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A novel fire-retardant polyacrylonitrile-based gel electrolyte for lithium batteries
PII:
Electrochimica Acta, Vol. 43, Nos 10±11, pp. 1193±1197, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013±4686/98 $19.00 + 0.00 S0013-4686(97)10019-6
A novel ®re-retardant polyacrylonitrile-based gel electrolyte for lithium batteries Hiroyuku Akashi*, Koji Sekai and Ko-ichi Tanaka Department of Materials Science Research Center, Sony Corp., 174 Fujitsuka-cho, Hodogaya-ku, Yokohama-shi, Japan (Received 23 September 1996; accepted 15 April 1997) AbstractÐA novel ®re-retardant gel electrolyte with ¯ammable solvents has been achieved for use with lithium batteries. We report that by optimizing of the ratio of polyacrylonitrile, ethylene carbonate, propylene carbonate, and LiPF6, the gel electrolyte shows a remarkable ®re-retardance without the need to add commercial reagents for ®re-retardant property. The linear burning ratios determined by a burning test with a butane-gas burner indicate that the burning behavior of the gel electrolyte strongly depends on the host polymer and lithium salt used; the PAN-based gel electrolyte containing LiPF6 speci®cally shows ®reretardant. The thermogravimetric analysis also indicates that LiPF6 lowers the carbonizing point of the PAN-based gel electrolyte and increases a residue of a carbonaceous material remaining after burning. The correlation of the linear burning rates and the carbonizing points suggests that the ®re-retardant property of the PAN-based gel electrolyte results from the carbonaceous layer formed on the surface. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: lithium batteries, gel electrolytes, thermogravimetric analysis, polyacrylonitrile, ®re-retardant.
1. INTRODUCTION Much attention has been focused on lithium plastic batteries [1±5] since a gel electrolyte is expected to provide a safer advanced battery with high energy density. Many gel electrolytes oer advantages such as high ionic conductivity with small temperature dependence, leak free, and high mechanical strength [6±9], however because most gel electrolytes contain a ¯ammable solvent, their use in lithium batteries cannot be considered safe. The ¯ammability of an electrolyte material may cause side-accidents when a lithium battery is broken or improperly heats up by another accident. Hence a ®re-retardant property should be highly pro®table for the electrolyte materials although little attention has been given to this point. A number of signi®cant developments in ®re-retardant polymers have been made [10]. In general, ®re-retardant polymers are obtained by adding a reagent based on a phosphide or a halogenide [10], *Author to whom correspondence should be addressed
although it is peculiar to add the reagents to a ¯ammable electrolyte material because of its low chemical stability. Thus it should be better to achieve the ®re-retardant gel electrolyte without any other additives. We report here a novel polyacrylonitrile (PAN)based gel electrolyte with remarkable ®re-retardance. Our concept for a ®re-retardant gel electrolyte is basically based on pyrolysis of a gel electrolyte. If a carbonaceous layer timely forms on the surface of the burning material and succeeds in controlling the diusion of oxygen and evaporation of ¯ammable vapor, the burning will terminate. It is well-known that PAN is a typical ladder polymer which carbonizes above 2008C in air to form a carbonaceous material. As many researchers have reported, a PAN-based gel electrolyte shows relatively high ionic conductivity at room temperature, ca. 3 mS/cm, and has a wide potential window over 4.5 V [8, 9]. Therefore, we paid particular attention to a carbonizing point near the boiling point of solvents such as ethylene carbonate (EC) and propylene carbonate (PC).
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H. Akashi et al. 2. EXPERIMENTAL 2.1. Preparation of the gel electrolyte
Polyacrylonitrile (Mw = 150,000, Polyscience) was dried under vacuum at 808C for 24 h and stored in a glovebox at a dewpoint lower than ÿ508C. Ethylene carbonate (battery grade, Mitsubishi Chemical), propylene carbonate (battery grade, Mitsubishi Chemical) were also used as solvents without further puri®cation. Lithium salts (battery grade, Tomiyama) were used as received. PAN-based gel electrolyte was prepared from a solution of polyacrylonitrile, ethylene carbonate, propylene carbonate, and lithium salt. The components were stirred and re¯uxed over a hot plate held at 1008C until completely dissolved. When a viscous and colorless solution was obtained, the solution was poured into a glass tube (diameter: 1.3 cm, length: 14 cm), then cooled to room temperature. All procedures took place in a dry room at a dew point as low as ÿ408C. The prepolymer of P(EO0.8/PO0.2) triacrylate was prepared by ring-opening polymerization and esteri®cation, as reported by Watanabe et al. [11]. A weighed amount of salt, the solvents, the polyether triacrylate, and 2,2-dimethoxy-2-phenylacetophenone (photo-initiator, 0.05 wt.% based on the polyether triacrylate) were mixed to make a homogeneous solution. The obtained viscous solution was poured into the glass tube and cross linked by irradiation with the uv light (HC-0411, Toshiba Lightech) for 1 min. After irradiation, a transparent and self-standing gel electrolyte was obtained.
Fig. 1. Schematic layout of the burning test.
burnt-out, the linear burning rate was calculated by using the recorded burning time and the length of the burnt area. 2.3. Thermal analysis Thermogravimetric analysis (TGA) was carried out using Rigaku TAS200 - TG8101D. Flowing nitrogen was used in all cases at 80 cc/min. A cylindrical aluminum pan (diameter: 5 mm, height: 2.5 mm) was used as a sample holder without sealing. A heating rate of 1008C/min was used. The carbonizing point of the gel electrolyte was taken to be the mid point of the reaction interval at a temperature from 2008C to 2508C. The residue of the gel electrolyte was taken at 5008C. 3. RESULTS AND DISCUSSION
2.2. Linear burning rate for the gel electrolyte
3.1. Linear burning rate of the gel electrolyte
Figure 1 shows a schematic layer of the burning test based on the UL 94 HB test [12]. Each sample was marked with two lines perpendicular to the longitudinal axis of the bar, 25 mm and 100 mm from the end to be ignited. The sample was positioned on a stainless-steel net. The burner was adjusted to produce a blue ¯ame 9 2 0.5 cm long by controlling the gas supply. Top of the ¯ame was inclined toward the end of the sample at an angle of approximately 458 to the horizontal and applied to the depth of 6 21 mm from the end of the sample. The test ¯ame was applied for 30 s without changing its position, removed after 30 s or as soon as the combustion front of the sample reached the 25 mm mark (if less than 30 s). The burning behavior of the gel was simply determined by how much of the gel burned. If the combustion front of the gel did not reach the 25 mm mark, the gel was regarded as ®re-retardant. If the combustion passed stopped between the 25 mm and the 100 mm mark, the gel was regarded as selfextinguishing. If the combustion passed over the 100 mm mark, the gel was regarded as burnt-out. When the gel electrolyte was self-extinguishing or
Figure 2 shows the linear burning rates for gel electrolytes with dierent salt species. The PANbased gel electrolytes containing LiPF6 proved ®reretardant or self-extinguishing. The ®re-resistance tended to depend on the concentration of LiPF6Ð the ®re-retardant property became more remarkable as the salt concentration was increased. In fact, when we cut the scorched part of the samples in two and observed its cross-section, a carbonaceous material was found to have formed a thin layer on the surface. On the other hand, all of the gel electrolytes without LiPF6 proved to be burnt-out despite the same salt concentration (Table 1). This result suggests that LiPF6 contributes to be the ®reretardant property of the ¯ammable gel electrolytes. The eect of the host polymer on burning behavior was investigated using the linear burning rate. We selected a P(EO0.8/PO0.2)-based gel electrolyte as the reference gel electrolyte, since the P(EO0.8/ PO0.2)-based gel electrolyte was of a gel electrolyte which had been studied by many researchers. The linear burning rate of a P(EO0.8/PO0.2)-based gel electrolyte with LiPF6 is also displayed in Fig. 2. However, the linear burning range of the P(EO0.8/
Novel ®re-retardant gel electrolyte
Fig. 2. Typical burning rate for the PAN-based gel electrolytes with dierent salt species. The data of a P(EO0.8/ PO0.2)-based gel electrolyte with LiPF6 is shown for comparison. (a) salt-free, PAN:EC:PC:LiX = 11:55:28:7; (b) X = LiBF4, (c) X = LiClO4, (e) X = LiPF6, (g) X = LiCF3SO3, (d) PAN:EC:PC:LiPF6=12:57:29:2, (f) (h) P(EO0.8/ PAN:EC:PC:LiPF6=10:54:27:9, PO0.2):EC:PC:LiPF6=38:38:19:5.
PO0.2)-based gel electrolyte showed the lowest at 10.5 mm/min, this gel electrolyte was eventually burned out. It follows from these results that the ®re-retardant property of the gel electrolyte is speci®cally obtained in the PAN-based gel electrolyte containing EC, PC, and LiPF6. So far as we are concerned, this is a ®rst report that a gel electrolyte has achieved ®re retardance without adding any commercial reagents. 3.2. Carbonizing reaction of the gel electrolyte Figure 3 shows typical Tg curves for the PANbased gel electrolytes with dierent salt species and with no salt. Two reaction intervals were clearly observed in the Tg curves of the PAN-based gel electrolyte with no salt. The weight loss is observed in temperatures from 100 to 2508C suggests evaporation of the mixed solvents. The other loss observed from 350 to 4008C suggests decomposition of PAN.
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Fig. 3. Tg curves for the PAN-based gel electrolyte with several salt species. (a) PAN:EC:PC = 11:59:30, (c) PAN:EC:PC:LiX = 11:55:28:7; (b) X = LiBF4, X = LiClO4, (e) X = LiPF6, (g) X = LiCF3SO3.
More of the residue of the PAN-based gel electrolytes with a lithium salt, except for the gel electrolyte with LiClO4, remained than of the salt-free gel electrolyte. Dissolving a lithium salt into the gel electrolyte reduces the weight loss in temperatures from 350 to 4008C. These results suggest that the carbonizing reaction of the PAN-based gel electrolyte proceeds more eectively when a lithium salt is added. On the other hand, the smallest residue, observed in the gel electrolyte with LiClO4, results from an explosion caused from a rapid oxidation of the salt. The carbonizing points of the gel electrolyte were found to depend on the salt species. As listed in Table 1, the carbonizing point of the gel electrolyte containing LiPF6 was lower than the boiling point of the mixed solvents (2318C). On the contrary, the carbonizing point of the other PAN-based gel electrolytes was higher than the boiling point of the mixed solvents. In addition, the carbonizing point of the gel electrolyte containing LiPF6 tends to fall as the salt concentration increases and also the residue increases, as shown in Figs 4 and 5. It follows from these results that LiPF6 interestingly accelerates the carbonizing reaction of the PAN-based gel electrolyte. Thermal stability of the PAN-based gel
Table 1. Component ratio, linear burning rate, carbonizing point, and residue of the gel electrolytes Component ratio (mol%) (a) PAN:EC:PC (salt-free) = 11:59:30 (b) PAN:EC:PC:LiBF4=11:55:28:7 (c) PAN:EC:PC:LiClO4=11:55:28:7 (d) PAN:EC:PC:LiPF6=12:57:29:2 (e) PAN:EC:PC:LiPF6=11:55:28:7 (f) PAN:EC:PC:LiPF6=10:54:27:9 (g) PAN:EC:PC:LiCF3SO3=11:55:28:7 (h) P(EO0.8/PO0.2):EC:PC:LiPF6=38:38:19:5 a
Salt concn.a (mol/kg) Ð 0.87 0.88 0.25 0.87 1.23 0.88 0.82
Linear burning rate Carbonizing point (mm/min) (8C) 37.3b 40.8 33.3 31.4* Ð Ð 38.4 10.5
230.6 242.3 249.1 224.7 226.5 225.2 244.9 234.1
Residue (%) 6.48 13.6 3.84 8.97 14.9 15.4 12.6 5.96
Molality estimated by weight of a solvent immersing PAN. The linear burning rate from the PAN-based gel electrolyte; PAN:PC = 14:86 because the mixture of PAN:EC:PC = 11:59:30 did not form a gel. *Self-extinguishing. b
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Fig. 4. Tg curves for the PAN-based gel electrolyte with dierent concentrations of LiPF6. (a) PAN:EC:PC = (e) 11:59:30, (d) PAN:EC:PC:LiPF6=12:57:29:2, PAN:EC:PC:LiPF6=11:55:28:7, (f) PAN:EC:PC:LiPF6= 10:54:27:9.
electrolyte is also found to be improved by the other lithium salt. A typical Tg curve of the P(EO0.8/PO0.2)-based gel electrolyte is shown in Fig. 6. In spite of the P(EO0.8/PO0.2)-based gel electrolyte containing LiPF6, the carbonizing point of the electrolyte shifted to a higher temperature than the boiling point of the mixed solvent. The residue of the gel electrolyte is also signi®cantly less than that of the PAN-based gel electrolyte containing LiPF6. These results can be explained by the dierent manner of pyrolysis of a host polymer. PAN is a well-known raw material for a carbon ®ber which forms carbonaceous material easily by its cross-linking reaction above 2008C. On the other hand, a P(EO0.8/ PO0.2) hardly forms any carbonaceous material. In fact, no carbonaceous material was observed on the stainless-steel net after the burning test. These results prove that LiPF6 accelerates the pyrolytic reaction of PAN.
Fig. 5. LiPF6 concentration dependence of carbonizing point and residue of PAN-based gel electrolytes. Circles and triangles correspond to the carbonizing points and the amount of residue, respectively.
Fig. 6. Comparison of Tg curves for the PAN-based gel electrolyte and the P(EO0.8/PO0.2)-based gel electrolyte. The data for a mixed solvents (EC:PC = 2.1) are indicated as reference. (a) PAN:EC:PC = 11:59:30, (e) PAN:EC: (h) P(EO0.8/PO0.2):EC:PC: PC:LiPF6=11:55:28:7, LiPF6=38:38:19:5.
3.3. Correlation between the burning behavior and the carbonization of the gel electrolyte The correlation between the linear burning rates and the carbonizing points of the gel electrolytes is shown in Fig. 7. The burning behavior obviously depends on the carbonizing point and the boiling point of the mixed solvents in both types of gel electrolyte. When the carbonizing point is lower than the boiling point, the gel electrolyte shows a remarkable ®re-retardant or self-extinguishing property. On the other hand, when the carbonizing point is higher than the boiling point of the mixed solvent, the gel electrolyte has a tendency to burn
Fig. 7. Correlation between carbonizing point and linear burning rate for the PAN-based gel electrolytes and the P(EO0.8/PO0.2)-based gel electrolyte. The broken line corresponds to the boiling point of the mixed solvents (EC:PC = 2.1). Open symbols are ®re-retardant or selfextinguishing samples, solid symbols are ¯ammable samples. (a) PAN:EC:PC = 11:59:30, PAN:EC:PC: LiX = 11:55:28:7, (b) X = LiBF4, (c) X = LiClO4, (e) X = LiPF6, (g) X = LiCF3SO3, (d) PAN:EC:PC: LiPF6=12:57:29:2, (f) PAN:EC:PC:LiPF6=10:54:27:9, (h) P(EO0.8/PO0.2:EC:PC:LiPF6=38:38:19:5.
Novel ®re-retardant gel electrolyte out. These results indicate that the ®re-retardant and self-extinguishing property is governed by the carbonaceous layer formed on the surface of the gel electrolyteÐthe carbonaceous layer make possible the ®re-retardant or self-extinguishing property by cutting o the supply of oxygen and diusion of some ¯ammable vapor from the burning part of the PAN-based gel electrolyte. It is also of interesting to point out that the linear burning rates tend to decrease as the carbonizing points increase in the PAN-based gel electrolytes without LiPF6. This relation is commonly observed in a commercial ®re-retardant polymer [6] suggesting the thermal stability of the gel electrolyte is largely responsible for ®re retardance. 4. CONCLUSION The results demonstrate a novel aspect of a PAN-based gel electrolyte that is expected to provide a safe performance and high energy density for lithium secondary batteries. Three main conclusions can be drawn from the present study. 1. The ®re-retardant property of a PAN-based gel electrolyte is obtained by complexing PAN, EC, PC, and LiPF6 without adding any ®re-retardant reagents. 2. Thermogravimetry measurements reveal that LiPF6 speci®cally promotes a carbonizing reaction of the PAN-based gel electrolytes more eectivelyÐthe salt reduces the carbonizing point and increases the residue of the carbonaceous material. 3. The correlation between linear burning rate and carbonizing points of the PAN-based gel electrolytes indicate that the ®re-retardant or self-extinguishing property is governed by the carbonaceous layer formed on the surface of a
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gel electrolyteÐthe ®re-retardant or self-extinguishing property results from the carbonaceous layer that cuts o the supply of O2 and diusion of ¯ammable vapor from a ¯ame.
ACKNOWLEDGEMENTS We thank Y. Inagaki for helpful discussion. REFERENCES 1. J. M. MaCallum and C. A. Vincent, Polymer Electrolyte Reviews 1 and 2, Elsevier Appl. Sci., London (1987 and 1989). 2. B. Scrosati, Application of Electroactive Polymers, Chapman & Hall, London (1993). 3. M. Gauthier, A. Belanger, P. Bouchard, B. Kapfer, S. Ricard, G. Vassort, M. Armand, J. Y. Sanchez and L. Krause, J. Pwr. Sources 54, 163 (1995). 4. J. M. Tarascon, C. Schmutz, A. S. Gozdz, P. C. Warren and F. K. Shokoohi, Mat. Res. Soc. Sympo. Proc. 369, 595 (1995). 5. F. Croce, S. Passerini and B. Scrosati, J. Electrochem. Soc. 1405, 6 (1994). 6. M. Watanabe, M. Kanba, K. Nagaoka and I. Shinohara, J. Polym. Sci., Polym. Phys. ed., 21, 939 (1983). 7. F. Croce, S. D. Brown, S. G. Greenbaum, S. M. Slane and M. Salomon, Chem. Mat. 5, 1268 (1993). 8. D. Peramunage, D. M. Pasquariello and K. M. Abraham, J. Electrochem. Soc. 1789, 6 (1995). 9. F. Croce, F. Gerace, G. Dautzemberg, S. Passerini, G. B. Appetecchi and B. Scrosati, Electrochim. Acta. 2187, 14 (1994). 10. H. Nishizawa, Polymer no Nannenka (Fire-retardent polymers), Taiseisha, Tokyo (1988). 11. M. Watanabe and A. Nishimoto, Solid State Ionics 79, 306 (1995). 12. Underwriters Laboratories inc., Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, UL94, 4th edn. (1994).
Electrochimica Acta, Vol. 43, Nos 10±11, pp. 1193±1197, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013±4686/98 $19.00 + 0.00 S0013-4686(97)10019-6
A novel ®re-retardant polyacrylonitrile-based gel electrolyte for lithium batteries Hiroyuku Akashi*, Koji Sekai and Ko-ichi Tanaka Department of Materials Science Research Center, Sony Corp., 174 Fujitsuka-cho, Hodogaya-ku, Yokohama-shi, Japan (Received 23 September 1996; accepted 15 April 1997) AbstractÐA novel ®re-retardant gel electrolyte with ¯ammable solvents has been achieved for use with lithium batteries. We report that by optimizing of the ratio of polyacrylonitrile, ethylene carbonate, propylene carbonate, and LiPF6, the gel electrolyte shows a remarkable ®re-retardance without the need to add commercial reagents for ®re-retardant property. The linear burning ratios determined by a burning test with a butane-gas burner indicate that the burning behavior of the gel electrolyte strongly depends on the host polymer and lithium salt used; the PAN-based gel electrolyte containing LiPF6 speci®cally shows ®reretardant. The thermogravimetric analysis also indicates that LiPF6 lowers the carbonizing point of the PAN-based gel electrolyte and increases a residue of a carbonaceous material remaining after burning. The correlation of the linear burning rates and the carbonizing points suggests that the ®re-retardant property of the PAN-based gel electrolyte results from the carbonaceous layer formed on the surface. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: lithium batteries, gel electrolytes, thermogravimetric analysis, polyacrylonitrile, ®re-retardant.
1. INTRODUCTION Much attention has been focused on lithium plastic batteries [1±5] since a gel electrolyte is expected to provide a safer advanced battery with high energy density. Many gel electrolytes oer advantages such as high ionic conductivity with small temperature dependence, leak free, and high mechanical strength [6±9], however because most gel electrolytes contain a ¯ammable solvent, their use in lithium batteries cannot be considered safe. The ¯ammability of an electrolyte material may cause side-accidents when a lithium battery is broken or improperly heats up by another accident. Hence a ®re-retardant property should be highly pro®table for the electrolyte materials although little attention has been given to this point. A number of signi®cant developments in ®re-retardant polymers have been made [10]. In general, ®re-retardant polymers are obtained by adding a reagent based on a phosphide or a halogenide [10], *Author to whom correspondence should be addressed
although it is peculiar to add the reagents to a ¯ammable electrolyte material because of its low chemical stability. Thus it should be better to achieve the ®re-retardant gel electrolyte without any other additives. We report here a novel polyacrylonitrile (PAN)based gel electrolyte with remarkable ®re-retardance. Our concept for a ®re-retardant gel electrolyte is basically based on pyrolysis of a gel electrolyte. If a carbonaceous layer timely forms on the surface of the burning material and succeeds in controlling the diusion of oxygen and evaporation of ¯ammable vapor, the burning will terminate. It is well-known that PAN is a typical ladder polymer which carbonizes above 2008C in air to form a carbonaceous material. As many researchers have reported, a PAN-based gel electrolyte shows relatively high ionic conductivity at room temperature, ca. 3 mS/cm, and has a wide potential window over 4.5 V [8, 9]. Therefore, we paid particular attention to a carbonizing point near the boiling point of solvents such as ethylene carbonate (EC) and propylene carbonate (PC).
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H. Akashi et al. 2. EXPERIMENTAL 2.1. Preparation of the gel electrolyte
Polyacrylonitrile (Mw = 150,000, Polyscience) was dried under vacuum at 808C for 24 h and stored in a glovebox at a dewpoint lower than ÿ508C. Ethylene carbonate (battery grade, Mitsubishi Chemical), propylene carbonate (battery grade, Mitsubishi Chemical) were also used as solvents without further puri®cation. Lithium salts (battery grade, Tomiyama) were used as received. PAN-based gel electrolyte was prepared from a solution of polyacrylonitrile, ethylene carbonate, propylene carbonate, and lithium salt. The components were stirred and re¯uxed over a hot plate held at 1008C until completely dissolved. When a viscous and colorless solution was obtained, the solution was poured into a glass tube (diameter: 1.3 cm, length: 14 cm), then cooled to room temperature. All procedures took place in a dry room at a dew point as low as ÿ408C. The prepolymer of P(EO0.8/PO0.2) triacrylate was prepared by ring-opening polymerization and esteri®cation, as reported by Watanabe et al. [11]. A weighed amount of salt, the solvents, the polyether triacrylate, and 2,2-dimethoxy-2-phenylacetophenone (photo-initiator, 0.05 wt.% based on the polyether triacrylate) were mixed to make a homogeneous solution. The obtained viscous solution was poured into the glass tube and cross linked by irradiation with the uv light (HC-0411, Toshiba Lightech) for 1 min. After irradiation, a transparent and self-standing gel electrolyte was obtained.
Fig. 1. Schematic layout of the burning test.
burnt-out, the linear burning rate was calculated by using the recorded burning time and the length of the burnt area. 2.3. Thermal analysis Thermogravimetric analysis (TGA) was carried out using Rigaku TAS200 - TG8101D. Flowing nitrogen was used in all cases at 80 cc/min. A cylindrical aluminum pan (diameter: 5 mm, height: 2.5 mm) was used as a sample holder without sealing. A heating rate of 1008C/min was used. The carbonizing point of the gel electrolyte was taken to be the mid point of the reaction interval at a temperature from 2008C to 2508C. The residue of the gel electrolyte was taken at 5008C. 3. RESULTS AND DISCUSSION
2.2. Linear burning rate for the gel electrolyte
3.1. Linear burning rate of the gel electrolyte
Figure 1 shows a schematic layer of the burning test based on the UL 94 HB test [12]. Each sample was marked with two lines perpendicular to the longitudinal axis of the bar, 25 mm and 100 mm from the end to be ignited. The sample was positioned on a stainless-steel net. The burner was adjusted to produce a blue ¯ame 9 2 0.5 cm long by controlling the gas supply. Top of the ¯ame was inclined toward the end of the sample at an angle of approximately 458 to the horizontal and applied to the depth of 6 21 mm from the end of the sample. The test ¯ame was applied for 30 s without changing its position, removed after 30 s or as soon as the combustion front of the sample reached the 25 mm mark (if less than 30 s). The burning behavior of the gel was simply determined by how much of the gel burned. If the combustion front of the gel did not reach the 25 mm mark, the gel was regarded as ®re-retardant. If the combustion passed stopped between the 25 mm and the 100 mm mark, the gel was regarded as selfextinguishing. If the combustion passed over the 100 mm mark, the gel was regarded as burnt-out. When the gel electrolyte was self-extinguishing or
Figure 2 shows the linear burning rates for gel electrolytes with dierent salt species. The PANbased gel electrolytes containing LiPF6 proved ®reretardant or self-extinguishing. The ®re-resistance tended to depend on the concentration of LiPF6Ð the ®re-retardant property became more remarkable as the salt concentration was increased. In fact, when we cut the scorched part of the samples in two and observed its cross-section, a carbonaceous material was found to have formed a thin layer on the surface. On the other hand, all of the gel electrolytes without LiPF6 proved to be burnt-out despite the same salt concentration (Table 1). This result suggests that LiPF6 contributes to be the ®reretardant property of the ¯ammable gel electrolytes. The eect of the host polymer on burning behavior was investigated using the linear burning rate. We selected a P(EO0.8/PO0.2)-based gel electrolyte as the reference gel electrolyte, since the P(EO0.8/ PO0.2)-based gel electrolyte was of a gel electrolyte which had been studied by many researchers. The linear burning rate of a P(EO0.8/PO0.2)-based gel electrolyte with LiPF6 is also displayed in Fig. 2. However, the linear burning range of the P(EO0.8/
Novel ®re-retardant gel electrolyte
Fig. 2. Typical burning rate for the PAN-based gel electrolytes with dierent salt species. The data of a P(EO0.8/ PO0.2)-based gel electrolyte with LiPF6 is shown for comparison. (a) salt-free, PAN:EC:PC:LiX = 11:55:28:7; (b) X = LiBF4, (c) X = LiClO4, (e) X = LiPF6, (g) X = LiCF3SO3, (d) PAN:EC:PC:LiPF6=12:57:29:2, (f) (h) P(EO0.8/ PAN:EC:PC:LiPF6=10:54:27:9, PO0.2):EC:PC:LiPF6=38:38:19:5.
PO0.2)-based gel electrolyte showed the lowest at 10.5 mm/min, this gel electrolyte was eventually burned out. It follows from these results that the ®re-retardant property of the gel electrolyte is speci®cally obtained in the PAN-based gel electrolyte containing EC, PC, and LiPF6. So far as we are concerned, this is a ®rst report that a gel electrolyte has achieved ®re retardance without adding any commercial reagents. 3.2. Carbonizing reaction of the gel electrolyte Figure 3 shows typical Tg curves for the PANbased gel electrolytes with dierent salt species and with no salt. Two reaction intervals were clearly observed in the Tg curves of the PAN-based gel electrolyte with no salt. The weight loss is observed in temperatures from 100 to 2508C suggests evaporation of the mixed solvents. The other loss observed from 350 to 4008C suggests decomposition of PAN.
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Fig. 3. Tg curves for the PAN-based gel electrolyte with several salt species. (a) PAN:EC:PC = 11:59:30, (c) PAN:EC:PC:LiX = 11:55:28:7; (b) X = LiBF4, X = LiClO4, (e) X = LiPF6, (g) X = LiCF3SO3.
More of the residue of the PAN-based gel electrolytes with a lithium salt, except for the gel electrolyte with LiClO4, remained than of the salt-free gel electrolyte. Dissolving a lithium salt into the gel electrolyte reduces the weight loss in temperatures from 350 to 4008C. These results suggest that the carbonizing reaction of the PAN-based gel electrolyte proceeds more eectively when a lithium salt is added. On the other hand, the smallest residue, observed in the gel electrolyte with LiClO4, results from an explosion caused from a rapid oxidation of the salt. The carbonizing points of the gel electrolyte were found to depend on the salt species. As listed in Table 1, the carbonizing point of the gel electrolyte containing LiPF6 was lower than the boiling point of the mixed solvents (2318C). On the contrary, the carbonizing point of the other PAN-based gel electrolytes was higher than the boiling point of the mixed solvents. In addition, the carbonizing point of the gel electrolyte containing LiPF6 tends to fall as the salt concentration increases and also the residue increases, as shown in Figs 4 and 5. It follows from these results that LiPF6 interestingly accelerates the carbonizing reaction of the PAN-based gel electrolyte. Thermal stability of the PAN-based gel
Table 1. Component ratio, linear burning rate, carbonizing point, and residue of the gel electrolytes Component ratio (mol%) (a) PAN:EC:PC (salt-free) = 11:59:30 (b) PAN:EC:PC:LiBF4=11:55:28:7 (c) PAN:EC:PC:LiClO4=11:55:28:7 (d) PAN:EC:PC:LiPF6=12:57:29:2 (e) PAN:EC:PC:LiPF6=11:55:28:7 (f) PAN:EC:PC:LiPF6=10:54:27:9 (g) PAN:EC:PC:LiCF3SO3=11:55:28:7 (h) P(EO0.8/PO0.2):EC:PC:LiPF6=38:38:19:5 a
Salt concn.a (mol/kg) Ð 0.87 0.88 0.25 0.87 1.23 0.88 0.82
Linear burning rate Carbonizing point (mm/min) (8C) 37.3b 40.8 33.3 31.4* Ð Ð 38.4 10.5
230.6 242.3 249.1 224.7 226.5 225.2 244.9 234.1
Residue (%) 6.48 13.6 3.84 8.97 14.9 15.4 12.6 5.96
Molality estimated by weight of a solvent immersing PAN. The linear burning rate from the PAN-based gel electrolyte; PAN:PC = 14:86 because the mixture of PAN:EC:PC = 11:59:30 did not form a gel. *Self-extinguishing. b
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Fig. 4. Tg curves for the PAN-based gel electrolyte with dierent concentrations of LiPF6. (a) PAN:EC:PC = (e) 11:59:30, (d) PAN:EC:PC:LiPF6=12:57:29:2, PAN:EC:PC:LiPF6=11:55:28:7, (f) PAN:EC:PC:LiPF6= 10:54:27:9.
electrolyte is also found to be improved by the other lithium salt. A typical Tg curve of the P(EO0.8/PO0.2)-based gel electrolyte is shown in Fig. 6. In spite of the P(EO0.8/PO0.2)-based gel electrolyte containing LiPF6, the carbonizing point of the electrolyte shifted to a higher temperature than the boiling point of the mixed solvent. The residue of the gel electrolyte is also signi®cantly less than that of the PAN-based gel electrolyte containing LiPF6. These results can be explained by the dierent manner of pyrolysis of a host polymer. PAN is a well-known raw material for a carbon ®ber which forms carbonaceous material easily by its cross-linking reaction above 2008C. On the other hand, a P(EO0.8/ PO0.2) hardly forms any carbonaceous material. In fact, no carbonaceous material was observed on the stainless-steel net after the burning test. These results prove that LiPF6 accelerates the pyrolytic reaction of PAN.
Fig. 5. LiPF6 concentration dependence of carbonizing point and residue of PAN-based gel electrolytes. Circles and triangles correspond to the carbonizing points and the amount of residue, respectively.
Fig. 6. Comparison of Tg curves for the PAN-based gel electrolyte and the P(EO0.8/PO0.2)-based gel electrolyte. The data for a mixed solvents (EC:PC = 2.1) are indicated as reference. (a) PAN:EC:PC = 11:59:30, (e) PAN:EC: (h) P(EO0.8/PO0.2):EC:PC: PC:LiPF6=11:55:28:7, LiPF6=38:38:19:5.
3.3. Correlation between the burning behavior and the carbonization of the gel electrolyte The correlation between the linear burning rates and the carbonizing points of the gel electrolytes is shown in Fig. 7. The burning behavior obviously depends on the carbonizing point and the boiling point of the mixed solvents in both types of gel electrolyte. When the carbonizing point is lower than the boiling point, the gel electrolyte shows a remarkable ®re-retardant or self-extinguishing property. On the other hand, when the carbonizing point is higher than the boiling point of the mixed solvent, the gel electrolyte has a tendency to burn
Fig. 7. Correlation between carbonizing point and linear burning rate for the PAN-based gel electrolytes and the P(EO0.8/PO0.2)-based gel electrolyte. The broken line corresponds to the boiling point of the mixed solvents (EC:PC = 2.1). Open symbols are ®re-retardant or selfextinguishing samples, solid symbols are ¯ammable samples. (a) PAN:EC:PC = 11:59:30, PAN:EC:PC: LiX = 11:55:28:7, (b) X = LiBF4, (c) X = LiClO4, (e) X = LiPF6, (g) X = LiCF3SO3, (d) PAN:EC:PC: LiPF6=12:57:29:2, (f) PAN:EC:PC:LiPF6=10:54:27:9, (h) P(EO0.8/PO0.2:EC:PC:LiPF6=38:38:19:5.
Novel ®re-retardant gel electrolyte out. These results indicate that the ®re-retardant and self-extinguishing property is governed by the carbonaceous layer formed on the surface of the gel electrolyteÐthe carbonaceous layer make possible the ®re-retardant or self-extinguishing property by cutting o the supply of oxygen and diusion of some ¯ammable vapor from the burning part of the PAN-based gel electrolyte. It is also of interesting to point out that the linear burning rates tend to decrease as the carbonizing points increase in the PAN-based gel electrolytes without LiPF6. This relation is commonly observed in a commercial ®re-retardant polymer [6] suggesting the thermal stability of the gel electrolyte is largely responsible for ®re retardance. 4. CONCLUSION The results demonstrate a novel aspect of a PAN-based gel electrolyte that is expected to provide a safe performance and high energy density for lithium secondary batteries. Three main conclusions can be drawn from the present study. 1. The ®re-retardant property of a PAN-based gel electrolyte is obtained by complexing PAN, EC, PC, and LiPF6 without adding any ®re-retardant reagents. 2. Thermogravimetry measurements reveal that LiPF6 speci®cally promotes a carbonizing reaction of the PAN-based gel electrolytes more eectivelyÐthe salt reduces the carbonizing point and increases the residue of the carbonaceous material. 3. The correlation between linear burning rate and carbonizing points of the PAN-based gel electrolytes indicate that the ®re-retardant or self-extinguishing property is governed by the carbonaceous layer formed on the surface of a
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gel electrolyteÐthe ®re-retardant or self-extinguishing property results from the carbonaceous layer that cuts o the supply of O2 and diusion of ¯ammable vapor from a ¯ame.
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