Effect of yttrium content in the La2-xYxMgNi9 battery anode alloys on the structural, hydrogen storage and electrochemical properties.

The present work focuses on the studies of influence of yttrium on the crystal structure, hydrogenation properties and electrochemical behaviors of the PuNi3-type La2-xYxMgNi9 (x = 0.25; 0.50; 0.75; and 1.00) intermetallic alloys used as anodes of the Ni-MH batteries where up to 1/2 part of lanthanum was replaced by yttrium. X-ray diffraction studies revealed that all studied alloys are two-phase and contain PuNi3-type AB3 intermetallics (major phase) and Gd2Co7-type A2B7-3R compounds (secondary phase). Unit cell constants and cell volumes for the crystal structures of the AB3 intermetallics linearly decrease following an increase in Y content. Interestingly, in the LaMgNi4 Laves type structure layer yttrium occupies not only the 6c site, but also partially fills the 3a site in the LaNi5 layer. Neutron diffraction studies confirmed that the saturated La1.5Y0.5MgNi9D12.4 hydride containing approximately 1 at. H/at. Me, crystallizes with a trigonal unit cell (space group R3̄m; a = 5.3681(2) Å, c = 26.437(4) Å) and is formed via an anisotropic expansion of the original intermetallic lattice. The studied hybrid structure is composed of LaNi5D5.2 and LaMgNi4D7.2 slabs with a similar hydrogen content. Interestingly, the H-caused expansion of the AB2 and AB5 layers is slightly uneven (23.2% and 27.7%, respectively). In the whole broad substitution range of yttrium for lanthanum, La2-xYxMgNi9 alloys, independent on the content of Y, form intermetallic hydrides with a high reversible hydrogen storage capacity of ∼1.5 wt% H, while the properties of the obtained hydrides are directly related to the substitution extent Y → La. Indeed, the most rich in yttrium LaYMgNi9 alloy at 20 °C shows a more than 10 times higher equilibrium pressure of hydrogen desorption as compared to the alloy with the smallest Y content, La1.75Y0.25MgNi9. A partial substitution of Y for La increases the electrochemical discharge capacity of La2.25Y0.75MgNi9 alloy to reach ∼450 mA h g-1 at a discharge current density of 10 mA g-1. The addition of Y greatly improves the electrochemical cycling performance, with remaining electrochemical capacity of up to 60% of the initial value, after performing 500 cycles, and is much superior as compared to the Y-free La2MgNi9-type anode. Thus, tailoring yttrium content in the alloys allows improvements of the performance of the studied alloys used as hydrogen storage and battery electrode materials.