Microstructure Characteristics And Electrochemical Properties Of As-cast And Melt‐Spun Mg₂NiFe_x （x=0～0.20）

After years of development,the performance of solid hydrogen storage materials has been greatly improved.Mg-based hydrogen storage materials represented by Mg-Ni alloys have high theoretical hydrogen storage capacity and low alloy density,and are considered to be promising anode materials for Ni/MH batteries.However,Mg-Ni alloys have poor hydrogen absorption and desorption kinetics at room temperature and low temperature,and their hydrides need to be absorbed and desorbed at high temperature,which hinders the large-scale application of Mg-based hydrogen storage materials in the field of Ni/MH batteries.From previous studies,it is found that the properties of transition metal elements Fe and Ni are similar.The use of transition metal elements Fe to diversify Mg₂Ni alloys can destroy the stability of metal hydrides and catalyze the hydrogen absorption and desorption properties of alloys.At the same time,it can also realize the coating of alloy particles and significantly improve the corrosion resistance of the alloy.In this paper,based on the as-cast Mg₂Ni alloy,the effects of the addition of transition element Fe and melt-spinning treatment on the microstructure,electrochemical properties and kinetic properties of Mg-Ni alloys were studied,which provided a theoretical basis for the practical application of Mg-Ni hydrogen storage alloys.It was found that no new phase or Fe element containing Fe was formed in the non-stoichiometric Mg₂Ni Fe_x（x=0～0.20）as-cast alloy.The added Fe element is dissolved in the Mg₂Ni phase,and the substitution of Ni is realized.The Mg₂（Ni,Fe）phase is formed and uniformly distributed in the main phase of the alloy,which effectively improves the hydrogen storage performance of the alloy.The electrochemical performance test shows that the addition of Fe has a significant improvement effect on the maximum electrochemical capacity of the alloy.The maximum discharge capacity of the as-cast Mg₂Ni Fe_x（x=0～0.20）alloy at room temperature shows a gradually increasing trend with the increase of Fe content,from 15.08 m Ah/g at x=0 to 21.36 m Ah/g at x=0.20,an increase of 41.64%.However,the cycle stability of the alloy decreased significantly.After 50 charge-discharge cycles,the capacity retention rate of the alloy was only 47.75%.The kinetic performance test shows that the addition of Fe element is beneficial to improve the charge transfer rate on the surface of the alloy electrode and reduce the resistance of hydrogen atoms from the surface of the alloy to the internal diffusion of the alloy,thus improving the kinetic performance of the alloy.On this basis,the Mg₂NiFe_x（x=0～0.20）alloys were melt-spun by 15 m/s,30 m/s and 45m/s.The effects of different melt-spinning speeds on the microstructure,electrochemical properties and kinetic properties of Mg₂Ni Fe_x（x=0～0.20）alloys were studied.It was found that the melt-spinning treatment does not make the Mg₂Ni Fe_x（x=0～0.20）hydrogen storage alloy form a multiphase structure,and the phase composition of the melt-spun alloy and the as-cast alloy is basically similar.As the melt-spinning speed increases,the intensity of the diffraction peaks gradually weakens,and some diffraction peaks even disappear,which means that the alloy changes from crystalline to amorphous/nanocrystalline structure.The microstructure of the alloy matrix is also gradually refined with the increase of the melt-spinning speed,and some fine grains are fused to form flocculent substances,indicating that the amorphous phase may be formed in the matrix.Electrochemical performance tests show that the melt-spinning treatment improves the cycle stability of the alloy,but reduces the maximum discharge capacity of the alloy.The capacity retention rate of the Mg₂Ni Fe_0.15alloy with a melt-spinning rate of 30 m/s reaches 92.75%after 50 cycles,while the capacity retention rate of the as-cast alloy after 50 cycles is only 67.03%,which is increased by25.72%.The dynamic performance test shows that the amorphous phase formed by the melt-spinning treatment hinders the diffusion of hydrogen atoms in the alloy and has an adverse effect on the dynamic performance of the alloy.