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Microporous Polymeric Composite Electrolytes from Microemulsion Polymerization Wu Xu, Kok-Siong Siow,* Zhiqiang Gao, Swee-Yong Lee, Pei-Yong Chow, and Leong-Ming Gan*,† Department of Chemistry, National University of Singapore (NUS), 10 Kent Ridge Crescent, Republic of Singapore 119260, Singapore Received October 13, 1998. In Final Form: March 30, 1999 A new kind of microporous polymeric electrolytes was prepared from microemulsion polymerization of the system containing acrylonitrile (AN), 4-vinylbenzenesulfonic acid lithium salt (VBSLi), ethylene glycol dimethacrylate (EGDMA), ω-methoxy poly(ethyleneoxy)40 undecyl-R-methacrylate (C1-PEO-C11-MA40), and water. The polymerized-microemulsion solids or membranes have open-cell porous microstructures. The water content in membranes can readily be exchanged with many organic solvents such as γ-butyrolactone (BL), a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) or propylene carbonate (PC) and EC. The membranes can also be filled with electrolyte solutions such as 1 M LiBF4/BL, 1 M LiSO3CF3/PC-EC, or 1 M LiClO4/EC-DMC to form polymeric composite electrolytes. Such composite electrolytes, exhibiting ionic conductivities in the area of 10-3 S cm-1 (25 °C), may be used in many electrochemical devices.
Introduction Polymeric electrolytes have many advantages over inorganic electrolytes because of their lightweight, easy film formation, good contact with electrodes, and no leakage. They can be used in lithium/lithium ion rechargeable batteries, electrochromic display devices, sensors, and other electrochemical devices.1 In the last 2 decades, many kinds of polymeric electrolytes have been synthesized2-10 and those with a good ionic conductive characteristic, high cationic transference number, wide electrochemical stability window, and strong mechanical property have been successfully applied in lithium/lithium ion batteries.11,12 * To whom correspondence should be addressed. † NUS and Institute of Materials Research and Engineering (IMRE), Singapore. (1) Armand, M. B.; Chabagno, J. M.; Dulcot, M. In Extended Abstracts, The 2nd International Conference on Solid Electrolytes, St. Andrews, Scotland, Sept 1978. (2) MacCallum, J. R., Vincent, C. A., Eds. Polymer Electrolyte Reviews, Vol. 1 and 2; Elsevier Applied Science: London and New York, 1987 and 1989. (3) Gray, F. M., Ed. Solid Polymer Electrolytes: Fundamental and Technological Applications; VCH Publishers, New York, 1991. (4) Takeoka, S.; Ohno, H.; Tsuchida, E. Polym. Adv. Technol. 1992, 4, 53. (5) Greenbaum, S., Ed. The Fourth International Symposium on Polymer Electrolytes (ISPE-4), Newport, Rhode Island, June 1994; Electrochim. Acta 1995, 40 (13-14). (6) Farrington, G. C., Vincent, C. A., Eds. Proceedings of the Electronic Conference on Solid Electrolytes Science and Technology; (accessed on internet June 1995); Solid State Ionics 1996, 85. (7) Thomas, J., Ed. The Fifth International Symposium on Polymer Electrolytes (ISPE-5), Uppsala, Sweden, Aug 1996; Electrochim. Acta 1995, 43 (10-11). (8) Appetechi, G. B.; Dautzenberg, G.; Scrosati, B. J. Electrochem. Soc. 1996, 143, 6. (9) Jiang, J.; Carroll, B.; Abraham, K. M. Electrochim. Acta 1997, 42, 2667. (10) Xu, W.; Siow, K. S.; Gao, Z.; Lee, S. W. Chem. Mater. 1998, 10, 1951. (11) Alamgir, M.; Abraham, K. M. In Lithium Batteries, New Materials: Developments and Perspectives; Pistoia, G., Ed.; Elsevier: Amsterdam, 1994; Chapter 3, Vol. 5. (12) Megahed, S., Barnett, B. M., Xie, L., Eds. Rechargeable Lithium and Lithium-Ion Batteries, PV94-28; The Electrochemical Society Proceedings Series; Electrochemical Society: Pennington, NJ, 1994.
The porous polypropylene (PP) and polyethylene (PE) films are now widely used in the battery industry as segregation membranes. Both of them, however, cannot hold the electrolyte solutions because of their poor polarity. We report here a new type of porous polymeric composite electrolytes formed by microemulsion polymerization. The formation of porous polymeric membranes or materials by polymerizing microemulsions containing monomers is an area of current research interests.13-16 Microemulsions, in contrast to emulsions, are thermodynamic stable transparent isotropic liquids consisting of aqueous and oil phases stabilized by a surfactant or a combination of a surfactant and a cosurfactant. Depending on the composition of a ternary system of oil, water, and surfaceactive agent(s), the microstructure of a microemulsion may exist as water droplets dispersed in oil (w/o) at low water contents, oil droplets dispersed in water (o/w) at high water contents, or a bicontinuous microstructure where oil and water coexist at intermediate water contents. The formation and the characterization of microstructures derived from the polymerization of various microemulsions containing oil monomers, aqueous monomers, and nonpolymerizable or polymerizable surfactants have been greatly studied in recent years.17-31 We present here our latest study on microporous polymeric composite (13) Candau, F. In Polymerization in Organized Media, Paleos, C. M., Ed.; Gordon and Breach Science: Newark, NJ, 1992; pp 215-282. (14) Gan, L. M.; Chew, C. H. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp M43214331. (15) Puig, J. E. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp M4333-4341. (16) Candau, F.; Anquetil, J. Y. In Micelles, Microemulsions and Monolayers; Shah, D. O., Ed.; Marcel Dekker: New York, 1998; pp 193-213. (17) Qutubuddin, S.; Hague, E.; Benton, W. J.; Fendler, E. J. In Polymer Associated Structures: Microemulsions and Liquid Crystals; El-Nokaly, M. A., Ed.; ACS Symposium Series 384; American Chemical Society: Washington, DC, 1989; p 64. (18) Menger, F. M.; Tsuno, T.; Hammond, G. S. J. Am. Chem. Soc. 1990, 112, 1263. (19) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1993, 34, 3305. (20) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. J. Appl. Polym. Sci. 1993, 47, 499.
10.1021/la981430u CCC: $18.00 © 1999 American Chemical Society Published on Web 06/12/1999
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electrolytes from the microemulsion polymerization of 4-vinylbenzenesulfonic acid lithium salt (VBSLi), acrylonitrile (AN), and a polymerizable nonionic surfactant, ω-methoxy poly(ethyleneoxy)n undecyl-R-methacrylate (abbreviated as C1-PEO-C11-MA-40). Experimental Section Materials. Acrylonitrile (AN) and ethylene glycol dimethacrylate (EGDMA) from Merck were purified by distillation under reduced pressure immediately prior to use. Lithium salt of 4-vinylbenzenesulfonic acid (VBSLi) was obtained by acidification of 4-vinylbenzenesulfonic acid sodium salt (VBSNa, Fluka), followed by neutralization with lithium hydroxide (BDH) aqueous solution and removal of water under high vacuum. Ammonium persulfate (APS) from Fluka and N,N,N′,N′-tetramethylethylenediamine (TMEDA) from Aldrich were used as received. Water was purified by a Milli-Q water purification system with a resistivity of 18.2 MΩ cm. The polymerizable nonionic surfactant C1-PEO-C11-MA-40 was synthesized following the procedures described by Liu et al.32 Ethylene carbonate (EC, Fluka), propylene carbonate (PC, BDH), γ-butyrolactone (BL, Fluka), and dimethyl carbonate (DMC, Merck) were purified by refluxing with calcium oxide and then distilled under reduced pressure or at normal atmosphere. Lithium perchlorate (LiClO4, Fluka) and lithium trifluoromethane sulfonate (LiSO3CF3, Aldrich) were dried in a vacuum oven at 130 °C for 24 h before use. Lithium tetrafluoroborate (LiBF4, Aldrich) was used as received. These treated solvents and lithium salts were stored in a drybox filled with purified argon. Phase Behavior of Microemulsion Systems. The transparent regions of the microemulsion systems consisting of AN, VBSLi, EGDMA, C1-PEO-C11-MA-40, and water were determined at room temperature (26 °C). The molar ratio of AN to VBSLi was fixed at 9:1, and EGDMA added was 5 wt % based on the total weight of AN and VBSLi. Systematic titrations of each mixture of AN, VBSLi, EGDMA, and C1-PEO-C11-MA-40 were made by adding water to each culture tube (with a screw cap) to determine the boundary between clear and turbid regions. The variations of the ionic conductivity were monitored by a JENWAY 4330 conductivity and pH meter with a conductivity cell constant of 1.02 cm-1. The established microemulsion region is represented by the shaded area in Figure 1. Microemulsion Polymerization. The compositions of microemulsions were systematically chosen for polymerization to obtain membranes from compositions along line P in Figure 1 and those shown in Table 1. APS and TMEDA in an equal molar ratio were used together as a redox initiator at 0.5 wt % based on the total weight of the monomers used. The microemulsion compositions containing APS in test tubes were first purged with nitrogen for about 15 min and then TMEDA was injected into each microemulsion system with good mixing. The polymerization was carried out at room temperature (26 °C) in an anaerobic chamber filled with nitrogen. When the viscosity of the sample increased significantly, it was immediately spread evenly between (21) Gan, L. M.; Chieng, T. H.; Chew, C. H.; Ng, S. C. Langmuir 1994, 10, 4022. (22) Qutubuddin, S.; Lin, C. S.; Tajuddin, Y. Polymer 1994, 35, 4606. (23) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C. Polymer 1995, 36, 1941. (24) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1995, 36, 2637. (25) Gan, L. M.; Li, T. D.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1995, 11, 3316. (26) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Lee, L.; Ng, S. C.; Pey, K. L.; Grant, D. Langmuir 1995, 11, 3321. (27) Chew, C. H.; Li, T. D.; Gan, L. M.; Teo, W. K. J. Macromol. Sci., A: Pure Appl. Chem. 1995, A32 (2), 211. (28) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C.; Pey, K. L. Langmuir 1996, 12, 319. (29) Liu, J.; Chew, C. H.; Wang, S. Y.; Gan, L. M. J. Macromol. Sci., A: Pure Appl. Chem. 1996, A33, 1181. (30) Li, T. D.; Gan, L. M.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1996, 12, 5863. (31) Gan, L. M.; Liu, J.; Poon, L. P.; Chew, C. H.; Gan, L. H. Polymer 1997, 38, 5339. (32) Liu, J.; Chew, C. H.; Gan, L. M. J. Macromol. Sci., A: Pure Appl. Chem. 1996, A33, 337.
Figure 1. The partial phase diagram of the system containing AN/VBSLi/EGDMA/C1-PEO-C11-MA-40/water at 26 °C. The AN/VBSLi molar ratio was fixed at 9:1 and EGDMA was at 5.0 wt % based on the total weight of AN and VBSLi. The microemulsion region is represented by the shaded area. Table 1. Microemulsion Compositions Used for Preparing Membranesa compositions (wt %) sample no. 1 2 3 4 5
AN
appearance C1-PEOVBSLi EGDMA C11-MA-40 water BP AP
28.62 11.48 27.27 10.67 25.48 10.17 23.87 9.35 22.08 8.82
2.15 2.04 1.90 1.79 1.65
42.03 39.95 37.39 34.99 32.42
15.72 20.06 25.07 29.99 35.03
C C C C C
O Tl Tl Tl Tl
a The AN/VBSLi molar ratio was fixed at 9:1, EGDMA was added at 5.0 wt % based on the total weight of AN and VBSLi. BP, before polymerization; AP, after polymerization at 26 °C by redox initiation; C, clear; O, opaque; Tl, translucent.
two well-cleaned glass plates. The glass plates were assembled slowly to avoid trapping air bubbles. The glass plate assembly was left standing at room temperature overnight before putting it in an oven at 50 °C for 24 h. The membrane was removed from the glass plates, extracted with 95% ethanol for 24 h, and then dried in a vacuum oven at 100 °C for 48 h. The IR spectra of the polymerized solids were taken with a BIO-RAD FT-IR spectrophotometer. Elemental analyses were performed by using a Perkin-Elmer elemental analyzer 2400 series II for CHNS/O and a Thermal Jarrell Ash IRIS/AP for Li. Pore Continuity. The continuity of the pore structures of the polymer membranes after ethanol extraction was confirmed from the drying rate of water desorption from the polymerized materials, using a DuPont Instruments TGA 2960 thermogravimetric analyzer. The polymer sample was first swollen with water for overnight, then dried in a stream of dry nitrogen isothermally at 73 °C for 5 h, and heated to 103 °C at a temperature ramp of 2 °C min-1. It was kept at this temperature for 1 h to get the final weight of the polymer which was free from any moisture. Morphology Observation. The resulting polymerized solids before and after extraction with ethanol were first frozen in liquid nitrogen and fractured mechanically. They were then vacuumdried at room temperature for 24 h before coated with gold using a coating machine of BAL-TEC sputter coater SCD005. The morphologies of gold-coated samples were examined with a Philips XL30 scanning electron microscope (SEM). Swelling Equilibrium. The extracted polymer sample was cut into small pieces and dried to a constant weight in a vacuum oven at 100 °C. They were then swollen in water at room temperature until they reached an equilibrium. The equilibrium swelling of the polymer samples with organic solvents such as EC-DMC (1:1 by volume), BL and PC-EC (1:1 by volume), and electrolyte solutions such as 1 M LiClO4/EC-DMC, 1 M LiBF4/
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Figure 2. The conductivity variation of precursor microemulsions as a function of the water content. BL, and 1 M LiSO3CF3/PC-EC was also measured. These swelling processes were carried out at about 50 °C in a drybox filled with purified argon. The equilibrium solvent content (ESC) in the polymer sample is defined as a percentage by
ESC(%) ) (Ws - W0)/Ws × 100%
(1)
where Ws and W0 are the weight of the swollen polymer sample at equilibrium swelling and the dried polymer sample, respectively. Ionic Conductivity. The ionic conductivities of the polymer membranes in equilibrium swelling with water, organic solvents, or electrolyte solutions of lithium salts were determined at room temperature, using a Zahner Elektrik Impedance Measurement Unit (IM6). It was interfaced with a personal computer, over a frequency range from 1 MHz to 1 Hz. A 5 mV ac amplitude was used and 10 points/decade were taken.
Results and Discussion Characterization of Microemulsion Systems. Figure 1 shows the partial phase diagram of the system containing AN/VBSLi/EGDMA/C1-PEO-C11-MA-40/water. A narrow belt of the transparent microemulsion region (the shaded area) is obtained. The microemulsion region shifts to the region of higher surfactant concentrations. The conductivity measurement has been used to roughly locate the various regions of different types of these ionic microemulsions. It is known that water-in-oil (w/o) and oil-in-water (o/w) microemulsions show respectively a very low and a very high conductivity. However, the transition from a w/o to a bicontinuous microemulsion should exhibit a rather sharp increase in conductivity. The variation of conductivity of the microemulsion as a function of the water content is shown in Figure 2. The initially low conductivity increases slowly in the low water regions ( PC > BL > DMC. Although the addition of the low polar DMC into EC will lead the polarity of EC-DMC to decrease, the polarity of the mixture solvent EC-DMC may still be slightly higher than that of BL. The higher dielectric constant of the solvent would lead the membrane to have more free ion carriers. As a result, the polymerized-microemulsion membrane saturated with EC-PC shows a higher ionic conductivity than that with EC-DMC, and the latter has a slightly higher ionic conductivity than that with BL. On the other hand, the conductivities of the polymerizedmicroemulsion membranes saturated with electrolyte solutions decrease from 1 M LiClO4/EC-DMC to 1 M LiBF4/BL and to 1 M LiSO3CF3/PC-EC. It is attributed to the decrease of a different degree of dissociation of the three lithium salts in organic solvents. The degree of (39) Dean, J. A., Ed. Handbook of Organic Chemistry; McGraw-Hill: Singapore, 1987.
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dissociation of various lithium salts has been studied in nonaqueous aprotic solvents showing the following order: 40,41 LiClO > LiBF > LiSO CF . This is consistent with 4 4 3 3 the trend for the observed conductivities from the polymerized-microemulsion membranes containing lithium salts in these organic solvents. The magnitude of the ionic conductivity of about 10-3 S cm-1 at 25 °C for the polymerized-microemulsion membranes saturated with 1 M LiClO4/EC-DMC may be suitable for further development of their potential applications as polymeric electrolytes in electrochemical devices, such as lithium/lithium ion rechargeable batteries, electrochromic display devices, and sensors. The electrochemical properties of the composite polymeric electrolytes are currently under investigation. Conclusions Microporous polymeric electrolytes containing AN, VBSLi, EGDMA, and C1-PEO-C11-MA-40 have been prepared via microemulsion polymerization. The specific conductivity of the precursor microemulsions reduces markedly after polymerization by transforming liquid microemulsions to solid polymeric materials. However, the extent of the conductivity retention (σf/σi) of the systems before and after polymerization increases with the increase of the water content in precursor microemulsions. The polymerized-microemulsion solids exhibit open-cell microstructures. The ionic conductivities for the polymeric membranes as synthesized are about 10-3 S cm-1. As the water in the membranes is replaced by the polar organic solvents such as BL, PC-EC, or EC-DMC, the conductivities of the membranes decrease sharply by about 2 orders of magnitude. However, their conductivities can be increased or restored up to about 10-3 S cm-1 after soaking the membranes in the electrolyte solutions of 1 M LiSO3CF3/PC-EC, 1 M LiBF4/BL, or 1 M LiClO4/ECDMC. It shows that the water content in these microporous membranes can be freely exchanged by organic solvents or electrolyte solutions. These ionic conductive materials can be further developed as composite polymeric electrolytes for potential applications in many electrochemical devices. Acknowledgment. The authors are grateful to the National University of Singapore for financial support for this work. The authors appreciate Ms. Quek Chai Hoon for her help in taking the SEM micrographs. LA981430U (40) Webber, A. J. Electrochem. Soc. 1991, 138, 2585. (41) Ue, M. J. Electrochem. Soc. 1994, 141, 3336.
