Description of the capacity degradation mechanism in LaNi5-based alloy electrodes

The mechanism of the capacity degradation of LaNi5-based alloy electrodes was investigated with a special focus on the influence of the alloy and surface composition, as well as the unique structure obtained by gas atomisation. The electrochemical properties, especially the cycle life curve (i.e. the capacity as a function of the cycle number of LaNi4.5Al0.5, LaNi2.5Co2.4Al0.1, (La + Mm)Ni3.5Co0.7Al0.35Mn0.4Zr0.05, and MmNi4.3Al0.2Mn0.5 alloy electrodes), was analysed and modelled. The capacity degradation upon cycling is determined by the chemical state of the alloy elements and the solubility of their oxides. The cycle life curves for the alloy electrodes without Co exhibited a rapid activation (3-4 cycles to reach maximum capacity), as well as rapid degradation (130-180 cycles for 50% maximum discharge capacity). LaNi2.5Co2.4Al0.1 and (La + Mm)Ni3.5Co0.7Al0.35Mn0.4Zr0.05 alloy electrodes activated after 7-10 cycles and showed very stable discharge behaviour (more than 400 cycles). The Co-containing alloy electrodes primarily lose the cycle stability because of mechanical decrepitation, whereas the alloys without Co suffer from selective dissolution of the unstable elements in the potential window, which was shown by our model of alloy degradation and confirmed by means of SEM, WDX, and ICP-OES data.