Phase Structure And Electrochemical Properties Of 2H- And 3R-type RE-Mg-Ni-Based Hydrogen Storage Alloys

Rare earth（RE）–Mg–Ni-based alloys are regarded as potential negative materials of nickel-metal hydride（Ni/MH）batteries with characters of high discharge capacity and high power ability.Rare earth–Mg–Ni-based alloys have kinds type of structures,AB₃,A₂B₇,A₅B₁₉ or AB₄ type phases,and each phase has two structural isomers:2H（Hexagonal）and 3R（Rhombohedral）type.Different types of the superlattice phases have close formation temperatures and chemical compositions.Therefore,the prepared alloys always have multiphase structure.Some studies,with effort,have obtained single phase alloys.However,the alloys still contained two configurations.In order to study the formational mechanism and electrochemical characteristics of single 2H-or 3R-type RE–Mg–Ni-based alloys,this work devotes to prepare the single 2H-and 3R-type A₂B₇ phase alloys,single 2H-and 3R-type A₅B₁₉ phase alloys,and studies the structural stabilities based on the volume difference of the two component subunits,[RENi₅]and[REMgNi₄],the effect of different rare earth elements on the formation of 2H-and 3R-type structures,and the effect of 2H-and 3R-type structures on the electrochemical properties of the alloys.Also,2H-type Ce₂Ni₇ or LaNi₅ secondary phase is introduced into the 2H-type A₅B₁₉phase alloy by adjusting molar ratio of sintering precursors and the maximum discharge capacity and high rate dischargeability are improved.Moreover,the formational mechanism of a new 3R-type AB₄ phase and its effect on the electrochemical performance are investigated.Single 2H-and 3R-type A₂B₇ phase La–Nd–Mg–Ni-based alloys are obtained by an induction melting method followed by step annealing treatments.The relationship between structural stability and the component subunit and the effect of structural stability on the hydrogen storage properties of the two compounds are investigated.It is found that the 3R-type compound always maintains a more complete crystal structure than the2H-type compound after hydrogenation/dehydrogenization or charge/discharge cycling.It is found that,after hydrogenation/dehydrogenization cycles,the volume of[A₂B₄]subunit in 3R-type structure shrinks while that of[A₂B₄]in 2H-type structure gets expanded.Meanwhile,the[AB₅]subunits in both structures expand,which have smaller initial volumes than[A₂B₄]subunits.The standard deviation of subunit volumesσ(V_[A2B4],V_[AB5])in 3R-type structure is always smaller than that of 2H-type structure,and theσ(V_[A2B4],V_[AB5])in 3R-type structure is only 0.45?³,far less than that of 2H-type compound（2.11?³）after 50 cycles.The lattice microstrain in 3R-type structure is quite lower than that in2H-type structure and the 3R-type structure possesses a superior structural stability and higher anti-pulverization ability.Transmission electron microscope and X-ray diffraction analyses show that the crystal structure of 3R-type compound is almost unchanged after50 cycles,while,severe lattice deformation is observed in 2H-type structure.After 50cycles,the capacity retention of 3R-type compounds is 100%while that of 2H-type compounds is only 87.44%.Single 2H-and 3R-type A₅B₁₉ phase La_0.60M_0.20Mg_0.20Ni_3.80（M=La,Pr,Nd,Gd）alloys are prepared by powder sintering method.Rietveld refinements of X-ray diffraction patterns find that La_0.80Mg_0.20Ni_3.80 compound has a single Pr₅Co₁₉-type structure.The Ce₅Co₁₉-type phase appears and increases with the decrease of atomic radius of M,until the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound shows a Ce₅Co₁₉-type single phase structure.The cycling stability and high rate dischargeability（HRD）of the alloy electrodes both improve with the increase of Ce₅Co₁₉-type phase.The capacity retention of La_0.60Gd_0.20Mg_0.20Ni_3.80compound at the 100^th cycle is high to 93.6%and the HRD reaches 66.9%at a discharge current density of 1500 mA/g.Moreover after 50 charge/discharge cycles,the Ce₅Co₁₉-type particle retains an intact crystal structure while severe amorphization occurs to Pr₅Co₁₉-type particle as shown in graphical abstract.The cohesive energy obtained from the First-principle calculations is analyzed combined with the experimental results.It is found that the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound with Ce₅Co₁₉-type single phase structure has the highest cohesive energy indicating a more stable structure.This work provides new insights into the superior composition-structure design of La-Mg-Ni system hydrogen storage alloys that may improve the cycling stability.The single Pr₅Co₁₉-type La_0.84Mg_0.16Ni_3.80 alloy has been prepared by powder sintering.To improve overall electrochemical properties of the single phase alloys,Ce₂Ni₇-type or LaNi₅ secondary phase is introduced into the single-phase alloy by adjusting molar ratio of precursors.The maximum discharge capacity increases from 338to 388 mAh/g and the discharge capacity retention at the 100^th cycle increases from 77%to 85.3%when Ce₂Ni₇-type phase abundance increases from 0 to 40.6 wt%.The HRD₁₅₀₀increases to 56%as the LaNi₅ phase abundance rises from 0 to 20.7 wt%.It is proved that the presence of Ce₂Ni₇-type phase in the alloy is more favorable to the maximum discharge capacity and cycling stability while the existence of LaNi₅ phase has a beneficial effect on the HRD.3R-type AB₄ phase La–Mg–Ni-based alloys are prepared through zoning annealing of La_0.78Mg_0.22Ni_3.90 polymorphic alloy.It is found that（La,Mg）₆Ni₂₄ phase forms form the peritectic reaction of the LaNi₅ phase and a liquid phase from（La,Mg）₅Ni₁₉ phase.Field-emission scanning electron microscope analysis find that the（La,Mg）₆Ni₂₄ phase shows diffuse distributing in the alloys and this particular morphology provide plenty of phase boundaries which offer more hydrogen diffusion routes.The high rate dischargeability（HRD）of the alloy electrodes increase with higher（La,Mg）₆Ni₂₄ phase abundance.When the abundance of（La,Mg）₆Ni₂₄ phase rises to 62.0 wt.%,the HRD of the alloy electrode at a discharge current density of 1500 mA/g(HRD₁₅₀₀)reaches 68.0%.Meanwhile,the surface exchange current density（I₀）and hydrogen diffusion coefficient（D）increase to 360.35 mA/g and 2.450×10^–11 cm²/s,respectively.