Improvement in cycle life performance of high capacity nickel-metal hydride battery

Hydrogen-absorbing alloys based on MmNi(5) (Mm, misch metal) have become widely employed in batteries for general use. Problems such as cycle life performance deterioration caused by corrosion of the alloys have been improved by altering alloy compositions and by surface treatments, but there remains room for further improvement. In this study, additives were investigated in order to improve the resistance to corrosion of an MmNi(5-x)M(x) (M = Co, Mn, Al) alloy surface. It was found that by mixing Y2O3 powder with the alloy powder in making a metal hydride (MPI) negative electrode, the elution of the alloy's constituent elements and the formation of hydroxides of rare earth metals were suppressed. When this MH electrode with additive was applied to a sealed Ni-MH cell, the increase in discharge reserve and the decrease in charge reserve caused by charge and discharge cycles were suppressed, giving a roughly doubled cycle lifetime of the cell. On the other hand, the charge and discharge characteristics of the cell were slightly deteriorated. A small amount of Y2O3 seems to dissolve in the alkaline electrolyte of the cell, be adsorbed on the alloy surface and form a protective film. This behaviour is considered to be the mechanism that improves the corrosion resistance of the alloy.