The structural and electrochemical properties of La_0.7Mg_0.3(Ni_0.85Co_0.15)_x (x = 3.0-5.0) hydrogen storage alloys

In this paper, the effect of the compositions on the structural and electrochemical characteristics of the La_0.7Mg_0.3(Ni_0.85Co_0.15)_x (x = 3.0, 3.5, 4.0, 5.0) hydrogen storage alloys have been investigated systematically. The results of the XRD Rietveld analyses show that the structures of the alloys change obviously with increasing x from 3.0 to 5.0. The main phase of the alloys with x = 3.0-3.5 is (La, Mg)Ni₃ phase (PuNi₃-type structure), but the main phase of the alloys with x = 4.0-5.0 is LaNi₅ phase (CaCu₅-type structure). Moreover, the phase abundance, lattice parameters and cell volumes of the (La, Mg)Ni₃ phase and the LaNi₅ phase change with increasing x. The electrochemical studies show that the maximum discharge capacity increases from 355.4 mAh/g (x = 3.0) to 395.6 mAh/g (x = 3.5) and then decreases to 226.8 mAh/g (x = 5.0). As the discharge current density is 1250 mA/g, the high rate dischargeability (HRD) increases from 67% (x = 3.0) to 81.3% (x = 3.5) and then decreases to 74.9% (x = 5.0). Furthermore, the exchange current density (I₀), the limiting current density (I_L) and the diffusion coefficient (D), of the alloy electrodes all first increase and then decrease with increasing x from 3.0 to 5.0, which is consistent with the variation of the HRD.