Study On Single-Phase Superlattice Structures And Capacity Fading Mechanism Of La-Y-Ni-Based Hydrogen Storage Alloys

The new La-Y-Ni-based hydrogen storage alloy is expected to become an important material for the development of hydrogen energy technology,and some progress has been made in its application and research,but the relationship between structure and performance of the alloys is not yet clear.This paper systematically studied the structures of AB₃,A₂B₇and A₅B₁₉La-Y-Ni-based single-phase alloys and their effects on solid/H₂and electrochemical properties,and revealed the capacity fading mechanism of the alloys deeply.It was hoped that it can provide scientific basis and technical assistance for the development of La-Y-Ni-based hydrogen storage alloy materials.The composition design of the ternary La-Y-Ni-based single-phase alloys was La Y₂Ni₉（AB₃-type）,La₂Y₄Ni₂₁（A₂B₇-type）and La₅Y₁₀Ni₅₇(A₅B₁₉-type),and the corresponding[AB₅]/[A₂B₄]precursor ratios were 1/1,2/1,and 3/1 respectively.According to the differential scanning calorimetry（DSC）analysis results of the precursor mixture and the basic principle of phase transition,three target single-phase alloys were prepared at 1173K,1353K and 1423K by powder sintering successively.The study on the solid/H₂reaction of three single-phase alloys found that the La₂Y₄Ni₂₁alloy had the largest hydrogen absorption capacity at 313K.With the increase of the[AB₅]/[A₂B₄]subunit ratio,the hydrogen absorption plateau pressure of the alloys increased in sequence,and the stability of corresponding hydride decreased in turn.The single/double platform produced during the hydrogen absorption of the alloys was related to their phase structure and phase abundance.In terms of electrochemical performance,as the proportion of[AB₅]subunits in the alloy increased,the maximum discharge capacity of the alloys gradually decreased,while their cycle stability showed an upward trend.The high-rate discharge（HRD）ability of the alloys was affected by the exchange current density I₀and the hydrogen diffusion coefficient D₀,and D₀played a major role.Among them,the HRD performance of the La₂Y₄Ni₂₁alloy was the best.The amorphization/pulverization and oxidation/corrosion of La-Y-Ni-based single-phase alloys in the cycle process were the main factors which caused their capacity degradation.No matter after electrochemistry or solid/H₂cycle for 50 weeks,the alloys had become amorphous.During the hydrogenation/dehydrogenation process of the alloys,the volume of the[AB₅]and[A₂B₄]subunit cells expanded/contracted,and these two subunits had different volume expansion/contraction rates in the H-dissolved solid solution（αphase）and the hydride（βphase）which could convert into each other.Both of them were closely related to the lattice strain that caused the pulverization of the alloys.The oxidation/corrosion of the alloys was mainly due to active La and Y elements during the electrochemical cycle.In general,with the increase in the ratio of[AB₅]/[A₂B₄]subunits,the degree of amorphization/pulverization and oxidation/corrosion of the three single-phase alloys decreased in turn.