Structural- and hydriding-properties of the Zr(V 0.25Ni 0.75) α (1≤ α≤4) alloy system

Abstract

Previous investigations on the Zr(V xNi 1− x ) 2 alloy system showed that Zr(V 0.25Ni 0.75) 2 has a very high reversible capacity (≈365 mA h g −1, discharge current i dis=100 mA g −1). This alloy does not crystallize in a perfect single phase, a second phase (Zr 7Ni 10) was observed. In order to understand the influence of the second phase and to optimize the alloy composition for high capacity, the influence of the stoichiometry was investigated. A series of alloys Zr(V 0.25Ni 0.75) α (1≤ α≤4) were prepared by r.f. levitation melting. The main phase in all the tested alloys was the cubic C15-Laves phase. A second phase was observed in the samples with α≤2.5, whereas the alloys with α>2.5 are single phase. The lattice parameter shows a significant step in the range of 2≤ α≤2.5. The electrochemically measured capacity is highest for α=2 ( C=1.0 H/M, 394 mA h g −1, i dis=5 mA g −1). The alloy system Zr(V 0.25Ni 0.75) α (1≤ α≤4) is very adaptable to changes in the stoichiometry. However, the distribution of A and B elements on the A and B sites is crucial for high capacity. A part of the vanadium atoms move from B sites to A sites in the overstoichiometric systems Zr(V 0.25Ni 0.75) α (2< α). The overstoichiometric (AB 3) alloy ZrV 1.5Ni 1.5 crystallizes in the cubic C15-Laves phase and shows a very high capacity of 800 mA h g −1 ( i dis=2 mA g −1). However, only 400 mA h g −1 can be discharged at a 20 mA g −1 discharge current.