Reduced graphene oxide-hollow carbon sphere nanostructure cathode material with ultra-high sulfur content for high performance lithium-sulfur batteries

Reduced graphene oxide-hollow carbon sphere nanostructure cathode material with ultra-high sulfur content for high performance lithium-sulfur batteries

Accepted Manuscript Title: Reduced graphene oxide-hollow carbon sphere nanostructure cathode material with ultra-high sulfur content for high performa...

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Accepted Manuscript Title: Reduced graphene oxide-hollow carbon sphere nanostructure cathode material with ultra-high sulfur content for high performance lithium-sulfur batteries Author: Shuangke Liu Yujie Li Xiaobin Hong Jing Xu Chunman Zheng Kai Xie PII: DOI: Reference:

S0013-4686(15)30871-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.11.101 EA 26101

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

2-9-2015 10-11-2015 19-11-2015

Please cite this article as: Shuangke Liu, Yujie Li, Xiaobin Hong, Jing Xu, Chunman Zheng, Kai Xie, Reduced graphene oxide-hollow carbon sphere nanostructure cathode material with ultra-high sulfur content for high performance lithium-sulfur batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.11.101 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.

Reduced graphene oxide-hollow carbon sphere nanostructure cathode material with ultra-high sulfur content for high performance lithium-sulfur batteries

Shuangke Liu*[email protected], Yujie Li, Xiaobin Hong, Jing Xu, Chunman Zheng*[email protected] and Kai Xie

College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, 410073, China.

Graphical abstract

we report a rGO-HCS composite to encapsulate sulfur with high sulfur contents from 70 to 90 wt% as sulfur cathode. The rGO-HCS/S composite loading 90 wt% sulfur delivers a high discharge capacity of 860 mAhg-1 (corresponding to1300 Wh kg-1 based on mass of the whole cathode) and obtains a high capacity retention of 77% after 78 cycles at a high sulfur areal density of 3 mg cm-2.The results indicate the great potential of the rGO-HCS nanostructure as promising sulfur host loading high content of sulfur with high gravimetric and volumetric energy density.

Highlights

1. A rGO-HCS composite loading ultra-high sulfur content of 90wt% was prepared. 2. The high sulfur content offers high gravimetric and volumetric energy density. 3. The thick electrode with 3 mg cm-2 sulfur shows high energy density and good cycling stability. 4. The rGO-HCS nanostructure design promises excellent performance with high sulfur loading.

Keyword: lithium-sulfur batteries, sulfur cathode, ultra-high sulfur content, high-performance

Abstract: Lithium-sulfur (Li-S) batteries have been regarded as promising next generation rechargeable energy storage system, due to the high theoretical specific capacity and energy density. However, the low sulfur content in the cathode electrode greatly decrease the practical energy density of the Li-S batteries. Herein, we report a reduced graphene oxide-hollow carbon sphere (rGO-HCS) composite to encapsulate sulfur with high sulfur contents from 70 to 90 wt% as sulfur cathode. The rGO-HCS/S composite loading 90 wt% sulfur delivers the highest energy density of 1281 Wh kg-1 based on the mass of the whole cathode electrode, even at a high sulfur areal density of 3 mg cm-2, it could still reach a high discharge capacity of 860 mAhg-1 (corresponding to1300 Wh kg-1 based on mass of the whole cathode) and obtain a high capacity retention of 77% after 78 cycles.These results indicate the great potential of the rGO-HCS nanostructure as promising sulfur host loading high content of sulfur with high gravimetric and volumetric energy density.

1 Introduction Lithium-sulfur (Li-S) batteries have been regarded as one of the most promising rechargeable energy storage systems for next generation electric and plug-in hybrid vehicles, due to the high theoretical specific capacity of 1672 mAh g-1 and high energy density of 2600 wh kg-1 of sulfur[1]. Additionally, element sulfur is naturally abundant with low cost and environmentally benign. However, the practical application of Li-S batteries is facing some challenges regarding sulfur cathode: (1) the insulating characteristic of sulfur (5×10–30 S cm-1 at 25℃) induces the low capacity and poor rate performance, which usually needs a large amount of conductive materials to improve; (2) the dissolution and shuttle effect of long chain polysulfides in electrolyte lead to loss of active materials and deteriorate the cycling performance[5]; (3) the large volume changes during charge-discharge process result in structural pulverization of electrode and poor cycling performance[7].

In the past decade, great efforts have been paid to address these issues by encapsulating sulfur into conductive materials, such as porous carbon[8], hollow carbon spheres[12], carbon nanotubes[17], graphene[21], and conductive polymers[31]. These conductive matrixes with high electrical conductivity, large specific surface area and abundant porosity greatly improve the reversible capacity, cycling performance and rate capability of the sulfur cathodes. However, in most cases, the sulfur content of the cathode composites is usually less than 80 wt% [8]and rarely reaches 90 wt%[20]. Moreover, additional carbon agents and binders are introduced during the electrode fabrication process, which further decrease the sulfur content in the whole electrode. The large amounts of these conductive materials and binders are usually electrochemical inactive in sulfur cathode and inevitably reduce the energy density both in mass and volume of the whole electrode.

Generally, the higher sulfur content, the poorer electrochemical performance of the cathode materials based on sulfur, probably due to the poor electronic conductivity and large volume expansion during cycling of the electrodes. Increasing the sulfur content/loading in the cathode electrode as many as possible while keep good electrochemical performance is essential to improve both the gravimetric and volumetric energy density of the Li-S batteries.

Some recent work have made efficient attempts to encapsulate large amounts of sulfur from 80% to 90% into Ketjen Black[33], CNTs[20], graphene[25], and hollow carbon spheres[16] to improve energy density of the Li-S batteries. Wang et al[33] reported a multi-core/shell C-PANI/[email protected] composite with sulfur content of 87 wt% and it delivered a high discharge capacity of 835 mAhg-1 after 100 cycles at 0.2C rate. Cheng et al[20] mixed 90 wt% sulfur with alighed CNTs by ball milling and the obtained CNTs/sulfur composite offered a low discharge capacity of 611 mAhg-1 at 0.1C rate. Liu et al[26] prepared a [email protected] composite with high sulfur content of 91wt%, however, a low discharge capacity of 430 mAhg-1 was retained after 100 cycles at 0.2C rate. It is still a big chanllenge to obtain good electrochemical performance with high sulfur content.

Herein, we encapsulted sulfur into a reduced graphene oxide-hollow carbon sphere (rGO-HCS) composite with high sulfur content from 70 to 90 wt% by a simple melt-diffusion method. Sulfur was infiltrated into and distributed homogeneously among the rGO-HCS composite without aggregation even when the sulfur content is up to 90wt%. The rGO-HCS/S composite loading 90 wt% sulfur delivers the highest discharge capacity of 610 mAh g-1 (with energy density of 1281 Wh kg-1) based on the mass of the whole cathode electrode, much higher than that of the composites loading 70 wt% and 80 wt% sulfur. Moreover, even when the sulfur areal density

increases to 3 mg cm-2, the rGO-HCS/S composite loading 90 wt% sulfur could still reach a high energy density of 1300 Wh kg-1 at a power density of 240 W kg-1 and obtain a good capacity retention of 73.3% after 100 cycles. The ultrahigh content of sulfur in the rGO-HCS composite renders remarkable increase of both gravimetric and volumetric energy density, which was probably ascribed to the 3D conductive graphene networks, hierarchical porous structure with large pore volume, homegeneous distribution and efficient confinement of sulfur in the rGO-HCS nanostructure.

2 Experimental section Synthesis of rGO-HCS composite

The rGO-HCS composite was synthesized by a hydrothermal self-assembly method[35] followed by high temperature annealing and HF etching, which was reported previously by our group[36]. Briefly, the [email protected] nanospheres were prepared by a stober method[37] and the GO was synthesized by an improved Hummers method[38]. Then, 1.5g [email protected] nanoparticles were sonicated in 60 ml water for 3h, then 30ml well dispersed GO aqueous solution (10.0 mg ml-1) and 30ml ascorbic acid solution was added and stirred for 1h, finnally,the mixed solution was heated for 12h at 180℃ in a sealed autoclave to form a 3D [email protected] gel.The gel was freeze dried and then annealed at 900℃ for 2h under N2 protection, after that, the obtained powders were finely ground and etched in 10wt% HF solution to obtain the rGO-HCS composite.

Synthesis of rGO-HCS/Scomposites

The rGO-HCS/S composites were prepared by a facile melt-diffusion method. Breifly, the

rGO-HCS composite was mixed with sulfur powder in the expect ratio by finely grinding and then heated to 155℃ for 8h under N2 atmosphere. The obtained rGO-HCS/S composites were named as rGO-HCS-0.7S, rGO-HCS-0.8S, and rGO-HCS-0.9S according to the designed sulfur contents in the composites.

Material characterization

The structure of the rGO-HCS/sulfur composites were measured by XRD (SIEMENS D-500) using Cu Ka radiation, ranging from 10˚ to 60˚ at a step of 8˚ min-1. The micro morphologies of the obtained composites were studied using a field emission scanning electron microscope (HITACHI S4800, Japan). The nitrogen adsorption-desorption analysis was carried out at 77.3K on a V-Sorb 2800 equipment. The thermogravimetric analysis (TGA) was performed with a TGA-600 with a heating rate of 10℃ min-1 under a N2 atmosphere to confirm the sulfur contents in the composite.The tap density of the composites was determined by dividing the mass of the composite into its tap volume.

Electrochemical characterization

Electrochemical experiments were performed using 2016 type coin cells. The working electrode was prepared by mixing 80 wt% cathode materials with 12 wt% Super P and 8 wt% LA133 aqueous binder using water and isopropanol as the solvent. After mixing well, the slurry was pasted on Al foil and dried overnight at 60℃ in a vacuum oven. 0.5M lithium bistrifluoromethanesulfonylimide

(LiTFSI)

in

a

mixed

solvent

of

1,3-dioxolane

and

1,2-dimethoxyethane (DOL/DME, 1 : 1, v/v) with 0.5 M LiNO3, purchased from Fosai New Material Co., Ltd (Suzhou), was used as the electrolyte, lithium metal foil was used as the anode

and the polypropylene membranes (Celgard Inc.) were used as the separators. Galvanostatic charge-discharge measurements were performed using a battery tester (LAND CT-2001A, Wuhan, China) at room temperature in a potential range of 1.7~2.8 V (vs. Li+/Li) at current densities of different C rates (1 C=1672 mA g-1). Cyclic voltammetry (CV) was performed with an electrochemical workstation (CHI 660C) between 1.7 and 2.8 V at a sweep rate of 0.1 mV s-1. Electrochemical impedance (EIS) analyses were conducted using the same equipment from 100 kHz to 100 mHz.

The energy/power density was calculated based on the mass of the whole cathode electrode, including the rGO-HCS/S composite, carbon Super P, and the binder. The energy density was calculated on equation (1): E

ms CsV mt

ms is the mass of the sulfur in the cathode, Cs is the specific capacity based on sulfur , V is the average discharge voltage of the cathode (2.1V), mt is the total mass of the cathode.

3 Results and discusssion The rGO-HCS composite was prepared by a hydrothermal self-assmebly method followed by high temperature carbonization and HF ethching, which was reported in our previous work[36]. Sulfur was encapsulated into the rGO-HCS composite by a facile melt infiltration method under N2 protection to produce rGO-HCS/S composites, the sulfur contents can be easily controlled by tailoring the proportion of sulfur and the rGO-HCS composite. Fig. 1 displays the scanning electron microscope (SEM) images of the rGO-HCS, rGO-HCS-0.7S, rGO-HCS-0.8S, and

rGO-HCS-0.9S composites. From Fig. 1a, we can see that the hollow carbon spheres (HCS) with diameter of ~180 nm interconnect with the thin graphene networks, the HCS could effectively prevent the re-stacking of the graphene sheets and provide enough pore space to accommodate sulfur infiltration, while the graphene networks could act as the fast electronic conductive channels and flexible mechanical scaffolds. After finely grounding and annealing under 155℃, sulfur was infiltrated and distributed into the rGO-HCS composite. From Fig.1b-d, we could see that the rGO-HCS/S composites still remained the original morphology without bulk sulfur aggregations obeserved, indicating the complete encapsulation of sulfur into the rGO-HCS nanostructure. In addition, with the increase of sulfur content from 70 wt% to 90 wt%, more particles with different contrast are presented in the images, indicating more sulfur diffused into the HCS.

In order to understand the sulfur distribution in the rGO-HCS nanostructure, TEM and STEM analyses were performed. Fig. 2a clearly shows the hollow structure of the carbon spheres, and confirms that hollow carbon spheres are connected with thin graphene sheets. Moreover, the dark area inside the top HCS indicates a heavy element distribution, which should be attributed to sulfur, in agreement with the STEM image and corresponding elemental mappings (Fig. 2b-d). The strong signals of S element inside the top HCS (Fig. 2d) directly reveals sulfur could completely infiltrate into the HCS through micropores in the shells by capillary force. The S signals inside the other two HCSs is weaker and almost no signals appears on the graphene sheets, demonstrating less sulfur was infiltrate into the inner part of these HCSs or distributed on the graphene sheets.

The X-ray diffraction patterns in Fig. 3a confirmed the orthorhombic phase of sulfur in the

rGO-HCS/S composites. Thermogravimetric analysis (TGA) was performed under nitrogen (N2 ) atmosphere to determine the sulfur content in the rGO-HCS/S composites with different sulfur contents, as shown in Figure 3b. The weight loss between 200 and 300℃ is ascribed to the sublimation of sulfur, the sulfur contents in the rGO-HCS-0.7S, rGO-HCS-0.8S, and rGO-HCS-0.9S are calculated to be 71, 79.6 and 90 wt%, in agreement with the designed compositions.

Fig. 4a shows the N2 isothermal adsorption curves and Fig. 4b shows the pore size distribution of the rGO-HCS composite and the rGO-HCS/S composites. The pore size of the rGO-HCS composite ranges from micropores to mesopores, the low pressure (P/P0) adsorption curve symbolizes the existence of micropores, the following slop at medium pressure indicates the presence of mesopores and the high pressure loop reveals rich macropores in the composite. The rGO-HCS composite has a high Brunauer–Emmett–Teller (BET) specific surface area of 430.12 m2 g−1 and a large pore volume of 2.23 cm3 g−1. After sulfur impregnation, the N2 isothermal adsorption curves of the three samples with different sulfur contents become flat and lose the porous characters from micro to macro pores, indicating sulfur is infused into the pore structure during heatreatment.The BET specific surface area sharply decreased to 22.1, 14.0, and 2.0 m2 g−1 and the pore volume dropped rapidly to 0.265, 0.158, and 0.016 cm3 g−1 for rGO-HCS-0.7S, rGO-HCS-0.8S and rGO-HCS-0.9S, respectively. The inset of Fig. 4b indicates that the rGO-HCS-0.7S and rGO-HCS-0.8S still remain a tiny volume from mesopores to macropores while there are no mesopores and macropores distributed in the rGO-HCS-0.9S, indicating the pores among the rGO-HCS-0.9S are fully filled by sulfur. Accompanied by the decrease of the BET surface area, the tap density of the rGO-HCS/S composites increase from 0.31 to 1.05 g cm-3

when the sulfur content increases from 70 wt% to 90 wt%. It is noted that the tap density of rGO-HCS-0.9S (1.05 g cm-3) almost reaches that of the pure sulfur (1.11 g cm-3), indicating a high volume energy density of the rGO-HCS-0.9S composite.

To study the electrochemical performance of the high-sulfur-content composites, the three rGO-HCS/S composites were assembled into 2016 type coin cells with lithium foil as the anode. The areal density of sulfur is about 1 mg cm-2. Fig. 5a shows the charge/discharge potential profiles of the three samples at a current density of 0.1C. All curves display two typical plateaus at 2.3V and 2.1V during discharge process, which correspond to the formation of long-chain Li2Sn (2
after 100 cycles. Both the three electrodes show high and stable Coloumbic efficiency up to 96% during the 100 cycles expect for the initial cycles (see Fig. S2), indicating the effectiveness of the rGO-HCS nanostructure design to suppress polysulfide shuttling. The overlap of the CV curves at different cycles (Fig. S2) further indicates the rGO-HCS-0.9S composite electrode with ultra-high sulfur content still has excellent electrochemical stability.

Though the rGO-HCS-0.7S composite electrode shows the best electrochemical performance if the capacity is calculated on mass of sulfur, however, if the capacity is calculated on mass of the whole electrode, the rGO-HCS-0.9S will demonstate the highest discharge capacity of ~610 mAhg-1 and energy density of 1281 Wh kg-1, much higher than those of rGO-HCS-0.7S (~500 mAhg-1 and 1050 Wh kg-1) and rGO-HCS-0.8S (530 mAhg-1 and 1113 Wh kg-1), and also better than some previous studies with 90 wt% sulfur loading [20].Considering the rGO-HCS-0.9S composite has the highest tap density (almost reaches that of pure sulfur), we could conclude that the rGO-HCS-0.9S composite delivers the highest energy density both on mass and volume. This reveals the importance of high sulfur content on achieving both high gravimetric and volumetric energy density for Li-S batteries.

To further study the electrochemical performance of the rGO-HCS/S composites electrodes, the rate capability was tested upon cycling from 0.1C to 4C rate, as shown in Fig. 5c. The rGO-HCS-0.7S shows the best rate performance, with discharge capacities of 1065, 910, 858, 828, and 803 mAh g-1 at 0.1, 0.2, 0.5, 1C, and 2C rate, respectively. Even at a high rate of 4C, it still delivers a high discharge capacity of 735 mAh g-1.With the increase of sulfur content, the discharge capacity drops with different extents at different C rates.The rGO-HCS-0.8S and rGO-HCS-0.9S show discharge capacities of 944, 914 mAh g-1 at 0.1C, respectively.The

capacities were 568 and 383 mAh g-1 at 4C rate, much remarkable difference compared to that at 0.1C rate. This indicate the sulfur content significantly influences the rate performance of the electrodes, especially at high C rates. It is well known that the rate performance is mainly rest with the electronic and ionic condutivity of the electrode materials, with the increase of sulfur content, the rGO-HCS/S composite has poorer electronic conductivity and limited ion transport channels, resulting a strong polarization at large current density and a notable drop of the rate performance. Fig. 5d shows the electrochemical impedance spectroscopy (EIS) of the three electrodes, all spectras demonstrate two depressed semicircles in the high and middle frequency regions followed by an inclined line in the low frequency region. The plots are fitted with equivalent circuit model (Fig. S5), in which the high frequency semicircle represents charge transfer resistance, the middle frequency semicircle is attributed to the formation of insoluble polysulfide species[40]. The results confirm that the rGO-HCS-0.9S electrode has the largest solution, charge transfer and insoluble polysulfides formation resistances, indicating the lowest electronic and ionic conductivity of the rGO-HCS-0.9S composite compared to the other two electrodes.

Though the rGO-HCS-0.9S composite electrode has the poorest rate performance among the three samples, considering it has a high sulfur content of 90 wt%, it still delivers decent discharge capacities of 705 mAh g-1 at 1C and 608 mAh g-1 at 2C, superior to that of recently reported composite cathodes [20,26], which also possess high sulfur content of ~90 wt%. Previous studies[40] indicate that a three dimentional porous current collector could effectively improve the rate performance. In the rGO-HCS/S composite with high sulfur content, the 3D conductive graphene networks could provide rapid electron transportation and effectively improve the rate performance.

Despite the rGO-HCS-0.9S composite has a high sulfur content of 90 wt%, the sulfur areal density of the rGO-HCS-0.9S electrode is only ~1 mg cm-2 in the above discussions, in order to further increase the energy density of the Li-S batteries, we pasted a thick rGO-HCS-0.9S electrode with sulfur areal density of ~3 mg cm-2. Fig. 6a shows the comparative rate performance of the two rGO-HCS-0.9S electrodes with sulfur areal density of ~1mg cm-2 and ~3mg cm-2, named as typcial electrode and thick electrode. The typical electrode and the thick electrode show discharge capacities of ~800 mAhg-1 at 0.1C rate, the typical electrode shows discharge capacities of ~800, ~760, ~700 and ~650 mAhg-1 at 0.2C, 0.5C, 1C and 2C rate, while the thick electrode delivers discharge capacities of ~750, ~550, ~150 and ~100 mAh g-1 at 0.2C, 0.5C, 1C and 2C rate. These results indicate that the thickness of the electrode has small impact on the discharge capacity at low C rate (0.1C), however, with increase of C rates (0.2~4C), the discharge capacity decrease rapidly for the thick electrode. This is because the thicker of the electrode, the longer diffusion length of ions and the larger resistance of the battery, thus the rate performance drops rapidly. Fig.6b shows the cycling performance and corresponding Coloumbic efficiency of the thick electrode at 0.1C rate, after the initial activation process, the thick electrode reaches the highest discharge capacity of 860 mAh g-1 at the 5th cycle and maintains 662 mAh g-1 at the 78th cycle with a high capacity retention of 77%. Moreover, the thick electrode demonstrates high Coloumbic efficiency of 97% up to 78 cycles. Fig. S4 shows the cycling stability of the rGO-HCS-0.9S thick electrode at 0.1C and 0.2C, respectively.The discharge capacity were 590 mAhg-1 after 90 cycles at 0.2C rate with a capacity retention of 73.3%. The thick electrode demonstrates comparable discharge capacities and cycling stability at low C rate compared to the typical electrode.The energy density of the thick electrode was 1300 wh kg-1 at a power density of

240 W kg-1 (0.1C), which were calculated based on the whole mass of the cathode electrode according to equation (1) . The high sulfur content of the rGO-HCS-0.9S affords dense cathode electrode with high energy density, which are several times higher than commercial cathode materials of Li-ion batteries. The high energy density and good cycling stability of the rGO-HCS-0.9S composite suggest that the novel rGO-HCS nanostructure could not only offer hierarchical pores with large pore volume to load ultrahigh content of sulfur, but also provide a 3D conductive graphene network for rapid electron and ion transfer, efficient confinement of long-chian polysulfide and suppression of volume change.

4 Conclusions In conclusion, the composites with high sulfur contents from 70wt% to 90wt% were prepared by encapsulate sulfur into the rGO-HCS composite via a simple melt-diffusion method. The rGO-HCS/S composite loading 90 wt% sulfur delivers the highest discharge capacity of 610 mAh g-1 (energy density of 1281 Wh kg-1) based on the mass of the whole cathode electrode, much higher than that of the composites loading 70 wt% and 80 wt% sulfur. Moreover, even when the sulfur areal density increases to 3 mg cm-2, the rGO-HCS/S composite loading 90 wt% sulfur could still reach a high energy density of 1300 Wh kg-1 at a power density of 240 W kg-1 and obtain a high capacity retention of 77% after 78 cycles. The ultrahigh content of sulfur in the rGO-HCS composite renders remarkable increase of both gravimetric and volumetric energy density, which indicates the great potential of the rGO-HCS nanostructure as promising sulfur host towards Li-S batteries with high energy density.

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Fig.1 SEM images of (a) the rGO-HCS composite, (b) the rGO-HCS-0.7S composite, (c) the rGO-HCS-0.8S composite and (d) the rGO-HCS-0.9S composite

Fig.2 (a) TEM image and corresponding elemental maps of the rGO-HCS-0.9S composite. (b) STEM image of the selected region, EDS maps of (c) C, (d) S elements in the selected region of the rGO-HCS-0.9S composite

Fig.3 (a) XRD patterns and (b) TG curves of the rGO-HCS/S composites

Fig.4 (a) the N2 isothermal adsorption curves and (b) the pore size distribution curves of the rGO-HCS composite and the rGO-HCS/S composites

Fig.5 electrochemical properties of the rGO-HCS composites with different sulfur contents: (a) the charge-discharge curves, (b) the cycling performance, (c) the rate capability, and (d) the Nyquist plots of AC impedance spectras

Fig.6 (a) rate capability of the rGO-HCS-0.9S electrodes with different areal density of sulfur, (b) cycling stability of the rGO-HCS-0.9S thick electrode at 0.1C