In this paper, the structural and electrochemical properties of La 0.7 Mg 0.3 (Ni 0.85 Co 0.15 ) x (x ϭ 2.5,3.0,3.5,4.0,4.5,5.0) hydrogen storage alloy electrodes were studied systematically. X-ray diffraction Rietveld analyses show that all these alloys consist of a (La, Mg)Ni 3 phase with the PuNi 3 -type rhombohedral structure and a LaNi 5 phase with the CaCu 5 -type hexagonal structure. The (La, Mg)Ni 3 phase abundance first increases to a high percentage ͑ϳ75%͒ and then decreases with increasing x. In contrast, the LaNi 5 -phase abundance first remains low and almost unchanged and then increases to a high percentage ͑ϳ70%͒ with increasing x. The pressure-composition isotherms shows that for each isotherm the plateau region widens and the plateau pressure is maintained almost unchanged when x increases from 2.5 to 3.5. However, as x increases further, the plateau region is shortened and becomes flatter, and the plateau pressure increases with x. Electrochemical studies indicate that all important electrochemical properties, including the maximum discharge capacity, the high rate dischargeability, the exchange current density, and the limiting current density of the alloy electrodes increase as x increases from 2.5 to 3.5 and then decrease when x increases further from 3.5 to 4.5.

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