Synthesis, Modification And Electrochemical Properties Of Nickel Hydroxide

The increasing concerns over air pollution and depletion of natural energy resources reserves have spurred renewed interest in electric vehicles (EV), where high power batteries are playing important role. MH/Ni batteries named as high specific energy green power sources are considered to be one of the most promising choices for EV and hybrid electric vehicle (HEV) applications. Furthermore, MH/Ni batteries are the first one which was used as power batteries in commercial HEVs. These batteries are usually designed with capacity limited by positive electrode for reasons of proper gas recombination reactions and battery safety. It follows that increasing the comprehensive performance of the nickel hydroxide electrode is essential for raising the performance of MH/Ni batteries. It’s the purpose of this thesis that increasing the high rate dischargeability and energy density of nickel hydroxide electrode by the synthesis of plate like nano-scale nickel hydroxide, nickel hydroxide/carbon composites and nanosheets ofα-Ni/Co hydroxides through various methods and modifying macro-spherical nickel hydroxide with cobalt oxyhydroxide via a new method.1. Owing to the special structural characteristics, nickel hydroxide with nanostructural multiphase was considered to have better electrochemical properties than that of spherical nickel hydroxide. However, as far as we know, few researches touch on the synthesis of a series of nano-scale nickel hydroxides and the systematical investigation of the structure/function relationships in nano-scale nickel hydroxides. For the purpose of understanding the structure/function relationships in nano-scale nickel hydroxides, a series of large-scale nickel hydroxide nanoplates with differences in size and crystallinity were synthesized in this work. Through cyclic voltammetry and rate-discharge electrochemistry tests combining the XRD、SEM and HR-TEM analysis of the structure of nano-scale nickel hydroxides, the conclusions was obtained as followings:①Nickel hydroxides with nanostructure have higher electrochemical reaction activity and are more easily activated than macro-spherical nickel hydroxide. However, not all nano-scale nickel hydroxides behave better electrochemical properties than macro-spherical Ni(OH)2.②Electrochemical activity and specific capacity of nano-scale nickel hydroxides could be improved by reducing the crystalline in the direction perpendicular to the axis of [100].③Not only the size of single nanometer nickel hydroxide but also the defects in crystals and un-perfect crystallization of single particles have influence on its electrochemical properties.2. Numerous researches have indicated that nanometer nickel hydroxides have better electrochemical reaction activity, higher charging efficiency and higher discharge potential than macro-spherical Ni(OH)2. However, the nature that nanometer Ni(OH)2 is prone to aggregate, which causes nanometer Ni(OH)2 to be difficult to mix with conductive materials effectively, blocks the behaving of the characteristics of nanometer Ni(OH)2. To overcome this problem, a new kind of composite was synthesized by loading the nanometer Ni(OH)2 uniformly onto the surface of nano-scale spherical carbon particles via sonochemical technique. In this way, not only the dispersion problem of nanometer Ni(OH)2 is resolved which guarantees the behaving of the characteristics of nanometer Ni(OH)2, but also the conductivity of nanometer Ni(OH)2 is improved intensively. In addition, the electrochemical tests show that this kind of material behaves excellent high rate dischargeability, and the utilization of the active material is raised.3. The number of theoretical exchangeable electrons per nickel atom inα-Ni(OH)2 reaches about 1.7 which is far larger than that ofβ-Ni(OH)2 (1). Therefore, synthesing stabilizedα-Ni(OH)2 is an effective way to increase the specific capacity of Ni(OH)2 cathode. Here, as far as we known, the partial cation exchange method was firstly and successfully adopted for the synthesis of a series of 2D Co-Ni bimetallic hydroxides. The cyclic voltammetry tests show that the synthesizedαphase Co-Ni bimetallic hydroxides have lower resistance and better reversibility than that ofβphase Co-Ni bimetallic hydroxides prepared by co-deposition method. In addition, the charge-discharge tests display that the synthesized a phase Co-Ni bimetallic hydroxides have higher specific capacity than the theoretical specific capacity ofβ-Ni(OH)2 and demonstrate outstanding cycling durability even after 500 cycles. Furthermore, the cation exchange process is reversible. This simple method gives a novel way to construct nanostructural Co-Ni hydroxides with different compositions which behave excellent electrochemical properties different from that of nanostructural Co-Ni hydroxides prepared by traditional method.4. Modifying the surface of macro-spherical Ni(OH)2 powder by forming a conductive network with metal cobalt, cobalt oxides or cobalt hydroxides can enhance the conductivity of macro-spherical Ni(OH)2 powder intensely. The coating methods regularly used only include plating and precipitation. Here, we successfully introduced cation-exchange method as a simple and versatile way to modify the surface layer of macro-spherical Ni(OH)2 particles to form the core-shell Ni(OH)2@CoOOH. Because the cation-exchange process takes place in situ, the shape and morphology of the precursor can ben preserved well in the resulting core-shell Ni(OH)2@CoOOH. The prepared core-shell structured Ni(OH)2@CoOOH exhibits unusual firm combination between shell CoOOH and the core of Ni(OH)2, which are considered to be linked by a traditional phase of a-Ni(OH)2. This type of structure not only enhances the firm combination between shell CoOOH and the core of Ni(OH)2 leading outstanding electrochemical cycling durability of Ni(OH)2@CoOOH, but also increases the specific capacity which is higher than the theoretical specific capacity ofβ-Ni(OH)2.The conductivity of starting Ni(OH)2 is improved highly by the modification of CoOOH in the surface of Ni(OH)2. The electrochemical tests show that the as-synthesized core-shell Ni(OH)2@CoOOH can be activated easily in a few cycles using currents of 0.5-2C when used as positive electrode material in MH-Ni batteries.In addition, the reaction mechanism concerned in the synthesis process was discussed.Given to the simple synthesis process of Ni(OH)2@CoOOH and the advantages of it, such as the high conductivity, easy to be activated, larger specific capacity, outstanding cycling durability, the produce of Ni(OH)2@CoOOH is magnified to tens of grams for exploring the feasibility in industrial production. When obtained Ni(OH)2@CoOOH sample was chosen as cathode material in a 6Ah prismatic power battery, discharge efficiency of 74.19% at the discharge current rate of 30C was achieved as compared to the discharge efficiency (1.86%) of a 6Ah prismatic power battery with normal commercial spherical Ni(OH)2 as positive material and with adding of CoO as conductive additive in the paste for producing CoOOH.