| La-Mg-Ni-based hydrogen storage alloys are a class of high-capacity electrode alloys which have been developed in recent years. As the candidate cathode materials of metal hydride nickel(MH-Ni) batteries, they have very important application prospect. However, at present, how to improve the comprehensive electrochemical properties of the alloy electrode, especially the cycle stability, is the key problem of resolving their practical application. This paper mainly takes A2B7-type alloys as the research object, and systematically investigated the effects of different elements partially substituting for La on their phase structure and electrochemical properties.The phase structure and electrochemical properties of over-stoichiometric La0.75Mg0.25-(Ni0.90Mn0.05Fe0.05)x(x = 3.3, 3.5, 3.8) hydrogen storage alloys were studied. When the stoichiometric ratio x was 3.3 or 3.5, the alloy mainly consisted of La Ni5 phase and(La,Mg)2Ni7 phase. However, when the x was 3.8, the alloy mainly consisted of La Ni5 phase and(La,Mg)Ni3 phase. As x increased from 3.3 to 3.8, the high rate dischargeability(HRD) of alloy electrodes was remarkably enhanced, the HRD1800 increased from 10.7% to 39.6%; The cyclic life and charge retention of alloy electrodes first increased and then decreased. When x = 3.5, they reached the optimum values. The exchange current density I0 and hydrogen diffusion coefficient D both increased with the increase of the stoichiometric ratio, and a linear correlation between the HRD1800 and the corresponding I0 was found.The A2B7-type hydrogen storage alloys La0.75R0.05Mg0.20Ni3.40Al0.10(R = La, Nd and Sm) mainly consisted of(La, Mg)2Ni7 phase, La Ni5 phase and(La,Mg)5Ni19 phase. Nd substitution for La remarkably promoted the formation of La Ni5 phase, while Sm was beneficial for the formation of(La,Mg)5Ni19 phase. At 1800 m A/g discharge current density, the HRD1800 of alloy electrodes increased by 13.9% and 6.5% with La substituted by Nd and Sm, respectively. The electrochemical kinetic measurements revealed that the exchange current density I0, charge transfer resistance R and hydrogen diffusion coefficient D for the alloy electrode were all facilitated with Nd/Sm partial substitution for La.The studies on the micro-structure and electrochemical properties of A2B7-type(La,Mg)1-xZrxNi3.5-2xMn2x(x = 0.00, 0.10, 0.20) hydrogen storage alloys have shown that(La,Mg)2Ni7 alloy was composed of La Ni5 phase,(La,Mg)2Ni7 phase and a small amount of(La,Mg)5Ni19 phase. The La Mg Ni4 phase appeared after Zr and Mn replaced La and Ni, respectively. Zr and Mn made the activation, high rate dischargeability and charge retention of alloy electrodes improve.In order to improve the electrochemical properties of La-Mg-Ni-based alloys, especially the high temperature electrochemical performances, yttrium(Y) element was used as the partial substitution element for lanthanum(La) and the alloys La0.80-xYxMg0.20Ni2.85Mn0.10Co0.55Al0.10(x = 0.00, 0.05 and 0.10) alloys were prepared. The XRD and SEM results revealed that the alloys were composed of La Ni5 and(La, Mg)2Ni7 phases, and the introduction of Y promoted the formation of(La, Mg)2Ni7 phase, thus causing the increase of discharge capacity of alloy electrodes. When the Y content x increased from 0.00 to 0.10, the HRD1800 of the alloy electrodes showed a sharp increase from 23.6% to 39.7% at 298 K. In addition, Y substitution for La improved the discharge kinetics of the alloy electrodes at higher temperatures. As the temperature increased from 298 K to 328 K, the exchange current density I0 increased and hydrogen diffusion coefficient D decreased. The high rate dischargeability of alloy electrodes reached their optimum values at 318 K. |
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