Interaction of Poly(vinylidene fluoride) with Graphite Particles. 2. Effect

Apr 15, 2004 - The homogeneity of PVDF distribution has been correlated with the electrochemical performance of lithium ion battery. .... particles, p...
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Interaction of Poly(vinylidene fluoride) with Graphite Particles. 2. Effect of Solvent Evaporation Kinetics and Chemical Properties of PVDF on the Surface Morphology of a Composite Film and Its Relation to Electrochemical Performance Mikyong Yoo,† Curtis W. Frank,*,†,‡ Shoichiro Mori,§,| and Shoji Yamaguchi§ Departments of Materials Science and Engineering and Chemical Engineering, Stanford University, Stanford, California 94305, and Mitsubishi Chemical Corporation, Tsukuba Research Center, 8-3-1 Chuo, Ami, Inashiki, Ibaraki 300-03, Japan Received July 8, 2003. Revised Manuscript Received November 26, 2003

We have determined the dependence of the surface distribution of poly(vinylidene fluoride) (PVDF) on molecular weight and functionality of PVDF and solvent evaporation kinetics in a graphite composite film analogous to that used as an anode in a lithium ion battery. The homogeneity of the PVDF surface distribution on the graphite particles is determined from fluorine dot mappings, which are detected using energy-dispersive spectroscopy and electron probe X-ray microanalysis. The results are quantified with a standard deviation method and a spatial autocorrelation function approach, which yield consistent results. We have also examined the electrochemical performance of the anodes with cyclic voltammetry and impedance spectroscopy and correlated the electrochemical properties with the homogeneity of PVDF distribution in the final film. Hydroxyl-modified PVDF shows more homogeneous distribution compared to unmodified PVDF. Furthermore, the homogeneity of PVDF distribution on the graphite increases as the solvent evaporation rate increases. Samples with homogeneous PVDF distribution show higher electrochemical capacity and lower resistance, which can be explained in terms of solid electrolyte interphase film formation, binding capability, binder swelling, and electrochemically active sites of the modified PVDF.

Introduction Despite recent advances in lithium ion rechargeable batteries, there remain challenging problems related to the fabrication of the anode and cathode, which are both composite materials. The anode consists predominantly of graphite particles (90-95 wt %) bound together by a polymeric binder such as poly(vinylidene fluoride) (PVDF). By contrast, the cathode consists of 80-90% spinel metal oxides along with 5-10% carbon black or graphite powder and 5-10% binder. Although the polymeric binder is a minor component, it has a significant effect on the performance of the lithium ion battery. For example, it is well-known that the surface chemistry and morphology of the carbon particles play important roles in the charge/discharge capacity and cycling performance.1,2 It has been reported that the polymer binder covers up to 70% of the graphite surface,3 so that it follows that the type and amount of binder can affect * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Materials Science and Engineering, Stanford University. ‡ Department of Chemical Engineering, Stanford University. § Mitsubishi Chemical Corporation. | Currently at National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. (1) Chusid, O.; Ely, Y. E.; Aurbach, D.; Babai, M.; Carmeli, Y. J. Power Sources 1993, 43, 47. (2) Peled, E.; Menachem, C.; BarTow, D.; Melman, A. J. Electrochem. Soc. 1996, 143, L4.

the electrochemical characteristics of the carbon electrode.1,4-6 Thus, it is important to understand the binder/particle interaction mechanism, which will lead to the final surface morphology, and its relationship to the electrochemical behavior of the composite anodes. In our previous work,7 we examined the relationship between processing parameters of a graphite particle/ PVDF binder slurry and the surface properties of the resulting composite film prepared by doctor blading and thermal annealing. We have demonstrated that the viscosity of the slurry depends on the interaction between the graphite particles and PVDF binder. Moreover, we found that the slurry viscosity correlates with the homogeneity of the PVDF distribution on the surface of the final composite films; samples with higher slurry viscosity show more homogeneous PVDF distribution. The degree of homogeneity of PVDF distribution was quantified with a standard deviation method and an autocorrelation method devised earlier.7 In the former procedure, we drew at least 10 random lines on a (3) Hirasawa, K. A.; Nishioka, K.; Sato, T.; Yamaguchi, S.; Mori, S. J. Power Sources 1997, 69, 97. (4) Tran, T. D.; Feikert, J. H.; Mayer, S. T.; Song, X.; Kinoshita, K. Carbonaceous materials as lithium intercalation anodes; Electrochemical Society: Symposium on Rechargeable Lithium and Lithium-ion Batteries, Miami Beach, Fl., 1994; Vol. 94. (5) Ohta, N.; Sogabe, T.; Kuroda, K. Carbon 2001, 39, 1434. (6) Zhang, S. S.; Jow, T. R. J. Power Sources 2002, 109, 422. (7) Yoo, M.; Frank, C. W.; Mori, S. Chem. Mater. 2003, 15, 850.

10.1021/cm0304593 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/15/2004

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mapping of fluorine density determined from electron probe X-ray microanalysis (EPMA) or energy-dispersive spectroscopy (EDS) and evaluated the distribution for distances between the dots falling on those lines. Then we calculated the standard deviation. In this way, a small standard deviation reflects more homogeneous distribution of dots and vice versa. This approach was verified with an autocorrelation method, from which we formulated the radial spacing between clusters and the area of a cluster. As the values of these two properties decrease, the degree of homogeneity increases. In this paper, we examine the PVDF distribution using a PVDF sample modified with hydroxyl functional groups and one with higher molecular weight, both of which showed higher interaction with graphite particles in slurries. In addition, we extend the studies with different casting solvents, which can influence the PVDF surface distribution. In the electrode preparation process, casting solvent is used to adjust the slurry viscosity to achieve successful casting. Before thermal annealing, the solvent influences the configuration of a polymer chain in solution, which ranges from a compact coil in a poor solvent to an extended swollen chain in a good solvent; the configuration influences the final polymer morphology in films. For example, it has been shown that polymersolvent complex formation can lead to spheroidal or fibrillar-like morphology in PVDF/diester gels.8 In addition, the number of nucleation sites and the size of crystallites depend on the solvation condition.9 During thermal annealing, the suspending solvent is evaporated at high temperature. The solvent evaporation process is of considerable interest because nonequilibrium effects may influence the configuration of the PVDF binder chains on the graphite particles. For example, it has been reported that the solvent evaporation rate affects the resulting morphology for polymer films,10 polymer blends,11-13 crystalline polymer membranes,14 and block copolymers.15 Recently, the effect of the binder morphology on the cycling performance was studied for a tin electrode,16 where it was found that different PVDF morphologies resulting from the casting solvent (NMP and decane) influenced the cycling capacity. Wachtler et al.16 explained the binder morphology effect in terms of the porosity of the electrode. As the porosity increases, the capacity increases and the resistance of the electrode decreases due to rapid lithium intercalation-deintercalation through the pores. This is not consistent with what has been observed for carbon electrodes, however.17 The different carbon morphology, for example, disoriented graphite particle stacks when silica binder (8) Dikshit, A. K.; Nandi, A. K. Macromolecules 2000, 33, 3495. (9) Benz, M.; Euler, W. B.; Gregory, O. J. Langmuir 2001, 17, 239. (10) Shi, Y.; Liu, J.; Yang, Y. J. Appl. Phys. 2000, 87, 4254. (11) Barnes, M. D.; Ng, K. C.; Fukui, K.; Sumpter, B. G.; Noid, D. W. Macromolecules 1999, 32, 7183. (12) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Macromolecules 1994, 27, 6532. (13) Woo, E. M.; Su, C. C. Polymer 1996, 37, 5189. (14) Young, T. H.; Huang, J. H.; Chuang, W. Y. Eur. Polym. J. 2002, 38, 63. (15) Zhang, Q. L.; Tsui, O. K. C.; Du, B. Y.; Zhang, F. J.; Tang, T.; He, T. B. Macromolecules 2000, 33, 9561. (16) Wachtler, M.; Wagner, M. R.; Schmied, M.; Winter, M.; Besenhard, J. O. J. Electroanal. Chem. 2001, 510, 12. (17) Aurbach, D.; Levi, M. D.; Lev, O.; Gun, J.; Rabinovich, L. J. Appl. Electrochem. 1998, 28, 1051.

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is used and oriented ones when PVDF is used, leads to different porosity in the carbon electrode. With a porous structure, a less passivating and more resistive solid electrolyte interphase (SEI) film is formed, leading to poor electrochemical performance.17 The SEI film, which is a passivating film deposited on the lithium-intercalated graphite anode during the first electrochemical cycle, is formed as a result of chemical reaction between lithium and the electrolyte.18,19 The SEI film formation results in the irreversible capacity in the first cycle, but ensures the stability and the cycleability of the anode for further cycles. This is because, once formed, it prevents further co-intercalation of solvent into the graphite layers, which can potentially lead to graphite exfoliation.19 The morphology of the SEI film depends on the binder since the binder covers the electrochemically active surface area; surface covered by binder leads to reduced SEI film formation.20 These studies16,17 have shown the effect of electrode structure, which results from the binder morphology, on the electrochemical performance. However, the effect of the binder surface distribution on the electrochemical behavior has not been extensively studied. Since the electrodes are sufficiently porous when the PVDF binder is used,17 Li+ ion intercalation readily occurs through the PVDF binder, which makes the porosity less important in graphite/PVDF composite films. Recently, it has been reported that the portion of the graphite particles covered by PVDF binder does not form SEI at low temperature because the PVDF binder prevents contact between the lithiated graphite particles and the electrolyte.21 Thus, the PVDF surface morphology can influence SEI formation, leading to altered electrochemical performance. Our objectives in this study were to examine the binder distribution on the surface of the graphite particles in terms of the molecular weight and functionality of the PVDF and the casting solvent and to correlate the PVDF distribution with the electrochemical behavior of graphite/PVDF anodes. The anode slurry containing graphite particles, polymeric binder, and casting solvent is a complex system that is subjected to many processing parameters; the goal is to control the final electrochemical performance. Experimental Section Composite Film Preparation. Eight different carbon particles were utilized in these experiments, including seven synthetic graphite particles and one amorphous carbon. MBC-N from Mitsubishi Chemical has characteristics between synthetic graphite and amorphous carbon. BET (BrunauerEmmett-Teller) surface area ranges from 2.8 to 20 m2/g for the eight types. Information on type of carbon, average particle size, and BET surface area was provided by manufacturers, as listed in Table 1. Three different PVDF samples with different molecular weights and functionality were used, as listed in Table 2. Slurries were prepared by mixing carbon (18) Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E. J. Electrochem. Soc. 1987, 134, 1611. (19) Fong, R.; Vonsacken, U.; Dahn, J. R. J. Electrochem. Soc. 1990, 137, 2009. (20) Winter, M.; Besenhard, J. O. In Lithium Ion Batteries: Fundamentals and Performance; Wakihara, M., Yamamoto, O., Eds.; Wiley-VCH: New York, 1998; pp 127-155. (21) Yamaki, J.; Takatsuji, H.; Kawamura, T.; Egashira, M. Solid State Ionics 2002, 148, 241.

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Table 1. Information on Eight Carbon Particles That Were Used for Experimentsa

MPG-V2 MBC-N MCMB SFG44 SFG75 KS15 SFG15 KS6 a

manufacturer

type of carbon

average particle size (µm)

BET surface area (m2/g)

Mitsubishi Chemical Co., Japan Mitsubishi Chemical Co., Japan Oosaka Gas Chem., Japan Timcal Co. Ltd., Switzerland Timcal Co. Ltd., Switzerland Timcal Co. Ltd., Switzerland Timcal Co. Ltd., Switzerland Timcal Co. Ltd., Switzerland

synthetic graphite amorphous carbon synthetic graphite synthetic graphite synthetic graphite synthetic graphite synthetic graphite synthetic graphite

18.4 18.0 7.8 22.0 27.0 7.7 8.1 3.3

2.8 5.0 3.3 5.0 3.5 12.0 8.8 20.0

Average particle sizes and BET surface areas were provided by the manufacturers. Table 2. Functionality, Molecular Weight, and Head-to-Head Defects of PVDF Bindera

PVDF350 PVDF350M PVDF500 a

manufacturer, product name

functionality

molecular weight

H-H defects (%) (from 19F NMR)

Kureha, KF1300 Atofina, MKB212A Atofina, Kynar301F

N/A 1 wt % -OH (a little -COOH) N/A

∼350000 ∼350000 ∼500000

4.22 ((0.53) 5.54 ((0.67) 6.55 ((1.24)

The molecular weights of PVDF were provided by the manufacturers, and the head-to-head defects were calculated using 19F NMR.

particles, 1-methyl-2-pyrrolidinone (NMP) (99.9+%, Mitsubishi Chemical Corp.) as a carrier, and 10 wt % solution of PVDF binder in the same solvent. We also used two other solvents, N,N-dimethylacetamide (DMA) (99.9+%, Aldrich) and N,Ndimethylformamide (DMF) (99.9+%, Aldrich), to study the effects of solvent evaporation on the PVDF morphology. The solid concentration of the slurries was 40 wt %. We spread the slurry using the doctor-blade method on a sheet of copper foil and dried it in an oven at 83 °C in air for 2 h to form 95/5 wt % of graphite/PVDF composite film with a thickness of 34.5 ((0.5) µm. Distribution of PVDF on the Surface of Carbon Particles. The spatial distribution of PVDF on the graphite particles was evaluated by fluorine mapping using energydispersive spectroscopy (EDS) (JSM-5600LV, JEOL) with a gun voltage of 10 kV. For studies of the solvent effect on the PVDF distribution, the fluorine of the PVDF was mapped with electron probe X-ray microanalysis (EPMA) (JXA-733, JEOL) with 50 nA of current. EDS offers a better secondary ion image and fast qualitative analysis, while EPMA has higher resolution and can detect most elements. We determined the degree of homogeneity of the PVDF distribution by quantifying the image of fluorine dots using a standard deviation method and spatial autocorrelation function approach, which have been discussed in detail previously.7 Four images for each sample were used for the autocorrelation method. Solvent Evaporation Kinetics. Solvent evaporation kinetics of NMP, DMA, and DMF in composite materials was detected with thermogravimetric analysis (TGA) (951 TGA, DuPont Instruments), through monitoring weight percentage loss as a function of time. Samples were heated to 83 °C at a scanning rate of 5 °C/min and held isothermally for 2 h to provide a uniform thermal history for all samples. Electrochemical Measurements. The electrochemical behavior of the carbon electrodes was examined using a conventional three-electrode cell with lithium foil as reference and counter electrodes. The reference electrode is used for measuring the working electrode potential, and the counter electrode plays a role as a conductor that completes the cell circuit, maintaining the correct current. The area of the carbon electrodes was 1 × 1 cm2, and the active mass of the electrodes was approximately 3 mg. The electrolyte solution was 1 M LiClO4 dissolved in a 3:7 (by volume) mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (Battery grade, Mitsubishi Chemical Corp.). Cyclic voltammetry and impedance spectroscopy were carried out on an electrochemical interface system (1280B, Solartron). In cyclic voltammetry, current in a cell is measured as a function of potential. The potential of an electrode in solution is linearly cycled from the starting potential to the final potential and back to the starting point. With this measurement, we can study the electroactivity of materials

and the surface of electrodes, for example, the SEI formation. Cyclic voltammetry was measured between 1.5 and 0.01 V versus Li/Li+ at 1 mV/s. Impedance spectroscopy may be used to analyze electrochemical processes occurring at electrode/electrolyte interfaces. Electrochemical impedance is usually measured by applying an ac potential to the electrochemical cell and measuring the current through the cell. If we apply a sinusoidal potential, the response to this potential is an ac current signal with a shift in phase (φ),22

E(t) ) Eo sin(ωt)

(1)

I(t) ) Io sin(ωt - φ)

(2)

where E(t) is the potential at time t, Eo is the amplitude of the potential, I(t) is the response current, Io is the amplitude of the current, and ω is the radial frequency. From an expression analogous to Ohm’s Law, we can calculate the impedance of the system as

Z)

Eo sin(ωt) E(t) ) ) Zo (cos φ + i sin φ) I(t) Io sin(ωt - φ)

(3)

The impedance is expressed in terms of a magnitude, Zo, and a phase shift, φ. In the last part of eq 3, Euler’s relationship is used to express the equation as a complex function. Data are typically presented as Nyquist plots where -Z′′ (imaginary part) is plotted versus Z′ (real part of impedance) (see Figure 9). In many systems, the impedance varies as the frequency of the applied voltage changes in a way that provides valuable insights into its physical and chemical properties. ac techniques also use low perturbation signals (mV) that do not disturb the system under testing, unlike dc techniques that generally require large perturbation signals. After cyclic voltammetry, the electrode potential was stepped at a given potential and kept for a sufficient time until the current became negligible (typically DMA > DMF for all the samples, which is in the order of increasing solvent evaporation rate. For different graphite particles, they decrease in the order MCMB > SFG75 > SFG44. Note that this order coincides with an increase of the BET surface area of the graphite particles. As the surface area increases, the interaction between graphite particles and PVDF increases, leading to a more flocculated structure with a small amount of PVDF left in the casting solvent. If there is significant PVDF un-adsorbed to graphite particles in the casting solvent, a higher activation energy is apparently needed for the solvent to evaporate due to the interaction between solvent and polymer. Therefore, as the surface area of the graphite particles increases, the activation energy for solvent evaporation decreases. Electrochemical Behavior. Figure 7 shows cyclic voltammograms for the first and third cycles of SFG15 graphite samples with different PVDF binder and casting solvents in LiClO4/EC + EMC. We chose the

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Figure 7. Cyclic voltammograms in the first (dashed line) and third (solid lines) cycles of SFG15 samples with different PVDF in 1 M LiClO4/EC + EMC (3:7). NMP is used for (a) PVDF350, (b) PVDF350M, and (c) PVDF500 samples and DMF for (d) PVDF350. ν ) 1 mV/s.

SFG15 sample to correlate the electrochemical behavior of composite films with the PVDF morphology for different PVDF and casting solvent (Figures 3 and 4). For all the electrochemistry experiments, we adopted Aurbach’s method of choosing the geometric area as the characteristic area23 since the effective area of the carbon electrode is difficult to estimate accurately due to its porosity. Moreover, the variable influence of binder types and wettability between electrolyte and carbon surface further complicate determination of the effective area. For comparison, the currents were normalized by the mass of the electrodes. A large cathodic current rise observed at potentials close to 0.0 V is assigned to lithium intercalation, and the anodic peak at about 0.30.5 V is due to the lithium deintercalation.24,25 However, in the first scan, the value of the cathodic peak is greater than that of the anodic peak; therefore, a fraction of the cathodic current is consumed by irreversible processes such as solvent decomposition. A peak at potentials between 0.8 and 0.2 V in the Li+ insertion half-cycle corresponds to the solid electrolyte interphase (SEI) formation.25 This peak is not shown after the first cycle; the SEI formation on graphite is almost completed during the first cathodic process of graphite electrodes. Figure 7 shows that the cathodic peak varies with the binder and casting solvent used for electrode fabrication. PVDF350 cast with DMF (Figure 7d) shows a much larger cathodic peak, followed by PVDF350M, PVDF350, and PVDF500. This can be attributed to the distribution of PVDF binder on the surface of electrodes, which will be discussed later. Figure 8 shows a typical impedance spectrum and an equivalent circuit used for modeling the spectrum. RS (a) represents the resistance of the solution electrolyte, and Rf (b) and Rct (c) are the resistance across the SEI (23) Aurbach, D.; Eineli, Y.; Markovsky, B.; Zaban, A.; Luski, S.; Carmeli, Y.; Yamin, H. J. Electrochem. Soc. 1995, 142, 2882. (24) Funabiki, A.; Inaba, M.; Ogumi, Z. J. Power Sources 1997, 68, 227. (25) Gnanaraj, J. S.; Levi, M. D.; Levi, E.; Salitra, G.; Aurbach, D.; Fischer, J. E.; Claye, A. J. Electrochem. Soc. 2001, 148, A525.

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film and the charge-transfer resistance at the electrodes, respectively. Cf (b) and CPE (c) are the capacity and constant phase element associated with Rf and Rct, respectively. W (d) is the Warburg impedance arising from the semi-infinite diffusion of the intercalated lithium in the graphite electrode, and Cint (e) is the internal capacitance. At high frequency, the flat semicircle shown in Figure 8 can be interpreted with a “Voigt”-type analogue of three to five RC circuits in series to model the highly complex and heterogeneous nature of the SEI.26 Therefore, the total resistance of the SEI film is represented as Rf ) Rf1 + Rf2 + Rf3. This is attributed to Li+ ion migration through the SEI film that covers graphite electrodes in solution. The second flat semicircle shown in the low-frequency region is attributed to the interfacial charge transfer, which is related to the interparticle contact; it strongly depends on the physical structure such as geometry and morphology of the electrodes.26 We used a constant phase element (CPE) for the second semicircle instead of a capacity to model the depressed semicircle. The CPE can be caused by many factors: a porous electrode structure,27 overlapping of several semicircles of different relaxation times,27 and the rough surface of electrodes.24 At lower frequency (in (d) region), the electrochemical intercalation process is under a semi-finite Warburg-type solid-state Li+ diffusion control, ideally with a 45° straight line to Z′ vs -Z′′ axes, which is not significantly influenced by the potential. At the lowest frequency, the spectra become steep lines reflecting the accumulation of Li+ ion in graphite particles (Cint). Figure 9 shows the impedance spectra of the SFG15/ PVDF350M sample cast with NMP at various potentials during intercalation. At open-circuit potential, the electrode exhibits an almost straight line, which indicates that the lithium intercalation did not occur at this potential and has only a bulk solution resistance. As the intercalation progresses, two depressed semicircles and an inclined line appear at potentials at and below 0.3 V. These semicircles depend on the potential; as the Li+ ion intercalation progresses (potential decreases), the first semicircle becomes bigger and the second smaller, which will be shown in Figure 10. Also, an increase of the electrolyte resistance (RS, see Figure 8) is observed upon the decrease of the potential, which may result either from the decreased active surface accessible to the ions or from electrolyte consumption or from increased concentration of soluble organic species which may hinder ion motion.28 The resistances with different PVDF binder and casting solvent at various potentials are quantified using the equivalent circuit of Figure 8, with the result shown in Figure 10. As the Li+ ion intercalation progresses (potential decreases), the resistance of Li+ ion migration through the SEI film (Rf) increases while the charge-transfer resistance (Rct) decreases. These phenomena are in agreement with the literature.29-32 (26) Aurbach, D.; Markovsky, B.; Levi, M. D.; Levi, E.; Schechter, A.; Moshkovich, M.; Cohen, Y. J. Power Sources 1999, 82, 95. (27) Macdonald, J. R. Impedance Spectroscopy: Emphasizing Solid Materials and Systems; Wiley: New York, 1987. (28) Yazami, R.; Reynier, Y. F. Electrochim. Acta 2002, 47, 1217. (29) Aurbach, D.; Markovsky, B.; Weissman, I.; Levi, E.; EinEli, Y. Electrochim. Acta 1999, 45, 67. (30) Levi, M. D.; Aurbach, D. J. Phys. Chem. B 1997, 101, 4641.

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Figure 8. Equivalent circuit used for modeling impedance spectra.

Figure 9. Nyquist plots measured by impedance spectroscopy at various potentials. Open circuit potential (OCV) was 3.4059 V. The plot at OCV is cut at a frequency of 30 mHz, and frequencies are shown in the plot of E ) 0.01 V.

The increase in Rf upon the decrease of potential is due to an interfacial phenomenon in the boundary between the surface films and the active mass, which is confirmed by the reversible contraction of semicircles upon deintercalation.26 Also, as Li+ ion intercalates, the interlayer spacing increases, leading to highly stressed SEI films, which influences the conductivity.25 The PVDF350/DMF sample shows the lowest Li+ migration resistance, followed by PVDF350M, PVDF350, and PVDF500. The decrease in the Rct upon decrease of potential in Figure 10b can be explained with ButlerVolmer-type kinetics;22,25 as the overpotential becomes smaller, the charge-transfer resistance becomes smaller. This charge-transfer resistance (Rct) is also related to the exchange current (io) at low overpotentials with the following equation:22 (31) Chang, Y. C.; Jong, J. H.; Fey, G. T. K. J. Electrochem. Soc. 2000, 147, 2033. (32) Umeda, M.; Dokko, K.; Fujita, Y.; Mohamedi, M.; Uchida, I.; Selman, J. R. Electrochim. Acta 2001, 47, 885.

Figure 10. Dependence of Rf and Rct on the potential derived from impedance spectra with SFG15 graphite, different PVDF, and casting solvents.

Rct )

RT nFio

(5)

where R is the gas constant, T is the absolute temperature, n is the number of electrons transferred, and F is Faraday’s constant. The exchange current is inversely proportional to the charge-transfer resistance, leading to the highest exchange current with the PVDF350/ DMF sample, which means that this sample is more favorable toward the Li+ ion charge-transfer reaction. This is consistent with the result in Figure 7, which

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shows the highest capacity for the PVDF350/DMF sample. Discussion Solvent Evaporation Rate Effect on PVDF Distribution. For a single-component polymer film on a solid substrate, it has been shown that the morphology of the film cast from solution is influenced by polymersolvent complex formation,8,10 solution preparation temperature,9 solvating power,9 solvent evaporation temperature,14 and solvent evaporation rate.10,11 Polymersolvent complex formation affects the polymer conformation; if the solvent molecules disrupt the dipole and van der Waals interactions that hold the polymer chains together, polymer chains become more disentangled. Therefore, increased polymer-solvent complexation gives a rough surface.8,10 The measured intrinsic viscosities of PVDF350 in the three kinds of solvents are 1.54, 1.31, and 1.35 (g/dL) for NMP, DMA, and DMF, respectively. This means that, in NMP solvent, PVDF350 is swollen slightly more than in the other solvents, which shows more inhomogeneous distribution in Figure 4. However, all the solvents used in this study may be classified as good solvents for PVDF. Each has a CdO functional group, which is the main factor for polymersolvent interaction via the interaction of the CdO dipole with the CH2CF2 dipole or by limited hydrogen bonding.8,9 Therefore, we need to consider another cause for the different morphology of PVDF with the different solvents. If the rate for solvent evaporation from inside the film is slower, this allows the polymer chains to have more thermal energy to relax into a more thermodynamically favorable conformation.10 Moreover, if the solvent evaporation is slow, solvent-induced crystallinity can occur, leading to the inhomogeneous distribution.13 On the other hand, rapid solvent evaporation in thin blend films, which can cause significant trapped chain entanglement, leads to a rather homogeneous distribution.12,13 Therefore, when we used NMP solvent with the slowest evaporation rate, we observed the more inhomogeneous distribution on the surface of graphite particles, as shown in Figure 4. Correlation of PVDF Distribution with Electrochemical Behavior. We have demonstrated that the capacity, the Li+ ion migration resistance, and the charge-transfer resistance depend on the PVDF and casting solvent used for anode preparation. This is ascribed to different surface distribution of PVDF. When DMF was used as a casting solvent, it shows more homogeneous distribution of PVDF compared to NMP. Also, the modified PVDF, PVDF350M, shows a more homogeneous PVDF distribution compared to other PVDFs. As shown in Figures 7 and 10, PVDF350/DMF and PVDF350M show large cathodic peaks and low resistances. We can explain this by postulating that a thin SEI film had formed on the surface of graphite particles homogeneously covered by PVDF binder, leading to the low resistance. Moreover, the active surface area of graphite accessible by Li+ ion (the edge planes of graphite) decreased with the inhomogeneous distribution of PVDF since PVDF preferentially deposits on the polar edges of graphite particles.7 While PVDF500 shows a homogeneous distribution in Figure 3 for some graphite samples, it shows the most

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inhomogeneous PVDF distribution for SFG15 samples compared to PVDF350 and PVDF350M, thus leading to the highest Rf and Rct and the lowest capacity. This is attributed to the higher molecular weight of PVDF500, which may thickly cover a large part of graphite surface. Also the high molecular weight PVDF can exhibit the same effect as does the high concentration of binder in the anode, which has shown low reversibility of intercalation-deintercalation.1 Besides the distribution of PVDF on the graphite surface, we need to consider other effects on the electrochemical performance of the composite anodes. First, porosity plays an important role for Li+ ion intercalation-deintercalation kinetics. Voids within the active mass resulting from disoriented graphite particles lead to poor electrical contact among the particles and a less passivating and more resistive SEI film, which results in poor electrochemical performance.17 On the other hand, a too compact electrode structure hinders Li+ ion migration. In our study, we used PVDF binder, with which the electrodes are sufficiently porous, allowing the solution to percolate and reach the entire active mass of the electrode.17 Therefore, this effect may be the same for different PVDF binders. Second, the binding capability of PVDF is important for the graphite anodes because Li+ ion intercalation-deintercalation involves volume expansion of graphite particles.17 To overcome the volume expansion, flexibility of binders is needed as well as high binding, which contributes to the stability of the graphite particles in the electrodes and prevents the exfoliation of graphite. PVDF350 shows higher adhesion strength between graphite and PVDF and to the substrate,33 which might contribute to the lower Rct than PVDF350M in Figure 10b. Next, we need to consider the swelling of PVDF in electrolyte solvents. If PVDF is swollen in electrolyte, the adhesion of the electrode materials to the other particles and to the copper current collector deteriorate, which will lead to internal contact loss and, as a consequence, an increase in the contact resistance. In our previous experiment,33 the composite anode with PVDF350M showed the least electrolyte uptake of 27.64 wt %, whereas PVDF350 and PVDF500 showed 38.56% and 38.66%, respectively. Therefore, the higher electrolyte uptake of PVDF350 and PVDF500 may explain the low capacity and high resistance, as shown in Figures 7 and 10. Finally, the good electrochemical performance of PVDF350M can be ascribed to the oxygen in the hydroxyl and carboxylic acid functionality since it has been reported that the reversible capacity for anodes with polyimide binder is proportional to the amount of oxygen atoms in the polyimide, which are electrochemically active sites.5 Summary In this paper, we have investigated the different surface morphology of the polymeric binder due to the molecular weight and functionality of PVDF and the solvent evaporation kinetics and correlated this with the electrochemical behavior of the composite anodes. We altered the interaction of PVDF with graphite by using (33) Yoo, M.; Frank, C. W.; Mori, S.; Yamaguchi, S. Polymer 2003, 44, 4197.

Interaction of PVDF with Graphite Particles

PVDF modified with hydroxyl functional group or with higher molecular weight PVDF, leading to an improved distribution of PVDF on the surface of graphite particles in the final composite films. We also used three different good solvents for PVDF as casting solvents and determined that the surface morphology of PVDF depends on the solvent evaporation rate. The degree of homogeneity increases as the solvent evaporation rate increases because the rapid solvent evaporation does not allow the polymer to be rearranged. The morphology of PVDF on the surface of particles is correlated with the electrochemical behavior of the composite anodes; anodes with more homogeneous PVDF distribution show

Chem. Mater., Vol. 16, No. 10, 2004 1953

higher capacity and lower resistance. This is probably attributed to the thin SEI film formed on the graphite surface. This study demonstrates that controlling processing parameters and the interaction between graphite particles and PVDF is important for obtaining desirable electrochemical properties of the composite films. Acknowledgment. This work was supported by Mitsubishi Chemical Corporation in Japan. CM0304593