An Investigation On The Phase Struture And Electrochemical Properties Of The La-Mg-Ni-based A₂B₇-type Hydrogen Storage Electrode Alloys

In this thesis, based on the review of research and development of the non-AB₂ type and non-AB₅ type rare earth-based hydrogen storage alloys, the La-Mg-Ni-based PuNi3-type alloys were selected as the starting point of this study. By means of XRD and SEM analysis, and the electrochemical test methods including the galvanostatic charge-discharge, linear polarization, EIS and potentialstatic discharge etc., the relationship among heat treatment, compositions, phase structure and electrochemical properties was studied systematically for the alloys. Firstly, the effect of annealing treatment on the structure and electrochemical properties of La_0.67Mg_0.33Ni_2.5Co_0.5 alloys was studied. On the basis of heat treatment of PuNi3-type alloys, it is found that the La-Mg-Ni-based A₂B₇-type alloys show much better overall electrochemical properties than the PuNi₃-type alloys. Consequently, the La-Mg-Ni-based A₂B₇-type alloys were selected as the object of study. And the influence of annealing temperature and element substitution on the structure and electrochemical properties of A₂B_y-type alloys were further investigated. Besides, the effect of heat treatment on the La-Mg-Ni-based alloys with two different compositions of Ni/(La,Mg)=3.8 and 4.0 was studied preliminarily.For the La(0.67)Mg(0.33)Ni_2.5Co_0.5 alloys, the effect of annealing temperature (1023K, 1073K, 1123K and 1173K) on the structure and electrochemical properties was investigated systematically. The results indicate that all the alloys consist of complex phases such as (La,Mg)(Ni,Co)3 phase (PuNi3-type), (La,Mg)2(Ni,Co)7 phase (Ce₂Ni₇-type), La(Ni,Co)₅ phase (CaCu₅-type) and LaMg(Ni,Co)4 phase (MgCu₄Sn-type). Annealing treatment at 1123K makes for the formation of PuNi3-type phase best in the alloy. While annealing treatment at 1173K makes for the formation of Ce₂Ni₇-type phase, and the main phase in alloy become (La,Mg)2(Ni,Co)7 phase. There is little effect on lattice parameters of the phases in alloy with annealing treatment. The plateau for desorption hydrogen has become flatter and wider after annealing treatment. And the discharge capacity of the alloy increases from 315.6mAh/g (as-cast) to 402.5mAh/g (1173K) with increasing annealing temperature. The Ce₂Ni₇-type alloy and PuNi3-type alloy have the same hydrogen absorption/desorption characteristic and electrode activation property. Furthermore, the Ce₂Ni₇-type alloy electrode shows better cyclic stability and kinetic properties than the PuNi3-type alloy electrode. And the capacity retention rate of the alloy annealed at 1173K reaches 92.9% after 70 charge/discharge cycles (S70).For the A₂B₇-type La_1.5Mg_0.5Ni_7.0 alloy, the influence of annealing temperature (1073K, 1123K and 1173K) on the structure and electrochemical properties was studied. The results indicate that the as-cast alloy consists mainly of LaNi₅ phase, LaMgNi₄ phase, (La,Mg)Ni3 phase and Gd₂Co₇-type phase. After heat treatment, the alloys consist of Gd₂Co₇-type phase, Ce₂Ni₇-type phase and (La,Mg)Ni₃ phase, and the (La, Mg)Ni₃ phase content decreases and the Ce₂Ni₇ type phase content increases with increasing the annealing temperature. The annealing treatment can obviously improve the overall electrochemical properties of alloy electrodes. The discharge capacity of the alloy annealed at 1073K is the highest (391.2mAh/g).With the increase of annealing temperature, the cyclic stability of the alloy electrodes increases and the cycle life of the alloy annealed at 1173K is thelongest (Si5o=82%). The high rate dischargeability (HRD) of the alloy electrodes increase with increasing annealing temperature and the alloy annealed at 1173K is the best (HRD9oo=89.0%). The Gd2Co7-type alloys and Ce2Ni7-type alloys have the same hydrogenation properties and electrochemical properties.Based on these results, La in the Lai.sMgo.5Ni7.o alloy was partly substituted by Y, and the structure and electrochemical properties of the La3-xYxMgNii4 (x=0, 1, 1.5, 2) alloys were investigated. It is found that all these alloys consist mainly of Gd2Co7-type phase and Ce2Ni7-type phase. The Gd2Co7-type phase content increases with increasing yttrium content while the Ce2Ni7-type phase content decreases. The increase of Y substitution in the alloys leads to a decrease in the cell volume. With increasing Y content, the plateau pressure for desorption hydrogen increases gradually and the discharge capacity of the alloy electrodes decreases from 363.3mAh/g (x=0) to 105.2mAh/g (x=2). But the cyclic stability of the alloys has been improved markedly with increasing x and the alloy with x=2 shows the best cycle stability (Sioo=86.2%). In addition, the high rate dischargeability (HRD) of the alloy electrodes decreases gradually with increasing x. The decrease of HRD is related to the decrease of both the charge-transfer velocity on the surface of electrode alloy and the hydrogen diffusion velocity in the bulk of alloy.In order to improve the overall properties of the alloy, Pr was selected as the substitution element for La in Lao.8Mgo.2Ni3 2C00.4AI0.2 alloy and the influence of Pr content on the structure and electrochemical properties of Lao.8-xPrxMgo.2Ni3.2Coo.4Alo.2 (x=0⁰.4) alloys was investigated. It is found that all the alloys consist mainly of LaNis phase, CesCoi9-type phase and PrsCo^-type phase. With increasing praseodymium content, the LaNis phase content decreases and the PrsCo^-type phase content increases, but the CesCoi9-type phase content increases first and then decreases with increasing x. The plateau pressure for desorption hydrogen decreases first and then increases with the increase of Pr content. The maximum discharge capacity of the alloy electrodes increases first and then decreases with the increase of Pr substitution in the alloys, and the discharge capacity of the alloy with x=0.15 is the highest (372.0mAh/g). Both the cyclic stability and high rate dischargeability (HRD) of the alloy electrodes have been improved markedly with increasing x. After 100 charge/discharge cycles (Si00), the capacity retention rate of the alloy with x=0.4 is 85.6%.For the La^MgNiig and LasMgNi24 alloys that were annealed at 1123K and 1173K for 24h and then water-quenched, the effect of annealing temperature on the structure and electrochemical properties was studied. The results indicate that all these alloys consist of LaNis phase, CesCo^-type phase and PrsCoisrtype phase, and also there is a small quantity of uncertain phase. The PrsCoig-type phase is prone to forming in the LasMgNi24 alloy. Electrochemical studies reveal that there is little effect on the electrode activation property of the alloy with difference annealing temperature. The discharge capacity of the two alloys decreases with increasing the annealing temperature. However, the cyclic stability of the alloys has been improved markedly with the increase of annealing temperature. The discharge capacity of the LasMgNi24 alloy annealed at 1223K is the highest (368.0mAh/g) and the cycle life of the La5MgNi24 alloy annealed at 1273K is the longest (Sioo=75.7%).