Phase Structure and Electrochemical Properties of La--Mg--Ni System AB3 Type Hydrogen Storage Electrode Alloys
La-Mg-Ni系AB3型贮氢电极合金的相结构与电化学性能

XRD Rietveld analysis shows that LaxMg3-xNi9 (x=1.0---2.3) alloys consist of a main phase with hexagonal PuNi3-type structure and a few impurity phases (mainly LaNi5 and MgNi2, increasing x leads to an increase in both the lattice parameters and the unit cell volume of the main phase. The hydride of the alloys preserves the PuNi3 type structure, but shows a large unit cell volume expansion. The electrochemical measures indicate that the desorption plateau pressure Prm of the alloys decreases noticeably as x increases, while the maximum discharge capacity Cmax increases from 88.3 (x=1.0) to 397.5 (x=2.0), and then decreases to 230 mA.h/g (x=2.3). For the alloys with Cmax>348 mA.h/g (x=1.7-2.2), the high-rate dischargeability (HRD) of the electrodes at i=400-1200 mA/g decreases with increaseing x. The slower decrease of HRD is mainly attributed to the decrease of eletrocatalytic activity due to the charge--transfer reaction, and the more rapid decrease of HRD of the alloys with $x>$2.0 is related to the lower hydrogen diffusion rate in the bulk of alloy. The rate of capacity retention (S100) of the alloys after 100 charge/ discharge cycles is around 55.7%-62.9%, the rather fast cycling capacity degradation is mainly due to the corrosion of La and Mg and the large unit cell volume expansion in the hydride phase.