Phase decomposition and electrochemical properties of single phase La1.6Mg0.4Ni7 alloy

The Ce2Ni7-type (hexagonal, 2H) single phase La1.6Mg0.4Ni7 alloy has been obtained by annealing the induction melting as-cast sample at 1223 K for 12 h. The relationship between phase structural stability and volume change rate of the three kinds of slabs in Ce2Ni7-type structure is studied. It is found that the volume change rate of Mg-containing [La1.22Mg0.78Ni4] slab after hydrogenation/dehydrogenation is larger than that of [LaNi5] I (outer) and [LaNi5] II (inner) slabs, and the consecutive cell volume change of [La1.22Mg0.78Ni4] slab ultimately results in the decomposition of Ce2Ni7-type phase La1.6Mg0.4Ni7 to amorphous La and Mg phases, nanocrystalline Ni, and CaCu5-type LaNi5 phases, as well as the reduction of electrochemical discharge capacity. Electrochemical studies show that the single phase alloy electrode possesses good discharge capacity (400 mAh g−1) and cycling stability (84.2% after 100 cycles). The improvement in phase structure stability and the cycling stability of the superlattice structure alloys can be achieved by inhibiting the significant volume change of Mg-containing slabs during hydrogenation/dehydrogenation.