Three-dimensional graphene hollow spheres with high sulfur loading for high-performance lithium-sulfur batteries

Accepted Manuscript Title: Three-dimensional graphene hollow spheres with high sulfur loading for high-performance lithium-sulfur batteries Author: Zhen Wu Wei Wang Yanting Wang Chen Chen Kelun Li Gangjin Zhao Chunyu Sun Wenjian Chen Lubin Ni Guowang Diao PII: DOI: Reference:

S0013-4686(16)32619-6 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.072 EA 28544

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-9-2016 11-11-2016 11-12-2016

Please cite this article as: Zhen Wu, Wei Wang, Yanting Wang, Chen Chen, Kelun Li, Gangjin Zhao, Chunyu Sun, Wenjian Chen, Lubin Ni, Guowang Diao, Three-dimensional graphene hollow spheres with high sulfur loading for high-performance lithium-sulfur batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.072 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.

Three-dimensional graphene hollow spheres with high sulfur

loading for high-performance lithium–sulfur batteries

Zhen Wu, Wei Wang, Yanting Wang, Chen Chen, Kelun Li, Gangjin Zhao, Chunyu Sun,

Wenjian Chen, Lubin Ni,* Guowang Diao,*

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002,

Jiangsu, People’s Republic of China.

*Corresponding Author. Email address: [email protected] (L. Ni). [email protected] (G.

Diao).

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ABSTRACT: Lithium-sulfur batteries have currently attracted wide interest due to their high theoretical capacity, but the practical applications are being hampered by capacity decay, mainly attributed to the polysulfide shuttle. Here, we have designed a novel three-dimensional (3D) nanostructure of graphene hollow spheres (HGs) as the sulfur host. The 3D HGs were successfully prepared via a self-assembly method of wrapping graphene oxide (GO) on SiO2 spheres, and then followed by carbonization and etching of the SiO2. The impregnation of sulfur into the hollow graphene spheres lead to obtaining the HGs/S cathode, which reaches up a high sulfur loading of 90 wt% in the composite (72 wt % in the whole cathode). The HGs/S cathode material remains a high discharge capacity of 810 mAh g-1 after 200 cycles at 0.5 C rate. Furthermore, it demonstrates a low capacity-decay rate of 0.083 % per cycle after 600 cycles at 1 C rate. Compared with pristine reduced graphene oxide/sulfur composites (RGO/S), the as-prepared 3D self-assembled graphene hollow spheres HGs/S exhibit significantly improved electrochemical performances in terms of high specific capacity, remarkable rate capability and excellent cycling stability. These synergistic effects are achieved by more effective 3D ion/electron transport pathways, and efficient confinement of polysulfide dissolution and shuttling.

Keywords: lithium-sulfur battery, graphene, hollow spheres, high sulfur content.

1. Introduction Due to the increasing demand for electronics devices and electric vehicles, lithium-sulfur (Li-S) batteries as a promising energy storage system have recently attracted a large amount of attention.[1−2] Li-S batteries deliver a high theoretical specific capacity of 1675 mAh g-1, 2

which is about five times higher than those of available lithium-insertion compound electrodes (e.g., layered LiCoO2, and olivine LiFePO4 cathodes).[3−6] In addition, sulfur is non-toxic and earth abundant, which also make it an attractive electrode material for energy storage. However, the practical applications of lithium−sulfur batteries are still impeded by the low electrical/ionic conductivities of sulfur, dissolution of intermediate lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) and large volume change of sulfur cathode during charge/discharge. All these drawbacks mainly cause the low Coulombic efficiency, low utilization of sulfur and rapid capacity decay. To overcome the dissolution of lithium polysulfides (LiPS), great efforts have been devoted to avoid the shuttle effect by physically confining the polysulfides (PS) within the cathode structure. Initially, carbonaceous materials are regarded as the ideal choice for loading sulfur, such as mesoporous carbons, hollow porous carbon spheres, carbon nanotubes/fibers, etc.[7−12] which can not only significantly increase the conductivity of sulfur, but also can alleviate the dissolution of polysulfides into organic liquid electrolytes. Subsequently, some polar materials have been proven to strongly adsorb and entrap polysulfides by forming chemical bonds with them in recent years. This type of polar compounds including metal oxides,[13−15] N-doped graphene/carbon matrixes,[16−19] and metal−organic frameworks[20] have been systematically studied so far, significantly suppressing the polysulfide shuttle at the molecular level. However, they are hard to achieve high sulfur-loading cathode with an ultrahigh surface area and enough active sites for the LiPS adsorption.[21] Due to high electrical conductivity, extraordinary mechanical properties and ultrahigh theoretical specific surface area (2630 m2 g-1)，graphene is currently considered as one of the most efficient sulfur hosts for Li-S batteries.[22] For example, Zhao et al. reported an intrinsically unstacked double-layer graphene via template-directed chemical vapour 3

deposition (CVP) and then impregnating sulfur into abundant mesopores, which showed excellent high-rate performance.[23] Recently, a graphene-based dual-spatial reservoir exhibited the improved cycling stability developed by Zhang et al., because the micropores could hold redundant polysulfides effectively.[24] Nevertheless, the physical confinement of polysulfides within these open porous graphene materials has been still limited and give rise to rapid capacity decay in the subsequent cycles. Moreover, the pristine graphene-based materials cannot accommodate a high mass loading of sulfur. Very recently, Liu et al. have combined graphene sheets with hollow carbon sphere as a high-performance Li–S battery cathode host in order to enhance the confinement of polysulfides and build a 3D conductive network in the electrode. This 3D nanostructure of graphene composites showed promising potential in teams of the high Coulombic efficiency, superior rate capability and excellent cycling performance.[25] Herein, we introduce a novel bottom-up approach for the fabrication of hollow graphene nanospheres cluster (HGs) as sulfur hosts in Li-S batteries. First, the core-shell SiO2@GO spheres with a diameter of 200 nm were prepared by adjusting the ionic strength in HCl aqueous solution, resulting in that GO film tightly wrapped on SiO2 spheres. Second, carbonization followed by HF etching of silica templates produced uniform hollow graphene nanospheres (HGs) with 3D conductive interlinked networks.[26] Finally, the sulfur is highly impregnate into the 3D nanostructure by melt-diffusion method to get the core-shell HGs/S cathode. In comparison with pristine reduced graphene oxide/sulfur composites (RGO/S), the hollow graphene nanosphere composites HGs/S with 90% sulfur loading demonstrates very good electrochemical performance, with a good capacity retention of 82 % (910 mAh g-1) after 200 cycles at a 0.2 C rate. The capacity decay over 200 cycles at 0.5 C and even over 600 cycles at 1C are found to be 0.03% and 0.08% per cycle, respectively.

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Experimental Section 2.1 Synthesis of Graphene Oxide. Graphene Oxide was prepared by a improved Hummers method.[27] Firstly, 3 g of graphite powder and 18 g KMnO4 were placed in a three-necked flask. A 9:1 mixture of concentrated H2SO4/H3PO4 (360 : 40 mL) was added, producing a slight exotherm to 35-40 °C. Then the mixture was heated to 50 °C and stirred for another 12 h. After cooling to room temperature, the reaction was poured onto ice (400 mL) with 30% H2O2 (3 mL). Afterwards, the suspension was centrifuged, and washed with 30% HCl and deionized water for several times. Finally, the resulting suspension was filtered and vacuum-dried overnight at 80 °C. 2.2 Synthesis of SiO2 Spheres. Silica spheres were synthesized according to the previous literature.[28] Typically, 5.0 mL of ammonia (32 wt%) was added into the mixture of water (34 mL) and ethanol (184 mL). 7 mL of tetraethylorthosilicate (TEOS) was slowly injected into the above solution and stirred for 2 hour at room temperature. The silica colloid were collected by centrifugation, and washed several times. 2.3 Synthesis of Hollow Graphene Spheres HGs. SiO2 particles were dispersed in water (100 mL). For Graphene oxide coating, the prepared solution of SiO2 spheres was mixed with 50 mL GO aqueous dispersion (4 mg/mL) under magnetic stirring. The addition of concentrated HCl solution (9 mL) to the mixture solution led to adjust the ionic strength in solutions. After stirring for 0.5 h, the mixture was filtered and vacuum-dried at 80 oC for 24 h. Afterwards, the obtained SiO2@GO composite calcined at 1000 °C for 2 h in Argon atmosphere and HF (10 wt%) etching, in order to achieve hollow graphene spheres. 2.4 Synthesis of HGs/S and RGO/S composites. To prepare the HGs/S composite, the synthesized hollow graphene spheres were mixed with sublimed sulfur in a mass ratio of MHGs : MS = 10 : 90, than heated to 155 °C for 20 h in sealed Teflon autoclave. The pristine 5

reduced graphene oxide/sulfur composite (RGO/S) was synthesized in the similar method, except that RGO and S in a mass ratio of 20: 80. 2.5 Characterization. TEM (Transmission Electron Microscopy) was conducted on a Philips TECNAI-12 instrument. SEM was carried out with Hitachi S-4800 (Japan). High-resolution TEM (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were conducted using a FEI Tecnai G2 F30 STWIN (USA) operating at 200 kV. X-ray powder diffraction (XRD) data were obtained with a graphite monochromator and Cu Kα radiation (λ = 0.1541 nm) on a D8 advance super speed powder diffractometer (Bruker). The thermogravimetric analyses (TG) were collected with a Netzsch TG209 F1 instrument with a 10 °C min-1 from 50 to 500 °C in flowing nitrogen atmosphere. Cyclic voltammetry (CV) measurements were performed using an electrochemical workstation (CHI660 E, Chenghua, China) at a scan rate of 0.1 mV s−1 between 1.7 and 2.8 V. The nitrogen adsorption isotherm was obtained at 77 K using a Beishide 3H-2000PS2 pore size analysis instrument. The average pore sizes were determined by the Barrett−Joyner−Halenda (BJH) method from the adsorption isotherm. Conductivity was measured using S2T-2A four point probe (Suzhou Tongchuang Electronics). 2.6 Electrochemical measurements. For the preparation of working electrode, a mixture of 80 wt% of active material, 10 wt% Super-P and 10 wt% PVDF was added in a N-methyl pyrrolidone (NMP) to form a homogeneous slurry. The slurry was coated onto carbon paper with the sulfur loading of 2-2.5 mg/cm2. The electrode film was dried in an oven at 60 °C overnight, and cut into round disks with a diameter of 15 mm. The 2032-type coin cells were assembled using Celgard 2300 membrane as separator and Li metal as anode. The electrolyte was composed of 0.5 M LiCF3SO3 and 0.5 M LiNO3 in a mixture of 1,3-dioxolane (DOL) and dimethoxymethane (DME) (1:1, v/v). Galvanostatic discharge-charge tests were performed on a Neware Battery Measurement System (Neware, China) with cutoff voltages of 2.8-1.7 V. 6

The current density was based on the weight of sulfur (1 C = 1675 mA g-1), and the specific capacity was calculated on the mass of sulfur.

3. Results and discussion The synthesis process of the HGs-S85 composite is illustrated in Fig. 1a. The SiO2 spheres used as a hard template were mixed with graphene aqueous dispersion. Graphene oxide (GO) would tend to lose electrostatic repulsive force in high concentration ionic solutions and precipitate out with GO coating on the outer surface of SiO2 sphere, [29] when moderate concentrated hydrochloric acid as an ionic solution was added in the mixture. Therefore, GO wrapped SiO2 sphere was fabricated by engineering the ionic strength in HCl aqueous solution, leading to the formation of core-shell structure of SiO2@GO composite. Next, the GO shell was carbonized under an argon atmosphere and the SiO2 was then removed by hydrofluoric acid solution, thus creating the hollow graphene spheres (HGs). Finally, sulfur was impregnated into pre-synthesized HGs hosts by the melt-diffusion method. The HGs/S composite has the following benefits compared to normal graphene-based materials as schematically illustrated in Fig. 1b: (1) the HGs provides enough space for a high sulfur loading for high-rate lithium-sulfur batteries. (2) Closely linked, the HGs/S supplies an efficient conductive network for the electron transportation and then promotes the utilization rate of sulfur. (3) Sulfur is contained within the interior of the pore structure of HGs/S, which is primarily responsible for enhancing polysulfides confinement. (4) The sufficient empty space in the HGs/S is present to allow for volume expansion of sulphur during lithiation, which can maintain its structural integrity and avoid loss of electrical contact with the current collector.

The scanning electron microscopy (SEM) and transmission electron microscope (TEM) 7

were carried out to confirm the core-shell nanostructure of the SiO2@GO composite. As displayed in Figs. 2a and 2d, SiO2 spheres with a uniform diameter of ~ 200 nm were tightly coated by thin graphene sheets. After treatment in aqueous HF solution, the hollow graphene spheres shell clearly appeared as the SiO2 were completely etched and removed (Figs. 2b and 2e). These hollow spheres are closely linked to construct 3D conductive network with efficient space-packing arrangement which are advantageous for the cycling stability of cathode materials. Sulfur loading into hollow graphene spheres was carried out by a classical melt-diffusion method. As shown in Fig. 2c, there is no obvious sulfur deposition on the wall of graphene spheres, indicating that sulfur has been completely infiltrated into the void space of the HGs. Because sulfur is heavier than graphene, the dark region from TEM image can be ascribed to sulfur as shown in Fig. 2f. Energy dispersive X-ray (EDX) elemental mapping also give visible evidence of the sulfur diffusion (Figs. 2g-2i). These features verified that sulfur was effectively confined within the hollow graphene spheres nanostructure. Moreover, the morphology of RGO/S reference composite was also characterized further by SEM, TEM, and EDX (Figs. S1-S2). Comparing with the 3D HGs/S composite, the RGO/S composite displayed carbon- sulfur layered structure, in which the sulfur particles were distributed freely on the surface of the rGO nanosheets (Figs. S1-S2). A nitrogen adsorption/desorption measurement is very important for calculating the pore size distribution in porous graphene materials. The pore distribution of HGs/S and RGO/S composites (Fig. S3) both showed mesopores in 3.8 and 5.4 nm, respectively, further indicating that 3D HGs/S composite can be used as polysulfide diffusion inhibitor by confining polysulfides within these smaller mesopores. To investigate the electrical conductivities of HGs/S and RGO/S composites by four probe method, HGs/S shows higher electrical conductivity (1.60 S cm-1) than RGO/S (0.047 S cm-1), confirming the existence of 3D conductive network in the HGs/S composite.

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Thermogravimetric analysis (TGA, Fig. 3a) showed that 90% sulfur loading in HGs/S is higher than 80% in RGO/S composite. The X-ray diffraction pattern of HGs/S composite revealed the existence of elemental sublimed sulfur (Fig. 3b). Moreover, two broad diffraction peaks around 26° and 42° for HGs correspond to the (002) and (100) planes of graphene (RGO), which confirmed the presence of hollow graphene spheres (Fig. 3b).

In order to evaluate the electrochemical performance of the HGs/S composite as cathode material in applications of lithium-sulfur batteries, long-term cycling stability of HGs/S cathode is an essential prerequisite to realistic applications of Li-S batteries. The 2032 type of coin cells were fabricated using lithium foil as the anode, 100 L of 0.5 M lithium trifluoromethane sulfonate (LiCF3SO3) and 0.5 M LiNO3 in a mixed solvent of 1,3-dioxolane and 1,2-dimethoxyethane (DOL/DME, 1:1, v/v) as the electrolyte. The composites were mixed with carbon black (Super P) and binder polyvinylidene fluoride (PVDF) (80:10:10 by weight) onto a carbon paper current collector to prepare the cathode electrodes. The electrolyte/sulfur (E/S) ratio is around 25 mL g-1. The areal sulfur loading of Li‒HGs/S cell could be easily accessible to 2~2.5 mg cm-2. Fig. 4a shows the typical CV curves of HGs/S cathode in Li–S batteries for first 7 cycles at a sweeping rate of 0.1 mV s-1. In the first CV cycle, the cathodic peak at 2.25 V is attributed to the reduction of S8 to long-chain polysulfides (Li2Sx, 4 ≤ x≤ 8). The second cathodic peak at 2.01 V corresponds to the further reduction of the long-chain polysulfides to Li2S2/Li2S. The delithation process from Li2S to Li2Sx/S8 is responsible for the strong anodic peak at 2.41 V. Furthermore, two cathodic peak slightly shifted to higher potentials at 2.28 and 2.04 V when the scan cycle number increased, suggesting that the 3D conductive HGs network can cause a reduction of polarization during charging and discharging process and relatively good capacity retention. Next, we also investigated the rate performances of HGs/S at various current densities from 0.1 to 2 C (Fig. 9

4b). At 0.1 C, the initial specific capacity of HGs/S cathode was 1342 mA g−1, approximately up to 80.1% of theoretical capacity (1675 mAh g−1) (Fig. 4c). When the current rate was increased to 0.2, 0.5, 1C and 2 C, the reversible specific capacities were reduced gradually to 1130, 1030, 908 and 676 mA g−1, respectively (Fig. 4c). Noticeably, the original capacity of HGs/S composite can be largely recovered when the rate is abruptly switched back to 0.2 and 0.1C, indicating that the high stability of the electrode and structural reliability of the HGs/S composite. Fig. 4c shows the typical discharge–charge voltage profiles at various rates from 0.1 to 2 C in the potential range of 2.8–1.7 V. All the discharge profiles demonstrate two-plateau and the excellent electronic/ionic transport properties. As shown in Fig. 4d, the Li‒HGs/S cell also exhibits excellent cycling stability at 0.2 C in comparison with the RGO/S composite. The HGs/S composite delivered an initial discharge specific capacity of 1139 mAh g−1, and the capacity retention of 80 % was still retained after 200 cycles with a low capacity fade rate of 0.092% per cycle. The Coulombic efficiency remains above 95% throughout. Moreover, the obvious capacity fade in the first several cycles is possibly attributed to redistribution of the sulfur inside HGs during charging and discharging process. In contrast, the RGO/S composite electrode has worse physical confinements of polysulfides and suffers from rapid capacity decay, giving rise to lower capacity of 374 mA hg-1 and lower retention 32 % at 0.2 C after 200 cycles (Fig. 4d).

Next, we fabricated two transparent soft-pack cells based on HGs/S and RGO/S, respectively, in order to visually observe the polysulfide dissolution behavior in the cells. Because the electrolyte for Li-S batteries is colorless but lithium polysulfide presents dark yellow color. Photos of two transparent cells before discharge are shown in Fig. 5a-5b, respectively, and then we used two Li-S cells of HGs/S and RGO/S in series to light up a 10

light-emitting diode (LED) for 5 min (Fig. 5e). The electrolyte in the cell with the RGO/S cathode turns brown after discharge due to the dissolution of lithium polysulfide (Fig. 5c). In contrast, the cell with the HGs/S cathode remains colorless even after discharge, indicating that the 3D hollow graphene spheres could better immobilize sulfur and confine the polysulfide within closed mesopores (Fig. 5d). This also can be further verified by a self-discharge test (Fig. S4). As shown in Fig. S4, two cells assembled with the HGs/S and RGO/S electrodes were first discharged to 720 mAh g-1 at 0.2 C (the coexistence of LiPS and Li2S2 at the middle of the third voltage plateau 2.1 V) and then rested for 50 h. Afterwards, the HGs/S electrode continued to deliver a higher capacity of 425 mAh g-1, whereas the RGO/S electrode only delivered a capacity of 285 mAh g-1.

To further demonstrate the structural advantages of the HGs/S composite, long-term cycling stability and Coulombic efficiency of the Li-S cells based on the HGs/S nanocomposite were also performed at practical level current density of 837.5 (0.5C), 1675 (1C) mA g-1 (Figs. 6a-6b). The cell was first cycled at 0.2 C rate for four times, and then is able to deliver an initial discharge capacity of 870 mAh g−1 (areal capacity of 2.1 mAh cm-2, Fig. S5) when the rate is changed to 0.5 C. A specific capacity of around 810 mAh g−1 (areal capacity of 1.9 mAh cm-2, Fig. S5) was still obtained at 0.5 C after 200 cycles, corresponding to good capacity retention of 93.1% (capacity decay of only 0.03% per cycle, Fig. 6a). Over a long-term cycling of 600 cycles at 1C, it can still maintain a specific capacity of 390 mA h g -1 (areal capacity of 0.9 mAh cm-2, Fig. S5) with a capacity decay as low as 0.08% per cycle, meanwhile an average Coulombic efficiency of 98 % was also achieved (Fig. 6b). In addition, because the total mass of the electrode materials actually includes HGs, conductive additives Super-P and binders PVDF (Mtotal = MHGs/S + MSuper-P + Mbinder), the specific capacity based on the total electrode mass can retain around 644 mAh g-1 (0.2C) and 584 mAh g-1 (0.5C) after 11

200 cycles, 273 mAh g−1 even at 1C over 600 cycles (Fig. S6-S8), which are still superior to all lithium-insertion compound electrodes.[30] Furthermore, the electrochemical performances for recently reported high-sulfur-loading carbon-sulfur composites are summarized in Table S1. The 3D HGs/S composite presents the high-level performance with the large discharge capacity, high-rate capability and long cyclability, which is highly comparable or even superior to their performances of some high-sulfur-loading carbon–sulfur composites.[31-36]

4. Conclusions In summary, we have designed and synthesized a novel structure of three-dimensional (3D) hollow graphene spheres (HGs) as an efficient sulfur host in improved performance Li-S battery. The SiO2 nanospheres were successfully wrapped up by graphene oxide (GO) film by engineering the ionic strength in solutions and then followed by carbonization and etching of the SiO2, leading to build uniform hollow graphene nanospheres (HGs) with 3D conductive interlinked networks. The novel 3D HGs nanostructure is used to highly impregnate sulfur via melt-diffusion method to form the core-shell HGs/S cathode. Rationally designed and controlled nanostructured HGs/S has significant functional benefits for Li-S cell: (1) The HGs provide enough space for a high sulfur-loading (90% wt) for high-rate lithium-sulfur batteries. (2) The uniform 3D conductive network in the HGs/S can construct highly-efficient 3D electron transfer pathways and ion diffusion channels. (3) The HGs/S hollow nanosphere can effectively confine the polysulfides physically within the interior of the pore structure of HGs/S, thus not only controlling the volume expansion of sulfur and polysulfide shuttle effect during the lithiation, but also maintain its structural integrity and avoiding loss of electrical contact with the current collector. Compared with pristine reduced graphene oxide/sulfur composites (RGO/S), HGs/S exhibit significantly improved electrochemical performances in terms of high specific capacity, remarkable rate capability and excellent cycling stability. 12

High discharge specific capacities of 1342, 1130, 1030, 908, and 676 mAh g-1 can be achieved at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. Furthermore, a low capacity decay rate of 0.03 % per cycle after 200 cycles at 0.5 C rate and 0.08 % per cycle after 600 cycles at 1.0 C rate have also been found. In short, we believe that the 3D HGs/S nanostructure with high loading of sulfur (90 wt% in the HGs composite and 72 wt % in the total electrode materials) not only results open up a rational approach to the synthesis of shape-controlled graphene materials, but also shed new light on the practical application of high-energy density and long-life lithium-sulfur batteries .

Acknowledgements The authors acknowledge the support of the National Natural Science Foundation of China (Grant no. 21273195, no. 21401162), cooperation fund from Yangzhou Government and Yangzhou University (Grant no. 2012038-9), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant no. 14KJB430024), the Jiangsu Provincial Postdoctoral Sustentation Fund (Grant no. 1402015B). Financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Natural Science Foundation of Education Committee of Jiangsu Province (no. 12KJB150023) is gratefully acknowledged. We also thank the testing center of Yangzhou University for SEM and TEM measurements.

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Fig. 1 a) Schematic illustration of the synthetic process of the HGs/S composite. b) Advantages of the HGs/S composite over RGO/S.

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Fig. 2 a) SEM and TEM images of a, d) core-shell SiO2@GOspheres. b, e) HGs. and c ,f) HGs/S. g) Dark field STEM image of HGs/S composite. h) EDX elemental mapping on C, and S.

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Fig. 3 a) TGA curves of RGO/S and HGs/S composites. b) XRD patterns of sublimed sulfur, HGs and HGs/S.

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Fig. 4 a) Cyclic voltammetry curves of HGs/S in the voltage range of 1.7-2.8 V at a sweeping rate of 0.1 mV s-1. b) Rate performance of HGs/S at different C-rate, ranging from 0.1C to 2C. c) Voltage profiles at various C rates. d) Cycling capacity and Coulombic efficiency of HGs/S in comparison with RGO/S at a current density of 0.2 C.

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Fig. 5 Photos of the transparent soft pack cells of a) HGs/S composite before discharge. b) RGO/S composite before discharge. c) HGs/S composite after discharge. d) RGO/S composite after discharge. e) Light-emitting diode (LED) demonstration: two Li-S cells of HGs/S and RGO/S in series are discharged to light up a light-emitting diode (LED) for 5 min.

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. Fig. 6 Cycling performance (red curve) and Coulombic efficiency (blue curve) of of HGs/S composite at a current rate of a) 0.5 C over 200 cycles. b) 1 C over 600 cycles.

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