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Surface modification of natural graphite particles for lithium ion batteries
Solid State Ionics 135 (2000) 209–212 www.elsevier.com / locate / ssi
Surface modification of natural graphite particles for lithium ion batteries a, a a b b b T. Tsumura *, A. Katanosaka , I. Souma , T. Ono , Y. Aihara , J. Kuratomi , M. Inagaki c a NARD Institute, 2 -6 -1, Nishinagasu-cho, Amagasaki 660 -0805, Japan Yuasa Corporation, 4 -5 -1, Ohgi-cho, Odawara Kanagawa 250 -0001, Japan c Aichi University of Technology, 1247, Yachigusa, Yachigusa-cho Toyota 470 -0392, Japan b
Abstract The surface of natural graphite particles for the anode of dry-type polymer lithium ion batteries was modified by means of carbon coating and polyethyleneoxide (PEO) grafting. Carbon coating on graphite particles was achieved by a simple mixing of graphite particles with polyvinylchloride powders and heating up to 5008C under a nitrogen gas flow. The graphite particles obtained were coated with carbon homogeneously. The carbon layer was oxidized by heating at 4008C in static air to form carboxyl group and then PEO was grafted by esterification of the carboxyl group with terminal hydroxyl group of PEO. The dispersibility of the PEO-grafted particles in water was improved compared to that of untreated graphite particles. 2000 Elsevier Science B.V. All rights reserved. Keywords: Surface; Modification; Coating; Grafting; Dry-type polymer lithium ion battery Materials: Graphite; Polyvinylchloride; Polyethyleneoxide
1. Introduction Lithium ion batteries using a solid polymer electrolyte have been widely studied in order to obtain safe batteries and to allow all types cell construction in a variety of sizes and shapes [1–6]. In dry-type polymer battery system, the volume change of carbon during the charge–discharge cycle causes a cleavage of interface between the carbon anode and the polymer electrolyte and is thought to decrease the capacity with the increase of the cycle number. *Corresponding author. Fax: 181-6-6482-7012. E-mail address: [email protected] (T. Tsumura).
In the present paper, polyethyleneoxide (PEO) was grafted on the graphite particles to improve the wettability of the graphite surface to the polymer electrolyte, PEO [7–10]. There are few acidic groups, which provide a site for the grafting of polymer, on the graphite particles. Therefore, graphite particles were coated with low-crystalline carbon which was formed by a mechanical mixing with polyvinylchloride (PVC) powders (50:50, w / w) and heating to a temperature of 5008C in a nitrogen gas flow [11–14]. Then the carbon-coated particles obtained were oxidized at 4008C in static air to form acidic groups on the carbon surface. The grafting of PEO (MW 4000) onto the sample particles was
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00365-9
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T. Tsumura et al. / Solid State Ionics 135 (2000) 209 – 212
achieved by esterification of the carboxyl group with terminal hydroxyl group of PEO. Dispersibility of particles in water was investigated to evaluate wettability, and characterization of samples was carried out by means of a scanning electron microscope (SEM) and laser Raman spectroscopy. Graphite particles were covered with carbonized PVC homogeneously and the weight of residue carbon was about 13 wt.% of PVC mixed. By air oxidation, 3?10 25 mol g 21 of carboxyl group was formed on sample. The weight gain of PEO-grafted sample was 1.8%, i.e. 15% of carboxyl groups on the carbon surface reacted with PEO. The dispersibility of the PEO-grafted particles in water was improved compared to that of pristine graphite particles.
2. Experimental Coating of graphite powders with carbonized PVC and grafting of PEO onto the carbon-coated graphite particles was achieved as follows; natural graphite particles (average particle diameter of 18.6 mm) and PVC powders (average polymerization degree of 600) were obtained from Kansainetukagaku and Sinetsukagakukougyo, respectively. Natural graphite particles were mixed with 50 wt.% PVC powders by shaking in a bottle. The mechanical mixture was heated to 2508C, kept at 2508C for 30 min and then heated to 5008C and kept at 500 o C for 1 h in nitrogen gas flow at the heating ratio of 108C min 21 . The reaction products were observed by SEM to evaluate the appearance of the surface. Carbon-coated graphite particles obtained were oxidized by means of heating at 4008C for 4 h in static air. The amount of carboxyl groups on the carbon surface was determined by titration (sodium hydrogencarbonate method [15]). Grafting of PEO onto the oxidized particles was achieved by esterification of carboxyl group on the coated carbon layer with terminal hydroxyl group of PEO. The sample powder and PEO (MW 4000) powder were dried under vacuum at 1108C for 24 h before use. First, the carboxyl group was converted into an acid chloride (COCl) group as follows. To a dispersion of the oxidized sample (|5 g) in 30 ml of anhydrous benzene, 10 ml of SOCl 2 was added and
the mixture was stirred at 808C for 2 h. After the reaction, benzene and unreacted SOCl 2 were distilled away under reduced pressure. Then, the grafting of PEO was done by esterification of the COCl group with terminal hydroxyl group of PEO. The particles obtained (|5 g) was dispersed in 30 ml of anhydrous benzene in which PEO (|6 g) and anhydrous pyridine (|0.1 g) was dissolved and the mixture was stirred at 808C for 24 h. The product filtered was washed with successive, benzene, acetone, and water, and dried at 1008C under reduced pressure. The laser Raman spectra were measured on the pristine graphite sample, the carbon-coated sample, the oxidized sample, the PEO used, and the PEOgrafted sample. The dispersibility of the pristine, carbon-coated, oxidized and PEO-grafted samples in water was observed. A 0.1-g amount of sample powder was dispersed in 10 ml of water and stability of dispersion was compared among the four samples.
3. Results and discussion
3.1. Coating of graphite particles with carbonizedPVC The weight of residue carbon from PVC is about 13.4% of PVC mixed. There are few carboxyl groups on the surface of the carbon-coated sample. SEM micrographs of pristine graphite particles and carbon coated graphite particles are shown in Fig. 1. The edges of the pristine graphite particles look sharp (Fig. 1a), but those of the carbon coated particles look blurred and the particles are apparently covered with carbonized-PVC (Fig. 1b). The coatedcarbon layer on graphite particle seems to be homogeneous and thin.
3.2. Oxidation of sample powders By air oxidation, 7 wt.% of the sample is burned out and 3?10 25 mol g 21 of surface carboxyl group is formed. The weight loss of the sample is reasonably supposed to be caused by burning of carbonizedPVC, because natural graphite powders do not show any weight loss after heating up to 4008C in static air. After oxidation, there was no visible change of
T. Tsumura et al. / Solid State Ionics 135 (2000) 209 – 212
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Fig. 1. SEM photographs of pristine graphite particle (a) and carbonized PVC coated graphite particle (b).
the surface structure of the carbon-coated graphite under SEM observation.
3.3. Grafting of PEO on sample powders The grafting of PEO (MW 4000) onto the sample particles was achieved by esterification of carboxyl group on the sample surface with terminal hydroxyl group of PEO. The weight gain of the product was 1.8%, that is, 15% of carboxyl groups on the carbon surface had reacted with PEO. Dispersion of the PEO-grafted sample is stable, and so the hydrophilicity of the PEO-grafted particles is higher than that of pristine graphite particles, carbon-coated particles, and oxidized particles.
3.4. Laser Raman spectroscopic study Raman spectra of the samples are shown in Fig. 2. The spectrum of the pristine graphite particles exhibits a distinct G band at 1580 cm 21 which is attributed to basal plane of graphite and a small D band at 1360 cm 21 which is attributed to edge plane of graphite as shown in Fig. 2a [16–18]. The G and D bands become broad and small after coating of
Fig. 2. Laser Raman spectra of samples; (a) pristine graphite, (b) carbon-coated graphite, (c) oxidized sample, (d) PEO and (e) PEO-grafted graphite.
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T. Tsumura et al. / Solid State Ionics 135 (2000) 209 – 212
carbonized PVC layer (Fig. 2b). This indicates that an amorphous carbon layer covers the graphite particles. There is no major change in spectrum between the carbon-coated and oxidized samples as shown in Fig. 2b and c. The spectrum of the PEOgrafted sample shows only a sharp G band at 1580 cm 21 but no peaks attributed to PEO grafted. PEO grafted on the surface may be burned out by laser beam irradiation, because PEO decomposes above 2108C. We have no direct evidence of grafting of PEO yet.
grafted sample are 330 mAh g 21 and 98%, respectively, and these values are higher than those of pristine graphite. The details of the electrochemical tests on the PEO-grafted graphite particles will be published elsewhere.
Acknowledgements We has been used some technologies developed by New Energy and Industrial Technology Development Organization (NEDO).
4. Conclusion References Surface modification of natural graphite particles was achieved by means of carbon coating and PEO grafting. The hydrophilicity of the graphite particles was increased by this modification. The electrochemical performance of the modified graphite particles in dry-type polymer lithium battery is currently under investigation. Fig. 3 shows the fourth charge– discharge curves of samples. The fourth charge capacity and charge–discharge efficiency of PEO-
Fig. 3. Fourth charge–discharge curves of samples; . . . . . . pristine graphite, ——— PEO-grafted graphite.
[1] K. Zaghib, Y. Choquette, A. Guerfi, M. Simoneau, A. Belanger, M. Gauthier, J. Power Sources 68 (1997) 368. [2] K.M. Abraham, Electrochim. Acta 38 (1993) 1233. [3] Z. Takehara, K. Kanamura, Electrochim. Acta 38 (1993) 1169. [4] R. Yazami, K. Zaghib, M. Deschamps, J. Power Sources 52 (1994) 55. [5] D. Fauteux, Electrochim. Acta 38 (1993) 1119. [6] E. Peled, D. Golodnitsky, G. Ardel, J. Electrochem. Soc. 144 (1997) L208. [7] N. Tsubokawa, Prog. Polym. Sci. 17 (1992) 417. [8] C.U. Pittman Jr., G.-R. He, B. Wu, S.D. Gardner, Carbon 35 (1997) 333. [9] K. Fujiki, N. Tsubokawa, Y. Sone, Polym. J. 22 (1990) 661. [10] N. Tsubokawa, S. Yoshikawa, K. Maruyama, T. Ogasawara, K. Saitoh, Polym. Bull. Soc. 39 (1997) 217. [11] M. Inagaki, H. Miura, H. Konno, J. Eur. Ceram. Soc. 18 (1998) 1011. [12] M. Inagaki, Y. Okada, H. Miura, H. Konno, Carbon 37 (1999) 329. [13] M. Inagaki, Y. Okada, V. Vignal, H. Konno, K. Oshida, Carbon 36 (1998) 1706. [14] H. Tu, J. Wang, Polym. Degrad. Stabil. 54 (1996) 195. [15] D. Rivin, Rubber Chem. Technol. 36 (1963) 729. [16] K. Nakamura, M. Kitajima, Appl. Phys. Lett. 59 (1991) 1550. [17] E. Asari, M. Kitajima, K. Nakamura, T. Kawabe, Phys. Rev. B47 (1993) 11143. [18] E. Asari, I. Kamioka, K. Nakamura, T. Kawabe, M. Kitajima, Phys. Rev. B49 (1994) 1011.
Surface modification of natural graphite particles for lithium ion batteries a, a a b b b T. Tsumura *, A. Katanosaka , I. Souma , T. Ono , Y. Aihara , J. Kuratomi , M. Inagaki c a NARD Institute, 2 -6 -1, Nishinagasu-cho, Amagasaki 660 -0805, Japan Yuasa Corporation, 4 -5 -1, Ohgi-cho, Odawara Kanagawa 250 -0001, Japan c Aichi University of Technology, 1247, Yachigusa, Yachigusa-cho Toyota 470 -0392, Japan b
Abstract The surface of natural graphite particles for the anode of dry-type polymer lithium ion batteries was modified by means of carbon coating and polyethyleneoxide (PEO) grafting. Carbon coating on graphite particles was achieved by a simple mixing of graphite particles with polyvinylchloride powders and heating up to 5008C under a nitrogen gas flow. The graphite particles obtained were coated with carbon homogeneously. The carbon layer was oxidized by heating at 4008C in static air to form carboxyl group and then PEO was grafted by esterification of the carboxyl group with terminal hydroxyl group of PEO. The dispersibility of the PEO-grafted particles in water was improved compared to that of untreated graphite particles. 2000 Elsevier Science B.V. All rights reserved. Keywords: Surface; Modification; Coating; Grafting; Dry-type polymer lithium ion battery Materials: Graphite; Polyvinylchloride; Polyethyleneoxide
1. Introduction Lithium ion batteries using a solid polymer electrolyte have been widely studied in order to obtain safe batteries and to allow all types cell construction in a variety of sizes and shapes [1–6]. In dry-type polymer battery system, the volume change of carbon during the charge–discharge cycle causes a cleavage of interface between the carbon anode and the polymer electrolyte and is thought to decrease the capacity with the increase of the cycle number. *Corresponding author. Fax: 181-6-6482-7012. E-mail address: [email protected] (T. Tsumura).
In the present paper, polyethyleneoxide (PEO) was grafted on the graphite particles to improve the wettability of the graphite surface to the polymer electrolyte, PEO [7–10]. There are few acidic groups, which provide a site for the grafting of polymer, on the graphite particles. Therefore, graphite particles were coated with low-crystalline carbon which was formed by a mechanical mixing with polyvinylchloride (PVC) powders (50:50, w / w) and heating to a temperature of 5008C in a nitrogen gas flow [11–14]. Then the carbon-coated particles obtained were oxidized at 4008C in static air to form acidic groups on the carbon surface. The grafting of PEO (MW 4000) onto the sample particles was
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00365-9
210
T. Tsumura et al. / Solid State Ionics 135 (2000) 209 – 212
achieved by esterification of the carboxyl group with terminal hydroxyl group of PEO. Dispersibility of particles in water was investigated to evaluate wettability, and characterization of samples was carried out by means of a scanning electron microscope (SEM) and laser Raman spectroscopy. Graphite particles were covered with carbonized PVC homogeneously and the weight of residue carbon was about 13 wt.% of PVC mixed. By air oxidation, 3?10 25 mol g 21 of carboxyl group was formed on sample. The weight gain of PEO-grafted sample was 1.8%, i.e. 15% of carboxyl groups on the carbon surface reacted with PEO. The dispersibility of the PEO-grafted particles in water was improved compared to that of pristine graphite particles.
2. Experimental Coating of graphite powders with carbonized PVC and grafting of PEO onto the carbon-coated graphite particles was achieved as follows; natural graphite particles (average particle diameter of 18.6 mm) and PVC powders (average polymerization degree of 600) were obtained from Kansainetukagaku and Sinetsukagakukougyo, respectively. Natural graphite particles were mixed with 50 wt.% PVC powders by shaking in a bottle. The mechanical mixture was heated to 2508C, kept at 2508C for 30 min and then heated to 5008C and kept at 500 o C for 1 h in nitrogen gas flow at the heating ratio of 108C min 21 . The reaction products were observed by SEM to evaluate the appearance of the surface. Carbon-coated graphite particles obtained were oxidized by means of heating at 4008C for 4 h in static air. The amount of carboxyl groups on the carbon surface was determined by titration (sodium hydrogencarbonate method [15]). Grafting of PEO onto the oxidized particles was achieved by esterification of carboxyl group on the coated carbon layer with terminal hydroxyl group of PEO. The sample powder and PEO (MW 4000) powder were dried under vacuum at 1108C for 24 h before use. First, the carboxyl group was converted into an acid chloride (COCl) group as follows. To a dispersion of the oxidized sample (|5 g) in 30 ml of anhydrous benzene, 10 ml of SOCl 2 was added and
the mixture was stirred at 808C for 2 h. After the reaction, benzene and unreacted SOCl 2 were distilled away under reduced pressure. Then, the grafting of PEO was done by esterification of the COCl group with terminal hydroxyl group of PEO. The particles obtained (|5 g) was dispersed in 30 ml of anhydrous benzene in which PEO (|6 g) and anhydrous pyridine (|0.1 g) was dissolved and the mixture was stirred at 808C for 24 h. The product filtered was washed with successive, benzene, acetone, and water, and dried at 1008C under reduced pressure. The laser Raman spectra were measured on the pristine graphite sample, the carbon-coated sample, the oxidized sample, the PEO used, and the PEOgrafted sample. The dispersibility of the pristine, carbon-coated, oxidized and PEO-grafted samples in water was observed. A 0.1-g amount of sample powder was dispersed in 10 ml of water and stability of dispersion was compared among the four samples.
3. Results and discussion
3.1. Coating of graphite particles with carbonizedPVC The weight of residue carbon from PVC is about 13.4% of PVC mixed. There are few carboxyl groups on the surface of the carbon-coated sample. SEM micrographs of pristine graphite particles and carbon coated graphite particles are shown in Fig. 1. The edges of the pristine graphite particles look sharp (Fig. 1a), but those of the carbon coated particles look blurred and the particles are apparently covered with carbonized-PVC (Fig. 1b). The coatedcarbon layer on graphite particle seems to be homogeneous and thin.
3.2. Oxidation of sample powders By air oxidation, 7 wt.% of the sample is burned out and 3?10 25 mol g 21 of surface carboxyl group is formed. The weight loss of the sample is reasonably supposed to be caused by burning of carbonizedPVC, because natural graphite powders do not show any weight loss after heating up to 4008C in static air. After oxidation, there was no visible change of
T. Tsumura et al. / Solid State Ionics 135 (2000) 209 – 212
211
Fig. 1. SEM photographs of pristine graphite particle (a) and carbonized PVC coated graphite particle (b).
the surface structure of the carbon-coated graphite under SEM observation.
3.3. Grafting of PEO on sample powders The grafting of PEO (MW 4000) onto the sample particles was achieved by esterification of carboxyl group on the sample surface with terminal hydroxyl group of PEO. The weight gain of the product was 1.8%, that is, 15% of carboxyl groups on the carbon surface had reacted with PEO. Dispersion of the PEO-grafted sample is stable, and so the hydrophilicity of the PEO-grafted particles is higher than that of pristine graphite particles, carbon-coated particles, and oxidized particles.
3.4. Laser Raman spectroscopic study Raman spectra of the samples are shown in Fig. 2. The spectrum of the pristine graphite particles exhibits a distinct G band at 1580 cm 21 which is attributed to basal plane of graphite and a small D band at 1360 cm 21 which is attributed to edge plane of graphite as shown in Fig. 2a [16–18]. The G and D bands become broad and small after coating of
Fig. 2. Laser Raman spectra of samples; (a) pristine graphite, (b) carbon-coated graphite, (c) oxidized sample, (d) PEO and (e) PEO-grafted graphite.
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T. Tsumura et al. / Solid State Ionics 135 (2000) 209 – 212
carbonized PVC layer (Fig. 2b). This indicates that an amorphous carbon layer covers the graphite particles. There is no major change in spectrum between the carbon-coated and oxidized samples as shown in Fig. 2b and c. The spectrum of the PEOgrafted sample shows only a sharp G band at 1580 cm 21 but no peaks attributed to PEO grafted. PEO grafted on the surface may be burned out by laser beam irradiation, because PEO decomposes above 2108C. We have no direct evidence of grafting of PEO yet.
grafted sample are 330 mAh g 21 and 98%, respectively, and these values are higher than those of pristine graphite. The details of the electrochemical tests on the PEO-grafted graphite particles will be published elsewhere.
Acknowledgements We has been used some technologies developed by New Energy and Industrial Technology Development Organization (NEDO).
4. Conclusion References Surface modification of natural graphite particles was achieved by means of carbon coating and PEO grafting. The hydrophilicity of the graphite particles was increased by this modification. The electrochemical performance of the modified graphite particles in dry-type polymer lithium battery is currently under investigation. Fig. 3 shows the fourth charge– discharge curves of samples. The fourth charge capacity and charge–discharge efficiency of PEO-
Fig. 3. Fourth charge–discharge curves of samples; . . . . . . pristine graphite, ——— PEO-grafted graphite.
[1] K. Zaghib, Y. Choquette, A. Guerfi, M. Simoneau, A. Belanger, M. Gauthier, J. Power Sources 68 (1997) 368. [2] K.M. Abraham, Electrochim. Acta 38 (1993) 1233. [3] Z. Takehara, K. Kanamura, Electrochim. Acta 38 (1993) 1169. [4] R. Yazami, K. Zaghib, M. Deschamps, J. Power Sources 52 (1994) 55. [5] D. Fauteux, Electrochim. Acta 38 (1993) 1119. [6] E. Peled, D. Golodnitsky, G. Ardel, J. Electrochem. Soc. 144 (1997) L208. [7] N. Tsubokawa, Prog. Polym. Sci. 17 (1992) 417. [8] C.U. Pittman Jr., G.-R. He, B. Wu, S.D. Gardner, Carbon 35 (1997) 333. [9] K. Fujiki, N. Tsubokawa, Y. Sone, Polym. J. 22 (1990) 661. [10] N. Tsubokawa, S. Yoshikawa, K. Maruyama, T. Ogasawara, K. Saitoh, Polym. Bull. Soc. 39 (1997) 217. [11] M. Inagaki, H. Miura, H. Konno, J. Eur. Ceram. Soc. 18 (1998) 1011. [12] M. Inagaki, Y. Okada, H. Miura, H. Konno, Carbon 37 (1999) 329. [13] M. Inagaki, Y. Okada, V. Vignal, H. Konno, K. Oshida, Carbon 36 (1998) 1706. [14] H. Tu, J. Wang, Polym. Degrad. Stabil. 54 (1996) 195. [15] D. Rivin, Rubber Chem. Technol. 36 (1963) 729. [16] K. Nakamura, M. Kitajima, Appl. Phys. Lett. 59 (1991) 1550. [17] E. Asari, M. Kitajima, K. Nakamura, T. Kawabe, Phys. Rev. B47 (1993) 11143. [18] E. Asari, I. Kamioka, K. Nakamura, T. Kawabe, M. Kitajima, Phys. Rev. B49 (1994) 1011.