Substitution effect of elements in Zr-based alloys with Laves phase for nickel-hydride battery

By examining the electrochemical and thermodynamic properties of Zr(1-x)A'(x)(VyNizMnuB'(v))(2+alpha) (0 less than or equal to x less than or equal to 0.1; 0.10 less than or equal to y less than or equal to 0.15; 0.41 less than or equal to z less than or equal to 0.62; 0.28 less than or equal to u less than or equal to 0.39; 0 less than or equal to v less than or equal to 0.05; 0 less than or equal to alpha less than or equal to 0.02; y + z + u + v = 1; A' = Ti, Nb; B' = Co, Cr, Cu, Fe) alloy systems with C15-type Laves phase structure, the results described below were obtained. The alloy electrodes were completely activated even by one or two charge-discharge cycles at a current density of 17 mA g(-1). Some of the alloys had discharge capacities larger than 400 mA h g(-1) at 17 mA g(-1). They also had capacities larger than 380 mA h g(-1) even at 220 mA g(-1). Moreover, the high rate capability for discharge increased as the absolute value of the partial molar enthalpy Delta H-o for hydride formation decreased. The Delta H-o values of such alloys with excellent electrochemical properties were found to range from -36 to -42 kJ mol H-2(-1). On the other hand, the substitution effect of the elements in the alloys was qualitatively and quantitatively evaluated as a function of the atomic radii and electronegativities of the constituent elements. Delta H-o, which shows the stability of the absorbed hydrogen, had a good correlation individually with the average electronegativity chi(av) and the average atomic radius r(av). The correlation was further improved when these two factors were considered together. The cell volume of C15 type had a correlation with Delta H-o and a good correlation with both the average atomic radius and the difference Delta(chi B-A) in electronegativity between the A and B atoms. From the above description it can be concluded that the ranges of r(av) and chi(av) of the alloys with excellent capacity and high rate efficiency are 1.516 less than or equal to r(av) less than or equal to 1.525 and 1.621 less than or equal to chi(av) less than or equal to 1.640 respectively.