Research Structure And Properties Of The La₄MgNi₁₉Hydrogen Storage Alloys

In this paper, based on the review of the research and development of La-Mg-Ni typehydrogen storage electrode alloys, the La₄MgNi₁₉alloys were investigated as the studyobject of this study. By means of X-ray diffractometer and scanning electron microscopeanalyses, and by CS310testing electrochemical of alloys. Choose Co, Fe and Mnsubstitution for Ni on the structure, the hydrogen storage and electrochemical properties ofLa₄MgNi_19-xR_x（x=0².0） alloys, the environment test temperature and the heat treatment onthe crystal structure and electrochemical properties of alloys were researched.XRD results show that La₄MgNi_19-xR_x（R=Co，Fe，Mn，x=0².0） alloys are mainlycomposed of LaNi₅type and La₄MgNi₁₉type phases. With the increasing of x, theabundance of LaNi₅phase in the R=Co alloy decreases, but that of La₄MgNi₁₉phase inalloys increases; at the same time, the phase abundances in R=Fe，Mn alloys show theopposite trend, and LaNi2phase appears in La₄MgNi₁₇Mn₂alloy. The microstructureobservation show that all of the alloys are dendritic structure, which becomes finer with theincrease of x value. The electrochemical tests show that La₄MgNi_19-xR_xalloys possess goodactivation performance and high rate discharge properties (HRD₉₀₀>85%), but the cyclestability need to be improved (S₁₀₀<60%). With the increase of x, the C_max of R=Co alloysgenerally increased from359.23mAh/g （x=0） to380.85mAh/g （x=2）, but the cycle lifedeclined from61.46%（x=0） to59.56%（x=2）; as for R=Fe alloys, the maximum dischargecapacity gradually decreased from359.23mAh/g （x=0） to333.77mAh/g（x=2）, but thevalue of cycle life went up first and then down; as for R=Mn alloy, the maximum dischargecapacity decrease after rising first, and the cycle life has showed a trend of decrease, reducefrom61.46%（x=0） to39.97%（x=2）. It is concluded that the phase abundance in La₄MgNi₁₉alloys and the appearance of new phase are the important factors which affect dischargecapacity and cycle stability of alloy electrode. Through the study on the dynamicsperformance of alloy electrodes, it is found that the good high rate discharge performance ofR=Co alloys is mainly due to its better electrocatalytic activity on the surface of alloy andhigher hydrogen diffusion rate in the bulk of alloy, and the HRD of R=Fe, Mn alloys arecontrolled by the former and the latter respectively. Compared completely, the R=Co alloysshow good Comprehensive electrochemical properties.For the as cast and the annealed La₄MgNi_17.5Co_1.5alloys, it is found that all of thealloys are mainly composed of LaNi₅type and La₄MgNi₁₉type phases. Heat treatment caneffectively improve the comprehensive electrochemical properties of alloys, and when thetemperature increases, the maximum discharge capacity and cycle stability of the alloy sshow a trend of rise after falling. It is concluded that heat treatment can improve thehomogenization of the alloys, and reduce the segregation phase in the alloys, which lead tothe improvement of the maximum discharge capacity and cycle life with higher annealingtemperature. However, with further increasing of temperature, the maximum discharge capacity and cycle life decline because of the volatilization of Mg element which leads tothe appearance of lower absorbing hydrogen phase. Among the alloys studied, the alloyannealed at1123K shows the best overall electrochemical properties: C_max=375.33mAh/g,HRD₉₀₀=94.83%, S₂₀₀=65.52%.The electrochemical performances of La₄MgNi_17.5Co_1.5alloys tested at Differenttemperature （283K³08K） are systematically studied. The results show that the increasingof temperature from283K to308K leads to the improvement of activation property of alloys,and the C_maxof alloys reaches the extreme value at308K （379.25mAh/g）. With theincreasing of temperature, the HRD of alloys increases but the cycle stability (S₁₀₀)decreases evidently from80.19%（283K） to52.04%（308K）. It suggests that the rise oftemperature accelerates the corrosion rate of alloy electrodes, and the bigger expansion orcontraction during the charge/discharge process increases the pulverization of alloy particles,which lead to the declination of cycle life with the rise of testing temperature.