Studies On Phase Transformation And Electrochemical Properties Of Ce₂Ni₇-type La–Mg–Ni-based Hydrogen Storage Alloys

In this paper,Ce₂Ni₇-type hydrogen storage alloys were prepared by a powder sintering method with adjusting the different mole ratio of LaNi5 and LaMgNi4 precursors,meanwhile the Ce₂Ni₇-type La_0.6Nd_0.15Mg_0.25Ni_3.4 and La_0.6Nd_0.15Mg_0.25Ni_3.2Al_0.1 alloys and containing Gd2Co7-and Ce₂Ni₇-type La_0.6Nd_0.15Mg_0.25Ni_3.2Mn0.1 alloy electrode were prepared through induction melting followed by zoning annealing method.The effects of phase structure transformation in the process of sintering and annealing,different phase structure and phase composition on electrochemical properties of alloy electrodes were studied.Researchs show that as for the process of preparing alloys by powder sintering method,when molar ratio of precursors La Ni5 to LaMgNi4 is 0.87,the alloy has La Ni2 phase and PuNi3-type phase.When the molar ratio of precursors is 1,excessive LaNi5 phase and?La,Mg?Ni3 phase react to?La,Mg?2Ni7 phase by petiactic reaction,meanwhile containing PuNi3-type phase.When the value of LaNi5/LaMgNi4 continues to increase to 1.12,phase structure in the alloy is?La,Mg?2Ni7 phase and?La,Mg?5Ni19 phase.Electrochemical studies show that?La,Mg?2Ni7 phase possesses superior discharge capacity and cycling stability.In addition,?La,Mg?2Ni7 phase also is helpful to improve the discharge performance.It is found that Ce₂Ni₇-type phase is formed from A5B19-type phase by petiactic reaction with the change of the annealing temperature and time in the annealing process.The as-cast La_0.6Nd_0.15Mg_0.25Ni_3.4 alloy consists of LaNi5,Gd2Co7,Ce₂Ni₇,MgCu4 Sn and Ce5Co19 phases,then Ce₂Ni₇-type single-phase alloy was prepared when annealing 14 h at 1248 K.The analysis of the as-cast La_0.6Nd_0.15Mg_0.25Ni_3.2Al_0.1 alloy showed that it contains LaNi5,Ce₂Ni₇,Ce5Co19,Pr5Co19 and MgCu4 Sn phases,finally under the condition of annealing 14 h at 1223 K,Ce₂Ni₇-type single-phase structure alloy was obtained.The as-cast La_0.6Nd_0.15Mg_0.25Ni_3.2Mn0.1 alloy was composed of LaNi5,Gd2Co7,Ce₂Ni₇,Pr5Co19 and MgCu4 Sn phases,and Ce₂Ni₇-type single-phase alloy was obtained annealing at 1248 K for 10 h.The discharge capacities of the above three kinds of Ce₂Ni₇-type single-phase alloy electrodes were 395.9 mAh/g,384.7 mAh/g and 396.3 mAh/g,respectively.The cycling retention rate after 100 cycles of Ce₂Ni₇-type La_0.6Nd_0.15Mg_0.25Ni_3.4 single-phase alloy electrode is 89.1%.The cycling property at the 100 th cycle of Ce₂Ni₇-type single-phase La_0.6Nd_0.15Mg_0.25Ni_3.2Al_0.1 alloy increases from 77.0% to 90.5%.The cycle capacity retention rate after 100 charge/discharge cycles of the Ce₂Ni₇-type single-phase La_0.6Nd_0.15Mg_0.25Ni_3.2Mn0.1 alloy increases from 64.0% to 89.0%,while its discharge capacity at the 100 th cycle is 352.7 mAh/g,which is mainly due to the variable quantity of internal stress for Ce₂Ni₇-type single-phase alloy in the process of charge/discharge is small,so the ability of resisting pulverization and crystallization was enhanced.