The Effect of Rapid Solidification on Ti-Zr Alloys for Application in Ni-MH Battery Anodes

Metal hydrides find applications in the storage of hydrogen and also as anode materials for the nickel-metal hydride batteries(Ni-MH). The properties of these materials can be tuned by selecting appropriate elemental composition and by controlling the microstructure. The overall goal in the development of advanced metal hydrides is to obtain materials with high hydrogen storage capacity, fast and completely reversible hydrogen absorption and desorption, and tunable phase stability (with an adsorption/desorption pressure around 1 bar at ambient conditions for the Ni-MH batteries), as well as good stability in the working environment (6-9 M KOH electrolyte in Ni-MH batteries). The goal of the present work was to study the effect of rapid solidification(RS) as a nanostructuring technology applied to obtain novel and improved anode materials for the Ni-MH battery. The focus of the work was on the TiZr-based AB2 Laves phase alloys, which contain significantly less rare earth metals (RE) compared to the commercialized alloys used today, which has the composition (RE(Ni,Co)5). The alloys were melt spun to obtain a grain refinement, and consequently improve the discharge capacity and high rate discharge performance of the alloys. Based on earlier experiments performed at IFE, two basic alloys with compositions (TixZr1-xLay)(Ni1.2Mn0.7V0.12Fe0.12) were chosen. Particular compositions were selected as x=0.15, and y=0.03 (Alloy 1), and x=0.2 and y=0.01 (Alloy 2). Both alloys contain minor amounts of La to help achieving fast activation of the material. The alloys were melt spun, with wheel surface speeds varying from 3.1-62.8 m/s. As the melt spinning process proved to modify the alloy composition, resulting in depletion of Mn which is easily vaporized, the effect of excess Mn addition to mitigate this problem was also studied for alloy 1. The effect of RS on the microstructure and phase compositions was characterized by using SEM, EDS, AES and XRD. Furthermore, the electrochemical properties were studied through electrochemical cycling performed at different current densities as well as by electrochemical impedance spectroscopy and PCT. The SEM studies showed a successful refinement of the microstructure by RS. The grain size gradually decreased from the original 2 μm in the as cast alloys to around 250 nm for the alloys melt spun with the highest cooling rate. The morphology of alloy 1 showed an isotropic reduction in grain size with RS, with a related formation of