Study on phase formation mechanism and electrochemical properties of La0.75−xNdxMg0.25Ni3.3 (x = 0, 0.15) alloys prepared by powder sintering

An investigation of the phase formation mechanism and electrochemical properties of the La0.75−xNdxMg0.25Ni3.3 (x = 0, 0.15) alloys has been conducted in this paper. As explored by Rietveld analysis of the XRD data, the La0.75Mg0.25Ni3.3 alloy is composed of Ce2Ni7-type and Gd2Co7-type phases accompanied by minor CaCu5-type phase. The partial substitution Nd for La benefits the formation of Gd2Co7-type phase, indicating that the atoms with smaller radius favor the formation of the Gd2Co7-type phase. However, CaCu5-type phase disappears with Nd substitution. Because part of the replaced La atoms transform into [A2B4] subunits with the Ni atoms, and the formed [A2B4] subunits crystallize with [AB5] subunits to produce A2B7-type phase, resulting in the depletion of the minor CaCu5-type phase. The electrochemical P-C isotherms present that the La0.60Nd0.15Mg0.25Ni3.3 alloy only containing A2B7-type phase has one single plateau while the La0.75Mg0.25Ni3.3 alloy which contains minor LaNi5 phase has two plateaus. It is beneficial for the elevation of equilibrium plateau pressure ascribing to the contraction in cell volumes by Nd substitution. Electrochemical measurements show that the substitution of La by Nd improves the discharge capacity and the high rate dischargeability owing to the increase of A2B7-type phase abundance.