Modulating superlattice structure and cyclic stability of Ce2Ni7-type LaY2Ni10.5-based alloys by Mn, Al, and Zr substitutions

Abstract

It is a great challenge to achieve high capacity and long cyclic life for hydrogen storage electrode alloys in Ni-MH batteries, especially for superlattice alloys. This work investigates three series of single-phase LaY2Ni10.5-based alloys to tune the Ce2Ni7-type superlattice structure and electrochemical hydrogen storage properties by Mn, Al, and Zr substitutions. The Mn, Al substitutions for Ni could reduce the volume difference between [A2B4] and [AB5] subunits, thus greatly improving the cyclic performance. The maximum discharge capacity for the LaY2Ni9.7Mn0.5Al0.3 alloy is 384.1 mAh g−1, and the capacity retention S200 is as high as 76.1%. However, the partial replacement of Y with Zr dramatically reduces the discharge capacity and cyclic stability, although the subunit volume difference is even zero in the LaY1.75Zr0.25Ni9.7Mn0.5Al0.3 alloy. Detailed structural analysis shows that LaY1.75Zr0.25Ni9.7Mn0.5Al0.3 alloy exhibits greater a-axis expansion (Δa/a = 4.52%) and cell volume change (Δc/c = 15.46%) during charging than the Mn, Al-substituted alloys. The unusual expansion at the basal plane of [A2B4] and [AB5] subunits should be responsible for a considerable lattice strain (1.42%) and poor cyclic performance in the Zr-substituted alloys. This work provides new insights into the design of high capacity and long-life superlattice electrode alloys.