Morphology-Dependent Li Storage Performance of Ordered

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Morphology Dependent Li Storage Performance of Ordered Mesoporous Carbon as Anode Material Min-Sik Kim, Dhrubajyoti Bhattacharjya, Baizeng Fang, Dae-Soo Yang, Tae Sung Bae, and Jong-Sung Yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401150t • Publication Date (Web): 13 May 2013 Downloaded from http://pubs.acs.org on May 21, 2013

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Morphology Dependent Li Storage Performance of Ordered Mesoporous Carbon as Anode Material Min-Sik Kim,† Dhrubajyoti Bhattacharjya,† Baizeng Fang,† Dae-Soo Yang,† Tae-Sung Bae ‡ and Jong-Sung Yu *† †

Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 339-700, Republic of Korea ‡ Korea Basic Science Institute, Jeonju, Jeonbuk 561-756, Republic of Korea

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Abstract: Rod-shaped ordered mesoporous carbons (OMCs) with different lengths, prepared by replication method using the corresponding size-tunable SBA-15 silicas with the same rodlike morphology as templates, are explored as anode material for Li-ion battery. All of the as-synthesized OMCs exhibit much higher Li storage capacity and better cyclability along with comparable rate capability as compared with commercial graphite. Particularly, the OMC-3 with the shortest length demonstrates the highest reversible discharge capacity of 1012 mAh g-1 at 100 mA g-1 and better cyclability with 86.6 % retention of initial capacity after 100 cycles. Although the coulombic efficiencies of all the OMCs are relatively low at the beginning, they improve promptly and after 10 cycles reach the level comparable to commercial graphite. Based on their specific capacity, cycle efficiency and rate capability, the OMC-3 outperforms considerably its carbon peers, OMC-1 and OMC-2 with longer length. This behaviour is mainly attributed to higher specific surface area, which provides more active sites for Li adsorption and storage along with the larger mesopore volume and shorter mesopore channels, which facilitate fast Li ion diffusion and electrolyte transport. The enhancement in reversible Li storage performance with decrease in the channel length is also supported by low solid electrolyte interphase resistance, contact resistance and Warburg impedance in electrochemical impedance spectroscopy.

1. INTRODUCTION Shortage of fossil fuels, nuclear waste, environmental issues and global warming linked to CO2 emissions have directed the global research efforts towards development of renewable green energy production, conversion and storage.1 These important energy and environmental issues along with rapid development of electric automotive and portable electronics industry have made it imperative to develop highly efficient energy storage systems, and to reach this goal, batteries and supercapacitors are being extensively studied around the world.2 Among the myriad of available battery chemistries, rechargeable lithium ion batteries (LIBs) have been considered as the most promising energy storage system for a wide variety of applications. LIBs have revolutionized portable electronic devices like cellular phones, laptops, and digital cameras and also are being considered as replacement of current Ni-MH battery technology in hybrid electric vehicle market.2 LIBs have many advantages like high energy density, high operating voltage, low self-discharge, wide temperature window and long lifespan.3,4 However, this technology now stands at a point where, due to an intrinsic limitation associated with diffusion rate of lithium ion and electron transport in electrode materials, it is finding difficulty in meeting demands for high energy load.5,6 The exploration for new electrode materials for improvement in both electrochemical properties and cost effectiveness for LIBs is always a continuous quest. Anode is one of the crucial parts of LIB fabrication, which involves lithium ion insertion and desertion within the electrode. Carbon materials, metals or metallic alloys and metal oxides or their composites are broad category anode materials.2 Among them, carbons are most popular due to their low working voltage with respect to lithium, high coulombic efficiency, high energy density and low cost.7,8 The most common carbon material used as a LIB anode is graphite due To whom correspondence [email protected] (J.-S.Y.)

should

be

addressed.

Email:

to its high cycle efficiency and good rate capability. However, graphitic carbon can store only one lithium ion for every six carbon atoms leading to low theoretical capacity of 372 mAh g-1.2 Moreover, low surface area of graphite leads to lower Li ion diffusion rate and hence lower power density.2 Disordered carbon materials can store two or three times more lithium ions because of their turbostatic disorder.8 Therefore, various nanostructured carbon materials such as carbon nanofibers,9 carbon nanocomposites,10 carbon nanotubes,11 multi-layered graphenes12 have been employed as anode for LIBs because these materials reveal large surface area and high surface activity, offering high lithium uptake capability. In order to power hybrid electric vehicles or high energy consuming electronics, it is highly needed to develop anode materials having larger capacities and higher Li diffusion rate. In achieving this target, nanostructured porous carbon materials can be very advantageous due to their high surface area and numerous active sites for Li ion adsorption and storage along with different pore sizes ranging from nanometer to micrometer scale .13-24 The nanostructured carbon materials possess novel size and surface dependent properties such as large surface area and high pore volume for excellent Li diffusion and large electrode/electrolyte interface, structural flexibility for buffering volume change during Li intercalation/deintercalation, and high electrical conductivity for enhancement of energy storage performance.25-27 Among them, microporous carbons (pore size < 2nm) have demonstrated larger capacities than traditional graphitic carbons although in many cases, they suffer from high irreversibility, which is due to the increase in solid electrolyte interface (SEI) area and/or due to interaction of Li ions with the carbon surface functional groups.27,28 Mesoporous (2 nm < pore size < 50 nm ) and macroporous (pore size > 50 nm) carbons with high surface area have also been explored as anode materials for LIBs.29,30 These high surface area carbons are usually synthesized by “nanocasting” method, which first involves the creation of a sacrificial porous silica template, followed by impregnation and subsequent carbonization of appropriate carbon precursor to form

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a carbon/template composite and then removal of the silica template, resulting in a highly porous carbon as an inverse replica of the parent template.31-35 Stein et al. reported three dimensionally interconnected ordered macroporous carbon achieving Li storage capacity of 435 mAh g-1.36 Ordered multimodal porous carbon with hierarchical meso-and macropore structure as anode material developed by our group revealed a capacity of 900 mAh g-1.37 Recently, hollow core mesoporous shell carbon with hierarchical nanostructure prepared by using nanocasting method has shown ultra-high reversible capacity of 1387 mAh g-1 at 100 mA g-1.38 Ordered mesoporous carbon (OMC), particularly CMK-3, synthesized using ordered silica SBA-15 template, is also a promising candidate for high Li storage capacity in the order of 1100 mAh g-1 as reported by Zhou et al.39 Several other research groups have also explored this interesting material as a support for making composite with inorganic oxide materials and found very good performance as electrode material for LIB.40-43 The morphology of CMK-3 is a reverse hexagonal structure of rod-like ordered SBA-15. Recently, we have developed a strategy to tune the size of SBA-15 silica from rod-like morphology to prism like morphology by changing HCl concentration in the reaction medium, which in turn can be used as template for size tunable synthesis of OMC with the inverse replica of corresponding template.44 This size tuning dramatically changes the surface area and mesopore volume of the synthesized OMCs. Therefore it will be very interesting and significant to study the change in Li storage performance with change in length of OMCs, which is, to the best of our knowledge, is not reported. In this study, rodshaped OMCs with three different sizes are synthesized by using the corresponding SBA-15 templates and explored as anode materials for LIB. It is found that, the OMCs have demonstrated higher Li storage capacity than commercial graphite with very good cycling and rate performance. Interestingly, the capacity, cycle efficiency and rate capability are found to be related to the specific surface area, mesopore volume and mesopore channel length of the OMCs, which are considered to be the key parameters for the Li ion diffusion and electrolyte movement. The OMC with shorter mesopore channels is found to have higher surface area and mesopore volume and facilitate fast Li ion diffusion and electrolyte transport, correspondingly exhibiting enhancement of Li storage capacity, cycle efficiency and rate capability.

2. EXPERIMENTAL SECTION 2.1. Preparation of OMCs with different lengths. OMCs were synthesized by a “nanocasting method” using SBA15 as a template and phenol/paraformaldehyde resin as carbon precursor. Conventional ordered mesoporous silica SBA-15 with rodlike morphology was used as a hard template and size of which was tuned by control of HCl concentration under hydrothermal conditions according to the established procedures.37,44 A typical synthesis route for the OMC is shown in Scheme 1, which includes impregnation of carbon precursor into the porous template followed by polymerization, carbonization and removal of the template to have template-free carbon replica. Typically, 0.374 g of phenol was introduced into 1.0 g of SBA-15 template by heating at 100 oC for 12 h under vacuum. The resulting phenolincorporated SBA-15 template was then reacted with paraformaldehyde (0.238 g) under vacuum at 130 oC for 24 h to produce a composite of phenol-paraformaldehyde resin/SBA-15. The composite was heated at 1 oC min-1 to 160 oC and held for 5 h under a nitrogen flow. The temperature was then ramped at 5 oC min-1 to 950 oC and held for 7 h to carbonize the cross-linked phenol resin inside the mesopores of the SBA-15 structure. After carbonization, SBA-15 template was removed by dissolving in 3.0

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Scheme 1. A typical scheme for the synthesis of rod-shaped OMC.

M NaOH and washing in EtOH–H2O solution with volume ratio of EtOH to H2O = 1: 1 to get the silica-free OMC. Three types of OMCs with different length were prepared by simply using SBA15 templates with the corresponding length with keeping all other parameter intact, and were designated as OMC-1 (1.2 to 1.9 µm in length), OMC-2 (0.8 to 1.2 µm), and OMC-3 (0.5 to 0.8 µm) in decreasing order of length. 2.2. Surface characterization. The surface morphologies and structures of OMC materials were examined by X-ray diffractometer (XRD) with small-angle X-ray scattering (SAXS) unit, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SAXS patterns were recorded on a Rigaku Smartlab X-ray diffractometer with Cu Kα radiation (λ=1.5406 Å) operating at 40 kV and 30 mA. The SEM images were obtained using a Hitachi S-4700 field emission electron microscope operated at an acceleration voltage of 10 kV. The TEM analysis was carried out by EM 912 Omega transmission electron microscope operated at 120 kV. Nitrogen sorption isotherms were measured on a KICT SPA-3000 Gas Adsorption Analyzer at 77 K. Before the measurements, the samples were outgassed at 150 oC at 20 μTorr pressure for 12 h. The Brumauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas. The total pore volume (Vtotal) was determined from the amount of gas adsorbed at the relative pressure of 0.99, while the mesopore volume (V meso) and micropore volumes (Vmicro) of the porous carbons were calculated from analysis of the adsorption isotherms using the Horvath– Kawazoe (HK) method. 2.3. Electrode preparation and cell configuration. Electrochemical measurements were carried out by using a CR2032 coin-type test cell (Hohsen Corporation, Japan), where OMC samples and commercial graphite (for comparison) were used as anode, Li metal foil (99.9% purity and 150 m thickness) as cathode, 1.0 M LiPF6 in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1 in volume) as electrolyte, and Celgard 2400 membrane as separator. For electrode fabrication, OMC sample or graphite was mixed with acetylene black (as a conductivity enhancer) and PVdF (Polyvinylidene fluoride) binder at a weight ratio of 8:1:1 in a solvent (i.e., N-Methyl-2pyrrolidone) to generate homogeneous slurry. The slurry was then uniformly pasted with 30 m thickness on Cu foil. The asprepared working electrodes were dried at 120 oC in a vacuum oven and pressed at a pressure around 4000 psi. The entire cell assembly was conducted in an Ar-filled glove box with the oxygen and the humidity level of 1 ppm or less, respectively within the chamber. The assembled cells were aged overnight before testing. The charge–discharge behaviour of the coin cells was characterized in a BaSyTec multichannel battery test system at a room temperature. The instrument was programmed to read in each 10 s step. The cells were cycled in the voltage range 3.0 – 0.01 V (vs. Li/Li+) at a rate of 100 mA g-1 during an initial formation process and at different rates in the following cycles. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a Biologic VSP electrochemical workstation in the frequency range of 100 kHz to 10 mHz at zero-bias potential with ac amplitude of 10 mV and dc potential of 100 mV. Impedance parameters were analyzed by fitting the spectra to the proposed equivalent circuit using ZView software.

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3. RESULTS AND DISCUSSION Figure 1a, c and e show representative SEM images of the assynthesized rodlike OMCs with different lengths. All the OMCs have retained the inverse morphology of the corresponding SBA15 silica templates. OMC-1 has a length of ca. 1.6 µm and a diameter of ca. 0.2 µm (Figure 1a), while OMC-2 has shorter length of ca. 1.0 μm and larger diameter of ca. 0.3 µm (Figure 1c). OMC-3, where the length shrinks much more, shows almost spherical morphology with a diameter of ca. 600 nm (Figure 1e). The sizes of the three OMCs were influenced by the sizes of SBA15 silica template, which were regulated by changing HCl concentration in the synthesis medium. The TEM images in Figure 1b, d and f show that all the OMCs have ordered mesopore arrays and 2-D hexagonal pore structure. Figure 2a shows the SAXS patterns of OMCs, which exhibit peaks that can be indexed as (100), (110) and (200) reflections associated with a 2-D hexagonal symmetry (p6mm).45,46 The lowangle (100) diffraction peak at 2θ = 0.98, 0.94 and 0.92o for OMC-1, OMC-2 and OMC-3 (Figure 2a), respectively confirms that the ordered mesoporous structure of the as-synthesized OMCs is still retained after carbonization and silica removal. The shift in peak position of (100) plane towards lower angle with decrease in length of the OMCs can be correlated to the increase in pore size with decreasing channel length of the OMCs.45,46 The surface areas and pore structures of the OMCs were further analyzed by a N2 adsorption analyzer. The N2 adsorptiondesorption isotherms and BJH desorption pore size distributions of OMC materials are given in Figure 2b, where it can be clearly seen that all the OMCs exhibit type IV isotherms with H4-type hysteresis loop. Type-IV isotherms are characteristics typically shown by mesoporous materials, and the hysteresis is due to the capillary condensation of nitrogen in mesopores. The hysteresis loop of type H4 is indicative of narrow slit-like pores, which is in accordance with the observation made from the TEM image for all the OMCs (Figure 1). The BET surface area and mesopore volume of the various OMCs are summarized in Table 1. The OMC-1,

Figure 1. Typical SEM and TEM images of OMC-1(a, b), OMC-2(c, d) and OMC-3(e, f).

Figure 2. Small-angle XRD patterns of OMCs (a) and the N2 adsorptiondesorption isotherms at 77 K for (■) OMC-1, (▲) OMC-2 and (●) OMC3 (b). Table 1. Structural properties of OMC carbons synthesized with various SBA-15 templates. Sample

SBET

Vmicro

Vmeso

Vtotal

Pore size

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

(cm3 g-1)

(nm)

OMC-1

711

0.4

0.7

1.1

3.2

OMC-2

1022

0.5

0.9

1.4

3.3

OMC-3

1220

0.5

1.1

1.6

3.4

OMC-2 and OMC-3 show surface areas of 711, 1022 and 1220 m2 g-1, respectively. In addition, it is found that the decrease in length of the OMC results in the increase in mesopore volume as well as surface area, namely the OMC-3 with the shortest length, has the largest surface area and mesopore volume. This is consistent with those observed from the TEM, SEM images and also with SAXS patterns (Figure 1 and 2a). The larger surface area and mesopore volume are expected to provide OMC-3 with enhanced Li storage capacity, while the unique structural properties such as short channel length and large mesopore volume can favour facile transfer of Li ions and electrolyte ions with improved rate capability in the OMC-3 mesopore structure. Figure 3 represents the galvanostatic charge/discharge curves of the as-prepared OMCs at a constant current of 100 mA g-1 with a potential window from 0.01 V to 3.0 V (vs. Li/Li+). It can be observed that the first charging capacity of the OMCs are very high (OMC-1/ OMC-2/ OMC-3: 1402 / 1980 / 2385 mAh g-1) with a plateau in ca. 0.9 V (vs. Li/Li+). This plateau formation is quite common in all carbon-based anode materials and is primarily due to the reaction of lithium with the electrolyte, which causes decomposition of the electrolyte and formation of solid electrolyte interphase (SEI) passivation layer on the surface of the carbon electrode.47,48 The first discharge capacities of OMC-1, OMC-2, OMC-3 were found to be 526, 815, 1012 mAh g-1, respectively. This result shows that the initial coulombic efficiencies of OMC-1, OMC-2 and OMC-3 are only 37, 41 and 42 %, respectively, which is quite low. Porous carbonaceous materials are notorious for low initial coulombic efficiencies, mainly due to high specific surface areas and irreversible lithium insertion into the microporous active sites, for example at the vicinity of residual H atoms causing breakdown of the electrolyte and the creation of the SEI films at the electrode/electrolyte interface.49 High irreversible capacity is caused mainly by micropores which led to an increased SEI area,2 and since all the OMCs have similar amount of micropore volume, they have almost similar irreversible capacity. However, the reversible capacity was almost maintained from the second charge/discharges cycles, suggesting a good cycling performance for the OMC electrodes. The coulombic efficiency increases promptly to the 90.8, 89.9 and 90.4 %, respectively, during the 2nd cycle and increases further to more than 95 % after 10th cycle and over 98 % after the 30th cycle. This improvement suggests

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Figure 4. Galvanostatic charge/discharge curves at 100 mAh g-1 for commercial graphite and various OMCs for the 2 nd cycle.

and OMC-2. Since the micropore volume is almost identical for all of the OMCs, the increase in discharge capacity from OMC-1 to OMC-3 can be directly correlated with the increase in surface area caused especially by the increase in mesopore volume and pore size with the decrease in length of OMC. In addition, shortened mesopore channel length in OMC along with increased mesopore volume and pore size from OMC-1 to OMC-3 in turn favors facile Li diffusion through mesopores to access more surface active sites for Li ion adsorption, likely resulting in higher Li storage capacity and rate performance. Figure 5 represents the galvanostatic cycling performance for various OMCs and commercial graphite at 100 mA g-1. All the OMCs were found to be very stable up to the 100th cycle with a minor reduction in the reversible capacity. The OMC-3 reveals higher initial reversible capacity and much higher Li storage capacity at the 100th cycle compared with the other two OMCs (i.e., OMC-2 and OMC-1) and commercial graphite, i.e., OMC-3, OMC-2, OMC-1 and graphite electrode have shown specific capacity of ca. 876, 664, 355 and 253 mAh g-1, respectively after the 100th cycle which corresponds to 86.6%, 85.6 %, 75.6% and 82.9% capacity retention from the 2nd discharge value. Cycling performance depends on reversible diffusion of lithium ion within porous carbon electrode materials. The decrease in channel length of OMC reduced the travel path favoring the unhindered mass transfer during insertion/extraction of lithium through the pores of OMC, thereby increasing capacity retention ability from OMC-1 to OMC-3. In present day scenario, it is becoming a mandatory electrochemical feature of LIBs to exhibit good rate capability to meet the requirement for high energy applications such as hybrid electric vehicles. The rate performances of OMC-1, OMC-2,

Figure 3. Galvanostatic charge/discharge curves at 100 mA g-1 for OMC-1 (a), OMC-2 (b) and OMC-3 (c).

that the SEI layer becomes steady for subsequent lithium insertion and extraction, which is consistent with the fading of the plateau from the second cycle. Figure 4 shows galvanostatic charge-discharge profiles obtained in the second charge/discharge cycle for various OMCs and the commercial graphite at a charge/discharge rate of 100 mA g-1. The OMC-3, OMC-2, OMC-1 and graphite exhibits reversible Li storage capacity of ca. 1011, 781, 469 and 305 mAh g-1, respectively. Evidently, the capacity of the OMC-3, OMC-2 and OMC-1 is ca. 3.3, 2.6 and 1.5 times that of graphite, respectively. It is also evident that discharge capacity for OMC-3 with shortest length is much higher than those observed for the longer OMC-1

Figure 5. Cycling performance and coulombic efficiency of commercial graphite and various OMCs at a specific current of 100 mA g-1.

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Figure 6. Rate performances of commercial graphite and OMCs at different current densities from 100 to 1000 mA g-1 and then back to 100 mA g-1. OMC-3 and commercial graphite anode at different current densities are illustrated in Figure 6. It is evident that all the assynthesized OMCs demonstrate an excellent rate capability, delivering a high fraction of its total capacity even at a rate of 1000 mA g-1 (i.e., the capacities of OMC-1, OMC-2 and OMC-3 are 181, 378, 520 mAh g-1 at 1000 mA g-1, which account for 34, 46.4 and 51% of respective value at 100 mA g-1). The capacity retention (i.e., 51%) of OMC-3 is much higher than that of commercial graphite (i.e. 13%). Moreover, the electrode recommences near the full original capacity after returning to the initial 100 mA g-1 regime (889 mAh g-1 i.e. 12 % decrease, which is comparable to 275 mAh g-1 i.e. 10 % decrease for graphite sample), which confirms the exceptional capability of the OMC electrode to keep its integrity not only for a long number of cycles, but as well at high rates. Abundance of micropores in OMCs provide more active sites for Li ion adsorption resulting in high Li storage capacity especially at relatively low charge/discharge rates, in which most of the micropores are accessible to Li species if the sizes of the micropores are larger enough than Li species. However, at a high rate, only small portion of these micropores are still accessible to Li species and would contribute to Li storage if there is no fast pathway constructed for fast mass transport by larger pores (i.e., mesopore). In case of OMC samples with greater amount of mesopores, portion of the micropores, which can be still accessible and would contribute to Li storage at high rates would increase with increasing mesopore volume for fast mass transport. That is probably why rate capability has increased with increase in mesopore volume from OMC-1 to OMC-3. Three major factors, namely, reversible storage capacity, cyclability and rate capability are often used to evaluate LIB performance, which are found to be strongly correlated to the surface area and pore structure of the active electrode materials. Behavior of Li ion storage performance in this study implies the importance of mesopores and well-developed pore structures in Li storage capacity. EIS is a vital tool to understand this surface dependent LIB performance. EIS measurements were conducted for the assynthesized OMCs after the performing up to 100 cycles of galvanostatic charge–discharge at 100 mA g-1 rate. Representative Nyquist plots are shown in Figure 7, where two semicircles in the high-medium frequency regime and an inclined line in low frequency are observed for each of the electrodes. The 1st semicircle corresponds to the sum of contact resistance and SEI resistance (RSEI), 2nd semicircle represents charge transfer resistance (Rct), while the inclined line accounts for characteristic Warburg behavior related to the mass transfer resistance of lithium ion within the pores of the mesoporous carbon.49,50 After neglecting usually small contact resistance parameter,51 an

Figure 7. Typical Nyquist plots recorded at various OMC electrode materials.

Table 2. Kinetic parameters derived from the Nyquist plot for the various OMCs. Sample

Rs/Ω

RSEI/ Ω

CSEI/ F

Rct/ Ω

ZW/ Ω·s-1/2

Cdl/F

OMC-1

41.1

35.01

3.3ⅹ 10-5

12.7

65.2

2.0ⅹ 10-6

OMC-2

23.0

18.41

1.7ⅹ 10-5

11.6

42.0

1.2ⅹ 10-6

OMC-3

16.0

12.4

2.4ⅹ 10-5

8.09

13.0

0.8ⅹ 10-6

RS:Solution resistance, RSEI:SEI resistance, CSEI:SEI capacitance, Rct:Charge transfer resistance, ZW:Warburg impedance, Cdl:Interfacial capacitance.

equivalent circuit is proposed as shown in Figure 7 (inset) for the analysis of impedance spectra. Some significant kinetic parameters are derived by fitting the impedance spectra to the proposed equivalent circuit using the ZView software, and listed in Table 2. A much smaller Rct and ZW were found for OMC-3 confirming faster charge and mass transfer in lithium insertion/extraction process and facile charge transfer at the electrode/electrolyte interface. The unique nanostructure consisting of channel-like ordered mesopores with not only relatively large mesopore volume, but also with short Li ion diffusion distance is probably the main reason behind faster mass transport and electrolyte movement in OMC-3 compared to those for its carbon peers, OMC-1 and OMC-2 with longer length. The change in value of Rct and ZW is in accordance with change in specific surface area, mesopore volume and channel length. Although Li ion species can penetrate into the mesopore channels (larger than 3.0 nm in diameter) of OMCs with no geometrical hindrance, it is likely that Li ion species have less resistance in shorter channels, which can lead to a higher rate capability compared to that of longer channels. This is in good agreement with the literature, where it was reported that mesoporous carbon with short lengths showed much higher specific capacitance and oxygen reduction electrocatalytic activity along with better performance retention ability than the samples with longer lengths.52-54.

4. CONCLUSIONS In this study, the OMCs with three different mesopore channel lengths were fabricated and explored as anode material in LIB. The synthesized OMCs possess distinctive structural features such as well-developed 2D hexagonal mesopores, large specific surface area, large mesopore volume and unique hierarchical porosities (i.e., meso- and micropores), allowing facile mass transport and electrolyte movement. Compared with the commercial graphite, all OMC materials have not only demonstrated higher Li storage capacity, but also better cycling performance and rate capability.

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Comparison of Li ion battery performance was drawn for change in length in three different OMCs in terms of specific capacity, cyclability, and rate capability, wherein OMC-3 with the smallest size possessed largest surface area and has shown the highest specific capacity along with higher cycle stability and rate capability compared to the other two OMCs. Variation in mesopore channel length and volume of OMCs changes the diffusion path for Li ions and thereby changes the reversible capacity, cyclic and rate performance. This behavior is also supported well by EIS spectra in terms of change in SEI resistance, charge transfer resistance and Warburg impedance. We believe that this systematic study will help to understand the correlation between surface structures and LIB performance which is much needed to develop high energy LIB in accordance with present day requirement.

ASSOCIATED CONTENT Supporting Information Pore size distribution curves, SEM and TEM images, and galvanostatic charge-discharge curves. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax: +82 44-860-1331. Tel: +82 44-860-1494.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by NRF grant (NRF 2010-0029245) and Global Frontier R&D Program on Center for Multiscale Energy System (NRF 2011-0031571) funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea. The authors also would like to thank Korean Basic Science Institute at Jeonju and Chuncheon for SEM, UHRSEM and TEM analysis.

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Graphic abstract Rod-shaped OMCs with three different sizes are prepared by using the corresponding SBA-15 template and explored as anode materials for LIB. Interestingly, all the OMCs have not only demonstrated higher Li storage capacity than commercial graphite, but also very good cycling performance and rate capability.

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