Study On Microstructure And Electrochemical Performance Of La_(0.7)mg_(0.3)(ni_(0.9)co_(0.1))_x Hydrogen Storage Alloys

摘要

Hydrogen storage alloys La_(0.7)Mg_(0.3)(Ni_(0.9)Co_(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 6h. 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_2 Ni_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 mAhg~(-1) (x = 3.0) to 392.1 mAhg~(-1) (x = 3.5) and then decreases to 344.1 mAhg~(-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).