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The effect of the binder morphology on the cycling stability of Li–alloy composite electrodes
Journal of Electroanalytical Chemistry 510 (2001) 12 – 19 www.elsevier.com/locate/jelechem
The effect of the binder morphology on the cycling stability of Li–alloy composite electrodes Mario Wachtler a,*, Markus R. Wagner a, Mario Schmied b, Martin Winter a, Ju¨rgen O. Besenhard a a
Institute for Chemical Technology of Inorganic Materials, Graz Uni6ersity of Technology, Stremayrgasse 16, 8010 Graz, Austria b Research Institute for Electron Microscopy, Graz Uni6ersity of Technology, Steyrergasse 17, 8010 Graz, Austria Received 18 April 2001; accepted 21 April 2001
Abstract It is demonstrated that the design of the composite electrode, or more precisely the morphology and distribution of the binder poly(vinylidine fluoride) (PVdF) within the composite electrode, has a significant impact on the cycling performance of Li storage alloy (Sn/SnSb) electrodes. Different binder morphologies and distributions have been obtained by using different solvents for the slurry preparation, such as 1-methyl-2-pyrrolidinone (NMP), in which PVdF is dissolved, yielding electrodes with a homogeneously and finely distributed binder, or decane, in which PVdF is only dispersed, yielding electrodes in which the original particle morphology of the binder powder is preserved. In constant current cycling tests carried out in an excess of electrolyte, the electrodes with the ‘dispersed’ binder show far better cycling capacities and stabilities than those with the ‘dissolved’ binder. This is explained by the different binding strengths, swelling behaviour in the electrolyte, electrode porosities, and possible ‘buffer’ effects of the compact and the finely distributed binders. © 2001 Published by Elsevier Science B.V. Keywords: Composite electrode; Binder distribution; Cycling stability; Li storage alloy; Sn/SnSb; PVdF
1. Introduction The key issue for the application of lithium storage metals and alloys in rechargeable Li-ion cells is the control of their volume changes which occur during cycling. Only by a proper choice of the active material and a proper design of its morphology is it possible to obtain a material, which can be cycled several times [1]. However, the volume changes are not only a challenge to the active material itself, but also to the protective film (the solid electrolyte interphase) which is formed at the electrode electrolyte interface [2], and to the maintenance of the integrity of the composite electrode during repeated cycling. Especially, the binder has to meet strict requirements. On the one hand it must counteract the dispersive forces caused by the expanding metallic host matrix and withstand extensive swelling in the polar organic electrolyte, since both * Corresponding author. Tel.: + 43-316-8738760; fax: +43-3168738272. E-mail address: [email protected] (M. Wachtler).
effects would result in a loss of internal contact; on the other hand it must retain a certain flexibility, since too rigid a system would simply crack and crumble. Obviously, the chemistry of the binder and — as will be shown here — also its distribution within the composite electrode play a vital role. Most of the hitherto published work on binders deals with the testing of new binder materials and their interactions with the active material and the electrolyte (some recent articles are e.g. Refs. [3–8]). In addition, it was found that the mixing method for the preparation of the electrode slurry [9] and the mixing sequence of the slurry components [10] had some influence on the capacities and on the cycling stabilities. Yang et al. [6,7] compared the cycling behaviour of Sn/SnSb-based composite electrodes with poly(vinylidene fluoride) (PVdF) and high-density polyethylene (PE-HD) as the binder and found that the composite electrodes with PE-HD showed a better cycling performance than those with PVdF. They explained this by the stronger swelling of PVdF in the polar electrolyte, which together with the large expansion of the active
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material results in fast loss of internal contact. What appears interesting is the different preparation of the composite electrodes: PVdF was dissolved in 1-methyl2-pyrrolidinone (NMP), whereas PE-HD was used dry without any solvents. The question arises as to whether the different cycling stability is solely a matter of the chemistry of the binder or whether it is also influenced by the different processing methods, which surely affect the distribution of the binder within the electrode. To elucidate this point, composite electrodes were investigated, which have exactly the same composition and the same binder, but which were processed in different ways (cf. also Ref. [11]).
2. Experimental
2.1. Li –alloy composite electrodes Sub-micron crystalline Sn/SnSb powder (82 wt%) [1] acting as active material, Ni powder (10 wt%) [1] acting as conductive additive, and PVdF (8 wt%; Aldrich, Mw = 534 000) acting as binder were blended, and NMP (Aldrich, 99%; Fluka, p.a.) or decane (Aldrich, 99+ %) were added to give a slurry. After thorough mixing under ultrasonic conditions, the slurry was painted onto both sides of a stainless steel mesh of wire diameter 25 mm and mesh width 67 mm (mesh 280) (Spo¨ rl, Germany) acting as the current collector. 2.29 0.1 mg composite material coated a geometric area of (2 × ) 16 mm2. The composite electrodes were pre-dried at 60 °C, pressed with 8 t cm − 2, and rigorously dried at 100–120 °C in a dynamic medium–high vacuum.
2.2. Constant current cycling Constant current cycling tests were carried out in laboratory-type glass cells with an excess ( 7 ml) of a 1 M LiPF6 +ethylene carbonate (EC)+ diethyl carbonate (DEC) electrolyte (Merck Selectipur® LP40) with Li metal foil as counter and reference electrodes, at current densities of 0.5 mA cm − 2, which corresponds to 89 mA g − 1 (with respect to the mass of active material), and with cut-off potentials of 20 and 1200 mV versus Li Li+. The composite electrodes with different amounts of PVdF were cycled in a 1 M LiClO4 (Merck Selectipur®)+propylene carbonate (PC) (Merck Selectipur®) electrolyte at current densities of 0.3 mA cm − 2 with cut-off potentials of 100 and 1200 mV versus Li Li+. The terms ‘charge’ and ‘discharge’ refer to the electrode being the anode in a full cell (and not the cathode in a half cell, as is actually the case here), i.e. ‘charge’ means Li-uptake and ‘discharge’ means Lirelease.
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2.3. Scanning electron microscopy Scanning electron microscope (SEM) investigations were performed with a DSM 982 Gemini digital scanning electron microscope (Zeiss, Germany). The composite materials were removed from the stainless steel substrate and analysed using 5 kV acceleration voltages for imaging with magnifications up to 30 000×.
3. Results and discussion Two types of Sn/SnSb composite electrodes were compared which have exactly the same composition but which were processed in two different ways. For the first type, NMP was used for the slurry preparation, as an example of a solvent, which dissolves PVdF. For the second type, decane was used, as an example of a dispersant, which does not substantially dissolve PVdF. The rest of the composite electrode preparation was the same (as outlined in Section 2). The results obtained may be generalised to all other solvents like NMP, in which PVdF is dissolved, or like decane, in which PVdF is not dissolved but only dispersed. Furthermore, composite electrodes which have been prepared dry, i.e. without any solvent added, should behave similarly to those prepared with decane — provided that similar mixing can be achieved during liquid and dry processing. SE micrographs of the uncycled composite electrodes are shown in Fig. 1 (processing with NMP) and Fig. 2 (processing with decane). In the electrodes processed with NMP the binder is homogeneously distributed throughout the composite electrodes, and connects the metal/alloy particles via a thin net of fine polymer threads. In the composite electrodes processed with decane the original powder morphology of the binder with particle diameters of 200–300 nm has been maintained. Furthermore, it can be seen that with the present mixing method (stirring and ultrasonication) it was not possible to obtain a uniform distribution of the binder particles within the composite electrode, but that the binder forms aggregates (indicated with an arrow in Fig. 2a). This inhomogeneous binder distribution is maintained during cycling, however, the active material appears more amorphous, which is attributed to morphological and structural changes of the active material caused by the alloying and de-alloying reactions during cycling and to the formation of a solid electrolyte interphase (SEI) [12]. Constant current cycling tests (carried out in an excess of electrolyte) show that the different processing 1
For a detailed discussion of the observed reversible and irreversible capacities of Sn/SnSb composite electrodes the reader is referred to Ref. [2].
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has a marked effect on the cycling performance (Fig. 3a).1 In the case of the ‘dissolved’ binder (processing with NMP) the reversible capacity rapidly drops to 150–200 mA h g(AM) − 1 (AM = active material) within the first few cycles. By contrast, the ‘dispersed’ binder (processing with decane) yields quite stable reversible capacities of 600 mA h g(AM) − 1. Obviously the ‘dissolved’ binder is too finely distributed to provide enough strength to withstand the expansion of the active material and thus to maintain the integrity of the composite material. In addition, it might be possible that the ‘dissolved’ binder covers large parts of the particles of the active material, resulting in a partial isolation with only a few points of electrical contact between the single particles of the active material. During expansion, these few points of contact could easily be lost, leading to a total isolation of the particle. A crucial point in this respect is also the ‘swelling’ of the binder in the electrolyte. The swelling behaviour of polymers has been the subject of many investigations especially in view of their potential use for gel-type polymer electrolytes. It was found that the amorphous
domains in the polymer account for the trapping of electrolyte (swelling), and the crystalline regions provide the mechanical strength [13]. Whereas for a polymer used for electrolytes a high electrolyte uptake is desired (to obtain high ionic conductivities), this is not the case for a polymer used as binder. Though a certain electrolyte uptake is necessary to obtain a good contact between the active material and the organic electrolyte, it should not result in such an expansion of the composite material that the electronic contact between the particles of the active material is lost. In addition, the electrolyte would be decomposed inside the electrode and an electronically insulating SEI would be formed. The ‘dissolved’ binder with its small threads with diameters of 30 nm and less is surely more sensitive towards swelling, than the ‘dispersed’ binder with its compact particles with diameters of 200 –300 nm. Furthermore, X-ray diffraction investigations, which are not reproduced here, have shown that the raw binder powder and the ‘dispersed’ binder are partly crystalline, whereas the ‘dissolved’ binder is largely amorphous. These crystalline regions act as an addi-
Fig. 1. SE micrographs of a Sn/SnSb composite electrode with a homogeneous distribution of PVdF, obtained by processing with NMP. (a) Survey image: (1) electrode surface; (2) inner face; (3) interface between electrode material and current collector. (b) Details of an inner face.
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Fig. 1. (Continued)
tional barrier against swelling [14]. However, swelling of the ‘dispersed’ binder in the long run may be yet another explanation for the capacity decrease above the 30th cycle, besides the isolation of active material owing to cracking and SEI formation, accumulation of electrolyte decomposition products in the electrolyte, etc. [2]. Another point is the porosity of the electrodes. Pores ease the access of electrolyte to the electrode interior and thus to the active material, and they resemble a void space where the active material (when it takes up Li) and the binder (when it swells) can expand into. From the SE micrographs, it appears that the composite material with the ‘dispersed’ binder has larger pores, than that with the ‘dissolved’ binder. However, the actual porosity has not been quantified, and it has to be regarded that it will immediately change as soon as the active material starts to expand. Therefore, the influence of porosity is difficult to assess. Finally, considering its softness, the binder-especially the dispersed binder particles — may play the role of a ‘buffer’, in the sense that the hard active material can
expand in a soft and ductile surrounding, which to some extent absorbs the volume increase and thus reduces the mechanical strain in the overall composite electrode. This ‘buffer’ action of the binder is even more evident in view of the volumetric composition, since, due to the different (theoretical) densities a mass ratio of Sn/SnSb:Ni:PVdF of 82:10:8 corresponds to a volume ratio of roughly 67:6:27 (for the unlithiated metal host and the un-swollen binder). The present findings differ from the results obtained with carbonaceous anode materials or with oxidic cathode materials, where the method of choice is to use NMP, without similar negative impacts on the cycling stability (even for a similar cell design as used here). A possible explanation for this is the considerably different dimensional stabilities of carbon/cathode materials and metals/alloys during lithium uptake and removal. The expanding metal/alloy host expands the whole composite material and thus eases the access of electrolyte to the electrode interior. In order to check other possible influences on the cycling performance a few additional experiments have
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been carried out: It made no significant difference whether the dry PVdF powder was blended with the metal powders and then the solvent NMP was added, or whether the PVdF was first dissolved in NMP and then the binder solution obtained was added to the powder. Obviously, during the mixing of the slurry by ultrasonication the PVdF is rapidly dissolved, and the same slurry is obtained as in the case of the pre-dissolved PVdF. It has been reported that the purity of NMP has a significant influence on the properties of PVdF [15]. To exclude the fact that the poor cycling stability of the composite electrodes processed with NMP is only an accidental result of an impure NMP solvent, NMP from two different suppliers and freshly distilled NMP were compared. Though some small differences in the initial capacities were found, which however were within the range of reproducibility, the rapid capacity fade in the first cycles remained, and all electrodes behaved far worse than those processed with decane.
Also rigorous drying in dynamic vacuum of the PVdF powder before use did not improve the results. Tests with PVdF and PVdF-copolymers from another supplier, which are not presented here, gave the same trends of the cycling stabilities. Furthermore, it is clear that the swelling behaviour of polymers depends on the electrolyte solution [14,16]. The marked difference between the composite electrodes with ‘dissolved’ and ‘dispersed’ binder was also found for a number of other PC or EC+ DEC based electrolytes (cf. e.g. the results in 1 M LiPF6 + EC-DEC in Fig. 3 and those in 1 M LiClO4 + PC in Fig. 4). Systematic tests with electrolytes based on other solvents have, however, not been carried out so far. As has been mentioned above, in the composite electrodes processed with decane the binder is inhomogeneously distributed, i.e. there are regions with binder particle aggregates and regions which are apparently free of binder. It might appear that no binder at all is necessary for the successful cycling of Li–alloy com-
Fig. 2. SE micrographs of a Sn/SnSb composite electrode with an inhomogeneous distribution of PVdF, obtained by processing with decane. (a) Survey image: (1) electrode surface; (2) inner face; (3) interface between electrode material and current collector. (b) Detail of an inner face: (1) metallic particles; (2) binder particles.
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Fig. 2. (Continued)
posite electrodes. That this is not the case may be gathered from Fig. 4, where composite electrodes with different amounts of binder are compared. When the amount of binder is decreased, the capacity and the cycling stability decrease. A further increase of the amount of binder (not shown here), does not significantly change the cycling stability, but only results in lower capacities with respect to the mass of the whole composite material, since the percentage of active material decreases. In spite of the promising results, at the present stage of investigations the processing with decane is not the final solution to all problems related to electrode fabrication. The first reason for this is the behaviour of the composite electrode in the first cycle. As can be seen in Fig. 3b the charge curve for the first Li uptake is not smooth but exhibits voltage spikes (an extreme example has been chosen for Fig. 3b). They are attributed to a local break-up of the composite material during the first expansion of the active material and the concurrent sudden access of electrolyte to the electrode interior. There, the elec-
trolyte is decomposed, which in the case of common electrolyte solvents such as EC and PC is accompanied by gas formation. The electrode break-up is caused by an inhomogeneous distribution of mechanical strain, which in turn is caused by the inhomogeneous binder distribution. It should be possible to improve this situation by a further optimisation of the slurry homogenisation (e.g. better mixing or the use of other dispersants), which results in a better distribution of the binder in the composite electrode. The second reason is the poor adhesion of the composite material to the current collector before pressing. The composite material resembles more a loose powder aggregate than a plastic metal/polymer composite, and the slightest mechanical shock results in the composite material falling off the current collector. This surely poses a problem to the use of decane for technical coating procedures. To overcome this problem one might think of a combined binder system, e.g. mixtures of different polymers, where one component is dissolved and accounts for adhesion, and the other is dispersed.
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Finally, it has to be pointed out that these findings are strongly influenced by the present experimental set-up, i.e. by the present components of the composite electrode (both chemistry and morphology), the present design and processing of the composite electrode, and the present cell design, and that they may differ in other cases. Especially the cell design will have a major influence, since the composite electrode is placed in an excess of electrolyte without any mechanical support, which could counteract the expansion of the electrode material; i.e. there is an almost infinite amount of electrolyte to swell the polymer, and if once a particle is disconnected it remains disconnected as there is no back-driving force. A close-packed configuration with separator materials or other mechanical support (e.g.
the presence of low pressure in the cell) and with limited amounts of electrolyte, which are characteristic of commercial cell designs, surely aid the binder in providing the mechanical stability of the composite electrode.
4. Conclusions Two types of Sn/SnSb composite electrodes with identical compositions were compared, where: (i) NMP (in which PVdF is dissolved); or (ii) decane (in which PVdF is only dispersed) were used for the slurry preparation. In case (i) the binder was homogeneously distributed within the composite electrode, forming thin
Fig. 3. Constant current cycling tests of Sn/SnSb composite electrodes with ‘dispersed’ PVdF (processing with decane, circles) and ‘dissolved’ PVdF (processing with NMP, triangles). (a) Charge (solid symbols) and discharge (open symbols) capacities (given with respect to the mass of active material (AM)). (b) Charge/discharge curves for the first three cycles. Electrode composition: 82 wt% Sn/SnSb; 10% Ni; 8% PVdF; electrolyte: 1 M LiPF6 +EC+ DEC; cycling parameters: 0.5 mA cm − 2; 20 – 1200 mV vs. Li Li+.
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Fig. 4. Constant current cycling tests of Sn/SnSb composite electrodes with different amounts of PVdF binder, processed with decane or NMP. Solid symbols: charge capacities, open symbols: discharge capacities. Electrode composition: (90 − x) wt% Sn/SnSb, 10% Ni, x% PVdF; electrolyte: 1 M LiClO4 +PC; cycling parameters: 0.3 mA cm − 2, 100 – 1200 mV vs. Li Li+.
threads of diameters of 30 nm and less between the active materials. In case (ii) the original particle morphology of the binder with diameters of 200 – 300 nm was maintained, and the binder was inhomogeneously distributed within the composite electrode. The cycling stability of the second type of electrodes was far better than that of the first one. This is explained with the higher binding strength and the different swelling behaviour of the compact binder particles compared to the finely distributed binder, with the different electrode porosities, and with a possible ‘buffer’ effect of the soft binder particles. Strictly speaking these results hold true only for the present composite electrode design and the present cell design with a large excess of electrolyte, and the effects are surely less pronounced in real cells with a closepacked configuration and a limited amount of electrolyte. Nevertheless, the results clearly show, to what extent the cycling behaviour can be determined by the design of the composite electrode, especially in the case of Li storage alloys, which undergo major volume changes during cycling. The impact of the design of the composite electrode should always be checked before ruling out a certain active material because of its apparently small cycling capacities and stabilities.
Acknowledgements Financial support by the Austrian Science Fund and the Oesterreichische National Bank in Project 12768 and in the Special Research Program ‘Electroactive Materials’ as well as support by Mitsubishi Chemical .
Co. (Japan) are gratefully acknowledged. Furthermore, the authors thank Merck (Germany) for the donation of electrolytes.
References [1] J. Yang, M. Wachtler, M. Winter, J.O. Besenhard, Electrochem. Solid-State Lett. 2 (1999) 161. [2] M. Wachtler, J.O. Besenhard, M. Winter, J. Power Sources 94 (2001) 189. [3] W.F. Liu, X.J. Huang, G.B. Li, Z.X. Wang, H. Huang, Z.H. Lu, R.J. Xue, L.Q. Chen, J. Power Sources 68 (1997) 344. [4] D. Aurbach, M.D. Levi, O. Lev, J. Gun, L. Rabinovich, J. Appl. Electrochem. 28 (1998) 1051. [5] M.N. Richard, J.R. Dahn, J. Power Sources 83 (1999) 71. [6] J. Yang, Y. Takeda, N. Imanishi, T. Ichikawa, O. Yamamoto, J. Power Sources 79 (1999) 220. [7] J. Yang, Y. Takeda, N. Imanishi, T. Ichikawa, O. Yamamoto, Solid State Ionics 135 (2000) 175. [8] M. Gabersˇcˇ ek, M. Bele, J. Drofenik, R. Dominko, S. Pejovnik, Electrochem. Solid-State Lett. 3 (2000) 171. [9] Z. Liu, A. Yu, J.Y. Lee, J. Power Sources 74 (1998) 228. [10] K.M. Kim, W.S. Jeon, I.J. Chung, S.H. Chang, J. Power Sources 83 (1999) 108. [11] M.R. Wagner, M. Wachtler, M. Schmied, P. Preishuber-Pflu¨ gl, F. Stelzer, J.O. Besenhard, M. Winter, IMLB10 — 10th International Meeting on Lithium Batteries, Como, Italy, May 28 –June 2, 2000, poster no. 73. [12] I. Rom, M. Wachtler, I. Papst, M. Schmied, J.O. Besenhard, F. Hofer, M. Winter, Solid State Ionics, in press. [13] J.-M. Tarascon, A.S. Gozdz, C. Schmutz, F. Shokoohi, P.C. Warren, Solid State Ionics 86 – 88 (1996) 49. [14] M.T. Burchill, Prog. Batt. Battery Mater. 17 (1998) 144. [15] B. Barrie`re, Y. Miyaki, M. Despotopoulou, M. Burchill, IMLB10 — 10th International Meeting on Lithium Batteries, Como, Italy, 28 May – 2 June, 2000, poster no. 349. [16] S. Sugawara, in: N. Oyama (Ed.), Porima Batteri no Saishin Gijutsu, Shi Emu Shi, Tokyo, 1998, p. 185.
The effect of the binder morphology on the cycling stability of Li–alloy composite electrodes Mario Wachtler a,*, Markus R. Wagner a, Mario Schmied b, Martin Winter a, Ju¨rgen O. Besenhard a a
Institute for Chemical Technology of Inorganic Materials, Graz Uni6ersity of Technology, Stremayrgasse 16, 8010 Graz, Austria b Research Institute for Electron Microscopy, Graz Uni6ersity of Technology, Steyrergasse 17, 8010 Graz, Austria Received 18 April 2001; accepted 21 April 2001
Abstract It is demonstrated that the design of the composite electrode, or more precisely the morphology and distribution of the binder poly(vinylidine fluoride) (PVdF) within the composite electrode, has a significant impact on the cycling performance of Li storage alloy (Sn/SnSb) electrodes. Different binder morphologies and distributions have been obtained by using different solvents for the slurry preparation, such as 1-methyl-2-pyrrolidinone (NMP), in which PVdF is dissolved, yielding electrodes with a homogeneously and finely distributed binder, or decane, in which PVdF is only dispersed, yielding electrodes in which the original particle morphology of the binder powder is preserved. In constant current cycling tests carried out in an excess of electrolyte, the electrodes with the ‘dispersed’ binder show far better cycling capacities and stabilities than those with the ‘dissolved’ binder. This is explained by the different binding strengths, swelling behaviour in the electrolyte, electrode porosities, and possible ‘buffer’ effects of the compact and the finely distributed binders. © 2001 Published by Elsevier Science B.V. Keywords: Composite electrode; Binder distribution; Cycling stability; Li storage alloy; Sn/SnSb; PVdF
1. Introduction The key issue for the application of lithium storage metals and alloys in rechargeable Li-ion cells is the control of their volume changes which occur during cycling. Only by a proper choice of the active material and a proper design of its morphology is it possible to obtain a material, which can be cycled several times [1]. However, the volume changes are not only a challenge to the active material itself, but also to the protective film (the solid electrolyte interphase) which is formed at the electrode electrolyte interface [2], and to the maintenance of the integrity of the composite electrode during repeated cycling. Especially, the binder has to meet strict requirements. On the one hand it must counteract the dispersive forces caused by the expanding metallic host matrix and withstand extensive swelling in the polar organic electrolyte, since both * Corresponding author. Tel.: + 43-316-8738760; fax: +43-3168738272. E-mail address: [email protected] (M. Wachtler).
effects would result in a loss of internal contact; on the other hand it must retain a certain flexibility, since too rigid a system would simply crack and crumble. Obviously, the chemistry of the binder and — as will be shown here — also its distribution within the composite electrode play a vital role. Most of the hitherto published work on binders deals with the testing of new binder materials and their interactions with the active material and the electrolyte (some recent articles are e.g. Refs. [3–8]). In addition, it was found that the mixing method for the preparation of the electrode slurry [9] and the mixing sequence of the slurry components [10] had some influence on the capacities and on the cycling stabilities. Yang et al. [6,7] compared the cycling behaviour of Sn/SnSb-based composite electrodes with poly(vinylidene fluoride) (PVdF) and high-density polyethylene (PE-HD) as the binder and found that the composite electrodes with PE-HD showed a better cycling performance than those with PVdF. They explained this by the stronger swelling of PVdF in the polar electrolyte, which together with the large expansion of the active
0022-0728/01/$ - see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 7 2 8 ( 0 1 ) 0 0 5 3 2 - 0
M. Wachtler et al. / Journal of Electroanalytical Chemistry 510 (2001) 12–19
material results in fast loss of internal contact. What appears interesting is the different preparation of the composite electrodes: PVdF was dissolved in 1-methyl2-pyrrolidinone (NMP), whereas PE-HD was used dry without any solvents. The question arises as to whether the different cycling stability is solely a matter of the chemistry of the binder or whether it is also influenced by the different processing methods, which surely affect the distribution of the binder within the electrode. To elucidate this point, composite electrodes were investigated, which have exactly the same composition and the same binder, but which were processed in different ways (cf. also Ref. [11]).
2. Experimental
2.1. Li –alloy composite electrodes Sub-micron crystalline Sn/SnSb powder (82 wt%) [1] acting as active material, Ni powder (10 wt%) [1] acting as conductive additive, and PVdF (8 wt%; Aldrich, Mw = 534 000) acting as binder were blended, and NMP (Aldrich, 99%; Fluka, p.a.) or decane (Aldrich, 99+ %) were added to give a slurry. After thorough mixing under ultrasonic conditions, the slurry was painted onto both sides of a stainless steel mesh of wire diameter 25 mm and mesh width 67 mm (mesh 280) (Spo¨ rl, Germany) acting as the current collector. 2.29 0.1 mg composite material coated a geometric area of (2 × ) 16 mm2. The composite electrodes were pre-dried at 60 °C, pressed with 8 t cm − 2, and rigorously dried at 100–120 °C in a dynamic medium–high vacuum.
2.2. Constant current cycling Constant current cycling tests were carried out in laboratory-type glass cells with an excess ( 7 ml) of a 1 M LiPF6 +ethylene carbonate (EC)+ diethyl carbonate (DEC) electrolyte (Merck Selectipur® LP40) with Li metal foil as counter and reference electrodes, at current densities of 0.5 mA cm − 2, which corresponds to 89 mA g − 1 (with respect to the mass of active material), and with cut-off potentials of 20 and 1200 mV versus Li Li+. The composite electrodes with different amounts of PVdF were cycled in a 1 M LiClO4 (Merck Selectipur®)+propylene carbonate (PC) (Merck Selectipur®) electrolyte at current densities of 0.3 mA cm − 2 with cut-off potentials of 100 and 1200 mV versus Li Li+. The terms ‘charge’ and ‘discharge’ refer to the electrode being the anode in a full cell (and not the cathode in a half cell, as is actually the case here), i.e. ‘charge’ means Li-uptake and ‘discharge’ means Lirelease.
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2.3. Scanning electron microscopy Scanning electron microscope (SEM) investigations were performed with a DSM 982 Gemini digital scanning electron microscope (Zeiss, Germany). The composite materials were removed from the stainless steel substrate and analysed using 5 kV acceleration voltages for imaging with magnifications up to 30 000×.
3. Results and discussion Two types of Sn/SnSb composite electrodes were compared which have exactly the same composition but which were processed in two different ways. For the first type, NMP was used for the slurry preparation, as an example of a solvent, which dissolves PVdF. For the second type, decane was used, as an example of a dispersant, which does not substantially dissolve PVdF. The rest of the composite electrode preparation was the same (as outlined in Section 2). The results obtained may be generalised to all other solvents like NMP, in which PVdF is dissolved, or like decane, in which PVdF is not dissolved but only dispersed. Furthermore, composite electrodes which have been prepared dry, i.e. without any solvent added, should behave similarly to those prepared with decane — provided that similar mixing can be achieved during liquid and dry processing. SE micrographs of the uncycled composite electrodes are shown in Fig. 1 (processing with NMP) and Fig. 2 (processing with decane). In the electrodes processed with NMP the binder is homogeneously distributed throughout the composite electrodes, and connects the metal/alloy particles via a thin net of fine polymer threads. In the composite electrodes processed with decane the original powder morphology of the binder with particle diameters of 200–300 nm has been maintained. Furthermore, it can be seen that with the present mixing method (stirring and ultrasonication) it was not possible to obtain a uniform distribution of the binder particles within the composite electrode, but that the binder forms aggregates (indicated with an arrow in Fig. 2a). This inhomogeneous binder distribution is maintained during cycling, however, the active material appears more amorphous, which is attributed to morphological and structural changes of the active material caused by the alloying and de-alloying reactions during cycling and to the formation of a solid electrolyte interphase (SEI) [12]. Constant current cycling tests (carried out in an excess of electrolyte) show that the different processing 1
For a detailed discussion of the observed reversible and irreversible capacities of Sn/SnSb composite electrodes the reader is referred to Ref. [2].
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has a marked effect on the cycling performance (Fig. 3a).1 In the case of the ‘dissolved’ binder (processing with NMP) the reversible capacity rapidly drops to 150–200 mA h g(AM) − 1 (AM = active material) within the first few cycles. By contrast, the ‘dispersed’ binder (processing with decane) yields quite stable reversible capacities of 600 mA h g(AM) − 1. Obviously the ‘dissolved’ binder is too finely distributed to provide enough strength to withstand the expansion of the active material and thus to maintain the integrity of the composite material. In addition, it might be possible that the ‘dissolved’ binder covers large parts of the particles of the active material, resulting in a partial isolation with only a few points of electrical contact between the single particles of the active material. During expansion, these few points of contact could easily be lost, leading to a total isolation of the particle. A crucial point in this respect is also the ‘swelling’ of the binder in the electrolyte. The swelling behaviour of polymers has been the subject of many investigations especially in view of their potential use for gel-type polymer electrolytes. It was found that the amorphous
domains in the polymer account for the trapping of electrolyte (swelling), and the crystalline regions provide the mechanical strength [13]. Whereas for a polymer used for electrolytes a high electrolyte uptake is desired (to obtain high ionic conductivities), this is not the case for a polymer used as binder. Though a certain electrolyte uptake is necessary to obtain a good contact between the active material and the organic electrolyte, it should not result in such an expansion of the composite material that the electronic contact between the particles of the active material is lost. In addition, the electrolyte would be decomposed inside the electrode and an electronically insulating SEI would be formed. The ‘dissolved’ binder with its small threads with diameters of 30 nm and less is surely more sensitive towards swelling, than the ‘dispersed’ binder with its compact particles with diameters of 200 –300 nm. Furthermore, X-ray diffraction investigations, which are not reproduced here, have shown that the raw binder powder and the ‘dispersed’ binder are partly crystalline, whereas the ‘dissolved’ binder is largely amorphous. These crystalline regions act as an addi-
Fig. 1. SE micrographs of a Sn/SnSb composite electrode with a homogeneous distribution of PVdF, obtained by processing with NMP. (a) Survey image: (1) electrode surface; (2) inner face; (3) interface between electrode material and current collector. (b) Details of an inner face.
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Fig. 1. (Continued)
tional barrier against swelling [14]. However, swelling of the ‘dispersed’ binder in the long run may be yet another explanation for the capacity decrease above the 30th cycle, besides the isolation of active material owing to cracking and SEI formation, accumulation of electrolyte decomposition products in the electrolyte, etc. [2]. Another point is the porosity of the electrodes. Pores ease the access of electrolyte to the electrode interior and thus to the active material, and they resemble a void space where the active material (when it takes up Li) and the binder (when it swells) can expand into. From the SE micrographs, it appears that the composite material with the ‘dispersed’ binder has larger pores, than that with the ‘dissolved’ binder. However, the actual porosity has not been quantified, and it has to be regarded that it will immediately change as soon as the active material starts to expand. Therefore, the influence of porosity is difficult to assess. Finally, considering its softness, the binder-especially the dispersed binder particles — may play the role of a ‘buffer’, in the sense that the hard active material can
expand in a soft and ductile surrounding, which to some extent absorbs the volume increase and thus reduces the mechanical strain in the overall composite electrode. This ‘buffer’ action of the binder is even more evident in view of the volumetric composition, since, due to the different (theoretical) densities a mass ratio of Sn/SnSb:Ni:PVdF of 82:10:8 corresponds to a volume ratio of roughly 67:6:27 (for the unlithiated metal host and the un-swollen binder). The present findings differ from the results obtained with carbonaceous anode materials or with oxidic cathode materials, where the method of choice is to use NMP, without similar negative impacts on the cycling stability (even for a similar cell design as used here). A possible explanation for this is the considerably different dimensional stabilities of carbon/cathode materials and metals/alloys during lithium uptake and removal. The expanding metal/alloy host expands the whole composite material and thus eases the access of electrolyte to the electrode interior. In order to check other possible influences on the cycling performance a few additional experiments have
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been carried out: It made no significant difference whether the dry PVdF powder was blended with the metal powders and then the solvent NMP was added, or whether the PVdF was first dissolved in NMP and then the binder solution obtained was added to the powder. Obviously, during the mixing of the slurry by ultrasonication the PVdF is rapidly dissolved, and the same slurry is obtained as in the case of the pre-dissolved PVdF. It has been reported that the purity of NMP has a significant influence on the properties of PVdF [15]. To exclude the fact that the poor cycling stability of the composite electrodes processed with NMP is only an accidental result of an impure NMP solvent, NMP from two different suppliers and freshly distilled NMP were compared. Though some small differences in the initial capacities were found, which however were within the range of reproducibility, the rapid capacity fade in the first cycles remained, and all electrodes behaved far worse than those processed with decane.
Also rigorous drying in dynamic vacuum of the PVdF powder before use did not improve the results. Tests with PVdF and PVdF-copolymers from another supplier, which are not presented here, gave the same trends of the cycling stabilities. Furthermore, it is clear that the swelling behaviour of polymers depends on the electrolyte solution [14,16]. The marked difference between the composite electrodes with ‘dissolved’ and ‘dispersed’ binder was also found for a number of other PC or EC+ DEC based electrolytes (cf. e.g. the results in 1 M LiPF6 + EC-DEC in Fig. 3 and those in 1 M LiClO4 + PC in Fig. 4). Systematic tests with electrolytes based on other solvents have, however, not been carried out so far. As has been mentioned above, in the composite electrodes processed with decane the binder is inhomogeneously distributed, i.e. there are regions with binder particle aggregates and regions which are apparently free of binder. It might appear that no binder at all is necessary for the successful cycling of Li–alloy com-
Fig. 2. SE micrographs of a Sn/SnSb composite electrode with an inhomogeneous distribution of PVdF, obtained by processing with decane. (a) Survey image: (1) electrode surface; (2) inner face; (3) interface between electrode material and current collector. (b) Detail of an inner face: (1) metallic particles; (2) binder particles.
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Fig. 2. (Continued)
posite electrodes. That this is not the case may be gathered from Fig. 4, where composite electrodes with different amounts of binder are compared. When the amount of binder is decreased, the capacity and the cycling stability decrease. A further increase of the amount of binder (not shown here), does not significantly change the cycling stability, but only results in lower capacities with respect to the mass of the whole composite material, since the percentage of active material decreases. In spite of the promising results, at the present stage of investigations the processing with decane is not the final solution to all problems related to electrode fabrication. The first reason for this is the behaviour of the composite electrode in the first cycle. As can be seen in Fig. 3b the charge curve for the first Li uptake is not smooth but exhibits voltage spikes (an extreme example has been chosen for Fig. 3b). They are attributed to a local break-up of the composite material during the first expansion of the active material and the concurrent sudden access of electrolyte to the electrode interior. There, the elec-
trolyte is decomposed, which in the case of common electrolyte solvents such as EC and PC is accompanied by gas formation. The electrode break-up is caused by an inhomogeneous distribution of mechanical strain, which in turn is caused by the inhomogeneous binder distribution. It should be possible to improve this situation by a further optimisation of the slurry homogenisation (e.g. better mixing or the use of other dispersants), which results in a better distribution of the binder in the composite electrode. The second reason is the poor adhesion of the composite material to the current collector before pressing. The composite material resembles more a loose powder aggregate than a plastic metal/polymer composite, and the slightest mechanical shock results in the composite material falling off the current collector. This surely poses a problem to the use of decane for technical coating procedures. To overcome this problem one might think of a combined binder system, e.g. mixtures of different polymers, where one component is dissolved and accounts for adhesion, and the other is dispersed.
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Finally, it has to be pointed out that these findings are strongly influenced by the present experimental set-up, i.e. by the present components of the composite electrode (both chemistry and morphology), the present design and processing of the composite electrode, and the present cell design, and that they may differ in other cases. Especially the cell design will have a major influence, since the composite electrode is placed in an excess of electrolyte without any mechanical support, which could counteract the expansion of the electrode material; i.e. there is an almost infinite amount of electrolyte to swell the polymer, and if once a particle is disconnected it remains disconnected as there is no back-driving force. A close-packed configuration with separator materials or other mechanical support (e.g.
the presence of low pressure in the cell) and with limited amounts of electrolyte, which are characteristic of commercial cell designs, surely aid the binder in providing the mechanical stability of the composite electrode.
4. Conclusions Two types of Sn/SnSb composite electrodes with identical compositions were compared, where: (i) NMP (in which PVdF is dissolved); or (ii) decane (in which PVdF is only dispersed) were used for the slurry preparation. In case (i) the binder was homogeneously distributed within the composite electrode, forming thin
Fig. 3. Constant current cycling tests of Sn/SnSb composite electrodes with ‘dispersed’ PVdF (processing with decane, circles) and ‘dissolved’ PVdF (processing with NMP, triangles). (a) Charge (solid symbols) and discharge (open symbols) capacities (given with respect to the mass of active material (AM)). (b) Charge/discharge curves for the first three cycles. Electrode composition: 82 wt% Sn/SnSb; 10% Ni; 8% PVdF; electrolyte: 1 M LiPF6 +EC+ DEC; cycling parameters: 0.5 mA cm − 2; 20 – 1200 mV vs. Li Li+.
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Fig. 4. Constant current cycling tests of Sn/SnSb composite electrodes with different amounts of PVdF binder, processed with decane or NMP. Solid symbols: charge capacities, open symbols: discharge capacities. Electrode composition: (90 − x) wt% Sn/SnSb, 10% Ni, x% PVdF; electrolyte: 1 M LiClO4 +PC; cycling parameters: 0.3 mA cm − 2, 100 – 1200 mV vs. Li Li+.
threads of diameters of 30 nm and less between the active materials. In case (ii) the original particle morphology of the binder with diameters of 200 – 300 nm was maintained, and the binder was inhomogeneously distributed within the composite electrode. The cycling stability of the second type of electrodes was far better than that of the first one. This is explained with the higher binding strength and the different swelling behaviour of the compact binder particles compared to the finely distributed binder, with the different electrode porosities, and with a possible ‘buffer’ effect of the soft binder particles. Strictly speaking these results hold true only for the present composite electrode design and the present cell design with a large excess of electrolyte, and the effects are surely less pronounced in real cells with a closepacked configuration and a limited amount of electrolyte. Nevertheless, the results clearly show, to what extent the cycling behaviour can be determined by the design of the composite electrode, especially in the case of Li storage alloys, which undergo major volume changes during cycling. The impact of the design of the composite electrode should always be checked before ruling out a certain active material because of its apparently small cycling capacities and stabilities.
Acknowledgements Financial support by the Austrian Science Fund and the Oesterreichische National Bank in Project 12768 and in the Special Research Program ‘Electroactive Materials’ as well as support by Mitsubishi Chemical .
Co. (Japan) are gratefully acknowledged. Furthermore, the authors thank Merck (Germany) for the donation of electrolytes.
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