Nanocrystalline LaNi5-type electrode materials for Ni-MHx batteries

Abstract

LaNi5-based hydrogen storage alloys are a promising choice for hydrogen energy systems. In this work, nanocrystalline La(Ni,M)5 (M=Mn, Al, and Co) alloys have been synthesized by mechanical alloying (MA) followed by annealing. The discharge capacity of electrodes prepared by application of MA and annealed LaNi5 alloy powder is low. It was found that the partial substitution of Ni by Al or Mn in La(Ni,M)5 alloy leads to an increase in discharge capacity. On the other hand, cobalt substituting nickel in LaNi4−xMn0.75Al0.25Cox greatly improves the cycle life of this material. For x=0.25 the discharge capacities up to ∼260mAhg−1 were measured at current density of charging and discharging of i=40mAg−1. The studies show that electrochemical properties of Ni–MHx batteries are a function of chemical composition and microstructure of the electrode materials used. XPS studies showed that the shape of the valence band measured for the arc-melted (polycrystalline) LaNi5 is practically the same as that reported earlier for the single crystalline sample. The substitution of Ni by Al in LaNi5 leads to significant modifications of the electronic structure of the polycrystalline sample. Furthermore, the XPS valence band of the MA nanocrystalline LaNi4Al1 alloy is considerably broader compared to that measured for the polycrystalline sample. The strong modifications of the electronic structure and the significant surface segregation of lanthanum atoms in the MA nanocrystalline LaNi5-type alloys could significantly influence its hydrogenation properties.