Effect of yttrium content in the La_2−<i>x</i>Y_<i>x</i>MgNi₉ 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 La_2-<i>x</i>Y_<i>x</i>MgNi₉ (<i>x</i> = 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 6<i>c</i> site, but also partially fills the 3<i>a</i> site in the LaNi₅ layer. Neutron diffraction studies confirmed that the saturated La_1.5Y_0.5MgNi₉D_12.4 hydride containing approximately 1 at. H/at. Me, crystallizes with a trigonal unit cell (space group <i>R</i>3̄<i>m</i>; <i>a</i> = 5.3681(2) Å, <i>c</i> = 26.437(4) Å) and is formed <i>via</i> an anisotropic expansion of the original intermetallic lattice. The studied hybrid structure is composed of LaNi₅D_5.2 and LaMgNi₄D_7.2 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, La_2-<i>x</i>Y_<i>x</i>MgNi₉ 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, La_1.75Y_0.25MgNi₉. A partial substitution of Y for La increases the electrochemical discharge capacity of La_2.25Y_0.75MgNi₉ 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.