Research On The Cycle Stability Of Magnisium-Based Hydrogen Storage Electrode Alloys By Multi-Components Alloying

To apply Mg-based amorphous alloys to new type rechargeable Ni/MH batteries, the research and development on Mg-based hydrogen storage electrode alloys have been reviewed in detail. In this paper, MgiNi and MgsoNiso were basic compositions. The alloys were mainly prepared by ball milling, and/or the sintering treatment being combined with. To improve the cycle stability of the Mg-based alloys, multi-component alloying was adopted. By that method, this paper studied the effects of alloy elements on the phase formation and electrochemical properties of the storage hydrogen alloys. And the degradation kinetics of discharge capacity for amorphous Mg-based alloy with the best properties was analysed finally.Among MgiNi type alloys which were continuously milled 160h, their phase structures and discharge capacity were mainly determined by the Ti content, and the discharge capacity of (Mg0.5Ti0.5)2Ni alloy was 353.2mAh/g. The other Mg2Ni type alloys were prepared by sintering and followed by ball milling. After Mg being partially substituted by Mn, the alloys' maximal discharge capacities were higher than that of Mg2Ni alloy. With Mn content increased, their cycle stabilities were improved gradually, while the maximal discharge capacity and high-rate dischargeability of them were declined gradually, on the contrary. And the alloy Mg1.8Al0.15Mn0.05Ni showed the best synthetical properties.In Mg50Ni50 series alloys, Ti effectively improved the cycle stability, while the alloys' discharge capacity did not decline quickly. After the metal oxide CeO2,TiO2, V2O5, ZnO etc. were ball milled with ternary Mg40Ti10Ni50 alloy, the cycle stability was improved at a certain extent, but the discharge capacity of them declined dramatically. Mg40Ti10Ni50 alloy being ball milled with AB2 type alloy ZrMno.sVo.iCoo |Ni, 2at weight ratio of 20:1, the cycle stability was improved about 7.2%, and the discharge capacity did not decline very much.The quaternary Mg-Ti-M-Ni alloys had preferably cycle stability, the properties ofinMg35Ti5Cu5Ni55 and Mg35Ti10Mn5Ni50 were relatively ideal. The content of Ni in alloy had directly influence on cycle stability, the more Ni, the better cycle stability, but the much worse discharge capacity.Quinary Mg45Ti5Mn2.5M2.sNi45 ball milled electrode alloys (M=Al,Cu,V,Zr) had superior discharge capacity and cycle stability than quaternary Mg45TUMnsNi45 alloy, where Mg45Ti5Mn2.5Al2.5Ni45 showed the best synthetical properties. Hexahydroxy alloys which sixth element M(M=Cu,V,Zn,Zr) partially substitutes Mg or Ni for Mg45Ti5Mni.5Al2.5Ni45 didn't show ideal properties in this work. After many experiments, some rules were found about the composition and technology optimization of Mg-Ti-Mn-Al/Cu-Ni alloys.The dependence of the discharge capacity was studied in the range of current density from 5 to 250 mA/g for Mg45Ti.sMn2.5Al2.5Ni45 electrode alloy by charge-discharge cycle tests. The amount of hydrogen used for the discharge decreased with cycles. The rate of capacity degradation at a given cycle increased with increasing discharge current density.The capacity degradation curves could be expressed as follows: , where (t) is the discharge capacity at time /. that is the period of time in which the electrode was immersed in the electrolyte solution, the ideal discharge capacity expected for the virgin electrode, the final steady discharge capacity after a number of cycles, and k the apparent rate constant for the degradation. The rate constant increased linearly with the current density. There are two periods during Mg-based electrode alloy's discharge capacity degradation: period of discharge capacity declining dramatically and period of discharge capacity maitaining. The former was aroused by the formation of Mg(OH)2, the later was determined by corrosion and passivation of Mg-based alloy, and the surface processes play dominant roles in affecting the Mg-based electrode alloy properties.