Synthesis of Ti-Zr-Ni amorphous and quasicrystal powders by mechanical alloying, and their electrochemical properties Akito Takasaki a) Department of Engineering Science and Mechanics, Shibaura Institute of Technology, Toyosu, Koto-ku, Tokyo 135-8548, Japan Tetsuya Okuyama Graduate School, Shibaura Institute of Technology, Toyosu, Koto-ku, Tokyo 135-8548, Japan Janusz S. Szmyd Faculty of Energy and Fuels, AGH–University of Science and Technology, Krakow 30-059, Poland (Received 19 January 2010; accepted 31 March 2010) Mechanical alloying of Ti 45 Zr 38–x Ni 17þx and Ti 45–x Zr 38 Ni 17þx (0  x  8) elemental powders produced an amorphous phase, but subsequent annealing converted the amorphous phase into an icosahedral quasicrystal phase, along with a Ti 2 Ni-type phase. The discharge capacities, measured in a three-electrode cell at room temperature for both the amorphous and quasicrystal electrodes, increased with increasing Ni substitution for Zr or Ti. The highest discharge capacities, which were about 60 mAh/g for the amorphous electrode and 100 mAh/g for the quasicrystal electrode, were obtained from (Ti 45 Zr 30 Ni 25 ) after substitution of Ni for Zr. For the Ti 45 Zr 30 Ni 25 composition, the discharge performance of the quasicrystal electrode was stable over charge/discharge cycling, but that of the amorphous electrode gradually decreased with cycling. The structure of the quasicrystal phase in the electrodes was stable, even after 15 charge/discharge cycles, but the amorphous phase converted to a (Ti, Zr)H 2 f.c.c. hydride. I. INTRODUCTION LaNi 5 and MmNi 5 intermetallic compounds, and their modified forms after the substitution of other metals for La (Mm) or Ni, or after the addition of a third or fourth element, have been studied by numerous researchers. This is because LaNi 5 is currently used as an anodic material for nickel/metal hydride (Ni/MH) secondary batteries, which do not contain the toxic cadmium pre- sent in alkaline batteries. The H/M (number of hydrogen atoms per metal atom) ratio of LaNi 5 is 1.0 (the hydride form of LaNi 5 is LaNi 5 H 6 ), which corresponds to a gravimetric hydrogen capacity of 1.4 wt%. Furthermore, the theoretical charge capacity of LaNi 5 estimated from its chemical composition is 348 mAh/g. The actual dis- charge capacity was about 330 mAh/g, which is almost identical to the theoretical capacity, indicating that most of the hydrogen absorbed in the compound can easily be removed electrochemically. Ti-Zr-Ni icosahedral (i) quasicrystal phases, which have a new type of translational long-range order and display noncrystallographic rotational symmetry, are believed to possess a large number of tetrahedral intersti- tial sites in the cluster, 1 which makes the i-phase alloys attractive candidate materials for hydrogen storage. One of the authors previously reported that Ti 45 Zr 38 Ni 17 quasicrystal powders could be synthesized by mechan- ical alloying (MA) and subsequent annealing, and that the hydrogen capacity of the i-phase powder reached about 2.8 wt%, 2 which is superior to that of LaNi 5 . We have recently reported electrochemical hydrogenation/ dehydrogenation properties of Ti 45 Zr 38 Ni 17 amorphous and i-phase electrodes synthesized by MA and subse- quent annealing. 2–7 The maximum discharge capacity of the i-phase electrode at room temperature (298 K) was 23.9 mAh/g at a current density of 15 mA/g, while that of the amorphous electrode with an identical composition was a much lower 5.9 mAh/g. The theoretical charge capacity of the Ti 45 Zr 38 Ni 17 i-phase electrode estimated from its chemical composition is 795 mAh/g, if one assumes the maximum H/M ratio of 1.9. 8 We previously reported an H/M ratio for the Ti 45 Zr 38 Ni 17 i-phase of 1.4, 2 which corresponds to a theoretical capacity of 570 mAh/g, which was still superior to that of LaNi 5 (348 mAh/g). It is likely that some hydrogen atoms remained at interstitial (tetrahedral) sites in the qua- silattice because of strong chemical interaction with the metal atoms surrounding the interstitial sites. Hydrogen atoms in the quasilattice are reported to preferentially exist near Ti and Zr atoms, 9,10 which have strong chem- ical affinities with hydrogen atoms. If one can reduce a) Address all correspondence to this author. e-mail: takasaki@sic.shibaura-it.acjp DOI: 10.1557/JMR.2010.0202 J. Mater. Res., Vol. 25, No. 8, Aug 2010 © 2010 Materials Research Society 1575