Microstructural Evolution And Improvement Of Electrochemical Properties For Ce-added A₂B₇-type La-Mg-Ni Hydrogen Storage Alloys

La-Mg-Ni based A₂B₇-type hydrogen storage alloys with excellent electrochemical performances,such as good activation behavior,high discharge capacity and good dynamic performance,have a very good development prospect in the application field of nickel metal hydride battery cathode materials for the hybrid electric vehicles（HEV）.But charge and discharge cyclic stability of the alloys still remains needed to be further improved,especially the low temperature electrochemical performances could not meet its applications in practice.At present,methods including element substitution and microstructure modification such as annealing treatment,high energy ball milling,surface coating and melt spinning have been adopted to improve the charge and discharge cycle stability of the alloys,and decades of research have shown that the element substitution and melt spinning have been considered as two kinds of effective means.So far,there is still a lack of systematic research about the effect of Ce-addition on the electrochemical properties for the La-Mg-Ni based A₂B₇-type hydrogen storage alloys.In this paper,the improvement of electrochemical performance for the alloys by adding alloying elements and microstructure refinement is mainly studied.Meanwhile,the low temperature electrochemical performances of the alloys at 268,258 and248K have been investigated.Finally,La-Mg-Ni based A₂B₇-type hydrogen storage alloys with excellent electrochemical performance have been obtained,and which lays a theoretical foundation for their application in the field of HEV.In this paper,ingots of the La-Mg-Ni based A₂B₇-type hydrogen storage alloys were prepared by induction melting according to the nominal compositions under argon atmosphere,afterwards,the samples were annealed at 1173K in a vacuum furnace for 8 hours.The melt-spun ribbons were obtained by a water-cooled rotating copper roller,and the spinning rates expressed by the linear velocity of the copper roller were 10,20,and 30m/s.The component of the samples was analyzed by the method of inductively coupled plasma emission spectrometry.The structure of the alloys was investigated by X-ray diffraction（XRD）method.The morphology was observed through a scanning electron microscopy（SEM）.The LANHE CT2001A apparatus was used to test the electrochemical properties,such as the activity cyclic number（N_a）,the maximum discharge capacity(C_max),the cyclic stability（S_n）,and the high rate dischargeability（HRD）.The CHI660E electrochemical work station was used to test the kinetic parameters,such as the corrosion current density(I_corr),the exchange current density（I₀）,and the hydrogen diffusion coefficient（D₀）.The effect of Co substation for Ni on structure and properties of the annealed La_0.75Mg_0.25Ni_3.5 alloys is studied at 298K firstly.The C_max and S₁₀₀00 of alloy reach the optimum values of 380.2mAh/g and 62.39%when the contents of Co is 0.5at.%,and the energy density of the alloy is also optimal.Based on the previous results,the effects of Ce substitution for La on the structure and electrochemical properties of the annealed La_0.75-xCe_xMg_0.25Ni₃Co_0.5（x=0,0.05,0.1,0.15,0.20at.%）alloy is investigated at 298K,and the results show that the La_0.75-xCe_xMg_0.25Ni₃Co_0.5 alloys contain three major phases of（La,Mg）Ni₃,（La,Mg）₂Ni₇,and LaNi₅.When Ce content increases,the abundance of（La,Mg）Ni₃and（La,Mg）₂Ni₇ phase reduces and the abundance of LaNi₅ phase increases,meanwhile,the lattice parameter and cell volume all decrease.The La_0.75-xCe_xMg_0.25Ni₃Co_0.5.5 alloys could be activated within 2 cycles,and the S₁₀₀ rises from 62.3%to 84.94%when Ce content increases.When Ce content is 0.1at.%,C₁₀₀00 of the La_0.65Ce_0.1Mg_0.25Ni₃Co_0.5.5 alloy reaches the maximum value of 259mAh/g,and the HRD parameters of the alloy reach the optimum values of 98.33,94.90,88.45,82.87,70.71 and 58.95%when i=300,600,900,1200,1500,3000 mA/g,respectively.Therefore,it is seen that the comprehensive electrochemical properties of the alloys reach the optimal status when Ce content is 0.1at.%.The influence of melt spinning on the structural and electrochemical performances of La_0.65Ce_0.1Mg_0.25Ni₃Co_0.5.5 alloy electrodes at 298K is investigated.The Rietveld refinement result reveals that the melt-spun alloys are mainly composed of（La,Mg）Ni₃,（La,Mg）₂Ni₇and LaNi₅ phase,as the melt spinning rate is enhanced from 0（the as-cast is defined as the melt spinning rate of 0）to 30m/s,the abundance of LaNi₅ phase increases while that of（La,Mg）Ni₃ and（La,Mg）₂Ni₇ phase shares a reverse trend.S₁₀₀ of the alloy increases with the rising of melt spinning rate.When the melt spinning rate is 10m/s,C₁₀₀ for the La_0.65Ce_0.1Mg_0.25Ni₃Co_0.5 alloy reaches the maximum value of 268.3mAh/g,and the HRD performances of the alloy also reaches the optimum value.The low temperature electrochemical properties of the La_0.65Ce_0.1Mg_0.25Ni₃Co_0.5 alloys at268,258 and 248K are further studied.It is concluded that S₁₀₀ of the alloy reaches the best values of 87.88%（268K）,90.14%（258K）,and 92.04%（248K）when the melt spinning rate is30m/s.When the spinning rate is 10m/s,C₁₀₀ for the alloys reach the maximum values of265.1mAh/g（268K）,253.4mAh/g（258K）,and 245.9mAh/g（248K）,and the HRD properties of the alloys are the best as well.Thus the melt-spun La_0.65Ce_0.1Mg_0.25Ni₃Co_0.5 alloy exhibits optimum electrochemical properties when the spinning rate is 10m/s.