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

• Hydrogenation properties of pseudobinary Ti(Ni,Cu) compounds are deeply investigated.
• Cu-substitution destabilizes the hydride phase and opens a wide miscibility gap.
• TiNi0.8Cu0.2 exhibits a high discharge capacity of 300 mAh g−1with good cycle life.
• Electrochemical discharge capacities are two-fold higher than for binary TiNi.
• Ti(Ni,Cu) compounds are promising rare-earth free anodes 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 (RT). 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 TiNi0.8Cu0.2 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.

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