Hydrogen Storage Properties Of La-Y-Ni A₂B₇ Alloy And Effect Of Rare Earth Element Y

Exploring new hydrogen storage alloy anode materials is the key to the development of high-capacity and high-power Ni-MH batteries.Although the La-Mg-Ni superlattice hydrogen storage alloy has high electrochemical discharge capacity and good dynamic performance,the magnesium element in the alloy system is easy to corrode,volatilized and has hidden danger in the process of industrial smelting,which restricts the outstanding problems of its practical application.In this paper,on the basis of a systematic review of the current research status of La-Mg-Ni based hydrogen storage alloys at home and abroad and previous works,a new type of magnesium free super lattice structure A₂B₇ La_1-xY_xNi_3.5?x=0-1?,La_1-xY_xNi_3.25Mn_0.15Al_0.1?x=0-1?and La_0.15Y_0.85Ni_3.35-xMn_0.15Al_x?x=0-0.5?annealed alloys are designed with the replacement of the rare earth element Y with Mg.A variety of experimental and analytical methods are used to systematically study the effects of A-side rare earth element Y,B-side Mn and Al elements on the microstructure,hydrogen storage behavior and electrochemical performance of the annealed alloy.The following results are obtained:Firstly,the effects of the content of different Y elements at the A-side on the phase structure,hydrogen storage and electrochemical properties of La_1-x-x Y_xNi_3.5?x=0-1?annealed alloy are studied systematically.The results show that the microstructure of the annealed alloys is a multiphase structure.Except for x=0.67,the main phase of the alloy is AB₅-type phase,while the main phases of the other alloys are A₂B₇-type phases.The PCT curves of the alloys have higher plateau pressure,larger slope of the platform,and obvious hysteresis of hydrogen absorption and desorption.The plateau pressure of hydrogenation is in the range of0.445-3.190 MPa and the maximum hydrogen absorption amount is 1.033-1.315 wt%.The hydrogen absorption rate of the alloy is controlled by the step of the diffusion of hydrogen atom in the alloy bulk phase.The results of electrochemical analysis show that the discharge capacity of the alloy electrode is 83.3-184.4 mAh/g and the capacity retention rate is S₁₀₀=74-95%after 100 cycles of charge and discharge.Secondly,the effects of Y elements at the A-side,Mn and Al elements at the B-side on thephasestructure,hydrogenstorageandelectrochemicalpropertiesof La_1-xY_xNi_3.25Mn_0.15Al_0.1?x=0-1.0?annealed alloy are studied systematically.The results show that the main phase of all the annealed alloys was Ce₂Ni₇-type.With the increase of Y content,phase abundance of the Ce₂Ni₇-type phase increases firstly and then decreases,meanwhile the cell volumes decrease gradually.The x=0.0-0.25 alloys are easy to form hydrogen-induced amorphization?HIA?,resulting in the disappearance of the plateau of PCT curves.When x?0.5,the alloys can effectively avoid the HIA,the PCT curves obviously present an approximately single platform,the maximum hydrogen storage capacity and the plateau pressure of hydrogenation is in the range of 1.418¹.48wt.%and 0.026-0.097MPa,respectively.Therefore,the hydrogen storage performance has been greatly improved.The electrochemical measurement results showed that the x=0.67-0.75 alloys exhibited the high discharge capacity of 376³81mAh/g and the cyclic stability of 80⁸5%after 100 cycles.The high rate discharge ability(HRD₉₀₀)of the alloys are 64.5⁸5.7%and the control step of the reaction kinetics of the electrode is mainly controlled by the diffusion rate of the hydrogen atom in body phase of the alloys.In summary,when the rare earth Y content is x=0.67,the alloy has good hydrogen storage and comprehensive electrochemical properties.Finally,the effects of different Al elements at B-side on the phase structure and electrochemical performance of La_0.15Y_0.85Ni_3.35-xMn_0.15Al_x?x=0-0.5?annealed alloy are studied systematically.The results show that the main phase of all the annealed alloys is2H-Ce₂Ni₇ type.With the increase of Al content,the phase abundance of 2H-Ce₂Ni₇ type phase decrease firstly,then increase and last decrease.When x=0.2,the phase abundance of2H-Ce₂Ni₇ type phase is the highest?79.83 wt.%?.The electrochemical measurement results showed that the alloy electrodes have good activation properties,the maximum discharge capacity is 368.8 mAh/g?x=0.2?,the capacity retention rate S₁₀₀ is 41.68-93.95%after 100cycles and the high rate discharge ability(HRD₉₀₀)of the alloys are 65.92-89.62%,the kinetics control step of the alloy electrode is mainly controlled by the diffusion rate of the hydrogen atom in body phase of the alloys.