Phase Structure And Electrochemical Properties Of Re-Mg Based Composite Hydrogen Storage Alloys

AB₅-type rare earth-based alloy, a kind of hydrogen storage alloy used as negative electrode materials of the nickel/metal hydride (Ni/MH) secondary battery, has easy initial activation, long cycle life and low cost, but still a small discharge capacity, poor high-rate dischargeability (HRD) and poor property at low temperature. Therefore, it is meaningful both in theory and in practice how to increase its discharge capacity and to improve other electrochemical properties. Mg-based hydrogen storage alloys are remarkable because of its relatively high capacity, light weight and low material cost. For improving the electrochemical characteristics of AB₅-type hydrogen storage alloys, it is an effective method to prepare the composite hydrogen storage alloys using AB₅-type rare earth-based alloy and Mg-based alloy. In this paper, a new type of AB5-x mass%LaMg₃ (x=2, 3, 4, 5, 6, 7, 8) composite hydrogen storage alloys were prepared by sintering the powder mixtures of a commercial AB₅ alloy and LaMg₃ alloy as addition, the influence of sintering temperature and additive mass of LaMg3 on the phase structure and electrochemical characteristics of the composite hydrogen storage alloys were also studied. It was shown that AB₅-x mass%LaMg₃ (x=2, 3, 4, 5, 6, 7, 8) composite alloys have mult-phase structure, the matrix phase is CaCu₅ structure of AB5 alloy, the second phase is LaNi₃ phase. The discharge capacity, discharge capacity at low temperature and high-rate dischargeability of AB₅ alloy electrodes were greatly improved after sintering treatment at 1123 K by adding 5 mass%LaMg₃ alloy. The maximum discharge capacity of the composite alloy electrodes increased from 325 mAh/g for x=0 to 358 mAh/g for x=5, and the HRD of the composite alloy for x=5 at the current density of 1200 mA/g was 30% of that of the alloy at 60mA/g. The discharge capacity of AB₅-5 mass%LaMg₃ composite alloy electrode at 233 K was up to 174 mAh/g. But cycle stability of this alloy was poor. Its discharge capacity after 200 cycles was 60.5% of the max discharge capacity. In order to improve the cycle stability of AB₅-5 mass%LaMg3 composite alloys, the influence of vapour deposited coating on phase structure and electrochemical performance was researched. It was found that the cycle stability was improved evidently after vapour deposited Al coating. After 200 cycles, the capacity retention was increased from 60.5% to 68.4%.