An Investigation On The Phase Struture And Electrochemical Properties Of The La-Mg-Ni-based A₂B₇ And A₅B₁₉ Type Hydrogen Storage Electrode Alloys

In this thesis, previous research works on non-AB₅ type RE-based hydrogen storage alloys have been extensively reviewed. On this basis, the La-Mg-Ni based A₂B₇ and A₅B₁₉ -type hydrogen storage electrode alloys were selected as the subject of this study, by means of XRD, SEM and EPMA analysis and the electrochemical test methods. The purpose of this paper is obtained the single-phase A₂B₇-type and A₅B₁₉-type hydrogen storage alloys by heat treatment, it is hoped to improve the cycle stability by the different phase structure.For the La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1alloy, the effect of the liquid encapsulated annealing treatment temperature on structure and electrochemical properties of the alloy were also investigated systematically in this paper. Experiment results showed that all alloys were composed of Ce₂Ni₇-type,Gd₂Co₇-type,LaNi₅-type,LaNi₃-type. After the alloy was annealed by the Liquid Encapsulated protects the Mg element volatility, the alloy not to obtain the sole A₂B₇ phase structure. The electrochemistry performance of alloy annealed at 1173K for 8h is quite good, the maximum discharge capacity is 362.60mAh/g, the cyclic stability S100= 79.14%. The high-rate discharge (HRD) of Alloy electrode (HRD₉₀₀) is 75.6%, The charge-transfer on the alloy surface was mainly controlling factor for the high-rate discharge ability. Alloy annealed by Liquid Encapsulated has not obtained the sole A₂B₇ phase structure alloy, but the electrochemistry performance of alloy better than the cast alloy's, The influence of the sole A₂B₇ phase structure on the electrochemistry performance is very important. Therefore, the annealing process and the alloying elements to be further improved.The La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1 alloy was annealed by the liquid encapsulated and the conventional and the two-steps annealing treatment. Experiment results showed that the phase structure of alloy electrode has not changed, only each abundance has changed. The electrochemistry performance of first two kind of annealing process alloys were the same because of the phases distributes were the same, the CaCu₅ abundance of two-steps annealing alloy increases obviously, alloy through three kinds of annealing craft, has not obtained the sole A₂B₇ phase structure. Considered as the influence of alloying elements designed the La_0.83Mg_0.17Ni_3.4Mn_0.1 alloy, the La_0.83Mg_0.17Ni_3.4Mn_0.1 alloy was annealed by the liquid encapsulated and the conventional and the two-steps annealing treatment, all alloys were composed of CaCu₅-type LaNi₅ phase, Ce₂Ni₇-type and Gd₂Co₇-type (La,Mg)₂Ni₇ phase, the LaNi₅ abundance of the alloy annealed by the liquid encapsulated is 8.48%, that more than the LaNi₅ abundance of the conventional annealing alloy in 4.39%. The LaNi₅ abundance of the alloy annealed by the two-steps is 6.81%, that less than the CaCu₅ abundance of the La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1 alloy. The LaNi₅ abundance of the La_0.83Mg_0.17Ni_3.4Mn_0.1 annealed by the conventional annealing is only 4.39%, that less than the LaNi₅ abundance of the La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1 alloy annealed by the conventional annealing. The La_0.83Mg_0.17Ni_3.4Mn_0.1 alloy annealed by the conventional annealing at 1173K for 8h, the electrochemistry performance of alloy is best, the maximum discharge capacity is 386.12 mAh/g, the capacity retention rate of 100 cyclic was more than 89.10%, its electrochemistry performance better than the La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1 alloy. From that we can see the influence of sole phase on the electrochemistry performance to be important.Based on optimizes the alloy ingredient, For the La_0.83Mg_0.17NixMn_0.1 (x=3.4,3.6)alloy, the effect of the annealing treatment temperature on structure and electrochemical properties of the alloy were also investigated systematically. The research indicated that the La_0.83Mg_0.17NixMn_0.1 (x=3.4,3.6) alloy annealed at 1173K for 8h to be possible to obtain more single type A₂B₇ and A₅B₁₉ phase composition. The CaCu₅ abundance of the La_0.83Mg_0.17NixMn_0.1 (x=3.4,3.6) alloy annealed at 1173K for 8h is very small, but has not obtained completely sole A₂B₇ and A₅B₁₉ phase composition. The activation properties of single type A₂B₇ and A₅B₁₉ phase composition alloys are the same. The maximum discharge capacity (386.12 mAh / g) of A2B7 type phase alloy is slightly higher than the maximum discharge capacity (371.38 mAh / g) of A₅B₁₉ type phase alloy. The cyclic stability of alloy electrode, both A₂B₇ and A₅B₁₉-type phase alloy are all very well. The capacity retention rate of 100 cyclic was more than 85%. The high-rate discharge (HRD₉₀₀ = 85%)of A₅B₁₉ phase alloy was higher than the high-rate discharge (HRD₉₀₀ = 76.6%) of A₂B₇ phase alloy. The charge-transfer on the alloy surface was mainly controlling factor for the high-rate discharge ability.The effect of rapidly quenching melt on the phase structure and electrochemical properties of La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1 hydrogen storage alloys was investigated systematically. The XRD analysis indicated that with increase of quenching rate, the phase composition of the La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1 alloy has not changed, but the each alloy phase abundance has obviously to change, with increase of quenching rate, LaNi₃ phase and Ce₂Ni₇-typeαLa2Ni7 phase is decrease gradually, but LaNi₅ and Gd₂Co₇-typeβLa₂Ni₇ phase is increase gradually. The structure of the La_0.83Mg_0.17Ni_3.25Al_0.15Mn_0.1 hydrogen alloy rapidly quenching is columnar crystals and the tiny crystal grain. With increase of quenching rate, the activation property of the alloy electrode is reduce, the maximum discharge capacity C_max decrease with the increase of quenching rate, but the cyclic stability to enhance.