Inhibition mechanism of capacity degradation in Mg-substituted LaY2-x Mg x Ni9 hydrogen storage alloys

Highlights: • Mg substitution destabilizes LaY₂Ni₉ phase and reduces maximum discharge capacity. • Inhibition of amorphization is confirmed by TEM and amorphization formation criterion. • Inhibition of the pulverization and amorphization enhances capacity retention. • LaY_1.25Mg_0.75Ni₉ alloy exhibits good overall electrochemical properties. -- Abstract: To elucidate the effect of Mg substitution on the electrochemical properties of LaY₂Ni₉ alloys and its thermodynamic mechanisms, the cyclic stability, maximum discharge capacity (MDC), microstructure evolution, hydrogen induced amorphization (HIA), pulverization, formation energy, and amorphization formation criteria of LaY_2-xMg_xNi₉ (x = 0, 0.25, 0.50, 0.75, and 1.00) alloys are investigated. The calculated formation energy indicates that Mg substitution destabilizes LaY₂Ni₉ phases and is responsible for decrement of phase fraction of LaY₂Ni₉ phases. The consumption of LaY₂Ni₉ phase and the generation of (La, Y)₂Ni₇ phase improve the cyclic stability by inhibiting the pulverization but reduce the MDC of the alloys. The amorphization formation criterion demonstrates that Mg substitution inhibits the HIA as verified by transmission electron microscopy results, which benefits both the MDC and cyclic stability of the alloys. The LaY_1.25Mg_0.75Ni₉ alloy exhibits good overall electrochemical properties with the MDC of 308.4 mAh/g and capacity retention after 100 cycles of 69.0%.