Study on microstructure and electrochemical performance of La0.7Mg0.3(Ni0.9Co0.1)x hydrogen storage alloys

Hydrogen storage alloys La0.7Mg0.3(Ni0.9Co0.1)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)Ni3 phase with the PuNi3-type rhombohedral structure, a LaNi5 phase with the CaCu5-type hexagonal structure and a (La, Mg)2Ni7 phase with the Ce2Ni7-type hexagonal structure. The main phase of the alloys with x = 3.0 and 3.1 is (La, Mg)Ni3 phase (PuNi3-type structure), the main phase of the alloys with x = 3.3, 3.4 and 3.5 is (La, Mg)2Ni7 phase (Ce2Ni7-type structure), and the main phase of the alloys with x = 3.7 and 3.8 is LaNi5 phase (CaCu5-type structure). Moreover, the lattice parameters of the (La, Mg)Ni3 phase, (La, Mg)2Ni7 phase and LaNi5 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).