Surface modification of Zr 0.9 Ti 0.1 Ni 1.1 Co 0.1 Mn 0.5 V 0.2 Cr 0.1 alloy was accomplished by mechanical grinding (MG). The decrepitation of alloy particles gave rise to a new surface. The effect of MG was systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), pressure-composition (PC) isotherms and electrochemical impedance spectroscopy (EIS). Initial activation and rate capability of a negative electrode made from this alloy were significantly improved by MG treatment at 300 rpm for 0.5-1 h, but prolonged MG treatment (10 h) reduced the nominal discharge capacity from 350 mAh g -1 to 180 mAh g -1 . Under these conditions the alloy particles disintegrate and become nanocrystalline, which reduces the discharge capacity owing to the change in the stereology of tetrahedral interstices available for hydrogen storage. The data based on PC isotherms and hydriding kinetics indicates that the equilibrium hydrogen pressure increases and the hydriding rate and storage capacity are significantly reduced by prolonged MG treatment. EIS data reveal that the improved rate capability can be ascribed to an enhanced charge-transfer reaction which is the rate-determining step in the hydriding and dehydriding reactions.

A negative electrode consisting of Laves-phase alloy particles with a composition of Zr 0.9 Ti 0.1 Ni 1.1 Co 0.1 Mn 0.5 V 0.2 Cr 0.1 requires more than 20 activation cycles to attain a nominal discharge capacity ͑ca. 350 mAh g −1 ͒. The activation cycles could be reduced to several times only by mechanical modification with planetary ballmilling and further shortened by mechanochemical modification using a small amount of Al or Mg powder as a modifier. Such mechanical and mechanochemical modifications are also effective for improving high-rate dischargeability. The best result was obtained by grinding the Laves-phase alloy with 1 wt % modifier for 1 h at 300 rpm. The reason for quick activation and improved high-rate dischargeability is discussed on the basis of electrochemical impedance, crystallographic structure, particle size, and specific surface area. .Zr-Ti based Laves-phase alloys are a promising negative electrode material for nickel-metal hydride ͑Ni-MH͒ batteries because of their high energy density and superior cycle stability. In contrast, this type of alloy requires many cycles for activation and it is characterized by insufficient rate capability. To solve the above problems, chemical 1-3 and physical 4,5 modification of alloys has been proposed by many researchers. Because the physical modification is simple and economical, we have examined the effect of mechanical grinding ͑MG͒ and disclosed the shortened activation cycles and improved high-rate dischargeability of the MH electrode.In this study, a small amount of Al or Mg powder was employed as a surface modifier in mechanical grinding ͑MG͒ of a Zr-Ti-based hydrogen storage alloy in order to further improve the performance of the negative electrode. Such modifiers are selected on the basis of high reducing power ͓E Al 3+ /Al 0 = −1.66 V, E Mg 2+ /Mg 0 = −2.36 V vs normal hydrogen electrode ͑NHE͔͒ 6 for producing active surface. During the MG treatment, decrepitation of alloy particles gave rise to fresh surface and large effective surface area. During the mechanochemical modification using a surface modifier, chemical effects in addition to the mechanical effects described above could be expected. The chemical effects ascribed to the modifier were systematically investigated through the electrochemical impedance and specific surface area evaluated by the Brunauer-Emmet-Teller ͑BET͒ method. ExperimentalLaves-phase alloy ͑Zr 0.9 Ti 0.1 Ni 1.1 Co 0.1 Mn 0.5 V 0.2 Cr 0.1 ͒ prepared by induction melting and pulverized to less than 100 m by repeated hydride-dehydride cycles was used as a standard sample. This alloy is supplied from the Osaka National Research Institute to standardize the characterization method of Laves-phase alloy electrode as an anode in a MH battery. Initially delivered Zr-Mn-based Laves-phase alloy as a common sample did not give a sufficient cycle life. The composition of Laves-phase alloy used in this experiment was adjusted to give a high energy density and long cycle life by introducing Ti, Co, V, and Cr with reference to prev...