Effects of Ti substitution for Zr on the electrochemical characteristics and structure of AB2-type Laves-phase alloys as metal hydride anodes

The structural composition and electrochemical capacity of four AB(2) Laves-type intermetallic alloys with various Ti/Zr ratios (TixZr1-xLa0.03Ni1.2Mn0.7V0.12Fe0.12, x = 0.12, 0.15, 0.18, and 0.22) were investigated in this study. The alloys were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. These data revealed the coexistence of the main phase of the C15-type FCC Laves-type AB(2) compounds with a secondary La-Ni intermetallic that was present in minor amounts. Increasing the Ti substitution for Zr caused the gradual shrinkage of the unit cells of the C15 phase. The neutron powder diffraction studies demonstrated that in the trihydride (Ti, Zr, V)(Ni, Mn, Fe, V)(2)D-3.2, D atoms filled A(2)B(2) tetrahedra, whereas V atoms occupied not only conventional 16d sites but also partially replaced Zr/Ti at 8a sites. All studied alloys showed similar activation behaviors, wherein four cycles were required to realize the highest electrochemical capacity of the anodes. The Ti12/Zr88 alloy demonstrated excellent full discharge capacity that reached 466 mAh/g. The Ti22/Zr78 alloy electrode exhibited good cycling stability (retention rate of similar to 71%) after 500 cycles and a superior high-rate discharge capability (retention rate of similar to 71%) at a discharge current density of 400 mA/g. The cycling of the Ti22/Zr78 alloy electrode was studied by electrochemical impedance spectroscopy (EIS) to understand the reasons for the deterioration of cycling capacity, which was related with the pulverization of the alloys and increase in irreversible capacity. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

The study investigated the structural composition and electrochemical capacity of four AB(2) Laves-type intermetallic alloys with different Ti/Zr ratios, revealing the gradual shrinkage of the unit cells of the C15 phase with increased Ti substitution for Zr. All studied alloys showed similar activation behaviors, requiring four cycles to achieve the highest electrochemical capacity of the anodes.