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Nuclear magnetic resonance studies of nanocomposite gel electrolytes
Electrochimica Acta 48 (2003) 2113 /2121 www.elsevier.com/locate/electacta
Nuclear magnetic resonance studies of nanocomposite gel electrolytes Sabina Abbrent a, Song H. Chung b, Steve G. Greenbaum a,*, Jacob Muthu c, Emmanuel P. Giannelis c a
Physics Department, Hunter College of City University of New York, 695 Park Avenue, New York, NY 10021, USA b Chemistry and Physics Department, William Paterson University, Wayne, NJ 07470, USA c Materials Science and Engineering Department, Cornell University, Ithaca, NY 14853, USA Received 19 May 2002; accepted 25 November 2002
Abstract Polymer gel electrolytes based on poly(vinylidene fluoride co-hexafluoropropylene), and containing layered nanoparticles such as fluorohectorite were investigated by pulsed gradient spin-echo nuclear magnetic resonance (NMR), as well as line shape and relaxation studies. Diffusion coefficients of Li ions, anions, and electrolyte solvent were measured by 7Li, 19F, and 1H NMR, respectively. All results indicate heterogeneous ionic environments in the nanocomposite materials. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polymer gel electrolyte; PVdF; Fluorohectorite; NMR; Diffusion
1. Introduction The desire to increase energy density and safety of lithium ion batteries has led to the development of polymer gel electrolytes, in which the liquid electrolyte is entrapped [1]. One such composition that has received considerable attention is based on a commercial copolymer poly(vinylidene fluoride co- hexafluoropropylene (PVdF-HFP) [2]. Another topic of interest in lithium battery research concerns the addition of nanoscale inorganic oxides such as Al2O3, SiO2 or TiO2 [3 /5] to polymer electrolytes to improve mechanical as well as, in some cases, electrochemical properties [6]. Such ‘‘fillers’’ have also been incorporated into gel electrolytes in order to preserve a porous structure that maximizes the absorption of the liquid electrolyte [2]. Anisotropic fillers such as layered silicates are of particular interest in nanocomposite formation because
* Corresponding author. Tel.: /1-212-772-4973; fax: /1-212-7725390. E-mail address: [email protected] (S.G. Greenbaum).
of their ability to intercalate organic compounds. For example, intercalation of poly(ethylene oxide) into Liexchanged montmorillonite was found to yield a polymer electrolyte with interesting properties, including complete suppression of the crystalline phase known to limit the ionic conductivity of these materials [7]. Layered silicate polymer nanocomposites also exhibit stiffness, strength, and stability in two dimensions rather than one due to layer orientation [8]. The purpose of this investigation is to ascertain the effects of filler type and concentration on ionic and molecular transport in composite gel electrolytes. Of particular interest in the present case is the extent to which the nanoscopic filler particles interact with the ions, in addition to whatever structural enhancement role they may play. The principal tool employed is pulsed gradient spin echo nuclear magnetic resonance (PGSE NMR), with which molecular self-diffusion coefficients can be determined. Depending on which nucleus is being probed, one can monitor the mobility of the cations (7Li), anions (19F), or solvent molecules (1H). Additional NMR spectral and relaxation measurements also shed light on the nature of the ion-filler interaction.
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00193-2
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2. Experimental 2.1. Sample preparation The polymer poly(vinyledene fluoride)-co-hexafluoro propylene (PVDF-co-HFP) (Aldrich) was used as received. Lithium trifluoromethono sulfate (Aldrich) was dried at around 140 8C in vacuum for overnight before use. Propylene carbonate (PC) (Aldrich) was stored in ˚ ) and ethylene carbonate molecular sieves (Linde 4 A (EC) (Aldrich) was used as received. The silicate used in the present study was Li-fluorohectorite (supplied by Corning), barium titanate (BaTiO3) (Corning) and a synthetic fluromica clay compound (Co-op Chem. Co., Japan). 1 M Lithium triflate (LiCF3SO3) solutions in PC and EC (1:1) were prepared (Table 1). The desired amount of the filler was added to the electrolyte and stirred overnight at room temperature. The polymer host PVDF-co-HFP was mixed with the nanocomposite electrolyte and 10 ml of tetrahydrofuran (THF) was added to the sample and the whole mixture was stirred at 50 8C in a tightly capped vial. After cooling to room temperature, the highly viscous gel solution was cast on a glass petri dish and allowed to dry inside a dessicator. The whole procedure was conducted inside a glove box. 2.2. Nuclear magnetic resonance The NMR spectra were acquired on a Chemagnetics CMX-300 spectrometer used in conjunction with Japan Magnet Technology Bo /7.1T superconducting magnet. In this field, 1H, 7Li and 19F resonances occur at vo/ 2p /301.02, 116.99 and 283.22 MHz, respectively. Details regarding the PGSE NMR experiments are described elsewhere [9]. The NMR diffusion measurements use the Hahn spin-echo [10] pulse sequence with a pair of square shaped gradient field pulses of magnitude g and duration d. The echo amplitude A: A(g) exp [ g2 Dg2 d2 (D (d=3))]
(1)
is attenuated by an amount dependent on how much the position of the spins has changed by process of selfTable 1 Composition of samples studied in this investigation Sample number Composition 0 (Reference) 1 2 3 4 5 6 7 8
Liquid electrolyte (1 M LiCF3SO3 in EC:PC (1:1)) Pure polymer electrolyte (/PVdF-co-HFP) (PPE) PPE/1 wt.% LiFluorohectorite (FH) PPE/3 wt.% FH PPE/5 wt.% FH PPE/7 wt.% FH PPE/10 wt.% FH PPE/5 wt.% BaTiO3 PPE/10 wt.% Clay
diffusion D in the time interval D and for a series of gradient strengths g /0.2 /1.2 T/m. All spectral and PSEG measurements were performed at a temperature range 294 /366 K. The typical reproducibility of the D measurements is about 9/3%. Spectra at the appropriate resonance frequency were obtained by collecting the free induction decay (FID) following a single p/2 pulse and Fourier transforming the data. Full widths at half maximum (FWHM) were then determined from expanded spectral plots. The spin-lattice relaxation time, T1, was determined by sampling the amplitude of the FID following an inversion recovery sequence [7] (p /t/p/2) for about 15 values of t. The time between repetitions of the pulse sequence was always greater than 5 T1. Throughout the temperature range of investigation the recovery is exponential and a well-defined T1 can be obtained by fitting the data to the equation: [A A(t)]=A C exp( t=T1 );
(2)
where A and C are fitting constants. The uncertainty in the determination of T1 is about Ö/5%. 2.3. Conductivity measurements Prior to measurements, the polymer electrolyte films were dried at 60 8C in a vacuum oven for 24 h. The ionic conductivity study was performed using an impedance analyzer. Polymer electrolyte samples of known thickness were sandwiched between the two stainless steel (316) electrodes under spring loaded conductivity cell. The impedance measurements were carried out in the frequency range from 5 Hz to 13 MHz. The temperature dependent ionic conductivity of the nanocomposite gel polymer electrolyte was measured in the range from room temperature to 100 8C. The temperature was controlled using a Lakeshore temperature controller.
3. Results and discussion 3.1. NMR spectra All 7Li spectra exhibit a single peak and there is no shift or splitting observed for any of the samples. A representative spectrum is shown in Fig. 1. The 19F spectra exhibit one large single peak representing the fluorine atoms of the anion, and several smaller and broader peaks arising from the fluorine atoms from the HFP copolymer and from the PVdF polymer backbone, as depicted in Fig. 2. The 1H spectra typically exhibit two single peaks, as shown in Fig. 3. The smaller peak at /1 kHz represents the CH3 group of the PC molecule, while the larger peak (at 0 Hz) is a summation of the CH and CH2 protons, that are present in the solvents (EC and PC) and that are unresolved due to broadening by
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Fig. 1. 7Li NMR spectrum of Sample 4.
Fig. 2.
19
F NMR spectrum of Sample 4.
restricted molecular motion in the gel (see Fig. 3a, b). The protons from the polymer backbone are not observed here because their line-width is much broader that the spectral window used to acquire the data in Fig. 3. Fig. 4 displays the spectral line-widths at room temperature of the various samples. There are no observed changes in line-width with increasing amount of FH for either 7Li or 19F, while the proton spectra narrow significantly with increasing FH concentration, except for the sample with 10% FH, which exhibits more restricted ionic and molecular motion. This suggests higher concentration and hence mobility of the solvent in the electrolyte, attributable to greater solvent uptake, with surprisingly little impact on the ionic species.
Fig. 3. (a) 1H NMR spectrum of the Reference sample (liquid), (b) 1H NMR spectrum of Sample 4.
Lithium-7 and 19F variable temperature line-width data are presented in Fig. 5. The filler-free gel and those containing FH and BaTiO3, exhibit very little temperature dependence over the range studied while the clay sample gives a sloped line. The weak temperature dependence is attributed to the relatively high temperature range in which the measurements were made, corresponding to the so-called extreme narrowing region [10]. There is only a slight temperature dependence observed for 19F line widths for samples with FH, BaTiO3, and clay fillers and 7Li line widths in the clay sample. Apparently the addition of the FH filler has little of an impact on the environment of the anion, while the BaTiO3 and clay fillers give a large increase in room temperature line widths, suggesting impeded
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3.2. Spin-relaxation (T1) measurements
Fig. 4. Line-widths of spectra obtained at room temperature for the eight tested samples; m stand for 19F, k for 7Li and % for 1H.
anion motion. This is also consistent with 7Li and diffusivity results discussed below.
19
F
As stated previously, there is one resolved component in the 7Li spectra, while the 19F and 1H spectra both show two peaks. For 19F, the relaxation behaviors of the two distinct peaks reflect dynamics of the anion and the polymer host. In the 1H measurements the CH3, CH2, and CH peaks from the solvent all exhibit similar relaxation behavior. The 19F peak from the polymer shows the lowest relative T1 values, and no variation between the samples. The T1 values for the anion follow similar behavior to that observed in the diffusion results, to be discussed later. The two solvent peaks show very similar and also the highest T1 values, which is behavior expected of mobile solvent molecules (the molecular reorientation time is too rapid to provide an efficient relaxation mechanism). The 7Li T1 values seem to be mostly independent of the concentration of the filler (T1 values being very similar for all FH samples), but they vary
Fig. 5. Temperature dependence of line-widths for (a) Sample 1, (b) Sample 6, (c) Sample 7 and (d) Sample 8, where m stand for 19F, k for 7Li and % for 1H.
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Fig. 6. Spin relaxation (T1) for the samples at room temperature; m representing the anion 19F, k 7Li, % solvent 1H, I 1H from CH3 group of PC and j polymer 19F.
between the different fillers, being lowest for the clay sample and highest for the FH samples, as summarized in Fig. 6. There is almost no temperature dependence of T1 observed for 7Li in the filler-containing samples, over the entire range of the measurements. This result could be an indication of a very broad T1 minimum and, therefore, suggests a very broad distribution of ionic motional correlation times. The filler-free sample and the reference exhibit some temperature dependence but no T1 minimum in the temperature range investigated, as shown in Fig. 7. The T1 results thus suggest that the presence of the filler leads to a heterogeneous cation environment, which in turn, yields a broad distribution of correlation times. There is some Arrhenius temperature dependence observed for the 19F anion T1 values, the corresponding activation energy values being largest
Fig. 7. Temperature dependence of the spin relaxation (T1) for 7Li; symbols representing: m Sample 1, k Sample 2, % Sample 3, I Sample 4, j Sample 5, I Sample 6, I Sample 7, I Sample 8 and ' the reference.
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Fig. 8. Temperature dependence of the spin relaxation (T1) for 19F; symbols representing: m Sample 1, k Sample 2, % Sample 3, I Sample 6, I Sample 7, I Sample 8 and ' the reference.
for the filler-free sample and decreasing with increasing amount of the FH filler, as displayed in Fig. 8. Because of the relatively weak 19F contribution from the polymer it is difficult to conclude anything about the polymer dynamics from the T1 results. 3.3. Diffusion measurements In general, it is possible to determine self-diffusion coefficients (D) corresponding to each peak for spectra that exhibit distinct peaks. The room temperature diffusion results are summarized in Fig. 9. The highest D’s are obtained for the protons, the solvent diffusing at the fastest rate. The D’s for 7Li and anion 19F generally follow the same trend, which in turn appears to be governed by the solvent mobility. However, there are some variations in the degree to which the behavior of all the diffusing species are coupled, depending on the amount and type of the filler present. With increasing
Fig. 9. Diffusion coefficients for the used samples at room temperature; symbols stand for m 19F, k 7Li and % 1H.
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amount of FH there is a larger difference between the D’s of the ions, where the fluorine diffusion is significantly restrained in samples with increased filler concentration. The BaTiO3 containing sample exhibits lowest D’s, for all three diffusing species (solvent, cation, and anion). The cation and anion exhibit nearly equal D’s in the liquid reference, the gel without fillers, and the clay filler sample. However, the other fillers appear to inhibit anion diffusion relative to cation diffusion. Sample 5 (7% FH) exhibits somewhat elevated D’s relative to other filler-containing materials, but this is attributed to partial phase separation visually observed for this sample. All samples exhibit approximately Arrhenius temperature dependence of the D-values (Table 2). Lithium shows very similar activation energies of the diffusion in all the samples. The lowest values of D are observed for the BaTiO3 sample, and there is in general only a slight decrease in D for the FH samples as compared with the filler-free sample, as shown in Fig. 10. An increased concentration of FH results in decreased anion Ds and lower activation energy values, as shown in Fig. 11. Some of the data from Figs. 10 and 11 are replotted for clarity in Fig. 12a, in which the filler-free and 10% FH samples are directly compared. Clearly, the anion diffusion is reduced in the filler-containing sample. On the other hand, the clay-containing sample exhibits nearly equal diffusion anion and cation D-values over the entire temperature range. The BaTiO3 and clay samples exhibit consistently lower ionic diffusivities and mobilities (7Li and 19F line widths). They are also the least conductive samples, as discussed below. Thus, there appears to be strong, and undesirable, interactions between the filler particles and the ions in the case of the BaTiO3 and clay fillers. 3.4. Conductivity measurements The ac conductivity results are plotted in Fig. 13 and show, similar to the diffusion measurements, the best results for the gel polymer without any filler. With increasing content of the FH filler the conductivity decreases. The conductivities follow Arrhenius behavior. Table 2 Activation energy (kJ/mol) for the temperature dependence of diffusion coefficients as obtained from the Arrhenius plots Sample number Ea (kJ/mol) for D of 1 3 4 5 6 7 8
9.9400 6.9800 7.8000 3.5100 9.5000 7.7000
19
F Ea (kJ/mol) for D of 7Li 7.2000 5.9400 6.1000 5.5000 5.9000 6.7000 7.0000
Fig. 10. Temperature dependence of diffusion coefficients for 7Li; symbols representing Sample 1 (m), Sample 2 (k), Sample 3 (%), Sample 4 (I), Sample 5 (j), Sample 6 (I), Sample 7 (I) and Sample 8 (I).
Fig. 11. Temperature dependence of diffusion coefficients for 19F; symbols representing Sample 1 (m), Sample 2 (k), Sample 3 (%), Sample 6 (I), Sample 7 (I) and Sample 8 (I).
It is instructive to compare the experimental conductivity values with the results calculated from diffusion coefficients via the Nernst/Einstein relation: lj
z2j F 2 RT
Dj
(3)
where lj is the contribution from species j with charge zj and diffusion coefficient Dj at temperature T. F is the Faraday constant, R the gas constant. When the conductivity values obtained by the ac measurements are compared with the calculated ones (Table 3), the calculated are usually higher [11]. This is caused by the different physical quantities measured by the two techniques, where in conductivity measurements only the charged species movement is measured, while the diffusion measurements register all moving species of the resonating species regardless of the charge state. With, for example, ion-pairs present in a sample, the
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Fig. 12. Temperature dependence for (a) Sample 1, (b) Sample 6 and (c) Sample 8; symbols representing k 7Li and m
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19
F.
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observed in the case of the pure gel electrolyte. Filler containing samples exhibit even wider discrepancy between calculated and measured conductivities (Fig. 14). It was expected that the filler material would decrease ion pairing and bring the measured and calculated values into closer agreement. That the opposite effect is observed suggests that the fillers do not, in fact, play a significant role in reducing ion pairing. Furthermore, it is speculated that the large observed discrepancy can be attributed to material heterogeneity in the filler-containing gels [12]. In particular, not all micron-scale ion diffusion processes lead to macroscopic ion transport in a heterogeneous conducting medium with many phase boundaries. Fig. 13. Conductivity values resulting from impedance measurements: symbols representing: Sample 1 (m), Sample 2 (k), Sample 3 (%), Sample 4 (I), Sample 5 (j), Sample 6 (I), Sample 7 (I) and Sample 8 (I).
Table 3 Molar ionic conductivity values for the diffusing species as calculated by the Nernst /Einstein equation Sample number
lF (S/cm mol)
lLi (S/cm mol)
s (S/cm)
1 2 3 4 5 6 7 8
1.30 0.98 1.05 0.80 1.19 0.73 0.34 0.68
1.30 1.20 1.01 1.17 1.47 1.07 0.51 0.74
2.6 2.18 2.06 1.97 2.66 1.80 0.85 1.42
calculated conductivity values will be higher than the measured ones because NMR is sensitive to the motion of the ion-pair while conductivity is not. This is in fact
4. Conclusions Gel electrolytes with nanoscopic fillers have superior mechanical properties as compared with gels without the filler. This is attributed in part, to the layered nature of the fillers employed in this investigation. The effect of the fillers on electrical properties is to decrease the conductivity by a modest amount. NMR relaxation measurements of filler-containing gels indicate that the Li environment is very heterogeneous. Diffusion results show that both anionic and cationic mobilities are impeded by the presence of the filler, but this effect is generally greater for the anions. The calculated conductivities obtained from the Nernst /Einstein equation are higher than the measured ones, which is expected. However, the larger differences observed for the filler containing samples cannot be explained by the ion pairing only, and are also attributed to material heterogeneity.
Acknowledgements This work was supported by grants from the US Air Force Office of Scientific Research.
References
Fig. 14. Measured (empty symbols) vs. calculated (filled symbols) conductivity values for Sample 1 (m), Sample 6 (j), Sample 7 (%) and Sample 8 (I).
[1] K.M. Abraham, M.J. Alamgir, J. Electrochem. Soc. 106 (1990) 1657. [2] J.-M. Tarascon, A.S. Gozdz, C.N. Schmutz, F.K. Shokoohi, P.C. Warren, Solid State Ionics 86 (1996) 49. [3] M. Forsyth, D.R. MacFarlane, A. Best, J. Adebahr, P. Jakobsson, A.J. Hill, Solid State Ionics 147 (2002) 203. [4] H.Y. Sun, H.J. Sohn, O. Yamamoto, N. Imanishi, J. Electrochem. Soc. 137 (1990) 3401. [5] A.C. Bloise, C.C. Tambelli, R.W.A. Franco, J.P. Donoso, C.J. Magon, M.F. Souza, A.V. Rosario, E.C. Pereira, Electrochim. Acta 46 (2001) 1571.
S. Abbrent et al. / Electrochimica Acta 48 (2003) 2113 /2121 [6] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nature 394 (1998) 456. [7] E. Hackett, E. Manias, E.P. Giannelis, Chem. Mater. 12 (8) (2000) 2161. [8] J.W. Gilman, Appl. Clay Sci. 15 (1999) 31. [9] S.H. Chung, S. Bajue, S.G. Greenbaum, J. Chem. Phys. 112 (2000) 8515.
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[10] C.P. Slichter, Principles of Magnetic Resonance, Springer, Berlin, 1990. [11] A. Reiche, A. Weinkaupf, B. Sandner, F. Rittig, G. Fleischer, Electrochim. Acta 45 (2000) 1327. [12] K. Hayamizu, Y. Aihara, S. Arai, W.S. Price, Electrochim. Acta 45 (2000) 1313.
Nuclear magnetic resonance studies of nanocomposite gel electrolytes Sabina Abbrent a, Song H. Chung b, Steve G. Greenbaum a,*, Jacob Muthu c, Emmanuel P. Giannelis c a
Physics Department, Hunter College of City University of New York, 695 Park Avenue, New York, NY 10021, USA b Chemistry and Physics Department, William Paterson University, Wayne, NJ 07470, USA c Materials Science and Engineering Department, Cornell University, Ithaca, NY 14853, USA Received 19 May 2002; accepted 25 November 2002
Abstract Polymer gel electrolytes based on poly(vinylidene fluoride co-hexafluoropropylene), and containing layered nanoparticles such as fluorohectorite were investigated by pulsed gradient spin-echo nuclear magnetic resonance (NMR), as well as line shape and relaxation studies. Diffusion coefficients of Li ions, anions, and electrolyte solvent were measured by 7Li, 19F, and 1H NMR, respectively. All results indicate heterogeneous ionic environments in the nanocomposite materials. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polymer gel electrolyte; PVdF; Fluorohectorite; NMR; Diffusion
1. Introduction The desire to increase energy density and safety of lithium ion batteries has led to the development of polymer gel electrolytes, in which the liquid electrolyte is entrapped [1]. One such composition that has received considerable attention is based on a commercial copolymer poly(vinylidene fluoride co- hexafluoropropylene (PVdF-HFP) [2]. Another topic of interest in lithium battery research concerns the addition of nanoscale inorganic oxides such as Al2O3, SiO2 or TiO2 [3 /5] to polymer electrolytes to improve mechanical as well as, in some cases, electrochemical properties [6]. Such ‘‘fillers’’ have also been incorporated into gel electrolytes in order to preserve a porous structure that maximizes the absorption of the liquid electrolyte [2]. Anisotropic fillers such as layered silicates are of particular interest in nanocomposite formation because
* Corresponding author. Tel.: /1-212-772-4973; fax: /1-212-7725390. E-mail address: [email protected] (S.G. Greenbaum).
of their ability to intercalate organic compounds. For example, intercalation of poly(ethylene oxide) into Liexchanged montmorillonite was found to yield a polymer electrolyte with interesting properties, including complete suppression of the crystalline phase known to limit the ionic conductivity of these materials [7]. Layered silicate polymer nanocomposites also exhibit stiffness, strength, and stability in two dimensions rather than one due to layer orientation [8]. The purpose of this investigation is to ascertain the effects of filler type and concentration on ionic and molecular transport in composite gel electrolytes. Of particular interest in the present case is the extent to which the nanoscopic filler particles interact with the ions, in addition to whatever structural enhancement role they may play. The principal tool employed is pulsed gradient spin echo nuclear magnetic resonance (PGSE NMR), with which molecular self-diffusion coefficients can be determined. Depending on which nucleus is being probed, one can monitor the mobility of the cations (7Li), anions (19F), or solvent molecules (1H). Additional NMR spectral and relaxation measurements also shed light on the nature of the ion-filler interaction.
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00193-2
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2. Experimental 2.1. Sample preparation The polymer poly(vinyledene fluoride)-co-hexafluoro propylene (PVDF-co-HFP) (Aldrich) was used as received. Lithium trifluoromethono sulfate (Aldrich) was dried at around 140 8C in vacuum for overnight before use. Propylene carbonate (PC) (Aldrich) was stored in ˚ ) and ethylene carbonate molecular sieves (Linde 4 A (EC) (Aldrich) was used as received. The silicate used in the present study was Li-fluorohectorite (supplied by Corning), barium titanate (BaTiO3) (Corning) and a synthetic fluromica clay compound (Co-op Chem. Co., Japan). 1 M Lithium triflate (LiCF3SO3) solutions in PC and EC (1:1) were prepared (Table 1). The desired amount of the filler was added to the electrolyte and stirred overnight at room temperature. The polymer host PVDF-co-HFP was mixed with the nanocomposite electrolyte and 10 ml of tetrahydrofuran (THF) was added to the sample and the whole mixture was stirred at 50 8C in a tightly capped vial. After cooling to room temperature, the highly viscous gel solution was cast on a glass petri dish and allowed to dry inside a dessicator. The whole procedure was conducted inside a glove box. 2.2. Nuclear magnetic resonance The NMR spectra were acquired on a Chemagnetics CMX-300 spectrometer used in conjunction with Japan Magnet Technology Bo /7.1T superconducting magnet. In this field, 1H, 7Li and 19F resonances occur at vo/ 2p /301.02, 116.99 and 283.22 MHz, respectively. Details regarding the PGSE NMR experiments are described elsewhere [9]. The NMR diffusion measurements use the Hahn spin-echo [10] pulse sequence with a pair of square shaped gradient field pulses of magnitude g and duration d. The echo amplitude A: A(g) exp [ g2 Dg2 d2 (D (d=3))]
(1)
is attenuated by an amount dependent on how much the position of the spins has changed by process of selfTable 1 Composition of samples studied in this investigation Sample number Composition 0 (Reference) 1 2 3 4 5 6 7 8
Liquid electrolyte (1 M LiCF3SO3 in EC:PC (1:1)) Pure polymer electrolyte (/PVdF-co-HFP) (PPE) PPE/1 wt.% LiFluorohectorite (FH) PPE/3 wt.% FH PPE/5 wt.% FH PPE/7 wt.% FH PPE/10 wt.% FH PPE/5 wt.% BaTiO3 PPE/10 wt.% Clay
diffusion D in the time interval D and for a series of gradient strengths g /0.2 /1.2 T/m. All spectral and PSEG measurements were performed at a temperature range 294 /366 K. The typical reproducibility of the D measurements is about 9/3%. Spectra at the appropriate resonance frequency were obtained by collecting the free induction decay (FID) following a single p/2 pulse and Fourier transforming the data. Full widths at half maximum (FWHM) were then determined from expanded spectral plots. The spin-lattice relaxation time, T1, was determined by sampling the amplitude of the FID following an inversion recovery sequence [7] (p /t/p/2) for about 15 values of t. The time between repetitions of the pulse sequence was always greater than 5 T1. Throughout the temperature range of investigation the recovery is exponential and a well-defined T1 can be obtained by fitting the data to the equation: [A A(t)]=A C exp( t=T1 );
(2)
where A and C are fitting constants. The uncertainty in the determination of T1 is about Ö/5%. 2.3. Conductivity measurements Prior to measurements, the polymer electrolyte films were dried at 60 8C in a vacuum oven for 24 h. The ionic conductivity study was performed using an impedance analyzer. Polymer electrolyte samples of known thickness were sandwiched between the two stainless steel (316) electrodes under spring loaded conductivity cell. The impedance measurements were carried out in the frequency range from 5 Hz to 13 MHz. The temperature dependent ionic conductivity of the nanocomposite gel polymer electrolyte was measured in the range from room temperature to 100 8C. The temperature was controlled using a Lakeshore temperature controller.
3. Results and discussion 3.1. NMR spectra All 7Li spectra exhibit a single peak and there is no shift or splitting observed for any of the samples. A representative spectrum is shown in Fig. 1. The 19F spectra exhibit one large single peak representing the fluorine atoms of the anion, and several smaller and broader peaks arising from the fluorine atoms from the HFP copolymer and from the PVdF polymer backbone, as depicted in Fig. 2. The 1H spectra typically exhibit two single peaks, as shown in Fig. 3. The smaller peak at /1 kHz represents the CH3 group of the PC molecule, while the larger peak (at 0 Hz) is a summation of the CH and CH2 protons, that are present in the solvents (EC and PC) and that are unresolved due to broadening by
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Fig. 1. 7Li NMR spectrum of Sample 4.
Fig. 2.
19
F NMR spectrum of Sample 4.
restricted molecular motion in the gel (see Fig. 3a, b). The protons from the polymer backbone are not observed here because their line-width is much broader that the spectral window used to acquire the data in Fig. 3. Fig. 4 displays the spectral line-widths at room temperature of the various samples. There are no observed changes in line-width with increasing amount of FH for either 7Li or 19F, while the proton spectra narrow significantly with increasing FH concentration, except for the sample with 10% FH, which exhibits more restricted ionic and molecular motion. This suggests higher concentration and hence mobility of the solvent in the electrolyte, attributable to greater solvent uptake, with surprisingly little impact on the ionic species.
Fig. 3. (a) 1H NMR spectrum of the Reference sample (liquid), (b) 1H NMR spectrum of Sample 4.
Lithium-7 and 19F variable temperature line-width data are presented in Fig. 5. The filler-free gel and those containing FH and BaTiO3, exhibit very little temperature dependence over the range studied while the clay sample gives a sloped line. The weak temperature dependence is attributed to the relatively high temperature range in which the measurements were made, corresponding to the so-called extreme narrowing region [10]. There is only a slight temperature dependence observed for 19F line widths for samples with FH, BaTiO3, and clay fillers and 7Li line widths in the clay sample. Apparently the addition of the FH filler has little of an impact on the environment of the anion, while the BaTiO3 and clay fillers give a large increase in room temperature line widths, suggesting impeded
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3.2. Spin-relaxation (T1) measurements
Fig. 4. Line-widths of spectra obtained at room temperature for the eight tested samples; m stand for 19F, k for 7Li and % for 1H.
anion motion. This is also consistent with 7Li and diffusivity results discussed below.
19
F
As stated previously, there is one resolved component in the 7Li spectra, while the 19F and 1H spectra both show two peaks. For 19F, the relaxation behaviors of the two distinct peaks reflect dynamics of the anion and the polymer host. In the 1H measurements the CH3, CH2, and CH peaks from the solvent all exhibit similar relaxation behavior. The 19F peak from the polymer shows the lowest relative T1 values, and no variation between the samples. The T1 values for the anion follow similar behavior to that observed in the diffusion results, to be discussed later. The two solvent peaks show very similar and also the highest T1 values, which is behavior expected of mobile solvent molecules (the molecular reorientation time is too rapid to provide an efficient relaxation mechanism). The 7Li T1 values seem to be mostly independent of the concentration of the filler (T1 values being very similar for all FH samples), but they vary
Fig. 5. Temperature dependence of line-widths for (a) Sample 1, (b) Sample 6, (c) Sample 7 and (d) Sample 8, where m stand for 19F, k for 7Li and % for 1H.
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Fig. 6. Spin relaxation (T1) for the samples at room temperature; m representing the anion 19F, k 7Li, % solvent 1H, I 1H from CH3 group of PC and j polymer 19F.
between the different fillers, being lowest for the clay sample and highest for the FH samples, as summarized in Fig. 6. There is almost no temperature dependence of T1 observed for 7Li in the filler-containing samples, over the entire range of the measurements. This result could be an indication of a very broad T1 minimum and, therefore, suggests a very broad distribution of ionic motional correlation times. The filler-free sample and the reference exhibit some temperature dependence but no T1 minimum in the temperature range investigated, as shown in Fig. 7. The T1 results thus suggest that the presence of the filler leads to a heterogeneous cation environment, which in turn, yields a broad distribution of correlation times. There is some Arrhenius temperature dependence observed for the 19F anion T1 values, the corresponding activation energy values being largest
Fig. 7. Temperature dependence of the spin relaxation (T1) for 7Li; symbols representing: m Sample 1, k Sample 2, % Sample 3, I Sample 4, j Sample 5, I Sample 6, I Sample 7, I Sample 8 and ' the reference.
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Fig. 8. Temperature dependence of the spin relaxation (T1) for 19F; symbols representing: m Sample 1, k Sample 2, % Sample 3, I Sample 6, I Sample 7, I Sample 8 and ' the reference.
for the filler-free sample and decreasing with increasing amount of the FH filler, as displayed in Fig. 8. Because of the relatively weak 19F contribution from the polymer it is difficult to conclude anything about the polymer dynamics from the T1 results. 3.3. Diffusion measurements In general, it is possible to determine self-diffusion coefficients (D) corresponding to each peak for spectra that exhibit distinct peaks. The room temperature diffusion results are summarized in Fig. 9. The highest D’s are obtained for the protons, the solvent diffusing at the fastest rate. The D’s for 7Li and anion 19F generally follow the same trend, which in turn appears to be governed by the solvent mobility. However, there are some variations in the degree to which the behavior of all the diffusing species are coupled, depending on the amount and type of the filler present. With increasing
Fig. 9. Diffusion coefficients for the used samples at room temperature; symbols stand for m 19F, k 7Li and % 1H.
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amount of FH there is a larger difference between the D’s of the ions, where the fluorine diffusion is significantly restrained in samples with increased filler concentration. The BaTiO3 containing sample exhibits lowest D’s, for all three diffusing species (solvent, cation, and anion). The cation and anion exhibit nearly equal D’s in the liquid reference, the gel without fillers, and the clay filler sample. However, the other fillers appear to inhibit anion diffusion relative to cation diffusion. Sample 5 (7% FH) exhibits somewhat elevated D’s relative to other filler-containing materials, but this is attributed to partial phase separation visually observed for this sample. All samples exhibit approximately Arrhenius temperature dependence of the D-values (Table 2). Lithium shows very similar activation energies of the diffusion in all the samples. The lowest values of D are observed for the BaTiO3 sample, and there is in general only a slight decrease in D for the FH samples as compared with the filler-free sample, as shown in Fig. 10. An increased concentration of FH results in decreased anion Ds and lower activation energy values, as shown in Fig. 11. Some of the data from Figs. 10 and 11 are replotted for clarity in Fig. 12a, in which the filler-free and 10% FH samples are directly compared. Clearly, the anion diffusion is reduced in the filler-containing sample. On the other hand, the clay-containing sample exhibits nearly equal diffusion anion and cation D-values over the entire temperature range. The BaTiO3 and clay samples exhibit consistently lower ionic diffusivities and mobilities (7Li and 19F line widths). They are also the least conductive samples, as discussed below. Thus, there appears to be strong, and undesirable, interactions between the filler particles and the ions in the case of the BaTiO3 and clay fillers. 3.4. Conductivity measurements The ac conductivity results are plotted in Fig. 13 and show, similar to the diffusion measurements, the best results for the gel polymer without any filler. With increasing content of the FH filler the conductivity decreases. The conductivities follow Arrhenius behavior. Table 2 Activation energy (kJ/mol) for the temperature dependence of diffusion coefficients as obtained from the Arrhenius plots Sample number Ea (kJ/mol) for D of 1 3 4 5 6 7 8
9.9400 6.9800 7.8000 3.5100 9.5000 7.7000
19
F Ea (kJ/mol) for D of 7Li 7.2000 5.9400 6.1000 5.5000 5.9000 6.7000 7.0000
Fig. 10. Temperature dependence of diffusion coefficients for 7Li; symbols representing Sample 1 (m), Sample 2 (k), Sample 3 (%), Sample 4 (I), Sample 5 (j), Sample 6 (I), Sample 7 (I) and Sample 8 (I).
Fig. 11. Temperature dependence of diffusion coefficients for 19F; symbols representing Sample 1 (m), Sample 2 (k), Sample 3 (%), Sample 6 (I), Sample 7 (I) and Sample 8 (I).
It is instructive to compare the experimental conductivity values with the results calculated from diffusion coefficients via the Nernst/Einstein relation: lj
z2j F 2 RT
Dj
(3)
where lj is the contribution from species j with charge zj and diffusion coefficient Dj at temperature T. F is the Faraday constant, R the gas constant. When the conductivity values obtained by the ac measurements are compared with the calculated ones (Table 3), the calculated are usually higher [11]. This is caused by the different physical quantities measured by the two techniques, where in conductivity measurements only the charged species movement is measured, while the diffusion measurements register all moving species of the resonating species regardless of the charge state. With, for example, ion-pairs present in a sample, the
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Fig. 12. Temperature dependence for (a) Sample 1, (b) Sample 6 and (c) Sample 8; symbols representing k 7Li and m
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19
F.
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observed in the case of the pure gel electrolyte. Filler containing samples exhibit even wider discrepancy between calculated and measured conductivities (Fig. 14). It was expected that the filler material would decrease ion pairing and bring the measured and calculated values into closer agreement. That the opposite effect is observed suggests that the fillers do not, in fact, play a significant role in reducing ion pairing. Furthermore, it is speculated that the large observed discrepancy can be attributed to material heterogeneity in the filler-containing gels [12]. In particular, not all micron-scale ion diffusion processes lead to macroscopic ion transport in a heterogeneous conducting medium with many phase boundaries. Fig. 13. Conductivity values resulting from impedance measurements: symbols representing: Sample 1 (m), Sample 2 (k), Sample 3 (%), Sample 4 (I), Sample 5 (j), Sample 6 (I), Sample 7 (I) and Sample 8 (I).
Table 3 Molar ionic conductivity values for the diffusing species as calculated by the Nernst /Einstein equation Sample number
lF (S/cm mol)
lLi (S/cm mol)
s (S/cm)
1 2 3 4 5 6 7 8
1.30 0.98 1.05 0.80 1.19 0.73 0.34 0.68
1.30 1.20 1.01 1.17 1.47 1.07 0.51 0.74
2.6 2.18 2.06 1.97 2.66 1.80 0.85 1.42
calculated conductivity values will be higher than the measured ones because NMR is sensitive to the motion of the ion-pair while conductivity is not. This is in fact
4. Conclusions Gel electrolytes with nanoscopic fillers have superior mechanical properties as compared with gels without the filler. This is attributed in part, to the layered nature of the fillers employed in this investigation. The effect of the fillers on electrical properties is to decrease the conductivity by a modest amount. NMR relaxation measurements of filler-containing gels indicate that the Li environment is very heterogeneous. Diffusion results show that both anionic and cationic mobilities are impeded by the presence of the filler, but this effect is generally greater for the anions. The calculated conductivities obtained from the Nernst /Einstein equation are higher than the measured ones, which is expected. However, the larger differences observed for the filler containing samples cannot be explained by the ion pairing only, and are also attributed to material heterogeneity.
Acknowledgements This work was supported by grants from the US Air Force Office of Scientific Research.
References
Fig. 14. Measured (empty symbols) vs. calculated (filled symbols) conductivity values for Sample 1 (m), Sample 6 (j), Sample 7 (%) and Sample 8 (I).
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