Experimental Py-BTE was synthesized starting from 1,2-bis(5-chloro-2-methyl-3-thienyl) cyclopentene [11b], which was lithiated with n-BuLi in THF at ±78 C, then treated with B(OBu)3 to provide the bis(boronic acid) derivatives. This was then used in the Suzuki coupling reaction with 4-bromopyridinium hydrochloride to provide Py-BTE [11a]. 1H NMR spectra were recorded on a Bruker AM-500 spectrometer. UV-vis spectra were recorded on a Varian Cary 500 apparatus. IR spectra were recorded on a Nicolet FT-IR20SX apparatus. Photo-luminescent spectra were recorded on Varian Cary Eclipse apparatus. Elemental analysis data were obtained on a Perkin Elmer 240c instrument. Mass spectra and time-of-flight (TOF)-mass spectra were obtained at 70 eV on VG 12±250 (VG Mass lab) and Mariner API TOF spectrometers (Turbo ion spray (TIS) ion source, PE Corp.). The optical switch experiments were carried out using a photochemical reaction apparatus (British Applied Photophys. Limited) with a 200 W Hg lamp. The distance between the sample and the lamp is 20 cm; in the front of the sample there is a cut-filter (Type FAL, transmittivity 0.7, half-width 66 nm, Lambda Physics, Germany) or a quartz cell (2 cm 2 cm) containing water. Py-BTE: 1H NMR (500 MHz, CDCl3, ppm): d 8.52 (d, 4H, pyridyl a-H), 7.33 (d, 4H, pyridyl b-H), 7.22 (s, 2H, thienyl C±H), 2.85 (t, 4H, ±CH2±), 2.12 (m, 2H, ±CH2±), 2.02 (s, 6H, ±CH3). High-resolution mass spectrometry (HRMS): calculated for C25H22N2S2: 414.58; found: 414.1. Element analysis: calculated for C25 H22N2S2: C 72.46, H 5.31, N 6.76; found: C 72.38, H 5.28, N 6.72 %. Received: May 21, 2003 Final version: September 18, 2003
±
[1] a) M. Irie, Chem. Rev. 2000, 100, 1685. b) B. L. Feringa, Molecular Switches, Wiley-VCH, Weinheim 2001. [2] M. Raymo, S. Giordani, J. Am. Chem. Soc. 2002, 124, 2004. [3] X. Guo, D. Zhang, T. Wang, D. Zhu, Chem. Commun. 2003, 914. [4] a) A. Credi, V. Balzani, S. J. Langford, J. F. Stoddart, J. Am. Chem. Soc. 1997, 119, 2679. b) J.-M. Lehn, Science 2002, 295, 2400. c) J. L. Bahr, G. Kodis, L. Garza, S. Lin, A. L. Moore, T. A. Moore, D. Gust, J. Am. Chem. Soc. 2001, 123, 7124. [5] a) A. P. de Silva, N. D. McClenaghan, J. Am. Chem. Soc. 2000, 122, 3965. b) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher, T. E. Rice, Chem. Rev. 1997, 97, 1515. c) G. J. Brown, A. P. de Silva, S. Pagliari, Chem. Commun. 2002, 2461. [6] a) T. B. Norsten, N. R. Branda, Adv. Mater. 2001, 13, 347. b) E. Murguly, T. B. Norsten, N. R. Branda, Angew. Chem. Int. Ed. 2001, 40, 1752. c) A. J. Myles, T. J. Wigglesworth, N. R. Branda, Adv. Mater. 2003, 15, 745. [7] S. L. Gilat, S. H. Kawai, J. M. Lehn, Chem. Eur. J. 1995, 1, 275. [8] a) M. Munakata, L. P. Wu, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga, K. Furuichi, J. Am. Chem. Soc. 1996, 118, 3305. b) M. Irie, K. Uchida, T. Eriguchi, Bull. Chem. Soc. Jpn. 1998, 71, 985. c) K. Matsuda, K. Takayama, M. Irie, Chem. Commun. 2001, 363. [9] M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, Nature 2002, 420, 759. [10] a) B. Z. Chen, M. Z. Wang, Y. Q. Wu, H. Tian, Chem. Commun. 2002, 1060. b) H. Tian, B. Z. Chen, H. Y. Tu, K. Müllen, Adv. Mater. 2002, 14, 918. c) Q. F. Luo, B. Z. Chen, M. Z. Wang, H. Tian, Adv. Funct. Mater. 2003, 13, 233. [11] a) B. Qin, R. X. Yao, X. L. Zhao, H. Tian, Org. Biomol. Chem. 2003, 1, 2187. b) L. N. Lucas, J. J. D. Jong, J. H. van Esch, R. M. Kellogg, B. L. Feringa, Eur. J. Org. Chem. 2003, 155. [12] For 1H NMR data, absorption and fluorescence spectra of the compound, as well as fluorescent probing for various metal ions, see supporting information available from the author or on WileyInterscience (www.interscience.wiley.com). [13] M. Irie, in Molecular Switches (Ed: B. L. Feringa), Wiley-VCH, Weinheim 2001, Ch. 2.
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DOI: 10.1002/adma.200306125
Lithium Storage in Ordered Mesoporous Carbon (CMK-3) with High Reversible Specific Energy Capacity and Good Cycling Performance By Haoshen Zhou,* Shenmin Zhu, Mitsuhiro Hibino, Itaru Honma, and Masaki Ichihara There is a large and increasing demand for portable electronic devices with high energy capacity, such as rechargeable batteries and fuel cells. The lithium rechargeable battery, which appears to be the most promising system, has attracted remarkable attention owing to its high energy density. However, it is still necessary to improve the electrode materials to obtain the highest possible energy capacity. Carbonaceous material is a major material for the negative electrode in the rechargeable lithium battery because it exhibits both higher specific capacity and more negative redox potential than most metal oxides, chalcogenides, and polymers.[1,2] The quality of lithium intercalation and deintercalation strongly depends on the crystalline phase, microstructure, and micromorphology of the carbonaceous materials. The intercalation and deintercalation processes can be expressed as[1,2] 6C + xLi + xe± > LixC6
(1)
Here x is the stoichiometric factor in LixC6. Generally, the carbonaceous materials can be divided into graphitic carbon, with a perfect stacking order of the crystallographic layer structure in the c-direction, and non-graphitic carbon without crystallographic order in the c-direction. The intercalation of lithium ions into graphitic carbon is almost reversible, and the stoichiometric factor x in LixC6 for graphitic carbon is about 1.0. However, non-graphitic carbon, which is synthesized at rather low temperatures (500±1000 C), can be classified into ªlow specific energy capacity carbonº, where x is about 0.5± 0.8, and ªhigh specific energy capacity carbonº, where x is about 1.2±3.0.[1,2] Although the high specific energy capacity carbon provides a remarkably high specific energy capacity, fatal problems hinder practical application in rechargeable lithium batteries: 1) a high irreversible specific capacity not only in the initial insertion±extraction cycle but also in subsequent cycles;[3±6] 2) poor cycling performance;[3±5,7] and 3) too low an intercalating potential, which is near to or exceeds 0 V (vs. Li+/Li)[4,7,8] for intercalation of several hundred milliampere hours per gram (mAh/g).
± [*]
Dr. H. Zhou, Dr. S. Zhu, Dr. M. Hibino, Dr. I. Honma Energy Electronics Institute National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba 305-8568 (Japan) E-mail:
[email protected] Dr. M. Ichihara Material Design and Characterization Laboratory Institute for Solid State Physics, University of Tokyo 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581 (Japan)
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rescence spectra of the photochromic molecule Py-BTE under sequential alternating UV/vis light irradiation, a complicated molecular switch with four optical outputs responding to four inputs (not all optical) is realized by a single compound PyBTE. Although the present system is in solution and the inputs are not all optical, the concept shown here may be useful in the future to design ªwetº computers that work more like the brain, relying on membrane-bound molecular processors, which are in nature similar to our brains.[5]
Intensity (a.u)
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Recently, in the search for potential high spe(c) (a) cific capacity carbonaceous materials, the electrochemical storage of lithium by intercalation and deintercalation in multi-walled (MWNTs) and single-walled (SWNTs) carbon nanotubes[9,10] has attracted attention. However, a high irreversible capacity Cirr (460±1080 mAh/ g) together with a relatively low reversible ca0.5 1.0 1.5 2.0 2.5 3.0 pacity Cre (100±400 mAh/g) has been observed. 2θ A large hysteresis between the initial intercala(d) tion and deintercalation processes has also been observed just as in other traditional high (b) specific capacity carbonaceous materials. Now much more effort is being made to find and synthesize high specific capacity carbonaceous materials with high reversible capacity and excellent cycle performance. Here we report, for the first time to our knowledge, a high reversible specific capacity 10 20 30 40 50 60 70 (850±1100 mAh/g) from ordered mesoporous 2θ carbon (CMK-3), synthesized using ordered silFig. 1. a) Small-angle and b) wide-angle XRD patterns of CMK-3. c,d) TEM images of CMK-3 ica SBA-15 as a template, with good discharge along (c) and perpendicular to (d) the direction of the hexagonal pore arrangement. (reduction) and charge (oxidation) cycle performance. Ordered mesoporous carbon, first reported by Ryoo et al.[11] in 1999, who carbonized sucrose in0.2 side the pores of the mesoporous silica template, has attracted 0 5th Cycle much attention for potential applications such as hydrogen -0.2 storage, as an adsorbent, and as a catalyst support, in addition -0.4 to electrochemical double-layer capacitors (EDLCs).[12±20] 1st Cycle -0.6 The morphology of CMK-3 carbon is the reverse hexagonal [11±13] structure of ordered SBA-15; the diffraction peaks of -0.8 [100], [110], and [200] in the hexagonal structure can be -1 observed in the small-angle X-ray diffraction (XRD) pattern -1.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 shown in Figure 1a. The transmission electron microscopy E (V vs. Li/Li+) (TEM) images shown in Figures 1c,d confirmed ordered me[18] soporous carbon with uniform pore size of about 4 nm. The Fig. 2. Cyclic voltammograms of CMK-3 in 1 M LiClO4 with EC + DMC electrolyte in a potential window from 0.01 V to 1.5 V (vs. Li/Li+) at the scan rate two broad diffraction peaks of [002] and [100] of the graphite of 0.1 mV/s. structure can also be observed in the wide-angle XRD pattern shown in Figure 1b. These broad peaks indicate that CMK-3 and the main intercalation part is below 0.4 V. The extraction includes very small amounts of stacked crystalline graphite of lithium from carbon occurred at 0.1±0.5 V with a broad phase. The d-spacing of [002] is ca. 0.36 nm, which is a little shoulder. These results were similar to those of non-graphitic larger than that (ca. 0.33 nm) of pure graphite carbon. carbon. The unit length of the hexagonal CMK-3 mesoporous The galvanostatic charge±discharge curves of CMK-3 at a carbon is about 10.5 nm. The uniform pore size is about constant current of 100 mA/g with a potential window from 3.9 nm according to the N2 adsorption±desorption isotherm 0.01 V to 3 V (versus Li+/Li) are shown in Figure 3. The first curve. So the thickness of the carbon wall is about discharge (reduction) process shows an enormous specific ca10.5 ± 3.9 = 6.6 nm. The Brunauer±Emmett±Teller (BET) surpacity of about 3100 mAh/g, which means the stoichiometric face area is about 1030 m2/g and the total pore volume is factor x in LixC6 is very high (x = 8.4). However, the reversible 0.87 cm3/g. capacity (oxidation) in the first process, denoted Cre, gives The cyclic voltammetry of ordered mesoporous CMK-3 at only about 1100 mAh/g (x = 3.0 for LixC6). This large loss of 0.1 mV/s scan rate in the potential range from 0.01 V to 1.5 V capacity, which is called the irreversible capacity Cirr, is about (versus Li/Li+) is shown in Figure 2. In the first cycle, more 2000 mAh/g. The ratio Cre/(Cre + Cirr) is about 34 %. The Cirr charge is devoted to the reduction process than to the oxidais related to the duty surface of CMK-3 ([H], [O] site, etc.), tion process. Above 1.0 V (versus Li/Li+), an EDLC charge± solid electrolyte interface (SEI) formation, and corrosion-like discharge process can be observed.[18] The reduction of lithireactions of LixC6.[1,10] There is also large hysteresis between um to carbon can be observed below 1.0 V (versus Li/Li+),
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E (V vs. Li/Li+)
3
Specific Energy Capacity (mAh/g)
Cirr
Cre
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Fig. 3. Galvanostatic discharging and charging processes of CMK-3 at a constant current of 100 mA/g. D1, C1, and D2 are the first discharge, first charge, and second discharge, respectively.
Fig. 4. Discharge and charge cycle performance of CMK-3 at a constant current of 100 mA/g.
the discharging and charging processes. This is a common phenomenon for high specific capacity carbonaceous materials that are prepared at low temperatures of 500±700 C.[1,2] The extent of this hysteresis is proportional to the hydrogen content of the carbonaceous materials.[1,2] The reversible capacity Cre of ordered mesoporous carbon CMK-3 is about 850±1100 mAh/g (LixC6, x = 2.3 to 3.0) in 20 cycles. The charge±discharge cycle performance is shown in Figure 4. In the second cycle, the ratio Cre/(Cre + Cirr) increases to 83 %, in the subsequent four or five cycles, the ratio Cre/(Cre + Cirr) increases to nearly 90 % and 93 %, respectively. The average loss v of each cycle for N cycles can be defined as
cess are larger than those of CMK-3. Both the large absolute value (LixC6, x = 2.3 to 3.0 in the first 20 cycles) of reversible capacity and small v (1.5 %) make CMK-3 potentially interesting for high specific capacity anode carbonaceous materials for lithium rechargeable batteries. According to the shape of the (002) XRD peak in the graphite structure, the ordered mesoporous carbon looks like ªsoft carbonº or ªsoft carbonº combined with ªhard carbonº. The stacking length Lc in the c-direction can be determined from the (002) Bragg peak by the Scherrer formula:[4]
CN = C1/(1 + Nv)
(2)
Here, C1 and CN are the specific energy capacities of cycles 1 and N, respectively. Considering the charging (oxidation) process, we find that v is as low as about 1.5 % for 20 cycles. Although some initial specific energy capacity is lost over the first few cycles, it still reaches a reversible high capacity plateau near 850±900 mAh/g after 20 cycles. In fact, the high specific capacities in the various electrode carbonaceous materials are only maintained for the first few cycles.[21] As shown in Table 1, although the ratio Cre/(Cre + Cirr) in the first cycle of PVC700 (from poly(vinyl chloride) at 700 C)[3,4] and OXY700 (from Oxychem phenolic resin at 700 C)[3] are larger than that of CMK-3, their absolute values are much smaller, and their average losses v for the charging (oxidation) pro-
L= k/(Bcosh)
(3)
Here, L, k, and h are crystalline lattice length, X-ray wavelength, and the Bragg angle, respectively. The Lc of CMK is about 1.67 nm, which indicates about 4 or 5 graphite layers stacked in the c-direction. The extent of the graphite sheet La can also be estimated from the (100) Bragg peak by the Scherrer formula. The La of CMK is about 1.96 nm. So, these 1.67 nm by 1.96 nm crystalline graphite stacking blocks are encapsulated in the 6.7 nm mesoporous framework. Such stacked graphitic blocks have been observed by TEM. Although the soft and hard carbon heated below 700 C shows a high irreversible specific capacity and hysteresis of the discharge±charge process, treatment at higher temperatures (800±1000 C) results in a dramatic decrease of both the irreversible and reversible specific capacities, in addition to the elimination of the hysteresis.[3] Such high irreversible specific capacities and hysteresis are considered to result from both
Table 1. Charge and discharge cycle performance of CMK-3 compared with that of other carbonaceous materials.
CMK-3 PVC700 [a] OXY700 [a] MWNT900 [10] SWNT [23]
Cirr-1st
Total surface area [m2/g]
[mAh/g]
1147 ~25 [3] ~240 [3] 204 ~300
2007 230 [6] 260 [3,6] 510 [b] 1413.8
Cre-1st [mAh/g] 1100 710 [6] 630 [3,6] 305 [b] 595.2
Cre-1st
Cre-1st/ Cre-5th/ (Cre-1st+Cirr-1st) Cre-1st [mAh/mL] [%] [%] 550 461 [3,6] 315 [3,6] ± ±
35 75 71 37 30
94 76 [c] 75 [c] 87 ±
Average loss v for charging process in the first 5 cycles [%] 1.5 6.3 [c] 6.6 [c] 2.9 ±
[a] The values were measured at 30 C at a current of about 37 mA/g. [b] The values were measured after degassing at a current of 24 mA/g. [c] The values are calculated according to cycle curves in [6].
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solid electrolyte interface (SEI) formation and the H (or O) in the carbonaceous materials.[3,4] For the latter, the lithium atoms are considered to bind quasi-reversibly on the hydrogen-terminated edges of the grapheme fragments in carbonaceous materials. So, hydrogen-terminated edges can be eliminated by the higher-temperature treatment (800±1000 C).[3,4] Generally, the high specific capacity non-graphitic carbon shows high irreversible capacity, even at higher number cycles, and poor cycling performance. Why does ordered mesoporous carbon give low irreversible discharging±charging capacity at higher number cycles and good cycling performance even with capacities as high as 850±1100 mAh/g? The only difference between CMK-3 and traditional non-graphitic carbonaceous materials is that there are many uniform mesopores with a diameter of 3.9 nm and surface area of 1030 m2/g in CMK-3. Such uniform mesopores and high surface area provide an idea candidate for EDLCs.[18] The electrochemical double layer capacitance of CMK-3 is higher than those of both MWNTs and SWNTs.[18] However, the large surface area, which forms a solid electrolyte interface to inhibit reversible faradaic reaction, gives high irreversible specific capacity.[22,23] This is why the irreversible capacity is very high in the first cycle. After the first several cycles, the irreversible capacity becomes very low, and Cre/(Cre + Ciir) is very high, over 90 %, with a high reversible capacity. The exact mechanism of high specific reversible capacity in CMK-3 is not clear. There are several models for excess Li capacity (x > 1.0 for LixC6) observed in carbonaceous materials, including formation of lithium multi-layers on grapheme sheets,[24] Li2 covalent molecules,[25] Li±C±H bonds,[26] and metallic lithium clusters in microcavities.[1,2] Ultramicropores with size of 0.7±0.9 nm surrounded by small graphite stacking blocks are assumed to be able to trap lithium in a metal lithium cluster.[1,2] How about the uniform nanoscopic cavities with a size of 3±4 nm in a special structure such as CMK-3? Both materials (CMK-3 and carbon nanotubes) have uniform pores. Does a similar phenomenon exist in both MWNTs and SWNTs? MWNTs and SWNTs also show potential high specific capacity as the anode of the rechargeable lithium battery and EDLCs.[9,10] The reversible capacity Cre of CMK-3 is much higher than those of both MWNTs and SWNTs.[23,27,28] Here we compare the microstructure, micromorphology, and electrochemical properties of CMK-3, PVC700,[3,4] OXY700,[3] MWNT 900 C,[9,10] and SWNTs.[23] The results are summarized in Table 1. It is surprising to find that CMK-3 looks like MWNTs and SWNTs, with a lower ratio of Cre/(Cre + Cirr) in the first cycle and a lower v value, not like PVC700 and OXY700, with a higher ratio of Cre/(Cre + Cirr) in the first cycle and a higher v value. In fact, both the ordered mesoporous carbon and CNTs have pores of a similar size, but CMK-3 has a 3D ordered pore structure and much more uniform pores than those of MWNTs and SWNTs. Is this high capacity caused by the 3D ordered structure of CMK-3? In contrast, SWNTs and MWNTs are collections of
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1D nanotubes. The specific capacity of individual SWNTs and densely packed bundles of SWNTs formed by ball milling have been investigated.[23] The densely packed bundle of SWNTs shows a higher reversible specific capacity, Li2.7C6, than the Li1.6C6 of the individual SWNTs.[23] So, the value of the densely packed bundle of SWNTs, which can be considered as a random 3D pore with a pore size of several nanometers, is near our reversible specific capacity results. Although the exact mechanism of high reversible specific capacity in CMK-3 is still not clear, it suggests that the high reversible specific capacity is possibly related to the 3D ordered structure of CMK-3. Perhaps ordered mesoporous structure plays a key role in this new phenomenon. In ordered mesoporous TiO2, some interesting phenomena based on the 3D mesoporous structure, which have not been completely explained, have also been observed.[29] In conclusion, we have observed for the first time a high specific energy capacity of about 1100 mAh/g (Li3C6) for lithium storage in the ordered mesoporous carbon CMK-3. After the first cycle, the discharge and charge remained at a reversible capacity level (LixC6: x = 2.3 to 3.0) with a good cycle performance.
Experimental Mesoporous silicate SBA-15 was synthesized by the process under acid conditions described elsewhere [19]. The carbon production was similar to the synthesis method described by Ryoo except for the method of carbonization [11,18]. Generally, commercially available sucrose (C12H22O11, 1.25 g) was added to a solution by mixture of H2SO4 (0.14 g) in H2O (5 g). After complete mixing, the mixture was put into a drying oven for 6 h at 373 K, then the temperature was increased to 433 K and held there for 6 h. After addition of sucrose (0.8 g), H2SO4 (0.09 g), and H2O (5 g), the mixture was treated again at 373 K and 433 K successively. The carbonization was complemented by pyrolysis with heating to 1073 K under a nitrogen gas environment. The silica template was removed using 5 wt.-% hydrofluoric acid at room temperature. The fundamental characteristics of the mesoporous carbon were investigated by X-ray diffraction (XRD), isotherm N2 adsorption±desorption and transmission electron microscopy (TEM). For electrochemical measurements, the samples were mixed and ground with 5 wt.-% Teflon (poly(tetrafluoroethylene)) powder as a binder. The mixture was spread and pressed on a copper mesh. The CMK-3 used for charge±discharge galvanostatic measurements was treated at 150 C for 5 h in a vacuum. The charge±discharge performance of the material was investigated in a three-electrode cell using lithium metal as counter and reference electrodes. The electrolyte was 1 M LiClO4 in ethylene carbonate (EC) with diethyl carbonate (DEC) (volume ratio EC/DEC = 1). Cell assembly was carried out in a glove box under an argon atmosphere. Received: September 5, 2003
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[21] T. Iijima, K. Suzuki, Y. Matsuda, Synth. Met. 1995, 73, 9. [22] F. Chevallier, L. Aymard, J.-M. Trarascon, J. Electrochem. Soc. A 2001, 148, 1216. [23] B. Gao, C. Bower, J. D. Lorentzen, L. Fleming, A. Kleinhammes, X. P. Tang, L. E. McNeil, Y. Wu, O. Zhou, Chem. Phys. Lett. 2000, 327, 69. [24] R. Yazami, M. Munshi, in Handbook of Solid State Batteries and Capacitors (Ed: M. Munshi), World Scientific, Singapore 1995, p. 425. [25] K. Sato, M. Noguchi, A. Demachi, N. Oki, M. Endo, Science 1994, 264, 556. [26] J. Dahn, T. Zhang, Y. Liu, J. Xue, Science 1995, 270, 590. [27] B. Gao, A. Kleinhammes, X. P. Tang, C. Bower, L. Fleming, Y. Wu, O. Zhou, Chem. Phys. Lett. 1999, 307, 153. [28] F. Leroux, K. Metenier, S. Gautier, E. Frackowiak, S. Bonnamy, F. Beguim, J. Power Sources 1999, 81±82, 317. [29] a) L. Kavan, J. Rathousky, M. Grätzel, V. Shklover, A. Zukal, J. Phys. Chem. B 2000, 104, 12 012. b) L. Kavan, J. Rathousky, M. Grätzel, V. Shklover, A. Zukal, Microporous Mesoporous Mater. 2001, 44±45, 653.
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