Effect of Al incorporation on the degradation in discharge capacity and electrochemical kinetics of La–Mg–Ni-based alloys with A2B7-type super-stacking structure

Abstract

The microstructure and electrochemical properties, especially the degradation in discharge capacity and kinetics performance of La0.80Mg0.20Ni2.95Co0.70−xAlx (x=0, 0.05, 0.10, 0.15) alloys were studied. The alloys mainly contained (La,Mg)2Ni7 and LaNi5 phases. Al element tended to enter LaNi5 phase rather than (La,Mg)2Ni7 phase, leading to the increase in LaNi5 phase abundance and decrease in (La,Mg)2Ni7 phase abundance. Moreover, the cell volumes of both phases increased with increasing Al content. The alloys were seriously pulverized and oxidized during cycling, causing serious degradation in both discharge capacity and electrochemical kinetics. The addition of Al relieved the alloy pulverization by decreasing the cell volume expansion rate after hydrogenation. The alleviated pulverization together with the Al oxide layer on the alloy surface retarded the alloy corrosion/oxidation during charge/discharge cycles in alkaline electrolyte. Thus, the capacity cycling retention and the HRD1500 cycling retention at the 150th cycle were increased from 61.9% (x=0) to 81.3% (x=0.15) and from 88.3% (x=0) to 95.7% (x=0.15), respectively.