Crucial role of Mg on phase transformation and stability of Sm–Mg–Ni-based AB2 type hydrogen storage alloy

Rare earth–Mg–Ni-based alloys are promising candidate for high-capacity hydrogen storage materials, and Mg is the key elements to maintain its crystal structure stable. In order to understand the role of Mg element in rare earth–Mg–Ni based alloys, the alloys with the composition of RE1–xMgxNi1.97Co0.03Al0.07 (RE = Sm0.80Y0.20; x = 0.35, 0.44, 0.52) were synthesized by powder sintering treatment. The increasing Mg content promotes the phase transformation from SmNi2 (Fd-3 m group) phase to MgCu4Sn-type (F-43 m group) phase. First-principle calculations and XRD simulations show that Mg element is reasonably approximated as the para-position substitution of rare earth elements at 4c sites when it enters into the unit cell of SmNi2 phase. The Sm0.38Y0.10Mg0.52Ni1.93Co0.03Al0.05 alloy with Mg fully occupying the 4c sites forms a stable structure, and it shows flat hydrogen absorption/desorption platforms, with a hydrogen absorption capacity of 1.12 wt%. Meanwhile, it has more excellent cycling stability. After 20 cycles of hydrogen absorption, the crystal structure of the alloy still maintains MgCu4Sn-type phase, and the capacity retention rate can reach 94.42 %.