Relationship between composition, volume expansion and cyclic stability of AB(5)-type metalhydride electrodes

Metal hydride alloys as electrode material for battery application contain up to 15 at.% cobalt. Alloys without cobalt show a much shorter cycle life compared to cobalt containing alloys. The mechanism of how cobalt influences the cycle life is still not well understood. The aim of this work is to investigate the influence of cobalt on the properties of the electrode. A series of alloys with different cobalt content and several other substituents for nickel (Fe, Cu,...) were prepared in two different ways. A set of samples was conventionally melted. A second set of samples was prepared by gas atomization. The volume expansion upon hydriding was analyzed by means of X-ray diffraction. Electrochemical measurements, e.g., discharge capacity as a function of cycle number, were performed. The volume expansion upon hydriding decreases with increasing cobalt content of the alloy. Cobalt substitution for nickel improves the cycle life of an electrode, especially at elevated temperatures (40 degrees C). However, alloys where cobalt is partially substituted by iron show an even better cyclic stability and rate capability.