Preparation And Electrochemical Properties Of Low Co Rare Earth ?La,Ce,Y??Ni,Co,Fe,Mn,Al?_5-5.6 Hydrogen Storage Alloys

Typical commercial rare earth AB5 hydrogen storage alloys usually contain 10wt.%Co element to maintain the charge and discharge cycle life.However,the commercial AB5 hydrogen storage alloy with high Co content is very difficult to adapt to the increasingly competitive market demand for nickel-metal hydride batteries due to the high cost of Co and facing the competition of lithium ion batteries,so it is still necessary to develop low-cost hydrogen storage alloy electrode materials,however,the AB5-type hydrogen storage alloys,which generally contain low-Co and Co-free,still generally face the weakness with insufficient cycle life.In this paper,low-Co and Co-free rare earth AB5.0-5.6 hydrogen storage alloys were studied.By adjusting the alloying elements on both sides of A and B,the effects of alloying,chemical composition ratio and preparation process on the microstructure,phase structure and electrochemical properties of the AB5.0-5.6 alloys were investigated.The influence of microstructure,phase structure and electrochemical properties,Through the initial optimization of the composition and properties of the alloys,the cycle stability of the alloy electrode were greatly improved.The research had reached the following conclusions.1.The effect of adding Y element on the electrochemical properties of La0.7-xYxCe0.3Ni3.9Co0.45Mn0.35Al0.3(x=0-0.4)alloy was studied by using a commercial AB5 alloy.The results showed that the cycle life of the alloy electrode was significantly improved after the addition of the Y element.When XY=0.2,the electrochemical performance of the alloy was the best,the maximum discharge capacity was 333.2 mAh/g,the capacity retention rate after 100 cycles was S100=92.50%,and the high rate performance HRD900=92.9%.On this basis,the effects of changing the content of Mn and Fe at the B side on the electrochemical properties of the Co-free Melt-Spun alloy(LaCeY)(NiFeAlMn)5.6(lOm/s)alloy were studied.The results showed that all the Melt-Spun alloys were CaCus type single-phase structure.The hydrogen absorption platform of the alloy were obviously reduced with the increase of Mn content.The alloy electrode had the maximum discharge capacity of 305.5 mAh/g when XMn=0.95.The effect of Fe on the hydrogen absorption platform of the alloy was not very obvious,but it had significant impact on the capacity and cycle stability of the alloy.The discharge capacity of the alloy electrode gradually decreased and the cycle stability gradually increased with the increase of the Fe content.When XFe = 0.5,the alloy discharge capacity was 290.5 mAh/g and the capacity retention rate S100 after 100 cycles reached about 97%.2.The changes of microstructure,phase structure and electrochemical properties of the alloy La0.55Ce0.25Y0.2(NiFeAlMn)5.0-5.6 under different preparation process were studied.The results showed that all the Melt-Spun(10m/s)alloy structure was CaCu5 type single-phase structure and the as-cast and annealed alloy(1273K×15h)were all composed of the main phase of CaCu5 type structure and the second phase of Mn-rich element.The annealing treatment dissolved a small amount of the second phase and the substrate composition tended to be uniform.As the composition ratio increased,the discharge capacity of the alloy electrode decreased,the cycle stability increased and the high rate performance decreased.For the same stoichiometry,the change law of the cycle stability S100 was followed by annealed alloy>Melt-Spun alloy>as-cast alloy and the high rate performance was just the opposite.The properties of the annealed AB5.5?5.6 alloy was relatively better among them,which the electrode capacity was 290?302mAh/g and the capacity retention rate S100 was about 97%.Stoichiometric ratio composition was beneficial to improve electrode cycle stability,but it could reduce the dynamic performance of the electrode reaction.3.Based on the influence of the alloying,over-stoichiometric ratio and preparation process on the electrochemical properties of the alloy,the annealed alloy La0.6?0.7Ce0.2?0.3Y0.1?0.15(Ni4.25?4.44Fe0.1-0.3Co0?0.2Mn0.5?0.9Al0.02?0.25)5.3-5.6 was designed and prepared by further regulating the A-side and B-side components.The results showed that the micro structure of the annealed alloy consisted of CaCu5 type main phase and a small amount of second phase.The cell volume of the CaCu5 type main phase gradually increased and the abundance of the second phase phase increased with the increase of the stoichiometric ratio.The platform pressure and maximum hydrogen absorption of the alloy all decreased with the increase of the stoichiometric ratio.With the increase of the stoichiometric ratio,the capacity of the alloy electrode decreased successively and the cycle stability S100 increased gradually.The stability of the alloy electrode S100 was further increased to 97?99%while adding a small amount of Co(xco=0.2),nevertheless,the addition of the Co element deteriorated the high rate discharge performance of the alloy.