The structure and electrochemical properties of La₂MgNi_x (x = 8.7-9.9) hydrogen storage electrode alloys

In this paper, the effect of Ni content on the crystal structure and electrochemical properties of the La₂MgNi_x (x = 8.7-9.9) alloys was investigated. All the alloys are composed of a main phase with hexagonal PuNi₃-type structure and a small quantity of impurity phases (LaNi₅, MgNi₂). The increase of Ni content in the alloys leads to a decrease in both the cell volume and the hydride stability, and leads to a noticeable decrease in cell volume expansion rate (ΔV/V) on hydriding. As Ni content increases, the discharge capacity of the alloys at 50 mA/g increases slightly and passes through a maximum of 411.5 mAh/g at x = 9.0, the high-rate dischargeability of the alloy electrodes at 800 mA/g (HRD₈₀₀) improves greatly from 38.8 (x = 8.7) to 84.4% (x = 9.9). The increase of high-rate dischargeability of the Ni-rich alloys is ascribed to the increase of the electrocatalytic activity of the alloy electrode and the higher diffusion rate of hydrogen in the bulk of the alloys. In the composition range of x = 8.7-9.6, the cycling stability (S₁₀₀) of the alloys increases with increasing of Ni content, increasing from 54.1 (x = 8.7) to 63.4% (x = 9.6). The improvement in cycling stability is mainly attributed to the lower cell volume expansion of the Ni-rich alloys on hydriding.