Sulfur Nanoscroll Material and its Application in Lithium Sulfur Batteries

Accepted Manuscript Title: A Novel One-dimensional Reduced Graphene Oxide/Sulfur Nanoscroll Material and its Application in Lithium Sulfur Batteries Author: Pengjian Zuo Wei Zhang Junfu Hua Yulin Ma Chunyu Du Xinqun Cheng Yunzhi Gao Geping Yin PII: DOI: Reference:

S0013-4686(16)32537-3 http://dx.doi.org/doi:10.1016/j.electacta.2016.11.179 EA 28469

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

12-9-2016 29-11-2016 29-11-2016

Please cite this article as: Pengjian Zuo, Wei Zhang, Junfu Hua, Yulin Ma, Chunyu Du, Xinqun Cheng, Yunzhi Gao, Geping Yin, A Novel One-dimensional Reduced Graphene Oxide/Sulfur Nanoscroll Material and its Application in Lithium Sulfur Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.11.179 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A

Novel

One-dimensional

Reduced

Graphene

Oxide/Sulfur Nanoscroll Material and its Application in Lithium Sulfur Batteries

Pengjian Zuo*, Wei Zhang, Junfu Hua, Yulin Ma, Chunyu Du, Xinqun Cheng, Yunzhi Gao, Geping Yin

a

Institute of Advanced Chemical Power Source, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China

* Corresponding author: [email protected] Tel: 86-451-86403961; Fax: 86-451-86403961

1

Graphical abstract Sonication Shock cooling / Freeze drying

RGO S nanoparticles

Annealing

RGO/SNS

2000

100

Charge Discharge Coulombic efficiency

1500 0.1C

1000

2C

60 40

500 0 0

80

20

10

20

30

40 50 60 Cycle Number

70

80

90

0

100

Coulombic Efficiency (%)

-1

Specific Capacity(mAh·g )

GO

RGO/SNP

A Novel One-dimensional Reduced Graphene Oxide/Sulfur Nanoscroll Material and its Application in Lithium Sulfur Batteries

Pengjian Zuo, Wei Zhang, Junfu Hua, Yulin Ma, Chunyu Du, Xinqun Cheng, Yunzhi Gao, Geping Yin

2

Highlights  1-D rGO/sulfur nanoscrolls were synthesized by freeze-casting strategy  Sulfur was wrapped by scrolled stretchy graphene with high electronic conductivity  RGO/SNS show a capability absorbing polysulfies compared with conventional RGO/S  RGO/S nanoscrolls demonstrate high discharge capacity and rate performance

3

Abstract：One-dimensional (1D) reduced graphene oxides possess unique structure properties in comparison with conventional graphene sheet material and show great potential in application of high-efficiency energy storage and conversion system. Lithium sulfur batteries have been considered as the promising candidates of high-energy rechargeable batteries due to the high theoretical energy density and cost-efficiency. However, their practical application has been hampered by rapid capacity fading during cycling. In this work, one-dimensional reduced graphene oxide/sulfur nanoscrolls show promise for uniformly confining elemental sulfur in their internal cavities and absorbing polysulfide intermediates. The remarkable electrochemical performance of the reduced graphene oxide/sulfur nanoscroll materials is mainly attributed to the desirable structural features, where sulfur is encapsulated into the RGO nanoscrolls and kept close contact with RGO. The RGO nanoscrolls not only provide the electron transport pathway, but effectively alleviate the dissolution and shuttling of the polysulfides during cycling for lithium sulfur batteries.

Keywords: reduced graphene oxides; one-dimensional structure; sulfur-contained nanoscrolls; electrochemical performance; lithium sulfur batteries

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1. Introduction The explosive growth in portable electronic devices and electric vehicles has promoted strenuous efforts in exploring high-energy storage systems. Lithium sulfur (Li-S) batteries have been receiving tremendous interests because of its high theoretical specific capacity of 1675 mAh g-1 regarding sulfur and energy density of 2600 Wh kg-1 based on the reaction of lithium with sulfur to form lithium sulfide, which are 3-5 fold higher than those of lithium ion batteries.[1-3] Moreover, sulfur as the active material in cathode for lithium sulfur battery is cost-effective and environmentally friendly.[3, 4] However, to date, lithium sulfur batteries still suffer from some issues that limit its immediate commercial applications. For example, the intrinsic insulating characteristics of sulfur and its reduced products results in poor rate performance and low utilization of active material.[5, 6] In addition, the continuous capacity loss during cycling can be resulted from the high solubility of the reduction intermediate species of Li2Sn (4≤n≤8) in conventional non-aqueous organic electrolyte and the electrode pulverization caused by volume expansion during discharge.[7-10] Moreover, shuttling of dissolved intermediate polysulfides between the electrodes during cycling will cause low Coulombic efficiency and instability of the lithium metal by forming solid Li2S or Li2S2 products, resulting in poor cycling stability.[11, 12] In order to address the aforementioned problems, various strategies on the physical and chemical encapsulation of active sulfur into various porous host 5

materials have been employed. [3, 13, 14] Carbon-based materials, including micro/mesoporous carbon,[10, 15-20] carbon nanofibers,[21, 22] carbon nanotubes,[6, 23, 24] and porous hollow carbon,[25, 26] have been utilized as host matrix for sulfur to improve the conductivity and suppress the shuttling of polysulfide intermediates. Graphene as a unique two-dimensional (2D) carbon material has been investigated intensively in various electrochemical devices.[27-31] Although some achievement has been made for sulfur-graphene application as cathode in lithium sulfur batteries, the cyclic stability was also unsatisfied due to the weak interactions related to physical absorption between sulfur and graphene. To enhance the interactions between sulfur and graphene, the functionalized graphenes, such as graphene oxide,[4] hydroxylated graphene and phenyl sulfonated graphene,[5, 32] were employed to immobilize active sulfur materials by the strong interaction between surface functional groups and intermediate polysulfides. Nevertheless, the 2D structure of graphene is not effective enough in accommodating the soluble polysulfides. Recently, a series of wrapped-type architectures have been deliberately designed to inhibit the dissolution of polysulfides by the physical barrier mechanism.[7, 33-36] Graphene

nanoscroll

(GNS)

is

an

increasingly important

carbonaceous

nanomaterial based on graphene sheets, showing a helical scroll structure.[37-39] It can be regarded as the resultant one-dimensional (1D) structure that is shaped by continuously rolling of flat graphene sheet, which is different from the multiwalled carbon nanotube (MWCNT) with seamless concentric structure. Unlike MWCNT possessing a weak interaction between the carbon walls, GNS has a continuous curly 6

structure. Remarkably, GNS with continuous π electrons performs some different properties from MWCNT in electronic properties, providing the uninterrupted conducting pathways, while π electrons in MWCNT are interrupted by the interlayer spacing between their concentric walls.[40, 41] As a result, GNS showed better capability to afford current density in comparison with MWCNT and has been studied as electrodes for supercapacitor and battery application.[41-43] The unique 1D structure and properties of GNS make it promising to encapsulate sulfur for enhancing the electronic conductivity and suppress the polysulfide dissolution in electrolyte. Herein, we are motivated to design a 1D tubular structure reduced graphene oxide (RGO)/S nanoscroll materials (denoted as RGO/SNS) by an environmentally friendly freeze-casting strategy without any toxic organic solvents. In this method, the dispersion (Figure S1) of prepared RGO (Figure S2) and sulfur nanoparticle (SNP) was cold-quenched with liquid nitrogen, followed by a freeze-drying and heat-treatment process. In this unique architecture, the sulfur was encapsulated into the cavities of RGO nanoscrolls. The interconnecting RGO nanoscrolls with an excellent electronic conductivity can provide an electronic conductive network, and as a carbon coating to effectively inhibit the dissolution of lithium polysulfides in liquid electrolyte by a physical restraint. The residual oxygen-containing groups on RGO surface are also beneficial to immobilize polysulfides by the strong chemical interaction.[4, 32, 44, 45] Moreover, the open topology of RGO nanoscrolls can not only make Li+ easily diffuse into the cavities of RGO nanoscrolls, but accommodate the contraction/expansion of active materials during cycling without any breakage of 7

tubular walls by radial expansion. As a consequence, a excellent electrochemical performance of RGO/S nanoscrolls as cathode can be achieved for lithium sulfur batteries.

2. Experimental 2.1 Preparation of RGO dispersion and RGO/S nanoscrolls Graphite oxide (GO) was prepared by two-step oxidation method as described in our previous study[46]. 200 mg sulfur nanoparticles (SNPs) (Beijing DK Nanotechnology Co., LTD) were dispersed in 116 mL deionized water with an ultrasonic assistance. Then, 50 mL RGO dispersion (1 mg mL-1) was injected into above sulfur suspension under vigorous magnetic stirring, followed by 2 h ultrasonication. The mixed aqueous suspension of RGO and SNPs was rapidly cooled with liquid nitrogen in a vacuum cup. The water was removed by a freeze-drying equipment, and a black gray pristine RGO nanoscrolls with an encapsulated SNPs (RGO/SNP nanoscrolls) were obtained. After further heat treatment at 180℃ for 2 h under argon atmosphere to remove the unencapsulated sulfur, the final product of RGO/SNS material was prepared successfully. In addition, a conventional reduced graphene oxide/sulfur composite (denoted as RGO/SCP) with a 2D structure was also prepared for comparison. RGO powder and sulfur were mixed in the weight ratio of 1:3 through grinding, then the mixture was dried at 155℃ for 12 h to allow the melted sulfur to infiltrate into the layer of RGO. RGO powders were prepared by dropping the concentrated hydrochloric acid in the stable RGO dispersion and washing with deionized water. After drying in freeze dryer, the RGO powders were 8

further held at 180℃ for 2 h in flowing argon. 2.2 General characterization The X-ray diffraction patterns of the samples were recorded using an X-ray powder diffractometer with Cu-Kα radiation. Raman spectroscopy was performed using a HR800 Raman machine. Thermogravimetric analysis (TGA) was performed via a simultaneous thermal analysis (DSC, Netzsch STA449F3) in argon at 10 ℃ min-1 in order to confirm the content of sulfur in the samples. The morphology and structure was examined by field-emission scanning electron microscope (FESEM, FEI Helios Nanolab600i) with energy dispersive spectroscopy (EDS) equipment and transmission electron microscope (TEM, H-7650). X-ray photoelectron spectra (XPS) were performed on a PHI model 5700 spectrometer. 2.3 Electrochemical measurements The electrochemical properties were examined using the 2025 coin-type cells. The slurry containing as-prepared RGO/S composite materials (80 wt%), acetylene carbon black (10 wt%) and polyvinylidene fluoride (PVDF) binder (10 wt%) in NMP was pasted onto Al foil current collector and dried at 50℃ for 12 h in a vacuum oven to obtain the working electrodes. Lithium foil was used as counter electrode, and a Celgard 2400 microporous polypropylene membrane was used as the separator. The electrolyte was a solution of 1 M LiTFSI in a solvent of DOL:DME (1:1 in a volume ratio) with 0.4 M LiNO3 as additive. The cells were assembled in a high-purity argon-filled glove box (moisture and oxygen levels less than 1ppm). As a comparison, the performance of conventional RGO/S composite (RGO/SCP) was also evaluated 9

by the same method. Galvanostatic cycling was carried out on a battery test system (Neware, BST-5V5mA) with a voltage ranging from 1.7 to 2.8 V. Cyclic voltammetry curves were recorded at a scanning rate of 0.05 mV·s-1 at 1.5-2.8 V using AUTOLAB PGSTAT302N instrument. The electrochemical impedance spectra with frequency range of 10 kHz-10 mHz was conducted using an electrochemical workstation of PARSTAT 2273. All of the measurements were carried out at room temperature.

3. Results and discussion The fabrication process of RGO/SNP and RGO/SNS was shown in Figure 1. RGO sheets were crumpled into 1D tubular structure by using an environmentally friendly freeze-casting strategy for RGO dispersion (Figure 2a). The 1D RGO nanoscroll structure can be illustrated more clearly from the high resolution SEM image, and the diameter distribution of about 300~700 nm for the RGO nanoscrolls can be found (Figure 2b). Figure 2c shows the TEM image of RGO nanoscrolls, and the RGO nanoscroll reveals a typical scrolling morphology, and the scroll-like configurations of the RGO were further identified (Figure 2d). Fig.1 Fig.2 The particle size of pristine sulfur ranges from ~50 to 300 nm (Figure S3). From the SEM images of RGO/SNP nanoscrolls (Figure 3a and b), it can be clearly observed that the tubular structure for the composite can be maintained, and SNPs (red arrow direction) are well wrapped by the RGO sheets. From the element mapping (Figure S4), we can found that the carbon and sulfur are well distributed in the 10

RGO/SNP nanoscrolls. The TEM observation was conducted in order to further verify the fact that the SNPs exist inside the RGO nanoscrolls. As seen in Figure 3c and d, RGO sheet shows a nanoscroll structure wrapping some nanoparticles which are identified as SNPs by EDS analysis (Figure 3f). The zoom-in TEM image (Figure 3e) shows that the SNPs are wrapped well by the RGO sheet. The morphology characterization reveals that the unique RGO/SNP nanoscroll structure has been successfully synthesized by a simple rapid cooling process, where SNPs are wrapped well by RGO sheets. Fig.3 A further heat-treatment for the pristine RGO/SNP nanoscrolls at 180 ℃ under flowing argon was conducted to remove the SNPs not well wrapped by RGO nanoscrolls and enhance the contact between active sulfur material and RGO, and the final product of RGO/SNS materials were obtained. In this work, two kinds of RGO/SNS samples with sulfur content of 54 wt% and 78 wt% (Figure S5, Figure S11a) were obtained depending on heat-treatment time. When the heat-treatment was employed, the SNPs on the external surface of RGO nanoscrolls volatilized and were taken away by flowing argon. At the same time, the interior SNPs encapsulated by RGO nanoscrolls melted and diffused into the cavities of RGO sheet with the assistance of strong adsorption effect derived from both relatively large surface area and oxygen groups on RGO surface, and an intimate contact between RGO and sulfur was obtained.[4] Moreover, the electronic conductivity of the sample can be improved by this low-temperature heat treatment (Table S1). From the morphology images 11

(Figure 4a and b), it is shown that the 1D tubule-like nanoscroll structure for RGO/SNS is still maintained even after the heat-treatment. The element mapping reveals that the carbon and sulfur are uniformly distributed in RGO/SNS samples after an annealing process(Figure S6). Compared with the pristine RGO/SNP nanoscrolls, the large SNPs which were encapsulated among the RGO nanoscrolls disappear even though a few residual smaller S particles (red arrow direction) still can be observed from the high magnification TEM image (Figure 4c). The HAADF-STEM was employed to further confirm the microstructure of RGO/SNS (Figure 4d), and the elemental mapping of carbon and sulfur, corresponding the rectangle region where no obvious large-size sulfur particles can be observed, reveals a homogeneous sulfur distribution in the RGO nanoscrolls. It can be deduced that when the synthesized RGO/SNP nanoscrolls were heated in flowing argon, some SNPs inside the RGO nanoscrolls melted and spread to RGO sheet, just as illustrated schematically in Figure 1. In the unique nanoscroll structure of RGO/SNS, the more close electronic contact of RGO with sulfur can be constructed consequently resulting in the improvement of the active material utilization. The dissolution of polysulfide intermediates into the non-aqueous electrolyte during cycling of RGO nanoscrolls with a relatively closed structure can be effectively suppressed. Moreover, the functional groups on the RGO surface which have strong adsorbing capability to sulfur and polysulfides can also result in great improvement on electrochemical performance. Fig.4 12

The sharp diffraction peaks of SNPs (Figure 5a) means that the SNPs are well crystalline (orthorhombic sulfur, PDF#08-0247). Besides the characteristic peaks of elemental sulfur in the RGO/SNP nanoscroll composite, the broad peak at around 24.6° is attributed to RGO (Figure S7). Compared with RGO/SNP nanoscroll composite, a weaker characteristic diffraction peak on crystalline sulfur can be detected for the RGO/SNS samples, implying that most of the sulfur diffused into the cavities of RGO and distributed in a highly dispersing state. The diffraction peaks of sulfur are not perfectly matching with those of RGO/SNP nanoscrolls (Figure 5a, inset), indicating that the possible crystalline conversion of sulfur has occurred, which is mainly caused by the coexistence of RGO during annealing process.[45] Figure 5b shows the Raman spectra of graphite, graphene oxide, reduced graphene oxide nanoscrolls, RGO/SNP and RGO/SNS. The Raman spectrum of graphite indicates a sharp and strong G-band at around 1582 cm−1 arising from the hexagonal vibration and a very weak D-band at around 1354 cm−1 related to the defects and sp3-hydridized bond, which indicates that the pristine graphite is of highly crystalline with few defects.[47] After reduction with NaBH4, the ID/IG ratio of RGO increases notably in comparison with GO, indicating the increasing amount of smaller graphitic domains upon reduction of GO [48-50]. The RGO/SNP nanoscrolls show a very similar spectrum to RGO. The two main peaks (1352 cm−1 and 1572 cm−1) are in agreement with the RGO spectrum, and the peak located at 470 cm−1 comes from the sulfur. The weak Raman signal of sulfur shows that most SNPs are well wrapped by RGO nanoscrolls. It should be noted that the peaks related to sulfur disappears in the 13

RGO/SNS sample, revealing that sulfur is completely encapsulated inside the RGO scrolls with a nanoscale size through annealing process.[51] From the XPS results, the RGO/SNS reveals strong sulfur signals at 227.4 eV for S 2s and 163.9 eV for S 2p from the wide-scan survey spectra (Figure 5c). In comparison with the XPS of RGO (Figure S8), an additional small shoulder peak at 168.8 eV is ascribed to the sulfate species related to the sulfur oxidation.[5] From the fitted spectrum (Figure 5d), the S 2p3/2 has the binding energies of 163.9 and 164.3 eV, which can be attributed to S-S and S-O species, respectively. The S-O bonding indicates the strong interaction between sulfur and RGO in the as-prepared RGO/SNS. As a result, the sulfur maintains close contact with RGO by S-O bonding, which can effectively immobilize polysulfides on RGO surface and prevent the diffusion of lithium polysulfides into the electrolyte.[52, 53] Fig.5 The cyclic voltammetry (CV) curves of the RGO/SNS samples in the initial four cycles are shown in Figure 6a. In the cathodic scanning process, It can be clearly observed that there are two main peaks at about 2.2 V and 2.0 V, corresponding to reduction of sulfur to soluble long-chain polysulfide intermediates (Li2Sn, 4≤n≤8) and the formation of insoluble Li2S2 and Li2S. Additionally, the anodic peak at 2.5 V corresponds to the oxidation of Li2S and Li2S2 to sulfur or Li2S8.[12, 54] Figure 6b shows the cycling performance of the RGO/SNS and RGO/SCP samples at 0.2 C. It can be seen that the RGO/SNS electrode exhibits excellent cycling performance. The initial discharge capacity and Columbic efficiency of the RGO/SNS is 1198 mAh·g−1 14

and 85.4%, respectively. After 100 cycles, the RGO/SNS electrode still has a discharge capacity of 684 mAh·g−1, showing a capacity retention of approximately 57%. However, the typical RGO/SCP without tubular structure shows lower specific capacity and poor stability related to the opened structure (Figure S9). After 100 cycles, the specific capacity of the RGO/SCP decreases from 1033 to 455 mAh·g−1 at 0.2 C mainly due to the dissolution of polysulfide intermediates into the non-aqueous electrolyte and the resultant shuttling effect.[55] The relatively high capacity and cyclic stability of RGO/SNS cathode suggest that the 1D RGO nanoscroll structure plays an important role in retarding polysulfide dissolution by the physical restraint. Moreover, the residual oxygen groups on the RGO surface can also capture the migrating Li2Sn by the chemical interaction, which has been reported in previous works.[4, 56] The interconnected RGO nanoscrolls provide both a conductive framework and fast lithium ion transfer pathways, resulting in the excellent electrochemical cyclic performance of the lithium sulfur battery.

The charge-discharge experiments of RGO/SNS at various current rates (0.2-2 C) in 1.7-2.8 V were performed, and the results are shown in Figure 6c. Consisting with the CV curves, these charge-discharge profiles show two apparent discharge plateaus, indicating a typical multistep lithiation process, and one charge plateau corresponding to the oxidation of polysulfides. It can be seen that the voltage hysteresis increases with the increasing charge/discharge rates, revealing the increasing polarization during cycling. Figure 6d depicts the rate capacity of RGO/SNS. The RGO/SNS shows the capacities of around 810, 719 and 634 mAh·g−1 at 0.5, 1 and 2 C, 15

respectively. Markedly, the sample shows a better capacity retention at higher rates, and even no obvious degradation can be found at 2 C. From the corresponding charge-discharge curves as shown in Figure 6c, the discharge capacities of 710, 770 and 835 mAh·g−1 are obtained when the current density is reset back to 1 C, 0.5 C and 0.2 C, respectively, without abrupt capacity fading. The unique nanoscroll structure of RGO/SNS results in the outstanding rate performance and cyclability compared with previous literatures (Table S2). Here, RGO nanoscrolls with a high electronic conductivity can form an interconnected conductive framework, and the highly dispersed sulfur on the RGO surface ensures the close contact between the RGO conductive matrix and sulfur. Moreover, the topological open structure of RGO nanoscrolls allow complete electrolyte penetration and provide fast lithium ion transport pathways.

Fig.6 The cyclic stability of RGO/SNS at 2 C is shown in Figure 7a. A relatively stable capacity of 630 mAh·g−1 can be obtained at the rate of 2 C after the activation at 0.1 C. After 100 cycles, the RGO/SNS still retains a reversible capacity of 505 mAh·g−1, revealing the good cyclic performance at a relatively high current rate. Electrochemical impedance spectra measurements were further carried out to gain additional insight into the electrochemical reaction process of RGO/SNS. The Nyquist plots of RGO/SNS electrode before and after cycling from 10 kHz to 10 mHz in a fully charged state are shown in Figure 7c. The Nyquist plots consist of one or two flattened semicircles and an inclined line. For lithium sulfur batteries, the high 16

frequency semicircle reflects the charge transfer resistance (Rct) in the interface of conductive material, dominating the reduction process during the upper voltage plateau, while the middle frequency semicircle corresponds to mass transport in the cathodic electrode which is a controlling step during the lower voltage plateau. Moreover, it should be mentioned that the charge transfer in interface and the mass transport in cathode are all related to the electrochemical reactions.[57, 58] It can be easily observed that the high frequency semicircle of the RGO/SNS electrode is compressed obviously from the 1st to the 5th cycle, indicating a continuously decreased interfacial charge transfer resistance in the initial five cycles, which is in good agreement with the decreased polarization potential during cycling (Figure 7b and inset). The little change in the semicircle in the mid-frequency range indicates a relatively stable Li2S2/Li2S film on the graphene surface.[59] Moreover, It can also be seen that the curves of the 5th and 50th cycle are nearly identical, implying a good electrochemical stability during subsequent cycling, which has been further verified for the RGO/SNS materials even with higher sulfur content up to 78% (Figure S10). Therefore, the sulfur-wrapped RGO with unique nanoscroll architecture demonstrates high discharge capacity and good electrochemical cyclic stability as cathode for high-energy Li-S batteries. Fig.7

4. Conclusions In summary, the unique 1D RGO/SNS materials were successfully synthesized by shock cooling of RGO/sulfur dispersion with the assistance of liquid nitrogen. 17

Compared with the typical RGO/S composite (RGO/SCP) without nanoscroll structure, the RGO/SNS materials show high reversible charge/discharge capacity and good cyclic capablity as well as excellent rate performance as cathode materials for Li-S batteries. The improved electrochemical performance of RGO/SNS can be ascribed to the unique 1D RGO nanoscroll structure. In this unique architecture, relatively closed structure of RGO nanoscrolls not only can mitigate the dissolution of polysulfides, but supply enough room to accommodate the volume changes of sulfur during charge/discharge process. The residual oxygen-containing groups on RGO surface are also helpful to capture the migrating lithium polysulfides of Li 2Sn by the chemical interaction. Moreover, the cross-linked RGO nanoscrolls with a superior electric conductivity can supply a electronic conductive framework and fast lithium ion diffusion pathways, leading to the excellent rate capability. Therefore, RGO nanoscrolls are promising candidates for encapsulating sulfur as cathode to release high specific capacity and good cycling performance for high performance Li-S batteries.

Acknowledgements This work was partially supported by The Natural Science Foundation of China (no. 50902038, 51202047), Heilongjiang Science & Technology Key Bidding Program (no. GA14A102) and Heilong Jiang Postdoctoral Funds for Scientific Research Initiation (no. LBH-Q12084).

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References [1] X. Ji, L.F. Nazar, Advances in Li-S batteries, Journal of Materials Chemistry, 20 (2010) 9821-9826. [2] J. Wang, X. Sun, Understanding and recent development of carbon coating on LiFePO4 cathode materials for lithium-ion batteries, Energy & Environmental Science, 5 (2012) 5163-5185. [3] X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries, Nat Mater, 8 (2009) 500-506. [4] L.W. Ji, M.M. Rao, H.M. Zheng, L. Zhang, Y.C. Li, W.H. Duan, J.H. Guo, E.J. Cairns, Y.G. Zhang, Graphene Oxide as a Sulfur Immobilizer in High Performance Lithium/Sulfur Cells, Journal of the American Chemical Society, 133 (2011) 18522-18525. [5] C.X. Zu, A. Manthiram, Hydroxylated Graphene-Sulfur Nanocomposites for High-Rate Lithium-Sulfur Batteries, Advanced Energy Materials, 3 (2013) 1008-1012. [6] W. Ahn, K.-B. Kim, K.-N. Jung, K.-H. Shin, C.-S. Jin, Synthesis and electrochemical properties of a sulfur-multi walled carbon nanotubes composite as a cathode material for lithium sulfur batteries, Journal of Power Sources, 202 (2012) 394-399. [7] H. Xu, Y. Deng, Z. Shi, Y. Qian, Y. Meng, G. Chen, Graphene-encapsulated sulfur 19

(GES) composites with a core–shell structure as superior cathode materials for lithium–sulfur batteries, Journal of Materials Chemistry A, 1 (2013) 15142. [8] J. Shim, K.A. Striebel, E.J. Cairns, The Lithium/Sulfur Rechargeable Cell: Effects of Electrode Composition and Solvent on Cell Performance, Journal of The Electrochemical Society, 149 (2002) A1321-A1325. [9] L. Yin, J. Wang, F. Lin, J. Yang, Y. Nuli, Polyacrylonitrile/graphene composite as a precursor to a sulfur-based cathode material for high-rate rechargeable Li-S batteries, Energy & Environmental Science, 5 (2012) 6966-6972. [10] G. He, X. Ji, L. Nazar, High "C" rate Li-S cathodes: sulfur imbibed bimodal porous carbons, Energy & Environmental Science, 4 (2011) 2878-2883. [11] Z. Xiao, Z. Yang, L. Wang, H. Nie, M.e. Zhong, Q. Lai, X. Xu, L. Zhang, S. Huang, A Lightweight TiO2/Graphene Interlayer, Applied as a Highly Effective Polysulfide Absorbent for Fast, Long-Life Lithium–Sulfur Batteries, Advanced Materials, (2015) n/a-n/a. [12] B. Ding, G. Xu, L. Shen, P. Nie, P. Hu, H. Dou, X. Zhang, Fabrication of a sandwich structured electrode for high-performance lithium-sulfur batteries, Journal of Materials Chemistry A, 1 (2013) 14280-14285. [13] A. Ghosh, S. Shukla, G.S. Khosla, B. Lochab, S. Mitra, Sustainable Sulfur-rich Copolymer/Graphene Composite as Lithium-Sulfur Battery Cathode with Excellent Electrochemical Performance, Scientific reports, 6 (2016) 25207. [14] S. Shukla, A. Ghosh, U.K. Sen, P.K. Roy, S. Mitra, B. Lochab, Cardanol benzoxazine-Sulfur Copolymers for Li-S batteries: Symbiosis of Sustainability and 20

Performance, ChemistrySelect, 1 (2016) 594-600. [15] C. Liang, N.J. Dudney, J.Y. Howe, Hierarchically Structured Sulfur/Carbon Nanocomposite Material for High-Energy Lithium Battery, Chemistry of Materials, 21 (2009) 4724-4730. [16] J. Schuster, G. He, B. Mandlmeier, T. Yim, K.T. Lee, T. Bein, L.F. Nazar, Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for Lithium–Sulfur Batteries, Angewandte Chemie International Edition, 51 (2012) 3591-3595. [17] S.-R. Chen, Y.-P. Zhai, G.-L. Xu, Y.-X. Jiang, D.-Y. Zhao, J.-T. Li, L. Huang, S.-G. Sun, Ordered mesoporous carbon/sulfur nanocomposite of high performances as cathode for lithium–sulfur battery, Electrochimica Acta, 56 (2011) 9549-9555. [18] B. Zhang, X. Qin, G.R. Li, X.P. Gao, Enhancement of long stability of sulfur cathode by encapsulating sulfur into micropores of carbon spheres, Energy & Environmental Science, 3 (2010) 1531-1537. [19] Y. Liu, B. Chen, F. Cao, X. Zhao, J. Yuan, Synthesis of nanoarchitectured LiNi0.5Mn0.5O2 spheres for high-performance rechargeable lithium-ion batteries via an in situ conversion route, Journal of Materials Chemistry, 21 (2011) 10437-10441. [20] H. Chen, Q. Li, N. Teng, D. Long, C. Ma, Y. Wei, J. Wang, L. Ling, Simultaneous micropore development and nitrogen doping of ordered mesoporous carbons for enhanced supercapacitor and Li-S cathode performance, Electrochimica Acta, 214 (2016) 231-240. [21] J. Guo, Y. Xu, C. Wang, Sulfur-Impregnated Disordered Carbon Nanotubes 21

Cathode for Lithium–Sulfur Batteries, Nano Letters, 11 (2011) 4288-4294. [22] G. Liang, J. Wu, X. Qin, M. Liu, Q. Li, Y.B. He, J.K. Kim, B. Li, F. Kang, Ultrafine TiO2 Decorated Carbon Nanofibers as Multifunctional Interlayer for High-Performance Lithium-Sulfur Battery, ACS Appl Mater Interfaces, 8 (2016) 23105-23113. [23] L. Yuan, H. Yuan, X. Qiu, L. Chen, W. Zhu, Improvement of cycle property of sulfur-coated multi-walled carbon nanotubes composite cathode for lithium/sulfur batteries, Journal of Power Sources, 189 (2009) 1141-1146. [24] W. Deng, A. Hu, X. Chen, S. Zhang, Q. Tang, Z. Liu, B. Fan, K. Xiao, Sulfur-impregnated 3D hierarchical porous nitrogen-doped aligned carbon nanotubes as high-performance cathode for lithium-sulfur batteries, Journal of Power Sources, 322 (2016) 138-146. [25] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Porous Hollow Carbon@Sulfur Composites for High-Power Lithium–Sulfur Batteries, Angewandte Chemie International Edition, 50 (2011) 5904-5908. [26] Y. Qu, Z. Zhang, K. Du, W. Chen, Y. Lai, Y. Liu, J. Li, Synthesis of nitrogen-containing hollow carbon microspheres by a modified template method as anodes for advanced sodium-ion batteries, Carbon, 105 (2016) 103-112. [27] J.J. Wang, Y.L. Li, X.L. Sun, Challenges and opportunities of nanostructured materials for aprotic rechargeable lithium-air batteries, Nano Energy, 2 (2013) 443-467. [28] W. Chen, S. Li, C. Chen, L. Yan, Self-Assembly and Embedding of Nanoparticles 22

by In Situ Reduced Graphene for Preparation of a 3D Graphene/Nanoparticle Aerogel, Advanced Materials, 23 (2011) 5679-5683. [29] H.F. Xiang, K. Zhang, G. Ji, J.Y. Lee, C.J. Zou, X.D. Chen, J.S. Wu, Graphene/nanosized silicon composites for lithium battery anodes with improved cycling stability, Carbon, 49 (2011) 1787-1796. [30] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, The role of graphene for electrochemical energy storage, Nature materials, 14 (2015) 271-279. [31] J.-Z. Wang, L. Lu, M. Choucair, J.A. Stride, X. Xu, H.-K. Liu, Sulfur-graphene composite for rechargeable lithium batteries, Journal of Power Sources, 196 (2011) 7030-7034. [32] L. Zhou, X. Lin, T. Huang, A. Yu, Binder-free phenyl sulfonated graphene/sulfur electrodes with excellent cyclability for lithium sulfur batteries, Journal of Materials Chemistry A, 2 (2014) 5117-5123. [33] S. Evers, L.F. Nazar, Graphene-enveloped sulfur in a one pot reaction: a cathode with good coulombic efficiency and high practical sulfur content, Chemical Communications, 48 (2012) 1233-1235. [34] F.-f. Zhang, X.-b. Zhang, Y.-h. Dong, L.-m. Wang, Facile and effective synthesis of reduced graphene oxide encapsulated sulfur via oil/water system for high performance lithium sulfur cells, Journal of Materials Chemistry, 22 (2012) 11452. [35] H.W. Wu, Y. Huang, M. Zong, X. Ding, J. Ding, X. Sun, Electrostatic self-assembly of graphene oxide wrapped sulfur particles for lithium-sulfur batteries, Materials Research Bulletin, 64 (2015) 12-16. 23

[36] H. Wang, Y. Yang, Y. Liang, J.T. Robinson, Y. Li, A. Jackson, Y. Cui, H. Dai, Graphene-wrapped sulfur particles as a rechargeable lithium-sulfur battery cathode material with high capacity and cycling stability, Nano Lett, 11 (2011) 2644-2647. [37] J. Zheng, H. Liu, B. Wu, Y. Guo, T. Wu, G. Yu, Y. Liu, D. Zhu, Production of High-Quality Carbon Nanoscrolls with Microwave Spark Assistance in Liquid Nitrogen, Advanced Materials, 23 (2011) 2460-+. [38] X. Xie, L. Ju, X. Feng, Y. Sun, R. Zhou, K. Liu, S. Fan, Q. Li, K. Jiang, Controlled Fabrication of High-Quality Carbon Nanoscrolls from Monolayer Graphene, Nano Letters, 9 (2009) 2565-2570. [39] S. Yoo, J. Lee, J.M. Kim, C.-Y. Seong, K.-d. Seong, Y. Piao, Well-dispersed sulfur wrapped in reduced graphene oxide nanoscroll as cathode material for lithium– sulfur battery, Journal of Electroanalytical Chemistry, 780 (2016) 19-25. [40] Z. Xu, B. Zheng, J. Chen, C. Gao, Highly Efficient Synthesis of Neat Graphene Nanoscrolls from Graphene Oxide by Well-Controlled Lyophilization, Chemistry of Materials, 26 (2014) 6811-6818. [41] J.P. Zhao, B.J. Yang, Z.M. Zheng, J. Yang, Z. Yang, P. Zhang, W.C. Ren, X.B. Yan, Facile Preparation of One-Dimensional Wrapping Structure: Graphene Nanoscroll-Wrapped of Fe3O4 Nanoparticles and Its Application for Lithium-Ion Battery, Acs Applied Materials & Interfaces, 6 (2014) 9890-9896. [42] T. Fan, W. Zeng, Q. Niu, S. Tong, K. Cai, Y. Liu, W. Huang, Y. Min, A.J. Epstein, Fabrication of high-quality graphene oxide nanoscrolls and application in supercapacitor, Nanoscale Research Letters, 10 (2015). 24

[43] M. Yan, F. Wang, C. Han, X. Ma, X. Xu, Q. An, L. Xu, C. Niu, Y. Zhao, X. Tian, P. Hu, H. Wu, L. Mai, Nanowire Templated Semihollow Bicontinuous Graphene Scrolls: Designed Construction, Mechanism, and Enhanced Energy Storage Performance, Journal of the American Chemical Society, 135 (2013) 18176-18182. [44] L. Zhang, L. Ji, P.-A. Glans, Y. Zhang, J. Zhu, J. Guo, Electronic structure and chemical bonding of a graphene oxide-sulfur nanocomposite for use in superior performance lithium-sulfur cells, Physical Chemistry Chemical Physics, 14 (2012) 13670-13675. [45] J. Zhang, Z.M. Dong, X.L. Wang, X.Y. Zhao, J.P. Tu, Q.M. Su, G.H. Du, Sulfur nanocrystals anchored graphene composite with highly improved electrochemical performance for lithium-sulfur batteries, Journal of Power Sources, 270 (2014) 1-8. [46] W. Zhang, P. Zuo, C. Chen, Y. Ma, X. Cheng, C. Du, Y. Gao, G. Yin, Facile synthesis of binder-free reduced graphene oxide/silicon anode for high-performance lithium ion batteries, Journal of Power Sources, 312 (2016) 216-222. [47] H.-K. Jeong, Y.P. Lee, R.J.W.E. Lahaye, M.-H. Park, K.H. An, I.J. Kim, C.-W. Yang, C.Y. Park, R.S. Ruoff, Y.H. Lee, Evidence of Graphitic AB Stacking Order of Graphite Oxides, Journal of the American Chemical Society, 130 (2008) 1362-1366. [48] Y. Xu, K. Sheng, C. Li, G. Shi, Highly conductive chemically converted graphene prepared from mildly oxidized graphene oxide, Journal of Materials Chemistry, 21 (2011) 7376-7380. [49] H. Wang, J.T. Robinson, X. Li, H. Dai, Solvothermal Reduction of Chemically Exfoliated Graphene Sheets, Journal of the American Chemical Society, 131 (2009) 25

9910-9911. [50] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon, 45 (2007) 1558-1565. [51] Z.Q. Li, L.W. Yin, Nitrogen-Doped MOF-Derived Micropores Carbon as Immobilizer for Small Sulfur Molecules as a Cathode for Lithium Sulfur Batteries with Excellent Electrochemical Performance, Acs Applied Materials & Interfaces, 7 (2015) 4029-4038. [52] S. Niu, W. Lv, C. Zhang, Y. Shi, J. Zhao, B. Li, Q.-H. Yang, F. Kang, One-pot self-assembly of graphene/carbon nanotube/sulfur hybrid with three dimensionally interconnected structure for lithium-sulfur batteries, Journal of Power Sources, 295 (2015) 182-189. [53] G. Zhou, L.-C. Yin, D.-W. Wang, L. Li, S. Pei, I.R. Gentle, F. Li, H.-M. Cheng, Fibrous Hybrid of Graphene and Sulfur Nanocrystals for High-Performance Lithium– Sulfur Batteries, ACS Nano, 7 (2013) 5367-5375. [54]

R.

Elazari,

G.

Salitra,

A.

Garsuch,

A.

Panchenko,

D.

Aurbach,

Sulfur-Impregnated Activated Carbon Fiber Cloth as a Binder-Free Cathode for Rechargeable Li-S Batteries, Advanced Materials, 23 (2011) 5641-5644. [55] M.C. Li, H.H. Song, X.H. Chen, J.S. Zhou, Z.K. Ma, Phenolic resin-grafted reduced graphene oxide as a highly stable anode material for lithium ion batteries, Physical Chemistry Chemical Physics, 17 (2015) 3250-3260. [56] N. Li, M. Zheng, H. Lu, Z. Hu, C. Shen, X. Chang, G. Ji, J. Cao, Y. Shi, 26

High-rate lithium-sulfur batteries promoted by reduced graphene oxide coating, Chemical Communications, 48 (2012) 4106-4108. [57] L. Yuan, X. Qiu, L. Chen, W. Zhu, New insight into the discharge process of sulfur cathode by electrochemical impedance spectroscopy, Journal of Power Sources, 189 (2009) 127-132. [58] W. Li, Q. Zhang, G. Zheng, Z.W. Seh, H. Yao, Y. Cui, Understanding the Role of Different Conductive Polymers in Improving the Nanostructured Sulfur Cathode Performance, Nano Letters, 13 (2013) 5534-5540. [59] K. Han, J.M. Shen, S.Q. Hao, H.Q. Ye, C. Wolverton, M.C. Kung, H.H. Kung, Free-Standing Nitrogen-doped Graphene Paper as Electrodes for High-Performance Lithium/Dissolved Polysulfide Batteries, Chemsuschem, 7 (2014) 2545-2553.

27

Figure captions Fig.1. Schematic diagram of the fabrication process of RGO/SNP and RGO/SNS. Fig.2. (a) and (b) SEM images of RGO nanoscrolls; b is the enlarged image of a. (c) Typical TEM image of the RGO nanoscrolls. (d) Zoom-in TEM image of the rectangle area in (c). Fig.3. (a) and (b) SEM images of RGO/SNP nanoscrolls at different magnification. (c) and (d) TEM images of the RGO/SNP nanoscrolls. (e) Zoom-in TEM image of the selected rectangle region in (c), and (f) energy dispersive spectroscopic (EDS) image of the selected rectangle region in (e). Fig.4. (a) SEM and (b and c) TEM images of RGO/SNS. (d) HAADF-STEM characterization of the RGO/SNS material with the elemental mapping of C and S (inset) for the rectangle region. Fig.5. (a) XRD patterns of S nanoparticles, RGO/SNP nanoscrolls and RGO/SNS. (b) Raman spectra for graphite, GO, RGO nanoscrolls, RGO/SNP nanoscrolls and RGO/SNS. (c) XPS survey spectra of RGO/SNS. (d) High resolution spectrum of S 2p for RGO/SNS. Fig.6. (a) The CV curves of the first four cycles of RGO/SNS electrode at a scan rate of 0.05 mV·s-1. (b) Cyclic performance of RGO/SNS electrode and RGO/SCP electrode. (c) Charge/discharge voltage profiles of as-prepared RGO/SNS electrode at different C rates, and (d) rate capability of RGO/SNS electrode at various cycling rates ranging from 0.2 C to 2 C. 28

Fig.7. (a) Cyclic performance and (b) Charge/discharge voltage profiles of the RGO/SNS electrode at a high rate of 2 C (the first two cycles were carried out at a current density of 0.2 C), and the inset shows the zoom-in discharge curves. (c) Nyquist plots of RGO/SNS electrode before cycling and after 1, 5, 50 cycles from 10 kHz to 10 mHz in the fully charged state.

29

Fig. 1

Sonication Shock cooling / Freeze drying

RGO S nanoparticles (SNP)

Annealing

GO

RGO/SNS

RGO/SNP

30

Fig. 2

31

Fig. 3

(b)

(d)

(e)

(c)

(f) S

Intensity (a.u.)

(a)

0

C O Cu 1

2

3

Energy (keV)

32

4

5

Fig. 4

33

Fig.5

(b)

(a)

D D

10

20

30

Intensity (a.u.)

Intensity (a.u.)

RGO/SNS

40

RGO/SNS

RGO/SNP nonoscrolls S nanoparticles 10

20

30

40

50

60

G

RGO/SNP nonoscrolls

RGO nanoscrolls GO Graphite

300

70

600

2 Theta (degree)

900

1200

1500

Raman shift /cm

(c)

1800

-1

(d)

Intensity (a.u.)

S 2p

Intensity (a.u.)

C 1s

O 1s S 2s 600

500

400

300

S 2p

200

100

S-S S-S sulphate species

S-O

S-O

170

168

166

164

Binding Energy (eV)

Binding Energy (eV)

34

162

Fig. 6

(b) 1st 2nd 3rd 4th

1.2

Current (mA)

-1

1.6

Specific Capacity(mAh·g )

(a)

0.8 0.4 0.0

-0.4 -0.8 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

1500 1200 900

RGO/SNS

600

RGO/SCP 300 0

0 10 20 30 40 50 60 70 80 90 100

Voltage (V)

Cycle Number

Voltage (V)

0.5C 0.2C 2C 1C

2.7 2.4 2.1 1.8 0

200

400

600

1800

-1

3.0

Specific Capacity(mAh·g )

(d)

(c)

800 1000 1200 -1

Specific Capacity(mAh·g )

35

Discharge Charge

1500 1200 0.2C

0.5C 0.2C 0.5C 1C 2C 1C

900 600 300 0

0

5

10 15 20 25 30 35 40

Cycle Number

1500

100

-1

Specific Capacity(mAh·g )

(a)

Charge Discharge Coulombic efficiency

0.1C 1000

500

80 60 40

2C

20 0

0

10

20

30

40

50

60

70

80

0 100

90

Cycle Number (b)

2.7

2.1 2.0 1.9 1.8 1.7 0

2.4 2.1 1.8 0

before cycle 1st 5th 50th

100

Z''(ohm)

Voltage (V)

(c) 120

5th 10th 30th 50th 100th

3.0

300

600

80 60 40 20

100 200 300 400 500 600 700 -1

Specific Capacity(mAh·g )

0

0

20

40

60

Z' (ohm)

36

80

100

120

Coulombic Efficiency (%)

Fig.7