Enhanced cycling stability and high rate dischargeability of (La,Mg)2Ni7-type hydrogen storage alloys with (La,Mg)5Ni19 minor phase

The A(2)B(7)-type lanthanum (La)-magnesium (Mg)-nickel (Ni)-based alloy with single (La,Mg)(2)Ni-7 phase and different amounts of (La,Mg)(5)Ni-19 minor phase was obtained by step-wise sintering. The impact of (La,Mg)(5)Ni-19 phase on the alloy's microstructure and electrochemical performance was subsequently studied. It was found that the average subunit volume in (La,Mg)(5)Ni-19 phase is smaller than that in (La,Mg)(2)Ni-7 phase, resulting in increases of strains inside the alloys and decreases of cell volumes. During battery charge/discharge, the (La,Mg)(5)Ni-19 phase network scattered in the alloys relieves internal stress, alleviates pulverization and oxidation of the alloys, stabilizes the stacking structures against amorphization, and finally improves the cycling stability of the alloys. Furthermore, (La,Mg)(5)Ni-19 phase with higher Ni content desorbs hydrogen ahead of (La,Mg)(2)Ni-7 phase. The reduced hydrogen pressure in (La,Mg)(5)Ni-19 phase can subsequently lead to the fast discharge of (La,Mg)(2)Ni-7 phase, thus making a remarkable improvement in high rate dischargeability at 1500 mA g(-1) from 46.2% to 58.9% with increasing (La,Mg)(5)Ni-19 phase abundance from 0 to 37.4 wt.%. Therefore, it is believed that A(2)B(7)-type La -Mg-Ni-based alloys with A(5)B(19)-type minor phase are promising prototypes for high-power and long-lifetime nickel/metal hydride battery electrode materials. (C) 2015 Elsevier B.V. All rights reserved.