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Structure of heat-treated mesoporous carbon and its electrochemical lithium intercalation behavior
Materials Chemistry and Physics 147 (2014) 1175e1182
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structure of heat-treated mesoporous carbon and its electrochemical lithium intercalation behavior Tomoki Tsumura a, *, Asuka Arikawa a, Taro Kinumoto a, Yasuhiko Arai a, Takahiro Morishita b, Hironori Orikasa b, Michio Inagaki c, Masahiro Toyoda a a b c
Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu Oita, Oita 870-1192, Japan Advanced Carbon Technology Center, Toyo Tanso Co., Ltd., 5-7-12 Takashima, Nishiyodogawa-ku, Osaka 555-0011, Japan Professor Emeritus of Hokkaido University, 228-7399 Nakagawa, Hosoe-cho, Kita-ku, Hamamatsu 431-1304, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Mesoporous carbon was heated in the temperature range between 1800 C and 3000 C. The growth of hexagonal carbon layers occurred in the mesopore carbon walls. Electrochemical lithium intercalation behavior was investigated for the graphitized mesoporous carbon.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2013 Received in revised form 27 June 2014 Accepted 4 July 2014 Available online 19 July 2014
Mesoporous carbon (MPC) was synthesized via the thermal decomposition of magnesium citrate in a flow of Ar gas, followed by dissolution of MgO nanoparticles in the carbonaceous pyrolysis product. The MPC was then heated at 1800, 2000, 2200, 2400, 2500, 2700 and 3000 C for 30 min. The MPC samples heated at 1800 and 2000 C having a mesopore structure with graphitized carbon walls exhibited an enhanced rate capability as a composite anode for lithium ion batteries. Heat treatment of the MPC from 2200 to 2700 C led to a partial collapse of micropores and mesopores, which decreased the charge/ discharge capacity. The MPC samples heated at 2700 and 3000 C having thick graphitized carbon layers and a mesopore structure with graphitized carbon walls showed charge/discharge curves with three lithium intercalation potential plateaus at approximately 0.2, 1.0, and 1.5 V vs. Li/Liþ. The MPC sample heated at 3000 C was the best anode material for lithium ion batteries among the MPC samples evaluated in this study because of its low irreversible capacity, high columbic efficiency, and high rate capability. © 2014 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Annealing Electrochemical techniques Electron microscopy
1. Introduction Lithium ion batteries (LIBs) have been used as power sources for portable electronics, and graphite has been extensively used as anodes for LIBs owing to its high columbic efficiency and flat reaction potential below 0.3 V vs. Li/Liþ [1e3]. Because of the high
* Corresponding author. Tel./fax: þ81 97 554 7906. E-mail address: [email protected] (T. Tsumura). http://dx.doi.org/10.1016/j.matchemphys.2014.07.001 0254-0584/© 2014 Elsevier B.V. All rights reserved.
functionalization and diversification of portable electronics and the expanding use of LIBs as energy storage devices for large-scale equipment such as electric vehicles and power generation systems for recyclable energy, there has been a great need to improve the energy and power densities of LIBs. Therefore, research for alternatives to graphite as anode materials has been active around the world [4,5]. Because carbon materials are non-toxic, inexpensive, and easily obtainable, mesoporous carbon (MPC), which has a high capacity and rate capability, has been proposed to replace graphite [6e9]. The capacity of MPC is beyond the theoretical
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capacity of 372 mAh g1 for graphite, because lithium ions are not only intercalated between the hexagonal carbon layers but also accommodated on defects, pores, and edges of the stacked hexagonal carbon layers. Although the relation between the structural parameters such as graphitization degree, number of stacked carbon layers, surface area, and pore distribution of MPC and its capacity has been investigated, the mechanism for the accommodation of lithium has not been clear until now. The high rate capability of MPC is reasonably supposed to be due to the short diffusion path in the carbon host material and the fast diffusion of lithium ions in the electrolyte through interconnected mesopores. Although they have these two advantages, i.e., large capacity and high rate performance, they also have some drawbacks, such as a large polarization between the charge and discharge potentials, also called hysteresis, indicating energy loss during the charge/ discharge cycle and a low electric conductivity attributed to a low graphitization degree. In order to improve the electric conductivity and electrochemical performance, there have been some reports discussing the graphitization of MPC and its electrochemical properties [10e12]. However, in most cases, MPC heated to temperatures above 1500 C have a surface area of less than 500 m2 g1 and most of the mesopores are thought to have collapsed [13e20]. Therefore, the effects of graphitized mesopore carbon walls on the electrochemical performance of MPC are not clear. In the present paper, the effects of the structural changes caused by the heat treatment of MPC on its electrochemical properties as an anode material for LIBs was investigated by using MPC prepared through the MgO-templating method developed in our group [21,22]. Since the entire mesopore structure remained at the temperatures up to 2000 C and part of the mesopore structure remained even after heat treatment at 3000 C, the electrochemical properties of mesopore carbon walls graphitized at different temperatures can be investigated clearly.
Thermal treatment of the MPC samples was carried out by using a graphitizing furnace at 1800, 2000, 2200, 2400, 2500, 2700, and 3000 C. The starting MPC was passed through a 100 mm sieve and set in a graphite crucible. The furnace tube was heated from room temperature to 1600 C, and evacuated by rotary vacuum pump down to 0.1 Torr. The furnace was then heated under Ar gas flow up to different target temperatures and maintained for 30 min at those temperatures. The heat-treated samples were gradually cooled down to room temperature in the furnace. The flow rate of Ar gas was 6 cm3 min1 and the heating rate was 20 C min1. The resultant samples after heat treatment were labeled according to their MPC-target temperature (MPC-1800, -2000, -2200, -2400, -2500, -2700, and -3000). 2.2. Characterization N2 gas adsorption/desorption isotherms for the samples degassed at 200 C under reduced pressure for 24 h were measured at 197 C by using an Autosorb-3B system, Quantachrome Instruments. BrunauereEmmetteTeller (BET) surface area and BarreteJoynereHalenda (BJH) pore distribution were determined from the N2 gas isotherms measured. Morphological changes after heat treatment were observed under a transmission electron microscope (TEM, JEM2100, JEOR Co. Ltd.). X-ray diffraction (XRD) patterns for the samples were recorded by using Ni-filtered CuKa radiation (RINT Ultima III, Rigaku Co. Ltd.). The accelerating voltage and tube current were 40 kV and 40 mA, respectively. The Raman spectra for the samples were measured by using an Ar laser (LabRAM ARAMIS, Horiba Co. Ltd.). The thermal stability in static air was investigated by thermogravimetric-differential thermal analysis (TG-DTA, Thermo plus TG8120, Rigaku Co. Ltd.). The heating rate was 10 C min1. 2.3. Electrochemical measurement
2. Experimental 2.1. Synthesis Mesoporous carbon (MPC) prepared thorough pyrolysis of magnesium citrate Mg3(C6H5O7)2 at 900 C under Ar gas flow was supplied by Toyo Tanso Co. Ltd. Detailed of the procedure for the synthesis of the starting MPC are reported elsewhere [21,22].
Electrochemical lithium intercalation/deintercalation for the samples was conducted by using an electrochemical cell (Takumi Giken Limited Private Co.), depicted in Fig. 1. The working electrode was a composite of the samples, carbon black, and polyvinylidene fluoride (PVDF) in a 8:1:1 mixing weight ratio coated on Cu foil. The counter electrode was lithium foil. A 1 M LiBF4 ethylenecarbonate/ diethylcarbonate, 1/1 vol% solution was used as the electrolyte. The
Fig. 1. Schematic illustration of the construction of the electrochemical cell used.
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cell was assembled in a glove box filled with dry Ar gas. Charge/ discharge curves were measured at a constant current of 40 mA g1 and the rate capability was investigated by increasing the current density in a step-by-step manner every five charge/discharge cycles from 40 to 100, 200, 500, and 1000 mA g1 3. Results and discussion 3.1. Structure After heating the starting MPC in a graphitization furnace in the temperature range between 1800 and 3000 C, the sample weight decreased linearly with increasing temperature. The weight of the MPC sample heated at 3000 C (MPC-3000) was 84 wt% of the initial weight, as shown Fig. 2. The weight loss was caused by the release of compounds containing hydrogen and oxygen, such as H2O, CO, and CO2. Fig. 3 shows the N2 gas adsorption/desorption isotherms for MPC samples heated at different temperatures and the starting MPC. There is a steep increase in the adsorption/desorption gas volume near a relative pressure of approximately 0 and a gradual increase at relative pressures ranging from 0.4 to 1.0 for the starting MPC, which indicates that the starting MPC had a certain amount of micropores and mesopores. With increasing heating temperature, the number of micropores and mesopores decreased, especially from 2500 to 3000 C, which shows a sharp decrease. BJH analysis for mesopore distribution is shown in Fig. 4. For the starting MPC, pores approximately 4 nm in diameter are observed. The pore diameter decreased from 4 to 3 nm during heat treatment up to 1800 C. For the MPC samples heated at temperatures above 2000 C, there was no change in the diameter but there was a change in the number of mesopores with diameters of 3 nm. Changes in BET surface area with increasing temperature are plotted in Fig. 5. The BET surface area for the MPC samples heated at temperatures up to 2000 C decreased gradually with increasing temperature. This decrease in BET surface area is due to the shrink of mesopore as shown in Fig. 4 and the decrease of micropore volume at a relative pressure of 0 in Fig. 3. At 2000 C, the BET surface area drastically decreased from 1700 m2 g1 for the MPC sample heated at 2000 C (MPC-2000) to 350 m2 g1 for the MPC sample heated at 3000 C (MPC-3000).
Fig. 2. Yield of the MPC samples heated at different temperatures.
Fig. 3. Nitrogen adsorption/desorption isotherms of MPC and MPC samples heated at different temperatures.
The Raman spectra of the MPC samples heated at the different temperatures are shown in Fig. 6. The starting MPC shows broad D and G bands at 1350 and 1600 cm1, respectively. The full width at half maximum of the two bands decreased with increasing temperature and an additional band, the D0 band, appears for the MPC samples heated at temperatures above 1800 C. Changes in the integral intensity ratio between the D and G bands for the heattreated MPC samples, ID/IG, which is used as a measure of the graphitization degree, are shown in Fig. 7. ID/IG decreases gradually with increasing temperature, indicating that the MPC samples were graphitized with increasing temperature. XRD patterns of the MPC samples heated at different temperatures are shown in Fig. 8. The starting MPC shows two broad peaks at 23 and 43 , assigned to the 002 and 10 diffractions, respectively. For the MPC samples heat treated at temperatures of 1800 and 2000 C, the full width at half maximum of the 10 diffraction peak decreased with increasing temperature, whereas the broad 002
Fig. 4. Pore size distributions of MPC and the MPC samples heated at different temperatures.
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Fig. 5. Change in BET surface area of the MPC samples with increasing heating temperatures.
diffraction line does not show any significant change. This indicates that a hexagonal carbon layer, or graphene, grew in the ab plane but that increased stacking of the carbon layers perpendicular to the ab plane, which is probably parallel to the mesopore carbon walls, does not occur. Upon heating from 2200 to 3000 C, a sharp 002 diffraction peak appears and increases gradually with increasing temperature. Orikasa and Morishita reported that the sharp 002 peak is due to small graphite flakes, which they observed under
Fig. 6. Raman spectra of MPC and the MPC samples heated at different temperatures.
Fig. 7. Change in the ID/IG ratio of Raman peak of MPC and the MPC samples heated at different temperatures.
TEM. They suggested that these graphite flakes were formed by vapor deposition or collapse of mesopores [23]. Furthermore, they pointed out that the sharp peak had at least three components with different d-spacings, but the origin of graphite flakes having two different d-spacings was not clear. For the broad 002 diffraction peak, the intensity increases, the width decreases, and the position
Fig. 8. XRD patterns of MPC and the MPC samples heated at different temperatures.
T. Tsumura et al. / Materials Chemistry and Physics 147 (2014) 1175e1182
shifts to a higher angle with increasing temperature. For the 10 peak, the intensity also increases with increasing temperature. These results indicate that stacking of the hexagonal carbon layers occurs in the [002] direction and that the hexagonal carbon layers grow in the ab plane. Fig. 9 shows TEM images for the starting MPC and MPC samples heated at 1800, 2000, 2200, 2400 and 3000 C. The starting MPC shows mesopores with a pore diameter below 10 nm and a carbon wall several nanometers thick without the formation of distinct long parallel lines that indicate stacked hexagonal carbon layers. After heating at 1800, 2000, or 2200 C, distinct curved long parallel lines form. For the MPC sample heated at 2400 C (MPC-2400), thick carbon walls grown in the [002] direction are observed. For the MPC sample heated at 3000 C (MPC-3000), many thicker graphitized stacking layers are observed. A pore structure with thin carbon walls, in which some carbon layers are stacked, is also visible under TEM observation, as shown in the TEM photo of MPC-3000. The temperature of thermal oxidation for graphitized carbon materials tends to be higher than that for low-crystalline carbon material in air. Therefore, graphitization behavior was also traced by evaluating changes in the thermal stability of the MPC samples with heat treatment. Fig. 10 shows the TG curves for the MPC sample heated under static air. The onset temperature of weight loss increased with increasing temperature. Weight loss at approximately 50 C, which is due to the release of adsorbed water molecules, is observed for the starting MPC and the MPC samples
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Fig. 10. TG curves of MPC and the MPC samples heated at different temperatures.
heated at temperatures below 2000 C. This result indicates that surfaces of the MPC samples heated at temperatures below 2000 C are hydrophilic and that some functional groups may be present on the surfaces. Fig. 11 shows differential thermal gravimetric (DTG) curves and DTA curves. Both the DTG peak and DTA peak, which are due to the combustion of carbon in air, shift to higher temperatures
Fig. 9. TEM images of MPC and the MPC samples heated at 1800, 2000, 2200, 2400, and 3000 C.
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Fig. 11. DTG and DTA curves of MPC and the MPC samples heated at different temperatures.
with increasing temperature. Fig. 12 shows the change in the temperature at the maximum point for the DTG and DTA peaks with increasing temperature. The temperature at the maximum of both peaks increases linearly with increasing temperature up to 2000 C. From 2000 to 2400 C, the temperatures at the maximum points are nearly-constant. The maximum temperature then
increases again with increasing temperature above 2500 C, and obvious broadening of the peak toward a higher temperature is observed for the MPC samples heated at 2700 and 3000 C. These results indicate that the thermal stability of the MPC samples gradually improved with increasing temperature except in the temperature range from 2000 to 2400 C. The MPC samples heated at temperatures above 2700 C contained a more thermally-stable component than a graphitized mesopore structure. The component is reasonably supposed to be small graphite flakes. From the structural characterization, the graphitization behavior of the MPC samples is explained as follows: (1) Graphitization of mesopore carbon walls occurred in conjunction with shrinkage of the mesopores and collapse of the micropores up to 2000 C; (2) the growth of hexagonal carbon layers occurred in the mesopore carbon walls in the temperature range from 2200 to 2400 C, with a partial collapse of mesopores and micropores. At 2400 C, vapor growth of graphite flakes may begin; (3) graphitization of mesopore carbon walls with a partial collapse of mesopores and micropores and vapor growth of graphite flakes above 2500 C occurs; (4) for the MPC samples heated at 2700 and 3000 C, the growth of small graphite flakes occurs. 3.2. Electrochemical properties
Fig. 12. Maximum temperature of DTG and DTA peaks of MPC and the MPC samples heated at different temperatures. Open circle: DTG, closed circle DTA.
Charge/discharge curves at 40 mA g1 for the starting MPC and the MPC samples heated at 2000, 2400, and 3000 C (MPC-2000, -2400, and -3000) are shown in Fig. 13. The first charge curves for all the samples show a plateau at approximately 1 V, which is not observed after the first charge. The capacity of the plateau decreased with increasing temperature. For the starting MPC, the capacity is more than 1500 mAh g1, whereas for the MPC sample heated at 3000 C (MPC-3000), it is approximately 1000 mAh g1.
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Fig. 13. Charge/discharge curves of MPC and the MPC samples heated at different temperatures. Insets are first charge curves.
The plateau is reasonably supposed to be due to the electrolyte decomposition at the surface, thus forming a solid electrolyte interface (SEI) [13e20]. With increasing temperature, the charge/ discharge capacities decrease, but the charge/discharge efficiency increases. Three plateaus of charge/discharge curves can be distinguished for the MPC sample heated at 3000 C (MPC-3000). The plateau below 0.3 V is lithium intercalation/deintercalation into the graphite structure [1e3]. This reaction may take place on the small graphite flakes in the samples. The other two plateaus at 1.0 and 1.5 V observed on the charge curves are thought to be due to lithium insertion/extraction in the graphitized mesopore carbon walls. Therefore, the charge/discharge curves do not show
hysteresis, which is observed in the charge/discharge curves for low-crystalline MPC. The carbon walls of mesopores graphitized at 2000 and 2400 C show no distinct any plateaus about they do show hysteresis on the charge/discharge curves. These results indicate that the electrochemical potential for the lithium insertion/extraction reaction on graphitized carbon layers with thicknesses of several nanometers is 1.0 and 1.5 V, which is somewhat higher than that of less than 0.3 V for graphite. Change in the charge capacity with increasing cycle number and the increase of the current density every five cycles for the starting MPC and the MPC samples heated at 2000, 2400 and 3000 C are shown in Fig. 14. The starting MPC shows a high capacity of more
Fig. 14. Change in charge capacity of MPC and the MPC samples heated at different temperatures with cycle number. Current density was increased step by step every 5 cycles, 40, 100, 200, 500, 1000 mA g1.
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than 1000 mAh g1 at the first and second cycle, but the capacity gradually decreased to less than 200 mAh g1 at the fifth cycle. The capacity decreased with every five-cycle increase in the current density, 150 mAh g1 at 100 mA g1 75 mAh g1 at 200 mA g1, 40 mAh g1 at 500 mA g1, and 20 mAh g1 at 1000 mA g1. For the MPC sample heated at 2000 C (MPC-2000), the retention ratio of the charge capacity with increasing cycle number and current density was rather high as compared to the starting MPC. From 2000 to 2400 C, the capacity decreased with increasing heating temperature. For the MPC samples heated at 2400e3000 C, the capacity increased with increasing temperature and the MPC sample heated at 3000 C (MPC-3000) showed good rate performance, although its capacity was less than that of MPC-2000. MPC3000 is therefore thought to be the most suitable for the anode of LIBs among the MPC samples evaluated in this study because of its low irreversible capacity, high columbic efficiency, and high rate capability. 4. Conclusions Mesoporous carbon (MPC) samples were heated at temperatures ranging from 1800 C to 3000 C for 30 min. For the MPC sample heated at temperatures up to 2000 C, the carbon walls of the mesopore structure were graphitized upon shrinkage of the mesopores, improving the rate capability of the composite anode for lithium ion batteries. For the MPC samples heated at 2200e2700 C, both the micropores and mesopores of the starting MPC partially collapsed, reducing the surface area, which led to a decrease in the charge/discharge capacity. Stacking of hexagonal carbon layers and annealing of the defects of the carbon walls also occurred with increasing temperature, and the formation of a small amount of graphitized thick carbon layers was observed under TEM. The MPC samples heated at 2700 and 3000 C were composed of thick graphitized layers and a mesoporous structure with graphitized thin carbon walls. The thick graphitized layers or the small graphite flakes showed lithium intercalation/deintercalation below 0.3 V vs. Li/Liþ and the mesoporous structure with
graphitized thin carbon walls showed a lithium insertion/extraction at 1.5 and 1.0 V vs. Li/Liþ with small polarization (without hysteresis). The MPC sample heated at 3000 C (MPC-3000) had the lowest capacity of approximately 500 mAh g1 at the fifth cycle, but it showed the lowest irreversible capacity and the highest columbic efficiency and rate capability. References [1] M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita, Carbon 38 (2000) 183e197. [2] T.P. Kumar, T.S.D. Kumari, A.M. Stephan, J. Indian Inst. Sci. 89 (2009) 393e424. [3] M. Noel, V. Suryanarayanan, J. Power Sources 111 (2002) 193e368. €m, J.-C. Jamas, J.-M. Tarascon, [4] D. Larcher, S. Beattie, M. Morcrette, K. Edstro J. Mater. Chem. 17 (2007) 3759e3772. [5] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, J. Power Sources 97e98 (2001) 235e239. [6] B. Guo, X. Wang, P.F. Fulvio, M. Chi, S.M. Mahurin, X.-G. Sun, S. Dai, Adv. Mater. 23 (2011) 4661e4666. [7] J. Yang, X.-Y. Zhou, J. Li, Y.-L. Zou, J.-J. Tang, Mater. Chem. Phys. 135 (2012) 445e450. [8] Z. Hou, F. Zeng, B. He, W. Tao, C. Ge, Y. Kuang, J. Zeng, Mater. Lett. 65 (2011) 897e900. [9] B. Fang, M.-S. Kim, J.H. Kim, S. Lim, J.-S. Yu, J. Mater. Chem. 20 (2010) 10253e10259. [10] H. Yamada, Y. Watanabe, I. Moriguchi, T. Kudo, Solid State Ionics 179 (2008) 1706e1709. [11] Z. Wu, W. Li, Y. Xia, P. Webley, D. Zhao, J. Mater. Chem. 22 (2012) 8835e8845. [12] F. Zeng, Z. Hou, B. He, C. Ge, J. Cao, Y. Kuang, Mat. Res. Bull. 47 (2012) 2104e2107. [13] M.B.V. Santos, E. Geissler, K. Laszlo, J.-N. Rouzaud, A.M. Alonso, J.M.D. Tascon, Carbon 50 (2012) 2929e2940. [14] M.B.V. Santos, A.M. Alonso, J.M.D. Tascon, J.-N. Rouzaud, C. Rochas, E. Geissler, K. Laszlo, J. Alloys Compd. 536S (2012) S464eS468. [15] S. Osswald, J. Chmiola, Y. Gogotsi, Carbon 50 (2012) 4880e4886. [16] M.F. Ottaviani, R. Mazzeo, Microporous Mesoporous Mater. 141 (2011) 61e68. [17] Y. Tian, Y. Song, Z. Tang, Q. Guo, L. Liu, J. Power Sources 184 (2008) 675e679. [18] S.-H. Chai, J.Y. Howe, X. Wang, M. Kidder, V. Schwartz, M.L. Golden, S.H. Overbury, S. Dai, D.-E. Jiang, Carbon 50 (2012) 1574e1582. [19] D. Zhai, H. Du, B. Li, Y. Zhu, F. Kang, Carbon 49 (2011) 718e736. [20] T.X. Nguyen, S.K. Bhatia, Carbon 44 (2006) 646e652. [21] T. Morishita, T. Tsumura, M. Toyoda, J. Przepiorski, A.W. Morawski, H. Konno, M. Inagaki, Carbon 48 (2010) 2690e2707. [22] T. Morishita, Y. Soneda, T. Tsumura, M. Inagaki, Carbon 44 (2006) 2360e2367. [23] H. Orikasa, T. Morishita, Tanso 254 (2012) 153e159.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structure of heat-treated mesoporous carbon and its electrochemical lithium intercalation behavior Tomoki Tsumura a, *, Asuka Arikawa a, Taro Kinumoto a, Yasuhiko Arai a, Takahiro Morishita b, Hironori Orikasa b, Michio Inagaki c, Masahiro Toyoda a a b c
Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu Oita, Oita 870-1192, Japan Advanced Carbon Technology Center, Toyo Tanso Co., Ltd., 5-7-12 Takashima, Nishiyodogawa-ku, Osaka 555-0011, Japan Professor Emeritus of Hokkaido University, 228-7399 Nakagawa, Hosoe-cho, Kita-ku, Hamamatsu 431-1304, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Mesoporous carbon was heated in the temperature range between 1800 C and 3000 C. The growth of hexagonal carbon layers occurred in the mesopore carbon walls. Electrochemical lithium intercalation behavior was investigated for the graphitized mesoporous carbon.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2013 Received in revised form 27 June 2014 Accepted 4 July 2014 Available online 19 July 2014
Mesoporous carbon (MPC) was synthesized via the thermal decomposition of magnesium citrate in a flow of Ar gas, followed by dissolution of MgO nanoparticles in the carbonaceous pyrolysis product. The MPC was then heated at 1800, 2000, 2200, 2400, 2500, 2700 and 3000 C for 30 min. The MPC samples heated at 1800 and 2000 C having a mesopore structure with graphitized carbon walls exhibited an enhanced rate capability as a composite anode for lithium ion batteries. Heat treatment of the MPC from 2200 to 2700 C led to a partial collapse of micropores and mesopores, which decreased the charge/ discharge capacity. The MPC samples heated at 2700 and 3000 C having thick graphitized carbon layers and a mesopore structure with graphitized carbon walls showed charge/discharge curves with three lithium intercalation potential plateaus at approximately 0.2, 1.0, and 1.5 V vs. Li/Liþ. The MPC sample heated at 3000 C was the best anode material for lithium ion batteries among the MPC samples evaluated in this study because of its low irreversible capacity, high columbic efficiency, and high rate capability. © 2014 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Annealing Electrochemical techniques Electron microscopy
1. Introduction Lithium ion batteries (LIBs) have been used as power sources for portable electronics, and graphite has been extensively used as anodes for LIBs owing to its high columbic efficiency and flat reaction potential below 0.3 V vs. Li/Liþ [1e3]. Because of the high
* Corresponding author. Tel./fax: þ81 97 554 7906. E-mail address: [email protected] (T. Tsumura). http://dx.doi.org/10.1016/j.matchemphys.2014.07.001 0254-0584/© 2014 Elsevier B.V. All rights reserved.
functionalization and diversification of portable electronics and the expanding use of LIBs as energy storage devices for large-scale equipment such as electric vehicles and power generation systems for recyclable energy, there has been a great need to improve the energy and power densities of LIBs. Therefore, research for alternatives to graphite as anode materials has been active around the world [4,5]. Because carbon materials are non-toxic, inexpensive, and easily obtainable, mesoporous carbon (MPC), which has a high capacity and rate capability, has been proposed to replace graphite [6e9]. The capacity of MPC is beyond the theoretical
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capacity of 372 mAh g1 for graphite, because lithium ions are not only intercalated between the hexagonal carbon layers but also accommodated on defects, pores, and edges of the stacked hexagonal carbon layers. Although the relation between the structural parameters such as graphitization degree, number of stacked carbon layers, surface area, and pore distribution of MPC and its capacity has been investigated, the mechanism for the accommodation of lithium has not been clear until now. The high rate capability of MPC is reasonably supposed to be due to the short diffusion path in the carbon host material and the fast diffusion of lithium ions in the electrolyte through interconnected mesopores. Although they have these two advantages, i.e., large capacity and high rate performance, they also have some drawbacks, such as a large polarization between the charge and discharge potentials, also called hysteresis, indicating energy loss during the charge/ discharge cycle and a low electric conductivity attributed to a low graphitization degree. In order to improve the electric conductivity and electrochemical performance, there have been some reports discussing the graphitization of MPC and its electrochemical properties [10e12]. However, in most cases, MPC heated to temperatures above 1500 C have a surface area of less than 500 m2 g1 and most of the mesopores are thought to have collapsed [13e20]. Therefore, the effects of graphitized mesopore carbon walls on the electrochemical performance of MPC are not clear. In the present paper, the effects of the structural changes caused by the heat treatment of MPC on its electrochemical properties as an anode material for LIBs was investigated by using MPC prepared through the MgO-templating method developed in our group [21,22]. Since the entire mesopore structure remained at the temperatures up to 2000 C and part of the mesopore structure remained even after heat treatment at 3000 C, the electrochemical properties of mesopore carbon walls graphitized at different temperatures can be investigated clearly.
Thermal treatment of the MPC samples was carried out by using a graphitizing furnace at 1800, 2000, 2200, 2400, 2500, 2700, and 3000 C. The starting MPC was passed through a 100 mm sieve and set in a graphite crucible. The furnace tube was heated from room temperature to 1600 C, and evacuated by rotary vacuum pump down to 0.1 Torr. The furnace was then heated under Ar gas flow up to different target temperatures and maintained for 30 min at those temperatures. The heat-treated samples were gradually cooled down to room temperature in the furnace. The flow rate of Ar gas was 6 cm3 min1 and the heating rate was 20 C min1. The resultant samples after heat treatment were labeled according to their MPC-target temperature (MPC-1800, -2000, -2200, -2400, -2500, -2700, and -3000). 2.2. Characterization N2 gas adsorption/desorption isotherms for the samples degassed at 200 C under reduced pressure for 24 h were measured at 197 C by using an Autosorb-3B system, Quantachrome Instruments. BrunauereEmmetteTeller (BET) surface area and BarreteJoynereHalenda (BJH) pore distribution were determined from the N2 gas isotherms measured. Morphological changes after heat treatment were observed under a transmission electron microscope (TEM, JEM2100, JEOR Co. Ltd.). X-ray diffraction (XRD) patterns for the samples were recorded by using Ni-filtered CuKa radiation (RINT Ultima III, Rigaku Co. Ltd.). The accelerating voltage and tube current were 40 kV and 40 mA, respectively. The Raman spectra for the samples were measured by using an Ar laser (LabRAM ARAMIS, Horiba Co. Ltd.). The thermal stability in static air was investigated by thermogravimetric-differential thermal analysis (TG-DTA, Thermo plus TG8120, Rigaku Co. Ltd.). The heating rate was 10 C min1. 2.3. Electrochemical measurement
2. Experimental 2.1. Synthesis Mesoporous carbon (MPC) prepared thorough pyrolysis of magnesium citrate Mg3(C6H5O7)2 at 900 C under Ar gas flow was supplied by Toyo Tanso Co. Ltd. Detailed of the procedure for the synthesis of the starting MPC are reported elsewhere [21,22].
Electrochemical lithium intercalation/deintercalation for the samples was conducted by using an electrochemical cell (Takumi Giken Limited Private Co.), depicted in Fig. 1. The working electrode was a composite of the samples, carbon black, and polyvinylidene fluoride (PVDF) in a 8:1:1 mixing weight ratio coated on Cu foil. The counter electrode was lithium foil. A 1 M LiBF4 ethylenecarbonate/ diethylcarbonate, 1/1 vol% solution was used as the electrolyte. The
Fig. 1. Schematic illustration of the construction of the electrochemical cell used.
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cell was assembled in a glove box filled with dry Ar gas. Charge/ discharge curves were measured at a constant current of 40 mA g1 and the rate capability was investigated by increasing the current density in a step-by-step manner every five charge/discharge cycles from 40 to 100, 200, 500, and 1000 mA g1 3. Results and discussion 3.1. Structure After heating the starting MPC in a graphitization furnace in the temperature range between 1800 and 3000 C, the sample weight decreased linearly with increasing temperature. The weight of the MPC sample heated at 3000 C (MPC-3000) was 84 wt% of the initial weight, as shown Fig. 2. The weight loss was caused by the release of compounds containing hydrogen and oxygen, such as H2O, CO, and CO2. Fig. 3 shows the N2 gas adsorption/desorption isotherms for MPC samples heated at different temperatures and the starting MPC. There is a steep increase in the adsorption/desorption gas volume near a relative pressure of approximately 0 and a gradual increase at relative pressures ranging from 0.4 to 1.0 for the starting MPC, which indicates that the starting MPC had a certain amount of micropores and mesopores. With increasing heating temperature, the number of micropores and mesopores decreased, especially from 2500 to 3000 C, which shows a sharp decrease. BJH analysis for mesopore distribution is shown in Fig. 4. For the starting MPC, pores approximately 4 nm in diameter are observed. The pore diameter decreased from 4 to 3 nm during heat treatment up to 1800 C. For the MPC samples heated at temperatures above 2000 C, there was no change in the diameter but there was a change in the number of mesopores with diameters of 3 nm. Changes in BET surface area with increasing temperature are plotted in Fig. 5. The BET surface area for the MPC samples heated at temperatures up to 2000 C decreased gradually with increasing temperature. This decrease in BET surface area is due to the shrink of mesopore as shown in Fig. 4 and the decrease of micropore volume at a relative pressure of 0 in Fig. 3. At 2000 C, the BET surface area drastically decreased from 1700 m2 g1 for the MPC sample heated at 2000 C (MPC-2000) to 350 m2 g1 for the MPC sample heated at 3000 C (MPC-3000).
Fig. 2. Yield of the MPC samples heated at different temperatures.
Fig. 3. Nitrogen adsorption/desorption isotherms of MPC and MPC samples heated at different temperatures.
The Raman spectra of the MPC samples heated at the different temperatures are shown in Fig. 6. The starting MPC shows broad D and G bands at 1350 and 1600 cm1, respectively. The full width at half maximum of the two bands decreased with increasing temperature and an additional band, the D0 band, appears for the MPC samples heated at temperatures above 1800 C. Changes in the integral intensity ratio between the D and G bands for the heattreated MPC samples, ID/IG, which is used as a measure of the graphitization degree, are shown in Fig. 7. ID/IG decreases gradually with increasing temperature, indicating that the MPC samples were graphitized with increasing temperature. XRD patterns of the MPC samples heated at different temperatures are shown in Fig. 8. The starting MPC shows two broad peaks at 23 and 43 , assigned to the 002 and 10 diffractions, respectively. For the MPC samples heat treated at temperatures of 1800 and 2000 C, the full width at half maximum of the 10 diffraction peak decreased with increasing temperature, whereas the broad 002
Fig. 4. Pore size distributions of MPC and the MPC samples heated at different temperatures.
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Fig. 5. Change in BET surface area of the MPC samples with increasing heating temperatures.
diffraction line does not show any significant change. This indicates that a hexagonal carbon layer, or graphene, grew in the ab plane but that increased stacking of the carbon layers perpendicular to the ab plane, which is probably parallel to the mesopore carbon walls, does not occur. Upon heating from 2200 to 3000 C, a sharp 002 diffraction peak appears and increases gradually with increasing temperature. Orikasa and Morishita reported that the sharp 002 peak is due to small graphite flakes, which they observed under
Fig. 6. Raman spectra of MPC and the MPC samples heated at different temperatures.
Fig. 7. Change in the ID/IG ratio of Raman peak of MPC and the MPC samples heated at different temperatures.
TEM. They suggested that these graphite flakes were formed by vapor deposition or collapse of mesopores [23]. Furthermore, they pointed out that the sharp peak had at least three components with different d-spacings, but the origin of graphite flakes having two different d-spacings was not clear. For the broad 002 diffraction peak, the intensity increases, the width decreases, and the position
Fig. 8. XRD patterns of MPC and the MPC samples heated at different temperatures.
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shifts to a higher angle with increasing temperature. For the 10 peak, the intensity also increases with increasing temperature. These results indicate that stacking of the hexagonal carbon layers occurs in the [002] direction and that the hexagonal carbon layers grow in the ab plane. Fig. 9 shows TEM images for the starting MPC and MPC samples heated at 1800, 2000, 2200, 2400 and 3000 C. The starting MPC shows mesopores with a pore diameter below 10 nm and a carbon wall several nanometers thick without the formation of distinct long parallel lines that indicate stacked hexagonal carbon layers. After heating at 1800, 2000, or 2200 C, distinct curved long parallel lines form. For the MPC sample heated at 2400 C (MPC-2400), thick carbon walls grown in the [002] direction are observed. For the MPC sample heated at 3000 C (MPC-3000), many thicker graphitized stacking layers are observed. A pore structure with thin carbon walls, in which some carbon layers are stacked, is also visible under TEM observation, as shown in the TEM photo of MPC-3000. The temperature of thermal oxidation for graphitized carbon materials tends to be higher than that for low-crystalline carbon material in air. Therefore, graphitization behavior was also traced by evaluating changes in the thermal stability of the MPC samples with heat treatment. Fig. 10 shows the TG curves for the MPC sample heated under static air. The onset temperature of weight loss increased with increasing temperature. Weight loss at approximately 50 C, which is due to the release of adsorbed water molecules, is observed for the starting MPC and the MPC samples
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Fig. 10. TG curves of MPC and the MPC samples heated at different temperatures.
heated at temperatures below 2000 C. This result indicates that surfaces of the MPC samples heated at temperatures below 2000 C are hydrophilic and that some functional groups may be present on the surfaces. Fig. 11 shows differential thermal gravimetric (DTG) curves and DTA curves. Both the DTG peak and DTA peak, which are due to the combustion of carbon in air, shift to higher temperatures
Fig. 9. TEM images of MPC and the MPC samples heated at 1800, 2000, 2200, 2400, and 3000 C.
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Fig. 11. DTG and DTA curves of MPC and the MPC samples heated at different temperatures.
with increasing temperature. Fig. 12 shows the change in the temperature at the maximum point for the DTG and DTA peaks with increasing temperature. The temperature at the maximum of both peaks increases linearly with increasing temperature up to 2000 C. From 2000 to 2400 C, the temperatures at the maximum points are nearly-constant. The maximum temperature then
increases again with increasing temperature above 2500 C, and obvious broadening of the peak toward a higher temperature is observed for the MPC samples heated at 2700 and 3000 C. These results indicate that the thermal stability of the MPC samples gradually improved with increasing temperature except in the temperature range from 2000 to 2400 C. The MPC samples heated at temperatures above 2700 C contained a more thermally-stable component than a graphitized mesopore structure. The component is reasonably supposed to be small graphite flakes. From the structural characterization, the graphitization behavior of the MPC samples is explained as follows: (1) Graphitization of mesopore carbon walls occurred in conjunction with shrinkage of the mesopores and collapse of the micropores up to 2000 C; (2) the growth of hexagonal carbon layers occurred in the mesopore carbon walls in the temperature range from 2200 to 2400 C, with a partial collapse of mesopores and micropores. At 2400 C, vapor growth of graphite flakes may begin; (3) graphitization of mesopore carbon walls with a partial collapse of mesopores and micropores and vapor growth of graphite flakes above 2500 C occurs; (4) for the MPC samples heated at 2700 and 3000 C, the growth of small graphite flakes occurs. 3.2. Electrochemical properties
Fig. 12. Maximum temperature of DTG and DTA peaks of MPC and the MPC samples heated at different temperatures. Open circle: DTG, closed circle DTA.
Charge/discharge curves at 40 mA g1 for the starting MPC and the MPC samples heated at 2000, 2400, and 3000 C (MPC-2000, -2400, and -3000) are shown in Fig. 13. The first charge curves for all the samples show a plateau at approximately 1 V, which is not observed after the first charge. The capacity of the plateau decreased with increasing temperature. For the starting MPC, the capacity is more than 1500 mAh g1, whereas for the MPC sample heated at 3000 C (MPC-3000), it is approximately 1000 mAh g1.
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Fig. 13. Charge/discharge curves of MPC and the MPC samples heated at different temperatures. Insets are first charge curves.
The plateau is reasonably supposed to be due to the electrolyte decomposition at the surface, thus forming a solid electrolyte interface (SEI) [13e20]. With increasing temperature, the charge/ discharge capacities decrease, but the charge/discharge efficiency increases. Three plateaus of charge/discharge curves can be distinguished for the MPC sample heated at 3000 C (MPC-3000). The plateau below 0.3 V is lithium intercalation/deintercalation into the graphite structure [1e3]. This reaction may take place on the small graphite flakes in the samples. The other two plateaus at 1.0 and 1.5 V observed on the charge curves are thought to be due to lithium insertion/extraction in the graphitized mesopore carbon walls. Therefore, the charge/discharge curves do not show
hysteresis, which is observed in the charge/discharge curves for low-crystalline MPC. The carbon walls of mesopores graphitized at 2000 and 2400 C show no distinct any plateaus about they do show hysteresis on the charge/discharge curves. These results indicate that the electrochemical potential for the lithium insertion/extraction reaction on graphitized carbon layers with thicknesses of several nanometers is 1.0 and 1.5 V, which is somewhat higher than that of less than 0.3 V for graphite. Change in the charge capacity with increasing cycle number and the increase of the current density every five cycles for the starting MPC and the MPC samples heated at 2000, 2400 and 3000 C are shown in Fig. 14. The starting MPC shows a high capacity of more
Fig. 14. Change in charge capacity of MPC and the MPC samples heated at different temperatures with cycle number. Current density was increased step by step every 5 cycles, 40, 100, 200, 500, 1000 mA g1.
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than 1000 mAh g1 at the first and second cycle, but the capacity gradually decreased to less than 200 mAh g1 at the fifth cycle. The capacity decreased with every five-cycle increase in the current density, 150 mAh g1 at 100 mA g1 75 mAh g1 at 200 mA g1, 40 mAh g1 at 500 mA g1, and 20 mAh g1 at 1000 mA g1. For the MPC sample heated at 2000 C (MPC-2000), the retention ratio of the charge capacity with increasing cycle number and current density was rather high as compared to the starting MPC. From 2000 to 2400 C, the capacity decreased with increasing heating temperature. For the MPC samples heated at 2400e3000 C, the capacity increased with increasing temperature and the MPC sample heated at 3000 C (MPC-3000) showed good rate performance, although its capacity was less than that of MPC-2000. MPC3000 is therefore thought to be the most suitable for the anode of LIBs among the MPC samples evaluated in this study because of its low irreversible capacity, high columbic efficiency, and high rate capability. 4. Conclusions Mesoporous carbon (MPC) samples were heated at temperatures ranging from 1800 C to 3000 C for 30 min. For the MPC sample heated at temperatures up to 2000 C, the carbon walls of the mesopore structure were graphitized upon shrinkage of the mesopores, improving the rate capability of the composite anode for lithium ion batteries. For the MPC samples heated at 2200e2700 C, both the micropores and mesopores of the starting MPC partially collapsed, reducing the surface area, which led to a decrease in the charge/discharge capacity. Stacking of hexagonal carbon layers and annealing of the defects of the carbon walls also occurred with increasing temperature, and the formation of a small amount of graphitized thick carbon layers was observed under TEM. The MPC samples heated at 2700 and 3000 C were composed of thick graphitized layers and a mesoporous structure with graphitized thin carbon walls. The thick graphitized layers or the small graphite flakes showed lithium intercalation/deintercalation below 0.3 V vs. Li/Liþ and the mesoporous structure with
graphitized thin carbon walls showed a lithium insertion/extraction at 1.5 and 1.0 V vs. Li/Liþ with small polarization (without hysteresis). The MPC sample heated at 3000 C (MPC-3000) had the lowest capacity of approximately 500 mAh g1 at the fifth cycle, but it showed the lowest irreversible capacity and the highest columbic efficiency and rate capability. References [1] M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita, Carbon 38 (2000) 183e197. [2] T.P. Kumar, T.S.D. Kumari, A.M. Stephan, J. Indian Inst. Sci. 89 (2009) 393e424. [3] M. Noel, V. Suryanarayanan, J. Power Sources 111 (2002) 193e368. €m, J.-C. Jamas, J.-M. Tarascon, [4] D. Larcher, S. Beattie, M. Morcrette, K. Edstro J. Mater. Chem. 17 (2007) 3759e3772. [5] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, J. Power Sources 97e98 (2001) 235e239. [6] B. Guo, X. Wang, P.F. Fulvio, M. Chi, S.M. Mahurin, X.-G. Sun, S. Dai, Adv. Mater. 23 (2011) 4661e4666. [7] J. Yang, X.-Y. Zhou, J. Li, Y.-L. Zou, J.-J. Tang, Mater. Chem. Phys. 135 (2012) 445e450. [8] Z. Hou, F. Zeng, B. He, W. Tao, C. Ge, Y. Kuang, J. Zeng, Mater. Lett. 65 (2011) 897e900. [9] B. Fang, M.-S. Kim, J.H. Kim, S. Lim, J.-S. Yu, J. Mater. Chem. 20 (2010) 10253e10259. [10] H. Yamada, Y. Watanabe, I. Moriguchi, T. Kudo, Solid State Ionics 179 (2008) 1706e1709. [11] Z. Wu, W. Li, Y. Xia, P. Webley, D. Zhao, J. Mater. Chem. 22 (2012) 8835e8845. [12] F. Zeng, Z. Hou, B. He, C. Ge, J. Cao, Y. Kuang, Mat. Res. Bull. 47 (2012) 2104e2107. [13] M.B.V. Santos, E. Geissler, K. Laszlo, J.-N. Rouzaud, A.M. Alonso, J.M.D. Tascon, Carbon 50 (2012) 2929e2940. [14] M.B.V. Santos, A.M. Alonso, J.M.D. Tascon, J.-N. Rouzaud, C. Rochas, E. Geissler, K. Laszlo, J. Alloys Compd. 536S (2012) S464eS468. [15] S. Osswald, J. Chmiola, Y. Gogotsi, Carbon 50 (2012) 4880e4886. [16] M.F. Ottaviani, R. Mazzeo, Microporous Mesoporous Mater. 141 (2011) 61e68. [17] Y. Tian, Y. Song, Z. Tang, Q. Guo, L. Liu, J. Power Sources 184 (2008) 675e679. [18] S.-H. Chai, J.Y. Howe, X. Wang, M. Kidder, V. Schwartz, M.L. Golden, S.H. Overbury, S. Dai, D.-E. Jiang, Carbon 50 (2012) 1574e1582. [19] D. Zhai, H. Du, B. Li, Y. Zhu, F. Kang, Carbon 49 (2011) 718e736. [20] T.X. Nguyen, S.K. Bhatia, Carbon 44 (2006) 646e652. [21] T. Morishita, T. Tsumura, M. Toyoda, J. Przepiorski, A.W. Morawski, H. Konno, M. Inagaki, Carbon 48 (2010) 2690e2707. [22] T. Morishita, Y. Soneda, T. Tsumura, M. Inagaki, Carbon 44 (2006) 2360e2367. [23] H. Orikasa, T. Morishita, Tanso 254 (2012) 153e159.