Changes of crystal structure and hydrogen storage performances for multiphase La_0.7Mg_0.3Ni₃ alloy upon gas–solid cycling

In order to examine the effects of structure stability on the degradation behaviors of multiphase La_0.7Mg_0.3Ni₃ alloy, changes of the crystal structure and hydrogen storage properties after gas–solid cycling were investigated in detail. The structural analysis identifies that (La,Mg)Ni₃ (PuNi₃-type) phase transforms to amorphous, i.e., hydrogen-induced amorphization(HIA) occurs whereas LaNi₅ (CaCu₅-type), (La,Mg)₂Ni₇ (Ce₂Ni₇-type), and (La,Mg)₅Ni₁₉ (Pr₅Co₁₉-type) phases still keep crystalline upon hydriding/dehydriding cycling. Partial amorphization remarkably affects both the gas–solid and electrochemical storage performances. The plateau of PCT curves becomes narrow and steep with cycling. Moreover, the maximum electrochemical capacity decreases notably after gas–solid hydrogenation repeats. The electrochemical capacity reduction could be ascribed to both drop of the maximum storage capacity and the slope of plateau induced by partial amorphization. For direct electrochemical cycling, it is suggested that the capacity decay is mainly attributed to HIA in the initial stage.