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SnSbx-based composite electrodes for lithium ion cells
Solid State Ionics 135 (2000) 175–180 www.elsevier.com / locate / ssi
SnSb x -based composite electrodes for lithium ion cells J. Yang*, Y. Takeda, N. Imanishi, T. Ichikawa, O. Yamamoto Department of Chemistry, Faculty of Engineering, Mie University Kamihamacho, Tsu, Mie 514 -8507, Japan
Abstract SnSb x alloy powders with different composition and particle size have been examined for use as Li-alloy composite anode materials in lithium ion cells. Rapid degradation of cycling performance of Li-alloy composite electrodes in 1 M LiClO 4 / EC 1 DEC (1:1) can be greatly suppressed by decreasing the particle size of SnSb x alloy hosts and choosing a suitable polymer binder. An increase of antimony content in SnSbx alloy to a certain extent also improved the cycling stability. In order to enhance coulombic efficiency at the first cycle, a given amount of lithium was introduced into the electrode before cycling by different methods. A remarkable improvement in the electrode performance can be achieved by adding a small amount of Li 2.6 Co 0.4 N powder into the SnSb x -based composite electrodes. 2000 Elsevier Science B.V. All rights reserved. Keywords: Lithium secondary cells; Lithium alloy composite electrodes; Ultrafine SnSb x powders; Cycling performance
1. Introduction Fast room temperature mobility of lithium and very high reversible capacity shows that some Lialloys such as Li–Al, Li–Sn and Li–Pb can be very attractive anode materials for lithium ion cells. Long cycle life of coin cell has been achieved in the basis of Mn-doped Li–Al anode at very low discharge rates [1]. Cycling stability of Li-alloy anodes under high Li-utilization, however, is often limited by the rapid mechanical disintegration due to drastic volume changes of metallic host matrices (Fig. 1). In order to enhance the dimensional and structural stability of the electrodes and thus to improve the cycling performance, many highly dispersed multiphase systems for Li-alloy composite anode materi*Corresponding author. Tel.: 1 81-59-231-9419; fax: 1 81-59231-9720. E-mail address: [email protected] (J. Yang).
als have been proposed. They can be mainly divided into two groups. One is generally composed of two phases: a reactant phase and a less active or inert phase (e.g. Al / Cu [2] and Sn 2 Fe / SnFe 3 C [3]). The
Fig. 1. Volume expansion after Li-insertion into host matrices (corresponding to 1 molar host material respectively).
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00361-1
176
J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
less active or inert phases in composite microstructures can reduce mechanical stress in the electrodes, buffer the volume effect and sometimes prevent the aggregation and recrystallization of highly divided active phase grains. Several such multiphase structures demonstrated improved or even excellent cycling performance [3–5]. The other group is composed of two or more reactant phases with different activity against lithium (e.g. wood’s alloys [6] and wide variostoichiometric range alloy [7]) that demonstrated a better dimensional stability. On the basis of ‘mixed-conducting matrix concept’ [8], an active phase in composite microstructures could become inert under controlled cut-off and served as mixedconducting matrix for reactant phases (e.g. LiCd x / Li 4.4 Sn [9]). It was found that electrodeposited multiphase SnSb x alloy film as host matrix had much better morphological and cycling stability than single phase Sn film [10]. However, the cycling tests were only confined to very thin thickness of the films ( , 4 mm) and limited charge input. These limitations of the bulk host matrix can be effectively overcome by the use of Li-alloy composite structures in which highly dispersed host powder and inert conducting additive are mechanically connected by flexible polymer binder. In this paper, electrochemical and cycling behaviors of SnSb x -based composite electrodes are studied.
2. Experimental
2.1. Metal and alloy powders Tin (99.8%, , 45 mm) and nickel powder (99.9%, 0.03 mm) are commercially available. SnSb x (x 5 0.13, 0.25 and 0.42) powders (400 mesh, , 38 mm) were prepared by melting and mechanical filing. High pure tin and antimony powders were stoichiometrically mixed and pressed, and then melted at 8508C under Ar atmosphere for 4 h. After cooling until room temperature, the alloy stick was polished to remove the surface layer and then filed to obtain SnSb x filings. Finally SnSb x powders (typically , 38 mm) were obtained from the filings by means of 400 mesh sieve. Two types of ultrafine SnSb x powders (nominal composition: SnSb 0.13 , particle size , 0.2 mm; SnSb 0.17 , , 0.8 mm) were synthesized by chemical precipitation from aqueous solutions containing the chlorides of the respective metals and complex agents with sodium borohydride as reductive agent [11]. Fig. 2 shows SEM photographs of these SnSb x powders. X-ray diffraction analysis indicated that all SnSb x powders consisted of Sn phase and intermetallic SnSb phase. Li 1.7 / SnSb 0.13 powder was prepared by electrochemical insertion of lithium into SnSb 0.13 ( , 0.2 mm) powder electrode containing 5% polyethylene with ultra-high molecular weight (stainless steel grid
Fig. 2. SEM photographs of SnSb x alloy powders with different particle size.
J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
as current collector). After Li-insertion, the electrode was soaked in high pure THF solution for 1 h and then washed by THF twice. After drying at 908C under vacuum, Li-alloy was detached from the current collector and ground to powder in a glove box.
2.2. Fabrication of electrodes Composite electrodes with a geometric area of 0.5 cm 2 were fabricated by following two methods: 1. For poly(vinylidene fluoride) (PVDF) binder, SnSb x powder, conducting additive Ni powder and PVDF dissolved in 1-methyl-2-pyrrolidinone were mixed to form slurry, which was painted on a stainless steel grid (250 mesh) as current collector. After solvent evaporation, the electrode was pressed under a pressure of 5 tons / cm 2 and dried at 1208C under vacuum. 2. For polyethylene (PE) with ultra-high molecular weight (m.p. 1448C, Aldrich) as binder, (Li)SnSb x , Ni and PE powders were mixed in an agate mortar in a glove box. The powder mixture was pressed onto the current collector by a pressure of 5 tons / cm 2 .
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3. Results and discussion A drastic volume effect of metallic host matrices during Li-insertion and Li-extraction requires choosing a suitable binder to maintain interparticle connection. Fig. 3 shows that the influence of polymer binder on the cyclability of SnSb x -based composite electrodes is striking. As PVDF is used as binder, inserted lithium can hardly be extracted, resulting in a very low coulombic efficiency. A further cycling is impossible. Comparably, high-density polyethylene is an appropriate binder for SnSb x host matrix. Its use increases the capacity and significantly improves the coulombic efficiency. It is believed that an interaction between weak-polar PVDF and polar solvents could cause swelling of PVDF binder and even decrease its binding ability. This, combined with great volume expansion of coarse SnSbx particles during Li-insertion, may produce a large irreversible deformation and result in subsequent electrical and interparticle disconnection in the electrode, so that lithium can not be extracted from isolated Li-alloy particles. On the other hand, highdensity polyethylene is chemically stable and rigid,
The typical weight of composite material of the electrodes was 7–10 mg. All electrode compositions referred to weight percentage and the capacity of the electrodes was calculated on the basis of the weight of active materials (excluding binder and Ni powder).
2.3. Electrochemical measurements Half-cell studies were performed in laboratory type glass cells with excess of organic electrolyte (1 M LiClO 4 / EC 1 DEC, 1:1) and battery grade lithium counter and reference electrodes. Composite electrodes were vertically placed in the electrolyte without any pressure. Cycling tests were carried out at charge and discharge current densities of 0.4 mA / cm 2 under a given cut-off. The rest time between charge and discharge was 1 min. Charging and discharging of the cell refer, respectively, to lithium insertion into and extraction from alloy composite electrodes.
Fig. 3. The first cycle curves of SnSb 0.13 -based composite electrodes with different binders. Electrode composition: 10% Ni, 10% PE and 80% SnSb 0.13 ( , 38 mm); 10% Ni, 9% PVDF and 81% SnSb 0.13 . ‘Time’ is regulated according to the weight of SnSb 0.13 alloy in the electrodes, correlating to the capacity at the same current density.
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Fig. 4. The first cycle curves of composite electrodes with different particle size and composition of metallic hosts. Electrode composition: 10% Ni, 10% PE and 80% SnSb 0.13 (or Sn).
which is favorable for the electrode morphological stability. Fig. 4 demonstrates the influence of the type and particle size of metallic hosts on the first cycle of the electrode. In the first charging process, 4 potential plateaus can be observed for the electrode based on coarse SnSb 0.13 powder (more clearly with PVDF as binder in Fig. 3). The first plateau near 0.8 V vs. Li / Li 1 represents Li-insertion into SnSb phase, forming Li 3 Sb phase and Sn phase [12,13]. The following plateaus correspond to Li-insertion into Sn and Li x Sn phases. With the decrease of the particle size, distinction of the potential plateaus becomes more different. As the particle size is close to nanoscale, the plateaus can no longer be distinguished. This could be attributed to poor crystallinity of the ultrafine powder and the fact that the proportion of the surface layer in the whole host material rises owing to large specific surface area. Generally, the surface contains minor oxide and may also absorb some impurity. It is more or less different from the alloy phase inside the particle. On the other hand, as Li–C and metallic lithium anodes, Li-insertion into metallic host matrices can also cause surface filming, forming a new interfacial phase. Surface filming consumes a small amount of charge, leading to a higher irreversible capacity. For this reason, the
coulombic efficiency is not high at the first cycle with the electrode based on ultrafine SnSb x powder (e.g. 82%, , 38 mm; 71%, , 0.8 mm; 63%, , 0.2 mm). It can also be observed from Fig. 4 that adding a small amount of antimony into tin to form SnSb phase essentially changes charging characteristics of Li-insertion. In the case of Sn host matrix, the potential for Li-insertion rapidly drops to below 0.4 V vs. Li / Li 1 . Several higher potential plateaus, respectively representing co-existence of two Li-poor phases under equilibrium or near-equilibrium conditions, can not be observed [14]. It implies that some lithium accumulation on the surface takes place due to not enough fast Li-diffusion in the metallic host. SnSb phase dispersed in Sn phase can greatly improve the kinetics of electrochemical Li-alloying process. A recent investigation showed that Li-diffusion is faster in SnSb phase than in Sn phase at ambient temperature [15]. The superiority of SnSb x over Sn host matrix in the cyclability has been revealed and discussed [10,11]. It was supposed that stepwise Li-insertion and Li-extraction related with the multiphase host buffered the volume changes and thus improved the cyclability. As shown in Fig. 5, increasing antimony content in SnSb x alloy to a certain extent can strengthen the buffering effect and enhance the cycling stability. However, it has to be pointed out
Fig. 5. The cyclability of composite electrodes consisting of 10% Ni, 10% PE and 80% SnSb x ( , 38 mm) with different Sb content.
J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
that low coulombic efficiency (mostly, h , 90%) and poor capacity retention remain a character of the electrode based on coarse SnSb x powder ( , 38 mm). An essential improvement in cycling performance can be achieved by decreasing the particle size of SnSb x powder to submicro-scale (Fig. 6). When lithium is inserted or extracted, ultrafine host particles have a weak tendency to further subdivision due to small absolute volume changes and small mechanical stress within the particles. On the other hand, an ultrafine powder has also relatively low packing density that leads to a small relative volume change during cycling. All of these favor the dimensional and conducting stability of the electrode and, therefore, enhance the electrode performance. The potential range for cycling is also an important factor determining the cyclability of Li-alloy composite electrodes. Fig. 7 reveals that cycling stability is quite poor in a high potential range. But it is noted that potential lower-limit until 0 V vs. Li / Li 1 is also unfavorable. The same tendency is also correlated with Sn-based composite electrodes. It appears that electrochemical cycling is more stable within Li-rich alloy phase zone than within Li-poor one. Li-insertion potential until 0 V vs. Li / Li 1 may cause more serious decomposition reaction of the electrolyte and thus degrade the electrode performance. In fact, a release of gases from the electrode can be observed at near 0 V. An obvious disadvantage of the use of ultrafine host powders is the low coulombic efficiency at the first cycle. When initially inserted lithium is partly extracted by controlling cut-off to improve the cyclability, the efficiency will become lower. This hinders practical application of Li-alloy composite electrodes. Two approaches have been taken to put a small amount of lithium into the electrodes before cycling to enhance the first efficiency. As shown in Fig. 8, electrochemical Li-insertion to form Li 1.7 / SnSb 0.13 powder makes the Li-alloy material lose some activity despite its high cycling efficiency. Thickened interphase layer and high interfacial polarization may be the reason for its low capacity. Another approach is mixing SnSb x powder with a small amount of Li-containing compounds to form a new composite system. It is found that a combination of SnSb x alloy with Li 2.6 Co 0.4 N [16] is successful. Fig. 9 exhibits a comparison of cycling behavior
179
Fig. 6. Cycling behavior of composite electrodes consisting of 10% Ni, 10% PE and 80% SnSb x with different particle sizes. Filled symbols: Li-insertion; Open symbols: Li-extraction.
Fig. 7. The cyclability of the composite electrode consisting of 10% Ni, 10% PE and 80% SnSb 0.13 ( , 0.2 mm) under different cut-off.
between SnSb 0.13 -based electrode and the new composite electrode. Owing to very small volume effect of Li 2.6 Co 0.4 N, its addition into SnSb 0.13 alloy powder can reduce the volume changes of the whole electrodes during cycling and thus improve the cycling performance. Moreover, the first coulombic efficiency can be regulated by controlling the content of Li 2.6 Co 0.4 N compound in the composite elec-
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J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
range for cycling. Non-polar high-density polyethylene is a more suitable binder for SnSb x host powder than weak-polar PVDF. With the decrease of the particle size of SnSb x powder, capacity retention and coulombic efficiency of the electrode can be significantly enhanced in the whole cycling process. But finer metallic host powder corresponds to a lower coulombic efficiency at the first cycle. A combination of SnSb x with Li 2.6 Co 0.4 N improves the electrode performance on all sides.
Acknowledgements
Fig. 8. Effect of Li-insertion into SnSb 0.13 ( , 0.2 mm) before cycling on the cycling performance. Filled symbols: Li-insertion; Open symbols: Li-extraction.
This research work was supported by Genesis Research Institute, INC., Nagoya, Japan.
References
Fig. 9. Effect of Li 2.6 Co 0.4 N as an electrode component on the cycling performance. Electrode composition: 10% Ni, 10% PE and 80% SnSb 0.13 ( , 0.2 mm); 10% Ni, 10% PE, 20% Li 2.6 Co 0.4 N and 60% SnSb 0.13 . Filled symbols: Li-insertion; Open symbols: Li-extraction.
trodes. A further study on this system is still in progress.
4. Conclusions Cycling performance of SnSb x -based composite electrodes is mainly dependent on polymer binder, particle size of SnSb x host matrix and the potential
[1] I. Nakane, T. Nohma, K. Teraji, N. Furukawa, IBA Los Angeles Meeting on Understanding Rechargeability of MnO 2 for Li-cells, May 12–13, 1989. [2] J.O. Besenhard, M. Hess, P. Komenda, Solid State Ionics 40–41 (1990) 525. [3] O. Mao, J.R. Dahn, 9th International Meeting on Lithium Batteries, Edinburgh 1998, Book of Abstracts, Poster 1, Tues 34. [4] M. Maxfield, T.R. Jow, S. Gould, M.G. Sewchok, L.W. Shacklette, J. Electrochem. Soc. 135 (1988) 299. [5] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [6] Z. Takehara, J. Power Sources 26 (1989) 257. [7] A. Lin, E. Peled, 6th International Meeting on Lithium Batteries, Muenster, Germany 1992, Extended abstract, p. 234. [8] B.A. Boukamp, G.C. Lesh, R.A. Huggins, J. Electrochem. Soc. 128 (1981) 725. [9] A. Anani, S. Crouch-Baker, R.A. Huggins, J. Electrochem. Soc. 135 (1988) 2103. [10] J. Yang, M. Winter, J.O. Besenhard, Solid State Ionics 90 (1996) 281. [11] J. Yang, M. Wachtler, M. Winter, J.O. Besenhard, Electrochem. Solid-State Lett. 2 (1999) 161. [12] J.O. Besenhard, J. Yang, M. Winter, J. Power Sources 68 (1997) 87. [13] M. Winter, J.O. Besenhard, J.H. Albering, J. Yang, M. Wachtler, Progress in Batteries and Battery Materials 17 (1998) 208. [14] R.A. Huggins, J. Power Sources 26 (1989) 109. [15] J.O. Besenhard, M. Wachtler, M. Winter, R. Andreaus, I. Rom, W. Sitte, J. Power Sources 81–82 (1999) 268. [16] M. Nishijima, T. Kagohashi, M. Imanishi, Y. Takeda, O. Yamamoto, S. Kondo, Solid State Ionics 83 (1996) 107.
SnSb x -based composite electrodes for lithium ion cells J. Yang*, Y. Takeda, N. Imanishi, T. Ichikawa, O. Yamamoto Department of Chemistry, Faculty of Engineering, Mie University Kamihamacho, Tsu, Mie 514 -8507, Japan
Abstract SnSb x alloy powders with different composition and particle size have been examined for use as Li-alloy composite anode materials in lithium ion cells. Rapid degradation of cycling performance of Li-alloy composite electrodes in 1 M LiClO 4 / EC 1 DEC (1:1) can be greatly suppressed by decreasing the particle size of SnSb x alloy hosts and choosing a suitable polymer binder. An increase of antimony content in SnSbx alloy to a certain extent also improved the cycling stability. In order to enhance coulombic efficiency at the first cycle, a given amount of lithium was introduced into the electrode before cycling by different methods. A remarkable improvement in the electrode performance can be achieved by adding a small amount of Li 2.6 Co 0.4 N powder into the SnSb x -based composite electrodes. 2000 Elsevier Science B.V. All rights reserved. Keywords: Lithium secondary cells; Lithium alloy composite electrodes; Ultrafine SnSb x powders; Cycling performance
1. Introduction Fast room temperature mobility of lithium and very high reversible capacity shows that some Lialloys such as Li–Al, Li–Sn and Li–Pb can be very attractive anode materials for lithium ion cells. Long cycle life of coin cell has been achieved in the basis of Mn-doped Li–Al anode at very low discharge rates [1]. Cycling stability of Li-alloy anodes under high Li-utilization, however, is often limited by the rapid mechanical disintegration due to drastic volume changes of metallic host matrices (Fig. 1). In order to enhance the dimensional and structural stability of the electrodes and thus to improve the cycling performance, many highly dispersed multiphase systems for Li-alloy composite anode materi*Corresponding author. Tel.: 1 81-59-231-9419; fax: 1 81-59231-9720. E-mail address: [email protected] (J. Yang).
als have been proposed. They can be mainly divided into two groups. One is generally composed of two phases: a reactant phase and a less active or inert phase (e.g. Al / Cu [2] and Sn 2 Fe / SnFe 3 C [3]). The
Fig. 1. Volume expansion after Li-insertion into host matrices (corresponding to 1 molar host material respectively).
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00361-1
176
J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
less active or inert phases in composite microstructures can reduce mechanical stress in the electrodes, buffer the volume effect and sometimes prevent the aggregation and recrystallization of highly divided active phase grains. Several such multiphase structures demonstrated improved or even excellent cycling performance [3–5]. The other group is composed of two or more reactant phases with different activity against lithium (e.g. wood’s alloys [6] and wide variostoichiometric range alloy [7]) that demonstrated a better dimensional stability. On the basis of ‘mixed-conducting matrix concept’ [8], an active phase in composite microstructures could become inert under controlled cut-off and served as mixedconducting matrix for reactant phases (e.g. LiCd x / Li 4.4 Sn [9]). It was found that electrodeposited multiphase SnSb x alloy film as host matrix had much better morphological and cycling stability than single phase Sn film [10]. However, the cycling tests were only confined to very thin thickness of the films ( , 4 mm) and limited charge input. These limitations of the bulk host matrix can be effectively overcome by the use of Li-alloy composite structures in which highly dispersed host powder and inert conducting additive are mechanically connected by flexible polymer binder. In this paper, electrochemical and cycling behaviors of SnSb x -based composite electrodes are studied.
2. Experimental
2.1. Metal and alloy powders Tin (99.8%, , 45 mm) and nickel powder (99.9%, 0.03 mm) are commercially available. SnSb x (x 5 0.13, 0.25 and 0.42) powders (400 mesh, , 38 mm) were prepared by melting and mechanical filing. High pure tin and antimony powders were stoichiometrically mixed and pressed, and then melted at 8508C under Ar atmosphere for 4 h. After cooling until room temperature, the alloy stick was polished to remove the surface layer and then filed to obtain SnSb x filings. Finally SnSb x powders (typically , 38 mm) were obtained from the filings by means of 400 mesh sieve. Two types of ultrafine SnSb x powders (nominal composition: SnSb 0.13 , particle size , 0.2 mm; SnSb 0.17 , , 0.8 mm) were synthesized by chemical precipitation from aqueous solutions containing the chlorides of the respective metals and complex agents with sodium borohydride as reductive agent [11]. Fig. 2 shows SEM photographs of these SnSb x powders. X-ray diffraction analysis indicated that all SnSb x powders consisted of Sn phase and intermetallic SnSb phase. Li 1.7 / SnSb 0.13 powder was prepared by electrochemical insertion of lithium into SnSb 0.13 ( , 0.2 mm) powder electrode containing 5% polyethylene with ultra-high molecular weight (stainless steel grid
Fig. 2. SEM photographs of SnSb x alloy powders with different particle size.
J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
as current collector). After Li-insertion, the electrode was soaked in high pure THF solution for 1 h and then washed by THF twice. After drying at 908C under vacuum, Li-alloy was detached from the current collector and ground to powder in a glove box.
2.2. Fabrication of electrodes Composite electrodes with a geometric area of 0.5 cm 2 were fabricated by following two methods: 1. For poly(vinylidene fluoride) (PVDF) binder, SnSb x powder, conducting additive Ni powder and PVDF dissolved in 1-methyl-2-pyrrolidinone were mixed to form slurry, which was painted on a stainless steel grid (250 mesh) as current collector. After solvent evaporation, the electrode was pressed under a pressure of 5 tons / cm 2 and dried at 1208C under vacuum. 2. For polyethylene (PE) with ultra-high molecular weight (m.p. 1448C, Aldrich) as binder, (Li)SnSb x , Ni and PE powders were mixed in an agate mortar in a glove box. The powder mixture was pressed onto the current collector by a pressure of 5 tons / cm 2 .
177
3. Results and discussion A drastic volume effect of metallic host matrices during Li-insertion and Li-extraction requires choosing a suitable binder to maintain interparticle connection. Fig. 3 shows that the influence of polymer binder on the cyclability of SnSb x -based composite electrodes is striking. As PVDF is used as binder, inserted lithium can hardly be extracted, resulting in a very low coulombic efficiency. A further cycling is impossible. Comparably, high-density polyethylene is an appropriate binder for SnSb x host matrix. Its use increases the capacity and significantly improves the coulombic efficiency. It is believed that an interaction between weak-polar PVDF and polar solvents could cause swelling of PVDF binder and even decrease its binding ability. This, combined with great volume expansion of coarse SnSbx particles during Li-insertion, may produce a large irreversible deformation and result in subsequent electrical and interparticle disconnection in the electrode, so that lithium can not be extracted from isolated Li-alloy particles. On the other hand, highdensity polyethylene is chemically stable and rigid,
The typical weight of composite material of the electrodes was 7–10 mg. All electrode compositions referred to weight percentage and the capacity of the electrodes was calculated on the basis of the weight of active materials (excluding binder and Ni powder).
2.3. Electrochemical measurements Half-cell studies were performed in laboratory type glass cells with excess of organic electrolyte (1 M LiClO 4 / EC 1 DEC, 1:1) and battery grade lithium counter and reference electrodes. Composite electrodes were vertically placed in the electrolyte without any pressure. Cycling tests were carried out at charge and discharge current densities of 0.4 mA / cm 2 under a given cut-off. The rest time between charge and discharge was 1 min. Charging and discharging of the cell refer, respectively, to lithium insertion into and extraction from alloy composite electrodes.
Fig. 3. The first cycle curves of SnSb 0.13 -based composite electrodes with different binders. Electrode composition: 10% Ni, 10% PE and 80% SnSb 0.13 ( , 38 mm); 10% Ni, 9% PVDF and 81% SnSb 0.13 . ‘Time’ is regulated according to the weight of SnSb 0.13 alloy in the electrodes, correlating to the capacity at the same current density.
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J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
Fig. 4. The first cycle curves of composite electrodes with different particle size and composition of metallic hosts. Electrode composition: 10% Ni, 10% PE and 80% SnSb 0.13 (or Sn).
which is favorable for the electrode morphological stability. Fig. 4 demonstrates the influence of the type and particle size of metallic hosts on the first cycle of the electrode. In the first charging process, 4 potential plateaus can be observed for the electrode based on coarse SnSb 0.13 powder (more clearly with PVDF as binder in Fig. 3). The first plateau near 0.8 V vs. Li / Li 1 represents Li-insertion into SnSb phase, forming Li 3 Sb phase and Sn phase [12,13]. The following plateaus correspond to Li-insertion into Sn and Li x Sn phases. With the decrease of the particle size, distinction of the potential plateaus becomes more different. As the particle size is close to nanoscale, the plateaus can no longer be distinguished. This could be attributed to poor crystallinity of the ultrafine powder and the fact that the proportion of the surface layer in the whole host material rises owing to large specific surface area. Generally, the surface contains minor oxide and may also absorb some impurity. It is more or less different from the alloy phase inside the particle. On the other hand, as Li–C and metallic lithium anodes, Li-insertion into metallic host matrices can also cause surface filming, forming a new interfacial phase. Surface filming consumes a small amount of charge, leading to a higher irreversible capacity. For this reason, the
coulombic efficiency is not high at the first cycle with the electrode based on ultrafine SnSb x powder (e.g. 82%, , 38 mm; 71%, , 0.8 mm; 63%, , 0.2 mm). It can also be observed from Fig. 4 that adding a small amount of antimony into tin to form SnSb phase essentially changes charging characteristics of Li-insertion. In the case of Sn host matrix, the potential for Li-insertion rapidly drops to below 0.4 V vs. Li / Li 1 . Several higher potential plateaus, respectively representing co-existence of two Li-poor phases under equilibrium or near-equilibrium conditions, can not be observed [14]. It implies that some lithium accumulation on the surface takes place due to not enough fast Li-diffusion in the metallic host. SnSb phase dispersed in Sn phase can greatly improve the kinetics of electrochemical Li-alloying process. A recent investigation showed that Li-diffusion is faster in SnSb phase than in Sn phase at ambient temperature [15]. The superiority of SnSb x over Sn host matrix in the cyclability has been revealed and discussed [10,11]. It was supposed that stepwise Li-insertion and Li-extraction related with the multiphase host buffered the volume changes and thus improved the cyclability. As shown in Fig. 5, increasing antimony content in SnSb x alloy to a certain extent can strengthen the buffering effect and enhance the cycling stability. However, it has to be pointed out
Fig. 5. The cyclability of composite electrodes consisting of 10% Ni, 10% PE and 80% SnSb x ( , 38 mm) with different Sb content.
J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
that low coulombic efficiency (mostly, h , 90%) and poor capacity retention remain a character of the electrode based on coarse SnSb x powder ( , 38 mm). An essential improvement in cycling performance can be achieved by decreasing the particle size of SnSb x powder to submicro-scale (Fig. 6). When lithium is inserted or extracted, ultrafine host particles have a weak tendency to further subdivision due to small absolute volume changes and small mechanical stress within the particles. On the other hand, an ultrafine powder has also relatively low packing density that leads to a small relative volume change during cycling. All of these favor the dimensional and conducting stability of the electrode and, therefore, enhance the electrode performance. The potential range for cycling is also an important factor determining the cyclability of Li-alloy composite electrodes. Fig. 7 reveals that cycling stability is quite poor in a high potential range. But it is noted that potential lower-limit until 0 V vs. Li / Li 1 is also unfavorable. The same tendency is also correlated with Sn-based composite electrodes. It appears that electrochemical cycling is more stable within Li-rich alloy phase zone than within Li-poor one. Li-insertion potential until 0 V vs. Li / Li 1 may cause more serious decomposition reaction of the electrolyte and thus degrade the electrode performance. In fact, a release of gases from the electrode can be observed at near 0 V. An obvious disadvantage of the use of ultrafine host powders is the low coulombic efficiency at the first cycle. When initially inserted lithium is partly extracted by controlling cut-off to improve the cyclability, the efficiency will become lower. This hinders practical application of Li-alloy composite electrodes. Two approaches have been taken to put a small amount of lithium into the electrodes before cycling to enhance the first efficiency. As shown in Fig. 8, electrochemical Li-insertion to form Li 1.7 / SnSb 0.13 powder makes the Li-alloy material lose some activity despite its high cycling efficiency. Thickened interphase layer and high interfacial polarization may be the reason for its low capacity. Another approach is mixing SnSb x powder with a small amount of Li-containing compounds to form a new composite system. It is found that a combination of SnSb x alloy with Li 2.6 Co 0.4 N [16] is successful. Fig. 9 exhibits a comparison of cycling behavior
179
Fig. 6. Cycling behavior of composite electrodes consisting of 10% Ni, 10% PE and 80% SnSb x with different particle sizes. Filled symbols: Li-insertion; Open symbols: Li-extraction.
Fig. 7. The cyclability of the composite electrode consisting of 10% Ni, 10% PE and 80% SnSb 0.13 ( , 0.2 mm) under different cut-off.
between SnSb 0.13 -based electrode and the new composite electrode. Owing to very small volume effect of Li 2.6 Co 0.4 N, its addition into SnSb 0.13 alloy powder can reduce the volume changes of the whole electrodes during cycling and thus improve the cycling performance. Moreover, the first coulombic efficiency can be regulated by controlling the content of Li 2.6 Co 0.4 N compound in the composite elec-
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J. Yang et al. / Solid State Ionics 135 (2000) 175 – 180
range for cycling. Non-polar high-density polyethylene is a more suitable binder for SnSb x host powder than weak-polar PVDF. With the decrease of the particle size of SnSb x powder, capacity retention and coulombic efficiency of the electrode can be significantly enhanced in the whole cycling process. But finer metallic host powder corresponds to a lower coulombic efficiency at the first cycle. A combination of SnSb x with Li 2.6 Co 0.4 N improves the electrode performance on all sides.
Acknowledgements
Fig. 8. Effect of Li-insertion into SnSb 0.13 ( , 0.2 mm) before cycling on the cycling performance. Filled symbols: Li-insertion; Open symbols: Li-extraction.
This research work was supported by Genesis Research Institute, INC., Nagoya, Japan.
References
Fig. 9. Effect of Li 2.6 Co 0.4 N as an electrode component on the cycling performance. Electrode composition: 10% Ni, 10% PE and 80% SnSb 0.13 ( , 0.2 mm); 10% Ni, 10% PE, 20% Li 2.6 Co 0.4 N and 60% SnSb 0.13 . Filled symbols: Li-insertion; Open symbols: Li-extraction.
trodes. A further study on this system is still in progress.
4. Conclusions Cycling performance of SnSb x -based composite electrodes is mainly dependent on polymer binder, particle size of SnSb x host matrix and the potential
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