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Single-ionic gel polymer electrolyte based on polyvinylidene fluoride and fluorine-containing ionomer
European Polymer Journal 40 (2004) 735–742 www.elsevier.com/locate/europolj
Single-ionic gel polymer electrolyte based on polyvinylidene fluoride and fluorine-containing ionomer Li-Ying Tian, Xiao-Bin Huang, Xiao-Zhen Tang
*
Institute of Polymer Materials, School of Chemistry and Chemical Technology, Shanghai Jiaotong University, Shanghai 200240, PR China Received 19 September 2003; received in revised form 9 November 2003; accepted 14 December 2003
Abstract The novel single-ionic conductive gel polymer electrolyte was prepared from polyvinylidene fluoride (PVDF), propylene glycol carbonate (PC) and a new fluorine-containing ionomer. Cation–carbonyl interaction behavior, morphology and ionic conductive properties of this gel polymer electrolyte were studied by infrared spectra analysis (IR), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and complex impedance analysis. The results showed that the fluorine-containing ionomer was miscible with both PVDF and PC, and that the carbonyl groups in the ionomer and PC could bond competitively with the cation. Both the content of fluorine-containing ionomer and the content of PC had a great effect on morphology and ionic conductive properties of the samples. For this new gel polymer electrolyte, an ionic conductivity of above 104 S cm1 at room temperature could be reached, and this electrolyte system was a single-ionic kind gel polymer electrolyte with the transport number of the sodium ion exceed 0.99 (tþ > 0:99). Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Gel polymer electrolyte; Polyvinylidene fluoride; Fluorine-containing ionomer; Ionic conductivity
1. Introduction There are many reports of high ionic conductivity gel polymer electrolytes (GPE) prepared from inorganic salts, organic additives and polymer matrices such as polyvinylidene fluoride (PVDF) [1–5]. While these types of GPEs were diionic electrolytes, for applications under a DC electric field, single-ionic conductors are required. Therefore, the prime concern in many works was the development of single-ionic type GPEs [6–8]. The initial perfluorosulfonic ionomer membrane prepared by DuPont, commercially called Nafion, was an excellent singleionic type polymer electrolyte material due to its high ionic conductivity and electrochemistry stability [9–11].
In this work, a series of single-ionic type gel polymer electrolytes were prepared from PVDF, propylene glycol carbonate (PC) and a novel fluorine-containing ionomer. In this ionomer, some polymer chain segments were similar to the structure of Nafion, whilst some other chain segments contained carbonyl groups, which could contribute good miscibility with both PVDF and PC. The complexation between the cation and carbonyl, morphology and ionic conductive property of these gel electrolytes were studied in detail.
2. Experimental 2.1. Chemicals
*
Corresponding author. Tel.: +86-21-54747142; fax: +86-2154743264. E-mail address: [email protected] (X.-Z. Tang).
Vinyl acetate (VA), C.P. grade, Shanghai No. 2 Chemical Factory, PR China, was distilled before use. Propylene glycol carbonate (PC), C.P. grade, Shanghai
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.12.006
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No. 2 Chemical Factory, PR China, was treated with CaH2 and then distilled under vacuum. Perfluoro(5methyl-3,6-dioxo-8-fluoride sulfonyl-1-octene) (PSVE), Shanghai 3F new materials Co., Ltd., purity >99.8% and Polyvinylidene fluoride (PVDF), Shanghai 3F new materials Co., Ltd., FR921, M w ¼ 900; 000, Xc ¼ 30% were used as received. All solvents were purified before use. 2.2. Synthesis of fluorine-containing ionomer A mixture of 50 g PSVE and 50 ml (15%) NaOH aqueous solution was stirred for 5 min, and then the reaction was stopped. A clear solution was obtained after the precipitated NaF was filtered, and the PH value of the solution was 7. The reaction between PSVE and NaOH was an exothermic reaction. It was proved by repeated experiments that, when the mole ratio of PSVE:NaOH was in the range of 1:1–2, all the NaOH would participate the reaction and would be turned into –SO3 Na and NaF, and the reacted system was always neutral. In this procedure, the group of SO2 F in PSVE was changed into SO3 Na, and defined as PSVE-Na. After that, 8.6 g VA was added; the solution was separated into two layers because VA could not dissolve in water. The solution was heated under reflux and 0.05 g (NH4 )2 S2 O8 in 1.0 ml water was added dropwise. The mixture was stirred under reflux for a further 5 h. The reaction was cooled down to room temperature and a clear solution was produced. The water was removed by distillation and the copolymer of VA and PSVE-Na was
CF2
NaOH
CF
H 2O
O CF2 CF
obtained and defined as VA-PSVE-Na. It was further dried at 60 °C under high vacuum for 24 h. The synthesis route was showed in Fig. 1. 2.3. Preparation of GPE membranes PVDF, PC and copolymer (VA-PSVE-Na) were dissolved in DMF, and stirred continuously until the mixture became a homogenous viscous liquid, after which the mixed solution was cast into an aluminum plate, and putted in an oven at 60 °C for 24 h. For simplicity, the electrolyte membranes were hereafter indicated by writing in the sequence of polymer, PC and the ionomer with the notation PVDF/PC/VA-PSVENa ¼ 1.5/0.5/1.5. The composition of the membranes was (in weight ratio) 1.5:0.5:1.5 (PVDF/PC/VA-PSVENa) respectively (Table 1). 2.4. Characterization Infrared spectra analysis was carried out on a Perkin–Elmer 963 FT-IR analyzer in the range of 400– 4000 cm1 . 13 C NMR was obtained using a Varian Mercury 400 spectrometer. Differential scanning calorimetry (DSC) data were collected on a Perkin–Elmer Pyris-1 differential scanning calorimeter using a heating rate of 20 °C/min from )70 °C to 200 °C under nitrogen. Scanning electron microscopy (SEM) was carried out on a HITACHI S2150 scanning electron microscope, and the images were taken at low vacuum (10 Pa) after sputtering a thin gold film on the sample. Complex
CF2
CF + NaF + HF O CF2
CF3
CF
CF3
O
O
CF2
CF2
CF2
CF2
SO2F
SO3Na
(NH4)2S2O8 CH2
CH
+
CF2
CF
O
O
C O
CF2
CH3
CF
CH2
CF3
CH n O
CF2
CF
m
O
C O
CF2
CH3
CF
CF3
O
O
CF2
CF2
CF2
CF2
SO3Na
SO3Na
Fig. 1. Synthesis route of VA-PSVE-Na (n:m ¼ 5:1).
L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
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Table 1 Component of PVDF/PC/VA-PSVE-Na GPEs Samples
PVDF/g
PC/g
VA-PSVE-Na/g
PVDF/VA-PSVE-Na ¼ 1.5/1.5 PC/VA-PSVE-Na ¼ 1.5/3.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/0.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/1.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/2.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/2.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/3.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/0.5/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.5/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/2.0/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/2.5/1.5
1.4982 0 1.4995 1.4997 1.5007 1.5003 1.4997 1.4983 1.5086 1.5041 1.5022 1.4914
0 1.5005 1.0130 1.0236 1.0043 0.9997 1.0051 1.0021 0.4990 1.5012 2.0012 2.4991
1.4930 3.0001 0.5002 1.0046 1.5069 2.0004 2.5050 3.0003 1.5028 1.4926 1.5001 1.4976
a
Transmittance / %
impedance analysis was carried out on SI 1260 Impedance/Gain-phase analyzer in the frequency range of 5–13 MHz and the temperature range from 25 to 100 °C, and the testing system was a cell of SSjgel electrolytejSS. RB could be obtained according to Cole–Cole plot, then ionic conductivity could be obtained from the equation r ¼ ð1=RB Þ d=s, where d and s were thickness and area of the sample. The polarization reversion experiment was measured using the same equipment as the complex impedance analysis with a polarization voltage of )3 V. After 1 h, the voltage was changed to +3 V and the current was measured.
b
c d
2000
1600
1200
800
Wave number / cm-1
3. Results and discussion 3.1. IR analysis
Fig. 2. Infrared spectrum of (a) PVAc, (b) VA-PSVE-Na, (c) PC, (d) PC/VA-PSVE-Na ¼ 1.5/1.5.
Infrared spectra of poly vinyl acetate (PVAc), VA-PSVE-Na, PC and VA-PSVE-Na/PC are shown in Fig. 2. Infrared spectra of PVAc (2a) and VA-PSVE-Na (2b) show that the peak at 1672 cm1 is due to carbonyl bonded with sodium ions of VA-PSVE-Na, and the peak at 1740 cm1 is due to free carbonyl of VA-PSVE-Na. Infrared spectra of PC (2c) show that the peak at about 1800 cm1 is due to free carbonyl of PC. The spectrum of VA-PSVE-Na/PC (2d) reveals the absorption at 1800 cm1 (free carbonyl of PC) and a shoulder peak around 1740 cm1 (free carbonyl of VA-PSVE-Na) [12,13]. In fact, with both carbonyl groups and sodium sulfonate groups in the side chains, VA-PSVE-Na shows good miscibility with PC. Comparing 2d to 2b, it is clearly shown that when PC is added into VA-PSVE-Na matrix, the interaction between the sodium ions and the carbonyl groups of the VA-PSVE-Na side chain is destroyed somewhat. This results in the weakening of the absorption peak at about 1672 cm1 . Fig. 3 shows infrared spectra of carbonyl groups with different VA-PSVE-Na content in the samples. With the
VA-PSVE-Na content in the complex increasing, the peak at 1672 cm1 assigned to the ‘‘bonded’’ carbonyl group of VA-PSVE-Na side chain increase in intensity, which meant that the interaction between the sodium ions and the carbonyl groups of VA-PSVE-Na became stronger with the increment of VA-PSVE-Na content. The effects of PC content on the spectra of the carbonyl groups are shown in Fig. 4. With increasing PC content, the peak at 1672 cm1 get weaker, while the peak around 1800 cm1 became stronger. This meant that with the increase of PC content, carbonyl bonded with sodium ions of VA-PSVE-Na is destroyed. Fig. 5 shows the SO 3 (asymmetric stretching) spectra of VA-PSVE-Na, PVDF/VA-PSVE-Na and PC/ VA-PSVE-Na. It can be seen that 5a and 5b are generally consistent. This result illustrates that the addition of the matrix polymer PVDF have no effect on the ion cluster structure of VA-PSVE-Na. Also, it is found that spectral band of SO 3 in 5c became broader and shift to lower wave number. This result can be explained
L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
Absorptivity / %
738
because in the sample of PC/VA-PSVE-Na the complexion between the carbonyl group in PC and the Naþ in VA-PSVE-Na weak the bonding force between the Naþ cation and the SO 3 anion. Therefore, the S–O group’s dipole polarization extent in the PC/VA-PSVENa sample is smaller than in the VA-PSVE-Na. In addition to this phenomenon, the absorption peak of 5c shifts to a lower wave number.
d c b a
1900
1800 1700 -1 Wavenumber / cm
1600
Absorptivity / %
Fig. 3. Infrared spectra of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series (carbonyl). (a) x ¼ 0:5, (b) x ¼ 1:0, (c) x ¼ 2:0, (d) x ¼ 2:5.
d c
3.2.
13
C-NMR analysis
Fig. 6 shows the 13 C-NMR spectra of the samples of liquid state PC, solid state VA-PSVE-Na and gel state PC/VA-PSVE-Na. The chemical shift at 155.41 ppm is assigned to the C atom of the PC carbonyl (6a) and the resonance at 174.24 ppm belongs to the C atom of VA carbonyl (6b). Fig. 6c, the complex of PC/VA-PSVE-Na, show the characteristic peak at 165.06 ppm (assigned to the C atom of VA carbonyl) and 157.10 ppm (C atom of PC carbonyl). Comparing 6c to 6a and 6b, it can be found that when the PC is mixed with VA-PSVE-Na, the C atom peak of the PC carbonyl shifts to the downfield, while the C atom peak of VA carbonyl shifts to the upfield. This result indicates that in the gel system the extensive interaction between the carbonyl of PC and the cation replace the interaction that existed within the structure of VA-PSVE-Na. This result is consistent with the FT-IR analysis.
b a 1900
1800 1700 Wavenumber / cm -1
1600
Fig. 4. Infrared spectra of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series (carbonyl). (a) y ¼ 0:5, (b) y ¼ 1:5, (c) y ¼ 2:0, (d) y ¼ 2:5:
3.3. DSC analysis Fig. 7 shows the DSC profiles of the PVDF/PC/ VA-PSVE-Na ¼ 1.5/1.0/x series. There is an endothermic peak between 150 and 175 °C, which is attributed to the melting of PVDF [14]. With the increase of VAPSVE-Na content, the endothermic peaks in the DSC
Transmittance / %
c
a b
b c
1100
a
1080
1060
1040
1020
1000
-1
Wave number / cm
Fig. 5. Infrared spectrum of (a) VA-PSVE-Na, (b)PVDF/ VA-PSVE-Na ¼ 1.5/1.5, (c) PC/VA-PSVE–Na(mas SO3 ).
200 180 160 140 120 100 80
60
40
20
0
δ / ppm
Fig. 6. 13 C-NMR spectra of (a) PC (liquid), (b) VA-PSVE-Na (solid), (c) PC/VA-PSVE-Na ¼ 1.5/3.0 (gel).
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d
d c b a
100
120
140
160
180
c
Heat Flow Endo Up / mW
Heat Flow Endo Up / mW
f e
b
a
-40
-20
o
Temperature/ C
0 Temperature/ °C
20
40
Fig. 7. DSC plots of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series: a ¼ 0:5, b ¼ 1:0, c ¼ 1:5, d ¼ 2:0, e ¼ 2:5, f ¼ 3:0.
Fig. 9. DSC plots of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series (low temperature): a ¼ 0:5, b ¼ 1:0, c ¼ 1:5, d ¼ 2:5.
curve shift to a lower temperature, revealing the presence of the VA-PSVE-Na influence on the crystallization of the PVDF, which is similar to the results of some blend systems reported before [15,16]. Depicted in Fig. 8 are the DSC plots for the PVDF/ PC/VA-PSVE-Na ¼ 1.5/y/1.5 series. The melting temperature kept decreasing with the increase in PC content, suggesting that the presence of PC also influenced the crystallization of PVDF. As expected, organic additives have great effect on the glass transition temperature of the polymer electrolytes. Fig. 9 shows that the sample’s Tg significantly decreases with the increase of PC content, which is attributed to the mobility increase of the polymer chain segment. The experimental results in Figs. 8 and 9 prove that the reorganization of the PVDF polymer chain happen due to the swelling by the solvent (PC).
3.4. SEM analysis SEM analysis can be used to study morphology structure of the GPE samples. Fig. 10 show the surface SEM images of the PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series films, where a macroscopic phase separation between the spherical polymer particles and void spaces can be noticed. Chemical structure of VA-PSVE-Na suggests that its fluorocarbon chain have good compatibility with PVDF. So, the spherical particles are attributed to the aggregation of PVDF and VA-PSVENa. The void spaces are certainly due to organic solvent (PC) [17]. When PC content is low, the GPE membranes phase separation is unclear. While, for the gel polymer electrolytes with higher PC contents, interconnected cavities are formed in which the electrolytes are free to flow as in a pure liquid phase.
3.5. Ionic conductivity
Heat Flow Endo Up/mW
e
d c
b a
100
120
140
160
180
Temperature/ °C
Fig. 8. DSC plots of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series (high temperature): a ¼ 0:5, b ¼ 1:0, c ¼ 1:5, d ¼ 2:0, e ¼ 2:5.
Ionic conductivity properties of the gel polymer electrolytes are given in Figs. 11–14. It can be found that an ambient ionic conductivity above 104 S cm1 can be reached for these new GPEs. Fig. 11 shows the effects of temperature on ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series. For many solid polymer electrolytes, both the dissociation of salts and the mobility of ions can be improved by the temperature, which results in an increase in ionic conductivity [18]. The ionic conductivity of the samples keeps increasing with the increase in temperature, as shown in Fig. 11. Coincide with results of many reference [19]. The effect of VA-PSVE-Na content on the ionic conductivity is shown in Fig. 12. Since VA-PSVE-Na acts as a polymeric salt in the GPEs, the number of
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L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
Fig. 10. SEM photos of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series.
-3.8
-3.8 -3.9
log (σ / S cm-1)
log (σ / S cm -1 )
-3.9 -4.0 -4.1 -4.2
-4.1
25 oC 40 oC 60 oC 80 oC 100 oC
-4.2 -4.3
-4.3 2.6
-4.0
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
1000 / ( T / K )
-4.4
0.5
1.0
1.5
2.0
2.5
Ionomer Content
Fig. 11. Effects of temperature on ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series.
Fig. 12. Effects of salt content on ionic conductivities of PVDF/ PC/VA-PSVE-Na ¼ 1.5/1.0/x series.
sodium ions definitely rises whilst the content of VAPSVE-Na increases, which results in the increase in the ionic conductivity. In this series, PC content is kept constant. When the content of VA-PSVE-Na increases to a certain high level, the dissociation of VA-PSVE-Na by PC would reach equilibrium, and then the number of sodium ions would reach a maximum, so that the increase in ionic conductivity slow. Fig. 13 shows the effects of temperature on the ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series. The ionic conductivities of this series keep
increasing with the increase in temperature, which is similar to the result of Fig. 11. From Fig. 14, it can be seen that the higher the PC content, the higher the ionic conductivity became. It also shows that when PC content is low, ionic conductivity increases rapidly, whilst when the PC content is high, the increase of ionic conductivity slow. In fact, the ionic conductivity properties of the gel electrolyte system are similar to those of a liquid electrolyte, and the ion transference mainly takes place in the liquid phase. Therefore, the increase of PC content is in favor of the
-3.2
0.07
-3.4
0.06
-3.6
0.05 I/mA
log (σ / S cm -1 )
L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
-3.8
0.04
-4.0
0.03
-4.2
0.02 0.01
-4.4 2.6
741
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
1000 / ( T / K )
Fig. 13. Effects of temperature on ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series: (j) y ¼ 0:5, (d) y ¼ 1:0, (N) y ¼ 1:5, (.) y ¼ 2:0, (r) y ¼ 2:5.
0.00
0
10
20
30 200 300 400 500 600 Time / min
Fig. 15. Current–time cure of electrolyte sample, after the polarity is reversed at 25 °C.
4. Conclusions -3.2
log (σ / S cm-1)
-3.4 -3.6 -3.8 o
25 C -4.0
o
40 C o
60 C
-4.2
o
80 C o
100 C
-4.4 0.5
1.0
1.5
2.0
2.5
PC content
Fig. 14. Effects of PC content on ionic conductivities of PVDF/ PC/VA-PSVE-Na ¼ 1.5/y/1.5 series.
mobility of the sodium ions, which can increase the ionic conductivity. However, when PC content is high enough, the dilution effect of PC on sodium ions would become more important, which go against ionic conductivity and result in slow increment of ionic conductivity. The method of polarity reversion is applied to determine the transport number of the gel polymer electrolyte samples in this paper. Fig. 15 shows the current–time cure of the electrolyte sample after the polarity is reversed at 25 °C. The transition time of the Naþ (sþ ) is 4.4 min, while after 600 min the current of the anion can not be detected. The transition time of the anion (s ) can be considered to be more P than 600 min. Then, from the equation: tþ ¼ ð1=si Þ= ð1=si Þ, the transport number of the sodium ion is calculated to exceed 0.99 (tþ > 0:99). This result manifests that the system is assigned as a single-ionic kind gel polymer electrolyte.
In this paper a novel fluorine-containing ionomer was synthesized. The gel electrolytes prepared from this new ionomer had good ionic conductors with high ionic conductivities of above 104 S cm1 at room temperature and the electrolyte samples were single-ionic kind gel polymer electrolytes with the transport number of the sodium ion exceed 0.99 (tþ > 0:99). This fluorinecontaining ionomer had a higher miscibility with PVDF and PC. The FT-IR analysis and NMR evidence suggested that not only the carbonyl groups in PC, but also the carbonyl groups of the ionomer could bond with sodium ions. The DSC and SEM analysis revealed that the GPEs were made of three phases: a polymer crystalline phase, an amorphous swollen phase and a liquid electrolyte phase. The investigation of the transport properties of the GPEs showed that both the content of the fluorine-containing ionomer and the content of PC had great effects on the ionic conductivity.
References [1] Saito Y, Capiglia C, Kataoka H, Yamamoto H, Ishikawa H. Conduction properties of PVDF-type polymer electrolytes with lithium salts, LiN(CF3 SO2 )2 and LiN(C2 F5 SO2 )2 . Solid State Ionics 2000;136–137:1161–6. [2] Boudin F, Andrieu X, Jehoulet C, Olsen II. Vinylidene fluoride––hexa fluoro propylene copolymers as hybrid electrolyte components for lithium batteries. J Power Sources 1999;81–82:804–7. [3] Song JY, Wang YY, Wan CC. Review of gel-type polymer electrolytes for lithium-ion batteries. J Power Sources 1999;77:183–97. [4] Periasamy P, Tatsumi K, Shikano M, Fujieda T, Saito Y. Studies on PVdF-based gel polymer electrolyte. J Power Sources 2000;88:269–73.
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Single-ionic gel polymer electrolyte based on polyvinylidene fluoride and fluorine-containing ionomer Li-Ying Tian, Xiao-Bin Huang, Xiao-Zhen Tang
*
Institute of Polymer Materials, School of Chemistry and Chemical Technology, Shanghai Jiaotong University, Shanghai 200240, PR China Received 19 September 2003; received in revised form 9 November 2003; accepted 14 December 2003
Abstract The novel single-ionic conductive gel polymer electrolyte was prepared from polyvinylidene fluoride (PVDF), propylene glycol carbonate (PC) and a new fluorine-containing ionomer. Cation–carbonyl interaction behavior, morphology and ionic conductive properties of this gel polymer electrolyte were studied by infrared spectra analysis (IR), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and complex impedance analysis. The results showed that the fluorine-containing ionomer was miscible with both PVDF and PC, and that the carbonyl groups in the ionomer and PC could bond competitively with the cation. Both the content of fluorine-containing ionomer and the content of PC had a great effect on morphology and ionic conductive properties of the samples. For this new gel polymer electrolyte, an ionic conductivity of above 104 S cm1 at room temperature could be reached, and this electrolyte system was a single-ionic kind gel polymer electrolyte with the transport number of the sodium ion exceed 0.99 (tþ > 0:99). Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Gel polymer electrolyte; Polyvinylidene fluoride; Fluorine-containing ionomer; Ionic conductivity
1. Introduction There are many reports of high ionic conductivity gel polymer electrolytes (GPE) prepared from inorganic salts, organic additives and polymer matrices such as polyvinylidene fluoride (PVDF) [1–5]. While these types of GPEs were diionic electrolytes, for applications under a DC electric field, single-ionic conductors are required. Therefore, the prime concern in many works was the development of single-ionic type GPEs [6–8]. The initial perfluorosulfonic ionomer membrane prepared by DuPont, commercially called Nafion, was an excellent singleionic type polymer electrolyte material due to its high ionic conductivity and electrochemistry stability [9–11].
In this work, a series of single-ionic type gel polymer electrolytes were prepared from PVDF, propylene glycol carbonate (PC) and a novel fluorine-containing ionomer. In this ionomer, some polymer chain segments were similar to the structure of Nafion, whilst some other chain segments contained carbonyl groups, which could contribute good miscibility with both PVDF and PC. The complexation between the cation and carbonyl, morphology and ionic conductive property of these gel electrolytes were studied in detail.
2. Experimental 2.1. Chemicals
*
Corresponding author. Tel.: +86-21-54747142; fax: +86-2154743264. E-mail address: [email protected] (X.-Z. Tang).
Vinyl acetate (VA), C.P. grade, Shanghai No. 2 Chemical Factory, PR China, was distilled before use. Propylene glycol carbonate (PC), C.P. grade, Shanghai
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.12.006
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No. 2 Chemical Factory, PR China, was treated with CaH2 and then distilled under vacuum. Perfluoro(5methyl-3,6-dioxo-8-fluoride sulfonyl-1-octene) (PSVE), Shanghai 3F new materials Co., Ltd., purity >99.8% and Polyvinylidene fluoride (PVDF), Shanghai 3F new materials Co., Ltd., FR921, M w ¼ 900; 000, Xc ¼ 30% were used as received. All solvents were purified before use. 2.2. Synthesis of fluorine-containing ionomer A mixture of 50 g PSVE and 50 ml (15%) NaOH aqueous solution was stirred for 5 min, and then the reaction was stopped. A clear solution was obtained after the precipitated NaF was filtered, and the PH value of the solution was 7. The reaction between PSVE and NaOH was an exothermic reaction. It was proved by repeated experiments that, when the mole ratio of PSVE:NaOH was in the range of 1:1–2, all the NaOH would participate the reaction and would be turned into –SO3 Na and NaF, and the reacted system was always neutral. In this procedure, the group of SO2 F in PSVE was changed into SO3 Na, and defined as PSVE-Na. After that, 8.6 g VA was added; the solution was separated into two layers because VA could not dissolve in water. The solution was heated under reflux and 0.05 g (NH4 )2 S2 O8 in 1.0 ml water was added dropwise. The mixture was stirred under reflux for a further 5 h. The reaction was cooled down to room temperature and a clear solution was produced. The water was removed by distillation and the copolymer of VA and PSVE-Na was
CF2
NaOH
CF
H 2O
O CF2 CF
obtained and defined as VA-PSVE-Na. It was further dried at 60 °C under high vacuum for 24 h. The synthesis route was showed in Fig. 1. 2.3. Preparation of GPE membranes PVDF, PC and copolymer (VA-PSVE-Na) were dissolved in DMF, and stirred continuously until the mixture became a homogenous viscous liquid, after which the mixed solution was cast into an aluminum plate, and putted in an oven at 60 °C for 24 h. For simplicity, the electrolyte membranes were hereafter indicated by writing in the sequence of polymer, PC and the ionomer with the notation PVDF/PC/VA-PSVENa ¼ 1.5/0.5/1.5. The composition of the membranes was (in weight ratio) 1.5:0.5:1.5 (PVDF/PC/VA-PSVENa) respectively (Table 1). 2.4. Characterization Infrared spectra analysis was carried out on a Perkin–Elmer 963 FT-IR analyzer in the range of 400– 4000 cm1 . 13 C NMR was obtained using a Varian Mercury 400 spectrometer. Differential scanning calorimetry (DSC) data were collected on a Perkin–Elmer Pyris-1 differential scanning calorimeter using a heating rate of 20 °C/min from )70 °C to 200 °C under nitrogen. Scanning electron microscopy (SEM) was carried out on a HITACHI S2150 scanning electron microscope, and the images were taken at low vacuum (10 Pa) after sputtering a thin gold film on the sample. Complex
CF2
CF + NaF + HF O CF2
CF3
CF
CF3
O
O
CF2
CF2
CF2
CF2
SO2F
SO3Na
(NH4)2S2O8 CH2
CH
+
CF2
CF
O
O
C O
CF2
CH3
CF
CH2
CF3
CH n O
CF2
CF
m
O
C O
CF2
CH3
CF
CF3
O
O
CF2
CF2
CF2
CF2
SO3Na
SO3Na
Fig. 1. Synthesis route of VA-PSVE-Na (n:m ¼ 5:1).
L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
737
Table 1 Component of PVDF/PC/VA-PSVE-Na GPEs Samples
PVDF/g
PC/g
VA-PSVE-Na/g
PVDF/VA-PSVE-Na ¼ 1.5/1.5 PC/VA-PSVE-Na ¼ 1.5/3.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/0.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/1.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/2.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/2.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/3.0 PVDF/PC/VA-PSVE-Na ¼ 1.5/0.5/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/1.5/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/2.0/1.5 PVDF/PC/VA-PSVE-Na ¼ 1.5/2.5/1.5
1.4982 0 1.4995 1.4997 1.5007 1.5003 1.4997 1.4983 1.5086 1.5041 1.5022 1.4914
0 1.5005 1.0130 1.0236 1.0043 0.9997 1.0051 1.0021 0.4990 1.5012 2.0012 2.4991
1.4930 3.0001 0.5002 1.0046 1.5069 2.0004 2.5050 3.0003 1.5028 1.4926 1.5001 1.4976
a
Transmittance / %
impedance analysis was carried out on SI 1260 Impedance/Gain-phase analyzer in the frequency range of 5–13 MHz and the temperature range from 25 to 100 °C, and the testing system was a cell of SSjgel electrolytejSS. RB could be obtained according to Cole–Cole plot, then ionic conductivity could be obtained from the equation r ¼ ð1=RB Þ d=s, where d and s were thickness and area of the sample. The polarization reversion experiment was measured using the same equipment as the complex impedance analysis with a polarization voltage of )3 V. After 1 h, the voltage was changed to +3 V and the current was measured.
b
c d
2000
1600
1200
800
Wave number / cm-1
3. Results and discussion 3.1. IR analysis
Fig. 2. Infrared spectrum of (a) PVAc, (b) VA-PSVE-Na, (c) PC, (d) PC/VA-PSVE-Na ¼ 1.5/1.5.
Infrared spectra of poly vinyl acetate (PVAc), VA-PSVE-Na, PC and VA-PSVE-Na/PC are shown in Fig. 2. Infrared spectra of PVAc (2a) and VA-PSVE-Na (2b) show that the peak at 1672 cm1 is due to carbonyl bonded with sodium ions of VA-PSVE-Na, and the peak at 1740 cm1 is due to free carbonyl of VA-PSVE-Na. Infrared spectra of PC (2c) show that the peak at about 1800 cm1 is due to free carbonyl of PC. The spectrum of VA-PSVE-Na/PC (2d) reveals the absorption at 1800 cm1 (free carbonyl of PC) and a shoulder peak around 1740 cm1 (free carbonyl of VA-PSVE-Na) [12,13]. In fact, with both carbonyl groups and sodium sulfonate groups in the side chains, VA-PSVE-Na shows good miscibility with PC. Comparing 2d to 2b, it is clearly shown that when PC is added into VA-PSVE-Na matrix, the interaction between the sodium ions and the carbonyl groups of the VA-PSVE-Na side chain is destroyed somewhat. This results in the weakening of the absorption peak at about 1672 cm1 . Fig. 3 shows infrared spectra of carbonyl groups with different VA-PSVE-Na content in the samples. With the
VA-PSVE-Na content in the complex increasing, the peak at 1672 cm1 assigned to the ‘‘bonded’’ carbonyl group of VA-PSVE-Na side chain increase in intensity, which meant that the interaction between the sodium ions and the carbonyl groups of VA-PSVE-Na became stronger with the increment of VA-PSVE-Na content. The effects of PC content on the spectra of the carbonyl groups are shown in Fig. 4. With increasing PC content, the peak at 1672 cm1 get weaker, while the peak around 1800 cm1 became stronger. This meant that with the increase of PC content, carbonyl bonded with sodium ions of VA-PSVE-Na is destroyed. Fig. 5 shows the SO 3 (asymmetric stretching) spectra of VA-PSVE-Na, PVDF/VA-PSVE-Na and PC/ VA-PSVE-Na. It can be seen that 5a and 5b are generally consistent. This result illustrates that the addition of the matrix polymer PVDF have no effect on the ion cluster structure of VA-PSVE-Na. Also, it is found that spectral band of SO 3 in 5c became broader and shift to lower wave number. This result can be explained
L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
Absorptivity / %
738
because in the sample of PC/VA-PSVE-Na the complexion between the carbonyl group in PC and the Naþ in VA-PSVE-Na weak the bonding force between the Naþ cation and the SO 3 anion. Therefore, the S–O group’s dipole polarization extent in the PC/VA-PSVENa sample is smaller than in the VA-PSVE-Na. In addition to this phenomenon, the absorption peak of 5c shifts to a lower wave number.
d c b a
1900
1800 1700 -1 Wavenumber / cm
1600
Absorptivity / %
Fig. 3. Infrared spectra of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series (carbonyl). (a) x ¼ 0:5, (b) x ¼ 1:0, (c) x ¼ 2:0, (d) x ¼ 2:5.
d c
3.2.
13
C-NMR analysis
Fig. 6 shows the 13 C-NMR spectra of the samples of liquid state PC, solid state VA-PSVE-Na and gel state PC/VA-PSVE-Na. The chemical shift at 155.41 ppm is assigned to the C atom of the PC carbonyl (6a) and the resonance at 174.24 ppm belongs to the C atom of VA carbonyl (6b). Fig. 6c, the complex of PC/VA-PSVE-Na, show the characteristic peak at 165.06 ppm (assigned to the C atom of VA carbonyl) and 157.10 ppm (C atom of PC carbonyl). Comparing 6c to 6a and 6b, it can be found that when the PC is mixed with VA-PSVE-Na, the C atom peak of the PC carbonyl shifts to the downfield, while the C atom peak of VA carbonyl shifts to the upfield. This result indicates that in the gel system the extensive interaction between the carbonyl of PC and the cation replace the interaction that existed within the structure of VA-PSVE-Na. This result is consistent with the FT-IR analysis.
b a 1900
1800 1700 Wavenumber / cm -1
1600
Fig. 4. Infrared spectra of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series (carbonyl). (a) y ¼ 0:5, (b) y ¼ 1:5, (c) y ¼ 2:0, (d) y ¼ 2:5:
3.3. DSC analysis Fig. 7 shows the DSC profiles of the PVDF/PC/ VA-PSVE-Na ¼ 1.5/1.0/x series. There is an endothermic peak between 150 and 175 °C, which is attributed to the melting of PVDF [14]. With the increase of VAPSVE-Na content, the endothermic peaks in the DSC
Transmittance / %
c
a b
b c
1100
a
1080
1060
1040
1020
1000
-1
Wave number / cm
Fig. 5. Infrared spectrum of (a) VA-PSVE-Na, (b)PVDF/ VA-PSVE-Na ¼ 1.5/1.5, (c) PC/VA-PSVE–Na(mas SO3 ).
200 180 160 140 120 100 80
60
40
20
0
δ / ppm
Fig. 6. 13 C-NMR spectra of (a) PC (liquid), (b) VA-PSVE-Na (solid), (c) PC/VA-PSVE-Na ¼ 1.5/3.0 (gel).
L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
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d
d c b a
100
120
140
160
180
c
Heat Flow Endo Up / mW
Heat Flow Endo Up / mW
f e
b
a
-40
-20
o
Temperature/ C
0 Temperature/ °C
20
40
Fig. 7. DSC plots of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series: a ¼ 0:5, b ¼ 1:0, c ¼ 1:5, d ¼ 2:0, e ¼ 2:5, f ¼ 3:0.
Fig. 9. DSC plots of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series (low temperature): a ¼ 0:5, b ¼ 1:0, c ¼ 1:5, d ¼ 2:5.
curve shift to a lower temperature, revealing the presence of the VA-PSVE-Na influence on the crystallization of the PVDF, which is similar to the results of some blend systems reported before [15,16]. Depicted in Fig. 8 are the DSC plots for the PVDF/ PC/VA-PSVE-Na ¼ 1.5/y/1.5 series. The melting temperature kept decreasing with the increase in PC content, suggesting that the presence of PC also influenced the crystallization of PVDF. As expected, organic additives have great effect on the glass transition temperature of the polymer electrolytes. Fig. 9 shows that the sample’s Tg significantly decreases with the increase of PC content, which is attributed to the mobility increase of the polymer chain segment. The experimental results in Figs. 8 and 9 prove that the reorganization of the PVDF polymer chain happen due to the swelling by the solvent (PC).
3.4. SEM analysis SEM analysis can be used to study morphology structure of the GPE samples. Fig. 10 show the surface SEM images of the PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series films, where a macroscopic phase separation between the spherical polymer particles and void spaces can be noticed. Chemical structure of VA-PSVE-Na suggests that its fluorocarbon chain have good compatibility with PVDF. So, the spherical particles are attributed to the aggregation of PVDF and VA-PSVENa. The void spaces are certainly due to organic solvent (PC) [17]. When PC content is low, the GPE membranes phase separation is unclear. While, for the gel polymer electrolytes with higher PC contents, interconnected cavities are formed in which the electrolytes are free to flow as in a pure liquid phase.
3.5. Ionic conductivity
Heat Flow Endo Up/mW
e
d c
b a
100
120
140
160
180
Temperature/ °C
Fig. 8. DSC plots of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series (high temperature): a ¼ 0:5, b ¼ 1:0, c ¼ 1:5, d ¼ 2:0, e ¼ 2:5.
Ionic conductivity properties of the gel polymer electrolytes are given in Figs. 11–14. It can be found that an ambient ionic conductivity above 104 S cm1 can be reached for these new GPEs. Fig. 11 shows the effects of temperature on ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series. For many solid polymer electrolytes, both the dissociation of salts and the mobility of ions can be improved by the temperature, which results in an increase in ionic conductivity [18]. The ionic conductivity of the samples keeps increasing with the increase in temperature, as shown in Fig. 11. Coincide with results of many reference [19]. The effect of VA-PSVE-Na content on the ionic conductivity is shown in Fig. 12. Since VA-PSVE-Na acts as a polymeric salt in the GPEs, the number of
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L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
Fig. 10. SEM photos of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series.
-3.8
-3.8 -3.9
log (σ / S cm-1)
log (σ / S cm -1 )
-3.9 -4.0 -4.1 -4.2
-4.1
25 oC 40 oC 60 oC 80 oC 100 oC
-4.2 -4.3
-4.3 2.6
-4.0
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
1000 / ( T / K )
-4.4
0.5
1.0
1.5
2.0
2.5
Ionomer Content
Fig. 11. Effects of temperature on ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/1.0/x series.
Fig. 12. Effects of salt content on ionic conductivities of PVDF/ PC/VA-PSVE-Na ¼ 1.5/1.0/x series.
sodium ions definitely rises whilst the content of VAPSVE-Na increases, which results in the increase in the ionic conductivity. In this series, PC content is kept constant. When the content of VA-PSVE-Na increases to a certain high level, the dissociation of VA-PSVE-Na by PC would reach equilibrium, and then the number of sodium ions would reach a maximum, so that the increase in ionic conductivity slow. Fig. 13 shows the effects of temperature on the ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series. The ionic conductivities of this series keep
increasing with the increase in temperature, which is similar to the result of Fig. 11. From Fig. 14, it can be seen that the higher the PC content, the higher the ionic conductivity became. It also shows that when PC content is low, ionic conductivity increases rapidly, whilst when the PC content is high, the increase of ionic conductivity slow. In fact, the ionic conductivity properties of the gel electrolyte system are similar to those of a liquid electrolyte, and the ion transference mainly takes place in the liquid phase. Therefore, the increase of PC content is in favor of the
-3.2
0.07
-3.4
0.06
-3.6
0.05 I/mA
log (σ / S cm -1 )
L.-Y. Tian et al. / European Polymer Journal 40 (2004) 735–742
-3.8
0.04
-4.0
0.03
-4.2
0.02 0.01
-4.4 2.6
741
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
1000 / ( T / K )
Fig. 13. Effects of temperature on ionic conductivities of PVDF/PC/VA-PSVE-Na ¼ 1.5/y/1.5 series: (j) y ¼ 0:5, (d) y ¼ 1:0, (N) y ¼ 1:5, (.) y ¼ 2:0, (r) y ¼ 2:5.
0.00
0
10
20
30 200 300 400 500 600 Time / min
Fig. 15. Current–time cure of electrolyte sample, after the polarity is reversed at 25 °C.
4. Conclusions -3.2
log (σ / S cm-1)
-3.4 -3.6 -3.8 o
25 C -4.0
o
40 C o
60 C
-4.2
o
80 C o
100 C
-4.4 0.5
1.0
1.5
2.0
2.5
PC content
Fig. 14. Effects of PC content on ionic conductivities of PVDF/ PC/VA-PSVE-Na ¼ 1.5/y/1.5 series.
mobility of the sodium ions, which can increase the ionic conductivity. However, when PC content is high enough, the dilution effect of PC on sodium ions would become more important, which go against ionic conductivity and result in slow increment of ionic conductivity. The method of polarity reversion is applied to determine the transport number of the gel polymer electrolyte samples in this paper. Fig. 15 shows the current–time cure of the electrolyte sample after the polarity is reversed at 25 °C. The transition time of the Naþ (sþ ) is 4.4 min, while after 600 min the current of the anion can not be detected. The transition time of the anion (s ) can be considered to be more P than 600 min. Then, from the equation: tþ ¼ ð1=si Þ= ð1=si Þ, the transport number of the sodium ion is calculated to exceed 0.99 (tþ > 0:99). This result manifests that the system is assigned as a single-ionic kind gel polymer electrolyte.
In this paper a novel fluorine-containing ionomer was synthesized. The gel electrolytes prepared from this new ionomer had good ionic conductors with high ionic conductivities of above 104 S cm1 at room temperature and the electrolyte samples were single-ionic kind gel polymer electrolytes with the transport number of the sodium ion exceed 0.99 (tþ > 0:99). This fluorinecontaining ionomer had a higher miscibility with PVDF and PC. The FT-IR analysis and NMR evidence suggested that not only the carbonyl groups in PC, but also the carbonyl groups of the ionomer could bond with sodium ions. The DSC and SEM analysis revealed that the GPEs were made of three phases: a polymer crystalline phase, an amorphous swollen phase and a liquid electrolyte phase. The investigation of the transport properties of the GPEs showed that both the content of the fluorine-containing ionomer and the content of PC had great effects on the ionic conductivity.
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