Three-Dimensional Network of N-Doped Carbon Ultrathin Nanosheets

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3D Network of N-Doped Carbon Ultrathin Nanosheets with Closely Packed Mesopores: Controllable Synthesis and Application in Electrochemical Energy Storage Shan Zhu, Jiajun Li, Liying Ma, Lichao Guo, Qunying Li, Chunnian He, Enzuo Liu, Fang He, Chunsheng Shi, and Naiqin Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02386 • Publication Date (Web): 19 Apr 2016 Downloaded from http://pubs.acs.org on April 20, 2016

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ACS Applied Materials & Interfaces

3D Network of N-Doped Carbon Ultrathin Nanosheets with Closely Packed Mesopores: Controllable Synthesis and Application in Electrochemical Energy Storage

Shan Zhua, Jiajun Lia, Liying Maa, Lichao Guoa, Qunying Lia, Chunnian Hea,b,c, Enzuo Liua,b, Fang Hea, Chunsheng Shia, and Naiqin Zhao* a, b,c

a

School of Materials Science and Engineering and Tianjin Key Laboratory of

Composites and Functional Materials, Tianjin University, Tianjin 300350, China b

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin

300350, China c

Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of

Education, Tianjin, 300350, China

*

Corresponding author: Email: [email protected] (N. Zhao) Tax: 86 02227404724 Tel: 86 22 27406693

1

ACS Paragon Plus Environment


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Abstract A flexible one-pot strategy of fabricating 3D network of nitrogen-doped (N-doped) carbon ultrathin nanosheets with closely packed mesopores (N-MCN) via in situ template method is reported in this research. The self-assembly soluble salts (NaCl and Na2SiO3) serve as hierarchical templates and support the formation of 3D glucose-urea complex. The organic complex is heat-treated to obtain 3D N-doped carbon network constructed by mesoporous nanosheets. Especially, both of the mesoporous structure and doping content can be easily tuned by adjusting the ratio of raw materials. The large specific surface area and closely packed mesopores facilitate the lithium ion intercalation/deintercalation accordingly. Besides, the nitrogen-content improves the lithium storage ability and capacitive properties. Due to the synergistic effect of hierarchical structure and heteroatoms composition, the 3D N-MCN shows excellent performances being as the electrode of lithium ion battery and supercapacitor, such as ultrahigh reversible storage capacity (1222 mAh g-1 at 0.1 A g-1), stable long cycle performance at high current density (600 cycles at 2 A g-1) and high capacitive properties (225 F g-1 at 1 A g-1 and 163 F g-1 at 50 A g-1).

Keywords: 3D Network; Nitrogen-Doping; Mesoporous Carbon Nanosheets; In-Situ Salt Templates; Lithium Ion Battery; Supercapacitor

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1. Introduction Both of lithium ion batteries (LIBs) and supercapacitors play crucial roles in meeting the ever-increasing energy storage demand.1-8 For LIB electrodes, the pivotal challenge is to improve the lithium storage capacity because the upper limit capacitance of commercial graphite anode is confined to 372 mAh g-1. Another critical issue is the transport kinetics which has a huge influence on the power density of devices.9-11 In view of the above mentioned facts, a high surface area and abundant channels for the electrolyte/electrode accessible contact are required for the anode materials in addition to a high transfer efficiency of ion.12-14 Furthermore, the long life performance is also an urgent problem since the volume expansion caused by lithium ion intercalation introduces the instability after cycles of processing.15,16 As for supercapacitors, a high surface area is essential to achieve high capacitance since the carbon-based electrodes store charge in electrochemical double layers. Moreover, a well-developed pore system is favorable for the sufficient contact between electrolyte and electrode, thus improving a high rate capacitance as a result.17-20 Owing to high specific surface area (SSA), abundant pores, electric conductivity and light weight, mesoporous carbon materials are of great interest in various fields especially for LIBs and supercapacitors.21-26 However, conventional mesoporous carbon materials suffer from the inherent drawbacks caused by the bulk structure of hexagonal or cubic shape: the core is inaccessible to transfer mass efficiently and the solid-electrolyte interface (SEI) layers formed during cycles of processing have a tendency to block the internal channels, which consequently brings about the limited 3



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applications.23,27,28 Therefore, researchers endeavor to reduce the thickness of mesoporous carbon matrix from bulk to ultrathin nanostructures, such as 2D mesoporous carbon nanosheets and etched graphene.29,30 However, due to the Van der Waals interaction these ultrathin layers are prone to restack and the rate performance of LIBs and supercapacitors are significantly deteriorated as a result. According to previous reports, constructing 2D sheets into 3D network will not only enhance the ion transportation but also tackle the aggregation problem.30,31 Thus, 3D network of mesoporous nanosheets would be an optimized structure for electrode materials of LIBs and supercapacitors. Nevertheless, there are few reports mentioned such novel structures which leaves a great research capability for further development. Besides of the optimized structures, the suitable components also play a significant role in carbonaceous materials for electrochemical energy storage applications. A high-level of nitrogen-doping efficiency is reasonable to achieve a high lithium storage because the doped heteroatoms are capable of increasing the active sites.32,33 Meanwhile, the nitrogen-content functional groups greatly increase the capacitive properties of carbon-based materials by introducing additional pseudocapacitance and quantum capacitance.34,35 However, the traditional strategies of doping nitrogen atoms often involve some rigorous experimental conditions, such as arc, plasma or high temperature ( >900 ℃). And the reagents are not easy to control and sometimes they are even dangerous such as polypyrroleor ammonia gas.36-38 Also,

some

researches

fabricate

doped

carbon

by

pyrolysizing

nitrogen-content materials or biomass, like protein or silk.39,40 However, these 4


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methods have difficulty in tuning the nitrogen content. Therefore, a controllable and facile method utilizing cheap and green raw materials becomes essential to produce N-doped carbons for advanced LIB anode and supercapacitor electrode. In this paper, we develop a one-pot strategy to fabricate 3D network constructed by N-doped carbon ultrathin nanosheets with closely packed mesopores (3D N-MCN) as a superior LIB anode and supercapacitor electrode. This method applies water-soluble salts as in situ templates, which is based on our previous work.41 A combination of glucose (C6H12O6), urea (CH4N2O), sodium chloride (NaCl) and sodium silicate (Na2SiO3) are dissolved, freeze-dried and heated under the condition of Ar. During the process, the salts crystallize and provide hierarchical templates for C6H12O6 and CH4N2O. And the organic complex converts into a nitrogen-doped carbon matrix. After removing the templates by water-washing, the obtained 3D N-MCN presents an interconnected cubic-like macropores network, in which closely packed mesopores are distributed vertically on the ultrathin nitrogen-doped carbon nanosheets. Especially, not only the pore size of mesopores but also the N-doping content can be tuned in a wide range by adjusting the ratio of various raw materials. As LIB anode, this 3D N-doped carbon network exhibits an ultrahigh lithium capacity (1222 mAh g-1 at 0.1 A g-1), a long cycling stability at high rates (a high capacity of 604 mAh g-1 is obtained at 2 A g-1 after 600 cycles) and excellent rate performance. Moreover, applied as the supercapacitor electrode 3D N-MCN presents superior capacitive property (225 F g-1 at 1 A g-1) and high capacitance retention (163 F g-1 at 50 A g-1). Furthermore, this one-step method based on green and cheap raw materials 5



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is suitable for scalable industrial production.

2. Experimental 2.1 Synthesis N-MCN To prepare N-MCN, glucose (1.25 g), urea (1.25 g), sodium silicate (0.3 g) and sodium chloride (20 g) were dissolved in 75 ml of deionized water. The resulting mixed solution was freeze-dried at -50 oC in vacuum and then ground by agate mortar to obtain very fine composite powder (~100 mesh). After that, the composite powders were heated at 700 oC for 2 h in a tube furnace under flowing Ar atmosphere (200 ml min-1). Once cooled to room temperature, the obtained powder was treated with deionized water to dissolve salts, and then pure N-MCN was dried at 80 oC overnight. Following the same procedure, a series of samples were prepared to reveal the tenability of structure and content of N-MCN. The detailed information of which is listed in Table S1. For comparison, three different samples were also prepared to demonstrate the influence of mesostructure and doping content including 3D N-doped carbon nanosheets without mesopores, undoped mesoporous carbon nanosheets (MCN) and carbon nanosheets (CN). 2.2 Characterization Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) were performed on FEI Tecnai G2 F20 TEM. Scanning Electronic Microscopy (SEM) images were tested by Hitachi S4800. X-ray photoelectron spectroscopy (XPS) was carried out on PHI5000VersaProbe. Raman spectra were recorded on the Lab 6


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RAMHR Raman spectrometer using laser excitation at 514.5 nm from an argon ion laser source. X-ray diffraction (XRD) measurements were taken on a Rigaku D/max diffractometer with Cu Kα raidiation. Brunauer-Emmett-Teller (BET) surface areas and porosities of the products were determined by nitrogen adsorption and desorption using a Micromeritics ASAP 2020 analyzer. 2.3 Electrochemical Measurements For the battery test, the slurry of 80 % N-MCN, 10 % carbon black and 10 % PVDF in N-methylpyrrolidone was coated and dried on copper foil. The mass loading of active materials was about 0.54 mg cm-2. The supercapacitor electrodes were made by means of the following steps: 80 % active materials (N-MCN and CN), 10 % conductivity agent (carbon black) and 10 % binder (polytetrafluoroethylene, PTFE) were blended with ethanol as solvent. Electrode film prepared by coating the mixture on a nickel foam was first vacuum-dried at 80 ℃ overnight and then pressured at 8 MPa for 3 min. The loading mass of the active materials on each current collector was 1.0-2.0 mg with the area of 0.81 cm2. Coin cells (CR2032) were fabricated utilizing lithium metal as the counter electrode, Celgard 2400 as the separator and LiPF6 (1M) in

ethylene

carbonate/dimethyl

carbonate/diethyl

carbonate

(EC/DMC/DEC,

1:1:1vol %) as the electrolyte. The cell assembly was conducted in an Ar-filled glovebox. Cyclic voltammetry (CV) measurement was conducted at 0.1 mV s-1 within the range of 0.005-3.0 V on a CHI660D electrochemical workstation. The cycle life and rate capability of the cells were tested within a fixed voltage window of 0.005-3.00V (vs Li+/Li) by using a battery testing system (LAND CT 2001A, China). 7



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The capacitive properties were tested in three-electrode configuration in 6 M KOH. The counter electrode is Pt plate, and the reference electrode is Hg/HgO electrode. All the electrochemical experiments were carried out using a CHI660D electrochemical workstation.

3. Results and Discussion 3.1 Structural Characterizations of the 3D N-MCN The overall synthetic strategy is illustrated in Figure 1. At the beginning, the mixture of carbon source (C6H12O6), the nitrogen source (CH4N2O) and the dual templates (Na2SiO3 and NaCl) were dissolved and freeze-dried to enable the self-assembly process. In the obtained complex, the NaCl crystal with uniform particles (1~2 µm) provided platforms for assembling Na2SiO3 and organics (C6H12O6-CH4N2O). Consequently, C6H12O6 carbonized into a carbon framework after pyrolysized at 700 oC, as well as CH4N2O decamped as nitrogenous group and doped nitrogen atoms into carbon. Then, the stable salt templates were removed by treating with deionized water, and the 3D N-MCN was obtained by the filtration and drying process.

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Figure 1. Schematic illustration of the one-step synthesis process of 3D N-MCN by using NaCl and Na2SiO3 as in situ templates. (a) Raw materials are dissolved together and freeze-dried in vacuum. (b) Glucose-urea-Na2SiO3 was assembled at each face of NaCl crystal. Then, the obtained sample is pyrolysised under Ar. (c) Removing the templates to get 3D N-MCN, which possess closely packed mesopores that facilitate the mass transportation. Before template-removing process, the morphology of N-MCN presents a cubic-like shape with 1 µm on one side due to the characteristics of NaCl crystal formed in the freeze-drying process, seen in Figure 2a.29,40 If the dying is carried out at 80 oC, the high thermodynamic energy enables NaCl to grow into huge particles, which accelerates the carbon matrix to 2D micro-scale sheet (Figure S1). When we zoom on the surface of coated NaCl cube (Figure 2b), a large number of smaller particles (Na2SiO3) embedded in the organic compounds can be observed, corresponding to the Figure 1b. After removing templates by washing, the remnant is the 3D interpenetrating network structure with the macropores (~1 µm in diameter) (Figure 2c), which is the inverse replication of NaCl crystals assembly. Without the help of NaCl, the final product is carbon bulk (Figure S2). Comparing to this carbon bulk, the 3D network-type morphology of N-MCN can greatly improve the specific surface area and facilitate the transportation of both electrons and ions.30,31,41 In the 9



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carbon nanosheets (as the walls of macropores), there exist closely packed mesopores (5~7 nm) as shown in the SEM (Figure 2d) and TEM images (Figure 2e, f), which is attributed to the uniform arrangement of salt particles. In HRTEM image, it is evident that the thickness of carbon nanosheets is less than 5 nm (Figure S3) and the pores can penetrate the whole layer of the matrix (Figure 2d, f). According to Figure S4, MCN displays very similar morphology to N-MCN, yet N-CN and CN only possess macro-structure without mesopores (Figure S5). The divergences between two types of samples not only highlight the mesopore-forming role of Na2SiO3, but also reflects the self-assembly process of in situ salt templates.

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Figure 2. (a) SEM of coated templates. (b) The Na2SiO3 particles embedded in the glucose-urea complex and coated at the surface of NaCl. (c,d) SEM, (d,e,f) TEM images of N-MCN. Scale bars: (a) 2 µm, (b,d) 200 nm, (c) 1 µm, (e) 200 nm, (f) 50 nm. Furthermore, Raman spectrum (Figure 3a) obtained for N-MCN presents three peaks at about 1361 cm-1 (D-band), 1595 cm-1 (G-band), and 2863 cm-1 (2D-band). Compared to the value of CN (1356, 1600, 2860 cm-1), the slight shift of peaks is caused by the doping of N atoms. The crystallization degree of carbon can be evaluated by the intensity ratio of D peak to G peak (ID/IG). The ID/IG of CN is calculated to be 0.85, implying that the obtained nanosheets mainly comprise the 11



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partially graphitic carbon.42 By comparison, N-MCN exhibits a larger ID/IG of 0.92, indicating more disorder and defects in the structure, due to the successful introduction of N atoms. Furthermore, the 2D peaks in both of N-MCN and CN are sensitive to the thickness of carbon layers, reflecting the ultrathin characteristic of the nanosheets. In the powder XRD results, only two broad diffraction peaks at around 26° and 42° are observed in N-MCN, representing the (002) and (100) reflections of graphite, respectively (Figure 3b),42 which demonstrate the complete elimination of NaCl and Na2SiO3. Especially, the angle of N-MCN corresponding to (002) is 25.5°, and CN is 25.3°. According to the Bragg equation, a lower angle represents an expended crystal plane for the N-MCN, suggesting that the nitrogen atoms can enlarge the interplanar spacing of carbon. Moreover, the elemental mapping images (carbon, nitrogen and oxygen) clearly reveal the homogeneous heteroatom-doping in carbon matrix (Figure 3c).

Figure 3. (a) Raman spectra of N-MCN and CN. (b) XRD spectra of N-MCN and CN and (c) STEM image of N-MCN and its corresponding element mapping results of C, 12


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N, O. To figure out the influence of the templates on the structure, both of N2 adsorption-desorption isotherms (Figure 4) and TEM (Figure S6) were conducted. Given the amount of NaCl, the observed mesopores size in N-MCN rise from 0, 5, 8 to 27 nm by elevating the weight ratio of Na2SiO3 from 0 %, 2 %, 4 % to 6 % (Figure 4), and the corresponding sample are labeled as N-CN, N-MCN, N-MCN-8 and N-MCN-27. Firstly, the N2 adsorption-desorption isotherm curves of N-CN is typeⅡ, corresponding to few mesopores in pore size distribution (PSD) result (Figure 4a, b). However, the curve of N-MCN represents typical Ⅳ, which has a hysteresis loop at a relative pressure in the range of 0.40-0.97 and suggests a narrow mesopore size distribution (Figure 4b). The addition of Na2SiO3 not only results in the formation of abundant mesopores, but also raises the SSA (based on the Brunauer-Emmett-Teller method) to 900 m2 g-1 (almost two-fold of N-CN with 465 m2 g-1). Elevating the ratio of Na2SiO3 from 2 % to 4 %, the mesopore size ascents from 5 to 8 nm, yet the SSA drops down to 692 m2 g-1. This phenomenon is caused by that smaller mesopores converge into bigger ones and reduce the SSA (Figure 4c). From the PSD results (Table S2), the co-occurrence of micropores volume decreasing and mesopores increasing is evident, further proving that the formation of mesopores is caused by the consumption of micropores.40 Comparing to N-MCN (Figure 2 and Figure S6), the nanosheets thickness of N-MCN-27 is increased because the accumulating Na2SiO3 expands the space between NaCl particles which leads to more organic complex coming into the gap. Moreover, the network keeps its stability even when the 13



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mesopores increase to 27 nm, clearly demonstrating the mechanical stability of this 3D framework (Figure S6). However, if given the Na2SiO3 greater weight (the weight ratio is above 10 %), the formed nanosheets become too thick to be supported for constructing 3D network, resulting in the architecture collapse into stacked 2D flat mesopores thick sheets (Figure S7). Notably, the existed peaks below 2 nm in all PSD results stem from the carbonization of glucose.43

Figure 4. (a)Adsorption-desorption isotherms and (b) pore size distributions of a series of N-MCN based on different mass ratio of Na2SiO3 and NaCl from 0 %, 2 % 14


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to 4 % and 6 %. (c) The relation among specific surface area, total pores volume and micropores volume. Moreover, the nitrogen doping level can be adjusted in a great extent by controlling the ratio between carbon and nitrogen source (Figure 5). Given the amount of glucose, the weight ratio of urea rise from 0 %, 40 %, 100 % to 160 % (Figure 5a), and the corresponding samples are labeled as MCN, N-MCN-L, N-MCN and N-MCN-H (-L and -H represent low and high nitrogen content, respectively). Obviously, the adding of urea can introduce nitrogen atoms into the carbon matrix from the evidence of 6.9 at% nitrogen of N-MCN. Moreover, the peak N 1s can be separated into three peaks (Figure 5b): N-6 (398.2 eV), N-5 (399.7 eV) and N-Q (400.8eV).39,44 The high content and various situations of the nitrogen atoms can create a significant amount of defects serving as additional active sites for lithium storage.45 In general, the ratios of three N-doping types remain at 1:1:2 for the various amount of nitrogen sources (Figure S8). Furthermore, the amount of oxygen-containing functional groups (C-O and C=O) is increased by the additional oxygen-content produced by the urea decomposition (Figure 5c). Because of the pseudocapacitive effect, the nitrogen- and oxygen-containing functional groups can also contribute to the capacitive properties.45-47

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Figure 5. (a) XPS spectra of MCN and N-MCN with different N-doping levels. (b) N 1s peak of N-MCN. (c) The relation among N-doping and O-doping levels with the ratio of urea and glucose. 3.2 Electrochemical properties of the 3D N-MCN The performance of N-MCN as the LIB anode is investigated in a half-cell configuration countered with metallic lithium in 1 M LiPF6. Firstly, the CV experiments were carried out to study the lithium-ion insertion/extraction reactions (Figure 6a). Three peaks appear at 0.63, 1.30 and 1.44 V in the 1st cycle, and disappear at the 2nd cycle, which is related to the irreversible consumption of charge 16


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via forming the SEI layer.30,32,46 In the cathodic scan, the peak near to 0 V is ascribed to the Li+ intercalation into N-MCN. Meanwhile, for the anodic scan, the peak at 0.2 V represents Li+ de-intercalation process from the graphitic layers; another peak at 1.05 V attributes to the delithiation from the active sites of mesoporous structure.27,28,30 In the charge/discharge voltage profiles (Figure S8) all voltage plateaus are in good agreement with that of CV curves (Figure 6a). Furthermore, the overlapping of CV curves indicates the nearly same electrochemical kinetics of N-MCN during the subsequent cycles. Besides, the CV data at various sweep rates (Figure S10) show that the total stored charge comprises three components: the faradaic process from the lithium ion insertion, the faradaic contribution derived from the surface atoms charge-transfer process (pseudocapacitance), and the nonfaradaic contribution from the double-layer effect. Especially, the capacitive component is dominant at the higher potential; the current comes primarily from the lithium ion insertion at the low potential (Figure S10). Figure 6b exhibits the galvanostatic cycle behaviors of 3D N-MCN and CN at the current density of 0.2 A g-1, together with the Coulombic efficiency of N-MCN. The N-MCN achieves a charge specific capacity of 1240 mAh g-1 at the 1st cycle. Due to the inherent vulnerability of heteroatom-doped porous carbon anodes, the initial Coulombic efficiency is unremarkable.27,28,42 However, this value increases rapidly and retains above 97% after six cycles. This fast stabilization demonstrates a facile lithium ion insertion and extraction process, which derives from the high efficient electron and ion transport in 3D N-MCN.9,27,29 Moreover, the N-MCN exhibits a 17



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discharge and charge capacities of 1026 and 1037 mAh g-1 at the 200th cycle. Such a high lithium storage ability is also approved by the galvanostatic testing at 0.1 A g-1 (Figure S11). In contrast, the CN electrode only has 413 and 414 mAh g-1. However, the N-MCN presents a certain degree of fluctuation in the process, which derives from two aspects: (1) the abundant vacancies and edges in the carbon matrix may increase the possibility of lithium-cluster formation and have negative effect on the stability of the anode;48 (2) the introduction of nitrogen atom disturbs the SEI layer and induces the fluctuation.47

Figure 6. (a) Cyclic voltammogram curves of N-MCN at 0.1 mV s-1. (b) Cycle performance and Coulombic efficiency of the N-MCN electrode at a current rate of 0.2 A g-1. (c) Rate capabilities and cycle performance of N-MCN, N-MCN-27 and CN cycled at different current rates. (d) Cycle performance and Coulombic efficiency of the N-MCN electrode at a current rate of 2 A g-1.

The durable and stable rate capacity of the N-MCN electrodes is observed at 18


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different charge/discharge rates from 0.1 to 10 A g-1 (Figure 6c). A high initial reversible capacity of 1406 mAh g-1 is obtained at 0.1 A g-1, being consistent with the galvanostatic tests. In the first rate cycle, the average reversible capacities are 1071, 852, 731, 633, 543 and 419 mAh g-1 for 0.1, 0.2, 0.5, 1, 2, and 5 A g-1, respectively. Even at a higher current rate of 10 A g-1, the reversible capacities are around 344 mAh g-1 and still close to the theoretical capacity of graphite. Remarkably, when the current rate restored from 10 A g-1 to 0.1 A g-1 after two rate cycles, a capacity of 1222 mAh g-1 is recoverable and sustainable up to the 310th cycle. According to our best knowledge, the carbon-based anodes enduring such a high capacity at high rate is rarely reported previously (Table S3). These results demonstrate that the N-MCN exhibits an expedited charge-transport process and the 3D structure of carbon network maintain extraordinarily stable behavior even under a high rate cycle.27,28 As a comparison, the performance of N-MCN-27 was also investigated to explore the influence of mesopores with various dimensions. At 10 A g-1, only 158 mAh g-1 is maintain for the N-MCN-27 (a half of N-MCN). In addition, the specific capacitance of N-MCN-27 is only retained around 35% when the current density rolls back to 0.1 A g-1. Since the nitrogen content of N-MCN-27 (6.1 at. %) (Figure S12) is similar to that of N-MCN (6.9 at. %), the decreasing of lithium storage ability is mainly caused by the structural difference. In N-MCN-27, the structure with larger pores is prone to be

irreversibly

collapsed

and

lose

its

stability

in

the

lithium

ion

intercalation/de-intercalation process. The cycling performance of 3D N-MCN at a high rate of 2 A g-1, was measured 19



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for 600 cycles (Figure 6d). The first charge capacity is 864 mAh g-1. After 600 cycles, the reversible capacity is 604 mAh g-1, still remarkably larger than the theoretical capacity of graphite. For compassion, the reversible capacity of CN electrode declines to ~200 mAh g-1 after 200 cycles (Figure S13), and a rapid fade of N-MCN-27 is observed in the long cycle test at 2 A g-1 (Figure S14). Moreover, it can be seen from the EIS results in Figure S15 that there is a semicircle across the medium-frequency region which represents the charge-transfer impedance (Rct) between the electrode and the electrolyte. Obviously, the doped samples exhibit lower Rct and N-MCN possesses the lowest resistance for contact and charge transfer, which implies the superior rate performance of 3D hierarchical architecture combining with heteroatoms introduction. The N-MCN enjoys the lowest resistance for contact and charge transfer. It can be confirmed that the 3D N-MCN has superior reversible lithium storage capacity and outstanding long-life cycling stability at high rates, which can be contributed to the following advantages. Firstly, both of the strongly connected 3D network and open ion channels are profitable for the process controlled by mass transportations. Secondly, the abundant hierarchical mesopores well-distributed on the ultrathin networks enable lithium ions electrochemically adsorbed on and intercalated into both sides of the carbon nanosheets, which gives rise to a high lithium storage as a result. The mesopore frame provides an excellent volume flexibility during the ions intercalation as well. Thirdly, by comparison of the doped and undoped samples in Figure S15 and Figure S16, it should be emphasized that N-doping plays a significant role which can be described in two aspects: (1) N-doping improves the conductivity 20


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supported by the EIS experiments; (2) leads to the increase of d-spacing between graphitic layers and facilitate the penetration of lithium ion. Therefore, we believe that the synergetic effects of the novel unique structure and the profitable heteroatom-doping mechanism remarkably enhance lithium storage capability and cycling stability of N-MCN.

Figure 7. (a) CV curves of N-MCN and CN at 5 and 50 mV s-1; (b) galvanostatic charge/discharge profiles of N-MCN at various current densities; (c) specific capacitance of N-MCN and CN at various current densities; (d) EIS results of N-MCN and CN.

Meanwhile, it has been investigated that the features consisting of high SSA, well-developed pores and N-doping endow N-MCN with great potential for the superior capacitive performances in a three-electrode system using 6 M KOH as the electrolyte. The CV curves of N-MCN and CN (Figure 7a) obtained at the scan rates of 5 and 50 mV s-1 demonstrate typical capacitive behaviors with the rectangular-like 21



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shapes.37,39,40 The important role of nitrogen-doping is further emphasized by plotting the CV curves of N-MCN and MCN together (Figure S17a, b). Apparently the areas under the CV curves are larger in N-MCN which will indicate a higher electrochemical capacitive property. Moreover, the hump at about -0.6 V in the CV curves of N-MCN caused by the introduced nitrogen atoms represents the Faradic reactions at different sites, such as the doped nitrogen or the carbon adjacent to the nitrogen atom.33,34 Furthermore, the good capacitance (123 F g-1) of MCN at a current density of 50 A g-1 certifies that the mesopores will be able to enhance the mass transportation and result in a good rate stability (Figure S17c, d). From Figure S18, it can be seen that the CV curve of N-MCN maintains a rectangular shape even at a high scan rate of 1000 mV s-1 reflecting the good electrical conductivity. In addition, the galvanostatic charge/discharge cycling experiments were performed which can be seen from Figure 7b. On the basis of the discharging curve, the specific capacitance of N-MCN is calculated to be 225 F g-1 at 1 A g-1 (Figure 7c), which is two-fold of CN (121 F g-1). More importantly, its capacitance remains at 163 F g-1 even at a high rate of 50 A g-1. As shown in Figure 7d, the EIS measurements on N-MCN and CN exhibit the medium-to-high frequency semicircle and low-frequency linear tail. The intercept of the real part (Z’) of N-MCN is smaller than those of CN, which is the reflection of a smaller value of the substrate intrinsic resistance, electrolyte resistance and interface contact resistance. Meanwhile, the nearly vertical line in the low frequency region shows that the N-MCN electrode has an excellent ion diffusion behavior. Such capacitive performances of N-MCN can be attributed to the combined effect of two 22


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main factors: (1) the high heteroatom doping level introduces the pseudocapacitance and improves the electron conductivity into the carbon matrix;33,34 (2) the abundant mesopores and the unique 3D structure allow for an effective ion migration, thereby generating the reversible capacitive behavior even at high rates.

4

Conclusions In conclusion, we reported an in situ template method to produce 3D network of

N-doped carbon ultrathin nanosheets with closely packed mesopores for high performance lithium ion battery and supercapacitor. In this preferential structure, the ultrathin nanosheets provide the high surface area for both of lithium ion adsorption and charges storage; the obtained 3D network prevents the carbon layers from restacking issue. More importantly, the dense-distributed mesopores not only facilitate the mass transportation for improving the rate performance, but also endow the matrix with volume flexibility. Meanwhile, the controllable nitrogen-doping achieved by one-pot pyrolysis process enhances the electrochemical properties remarkably, which will improve the conductivity, add active site for lithium ion storage and introduce additional specific capacitance. Consequently, the unique N-doped carbon nanostructure exhibits a superior electrochemical energy storage performance: (1) for LIBs, ultrahigh storage capacity (1222 mAh g-1) and great long cycle performance at high rate (600 cycles at 2 A g-1); (2) for supercapacitors, high specific capacitance (225 F g-1 at 1 A g-1) and remarkable rate performance (163 F g-1 at 50 A g-1). Besides, it is believed that this simple strategy based on salts templates is applicable for 23



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scalable production and also can be extended to other elements (such as boron) doping carbon materials with immense potential for various applications.

Supporting Information Detailed preparation of samples, morphology characterizations for the samples (SEM and TEM), physical and chemical characteristics (the PSD results and the XPS results), additional electrochemical properties such as rate capability and cyclability.

Acknowledgements This work was supported by the National Natural Science Foundation of China No. 51472177, No. 51422104, the State Key Program of National Natural Science of China No. 51531004 and China-EU Science and Technology Cooperation Project SQ2013ZOA100006.

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Formation of Small Li Clusters on Graphene for the Anode of Lithium-Ion 30


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Batteries. ACS Appl. Mater. Interfaces 2013, 5 (16), 7793-7797.

31



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Graphical Abstract 35x15mm (300 x 300 DPI)


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Three-Dimensional Network of N-Doped Carbon Ultrathin Nanosheets

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