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Electroplating synthesis and electrochemical properties of macroporous Sn–Cu alloy electrode for lithium-ion batteries
Electrochimica Acta 52 (2007) 6741–6747
Electroplating synthesis and electrochemical properties of macroporous Sn–Cu alloy electrode for lithium-ion batteries Fu-Sheng Ke, Ling Huang ∗ , Jin-Shu Cai, Shi-Gang Sun ∗ Department of Chemistry, College of Chemistry and Chemical Engineering, State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, PR China Received 23 November 2006; received in revised form 25 April 2007; accepted 25 April 2007 Available online 6 May 2007
Abstract Macroporous material of Sn–Cu alloy of different pore sizes designated as anode in lithium-ion batteries were fabricated through colloidal crystal template method. The structure and electrochemical properties of the macroporous Sn–Cu alloy electrodes were examined by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and galvanostatic cycling. The results demonstrated that the electrodes of macroporous Sn–Cu alloy with pore size respectively of 180 and 500 nm can deliver reversible capacity of 350 and 270 mAh g−1 up to 70th cycles of charge/discharge. The cycle performance of the macroporous Sn–Cu alloy of 180 nm in pore size is better than that of the macroporous Sn–Cu alloy with 500-nm-diameter pores. It has revealed that the porous structure of the macroporous Sn–Cu alloy material is of importance to strengthen mechanically the electrode and to reduce significantly the effect of volume expansion during cycling. © 2007 Elsevier Ltd. All rights reserved. Keywords: Electroplating; Colloidal crystal template; Macroporous; Tin–copper alloy; Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIB) are widely used as a power supply for portable electronic devices, such as mobile phones and notebook computers. To meet with the rapid increasing demand on specific energy density and safety of the LIB for diverse applications, the exploitation of new electrode materials with better performances has become the key issue in development of the LIB. The graphite material that was used as anode of Li-ion batteries mainly in present commercialized production has already approached its theoretical capacity limit of C6 Li (372 mAh g−1 ) [1]. Extensive efforts have been paid to explore different types of materials (such as Sn, Sb, Si, Al, Ge) to be employed as anode electrode in LIB, which are promising to exceed the capacity of carbon materials. Tin-based materials were widely studied among them as alternative anode material to carbon for lithium-ion batteries thanks to its much higher the-
∗
Corresponding authors. Tel.: +86 592 2181436; fax: +86 592 2183629. E-mail addresses: [email protected] (L. Huang), [email protected] (S.-G. Sun). 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.100
oretical capacity (991 mAh g−1 ) than that of the carbon material [2]. However, a pure tin electrode suffers severely from its poor cycleability due to mechanical fatigue caused by volume change during lithium insertion and extraction processes [2,3]. In order to improve the cycleability property of tin anode, modification of the electrode structure is a vital factor [4]. Many attempts have demonstrated that both the size of the particles constituting of the electrode and the size of the grains within these particles play critical roles in electrochemical performance [3–6]. It exists nevertheless some unsolved problems, such as the aggregation of tin during cycling. Extensive attention has been paid in past years on tin-based intermetallic compounds, i.e. Snx My (M: inactive element), including Sn–Co [7–9], Sn–Ni [10,11], Sn–Cu [12–18], etc. It has demonstrated that these materials exhibited longer cycleability than that of pure tin electrode. Winter and Besenhard [19] inferred that the inactive element can buffer the large volume change and as a barrier against the aggregation of Sn into large grains during Li-ion insertion and extraction processes. However, long-term charge/discharge cycling will still lead the rapid loss in reversible capacity and rechargeability. Among tin-based intermetallic compounds, the Sn–Cu intermetallic compounds were suggested as the promising alternative
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anode materials [12–18]. In particular, lithium can be reversibly inserted into Cu6 Sn5 to form Li13 Cu6 Sn5 (200 mAh g−1 ), which allowed the battery to be cycled in restricted voltage region. However, a fairly large volume change leads to poor cycle performance, and gives rise to a cracking and crumbling of the metallic host. To overcome this problem, Shin and Liu [18] have fabricated three-dimensional (3D) porous Cu–Sn alloy through hydrogen bubbles as template by using electrochemical deposition. Porous materials were received recently considerable attentions as promising new anode materials for lithium-ion batteries [9,11,18,20–26]. The advantages of porous electrodes consist in: (i) the large open pores that allow easily transport of liquid electrolyte; (ii) a large number of active sites for chargetransfer reactions due to the high surface area of materials; (iii) the continuous network that is expected to improve electrical conductivity; and (iv) numerous pores that can buffer the large volume change caused by disintegration of the structure [20]. Porous materials can be fabricated by various methods, such as infiltration, electroplating, ionic spraying, laser spraying. In comparison, electroplating presents significant advantages, particular for the deposition of thin films of macroporous materials. The microstructure of the deposits can also be drastically adjusted by well-controlling the electroplating conditions, i.e. the chemical composition of plating solutions (e.g. by addition of leveling and complexing agents), the solution temperature, the stirring conditions and the plating current densities. In this paper we have fabricated electrodes of macroporous Sn–Cu alloy with different pore size (180 and 500 nm) by eletrochemical plating method and using colloidal crystal as template. The structure and electrochemical properties of electroplating synthesized macroporous Sn–Cu alloy electrodes were investigated by various techniques. It has demonstrated that the cycling performance of a lithium ion battery employing the macroporous Sn–Cu alloy as anode can be significantly improved.
bath for 2 days to dissolve away the PS template. Finally, the working electrodes were subjected to an anhydrous ethanol sonication bath for 1 min, and then dried at 120 ◦ C and 10−3 Torr for 10 h. It has measured that the mass of the electrodeposited macroporous Sn–Cu alloy is about 0.5 mg, which gives rise to a film thickness about 1 m in considering that the macroporous Sn–Cu alloy film electrode keeps the same geometric surface area of the substrate, i.e. 0.785 cm2 . The as-prepared samples were characterized by field emission scanning electron microscopy (FE-SEM, LEO-1530 SEM system), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD, Philips X’Pert Pro Super X-ray diffractometer, Cu K␣ radiation) and X-ray photoelectron spectroscopy (PHI Quantum 2000 Scanning ESCA Microprobe). Electrochemical charge–discharge behaviors were investigated directly using coin cells (type CR2025) assembled in an argon-filled glove box. The cell was made from a macroporous Sn–Cu alloy cathode and a lithium anode. The electrodes were separated by a separator material (Celgard 2400). The electrolyte is consisted of a solution of 1 M LiPF6 in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) 1:1:1 (vol%) obtained from Zhangjiagang Guotai-huarong New Chemical Materials Co., Ltd. It has measured that the water content in electcrolyte is less than 8 ppm. The cells were galvanostatically charged and discharged in a battery test system (NEWARE BTS-610, Neware Technology Co., Ltd. China) at constant current density of 160 mA g−1 (about 0.3 C) with a cut-off voltage of 0.05 to 1.0 V(versus Li/Li+ ) at room temperature. The cycled cells were taken apart in an argonfilled glove box. In order to remove electrolyte and to avoid LiPF6 deposition, the cycled electrodes were washed in DMC three times, then dried under vacuum at ambient temperature, and finally examined using SEM. 3. Results and discussion
2. Experimental Polystyrene (PS) spheres of 180 and 500 nm in diameter were synthesized by using an emulsion polymerization followed by a seed growth polymerization technique according to literature [9,11,27,28]. The detailed preparation procedure was described in a previous paper [9,11]. The PS spheres of 500 nm in diameter were synthesized by using the 180 nm PS spheres as a seed solution. The PS spheres were assembled onto a Ni-coated Cu sheet that was prepared by self-sedimentation in a slowly evaporating dispersion of PS spheres in ethanol, as described by Bartlett et al. [29]. Electroplating was carried out on an electrochemistry working station CHI660A (Chenhua Co., Shanghai). Sn–Cu alloy was deposited onto PS templates at a constant current density of 0.50 A dm−2 from a plating solution of 15 g L−1 SnCl2 ·2H2 O, 4 g L−1 CuSO4 ·2H2 O, 200 g L−1 K4 P2 O7 , and 5 mL L−1 additive agent (provided by Dr. Fan) at room temperature, without stirring. The pH of the solution was adjusted to 8.5. All these reagents are analytical reagents, and were purchased from Sinapharm Chemical Reagent Co., Ltd. After electrodeposition, the working electrodes were immersed in terahydrofuran (THF)
Fig. 1 shows SEM images of the template of PS spheres described in detail in Ref. [11]. In brief, we can observe that the PS spheres are well packed together. The average diameter of the sphere is measured about 180 nm (Fig. 1a) and 500 nm (Fig. 1b), which are in consistent with the value measured from TEM result (not shown). The PS spheres were accumulated into well-ordered areas by lateral capillary forces [30] when the ethanol film becomes as thin as the particle diameter. The lateral capillary force acts solely on particles protruding out of the ethanol surface. Due to meniscus forming between two particles strong capillary forces arises. Their lateral projection drags the particles together. Using ethanol as disperse media can accelerate PS spheres sedimentation. We observe also that the template of PS spheres of 500 nm in diameter illustrates a more order than the template of PS spheres of 180 nm in diameter. Although selfsedimentation on relatively rough substrate may contain some defects, a “faultless” colloidal crystal template is not necessary in the preparation of porous electrodes for lithium-ion batteries [25]. The electrochemical plating of Sn–Cu alloy was carried out on the prepared templates. In order to make the solution ade-
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Fig. 1. SEM images of the template of PS spheres deposited on a Ni-coated Cu sheet (a) 180 nm; (b) 500 nm. The inset shows a corresponding high-magnification image.
Fig. 2. SEM images of macroporous Sn–Cu alloy with 180-nm-diameter pores (a) and 500-nm-diameter pores (b).
quately infiltrating into voids of the template, the PS sphere template films were kept in plating solution for 15 min before the electroplating. After passing a current, the Sn–Cu alloy nuclei can be nucleated on electrode surface unoccupied by PS spheres, and grown three dimensionally. The Sn–Cu alloy grew layer by layer from the substrate and filled the voids between the closepacked PS latex spheres. Some air bubbles may be arisen from the working electrode during electrodeposition, if the current density is too large. So a constant current of 0.5 A dm−2 was applied during the deposition, which prevents also the fragile templates from breaking. After the deposition of Sn–Cu alloy the PS spheres were dissolved and removed by socking the electrodes in THF for 48 h. Fig. 2 displays SEM images of the macroporous Sn–Cu alloy after removing PS template. We observe holes that are distributed on the Sn–Cu alloy layer and are interconnected to form relatively uniform pores. Such holes appear because the impregnating solid material cannot grow at the contact area of a neighboring pair of colloidal spheres in PS template. The average pore size are about 180 nm (Fig. 2a) and 500 nm (Fig. 2b), respectively. It is evident that the pores match the size of the starting PS spheres, thus indicating that shrinkage of the alloy structure is negligible [31,32]. The continuous framework would yield advanced electronic and ionic conductivity. The voids in these samples ensure electrolyte accessing to the large inner surfaces inside the macroporous alloy. The thickness of the macroporous Sn–Cu alloy films prepared by electrodeposition can be adjusted from tens of nanometer to several micrometers
by changing the deposition time. However, the deposition time could not be overlong, otherwise, the upper layer PS spheres will be wrapped and caused difficulty of removing the PS sphere template by wet etching. In concerning the deposition mechanism, Bartlett et al. [33] inferred that the electrochemical deposition is initiated at electrode surface and grew out through the overlying template. To confirm the chemical composition of the macroporous Sn–Cu alloy, at the same time, to avoid the disturbance of the substrate (Cu), Sn–Cu alloy deposit on stainless steel sheet was prepared by electrodeposition under the same conditions. The Sn–Cu alloy was determined by EDS and XRD. Fig. 3 illustrates the EDS spectrum of the Sn–Cu alloy, which indicates
Fig. 3. EDS spectrum of the Sn–Cu alloy deposited on stainless steel sheet.
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Fig. 4. XRD patterns of the Sn–Cu alloy deposited on stainless steel sheet.
that the average atomic ratio of Sn:Cu is about 50.15:49.85 for this alloy. The XRD pattern of the Sn–Cu alloy is shown in Fig. 4, which indicates the presence of two different phase, i.e. the main electrodeposited product of Cu6 Sn5 (JCPDS no. 45-1488) with monoclinic structure (space group, C2/c) that is evidenced by the main peaks of 2θ at 30.04, 42.90 and 43.24◦ , and a minor quantity of Sn (JCPDS no. 65-0296) with tetragonal structure
(space group, I41/amd). The Sn–Cu alloy is present as a monoclinic solid solution, metallic tin as solvent and copper as solute, and copper atoms partly replace the tin atoms in the Sn–Cu alloy. From XPS analysis (Fig. 5), we obtained the XPS survey spectrum of Cu (Fig. 5a), and Sn (Fig. 5b). The Cu 2p exhibits a doublet (i.e. 2p1/2 and 2p3/2 ) with a spin–orbit splitting, in agreement with literature [34], of about 19.8 eV. The peaks at 952.0 and 932.2 eV are associated with metallic Cu (XPS value of pure Cu 2p3/2 = 932.7 eV) [34]. The Sn 3d region exhibits a welldefined doublet with a spin–orbit splitting of about 8.42 eV [34]. It can be deconvoluted into four peaks at 493.33 and 494.51 eV for Sn 3d3/2 , 484.91 and 486.01 eV for Sn 3d5/2 . The signals at 494.51 and 486.01 eV are assigned to tin oxide. The peak at 493.33 eV is attributed to pure Sn [35]. The binding of 484.91 eV for Sn 3d5/2 is close to the literature value of pure Sn (485.0 eV) [34]. The electrochemical properties of electrodes of macroporous Sn–Cu alloy with 180 and 500 nm in pore size are illustrated in Figs. 6 and 7, respectively, in which the 1st, 2nd, 10th, 30th, 50th, and 70th charge–discharge curves are displayed. Lithium ions intercalation and extraction from the macroporous electrodes are associated with processes of charge and discharge of the anode, respectively. Large irreversible capacity observed in the first cycle may be caused by the existence of possible oxide impurities on electrode surface and the formation of solid electrolyte interphase (SEI), since large specific area of the electrode can consume much capacity for the formation of SEI. Pereira
Fig. 5. XPS spectra of (a) Cu 2p and (b) Sn 3d of the macroporous Sn–Cu alloy.
Fig. 6. (a) Charge/discharge curves of the macroporous Sn–Cu alloy with 180-nm-diameter pores. (b) Capacity vs. cycle number of the macroporous Sn–Cu alloy with 180-nm-diameter pores.
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Fig. 7. (a) Charge/discharge curves of the macroporous Sn–Cu alloy with 500-nm-diameter pores. (b) Capacity vs. cycle number of the macroporous Sn–Cu alloy with 500-nm-diameter pores.
et al. [36] have suggested that the reaction at a potential higher than 1.5 V (versus Li/Li+ ) during the first Li insertion might be rooted in electrolyte decomposition that is activated by pure tin of relatively high surface area acting as a catalyst. The reaction at potential between 1.5 and 0.9 V (versus Li/Li+ ) during the first charge represents the reduction of tin oxide. The potential plateau associated with these reactions disappears after the first cycle. The reaction around 0.7 V (versus Li/Li+ ) is assigned to the formation of SEI. It appears a long plateau around 0.35 V (versus Li/Li+ ), and the electrode potential is decreased gradually until 0.05 V. This is ascribed to the formation reaction of Lix Sn (x = 1–3). Figs. 6b and 7b show the cycling behaviors of electrodes of macroporous Sn–Cu alloy of both 180 and 500 nm in pore size. At the first cycle, the charge capacity (intercalation) and the discharge capacity are 810 and 445 mAh g−1 in Fig. 6b, 846 and 551 mAh g−1 in Fig. 7b, respectively. The macroporous Sn–Cu alloy with 500-nm-diameter pores electrode show higher capacity than that of the 180 nm pore size macroporous Sn–Cu alloy electrode. It may be attributed to that the electrolyte may be penetrated more easily into the large pore size during the first charge/discharge. Thus, most active materials can react with lithium ions. However, both electrodes exhibit large irreversible capacity at initial charge–discharge cycle. The loss of irreversible capacity may be attributed to: (i) the formation of lithium oxides (Li2 O) from a small quantity SnOx at surface of the alloy; (ii) the formation of a SEI layer on the surface with large specific surface area (due to the macroporous structure); (iii) the decomposition of electrolyte at electrode/electrolyte interphase; and (iv) the presence of pure tin which is very easily cracking and crumbling during the lithium ion insertion/extraction. The columbic efficiency in the charge–discharge processes is maintained as high as 92% except for the initial several cycles in Fig. 6b. It has been measured that the macroporous Sn–Cu alloy electrode material has delivered a reversible capacity about 410 mAh g−1 . As shown in Fig. 6b, the cycling performance of the macroporous Sn–Cu alloy with 180-nm-diameter pores electrode is excellent, the reversible capacity can be maintained 350 mAh g−1 after 70 cycles of charge/discharge. The reversible capacity of the macroporous Sn–Cu alloy with 500-nm-diameter pores electrode can be also maintained 270 mAh g−1 up to 70th cycle in Fig. 7b. It is evi-
dent that the former demonstrates a better cycle performance than that of the latter. It can be attributed to that the pore wall of the macroporous Sn–Cu alloy with 500-nm-diameter pores is very thin, which is easily cracking and crumbling during lithium insertion/extraction. In comparison, for a Sn–Cu alloy anode without the macroporous structure, the reversible capacity only maintained 220 mAh g−1 after 50 cycles [15]. Moreover, in our study, the cycling performance of the macroporous Sn–Cu alloy electrode fabricated by PS crystal template has been improved, which is better than that of synthesis 3D porous Cu–Sn [18] alloy by using hydrogen bubbles as template. This may be attributed to the stronger integrated strength, which is between active materials and current collector, than that of using hydrogen bubbles as template. SEM images of the macroporous Sn–Cu alloy with 180nm-diameter pores electrodes at different stage of charging are shown in Fig. 8. At 1.5 V the macroporous structure is resembled to that of the original uncycled material. However, some reactions may be occurred in some active sites. It is in consistent with the result of charge–discharge curves (Fig. 6a). At 1.0 V, the diameter of pores is diminished a little due to the reaction of part of tin oxide with Li. The pore size is sequentially diminished at 0.6 V, at the same time, some insulated substances are overlay on the electrode surface, which may be attributed to the formation of SEI. When charging to 0.05 V, the porous structure was almost disappeared. After the completion of cycle, however, the porous structure is appeared again as shown in Fig. 8e. Fig. 8f displays the structure of the electrode after 40 cycles, in which porous structure can be still observed in the electrode. The porous structure is remained within the electrode. Due to the insulated substance, it is very difficult to observe the detailed structure changing. To some extent, the porous structure can partly accommodate the volume expansion and prolong the cycleability. The good capacity retention and cycleability of the macroporous Sn–Cu alloy electrodes may be attributed to the porous structure and the addition of Cu. The macroporous structure can restrain the pulverization of electrode during charge/discharge cycles, and can partly accommodate the volume expansion. As a result the cycle life of the electrode is significantly improved and the diffusion of Li insertion/extraction of the macroporous materials is enhanced. Such properties can improve also the
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Fig. 8. SEM images acquired at different stages of charging of the macroporous Sn–Cu alloy with 180-nm-diameter pores electrodes at (a) 1.5 V, (b) 1.0 V, (c) 0.6 V, (d) 0.05 V, and after the first cycle (e) and the 40th cycle (f).
chargeability/dischargeability at large current densities. The addition of Cu as matrix in plated film relaxes also the mechanical stress that is resulted from volume expansion and contraction during Li insertion and desertion processes. 4. Conclusion The current paper reports the fabrication and electrochemical properties of an anode material with a macroporous structure for lithium-ion batteries. The electrodes of macroporous Sn–Cu alloy of different pore size were prepared by electroplating into the interstitial spaces of a template formed by PS latex spheres self-sedimentation on a relatively rough Ni-coated Cu foil surface. When the macroporous Sn–Cu alloy is used as negative electrode (anode) in a rechargeable lithium ion battery, the cycling performance of the battery has been significantly enhanced in comparison with the anode of Sn–Cu alloy without the macroporous structure. The results show that the
cycle performance of the small pore size macroporous Sn–Cu alloy is better than that of the large pore size macroporous Sn–Cu alloy. The study demonstrated that the Sn–Cu alloy with macroporous structure can be easily and conveniently mass produced by a general template electroplating technique on relatively rough substrate (Ni-coated Cu sheet). This method could be extended to fabricate macroporous materials of other metal, alloy and oxide. It is anticipated that the strategy of preparing macroporous Sn–Cu alloy can be also applied to synthesize other promising macroporous materials for lithium-ion batteries. Acknowledgements This work was supported by Major State Basic Research Development Program of China (2002CB2118004). The authors thank Dr. X.Y. Fan for the offer of additive agents for electroplating.
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Electroplating synthesis and electrochemical properties of macroporous Sn–Cu alloy electrode for lithium-ion batteries Fu-Sheng Ke, Ling Huang ∗ , Jin-Shu Cai, Shi-Gang Sun ∗ Department of Chemistry, College of Chemistry and Chemical Engineering, State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, PR China Received 23 November 2006; received in revised form 25 April 2007; accepted 25 April 2007 Available online 6 May 2007
Abstract Macroporous material of Sn–Cu alloy of different pore sizes designated as anode in lithium-ion batteries were fabricated through colloidal crystal template method. The structure and electrochemical properties of the macroporous Sn–Cu alloy electrodes were examined by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and galvanostatic cycling. The results demonstrated that the electrodes of macroporous Sn–Cu alloy with pore size respectively of 180 and 500 nm can deliver reversible capacity of 350 and 270 mAh g−1 up to 70th cycles of charge/discharge. The cycle performance of the macroporous Sn–Cu alloy of 180 nm in pore size is better than that of the macroporous Sn–Cu alloy with 500-nm-diameter pores. It has revealed that the porous structure of the macroporous Sn–Cu alloy material is of importance to strengthen mechanically the electrode and to reduce significantly the effect of volume expansion during cycling. © 2007 Elsevier Ltd. All rights reserved. Keywords: Electroplating; Colloidal crystal template; Macroporous; Tin–copper alloy; Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIB) are widely used as a power supply for portable electronic devices, such as mobile phones and notebook computers. To meet with the rapid increasing demand on specific energy density and safety of the LIB for diverse applications, the exploitation of new electrode materials with better performances has become the key issue in development of the LIB. The graphite material that was used as anode of Li-ion batteries mainly in present commercialized production has already approached its theoretical capacity limit of C6 Li (372 mAh g−1 ) [1]. Extensive efforts have been paid to explore different types of materials (such as Sn, Sb, Si, Al, Ge) to be employed as anode electrode in LIB, which are promising to exceed the capacity of carbon materials. Tin-based materials were widely studied among them as alternative anode material to carbon for lithium-ion batteries thanks to its much higher the-
∗
Corresponding authors. Tel.: +86 592 2181436; fax: +86 592 2183629. E-mail addresses: [email protected] (L. Huang), [email protected] (S.-G. Sun). 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.100
oretical capacity (991 mAh g−1 ) than that of the carbon material [2]. However, a pure tin electrode suffers severely from its poor cycleability due to mechanical fatigue caused by volume change during lithium insertion and extraction processes [2,3]. In order to improve the cycleability property of tin anode, modification of the electrode structure is a vital factor [4]. Many attempts have demonstrated that both the size of the particles constituting of the electrode and the size of the grains within these particles play critical roles in electrochemical performance [3–6]. It exists nevertheless some unsolved problems, such as the aggregation of tin during cycling. Extensive attention has been paid in past years on tin-based intermetallic compounds, i.e. Snx My (M: inactive element), including Sn–Co [7–9], Sn–Ni [10,11], Sn–Cu [12–18], etc. It has demonstrated that these materials exhibited longer cycleability than that of pure tin electrode. Winter and Besenhard [19] inferred that the inactive element can buffer the large volume change and as a barrier against the aggregation of Sn into large grains during Li-ion insertion and extraction processes. However, long-term charge/discharge cycling will still lead the rapid loss in reversible capacity and rechargeability. Among tin-based intermetallic compounds, the Sn–Cu intermetallic compounds were suggested as the promising alternative
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anode materials [12–18]. In particular, lithium can be reversibly inserted into Cu6 Sn5 to form Li13 Cu6 Sn5 (200 mAh g−1 ), which allowed the battery to be cycled in restricted voltage region. However, a fairly large volume change leads to poor cycle performance, and gives rise to a cracking and crumbling of the metallic host. To overcome this problem, Shin and Liu [18] have fabricated three-dimensional (3D) porous Cu–Sn alloy through hydrogen bubbles as template by using electrochemical deposition. Porous materials were received recently considerable attentions as promising new anode materials for lithium-ion batteries [9,11,18,20–26]. The advantages of porous electrodes consist in: (i) the large open pores that allow easily transport of liquid electrolyte; (ii) a large number of active sites for chargetransfer reactions due to the high surface area of materials; (iii) the continuous network that is expected to improve electrical conductivity; and (iv) numerous pores that can buffer the large volume change caused by disintegration of the structure [20]. Porous materials can be fabricated by various methods, such as infiltration, electroplating, ionic spraying, laser spraying. In comparison, electroplating presents significant advantages, particular for the deposition of thin films of macroporous materials. The microstructure of the deposits can also be drastically adjusted by well-controlling the electroplating conditions, i.e. the chemical composition of plating solutions (e.g. by addition of leveling and complexing agents), the solution temperature, the stirring conditions and the plating current densities. In this paper we have fabricated electrodes of macroporous Sn–Cu alloy with different pore size (180 and 500 nm) by eletrochemical plating method and using colloidal crystal as template. The structure and electrochemical properties of electroplating synthesized macroporous Sn–Cu alloy electrodes were investigated by various techniques. It has demonstrated that the cycling performance of a lithium ion battery employing the macroporous Sn–Cu alloy as anode can be significantly improved.
bath for 2 days to dissolve away the PS template. Finally, the working electrodes were subjected to an anhydrous ethanol sonication bath for 1 min, and then dried at 120 ◦ C and 10−3 Torr for 10 h. It has measured that the mass of the electrodeposited macroporous Sn–Cu alloy is about 0.5 mg, which gives rise to a film thickness about 1 m in considering that the macroporous Sn–Cu alloy film electrode keeps the same geometric surface area of the substrate, i.e. 0.785 cm2 . The as-prepared samples were characterized by field emission scanning electron microscopy (FE-SEM, LEO-1530 SEM system), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD, Philips X’Pert Pro Super X-ray diffractometer, Cu K␣ radiation) and X-ray photoelectron spectroscopy (PHI Quantum 2000 Scanning ESCA Microprobe). Electrochemical charge–discharge behaviors were investigated directly using coin cells (type CR2025) assembled in an argon-filled glove box. The cell was made from a macroporous Sn–Cu alloy cathode and a lithium anode. The electrodes were separated by a separator material (Celgard 2400). The electrolyte is consisted of a solution of 1 M LiPF6 in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) 1:1:1 (vol%) obtained from Zhangjiagang Guotai-huarong New Chemical Materials Co., Ltd. It has measured that the water content in electcrolyte is less than 8 ppm. The cells were galvanostatically charged and discharged in a battery test system (NEWARE BTS-610, Neware Technology Co., Ltd. China) at constant current density of 160 mA g−1 (about 0.3 C) with a cut-off voltage of 0.05 to 1.0 V(versus Li/Li+ ) at room temperature. The cycled cells were taken apart in an argonfilled glove box. In order to remove electrolyte and to avoid LiPF6 deposition, the cycled electrodes were washed in DMC three times, then dried under vacuum at ambient temperature, and finally examined using SEM. 3. Results and discussion
2. Experimental Polystyrene (PS) spheres of 180 and 500 nm in diameter were synthesized by using an emulsion polymerization followed by a seed growth polymerization technique according to literature [9,11,27,28]. The detailed preparation procedure was described in a previous paper [9,11]. The PS spheres of 500 nm in diameter were synthesized by using the 180 nm PS spheres as a seed solution. The PS spheres were assembled onto a Ni-coated Cu sheet that was prepared by self-sedimentation in a slowly evaporating dispersion of PS spheres in ethanol, as described by Bartlett et al. [29]. Electroplating was carried out on an electrochemistry working station CHI660A (Chenhua Co., Shanghai). Sn–Cu alloy was deposited onto PS templates at a constant current density of 0.50 A dm−2 from a plating solution of 15 g L−1 SnCl2 ·2H2 O, 4 g L−1 CuSO4 ·2H2 O, 200 g L−1 K4 P2 O7 , and 5 mL L−1 additive agent (provided by Dr. Fan) at room temperature, without stirring. The pH of the solution was adjusted to 8.5. All these reagents are analytical reagents, and were purchased from Sinapharm Chemical Reagent Co., Ltd. After electrodeposition, the working electrodes were immersed in terahydrofuran (THF)
Fig. 1 shows SEM images of the template of PS spheres described in detail in Ref. [11]. In brief, we can observe that the PS spheres are well packed together. The average diameter of the sphere is measured about 180 nm (Fig. 1a) and 500 nm (Fig. 1b), which are in consistent with the value measured from TEM result (not shown). The PS spheres were accumulated into well-ordered areas by lateral capillary forces [30] when the ethanol film becomes as thin as the particle diameter. The lateral capillary force acts solely on particles protruding out of the ethanol surface. Due to meniscus forming between two particles strong capillary forces arises. Their lateral projection drags the particles together. Using ethanol as disperse media can accelerate PS spheres sedimentation. We observe also that the template of PS spheres of 500 nm in diameter illustrates a more order than the template of PS spheres of 180 nm in diameter. Although selfsedimentation on relatively rough substrate may contain some defects, a “faultless” colloidal crystal template is not necessary in the preparation of porous electrodes for lithium-ion batteries [25]. The electrochemical plating of Sn–Cu alloy was carried out on the prepared templates. In order to make the solution ade-
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Fig. 1. SEM images of the template of PS spheres deposited on a Ni-coated Cu sheet (a) 180 nm; (b) 500 nm. The inset shows a corresponding high-magnification image.
Fig. 2. SEM images of macroporous Sn–Cu alloy with 180-nm-diameter pores (a) and 500-nm-diameter pores (b).
quately infiltrating into voids of the template, the PS sphere template films were kept in plating solution for 15 min before the electroplating. After passing a current, the Sn–Cu alloy nuclei can be nucleated on electrode surface unoccupied by PS spheres, and grown three dimensionally. The Sn–Cu alloy grew layer by layer from the substrate and filled the voids between the closepacked PS latex spheres. Some air bubbles may be arisen from the working electrode during electrodeposition, if the current density is too large. So a constant current of 0.5 A dm−2 was applied during the deposition, which prevents also the fragile templates from breaking. After the deposition of Sn–Cu alloy the PS spheres were dissolved and removed by socking the electrodes in THF for 48 h. Fig. 2 displays SEM images of the macroporous Sn–Cu alloy after removing PS template. We observe holes that are distributed on the Sn–Cu alloy layer and are interconnected to form relatively uniform pores. Such holes appear because the impregnating solid material cannot grow at the contact area of a neighboring pair of colloidal spheres in PS template. The average pore size are about 180 nm (Fig. 2a) and 500 nm (Fig. 2b), respectively. It is evident that the pores match the size of the starting PS spheres, thus indicating that shrinkage of the alloy structure is negligible [31,32]. The continuous framework would yield advanced electronic and ionic conductivity. The voids in these samples ensure electrolyte accessing to the large inner surfaces inside the macroporous alloy. The thickness of the macroporous Sn–Cu alloy films prepared by electrodeposition can be adjusted from tens of nanometer to several micrometers
by changing the deposition time. However, the deposition time could not be overlong, otherwise, the upper layer PS spheres will be wrapped and caused difficulty of removing the PS sphere template by wet etching. In concerning the deposition mechanism, Bartlett et al. [33] inferred that the electrochemical deposition is initiated at electrode surface and grew out through the overlying template. To confirm the chemical composition of the macroporous Sn–Cu alloy, at the same time, to avoid the disturbance of the substrate (Cu), Sn–Cu alloy deposit on stainless steel sheet was prepared by electrodeposition under the same conditions. The Sn–Cu alloy was determined by EDS and XRD. Fig. 3 illustrates the EDS spectrum of the Sn–Cu alloy, which indicates
Fig. 3. EDS spectrum of the Sn–Cu alloy deposited on stainless steel sheet.
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Fig. 4. XRD patterns of the Sn–Cu alloy deposited on stainless steel sheet.
that the average atomic ratio of Sn:Cu is about 50.15:49.85 for this alloy. The XRD pattern of the Sn–Cu alloy is shown in Fig. 4, which indicates the presence of two different phase, i.e. the main electrodeposited product of Cu6 Sn5 (JCPDS no. 45-1488) with monoclinic structure (space group, C2/c) that is evidenced by the main peaks of 2θ at 30.04, 42.90 and 43.24◦ , and a minor quantity of Sn (JCPDS no. 65-0296) with tetragonal structure
(space group, I41/amd). The Sn–Cu alloy is present as a monoclinic solid solution, metallic tin as solvent and copper as solute, and copper atoms partly replace the tin atoms in the Sn–Cu alloy. From XPS analysis (Fig. 5), we obtained the XPS survey spectrum of Cu (Fig. 5a), and Sn (Fig. 5b). The Cu 2p exhibits a doublet (i.e. 2p1/2 and 2p3/2 ) with a spin–orbit splitting, in agreement with literature [34], of about 19.8 eV. The peaks at 952.0 and 932.2 eV are associated with metallic Cu (XPS value of pure Cu 2p3/2 = 932.7 eV) [34]. The Sn 3d region exhibits a welldefined doublet with a spin–orbit splitting of about 8.42 eV [34]. It can be deconvoluted into four peaks at 493.33 and 494.51 eV for Sn 3d3/2 , 484.91 and 486.01 eV for Sn 3d5/2 . The signals at 494.51 and 486.01 eV are assigned to tin oxide. The peak at 493.33 eV is attributed to pure Sn [35]. The binding of 484.91 eV for Sn 3d5/2 is close to the literature value of pure Sn (485.0 eV) [34]. The electrochemical properties of electrodes of macroporous Sn–Cu alloy with 180 and 500 nm in pore size are illustrated in Figs. 6 and 7, respectively, in which the 1st, 2nd, 10th, 30th, 50th, and 70th charge–discharge curves are displayed. Lithium ions intercalation and extraction from the macroporous electrodes are associated with processes of charge and discharge of the anode, respectively. Large irreversible capacity observed in the first cycle may be caused by the existence of possible oxide impurities on electrode surface and the formation of solid electrolyte interphase (SEI), since large specific area of the electrode can consume much capacity for the formation of SEI. Pereira
Fig. 5. XPS spectra of (a) Cu 2p and (b) Sn 3d of the macroporous Sn–Cu alloy.
Fig. 6. (a) Charge/discharge curves of the macroporous Sn–Cu alloy with 180-nm-diameter pores. (b) Capacity vs. cycle number of the macroporous Sn–Cu alloy with 180-nm-diameter pores.
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Fig. 7. (a) Charge/discharge curves of the macroporous Sn–Cu alloy with 500-nm-diameter pores. (b) Capacity vs. cycle number of the macroporous Sn–Cu alloy with 500-nm-diameter pores.
et al. [36] have suggested that the reaction at a potential higher than 1.5 V (versus Li/Li+ ) during the first Li insertion might be rooted in electrolyte decomposition that is activated by pure tin of relatively high surface area acting as a catalyst. The reaction at potential between 1.5 and 0.9 V (versus Li/Li+ ) during the first charge represents the reduction of tin oxide. The potential plateau associated with these reactions disappears after the first cycle. The reaction around 0.7 V (versus Li/Li+ ) is assigned to the formation of SEI. It appears a long plateau around 0.35 V (versus Li/Li+ ), and the electrode potential is decreased gradually until 0.05 V. This is ascribed to the formation reaction of Lix Sn (x = 1–3). Figs. 6b and 7b show the cycling behaviors of electrodes of macroporous Sn–Cu alloy of both 180 and 500 nm in pore size. At the first cycle, the charge capacity (intercalation) and the discharge capacity are 810 and 445 mAh g−1 in Fig. 6b, 846 and 551 mAh g−1 in Fig. 7b, respectively. The macroporous Sn–Cu alloy with 500-nm-diameter pores electrode show higher capacity than that of the 180 nm pore size macroporous Sn–Cu alloy electrode. It may be attributed to that the electrolyte may be penetrated more easily into the large pore size during the first charge/discharge. Thus, most active materials can react with lithium ions. However, both electrodes exhibit large irreversible capacity at initial charge–discharge cycle. The loss of irreversible capacity may be attributed to: (i) the formation of lithium oxides (Li2 O) from a small quantity SnOx at surface of the alloy; (ii) the formation of a SEI layer on the surface with large specific surface area (due to the macroporous structure); (iii) the decomposition of electrolyte at electrode/electrolyte interphase; and (iv) the presence of pure tin which is very easily cracking and crumbling during the lithium ion insertion/extraction. The columbic efficiency in the charge–discharge processes is maintained as high as 92% except for the initial several cycles in Fig. 6b. It has been measured that the macroporous Sn–Cu alloy electrode material has delivered a reversible capacity about 410 mAh g−1 . As shown in Fig. 6b, the cycling performance of the macroporous Sn–Cu alloy with 180-nm-diameter pores electrode is excellent, the reversible capacity can be maintained 350 mAh g−1 after 70 cycles of charge/discharge. The reversible capacity of the macroporous Sn–Cu alloy with 500-nm-diameter pores electrode can be also maintained 270 mAh g−1 up to 70th cycle in Fig. 7b. It is evi-
dent that the former demonstrates a better cycle performance than that of the latter. It can be attributed to that the pore wall of the macroporous Sn–Cu alloy with 500-nm-diameter pores is very thin, which is easily cracking and crumbling during lithium insertion/extraction. In comparison, for a Sn–Cu alloy anode without the macroporous structure, the reversible capacity only maintained 220 mAh g−1 after 50 cycles [15]. Moreover, in our study, the cycling performance of the macroporous Sn–Cu alloy electrode fabricated by PS crystal template has been improved, which is better than that of synthesis 3D porous Cu–Sn [18] alloy by using hydrogen bubbles as template. This may be attributed to the stronger integrated strength, which is between active materials and current collector, than that of using hydrogen bubbles as template. SEM images of the macroporous Sn–Cu alloy with 180nm-diameter pores electrodes at different stage of charging are shown in Fig. 8. At 1.5 V the macroporous structure is resembled to that of the original uncycled material. However, some reactions may be occurred in some active sites. It is in consistent with the result of charge–discharge curves (Fig. 6a). At 1.0 V, the diameter of pores is diminished a little due to the reaction of part of tin oxide with Li. The pore size is sequentially diminished at 0.6 V, at the same time, some insulated substances are overlay on the electrode surface, which may be attributed to the formation of SEI. When charging to 0.05 V, the porous structure was almost disappeared. After the completion of cycle, however, the porous structure is appeared again as shown in Fig. 8e. Fig. 8f displays the structure of the electrode after 40 cycles, in which porous structure can be still observed in the electrode. The porous structure is remained within the electrode. Due to the insulated substance, it is very difficult to observe the detailed structure changing. To some extent, the porous structure can partly accommodate the volume expansion and prolong the cycleability. The good capacity retention and cycleability of the macroporous Sn–Cu alloy electrodes may be attributed to the porous structure and the addition of Cu. The macroporous structure can restrain the pulverization of electrode during charge/discharge cycles, and can partly accommodate the volume expansion. As a result the cycle life of the electrode is significantly improved and the diffusion of Li insertion/extraction of the macroporous materials is enhanced. Such properties can improve also the
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Fig. 8. SEM images acquired at different stages of charging of the macroporous Sn–Cu alloy with 180-nm-diameter pores electrodes at (a) 1.5 V, (b) 1.0 V, (c) 0.6 V, (d) 0.05 V, and after the first cycle (e) and the 40th cycle (f).
chargeability/dischargeability at large current densities. The addition of Cu as matrix in plated film relaxes also the mechanical stress that is resulted from volume expansion and contraction during Li insertion and desertion processes. 4. Conclusion The current paper reports the fabrication and electrochemical properties of an anode material with a macroporous structure for lithium-ion batteries. The electrodes of macroporous Sn–Cu alloy of different pore size were prepared by electroplating into the interstitial spaces of a template formed by PS latex spheres self-sedimentation on a relatively rough Ni-coated Cu foil surface. When the macroporous Sn–Cu alloy is used as negative electrode (anode) in a rechargeable lithium ion battery, the cycling performance of the battery has been significantly enhanced in comparison with the anode of Sn–Cu alloy without the macroporous structure. The results show that the
cycle performance of the small pore size macroporous Sn–Cu alloy is better than that of the large pore size macroporous Sn–Cu alloy. The study demonstrated that the Sn–Cu alloy with macroporous structure can be easily and conveniently mass produced by a general template electroplating technique on relatively rough substrate (Ni-coated Cu sheet). This method could be extended to fabricate macroporous materials of other metal, alloy and oxide. It is anticipated that the strategy of preparing macroporous Sn–Cu alloy can be also applied to synthesize other promising macroporous materials for lithium-ion batteries. Acknowledgements This work was supported by Major State Basic Research Development Program of China (2002CB2118004). The authors thank Dr. X.Y. Fan for the offer of additive agents for electroplating.
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