(400f) Computational Study of Zr2Ni7 and Zr7Ni10 Defect Models for Nickel-Metal Hydride Batteries

Authors: 
Wong, D. F., Wayne State University
Young, K., BASF Battery Materials-Ovonic
Ng, K. Y. S., Wayne State University

Zr-Ni-based alloys as nickel-metal hydride battery anode materials offer low-cost, flexible and tunable battery performance.  The defect models of Zr2Ni7 and Zr7Ni10 are discussed here. High nickel-content alloys such as Zr2Ni7 are able to yield metallic Ni clusters in the surface oxide layer that enable high-rate discharge capability.  Zr7Ni10 is an important secondary phase found in multi-phased AB2 Laves-phase-based metal hydride alloys, and the synergetic effect between the Zr-Ni and the Laves phases allows access to the high hydrogen storage of the Zr-Ni phases despite the lower absorption/desorption kinetics.  As binary intermetallic compounds, Zr2Ni7 is known as a “line compound”, while Zr7Ni10 shows solubility with Zr-rich stoichiometries. The ability to tune the ratio between hydride formers such as Zr and hydride modifiers such as Ni while maintaining the structure of the alloys is an important feature for designing battery materials for a desired application, and can strongly affect battery performance properties.  Defect models in Zr2Ni7 and Zr7Ni10 computed using a combination of density functional theory and statistical mechanics offer a starting point for understanding the nature of the Zr-Ni solubility limits.  Vacancy and anti-site defect formation energies are calculated and reported for Zr-rich, Ni-rich, and stoichiometric compounds of Zr2Ni7 and Zr7Ni10, and the implications of the defect models on nickel-metal hydride anode design and performance are discussed.