A Study On The Phase Structures And Electrochemical Properties Of La-Mg-Ni Based A₂B₇-type Hydrogen Storage Alloys

In recent years,environmental and energy problems seriously restricts the sustainable development of economy all over the world.In the study of the new energy boom,the hydrogen,as a kind of excellent secondary energy,has drawn more and more people's attention.The development of nickel/metal hydride rechargeable batteries effectively promotes the advancement of the hydrogen technology,having been widely used in new energy vehicles,electric tools and household appliances and other fields.The properties of Ni/MH battery mainly depend on the negative electrode,hydrogen storage alloys.The AB₅-type rare earth alloys,as studied maturely and mainly used for industrialization,will not meet the needs of the application for the disadvantage of low capacity.The A₂B₇-type alloys,holding an high theoretical discharge capacity of 420mAh/g,is considered to be the most potential Ni/MH batteries cathode materials.The existing problem of A₂B₇-type alloys is that their poor electrochemical cycle stability and kinetics cannot meet the requirements of practical application,so that improving the cycle stability performance and the electrochemical kinetics are the key of the correlational research.Therefore,substituting Ni with a little amount of Ti,Cu and B,the hydrogen storage alloys were prepared by solid diffusion reaction method,and the effects of elements replacement on the structures and electrochemical hydrogen storage performance of the alloy electrodes had been systematically investigated.The phase structures were analyzed by using XRD,the microtopography observed by SEM,and the ingredients in micro zone were tested by EDS,respectively.The electrochemical performance and the kinetics of the alloy electrodes were studied in-depth by using constant current charge-discharge test,Tafel polarization curve test,linear polarization curve and electrochemical impedance spectroscopy test,et al.The result for La0.75Mg_0.25Ni_3.2-xMn_0.3Ti_x?x=0,0.05,0.1,0.15,0.2?alloys,the purity of?La,Mg?₂Ni₇ phase raises and the maximum discharge capacity continues to decline from 369.3mAh/g at the value of x is 0 to 307.3mAh/g as the x value is 0.2,and the cycle stability could be enhanced only when the content of Ti is a few.The kinetics of alloy electrodes firstly increases and then decreases,and the high rate discharge capacity would be best when the x value is 0.1,with a number of 59.8% when the current density is 1200mA/g.All the La_0.75Mg_0.25Ni_3.2-xMn_0.3Cu_x?x=0,0.05,0.1,0.15,0.2?alloys holding good activation performance,although the Cu joining has produced extremely adverse effect on the maximum discharge capacity of alloys,but Cu can play a very positive role in the improvement of the cycle stability,so that the cycle stability of alloy has been gradually increased.With the value of Cu replacement rising,the S100 of the alloy electrodes constantly increase from 53.2%?x=0?to 72.1%?x=0.2?.Moreover,the kinetics of the alloys could be improved at the low amount of the Cu.The La_0.75Mg_0.25Ni_3.2-xMn_0.3B_x?x=0,0.05,0.1,0.15,0.2?alloys have the more complex phase structures,the La₃Ni₁₃B₂ phase appearing after B adding and the cell volume of the primary phases expanding.The adding B makes a reducing in the Cmax of the alloy electrodes,and the cycle stability firstly raise up but then bring down.The research indicates that B can make a completely excellent contribution to elevating the kinetics of the alloys,the exchange current density going to be more and more big and the charge transfer resistance on the alloy surface continuously decreasing with the increasing quantity of the B substituting.That's to say,the kinetics of alloys are keeping improving,as the count of HRD₁₂₀₀ rapidly growing up from 51.8% to 61.5% when the x number ranging from 0 to 0.2.