Destructive physical analysis results of Ni/H 2 cells cycled in low earth orbit regime (II)

Abstract

Six 48-Ah individual pressure vessel Ni/H 2 cells containing 26 and 31% KOH electrolytes have been on a low earth orbit cycle-life test. Three cells containing 31% KOH have failed after an average of 6400 cycles while the other three with 26% KOH have cycled for an average of 19 500 cycles. We have carried out post-cycle characterization tests and destructive physical analyses (DPA) of all cells. The DPA included visual inspections, measurements of electrode thickness, scanning electron microscopy, chemical analyses, and measurements of nickel electrode capacity in an electrolyte flooded cell. A gradual decrease of the usable cell capacity at high rate (1.4 C) was the failure mode of all cells tested. Decrease of the usable capacity was due to decrease in the utilization of the active material with cycling mainly due to build-up of low rate capacity (residual capacity) and undischargeable active material. Cycle life of one of the cells might have been shortened prematurely due to the breakage of the cell core. Many gradual changes occurred with cycling at a rate which is independent of the KOH concentration. Each of these individual changes are difficult to be attributed to be the direct cause of the utilization decrease. However, an involved interaction of the changes might be responsible for the decrease.