Effects Of Partial Substitution Of A-side Elements On Electrochemical Properties Of Superlattice La-Mg-Ni-Based Hydrogen Storage Alloys

In order to meet the energy demand,human beings have invested a lot of efforts in the utilization of clean and renewable energy.In the past decade,a new type of hydrogen storage alloys,AB3.0-4.0 type superlattice alloys,has been developed.Such alloys have a superlattice structure consisting of[A2B4]and[AB5]sublattices stacked alternately along the c-axis.The coexistence of[A2B4]and[AB5]sublattices endows the alloy system with good activation properties and high energy density.However,the complex stacking structure also adversely affects the structural stability of superlattice alloys.Element substitution is an effective method to improve hydrogen storage alloys.In this paper,as-cast multiphase La-Mg-Ni-based hydrogen storage alloys,AB3-type single-phase La0.7R0.1Mg0.2Ni3（R=La,Pr,Nd,Gd）hydrogen storage alloys,A2B7-type single phase La0.75-xGdxMg0.25Ni3.5（x=0,0.05,0.1,0.15）alloy were prepared by strictly controlling the preparation temperature and element composition.The structure of the alloy was studied by XRD,SEM,TEM and other structural characterization methods,combined with the electrochemical properties of the alloy,the effect of element substitution on the structure and properties of the superlattice alloy was analyzed.The research results are as follows:In the as-cast multiphase La-Mg-Ni-based hydrogen storage alloys,the unit cell parameters each phase decrease in the order of La>Pr>Nd>Gd.All alloys have good activation properties.The discharge capacity increases in the order of Gd>Nd>Pr>La.The cycle stability of the alloy is best when La is partially substituted by Pr,which is attributed to the reduction of internal stress and pulverization by a small amount of LaNi5 phase.In AB3-type single-phase La-Mg-Ni-based hydrogen storage alloys,when Gd element is substituted,the sublattice volume difference between[A2B4]and[AB5]is significantly reduced,and it has stronger oxidation/corrosion resistance;During the hydrogenation/dehydrogenation process,the sublattice volume mismatch of the R=Gd alloy is small.Correspondingly,the R=Gd alloy has smaller microstrain and better crystal structure after 100 cycles,and the capacity retention is improved from 52.0%（R=La）to 67.6%（R=Gd）.However,limited by the structural defects of the AB3-type phase structure itself,the cycle stability of the alloy is still unsatisfactory.For A2B7-type single-phase La0.75-xGdxMg0.25Ni3.5（x=0,0.05,0.1,0.15）series alloys,the crystal volumes of[A2B4]and[AB5]sublattice of La0.6Gd0.15Mg0.25Ni3.5 alloy are almost equal.The sublattice volume mismatch between[A2B4]and[AB5]of the La0.6Gd0.15Mg0.25Ni3.5 alloy during the hydrogenation/dehydrogenation process is only half that of the original alloy,and it has a stronger oxidation/corrosion resistance.The capacity retention is improved from 82.1%（La0.75Mg0.25Ni3.5）to 88.2%（La0.60Gd0.15Mg0.25Ni3.5）without sacrificing the maximum discharge capacity,while improving the high-rate discharge capability.It provides theoretical guidance for the preparation of La-Mg-Ni-based hydrogen storage alloy electrode materials with good electrochemical performance and structural stability.