Effect Of Alloying (Nd,Co,Zr)and Preparation Process On Microstructure And Electrochemical Properties Of La-Y-Ni Based A₂B₇-type Hydrogen Storage Alloys

The rare earth AB₅-type multi-component hydrogen storage alloys has the advantages of good discharge performance,cycle life,good environmental compatibility and so on.It is the cathode material for the main commercial nickel/metal hydride(Ni/MH)secondary batteries at present.However,the low capacity of commercial AB₅ rare earth alloys limits the use of nickel metal hydride batteries in high energy density devices.A₂B₇-type rare earth hydrogen storage alloys have higher discharge capacity than AB₅ type.In this paper,La_0.35Y_0.65Ni_3.23Mn_0.17Al_0.10hydrogen storage alloys of La-Y-Ni based A₂B₇-type were taken as the research objects.Nd element was used to replace La and Co and Zr elements were used to replace Ni in alloying.The properties of the alloys were characterized by XRD,SEM/EDS,Sietveld apparatus,electrochemical test and so on.The effects of Nd/Co/Zr alloying on the microstructure,phase structure and electrochemical properties of rare earth superslattice A₂B₇-type hydrogen storage alloys were studied.The main research results are as follows:Microstructure and electrochemical properties of La_0.35-xNd_xY_0.65Ni_3.03Mn_0.17Al_0.1Co_0.2(x=0-0.35)annealed alloys of Nd replacing La were investigated.The results showed that the increase of the Nd element in the alloys was helpful for forming the phase of Ce₂Ni₇-type.The addition of Nd increased the slope of the desorption platform,and the pressure of the desorption platform,which reduced the discharge capacity of the alloy electrodes.The cycle life of the alloy electrode was prolonged with the increase of Nd element.The alloy electrode had the longest cycle life(S₁₀₀=88.5%)and the best high rate performance(HRD₉₀₀=93.7%)at x=0.35.Based on the above,the La_0.15Nd_0.2Y_0.65Ni_3.03Mn_0.17Al_0.1Co_0.2 alloy was studied at different heat treatment temperature of 1143?1423 K.The results showed that annealing at high temperature tended to form 2H-Ce₂Co₇type single phase and 3R-Gd₂Co₇ phase was more stable at lower temperature.The annealed alloy at 1223 K had a highest discharge capacity(C_max=378.7 m Ah/g).As the annealing temperature increased from 1143 K to 1423K,the cycle life of the alloy electrodes decreased first and then increased.The annealed alloy at 1423 K had the best cycle stability(S₁₀₀=90.5%).On the basis of the above experiments,Nd_0.35Y_0.65Ni_3.23-xMn_0.17Al_0.1Co_x(x=0-1)alloys was designed.It was found that adding proper amount of Co element could effectively reduce the platform pressure of hydrogen desorbing,which was beneficial to increase the discharge capacity of the alloys electrodes.The alloy electrode had the maximum discharge capacity when x=0.3(C_max=369.6 m Ah/g).Therefore,replacement of Ni by Co increased the passivation of the alloys electrodes,which improved the cycling stability of the alloys electrodes and leaded to a higher charge transfer resistance,thus reducing the electrochemical kinetics of the electrodes.La_0.33Y_0.67Ni_3.13-xMn_0.17Al_0.1Ti_0.1Zr_x(x=0-0.17)alloys were designed,and the effects of Zr on the microstructure and properties of La-Y-Ni based A₂B₇-type alloys were studied under the two preparation conditions of arc melting and induction melting.It was found that under the present preparation conditions,the introduction of Zr induced the precipitation of the enriched phases in the alloys,which resulted in a significant decrease in the discharge capacity of the alloys.The cycle life of the alloys electrode increased first and then decreased with the increase of Zr content in the induction melting alloy annealed at 1223 K.When x=0.05,the cycle life of the alloy electrode was the longest after 100 times charges and discharges,and the cycle life of S₁₀₀ was 80.4%.At the end of the paper,the laboratory method of arc melting and induction melting process of A₂B₇-type La_0.35Y_0.65Ni_3.23Mn_0.17Al_0.10 alloys on organizational structure and electrochemical properties of alloys were studied.It was found that the microstructure and composition of the as-cast alloy were more homogeneous in induction melting.The higher the annealing temperature was,the less the impurity phases were in the as-cast alloy.The as-cast microstructure prepared by induction melting and arc melting precipitated Ce₂Ni₇-type single phase microstructure at 1253 K and 1223 K,respectively.The maximum discharge capacities of the two alloys electrodes prepared by induction smelting and arc smelting at 1223 K were close to each other,with C_max of about 375 m Ah/g.Compared with arc smelting alloy,induction smelting alloy had longer cycle life,and the maximum cycle stability S₁₀₀ could reach 88.6%,but its high rate performance was slightly worse than arc smelting alloy.For the induction smelting annealed alloy,with the increase of heat treatment temperature,the equilibrium hydrogen pressure in the PCT curve increased gradually,and the gas hydrogen storage capacity increased first and then decreased.