Synthesis of composite metal hydride alloy of A 2 B and AB type by mechanical alloying Sang Soo Han * , Nam Hoon Goo, Woon Tae Jeong, Kyung Sub Lee Department of Materials Science and Engineering, Hanyang University, 133-791 Seoul, South Korea Received 28 April 2000; accepted 1 June 2000 Abstract A composite alloy composed of Mg 2 Ni and TiNi phases has been synthesized directly from elemental powders of Mg, Ni and Ti by mechanical alloying. The alloyed powders are produced by milling for 20 h. Most of the powders are not a perfect composite state but a mixture of Mg 2 Ni and TiNi grains. The amount of the Mg 2 Ni phase is relatively less than that of the TiNi phase because more Mg forms a solid-solution with TiNi than Ti forms with Mg 2 Ni. The maximum discharge capacity of the composite electrode is 380 mAh g 1 at a discharge current density of 10 mA g 1 . This value is higher than that of a mechanically alloyed Mg 2 Ni electrode. The composite electrode shows improved cycle-life compared with single-phase Mg 2 Ni. For example, after 150 cycles the ratio of the discharge capacity to the maximum value is about 55% whilst the ratio for Mg 2 Ni is below 10%. The composite electrode also has a high-rate discharge capability which is about 100 mAh g 1 after 40 cycles, regardless of the discharge current density. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Composite metal hydride alloy; Mg 2 Ni; TiNi; Mechanical alloying 1. Introduction The speci®c capacity and speci®c energy of hydride electrode materials depend on the chemical composition and crystalline structure. Well-known hydrogen-storage materials include AB, A 2 B, AB 2 and AB 5 type intermetallic compounds. The corrosion stability of these materials is ascribed to the formation of a passive ®lm on the surface of the material, which can protect the bulk from being further corroded during repeated charging±discharging. If, however, the ®lm is too compact to allow hydrogen to diffuse into and out of the electrode, or if it has a low catalytic activity for the electrochemical reduction and oxidation of hydrogen, the attainable charge±discharge rates, as well as the energy and power densities, are sig- ni®cantly reduced. The characteristics of the electrode sur- face also determine the rate at which fresh electrodes can be activated. For a given electrode material, it is dif®cult to satisfy all the requirements necessary to produce an electrode with maximum performance. In order to overcome this problem, composite hydride materials have been intro- duced as a new class of electrode materials [1±5]. The composite is a material which consists of two or more hydrogen-storage alloys or intermetallic com- pounds. The major component is, in general, characterized by good hydrogen-storage properties and high corrosion resistance and, therefore, acts as the main hydrogen-storage medium. The minor component is used as a surface activator to improve the kinetics of hydrogen sorption±desorption, as well as to ease the initial activation of the major component. Cui et al. [1] successfully synthesized a new composite alloy Mg 2 Ni-40 wt.%Ti 2 Ni by a particle inlaying method which included mechanical alloying and sintering. It was shown that the discharge capacity of the electrode was effectively improved from 8 mAh g 1 for Mg 2 Ni to l65 mAh g 1 for the new composite electrode at ambient temperature. Terzieva et al. [2] also synthesized Mg-LaNi 5 composite materials to improve the absorption±desorption characteristics of magnesium towards hydrogen. Until recently, all preparations of composite metal hydrides were composed of the following two steps: (i) mechanical milling of each metal hydride and (ii) heat- treatment. These methods are very complex. In our study, the composite metal hydride is very simply synthesized by mechanical alloying of the elemental powders. Thus, Journal of Power Sources 92 (2001) 157±162 * Corresponding author. Tel.: 82-2-2281-4914; fax: 82-2-2281-4914. E-mail address: mithan@dreamwiz.com (S.S. Han). 0378-7753/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0378-7753(00)00516-4