Effects of charge rate and cycling on the morphology of Cd and Cd(OH)2 in sintered plate electrodes

Abstract

Optical and scanning electron microscopy have been used to study the growth and redistribution of Cd and Cd(OH)2 in sintered plate electrodes as a function of charge rate and cycle number. As expected, the growth of both components was found to increase with increasing cycle number and decreasing charge rate. Because the deposits, particularly after extended cycling, always contained appreciable quantities of Cd metal in both the charged and discharged state, the sizes of Cd(OH)2 crystallites were difficult to quantify. High charge and discharge rates promoted greater aggregation and redistribution of active material towards the electrode edge. This resulted in a considerable decrease in the available pore volume per unit mass of active material and in extreme cases to localized pore blockage. The trapping of Cd metal by highly crystalline, unchargeable hexagonal platelets of (δ-Cd(OH)2 resulted in about 50% of the active material becoming obsolete after 100 cycles at high charge and discharge rates. At this stage only the finely divided Cd metal in the electrode interior continued to function. Low charge rates gave deposits of more uniform size and distribution but these contained a high percentage of large Cd particles which discharged less efficiently than those produced at the high charge rate.