| The Ni/MH batteries are extensively used because of their high reliability, environment-friendly characteristic and high performance-cost ratio. Nickel hydroxide (Ni(OH)2) is used as the main active material in the positive electrode for Ni/MH batteries, and the improvement of it’s performance becomes one of the valid approaches to enhance Ni/MH battery’s whole performance.Ni(OH)2 has two polymorphs, namely α-Ni(OH)2 and P-Ni(OH)2. The theoretical specific capacity of α-Ni(OH)2 is 482mAh/g, which is 193mAh/g larger than that of β-Ni(OH)2 (289mAh/g). However, the α-Ni(OH)2 is unstable in strong alkaline solution, making it have not been put into use. In this paper, for a purpose of improving the stability of α-Ni(OH)2, we used partial substitution of nickel ion with other metal additives (singly doped and codoped) in the nickel hydroxide lattice to synthesize stabilized nanometer α-Ni(OH)2 or α/β mix structure Ni(OH)2. The preparation conditions are optimised and the mechanism is also discussed. The main endeavor and contributions of the dissertation are as follows:First, nanometer Cu-stabilized nickel hydroxide samples were synthesized by means of ultrasonic-assisted precipitation. X-ray diffraction (XRD), transmission electron microscopy (TEM) and laser particle size analyzer (PSD) were used to characterize the crystal structure, morphology and particle diameter of the prepared samples. The cyclic voltammetry (CV) was employed to characterize the effect of Cu-doping amount on the electrochemical performance of the samples. The charge-discharge properties and cycle performance of nanometer Ni(OH)2 composite electrode (made by mixing 8wt.% nanometer Ni(OH)2 with commercial micro-size (3 spherical nickel hydroxide) were measured by constant current charging/discharging test, using simulated battery system.The results proved that, the prepared samples were α-Ni(OH)2. The X-ray patterns were anisotropic broadening and the lattice distortion is also anisotropic. The particles were columnar or needle-like, and the average particle size is about 96～110 nm. With increase in Cu content, the average particle sizes decrease firstly and then increase, and the primary particles changed from columnar to needle-like. Complex electrodes were prepared by mixing 8wt.% nanometer samples with commercial micro-size spherical nickel hydroxide. The specific capacity increases initially and then decreases with increase in Cu content, the electrode containing 0.9wt.% Cu has the maximum discharge capacity of 310mAh/g at 0.2C. And it has a lower charging voltage, higher discharge voltage plateau, better cycle performance and larger proton diffusion coefficient than the other electrodes.Second, nanometer Cu/Al codoped and Cu singly doped Ni(OH)2 have been synthesized by the same method, the only difference is the dosage of buffer agent (sodium carbonate). The results show that the nanometer samples are α-phase structure with narrow particle size distribution, and the average particle size are about 95.8nm and 103.9nm. The tap density of Cu/Al codoped nano-Ni(OH)2 is higher than that of Cu singly doped nano-Ni(OH)2. The electrochemical performance test of complex electrodes indicate that, the electrochemical performance of Cu/Al codoped nanometer α-Ni(OH)2 is better than that of Cu singly doped nano-Ni(OH)2, the former’s discharge capacity reaches 330 mAh/g at 0.2C, and it exhibits higher electrochemical reaction reversibility, better cyclic stability and larger proton diffusion coefficient.Third, nano-Ni(OH)2 doped with cobalt was prepared by ultrasonic-assisted precipitation. The effects of cobalt doping amount, the concentration of buffer (sodium carbonate) and Ni2+ on the crystal structure, particle size and electrochemical performance of Ni(OH)2 were characterized. The results indicate that different Co-containing Ni(OH)2 were co-existence with α and β phase. As the doping ratio increases, the proportion of a-Ni(OH)2 increased initially and then decreased. The increase of buffer amount is beneficial to change the crystal structure, and form a-Ni(OH)2, a proper doping amount can show better electrochemical performance. A higher concentration of reactants is conducive to the formation of a-Ni(OH)2. The electrochemical tests of composite electrodes indicate that, the discharge capacities of electrodes increased initially and then decreased with the increase of buffer amount. When the concentration of Ni2+ is 0.4mol/L and Na2CO3 is 0.5g, the electrode (the content of Co is 1.02 wt.%) has the highest charge efficiency and the largest discharge capacity. The discharge capacity reaches 347mAh/g at 0.5C rate, which is 30% larger than that of spherical nickel electrode. We discussed the mechanism why composite electrode can remarkably increase discharge capacity, and put forward a point of view that the nanometer a-Ni(OH)2 act as activation center, playing a part of catalysis. |
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