Trans. Nonferrous Met. Soc. China 22(2012) 1995-1999

Phase structure and electrochemical properties of La_1.7+xMg_1.3-x(NiCoMn)_9.3(x=0-0.4) hydrogen storage alloys

WEI Fan-song¹, LI Li², XIANG Hong-fu¹, LI Hui¹, WEI Fan-na¹

1. School of Materials Science and Engineering,Jiangsu University of Science and Technology, Zhenjiang 212003, China;

2. Environment Monitoring Center of Zhenjiang City, Zhenjiang 212000, China

Received 1 August 2011; accepted 21 December 2011

Abstract: The phase structure and electrochemical properties of La_1.7+xMg_1.3-x(NiCoMn)_9.3(x=0-0.4) alloys were investigated. The XRD analysis reveals that the alloys consist of LaNi₅ phase and other phases, such as LaMg₂Ni₉ phase (PuNi₃ structure) and La₄MgNi₁₉ phases (Ce₅Co₁₉+Pr₅Co₁₉ structure, namely A₅B₁₉ type). With the increase of the x value, the LaMg₂Ni₉ phase fades away and La₄MgNi₁₉ phases appear, while the abundance of LaNi₅ phase firstly increases and then decreases. At the same time, the cell volume of LaNi₅ phase and LaMg₂Ni₉ phase decreases. The electrochemical measurement shows that alloy electrodes could be activated in 4-5 cycles, and with the increase of the x value, the maximum discharge capacity gradually increases from 330.9 mA·h/g (x=0) to 366.8 mA·h/g (x=0.4), but the high-rate dischargeability (HRD) and cyclic stability (S) decrease somewhat (x=0.4, HRD₆₀₀=82.32%, S₁₀₀=73.8%). It is found that the HRD is mainly controlled by the electrocatalytic activity on the alloy electrode surface, and the decline of cyclic stability is due to the appearance of A₅B₁₉ type phase with larger hydrogen storage capacity, which leads to larger volume expansion and more intercrystalline stress and then easier pulverization during charging/discharging.

Key words: hydrogen storage alloy; A₅B₁₉ type; crystal structure; electrochemical property; La-Mg-Ni system

1 Introduction

With the development of portable electronic devices and equipments, the demand for Ni/MH battery with higher capacity is ever growing. Mischmetal-based AB₅-type alloys are now widely used as the negative electrode materials for Ni/MH batteries, but their maximum discharge capacity is generally 330 mA·h/g or so which is approaching the theoretical capacity limited by single CaCu₅ type structure. It is difficult to meet the demand for increasing energy density of Ni/MH secondary battery [1]. In recent years, a great attention to La-Mg-Ni system alloys has been paid owing to the benefit of low cost and high discharge capacity. KADIR et al [2,3] revealed that RMg₂Ni₉ (R=rare earth, Ca or Y) alloys still held PuNi₃-type rhombohedral structure after hydriding, and their hydrogen storage capacity could reach 1.7%-1.8% (mass fraction) through the appropriate element substitution, which was significantly higher than that of the mischmetal-based AB₅-type alloys. Subsequently, CHEN et al [4] found that the discharge capacity of LaCaMg₂Ni₉ alloy was up to 360 mA·h/g, but the high-rate dischargeability (HRD) and cyclic stability were poor. KOHNO et al [5] investigated the properties of AB_3-3.5-type alloy electrode in La-Mg-Ni system, and found that at room temperature, the La_0.7Mg_0.3Ni_2.8Co_0.5 alloy had a discharge capacity of 410 mA·h/g which increased by 25% compared with the AB₅-type alloy, a better capacity retention rate after 30 cycles and a higher high-rate dischargeability, so it is considered to be a promising candidate for the negative electrode materials for Ni/MH batteries. However, considering the commercial applications, the overall electrochemical properties of such alloys have to be further improved. Researches have indicated that changing the stoichiometry [6,7] and partial substituting the elements of the alloy [8-14] are effective ways to improve the cycle life of alloy, at the same time, the cyclic stability can also be improved significantly by use of different material processing methods such as heat treatment [15,16] and rapid solidification [17,18]. In this work, La_1.7+xMg_1.3-xNi_7.05Co_1.98Mn_0.27 (x=0-0.4) alloys were prepared by vacuum melting followed with an annealing treatment, and the influence of mass ratio of La to Mg on the phase structure and electrochemical properties of the alloys was investigated in order to improve the overall electrochemical properties of the alloys.

2 Experimental

The alloy samples were prepared by vacuum levitation melting in argon atmosphere and remelted three times to ensure a high homogeneity. The purity of all starting elemental metals was higher than 99.9%. Half the as-cast ingot was annealed under vacuum at 1173 K for 12 h.

The alloy samples thus prepared were ground mechanically into powders below 50 μm and used for electrochemical tests and XRD analysis. The crystal structure of the alloy samples was determined by XRD analysis using a Rigaku D/max 2500/PC diffractometer with Cu K_α radiation, and the XRD patterns were obtained by the step scan mode with step size of  2θ=0.02 °.

For electrochemical tests, pellet type alloy electrodes (d=10mm) were prepared by cold pressing the mixture of the alloy powder with carbonyl Ni powder in a mass ratio of 1:4. The electrochemical tests were carried out at 298 K in a conventional tri-electrode cell consisting of a working electrode (MH electrode), a sintered Ni(OH)/NiOOH counter electrode and a Hg/HgO reference electrode, and the electrolyte was 6 mol/L KOH solution. The discharge capacity was determined galvanostatically by using an automatic charge/discharge unit (Land). Each electrode was charged at 100 mA/g for 4.5 h, and discharged at     60 mA/g to the cut-off potential of -0.7 V (vs Hg/HgO). The high-rate dischargeability (HRD) defined as C_n/(C_n+C₆₀) was determined by the ratio of the discharge capacity C_n (with n=300 or 600 mA/g, respectively) to the total discharge capacity defined as the sum of C_n and C₆₀, which was the additional capacity measured subsequently at 60 mA/g after C_n was measured. The cycling test was conducted at the charge/discharge current density of 100 mA/g. The cyclic capacity retention rate S₁₀₀ was defined as S₁₀₀=C₁₀₀/C_max, where C₁₀₀ was the discharge capacities at the 100th cycles. In evaluating the kinetic properties of the electrode reaction, the linear polarization curves of the electrode were plotted with a Solarton SI 1287 potentiostat by scanning the electrode potential at the rate of 0.1 mV/s from –5 to 5 mV(versus open circuit potential) at 50% depth of discharge (DOD). The potentiostatic discharge technique was used to evaluate the diffusion coefficient of hydrogen within the alloy bulk. After being fully charged and followed by a 30 min open circuit rest-period, the test electrodes were discharged with 600 mV potential-step for 2000 s on a Sloartron SI1287 potentiostat, and then the experimental data were analyzed by using the CorrWare electrochemical corrosion software.

3 Results and discussion

3.1 Phase structure

Figure 1 shows the XRD patterns of annealed La_1.7+xMg_1.3-x(NiCoMn)_9.3(x=0-0.4) alloys. It can be seen that all the alloys are composed of LaNi₅ phase (CaCu₅ structure, space group: P6/mmm) and some other phases, such as LaMg₂Ni₉ phase (PuNi₃ structure, space group: R-3m) and La₄MgNi₁₉ phases with A₅B₁₉ type structure (Ce₅Co₁₉+Pr₅Co₁₉, space groups are R-3m and P6₃/mmc respectively). With the increase of the x value, the diffraction peaks of the LaMg₂Ni₉ phase become lower, and when x=0.4, the peaks of the LaMg₂Ni₉ phase disappear, while those of La₄MgNi₁₉ phases appear. This indicates that the reduction of Mg content in the alloy can promote the increase of La₄MgNi₁₉ phases.

Fig. 1 XRD patterns of annealed La_1.7+xMg_1.3-x(NiCoMn)_9.3 (x=0-0.4) alloys

Figure 2 shows the XRD Rietveld analysis patterns of La_2.1Mg_0.9Ni_7.05Co_1.98Mn_0.27 alloys, in which the fitting factor (R_p) is 11.6 and goodness of fit (S) is 1.9. The fitting results reveal that the Mg occupies the 4f and 6c positions of La atom in A₅B₁₉ type structure, with the same occupation as Mg in PuNi₃ structure. The phase composition and lattice parameters of the phases in alloys were calculated by the Rietveld method and are listed in Table 1. It can be seen that as x increases, the abundance of LaMg₂Ni₉ phase gradually decreases from 61.29% (x=0) to none (x=0.4), but that of LaNi₅ phase firstly increases from 38.71% (x=0) to 63.32% (x=0.2) and then decreases to 26.31% (x=0.4). This indicates that the higher mass ratio of La to Mg easily leads to the transformation of LaMg₂Ni₉ phase into LaNi₅ phase and further into La₄MgNi₁₉ phase (Ce₅Co₁₉+Pr₅Co₁₉ type). It can also be seen from Table 1 that the cell volumes of LaMg₂Ni₉ phase and LaNi₅ phase tend down with the increase of x. For example, the volume of LaNi₅ phase decreases from 88.6254×10^-3 nm³ (x=0) to 88.4473×10^-3 nm³ (x=0.4), which may be due to the phase transformation and the variation of their abundance.

Fig. 2 X-ray diffraction patterns for La_2.1Mg_0.9Ni_7.05Co_1.98- Mn_0.27 alloy

3.2 Electrochemical properties

3.2.1 Characteristics of discharge platform

Figure 3 shows the electrochemical plateau curves for hydrogen desorption of La_1.7+xMg_1.3-x(NiCoMn)_9.3 (x=0-0.4) alloys. As can be seen from Fig.3, although all the alloys are composed of double phases or multiphases, their discharging plateau curves show a characteristics  of single platform, which may be ascribed to their approximate plateau pressures. When x=0.4, the plateau lowers obviously, and referring to the Rietveld analysis in Table 1, the total mass fraction of La₄MgNi₁₉ phase (Ce₅Co₁₉+Pr₅Co₁₉ type structure) with larger cell volume is up to 73.69%, which adds the hydrogen position in the alloy and then leads to a lower platform and a higher capacity.

Fig. 3 Electrochemical plateau curves for hydrogen desorption of La_1.7+xMg_1.3-x(NiCoMn)_9.3 (x=0-0.4) alloys at 298 K

3.2.2 Activation performance and discharge capacity

The electrochemical properties of the alloys are summarized in Table 2. It can be seen that alloy electrodes can be activated in 4-5 cycles, and with x increasing, the maximum discharge capacity gradually increases from 330.9 mA·h/g (x=0) to 366.8 mA·h/g (x= 0.4). This means that the higher mass ratio of La to Mg has a little influence on the activation performance, but can significantly enhance the original capacity, which may be related to the higher hydrogen storage capacity of La₄MgNi₁₉ phases (about 1.5%).

Table 1 Phase composition and lattice parameters of La_1.7+xMg_1.3-x(NiCoMn)_9.3(x=0-0.4) alloys

Table 2 Electrochemical properties of La_1.7+xMg_1.3-x(NiCoMn)_9.3(x=0-0.4) alloys

3.2.3 High-rate dischargeability

From Table 2, it is also found that the high-rate dischargeabilities of alloys at x=0 and x=0.2 are similar, and when the x value further increases to 0.4, the HRD decreases obviously. At the discharge rate of 300 mA/g, the HRD₃₀₀ of x=0 or x=0.2 alloy is around 97%, but that of x=0.4 alloy is only 92.69%. When the discharge current increases to 600 mA/g, the HRD₆₀₀ of x=0 or x=0.2 alloy is up to 93%, but that of x=0.4 alloy obviously decreases to 82.32%. It is known that the high-rate dischargeability of MH electrode is mainly influenced by the electrochemical reaction rate on the alloy surface and the diffusion rate of hydrogen in the bulk of the alloy [19]. To examine the factors of discharge kinetics in alloy electrodes, linear polarization and potential-step experiment were performed on the alloy electrodes, and then exchange current density (J₀) and hydrogen diffusion coefficient (D) were calculated respectively according to the obtained curves, as listed in Table 2, and the detailed calculating method can be seen in Ref. [20]. As shown in Table 2, with the increase of the x value, the exchange current density (J₀) of alloy electrodes firstly increases and then decreases, but the hydrogen diffusion coefficient (D) continuously decreases from 3.01×10^{-10 cm2}/s (x=0) to 2.51×10^{-10 cm2}/s (x=0.4). Comparing the HRD with the values of J₀ and D, it is found that the variation of J₀ is consistent with that of HRD values of the alloy electrodes. This indicates that the HRD is mainly controlled by the electrocatalytic activity on the alloy electrode surface.

3.2.4 Cyclic stability

Figure 4 shows the cyclic stability curves of alloy electrodes. From Table 2 and Fig. 4, it can be seen that the alloy with higher x value shows further poor cycling stability. For x=0.4 alloy, although its cycling stability declines obviously, the capacity retention rate (S₁₀₀) of the alloy after 100 cycles is still up to 73.8%. As seen from the above analysis, there are Ce₅Co₁₉+Pr₅Co₁₉ type phases with larger hydrogen storage capacity for x=0.4 alloy, which causes larger cell volume expansion and more intercrystalline stress, then resulting in easier pulverization during charge/discharge. It is well accepted that a higher degree of pulverization exposes more surface area to the corrosive electrolyte, then corrodes more and hence has a poor cycling stability.

Fig. 4 Cyclic stability curves of alloy electrodes at 298 K

4 Conclusions

1) The structure analysis reveals that all the alloys are composed of LaNi₅ phase (CaCu₅ structure) and other phases, such as LaMg₂Ni₉ phase (PuNi₃ structure) and La₄MgNi₁₉ phases with A₅B₁₉ type structure (Ce₅Co₁₉+Pr₅Co₁₉). With the increase of the x value, the LaMg₂Ni₉ phase disappears and La₄MgNi₁₉ phase appears, but the abundance of LaNi₅ phase increases firstly and then decreases. At the same time, the cell volume of LaNi₅ and LaMg₂Ni₉ decreases.

2) The electrochemical measurement shows that the increase of x value improves the activation and initial capacity, but lowers the high-rate dischargeability somewhat. It is found that the HRD is mainly controlled by the electrocatalytic activity on the alloy electrode surface, and the decline of cyclic stability is due to the appearance of A₅B₁₉ type phase with larger hydrogen storage capacity, which leads to larger volume expansion and intercrystalline stress and then easier pulverization during charge/discharge.

3) La_2.1Mg_0.9(NiCoMn)_9.3 alloy shows a high capacity (366.8 mA·h/g), reasonable cyclic stability  (S₁₀₀=73.8%) and good 1 C rate dischargeability (HRD₃₀₀= 92.69%), which is a promising candidate for a high capacity negative electrode material for Ni/MH batteries.

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La_1.7+xMg_1.3-x(NiCoMn)_9.3(x=0~0.4)贮氢合金的相结构与电化学性能

魏范松¹，黎  莉²，项宏福¹，李  惠¹，魏范娜¹

1. 江苏科技大学 材料科学与工程学院，镇江 212003；

2. 江苏省镇江市环境监测中心站，镇江 212000

摘  要：对La_1.7+xMg_1.3-x(NiCoMn)_9.3(x=0~0.4)贮氢合金相结构和电化学性能进行研究。结构分析表明，合金主要由LaNi₅相(CaCu₅结构)和其他相组成，如LaMg₂Ni₉相 (PuNi₃结构) 或La₄MgNi₁₉相(Ce₅Co₁₉+Pr₅Co₁₉结构)。随着x的增加，LaMg₂Ni₉相消失并出现La₄MgNi₁₉相，而LaNi₅相的含量则先增加后减小，且晶胞体积下降。电化学分析表明，合金电极只需4、5次循环即可活化；随着x的增加，最大放电容量逐渐增大，从x=0的330.9 mA·h/g增加到x=0.4的366.8 mA·h/g，但高倍率放电性能(HRD)和循环稳定性(S)则有所下降(x=0.4, HRD₆₀₀=82.32%, S₁₀₀=73.8% )。研究认为，HRD主要由合金电极表面的电催化活性控制，而循环稳定性的下降则是由于x=0.4合金中出现了具有较大吸氢量的Ce₅Co₁₉和Pr₅Co₁₉型结构相，导致吸氢膨胀率和晶间应力增大，使合金颗粒在吸放氢过程中较易粉化所致。

关键词：贮氢合金；A₅B₁₉型；晶体结构；电化学性能；La-Mg-Ni系

 (Edited by LI Xiang-qun)

Foundation item: Project (2008CL068L) supported by the Natural Science Research Project of Higher Education of Jiangsu Province, China; Project (50901036) supported by the National Natural Science Foundation of China

Corresponding author: WEI Fan-song; Tel: +86-511-85809119; E-mail: zju.wei@yahoo.com.cn

DOI: 10.1016/S1003-6326(11)61419-2