Effect Of Nd/Co Alloying And State Of Charge On The Cycling Stability Of La-Y-Ni AB_3.5-3.8 Hydrogen Storage Alloy Electrode

As the negative electrode material of the Nickel-Hydrogen battery,the properties and applications of hydrogen storage alloys are important factors restricting the development of Ni-MH batteries.The negative electrode material of traditional commercial nickel/metal hydride(Ni/MH)secondary battery is AB₅ type multiple hydrogen storage alloy.Rare earth AB₅ type multiple hydrogen storage alloy has the advantages of good discharge performance,long cycle life and good environmental compatibility.However,the low capacity of commercial AB₅ type alloy electrode limits the application of nickel-hydrogen battery in high energy density equipment.The A₂B₇hydrogen storage alloy with rare earth R-Mg-Ni super lattice structure has higher discharge capacity,but its cycle stability is still poor.The results show that the new type La-Y-Ni hydrogen storage alloy electrode has the characteristics of easy activation,high capacity and low hydrogen-induced amorphization tendency,and the A₅B₁₉ type alloy has more Ca Cu₅ structure unit stacking than the A₂B₇ type alloy.In theory,the structure is more stable during hydrogen absorption and discharge.The A₅B₁₉ type La_0.25Nd_0.2Y_0.55N_i3.52-xCo_xMn_0.18Al_0.1(x=0-1)hydrogen storage alloy is studied in this paper.The influence of Co elements and heat treatment on the phase structure and electrochemical properties of A₅B₁₉ alloy was systematically studied by electrochemical test and XRD?SEM/EDS and so on.On this basis,the effect of charge(electrochemical hydrogen absorption capacity)on the cycling stability and process behavior of the alloy electrode was preliminarily discussed with the La-Y-Ni A₂B₇type alloy as the object.And the changes of microstructure,surface morphology and composition,hydrogen-induced pulverization and strain accumulation of alloy electrode under different charge were analyzed.This article surrounds the above content,the main results obtained are as follows:(1)For La_0.25N_0.2Y_0.55Ni_3.52-xCo_xMn_0.18Al_0.1(x=0-1),The proper addition of co elements in the alloy is beneficial to the disappearance of the La Ni₅ phase and the formation of the A₅B₁₉ phase.With the increase of co content,the maximum discharge capacity capacity of the alloy electrode cmax increase first and then decrease,and the maximum value is 378.6 m Ah/g.The interaction between Co element and Ni element(Ni-Co film)obviously improves the corrosion resistance of alloy electrode,and the atomic radius of Co element is large,which reduces the strain of alloy electrode during hydrogen absorption and discharge.The cycling stability of alloy electrode S100 increased from 88.51%(x=0)to 94.44%(x=1)(2)Taking the La_0.25Nd_0.2Y_0.55Ni_3.32Co_0.2Mn_0.18Al_0.1 alloy with the substitution of Co element 0.2 as the research object,samples were subjected to different heat treatments 1173-1373 K.It turns out,annealing at too high or too low temperature will promote the formation of La Ni₅ phase.The alloy does not contain La Ni₅ phase1273 K annealing,with a maximum discharge capacity(C_max=378.6 m Ah/g),And has the best cycle stability(S₁₀₀=91.12%),The results show that the A₅B₁₉ phase and the A₂B₇ phase have high capacity and stability compared with the AB₅ phase.(3)Based on the previous work,we found that alloy La_0.33Y_0.67Ni_3.23Mn_0.17Al_0.1of charging load capacity(hydrogen production)has a large impact on its electrochemical cycle stability.When charging and discharging cycles with different charges,there is a period in which the capacity does not decay,and we regularly take it as the incubation period.Using this phenomenon,we used 95%charge and discharge of A₂B₇ La_0.33Y_0.67Ni_3.23Mn_0.17Al_0.1 alloy to study the surface morphology,composition and structure evolution behavior of the alloy electrode during the incubation period.The mechanism of electrode capacity attenuation is further analyzed.The results show that coarse flocculants have been formed on the surface of the sample after 16 cycles.And the particle size of the alloy has changed from48?74?m before the cycle to about 200?m.After 32 cycles,the surface of the sample is relatively smooth but high-power observation found that the surface precipitates fine needle,cracking of alloy particles and fresh surfaces;After 55 cycles,The needle is formed in large quantities,and the layer falls off on the surface of the alloy particles.Combined with the above results,it is found that although the apparent discharge capacity of the alloy electrode does not decrease during the incubation period,a large number of needles are still precipitated on the microscopic surface.This phenomenon and preliminary conclusions are important to understand the capacity decline of the electrode in the alloy system.