Investigation of the electronic properties of substituted LaNi5 compounds used as material for batteries

The electronic properties of LaNi5 and substituted LaNi4M (M = Mn, Fe, Co, Cu and Al) and LaNi3.55Mn0.4Al0.3Co0.75 compounds have been studied by X-ra y absorption (XAS) and photoemission (PES) spectroscopies in order to under stand 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 dom inated 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) calculat ions. Al substitution leads to a progressive filling of the Ni d band assoc iated with a decrease of the DOS at E-F. In LaNi3.55Mn0.4Al0.3Co0.75 (LaNi3 S), the XAS measurements indicate that the filling of the individual 3d sub bands 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 L aNi4Co. 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 particul ar stability of the compounds up to this hydrogen concentration, a feature which has been observed in the pressure-composition isotherms and which app ears to play an important role in the longer cycle life of these electrode materials.