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ZSM-5 based composite microporous polymer electrolyte with novel pore configuration and ionic conductivity
Solid State Ionics 177 (2006) 709 – 713 www.elsevier.com/locate/ssi
PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte with novel pore configuration and ionic conductivity Jingyu Xi, Xinping Qiu ⁎, Liquan Chen Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China Received 15 December 2005; accepted 22 January 2006
Abstract A novel composite microporous polymer electrolyte based on poly(vinylidene fluoride), poly(ethylene oxide), and microporous molecular sieves ZSM-5 (denoted as PVDF–PEO/ZSM-5) was prepared by a simple phase inversion technique. PEO can obviously improve the pore configuration, such as pore size, porosity, and pore connectivity of PVDF-based microporous membranes, results in a high room temperature ionic conductivity. Microporous molecular sieves ZSM-5 can further improve the mechanical strength of PVDF–PEO blends and form special conducting pathway in PVDF–PEO matrix by absorb liquid electrolyte in its two-dimensional interconnect channels. The high room temperature ionic conductivity combined with good mechanical strength implies that PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte can be used as candidate electrolyte and/or separator material for high-performance rechargeable lithium batteries. © 2006 Elsevier B.V. All rights reserved. Keywords: Microporous polymer electrolyte; PVDF–PEO blends; ZSM-5; Pore configuration; Ionic transport
1. Introduction Rechargeable lithium batteries (RLBs) using polymer electrolytes instead of traditional liquid electrolytes, are well adapted to various geometries and are cost competitive and safe compared with lithium ion batteries (LIBs) [1]. At present, polymer electrolytes mainly include three kinds [2,3]: solid polymer electrolytes (SPEs) [4], gel polymer electrolytes (GPEs) [5], and microporous polymer electrolytes (MPEs) [6–10]. From the aspect of industrialization, MPEs have significant advantages due to its high ionic conductivity and excellent mechanical properties. In 1994, Bellcore developed the microporous membrane based on PVDF–HFP copolymer, which showed favorable ionic conductivity (∼1 mS cmP− 1P) at room temperature after soaking with liquid electrolyte [7]. However, the dibutyl phthalate (DBP) extraction step is inconvenient, since it increases the cost of the process and presents safety concerns related to handling of large volume of volatile solvents [7,8]. To overcome this shortcoming, the phase inversion method, for which a microporous polymer matrix can be prepared by casting ⁎ Corresponding author. Tel.: +86 10 62794235; fax: +86 10 62794234. E-mail address: [email protected] (X. Qiu). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.01.032
a polymer solution and evaporating the solvent and non-solvent in turn, was developed and has been used successfully to prepare PVDF–HFP copolymer based microporous polymer electrolytes [8]. In previous work [11,12], we prepared a novel MPE based on the blends of poly(vinylidene fluoride) and poly(ethylene oxide) (PVDF–PEO) by phase inversion technique, in which the addition of PEO can obviously improve the pore configuration, such as pore size, porosity, and pore connectivity of PVDFbased microporous membranes, and consequently, the room temperature ionic conductivity was greatly enhanced. To further improve the mechanical properties of above PVDF–PEO blends based MPE, microporous molecular sieves ZSM-5 was selected as the filler. The effect of ZSM-5 on pore configuration and ionic transport properties of PVDF–PEO/ZSM-5 composite MPE was discussed in this paper. 2. Experimental Chaput et al. have reported that PVDF and PEO performed immiscible polymer blends [13]. However, in our previous study [11,12], we found PEO, PVDF, and glycerin can form homogeneous solution in DMF at temperature higher than
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70 °C. On the contrary, when temperature is lower than 70 °C, phase separation can be observed. PVDF–PEO/ZSM-5 composite microporous membranes were prepared by a phase inversion technique. PVDF (Mw = 900,000, obtained from Shanghai San Ai New Material Co., Ltd.), PEO (Mw = 1,000,000, obtained from Shanghai Liansheng Chem. Tech. Co., Ltd.), and ZSM-5 (Si / Al = 25, obtained from Nankai University Catalyst Company) were dissolved in a mixture of DMF (solvent) and glycerin (nonsolvent) (V / V = 10 / 1). After strongly stirring for 3∼4 h at 80 °C, the resulting homogeneous solution were cast onto a glass plate, and then placed in an oven at 120 °C for 24 h. In the drying process, solvent (DMF) and non-solvent (glycerin) evaporated in turn, and the location of glycerin formed micro-pores. Finally, we obtained white freestanding membranes with thickness ranging from 100 to 150 μm. TG analysis confirmed that both solvent and non-solvent evaporated completely during above preparing process. Elemental analysis results show that the composite microporous membrane has the same composition (PVDF and PEO) as the starting mixture. The composite microporous membranes used in this study were denoted as PVDF– PEO/x%ZSM-5, in which the weight ratio of PEO to PVDF is 0.5. Porosity of PVDF–PEO/x%ZSM-5 membranes was tested by immersing the membrane in 1-butanol for 2 h and calculated using the following equation: Porosityð%Þ ¼ 100 ðwt −w0 Þ=qV where wt and w0 are the weight of the wet and dry membrane, respectively. V is the apparent volume of the membrane. ρ is the density of 1-butanol. Liquid electrolyte uptake of PVDF–PEO/x%ZSM-5 membranes was measured as a function of dipping time in 1 mol L− 1 LiClO4/PC solution and calculated as follows: Weight uptakeð%Þ ¼ 100 ðwt −w0 Þ=w0 where wt and w0 are the weight of the wet and dry membrane, respectively. Pore distribution and pore structure in the surface and bulk of PVDF–PEO/x%ZSM-5 composite microporous membranes were studied by scanning electron microscopy (SEM) using Hitachi S-2150 instrument with gold sputtered coated films. To observe the cross-section of the samples, the membranes were broken in liquid nitrogen. Mechanical strength of PVDF–PEO/x%ZSM-5 composite microporous membranes were measured from stress to strain
Table 1 Physical properties of pure PVDF membrane and PVDF–50%PEO blends membrane Sample___________ Uptake (%)
Porosity (%)
Conductivity Tensile modulus (MPa) (S cm− 1)
PVDF PVDF– 50%PEO
42.1 84.3
1.18 × 10− 5 1.96 × 10− 3
140 212
85.6 28.5
Fig. 1. Surface SEM images of pure PVDF membrane (a) and PVDF–PEO blends membranes (b). Inset shows cross-section SEM images of the sample.
tests using Shimadzu AGS-10KNG instrument with the tensile speed of 1 mm/min. Ionic conductivity was measured by AC impedance spectroscopy carried out in the 1 MHz∼1 Hz frequency range by using a Solartron 1260 Impedance/Gain-Phase Analyzer coupled with a Solartron 1287 Electrochemical Interface. 3. Results and discussion In previous work, we have found that an adequate amount of PEO can obviously enhance the ionic conductivity of PVDFbased MPE by improve the pore configuration, especially pore connectivity, of PVDF–PEO blends (Table 1 and Fig. 1). Unfortunately, the mechanical strength of PVDF–PEO blends is very low compared with pristine PVDF, due to its high porosity (Table 1). To conquer this shortage, ZSM-5 molecular sieves were doped into PVDF–PEO blends to form PVDF–PEO/ ZSM-5 composite membrane. Fig. 2 shows the surface SEM images of as-prepared PVDF– PEO/x%ZSM-5 membranes. It can be seen that the pore distribution on surface of the membrane becomes sparser (in other word, pore size increases) compared with PVDF–PEO membrane (Fig. 1b). However, ZSM-5 content has little influence on pore distribution. Some small clusters of ZSM-5 can be observed in micro-pores when the content of ZSM-5 is relative higher (Fig. 2c–d). From the cross-section SEM images
J. Xi et al. / Solid State Ionics 177 (2006) 709–713
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Fig. 2. Surface SEM images of PVDF–PEO/x%ZSM-5 composite microporous membranes: (a) x = 5; (b) x = 10; (c) x = 20; (d) x = 30.
(Fig. 3), we can find that the micro-pores in the bulk of PVDF– PEO/x%ZSM-5 membranes are well interconnected for all ZSM-5 content in this study. Fig. 4 displays the mechanical strength of PVDF–PEO/x% ZSM-5 composite microporous membranes as a function of ZSM-5 content. Tensile modulus of PVDF–PEO/x%ZSM-5 membranes first increases with the increasing of ZSM-5 content and reaches the maximum (52.3 MPa) when ZSM-5 content is 15%. This phenomenon can be attributed to the Lewis acid– base interactions between the ether O (Lewis base) of PEO chains and the Lewis acid sites on the surface of ZSM-5 (resulting from the periodic replacement of [AlO4]− for [SiO4] in the framework) particles [14–16]. When the content of ZSM5 further increases, tensile modulus of PVDF–PEO/x%ZSM-5 membranes decreases, this can be explained by the aggregating of ZSM-5 particles at high loading content (Fig. 2c–d). Fig. 5 shows the porosity and liquid electrolyte (1 mol L− 1 LiClO4/PC solution) weight uptake of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content. As can be seen from Fig. 5(a), the porosity of PVDF– PEO/x%ZSM-5 composite microporous membranes decreases with the addition of ZSM-5, agrees with the change of pore distribution on the surface of PVDF–PEO/x%ZSM-5 mem-
branes observed from SEM images, as shown in Figs. 1(b) and 2. Liquid electrolyte weight uptake of PVDF–PEO/x% ZSM-5 composite microporous membranes also decreases with the increasing of ZSM-5 (Fig. 5b) and shows nearly the same change tendency as porosity. Fig. 6 displays the room temperature (25 °C) ionic conductivity of PVDF–PEO/x%ZSM-5 composite microporous membranes (soaking with 1 mol L− 1 LiClO4/PC solution) as a function of ZSM-5 content. It is worth to note that ionic conductivity shows completely different change tendency compared with the change tendency of porosity and liquid electrolyte weight uptake, as shown in Fig. 5. Ionic conductivity first decreases with the addition of ZSM-5, due to the decreasing of porosity (liquid electrolyte weight uptake). Porosity (liquid electrolyte weight uptake) and ionic conductivity of PVDF–PEO/5%ZSM-5 is about 85.5% (86.8%) and 78% of pristine PVDF–PEO, respectively. On the contrary, ionic conductivity of PVDF–PEO/x%ZSM-5 membranes begin to increase when ZSM-5 content is higher than 5%, despite the decreasing of porosity (liquid electrolyte weight uptake). Fig. 7 displays the proposed schematic representation of pore configuration exists in pure PVDF membrane, PVDF–PEO blends membrane, and PVDF–PEO/ZSM-5 composite
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(a)
Porosity / %
90
80
70
60 0
5
10
15
20
25
30
25
30
ZSM-5 content / %
(b)
Fig. 3. Cross-section SEM images of PVDF–PEO/x%ZSM-5 composite microporous membranes: (a) x = 10; (b) x = 30.
membranes. For pure PVDF system, the interconnectivity of each pore is very poor, as shown in Figs. 1(a) and 7(a), results in a very low ionic conductivity (∼10− 5 S cm− 1). PVDF–PEO blends system shows well pore interconnectivity, as shown in Figs. 1(b) and 7(b), due to the hydrogen bond interactions between PEO and glycerin (non-solvent) during the membrane preparation process [11,12], and hence, ionic conductivity can be increased for about two magnitudes. For PVDF–PEO/ZSM5 composite system, ZSM-5 can highly disperse into PVDF– PEO matrix (see Figs. 2 and 3). Although the porosity and
Weight uptake / %
220
180
160
0
5
10
15
20
ZSM-5 content / % Fig. 5. (a) Porosity of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content. (b) Liquid electrolyte (1 mol L− 1 LiClO4/PC solution) weight uptake of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content.
60
2.0
Conductivity / mS cm-1
Tensile modulus / MPa
200
50
40
30
1.9 1.8 1.7 1.6 1.5 0
20 0
5
10
15
20
25
30
5
10
15
20
25
30
ZSM-5 content / %
ZSM-5 content / % Fig. 4. Mechanical strength of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content.
Fig. 6. Room temperature (25 °C) ionic conductivity of PVDF–PEO/x%ZSM-5 composite microporous membranes (soaking with 1 mol L− 1 LiClO4/PC solution) as a function of ZSM-5 content.
J. Xi et al. / Solid State Ionics 177 (2006) 709–713
(a)
713
(b)
PVDF matrix
PVDF-PEO matrix
Pore configuration
PVDF-PEO/ZSM-5 matrix
(c)
ZSM-5
Fig. 7. Schematic representation of pore configuration exists in PVDF (a), PVDF–PEO (b), and PVDF–PEO/ZSM-5 (c) microporous membranes.
liquid electrolyte weight uptake of PVDF–PEO/ZSM-5 membrane is lower than that of PVDF–PEO blends (see Fig. 5), pore configuration of PVDF–PEO/ZSM-5 membrane is nearly the same as PVDF–PEO blends (see Fig. 3). ZSM-5 molecular sieves, usually known as shape-selective catalyst in a great deal of catalysis fields [15], have two-dimensional interconnect channels. ZSM-5 can absorb liquid electrolyte into its channels and these liquid electrolyte at ZSM-5 particles may form a special conducting pathway in PVDF–PEO matrix when its content is high enough, as shown in Fig. 7(c). This proposed ionic transporting model agrees well with the change tendency of ionic conductivity of PVDF–PEO/ZSM-5 (Fig. 6). 4. Conclusion PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte was prepared by phase inversion technique. PEO can obviously improve the pore configuration of PVDF-based microporous membranes, and hence, the room temperature ionic conductivity was greatly enhanced. ZSM-5 molecular sieves can further improve the mechanical strength of PVDF– PEO blends. In addition, by absorb liquid electrolyte, ZSM-5 particles can form a special conducting pathway in PVDF–PEO matrix when its content is high enough, which can also improve the ionic conductivity. Our experiment results show that PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte can be used as candidate electrolyte and/or separator material for high-performance rechargeable lithium batteries due to its high ionic conductivity and good mechanical strength.
Acknowledgements This work was financially supported by the State Key Basic Research Program of China (2002CB211803). J. Xi thanks the China Postdoctoral Science Foundation for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J.-M. Tarascon, M. Armand, Nature 414 (2001) 359. W.H. Meyer, Adv. Mater. 10 (1998) 439. K. Murata, S. Izuchi, Y. Yoshihisa, Electrochim. Acta 45 (2000) 1501. F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nature 394 (1998) 456. J.Y. Song, Y.Y. Wang, C.C. Wan, J. Power Sources 77 (1999) 183. C.L. Cheng, C.C. Wan, Y.Y. Wang, Electrochem. Commun. 6 (2004) 531. J.-M. Tarascon, A.S. Gozdz, C. Schmutz, F. Shokoohi, P.C. Warren, Solid State Ionics 86–88 (1996) 49. A.D. Pasquier, P.C. Warren, D. Culver, A.S. Gozdz, G.G. Amatucci, J.-M. Tarascon, Solid State Ionics 135 (2000) 249. F. Boudin, X. Andrieu, C. Jehoulet, I.I. Olsen, J. Power Sources 81–82 (1999) 804. L. Sannier, R. Bouchet, L. Santinacci, S. Grugeon, J.-M. Tarascon, J. Electrochem. Soc. 151 (2004) A873. J. Li, J. Xi, Q. Song, X. Tang, Chin. Sci. Bull. 50 (2005) 368. J. Xi, X. Qiu, J. Li, X. Tang, W. Zhu, L. Chen. J. Power Sources (in press) doi:10.1016/j.jpowsour.2005.08.009. S. Chaput, C. Carrot, M. Castro, F. Prochazka, Rheol. Acta 43 (2004) 417. Ch. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, 5th ed., Elsevier, Amsterdam, 2001. A. Corma, Chem. Rev. 97 (1997) 2373. J. Xi, S. Miao, X. Tang, Macromolecules 37 (2004) 8592.
PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte with novel pore configuration and ionic conductivity Jingyu Xi, Xinping Qiu ⁎, Liquan Chen Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China Received 15 December 2005; accepted 22 January 2006
Abstract A novel composite microporous polymer electrolyte based on poly(vinylidene fluoride), poly(ethylene oxide), and microporous molecular sieves ZSM-5 (denoted as PVDF–PEO/ZSM-5) was prepared by a simple phase inversion technique. PEO can obviously improve the pore configuration, such as pore size, porosity, and pore connectivity of PVDF-based microporous membranes, results in a high room temperature ionic conductivity. Microporous molecular sieves ZSM-5 can further improve the mechanical strength of PVDF–PEO blends and form special conducting pathway in PVDF–PEO matrix by absorb liquid electrolyte in its two-dimensional interconnect channels. The high room temperature ionic conductivity combined with good mechanical strength implies that PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte can be used as candidate electrolyte and/or separator material for high-performance rechargeable lithium batteries. © 2006 Elsevier B.V. All rights reserved. Keywords: Microporous polymer electrolyte; PVDF–PEO blends; ZSM-5; Pore configuration; Ionic transport
1. Introduction Rechargeable lithium batteries (RLBs) using polymer electrolytes instead of traditional liquid electrolytes, are well adapted to various geometries and are cost competitive and safe compared with lithium ion batteries (LIBs) [1]. At present, polymer electrolytes mainly include three kinds [2,3]: solid polymer electrolytes (SPEs) [4], gel polymer electrolytes (GPEs) [5], and microporous polymer electrolytes (MPEs) [6–10]. From the aspect of industrialization, MPEs have significant advantages due to its high ionic conductivity and excellent mechanical properties. In 1994, Bellcore developed the microporous membrane based on PVDF–HFP copolymer, which showed favorable ionic conductivity (∼1 mS cmP− 1P) at room temperature after soaking with liquid electrolyte [7]. However, the dibutyl phthalate (DBP) extraction step is inconvenient, since it increases the cost of the process and presents safety concerns related to handling of large volume of volatile solvents [7,8]. To overcome this shortcoming, the phase inversion method, for which a microporous polymer matrix can be prepared by casting ⁎ Corresponding author. Tel.: +86 10 62794235; fax: +86 10 62794234. E-mail address: [email protected] (X. Qiu). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.01.032
a polymer solution and evaporating the solvent and non-solvent in turn, was developed and has been used successfully to prepare PVDF–HFP copolymer based microporous polymer electrolytes [8]. In previous work [11,12], we prepared a novel MPE based on the blends of poly(vinylidene fluoride) and poly(ethylene oxide) (PVDF–PEO) by phase inversion technique, in which the addition of PEO can obviously improve the pore configuration, such as pore size, porosity, and pore connectivity of PVDFbased microporous membranes, and consequently, the room temperature ionic conductivity was greatly enhanced. To further improve the mechanical properties of above PVDF–PEO blends based MPE, microporous molecular sieves ZSM-5 was selected as the filler. The effect of ZSM-5 on pore configuration and ionic transport properties of PVDF–PEO/ZSM-5 composite MPE was discussed in this paper. 2. Experimental Chaput et al. have reported that PVDF and PEO performed immiscible polymer blends [13]. However, in our previous study [11,12], we found PEO, PVDF, and glycerin can form homogeneous solution in DMF at temperature higher than
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70 °C. On the contrary, when temperature is lower than 70 °C, phase separation can be observed. PVDF–PEO/ZSM-5 composite microporous membranes were prepared by a phase inversion technique. PVDF (Mw = 900,000, obtained from Shanghai San Ai New Material Co., Ltd.), PEO (Mw = 1,000,000, obtained from Shanghai Liansheng Chem. Tech. Co., Ltd.), and ZSM-5 (Si / Al = 25, obtained from Nankai University Catalyst Company) were dissolved in a mixture of DMF (solvent) and glycerin (nonsolvent) (V / V = 10 / 1). After strongly stirring for 3∼4 h at 80 °C, the resulting homogeneous solution were cast onto a glass plate, and then placed in an oven at 120 °C for 24 h. In the drying process, solvent (DMF) and non-solvent (glycerin) evaporated in turn, and the location of glycerin formed micro-pores. Finally, we obtained white freestanding membranes with thickness ranging from 100 to 150 μm. TG analysis confirmed that both solvent and non-solvent evaporated completely during above preparing process. Elemental analysis results show that the composite microporous membrane has the same composition (PVDF and PEO) as the starting mixture. The composite microporous membranes used in this study were denoted as PVDF– PEO/x%ZSM-5, in which the weight ratio of PEO to PVDF is 0.5. Porosity of PVDF–PEO/x%ZSM-5 membranes was tested by immersing the membrane in 1-butanol for 2 h and calculated using the following equation: Porosityð%Þ ¼ 100 ðwt −w0 Þ=qV where wt and w0 are the weight of the wet and dry membrane, respectively. V is the apparent volume of the membrane. ρ is the density of 1-butanol. Liquid electrolyte uptake of PVDF–PEO/x%ZSM-5 membranes was measured as a function of dipping time in 1 mol L− 1 LiClO4/PC solution and calculated as follows: Weight uptakeð%Þ ¼ 100 ðwt −w0 Þ=w0 where wt and w0 are the weight of the wet and dry membrane, respectively. Pore distribution and pore structure in the surface and bulk of PVDF–PEO/x%ZSM-5 composite microporous membranes were studied by scanning electron microscopy (SEM) using Hitachi S-2150 instrument with gold sputtered coated films. To observe the cross-section of the samples, the membranes were broken in liquid nitrogen. Mechanical strength of PVDF–PEO/x%ZSM-5 composite microporous membranes were measured from stress to strain
Table 1 Physical properties of pure PVDF membrane and PVDF–50%PEO blends membrane Sample___________ Uptake (%)
Porosity (%)
Conductivity Tensile modulus (MPa) (S cm− 1)
PVDF PVDF– 50%PEO
42.1 84.3
1.18 × 10− 5 1.96 × 10− 3
140 212
85.6 28.5
Fig. 1. Surface SEM images of pure PVDF membrane (a) and PVDF–PEO blends membranes (b). Inset shows cross-section SEM images of the sample.
tests using Shimadzu AGS-10KNG instrument with the tensile speed of 1 mm/min. Ionic conductivity was measured by AC impedance spectroscopy carried out in the 1 MHz∼1 Hz frequency range by using a Solartron 1260 Impedance/Gain-Phase Analyzer coupled with a Solartron 1287 Electrochemical Interface. 3. Results and discussion In previous work, we have found that an adequate amount of PEO can obviously enhance the ionic conductivity of PVDFbased MPE by improve the pore configuration, especially pore connectivity, of PVDF–PEO blends (Table 1 and Fig. 1). Unfortunately, the mechanical strength of PVDF–PEO blends is very low compared with pristine PVDF, due to its high porosity (Table 1). To conquer this shortage, ZSM-5 molecular sieves were doped into PVDF–PEO blends to form PVDF–PEO/ ZSM-5 composite membrane. Fig. 2 shows the surface SEM images of as-prepared PVDF– PEO/x%ZSM-5 membranes. It can be seen that the pore distribution on surface of the membrane becomes sparser (in other word, pore size increases) compared with PVDF–PEO membrane (Fig. 1b). However, ZSM-5 content has little influence on pore distribution. Some small clusters of ZSM-5 can be observed in micro-pores when the content of ZSM-5 is relative higher (Fig. 2c–d). From the cross-section SEM images
J. Xi et al. / Solid State Ionics 177 (2006) 709–713
711
Fig. 2. Surface SEM images of PVDF–PEO/x%ZSM-5 composite microporous membranes: (a) x = 5; (b) x = 10; (c) x = 20; (d) x = 30.
(Fig. 3), we can find that the micro-pores in the bulk of PVDF– PEO/x%ZSM-5 membranes are well interconnected for all ZSM-5 content in this study. Fig. 4 displays the mechanical strength of PVDF–PEO/x% ZSM-5 composite microporous membranes as a function of ZSM-5 content. Tensile modulus of PVDF–PEO/x%ZSM-5 membranes first increases with the increasing of ZSM-5 content and reaches the maximum (52.3 MPa) when ZSM-5 content is 15%. This phenomenon can be attributed to the Lewis acid– base interactions between the ether O (Lewis base) of PEO chains and the Lewis acid sites on the surface of ZSM-5 (resulting from the periodic replacement of [AlO4]− for [SiO4] in the framework) particles [14–16]. When the content of ZSM5 further increases, tensile modulus of PVDF–PEO/x%ZSM-5 membranes decreases, this can be explained by the aggregating of ZSM-5 particles at high loading content (Fig. 2c–d). Fig. 5 shows the porosity and liquid electrolyte (1 mol L− 1 LiClO4/PC solution) weight uptake of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content. As can be seen from Fig. 5(a), the porosity of PVDF– PEO/x%ZSM-5 composite microporous membranes decreases with the addition of ZSM-5, agrees with the change of pore distribution on the surface of PVDF–PEO/x%ZSM-5 mem-
branes observed from SEM images, as shown in Figs. 1(b) and 2. Liquid electrolyte weight uptake of PVDF–PEO/x% ZSM-5 composite microporous membranes also decreases with the increasing of ZSM-5 (Fig. 5b) and shows nearly the same change tendency as porosity. Fig. 6 displays the room temperature (25 °C) ionic conductivity of PVDF–PEO/x%ZSM-5 composite microporous membranes (soaking with 1 mol L− 1 LiClO4/PC solution) as a function of ZSM-5 content. It is worth to note that ionic conductivity shows completely different change tendency compared with the change tendency of porosity and liquid electrolyte weight uptake, as shown in Fig. 5. Ionic conductivity first decreases with the addition of ZSM-5, due to the decreasing of porosity (liquid electrolyte weight uptake). Porosity (liquid electrolyte weight uptake) and ionic conductivity of PVDF–PEO/5%ZSM-5 is about 85.5% (86.8%) and 78% of pristine PVDF–PEO, respectively. On the contrary, ionic conductivity of PVDF–PEO/x%ZSM-5 membranes begin to increase when ZSM-5 content is higher than 5%, despite the decreasing of porosity (liquid electrolyte weight uptake). Fig. 7 displays the proposed schematic representation of pore configuration exists in pure PVDF membrane, PVDF–PEO blends membrane, and PVDF–PEO/ZSM-5 composite
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(a)
Porosity / %
90
80
70
60 0
5
10
15
20
25
30
25
30
ZSM-5 content / %
(b)
Fig. 3. Cross-section SEM images of PVDF–PEO/x%ZSM-5 composite microporous membranes: (a) x = 10; (b) x = 30.
membranes. For pure PVDF system, the interconnectivity of each pore is very poor, as shown in Figs. 1(a) and 7(a), results in a very low ionic conductivity (∼10− 5 S cm− 1). PVDF–PEO blends system shows well pore interconnectivity, as shown in Figs. 1(b) and 7(b), due to the hydrogen bond interactions between PEO and glycerin (non-solvent) during the membrane preparation process [11,12], and hence, ionic conductivity can be increased for about two magnitudes. For PVDF–PEO/ZSM5 composite system, ZSM-5 can highly disperse into PVDF– PEO matrix (see Figs. 2 and 3). Although the porosity and
Weight uptake / %
220
180
160
0
5
10
15
20
ZSM-5 content / % Fig. 5. (a) Porosity of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content. (b) Liquid electrolyte (1 mol L− 1 LiClO4/PC solution) weight uptake of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content.
60
2.0
Conductivity / mS cm-1
Tensile modulus / MPa
200
50
40
30
1.9 1.8 1.7 1.6 1.5 0
20 0
5
10
15
20
25
30
5
10
15
20
25
30
ZSM-5 content / %
ZSM-5 content / % Fig. 4. Mechanical strength of PVDF–PEO/x%ZSM-5 composite microporous membranes as a function of ZSM-5 content.
Fig. 6. Room temperature (25 °C) ionic conductivity of PVDF–PEO/x%ZSM-5 composite microporous membranes (soaking with 1 mol L− 1 LiClO4/PC solution) as a function of ZSM-5 content.
J. Xi et al. / Solid State Ionics 177 (2006) 709–713
(a)
713
(b)
PVDF matrix
PVDF-PEO matrix
Pore configuration
PVDF-PEO/ZSM-5 matrix
(c)
ZSM-5
Fig. 7. Schematic representation of pore configuration exists in PVDF (a), PVDF–PEO (b), and PVDF–PEO/ZSM-5 (c) microporous membranes.
liquid electrolyte weight uptake of PVDF–PEO/ZSM-5 membrane is lower than that of PVDF–PEO blends (see Fig. 5), pore configuration of PVDF–PEO/ZSM-5 membrane is nearly the same as PVDF–PEO blends (see Fig. 3). ZSM-5 molecular sieves, usually known as shape-selective catalyst in a great deal of catalysis fields [15], have two-dimensional interconnect channels. ZSM-5 can absorb liquid electrolyte into its channels and these liquid electrolyte at ZSM-5 particles may form a special conducting pathway in PVDF–PEO matrix when its content is high enough, as shown in Fig. 7(c). This proposed ionic transporting model agrees well with the change tendency of ionic conductivity of PVDF–PEO/ZSM-5 (Fig. 6). 4. Conclusion PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte was prepared by phase inversion technique. PEO can obviously improve the pore configuration of PVDF-based microporous membranes, and hence, the room temperature ionic conductivity was greatly enhanced. ZSM-5 molecular sieves can further improve the mechanical strength of PVDF– PEO blends. In addition, by absorb liquid electrolyte, ZSM-5 particles can form a special conducting pathway in PVDF–PEO matrix when its content is high enough, which can also improve the ionic conductivity. Our experiment results show that PVDF–PEO/ZSM-5 based composite microporous polymer electrolyte can be used as candidate electrolyte and/or separator material for high-performance rechargeable lithium batteries due to its high ionic conductivity and good mechanical strength.
Acknowledgements This work was financially supported by the State Key Basic Research Program of China (2002CB211803). J. Xi thanks the China Postdoctoral Science Foundation for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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