Study On Phase Structure And Electrochemical Properties Of La_0.75Mg_0.25?Ni,Co?_x Hydrogen Storage Alloy

Nickel-hydrogen batteries have been successfully applied in the field of new energy sources due to advantages of pollution-free,high power,good stability and high energy density.Hydrogen storage alloys,as the anode materials of Ni-MH batteries,are particularly critical to enhance the competitiveness of China in the new energy.The new La-Mg-Ni hydrogen storage alloys have broad application prospects,but the cyclic stability is low.The research shows that the alloys can be improved by changing the stoichiometric ratio.However the research on stoichiometric ratio is focused on?3-3.5?,and that with stoichiometric ratio?3.6-4.0?is less now.In view of the above problems,the effects of different stoichiometric ratios,subsequent element substitution and heat treatment on the properties of La-Mg-Ni hydrogen storage alloys were studied.The as-cast La_0.75Mg_0.25(Ni_0.83Co_0.17)_x?x=3.6,3.7,3.8,3.9,4.0?alloys were studied.It is found that the alloys consisted of multiphase phases,including LaNi₅,A₅B₁₉ and CeNi₂phases.With the increase of stoichiometric ratio,the abundance of A5B19 phase increases first and then decreases,and reaches 64.51wt%at x=3.9.The maximum discharge capacity of alloy electrodes C_maxis the same as that of A₅B₁₉9 phase abundances.The maximum discharge capacity is 372.2mAh/g?x=3.9?at x=3.9.However,with the increase of stoichiometric ratio,the cyclic stability of alloy electrodes is not good.The capacity retention rate of 100 cycles is reduced from 68.92%?x=3.6?to 65.43%?x=4.0?.The experimental results show that the high rate discharge performance of alloy electrodes is controlled by the exchange current density on the surface of the electrodes,and the high rate discharge performance can be improved by properly increasing the metering ratio.Among the alloys studied,when x=3.9,the comprehensive performance is the best,when C_maxis 372.2 mAh/g,S₁₀₀ is 65.54%,and HRD₉₀₀ is 83.33%at 900 mA/g.The as-cast La_0.75Mg_0.25(Ni_0.83Co_0.17)_x alloys were annealed?1123K,6h?.After annealing,the maximum discharge capacity of the alloy electrode increases significantly compared with that of the as-cast alloy.The maximum value of C_max is 386.6mAh/g at x=3.9.The cyclic stability of alloy electrodes is the highest when x=3.7,and the cyclic stability of S₁₀₀ is 82.76%.The experimental results show that the high rate discharge performance of annealed alloy electrodes is mainly controlled by the diffusion rate of hydrogen atoms in the alloy electrodes.Among the annealed alloys,the comprehensive properties are the best when x=3.8,when C_maxax is 383.1mAh/g,S₁₀₀00 is 75.98%,HRD₉₀₀ is 92.8%.The as-cast and annealed La_0.75Mg_0.25?Ni,Co?_3.9?Ni/Co=1,2,5,9,19?alloys were studied.It is found that the maximum discharge capacity of alloy electrodes first increases and then decreases with the increase of Ni/Co ratio.The maximum values of C_max in both alloys were obtained at Ni/Co=5.Reducing the ratio of Ni/Co properly can improve the cycle stability of alloy electrode,but it is not conducive to the activation of alloy electrode.For example,when Ni/Co=1,the annealed alloy electrode S₁₀₀ reaches 87.76%,but the activation needs five charge-discharge cycles.The high rate discharge performance of alloy electrodes in both States is controlled by the exchange current density,which increases first and then decreases with the increase of Ni/Co value.The results show that the comprehensive performance of the alloy electrode is better when Ni/Co=5⁹.For example,when annealed alloy is Ni/Co=9,C_max is 363.9mAh/g,S₁₀₀ is 74.58%,HRD₉₀₀ is 90.46%.La_0.75-xCe_xMg_0.25Ni_3.25Co_0.65?x=0,0.05,0.10,0.15,0.20?alloys as cast and annealed were studied.It is found that adding Ce element could promote the formation of LaNi₅ phase in the alloys,and the cell volume and various parameters of the alloys decreased significantly.The maximum discharge capacity decreases with the increase of Ce substitution.The maximum discharge capacity of as-cast and annealed C_max alloys is obtained at x=0.The experimental results show that the cyclic stability of alloy electrodes can be significantly improved by adding Ce.For example,as-cast alloy S₁₀₀ increases gradually from 65.54%?x=0?to 81.19%at x=0.20.The high rate discharge performance of alloy electrodes first increases and then decreases with the increase of Ce replacement.The HRD₉₀₀ of as-cast and annealed alloys reaches the highest value at x=0.05,90.07%and 90.16%respectively.