The Phase Structures And Electrochemical Properties Of Melt-spun Low-Co And Co-free AB₅-type Hydrogen Storage Alloys

In this thesis, based on the review of the research and development of AB5 type low-Co and Co-free hydrogen storage electrode alloys, some ways for improving the overall properties of AB5 type low Co and Co-free electrode alloy with different Co contents(6wt%, 3.5wt% and 0) were proposed as the objects of this study. By means of XRD, EDS analyses and electrochemical measurements, the effects of the alloy preparation methods (melt-spinning with different cooling rates : 1, 3, 5, 10 and 15m/s) on the phase structure and electrochemical properties of some low-Co and Co-free alloys were studied systemically.For the low-Co MlNi_4.0Co_0.3Mn_0.4Al_0.3 alloys, the phase structure and electrochemical properties of these alloys were comparatively studied. It is found that all of the alloys are single CaCu₅ phase structure but the increase of cooling rate of the alloys leads to an increase in both crystallizability and composition homogeneity, and leads to a decrease in cell volume expansion rate (AV/V) on hydriding(18.2% to 16.1%). Compared to the as-cast alloy, the melt-spun alloys have a lower discharge capacity and inferior high-rate dischargeability, but a much improved cycling stability as the cooling rate increases. Among the alloys studied, the melt-spun alloy (10m/s) has the best overall electrochemical properties: its maximum discharge capacity C_max=314.30mAh/g, activation process needs only 2 cycles, the high-rate dischargeability HRD₆₀₀=76.20%, and the capacity retention rate after 300 charge-discharge cycles reaches 75.37%.The phase structure and electrochemical properties of the low-Co MlNi4.oCoo.2Mno.40Alo.30Cuo.10 alloys were also investigated. It is found all the melt-spun alloys prepared with all these methods are all single CaCu₅ phase structure and have a more homogeneous composition, and leads to a noticeable decrease in cell volume expansion rate (AV/V) on hydriding(17.4% to 14.4%). Compared to the as-cast alloy, the melt-spun alloys also have a lower discharge capacity and inferior high-rate dischargeability, but a much better cycling stability as the cooling rate increases. Among the alloys studied, the melt-spun alloy (10m/s) has the best overall electrochemical properties: its maximum discharge capacity C_max=310.42mAh/g, activation process needs only 2 cycles, the high-rate dischargeability HRD₆₀₀=68.92%, and the capacity retention rate after 300 charge-discharge cycles reaches 74.91%.For the Co-free Ml(Nio.8oAlo.o6 Si_0.02Fe_{0. 12}5.2 alloys, the effects of rapidsolidification on the phase structure and electrochemical properties of these alloys were studied. It is found that all of the alloys are single CaCus phase structure and their expansion rate (AV/V) decreased 17.9% from to 12.5%.After melt-spinning treatment, the microstructure of the alloy change from the coarse dendritic structure (as-cast) to the fine cellular structure and the melt-spun alloy have a more homogenenous composition. Compared to the as-cast alloy, the melt-spun alloys have a lower discharge capacity and inferior high-rate dischargeability, but a much better cycling stability as the cooling rate increases. Among the alloys studied, the melt-spun alloy (5m/s) has the best overall electrochemical properties: its maximum discharge capacity Cmax=302.51mAh/g, activation process needs only 2 cycles, the high-rate dischargeability HRD6oo-65.37%, and the capacity retention rate after 300 charge-discharge cycles reaches 75.61%.It is found that the improvement in cycling stability of the alloys is closely related to their lower cell volume expansion on hydriding and more uniform composition, and the decrease of HRD of the alloys is mainly attributed to the decrease of electrocatalytic activity for change-transfer reaction and the lowering of the diffusion rate of hydrogen in the alloy bulk.</sub>