Electrochemical Performances And Degradation Mechanism Of RE-Mg-Ni Based Alloys

Superlattice La-Mg-Ni based hydrogen storage alloys are considered as the next-generation negative electrode active materials used in nickel/metal hydride?Ni/MH? battery for their excellent hydrogen storage properties compared with the traditional AB5-type alloys. However, electrochemical discharge capacities of those alloys decay rapidly during the long-term hydrogen storage cyclings. To further improve the cycling stability of La-Mg-Ni based alloys, it is important to understand relationship between the degradation mechanism and the chemical composition/ microstructure. Thus, the internal relations of the microstructure, electrochemical performances and degradation behaviors of La-Mg-Ni based alloys were systematically studied in the present work.In this dissertation, microstructural evolution from the as-cast La_1.7Mg_0.3Ni₇ and La_1.6Mg_0.4Ni₇ alloy to annealed samples was studied, and the phase transformation reactions were discussed combined with the computed phase-diagram. The results show that microstructural diversification occurs synchronously in the corresponding micro-areas of the as-cast and annealed alloys, demonstrating a significant heredity effect between the initial and annealed microstructure. Increase cooling rate by a proper amount increases the abundances of the matrix phase-?La,Mg?Ni₃ and?La,Mg?₂Ni₇, and reduces the amount of fine LaNi₅ to boost the abundance of?La,Mg?₂Ni₇. Orientation relationships between?La,Mg?₂Ni₇ and?La,Mg?₅Ni₁₉, as well as LaNi₅ and?La,Mg?₅Ni₁₉ are confirmed to be [10-10]//[10-10] and?0001?//?0001?. New La-Mg-Ni phases growing by the ledge mode are coherent with the matrix.The intrinsic corrosion and pulverization behaviors of the annealed La₂MgNi₉, La_1.5Mg_0.5Ni₇ and La₄MgNi₁₉ alloys were characterized by immersion and gaseous cycling test respectively. Moreover, hydrogen induced amorphization?HIA? behaviors of these alloys during gaseous cycling were investigated by XRD and TEM. It is found that corrosion of the Mg-rich phase is obviously more serious than that in the Mg-poor phase. The intrinsic corrosion resistance of the three alloys follows the trend that La₂MgNi₉>La_1.5Mg_0.5Ni₇>La₄MgNi₁₉. In addition, the intrinsic pulverization inclination is confirmed to be in the order of La₂MgNi₉<La_1.5Mg_0.5Ni₇<La₄MgNi₁₉. It shows that trend of the micro-hardness of the various phases is consistent with that of the pulverization inclination, indicating it is the key issue as to the pulverization behaviors. Furthermore, HIA followed by decomposition into nanocrystalline LaNi₅, Ni, LaH₃ and MgH₂ has occurred in the three alloys after gaseous cyclings. The structural stability towards HIA of various compounds follows the order: La₂MgNi₉>La_1.5Mg_0.5Ni₇>La₄MgNi₁₉. HIA promotes thermal stability of the amorphous hydrides and leads to the decrease of the gaseous and electrochemical hydrogen storage capability. HIA and disproportionation have also been detected after electrochemical cycling. HIA of the La-Mg-Ni phases is thus considered to be non-thermally activated. Amorphization of the La-Mg-Ni phases becomes more severe as the hydrogenation content and cooling rate increased and the charging temperature decreased.Effects of the rapid quenching and annealing treatment on the microstructure and hydrogen storage performances of the La₂MgNi₉, La_1.5Mg_0.5Ni₇ and La₄MgNi₁₉ alloys have been studied in the present dissertation. Increase in the cooling rate of the rapid quenching technology promotes the grain refinement and abundance of LaNi₅, depresses the electrochemical discharge capacity but significantly improves the electrochemical stability. Annealing after rapid quenching enhances the discharge capacity and high rate dischargeability?HRD?, but decreases the electrochemical stability. Fine microstructure is favorable to the hydrogen diffusion ability and leads to synergetic hydrogenation process, but contributes to a higher charge-transfer resistance, which is considered to be the main reason ascribed to the poor HRD. The as-quenched alloy has an excellent anti-pulverization ability. Annealing after rapid quenching enhances the intrinsic corrosion resistance of the alloys, but destroys the anti-pulverization ability due to the formation of micro-ledges and oxide particles.Partial substitution of Ni by Co, Al and Zn in the La₂Mg(Ni_1-X,M_X)₉?x=0.1,0.15,0.2? alloys and total replacement of La by Y, Ce and Nd in a series of RE_1.5Mg_0.5Ni₇ alloys were investigated in this dissertation. A new A₇B₂₃ type structure has been confirmed in the La₂Mg(Ni_0.8,Co_0.2)₉ alloy, whose sub block-layer is composed of three [AB₅] and two [A₂B₄] unit block-layer, and stacks by A-B-A-B type along the c-axis. Increase of Co content promotes both the discharge capacity and cycling stability, but worsens HRD. Co-addition aggravates pulverization but improves the structural stability and intrinsic corrosion resistance. Increase of Al and Zn leads to increase of abundance of AB₅ and AB₂ type phases, and decreases the plateau and suppresses the gaseous and electrochemical capacities and HRD. Al is harmful to the anti-pulverization ability but promotes the intrinsic corrosion resistance. Substitution by Zn is found to enhance the anti-pulverization property as well as the corrosion resistance of the A₂B₇ type phase. Replacements of La by Y, Ce and Nd significantly affect microstructure of the alloys. Nd decreases the discharge capacity but enhances the cycling stability and the structural stability, especially for the AB₂ type phase which can keep the crystal state after gaseous cycling. Y element is found to significantly enhance the anti-corrosion ability. However, the Ce_1.5Mg_0.5Ni₇ and Y_1.5Mg_0.5Ni₇ alloys can hardly to be discharged in an electrochemical-environment due to their high hydrogen absorption plateau.