On the cycle life improvement of amorphous MgNi-based alloy for Ni–MH batteries

Abstract

Amorphous MgNi prepared by mechanical alloying has a high initial discharge capacity ( ∼ 440 mAh/g) but a low cycle life due to its oxidation and pulverization during cycling. In the present work, a Mg–Ni–Ti–Al quaternary alloy with enhanced cycle life has been elaborated. Al addition creates a synergetic effect with Ti presumably to form a protective layer limiting Mg(OH) 2 formation. The Mg 0.9Ti 0.1NiAl 0.05 electrode retains 67% of its initial discharge capacity ( ∼ 400 mAh/g) after 15 cycles compared to 34% for MgNi. The charging conditions have also a great influence on the electrode cycle life as demonstrated by the existence of a charge input threshold under which minor capacity decay occurs. The charge input threshold is higher for Mg 0.9Ti 0.1NiAl 0.05 electrode than for MgNi electrode, which may reflect the improvement of the alloy resistance to pulverization. In addition, we pointed out that electrode made from large particles (¿150 μ m) has a better cycle life. We have then conceived a Mg 0.9Ti 0.1NiAl 0.05 electrode with an appropriate particle size having a capacity decay rate as low as that observed for a commercial AB 5 alloy (i.e. ∼ 0.2 % per cycle) under controlled charging conditions ( C input ≤ 300 mAh/g).