Pulverization mechanism of the multiphase Ti–V-based hydrogen storage electrode alloy during charge/discharge cycling

Pulverization is an important key factor for the electrochemical cycle stability of many hydrogen storage alloys. In this paper, the pulverization mechanism of the multiphase Ti–V-based hydrogen storage alloy which mainly consists of a V-based solid solution phase with the BCC structure and a C14 Laves phase is studied based on a sample material of the Ti0.8Zr0.2V2.7Mn0.5Cr0.6Ni1.25Fe0.2 alloy. The microstructure of the alloy and the morphology change of the alloy electrode during the charge/discharge process were observed by transmission electron microscope, scanning electron microscope and atomic force microscope, etc. The effect of mechanical properties of the V-based phase and the C14 Laves phase on the pulverization behavior of the Ti–V-based alloy is discussed. The results show that microcracks initially occur at the phase boundary of the V-based phase and the C14 Laves phase and then extend to the C14 Laves phase in the charge/discharge process. The phase boundary is composed of a Ti segregated amorphous layer with a thickness of about 90 nm, mismatching with the crystallized V-base phase and C14 Laves phase. The toughness of the C14 Laves phase is much lower and the hardness is higher than that of the V-based phase. The weak bonding strength of the phase boundary, the lower toughness of the C14 Laves phase and the large volume expansion/contraction of the C14 Laves phase during charge/discharge cycling are the main factors that cause the pulverization of the Ti–V-based alloy.