Study On The Preparation Process And Properties Of A₅B₁₉-Type La₄MgNi₁₉ Hydrogen Storage Alloys

As the crucial application field for hydrogen energy,the A₅B₁₉-type La-Mg-Ni alloys,used as the negative materials for nickel/metal（Ni-MH）battery,are regarded as one of potential materials substituted for the commercial LaNi₅-type alloys,due to their high discharge capacity and good kinetics performance.Nowadays,smelting is still the common preparation method,and then followed by annealing for modification.However,during the preparation process,the volatilization of Mgand La results in the instability of components,and even there are still a few other phases in the annealed alloys.To solve the issues above,researchers focus their attention on the powder sintering process,and as reported,the single-phase La-Mg-Ni alloys have been successfully prepared in multiple experiments.Based on this sintering process,the vacuum sintering method by using quartz tube encapsulation is designed,and we choose the La-Ni and Mg-Ni intermetallics as materials,and the influences of sintering duration and temperatures with different combination materials,and Ni/La ratios on the phase structures and electrochemical performance are systematically investigated.The intermetallics including LaNi_3.1156 and Mg₂Ni are chosen as raw materials,the Ni/La ratio is 4.75,and the initial sintering parameter is 800℃,4 h.With the increase of sintering duration and temperatures,the phase structures of the alloys transform from multi-phases including LaMg₂Ni₉,LaNi₅ and LaNi₂（Ce Ni₂-type）into LaNi₅ single-phase.The alloy activation performance is affected minimally by the sintering duration,while it decreases significantly with the increase of temperatures.The maximum discharge capacity(C_max)increases with growing duration due to the increased abundance of LaNi₅ phase while decreases slightly with increasing temperatures,and among them,the alloy sintered at800℃for 4 h shows the highest capacity,which is 279.48 m Ah/g.The cycling stability decreases with the growing duration due to the decreased unit cell volume of LaNi₅ main-phase and reduction of low hydrogen-absorption phase,consequently the alloy anti-pulverization property decreases.while the cycling stability can hardly affected by the temperature,and S₁₀₀ remains around 60%.The high-rate performance improves remarkably with increasing duration and temperature.The analysis of kinetic performance shows that the high-rate performance is mainly controlled by the surface charge transfer rate when affected by the sintering time.However,when the temperature is different,it is mainly limited by the internal hydrogen diffusion rate.The intermetallics including LaNi_3.1156,LaNi₂ and MgNi₂ are chosen as raw materials,the Ni/La ratio is 4.75,and the initial sintering parameter is 900℃,6 h,then the time and temperatures are adjusted,respectively.The phase structure of all the alloys with different sintering parameters is almost single LaNi₅,only the alloy sintered at 850℃for 48 h contains a few LaNi₂（Ce Ni₂-type）phase.Since they are all LaNi₅ single-phase,the maximum discharge capacity of the alloys is similar,which is about 273～295 m Ah/g.Moreover,the alloy sintered at 900℃for 48 h shows the highest capacity,which is 294.83m Ah/g.The cycling stability decreases with the growing sintering duration and temperatures,which is related to the decreased unit cell volume of LaNi₅ phase or the reduction of phase interface.The high-rate performance improves remarkably with increasing duration and temperatures:with the growing sintering time at 900℃,the high-rate performance(HRD₉₀₀)increased from 71.79%to 83～91%,while it gradually increased from 80.65%to 95.45%with the increase of temperatures at 48 h.The kinetic analysis shows that the internal hydrogen diffusion rate is the key factor affecting the high rate performance of the alloys.Using LaNi_3.1156 and Mg₂Ni as raw materials,the Ni/La ratios are adjusted to 3.6～4.4,and the sintering parameter is 800℃,4 h.It is found that the phase structures of the alloys change significantly with the change of Ni/La ratios:when the ratios are 4.4 and 4.2,the main phases of the alloy are LaNi₅,La₄MgNi₁₉(Pr₅Co₁₉-type)and La₂Ni₇.When the ratio is4.0,the main phase are La₄MgNi₁₉(Pr₅Co₁₉-type)and LaNi₂（Ce Ni₂-type）,and when the ratio are 3.8 and 3.6,the main phase is La₂Ni₇.The activation performance of the alloys decreases with the decrease of the Ni/La ratios,and the number of activation increases from4 to 6～7 times.The maximum discharge capacity(C_max)declines with the decreased Ni/La ratios,which is due to the discharge capacity:La₄MgNi₁₉>La₂Ni₇>LaNi₂.Among them,the discharge capacity of the alloy with Ni/La=4.4 is the highest,which is 195.75 m Ah/g.Each alloy shows good cycling stability,and their cycling capacity retention(S₅₀)is higher than 82%.The Ni/La ratios have a significant influence on the high rate performance of the alloys:when the ratio is 4.0,HRD₉₀₀ is only 65.08%,while HRD₉₀₀ of other alloys is higher than 75%,and when the ratio is 4.4 and 3.8,the alloy HRD₉₀₀ is higher than 81%.The kinetic performance analysis shows that the high-rate performance of the alloys is subject to both the surface charge transfer rate and the internal hydrogen diffusion rate,but the latter plays a more critical role.