The Chemical Composition Of The Stoichiometry And Preparation Process On The Performance Of La-Mg-Ni System Hydrogen Storage Alloy Electrode

In this thesis, previous research works on La-Mg-Ni system alloys had been extensivelyreviewed，combined with the laboratory’s related experience and achievements. On this basis,the La-Mg-Ni based AB_3.0-type hydrogen storage alloys of La_0.6Gd_0.2Mg_0.2Ni_2.6Co_0.3Al_0.1andA₅B₁₉-type hydrogen storage alloys of La_0.68Gd_0.2Mg_0.12Ni₃.3Co_0.3Al_0.1were selected as thesubject of this study. By means of ICP, XRD, EPMA, Rietveld analysis and theelectrochemical test methods, the hydrogen storage alloys were investigated systemically.Through preparation high phase abundance of the AB_3.0-type alloys and A₅B₁₉-type alloyswith Gd and low Mg, combined with hydrogen storage alloys ofLa_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1had been produced by the relative higher abundance ofA₂B₇-type phase. Because the Mg element in the La-Mg-Ni system hydrogen storage alloysplayed a very important role and the content was sensitive, but because of its owncharacteristics（the higher saturation vapor pressure、 the lower melting point and boilingpoint）in the preparation process was difficult to control. According to the recent relevantliterature and reports, the gas of He on the inhibition of Mg element had good effect, so thisarticle will research on the effect and mechanism of He in the melting process of Mg element.The AB_3.0-type storage alloys of La_0.6Gd_0.2Mg_0.2Ni_2.6Co_0.3Al_0.1were heated treatment atdifferent tempaeratures（873K-1123K）. The research found that the annealed alloy consistedof Ce₂Ni₇-type phase、CaCu5-type phase and PuNi₃-type phase.It was good to improve thePuNi₃-type phase abundance with the increasing annealing temperature. When the annealingtemperature was at1073K, the PuNi₃-type phase abundance reach to a maximum value of86.95wt%. Meanwhile the discharge capacity of this alloy electrode reached a maximumvalue of349.99mAh/g, but the cycle life(S₁₀₀)reduced to69.57%. When the annealingtemperature reached at873K, the discharge capacity of this alloy electrode was only244.17mAh/g; cycle life(S₁₀₀) reach to82.32%.The A₅B₁₉-type storage alloy of La_0.68Gd_0.2Mg_0.12Ni_3.3Co_0.3Al_0.1were heated treatment atdifferent tempaeratures（1173K-1273K）.The research found that the as-cast alloy mainlyconsisted of CaCu5-type phase、Pr₅Co₁₉(Ce₅Co₁₉)-type phase and PuNi₃-type phase. Annealedalloys had multiple phase organization，which was composited of the main phase ofA₅B₁₉(Pr₅Co₁₉and Ce₅Co₁₉)-type phase. The main phase of A₅B₁₉(Pr₅Co₁₉and Ce₅Co₁₉)-typeincreased gradually with the increasing annealing temperature. The phase abundances reacheda maximum value of87.8wt%when the annealing temperature was at1273K. The as-castalloy electrode had the lowest discharge capacity （313.98mAh/g） and the worst cyclestability （69.57%）. Annealing temperature was not obvious influence in alloy electrodeactivation performance and high rate discharge characteristics, but the electrode capacity andcycle stability was influenced. when the annealing temperature reached at1273K,the alloyelectrode showed the good electrochemical performance. The alloy electrode discharge capacity was373.01mAh/g and the cycle life(S₁₀₀) was90.20%.According to the different stoichiometry ratio alloys of AB_3.0-type, A₂B₇-type and A₅B₁₉-type were investigated. When each phase achieved higher abundance, the best electrochemicalperformance was A₂B₇-type alloy electrode of La_0.68Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1, theelectrochemical capacity、cycle life (S₁₀₀)and high rate capability(HRD₉₀₀)were386.80mAh/g、91.50%and80.90%; followed by A₅B₁₉-type alloy electrode ofLa_0.68Gd_0.2Mg_0.12Ni_3.3Co_0.3Al_0.1alloy; and the worst electrochemical performance was theAB_3.0-type alloy electrode of La_0.6Gd_0.2Mg_0.2Ni_2.6Co_0.3Al_0.1alloy, its electrochemicalcapacity、cycle life(S₁₀₀) and high rate capability(HRD₉₀₀)were349.99mAh/g,69.57%and38.83%.Mg content and its electrochemical properties were relatively large differences duringmelting under different atmosphere of the La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1alloy. In the pure Ar0.3MPa melting protective atmosphere，the content of Mg was only0.54wt%. In the pure He0.3MPa melting protective atmosphere，the Mg content increased to1.09wt%. The effect ofgas of He as smelting protective was superior than that of the gas of Ar. At the same time withthe increased pressure protection of the pure gas of He，the as-cast alloy of Mg contentshowed a trend of increase.When the alloy of La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1melt under Ar0.3MPa and covering agent，the content of Mg was1.25wt%，and the alloy electrode had agood cycle stability (S₁₀₀)and capacity were91.73%and383.60mAh/g.