Doctor of Philosophy
Department of Materials Engineering
Chen, J., High energy storage materials for rechargeable nickel-metal hydride batteries, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 1999. http://ro.uow.edu.au/theses/1502
The properties of nickel hydroxide electrodes, metal hydride electrodes and nickel-metal hydride (Ni-MH) batteries were studied. Spherical nickel hydroxide powders having a high tapping density of 2.15 to 2.23 g/cm3 and a low pore volume of about 0.04 ml/g were prepared by a spraying technique. The effects of preparation conditions on the characteristics of the powders were investigated. The utilization of Ni(OH)2 at the temperature of 25°C was improved by the coprecipitation of 3.5 wt.% Co2+. The charging efficiency of the electrode at 60°C was improved by the addition of 3.0 wt.% Ca2+ owing to a large difference between the potentials of oxygen evolution and oxidation (Ni2+ to Ni3+). The cycle life was greatly increased by adding 3.6 wt.% Zn2+ due to the decreased amount of y-NiOOH, which is formed mainly at the end of the charging process and then results in the electrode swelling.
The influence of individual elements Ce, Pr or Nd as substituents for La in LaNi3.8Co0.5Mn0.4Alo.3 was observed. The Ce addition was the most effective in improving the properties of the alloy such as high-rate dischargeability and overpotential. The electrode performance, especially the electrode activation, was improved greatly by ball-milling the as-cast Zr0.5Tio.5(V0.25Mn0.i5Ni0.6)2 alloy with Ni powder because of the formation of a large interface, an oxygen depleted and a nickel-rich surface layer, but the cycle life decreased after long-time ball-milling because of the oxidation of the increased specific surface area
Rail-milling the Mg-based alloy, in particular by the addition of Co or Ni powder, was an effective method of improving the characteristics of hydrogen absorption/desorption for the changed amorphous structure. The crystallographic and electrochemical properties the non-stoichiometric Mg2Nix(0.9≤x≤1.1) were found to be strongly dependent on the value of x. The capacity decay of Mg-based alloys was caused mainly by the oxidation of Mg in the alloy. The partial replacement of Mg by Ti or Ce could depress the formation of Mg(OH)2 because of the improved protection provided by impervious TiO2 or CeO2. The Ni-coating on the surface of amorphous Mg-based alloy powders was also very useful in improving the electrode cycle life. A discharge capacity of 780 mAh/was achieved for Ni-coated Mg2Ni0.8Mn0.2, and after 20 charge-discharge cycles, the electrode retained a capacity of 72
The hydrogen diffusion coefficient (D) in the alloy electrodes studied above was determined using a potentiostatic discharge method. The D value was in the range of 10-8 to 10-9 cm2/s with the highest value for rare-earth system alloy and the lowest value Mg-based alloy. The temperature dependence of the D value can be described by the Arrhenius expression D = D0 e(-Ea/RT) . The larger is the D value, the lower is the activation energy Ea.
The optimum conditions for preparing Ni-MH cells with high performance required the controlling of the capacity ratio between the negative and the positive electrodes (rN/p) and the physical dimensions of these electrodes. Ni-MH batteries were constructed successfully with a high capacity of 1350 mAh (AA size) and high energy densities of 69.7 Wh/kg and 211 Wh/L by combining 3.6 wt.% Zn2+ added spherical Ni(OH)2powders and 10 wt.% nickel ball-milled Zr0.5Tio.5(V0.25Mn0.i5Ni0.6)2 alloy.