Structural and hydrogenation study on the ball milled TiH2-Mg-Ni

With the aim of further understanding for Ti-Ni-Mg alloys and their hydrogenation behavior, powders of TiH2, Mg and Ni with the molar ratio of 3:1:2 have been mechanically milled for 10 h, 20 h, 30 h, 40 h according to the stoichiometry (TiH2)1.5Mg0.5Ni. Microstructures of the milled sample were analyzed and their hydrogenation properties as negative electrodes for Ni-MH batteries were studied. Phase change with milling time revealed the fast formation of the Ti-Mg-H FCC phase. The alloying priority among Ti, Mg and Ni was demonstrated by comparing phase compositions in different milling time. Hydrogen capacities evaluated by both solid-gas reaction and electrochemical cycling under galvanostatic conditions show that overall capacities increase with milling time. Except for the sample milled 10 h, which hardly delivers any reversible hydrogen capacity, all samples exhibit excellent cycling stability after capacity drop in the first few cycles. The best discharge capacity 100 mAh/g is observed for the sample milled 40 h. The PCI (Pressure-Composition-Isotherms) and GITT (Galvanostatic Intermittent Titration Technique) curves indicate that all samples absorb hydrogen in solid solution. The measured capacities are concluded to be contributed by the Ti-Mg-H phase and the TiNi phase, while only the latter can provide reversible capacity.