Bioinspired Synthesis of Hierarchical Porous Graphitic Carbon

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Bioinspired synthesis of hierarchical porous graphitic carbon spheres with outstanding high-rate performance in lithium-ion batteries Su-Xi Wang, Shilin Chen, Qiliang Wei, Xikui Zhang, Siew Yee Wong, Shuhui Sun, and Xu Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504042s • Publication Date (Web): 17 Dec 2014 Downloaded from http://pubs.acs.org on December 24, 2014

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Chemistry of Materials

Bioinspired synthesis of hierarchical porous graphitic carbon spheres with outstanding high-rate performance in lithium-ion batteries Su-Xi Wang1, Shilin Chen1, Qiliang Wei2, Xikui Zhang1, Siew Yee Wong1, Shuhui Sun2 and Xu Li1,* 1

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602

2

Institut National de la Recherche Scientifique -Énergie Matériaux et Télécommunications, Varennes, QC J3X 1S2, Canada KEYWORDS: lithium-ion batteries, carbon spheres, high-rate performance, hydrothermal synthesis, biomimetic synthesis ABSTRACT: Inspired by the biomineralization of unicellular diatoms, a biomimetic approach based on template (pluronic F127 micelle cluster)-induced self-assembly of α-cyclodextrin is developed to create hierarchical porous graphitic carbon spheres via hydrothermal treatment followed by pyrolysis. The as-obtained carbon spheres combine the features required for high-power electrode materials in lithium ion batteries (LIBs), such as high degree of graphitization, large surface area with hierarchically distributed pore sizes as well as doping with heteroatoms, which synergistically contribute to their impressive electrochemical properties. When applied as anode for LIBs, the carbon spheres exhibit high reversible capacity (ca. 700 mA h g-1 at 50 mA g-1), good cycling stability and remarkably outstanding high-rate performance (ca. 600, 450 and 290 mA h g-1 obtained at a current density of 1, 10 and 30 A g-1, respectively), which is among the best of present pure carbon materials for LIBs applications. The fabrication process is straightforward and cost-effective, providing a new methodology for the tailored design of carbon materials with enhanced power densities for energy storage applications.

1. INTRODUCTION Over the past decade, lithium rechargeable batteries have been developed into the most widely used environmentally benign energy storage devices in place of the everdepleting conventional energy resources, owing to their advantages such as high energy density and electrical potential, no memory effect, low self-discharge rate and high stability.1,2 However, with the rapid evolution of various intelligent mobile electronic devices and popularization of electric or hybrid-electric vehicles (EVs or HEVs), traditional LIBs are faced with more stringent standards nowadays, especially the fast charge/discharge capabilities. The major challenge in the field of electrical energy storage today is to integrate the advantages of batteries and electrochemical capacitors by developing electrode materials that can achieve high rate performance of supercapacitors while delivering the energy densities of batteries.3-5 Carbon materials with various microtextures and wide availabilities have been holding the predominant position as anode materials for LIBs owing to their low cost and the highest degree of safety at different temperatures.6 During the past several years, porous carbons (PC) have sparked enormous interest for energy applications because their highly developed surface area and wellestablished three-dimensionally interconnected porous network not only offers the chemistry and structure to

store and insert the lithium ions, but also provides abundant channels for fast mass transport and shorter diffusion distances to the interior surfaces.7-20 A typical example of the application of porous carbon in LIBs is shown by Hu and co-workers who prepared macro- and mesoporous (with mesopore diameters around 7.3 nm) carbon monoliths with relatively higher proportions of graphitelike ordered carbon structure from mesophase pitch using porous silica as hard templates.7 The material exhibit high reversible capacity (540 mA h g-1 at 1 C, 1 C = 372 mA g-1) and good high-rate performance (260, 145 and 70 mA h g-1 at 10, 30 and 60 C, respectively) when applied as anode in LIBs. In order to further enhance high-rate capabilities, many hybrid PC architectures incorporated with homogenously dispersed metal/metal oxide nanoparticles have also been fabricated and studied in rechargeable lithium batteries.11-15 For example, He et al. prepared 2D porous graphitic carbon nanosheets encapsulated with Fe3O4 nanoparticles with the assistance of NaCl particles.11 The hybrid material which has a broad pore size distribution (1-235 nm) with the preponderance of mesopores (around 3.7 nm) features high rate capacities (858, 587, and 311 mA h g-1 at 5, 10, and 20 C, respectively, 1 C = 1 A g-1). In all of the aforementioned examples, high porosity, interconnected channel systems as well as well-graphitized carbon walls are the key ingredients that work synergistically to provide good capability for Li insertion/extraction at high current densities. It is therefore of great significance to

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find a direct, low-cost and effective approach to prepare hierarchical porous carbons with good conductivity intended for high-power LIB applications. In nature, a great number of micro-organisms are found to assemble biominerals into intricate three-dimensional (3D) structures. Among the most famous examples are unicellular algae–diatoms–that display a dazzling variety of complex hierarchical 3D architectures with great structural control over nano- to millimeter length scales.21-22 These organisms possess a cell wall composed of organic molecules or macromolecules as templates to induce and direct the precise precipitation of silica building blocks in the silica deposition vesicle (SDV) through hydrogen bonding or ionic interactions to form the complex structures. This natural phenomenon offers great inspiration to many biomimetic templating strategies based on selfassembled molecular precursors that allow the production of advanced nanomaterials with high accuracy under mild conditions to achieve superior properties for a wide range of applications.23-26 Herein, we present a biomimetic approach for fabricating hierarchical porous graphitic carbon spheres (GCS) from the aqueous mixture of α-cyclodextrin (α-CD), pluronic F127 triblock polymer and cobalt(II) gluconate via hydrothermal treatment followed by pyrolysis. In this method, the carbon scaffold is constructed via the wellordered self-assembly of α-CD which is induced and templated by the organic micro-vesicles, originated from the aggregation of F127 micelles in the presence of cobalt(II) gluconate, through hydrogen bonding interactions. These GCS are rich in micropores and mesopores, which provides a harmonious electrochemical environment that guarantees smooth ion transfer and high charge storage capability. Moreover, the highly crystallized carbon structures induced by catalytic graphitization as well as the heteroatom doping further contribute to their performance due to the improved electrical conductivity and electrochemical reactivity. When applied as anode material for LIBs, the carbon spheres exhibit high reversible capacity (ca. 700 mA h g-1 at 50 mA g-1), good cycling stability and remarkably outstanding high-rate performance (ca. 600, 450 and 290 mA h g-1 obtained at a current density of 1, 10 and 30 A g-1, respectively), which is among the best of present pure carbon materials for energy storage applications.7-10, 27-35 2. METHOD Preparation of graphitic carbon spheres (GCS). 30 mg F127, 10 mg cobalt(II) gluconate and 60 mg α-CD were dissolved in 20, 10 and 10 mL deionized (DI) water, respectively under stirring. Then the cobalt gluconate solution was injected into the F127 solution. After stirring for 15 min, the α-CD solution was added to the above mixture and stirring was continued at room temperature overnight. The solution was finally transferred into a 100 mL Teflon sealed autoclave chamber and subjected to hydrothermal treatment at 200 °C for 6 h. The as-prepared “soft” carbon spheres were collected and washed by DI

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water three times by centrifuging at 9000 rpm for 20 min. After washing, the dark precipitate was redispersed in DI water and freeze dried to yield black powders, which were further heated to 900 oC under argon at a heating rate of 5 o C min-1 and pyrolyzed at this temperature for 3 h to generate the graphitic carbon spheres, which were finally washed with 6 N HCl and DI water for three times and dried under vacuum at 60 oC overnight. Preparation of N-doped graphitic carbon spheres (NGCS). The as-prepared "soft” carbon spheres (60 mg) via hydrothermal treatment and urea (3.6 g) were dissolved in 10 mL DI water under sonication. After vacuum dried, the mixture was heated to 900 oC under argon at a heating rate of 5 oC min-1 and pyrolyzed at this temperature for 3 h to generate the N-doped graphitic carbon spheres. Finally, the graphitic carbon spheres were washed with 6 N HCl and DI water for three times and dried under vacuum at 60 oC overnight. Characterization. The size, morphology and microstructures of the carbon spheres were observed under a JEOL JSM 6700 field-emission scanning electron microscope (FESEM) at an accelerating voltage of 5 kV and a CM 300 FEG-Philips high-resolution transmission electron microscope (TEM). The TEM image of the F127 micelles was obtained via dropping the micellar solution on the copper grid followed by staining with 2 wt% phosphotungstic acid. The carbon structure was studied by Raman spectroscopy recorded on Jobin Yvon T64000 triple spectrograph micro-Raman system. The components of the carbon spheres were analyzed by X-ray diffraction (XRD) which was performed with a Bruker D8 Discover GADDS X-ray diffractometer with Cu Kα radiation. The surface chemistry of the samples was studied by X-ray photoelectron spectroscopy (XPS) analysis which was conducted on a VG ESCA LAB-220i XL X-ray photoelectron spectrometer with an exciting source of Al. The N2 adsorption/desorption isotherms at −196 °C were measured using a Micromeritics ASAP 2020 system. The Brunauer−Emmett−Teller (BET) surface area was evaluated using N2 adsorption data in the relative pressure (P/P0) range of 0.06−0.2 and the pore size distribution was calculated by analyzing the adsorption branch of the N2 isotherms using the non-local density functional theory (NLDFT) method. Dynamic light scattering (DLS) was performed with Malvern Zetasizer Nano-S using a HeNe laser (633 nm) to measure the nanoparticle size distribution of the micellar solution. Electrochemical measurements. Electrochemical performance of the carbon spheres was evaluated as anode in lithium-ion batteries. The working electrode was fabricated by coating the slurry of carbon spheres (80 wt %), carbon black (10 wt %), and polyvinylidene fluoride (PVDF) (10 wt %) in N-methyl pyrrolidinone (NMP) onto a copper foil (with a thickness of 15 µm). The coated copper foil was dried under vacuum at 120 °C overnight and then punched into discs of 15 mm in diameter. The asprepared electrode with thickness of 75-90 µm and active material loading of 4-5 mg was assembled into 2032 button cells in an argon-filled glove box with lithium foil,


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Figure 1. Schematic fabrication process for the hierarchical porous graphitic carbon spheres.

Celgard 2325 membrane and 1 M LiPF6 in ethylene carbonate / dimethyl carbonate (1:1 v/v ratio) as the counter electrode, separator and electrolyte, respectively. The charge-discharge testing was conducted on NEWARE battery tester at different current densities with a cutoff voltage window of 0.005−3.0 V. Rate capacities were obtained via discharging at 50 mA g-1 and charging at various current densities. The cyclic voltammetry test was performed on an electrochemical workstation (PGSTAT302, Autolab) within a voltage window of 0–3.0 V and at a scan rate of 0.01 V s-1. The electrochemical impedance spectroscopy (EIS) study was conducted using the same electrochemical workstation in a frequency range of 106 to 10-2 Hz and at the a.c. amplitude of 5 mV. 3. RESULTS AND DISCUSSION The hierarchical porous graphitic carbon spheres are derived from cyclodextrins (CDs), the most extensively studied self-assembled building blocks that can be threaded onto linear polymer to form necklace-like structures due to hydrogen-bonding interactions,36-38 and the biomimetic synthetic strategy is schematically depicted in Figure 1. First, the amphiphilic F127 is dissolved in water to selfassemble into micelles with hydrophilic PEO chains surrounding the hydrophobic micelle core. Then upon incorporation of cobalt(II) gluconate, the small micelles aggregate to form large spherical clusters due to hydrogen-bonding interactions between PEO chains and cobalt gluconate which bears multiple hydroxyl groups. Afterwards, α-CD is added to the solution and threaded onto PEO chains in the clusters. The subsequent hydrothermal process gives rise to “soft” carbon spheres with uniform dispersion of cobalt oxides. Finally, cobalt oxides are reduced to cobalt by carbon during the further annealing process at 900 oC under argon and aggregated to cobalt nanoparticles which function as catalyst for the graphitization of the amorphous carbon derived from α-CD, and

the soft template F127 is simultaneously burned out to generate mesopores in the carbon matrix. In this methodology, cobalt gluconate not only acts as the precursor of Co catalyst for low temperature graphitization, but is also responsible for the agglomeration of F127 micelles which act as templates to induce the precise threading of α-CD via hydrogen-bonding interactions to construct the carbon structure. The formation of spherical clusters in the micellar solution before hydrothermal treatment is verified by dynamic light scattering (DLS) and TEM analysis (see Figures S1 and S2 in the Supporting Information). As shown in the TEM image (Figure S1a), the F127 micelles are spherical nanoparticles with a size distribution around 10 nm. Addition of cobalt gluconate induces the aggregation of small micelles to large clusters with diameters ranging from 150 to 200 nm (Figure S1b). Further threading of α-CD onto the PEO chains in the clusters gives rise to spherical clusters with an increased diameter distribution from 200 to 300 nm (Figure S1c). The evolution of the particle sizes observed by TEM is in accordance with the results of DLS analysis shown in Figure S2. In order to further demonstrate the crucial role of cobalt gluconate in the particle formation, 10 and 20 mg of cobalt gluconate was added into F127 micelle solution, respectively. As revealed in the DLS profile shown in Figure S3, increased amount of cobalt gluconate leads to faster and more complete aggregation of the F127 micelles with larger size of the clusters (around 260 nm). A different reactant addition sequence in which cobalt gluconate was added after threading of α-CD onto PEO chains was also conducted. In contrast to form uniform carbon spheres with homogenously dispersed cobalt, the asprepared carbon spheres were not uniform and cobalt particles were observed aggregated outside the carbon spheres (Figure S5). Furthermore, it has also been demonstrated that the sizes and electrochemical performance of the prepared graphitic carbon spheres can be easily tai-



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lored via varying the proportions of cobalt gluconate in the micellar solutions (as evidenced by Figures S4 and S6). The size, morphology, and microstructure of the asprepared GCS were studied by transmission electron microscopy (TEM) analysis. Figures 2a-b present the TEM images of the spheres derived from hydrothermal treatment of the solution of α-CD, F127 and cobalt gluconate at 200 oC for 6 h. It reveals that these particles are solid spheres with smooth surfaces and have an average diameter of 330 nm with a narrow size distribution. As show in Figure 2c, further pyrolysis at 900 oC (with a heating rate of 5 oC min-1) for 3 h under argon leads to the formation of carbon spheres with a smaller average diameter of 250 nm, which are dotted with cobalt nanoparticles, as evidenced by the X-ray diffraction (XRD) patterns (Figure 4a). The magnified TEM images (Figures 2d-e) clearly reveal the interconnected mesoporous structure of the spheres, which could be ascribed to the decomposition of the soft template F127 as well as the internal void space generated from micelle stacking (as shown in Figure 1). Remarkably, as shown in the high-resolution image (Figure 2f), the porous carbon is composed of conductive graphitic layers with weak edge terminations and small crystalline domains with an interlayer d-spacing of 0.34 nm. The graphitic structures of the carbon walls are further confirmed by XRD and Raman analysis. The XRD patterns of the carbon spheres (Figure 4a) displayed an intensive peak centered at 25.3o, which can be indexed to the (002) diffraction of graphitic carbon. A representative Raman spectrum of the carbonized sample (Figure 4b) shows two bands centered at 1594.6 (G band) and 1347.8 cm-1 (D band), respectively. The G band is closely related to the graphitic carbon phase with sp2 electronic configuration. As can be seen in the spectrum, the sample exhibits a strong G-band signal with a low ID / IG ratio of 1.19. These results reveal the high graphitization of the GCS. Doping with heteroatoms (such as N, B) has been proven to be an enabling strategy to enhance both the capacitance and electrical conductivity of carbonaceous materials.27,29,30 To further enhance the electrochemical performance, the preformed "soft" carbon spheres by hydrothermal treatment are vacuum impregnated with urea aqueous solution prior to pyrolysis at 900 oC to generate N-doped GCS (NGCS). The EDX mapping images (Figures 3c-f) demonstrate the uniform distribution of the carbon, nitrogen, oxygen element as well as the cobalt nanoparticles in the carbon spheres. Figure 3a presents the morphology of the NGCS after removal of cobalt with HCl. As revealed by the high resolution image (Figure 3b), the NGCS exhibit a less ordered graphitic microstructure compared with the non-doped ones, which could be ascribed to the reactions between carbon and ammonia gas at high temperature in the nitrogen doping process which introduce large amount of topological defects and structural disorder to the carbon spheres. This can also be verified by the Raman spectrum of NGCS displayed in Figure 4b. The peak intensity ratio of D to G increased from 1.19 to 1.57 after doping with nitrogen. Furthermore, com-

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pared with the peak positions of GCS, red shift is observed with both the D-band (from 1347.8 to 1341.2 cm-1) and G-band (from 1594.6 to 1586.3 cm-1) of NGCS, which is consistent with the results of other nitrogen-doped carbon materials.39

Figure 2. (a) and (b) : TEM images of nanospheres after hydrothermal treatment; (c), (d), (e) and (f) : TEM images of o Co-GCS after pyrolysis at 900 C.

Figure 3. (a) and (b): TEM images of NGCS after removal of cobalt; STEM EDX mapping of NGCS before removal of cobalt: (c) C element, (d) N element (e) O element (f) Co element.

X-ray photoelectron spectroscopy (XPS) analysis was conducted to study the elemental composition and chem-


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ical states of the elements that exist in the NGCS. Figure 4c displays the survey spectrum (0-900 V) of the carbon sample, which basically includes C1s, N1s and O1s without any other impurities and the elements’ atomic ratios are 85.7, 7.8 and 6.5 %, respectively. As shown in Figure 4e, the N1s spectrum can be fitted into four component peaks located at 398.4, 399.6, 401.0 and 402.8 eV, which could be identified to pyridinic nitrogen (31.1 %), pyrrolic nitrogen (11.4 %), quaternary nitrogen (40.8 %) and oxidized nitrogen (16.7 %), respectively.40 As reported previously, only pentagonal pyrrolic nitrogen is formed at low temperature around 300 oC and it could be converted to pyridinic and quaternary nitrogen as the carbonization temperature rises.41 The quaternary nitrogen has been known to increase the electronic conductivity by generating excessive electrons, thereby contributes to the rate capability.42 The mechanism of Li storage in N-rich carbon is believed to relate to the strong electronegativity of nitrogen and the hybridization of nitrogen lone pair electrons with the π electrons in carbon, which makes favorable binding sites for Li storage.27 Moreover, the existence of N atoms also creates defects in the carbon and generates more active sites for the Li storage. The C1s spectrum of the NGCS ranging from 282 to 292 eV (Figure 4d) can be fitted into four peaks centered at 284.9, 286.3 and 287.2 and 288.9 eV, which could be assigned to sp2 C=C/sp3 C-C (84.0 %), CH3CN/C-O (9.2 %), C=O (3.1 %) and N-C=N (3.5 %), respectively.43

Figure 4. (a) XRD patterns of Co-GCS; (b) Raman spectra of GCS and NGCS. XPS of NGCS: (c) survey; (d) C1s; (e) N1s.

Figure 5. Nitrogen adsorption/desorption isotherms and the corresponding NLDFT pore size distribution of (a) GCS and (b) NGCS.

Figure 5 shows the N2 adsorption–desorption isotherms and the corresponding pore size distribution curves for the graphitic carbon spheres, with the porous structure parameters summarized in Table S1. The isotherm is of type IV with a hysteresis loop, and a large N2 uptake was observed at low relative pressures, indicating the presence of micropores with pore width less than 2 nm. The GCS have a Brunauer–Emmett–Teller (BET) surface area of 328 m2 g-1 and the micropore volume is calculated to be 0.13 cm3 g-1 using the non-local density functional theory (NLDFT) method, accounting for 46 % of the total pore volume (0.28 cm3 g-1). As revealed by the NLDFT pore size distribution, this material possesses plenty of mesomacropores with diameters ranging from 20-60 nm. After nitrogen doping via thermal annealing with urea, the NGCS exhibit a much higher BET surface area of 512 m2 g-1 and a greatly increased total pore volume of 0.85 cm3 g-1 as a consequence of the reactions between ammonia gas and carbon at elevated temperature. Notably, the micropore volume of NGCS (0.18 cm3 g-1) is slightly higher than that of GCS, however, the pore ratio is decreased to a low level of 21 %. The nitrogen doping process gave rise to a dramatically enhanced meso-macropore volume and a broader pore size distribution from 2 to 66 nm. The above BET reports disclose that this kind of material possesses numerous micropores which may bring about numerous accommodation sites for Li-ion storage and thereby ensures the good specific capacity. The mesopores may have partial action on Li storage, and can also provide optimal



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transport pathways, together with the macropores, to improve the Li-ion transfer rate. Thus, the hierarchical porous carbon spheres are expected to have favorable electrochemical properties, especially at high current densities.

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structural defects and higher pore volume bring NGCS more surface area to expose in the electrolyte. All these lead to the formation of thicker SEI layer on NGCS compared to the un-doped ones.46 As reported before, large irreversible capacity loss is an unavoidable phenomenon

Figure 6. (a) Cyclic voltammograms of the GCS and NGCS at the first cycle. The voltage window and scanning rate are 0.0–3.0 V -1 -1 and 0.01 mV s , respectively; (b) Initial discharge/charge curves of the GCS and NGCS electrodes at a current rate of 50 mA g ; -1 (c) Rate performance of the GCS and NGCS electrodes at different current densities (A g ); (d) Cyclic performance of the GCS -1 and NGCS electrodes cycled at current densities of 50 mA g-1 and 30 A g , respectively; (e) Electrochemical impedance spectroscopy (EIS) of GCS and NGCS before charge-discharge test; (f) EIS of GCS and NGCS after 50 charge-discharge cycles at a current -1 density of 50 mA g .

The electrochemical performance of the α-CD derived graphitic carbon spheres was investigated as anode in LIBs. Figure 6b shows the initial charge / discharge curves of the graphitic carbon spheres with and without nitrogen-doping at a current density of 50 mA g-1. Compared with GCS which exhibit an initial charge capacity of 552 mA h g-1, NGCS show a much higher value of 803 mA h g-1 but with a slightly lower columbic efficiency of 49 %. During the first discharge process of NGCS, the voltage drops slowly with two obvious plateaus at around 1.7 and 1.0 V, however, only one plateau is observed around 0.8 V in that of GCS. This can be also verified with the cyclic voltammograms of the two carbon samples at the first scanning cycle (Figure 6a). Compared with GCS which exhibit only one broad lithium intercalation peak, NGCS display three apparent peaks starting from 1.7, 1.0 and 0.5 V in the first reduction process. The voltage plateaus could be attributed to the irreversible Li-insertion into superfine pores, and the rapid formation of solid electrolyte interface (SEI) film caused by the electrolyte decomposition.44,45 Due to nitrogen doping, NGCS have higher electrochemical activity, leading to reactions with lithium ions more easily and sufficiently. Additionally, more

in porous carbonaceous electrodes in LIBs. Besides the formation of SEI layers, the special positions such as the vicinity of residual H atoms and deep-seated micropores also may lead to irreversible lithium insertion. In addition, there are unstable structures around the micropores, the interaction between lithium and the defects may break the graphite crystallites and change the tiny pore structures irreversibly, therefore, the reversible capacity fades.10 Compared with GCS, the surface area of NGCS is much larger, resulting in more side/edge carbon atoms and structural defects accounting for the higher irreversible capacity and lower initial coulombic efficiency. Remarkably, the graphitic porous carbon spheres exhibit impressive high-rate performance and cyclic stability, which is among the best of the present pure carbon anode materials.7-10,27-35 As shown in Figure 6c, GCS exhibit a reversible charge capacity of 432 mA h g-1 at a low current rate of 50 mA g-1 after 10 cycles. The capacity decreases very slowly with increasing the charging currents step by step. Even when the current density is increased 600-fold from 50 mA g-1 to 30 A g-1, 57 % of the original capacity (245 mA h g-1) is still retained. More attractively, a much higher reversible capacity of 704 mA h g-1 was obtained


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with the nitrogen-doped graphitic carbon spheres at a current density of 50 mA g-1. The rate capacities of NGCS have been increased more than 50 % in comparison with those of GCS at each stage below 5 A g-1. At the highest current density of 30 A g-1, the material still maintains a capacity of ca. 290 mA h g-1. Furthermore, the charge capacities of both the two carbon materials can be almost recovered to the original value once the current density is tuned back to 50 mA g−1 after cycling at different rates, demonstrating the good reversibility of the carbon materials. The outstanding high-rate performance of the carbon spheres can be attributed to the graphitic layers for improving electrical conductivity as well as the quantities of interconnected mesopores which provide short pathways for the lithium ion diffusion and electron transportation. In addition, it is likely that the quaternary nitrogen (N-Q) formed in NGCS contributes to the excellent rate capability, as N-Q has been known to increase the electronic conductivities by generating excessive electrons.41 Figure 6d displays the cyclic stability of the carbon samples evaluated at current densities of 50 mA g−1 and 30 A g-1 up to 50 cycles. Although the charge capacities decrease rapidly during the first several cycles at low current densities, high coulombic efficiencies (> 99%) were observed with both the two carbon samples after 15 cycles. Both GCS and NGCS retained more than 80% of the initial capacitance after 50 cycles. Notably, when evaluated at high current densities, both the two materials exhibit excellent electrochemical stability, highlighting the great potential of the materials for high-power LIB applications. The electrochemical impedance spectroscopy (EIS) study of the graphitic carbon spheres with and without nitrogen doping was also conducted before and after charge/discharge test to better understand their electrochemical performance (Figures 6e-f). The Nyquist plots for both GCS and NGCS consist of two semicircles in the high-to-medium frequencies and a straight sloping line in the low-frequency region. The semicircle at high frequency can be assigned to the solid electrolyte interphase (SEI) film resistance (RSEI), while that at mid-frequency is attributed to the charge transfer resistance (Rct). The linear region is associated with ionic diffusion impedance through the bulk of the active material. An equivalent circuit (inset of Figure 6e) was used to analyze the measured impedance data and the results from fitting the model are shown in Table S2. Rs denotes the ohmic resistance and the values of Rs are very close for both samples because the same electrolyte and identical cell conﬁgurations were employed. Besides, it can be seen that, before testing, the RSEI and Rct of the GCS are 8.47 and 1.67 Ω, respectively, higher than those of the NGCS electrode (RSEI and Rct of the NGCS are 3.78 and 1.16 Ω, respectively), which means that both SEI resistance and charge-transfer resistance are reduced by doping with nitrogen. This could be ascribed to the increased specific surface area and higher proportion of mesopores in NGCS as well as the enhanced electrochemical reactivity and conductivity caused by N-doping. After cycling test, both the SEI impedance and charge transfer resistances of the

two samples increase a bit, (RSEI of GCS and NGCS increased to 26.39 and 13.66 Ω, Rct of GCS and NGCS increased to 6.88 and 4.23 Ω, respectively), but still maintain at a low level, demonstrating the easy transportation of electrons and ions within the two carbon samples, especially the NGCS sample, which leads to their outstanding high-rate performance. 4. CONCLUSION In conclusion, this research represents a biomimetic strategy for the preparation of hierarchical porous graphitic carbon nanospheres via hydrothermal treatment of readily available α-CD and F127 in the presence of cobalt gluconate followed by annealing. In this methodology, the addition of cobalt gluconate not only leads to the agglomeration of F127 micelles which act as templates to induce the precise threading of α-CD via hydrogenbonding interactions to construct the carbon structure, but also act as the precursor of Co catalyst for low temperature graphitization. The micelle stacking and thermal decomposition of F127 are responsible for the formation of interconnected mesoporous architecture. The obtained nanocarbons combine the characteristics required for high performance electrode materials in LIBs, such as high degree of graphitization, large surface area with hierarchically distributed pore sizes as well as doping with heteroatoms, which synergistically contribute to their impressive electrochemical properties in terms of high reversible specific capacities, good cyclic stabilities and especially outstanding high-rate performance. Moreover, the fabrication process is cost-effective and straightforward, further demonstrating the considerable potential of these hierarchical porous graphitic carbon spheres for applications in a variety of high-power energy storage devices.

ASSOCIATED CONTENT Supporting Information. The dynamic light scattering (DLS) and TEM analysis to demonstrate the function of cobalt gluconate in the formation of spherical clusters in the micellar solution before hydrothermal treatment, the parameters of the porous structure and the influence of different proportions of cobalt gluconate on the electrochemical performance of the resultant carbon spheres, the porous structure parameters of the graphitic carbon spheres, the fitted electrochemical impedance parameters of the carbon spheres. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Address correspondence to [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

REFERENCES (1) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359–367.



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(2) Bruce, P. G.; Scrosti, B.; Tarascon, J. M. Angew. Chem. Int. Ed. 2008, 47, 2930–2946. (3) Zhang, H.; Yu, X.; Braun, P. V. Nat. Nanotechnol. 2011, 6, 277– 281. (4) Lee, S. W.; Yabuuchi, N.; Gallant, B. M.; Chen, S.; Kim, B.-S.; Hammond, P. T.; Shao-Horn, Y. Nat. Nanotechnol. 2010, 5, 531– 537. (5) Kang, B.; Ceder, G. Nature 2009, 458, 190–193. (6) Lahiri, I.; Choi, W. Crit. Rev. Solid State Mater. Sci. 2013, 38, 128–166. (7) Hu, Y.-S.; Adelhelm, P.; Smarsly, B. M.; Hore, S.; Antonietti, M. Adv. Funct. Mater. 2007, 17, 1873–1878. (8) Adelhelm, P.; Hu, Y.-S.; Chuenchom, L.; Antonietti, M.; Smarsly, B. M.; Maier, J. Adv, Mater. 2007, 19, 4012–4017. (9) Fang, Y.; Lv, Y.; Che, R.; Wu, H.; Zhang, X.; Gu, D.; Zheng, G.; Zhao, D. J. Am. Chem. Soc. 2013, 135, 1524–1530. (10) Song, R.; Song, H.; Zhou, J.; Chen, X.; Wu, B.; Yang, H. Y. J. Mater. Chem. 2012, 22, 12369–12374. (11) He, C.; Wu, S.; Zhao, N.; Shi, C.; Liu, E.; Li, J. ACS Nano 2013, 7, 4459–4469. (12) Yang, S. J.; Nam, S.; Kim, T.; Im, J. H.; Jung, H.; Kang, J. H.; Wi, S.; Park, B.; Park, C. R. J. Am. Chem. Soc. 2013, 135, 7394– 7397. (13) Xia,Y.; Xiao, Z.; Dou, X.; Huang, H.; Lu, X.; Yan, R.; Gan, Y.; Zhu, W.; Tu, J.; Zhang, W.; Tao, X. ACS Nano 2013, 7, 7083–7092. (14) Wang, L.; Zhuo, L.; Zhang, C.; Zhao, F. Chem. Eur. J. 2014, 20, 4308–4315. (15) Xiao, Y.; Cao, M.; Ren, L.; Hu, C. Nanoscale 2012, 4, 7469– 7472. (16) Liang, C.; Dudney, N. J.; Howe, J. Y. Chem. Mater. 2009, 21, 4724–4730. (17) Vu, A.; Li, X.; Phillips, J.; Han, A.; Smyrl, W. H.; Buhlmann, P.; Stein, A. Chem. Mater. 2013, 25, 4137–4148. (18) Xu, J. J.; Wang, Z.-L.; Xu, D.; Zhang, L.-L.; Zhang, X. B. Nat. Commun. 2013, 4, 2438. (19) Zhao, M.-Q.; Zhang, Q.; Huang, J.-Q.; Tian, G.-L.; Nie, J.-Q.; Peng, H.-J.; Wei, F. Nat. Commun. 2014, 5, 3410. (20) Mai, L.-Q.; Minhas-Khan, A.; Tian, X.; Hercule, K. M.; Zhao, Y.-L.; Lin, X.; Xu, X. Nat. Commun. 2013, 4. 2923. (21) Sumper, M.; Brunner, E. Adv. Funct. Mater. 2006, 16, 17–26. (22) Bauerlein, E. Angew. Chem. Int. Ed. 2003, 42, 614–641. (23) Yang, W.; Lopez, P. J.; Rosengarten, G. Analyst 2011, 136, 42– 53.

Page 8 of 9

(26) Liu, L.; Yang, L.-Q.; Liang, H.-W.; Cong, H.-P.; Jiang J.; Yu, S.-H. ACS Nano 2013, 7, 1368–1378. (27) Li, Z.; Park, J. H.; Cho, W. K.; Lee, H.-S.; Choi, I. S. Energy Environ. Sci. 2013, 6, 871–878. (28) Chen, L.; Wang, Z.; He, C.; Zhao, N.; Shi, C.; Liu, E.; Li, J. ACS Appl. Mater. Interfaces 2013, 5, 9537–9545. (29) Hu, C.; Xiao, Y.; Zhao, Y.; Chen, N.; Zhang, Z.; Cao, M.; Qu, L. Nanoscale 2013, 5, 2726–2733. (30) Qie, L.; Chen, W.-M.; Wang, Z.-H.; Shao, Q.-G.; Li, X.; Yuan, L.-X.; Hu, X.-L.; Zhang, W.-X.; Huang Y.-H. Adv. Mater. 2012, 24, 2047–2050. (31) Yang, Y.; Pang, R.; Zhou, X.; Zhang, Y.; Wu, H.; Guo S. J. Mater. Chem. 2012, 22, 23194–23200. (32) Zhou, G.; Wang, D.-W.; Shan, X.; Li, N.; Li, F.; Cheng, H.-M. J. Mater. Chem. 2012, 22, 11252–11256. (33) Yan, Y.; Yin, Y.-X.; Xin, S.; Guo, Y.-G.; Wan, L.-J. Chem. Commun. 2012, 48, 10663–10665. (34) Tang, K.; White, R. J.; Mu, X.; Titirici, M.-M.; van Aken, P. A.; Maier, J. ChemSusChem 2012, 5, 400–403. (35) Wang, K.; Li, Z.; Wang, Y.; Liu, H.; Chen, J.; Holmes, J.; Zhou, H. J. Mater. Chem. 2010, 20, 9748–9753. (36) Chen, G.; Jiang, M. Chem. Soc. Rev. 2011, 40, 2254–2266. (37) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325–327. (38) Li, J.; Ni, X.; Leong, K. Angew. Chem. Int. Ed. 2003, 42, 69– 72. (39) Wei, D.; Liu, Y.; Wang, Y.; Zhang, H.; Huang, L.; Yu, G. Nano Lett. 2009, 9, 1752–1758. (40) Chen, X. Y.; Xie, D. H.; Chen, C.; Liu, J. W. J. Colloid Interface Sci. 2013, 393, 241–248. (41) Su, F.; Poh, C. K.; Chen, J. S.; Xu, G.; Wang, D.; Li, Q.; Lin, J.; Lou, X. W. Energy Environ. Sci. 2011, 4, 717–724. (42) Jeong, H. M.; Lee, J. W.; Shin, W. H.; Choi, Y. J.; Shin, H. J.; Kang, J. K.; Choi, J. W. Nano Lett. 2011, 11, 2472–2477. (43) Pevida, C.; Drage, T. C.; Snape, C. E. Carbon 2008, 46, 1464– 1474. (44) Yao, J.; Wang, G. X.; Ahn, J.; Liu, H. K.; Dou, S. X. J. Power Sources 2003, 114, 292–297. (45) Guo, P.; Song, H.; Chen, X. Electrochem. Commun. 2009, 11, 1320–1324. (46) Du, M.; Sun, J.; Chang, J.; Yang, F.; Shi, L.; Gao, L. RSC Adv. 2014, 4, 42412–42417.

(24) Yu, S.-H. Top Curr. Chem. 2007, 271, 79–118. (25) Yi, J.; Jang, H. S.; Lee, J. S.; Park, W. I. Nano Lett. 2012, 12, 3743–3748.


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TOC Figure


9

Bioinspired Synthesis of Hierarchical Porous Graphitic Carbon

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