Effect of Gd and Co content on electrochemical and electronic properties of La$_{1.5}$Mg$_{0.5}$Ni$_7$ alloys: a combined experimental and first-principles study

Abstract

In this work, we investigate the effect of Gd and Co substitutions on the electrochemical and electronic properties of La$_{1.5}$Mg$_{0.5}$Ni$_7$ alloy. Two series of La$_{1.5-x}$Gd$_x$Mg$_{0.5}$Ni$_7$ ($x$ = 0.0, 0.25, 1.0) and La$_{1.5}$Mg$_{0.5}$Ni$_{7-y}$Co$_y$ ($y$ = 0.0, 0.5, 1.5) alloys are produced using mechanical alloying technique. The X-ray diffraction indicates multiphase character of the samples, with the majority of hexagonal Ce$_2$Ni$_7$-type and rhombohedral Gd$_2$Co$_7$-type structures of (La,Mg)$_2$Ni$_7$ phase. Partial substitutions of La by Gd or Ni by Co in La$_{1.5}$Mg$_{0.5}$Ni$_7$ phase result in increase of cycle stability of the metal hydride (MH$_x$) electrodes. All considered alloys reach the maximum discharge capacity after three charging-discharging cycles. Two optimal compositions, in respect of electrochemical properties, are subsequently investigated by the X-ray photoelectron spectroscopy (XPS). The experimental analysis of the valence band is further extended by the density functional theory (DFT) calculations. We also discuss the effects of alloying and site preference of dopants on the position of the van Hove-type singularity, as observed in the electronic densities of states (DOS) in proximity of the Fermi level.