Abstract

The electronic properties of LaNi₅ and substituted LaNi₄M (M = Mn, Fe, Co, Cu and Al) and LaNi_3.55Mn_0.4Al_0.3Co_0.75 compounds have been studied by X-ray absorption (XAS) and photoemission (PES) spectroscopies in order to understand the influence of the substitutions on the electrochemical properties of these materials in rechargeable batteries. The experimental results are compared with band structure calculations. The valence band of LaNi₅ is dominated by the Ni 3d band. Ni substitution by another 3d metal leads to the formation of an additional 3d subband which is above that of Ni for Mn, Fe and Co and below for Cu, in agreement with density of states (DOS) calculations. Al substitution leads to a progressive filling of the Ni d band associated with a decrease of the DOS at E_F. In LaNi_3.55Mn_0.4Al_0.3Co_0.75 (LaNi3S), the XAS measurements indicate that the filling of the individual 3d subbands is different from the corresponding ones in LaNi₄M single substituted compounds (M = Mn and Co). The total number of holes in the d bands of the transition elements of the LaNi3S compound is found to be the same as in LaNi₄Co. The complete filling of these states by 4 H atoms per formula unit in LaNi₄CoH₄ as well as in the LaNi3S hydride is associated with a particular stability of the compounds up to this hydrogen concentration, a feature which has been observed in the pressure–composition isotherms and which appears to play an important role in the longer cycle life of these electrode materials.