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Anodic performances of anisotropic carbon derived from isotropic quinoline pitch
Carbon 37 (1999) 323–327
Anodic performances of anisotropic carbon derived from isotropic quinoline pitch Isao Mochida *, Cha-Hun Ku, Seong-Ho Yoon, Yozo Korai Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received 27 March 1998; accepted 10 June 1998
Abstract The anodic performances of anisotropic carbon derived from quinoline pitch (CQP) to be used in the lithium ion battery were studied to clarify the influence of nitrogen in the carbon hexagonal sheet. The quinoline pitch was synthesized by using HF/BF as a catalyst. Nitrogen in the carbon existed as pyridinic, pyrrolic and quaternary types 3 to be converted in this sequence, depending upon HTT. Above 1000°C, nitrogen started to evolve, leaving vacancies in the sheet which were filled by graphitization at higher HTT. CQP prepared below 700°C showed much lower discharge capacity than that derived from naphthalene pitch (CNP) at the same temperature. Higher temperature treatment reduced the capacity as observed commonly in the anisotropic carbon, however, the extent of reduction was smaller. Hence, the capacity became the same at 800°C and larger at 1000 and 1200°C than those of CNP. The capacity of CQP after the heat-treatment at 1000°C, where the cycle stability was acceptable was 320 mAh g−1. The nitrogen in the ring disturbs the growth and stacking of hexagonal sheets by the carbonization below 700°C to reduce the capacity of CQP, whereas the vacancies due to nitrogen evolution by the heat-treatment at 1000°C appeared to provide new type sites for lithium charge. Such sites gave potentials of charge and discharge at 0.5–1.0 V reversibly and capacity of 468 mAh g−1. Higher temperature may remove such sites by graphitization. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Coke; B. Calcination; Carbonization; D. Electrochemical properties
1. Introduction Anodic carbon of better performance in the lithium ion battery has been extensively explored [1–5]. Larger capacity, better cycle stability and less undischargeable amount are the key factors. So far, the structures of carbon for better performance have been concerned principally in terms of the size of layered stacking and alignment of the hexagonal planes [6–10]. Very recently, boron within the hexagonal network has been examined from two points of graphitization (stacking extent) and electronic structure of hexagonal sheets [11–13]. * Corresponding author. Tel: +81 92 583 7797; Fax: +81 92 583 7798; e-mail: [email protected]
Other heteroatoms such as oxygen and chlorine are believed to increase the undischargeable amount regardless of their location within the hexagonal sheet or on its periphery. Some other heteroatoms such as nitrogen [14,15] and silicon [16 ] may play favorable roles. In the present study, anodic performances of anisotropic carbon prepared from isotropic quinoline pitch were examined to clarify the contribution of nitrogen atoms within the hexagonal sheets. Nitrogen in quinoline pitch derived carbon can be pyridinic, pyrrolic and quaternary N forms according to the extent of ring condensation by the carbonization. The nitrogen in the hexagonal sheet can also evolve above 1000°C to leave a vacancy in the sheet or fill the vacancy to develop the complete carbon hexagon for the graphitization. Hence, the anodic performances of the carbons derived from quinoline pitch are correlated to their structural factors
0008-6223/99/$ — see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0 0 0 8- 6 2 23 ( 9 8 ) 0 01 9 9 -7
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such as forms of nitrogen and numbers of its vacant site in the hexagonal sheet in addition to peripheral, graphitic and orientational factors of common carbon hexagons.
2. Experimental The isotropic quinoline pitch used in the present study synthesized from quinoline using HF/BF as a catalyst 3 was supplied by Mitsubishi Gas Chemical Co. The elemental analyses and softening point (by flow tester) of as-received pitch are C: 83.79, H: 4.32, N: 10.42 (wt%), H/C: 0.62, N/C: 0.11 (atomic ratio), and 244°C, respectively. The nitrogen content of the pitch was much higher than that of coal-tar pitch [17], favoring its significance in the resultant carbon, even if heat-treatment may reduce its content. 2.1. Carbonization and calcination Quinoline pitch was carbonized at 600°C for 30 min, and then calcined at 700–1200°C for 1 h after grinding. Quinoline pitch was carbonized through a liquid state to give an anisotropic coke of significant volume expansion. The heating rate was 10°C min−1 for both heattreatments. Anisotropic carbon was also prepared from naphthalene mesophase pitch under the same heattreatment conditions for comparison. 2.2. Structures and properties of coke The structure of carbons was analyzed by elemental analyses and X-ray diffraction. Specific resistivities of each carbon electrode were measured using the fourterminal method. Density was measured using the 2-butanol displacement method. 2.3. Electrochemical performances The carbon electrode was made by pressing carbon mixed with 10 wt% PTFE (polytetrafluoroethylene) as a binder on a stainless steel mesh current collector. They were dried at 200°C for 12 h in vacuum. The electrochemical cell used in this experiment consisted of three electrodes, carbon as working electrode, and lithium foil as counter and reference, respectively. The cell was kept in a LiPF /EC+DMC (vol. ratio 1:1) 6 electrolyte for 12 h to wet the electrode well with the electrolyte. A constant current of 0.2mA cm−2 was applied for standard charge and discharge, however, the limited potential method in which the potential was maintained at a fixed potential (in this study, 0 V ) for a long time (40 h) was also examined in order to obtain the maximum charging.
3. Results 3.1. Structures and properties of carbons prepared from quinoline pitch Table 1 shows the compositions of carbons derived from quinoline pitch (CQP) by the carbonization at 600–1200°C. The hydrogen content decreased rapidly between 600–700°C, and then gradually to 1200°C. On the other hand, nitrogen content decreased gradually at first, and then rapidly above 1000°C to 3% of atomic ratio to carbon at 1200°C. Figs. 1 and 2 show XRD patterns and Raman spectra, respectively, of carbons derived from quinoline and naphthalene pitches. The heat-treatment above 800°C provided similar crystallographic structures of carbons regardless of their starting materials although the (002) peak of naphthalene pitch derived carbons appeared more sharp than those of quinoline pitch based ones in the temperature range of 600–700°C. This means that nitrogen in the pitch has little effect on the microstructure of carbons carbonized above 800°C. Fig. 3 shows XPS spectra of N in CQP. Generally, 1s nitrogen exists in three types in carbonaceous substances, i.e. pyrrolic, pyridinic and quaternary types as shown in Fig. 3(a). Nitrogen existed as pyridinic or pyrrolic types in isotropic quinoline pitch. It principally remained as pyridinic or pyrrolic type even after the heat-treatment at 600°C. The long time heat-treatment did not change the types at 600°C. Nitrogen of pyrrolic type started to change to quaternary type by carbonization above 700°C. The pyridinic type of nitrogen was reduced from 1000°C, and then, the quaternary one also decreased by the heat-treatment at higher temperature together with the reduction of overall nitrogen content. Table 2 shows changes of specific resistivity and density of CQP. The very high resistivity of carbon calcined at 600°C decreased sharply to 20 V cm at 700°C, further to 4 V cm at 800°C and then gradually to 1.5 V cm at 1000°C. The resistivity stayed at almost the same value above 1000°C up to 1200°C. The density showed a sharp increase by the calcination from 600 to 1000°C and then gradually up to 1200°C. 3.2. Capacities of carbon anode derived from quinoline pitch by the limited potential method Discharge characteristics of carbons heat-treated at 600–1200°C are shown in Fig. 4, where the carbons were charged by limited potential and discharged by the constant current method in order to obtain the maximum capacity [18]. Discharge capacity decreased by raising HTT above 700°C as the discharge plateau at about 1 V was reduced. The carbon heat-treated at 600°C exhibited much lower capacity than that at 700°C. A longer heat-treatment for 20 h at 600°C very markedly
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I. Mochida et al. / Carbon 37 (1999) 323–327 Table 1 Elemental analyses of carbons prepared from quinoline isotropic pitch Code
CQP600-1 CQP600-20 CQP600-40 CQP700-1 CQP800-1 CQP1000-1 CQP1200-1
HTT (°C )
600 600 600 700 800 1000 1200
Holding time (h)
1 20 40 1 1 1 1
Fig. 1. Comparison of XRD patterns of carbons prepared from quinoline isotropic pitch (CQP) to the carbons from naphthalene mesophase pitch (CNP).
Elemental analyses (wt%)
Atomic ratio
C
H
N
H/C
N/C
83.35 84.17 83.87 88.11 89.40 91.71 96.00
3.09 2.25 2.20 0.87 0.54 0.29 0.14
9.86 10.00 10.01 9.42 8.78 7.05 3.47
0.44 0.32 0.31 0.12 0.07 0.03 0.02
0.10 0.10 0.10 0.09 0.08 0.07 0.03
Fig. 3. Types of nitrogen (a) and XPS spectra of N for carbons 1s prepared from quinoline isotropic pitch (CQP) (b). Table 2 Properties of carbons prepared from quinoline isotropic pitch Code
Specific resistivitya (V cm)
Densityb (g cm−3)
CQP600-1 CQP600-20 CQP600-40 CQP700-1 CQP800-1 CQP1000-1 CQP1200-1
>106 6×105 4×105 20 4 1.5 0.8
1.38 1.47 1.47 1.60 1.68 1.92 2.04
Fig. 2. Comparison of Raman spectra of carbons prepared from quinoline isotropic pitch (CQP) to the carbons from naphthalene mesophase pitch (CNP).
a Measured by the four-terminal method. b 2-Butanol displacement density.
increased the discharge capacity as shown in Fig. 5. Further, a longer time of heat-treatment for 40 h slightly decreased the capacity. The discharge of the carbons consisted basically of three types as observed with anisotropic carbons heat-treated at this same temperature range [19]. The major difference in the capacity appeared in the plateau at about 1 V of discharge potential. In contrast, longer heat-treatment at 700°C increased the capacity only a little, and then decreased it by further longer heat-treatment.
Discharge profiles of carbons prepared from naphthalene (CNP) and quinoline pitches (CQP) by the heattreatment at the same temperature are compared in Fig. 6(a)–(c). They were divided into three temperature ranges according to the type of nitrogen analyzed by XPS (Fig. 3). CQP600 contained pyridinic and pyrrolic nitrogens, CQP700 and CQP800 contained quaternary and pyridinic ones. Nitrogen started to evolve in CQP1000 and CQP1200. The CQP at 600°C showed very poor performance
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Fig. 4. Potential–capacity profiles for the first cycle of CQP at various temperatures. Limited potential charge: constant current charge with 0.2 mA cm−2 followed by constant potential charge at 0 V for 40 h.
Fig. 6. Potential–capacity profiles for the first cycle of CQP and CNP. (a) 600°C, (b) 700–800°C, (c) 1000–1200°C. Limited potential charging. Fig. 5. Change of potential–capacity profiles for the first cycle of CQP depending upon holding time. Limited potential method.
than that of CNP regardless of holding time as shown in Fig. 6(a). Carbons heat-treated at 700°C also showed similar trends, CQP showing much less discharge capacity at 1 V. Heat-treatment at 800°C, both provided similar capacities and profiles as shown in Fig. 6(b), CQP800 showing slightly lower discharge potential in the increasing potential range of about 0.5–1.0 V. Fig. 6(c) shows the difference in the performances of carbons heat-treated at 1000 and 1200°C. In this HTT region, the capacity of CQP was much larger than those of CNP due to larger capacity in the potential range of 0.5 to 1 V than those of CNP. Thus, CQP maintained a larger capacity in a higher discharge potential range which was reduced markedly for CNP heat-treated at the same HTT. Some mechanism to maintain the capacity of this range is involved in CQP. Fig. 7 illustrates the cycle stabilities of anisotropic CQP and CNP. The coulombic efficiency of CQP at the first cycle was worse than that of CNP, however, it became stable after the third cycle, and showed higher stationary capacity by 10% than that of CNP.
Fig. 7. Cycle stability of charge/discharge capacity of CQP1000 and CNP1000. They were charged and discharged by the constant current method (0.2 mA cm−2) at the potential range of 0.0–2.0 V vs Li/Li+.
4. Discussion The present study describes the influence of nitrogen atoms in the carbon on the anodic performances in the lithium ion battery. The influences were observed in two contrasting features, reducing or enhancing the capacity of the carbons heat-treated particularly at 700 and 1000°C, respectively, compared to those of nitrogen free
I. Mochida et al. / Carbon 37 (1999) 323–327
mesophase pitch derived carbon heat-treated at the respective same temperatures. The reduced and enhanced capacity was found at about 1 V and 0.5–1 V, respectively. Both capacities reflect the charge at 0 V. The insertion mechanism and site of lithium ion into anisotropic carbon have been extensively discussed [1,2,4,5,19–24]. There are three mechanisms to explain the charge/discharge profiles of anisotropic carbon [19]. Type I: Reversible charge onto the hexagonal carbon surface by partial charge-transferring (0.1–1.0 V ). Type II: Reversible charge in the interlayers of carbon sheets to form an intercalation compound of higher stage (0.0–0.25 V ). Type III: Charging at 0.0–0.1 V and discharging at about 1 V of lithium located at the edges of carbon clusters in face-to-face with each other. By comparing the performances of CQP to those of CNP, the reduced capacity of the former carbon heattreated at 700°C is ascribed to that of the third type of charge/discharge. Poorer orientation of the former carbon may be responsible, even if the carbon shows the anisotropy as indicated by XRD and Raman. The enhanced capacity of the quinoline pitch derived carbon heat-treated at 1000°C is ascribed to those of both first and third types of charge/discharge. The capacity of the first type can increase because the conversion of pyrrolic and pyridinic N to quaternary N allows the growth and enhances electron affinity of hexagonal sheets. Further higher heat-treatment temperature eliminates nitrogen from hexagonal sheets of not only pyridinic but also quaternary type and the capacity of CQP exceeds that of nitrogen free CNP. In addition, the profiles of the part of increased capacity appear to fit to Type III, which is charged and discharged at 0.0–0.1 V and about 1 V, respectively, even if the heattreatment is higher than expected to allow Type III of charge/discharge in the anisotropic carbon. The edges of oriented clusters in face-to-face are much less with CQP of poor orientation as described above and should also be reduced by such a heat-treatment at higher temperature. Hence, another site should be induced. In this heat-treatment, nitrogen evolves from carbon. Hence, the vacancies may be produced in the hexagonal sheet to contribute charge/discharge of lithium ion. The charge/discharge of such sites may be similar to that of Type III, although the vacancy can be stable for charge/discharge cycles, being different from the edge of the oriented cluster. The difference in the potential of charge and discharge may be due to a large overpotential. The higher temperature heat-treatment may remove
327
these sites, similarly reducing the capacity as observed with other anisotropic carbons [2].
Acknowledgements This study was supported by the Proposal-Based New Industry Creative Type Technology R&D Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
References [1] Sato K, Noguchi M, Demachi A, Oki N, Endo M. Science 1994;264:556. [2] Dahn JR, Zheng T, Liu Y, Xue JS. Science 1995;270:590. [3] Yata S, Kinoshita H, Komori M, Ando N, Kashiwamura T, Harada T, Tanaka K, Yamabe T. Synth. Met. 1994;62:153. [4] Mabuchi A, Tokumitsu K, Fujimoto H, Kasuh T. J. Electrochem. Soc. 1995;142:1041. [5] Tokumitsu K, Mabuchi A, Fujimoto H, Kasuh T. J. Electrochem. Soc. 1996;143:2235. [6 ] Dahn JR, Fong R, Spoon MJ. Phys. Rev. B. 1990;42:6424. [7] Tatsumi K, Akai T, Imamura T, Zaghib K, Iwashita N, Higuchi S, Sawada Y. J. Electrochem. Soc. 1923;1996:143. [8] Dahn JR, Sleigh AK, Shi H, Reimers JN, Zhong Q, Way BM. Electrochim. Acta. 1993;38:1179. [9] Imanishi N, Inagaki M. Carbon 1995;31:1107. [10] Zheng T, Reimers JN, Dahn JR. Phys. Rev. B. 1995;51:734. [11] Dahn JR, Reimers JN, Sleigh AK, Tiedje T. Phys. Rev. B. 1992;45:3773. [12] Way BM, Dahn JR, Tiedje T, Myrtle K, Kasrai M. Phys. Rev. B. 1992;46:1697. [13] Way BM, Dahn JR. J. Electrochem. Soc. 1994;141:907. [14] Weydanz WJ, Way BM, van Buuren T, Dahn JR. J. Electrochem. Soc. 1994;141:900. [15] Morita M, Hanada T, Tsutsumi H, Matsuda Y, Kawaguchi M. J. Electrochem. Soc. 1992;139:1227. [16 ] Wilson AW, Dahn JR. J. Electrochem. Soc. 1995;142:326. [17] Mochida I, Fei YQ, Korai Y, Fujimoto K, Yamashita R. Carbon 1989;27:375. [18] Ku C-H, Korai Y, Sakanishi K, Mochida I. Ext abstr of 23rd biennial conference on carbon. 1997:II.156. [19] Mochida I, Ku C-H, Korai Y. To be printed in J Power Sources. [20] Zheng T, McKinnon WR, Dahn JR. J. Electrochem. Soc. 1996;143:2137. [21] Matsumura Y, Wang S, Mondori J. Carbon 1995;33:1457. [22] Xing W, Xue JS, Zheng T, Gibaud A, Dahn JR. J. Electrochem. Soc. 1996;143:3482. [23] Mori Y, Iriyama T, Hashimoto T, Yamazaki S, Kawakami F, Shiroki H, Yamabe T. J. Power Sources 1995;56:205. [24] Liu Y, Xue JS, Zheng T, Dahn JR. Carbon 1996;34:193.
Anodic performances of anisotropic carbon derived from isotropic quinoline pitch Isao Mochida *, Cha-Hun Ku, Seong-Ho Yoon, Yozo Korai Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received 27 March 1998; accepted 10 June 1998
Abstract The anodic performances of anisotropic carbon derived from quinoline pitch (CQP) to be used in the lithium ion battery were studied to clarify the influence of nitrogen in the carbon hexagonal sheet. The quinoline pitch was synthesized by using HF/BF as a catalyst. Nitrogen in the carbon existed as pyridinic, pyrrolic and quaternary types 3 to be converted in this sequence, depending upon HTT. Above 1000°C, nitrogen started to evolve, leaving vacancies in the sheet which were filled by graphitization at higher HTT. CQP prepared below 700°C showed much lower discharge capacity than that derived from naphthalene pitch (CNP) at the same temperature. Higher temperature treatment reduced the capacity as observed commonly in the anisotropic carbon, however, the extent of reduction was smaller. Hence, the capacity became the same at 800°C and larger at 1000 and 1200°C than those of CNP. The capacity of CQP after the heat-treatment at 1000°C, where the cycle stability was acceptable was 320 mAh g−1. The nitrogen in the ring disturbs the growth and stacking of hexagonal sheets by the carbonization below 700°C to reduce the capacity of CQP, whereas the vacancies due to nitrogen evolution by the heat-treatment at 1000°C appeared to provide new type sites for lithium charge. Such sites gave potentials of charge and discharge at 0.5–1.0 V reversibly and capacity of 468 mAh g−1. Higher temperature may remove such sites by graphitization. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Coke; B. Calcination; Carbonization; D. Electrochemical properties
1. Introduction Anodic carbon of better performance in the lithium ion battery has been extensively explored [1–5]. Larger capacity, better cycle stability and less undischargeable amount are the key factors. So far, the structures of carbon for better performance have been concerned principally in terms of the size of layered stacking and alignment of the hexagonal planes [6–10]. Very recently, boron within the hexagonal network has been examined from two points of graphitization (stacking extent) and electronic structure of hexagonal sheets [11–13]. * Corresponding author. Tel: +81 92 583 7797; Fax: +81 92 583 7798; e-mail: [email protected]
Other heteroatoms such as oxygen and chlorine are believed to increase the undischargeable amount regardless of their location within the hexagonal sheet or on its periphery. Some other heteroatoms such as nitrogen [14,15] and silicon [16 ] may play favorable roles. In the present study, anodic performances of anisotropic carbon prepared from isotropic quinoline pitch were examined to clarify the contribution of nitrogen atoms within the hexagonal sheets. Nitrogen in quinoline pitch derived carbon can be pyridinic, pyrrolic and quaternary N forms according to the extent of ring condensation by the carbonization. The nitrogen in the hexagonal sheet can also evolve above 1000°C to leave a vacancy in the sheet or fill the vacancy to develop the complete carbon hexagon for the graphitization. Hence, the anodic performances of the carbons derived from quinoline pitch are correlated to their structural factors
0008-6223/99/$ — see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0 0 0 8- 6 2 23 ( 9 8 ) 0 01 9 9 -7
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I. Mochida et al. / Carbon 37 (1999) 323–327
such as forms of nitrogen and numbers of its vacant site in the hexagonal sheet in addition to peripheral, graphitic and orientational factors of common carbon hexagons.
2. Experimental The isotropic quinoline pitch used in the present study synthesized from quinoline using HF/BF as a catalyst 3 was supplied by Mitsubishi Gas Chemical Co. The elemental analyses and softening point (by flow tester) of as-received pitch are C: 83.79, H: 4.32, N: 10.42 (wt%), H/C: 0.62, N/C: 0.11 (atomic ratio), and 244°C, respectively. The nitrogen content of the pitch was much higher than that of coal-tar pitch [17], favoring its significance in the resultant carbon, even if heat-treatment may reduce its content. 2.1. Carbonization and calcination Quinoline pitch was carbonized at 600°C for 30 min, and then calcined at 700–1200°C for 1 h after grinding. Quinoline pitch was carbonized through a liquid state to give an anisotropic coke of significant volume expansion. The heating rate was 10°C min−1 for both heattreatments. Anisotropic carbon was also prepared from naphthalene mesophase pitch under the same heattreatment conditions for comparison. 2.2. Structures and properties of coke The structure of carbons was analyzed by elemental analyses and X-ray diffraction. Specific resistivities of each carbon electrode were measured using the fourterminal method. Density was measured using the 2-butanol displacement method. 2.3. Electrochemical performances The carbon electrode was made by pressing carbon mixed with 10 wt% PTFE (polytetrafluoroethylene) as a binder on a stainless steel mesh current collector. They were dried at 200°C for 12 h in vacuum. The electrochemical cell used in this experiment consisted of three electrodes, carbon as working electrode, and lithium foil as counter and reference, respectively. The cell was kept in a LiPF /EC+DMC (vol. ratio 1:1) 6 electrolyte for 12 h to wet the electrode well with the electrolyte. A constant current of 0.2mA cm−2 was applied for standard charge and discharge, however, the limited potential method in which the potential was maintained at a fixed potential (in this study, 0 V ) for a long time (40 h) was also examined in order to obtain the maximum charging.
3. Results 3.1. Structures and properties of carbons prepared from quinoline pitch Table 1 shows the compositions of carbons derived from quinoline pitch (CQP) by the carbonization at 600–1200°C. The hydrogen content decreased rapidly between 600–700°C, and then gradually to 1200°C. On the other hand, nitrogen content decreased gradually at first, and then rapidly above 1000°C to 3% of atomic ratio to carbon at 1200°C. Figs. 1 and 2 show XRD patterns and Raman spectra, respectively, of carbons derived from quinoline and naphthalene pitches. The heat-treatment above 800°C provided similar crystallographic structures of carbons regardless of their starting materials although the (002) peak of naphthalene pitch derived carbons appeared more sharp than those of quinoline pitch based ones in the temperature range of 600–700°C. This means that nitrogen in the pitch has little effect on the microstructure of carbons carbonized above 800°C. Fig. 3 shows XPS spectra of N in CQP. Generally, 1s nitrogen exists in three types in carbonaceous substances, i.e. pyrrolic, pyridinic and quaternary types as shown in Fig. 3(a). Nitrogen existed as pyridinic or pyrrolic types in isotropic quinoline pitch. It principally remained as pyridinic or pyrrolic type even after the heat-treatment at 600°C. The long time heat-treatment did not change the types at 600°C. Nitrogen of pyrrolic type started to change to quaternary type by carbonization above 700°C. The pyridinic type of nitrogen was reduced from 1000°C, and then, the quaternary one also decreased by the heat-treatment at higher temperature together with the reduction of overall nitrogen content. Table 2 shows changes of specific resistivity and density of CQP. The very high resistivity of carbon calcined at 600°C decreased sharply to 20 V cm at 700°C, further to 4 V cm at 800°C and then gradually to 1.5 V cm at 1000°C. The resistivity stayed at almost the same value above 1000°C up to 1200°C. The density showed a sharp increase by the calcination from 600 to 1000°C and then gradually up to 1200°C. 3.2. Capacities of carbon anode derived from quinoline pitch by the limited potential method Discharge characteristics of carbons heat-treated at 600–1200°C are shown in Fig. 4, where the carbons were charged by limited potential and discharged by the constant current method in order to obtain the maximum capacity [18]. Discharge capacity decreased by raising HTT above 700°C as the discharge plateau at about 1 V was reduced. The carbon heat-treated at 600°C exhibited much lower capacity than that at 700°C. A longer heat-treatment for 20 h at 600°C very markedly
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I. Mochida et al. / Carbon 37 (1999) 323–327 Table 1 Elemental analyses of carbons prepared from quinoline isotropic pitch Code
CQP600-1 CQP600-20 CQP600-40 CQP700-1 CQP800-1 CQP1000-1 CQP1200-1
HTT (°C )
600 600 600 700 800 1000 1200
Holding time (h)
1 20 40 1 1 1 1
Fig. 1. Comparison of XRD patterns of carbons prepared from quinoline isotropic pitch (CQP) to the carbons from naphthalene mesophase pitch (CNP).
Elemental analyses (wt%)
Atomic ratio
C
H
N
H/C
N/C
83.35 84.17 83.87 88.11 89.40 91.71 96.00
3.09 2.25 2.20 0.87 0.54 0.29 0.14
9.86 10.00 10.01 9.42 8.78 7.05 3.47
0.44 0.32 0.31 0.12 0.07 0.03 0.02
0.10 0.10 0.10 0.09 0.08 0.07 0.03
Fig. 3. Types of nitrogen (a) and XPS spectra of N for carbons 1s prepared from quinoline isotropic pitch (CQP) (b). Table 2 Properties of carbons prepared from quinoline isotropic pitch Code
Specific resistivitya (V cm)
Densityb (g cm−3)
CQP600-1 CQP600-20 CQP600-40 CQP700-1 CQP800-1 CQP1000-1 CQP1200-1
>106 6×105 4×105 20 4 1.5 0.8
1.38 1.47 1.47 1.60 1.68 1.92 2.04
Fig. 2. Comparison of Raman spectra of carbons prepared from quinoline isotropic pitch (CQP) to the carbons from naphthalene mesophase pitch (CNP).
a Measured by the four-terminal method. b 2-Butanol displacement density.
increased the discharge capacity as shown in Fig. 5. Further, a longer time of heat-treatment for 40 h slightly decreased the capacity. The discharge of the carbons consisted basically of three types as observed with anisotropic carbons heat-treated at this same temperature range [19]. The major difference in the capacity appeared in the plateau at about 1 V of discharge potential. In contrast, longer heat-treatment at 700°C increased the capacity only a little, and then decreased it by further longer heat-treatment.
Discharge profiles of carbons prepared from naphthalene (CNP) and quinoline pitches (CQP) by the heattreatment at the same temperature are compared in Fig. 6(a)–(c). They were divided into three temperature ranges according to the type of nitrogen analyzed by XPS (Fig. 3). CQP600 contained pyridinic and pyrrolic nitrogens, CQP700 and CQP800 contained quaternary and pyridinic ones. Nitrogen started to evolve in CQP1000 and CQP1200. The CQP at 600°C showed very poor performance
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Fig. 4. Potential–capacity profiles for the first cycle of CQP at various temperatures. Limited potential charge: constant current charge with 0.2 mA cm−2 followed by constant potential charge at 0 V for 40 h.
Fig. 6. Potential–capacity profiles for the first cycle of CQP and CNP. (a) 600°C, (b) 700–800°C, (c) 1000–1200°C. Limited potential charging. Fig. 5. Change of potential–capacity profiles for the first cycle of CQP depending upon holding time. Limited potential method.
than that of CNP regardless of holding time as shown in Fig. 6(a). Carbons heat-treated at 700°C also showed similar trends, CQP showing much less discharge capacity at 1 V. Heat-treatment at 800°C, both provided similar capacities and profiles as shown in Fig. 6(b), CQP800 showing slightly lower discharge potential in the increasing potential range of about 0.5–1.0 V. Fig. 6(c) shows the difference in the performances of carbons heat-treated at 1000 and 1200°C. In this HTT region, the capacity of CQP was much larger than those of CNP due to larger capacity in the potential range of 0.5 to 1 V than those of CNP. Thus, CQP maintained a larger capacity in a higher discharge potential range which was reduced markedly for CNP heat-treated at the same HTT. Some mechanism to maintain the capacity of this range is involved in CQP. Fig. 7 illustrates the cycle stabilities of anisotropic CQP and CNP. The coulombic efficiency of CQP at the first cycle was worse than that of CNP, however, it became stable after the third cycle, and showed higher stationary capacity by 10% than that of CNP.
Fig. 7. Cycle stability of charge/discharge capacity of CQP1000 and CNP1000. They were charged and discharged by the constant current method (0.2 mA cm−2) at the potential range of 0.0–2.0 V vs Li/Li+.
4. Discussion The present study describes the influence of nitrogen atoms in the carbon on the anodic performances in the lithium ion battery. The influences were observed in two contrasting features, reducing or enhancing the capacity of the carbons heat-treated particularly at 700 and 1000°C, respectively, compared to those of nitrogen free
I. Mochida et al. / Carbon 37 (1999) 323–327
mesophase pitch derived carbon heat-treated at the respective same temperatures. The reduced and enhanced capacity was found at about 1 V and 0.5–1 V, respectively. Both capacities reflect the charge at 0 V. The insertion mechanism and site of lithium ion into anisotropic carbon have been extensively discussed [1,2,4,5,19–24]. There are three mechanisms to explain the charge/discharge profiles of anisotropic carbon [19]. Type I: Reversible charge onto the hexagonal carbon surface by partial charge-transferring (0.1–1.0 V ). Type II: Reversible charge in the interlayers of carbon sheets to form an intercalation compound of higher stage (0.0–0.25 V ). Type III: Charging at 0.0–0.1 V and discharging at about 1 V of lithium located at the edges of carbon clusters in face-to-face with each other. By comparing the performances of CQP to those of CNP, the reduced capacity of the former carbon heattreated at 700°C is ascribed to that of the third type of charge/discharge. Poorer orientation of the former carbon may be responsible, even if the carbon shows the anisotropy as indicated by XRD and Raman. The enhanced capacity of the quinoline pitch derived carbon heat-treated at 1000°C is ascribed to those of both first and third types of charge/discharge. The capacity of the first type can increase because the conversion of pyrrolic and pyridinic N to quaternary N allows the growth and enhances electron affinity of hexagonal sheets. Further higher heat-treatment temperature eliminates nitrogen from hexagonal sheets of not only pyridinic but also quaternary type and the capacity of CQP exceeds that of nitrogen free CNP. In addition, the profiles of the part of increased capacity appear to fit to Type III, which is charged and discharged at 0.0–0.1 V and about 1 V, respectively, even if the heattreatment is higher than expected to allow Type III of charge/discharge in the anisotropic carbon. The edges of oriented clusters in face-to-face are much less with CQP of poor orientation as described above and should also be reduced by such a heat-treatment at higher temperature. Hence, another site should be induced. In this heat-treatment, nitrogen evolves from carbon. Hence, the vacancies may be produced in the hexagonal sheet to contribute charge/discharge of lithium ion. The charge/discharge of such sites may be similar to that of Type III, although the vacancy can be stable for charge/discharge cycles, being different from the edge of the oriented cluster. The difference in the potential of charge and discharge may be due to a large overpotential. The higher temperature heat-treatment may remove
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these sites, similarly reducing the capacity as observed with other anisotropic carbons [2].
Acknowledgements This study was supported by the Proposal-Based New Industry Creative Type Technology R&D Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
References [1] Sato K, Noguchi M, Demachi A, Oki N, Endo M. Science 1994;264:556. [2] Dahn JR, Zheng T, Liu Y, Xue JS. Science 1995;270:590. [3] Yata S, Kinoshita H, Komori M, Ando N, Kashiwamura T, Harada T, Tanaka K, Yamabe T. Synth. Met. 1994;62:153. [4] Mabuchi A, Tokumitsu K, Fujimoto H, Kasuh T. J. Electrochem. Soc. 1995;142:1041. [5] Tokumitsu K, Mabuchi A, Fujimoto H, Kasuh T. J. Electrochem. Soc. 1996;143:2235. [6 ] Dahn JR, Fong R, Spoon MJ. Phys. Rev. B. 1990;42:6424. [7] Tatsumi K, Akai T, Imamura T, Zaghib K, Iwashita N, Higuchi S, Sawada Y. J. Electrochem. Soc. 1923;1996:143. [8] Dahn JR, Sleigh AK, Shi H, Reimers JN, Zhong Q, Way BM. Electrochim. Acta. 1993;38:1179. [9] Imanishi N, Inagaki M. Carbon 1995;31:1107. [10] Zheng T, Reimers JN, Dahn JR. Phys. Rev. B. 1995;51:734. [11] Dahn JR, Reimers JN, Sleigh AK, Tiedje T. Phys. Rev. B. 1992;45:3773. [12] Way BM, Dahn JR, Tiedje T, Myrtle K, Kasrai M. Phys. Rev. B. 1992;46:1697. [13] Way BM, Dahn JR. J. Electrochem. Soc. 1994;141:907. [14] Weydanz WJ, Way BM, van Buuren T, Dahn JR. J. Electrochem. Soc. 1994;141:900. [15] Morita M, Hanada T, Tsutsumi H, Matsuda Y, Kawaguchi M. J. Electrochem. Soc. 1992;139:1227. [16 ] Wilson AW, Dahn JR. J. Electrochem. Soc. 1995;142:326. [17] Mochida I, Fei YQ, Korai Y, Fujimoto K, Yamashita R. Carbon 1989;27:375. [18] Ku C-H, Korai Y, Sakanishi K, Mochida I. Ext abstr of 23rd biennial conference on carbon. 1997:II.156. [19] Mochida I, Ku C-H, Korai Y. To be printed in J Power Sources. [20] Zheng T, McKinnon WR, Dahn JR. J. Electrochem. Soc. 1996;143:2137. [21] Matsumura Y, Wang S, Mondori J. Carbon 1995;33:1457. [22] Xing W, Xue JS, Zheng T, Gibaud A, Dahn JR. J. Electrochem. Soc. 1996;143:3482. [23] Mori Y, Iriyama T, Hashimoto T, Yamazaki S, Kawakami F, Shiroki H, Yamabe T. J. Power Sources 1995;56:205. [24] Liu Y, Xue JS, Zheng T, Dahn JR. Carbon 1996;34:193.