Synthesis of composite Mg-Ni-Ca metal hydride by mechanical alloying

Amorphous and composite Mg-Ni-Ca metal hydrides were synthesized by mechanical alloying. (Mg0.5Ca0.5)Ni2 yielded the composite of Mg2Ni and CaNi5, while MgNi0.95Ca0.05 produced the amorphous phase. The initial discharge capacities of the amorphous MgNi0.95Ca0.05 alloy and the composite (Mg0.5Ca0.5)Ni2 alloy were 460 and 210 mAh/g, respectively. After 20 cycles, the amorphous electrode retained 11% of the initial discharge capacity while the composite electrode maintained 64% of the initial discharge capacity. After 10 cycles, the XPS analysis showed that Mg spectra of the amorphous MgNi0.95Ca0.05 alloy was shifted to a lower binding energy because Mg reacted with OH− of the electrolyte. However, Mg spectra of the composite (Mg0.5Ca0.5)Ni2 alloy showed that the binding energy did not change during the cycle test. These results indicate that the amorphous phase had the highest discharge capacity and the composite formation improved the cyclic stability.