO3 may be better than that of MgSn(OH)6; Therefore, MgSn(OH)6-modified ZnO after calcination can further improve and enhance the electrochemical cycle stability of ZnO.(3)After the surface of calcium zincate modified by MgSn(OH)6, the cycle stability and the utilization of the electrode of calcium zincate has been improved to some extent. And the performance of zinc electrode showed a lower charge voltage and higher discharge voltage, charge current efficiency of zinc electrode increased, and the reversible stability of charge-discharge partly improved. Moreover, the corrosion resistance of modified calcium zincate in the electrolyte significantly enhanced; and the stability of modified calcium zincate electrode in alkaline electrolyte significantly enhanced. Some MgSn(OH)6 modified on the surface of calcium zincate was reduced to metallic tin in the charge-discharge process; and the tin simple substance has always been wrapped the surface of calcium zincate particles, which the tin can enhance the conductivity of zinc electrode, and decrease the electrode polarization, promote uniformity of zinc deposition on the zinc electrode. The modifier increased the utilizaion and the electrochemical activity of the zinc active electrode material. That the content of MgSn(OH)6 modified on calcium zincate reached the amount of 9wt.% is appropriate and the electrochemical performance of the zinc electrode played best with the content of MgSn(OH)6 modifid on the surface of calcium zincate particles. |
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