Effect Of Ball-milling With Ni Powder And Nano Carbon On The Electrochemical Properties Of Amorphous Mg Based Alloys

In this thesis, the recent research and development on Mg based hydrogen storage electrode alloys has been reviewed. On the basis of the review, amorphous Mg-Ni based alloys are selected as the object of this study. Amorphous Mg₂Ni_0.95Sn_0.05+x wt%Ni alloy is prepared by ball milling the mixture of as-cast Mg2Nio.95Sno.05 and Ni powder, the influence of Ni powder content and preparation conditions on the microstructures and electrochemical properties are studied. Further more, the amorphous system which has the highest dishcharge capacity, is chosen to be coated by carbon nanopowder and carbon nanotube(CNT) by ball milling technique to get better cycling stability. The main result are as follows:(1) The maximum discharge capacity of as-cast Mg₂Ni_0.95Sn_0.05 is very low(only 16mAh/g), and no significant improvement obtained even after ball milling without addition of Ni. As the content(x) of Ni increases, the maximum discharge capacity of the ball milled Mg₂Ni_0.95Sn_0.05+x wt%Ni first increases and then decreases and reaches the maximum capacity of 625.6mAh/g when x=75, the cycling stability increases and reaches the best cycling stability (the capacity maintain rate for 10 cycles is 70.6%) when x=125. With ball milling time(t) increasing, the maximum discharge capacity of ball milled Mg₂Ni_0.9Sn_0.05+75wt%Ni increases and reaches the maximum capacity of 670.1mA/g when t=200h, the cycling stability increases and then decreases and reaches the best cycling stability (the capacity maintain rate for 10 cycles is 64.7%) when f=100h.The microstructure analysis results show that the addition of Ni enhance the formation of amorphous phase; As the ball milling time increases, the particle size becomes smaller and the degree of amorphorization increases and then reaches a relative constant value, but further increasing the ball milling time leads to the increase of particle size.During the stage of short ball milling time, the electrochemical capacity is controlled by the degree of amorphorization, while during the stage of long ball milling time, it is controlled by the particle size.(2) On the basis of the above experimental results, we select amorphous Mg₂Ni_0.95Sn_0.05+75wt%Ni ball milled for 200h to composite with carbon nanopowder and carbon nanotube. The maximum capacity of alloy increases with additon of proper amount of carbon nanopowder.But The cycling stability has no improvement after addition of carbon nanopowder. With increasing content(y) of carbon nanopowder, the maximum discharge capacity increases first and thendecreases and reaches maximum capacity 726.8mAh/g when y=\0, while the cycling stability has little change and then decreases, and when the addtion amount(y) of carbon nanopowder reaches 20, the capacity maintain rate after 10 cycles decreases to 20%. The decay of cycling stability is relative to the serious electrode pulverzation resulting from the decrease of stickiness between particles, which may due to the addition of carbon nanopowder.(3) When CNT is added, the maximum capacity increases and the cycling stability improves.As the content(z) of CNT increases, the maximum discharge capacity increases first and then decreases and reaches maximum capacity of 750.3mAh/g when z=5. The cycling stability increases after addition of CNT, and the capacity maintain rates after 10 cycles are all about 54%, which increases 13% comparing to the alloy without coating CNT. CNT has excellent corrosion resistance, electrical conductivity and function of hydrogen passing channel. This characters result in improvement of electrochemical properties of alloy.