| Based on the review of the research and development of the rare earth-based hydrogen storage alloys with PuNi3 structure, the AB2C9-type La-Ti-Mg-Ni system hydrogen storage alloys were selected as the study object of this work. The effect of element substitution and annealing treatment on the microstructure, the hydrogen storage and electrochemical properties of La-Ti-Mg-Ni system alloys were investigated systematically by means of XRD, PCT, hydrogen absorption/desoprion kineti, SEM, positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) measurements and electrochemical analysis including galvanostatic charge-discharge and cyclic voltammetries.For the La2-xTixMgNi9 (x=0.1,0.2,0.3,0.4) alloys, LaNi5 phase (space group P6/mmm) with the hexagonal CaCu5-type structure, LaNi3 phase (space group R-3m) and LaMg2Ni9 phase (space group R3m) with rhombohedral PuNi3-type structure are the main phase. With x increasing, the lattice parameters of LaNi5 and LaNi3 phase remain almost unchanged, and the cell volume of LaMg2Ni9 phase becomes smaller as x is above 0.2 because the radius of Ti of 1.45A is smaller than that of La of 1.88A. As Ti content increases, the maximum hydrogen storage capacities decrease gradually from 1.51wt.%(x=0.1) to 1.22wt.%(x=0.4), the hydrogen absorption/desorption plateau pressures first decrease and then increase, and the La1.8MgTi0.2Ni9 alloy has the lowest plateau pressure, which indicates that substituting La by suitable Ti content can lower the plateau pressure of La2-xTixMgNi9 (x=0.1,0.2,0.3,0.4) alloys. Among all the studied hydrides, the most stable La1.9Ti0.1MgNi9 hydride shows the slowest hydrogen desorption rate. Electrochemical studies reveal that the overall electrochemical properties of the alloy electrode with x=0.2 are better, e.g., its maximum discharge capacity is 333.2mAh/g, and the HRD1100 (high rate dischargeability at 1100mA/g discharging current density) reaches 83.7%, but its discharge capacity reduces to only 203.7mAh/g after 50 charge/discharge cycles, which needs also to be further improved.In order to improve the overall hydrogen storage properties of the alloys, La2-xTixMgNi9(x=0.1,0.2,0.3) alloys were prepared by annealing treatment, and the effect of heat treatment on the phase structure, hydrogen storage and electrochemical properties of La2-xTixMgNi9 (x=0.1,0.2,0.3) alloys were investigated in detail. The results indicate that for LaNi5, LaNi3, LaMg2Ni9 and Ti2Ni phases, Ti2Ni phase appears at 900℃, while LaMg2Ni9 phase disappears in the La1.9Ti0.1MgNi9 alloy annealed at 900℃. The diffraction peaks of the phases become narrowed or sharper by thermal treatment meaning higher composition homogeneity, which favors not only the improvement of the maximum/effective hydrogen storage capacity and hydrogen absorption/desorption kinetic, but also the decrease of the stability of hydrides and hydrogen absorption/desorption plateau pressure. The hysteresis factor increases after heat treatment because the higher composition homogeneity of annealed alloys decreases the grain boundaries and lattice defects, and increases the obstruction in the process of hydriding and dehydriding. For the maximum/effective hydrogen storage capacities and hydrogen absorption/desorption rate, they reach the optimization at 800℃for La1.9Ti0.1MgNi9 alloy, while at 900℃for La1.8Ti0.2MgNi9 and La1.7Ti0.3MgNi9 alloys. The reason is that LaMg2Ni9 phase disappears and Ti2Ni appears for the La1.9Ti0.1MgNi9 alloy annealed at 900℃. To summarize the results obtained by electrochemical measurements, the electrochemical properties including the maximum discharge capacity, the cycling stability and the high rate dischargeability (HRD) have been markedly improved after annealing treatment, and the optimum alloy is found to be La1.8Ti0.2MgNi9 alloy annealed at 900℃. For the alloy electrode, the maximum discharge capacity and HRD at the discharge current density 1100mA/g(HRD1100) are 365.7mAh/g and 85.1%, respectively. And it undergoes 177 charge/discharge cycles when the discharge capacity reduces to 60% maximum discharge capacity, which is higher than that of its as-cast alloy electrode of 52 cycles. The improvement of cycling stability can be associated with two facts. One is the higher composition homogeneity of annealed alloys, which reduces the particle pulverization, and improves anti-oxidation ability. The other is the LaNi5 phase with catalysis activity, which makes the stable Ti2Ni phase with high discharge capacity absorb/desorb hydrogen reversibly. For further improving the overall properties of La1.8Tio.2MgNi9 alloy, Ni is substituted by Co, and the La1.8Tio.2MgNi9-xCox (x=0,0.1,0.2,0.3,0.4,0.5) alloys defined as Col, Col,Co2, Co3, Co4, Co5 alloys, respectively, were prepared. The microstructure and the hydrogen storage and electrochemical properties of the alloys were systematically studied. It is found that LaNi5 phase and LaMg2Ni9 phase are the main phase. Co addition has almost not change the lattice parameters of LaNi5 phase, but leads to the disappearance of LaNi3 phase. The order of the cell volume of LaMg2Ni9 phase with increasing x is CoO>Co5>Co3>Col>Co4>Co2. The higher hydrogen storage capacity of CoO, Col, Co3 and Co5 alloys is due to their larger cell volume when compared to Co2 and Co4 alloys. Among all the La1.8Tio.2MgNi9-xCox (x=0,0.1,0.2,0.3,0.4, 0.5) alloys, Co4 alloy shows the smallest hydrogen storage capacity and the lowest hysteresis factor, but the highest hydrogen desorption plateau pressure. For Co2 and Co5 alloy, the positron annihilation lifetime spectroscopy and coincidence Doppler broadening spetra show that the mean positron lifetimes increases and the number of high momentum electode decreases after the alloys absorb and desorb hydrogen repeatedly (10 cycles) comparing with those of the alloys without hydrogenation, which is mainly attributed to the increase of defects. The maximum discharge capacity has been markedly improved by partial substitution of Co for Ni, and increases from 333.2mAh/g (CoO) to 365.2mAh/g (Co1),364.9mAh/g (Co2),350.2mAh/g (Co3),353.5mAh/g (Co4) and 364.7mAh/g (Co5). Moreover, the addition of Co slows down the capacity degradation and prolongs the cycle life, and the optimum Co content is x=0.5. However, the discharge capacity of Co5 alloy electrode decreases to 218.7mAh/g after 80 charge/discharge cycles with the decay of discharge capacity in each cycle of 1.83mAh/g·cycle, so the cycling stability should be further improved.For La1.8Tio.2MgNi9-xAlx (x=0,0.1,0.2,0.3,0.4,0.5) alloys, the results obtained by XRD analyses and electrochemical measurements shows that LaMg2Ni9 phase appears in all alloys. The presence of Al leads to the replacement of La(Ni,Al)5 phase for LaNi5 phase. LaNi2 phase appears and LaNi3 phase disappears with further increasing x (x>0.2). With the increasing of Al content, the cell volume of the alloys first decreases to the smallest as x=0.3 and then increases to the largest when x is 0.4 and 0.5. All alloy electrodes are easily activated to the maximum discharge capacity within 2-3 cycles. With x increasing, the maximum discharge capacity first increases from 333.2mAh/g (x=0) to 357.7mAh/g (x=0.1) and then decreases gradually to 319.8mAh/g (x=0.5). La1.8Tio.2MgNi8.9Alo.1 alloy electrode exhibits relatively higher discharge capacity in activation may be due to its better crystallization compared with Lai.8Tio.2MgNi9 alloy electrode and the disappearance of LaNi3 and appearance of LaNi2 phase as x>0.2. The addition of Al leads to a noticeable improvement of cycling stability, and the cycle life of La1.8Tio.2MgNi8.7Alo.3 alloy electrode is longest, which may be associated with its suitable Al content and the smallest cell volume. Among the alloys studied, the La1.8Tio.2MgNi8.7Al0.3 alloy electrode show a relatively good overall properties with the maximum discharge capacity of 340.0mAh/g and the retention of discharge capacity of 60% after 100 charge/discharge cycles.
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