RELATIONSHIP BETWEEN COMPOSITION, VOLUME EXPANSION AND CYCLIC STABILITY OF AB(5)-TYPE METALHYDRIDE ELECTRODES

Metal hydride alloys as electrode material for battery application con tain up to 15 at.% cobalt. Alloys without cobalt show a much shorter c ycle life compared to cobalt containing alloys. The mechanism of how c obalt influences the cycle life is still not well understood. The aim of this work is to investigate the influence of cobalt on the properti es of the electrode. A series of alloys with different cobalt content and several other substituents for nickel (Fe, Cu,...) were prepared i n two different ways. A set of samples was conventionally melted. A se cond set of samples was prepared by gas atomization. The volume expans ion upon hydriding was analyzed by means of X-ray diffraction. Electro chemical 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 partiall y substituted by iron show an even better cyclic stability and rate ca pability.