The role of magnesium on properties of La3-xMgxNi9 (x=0, 0.5, 1.0, 1.5, 2.0) hydrogen storage alloys from first-principles calculations

Abstract

The present work gives the electronic structures of La3-xMgxNi9 (x = 0.0–2.0) alloys by first-principles calculations using the generalized gradient approximation of Perdew-Wang 91 (GGA-PW91) method within Cambridge Serial Total Energy Package (CASTEP), aiming at gaining insight into the hydrogen storage mechanism of La3-xMgxNi9 alloys modified by Mg. The results show that the La3-xMgxNi9 alloys consist predominantly of interactions between La-Ni, Ni-Ni or/and Mg-Ni. Among them, La-Ni interaction is the major factor controlling the structural stability of the alloys. Mg substitution increases the La-Ni bonding interactions to achieve stable Mg-containing metal matrices for reversible hydrogen absorption-desorption. This is particularly obvious at high Mg composition, as the La-Ni interactions gradually increase with Mg content. The increase of La-Ni interactions coupled with the decrease of Mg-Ni and Ni-Ni interactions will relieve the hydrogen-induced amorphization and disproportionation, and subsequently enhance the cyclic stability of La3-xMgxNi9 alloys at high Mg content. However, Mg substitution for La leads to a subsequent contraction in cell volume, dramatically reducing the reversible H capacity at high Mg composition such as LaMg2Ni9. Suitable Mg content in La-Mg-Ni systems, such as an approximately range x = 1.0–1.4 in La3-xMgxNi9 alloys, is required in trade-off between hydrogen storage capacity and cycle life.