Abstract
The structural composition and electrochemical capacity of four AB2 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 AB2 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)2D3.2, D atoms filled A2B2 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 ~71%) after 500 cycles and a superior high-rate discharge capability (retention rate of ~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.
Highlights
The Ni/MH batteries have for a long time been the most extensively used energy/power sources for hybrid electrical vehicles [1,2]
Ti/Zr ratios (TixZr1−xLa0.03Ni1.2Mn0.7V0.12Fe0.12, x = 0.12, 0.15, 0.18, and 0.22) de signated as Ti12/Zr88, Ti15/Zr85, Ti18/Zr82, and Ti22/Zr78 were synthesized by means of arc melting with enough constituent elements
Considering that Mn is consumed at high temperatures because of evaporation, an excess of 4% Mn was charged into the initial mixtures
Summary
The Ni/MH (nickel hydride) batteries have for a long time been the most extensively used energy/power sources for hybrid electrical vehicles [1,2]. Compared with C14-type alloys, the C15 structure offered a better HRD performance because of its enhanced hydrogen bulk diffusion rates, improved specific power, and a better performance at low temperature with the disadvantage of an inferior cycle life [18]. Zr–Ni and Ti–Ni phases play important roles in improving battery performances, including activation, HRD, and cycle life [19,20,21]. Considering that the atomic weight of Zr is considerably higher than that of Ti, the discharging capacity of the alloys was expected to be improved through the substitution of Zr by Ti. the Zr substitution by Ti led to a decreasing stability of the alloy’s hydride [16]. The impact of Ti/Zr ratios in the annealed Zrbased C15-type AB2 alloys was systematically studied to uncover a relationship between the phase- structural and chemical composi tions and the electrochemical performances
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