Effects Of Mg And Mn Substitution On Electrochemical Properties Of La-Y-Ni Hydrogen Storage Alloys

At present,the increasing application demands put forward higher requirements for the electrochemical capacity,cycle stability and high rate discharge performance of hydrogen storage alloy,and it is urgent to develop new hydrogen storage alloy electrode materials.The kind of the elements has significant influence on its phase compositon and electrochemical performance.At present,it is urgent to develop new negative electrode materials due to the increasing demand for applications.In this thesis,the La-Y-Ni system alloys which were chosen by the thermodynamic phase diagram were investigated by Mg or Mn substitution for Y or Ni in order to solve the hydrogen-induced amorphization（HIA）behavior of the alloys.The mechanism of Mg or Mn substitution on the phase composition and electrochemical properties of La-Y-Ni type hydrogen storage alloys was expounded.The main research contents and conclusions of this thesis are as follows.Firstly,a series of alloys were designed and prepared based on the ternary thermodynamic phase diagram of RE-Mg-Ni（RE=Nd,Y,and Ce）system.According to the electrochemical capacity of these alloys,the electrochemical capacity diagrams of RE-Mg-Ni（RE=Nd,Y,and Ce）system were calculated by matrix operation.The relationship between component,phase and electrochemical capacity was established,which was used to guide the design of hydrogen storage alloys.According to the electrochemical capacity diagram,the electrochemical capacity of long-period stacking structure phase and other magnesium rich phases in the magnesium rich angle region is low,which is not suitable for electrode materials.The AB₂-type alloys with Mg solid solubility have a higher discharge capacity,such as Nd Mg Ni₄ and YMg Ni₄alloys.The AB₂-type alloys without Mg solid solubility have a lower discharge capacity,such as Ce Ni₂ alloy.According to the analysis of Y-Mg-Ni ternary phase diagram and the mechanism of hydrogen-induced amorphous behavior of La Y₂Ni₉ alloy,Mg element is adopted to suppress the hydrogen-induced amorphous behavior by its dissolution in[YNi₂]unit.It is found that Mg substitution of Y inhibited the HIA behavior of La Y₂Ni₉ phase,which increased the maximum discharge capacity of the alloy by 37.1%.Moreover,Mg substitution also helps enhance the high rate discharge ability（HRD）by the formation of（La,Y,Mg）₂Ni₇ phase in La Y_1.25Mg_0.75Ni₉ alloy whose HRD₁₅₀₀ can reach 70.8%.La_5.42Y_18.50Ni_76.08 alloy with AB₃ and A₂B₇ multi-phase mixed structure were designed according to La-Y-Ni ternary phase diagram.The influences of Mn substitution of Ni on the HIA behavior and the electrochemical properties of this alloy have been studied.The increase amount of Mn is not only conducive to the formation of the La Y₂Ni₉ phase,but also significantly suppress the HIA of it.Therefore,the maximum discharge capacity of La_5.42Y_18.50Ni_70.08Mn₆ alloy increased by 37.9%compare with La_5.42Y_18.50Ni_76.08 alloy.It is also found that the suppression of the HIA improves the cyclic stability of the alloy electrodes.However,it also causes deterioration of the high rate discharge performance of the alloy electrodes.The HRD₉₀₀decreased from 35.8%to 7.7%.The La Y_1.25Mg_0.75Ni_9-xMn_x（x=0,0.5,0.7）and La_0.71Y_2.16-xMg_xNi_8.3Mn_0.7（x=0,0.75,1.00）alloys were designed to explore the mechanism of the substitution of Mg and Mn on the phase and electrochemical properties of La-Y-Ni alloys.The results show that the discharge capacity of the alloy electrodes increases first and then decreases under the combined effect of phase abundances and unit-cell volume.The discharge capacity of alloy electrode reaches the maximum value when Mn content x=0.5.But the values of HRD₉₀₀ decreased from 68.8%（x=0）to 36.6%（x=0.7）for the decrease of the hydrogen diffusion coefficient with the increase of Mn content.However,the effect of Mg content on the alloy electrodes is opposite to Mn.The maximum discharge capacity of the alloy electrodes reduced about 60.8%,but the values of HRD₉₀₀ increased from 7.7%to 82.3%with the increased Mg content.