The Preparation And Properties Of Novel AB₄-type Superlattice Structure Hydrogen Storage Alloy

Rare earth–Mg–Ni-based（RE–Mg–Ni）superlattice structure alloys are new generation negative electrode materials for nickel/metal hydride batteries owing to the advatages of high capacity,long cycling life and good rate performance.They can be devided into several types including AB₃,A₂B₇,A₅B₁₉ and AB₄ phases.Among above superlattice structure alloys,novel AB₄-type alloy is promising candidate resulting from long cyclability and high rate dischargeability.Herein,the AB₄-type RE–Mg–Ni-based alloys were prepared to study the formation condition,phase transformation mechanism and the properties of AB₄-type alloys.Moreover,this work reveals the interaction of[AB₅]and[A₂B₄]subunits inside structure and their impact on the structural stability.Besides,the effect of substituted element including Al,Sm and Nd to the alloy performance is studied.The work is expected to shed the light on the design of negative electrode materials for Ni/MH batteries with long cycling stability and rate capability performance.The ternary AB₄-type La–Mg–Ni alloy is prepared by induction melting method followed by annealing treatment at 1050℃.The preferable cycling stability and rate capability of AB₄-type is closely related with its superlattice structure built by[A₂B₄],[AB₅]-1 and[AB₅]-2 subunits along c axis.Based on the results of Rietveld refinement and calculation of hydrogenation/dehydrogenation cycling,it is found the[A₂B₄],[AB₅]-1 and[AB₅]-2 subunits act differently corresponding to the expansion/contraction during the hydrogen hydrogenation and dehydrogenation process,leading to the volume mismatch.The volume mismatch between[A₂B₄]and[AB₅]-1 subunits is deemed as the main factor that affects the structure stability.More importantly,the[AB₅]-2 subunit away from the[A₂B₄]subunit owns great ability of expansion and contraction which even has smaller volume than original volume（87.300（?）³）after dehydrogenation.Notably,[AB₅]-2 subunits act as transition relieving the mismatch of[A₂B₄]and[AB₅]-1 subunits,contributing to the enhanced structure stability and inhibited amorphization.Benefiting from the improved structural stability,the AB₄-type La–Mg–Ni alloy exhibits superior cycling stability that it delivers 340.0 m Ah/g corresponding to the capacity retention rate of 88.2%after 100 cycles.AB₄-type La_0.78Mg_0.22Ni_3.67Al_0.10 single phase alloy was prepared by induction melting method and followed annealing treatment at 950℃.The formation temperature of AB₄-type superlattice structure of Al-substituted alloy is lower than that of ternary La–Mg–Ni alloy,resulting from the lower melting point of Al compared to Ni.The maximum hydrogen absorption capacity of it is 1.50 wt%at 303 K.At 298 K,the alloy electrode delivers a high electrochemical capacity of 393 m Ah/g,which is comparable to the AB₃-type alloy.AB₄-type La_0.78Mg_0.22Ni_3.67Al_0.10 single phase alloy shows admirable discharge ability at both high temperature and low temperature.It gives 280 m Ah/g and 323 m Ah/g at corresponding temperature of 233 K and 323 K,respectively.Besides,the alloy exhibits superior cycling stability.It delivers 356 m Ah/g with retention rate of 90.6%,after 100 cycles.Further research reveals that element of alloy including La,Mg and Ni were oxidized forming La（OH）₃,Mg（OH）₂ and Ni（OH）₂.While it is Al₂O₃ for Al element.The formed Al₂O₃film on the alloy surface prevents the further oxidation/corrosion of alloy bulk.Moreover,the selective substituted occupation of Al promotes the accommodation of subunits volume mismatch and strengthens the structural stability.Hence,the conclusion can be driven that the La_0.78Mg_0.22Ni_3.67Al_0.10 single phase alloy has great ability of structure stability and antioxidation,showing improved cycling durability.The representative crystal structure of both 2H-type and 3R-type AB₄ superlattice structures are built to further analyze the characteristic of AB₄-type superlattice structure.According to the change of phase abundance in the annealing process at various temperatures,the formation condition and phase transformation mechanism of AB₄-type superlattice structure are studied.It is demonstrated that the AB₄ phase is formed by a peritectic reaction between Pr₅Co₁₉-type（2H）phase and one liquid phase in a range of1000–1010℃.The compound exhibits a high discharge capacity of 391.2 m Ah/g and can reach 286.2 m Ah/g even at a discharge current of 1800 m A/g.The superior high rate dischargeability performance could be attributed to its preferable charge transfer rate and hydrogen diffusion speed.Furthermore,the cycling life of the AB₄ compound is up to 83.3%after 200 cycles.The cycling stability and rate performance of AB₄-type single phase alloy are superior than Pr₅Co₁₉-type single phase alloy.The effect of Sm element and cooperative Sm and Nd element was studied based on thedesignedAB₄-typeLa_0.65Sm_0.15Mg_0.20Ni_3.50Al_0.12and La0.64Sm0.08Nd0.08Mg0.20Ni3.64Al0.10 alloys.The AB4-type La0.65Sm0.15Mg0.20Ni3.50Al0.12alloy shows preferable cycling stability that its capacity retention rate is up to 91.2%after100 electrochemical cycles,resulting from the substitution of Sm.The alloy performs commendable at low temperature.At 243 K,233 K and 223 K,the discharge capacities are corresponding to 259.8,220.3 and 82.0 m Ah/g,respectively.After substitution by cooperative Nd,the high rate dischargeability of obtained AB₄-type La_0.64Sm_0.08Nd_0.08Mg_0.20Ni_3.64Al_0.10 alloy is further promoted.At the discharge current density of 5C,the discharge capacity of AB₄-type alloy is up to 308.0 mAh/g.