Trans. Nonferrous Met. Soc. China 25(2015) 520-526

Effect of Sn substitution for Co on microstructure and electrochemical performance of AB₅ type La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys

Julio Cesar Serafim CASINI^1,2, Zai-ping GUO¹, Hua Kun LIU¹,

Eliner Affonso FERREIRA², Rubens Nunes FARIA², Hidetoshi TAKIISHI²

1. Institute for Semiconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia;

2. Materials Science and Technology Center, Nuclear and Energy Research Institute, University of Paulo, SP 05508-900, Brazil

Received 18 November 2013; accepted 11 March 2014

Abstract: The effects of substitution of Sn for Co on the microstructure, hydrogen storage and electrochemical discharge capacity of La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0, 0.1, 0.2, 0.3 and 0.5) alloys were investigated using X-ray diffraction (XRD), pressure composition isotherm (PCT) and electrochemical discharge cycle. XRD, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) tests showed that all of alloys are mainly composed of LaNi₅ and MgNi₂ phases, but when increasing the content of Sn in alloys, the LaNiSn phase appears and microstructure is refined. The PCT showed that increasing substitution of Sn for Co results in decrease of the maximum hydrogen storage capacity from 1.48% (x=0) to 0.85% (x=0.5). The electrochemical tests indicated that the maximum discharge capacity decreases from 337.1 mA·h/g (x=0) to 239.8 mA·h/g (x=0.5); however, the discharge capacity retention at the 100th cycle increases from 70.2% (x=0) to 78.0% (x=0.5).

Key words: hydrogen storage alloys; microstructure; Ni-MH batteries; Sn; Co; substitution

1 Introduction

Over the past years, LaNi₅-based hydrogen storage alloys for use in electrodes of rechargeable batteries have systematically incorporated specific alloying elements to improve the kinetics of hydrogen absorption and desorption, increase cycle life, improve corrosion resistance, etc. Cobalt, aluminium, manganese and magnesium are invariably present in the alloy composition [1].

What is known about Co is a very important element for improving the discharge capacity and prolonging the cycle lives of the electrodes alloys. For example, LIU et al [2,3] studied the effect of Co content on the structure, hydrogen storage and electrochemical performance of the La_0.7Mg_0.3Ni_3.4-xMn_0.1Co_x (x=0-1.6) alloys. Firstly, partial substitution of Ni by Co effectively improved the cyclic stability and the discharge capacity initially increased from 397.5 mA·h/g (x=0.00) to 403.1 mA·h/g (x=0.75) then a slight loss to 380.7 mA·h/g (x=1.60). Secondly, the appropriate substitution also increased the electrochemical kinetic performance of the alloy [2,3].

The typical commercial alloy, used as negative electrode of Ni-MH batteries, MmNi_3.55Co_0.75Mn_0.4Al_0.3, has high capacity and good cycling stability. It contains 10% of Co. However, Co takes up about 40% of the total cost of this alloy [4,5].

In the literature, there are several examples of Co-free and low-Co AB₅ type alloys. FERREIRA et al [6] investigated the influence of substitution of Co by Nb on La_0.7Mg_0.3Al_0.3Mn_0.4X_0.5Ni_3.8 (x=Co, Nb) alloys. Results showed that NbNi₃ phase appears on the microstructure and the discharge capacity decreases from 324 mA·h/g (Co) to 221 mA·h/g (Nb) [6].

Few investigations had been published based on substitution of Sn for Ni [7-10]. For example, KUMAR et al [10] reported a favourable charge-discharge cycling stability and good discharge capacity on the LaNi_4.25Co_0.5Sn_0.25 alloy. In this work, the microstructure, phase identification, hydrogen absorption isotherms, electrochemical capacity, cycling stability and high rate dischargeability of AB₅ type La_0.7Mg_0.3Al_0.3Mn_0.4- Co_0.5-xSn_xNi_3.8 hydrogen storage alloys are investigated systematically.

2 Experimental

The nominal composition of the studied alloys was designed as La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0, 0.1, 0.2, 0.3 and 0.5). The purities of all elements were at least 99.9%. The alloys were prepared by induction melting in a water-cooled copper crucible under the protection of argon atmosphere. The ingots were re-melted twice for homogeneity.

For X-ray diffraction (XRD), pressure composition isotherm (PCT) and electrochemical measurements, the alloys were mechanically pulverized and sieved through 200 mesh size (<75 μm).

Phase identification was performed on a GBC MMA X-ray diffractometer by Cu K_α radiation at 40 kV and 25 mA. The scan rate was 0.25 (°)/min with a step of 0.02° from 10° to 100°. The phase compositions were collected using Crystallographica-Search Match (CSM, PDF release 2002). The Rietveld refinement was obtained using Materials Studio 4.3.

In order to investigate the morphology and composition of the alloy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectrometry (EDS) was used.

The PCT tests were conducted using a gas reaction controller, which is a product of Advanced Materials Corporation, USA. Before the measurement, each sample was put into a vessel and was evacuated at 30 °C for 1 h using a rotary vacuum pump, and activated 5 cycles. The PCT tests were performed at 30 °C until 3 MPa of hydrogen pressure. Hydrogen gas with purity above 99.99% was supplied from a cylinder.

All test electrodes were prepared by mixing alloy powder and nickel powder (<3 μm) with mass ratio of 1:4. The mixture was cold pressed under a pressure of 15 MPa (about 1 cm² in area and 0.1 mm in thickness). The discharge capacity of each electrode was measured in a flooded cell configuration using Ni(OH)₂/NiOOH as a counter electrode and 6 mol/L KOH solution as the electrolyte. The system was charged at 100 mA/g for 5 h followed by a 10 min rest and then discharged at 50 mA/g to the cut-off potential of 0.8 V. The high rate dischargeabilities were calculated after electrochemical cells were completely active.

3 Results and discussion

3.1 Microstructure and phase identification

The XRD patterns of the La_0.7Mg_0.3Al_0.3Mn_0.4- Co_0.5-xSn_xNi_3.8 (x=0-0.5) hydrogen storage alloys are shown in Fig. 1. It could be seen that the alloys are mainly composed of LaNi₅ (CaCu₅-type hexagonal structure) and MgNi₂ (hexagonal structure), while the La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5Ni_3.8 alloy shows (La,Mg)Ni₃ (PuNi₃-type rhombohedral structure) phase. The LaNiSn phase is formed and increased gradually when increasing x.

Fig. 1 XRD patterns of La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) hydrogen storage alloys

The lattice parameters, unit cell volumes and the phase abundance of the alloys with different values of x obtained from Rietveld method of XRD analysis are listed in Table 1. It can be seen clearly that the parameters of a and c and the unit cell volumes of the LaNi₅ phase increase with increasing Sn content. Because the atomic radius of Co (1.35 ) is smaller than that (1.45 ) of Sn, the content of the LaNi₅ decreases from 75.44% (x=0) to 53.42% (x=0.5), while the content of LaNiSn increases from 4.44% (x=0.1) to 18.96% (x=0.5).

Figure 2 shows the SEM micrographs of the La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) hydrogen storage alloys. Combining the results of XRD, SEM micrographs and EDS (Table 2) analysis shows that the La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys are composed of La- and Ni-rich phase (LaNi₅ phase, light grey regions), Mg- and Ni-rich phase (MgNi₂ phase, dark regions) and La-, Sn- and Ni-rich phase (LaNiSn phase, white regions) with small quantity of Mg content. As shown in Fig. 2(a), the composition of La, Mg and Ni ((La,Mg)Ni₃ phase, dark grey regions) is similar to the results reported by ZHANG et al [11] on the La_1.3CaMg_0.7Ni_9-x(Al_0.5W_0.5)_x alloys. When increasing the content of Sn, the content of LaNiSn phase increases gradually and more fine grains are found, which is similar for LaNi_4.0Al_0.2Fe_0.4Cu_0.4-xSn_x alloys studied by REN et al [12]. The further research would be to study the influence of annealing treatment on these alloys.

Table 1 Characteristics of alloy phase in La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) hydrogen storage alloys

Fig. 2 SEM images of La_0.7Mg_0.3Al_0.3Mn_0.4- Co_0.5-xSn_xNi_3.8 alloys

Table 2 EDS results for different phases identified in SEM images

3.2 Pressure-composition-temperature (PCT) tests

PCT curves of the La_0.7Mg_0.3Al_0.3Mn_0.4- Co_0.5-xSn_xNi_3.8 (x=0-0.5) hydrogen storage alloys are shown in Fig. 3. It is seen that the maximum hydrogen storage capacity of the alloys decreases from 1.48% (x=0) to 0.85% (x=0.5), which could be attributed to the content of LaNi₅ reduced when Sn substituted Co. REN et al [12] reported that the substitution of Sn in LaNi_4.0Al_0.2Fe_0.4Cu_0.4-xSn_x alloys also decreased the hydrogen storage capacity.

Fig. 3 PCT curves of La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys at 30 °C (H/M represents the hydrogen storage in a metal alloy)

These support that the optimized content of LaNi₅ phase and (La,Mg)Ni₃ phase improved the hydrogen storage capacity [13].

The PCT isotherm absorption-desorption hysteresis factor (H_f) was calculated by the following formula:

                                (1)

where p_a and p_d are the absorption and desorption equilibrium pressures at 50% of hydrogen storage capacity. In Table 3, the slope factor (S_f) is the ratio of capacities between the middle region (defined between 0.01 and 0.5 MPa) and the entire reversible region. The data of H_f and S_f for each alloy are listed in Table 3.

Table 3 Maximum hydrogen storage capacity, H_f, and S_f of La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys

The hysteresis factor is an important factor that represents the reversibility of hydrogen absorption and desorption of the hydrogen storage alloys. It is clearly observed from Table 3 that H_f decreases from 1.06 (x=0) to 0.71 (x=0.5) with increase of Sn content in the alloys. On the other hand, the reversibility between hydrogen absorption and desorption is improved by increase of Sn content in the alloys and eventually results in the decrease of H_f. This suggests that increase of LaNi₅ cell volume may enhance the hydrogen storage capability. As indicated by HUANG et al [14], the increase of crystal cell volume reduces the pressure of hydrogen absorption and desorption of the alloys.

The slope factor can explain the plateau performance of hydrogen absorption. It is seen from Table 3 that S_f is enhanced from 38.85% (x=0) to 65.24% (x=0.5) with increase of Sn content in the alloys, which means that plateau performance of hydrogen absorption is weakened. HUANG et al [14] and ZHOU et al [15] showed that H_f and S_f are strongly influenced by homogenous composition. The homogenous composition and perfected structure improve the plateau performance of hydrogen absorption. It may be assumed that homogeneity of phases of the alloys decreases with increasing Sn content.

3.3 Electrochemical discharge characteristics

Figure 4 shows the activation profiles of the La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys electrodes. It can be seen that all electrodes can be easily activated to their maximum discharge capacity within five cycles.

The maximum discharge capacities (C_max) of the electrode alloys are summarized in Table 4. It can be seen clearly that electrochemical capacity decreases markedly from 337.1 mA·h/g (x=0) to 239.8 mA·h/g (x=0.5) with the increase in Sn content. However, a close inspection reveals that with small Sn content, such as La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.4Sn_0.1Ni_3.8 (x=0.1), the maximum discharge capacity only decreases by 1.5% and the capacity retention increases by 1.1% compared with Sn-free alloy (x=0). These results are similar to MlNi_(4.45-x)Mn_0.4Al_0.15Sn_x reported by MA et al [16]. The discharge capacity of the MlNi_(4.45-x)Mn_0.4Al_0.15Sn_x electrode alloys decreases from 343.2 mA·h/g (x=0) to 235.4 mA·h/g (x=0.5) with increase of Sn content.

Fig. 4 Activation profiles of La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys

Table 4 Electrochemical properties of La_0.7Mg_0.3Al_0.3Mn_0.4- Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys

As indicated by MUNGOLE et al [17], the decrease in the discharge capacity can be ascribed to the formation of the precipitate phase. In our case, we found that the formation and increase of the LaNiSn phase in AB₅ type hydrogen storage alloys resulted in the decrease of hydrogen storage capacity and also electrochemical discharge capacity as shown in Fig. 5, indicating the correlation between the maximum hydrogen storage and electrochemical discharge capacity of the La_0.7Mg_0.3Al_0.3- Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys.

Figure 6 shows the cycle life curves of the alloy electrodes. The discharge capacity retentions of the alloy electrodes after 100 charge-discharge cycles, C₁₀₀/C_max, are also listed in Table 4, where C₁₀₀ is the discharge capacity of the 100th cycle. It can be seen from Table 4 that the discharge capacity retentions of the electrodes increase from 70.2% (x=0) to 78.0% (x=0.5) after 100 charge-discharge cycles. The pulverization of the alloy particles during the electrochemical tests may be the main reason of efficacy loss of hydrogen storage capacity of the alloy electrode. The oxidation and corrosion layer increased resistance to hydrogen adsorption and desorption during the pulverization of the alloy in alkaline electrolyte. We also considered that the consequences of oxidation and corrosion layers created a barrier of diffusion of hydrogen atoms in the alloy electrode and decreased drastically property of hydrogen absorption and desorption during the charge-discharge cycling. Therefore, it can be concluded that with the increase of Sn content in the alloy, the corrosion and pulverization resistance of the alloys increases. As put forward by LIN et al [18], the evidence of increasing the cycle life was explained by formation of a passive SnO₂ layer on the surface of the alloy particles, preventing the pulverization of electrode materials during the cycling.

Fig. 5 Hydrogen storage and electrochemical discharge capacity of La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys

Figure 7 shows the high-rate dischargeability (HRD) of the La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) hydrogen storage alloy electrodes. The HRD, a main factor that represents the kinetic property of hydrogen storage alloy electrode, is calculated according to the following formula:

                       (2)

where C_d is a discharge capacity with a cut-off potential of 0.8 V at the discharge current density J_d; C₅₀ is the residual discharge capacity with a cut-off potential of 0.8 V at the discharge current density J= 50 mA/g also after the electrode has been fully discharged at J_d. The HRD at 1400 mA/g is also listed in Table 4. It is apparent from Fig. 7 that HRD decreased when Sn content increased in the alloys. Taking the discharge current density J_d=1000 mA/g as an example, the HRD decreased drastically from 72.5% (x=0) to 27.9% (x=0.5). These values have similarities for Ml_(4.45-x)Mn_0.4Al_0.15Sn_x studied by MA et al [16], who reported that the HRD decreased by almost five times when Sn content increased in alloys. It is believed that the decrease of HRD caused by Sn substitution may be attributed to a formation of SnO₂ as reported by LIN et al [18]. The further research would be to study the influence of Sn content on corrosion resistance of the alloys.

Fig. 6 Cyclic stability of La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys

Fig. 7 High-rate dischargeability of La_0.7Mg_0.3Al_0.3Mn_0.4- Co_0.5-xSn_xNi_3.8 (x=0-0.5) alloys

4 Conclusions

This paper investigated the influence of substitution of cobalt by tin on the microstructure, hydrogen storage performance and electrochemical properties of the La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5) hydrogen storage alloys. The increasing substitution of Sn for Co resulted in the increase of LaNiSn phase content and cell volume of LaNi₅ phase, and the decrease of LaNi₅ phase content. The hydrogen storage capacity and hysteresis factor of the hydrogen absorption and desorption decreased with enhancement of sloping factor when increasing the substitution amount of Sn for Co. When the substituted amount of Sn for Co increased from x=0 to x=0.5, the electrochemical measurements showed that the maximum discharge capacity decreased from 337.1 mA·h/g to 239.8 mA·h/g; the discharge capacity retention at the 100th cycle increased from 70.2% to 78.0%; at a discharge current density of 1400 mA/g, the high-rate dischargeability of the alloy electrodes decreased from 68.4% to 25.7%.

Acknowledgements

The authors wish to thank Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong and Institute Nuclear and Energy Research (IPEN), University of Paulo for the financial support and thanks are also given to National Council for Scientific and Technological Development – CNPQ – Brazil for the scholarships and financial support (CNPQ 472504/2010-0) granted to Julio Cesar Serafim CASINI.

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Sn替换Co对AB₅型La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0-0.5)合金显微结构和电化学性能的影响

Julio Cesar Serafim CASINI^1,2, Zai-ping GUO¹, Hua Kun LIU¹,

Eliner Affonso FERREIRA², Rubens Nunes FARIA², Hidetoshi TAKIISHI²

1. Institute for Semiconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia;

2. Materials Science and Technology Center, Nuclear and Energy Research Institute,

University of Paulo, SP 05508-900, Brazil

摘  要：采用X射线衍射方法、压力-成分等温线、电化学放电循环研究了AB₅型La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5-xSn_xNi_3.8 (x=0, 0.1, 0.2, 0.3, 0.5)合金中用Sn替换Co对其显微结构、储氢性能和电化学放电容量的影响。XRD、SEM及EDS测试结果表明，所有的合金都主要由LaNi₅和MgNi₂相组成，但随着合金中Sn含量的逐渐增加，出现LaNiSn相且显微结构得到细化。压力-成分等温线表明，随着合金中Sn含量的增加，合金的最大储氢容量从1.48%(x=0)降低到0.85%(x=0.5)。电化学测试结果表明，随着合金中Sn含量的增加，合金的最大放电容量从337.1 mA·h/g(x=0)降低到249.8 mA·h/g(x=0.5)；充放电循环100次的放电容量保持率从70.2%(x=0)增加到78.0%(x=0.5)。

关键词：储氢合金；显微组织；镍氢电池；Sn; Co；取代

(Edited by Hua YANG)

Corresponding author: Julio Cesar Serafim CASINI; Tel: +55-11-31339248; Fax: +55-11-31339018; E-mail: jcasini@ipen.br

DOI: 10.1016/S1003-6326(15)63633-0