An Investigation On The La-Ni-Sn-based AB_5+x Type Co-free Hydrogen Storage Electrode 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, the La-Ni-Sn-based AB_5+x type Co-free alloys were selected as the study object of this work. By means of XRD, SEM, XPS, AES, ICP analyses and electrochemical measurements, the effects of the alloy preparation methods (including heat-treatment and rapidly quenching) and the alloy stoichiometry on the crystal structure and electrochemical properties of some Co-free alloys were studied systemically.Based on the comparative study of the as-cast and melt-spun (1～5m/s) the LaNi_4.92Sn_0.33 alloys, it is found that the as-cast alloy consists of two similar CaCu₅ type phases with different Sn content and a few single Sn phase, and all the melt-spun alloys are single CaCu₅ type structure. After melt-spinning treatment, the microstructure of the alloys change from the coarse dendritic structure (as-cast) to the fine cellular structure and the melt-spun alloys have a more homogenous composition. With the increase of solidification rate, the cycling stabilities of the alloys are improved evidently (S₂₀₀ from 42.7% of as-cast alloy increases to 62.5-78% of melt-spun alloys), but their maximum discharge capacity and high-rate discharge-ability are lowered somewhat. It is found that the great improvement in cycling stability of the melt-spun alloys is mainly due to their lower overall volume expansionand pulverization on hydriding and more uniform composition, while the decrease in both the electrocatalytic activity of the alloy electrodes and the hydrogen diffusion rate in the alloy bulk are the main reason for their relatively lower high-rate dischargeability. Among the alloys studied, the melt-spun alloy (3m/s) has the best overall electrochemical properties: its maximum discharge capacity C_max=310.7 mAh/g, the high-rate dischargeability HRD₆₀₀=77.2%, and the capacity retention rate(S₂₀₀) after 200 cycles reaches 71.1%.For the annealed La(Ni,Sn)_5+x(x=0.0～0.35) alloys with different stoichiometry, it is found that all of the alloys are single CaCu₅ type structure. In the structure of over-stoichiometric alloys (x>0), part La atoms of la sites are replaced by the Ni-Ni dumbbells arranging along c axis, and the replaced fraction (y) increases with the increasing of x value, which leads to a lower cell volume expansion on hydriding, and greatly improves the cycling stability of the alloys, but the increase of x also leads to some decrease in their discharge capacity and high-rate dischargeability. Among the alloys studied, the LaNi4.92Sno.33 alloy has the best overall electrochemical properties: C_max=320.6 mAh/g, HRD₆₀₀=80.9%, S₂₀₀=79.5%.From the investigation on the mechanism of cycling capacity decay of the as-cast, melt-spun 3m/s and annealed LaNi4.92Sno.33 alloys, it is found that during the charge/discharge cycling process, the main reason for capacity decay of the alloys is that the oxidation-corrosion of the main hydrogen absorbing element La and the progressive pulverization of the alloys. The corrosion products on the cycled alloysurfaces are mainly consisted of La(OH)₃ with small amount of Ni(OH)₂ and SnO₂. However, as compared to the as-cast alloy, the melt-spun and annealed alloys show a lower pulverization degree and a much thinner corrosion layer on the alloy surface, indicating that the melt-spun and annealing treatments result in a lower volume expansion of the alloys on hydriding, and more uniform alloy composition, which improves the abilities of anti-pulverization and corrosion-resistance of the alloys, and hence leads to the great improvement in cycling stability.In order to obtain better understanding of the structural variation of the alloys during charge-discharge process, the phase composition, the crystal structure and the cell volume expansion rate of the as-cast, melt-spun and annealed LaNi4.92Sno.33 alloys during the charge/discharge process were studied by the means of the in situ XRD analysis. It is found that during charge/discharge process, the α←→(α+β)←→β phase transition are found, and there are a large discrete cell volume expansion and contraction during α←→β phase transformation. As compared to the as-cast alloy, the melt-spun and annealed alloys show a much lower discrete cell volume expansion (contraction) rate, the ratio of the discrete cell volume expansion to the overall volume expansion of the alloys is as following order: as-cast (74%) > annealed alloys (63%) > melt-spun alloys (60%). It is believed that the lower discrete cell volume expansion of the melt-spun and annealed alloys during α←→β phase transformation is the main reason for their improved pulverization resistance and cycling stability. In addition, in the domain of diffraction, it is also found that the crystallite size of α and β phases is related to the hydrogen content and the micro-strain or the dislocation deduced in the alloys.In order to improve the overall electrochemical properties of the alloys, the Sn in the LaNi_4.92Sn_0.33 alloy was partly substituted by each of the five conventional elements (M), and the annealed LaNi_4.92Sn_0.23M_0.1 (M= Mn,Fe,Co,Cu,Al) quaternary alloys were studied to examine the effects of element substitution on the phase structure and electrochemical properties of the alloys. It is found that the M=Fe,Co,Al alloys consists of a main phase with CaCu₅ type structure and some second phase, but the M= Mn, Cu alloys are single CaCu₅ type structure. The substitution of M for Sn leads to some increase in the maximum discharge capacity and the high-rate dischargeability of the alloys, but only the M=Mn, Cu alloys show a improved cycling stability. It is found that the substituted of Sn by Mn, Cu result in a lower dissolution amount of the La, Sn elements into electrolyte, and the improvement of corrosion resistance of the alloys, which leads to the improved cycling stability. Among the alloys studied, the M=Mn alloy has the best overall electrochemical properties: C_max=329mAh/g, HRD₆₀₀=89.1%, S₂₀₀=83.8%.