Study of the structure and the electrochemical properties of Ml(NiCoMgAl)5.1−xZnx hydrogen storage alloy

In this paper a new type of hydrogen storage alloy Ml(NiCoMgAl)5.1−xZnx (0⩽x⩽0.3) was obtained by vacuum inductive melting of La-rich mixed mischmetal elements with Ni and some other alloy elements such as Co, Mg, Al and Zn. The pressure–composition–temperature (P–C–T) tests show that the highest hydrogen concentration can reach 1.58 wt% for Zn-free (x=0) composition and then decreases from 1.44 wt% (x=0.2) to 1.19 wt% (x=0.3), respectively, with the increase of Zn in the alloys. The electrochemical tests show that the electrochemical capacity of alloys reaches a very high level, 380 mAh/g for x=0, and then decreases to 366 mAh/g (x=0.1), 345 mAh/g (x=0.2) and 271 mAh/g (x=0.3), respectively, with the increase of Zn in the alloys. The attenuation rate of capacity decreases from 16% (x=0) to 4% (x=0.3) after 100 charging/discharging cycles. Proper Zn content (0<x<0.2) influences the electrochemical capacity of alloys a little, but it improves the cycle stability and the discharge ability, especially the high rate discharge ability. Optic microscopy, SEM and EDX analyses indicate that the main part of alloys are Mg-free and Mg-contained LaNi5-based solid solutions. The latter is the key to the high electrochemical capacity. The addition of Zn makes the secondary phase change from LaNi2 to Co-rich phase in the alloys and also influences their quantity and shape.