sulfur nanocrystals for lithium–sulfur batteries

Accepted Manuscript One-step hydrothermal synthesis of three-dimensional porous graphene aerogels/sulfur nanocrystals for lithium-sulfur batteries Yong Jiang, Mengna Lu, Xuetao Ling, Zheng Jiao, Lingli Chen, Lu Chen, Pengfei Hu, Bing Zhao PII: DOI: Reference:

S0925-8388(15)01429-2 http://dx.doi.org/10.1016/j.jallcom.2015.05.125 JALCOM 34241

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

13 February 2015 4 May 2015 15 May 2015

Please cite this article as: Y. Jiang, M. Lu, X. Ling, Z. Jiao, L. Chen, L. Chen, P. Hu, B. Zhao, One-step hydrothermal synthesis of three-dimensional porous graphene aerogels/sulfur nanocrystals for lithium-sulfur batteries, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.05.125

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One-step hydrothermal synthesis of three-dimensional porous graphene aerogels/sulfur nanocrystals for lithium-sulfur batteries Yong Jianga, Mengna Lua, Xuetao Linga, Zheng Jiaoa, Lingli Chena, Lu Chena, Pengfei Hub, Bing Zhao a,* a

School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, PR China b

Instrumental Analysis and Research Center, Shanghai University, Shanghai 200444, PR China * Correspondence author. Tel./fax: +86 21 66137798.

E-mail address: [email protected] (B. Zhao)

ABSTRACT Lithium-sulfur (Li-S) batteries are receiving significant attention as a new energy source because of its high theoretical capacity and specific energy. However, the low sulfur loading and large particles (usually in submicron dimension) in the cathode greatly offset its advantage in high energy density and lead to the instability of the cathode and rapid capacity decay. Herein, we introduce a one-step hydrothermal synthesis of three-dimensional porous graphene aerogels/sulfur nanocrystals to suppress the rapid fading of sulfur electrode. It is found that the hydrothermal temperature and viscosity of liquid sulfur have significant effects on particle size and loading mass of sulfur nanosrystals, graphitization degree of graphene and chemical bonding between sulfur and oxygen-containing groups of graphene. The hybrid could deliver a specific capacity of 716.2 mAh·g-1 after 50 cycles at a current density of 100 mA·g-1 and reversible capacity of 517.9 mA h·g-1 at 1 A·g-1. The performance we demonstrate herein suggests that Li-S battery may provide an opportunity for development of rechargeable battery systems. Keywords: hydrothermal synthesis; three-dimensional; graphene aerogel; sulfur nanocrystals; lithium-sulfur batteries Page 1 of 21

1. Introduction With the rapid development of electronic products and electrical tools, the next generation of rechargeable batteries demands high capacity and energy density. The lithium-sulfur

battery

is

currently

attracting

considerable

attention

as

a

high-energy-density storage and power device for sustainable electric vehicles [1]. Sulfur, one of the most abundant elements on earth, is an electrochemically active material that can accept up to two electrons per atom at 2.15 V vs. Li/Li+, which has a high theoretical capacity of 1675 mAh·g-1 and energy density of 2567 Wh·kg-1 [2]. Unlike conventional insertion cathode materials, sulfur is a poor conductivity material, and during cycling sulfur undergoes a series of compositional and structural changes, which involve soluble polysulfides, insoluble sulfides and volume expansion [3-6]. Theses issues result in loss of active materials, low utilization of the sulphur cathode, low overall coulombic efficiency and, ultimately, in severe capacity decay upon cycling. To address these issues, research and development efforts are focused on the improvement of the conductivity and prevention of the polysulfide dissolution and shuttling. Of notable success, the carbon-based sulfur composites are identified as the most effective strategy. Numerous reports of high-capacity Li-S batteries have confirmed that an ideal carbon matrix for sulfur-carbon composites needs to have (i) high electrical conductivity, (ii) electrochemical affinity for sulfur, (iii) small characteristic dimension of the sulfur particles to avoid pulverization and promote accessibility of liquid electrolyte to active material, and (iv) stable framework to

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sustain the strain generated by the volume changes of the active material during cycling.

[4]

Recently,

ordered

mesoporous

carbon

[7],

various

carbon

micro/nano-structured materials, such as mesoporous carbon spheres [8-10], hollow/porous carbon nanofibers [11-14], activated carbon fiber [15], and graphene [16-19] are used as a conductive matrix to constrain sulfur within the carbon frameworks. Most of the sulfur based cathodes can achieve high specific capacity in terms of sulfur as active materials. However, most of the as-prepared electrodes have extremely low sulfur areal mass loading, which would greatly led to low capacity of the battery systems and cannot comparable with the commercial lithium ion batteries. Therefore, it is a challenge to improve the utilization of sulfur and the mass loadings simultaneously. As one of the most promising conductive carbonaceous materials, graphene is a two-dimensional material with super electrical conductivity, high surface area, chemical stability, and mechanical strength and flexibility, making it become a useful substrate to anchor active materials for electrochemical energy storage applications [20]. Researchers have demonstrated that embedding sulfur in flexible graphene sheets to form two-dimensional graphene/S composites partially alleviates the polysulfide shuttle, and promotes long cycle life. However, graphene fails to trap the polysulfides due to its open structure, resulting in low coulombic efficiency and limited cyclic stability. Therefore, it is urgent to construct a three-dimensional (3D) structure of graphene that provides highly conducting network for charge transfer, flexible space accommodating the volumetric expansion during cycling as well as

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porous morphology to immobilize the polysulfide species. In addition, developing 3D structures with this extraordinary nanomaterial would further expand its applications. Recently, many literatures were reported about metal hydroxide and oxide nanocrystals with interesting morphologies and nanoscale sizes in graphene aerogels (GA). As a result of controlled nucleation and growth, the hydroxide or oxide nanomaterials are selectively formed on the surface of graphene with intimate interaction to the conducting graphene substrate. The strong electron coupling renders these electrically insulating materials conductive, which would significantly increase the specific capacity and rate capability of the GA-based hybrid materials. Although these developments are encouraging, sulfur loading in the 3D framework structure is unsatisfactory. In most reported cases, the areal loading of sulfur on the electrode was less than 2.0 mg·cm-2 and sulfur-containing composites had a sulfur fraction below 70 wt%. [13,20-23] , which are far below the present lithium ion batteries cathodes (such as LiFePO4 and LiCoO2) with areal loadings of 13~30 mg·cm-2. In addition, as a highly insulated material (5×10−30 S·cm−1 at 25 °C), the particle size of sulfur could largely influence its electrochemical performance. The previous synthesis processes of 3D conductive carbon matrix immobilized sulfur composites mainly include mixing, ball-milling, sulfur melting route, sulfur vaporizing route, and chemical reaction deposition strategy, of which the sulfur are incorporated after the formation of carbon matrix, and the particles are usually submicron in diameter. Moreover, it is a challenge to develop a simple method to synthesize 3D graphene/sulfur composites with strong adherence between graphene

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and sulfur. Therefore, it is necessary to develop new strategies that not only can improve the cyclic stability of Li-S batteries but also increase the sulfur loading and hence maintain their energy density advantage. In this paper, a light black 3D porous graphene aerogels/sulfur (GA-S) nanocomposite was obtained through a one-step hydrothermal process by using graphene oxide (GO) dispersion and sulfur/carbon disulfide (CS2) mixed solution. Active sulfur with particle sizes range from several nanometers to several decade nanometers was deposited in GA architectures. The oxygen-containing functional groups on the graphene surface play the role of immobilizers that keep intimate contact of the conducting matrix with sulfur species, and effectively confine the polysulfides from dissolving. The GA can not only provide a robust electron transport network but also accommodate the sulfur volumetric expansion/contraction. As a result, the GA-S hybrids exhibit outstanding reversible capacity and excellent rate performance (716.2 mAh·g-1 after 50 cycles at a current density of 100 mA·g-1 and reversible discharging capacity of 517.9 mA h·g-1 at 1 A·g-1). 2. Experimental 2.1 Materials preparation Firstly, GO sheets were prepared from natural graphite powder via a modified Hummer’s method as described in our previous work [24]. Then, GA-S were prepared by one-step hydrothermal method, which is an easily and controllable method. In briefly, 50 mL GO suspension (2 mg·mL-1) and 10 mL ethanol were mixed and stirred for 10 min, then 5 mL solution of sublimed S/CS2 (30 mg·mL-1) was added. After

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stirring for 1 h, the mixture suspension was then sealed in a 100 mL Teflon-lined stainless steel autoclave for hydrothermal reaction at 120, 150 and 180°C for 12 h. The as prepared wet hydrogels were washed by alcohol and distilled water several times, and finally freeze-dried under vacuum overnight. The GA-S hybrid samples are denoted as GA-S-X, while X represents hydrothermal temperature (120, 150 or 180°C). 2.2 Structural characterization X-ray power

diffraction

(XRD)

observations

were

carried

out

on

a

Rigaku-D/max-2550PC diffractometer with Cu-Kα radiation (λ=1.50405 Å). A thermogravimetric

analysis

(TGA)

apparatus

Germany

Netzsch-STA409PC

instrument was used for the thermal characterization. The powder morphology was observed by field-emission scanning electron microscopy (FESEM,

JEOL

JSM-6700F). The transmission electron microscope (TEM) observations were performed on a JEOL JEM-2010F electron microscope operating at 200 kV. Raman spectra were recorded on Renishaw in plus laser Raman spectrometer with a 514.5 nm laser (5 mW). X-ray photoelectron spectroscopy (XPS) was characterized by ESCA LAB 250Xi instrument with Al Kα radiation. 2.3 Electrochemical measurements. Monolithic GA-S were cut into small slices with a thickness of about 1 mm and diameter of about 10-12 mm, and then used as the cathode directly. 2032 type coin cells were assembled in an argon-filled glove box with lithium foil as the anode, the separator was purchased from Celgard (model 2400), the electrolyte was 1.0 M

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lithium bis(trifluoromethane sulfonylimide) (LiTFSI) in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME, 1:1 by volume ratio), and lithium nitrate (1 wt%) was added to the electrolyte as an additive to help passivate the surface of the lithium anode and reduce the shuttle effect. Galvanostatic measurements were conducted using a LAND CT2001A battery test system between 1.5 and 2.8 V (vs. Li/Li+). Cyclic voltammograms (CV) were recorded using a lithium foil as both counter and reference electrodes with a scan rate of 0.05 mV·s-1 between 1.5 and 2.8 V on CHI 660C electrochemical working station (CHI Instruments, Shanghai, China). 3. Results and discussion In this work, the one-step hydrothermal process was used for the preparation of 3D porous GA-S hybrid as shown in Scheme 1. In the hydrothermal reaction system, alcohol plays the role of improving the miscibility of the GO and S/CS2 solution. GO serve as the skeleton and sulfur would load on the skeleton under electrostatic force between the S and the hydroxyl and epoxide functional groups on the GO surface. Then, the chemical bonding was obtained between sulfur and graphene in the composite, which will be discussed in the XPS section. As it is well known, there is lots of water inside the graphene/S hydrogels, and the liquid extraction process has a great influence on the final microstructure. The surface tension of water induced by its evaporation definitely would cause substantial re-aggregation of graphene. While the freeze-drying could avoid this influence to obtain a large Brunauer−Emmett−Teller (BET) surface area and open framework of 3D graphene/S aerogels [25]. The powder XRD patterns of the GA-S samples are shown in Fig. 1. The broad diffraction peaks

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centered at 2θ = 24° could be recognized as (002) diffraction peak of graphene. All the other diffraction peaks can be indexed to orthorhombic sulfur (PDF #08-0247). The low diffraction intensities indicate that sulfur in the GA framework was very poorly crystallized, which was consistent with previous observations that confine sulfur was less crystalline [13]. The different peak intensities of S and graphene for the three samples are due to their different content of S, and the broadened diffraction peaks indicate the small size of the sulfur crystals. Thermal behavior and sulfur loading for GA-S were studied by using TGA analysis in a nitrogen environment as shown in Fig. 2. The main weight loss of sulfur is taken place between 240°C and 400°C, owing to the sublimation of incorporated sulfur. Based on the weight loss, the sulfur loading can be estimated to be about 49.05 %, 72.90 % and 73.69 % in GA-S-120, GA-S-150 and GA-S-180, respectively. As we all known that material chemical activity is affected by temperature and viscosity of the reaction system. High temperature and low viscosity would increase the mobility of the reactive ions and formation of crystal nuclei. While the viscosity of liquid sulfur at 150°C is ~7 mPa·s, which is the lowest viscosity value in temperature range from melting point to 250°C. Thus, the sulfur loading increases sharply along with the hydrothermal temperature rising from 120°C to 150°C. However, with further increase of the temperature to 180°C, the viscosity of liquid sulfur increases rapidly over three orders of magnitude [26], resulting in not less complete diffusion of sulphur into the open pore of grephene aerogels. So we observed that GA-S-180 sample has high sulfur content almost not more than that of GA-S-150.

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Morphologies of the GA-S hybrids were deeply characterized by scanning electron microscopy (SEM) as shown in Fig. 3. As it can be seen all the three hydrothermal samples have well-defined and interconnected 3D porous network with open pores. Pores with diameter in the range from several hundred nanometers to several micrometers are embedded with the ultrathin layer of aerogel matrix. However, we note that GA-S-150 shows strongly aggregated graphene with a loose structure and irregular arrangement, it must be because of sulfur’s maximum mobility. Fig. 3g was the digital photograph of as-prepared graphene aerogels with sulfur hybrids. And the energy-dispersive X-ray spectroscopy (Fig. 3h, h1 and h2) revealed the presence of C, O and S in the GA-S hybrid. As a result, we successfully demonstrated 3D interconnected porous architecture with a uniform deposition of sulfur particles, which could provide highly efficient conductive networks with enhanced surface area and short diffusion path length for lithium ion transport, and also have huge space to accommodate the sulfur volume expansion. The nature of the graphene in the GA-S composite was analyzed by Raman spectra as shown in Fig. 4. The relative intensity of the D peak at ~1335 cm-1 represented disordered carbon and the G peak at ~1580 cm-1 represented graphitic carbon [27]. In graphene with zero-dimensional pointlike defects, the distance between defects (written as LD), is a measure of the amount of disorder, which can be estimated from the intensity ratio of the D and G peaks [28]. We can easily observe that the ID/IG values of GA-S-120, GA-S-150 and GA-S-180 are 1.33, 1.57 and 1.23, respectively. It means that LD of GA-S-150 sample is about 7 nm, much lower than that of

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GA-S-120. The decreased LD for GA-S-150 is attributed to the increased number of isolated sp2 domains and again indicates the effectiveness of the graphene oxide reduction during the hydrothermal process. However, with the reduction degree further increased (as the hydrothermal temperature rising to 180°C), the graphene sheets agglomerate together in a compact and order manner. Therefore, the ID/IG has a decreasing trend when the temperature is above 180°C. The other possible reason may be due to smaller combine energy needed for sulfur and graphene at higher temperature, which can also be identified by XPS characterization. In order to observe the sizes of sulfur loading on graphene, the TEM characterizations were performed as shown in Fig. 5. It is found that sulfur particles are uniformly dispersed in the graphene matrix for all the three samples. The GA-S-120 and GA-S-150 both have a size of 30-50 nm, and the size of GA-S-180 sample is about 5 nm, which are in good agreement with XRD diffraction. The well-resolved lattice fringes in the high-resolution transmission electron microscopy (HRTEM) image correspond to an interplanar spacing of 0.34 nm (Fig. 5d), consistent with the (026) planes of the orthorhombic sulfur. The nanometer sizes of sulfur nanocrystals suggest that presence of oxygen-containing functional groups plays an important role in immobilizing sulfur and preventing the sulfur from growing into bulk particles [29]. Comparatively speaking, high temperature offers large binding energy, which would derive more bond connection between the sulfur and oxygen-containing functional groups on graphene. This force will reduce the aggregation of sulfur particles, thus small sulphur nanocrystals were obtained.

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The unique structure of the GA-S nanocomposite can improve the overall electrochemical performance when it is used as a cathode material for Li/S batteries. First, the small characteristic dimension of the sulfur particles generally means low volume expansion/contraction and reduction of pulverization during charge/discharge cycling. In addition, the partially reduced graphene with its large surface area along with ubiquitous cavities can establish more intimate electronic contact with S and avoid aggregation and loss of electrical contact with the current collector. Second, the low temperature hydrothermal-treated GA still contains various kinds of oxygen-containing functional groups. These functional groups have strong adsorbing ability to anchor S atoms and to effectively prevent the subsequently formed Li polysulfides from dissolving in the electrolyte, improving the utilization of active materials and suppressing shuttle effect [17]. In order to elucidate the chemical composition component and chemical bonding state of the GA-S hybrids, XPS characterization on the GO and GA-S samples were carried out. In a full scan of XPS spectrum (Fig. 6a), C 1s and O 1s peaks were observed in all four samples, while S 2p and S 2s peaks were observed for GA-S hybrids. This result further confirmed the existence of the S element in GA-S composites. The peak intensity of O 1s slips down with increasing the hydrothermal temperature, which means reduction degree of GA-S hybrids. In Fig. 6b, it can be seen that GO C 1s peaks at 284.6, 286.6, 287.5, and 289.0 eV correspond to carbon species of C-C/C=C, C-O, C=O, and O-C=O, respectively [30]. After hydrothermal, the peak intensities of carboxyl, carbonyl and epoxy groups for GA-S-180 were

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significantly reduced (Fig. 6c), indicating that the majority of GO has been reduced to graphene during the reaction process. Furthermore, the C-O peaks shifted toward lower energy to some extent (285.6 eV), this indicates that strong chemical interaction between S and the functional group of GO happens (possibly the C-O-S bond) and S can partially reduce the GO [17,31]. From the S 2p spectra in Fig. 6d-f, the 2p3/2(163.6, 164.8 eV) and 2p1/2 (164.3, 165.5 eV) with an energy separation of 1.2 eV are attributed to the S-S and S-O bond [32], the minor peak at 168.6 eV may be assigned to sulfate species. The peak intensities of S-O bond became strong with the hydrothermal temperature rising, the proportion were 16.1%，20.0% and 29.4% for GA-S-120, GA-S-150 and GA-S-180, respectively. This indicates a much better C-O-S bonding of sulfur and graphene at high hydrothermal temperature, which could restrain sulfur and polysulfides during discharge-charge process, thus improving the utilization of active materials and suppressing shuttle effect [33]. Galvanostatic discharge-charge experiments were carried out to evaluate the lithium storage properties of the GA-S composites. Fig. 7a shows the CV curves of the first three cycles of the GA-S-180 sample at a scan rate of 0.05 mV·s−1 between 1.5–2.8 V. During the cathodic scan, two discrete reduction peaks positioned around 2.25 and 1.97 V were observed, indicating a multiple-step reduction mechanism of sulfur within lithium. The first reductive peak at ∼2.25 V corresponds to the transformation of cyclo-octasulfur (S8) to long-chain soluble lithium polysulfides (Li2Sn, 3≤n≤6). Another cathodic peak at ~1.97 V can be attributed to a further

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reduction of polysulfide ions to insoluble Li2S2 and peak at 1.75 V should be ascribed to Li2S2 turn to Li2S [34]. In the subsequent anodic scan, only one sharp oxidation peak is observed at ~2.56 V that is attributed to the complete conversion of Li2S and polysulfides into elemental S. As the cycle number increased, the oxidation peak at 2.56 V becomes less significant, while another new one at 2.45 V grows higher in intensity. The oxidation peak at 2.45 V is associated with the formation of Li2Sn (n>2). During the anodic oxidation of lithium sulphides to polysulphides, partial unconstrained dissolution of polysulphide ions causes a reduction in the anodic current, which is stabilized after two cycles. In particular, after the second cycle, both the CV peak positions and peak currents undergo very small changes, indicating good reactive reversibility and cycling stability of the composite electrode. The CV results show that GA can help to prevent S from dissolving into the electrolyte because of its large surface along with some functional groups on the surface. These results are in agreement with our galvanostatic discharge-charge curves and other reports [8,17]. Fig. 7b depicts the first discharge/charge voltage profiles of electrodes at 100 mA·g-1 between 1.5 V to 2.8 V (The typical mass loading of the sulfur is ~ 5 mg·cm-2, and specific capacities are calculated based on the sulfur mass only). All the discharge curves show three plateaus in the voltage profile that are consistent with the peaks in the CV curves and are also well documented in the literature [9,17,35]. The GA-S-120, GA-S-150 and GA-S-180 samples deliver a high initial discharge capacity of 1661.9, 1365.5 and 1351.3 mAh·g-1 at 100 mA·g-1. The sample of GA-S-120 shows the highest specific capacity, which may be due to its lowest sulfur content (49.05 %).

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Obviously, a lower sulfur load should give higher sulfur utilization and faster kinetics (on the account of total specific capacity). Fig. 7c shows rate capability of the GA-S-120, GA-S-150 and GA-S-180 cathodes cycled at various current rates from 100 mA·g-1 to 1 A·g-1. It demonstrated that GA-S-180 have the best rate capability, which remains a discharge capacity of 517.9 mAh·g-1 at 1 A·g-1. After cycling at various rates, when the current switched abruptly from 1 A·g-1 to 100 mA·g-1 again, a reversible capacity of 795.8 mAh·g-1 can still be reversed, indicating robustness and stability of the cross-linked hybrid electrode. The rate capability is even better than that of the graphene wrapped sulfur nanoparticles composite [36]. This result demonstrates that the robust and highly porous structured graphene not only enables stable and continue pathway for rapid electron and ion transportation, but also keep cathode integrity and accommodate the volume change during high-rate charge/discharge processes. The actions of interactions between active materials and graphene on improving the lithium storage properties have also been discussed in the recently review [37] and previous papers [25]. As illustrated in Fig. 7d, the cycled performances of the hybrids were tested. All three materials show a drop in capacity during the first two cycles, followed by good stability on subsequent cycling. This is explained by the erosion of sulfur from the outer surface of the GA. It is also consistent with their initial charge/discharge profile (Fig. 7b). The overcharge capacity ranged from 295 to 480 mAh·g-1 is observed, which we attribute to the dissolution of the reduced surface polysulfide species which engage in the shuttle mechanism during the electrode redox reactions. After cycling

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for 50 cycles, the discharges capacities of 539.3, 481.1 and 716.2 mAh·g-1 were obtained. The specific capacity of GA-S-180 versus the electrode is much higher than that of many two-dimensional graphene/sulfur composites [16,33,36,40], and also higher than 3D graphene/sulfur composites whose sulfur were loaded after the formation of 3D graphene hydrogel/aerogel [25,41], especially considering the ultra-high sulfur loading in the system. By the way, the main cause of the capacity decay for GA-S-120 may be its weak bonding between the sulfur and graphene, which has been demonstrated in XPS. While for GA-S-150 sample, the significant amount of defects within graphene, and its loose structure and irregular arrangement lead to poor cycling performance. Generally speaking, the excellent cycling stability of the GA-S-180 composite electrode could ascribe to at least three aspects [7,17,42,43]. Firstly, the nanometer size of sulfur nanocrystals and stable open framework of graphene aerogel, which provides a three-dimensional, molecular-level capsule to contain the sulfur compounds. Therefore, mechanical stress arising from the electrochemical reaction is effectively alleviated. Secondly, the intimate contact of the S provided by the large surface area and the functional groups on graphene is favorable to good electron/ion accessibility. Thirdly, the highly reactive functional groups of graphene are known to bind favorably with sulfur and polysulphide anions, hence further inhibiting the extent of polysulphide dissolution and redox shuttle phenomenon.

4. Conclusions In conclusion, a high special capacity of three-dimensional graphene/sulfur cathode

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with high sulfur loading was prepared by a one-step hydrothermal process. The GA-S-180 hybrid could deliver a specific capacity of 716.2 mAh·g-1 after 50 cycles at a current density of 100 mA·g-1 and reversible capacity of 517.9 mA h·g-1 at 1 A·g-1. The nanometer size of sulfur nanocrystals, stable open framework of graphene aerogel, intimate contact of the S with graphene conductive matrix, and strong adsorbing ability of functional groups of graphene are demonstrated to be the main factors for its excellent rate capability and cycling performance. The unique properties of the hybrids plus their simple fabrication make this class of materials attractive for further investigation for Li-S batteries applications.

Acknowledgements This work was supported by the Natural Science Foundation of China (11275121, 21241002,

21371116),

Science

and

Technology

Committee

of

Shanghai

(13DZ1200502, 13DZ2295200), and Program for Innovative Research Team in University (IRT13078).

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Figure Captions Scheme 1 Schematic of the formation of GA-S hybrid. Fig. 1. XRD spectra of GA-S hybrids shows it matches the PDF card of 08-0247. Fig. 2. Thermogravimetric analysis of the GA-S hybrids in nitrogen with heating rate of 10 °C·min-1. Fig. 3. Low and high magnification SEM images of (a, b) GA-S-120, (c, d) GA-S-150 and (e, f) GA-S-180 hybrids. (g) Digital images of as-prepared GA-S hybrids. (h) The EDS spectrum (h1) and element mapping of sulfur (h2) for GA-S hybrid. Fig. 4. Raman spectra of GA-S hybrids with its characteristic D band (1335 cm-1) and G band (1580 cm-1) that are characteristic of graphene. Fig. 5. TEM images of (a) GA-S-120, (b) GA-S-150 and (c) GA-S-180 hybrids. (d) HRTEM image of GA-S-180 hybrid. Fig. 6. (a) XPS survey spectrum and C 1s XPS analysis of (b) GO and (c) GA-S-180 hybrid. S 2p XPS spectra of (d) GA-S-120, (e) GA-S-150 and (f) GA-S-180 hybrids. Fig. 7. (a) Typical CV curves of GA-S-180 hybrid cathode at a sweep rate of 0.05 mV·S-1. (b) Initial charge/discharge profiles of GA-S hybrids. (c) Capacity at different current densities of the GA-S hybrids. (d) Cyclic performance of the GA-S hybrids at 100 mA·g-1 for 50 cycles.

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Graphical abstract\

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Research Highlights · 3D porous GA/S nanocrystals are prepared by a one-step hydrothermal method. · The structure is affected by hydrothermal temperature and liquid sulfur’s viscosity. · The hybrid delivers a capacity of 716.2 mAh·g-1 after 50 cycles at 100 mA·g-1. · The nanosized S, strong adsorbability and intimate contact of GNS are main factors.

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