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Gel polymer electrolytes for electric double layer capacitors
Solid State Ionics 113–115 (1998) 103–107
Gel polymer electrolytes for electric double layer capacitors a, a b b Y. Matsuda *, K. Inoue , H. Takeuchi , Y. Okuhama a
Faculty of Engineering and High Technology Research Center, Kansai University, Yamate-cho 3 -3 -35, Suita, Osaka 564, Japan b Daiwa Fine Chemicals Co. Ltd., Shimozawadori 2 -1 -17, Hyogoku, Kobe, 652, Japan
Abstract Gel polymer electrolytes consisted of polyvinylpyrrolidone (PVP) and PVP–polyvinylacetate (PVP–PVAc) as base polymers, (C 2 H 5 ) 4 NBF 4 as an electrolyte salt and propylene carbonate (PC) as a plasticizer. The electric conductance of the gel polymer electrolytes, the capacitance and charge–discharge current efficiency of test capacitors with these electrolytes and activated carbon fiber were measured. The conductivity of the gel polymer electrolytes was measured in the range between 2 20 and 1 608C, and was around 10 23 S cm 21 at 258C. The discharge capacitance of one of the electrode of model capacitors with PVP–PVAc-based polymer was 0.74 F cm 22 (38 F g 21 ), and cycling current efficiency was over 90%. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Gel polymer electrolytes; Capacitance; Electric conductance; Charge–discharge current efficiency
1. Introduction Much attention has been focused on electric double-layer capacitors (EDLCs) because they have high energy density comparable to that of usual rechargeable batteries so that they have been used for rechargeable back-up sources. It is notable that EDLCs exhibit both high-rate charge–discharge capacity and long cycle life. Although there have been many reports on EDLCs containing liquid electrolyte systems, EDLCs with polymer electrolytes have so far been limited. The application of the polymer electrolytes to EDLCs will lead to a thin film cell and high reliability without leakage of liquid components. Recently, the application of some gel electrolytes has been reported [1–6]. These gel systems were found to show high conductivity and lead to large capacitance and charge–discharge *Corresponding author. Tel.: 1 81-6-368-1121; fax: 1 81-6339-4026.
cyclability. PVP was reported as one of the candidates of base polymer for polymer Li rechargeable batteries [7], but it has not been introduced as the base polymer for gel polymer electric double-layer capacitors. Furthermore, since PVAc is an inexpensive material, an EDCL with PVP–PVAc would be economically profitable. In the present work, gel electrolytes which consisted of poly(vinyl pyrrolidone) (PVP) or PVP–poly(vinyl acetate) (PVAc) co-polymer as a base polymer, tetraethylammonium as an electrolyte salt and propylene carbonate as a plasticizer were prepared and the conductivity was measured, and the capacity and charge–discharge current efficiency of test capacitors with these gel electrolytes were estimated. 2. Experimental The gel polymer electrolytes were prepared as follows. PVP and PVP–PVAc gel systems were prepared by mixing 0.3 g of PVP or PVP–PVAc, X
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00281-1
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Y. Matsuda et al. / Solid State Ionics 113 – 115 (1998) 103 – 107
mol of (C 2 H 5 ) 4 NBF 4 in 15 ml of PC (Mitsubishi Chemical Corp.) at 1108C, followed by evaporating the PC solvent in a vacuum oven under 1.0 MPa at 1108C for 6 h. In this procedure, around 70% of PC was evaporated under reduced pressure. The mean molecular weights of PVP(a), PVP(b) and PVP– PVAc were 40 000, 10 000 and 350 000, respectively, and the mixed ratio of PVP/ PVAc was 6 / 4. The ionic conductivity was measured with a 1-kHz ac using two Ni electrodes (surface area: 3.0 cm 2 ) and an LCR meter (Ando Denki 4306). A test cell of EDLCs was constructed with the gel polymer electrolyte and two electrodes made of activated carbon fiber cloth (44.1 mg, apparent surface area: 2.27 cm 2 ) as illustrated in Fig. 1. The activated carbon fiber cloth (Toyobo, BW552) was made from phenolic resin. The carbon fiber cloth was adhered to nickel current collector with an engineering plastic sheet (Sumitomo Bakelite Co. Ltd., FS 4654). The two electrodes were fixed face to face by Teflon plate. The distance between the electrodes was 2.0 mm. Each pair of the fixed electrodes was soaked in the precursor solution, followed by evaporating the
Fig. 1. Schematic diagram of apparatus for the test capacitor with PVP or PVP–PVAc system. (1) Ni wire; (2) Ni plate; (3) engineering plastic sheet; (4) gelled polymer electrolyte; (5) Teflon plate; (6) activated carbon fiber electrode; (7) Ni mesh.
PC solvent in the vacuum oven under the abovementioned conditions. The capacitor was first charged to 2.0 V, then discharged to 1.0 V at a constant current (1.0 or 2.0 mA). The current efficiency was defined as (QD/ QC) 3 100 where QD and QC are the amounts of electricity for discharging and charging, respectively. The leak current was measured at a floating voltage of 2 V. The electrochemical measurements were carried out in a dry Ar atmosphere at room temperature (20–258C).
3. Results and discussion Typical temperature dependence of the ionic conductivity for PVP(a) / TEABF 4 (X mol) / PC composites between 2 20 and 1 608C are shown in Fig. 2. The ionic conductivity increased with an increase in TEABF 4 content. The PVP(a) composites showed good conductivity: 0.96 3 10 23 S cm 21 at 258C, X 5 0.020 mol. Fig. 3 shows a similar relationship using PVP(b) whose Mw was ca. 10 000 and the effect of the difference of Mw of the base PVP polymers on the conductivity was small. In Fig. 4, the temperature dependence of ionic conductivity of PVP–PVAc / TEABF 4 (X mol) / PC is shown. The ionic conductivity was almost the same as that of PVP-based gel polymer electrolytes. Among the
Fig. 2. Temperature dependence of ionic conductivity for PVP(a) / TEABF 4 (X mol) / PC. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Y. Matsuda et al. / Solid State Ionics 113 – 115 (1998) 103 – 107
Fig. 3. Temperature dependence of ionic conductivity for PVP(b) / TEABF 4 (X mol) / PC. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 4. Temperature dependence of ionic conductivity for PVP– PVAc / TEABF 4 (X mol) / PC. n, X 5 0.005; d, X 5 0.010; m,X 5 0.015; s, X 5 0.020.
results in Figs. 2–4, the curves showed positive deviations from a linear relation, which is a characteristic feature for polymeric solid electrolyte systems [2,3,8,9]. The charge–discharge characteristics of EDLCs with the PVP(a) composites, PVP(a) / TEABF 4 (X mol) / PC, were investigated under a constant current condition (1.0 mA), where the operation voltage was between 1.0 and 2.0 V. The results are shown in Fig.
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Fig. 5. Discharge capacitance of test capacitors with PVP(K-30) (3.0 g) / TEABF 4 (X mol) / PC (4.5 ml); charge–discharge current, 1.0 mA; operation voltage, 1–2 V. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
5. The discharge capacitance increased with an increase in the TEABF 4 content. This is because total resistance decreased so that the concentration of carrier ions increased in gel polymer electrolyte with an increase in the TEABF 4 content, and ion-adsorption increased on the interphase between the electrodes and the electrolyte. The discharge capacitance of EDLCs with PVP(a) composites showed high capacitance: 0.740 F cm 22 (maximal value) and 0.707 F cm 22 (mean in cycles). These capacitances correspond to 38.1 F g 21 (maximal value) and 36.4 F g 21 (mean value), respectively. The data showed stable performance and high capacity. The cycling current efficiency is shown in Fig. 6 and high current efficiency over 90% was observed except for the initial 2 or 3 cycles and it was around 100% on a suitable condition. Rather small capacity was obtained by using capacitors with PVP(b)-based gel polymers as shown in Fig. 7. The discharge capacitance of capacitors with PVP–PVAc / TEABF 4 (X mol) / PC with charge–discharge cycle is shown in Fig. 8. The results were almost the same as that of the capacitors with PVP-based gel polymers. The current efficiency was between 90 and 100% and the performance was stable.
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Fig. 6. Cycling coulombic efficiency of test capacitors with PVP(a) / TEABF 4 (X mol) / PC; charge–discharge current, 1.0 mA; operation voltage, 1–2 V. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 8. Discharge capacitance of test capacitors with PVP–PVAc / TEABF 4 (X mol) / PC (4.5 ml); charge–discharge current, 1.0 mA; operation voltage, 1–2 V. d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 7. Discharge capacitance of test capacitors with PVP(b) / TEABF 4 (X mol) / PC; charge–discharge current, 1.0 mA; operation voltage, 1–2 V. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 9. Effect of current density on discharge capacitance of a model capacitor with PVP(a) / TEABF 4 (0.020 mol) / PC; charge– discharge current, s, 1.0 mA / 2.27 cm 2 and d, 2.0 mA / 2.27 cm 2 ; operation voltage, 1–2 V.
On the effect of charge–discharge current on the capacity, the capacity decreased on application of higher current as shown in Fig. 9. In Fig. 10, the leak
current in the model capacitors is shown. This leak current was small and it decreased to around 0.014 mA cm 22 for 3 h. The current value was similar to
Y. Matsuda et al. / Solid State Ionics 113 – 115 (1998) 103 – 107
107
was over 90% and around 100% was obtained on a suitable condition. The leak current was around 0.014 mA cm 22 which was similar to that of a model capacitor with PAN-based gel polymer.
Acknowledgements The authors are grateful for the Grant in Aid (260, No. 09215240) from the Ministry of Education, Science, Sports, and Culture, Japan, and the expenses for the faculty joint research programs in Kansai University.
References Fig. 10. Leak current in the model capacitors with PVP(a,b) and PVP–PVAc / TEABF 4 (0.020 mol) / PC; applied voltage, 2.0 V. s, PVP(a), n, PVP(b); d, PVP–PVAc.
that in capacitors with poly(acrylonitrile)-based gel polymers.
4. Conclusion The ionic conductivity of PVP and PVP–PVAcbased gel polymer electrolytes was around 1.0 mS cm 21 at 258C, and these are excellent ionic conductors. The mean charge–discharge capacity of the electrode of capacitors with these gel polymer electrolytes was around 0.7 F cm 22 (36 F g 21 ) and the charge–discharge current efficiency of the capacitors
[1] T. Kanbara, M. Inami, T. Yamamoto, J. Power Sources 36 (1991) 87. [2] Y. Matsuda, M. Morita, M. Ishikawa, M. Ihara, J. Electrochem. Soc. 140 (1993) L109. [3] M. Ishikawa, M. Morita, M. Ihara, Y. Matsuda, J. Electrochem. Soc. 141 (1994) 1730. [4] M. Ishikawa, M. Ihara, M. Morita, Y. Matsuda, Electrochim. Acta 40 (1995) 2217. [5] C. Arbizzani, M. Mastragostino, L. Meneghello, Electrochim. Acta 40 (1995) 2223. [6] M. Ishikawa, M. Morita, Y. Matsuda, in: F.M. Delnick, et al. (Eds.), Electrochemical Capacitors II, Electrochem. Soc., 1996, p. 325. [7] K.M. Abraham, M. Alamgir, J. Power Sources 43–44 (1993) 195. [8] M. Morita, T. Fukumasa, M. Motoda, H. Tsutsumi, Y. Matsuda, T. Takahshi, H. Ashitaka, J. Electrochem. Soc. 137 (1990) 340. [9] S. Sekido, T. Muranaka, Y. Yoshino, et al., National Technical Report, vol. 26, 1980, p. 220.
Gel polymer electrolytes for electric double layer capacitors a, a b b Y. Matsuda *, K. Inoue , H. Takeuchi , Y. Okuhama a
Faculty of Engineering and High Technology Research Center, Kansai University, Yamate-cho 3 -3 -35, Suita, Osaka 564, Japan b Daiwa Fine Chemicals Co. Ltd., Shimozawadori 2 -1 -17, Hyogoku, Kobe, 652, Japan
Abstract Gel polymer electrolytes consisted of polyvinylpyrrolidone (PVP) and PVP–polyvinylacetate (PVP–PVAc) as base polymers, (C 2 H 5 ) 4 NBF 4 as an electrolyte salt and propylene carbonate (PC) as a plasticizer. The electric conductance of the gel polymer electrolytes, the capacitance and charge–discharge current efficiency of test capacitors with these electrolytes and activated carbon fiber were measured. The conductivity of the gel polymer electrolytes was measured in the range between 2 20 and 1 608C, and was around 10 23 S cm 21 at 258C. The discharge capacitance of one of the electrode of model capacitors with PVP–PVAc-based polymer was 0.74 F cm 22 (38 F g 21 ), and cycling current efficiency was over 90%. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Gel polymer electrolytes; Capacitance; Electric conductance; Charge–discharge current efficiency
1. Introduction Much attention has been focused on electric double-layer capacitors (EDLCs) because they have high energy density comparable to that of usual rechargeable batteries so that they have been used for rechargeable back-up sources. It is notable that EDLCs exhibit both high-rate charge–discharge capacity and long cycle life. Although there have been many reports on EDLCs containing liquid electrolyte systems, EDLCs with polymer electrolytes have so far been limited. The application of the polymer electrolytes to EDLCs will lead to a thin film cell and high reliability without leakage of liquid components. Recently, the application of some gel electrolytes has been reported [1–6]. These gel systems were found to show high conductivity and lead to large capacitance and charge–discharge *Corresponding author. Tel.: 1 81-6-368-1121; fax: 1 81-6339-4026.
cyclability. PVP was reported as one of the candidates of base polymer for polymer Li rechargeable batteries [7], but it has not been introduced as the base polymer for gel polymer electric double-layer capacitors. Furthermore, since PVAc is an inexpensive material, an EDCL with PVP–PVAc would be economically profitable. In the present work, gel electrolytes which consisted of poly(vinyl pyrrolidone) (PVP) or PVP–poly(vinyl acetate) (PVAc) co-polymer as a base polymer, tetraethylammonium as an electrolyte salt and propylene carbonate as a plasticizer were prepared and the conductivity was measured, and the capacity and charge–discharge current efficiency of test capacitors with these gel electrolytes were estimated. 2. Experimental The gel polymer electrolytes were prepared as follows. PVP and PVP–PVAc gel systems were prepared by mixing 0.3 g of PVP or PVP–PVAc, X
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00281-1
104
Y. Matsuda et al. / Solid State Ionics 113 – 115 (1998) 103 – 107
mol of (C 2 H 5 ) 4 NBF 4 in 15 ml of PC (Mitsubishi Chemical Corp.) at 1108C, followed by evaporating the PC solvent in a vacuum oven under 1.0 MPa at 1108C for 6 h. In this procedure, around 70% of PC was evaporated under reduced pressure. The mean molecular weights of PVP(a), PVP(b) and PVP– PVAc were 40 000, 10 000 and 350 000, respectively, and the mixed ratio of PVP/ PVAc was 6 / 4. The ionic conductivity was measured with a 1-kHz ac using two Ni electrodes (surface area: 3.0 cm 2 ) and an LCR meter (Ando Denki 4306). A test cell of EDLCs was constructed with the gel polymer electrolyte and two electrodes made of activated carbon fiber cloth (44.1 mg, apparent surface area: 2.27 cm 2 ) as illustrated in Fig. 1. The activated carbon fiber cloth (Toyobo, BW552) was made from phenolic resin. The carbon fiber cloth was adhered to nickel current collector with an engineering plastic sheet (Sumitomo Bakelite Co. Ltd., FS 4654). The two electrodes were fixed face to face by Teflon plate. The distance between the electrodes was 2.0 mm. Each pair of the fixed electrodes was soaked in the precursor solution, followed by evaporating the
Fig. 1. Schematic diagram of apparatus for the test capacitor with PVP or PVP–PVAc system. (1) Ni wire; (2) Ni plate; (3) engineering plastic sheet; (4) gelled polymer electrolyte; (5) Teflon plate; (6) activated carbon fiber electrode; (7) Ni mesh.
PC solvent in the vacuum oven under the abovementioned conditions. The capacitor was first charged to 2.0 V, then discharged to 1.0 V at a constant current (1.0 or 2.0 mA). The current efficiency was defined as (QD/ QC) 3 100 where QD and QC are the amounts of electricity for discharging and charging, respectively. The leak current was measured at a floating voltage of 2 V. The electrochemical measurements were carried out in a dry Ar atmosphere at room temperature (20–258C).
3. Results and discussion Typical temperature dependence of the ionic conductivity for PVP(a) / TEABF 4 (X mol) / PC composites between 2 20 and 1 608C are shown in Fig. 2. The ionic conductivity increased with an increase in TEABF 4 content. The PVP(a) composites showed good conductivity: 0.96 3 10 23 S cm 21 at 258C, X 5 0.020 mol. Fig. 3 shows a similar relationship using PVP(b) whose Mw was ca. 10 000 and the effect of the difference of Mw of the base PVP polymers on the conductivity was small. In Fig. 4, the temperature dependence of ionic conductivity of PVP–PVAc / TEABF 4 (X mol) / PC is shown. The ionic conductivity was almost the same as that of PVP-based gel polymer electrolytes. Among the
Fig. 2. Temperature dependence of ionic conductivity for PVP(a) / TEABF 4 (X mol) / PC. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Y. Matsuda et al. / Solid State Ionics 113 – 115 (1998) 103 – 107
Fig. 3. Temperature dependence of ionic conductivity for PVP(b) / TEABF 4 (X mol) / PC. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 4. Temperature dependence of ionic conductivity for PVP– PVAc / TEABF 4 (X mol) / PC. n, X 5 0.005; d, X 5 0.010; m,X 5 0.015; s, X 5 0.020.
results in Figs. 2–4, the curves showed positive deviations from a linear relation, which is a characteristic feature for polymeric solid electrolyte systems [2,3,8,9]. The charge–discharge characteristics of EDLCs with the PVP(a) composites, PVP(a) / TEABF 4 (X mol) / PC, were investigated under a constant current condition (1.0 mA), where the operation voltage was between 1.0 and 2.0 V. The results are shown in Fig.
105
Fig. 5. Discharge capacitance of test capacitors with PVP(K-30) (3.0 g) / TEABF 4 (X mol) / PC (4.5 ml); charge–discharge current, 1.0 mA; operation voltage, 1–2 V. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
5. The discharge capacitance increased with an increase in the TEABF 4 content. This is because total resistance decreased so that the concentration of carrier ions increased in gel polymer electrolyte with an increase in the TEABF 4 content, and ion-adsorption increased on the interphase between the electrodes and the electrolyte. The discharge capacitance of EDLCs with PVP(a) composites showed high capacitance: 0.740 F cm 22 (maximal value) and 0.707 F cm 22 (mean in cycles). These capacitances correspond to 38.1 F g 21 (maximal value) and 36.4 F g 21 (mean value), respectively. The data showed stable performance and high capacity. The cycling current efficiency is shown in Fig. 6 and high current efficiency over 90% was observed except for the initial 2 or 3 cycles and it was around 100% on a suitable condition. Rather small capacity was obtained by using capacitors with PVP(b)-based gel polymers as shown in Fig. 7. The discharge capacitance of capacitors with PVP–PVAc / TEABF 4 (X mol) / PC with charge–discharge cycle is shown in Fig. 8. The results were almost the same as that of the capacitors with PVP-based gel polymers. The current efficiency was between 90 and 100% and the performance was stable.
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Fig. 6. Cycling coulombic efficiency of test capacitors with PVP(a) / TEABF 4 (X mol) / PC; charge–discharge current, 1.0 mA; operation voltage, 1–2 V. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 8. Discharge capacitance of test capacitors with PVP–PVAc / TEABF 4 (X mol) / PC (4.5 ml); charge–discharge current, 1.0 mA; operation voltage, 1–2 V. d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 7. Discharge capacitance of test capacitors with PVP(b) / TEABF 4 (X mol) / PC; charge–discharge current, 1.0 mA; operation voltage, 1–2 V. n, X 5 0.005; d, X 5 0.010; m, X 5 0.015; s, X 5 0.020.
Fig. 9. Effect of current density on discharge capacitance of a model capacitor with PVP(a) / TEABF 4 (0.020 mol) / PC; charge– discharge current, s, 1.0 mA / 2.27 cm 2 and d, 2.0 mA / 2.27 cm 2 ; operation voltage, 1–2 V.
On the effect of charge–discharge current on the capacity, the capacity decreased on application of higher current as shown in Fig. 9. In Fig. 10, the leak
current in the model capacitors is shown. This leak current was small and it decreased to around 0.014 mA cm 22 for 3 h. The current value was similar to
Y. Matsuda et al. / Solid State Ionics 113 – 115 (1998) 103 – 107
107
was over 90% and around 100% was obtained on a suitable condition. The leak current was around 0.014 mA cm 22 which was similar to that of a model capacitor with PAN-based gel polymer.
Acknowledgements The authors are grateful for the Grant in Aid (260, No. 09215240) from the Ministry of Education, Science, Sports, and Culture, Japan, and the expenses for the faculty joint research programs in Kansai University.
References Fig. 10. Leak current in the model capacitors with PVP(a,b) and PVP–PVAc / TEABF 4 (0.020 mol) / PC; applied voltage, 2.0 V. s, PVP(a), n, PVP(b); d, PVP–PVAc.
that in capacitors with poly(acrylonitrile)-based gel polymers.
4. Conclusion The ionic conductivity of PVP and PVP–PVAcbased gel polymer electrolytes was around 1.0 mS cm 21 at 258C, and these are excellent ionic conductors. The mean charge–discharge capacity of the electrode of capacitors with these gel polymer electrolytes was around 0.7 F cm 22 (36 F g 21 ) and the charge–discharge current efficiency of the capacitors
[1] T. Kanbara, M. Inami, T. Yamamoto, J. Power Sources 36 (1991) 87. [2] Y. Matsuda, M. Morita, M. Ishikawa, M. Ihara, J. Electrochem. Soc. 140 (1993) L109. [3] M. Ishikawa, M. Morita, M. Ihara, Y. Matsuda, J. Electrochem. Soc. 141 (1994) 1730. [4] M. Ishikawa, M. Ihara, M. Morita, Y. Matsuda, Electrochim. Acta 40 (1995) 2217. [5] C. Arbizzani, M. Mastragostino, L. Meneghello, Electrochim. Acta 40 (1995) 2223. [6] M. Ishikawa, M. Morita, Y. Matsuda, in: F.M. Delnick, et al. (Eds.), Electrochemical Capacitors II, Electrochem. Soc., 1996, p. 325. [7] K.M. Abraham, M. Alamgir, J. Power Sources 43–44 (1993) 195. [8] M. Morita, T. Fukumasa, M. Motoda, H. Tsutsumi, Y. Matsuda, T. Takahshi, H. Ashitaka, J. Electrochem. Soc. 137 (1990) 340. [9] S. Sekido, T. Muranaka, Y. Yoshino, et al., National Technical Report, vol. 26, 1980, p. 220.