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Expansion of mesocarbon microbeads
Carbon 44 (2006) 730–733 www.elsevier.com/locate/carbon
Expansion of mesocarbon microbeads Shubin Yang, Huaihe Song *, Xiaohong Chen Key Laboratory of Science and Technology of Controllable Chemical Reaction, Ministry of Education, Beijing University of Chemical Technology, 100029 Beijing, PR China Received 18 July 2005; accepted 19 September 2005 Available online 9 November 2005
Abstract Expanded mesocarbon microbeads (EMCMB) were prepared from graphitized mesocarbon microbeads by a chemical method. The expanded volume of EMCMB was significantly influenced by the reaction time, temperature, weight ratio of sulfuric acid and nitric acid, the maximum value of which was 4.1 ml/g. The morphology and structure characters of MCMB after chemical reaction and expansion were also investigated by SEM, BET and XRD measurements. The results show that the spherical, layered structure of MCMB could be verified convincingly by SEM micrographs of EMCMB, and the values of crystallite parameters of intercalated MCMB and EMCMB such as Lc and La decreased significantly compared with that of pristine MCMB, indicating that the stack height and stack width of carbon layers in a crystallite decreased after intercalation reaction and expansion. Meanwhile, EMCMB with appropriate porosity using as anode material for lithium-ion batteries exhibited an excellent high-rate discharge capacity and good cycle stability at the large current density. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon microbeads; Intercalation compounds; X-ray diffraction; Electrochemical properties
To improve the rate capability and cycle ability of lithium ion batteries using in electric vehicle fields, various kinds of carbonaceous materials, such as graphitized carbons, hard carbon spherules [1], three-dimensionally ordered carbons [2], etc., have been extensively studied to search for an ideal anode material with an excellent highrate performance. For the graphitized carbons, many researchers indicated that the solid-state diffusion of lithium ion in electrode is the rate-limiting step, and could be enhanced by increasing the porosity of electrodes [3]. However, the conventional graphitized carbons generally possess very low porosity, which largely limits the diffusion of lithium ion in the carbon electrodes and significantly affects their rate performances. For the non-graphitizable carbon materials, although lithium-ion transfer in the carbons is faster than that in graphite due to the higher porosities [1], they usually require additional binder and *
Corresponding author. Tel.: +86 10 64434916; fax: +86 10 64437587. E-mail address: [email protected] (H. Song).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.019
conducting agents to improve their electric conductivity, which leads to lower energy densities of batteries. And some of these materials exhibit potential hysteresis. Our research work focused on maintaining the electric conductivity and increasing the porosity of graphitized carbons for the purpose of designing carbon anode materials with high-rate capability. In this letter, expanded mesocarbon microbeads (EMCMB) with appropriate porosity were prepared from graphitized mesocarbon microbeads (MCMB). The influences of reaction time on the morphologies, structures and electrochemical properties of EMCMB were investigated by Scanning electron microscope (SEM), X-ray diffraction (XRD) and galvanostatic charge/discharge measurements. EMCMB was prepared by a modified method based on the previous work about the preparation of expanded graphite (EG) [4]. Graphitized MCMB (manufactured by Osaka Gas Corp.) was added into a mixture of concentrated sulfuric acid (98%) and nitric acid (65%) under magnetic stirring. After 30 min KMnO4 and FeCl3 were
S. Yang et al. / Carbon 44 (2006) 730–733 4.0 3.5
Vg (ml/g)
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
20
40
60
80
100
120
140
Reaction time (h)
Fig. 1. Effect of reaction time on the expanded volume of EMCMB.
gradually added into the mixture to participate in the reaction with MCMB. After reaction for certain hours, the mixture was filtered, washed and dried. Finally, the intercalated MCMB (IMCMB) was quickly sent to a tubular furnace pre-heated to 900 °C, worm-like EMCMB was generated. The expanded volume of MCMB is significantly influenced by reaction time, the results are shown in Fig. 1. It can be seen that with reaction time prolonging from 2 to 125 h, the expanded volume of EMCMB increases to the
731
highest value of 3.9 ml/g at 12 h, and then decreases. When reaction time is longer than 96 h, the expanded volume tends to a constant value of 2.8 ml/g. The reason for this should be that, when reaction time is too short, the intercalation reaction is incomplete. It is difficult to cause great volume change of MCMB by burst for low amounts of intercalation compounds in the carbon layers. Whereas when reaction time is too long, the distance between carbon layers becomes too wide to intercalate due to over oxidation of MCMB. Compared with the value of 1.50 m2/g of MCMB, the surface areas of EMCMB increase to about 13–18 m2/g depending on different reaction time. Fig. 2 shows the morphology variations of MCMB before and after expansion. It can be seen that MCMB was transformed into elliptical EMCMB from original regular spherule. Further investigation on the micrographs of EMCMB reveals that MCMB was extended along c-axis, most of the carbon sheets were basically parallel to each other which deformed in an irregular pattern. It is ascribed to the layered structure of MCMB, which allows additional atoms or molecules to occupy spaces between the carbon layers to form graphite intercalation compounds (GIC) under appropriate conditions. When GIC are given a thermal shock, they decompose and tear the layers apart, leading to an expansion along the c-axis. So the loading quantity in carbon layers significantly affects the expansion
Fig. 2. SEM micrographs of pristine MCMB (a) and EMCMB reacted for various time: (b) 12 h; (c) 24 h and (d) 96 h.
a g f e d
Intensity (CPS)
S. Yang et al. / Carbon 44 (2006) 730–733
Intensity (CPS)
732
a g ef d c b
c b
7
14 2-Theta (degree)
24
21
26
28
2-Theta (degree)
Fig. 3. XRD patterns of pristine MCMB (a), dry IMCMB reacted for various time ((b) 12 h, (c) 24 h, (d) 96 h)) and corresponding EMCMB ((e) 12 h, (f) 24 h and (g) 96 h).
28
Lc(nm) La(nm)
26
a
400
b
22 20 18
c
16 14 12
d
10 0
20
40
60
80
100
120
140
Reaction time (h)
Reversible capacity (mAh/g)
Lc and La (nm)
24
300
200
Pristine MCMB EMCMB reacted for 2 h EMCMB reacted for 12 h EMCMB reacted for 96 h
100
Fig. 4. Variations of the crystallite sizes, Lc and La (nm) of IMCMB (b,d) and EMCMB (a,c). 0 0
2
4
6
8
10
Cycle number
(a) 400
Reversible capacity (mAh/g)
rate and morphology of EMCMB. It can be seen from Fig. 2b–d that the expansion of EMCMB reacted for 12 and 24 h is remarkable, but the EMCMB reacted for 96 h has more cracks and lower expansion ratio due to over oxidation. The structure changes of MCMB were investigated by XRD. For pristine MCMB, the (0 0 2) diffraction peak appeared at 26.44° is strong in intensity and sharp in width. After intercalation and dryness, the (0 0 2) peak of IMCMB is shifted to the smaller angle and becomes dispersive, wide and significantly decreases in intensity (Fig. 3b– d). Moreover, with reaction time prolonging from 12 to 96 h, the intensity increases again, suggesting the quantity of intercalation compounds decreases with the increasing of reaction time in this range, which is well consistent with the changes of its expansion volume and morphology. Meanwhile, an additional peak around 2h = 12° in XRD patterns of IMCMB is observed, which is corresponding to the (0 0 1) peak of graphite oxide [5] resulting from oxidation during intercalation process. After expansion, the (0 0 2) peak is shifted back, but its intensity is far smaller than that of the pristine MCMB, implying that the ordered
300
200
Pristine MCMB EMCMB reacted for 2 h EMCMB reacted for 12 h
100
0 0
(b)
10
20
30
40
50
Cycle number
Fig. 5. Cycle performance of MCMB and EMCMB at the current density of (a) 0.2 mA/cm2 and (b) 0.8 mA/cm2, respectively.
S. Yang et al. / Carbon 44 (2006) 730–733
degree of MCMB decreased and cannot be recovered completely (Fig. 3e–f). Crystallite parameters Lc and La were calculated from the peak position and the half-height width of the (0 0 2) and (1 1 0) reflections using the Scherrer equation, respectively [6]. The results are shown in Fig. 4. It is observed that, after reaction, La and Lc of IMCMB decrease rapidly to the constant values of 22 and 10.5 nm (Fig. 4b and d), respectively. However, we cannot calculate the intercalation stage index from XRD patterns, suggesting the higher stage structure of IMCMB. In comparison, although the La and Lc of EMCMB remove back after expansion, they are still much smaller than that of MCMB (Fig. 4a and c). This observation suggests that the stacking height and width of graphite crystallite decreased through the processes, which would influence the reversible capacities of EMCMB [6]. Cycle performances of EMCMB electrodes at the charge/discharge current density of 0.2 and 0.8 mA/cm2 are illustrated in Fig. 5. It can be seen that, at low current density of 0.2 mA/cm2, the reversible capacity of pristine MCMB electrode fixed at 290 mA h/g. In contrast, the EMCMB reacted for 12 h displayed the high capacity of 310 mA h/g and excellent cycle performance. Contrary the reversible capacity of EMCMB reacted for 96 h was only 280 mA h/g. At higher current density of 0.8 mA/ cm2, the electrode composed of EMCMB reacted for 12 h showed the best cycle performance with reversible capacity of 260 mA h/g, which is higher than that of pristine MCMB electrode (227 mA h/g). This should be attributed to the special morphology and worm-like structure of EMCMB, which may efficiently relax the volume changes of EMCMB particles during cycling. In addition, the higher porosity could decrease the effective current density,
733
make more electrolytes immerge into electrode and was favorable for rapid transport of lithium ions in electrode. In summary, EMCMB, prepared from graphitized mesocarbon microbeads, exhibited an excellent high-rate charge–discharge capacity (260 mA h/g) and a good cycle stability when was used as anode material for lithium-ion batteries. It provides a new and effective method for improving the high-rate capability of MCMB. Acknowledgements This work was supported by Beijing Nova Plan of Science and Technology (Nos. 954811400 and 200309B) and Program for New Century Excellent Talents in University of China (NCET-04-0122). References [1] Wang Q, Li H, Chen LQ, Huang XJ. Novel spherical microporous carbon as anode material for Li-ion batteries. Solid State Ionics 2002;152–153:43–50. [2] Wang T, Liu XY, Zhao DY, Jiang ZY. The unusual electrochemical characteristics of a novel three-dimensional ordered bicontinuous mesoporous carbon. Chem Phys Lett 2004;389(4–6):327–31. [3] Sawai K, Ohzuku T. Factors affecting rate capability of graphite electrodes for lithium-ion batteries. J Electrochem Soc 2003;150(6): A674–A678. [4] Zheng GH, Wu JS, Wang WP, Pan CY. Characterizations of expanded graphite/polymer composites prepared by in situ polymerization. Carbon 2004;42(14):2839–47. [5] Matsuo Y, Niwa T, Sugie Y. Preparation and characterization of cationic surfactant-intercalated graphite oxide. Carbon 1999; 37(6): 897–901. [6] Fukuda K, Kikuya K, Isono K, Yoshio M. Foliated natural graphite as the anode material for rechargeable lithium-ion cells. J Power Sour 1997;69(1–2):165–8.
Expansion of mesocarbon microbeads Shubin Yang, Huaihe Song *, Xiaohong Chen Key Laboratory of Science and Technology of Controllable Chemical Reaction, Ministry of Education, Beijing University of Chemical Technology, 100029 Beijing, PR China Received 18 July 2005; accepted 19 September 2005 Available online 9 November 2005
Abstract Expanded mesocarbon microbeads (EMCMB) were prepared from graphitized mesocarbon microbeads by a chemical method. The expanded volume of EMCMB was significantly influenced by the reaction time, temperature, weight ratio of sulfuric acid and nitric acid, the maximum value of which was 4.1 ml/g. The morphology and structure characters of MCMB after chemical reaction and expansion were also investigated by SEM, BET and XRD measurements. The results show that the spherical, layered structure of MCMB could be verified convincingly by SEM micrographs of EMCMB, and the values of crystallite parameters of intercalated MCMB and EMCMB such as Lc and La decreased significantly compared with that of pristine MCMB, indicating that the stack height and stack width of carbon layers in a crystallite decreased after intercalation reaction and expansion. Meanwhile, EMCMB with appropriate porosity using as anode material for lithium-ion batteries exhibited an excellent high-rate discharge capacity and good cycle stability at the large current density. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon microbeads; Intercalation compounds; X-ray diffraction; Electrochemical properties
To improve the rate capability and cycle ability of lithium ion batteries using in electric vehicle fields, various kinds of carbonaceous materials, such as graphitized carbons, hard carbon spherules [1], three-dimensionally ordered carbons [2], etc., have been extensively studied to search for an ideal anode material with an excellent highrate performance. For the graphitized carbons, many researchers indicated that the solid-state diffusion of lithium ion in electrode is the rate-limiting step, and could be enhanced by increasing the porosity of electrodes [3]. However, the conventional graphitized carbons generally possess very low porosity, which largely limits the diffusion of lithium ion in the carbon electrodes and significantly affects their rate performances. For the non-graphitizable carbon materials, although lithium-ion transfer in the carbons is faster than that in graphite due to the higher porosities [1], they usually require additional binder and *
Corresponding author. Tel.: +86 10 64434916; fax: +86 10 64437587. E-mail address: [email protected] (H. Song).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.019
conducting agents to improve their electric conductivity, which leads to lower energy densities of batteries. And some of these materials exhibit potential hysteresis. Our research work focused on maintaining the electric conductivity and increasing the porosity of graphitized carbons for the purpose of designing carbon anode materials with high-rate capability. In this letter, expanded mesocarbon microbeads (EMCMB) with appropriate porosity were prepared from graphitized mesocarbon microbeads (MCMB). The influences of reaction time on the morphologies, structures and electrochemical properties of EMCMB were investigated by Scanning electron microscope (SEM), X-ray diffraction (XRD) and galvanostatic charge/discharge measurements. EMCMB was prepared by a modified method based on the previous work about the preparation of expanded graphite (EG) [4]. Graphitized MCMB (manufactured by Osaka Gas Corp.) was added into a mixture of concentrated sulfuric acid (98%) and nitric acid (65%) under magnetic stirring. After 30 min KMnO4 and FeCl3 were
S. Yang et al. / Carbon 44 (2006) 730–733 4.0 3.5
Vg (ml/g)
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
20
40
60
80
100
120
140
Reaction time (h)
Fig. 1. Effect of reaction time on the expanded volume of EMCMB.
gradually added into the mixture to participate in the reaction with MCMB. After reaction for certain hours, the mixture was filtered, washed and dried. Finally, the intercalated MCMB (IMCMB) was quickly sent to a tubular furnace pre-heated to 900 °C, worm-like EMCMB was generated. The expanded volume of MCMB is significantly influenced by reaction time, the results are shown in Fig. 1. It can be seen that with reaction time prolonging from 2 to 125 h, the expanded volume of EMCMB increases to the
731
highest value of 3.9 ml/g at 12 h, and then decreases. When reaction time is longer than 96 h, the expanded volume tends to a constant value of 2.8 ml/g. The reason for this should be that, when reaction time is too short, the intercalation reaction is incomplete. It is difficult to cause great volume change of MCMB by burst for low amounts of intercalation compounds in the carbon layers. Whereas when reaction time is too long, the distance between carbon layers becomes too wide to intercalate due to over oxidation of MCMB. Compared with the value of 1.50 m2/g of MCMB, the surface areas of EMCMB increase to about 13–18 m2/g depending on different reaction time. Fig. 2 shows the morphology variations of MCMB before and after expansion. It can be seen that MCMB was transformed into elliptical EMCMB from original regular spherule. Further investigation on the micrographs of EMCMB reveals that MCMB was extended along c-axis, most of the carbon sheets were basically parallel to each other which deformed in an irregular pattern. It is ascribed to the layered structure of MCMB, which allows additional atoms or molecules to occupy spaces between the carbon layers to form graphite intercalation compounds (GIC) under appropriate conditions. When GIC are given a thermal shock, they decompose and tear the layers apart, leading to an expansion along the c-axis. So the loading quantity in carbon layers significantly affects the expansion
Fig. 2. SEM micrographs of pristine MCMB (a) and EMCMB reacted for various time: (b) 12 h; (c) 24 h and (d) 96 h.
a g f e d
Intensity (CPS)
S. Yang et al. / Carbon 44 (2006) 730–733
Intensity (CPS)
732
a g ef d c b
c b
7
14 2-Theta (degree)
24
21
26
28
2-Theta (degree)
Fig. 3. XRD patterns of pristine MCMB (a), dry IMCMB reacted for various time ((b) 12 h, (c) 24 h, (d) 96 h)) and corresponding EMCMB ((e) 12 h, (f) 24 h and (g) 96 h).
28
Lc(nm) La(nm)
26
a
400
b
22 20 18
c
16 14 12
d
10 0
20
40
60
80
100
120
140
Reaction time (h)
Reversible capacity (mAh/g)
Lc and La (nm)
24
300
200
Pristine MCMB EMCMB reacted for 2 h EMCMB reacted for 12 h EMCMB reacted for 96 h
100
Fig. 4. Variations of the crystallite sizes, Lc and La (nm) of IMCMB (b,d) and EMCMB (a,c). 0 0
2
4
6
8
10
Cycle number
(a) 400
Reversible capacity (mAh/g)
rate and morphology of EMCMB. It can be seen from Fig. 2b–d that the expansion of EMCMB reacted for 12 and 24 h is remarkable, but the EMCMB reacted for 96 h has more cracks and lower expansion ratio due to over oxidation. The structure changes of MCMB were investigated by XRD. For pristine MCMB, the (0 0 2) diffraction peak appeared at 26.44° is strong in intensity and sharp in width. After intercalation and dryness, the (0 0 2) peak of IMCMB is shifted to the smaller angle and becomes dispersive, wide and significantly decreases in intensity (Fig. 3b– d). Moreover, with reaction time prolonging from 12 to 96 h, the intensity increases again, suggesting the quantity of intercalation compounds decreases with the increasing of reaction time in this range, which is well consistent with the changes of its expansion volume and morphology. Meanwhile, an additional peak around 2h = 12° in XRD patterns of IMCMB is observed, which is corresponding to the (0 0 1) peak of graphite oxide [5] resulting from oxidation during intercalation process. After expansion, the (0 0 2) peak is shifted back, but its intensity is far smaller than that of the pristine MCMB, implying that the ordered
300
200
Pristine MCMB EMCMB reacted for 2 h EMCMB reacted for 12 h
100
0 0
(b)
10
20
30
40
50
Cycle number
Fig. 5. Cycle performance of MCMB and EMCMB at the current density of (a) 0.2 mA/cm2 and (b) 0.8 mA/cm2, respectively.
S. Yang et al. / Carbon 44 (2006) 730–733
degree of MCMB decreased and cannot be recovered completely (Fig. 3e–f). Crystallite parameters Lc and La were calculated from the peak position and the half-height width of the (0 0 2) and (1 1 0) reflections using the Scherrer equation, respectively [6]. The results are shown in Fig. 4. It is observed that, after reaction, La and Lc of IMCMB decrease rapidly to the constant values of 22 and 10.5 nm (Fig. 4b and d), respectively. However, we cannot calculate the intercalation stage index from XRD patterns, suggesting the higher stage structure of IMCMB. In comparison, although the La and Lc of EMCMB remove back after expansion, they are still much smaller than that of MCMB (Fig. 4a and c). This observation suggests that the stacking height and width of graphite crystallite decreased through the processes, which would influence the reversible capacities of EMCMB [6]. Cycle performances of EMCMB electrodes at the charge/discharge current density of 0.2 and 0.8 mA/cm2 are illustrated in Fig. 5. It can be seen that, at low current density of 0.2 mA/cm2, the reversible capacity of pristine MCMB electrode fixed at 290 mA h/g. In contrast, the EMCMB reacted for 12 h displayed the high capacity of 310 mA h/g and excellent cycle performance. Contrary the reversible capacity of EMCMB reacted for 96 h was only 280 mA h/g. At higher current density of 0.8 mA/ cm2, the electrode composed of EMCMB reacted for 12 h showed the best cycle performance with reversible capacity of 260 mA h/g, which is higher than that of pristine MCMB electrode (227 mA h/g). This should be attributed to the special morphology and worm-like structure of EMCMB, which may efficiently relax the volume changes of EMCMB particles during cycling. In addition, the higher porosity could decrease the effective current density,
733
make more electrolytes immerge into electrode and was favorable for rapid transport of lithium ions in electrode. In summary, EMCMB, prepared from graphitized mesocarbon microbeads, exhibited an excellent high-rate charge–discharge capacity (260 mA h/g) and a good cycle stability when was used as anode material for lithium-ion batteries. It provides a new and effective method for improving the high-rate capability of MCMB. Acknowledgements This work was supported by Beijing Nova Plan of Science and Technology (Nos. 954811400 and 200309B) and Program for New Century Excellent Talents in University of China (NCET-04-0122). References [1] Wang Q, Li H, Chen LQ, Huang XJ. Novel spherical microporous carbon as anode material for Li-ion batteries. Solid State Ionics 2002;152–153:43–50. [2] Wang T, Liu XY, Zhao DY, Jiang ZY. The unusual electrochemical characteristics of a novel three-dimensional ordered bicontinuous mesoporous carbon. Chem Phys Lett 2004;389(4–6):327–31. [3] Sawai K, Ohzuku T. Factors affecting rate capability of graphite electrodes for lithium-ion batteries. J Electrochem Soc 2003;150(6): A674–A678. [4] Zheng GH, Wu JS, Wang WP, Pan CY. Characterizations of expanded graphite/polymer composites prepared by in situ polymerization. Carbon 2004;42(14):2839–47. [5] Matsuo Y, Niwa T, Sugie Y. Preparation and characterization of cationic surfactant-intercalated graphite oxide. Carbon 1999; 37(6): 897–901. [6] Fukuda K, Kikuya K, Isono K, Yoshio M. Foliated natural graphite as the anode material for rechargeable lithium-ion cells. J Power Sour 1997;69(1–2):165–8.