Effect And Synergistic Effect Of Y And Mn On Microstructure And Electrochemical Properties Of A₂B₇ La-Y-Ni Based Hydrogen Storage Alloy

Nowadays,AB_3.0³.8 type rare earth-La-Mg-Ni-based hydrogen storage alloy with superlattice structure has been extensively investigated because of its moderate hydrogen absorption-desorption plateau and higher electrochemical discharge capacity.However,lower melting point?648.8?and higher saturated vapor pressure of Mg have a negative influence on smelting process of La-Mg-Ni-based alloys,in which it is extremely easier to volatilize,to generate dust and to explode.This means that it not only involves great safety risk and increased preparation cost,but also makes it difficulty in controlling the Mg content,the phase composition of the alloy and the consistency between electrode and battery performance.The research and development?R&D?of new A₂B₇ type La-Y-Ni-based hydrogen alloy characterized by small hydrogen-induced amorphization tendency,easy activation,high electrochemical discharge capacity and relatively safe pyrometallurgical processing has been putting high value on and has excellent application prospects as a candidate of anode material for nickel-hydrogen battery.However,there is still need to improve charge-discharge cycle stability,and to optimize its performance and composition of La-Y-Ni-based alloy electrode due to the insufficient research on its hydrogen storage,electrochemical performance and alloying effect.It has been shown that Y and Mn element played important roles in optimizing the hydrogen storage and electrochemical properties of La-Y-Ni-based hydrogen storage alloys.Whereas,there is no literature on the synergy between Y element and Mn element,making a difference on the microstructure and electrochemical properties of alloys.Therefore,A₂B₇-type La_1-x-x Y_x Ni_3.4-z.4-z Mn_z Al_0.1?x=0¹;z=0⁰.5?alloy was prepared by vacuum arc melting furnace and heat treatment method.The microstructure and electrochemical properties of this alloy was then tested and analysed by material research methods such as SEM,XRD,EDS etc.and electrochemical performance test methods,aiming to investigate the synergistic effect of Y and Mn on the microstructure,phase structure,hydrogen storage and electrochemical properties of the alloy systematically.The results as follows:?1?The main phase of La_1-x-x Y_x Ni_3.4-z.4-z Mn_z Al_0.1?x=0.3¹;z=0⁰.5?alloy consisting of PuNi₃ phase,CeNi₃ phase,Ce₂Ni₇ phase,Gd₂Co₇ phase,Ce₅Co₁₉ phase and LaNi₅phase,was Ce₂Ni₇ phase.The cell parameters?a,c?and cell volume?V?of Ce₂Ni₇phase showed a gradually-decreased trend with the increase of Y content,and witnessed an opposite trend with the increase of Mn content.?2?When the content of Y element was lower?x<0.5?,it was necessary for the amorphization to be inhibited by adding Mn element during the process of hydrogen absorption-desorption.When the content of Y element was higher?x?0.7?,this amorphization phenomenon can be effectively inhibited by the Y element alone.The pressure of hydrogen absorption-desorption plateau gradually increased with the increase of Y content,and gradually decreased with the increase of Mn content.?3?La_1-x-x Y_x Ni_3.4-z.4-z Mn_z Al_0.1?x=0.3¹;z=0⁰.5?alloy electrodes had good activation properties and the maximum discharge capacity can be reached after 2⁵times activation processes;With the increase of single element or both of them,the maximum discharge capacity of the alloy electrode increased first and then decreased after peaking.When x=0.7 and z=0.15,the alloy electrode had its maximum discharge capacity at 390.4 mAh/g.The addition of Y element was adverse to the stability of charge-discharge cycle of the alloy electrode,while the influence that Mn had was closely related to the content of Y.The cycle stability of the alloy electrode can be deteriorated by adding Mn when Y content was lower or maximum?x?0.5,x=1?.The cycle stability can be improved by a small amount of Mn?z=0.15?and cut down by higher Mn?z>0.15?when Y content was higher?0.7?x?0.85?.When the content of Y element and Mn element increased simultaneously,the charge-discharge cycle stability(S₁₀₀)was on the in decline,from 95.09%to 15.60%.The high rate discharge ability was controlled by the diffusion of hydrogen atoms in the alloy phases.When x=0.7 and z=0.2,the high rate discharge ability(HRD₉₀₀)optimized at 87.73%.Overall,the performance of the La-Y-Ni hydrogen storage alloys can be further optimized through the synergy between the Y element and the Mn element.By coordinating the content of Y element and Mn element in the alloys,amorphization in the process of hydrogen absorption-desorption can be inhibited,hydrogen absorption-desorption performance can be improved,the maximum discharge capacity and stability of the charge-discharge cycle and ability of high rate discharge can be improved.