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Abstract:Crystal structure, pressure-composition isotherms and electrochemical properties of the Zr 0.6−x Ti 0.4 Nb x Ni (x = 0.01, 0.02, and 0.05) alloys were investigated. Their X-ray diffraction profiles demonstrated that all the Zr 0.6−x Ti 0.4 Nb x Ni alloys consisted of the primary phase with the B33-type orthorhombic structure and the secondary phase with the B2-type Ti 0.6 Zr 0.4 Ni cubic structure. Rietveld refinement demonstrated that the atomic fraction of the secondary phase increased with the Nb content. The Zr 0.6−x Ti 0.4 Nb x Ni alloys were lower in hydrogen storage capacity than the Nb-free Zr 0.6 Ti 0.4 Ni alloy due to an increase in the abundance of the secondary phase. In the charge-discharge tests with the Zr 0.6−x Ti 0.4 Nb x 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 Zr 0.58 Ti 0.4 Nb 0.02 Ni alloy negative electrode were 384 and 335 mAh g −1 , respectively, which were higher than those of the other Zr 0.6−x Ti 0.4 Nb x Ni and Zr 0.6 Ti 0.4 Ni alloy negative electrodes.

AB-type ZrTiNbNi alloys are good candidates as the negative electrode for Ni-metal hydride (MH) batteries because they have high discharge capacity around 330 mAh g ¹1 at 303 K. The Zr 0.49 Ti 0.5 Nb 0.01 Ni alloy consists of two phases, the primary-phase with B33-type orthorhombic structure and the secondary-phase with B2-type cubic structure. To compare electrochemical properties of each phase with the mother alloy, we synthesized the primary-phase and secondary phase alloys with compositions of Zr 0.54 Ti 0.47 Nb 0.01 Ni 0.98 and Zr 0.47 Ti 0.52 Nb 0.01 Ni, respectively. The discharge capacity was examined at 25 mA g ¹1 and 303 K, showing that the primary-phase alloy has the highest value of 362 mAh g ¹1 than the mother alloy (335 mAh g ¹1 ) and the secondary-phase alloy (253 mAh g ¹1 ). For cycle performance, all alloys were excellent (½95%) at 100 mA g ¹1 and 303 K. For high-rate dischargeability, the secondary-phase alloy was the best, probably because the stability of hydride for the secondary-phase alloy was lower than that for the mother and the primary-phase alloy.