Microstructure Changes of Amorphous Mg-Ni-Nd Alloys During Charge/Discharge Cycling and its Thermodynamic Verification

摘要

Amorphous Mg-based hydrogen-storage alloy (Mg70.6Ni29.4)90Nd10 is prepared by melt-spinning. Thephase structures of the ribbons before and after charge/discharge cycling are characterized by high- resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis,respectively. It is found that the amorphous structure begins to crystallize after four cycles ofhydrogenation/ dehydrogenation and a new nano-crystallized phase NdMg2Ni9 is detected withaverage grain size in the range 5 nm. Nano-size Mg2Ni phases appear after 60 cycles宎nd after 20cycles of charging/discharging stable Mg2Ni, -Mg and Nd2H5 phases are present. The followingresults of thermodynamics calculation verifies that the nano-crystallized phase NdMg2Ni9 is presentfirstly during the process of crystallization, because it free-energy (G=-15.2789KJ/mol孴=300K)is the lowest among Mg-Ni phase=-8.2694KJ/mol孴=300Kand Mg-Nd phase=- 13.29503KJ/mol孴=300K A good cycle life and high discharge capacity are also observed in the(Mg70.6Ni29.4)90Nd10 alloy electrode. It is suggested that homogeneous microstructure contributes to theenhancement in electrochemical characteristics; the presence of NdMg2Ni9 is beneficial for theimprovement in cycle life of (Mg70.6Ni29.4)90Nd10 alloy electrode.