Many extensive studies have been carried out on AB (TiFe), AB 2 (ZrCr 2 , TiCr 2 ), AB 5 (LaNi 5 ), and A 2 B (Mg 2 Ni) hydrogen storage alloys developed as the anodic materials of Ni/MH batteries. 1-4 Among these, Mg 2 Ni-type alloys are promising materials due to their high hydrogen storage capacity (up to 3.6 wt %), low material cost, rich mineral sources, and light weight. 5,6 Conventionally, the MH powders have been manufactured by arc melting or vacuum induction melting and then by mechanical pulverization. However, it is difficult to prepare Mg 2 Ni-type alloys by conventional melting processes due to their large differences in melting temperature. Moreover, they can absorb and desorb hydrogen at high temperature (above 300ЊC) and the discharge capacity is very low at low temperature (below 30ЊC), 7 which hinders the application of these alloys.When the mechanical alloying process is adopted to synthesize these alloys, the synthesized alloys show unusual characteristics, such as nanocrystalline structure, 8,9 amorphous materials 10,11 and alloys with extended solubilities at low temperature. 9,12 It is also reported that amorphous Mg-Ni alloys prepared by mechanical alloying have a high discharge capacity, and the nanostuctured Mg 2 Ni-type alloys prepared by the same method show the improved hydrogenation characteristics at low temperature. [9][10][11][13][14][15] However, the electrode characteristics of mechanically alloyed Mg 2 Ni-type metal hydrides have not been reported yet.In the present study the mechanical alloying process was introduced to synthesize nanocrystalline Mg 2 Ni-type metal hydrides. The synthesized, nanocrystalline Mg 2 Ni alloys were expected to lower the temperature required for hydrogenation and improve the electrochemical characteristics of metal hydride electrodes at low temperature (30ЊC). The partial substitution of alloying elements Co, Cu, and Mn for Ni was carried out in order to improve the electrode characteristics of mechanically alloyed Mg 2 Ni. Experimental The starting materials for the ballmilling were the mixtures of the elemental powders. To prepare the starting materials from elemental powders, the powders of Mg (<175 m, 99.9% purity) and Ni (<45 m, 99.9% purity) were mixed to give the desired composition. These starting materials were put into a CrNi steel bowl together with stainless steel balls (SUS304, 2/16 in. diam). The handling of the powders was done in an argon-filled glove box in order to prevent oxidation of powders. Mechanical alloying was carried out using a planetary ball mill (Fritsch Pulverisette 5) at a speed of 100 rpm. The weight ratios of ball to powder were 15:1 in the case of mechanical alloying and 50:1 in the case of short-term ballmilling which was followed by heat-treatment.The structures of the ballmilled powders were characterized by X-ray diffraction (XRD) analysis using Cu K␣ radiation ( ϭ 1.5418 Å). Microstructural studies were performed by scanning electron and transmission electron microscopy (SEM and TEM, respectively). In order t...