Effect of Gd and Co content on electrochemical and electronic properties of La1.5Mg0.5Ni7 alloys: A combined experimental and first-principles study

In this work we investigate the effect of Gd and Co substitutions on the electrochemical and electronic properties of La1.5Mg0.5Ni7 alloy. Two series of La1.5−xGdxMg0.5Ni7 (x = 0.0, 0.25, 1.0) and La1.5Mg0.5Ni7−yCoy (y = 0.0, 0.5, 1.5) alloys are produced using mechanical alloying technique. The X-ray diffraction indicates multi-phase character of the samples with majority of hexagonal Ce2Ni7-type and rhombohedral Gd2Co7-type structures of (La,Mg)2Ni7 phase. Partial substitutions of La by Gd or Ni by Co in La1.5Mg0.5Ni7 phase result in increase of cycle stability of the metal hydride (MHx) 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.