Study on microstructure and electrochemical performance of La 0.7Mg 0.3(Ni 0.9Co 0.1) x hydrogen storage alloys

Abstract

Hydrogen storage alloys La 0.7Mg 0.3(Ni 0.9Co 0.1) x ( x = 3.0, 3.1, 3.3, 3.4, 3.5, 3.7 and 3.8) were prepared by inductive melting followed by annealing treatment at 1173 K for 6 h. The effects of the stoichiometry ( x) on the structural and electrochemical characteristics of the alloys were investigated systematically. X-ray diffraction (XRD), optical morphology and energy dispersive spectrometry (EDS) analyses showed that these alloys have a multiphase structure which consists of a (La, Mg)Ni 3 phase with the PuNi 3-type rhombohedral structure, a LaNi 5 phase with the CaCu 5-type hexagonal structure and a (La, Mg) 2Ni 7 phase with the Ce 2Ni 7-type hexagonal structure. The main phase of the alloys with x = 3.0 and 3.1 is (La, Mg)Ni 3 phase (PuNi 3-type structure), the main phase of the alloys with x = 3.3, 3.4 and 3.5 is (La, Mg) 2Ni 7 phase (Ce 2Ni 7-type structure), and the main phase of the alloys with x = 3.7 and 3.8 is LaNi 5 phase (CaCu 5-type structure). Moreover, the lattice parameters of the (La, Mg)Ni 3 phase, (La, Mg) 2Ni 7 phase and LaNi 5 phase decrease monotonously with the increase of the value x. The electrochemical analysis shows that the maximum discharge capacity increases from 356.6 mA h g −1 ( x = 3.0) to 392.1 mA h g −1 ( x = 3.5) and then decreases to 344.1 mA h g −1 ( x = 3.8), and the alloys exhibit good cycling stability. As the discharge current density is 3000 mA g −1, the high-rate dischargeability (HRD) increases from 30.1% ( x = 3.0) to 56.1% ( x = 3.8). The low temperature dischargeability (LTD) increases from 24.3% ( x = 3.0) to 58.96% ( x = 3.7) and then decreases to 48.1% ( x = 3.8).