Function of aluminum in crystal structure of rare earth-Mg-Ni hydrogen-absorbing alloy and deterioration mechanism of Nd0.9Mg0.1Ni3.5 and Nd0.9Mg0.1Ni3.3Al0.2 alloys

We investigated in detail the effect of incorporating Al in the crystal structure of rare earth-Mg-Ni hydrogen-absorbing alloys, which were developed as candidate materials for the metal hydride (MH) negative electrode in commercial Ni-MH batteries, using synchrotron powder X-ray diffraction. Partially substituting the Ni part with Al changes the lattice parameter of the major A2B7 phase, eliminating a mismatch between the AB2 units and the AB5 units. The change of the lattice parameter in the alloy leads to good hydrogen reversibility and good durability. Furthermore, we observed the alloy after 30 hydrogen absorption-desorption cycles using a scanning transmission electron microscope image. Consequently, we found that the surface layer of the Nd0.9Mg0.1Ni3.5 alloy had an amorphous state, while the surface layer of the Nd0.9Mg0.1Ni3.3Al0.2 alloy did not. We confirmed that the deterioration mechanism of the Nd0.9Mg0.1Ni3.3Al0.2 alloy was due to partial expansion of the AB2 unit along the c-axis; the local area of deterioration expanded during the cycling period.