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 o r equal to 0.1; 0.10 less than or equal to y less than or equal to 0.1 5; 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 descr ibed below were obtained. The alloy electrodes were completely activat ed 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 4 00 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 f or discharge increased as the absolute value of the partial molar enth alpy Delta H-o for hydride formation decreased. The Delta H-o values o f such alloys with excellent electrochemical properties were found to range from -36 to -42 kJ mol H-2(-1). On the other hand, the substitut ion effect of the elements in the alloys was qualitatively and quantit atively evaluated as a function of the atomic radii and electronegativ ities of the constituent elements. Delta H-o, which shows the stabilit y of the absorbed hydrogen, had a good correlation individually with t he average electronegativity chi(av) and the average atomic radius r(a v). The correlation was further improved when these two factors were c onsidered 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 a nd B atoms. From the above description it can be concluded that the ra nges of r(av) and chi(av) of the alloys with excellent capacity and hi gh 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 eq ual to 1.640 respectively.