| La-Mg-Ni type hydrogen storage alloys with superlattices have attracted people'sattention because of their high capacity and favorable activation since their appearance.The discharge capacity of this type alloy reaches 400mAh/g, which is nearly 20 percenthigher than that of AB5 type alloy, 330mAh/g. The competibility of Ni/MH battery willbe stronger if this type alloy is applied to Ni/MH battery. The reason for this type alloypossessing high capacity is that the La-Mg-Ni type hydrogen storage alloys can beregarded as the alternant arrangement of the two substructure of AB5 and AB2, which hashigh capacity for storing hydrogen and then called the superlattices hydrogen storagealloys. This work aims to improve the cycling performance of La-Mg-Ni type alloys,substituting partial elements and annealing heat treatment. XRD (X-ray diffraction), DC5and P-C-T (Pressure Component Temperature) techniques were used to study the phasestructure and electrochemical properties of La-Mg-Ni type hydrogen storage alloys.After investigating the influence of experimental condition, parameter and dataprocessing method of modern X-ray diffractometer on peak positions, intensities andHWHM, XRD methods of correct measuring and characterization the microcosmicstructural parameters of AB5-type hydrogen storage alloys have been described andproposed. Then this mothed was applied to the study of structure of La-Mg-Ni typehydrogen storage alloy.Designing the La0.7Mg0.3(Ni1-x(Cox)3.5(x=0.00-0.20) alloys in order to optimizing thecontent of Co. The results indicate that the alloys were composed of the LaNi5 phase withthe CaCu5-type structure and the (La,Mg)2Ni7 phase with the Ce2Ni7-type structure. The maximum electrochemical discharge capacity increases with the increase of Co content.Moreover, the cycle stabilities of La0.7Mg0.3(Ni1-xCox)3.5 are improved remarkably by smallquantity replacement of Ni by Co, after 250 cycles, the discharge capacity increases from30% for x=0.00 to 65% for x=0.20. Mg is one of the based elements of this type alloys. Inorder to optimize the content of Mg, and design the La1-xMgxNi2.8Co0.7(x=0.10-0.25)alloys.X-ray diffraction (XRD) shows that all of the alloys consist of a main phase withCe2Ni7-type structure with small impurities, such as LaNi5-type phases. The dischargecapacity of alloys increases firstly and then decreases. Electrochemical studies indicate thatthe maximum discharge capacity firstly increases from 350.9mAh/g(x=0.10) to388.TmAh/g(x=0.20) and then decreases to 369.9mAh/g(x=0.25). However, withincreasing Mg substitution, the cycling durability of electrodes wasn't remarkablydeteriorated. In the face of High rate dischargeability, there is only little difference amongthe alloys when x≤0.2, but it descends rapidly when x＞0.2. The alloys exhibit a higherLTD-Low Temperature Dischargeability-(LTD233K＞70%) when x=0.2.The effect of several elements on the structure and performance of alloys was studiedsystematically through orthogonality. The result indicates that additive elements don'tchange the structure of alloys. The element La is one of the basic constituent of the alloys,the discharge capacity decreases with the increase of La content, and the rule wasn'tdiscovered between the cycling durability and La content. The discharge capacitydecreases because of addition of Pr and Nd. The decreases with the increase of Pr content,but increases with the increase of Nd content. Mg is the key element of the alloys. Thedischarge capacity increases with the increase of Mg content, but the rate of attenuationhas a decreasing trend. Ni is another basic content of alloys. The discharge capacityincreases with the decrease of Ni content. However, the rate of attenuation firstly increasesand then decreases with the increase of Ni content, and has a maximum when x=0.9. Co isa beneficial element of alloys. The addition of Co content not only enhances the dischargecapacity of alloys, but also improves the cycling durability. The discharge capacity ofalloys firstly increases and then decreases with the increase of ratio of B/A value, andreaches a maximum at x=3.5, at the same time, the rate of attenuation reaches a minimum. From the experiment, the optimization alloy is La0.5Pr0.2Nd0.1 Mg0.2(Ni0.85Co0.15)3.5.The La0.5Pr0.2Nd0.1Mg0.2(Ni0.85-xCo0.15Alx)3.5(x=0.01-0.04) alloys were prepared inorder to find the effects on the structure and property by the addition of Al. The resultsindicate that the alloys were composed of (La,Mg)2Ni7 phase with the Ce2Ni7-typestructure. The value of c/a increases with the increase of Al content. The increase of Alleads to some decrease in both the discharge capacity and the high-ratedischargeability(HRD) but leads to a significant improvement in cycling stability of alloys.The discharge capacity decreases from 394.6mAh/g (x=0.01) to 380.6mAh/g (x=0.04), andthe HRD decreases from 61% (x=0.01) to 35% (x=0.04) as the electrodes were dischargedat 1200mA/g. However, the rate of attenuation decreases from -0.32mAh/g·time (x=0.01)to -0.20mAh/g·time (x=0.04).For studying the effect of Mn, Mo, Ce elements on the structure and performance ofalloys, we designed three different types of alloys, that are, La0.4Pr0.3Nd0.1Mg0.2(Ni1-xCo0.15Al0.02Mnx)3.5 (x=0.01-0.04),La0.50Pr0.22Nd0.06Mg0.22(Ni0.84-xCo0.15Al0.01Moy)3.5 (y=0.00-0.04),La0.50-x CezPr0.2Nd0.1Mg0.2(Ni0.84Co0.15Al0.01)3.5(z=0.00-0.06). The results indicatethat all the three elements decrease the discharge capacity of alloys. Mn has little effect onthe cycling durability, but Mo and Ce deteriorate the cycling durability of alloys. The HRDof the alloys can be improved when y≤0.01,z≤0.04. However, the performance of HRDdecreases with continued increase of Mo and Ce content.The granularity of alloys powder is smaller, the initial discharge capacity and the rateof attenuation are bigger. Oxidation is the main reason of attenuation during the initialstages of cycling. But the pulverization is the main cause of attenuation during the belowstages of cycling.
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