Ti(Ni,Cu) pseudobinary compounds as efficient negative electrodes for Ni-MH batteries

The effect of Ni by Cu substitution on the structural, solid-gas and electrochemical hydrogenation properties of TiNi has been investigated. Pseudo-binary TiNi1_xCux (x <= 0.5) compounds have been synthesized by induction melting. They crystallize in B2 structure above 350 K and either in B19' (x < 0.1) or B19 (0.2 < x < 0.5) at room temperature (RI). For all compounds, Pressure-Composition Isotherms at 423 K exhibit a single slopping plateau pressure within the range 10(-3)-1 MPa of hydrogen pressure revealing a metal to hydride transformation. Both the hydrogenation capacity and the hydride stability decrease with Cu-content. The hydrided pseudobinary compounds crystallize in the tetragonal S.G. I4/ mmm structure as for TiNi hydride. The electrochemical discharge capacity increases with Cu content from 150 mAh g(-1) for TiNi up to 300 mAh g(-1) for TiNi0.8CuO0.2 and then decreases again for larger Cu amounts. Electrochemical isotherms and in-situ neutron diffraction measurements at RT demonstrate that such a capacity increase results from a metal to hydride phase transformation in which the hydride phase is destabilized by Cu substitution. The TiNi0.8Cu0.2 compound exhibits interesting cycling stability for 30 cycles and good high-rate capability at D/2 rate. This compound has promising electrochemical properties as compared to commercial LaNi5-type alloys with the advantage of being rare-earth metal free. (C) 2014 Elsevier B.V. All rights reserved.