Phase Structure And Electrochemical Properties Of High Capacity RE-Mg-Ni-Based Hydrogen Storage Alloys

AB₃-and A₂B₇-type RE–Mg–Ni-based hydrogen storage alloys are regareds as promising negative materials of nickel-metal hydride?Ni/MH?battery due to the advantage of the high power capacity.However,the poor cycling stability of the alloys is the issue to be sovled.The close composition and formational temperature of each superlattice structure in RE–Mg–Ni-based is the foot cause,which leads to the unclear relationship between the phase compositions and electrochemical properties of the alloys because of the complicated multiphase structure.Therefore,to fully reveal the relationship,the phase formation mechanism and electrochemical properties of AB₃-and A₂B₇-type single-phase superlattice La–Mg–Ni alloys are studied.Based on the results,the effects of addition of Nd and Sm,and Mg content on the phase formation and electrochemical properties of RE–Mg–Ni alloys are revealed.In addition,a new type Sm–Mg–Ni alloy has been prepared,and its phase structure and hydrogen storage characteristics are studied.PuNi₃-type La_0.67Mg_0.33Ni₃ and Ce₂Ni₇-type La_1.5Mg_0.5Ni₇ alloys have been prepared by an induction melting mehod followed by annealing treatment,and the formation mechanism and electrochemical properties of the single-phase alloys have been studied.It is found that the main phase of the as-cast La_0.67Mg_0.33Ni₃ and La_1.5Mg_0.5Ni₇ alloys are PuNi₃-and Ce₂Ni₇-type phases,respectively,and both of the alloys contain the minor CaCu₅-,Ce₅Co₁₉-,Gd₂Co₇-and MgCu₄Sn-type phases.Non-superlattice CaCu₅-and MgCu₄Sn-type phases can convert to superlattce phases at low temperatures,and the Ce₅Co₁₉-type minor phase transforms to the main phases at 900??C by petiactic reaction.The Gd₂Co₇-type phase can derectly transform to PuNi₃-type phase at 950??C by petiactic reaction for the La_0.67Mg_0.33Ni₃,but decompose to Ce₅Co₁₉-type phase firstly in the range of 925–935??C and then the new formed Ce₅Co₁₉-type phase petiacticly reacts to form the Ce₂Ni₇-type phase at 950??C for the La_1.5Mg_0.5Ni₇.Both of the single-phase La_0.67Mg_0.33Ni₃ and La_1.5Mg_0.5Ni₇ alloys show superior electrochemical discharge capacities,of which are401 and 394 mAh/g,respectively.The cycling life at the 100 thcycle of the La_1.5Mg_0.5Ni₇ alloy is much higher than that of the La_0.67Mg_0.33Ni₃ alloy,which is 83.7% and 76.3%,respectively.Besides,we found the the capacity attenuation of the single-phase La_1.5Mg_0.5Ni₇ alloy is mainly due to the loss of active material at the alloy surface caused by oxidization of La and Mg.Oxidation of La occurs prior to that of Mg.La hydroxide grows from nano-structured needles to larger-scaled rods then to unformed lamellar hydroxide,whereas the precipitation of Mg forms as irregular lamellae inlaid with hexagonal flakes.The effect of addition of rare metal Nd and Sm on phase structural formation and electrochemical properties of AB₃-and A₂B₇-type La–Mg–Ni-based alloys have been studied.It is found that partial substitution of Nd for La benefits the formation of Gd₂Co₇-type phase in the La0.55Nd0.12Mg0.33Ni₃.0 alloy.Nd mainly replace La in [AB₅]slabs in the PuNi₃-type structure,which increases the volume ratio of [A₂B₄] slabs in the cell and decreases the volume change of the [A₂B₄] slabs during hydrogenation/dehydrogenation process,thus contributing to the improvement in cycling stability.Moreover,Nd substitution helps to the enhancement in high rate dischargeability?HRD?.Addition of Sm into La_1.5Mg_0.5Ni₇ alloy promotes the Gd₂Co₇-type phase formation with increasing annealing period,which the phase abundance of Gd₂Co₇-type phase reaches to 73.2 wt.%.The A₂B₇-type La_0.60Sm_0.15Mg_0.25Ni₃.4 alloy is superior in discharge capacity?382 m Ah/g?and HRD?40% at a 1500 mA/g discharge current density?.Especially,the cycling stability of the alloy is enhanced to 87.7% at the 100 thcycle.The improved cycling stability of the alloy is mainly attributed to its slower growth of inner strain during charging/discharging,which improves anti-pulverization and anti-amorphization abilities,and drops oxidation degree of the alloy.The solid solubility of Mg in the Nd1-xMgxNi₃?x = 0.10–0.50?alloys and the effect of Mg content on crystal transformation and electrochemical characteristics of La_2-xMg_xNi₇?x= 0.40–0.60?alloys have been studied.It is found that the maximum solid solubility of Mg in the Nd_1-xMg_xNi₃ is 0.36 at 1183 K with the PuNi₃-type single-phase structure.The alloy inclines to form the superlattice structure with more ratios of [A₂B₄] subunits with the increasing Mg content,of which the alloy contains of the minor Ce₂Ni₇-type and MgNi2 phase as the secondary phase,being with less and more content of the solid solubility Mg,respectively.The PuNi₃-type single-phase Nd0.64Mg0.36Ni₃ alloy exhibits good overallelectrochemical properities,which the discharge capacity is 341 mAh/g,and the HRD1500 and cycling property are 62.2% and 84.5%,respectively.For the La2-xMgxNi7 alloys,the alloy with allotropic structures Ce₂Ni₇-?2H?and Gd₂Co₇-type?3R?phases exists in a range of Mg concentration from 0.48 to 0.50,and the Mg atoms tend to enter into the3R-type phase.Outside that range,further increasing and decreasing the Mg to 0.60 and0.40 are favorable for the formation of PuNi₃-and CaCu₅-type phases,respectively.The single-phase A₂B₇-type alloy with allotropic phases is superior in discharge capacity?388mAh/g?and cycling stability?78 % at the 100 th cycle?.Appearance of both of PuNi₃-type and CaCu₅-type phases reduce the cycling stability,however,CaCu₅-type phase can significantly improve the HRD of the alloy electrode.The hydrogen absorption/desorption process and hydrogen storage ability of a new Gd₂Co₇-type Sm_1.6Mg_0.4Ni₇ alloy as hydrogen storage materials have been studied.The Sm_1.6Mg_0.4Ni₇ alloy absorbs 1.88 wt.% H2 within 17 min at 298 K under 10 MPa H2.Meanwhile,the hydrogen absorption speed accelerates to 3.4 min after 20hydrogenation/dehydrogenation cycles with a 1.44 wt.% H2 under 3 MPa H2.Especially,the capacity retention rate of the alloy reaches 99.3% at the 100 thcycle.We found the hydrogen absorption/desorption of the alloy undergoes two equilibrium stages,relating to transformation of H2 between H-solid solution phase and hydride phase with a lower rate and higher enthalpy change at the lower concentration H2 stage,and the direct conversion between H2 and the hydride phase with a higher rate and lower enthalpy change at the higher concentration H2 stage.The two step mode lowers the inner-molecular strain and mismatch in subunit volumes of the alloy in hydrogen absorption/desorption,caused from the transformation of H2 at the lower concentration of H2 stage,thus leading excellent structural and cycling stabilities.