Study On The Structrue And Electrochemical Properties Of La-Mg-Ni Based New Type Hydrogen Storage Electrode Alloys

PuNi₃-type hydrogen storage alloy of La-Mg-Ni system is a kind of new type high capacity electrode alloy which was found in recent years, this kind of alloy possesses an important prospect of development and utilization. However, the cyclic life of alloy electrodes is poor, then it is the key question of unilization researches that how to improve the cyclic life of alloy electrodes. So in this paper, the PuNi₃-type alloy was of the jumping-off point of study of La-Mg-Ni system alloys. Firstly, element substitution and annealing treatment were conducted in order to improve the cyclic stability and high rate dischargeability (HRD) of PuNi₃-type alloys. On the basis of annealing treatment of PuNi₃-type alloys, it is found that A₂B₇-type alloys of La-Mg-Ni system show much better overall electrochemical properties than the PuNi₃-type alloys. Consequently A₂B₇-type alloys became the object of researches. The effect of element substitution and annealing treatment on alloy structure and electrochemical were investigated systemically, the study showed that A₂B₇-type alloy is an important new type candidate of Ni/MH battery negative electrode which processes a good commercialization prospect. Besides, in this paper, methods of material analysis, as ND (neutron diffraction), XRD (X-ray diffraction), particle analysis, HRTEM (high resolution transmission electron microscope) et al, and technology of electrochemical measurement like electrochemical impedance spectrum were introduced to investigate the capacity degradation rule of PuNi₃-type and Ce₂Ni₇-type alloys, and also to clarify the intrinsic reason of the better cyclic stability of Ce₂Ni₇-type alloys when compared with PuNi₃-type alloys.With respect to the PuNi₃-type alloy, La₂MgNi₉ alloy was as the basic component for study in this paper. The effect of the substitution of misch metal and Ti/Zr element on alloy phase structure and electrochemical properties were investigated systematically. It is found that misch metal like Ce, Pr, Nd had an distinct influence on the phase structure, also led to the anisotropic change of microstructure of unit cell and accelerated the pulverization of alloy particles. Misch metal could not improve cyclic stability of alloy electrodes effectively, but it could increase HRD of alloy electrodes remarkably. Ti/Zr elements could not dissolve into PuNi₃-type alloys, but formed stable Ti-Ni or Zr-Ni phase which was distributed like branch in alloys. Introduction of Ti/Zr elements could improve the cyclic stability and HRD of alloy electrode to some extent, but also reduced the discharge capacity of electrodes.To PuNi3-type alloy, the effect of annealing treatment temperature on structure and electrochemical properties of La_0.67Mg_0.33Ni_2.5Co_0.5 alloy were also investigated systematically in this paper. Annealing treatment had an important influence on the phase structure of alloys, annealing treatment at 1123K was favorite of the formation of PuNi₃-type, while annealing treatment at high temperature (> 1123K) would induce the phase transformation from PuNi₃-type to Ce₂Ni₇-type. At the same time, annealingtreatment would improve hydrogenation properties of alloys remarkably, Ce2Ni7-type alloy and PuNi3-type exhibited similar hydrogen absorption/desorption characteristic and electrode activation properties, and discharge capacity both electrodes could be up to 400mAh/g. However it was limited that the cyclic stability of PuNi3-type alloys were improved by annealing treatment. It should be noticed that Ce2Ni7-type electrode alloy showed better cyclic stability and kinetic properties than PuNi3-type one. The capacity retention rate after 70 charge/discharge cycles of Ce2Ni7-type alloy electrode was up to 92.9%, it has been proved to be a kind of high capacity alloy electrode with good applied prospect.Based on A2B7-type alloys, the effects of Mg content, Co content and rare earth elements on alloy structure and electrochemical properties were examined systematically. Experiment results showed that Mg content had an important influence on the phase structure of A2B7-type alloys. According to the different Mg content, alloys would present two different kinds of crystal structure with Ce2Ni7-type and Gd2Co7-type. High Mg content in alloys would lead to the decomposition of A2B7-type phase into PuNi3-type phase and LaNis phase. Because the ratio of CaCus unit and Laves unit in A2B7-type alloy and PuNi3-type alloy is different, the microstructure of both kinds of alloys exhibited different change rule. Mg atoms were distributed mainly at laves unit of A2B7-type alloy. The effect of Mg content on hydrogenation and electrochemical properties of alloys was distinct. When the Mg content was low, alloys would show hydrogen induced amorphous phenomenon with low discharge capacity, but cyclic stability was better;when Mg content increased, amorphous phenomenon disappeared, alloy electrodes exhibited high discharge capacity and good kinetic properties;when Mg content increased further,-and the overall electrochemical properties of alloy electrodes would be deteriorated. Co element in Ce2Ni7-type alloys could not change the phase structure of La1.5Mgo.5Ni7 alloy. Co atoms were only distributed at CaCus unit of Ce2Ni7-type unit cell, not at Laves unit. The special distribution of Co atoms in Ce2Ni7-type unit cell would lead to much more serious pulverization of alloy particle due to inconsistent volume expansion of two kinds of units, and then Co element could not improve cyclic stability of Ce2Ni7-type alloy electrodes. The effect of rare earth elements on electrochemical properties of A2B7-type alloy electrodes was similar to PuNi3-type alloy electrodes, and Ce element could deteriorate cyclic stability of A2B7-type alloy electrodes, and it should be used carefully.Based on the La1.5Mgo.5Ni7 alloy, the effect of annealing temperature on alloy phase structure and microstructure and electrochemical properties were investigated. As-cast alloy was a multi-phase structure;after annealing treatment, it was converted into double-phase structure with Gd2Co7-type and Ce2Ni7-type. Annealing treatment could obviously improve overall electrochemical properties of alloy electrodes. After annealing treatment at 1173K, the discharge capacity of electrodes could be up to 391.2mAh/g and the capacity retention rate after 150 charge/discharge cycles could reach 82.0%. Hydrogenation properties and electrochemical properties between Gd2Co7-type alloy and Ce2Ni7-type alloy was similar.The study of alloy structure, pulverization and corrosion clarified basicallycapacity degradation mechanism of PuNi3-type and Ce2Ni7-type alloys. Neutron diffraction analysis showed that volume expansion of Laves unit in unit cell was far beyond CaCus unit during hydrogenation process, what' s more, the expansion of Laves unit was anisotropic. The inconsistent expansion of two kinds of units would accelerate pulverization. In addition, PuNi3-type alloy has more Laves unit than Ce2Ni7-type one, so the volume expansion of the former was larger than the latter during hydrogenation process, which would lead to the more serious pulverization of PuNi3-type alloy when compared with Ce2Ni7-type alloy. During the electrochemical charge/discharge process, the more serious corrosion of PuNi3-type alloy in contrast to Ce2Ni7-type alloy was an important factor of the more poor cyclic stability of PuNi3-type alloy electrodes. The capacity degradation of Ce2Ni7-type alloy electrode resulted mainly from electrochemical corrosion. PuNi3-type alloy exhibited amorphous phenomenon in charge/discharge cycles as well as hydrogen absorption/desorption process, and amorphous phenomenon could cause the reduction of the reversible hydrogen capacity, which became the other important factor of discharge capacity degradation. In contrast to PuNi3-type alloy, Ce2Ni7-type didn' t exhibit obvious amorphous phenomenon, which was the main reason of better cyclic stability.