Phase Structure And Electrochemical Properties Of New-Type Ml-Mg-Based Hydrogen Storage Alloys

The alloys Ml_0.90Mg_0.10(NiCoMnAl)_5x (x = 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90), Ml_1-xMg_x(NiCoMnAl)_3.5 (x = 0.10, 0.20, 0.25, 0.30), La_0.60R_0.20Mg_0.20(NiCoMnAl)_3.5 (R = La, Ce, Pr, Nd) and AB_5–x mass%LaMg₃ (x = 0, 2, 3, 5, 6, 8) composite alloys were prepared and investigated and then the composite alloys were plated by a vacuum evaporation plating method in this paper. ICP, XRD, SEM-EDX, current static charge/discharge and test for kinetics were employed to determine the microstructure, charge/discharge characteristics and electrochemical kinetics properties. The ICP, SEM and XPS methods were used to test the difference in electrolyte composition, structure and elemental chemical state before and after cycling in order to find out the degradation mechanism of Ml–Mg-based alloys. Meanwhile, the techniques for formalization preparation of Ml–Mg-based alloys have been investigated. The properties of the AA-type high-capacity MH/Ni batteries assembled using the Ml–Mg-based alloys as negative electrode materials were tested and analyzed.The Ml_0.90Mg_0.10(NiCoMnAl)_5x alloys consist mainly of LaNi₅ phase and LaNi₃ phase. As the stoichiometry increases, the maximum discharge capacity of the alloy electrodes increases from 204 mAh/g (x = 0.60) to 375 mAh/g (x = 0.70) and then decreases to 343 mAh/g (x = 0.90). The high rate dischargeability under the mixed control of surface reaction and bulk diffusion increases and then decreases with increasing x. The low temperature dischargeability and the cycling stability increase as x increses. When x is 0.90, the capacity retention at the 200th cycle (S200) reaches 80.7%. In the Ml1-xMgx- (NiCoMnAl)_3.5 alloys, Rietveld refinement shows that Mg increases the relative content of LaNi₃ phase, and therefore the maximum discharge capacity, high rate dischargeability, low temperature dischargeability and charge retention increase first and then decrease. When Ml/Mg is 0.80/0.20, the alloy exhibits higher capacity (369 mAh/g) and better kinetics, high rate and low temperature dischargeability. However, the cycling stability is deteriorated with increasing Mg content. The partial substitution of La with Ce improves the kinetics, high rate and low temperature dischargeability; Nd substitution improves the cycling stability, capacity retention rate at the 100th cycle increasing by 13.4%. Study on the AB₅–x mass%LaMg₃ composite hydrogen storage alloys shows that the AB5–5 mass%LaMg₃ alloy prepared by sintering at 1123 K has larger discharge capacity and better high rate and low temperature dischargeability. The vacuum evaporation plating of Cu, Al and Ni was performed on the AB5–5 mass%LaMg₃ alloy, and the high rate dischargeability, low temperature dischargeability and cycling stability have been further improved.The alloy Ml_0.88Mg_0.12(NiMnCoAl)_3.5 with good overall electrochemical properties has been prepared by adopting a properly low Mg content and substituting pure La with La-rich mishmetal. The alloy consists mainly of CaCu5-type LaNi5 phase and Ce2Ni7-type La2Ni7 phase. In comparison with commercial AB5-type alloy, the alloy has some advantages, such as easier to activate, higher discharge capacity (about 16% higher than that of commercial AB5-type alloy), better high rate dischargeability, lower self-discharge rate and so on. However, its cycling stability needs to improve futher. Research on the degradation mechanism of Ml–Mg-based alloys shows that deformation of alloy structure caused by repeated in and out of hydrogen atoms and serious corrosion of La, Mg and Al are the two main reasons to the degradation. Especially, the corrosion substance forms a passivation layer at the surface of alloy particles, deteriorating the kinetics, lowering the electrochemical properties.The evaporation of Mg is reduced by adopting the methods of replacing Mg with Mg–RE(Ni) intermetallic alloy, employing special material-adding method and con- trolling melting power and time. The difference in alloy composition between batches has been eliminated. And finally the alloy with required composition has been prepared successfully. And also we have worked out rational parameters for melting and annealing and rational principle for cleaning the furnace. The Ml–Mg-based hydrogen storage alloys we prepared have been used to assemble AA-type 2400 mAh MH/Ni batteries. The measurement shows that batteries have good discharge plateau. The 0.5C charge/ discharge cycle life and IEC cycle life can both meet the requirement. The inner pressure during charge process is no more than 3.0 MPa, lower than that of batteries unsing commercial alloys, and it enhances the safety of the MH/Ni batteries.