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Binary Hierarchical Porous Graphene/Pyrolytic Carbon Nanocomposite Matrix Loaded with Sulfur as High Performance Li-S Battery Cathode Hang Zhang, Qiuming Gao, Weiwei Qian, Hong Xiao, Zeyu Li, Li Ma, and Xuehui Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03806 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Binary Hierarchical Porous Graphene/Pyrolytic Carbon Nanocomposite Matrix Loaded with Sulfur as High Performance Li-S Battery Cathode Hang Zhang, Qiuming Gao*, Weiwei Qian, Hong Xiao, Zeyu Li, Li Ma and Xuehui Tian Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, Beijing Advanced Innovation Center for Biomedical Engineering, School of Chemistry, Beihang University, Beijing 100191, P. R. China. KEYWORDS: hierarchical porous nanocomposite; graphene; pyrolytic carbon; cathode; Li-S battery.

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ABSTRACT: N, O-codoped hierarchical porous nanocomposite consisted of binary reduced graphene oxide and pyrolytic carbon from chitosan (rGO/PC) is fabricated. The optimized rGO/PC possesses micropores with size distribution concentrated around 1.1 nm and plenty of meso/macropores. The BET specific surface area is 480.8 m2 g-1, and impressively large pore volume of 2.14 cm3 g-1. Based on the synergistic effects of the following main factors: (i) the confined space effect in the hierarchical porous binary carbonaceous matrix, (ii) the anchor effects by strong chemical bonds with codoped N and O atoms; and (iii) the good flexibility and conductivity of rGO, the rGO/PC/S holding 75 wt.% S exhibits high performance as Li-S battery cathode. Specific capacity of 1625 mAh g-1 can be delivered at 0.1 C (1 C = 1675 mA g-1), while 848 mAh g-1 can be maintained after 300 cycles at 1 C. Even at high rate of 5 C, 412 mAh g-1 can be restrained after 1000 cycles.


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1. INTRODUCTION With the development of electric vehicles, unmanned aircrafts and other electric drive equipment, more reliable batteries owning high energy density, stable cyclic performance and low cost are increasingly required.1,

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Lithium-sulfur battery, considered as one of the most

promising energy storage systems, has attracted a great deal of attention for the high theoretical energy density and abundance of sulfur.3-5 However, there are still several issues to be solved. Firstly, the inherent insulation property of S and the final products Li2S2/Li2S results in low availability of the active material, especially at high current density. Secondly, the intermediate polysulfides could easily dissolve in electrolyte and migrate between the electrodes, i.e., “shuttle effect”, which forms solid Li2S2/Li2S on the surface of Li foil, leading to a remarkable loss of S. Moreover, a large volume variation up to 80% caused by the conversion between S and Li2S2/Li2S results in destruction of electrode structure and limited lifespan of Li-S battery.6, 7 Many efforts have been attempted to settle these challenges. Carbon materials, such as porous carbons8-10, hollow carbon composites11,

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, carbon nanotubes13,

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and graphenes15,

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, are

generally introduced for improvement of conductivity. Graphene, with a two dimensional layer structure composed of sp2-bonded C atoms, is regarded as an ideal candidate for highperformance S/C cathode.17 The large electrical conductivity (~106 S cm-1), high surface area, superior mechanical strength and flexibility nature of graphene are utilized to fabricate various structures for sulfur loading.18 Porous carbon, with hierarchical micro/mesoporous structure could be an effective sulfur loader to relieve the direct dissolution, and in this case the mesopores may offer sufficient space to accommodate S8 molecular and micropores restrain the escape of polysulfides.19, 20 Guo et al. pointed out that the cyclo-S8 molecule will convert to S2-4 molecules with short-chain like structure by loading sulfur into carbon micropores with the pore sizes of


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about 0.5 nm.21, 22 However, some complex procedures such as template method23 and chemical reagent activation method24 are required to prepare the micro/mesoporous structure. It is not efficient to satisfy the demand of practical utilization. Moreover, nonpolar carbon layer could not effectively immobilize polar polysufide molecules but only trap them physically.25 To intensify the binding force of sulfur and polysulfides on carbon surface, heteroatom doping is extensively accepted. Many researches have confirmed that the chemical bond between polysulfide and pyridinic nitrogen is much stronger than S-C bond.26-29 Some oxygen-containing function groups can make the similar contributions too. However, it is necessary to simplify the doped heteroatoms introduction process. Herein, a simple route to obtain N, O-codoped hierarchical porous reduced oxide graphene (rGO) and pyrolytic carbon (PC) composite material by heat-treatment of chitosan/graphene oxide composite is applied. The optimized rGO/PC nanocomposite possesses concentrated micropores and plenty of meso/macropores with large specific surface area and pore volume. When used as sulfur holder for Li-S battery cathode, excellent performance can be obtained for rGO/PC/S even at high scan rate and long cycle.

2. EXPERIMENTAL SECTION 2.1. Syntheses of rGO/PC/S, rGO/S, PC/S and rGO+PC/S. GO is prepared using the modified Hummers method, subsequently dispersed in deionized water by ultrasonication to form solution with concentration of 2 mg mL-1. Chitosan, dissolved in 2 wt. % acetic acid, is added into the GO suspension liquid dropwisely under intense magnetic stirring. The mass ratio of chitosan to GO is controlled as 0.15:1. The mixture is freeze-dried after ultrasonication and then calculated at 800oC in tube furnace for 2 h under Ar atmosphere. When it cools down to room temperature, the resultant rGO/PC is grinded with three times the mass of sulfur and sealed


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in Teflon kettle with inert gas. Then, it is heated to 155oC in an oven and maintained for 12 h. The final product rGO/PC/S loading of 75 wt. % S is obtained. The contrast sample rGO/S and PC/S are synthesized all the same with above except for the addition of chitosan or GO. The rGO and PC with the same ratio are physically grinded before mixed with S to obtain rGO+PC/S. 2.2. Characterization. The morphologies and microstructures are observed with the fieldemission scanning electron microscopy (SEM, JSM-7500F) and high-resolution transmission electron microscope (HRTEM, JEM-2100F). The compositions of the C, S, O and N of the composites are further determined using energy dispersive X-ray spectroscopy (EDS, JEM2100F). X-ray diffraction (XRD) analyses are carried out on Labx XRD-6000, Shimadzu Japan with Cu Kα radiation and the wavelength is 0.15418 nm. A scan rate of 6o min-1 is carried out within 2θ = 10 - 80o at room temperature. N2 adsorption/desorption isotherms are measured at 77 K with Micromeritics ASAP 2010 analyzer. The specific surface area values are calculated by Brunauer-Emmett-Teller (BET) method. The pore size distributions are analyzed with the Density Functional Theory (DFT). Raman spectra are obtained with Laser Raman Spectrometer (LabRAM HR800). Thermogravimetric analysis (TGA) is evaluated with SDTQ600 (TA Instruments, USA) in N2 with a heating rate of 10oC min-1 from room temperature to 500oC. Xray photo electron spectra (XPS) are performed by an X-ray photo electron spectrometer (KAlpha1063) with a monochromatic Al Kα X-ray source. The electrical resistivity of all samples is determined using a four point probe method (KDB-1). 2.3. Electrochemical Measurements. The slurry is prepared by mixing sample, acetylene black and poly(vinylidene fluoride) (PVDF) with mass ratio of 8:1:1 in N-methylpyrrolidone (NMP). After the mixture is sufficiently stirred, the homogeneous slurry is transferred onto cleaned Al foil and spread by scraper, which is dried in a vacuum oven at 60oC and cut to round


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plates with a diameter of 1.2 cm. The as-prepared electrodes are fabricated to 2032 coin cells in a glove box with Ar atmosphere. The average sulfur loading on current collector is about 1.2 mg cm-1. The electrode with more sulfur loading (3.2 mg cm-1) is also tested for its better application. Besides, the electrolyte used in our work is controlled at about 30 µL mg-1(S). The electrolyte is 1 M lithium bis-trifluoromethane sulfonylimide (LiTFSI) in 1, 3-dioxolane (DOL) and dimethoxymethane (DME) with a volume ratio of 1:1 containing 1 wt. % LiNO3 solution. Li foil is used as the counter electrode. Cyclic-voltammetry (CV) test is carried out with the scan rate of 0.2 mV s-1 between 1.7 - 2.8 V. Electrochemical impedance spectroscopy (EIS) is measured with the frequency range of 100 kHz - 0.01 Hz on the CHI660D electrochemical workstation. Galvanostatic charge-discharge process and performance at different current rates are tested on Land CT2001A instrument within voltage range of 1.7 - 2.8 V. 2.4. Visualized Adsorption Test. Li2S4 solution is prepared by dissolving Li2S and S at a molar ratio of 1:3 in DME. The concentration is controlled to 1 mmol L-1. For the adsorption test, rGO and rGO/PC samples with the same surface area (4 m2) are immersed into 10 mL Li2S4 solution with string before centrifugation. As to PC sample, 50 mg sample is used. Optical picture is taken with 1 mmol L-1 Li2S4/DME solution for comparison.

3. RESULTS AND DISCUSSION The synthesis process is illustrated in Figure 1a. Chitosan, possessing abundant hydroxyl and amino functional groups, could anchor tightly to graphene oxide (GO) nanosheets by chemical bond with oxygen-containing functional groups.30 Certain amount of chitosan/acetic acid solution is dropped into GO suspension liquid under intense stirring preceding to ultrasonic treatment. The tremellose mixture is subsequently freeze-dried and calcined in Ar atmosphere at 800oC. The rGO/PC/S composite is obtained through conventional melt-diffusion method.


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SEM images of rGO, rGO/PC and rGO/PC/S samples are exhibited in Figure 1b-d. The characteristic wrinkle layers for rGO sheets can be observed serving as the skeleton network. No carbon particles are observed on the surface of rGO matrix (Figure 1e), but can be displayed by TEM images (Figure 1f, g), illuminating that the PC nanoparticles inlay between the rGO sheets with a sandwich-like structure. No agglomerated sulfur particles are observed for rGO/PC/S, confirming uniform distribution of sulfur on the rGO sheets, which would be beneficial for a sufficient utilization as the Li-S battery cathode even at high current density. Very smooth surfaces could be found for the rGO matrix in the HRTEM image (Figure 1h). The sandwiched carbon particles with the sizes of about dozens of nanometers are found in the HRTEM images for the rGO/PC (Figure 1f) and rGO/PC/S (Figure 1j) samples, holding similar structure before and after sulfur loading. N2 adsorption-desorption isotherms of the rGO and rGO/PC samples were determined and presented in Figure 2a, where typical type-IV isotherms could be observed for both samples, indicating the mesoporous textures.31, 32 The larger hysteresis loop at high relative pressure (P/Po > 0.4) of desorption isotherm for rGO/PC indicates increased mesopores in the rGO/PC structure because of the introduction of pyrolytic carbon from the decomposition of chitosan. Some micropores are also occurred for both rGO and rGO/PC samples concluded from the isotherms at low relative pressure below 0.04.33, 34 It is further confirmed by the pore size distribution curves in Figure 2b. Compared to rGO, rGO/PC sample possesses more concentrated micropores with diameters about 1.1 nm and meso/macropores with diameter above 3 nm. As a result, the specific surface area increases from 337.6 m2 g-1 for rGO to 480.8 m2 g-1 for rGO/PC, mainly contributed by micro/mesopores (Table 1), so does the total pore volume, from 1.53 to 2.14 cm3 g-1. Moreover, without GO, the PC sample only possesses a specific surface area 0.38 m2 g-1 and


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pore volume less than 0.01 cm3 g-1 (Figure S1). These remarkable consequent increases by the introduction of chitosan make the rGO/PC composite suitable for high-loading and wellperforming sulfur matrix. The specific surface area and total pore volume decrease to 18.5 m2 g-1 and 0.11 cm3 g-1 for rGO/PC/S, greatly lower than that of rGO/PC as the pores filled with S. XRD is introduced to characterize rGO, rGO/PC, rGO/PC/S and sublimed sulfur. As shown in Figure 3a, the broad 2θ peak at around 25o assigned to the (002) plane of the hexagonal graphitic-type lattice could be observed for both rGO and rGO/PC. A small peak at about 27o may be ascribed to the addition of pyrolytic carbon. The increased diffraction intensity for rGO/PC around low-angle diffraction may be attributed to its abundant pore textures.25 Characteristic peaks for both carbon and sulfur (indicated by asterisk) could be observed after sulfur loading. Lower XRD peak intensities for rGO/PC/S compared to that of pure S also suggest that plenty of sulfur is hidden into the holes, which is in accordance with the BET results. Raman spectra were measured for all the rGO, PC, rGO/PC, rGO/S, PC/S and rGO/PC/S samples (Figure S2,3) with the spectra of the rGO, rGO/PC and rGO/PC/S samples in the range of 1000 - 1800 cm-1 given in Figure 3b for clarity. Strong characteristic peaks corresponding to sulfur can be observed in Figure S2 in PC/S (indicated by asterisk), illustrating a large amount of sulfur exposed because of lack of pores in PC, so does rGO/S sample with very weak peak intensity for sulfur due to a small amount of sulfur outside of the pores. As to rGO/PC/S, no peaks of sulfur can be observed as a result of enough space for sulfur holding. The D-band (peak at about 1330 cm-1, standing for the structural defects in the graphitic structure) and G-band (peak at about 1590 cm-1, corresponding to the tangential vibration of ordered C atoms)35, 36 can be observed for rGO, rGO/PC, rGO/S and rGO/PC/S. The weak corresponding peaks are ascribed to low atom activity for PC and sulfur coating for PC/S. The IG/ID value calculated by


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peak intensity of D and G-band for rGO/PC/S comes to 0.77, showing a certain amount of defects, mainly resulted from the heteroatom doping and hollows, existed in the composite, which can contribute for the sulfur and intermediate fixation. The S content in rGO/PC/S is 75.3 wt. % calculated by TGA (Figure 3c). C, N, O and S elements were examined by EDS in Figure 4, showing the evenly distribution of doped nitrogen atoms, oxygen-containing functional groups and sulfur, which would benefit for sufficient sulfur adsorption and fixation. Of particular note is that the chitosan was homogeneously dispersed in solution and chemical bonded with GO in the preparation process, when the chitosan was pyrolyzed and turned to the carbon sphere, partial N atoms could be released and embedded into the graphene skeleton. Thus, the N may disperse on both PC and rGO resulting in the homogeneous distribution of N in rGO/PC/S. XPS is used to detect the elemental status of rGO, rGO/PC and rGO/PC/S with the spectra analyses of rGO/PC and rGO/PC/S shown in Figure 5 for clarity. C, O and N elements are obviously observed in the XPS survey spectra of rGO/PC and rGO/PC/S. No N atoms are detected for rGO and the contents of O in rGO and rGO/PC are 4.78 and 5.52 at.%, respectively (Table S1). More O atoms are introduced for rGO/PC with the introduction of PC compared to that of rGO. The rGO/PC/S and the response peaks for S 2s and S 2p in the XPS survey spectrum of rGO/PC/S elucidate the efficient sulfur loading for rGO/PC/S (Figure 5a). The C 1s spectrum of rGO/PC/S (Figure 5b) is fitted by three peaks at 284.8, 285.8 and 286.5 eV, respectively, corresponding to three main states of C in the composite, i.e., graphitic C (C-C/C=C), chemical bonded with N (C-N) and O (C-O).37 The doped N atoms for rGO/PC/S mainly exist in three status in Figure 5c, i.e., pyridinic, pyrrolic and graphitic nitrogen, whose response peaks occur at 398.3, 399.3 and 401.1 eV, respectively.27, 38 Two speaks at 163.9 and 164.9 eV related to S 2p3/2


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and S 2p1/2 can be observed in Figure 5d for rGO/PC/S. A strong peak at 168.8 eV is attributed to the S-O bond, indicating a strong chemical anchor of sulfur onto the oxygen atoms of the composite, which will sufficiently subdue the diffusion of polysulfide intermediates to improve the cyclic stability. Besides, partial of the response around 169 eV could results from the oxidation by air.39, 40 Electrochemical properties were valued by CV and galvanostatic charge-discharge measurements. The CV curves of the rGO/PC/S composite cathode for the first three cycles at a scan rate of 0.2 mV s-1 are described in Figure 6a. Two reduction peaks occurred around 2.26 and 1.97 V corresponds to the S8 molecular reduced to long chain polysulfide (Li2Sx, 4 ≤ x ≤ 8) and further reduced to Li2S2/Li2S.41 And one broad anodic peak around 2.47 V stands for the inverse process in the anodic procedure. A notable shift of the anodic peak location comes up from the 1st cycle to the next one, from 2.47 to around 2.4 V, showing the more reversible oxidation after the activation in the first cycle. And the doublet at 2.37 and 2.43 V is associated with the reversible formation of polysulfides and the subsequent oxidation to S8.42 Profiles for the 2nd and 3rd cycle have been well overlapped, indicating the good electrochemical stability. Rate performances of rGO/PC/S and rGO/S at different current densities are tested from 0.1 to 5 C as shown in Figure 6b. The rGO/PC/S cathode exhibits a discharge capacity of 1625 mAh g-1 at the first discharge cycle, approximate to the theoretical specific capacity of sulfur, and above 1400 mAh g-1 for the next cycles at 0.1 C. The discharge capacity values come to 1178, 1103, 982 and 685 mAh g-1 for 0.5, 1, 2 and 5 C, respectively. When the current density comes back to 0.1 C, a specific capacity of 1225 mAh g-1 can be still delivered. In contrast, the rGO/S cathode exhibits a discharge capacity of 1127 mAh g-1 at the initial cycle at 0.1 C, while the specific capacity values at 0.5 - 5 C come to 854, 748, 618 and 261 mAh g-1, respectively. For


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comparison, physically mixed rGO+PC/S was also tested (Figure S3). A better performance is achieved than that of whether rGO or PC as sulfur matrix alone, revealing a synergetic effect between rGO and PC. However, it is still worse than that of rGO/PC/S, since the physically mixed PC cannot enlarge the specific surface area, pore volume and less doped N and O atoms are exposed. So forth, the performance improvement of rGO+PC/S is limited. The rGO/PC/S composite samples obtained by different mass ratios of chitosan to GO (0.08:1, 0.15:1 and 0.25:1) were determined (Figure S4). The best performance was gotten from sample with optimized precursor ratio of 0.15:1.The galvanostatic charge and discharge profiles at different current density for rGO/PC/S cathode are depicted in Figure 6c. The good remain of plateaus for both charge and discharge process suggests excellent performance at different current density of rGO/PC/S cathode. Cyclic performances at a scan rate of 1 C were measured and are shown in Figure 6d. The specific capacity comes to 848 mAh g-1 after 300 cycles at 1 C for the rGO/PC/S composite cathode, with an excellent retention of 79.6% and the Coulomb efficiency all above 95%. While for the rGO/S cathode, it turns to 372 mAh g-1 and 48.3%. The charge/discharge profiles of different cycles in Figure 6e demonstrate no plateau recession with the growing cyclic numbers. A test with larger current density of 5 C and longer cyclic spans of 1000 cycles is further carried out in Figure 6f. Specific capacity up to 412 mAh g-1 can be finally obtained with average fading rate of 0.026% per cycle, more excellent than that of the rGO/S composite, which decreases to lower than 100 mAh g-1 after 150 cycles. Additionally, the electrode with high content of sulfur loading (3.2 mg cm-1) was measured (Figure S5), the discharge specific capacities of 1071.9 and 639.9 mAh g-1 can be obtained at 0.1 and 1 C, respectively, and 410.2 mAh g-1 may be maintained after 200 cycles at 1 C, presenting a certain advantage in the practical utilization.


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The main factors for the outstanding performance of the rGO/PC/S composite can be explained as follows. The conjugate π-bond of graphene layers offers excellent electron transport pathway. Sulfur mainly spreads uniformly into sheets and pores with nanosized scale, which shortens electron transmission distance, resulting in a high utilization even at high current density. The hierarchical micro/mesoporous structure of the rGO/PC matrix can more efficiently encapsulate sulfur and polysulfides via physical constraint. The doped heteroatoms exist as strong polysulfide anchor because of its higher electronegativity and thus forming a kind of dipole-dipole interaction with the electropositive Li+ in polysulfides.28 Finally, the graphenebased matrix maintains a satisfiable flexibility, making it easy for the carbon framework to cushion the volume changes, ensuring the structure stable during repeated cycles. The strong adsorption effect for polysulfides can be confirmed by Figure 6g. The rGO and rGO/PC samples with same surface area of 4 m2 are immersed into 10 mL Li2S4/DME solution (1 mmol L-1) respectively before stirring and centrifugation. Almost total adsorption of polysulfides can be observed as the solution turns to nearly colorless with rGO/PC addition. While, light color still remained for rGO sample and obvious yellow color preserved for PC sample (Figure S6), suggesting enhanced affinity of Li2S4 molecules to N and O co-modified rGO/PC sample. Electrical resistivity tests for the rGO, rGO/PC, rGO/S and rGO/PC/S samples were conducted (Table S2). The electrical resistivity of rGO/PC (2.64 Ω·cm) is larger than rGO (1.96 Ω·cm) because of more defects. After sulfur loading, the electrical resistivity is enlarged because of the insulation nature of sulfur. However, rGO/PC/S exhibits a smaller electrical resistivity (5.75 Ω·cm) than rGO/S (7.19 Ω·cm) for possessing larger specific surface area and pore volume for sulfur loading, which relive the hindrance for electron transfer among the carbon skeleton.


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EIS of the rGO/PC/S fabricated coin cell were measured at fresh state and after 50 and 1000 charge-discharge cycles, as shown in Figure 6h. The semicircle at high frequency range corresponds to the charge transfer process happened on the electrode/electrolyte interface, noted as Rct. A noteworthy decrease from 78 to 20 Ω is emerged for the Rct value of the coin cell after 50 cycles. It could be attributed to a re-arrangement of sulfur and more sufficient contact between electrode material and electrolyte. The value of Rct comes to 33 Ω after 1000 cycles, which might be caused by the deposition of insoluble Li2S2/Li2S on the electrode surface. The straight line at low frequency range stands for the ion diffusion from electrolyte to the interface with electrode surface, which is driven by the concentration gradient. Obviously, the diffusion becomes more efficient after cycling. It might be caused by the sufficient electrolyte infiltration thorough the abundant porous structure and the accompanying ion buffer zone.

4. CONCLUSION In summary, a kind of N,O-codoped hierarchical micro/meso/macroporous binary reduced graphene oxide and pyrolytic carbon graphene nanocomposite can be simply obtained via heat treatment of chitosan/graphene oxide mixture. Plenty of porous texture helps to improve the specific surface area up to 480.8 m2 g-1, while pore volume comes to a very high value of 2.14 cm3 g-1. Sufficient space could be offered for sulfur loading with mass ratio of 75% and accommodate polysulfides from diffusion during the electrochemical reaction procedure. The codoped N and O atoms act as effective intermediates immobilization sites by forming chemical bonds with polysulfides, while graphene accelerating electron and ion transports and buffering volume change. Besides, the rGO/PC/S possesses a low electrical resistivity of 5.75 Ω·cm. Thus, three key factors of porosity, polarity and conductivity, which are important for excellent sulfur matrix, have been all modified, resulting in the superior performance for rGO/PC to rGO as the


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Li-S battery cathode. As a result, a remarkable discharge capacity above 1400 mAh g-1 at 0.1 C and 685 mAh g-1 at 0.5 C could be achieved. It could be maintained at 848 mAh g-1 after 300 cycles at 1 C, and the fading rate is 0.026% per cycle for 1000 cycles at 5 C, showing this composite excellent potential as Li-S battery cathode.


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ASSOCIATED CONTENT SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI:. Figure S1-S6: N2 adsorption/desorption isotherms for PC; Raman spectra of rGO, PC, rGO/PC, rGO/S, PC/S and rGO/PC/S; optimization for rGO/PC/S via rate capacitance; Cycle performance of high-loading cathode; Visualized adsorption test for PC; Table S1-S2: atom contents by XPS; electrical resistivities contrast. (PDF)

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by National Basic Research Programs of China (973 Program, No. 2014CB931800 and 2011CB935700), Chinese National Science Foundation (No. 21571010 and U0734002), Chinese Aeronautic Project (No. 2013ZF51069) and 111 Project (No. B14009).


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Figure Captions Figure 1. (a) Schematic preparation strategy of the rGO/PC/S composite, as well as (b-d) SEM, (e-g) TEM and (h-j) HRTEM images of GO, rGO/PC and rGO/PC/S. Figure 2. (a) N2 adsorption/desorption isotherms and (b) the pore size distribution for rGO, rGO/PC and rGO/PC/S. Figure 3. (a) XRD patterns of rGO, rGO/PC, rGO/PC/S and sulfur, (b) Raman spectra of rGO, rGO/PC and rGO/PC/S, and (c) TGA curve of rGO/PC/S in N2 with heating rate of 10oC min-1. Figure 4. (a) HRTEM image of rGO/PC/S, as well as (b-e) elements mapping images for C, O, S and N. Figure 5. (a) XPS survey spectra of rGO/PC and rGO/PC/S, as well as (b-d) the fitted profiles for C, N and S elements in rGO/PC/S. Figure 6. (a) CV profiles at a scan rate of 0.2 mV s-1 for the initial three cycles of the rGO/PC/S cathode, (b) the discharge specific capacity of rGO/S and rGO/PC/S cathodes at different rate conditions, (c) the galvanostatic charge/discharge profiles at different rate for the rGO/PC/S cathode, (d) the discharge specific capacities and efficiencies at 1 C of 300 cycles for the rGO/S and rGO/PC/S cathodes, (e) the charge/discharge profiles of different cycles for the rGO/PC/S cathode at 1 C of different cycles, (f) the discharge specific capacities and efficiencies at 5 C for the rGO/S and rGO/PC/S cathodes, (g) visualized adsorption of Li2S4 on rGO and rGO/PC with the same surface area, and (h) the EIS curves for the rGO/PC/S cathode with different cycles.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Table 1. The pore volumes and specific surface areas of rGO, rGO/PC and rGO/PC/S samples. Pore volume (cm3 g-1)

Specific surface area (m2 g-1)

Sample Micropore

Mesopore

Macropore

Micropore

Mesopore

Macropore

rGO

0.012

0.53

0.99

17.5

56.9

263.2

rGO/PC

0.048

0.75

1.35

73.3

103.9

303.6

0.038

0.069

3.1

4.3

11.1

rGO/PC/S 0.0028


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