Electrochemical properties of Nb-substituted Zr-Ti-Ni hydrogen storage alloy negative electrodes for nickel-metal hydride batteries

Abstract

Crystal structure, pressure-composition isotherms and electrochemical properties of the Zr0.6−x Ti0.4 Nbx Ni (x = 0.01, 0.02, and 0.05) alloys were investigated. Their X-ray diffraction profiles demonstrated that all the Zr0.6−x Ti0.4 Nbx Ni alloys consisted of the primary phase with the B33-type orthorhombic structure and the secondary phase with the B2-type Ti0.6 Zr0.4 Ni cubic structure. Rietveld refinement demonstrated that the atomic fraction of the secondary phase increased with the Nb content. The Zr0.6−x Ti0.4 Nbx Ni alloys were lower in hydrogen storage capacity than the Nb-free Zr0.6 Ti0.4 Ni alloy due to an increase in the abundance of the secondary phase. In the charge-discharge tests with the Zr0.6−x Ti0.4 Nbx Ni alloy negative electrodes, all the initial discharge curves had two potential plateaus due to the electrochemical hydrogen desorption of trihydride to monohydride and monohydride to alloy of the primary phase. The total discharge capacities at 333 and 303 K for the Zr0.58 Ti0.4 Nb0.02 Ni alloy negative electrode were 384 and 335 mAh g−1, respectively, which were higher than those of the other Zr0.6−x Ti0.4 Nbx Ni and Zr0.6 Ti0.4 Ni alloy negative electrodes.