Study On Electrochemical Properties And Phase Structure Of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3/Mg₂Ni Composite Hydrogen Storage Alloys Prepared By Two-step Melting

At present, rare earth hydrogen storage alloy type AB5 used wildly for rechargeable battery has low capacity and short cycle life. In order to develop a novel material with good electrochemical properties, a composite of rare earth hydrogen storage and Mg–based alloy are usually prepared by mechanical alloying (MA). However, there are two disadvantages of long grinding time and short cycle stability. In addition, phases can not be joined closely with each other together, particle surface is easy to be oxidized and polluted, as well as some impurity is also easy to be introduced into alloy in the processing.In order to improve the characteristics of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3, the composites of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3/x wt%Mg2Ni (x=0, 5, 10, 30) were synthesized by two-step re-melting in this work. Firstly, the master alloy ingots of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 and Mg2Ni were prepared separately,then were crushed and mixed in designed proportion. The mixture was re-molten and the composite was obtained. The electrochemical performance and PCT characteristic of the composites were tested by means of electrochemical measurement. Their thermodynamic properties were calculated and phase structures and components were analyzed by XRD, SEM and EDS, respectively. The effects of Mg2Ni content in the composites on the phase structures, electrochemical and thermodynamic properties were investigated.The results indicate that the cycle stability of composites is improved drastically compared with that of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3, as the capacity retention rate after 200 cycles increases 16% after it was compounded with Mg2Ni; the discharge capacity of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 is going to decline rapidly from the 133th cycle and the electrochemical capacity is only 49.6% of its initial capacity when reaching 150th cycle, whereas it will be approximately 89% for composite alloy. The composite with 5% Mg2Ni has an excellent comprehensive thermodynamic and electrochemical property. It is activated after only 6 cycles (Cn=6) at room temperature. The maximum discharge capacity (Cmax), high-rate discharge ability HRD600, capacity retention after 200 cycles, and discharge plateau voltage are 274.4mAh/g, 32.5%, 78.24%, and 1.022V, respectively. It is found that the alloy has a large effective hydrogen capacity resulted in its good discharge voltage plateau characteristic, such as wide plateau, gentle slope, and long continuous discharge time at the plateau voltage.The results show that there is a stronger and a narrower full width at half maximum peak in the XRD patterns of the main-phase LaNi5 in master alloyMm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 than that of it in the composites. It illustrates that the grain of the substrate is fined resulted in Mg2Ni adding. The results of SEM/EDS analysis show that, with x increasing, the microstructures of the composites varies gradually from the lamellar(x=5), acicular(x=10) to massive(x=30), Al-rich and Ni-rich phases exist in the substrate, and the grains of phase Mg2Ni grow slightly.It is also found that the composite contained 30wt% Mg2Ni is difficult to be activated at room temperature. Its maximum discharge capacity (Cmax) and capacity retention after 200 cycles are only 90.72 mAh/g and 61.86%, individually. The HRD900 is less than 15%. So, it is not of benefit to the improvement of composite properties by adding excessive content of Mg2Ni in it.