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Microstructure and electrochemical performances of La0.7Mg0.3Ni2.55−xCo0.45Alx (x = 0–0.4) hydrogen storage alloys prepared by casting and rapid quenching
Journal of Alloys and Compounds 425 (2006) 399–405
Microstructure and electrochemical performances of La0.7Mg0.3Ni2.55−xCo0.45Alx (x = 0–0.4) hydrogen storage alloys prepared by casting and rapid quenching Bao-wei Li a , Hui-ping Ren a , Yang-huan Zhang a,b,∗ , Xiao-ping Dong a , Jiang-yuan Ren a , Xin-lin Wang b b
a School of Material, Inner Mongolia University of Science and Technology, Baotou 014010, PR China Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, PR China
Received 3 November 2005; received in revised form 21 January 2006; accepted 24 January 2006 Available online 3 March 2006
Abstract In order to improve the cycle stability of La–Mg–Ni system (PuNi3 -type) hydrogen storage alloy, Ni in the alloy was partly substituted by Al, and La0.7 Mg0.3 Ni2.55−x Co0.45 Alx (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of substituting Ni with Al and rapid quenching on the microstructures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD show that the as-cast and quenched alloys have a multiphase structure, including the (La, Mg)Ni3 phase, the LaNi5 phase and the LaNi2 phase. The substitution of Al for Ni increases the amount of the LaNi2 phase in the as-cast alloys, but it has an inappreciable influence on the abundance of the LaNi2 phase in the as-quenched alloy. The results obtained by the electrochemical measurement indicate that the capacities of the as-cast and quenched alloys monotonously decrease and their cycle stabilities significantly increase with the increase of Al content. Rapid quenching leads to the decrease of the capacity and an obvious improvement of the cycle stability of the alloys. © 2006 Elsevier B.V. All rights reserved. Keywords: Substituting Ni with Al; La–Mg–Ni system electrode alloy; Rapid quenching; Microstructures; Electrochemical performances
1. Introduction Ni-MH batteries, taking hydrogen storage alloy as negative electrode, have been used widely by virtue of several of their advantages, such as high capacity, capable of performing a high rate charge/discharge, high resistance to overcharging and over-discharging, a long cycle life, environmental friendliness, and interchangeability with Ni–Cd batteries [1,2]. A series of metal hydride electrode materials have been discovered, including the rare-earth-based AB5 -type alloys [3], the AB2 -type Laves phase alloys [4], the V-based solid solution alloys [5], and the Mg-based alloys [6,7]. Among the hydrogen storage alloys mentioned above, the discharge capacity of ∗
Corresponding author at: Department of Functional Material Research, Central Iron and Steel Research Institute, 76 Xueyuan Nan Road, Haidian District, 100081 Beijing, PR China. Tel.: +86 10 62187570; fax: +86 10 62182296. E-mail addresses: [email protected], [email protected] (Y.-h. Zhang). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.060
the AB5 -type electrode alloys is comparatively low, the activation of the AB2 -type Laves phase electrode alloys is very difficult, the electrocatalytic activation of the V-based solid solution alloys is also difficult, and the cycle stability of the Mg-based electrode alloys is extremely poor. Especially, the electrochemical capacity of currently advanced AB5 -type materials has reached 310–330 mAh/g at 0.2–0.3 ◦ C rate and room temperature. It seems to be difficult to further improve the capacity of the AB5 -type alloys since the theoretical capacity of LaNi5 is about 372 mAh/g. Recently, the rechargeable Ni-MH cells are encountering serious competition from Li-ion cells since the Li-ion cells show higher energy density than the Ni-MH cells per unit weight or volume. Therefore, investigations of new type electrode alloys with higher capacity and longer cycle life are extremely important to exalt the competition ability of Ni-MH batteries in the rechargeable battery field. Several new and good hydrogen storage alloys were reported recently. One of the most promising candidates is the La–Mg–Ni system for increasing the capacity. After investigating the elec-
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trochemical performances of the La2 MgNi9 , La5 Mg2 Ni23 and La3 MgNi14 -type electrode alloys, Kohno et al. [8] found that the La5 Mg2 Ni23 -type electrode alloy La0.7 Mg0.3 Ni2.8 Co0.5 has a capacity of 410 mAh/g and good cycle stability during 30 charge–discharge cycles. Kadir et al. [9] investigated the structure of the R2 MgNi9 (R = La, Ce, Pr, Nd, Sm and Gd) alloys and the result obtained shows that the alloys have the PuNi3 type structure. Pan et al. [10,11] investigated the structures and electrochemical characteristics of the La0.7 Mg0.3 (Ni0.85 Co0.15 )x (x = 2.5–5.0) alloy system. The obtained results showed that the alloys (x = 2.5–3.5) have optimal synthetic electrochemical performances, and the alloy (x = 3.5) has a maximum discharge capacity of 398.4 mAh/g, but the cycle stability of the alloy needs to be improved further. Lei et al. [12] investigated the effect of La/Ni ratio on the structure and electrochemical properties of Lax Mg3−x Ni9 (x = 1.6–2.2) hydrogen storage electrode alloys. The researched results indicated that the discharge capacity of the alloy increases with the increase of the La/Ni ratio. A lot of the investigations on the La–Mg–Ni system electrode alloy were carried out by researchers and some important progresses have been obtained, but some key problems, such as the cycle stability of the alloy, need to be solved further. In our experiments, the effects of partial substitution of Al for Ni on the microstructure and electrochemical properties of the as-cast and quenched La–Mg–Ni system electrode alloys were investigated and some important results were obtained. 2. Experimental 2.1. Preparation of alloys The experimental alloys were melted in an argon atmosphere using a vacuum induction furnace. The purity of all the component metals (La, Ni, Co, Mg and Al) is at least 99.8%. In order to prevent the volatilization of magnesium during melting, a positive argon pressure of 0.1 MPa was applied. After induction melting, the melt was poured into a copper mould cooled by water, and a cast ingot was obtained. Part of the as-cast alloys was re-melted and quenched by melt-spinning with a rotating copper wheel. Flakes of the as-quenched alloys were obtained with different quenching rates. The quenching rate is expressed by the linear velocity of the copper wheel and the quenching rates used in the experiment are 15, 20, 25 and 30 m/s. The nominal composition of the investigated alloys is La0.7 Mg0.3 Ni2.55−x Co0.45 Alx (x = 0, 0.1, 0.2, 0.3, 0.4). These alloys are represented by M0 , M1 , M2 , M3 and M4 , respectively.
2.2. Electrode preparation and electrochemical measurement The fractions of the as-cast and quenched alloys, which were ground mechanically into powder below 200 mesh, were used for the preparation of the experimental electrode. Electrode pellets with 15 mm diameter were prepared by mixing 1 g alloy powder with fine nickel powder in a weight ratio of 1:1 together with a small amount of polyvinyl alcohol (PVA) solution as binder, and then compressing under a pressure of 35 MPa. After drying for 4 h, the electrode pellets were immersed in 6 M KOH solution for 24 h in order to wet fully the electrodes before the electrochemical measurement. The electrode pellet was fixed on the outgoing line of a negative electrode of an open tri-electrode cell and the electrochemical characteristics were measured. Ni(OH)2 /NiOOH is the positive electrode of the experimental cell, Hg/HgO the reference electrode and 6 M KOH solution the electrolyte. In order to reduce the ohmic drop between the working electrode and the reference electrode, a Luggin capillary was located close to the hydride electrode in the working electrode apartment. The voltage between the negative electrode and the reference electrode is defined as the discharge voltage. Every cycle was charged with constant
current of 100 mA/g for 5 h, resting 15 min and then discharged at 100 mA/g to −0.500 V cut-off voltage. The environment temperature of the measurement was kept at 30 ◦ C.
2.3. Microstructure determination and morphology observation The samples of the as-cast and quenched alloys were pulverized by mechanical grinding, and the sizes of the powder sample particles were less than 70 m. The phase structures and compositions of the alloys were determined by XRD diffractometer of D/max/2400. The diffraction was performed with Cu K␣1 radiation filtered by graphite. The experimental parameters for determining phase structure are 160 mA, 40 kV and 10◦ /min, respectively. The samples of the ascast alloys were directly polished, and the flakes of the as-quenched alloys were inlaid in epoxy resin for polishing. The samples thus prepared were etched with a 60% HF solution. The morphologies of the as-cast and quenched alloys were examined by SEM. The powder samples were dispersed in anhydrous alcohol for observing the grain morphology with TEM, and for determining crystalline state of the samples with selected area electron diffraction (SAD).
3. Results and discussion 3.1. Electrochemical performances of the alloys 3.1.1. Activation performance The activation number denoted by n was characterized by the number of charge–discharge cycles required for attaining the greatest discharge capacity through a charge–discharge cycle at a constant current density. The cycle number dependence of the discharge capacity of the as-cast and quenched alloys was illustrated in Fig. 1, the charge–discharge current density being 100 mA/g. It can be derived from Fig. 1 that all of the as-cast and quenched alloys have an excellent activation performance and could completely be activated after 2–3 cycles. Rapid quenching has an insignificant influence on the activation capability of the alloys. 3.1.2. Discharge capacity The Al content dependence of the maximum discharge capacity of the alloys was illustrated in Fig. 2. Fig. 2 indicates that the capacities of the as-cast and quenched alloys almost linearly decrease with the increase of Al content. When Al content increases from 0 to 0.4, the capacity of the as-cast alloy decreases from 396.4 to 324.4 mAh/g, and for as-quenched (30 m/s) alloy, from 364 to 304.5 mAh/g. The quenching rate has an obvious influence on the capacity of the alloys. The relationship between the quenching rate and the capacity of the alloy was shown in Fig. 3. Fig. 3 indicates that the capacities of the alloys decrease with the increase of the quenching rate. When the quenching rate increases from 0 (as-cast was defined as quenching rate of 0 m/s) to 30 m/s, the capacity of M0 alloy decreases from 396.4 to 364 mAh/g. For M4 alloys from 324.4 to 304.5 mAh/g. 3.1.3. Cycle life The cycle life, indicated by N, is characterized by the cycle number after which the discharge capacity of the alloy is reduced to 60% of the maximum capacity. The cycle number dependence of the discharge capacity of the as-cast and quenched alloys is shown in Fig. 4. Fig. 4 shows that the slopes of the curves of the as-cast and quenched (20 m/s) alloys obviously decrease with
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Fig. 3. Quenching rate dependence of the discharge capacity of the alloys.
Fig. 1. The activation capabilities of the as-cast and quenched alloys: (a) as-cast and (b) as-quenched (20 m/s).
Fig. 2. The Al content dependence of the discharge capacity of the alloys.
Fig. 4. The relationship between the cycle number and the discharge capacity of the alloys: (a) as-cast and (b) as-quenched (20 m/s).
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quenching on the electrochemical performances of the alloys, the microstructures of the as-cast and quenched alloys were analyzed further. 3.2. Microstructure
Fig. 5. The Al content dependence of the cycle life of the alloys.
3.2.1. Phase composition and structure The phase composition and structure of the as-cast and quenched alloys were analyzed by XRD and the results were shown in Fig. 7. It can be seen from Fig. 7 that the as-cast and quenched alloys have a multiphase structure, composing of the (La, Mg)Ni3 phase, the LaNi5 phase and the LaNi2 phase. The substitution of Al for Ni and rapid quenching have an inappreciable influence on the phase compositions of the alloys, but both changed the phase abundances of the alloys obviously. For as-cast alloys, the amount of the LaNi2 phase increases with the increase of the Al content and two main diffraction peaks have a tendency of overlapping.
the increase of Al content. This indicates the substitution of Al for Ni can improve the cycle stability of the alloys. In order to show the effect of the Al content on the cycle life of the alloys, the Al content dependence of the cycle life of the alloys was presented in Fig. 5. The results in Fig. 5 indicate that the cycle lives of the as-cast and quenched alloys increase significantly with the increase of the Al content. When Al content increases from 0 to 0.4, the cycle life of the as-cast alloy increase from 72 to 132 cycles, and for as-quenched (30 m/s) alloy, from 100 to 136 cycles. Obviously, the influence of the substitution of Al for Ni on the cycle life of the as-cast alloys is much stronger than on that of the as-quenched alloy. The quenching rate dependence of the cycle life of the alloy was illustrated in Fig. 6. Fig. 6 indicates that the quenching rate has an influence on the cycle life of the alloys, but it decreases gradually with the increase of the Al content. When the quenching rate increases from 0 to 30 m/s, the cycle life of M0 alloy increases from 72 to 100 cycles, and for M4 alloy from 132 to 136 cycles. In order to reveal the influence mechanisms of substituting Ni with Al and rapid
Fig. 6. The quenching rate dependence of the cycle life of the alloys.
Fig. 7. The X-ray diffraction patterns of the as-cast and quenched alloys: (a) as-cast and (b) as-quenched (20 m/s).
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Fig. 8. The morphologies of the as-cast and quenched (20 m/s) alloys taken by SEM: (a), (b), (c) as-cast M0 , M2 and M4 alloys; (d), (e), (f) as-quenched M0 , M2 and M4 alloys.
3.2.2. Microstructure and morphology The morphologies of the as-cast and quenched alloys were inspected by SEM, and the results were shown in Fig. 8. The results obtained by energy spectrum analysis of SEM indicate that all the experimental alloys are of multiphase structure, containing both the (La, Mg)Ni3 phase and the LaNi5 phase. Because the amount of the LaNi2 phase is small and it affects the (La, Mg)Ni3 phase in the process of growing, it is difficult to see the morphology of the LaNi2 phase. Fig. 8 shows that the grains of the as-cast alloys are very coarse and the composition homogeneity is very poor. Comparing with the morphologies of the as-cast alloys, the rapid quenching markedly refined the grains and significantly improved the composition homogeneity of the alloys. The grain morphologies of the as-quenched alloys are dendrite, which is different from the morphology of the asquenched AB5 -type electrode alloy because its morphology is columnar [13]. The substitution of Al for Ni lead to the grains of the as-quenched alloys obviously coarsen, which indicated that the substitution of Al for Ni decreases the nucleation ratio and increases the growing velocity in process of no-equilibrium crystalline of the alloys. The morphologies and the crystalline state of the as-quenched alloys were examined by TEM, and the results were shown in Fig. 9. The results in Fig. 9 indicate that as-quenched (20 m/s) M0 has a tendency of the formation of an amorphous phase and M2 alloys with the same quenching rate have a typical micro-crystal and nano-crystal structure.
This indicates that the substitution of Al for Ni is unfavourable to the formation of an amorphous phase in the alloys. The morphologies of the as-cast M4 alloy particles before and after electrochemical cycle as observed by SEM are shown in Fig. 10. We can see from Fig. 10 that the particle size and surface morphology of the M4 alloy have a change after electrochemical cycle, leading to a disappearance of the pointedness of the particle and to a slight decrease of the particle size. Worthy to be noticed is that an oxidation and corrosion layer can be seen on the surface of the alloy. It is the oxidation and corrosion layer that leads to some significant changes of the electrochemical performances of the alloy. The fact that the as-cast and quenched alloys have good activation performance is mainly ascribed to their multiphase structures. When hydrogen atoms enter the interstitial of the tetrahedron or octahedron of the alloy lattice, the producing of extra strain energy is inevitable. The phase boundary probable acts as a buffer area for release of the stress energy so that the lattice distortion and strain energy formed in the process of hydrogen absorption can be decreased. Moreover, the phase boundary provides good tunnels for diffusion of hydrogen atoms, the activation capability of the alloys being improved significantly [14]. The discharge capacity of the alloy is relevant to its crystal structure, phase composition and structure, grain size, composition homogeneity and surface state. The main reason of
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Fig. 9. The morphologies and SAD of the as-quenched (20 m/s) alloys taken by TEM: (a) and (c) morphology and SAD of M0 alloy; (b) and (d) morphology and SAD of M2 alloy.
substituting Ni with Al decreasing the capacity of the alloys is ascribed to a dense oxidation film formed on the surface of the alloy, which strongly restrains the diffusion of hydrogen atoms from the surface to inside of the alloy. Further, substituting Ni with Al increases the amount of LaNi2 phase. This is an important factor of the capacity decreased. The influences of rapid quenching on the capacity are complicated. Both the composition homogeneity and the grain refinement resulted from rapid quenching are favourable for the capacity on one hand, and on the other hand, the increase of the lattice stress produced by rapid quenched is unfavourable. Therefore, whether rapid quenching would increase or decrease the capacity of the alloy depends on the predominant one of the above. The results shown in Fig. 3 indicate that the effect of the lattice stress was predominated. The result in the literature [15] confirmed that the fundamental reasons for the capacity decay of the electrode alloy are the pulverization and oxidation of the alloy during charge–discharge cycle. The fact that the substituting Ni with Al enhances the cycle stability of as-cast alloys is due to the formation of a
dense oxide film on the alloy surface, which can significantly enhance the anti-oxidation and corrosion ability of the alloy electrode in alkaline electrolyte. The influence of the quenching rate on the cycle life of the as-quenched alloys is gradually decreased with the increase of Al content. This is attributed to substituting Ni with Al restraining the formation of an amorphous in the alloy because an amorphous phase can increase not only the anti-pulverization ability but also the anti-corrosion capability of the alloy [16]. The grain refinement resulted from rapid quenching has an imperceptible influence on the improvement of the anti-corrosion and oxidation capability of the alloys. Therefore, it seems to be self-evident that the function of the rapid quenching on the cycle life of alloys decreases with the increase of Al content. The pulverization of the alloy in process of charge–discharge cycle is an important reason of the loss efficient of the alloy electrode. The grain refinement can significantly improve the anti-pulverization capability of the alloy electrode. Therefore, the rapid quenching can increase the cycle stability of the alloys, but the function is very limited.
Fig. 10. The granular morphologies of as-cast M4 alloy before (a) and after (b) electrochemical cycle (SEM).
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4. Conclusions
References
1. The as-cast and quenched La0.7 Mg0.3 Ni2.55−x Co0.45 Alx (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys have a multiphase structure, including the (La, Mg)Ni3 phase, the LaNi5 phase and a small amount of the LaNi2 phase. The substitution of Al for Ni has an inappreciable influence on the phase compositions of the alloys, but increases the abundances of the LaNi2 phase in the as-cast alloys. The substitution of Al for Ni led to the grains coarsen of the as-quenched alloys, and it was unfavourable to the formation of an amorphous phase. 2. The substitution of Al for Ni decreases the capacity of the as-cast and quenched alloys, but significantly improves their cycle stabilities. When the amount of substituting Ni with Al increases from 0 to 0.4, the capacity of the as-cast alloy decreases from 396.5 to 324.4 mAh/g, and the cycle life increases from 72 to 132 cycles. For the as-quenched (20 m/s) alloys, the capacity decreases from 386.4 to 317.8 mAh/g, and the cycle life increases from 86 to 135 cycles. The substitution of Al for Ni has an insignificant influence on the activation capability of the alloys. 3. The effects of rapid quenching on the structure and electrochemical performances of the alloys are notable. The rapid quenching leads to the grain refinement of the alloy. The discharge capacity decreases and the cycle life increases significantly with the increase of the quenching rate.
[1] I. Uehara, T. Sakai, H. Ishikawa, J. Alloys Compd. 253/254 (1997) 635–641. [2] Q.D. Wang, C.P. Chen, Y.Q. Lei, J. Alloys Compd. 253/254 (1997) 629–634. [3] J.J.G. Willems, K.H.J. Buschow, J. Less-Common Met. 129 (1987) 13– 30. [4] S.R. Ovshinsky, M.A. Fetcenko, J. Ross, Science 260 (1993) 176– 181. [5] M. Tsukahara, T. Kamiya, K. Takahashi, A. Kawabata, S. Sakurai, J. Shi, H.T. Takeshita, N. Kuriyama, T. Sakai, J. Electrochem. Soc. 147 (2000) 2941–2944. [6] D. Sun, H. Enoki, F. Gingl, E. Akiba, J. Alloys Compd. 285 (1999) 279– 283. [7] T. Kohno, H. Yoshida, F. Kawashima, T. Inaba, I. Sakai, M. Yamamoto, M. Kanda, J. Alloys Compd. 311 (2000) L5–L7. [8] T. Kohno, H. Yoshida, F. Kawashima, T. Inaba, I. Sakai, M. Yamamoto, M. Kanda, J. Alloys Compd. 311 (2000) L5–L7. [9] K. Kadir, I. Uehara, T. Sakai, J. Alloys Compd. 257 (1997) 115– 121. [10] H. Pan, Y. Liu, M. Gao, Y. Lei, Q. Wang, J. Electrochem. Soc. 150 (2003) A565–A570. [11] H. Pan, Y. Liu, M. Gao, Y. Zhu, Y. Lei, Q. Wang, J. Alloys Compd. 351 (2003) 228–234. [12] B. Liao, Y.Q. Lei, L.X. Chen, G.L. Lu, H.G. Pan, Q.D. Wang, J. Power Sources 129 (2004) 358–367. [13] Y.-H. Zhang, G.-Q. Wang, X.-P. Dong, S.-H. Guo, X.-L. Wang, Int. J. Hydrogen Energy 30 (2005) 1091–1098. [14] P. Li, X.-L. Wang, Y.-H. Zhang, R. Li, J.-M. Wu, X.-H. Qu, J. Alloys Compd. 353 (2003) 278–282. [15] D. Chartouni, F. Meli, A. Z¨uttel, K. Gross, L. Schlapbach, J. Alloys Compd. 241 (1996) 160–166. [16] Y.-H. Zhang, G.-Q. Wang, X.-P. Dong, S.-H. Guo, J.-M. Wu, X.-L. Wang, J. Alloys Compd. 379 (2004) 298–304.
Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 50131040) and Key Technologies R&D Program of Inner Mongolia (Grant No. 20050205).
Microstructure and electrochemical performances of La0.7Mg0.3Ni2.55−xCo0.45Alx (x = 0–0.4) hydrogen storage alloys prepared by casting and rapid quenching Bao-wei Li a , Hui-ping Ren a , Yang-huan Zhang a,b,∗ , Xiao-ping Dong a , Jiang-yuan Ren a , Xin-lin Wang b b
a School of Material, Inner Mongolia University of Science and Technology, Baotou 014010, PR China Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, PR China
Received 3 November 2005; received in revised form 21 January 2006; accepted 24 January 2006 Available online 3 March 2006
Abstract In order to improve the cycle stability of La–Mg–Ni system (PuNi3 -type) hydrogen storage alloy, Ni in the alloy was partly substituted by Al, and La0.7 Mg0.3 Ni2.55−x Co0.45 Alx (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of substituting Ni with Al and rapid quenching on the microstructures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD show that the as-cast and quenched alloys have a multiphase structure, including the (La, Mg)Ni3 phase, the LaNi5 phase and the LaNi2 phase. The substitution of Al for Ni increases the amount of the LaNi2 phase in the as-cast alloys, but it has an inappreciable influence on the abundance of the LaNi2 phase in the as-quenched alloy. The results obtained by the electrochemical measurement indicate that the capacities of the as-cast and quenched alloys monotonously decrease and their cycle stabilities significantly increase with the increase of Al content. Rapid quenching leads to the decrease of the capacity and an obvious improvement of the cycle stability of the alloys. © 2006 Elsevier B.V. All rights reserved. Keywords: Substituting Ni with Al; La–Mg–Ni system electrode alloy; Rapid quenching; Microstructures; Electrochemical performances
1. Introduction Ni-MH batteries, taking hydrogen storage alloy as negative electrode, have been used widely by virtue of several of their advantages, such as high capacity, capable of performing a high rate charge/discharge, high resistance to overcharging and over-discharging, a long cycle life, environmental friendliness, and interchangeability with Ni–Cd batteries [1,2]. A series of metal hydride electrode materials have been discovered, including the rare-earth-based AB5 -type alloys [3], the AB2 -type Laves phase alloys [4], the V-based solid solution alloys [5], and the Mg-based alloys [6,7]. Among the hydrogen storage alloys mentioned above, the discharge capacity of ∗
Corresponding author at: Department of Functional Material Research, Central Iron and Steel Research Institute, 76 Xueyuan Nan Road, Haidian District, 100081 Beijing, PR China. Tel.: +86 10 62187570; fax: +86 10 62182296. E-mail addresses: [email protected], [email protected] (Y.-h. Zhang). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.060
the AB5 -type electrode alloys is comparatively low, the activation of the AB2 -type Laves phase electrode alloys is very difficult, the electrocatalytic activation of the V-based solid solution alloys is also difficult, and the cycle stability of the Mg-based electrode alloys is extremely poor. Especially, the electrochemical capacity of currently advanced AB5 -type materials has reached 310–330 mAh/g at 0.2–0.3 ◦ C rate and room temperature. It seems to be difficult to further improve the capacity of the AB5 -type alloys since the theoretical capacity of LaNi5 is about 372 mAh/g. Recently, the rechargeable Ni-MH cells are encountering serious competition from Li-ion cells since the Li-ion cells show higher energy density than the Ni-MH cells per unit weight or volume. Therefore, investigations of new type electrode alloys with higher capacity and longer cycle life are extremely important to exalt the competition ability of Ni-MH batteries in the rechargeable battery field. Several new and good hydrogen storage alloys were reported recently. One of the most promising candidates is the La–Mg–Ni system for increasing the capacity. After investigating the elec-
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trochemical performances of the La2 MgNi9 , La5 Mg2 Ni23 and La3 MgNi14 -type electrode alloys, Kohno et al. [8] found that the La5 Mg2 Ni23 -type electrode alloy La0.7 Mg0.3 Ni2.8 Co0.5 has a capacity of 410 mAh/g and good cycle stability during 30 charge–discharge cycles. Kadir et al. [9] investigated the structure of the R2 MgNi9 (R = La, Ce, Pr, Nd, Sm and Gd) alloys and the result obtained shows that the alloys have the PuNi3 type structure. Pan et al. [10,11] investigated the structures and electrochemical characteristics of the La0.7 Mg0.3 (Ni0.85 Co0.15 )x (x = 2.5–5.0) alloy system. The obtained results showed that the alloys (x = 2.5–3.5) have optimal synthetic electrochemical performances, and the alloy (x = 3.5) has a maximum discharge capacity of 398.4 mAh/g, but the cycle stability of the alloy needs to be improved further. Lei et al. [12] investigated the effect of La/Ni ratio on the structure and electrochemical properties of Lax Mg3−x Ni9 (x = 1.6–2.2) hydrogen storage electrode alloys. The researched results indicated that the discharge capacity of the alloy increases with the increase of the La/Ni ratio. A lot of the investigations on the La–Mg–Ni system electrode alloy were carried out by researchers and some important progresses have been obtained, but some key problems, such as the cycle stability of the alloy, need to be solved further. In our experiments, the effects of partial substitution of Al for Ni on the microstructure and electrochemical properties of the as-cast and quenched La–Mg–Ni system electrode alloys were investigated and some important results were obtained. 2. Experimental 2.1. Preparation of alloys The experimental alloys were melted in an argon atmosphere using a vacuum induction furnace. The purity of all the component metals (La, Ni, Co, Mg and Al) is at least 99.8%. In order to prevent the volatilization of magnesium during melting, a positive argon pressure of 0.1 MPa was applied. After induction melting, the melt was poured into a copper mould cooled by water, and a cast ingot was obtained. Part of the as-cast alloys was re-melted and quenched by melt-spinning with a rotating copper wheel. Flakes of the as-quenched alloys were obtained with different quenching rates. The quenching rate is expressed by the linear velocity of the copper wheel and the quenching rates used in the experiment are 15, 20, 25 and 30 m/s. The nominal composition of the investigated alloys is La0.7 Mg0.3 Ni2.55−x Co0.45 Alx (x = 0, 0.1, 0.2, 0.3, 0.4). These alloys are represented by M0 , M1 , M2 , M3 and M4 , respectively.
2.2. Electrode preparation and electrochemical measurement The fractions of the as-cast and quenched alloys, which were ground mechanically into powder below 200 mesh, were used for the preparation of the experimental electrode. Electrode pellets with 15 mm diameter were prepared by mixing 1 g alloy powder with fine nickel powder in a weight ratio of 1:1 together with a small amount of polyvinyl alcohol (PVA) solution as binder, and then compressing under a pressure of 35 MPa. After drying for 4 h, the electrode pellets were immersed in 6 M KOH solution for 24 h in order to wet fully the electrodes before the electrochemical measurement. The electrode pellet was fixed on the outgoing line of a negative electrode of an open tri-electrode cell and the electrochemical characteristics were measured. Ni(OH)2 /NiOOH is the positive electrode of the experimental cell, Hg/HgO the reference electrode and 6 M KOH solution the electrolyte. In order to reduce the ohmic drop between the working electrode and the reference electrode, a Luggin capillary was located close to the hydride electrode in the working electrode apartment. The voltage between the negative electrode and the reference electrode is defined as the discharge voltage. Every cycle was charged with constant
current of 100 mA/g for 5 h, resting 15 min and then discharged at 100 mA/g to −0.500 V cut-off voltage. The environment temperature of the measurement was kept at 30 ◦ C.
2.3. Microstructure determination and morphology observation The samples of the as-cast and quenched alloys were pulverized by mechanical grinding, and the sizes of the powder sample particles were less than 70 m. The phase structures and compositions of the alloys were determined by XRD diffractometer of D/max/2400. The diffraction was performed with Cu K␣1 radiation filtered by graphite. The experimental parameters for determining phase structure are 160 mA, 40 kV and 10◦ /min, respectively. The samples of the ascast alloys were directly polished, and the flakes of the as-quenched alloys were inlaid in epoxy resin for polishing. The samples thus prepared were etched with a 60% HF solution. The morphologies of the as-cast and quenched alloys were examined by SEM. The powder samples were dispersed in anhydrous alcohol for observing the grain morphology with TEM, and for determining crystalline state of the samples with selected area electron diffraction (SAD).
3. Results and discussion 3.1. Electrochemical performances of the alloys 3.1.1. Activation performance The activation number denoted by n was characterized by the number of charge–discharge cycles required for attaining the greatest discharge capacity through a charge–discharge cycle at a constant current density. The cycle number dependence of the discharge capacity of the as-cast and quenched alloys was illustrated in Fig. 1, the charge–discharge current density being 100 mA/g. It can be derived from Fig. 1 that all of the as-cast and quenched alloys have an excellent activation performance and could completely be activated after 2–3 cycles. Rapid quenching has an insignificant influence on the activation capability of the alloys. 3.1.2. Discharge capacity The Al content dependence of the maximum discharge capacity of the alloys was illustrated in Fig. 2. Fig. 2 indicates that the capacities of the as-cast and quenched alloys almost linearly decrease with the increase of Al content. When Al content increases from 0 to 0.4, the capacity of the as-cast alloy decreases from 396.4 to 324.4 mAh/g, and for as-quenched (30 m/s) alloy, from 364 to 304.5 mAh/g. The quenching rate has an obvious influence on the capacity of the alloys. The relationship between the quenching rate and the capacity of the alloy was shown in Fig. 3. Fig. 3 indicates that the capacities of the alloys decrease with the increase of the quenching rate. When the quenching rate increases from 0 (as-cast was defined as quenching rate of 0 m/s) to 30 m/s, the capacity of M0 alloy decreases from 396.4 to 364 mAh/g. For M4 alloys from 324.4 to 304.5 mAh/g. 3.1.3. Cycle life The cycle life, indicated by N, is characterized by the cycle number after which the discharge capacity of the alloy is reduced to 60% of the maximum capacity. The cycle number dependence of the discharge capacity of the as-cast and quenched alloys is shown in Fig. 4. Fig. 4 shows that the slopes of the curves of the as-cast and quenched (20 m/s) alloys obviously decrease with
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Fig. 3. Quenching rate dependence of the discharge capacity of the alloys.
Fig. 1. The activation capabilities of the as-cast and quenched alloys: (a) as-cast and (b) as-quenched (20 m/s).
Fig. 2. The Al content dependence of the discharge capacity of the alloys.
Fig. 4. The relationship between the cycle number and the discharge capacity of the alloys: (a) as-cast and (b) as-quenched (20 m/s).
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quenching on the electrochemical performances of the alloys, the microstructures of the as-cast and quenched alloys were analyzed further. 3.2. Microstructure
Fig. 5. The Al content dependence of the cycle life of the alloys.
3.2.1. Phase composition and structure The phase composition and structure of the as-cast and quenched alloys were analyzed by XRD and the results were shown in Fig. 7. It can be seen from Fig. 7 that the as-cast and quenched alloys have a multiphase structure, composing of the (La, Mg)Ni3 phase, the LaNi5 phase and the LaNi2 phase. The substitution of Al for Ni and rapid quenching have an inappreciable influence on the phase compositions of the alloys, but both changed the phase abundances of the alloys obviously. For as-cast alloys, the amount of the LaNi2 phase increases with the increase of the Al content and two main diffraction peaks have a tendency of overlapping.
the increase of Al content. This indicates the substitution of Al for Ni can improve the cycle stability of the alloys. In order to show the effect of the Al content on the cycle life of the alloys, the Al content dependence of the cycle life of the alloys was presented in Fig. 5. The results in Fig. 5 indicate that the cycle lives of the as-cast and quenched alloys increase significantly with the increase of the Al content. When Al content increases from 0 to 0.4, the cycle life of the as-cast alloy increase from 72 to 132 cycles, and for as-quenched (30 m/s) alloy, from 100 to 136 cycles. Obviously, the influence of the substitution of Al for Ni on the cycle life of the as-cast alloys is much stronger than on that of the as-quenched alloy. The quenching rate dependence of the cycle life of the alloy was illustrated in Fig. 6. Fig. 6 indicates that the quenching rate has an influence on the cycle life of the alloys, but it decreases gradually with the increase of the Al content. When the quenching rate increases from 0 to 30 m/s, the cycle life of M0 alloy increases from 72 to 100 cycles, and for M4 alloy from 132 to 136 cycles. In order to reveal the influence mechanisms of substituting Ni with Al and rapid
Fig. 6. The quenching rate dependence of the cycle life of the alloys.
Fig. 7. The X-ray diffraction patterns of the as-cast and quenched alloys: (a) as-cast and (b) as-quenched (20 m/s).
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Fig. 8. The morphologies of the as-cast and quenched (20 m/s) alloys taken by SEM: (a), (b), (c) as-cast M0 , M2 and M4 alloys; (d), (e), (f) as-quenched M0 , M2 and M4 alloys.
3.2.2. Microstructure and morphology The morphologies of the as-cast and quenched alloys were inspected by SEM, and the results were shown in Fig. 8. The results obtained by energy spectrum analysis of SEM indicate that all the experimental alloys are of multiphase structure, containing both the (La, Mg)Ni3 phase and the LaNi5 phase. Because the amount of the LaNi2 phase is small and it affects the (La, Mg)Ni3 phase in the process of growing, it is difficult to see the morphology of the LaNi2 phase. Fig. 8 shows that the grains of the as-cast alloys are very coarse and the composition homogeneity is very poor. Comparing with the morphologies of the as-cast alloys, the rapid quenching markedly refined the grains and significantly improved the composition homogeneity of the alloys. The grain morphologies of the as-quenched alloys are dendrite, which is different from the morphology of the asquenched AB5 -type electrode alloy because its morphology is columnar [13]. The substitution of Al for Ni lead to the grains of the as-quenched alloys obviously coarsen, which indicated that the substitution of Al for Ni decreases the nucleation ratio and increases the growing velocity in process of no-equilibrium crystalline of the alloys. The morphologies and the crystalline state of the as-quenched alloys were examined by TEM, and the results were shown in Fig. 9. The results in Fig. 9 indicate that as-quenched (20 m/s) M0 has a tendency of the formation of an amorphous phase and M2 alloys with the same quenching rate have a typical micro-crystal and nano-crystal structure.
This indicates that the substitution of Al for Ni is unfavourable to the formation of an amorphous phase in the alloys. The morphologies of the as-cast M4 alloy particles before and after electrochemical cycle as observed by SEM are shown in Fig. 10. We can see from Fig. 10 that the particle size and surface morphology of the M4 alloy have a change after electrochemical cycle, leading to a disappearance of the pointedness of the particle and to a slight decrease of the particle size. Worthy to be noticed is that an oxidation and corrosion layer can be seen on the surface of the alloy. It is the oxidation and corrosion layer that leads to some significant changes of the electrochemical performances of the alloy. The fact that the as-cast and quenched alloys have good activation performance is mainly ascribed to their multiphase structures. When hydrogen atoms enter the interstitial of the tetrahedron or octahedron of the alloy lattice, the producing of extra strain energy is inevitable. The phase boundary probable acts as a buffer area for release of the stress energy so that the lattice distortion and strain energy formed in the process of hydrogen absorption can be decreased. Moreover, the phase boundary provides good tunnels for diffusion of hydrogen atoms, the activation capability of the alloys being improved significantly [14]. The discharge capacity of the alloy is relevant to its crystal structure, phase composition and structure, grain size, composition homogeneity and surface state. The main reason of
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Fig. 9. The morphologies and SAD of the as-quenched (20 m/s) alloys taken by TEM: (a) and (c) morphology and SAD of M0 alloy; (b) and (d) morphology and SAD of M2 alloy.
substituting Ni with Al decreasing the capacity of the alloys is ascribed to a dense oxidation film formed on the surface of the alloy, which strongly restrains the diffusion of hydrogen atoms from the surface to inside of the alloy. Further, substituting Ni with Al increases the amount of LaNi2 phase. This is an important factor of the capacity decreased. The influences of rapid quenching on the capacity are complicated. Both the composition homogeneity and the grain refinement resulted from rapid quenching are favourable for the capacity on one hand, and on the other hand, the increase of the lattice stress produced by rapid quenched is unfavourable. Therefore, whether rapid quenching would increase or decrease the capacity of the alloy depends on the predominant one of the above. The results shown in Fig. 3 indicate that the effect of the lattice stress was predominated. The result in the literature [15] confirmed that the fundamental reasons for the capacity decay of the electrode alloy are the pulverization and oxidation of the alloy during charge–discharge cycle. The fact that the substituting Ni with Al enhances the cycle stability of as-cast alloys is due to the formation of a
dense oxide film on the alloy surface, which can significantly enhance the anti-oxidation and corrosion ability of the alloy electrode in alkaline electrolyte. The influence of the quenching rate on the cycle life of the as-quenched alloys is gradually decreased with the increase of Al content. This is attributed to substituting Ni with Al restraining the formation of an amorphous in the alloy because an amorphous phase can increase not only the anti-pulverization ability but also the anti-corrosion capability of the alloy [16]. The grain refinement resulted from rapid quenching has an imperceptible influence on the improvement of the anti-corrosion and oxidation capability of the alloys. Therefore, it seems to be self-evident that the function of the rapid quenching on the cycle life of alloys decreases with the increase of Al content. The pulverization of the alloy in process of charge–discharge cycle is an important reason of the loss efficient of the alloy electrode. The grain refinement can significantly improve the anti-pulverization capability of the alloy electrode. Therefore, the rapid quenching can increase the cycle stability of the alloys, but the function is very limited.
Fig. 10. The granular morphologies of as-cast M4 alloy before (a) and after (b) electrochemical cycle (SEM).
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4. Conclusions
References
1. The as-cast and quenched La0.7 Mg0.3 Ni2.55−x Co0.45 Alx (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys have a multiphase structure, including the (La, Mg)Ni3 phase, the LaNi5 phase and a small amount of the LaNi2 phase. The substitution of Al for Ni has an inappreciable influence on the phase compositions of the alloys, but increases the abundances of the LaNi2 phase in the as-cast alloys. The substitution of Al for Ni led to the grains coarsen of the as-quenched alloys, and it was unfavourable to the formation of an amorphous phase. 2. The substitution of Al for Ni decreases the capacity of the as-cast and quenched alloys, but significantly improves their cycle stabilities. When the amount of substituting Ni with Al increases from 0 to 0.4, the capacity of the as-cast alloy decreases from 396.5 to 324.4 mAh/g, and the cycle life increases from 72 to 132 cycles. For the as-quenched (20 m/s) alloys, the capacity decreases from 386.4 to 317.8 mAh/g, and the cycle life increases from 86 to 135 cycles. The substitution of Al for Ni has an insignificant influence on the activation capability of the alloys. 3. The effects of rapid quenching on the structure and electrochemical performances of the alloys are notable. The rapid quenching leads to the grain refinement of the alloy. The discharge capacity decreases and the cycle life increases significantly with the increase of the quenching rate.
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Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 50131040) and Key Technologies R&D Program of Inner Mongolia (Grant No. 20050205).