Investigation On The Electrochemical Properties Of Amorphous La-Mg-Ni System Hydrogen Storage Alloys

The AB₃-type La-Mg-Ni based hydrogen storage alloys were paid much attention as the candidates for negative electrode materials of Ni/MH secondary batteries due to their high discharge capacities. In this work, previous research on La-Mg-Ni based hydrogen storage alloys have been extensively reviewed. On this basis, the LaMg₁₂ alloy was selected as the subject of this study. The phase structure, electrochemical properties of LaMg₁₁Zr+200wt.%Ni alloys were studied systematically by means of XRD, SEM, DSC analysis and the electrochemical test methods which included galvanostatic charge-discharge, linear Polarization and potentialstatic discharge. Then, the effects and mechnisam of the amount of both B addition and inherent elements Zr,La,Mg on the phase structure and electrochemical properties of the LaMg₁₁Zr+200wt.%Ni hydrogen storage alloy electrodes were investigated systematically.The effect of ball-milling time on the phase structure and electrochemical properties of LaMg₁₁Ni+200wt.%Ni composite were studied. Results of XRD and SEM analyses showed that the melting LaMg₁₁Zr alloy had a multiphase structure and several characteristic diffraction peaks. The main phase was La2Mg17, and the other phase was Zr, the shape of alloy particles was polygonal or irregular. The alloys with Ni addition were amorphous after 20h ball-milling, and the shape of alloy particles had gradually become spherical or spherical-alike. It was clear that Ni addition make the amorphization of Mg-based alloy much more effective since it decreased the ball-milling time for completely amorphous. Electrochemical tests indicated that the discharge capacity and cycling stability of the alloy electrodes increased first and then decreased with the ball-milling time from 0 to 40h. During the whole procedure, 20h ball-milled alloy electrode exhibited the highest discharge capacity of 576.2 mAh/g, and the capacity retention rate S20 reached 55.4% after 20 charge–discharge cycles, and the dynamic performance was also improved.The hydrogen storage alloy LaMg₁₁Zr+200wt.%Ni, which was produced with La,Mg,Zr,Ni metallic powder by mechanical alloying (MA) method. The XRD and SEM analyses of the alloy showed that the alloy had an amorphous structure already after 20h ball-milling, with only one characteristic diffraction peak of LaMg, fine and homogeneous alloy particles as well as some colonies. Electrochemical tests showed that the maximum discharge capacity of alloy electrode was high, but the cycling stability and high-rate discharge performance were poor, the maximum discharge capacity reached 681.6 mAh/g, but the cycle capacity retention rate was only 32.1% after 20 weeks charge-discharge. This was mainly due to the size of alloy prepared by mechanical alloying were more susceptible to expansion and contraction, thus the electrode was more likely to be powered, and got serious corrosion. As a result, the amount of Mg participated in absorption and desorption of hydrogen reduced consequent the cycle life and high-rate discharge performance of alloy electrode decreased. So, the LaMg₁₁Zr+200wt.%Ni alloy electrode prepared by two-step method of melting and mechanical alloying (MA) displayed better comprehensive electrochemical performance than that by only mechanical alloying (MA) method.The phase structure and electrochemical properties of the LaMg₁₁Zr+200wt.%Ni+xwt.%B (x=0,2,5,10) alloys were systemically investigated. The results indicated that alloys were totally amorphous after 20h ball-milling and the formation as well as thermal stability of amorphous alloys was improved by B addition. All the alloy electrodes had good electrochemistry activation characteristics, the discharge capacity of the alloy electrodes varied with increasing the boron content, the LaMg₁₁Zr+200wt.%Ni+2wt.%B alloy electrode achieved the maximum discharge capacity of 614.2 mAh/g, which is 6.7% higher than that without B addition. At the meantime, the cycling stability of the composite electrodes was improved and the discharge capacity retention rate S30 increased from 44.4% (x=0) to 63.8% (x=5) and further improved to 70.4% (x=10) after 30 charge–discharge cycles. Moreover, due to B was an electron-deficient atom with special electrochemical properties and high electronegativity, it can be integrated into Mg thus to reduce the strength of Mg-H ionic bond and provide more active sites and proliferation of channel for the proliferation of surface charge and diffusion of hydrogen, further improved the electrochemical capacity and cycling stability of the alloys.The effect of Zr/La/Mg content on the structure and electrochemical properties of LaMg₁₁Zr+200wt.%Ni alloys were investigated. The structure analysis showed that alloys were amorphous after 20h ball-milling, and the Zr/La/Mg addition significantly refined the alloy particles. The electrochemical tests indicated that the maximum discharge capacity decreased from 576.2 mAh/g to 399.7 mAh/g when Zr increased from 0wt.% to 10wt.%. Moreover, the cycling stability was improved, the capacity retention rate S30 increased from 44.4% to 88.9% after 30 charge–discharge cycles, and the dynamic performance was also improved. The charging resistance of the alloy electrodes decreased with La addition; the maximum discharge capacity of the alloy electrodes went up then down with the increase of La. The La addition can improve the cycling stability and dynamic performance of alloy electrodes to some extent. The maximum discharge capacity of LaMg₁₁Zr+200wt.%Ni+ 5wt.%La alloy electrode achieved 597.2 mAh/g, and the capacity retention rate of the composites was 53.5% after 30 cycles. The alloys had good electrochemistry activation characteristics for the addition of Mg, and the higher Mg content, the higher maximum discharge capacities, but the cycling stability of alloy electrodes was deteriorated sharply. LaMg₁₁Zr+200wt.%Ni+10wt.%Mg alloy electrode could achieve the maximum discharge capacity of 994.8 mAh/g, however, the capacity retention rate after 30 charge–discharge cycles could be only 26.0%.