Phase transformation and electrochemical properties of La0.6R0.15Mg0.25Ni3.5 (R = La, Pr, Nd, Gd) alloys with multiphase structure

Abstract

As a promising Ni-MH battery electrode material, the La-Mg-Ni-based superlattice hydrogen storage alloys have attracted extensive attention due to their stable crystal structure, high energy density and good electrochemical properties. However, most of the existing studies are based on annealed alloys with simple phase compositions. In the present work, the effects of rare-earth elements and phase composition on the electrochemical properties and structure stability of a series of as-cast La0.6R0.15Mg0.25Ni3.5 (R = La, Pr, Nd, Gd) alloys are studied. The alloys contain (La, Mg)2Ni7, (La, Mg)5Ni19 and (La, Mg)Ni3 superlattice phases as well as LaNi5 and LaMgNi4 non-superlattice phases. As the atomic radius of the replacement element R decreases, the cell parameters of each phase in the alloys from large to small are: La> Pr> Nd> Gd. The activation cycle of alloys is one charge/discharge cycle, which demonstrate excellent activation property due to their multi-phase structure that offers many pathways and tunnels for the transfer of hydrogen atoms. Moreover, it is found that Pr enhances the cycling stability and structural stability as well as reduces the oxidation degree of the alloys owing to its function on suppressing LaNi5 non-superlattice phase which would cause an increase in internal stress and structure damage due to the continuous expansion and contraction of superlattice phase cell volume with the process of charge/discharge. Furthermore, the addition of Nd can accelerate the diffusion of hydrogen and increase the reaction speed of charge transfer at the electrode/electrolyte interface, resulting in an enhanced rate discharge performance.