Preparation Of Mg-Ni Hydrogen Storage Alloys And Their Microstructure And Electrochemical Properties

Magnesium based hydrogen storage alloys have become to be one of the mostpromising Ni-MH battery negative materials because of their advantages such as hightheory hydrogen storage capacity, low density, low cost, no pollution. However, lowpractical electrochemical capacity, dynamic performance，poor anti-corrosion abilityin alkaline solution and other factors limit its application in nickel-metal hydridebattery．In this paper，the MgxNi（x=1,1.5,2; at%）, Mg₂Ni+x wt%graphite（x=0,5,10）and Mg₂Ni_0.9M_0.1（M=Mo, Sn） alloys had been synthesized by mechanical alloying.The phase structure, surface configuration and the electrochemical performances ofthese alloys were analyzed. The influences of magnesium nickel ratio, element dopingand surface coating treatment on the Mg-Ni alloy electrode electrochemicalperformance were discussed. The obtained results show that:（1） After ball-milled for120h，the MgNi fully amorphous alloy powder isobtained at300r/min Speed，while the ball to powder ratio is20:1. Lowering of Mg/Ni ratio is useful to form amorphous phase for Mg-Ni alloy in the milling process. Inthe three alloys， amorphous Mg-Ni alloy showed the best electrochemicalperformance and the fine cycle stability. After10cycles the discharge capacity is still278mAh/g, in which the capacity remaining rate is64percent. The cyclicvoltammetry curves of the alloy electodes are measured，and the diffusion coefficientsof hydrogen in these electrodes are calculated. The results show that: The reaction ofMg-Ni hydrogen storage alloy electrode is diffusion-controlled process. The hydrogenshows a relatively strong diffusion ability in amorphous MgNi alloy electode，itsdiffusion coefficient is1.98×10^-8cm²/s.（2） Graphite was milled and coated on the surface of Mg₂Ni alloy, the resultsshow that the addition of graphite not only improves the discharge capacity of theMg₂Ni alloys, but also improves anti-corrosion performance. The maximum dischargecapacities of the alloy electrodes added5wt%and10wt%graphite are340mAh/gand323mAh/g, the capacity remaining rates after10cycles are62.9%and48.9%,after15cycles, they reach41.2%and37.2%，respectively. The analysis shows that thecontinuous refinement of the powder grains, powders of lamellar structure and thesurplus of nanocrystalline Ni are the main reasons that the alloys milled and coated with graphite exhibit excellent electrochemical performance.（3） The replacement of Mo and Sn can significantly improve the dischargecapacity and cycle stability of the Mg₂Ni alloys. Under the same milling time，thealloy powder particles which is the alternative of partial substitution of Sn on Ni issmaller than the substitution of Sn, the distribution of particle size is also moreuniform. Completely amorphous Mg₂Ni0.9Sn0.1alloy can be obtained after160hball-milled. The alloy electode shows the best anti-corrosion performance.