Comparison among Constituent Phases in Superlattice Metal Hydride Alloys for Battery Applications

The effects of seven constituent phases—CeNi3, NdNi3, Nd2Ni7, Pr2Ni7, Sm5Ni19, Nd5Co19, and CaCu5—on the gaseous phase and electrochemical characteristics of a superlattice metal hydride alloy made by induction melting with a composition of Sm14La5.7Mg4.0Ni73Al3.3 were studied through a series of annealing experiments. With an increase in annealing temperature, the abundance of non-superlattice CaCu5 phase first decreases and then increases, which is opposite to the phase abundance evolution of Nd2Ni7—the phase with the best electrochemical performance. The optimal annealing condition for the composition in this study is 920 °C for 5 h. Extensive correlation studies reveal that the A2B7 phase demonstrates higher gaseous phase hydrogen storage and electrochemical discharge capacities and better battery performance in high-rate dischargeability, charge retention, and cycle life. Moreover, the hexagonal stacking structure is found to be more favorable than the rhombohedral structure.