Investigation On The Structrual And Electrochemical Properties Of La-Y-Ni-Based Superlattice Alloys

The superlattice structures consist of[AB₅]and[A₂B₄]subunits with different stacking ratio and mode.It combinies the high rate capability of AB₅-type alloys with the high discharge capacity of AB₂-type alloys.The Rare earth-Mg-Ni-based superlattice alloys have been developed as the third-generation negative electrode materials for Ni-MH batteries.By replacing Mg with rare earth Y,La-Y-Ni-based superlattice alloys could overcome the problems of easy volatility of low melting point Mg elements and significant safety hazards in the preparation process.Although La-Y-Ni-based superlattice alloys have been widely concerned and studied,their cyclic stability still can not meet the requirements of commercial applications.In addition,La-Y-Ni-based alloys tend to form complex multi-phase and different polymorphs,so the relationships between alloy composition,structure and electrochemical properties are challenging to investigate accurately,and the capacity degradation mechanism is still unclear.In this paper,the single-phase A₂B₇-,A₅B₁₉-,and AB₄-type La-Y-Ni-based superlattice alloys were prepared by optimizing the annealing or quenching temperatures,and the single-phase generation mechanism and superlattice structure characteristics of each phase are systematically investigated.The effects of Mn,Al and Zr element substitutions on the superlattice structure and cyclic stability are investigated.The structural evolution of different superlattice structures during the charging/discharging processes and its effect on the electrochemical properties are revealed,which provides theoretical guidance for the development of high-performance rare-earth superlattice electrode alloys with high capacity and long life.The main conclusions are as follows:The A₂B₇-type superlattice structure is stacked by[AB₅]and[A₂B₄]subunits in the ratio of 2:1,including rhombohedral 3R-Gd₂Co₇ and hexagonal 2H-Ce₂Ni₇ phases.The structural stability of the individual phase after hydrogen absorption/desorption has not been comparatively studied in the A₂B₇ La-Y-Ni-based alloy.By gradually increasing the annealing temperature,the 3R-Gd₂Co₇ and 2H-Ce₂Ni₇ phases are separated for the first time in the A₂B₇-type La Y₂Ni_10.5 alloy,and the single-phase 3R-and 2H-La Y₂Ni_10.5 alloys were obtained at 1000°C and 1100°C,respectively.Y both the 3R and 2H phases preferably occupies the[A₂B₄]subunit,and the[A₂B₄]subunit volumes(V_[A2B4])of these two phases are 90.04?³and 88.98?³,respectively.and the subunit volume differences(（35）V_s=V_[A2B4]-V_[AB5])of the3R and 2H phases are 3.62?³ and 2.36?³,respectively.The smaller V_[A2B4]with（35）V_s render the 2H-La Y₂Ni_10.5 alloy better structural stability in hydrogen absorption/desorption,with a maximum hydrogen absorption content of 1.49 wt.%and reversible content of 1.29 wt.%.However,the discharge capacity（219.9 m Ah/g）and cycling stability(S₁₀₀=52.9%)of the2H-La Y₂Ni_10.5 alloy still need further improvement.To improve the electrochemical capacity and cycling stability of 2H-La Y₂Ni_10.5 alloy,three series of single-phase La Y₂Ni_10.5-xMn_x（x=0.4,0.5,0.6）,La Y₂Ni_10-yMn_0.5Al_y（y=0.1,0.3,0.5）and La Y_2-zZr_zNi_9.7Mn_0.5Al_0.3（z=0.1,0.2,0.25）with 2H-Ce₂Ni₇ phase are prepared.The selective occupation of Mn and Al in the[AB₅]subunit reduces the volume difference between the[A₂B₄]and[AB₅]subunits,thus improving the cyclic stability.The maximum discharge capacity of the La Y₂Ni_9.7Mn_0.5Al_0.3 alloy is 384.1 m Ah/g,and the capacity retention S₂₀₀ is as high as 76.1%.However,The Zr with a smaller atomic radius selectively occupies the[A₂B₄]subunit,making the volume difference La Y_1.75Zr_0.25Ni_9.7Mn_0.5Al_0.3 alloy zero,but significantly reduces the discharge capacity and cycling stability.Structural analysis of the charging and discharging process shows that the volume expansion of the[A₂B₄]and adjacent[AB₅]subunits along the a axis,causing a larger cell volume expansion ratio and greater lattice strain.In contrast,for the Mn,Al substituted alloys,the volume expansion due to subunit asynchronous hydrogen absorption occurs mainly in the c-axis direction.Therefore,reducing the subunit volume difference is not the only consideration to improve the stability of the superlattice structure.The A₅B₁₉-type superlattice structure is stacked by[AB₅]and[A₂B₄]subunits in the ratio of 3:1,and the La₅Ni₁₉ phase is thermodynamically unstable.The main phase of the current A₅B₁₉-type La-Y-Ni-based alloy is the rhombohedral 3R-Ce₅Co₁₉ phase.In order to improve the cyclic stability,the A₅B₁₉-type single-phase(La_0.33Y_0.67)₅Ni_17.6Mn_0.9Al_0.5 superlattice alloy with a hexagonal 2H-Pr₅Co₁₉ is prepared by quenching for the first time.By gradually increasing the quenching temperature,the crystallization of the hexagonal 2H-Ce₂Co₇ phase can be suppressed,the transformation of the rhombohedral 3R-Ce₅Co₁₉ phase into the2H-Pr₅Co₁₉ phase can be promoted,and the single-phase(La_0.33Y_0.67)₅Ni_17.6Mn_0.9Al_0.5 alloy with 2H-Pr₅Co₁₉ structure can be obtained at 1100℃.The single-phase(La_0.33Y_0.67)₅Ni_17.6Mn_0.9Al_0.5 alloy exhibits the best electrochemical hydrogen storage performance,including the highest discharge capacity C_max=368.8 m Ah/g,the best cycling stability S₂₀₀=73.4%and the best high rate performance HRD₁₅₀₀=58.6%.The[AB₅]subunits in the superlattice structure,including[AB₅]-1 and[AB₅]-2 two types.The lattice strain in the hydrogen absorption process originates from the asynchronous hydrogen absorption of[A₂B₄]with the non-neighboring[AB₅]-2 subunit,and its maximum lattice strain occurs in the?hydride phase formed at 50%of the charging depth.The AB₄-type superlattice structure is stacked by[AB₅]and[A₂B₄]subunits in the ratio of 4:1,and the AB₄-type La-Y-Ni-based alloys have not been reported.In this paper,AB₄-type La-Y-Ni-based superlattice alloys are prepared by quenching.For the first time,the rhombohedral 3R-La Ni₄ phase is discovered,generated from La Ni₅and La₅Ni₁₉ by the peritectic reaction.However,the hexagonal La Ni₄ phase is not found.The space group of the3R-La Ni₄ phase is R^—3m,with the cell parameters a=5.0435?,c=60.895?,and V=1341.483?³.The maximum hydrogen content of the single-phase La_0.4Y_0.6Ni₄ alloy is 1.54wt.%,but the hydrogen amorphization leads to reversible hydrogen content of only 0.95 wt.%and electrochemical capacity of only 45.8 m Ah/g.The addition of Al elements inhibits the generation of the 3R-La Ni₄ phase,while the addition of Mn elements inhibits the hydrogen amorphization of the 3R-La Ni₄ phase.The La_1.2Y_1.8Ni₁₁Mn alloy with a major 3R-La Ni₄phase of 91.4 wt.%is prepared at 1125°C by quenching,and its discharge capacity increases to 310.1 m Ah/g.