Properties of mechanically alloyed Mg–Ni–Ti ternary hydrogen storage alloys for Ni-MH batteries Ste ´phane Ruggeri a , Lionel Roue ´ a,* , Jacques Huot b , Robert Schulz b , Luc Aymard c , Jean-Marie Tarascon c a INRS-E ´ nergie et Mate ´riaux, 1650 Blvd. Lionel-Boulet, C.P. 1200, Varennes, Que., Canada J3X 1S2 b Institut de Recherche d’Hydro-Que ´bec, 1800 Blvd. Lionel-Boulet, C.P. 1000, Varennes, Que., Canada J3X 1S1 c Laboratoire de Re ´activite ´ et de Chimie des Solides (URA CNRS 1211), Universite ´ de Picardie Jules Verne, 33 rue Saint Leu, 80039 Amiens, France Received 23 July 2002; accepted 16 August 2002 Abstract MgNiTi x , Mg 1x Ti x Ni and MgNi 1x Ti x (with x varying from 0 to 0.5) alloys have been prepared by high energy ball milling and tested as hydrogen storage electrodes. The initial discharge capacities of the Mg–Ni–Ti ternary alloys are inferior to the MgNi electrode capacity. However, an exception is observed with MgNi 0.95 Ti 0.05 , which has an initial discharge capacity of 575 mAh/g compared to 522 mAh/g for the MgNi electrode. The Mg–Ni-Ti ternary alloys show improved cycle life compared to Mg–Ni binary alloys with the same Mg/Ni atomic ratio. The best cycle life is observed with Mg 0.5 Ti 0.5 Ni electrode which retains 75% of initial capacity after 10 cycles in comparison to 39% for MgNi electrodes, in addition to improved high-rate dischargeability (HRD). According to the XPS analysis, the cycle life improvement of the Mg 0.5 Ti 0.5 Ni electrode can be related to the formation of TiO 2 which limits Mg(OH) 2 formation. The anodic polarization curve of Mg 0.5 Ti 0.5 Ni electrode shows that the current related to the active/passive transition is much less important and that the passive region is more extended than for the MgNi electrode but the corrosion of the electrode is still significant. This suggests that the cycle life improvement would be also associated with a decrease of the particle pulverization upon cycling. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen storage alloys; Magnesium-based compounds; Mechanical alloying; Nickel-metal hydride battery 1. Introduction Recently, several studies have clearly demonstrated that magnesium-based alloys, particularly nanocrystalline and amorphous Mg–Ni type compounds prepared by mechanical alloying, are promising materials as a metal hydride (MH) electrode for Ni-MH batteries. For example, amorphous MgNi alloy obtained after 10 h of ball milling has an initial discharge capacity of 522 mAh/g [1] in comparison to 280– 320 mAh/g for commercial LaNi 5 -based alloys. In addition, such Mg-based alloys do not need an activation process unlike the conventional AB 5 -type and AB 2 -type hydrogen storage materials. Lastly, they possess the advantages of very low toxicity and low price (Mg is the sixth most abundant element in the earth’s crust). Nevertheless, their charge/discharge kinetics are not yet totally satisfactory and their practical discharge capacities have not attained their theoretical values. Furthermore, the cycle lifetimes of Mg- based electrodes are insufficient from the practical view- point. For example, the discharge capacity decay of MgNi electrode is over 70% after only 20 charge–discharge cycles [1]. Such degradation, which nullifies the practical use of magnesium-based alloys as negative electrode materials for Ni-MH batteries, must be drastically reduced. This is a considerable scientific challenge. The capacity degradation is associated with the irreversible oxidation of the alloy by the electrolyte (KOH) leading to the formation of a Mg(OH) 2 layer on the surface of the alloy particles [2–5]. This consumes active material, affects the charge transfer across the alloy/electrolyte interface [4] and may act as a barrier reducing hydrogen diffusion into and from the alloy bulk. Such deleterious phenomena are accentuated by the pulverization of the alloy during charge/discharge cycles which creates new active surface and consequently forms additional Mg(OH) 2 after contact with the electrolyte. A possible way to decrease the capacity loss of Mg-based alloys is to adjust the composition of the Mg–Ni alloys by Journal of Power Sources 112 (2002) 547–556 * Corresponding author. Tel.: þ1-450-929-8185; fax: þ1-450-929-8102. E-mail address: roue@inrs-ener.uquebec.ca (L. Roue ´). 0378-7753/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-7753(02)00451-2