scotch-tape-like exfoliation of graphite assisted with elemental sulfur and graphene–sulfur...

Energy &Environmental Science

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aCAS Key Laboratory of Materials for En

Ceramics, Chinese Academy of Science

[email protected]; Tel: +86-21-524116bState Key Laboratory of Rare Earth Materia

Chemistry and Molecular Engineering, PekincState Key Laboratory of Functional Materia

Microsystem and Information Technology,

200050, China

Cite this: DOI: 10.1039/c3ee24324a

Received 10th December 2012Accepted 7th February 2013

DOI: 10.1039/c3ee24324a

www.rsc.org/ees

This journal is ª The Royal Society of

Scotch-tape-like exfoliation of graphite assisted withelemental sulfur and graphene–sulfur composites forhigh-performance lithium-sulfur batteries†

Tianquan Lin,‡ab Yufeng Tang,‡a Yaoming Wang,a Hui Bi,a Zhanqiang Liu,a

Fuqiang Huang,*ab Xiaoming Xiec and Mianheng Jiangc

A new composite structure of graphene–sulfur with a high electrochemical performance is proposed.

Scotch-tape-like sulfur-assisted exfoliation of graphite is developed to produce the graphene–sulfur

composites and freestanding low-defect graphene sheets. The intimate interaction between sulfur and

graphene, attributed to the similar electronegativities of the two elements, is stronger than the van der

Waals forces between adjacent p–p stacked graphene layers. This causes cleavage of the graphene

layers when the sulfur molecules stick to the surface and edges of the graphite, similar to Scotch tape in

micromechanical exfoliation processes. This approach enables us to obtain graphene with an electrical

conductivity as high as 1820 S cm�1 and a Hall mobility as high as 200 cm2 V�1 s�1, superior to most

reported graphene. Furthermore, the graphene sheets which uniformly anchor sulfur molecules provide

a superior confinement ability for polysulfides, sufficient space to accommodate sulfur volumetric

expansion, a large contact area with the sulfur and a short transport pathway for both electrons and

Li+. The unique structure containing 73 wt.% sulfur exhibits excellent overall electrochemical properties

of 615 mA h g�1 at the 1 C (1 C ¼ 1675 mA g�1) rate after 100 cycles (corresponding average

Coulombic efficiency of over 96%) and 570 mA h g�1 at 2 C. These encouraging results represent that

sulfur molecules bound onto graphene sheets could be a promising cathode material for lithium

batteries with a high energy density.

Broader context

The Li–S battery exhibits a high theoretical capacity (1675 mA h g�1) and energy density (2600 W h kg�1) but its rapid capacity decay owing to polysulphidedissolution presents a signicant technical challenge. Here, we demonstrate the design of a unique composite of graphene–sulfur with a high electrochemicalperformance. The elemental sulfur, due to the intimate attraction between sulfur and graphite layers, plays a similar role to Scotch tape to assist withmicromechanical exfoliation during ball milling. This Scotch-tape-like sulfur-assisted exfoliation of graphite enables us to obtain freestanding low-defectgraphene sheets and sulfur molecules uniformly dispersed on the graphene surface. The graphene possesses an electrical conductivity as high as 1820 S cm�1,superior to most reported graphene. Furthermore, the graphene sheets which uniformly anchor sulfur molecules provide a superior connement ability forpolysuldes, sufficient space to accommodate sulfur volumetric expansion, a large contact area with the sulfur and a short transport pathway for both electronsand Li+. The unique structure containing 73 wt.% sulfur exhibits excellent electrochemical properties of 615 mA h g�1 at the 1 C rate aer 100 cycles. Theseencouraging results represent that sulfur molecules bound onto graphene sheets could be a promising cathode material for lithium batteries with a high energydensity.

1 Introduction

The lithium–sulfur battery, as a promising energy storagesystem, has attracted much attention due to its high theoretical

ergy Conversion, Shanghai Institute of

s, Shanghai 200050, China. E-mail:

20

ls Chemistry and Applications, College of

g University, Beijing 100871, China

ls for Informatics, Shanghai Institute of

Chinese Academy of Sciences, Shanghai

Chemistry 2013

specic capacity (1675 mA h g�1) and high energy density (2600W h kg�1) as well as its abundant and inexpensive sulfurmaterial.1,2 However, their practical applications are severelyrestrictive arising from the problems of the low electrical

† Electronic supplementary information (ESI) available: Synthesis of grapheneoxide, characterizations, thickness statistics, electronegativity of elements andcorresponding contact angle of sulfur, comparison of a set of electrochemicalperformances, electrochemical properties for composites with 82 wt.% sulfurand additional XPS, TEM, EDS analysis data are given. See DOI:10.1039/c3ee24324a

‡ T. Lin and Y. Tang contributed equally to this work.

Energy Environ. Sci.

http://dx.doi.org/10.1039/c3ee24324a

http://pubs.rsc.org/en/journals/journal/EE

Energy & Environmental Science Paper

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conductivity of sulfur, dissolution of polysuldes in the elec-trolyte and the volume expansion of sulfur during charge–discharge.3–6 These problems cause a poor cycle life, a lowspecic capacity and low energy efficiency. Various carbon-based sulfur composites (mesoporous carbon, carbon nano-tubes, active carbon, graphene, etc.) have been prepared ascathode materials.1,2,7–12 Graphene, as a star material, has actedas a carbon-based support material for lithium sulfur batteries,but the electrochemical properties are poor at high currentdensities (520mA h g�1 at 0.2 C aer 100 cycles).4,13–16 Therefore,it remains challenging to retain a high and stable capacity ofsulfur cathodes at high current densities. The key point is todevelop a new structure for carbon-based sulfur composites topromise rapid high-rate charge transfer and prevent poly-suldes being dissolved in the electrolyte.

Graphene, with a high surface area (�2600 m2 g�1) and highmobility of charge carriers (200 000 cm2 V�1 s�1), is an idealcandidate to anchor or wrap sulfur.4,13,14,16–18 The reported gra-phene–sulfur composites were prepared by impregnatingaggregated graphene sheets with melted sulfur or a sulfursolution to form graphene-wrapped sulfur particles (Scheme1a).4,14 This approach could improve the conductivity of sulfurbut is not effective for conning the polysuldes with carbonspecies. Polysulde clusters can still readily diffuse out of thegraphene and initiate the “shuttle” problem, which signicantlyundermines the cycling stability of the cell. Such a sulfur–gra-phene composite structure may be responsible for the unsat-ised electrochemical behavior of the lithium–sulfur batteries.

The ideal structure of a graphene–sulfur cathode is that S8single molecules are uniformly dispersed onto the graphenesurface, as depicted in Scheme 1b. The sulfur molecule (S8) is adouble-layer zigzag ring with four upper and four lower atoms.The four lone pairs of the S 3pz

2 electrons can interact with theantibonding conjugated p* states of the graphene plane.Furthermore, the larger electron density of polysulde (Sn

2�, 2 <n < 8) during discharge has a stronger interaction with theconjugated p* states of graphene than elemental sulfur.Therefore, graphene is a good choice to immobilize bothelemental sulfur and polysuldes due to their intimate inter-action. The unique structure has several advantages including asuperior connement ability for polysuldes, a large contactarea with the sulfur, sufficient space to accommodate sulfurvolumetric expansion and a short transport pathway for bothelectrons and Li+.

Scheme 1 Schematic graphene–sulfur composites: (a) graphene-wrappedsulfur particle and (b) sulfur molecules (S8) dispersed on graphene sheets.


The graphene–sulfur composite structure proposed inScheme 1b requires low-defect graphene sheets, which canensure not only a high electrical conductivity but also a largeattractive interaction between the sulfur molecules and gra-phene sheets. In the previous reports on graphene–sulfurcomposites, most of the graphene sheets were prepared bysolution exfoliation of graphite oxide (r-GO). The as-preparedgraphene has a poor electrical conductivity owing to the struc-tural defects formed during the vigorous exfoliation andreduction processes. This is an obstacle for its use as a goodconductor in electronics and energy storage devices.18 Mean-while, the conjugated p* states of graphene are disorganized bythe defects, which hinders the intimate interaction betweengraphene and the sulfur molecules. Considering the drawbacksof r-GO, we expect low-defect graphene with a high electricalconductivity to anchor single S8 molecules and polysuldes.

Based on basic chemistry knowledge, the chemical interac-tion between a sulfur molecule and a graphene sheet is strongerthan the van der Waals force between adjacent p–p stackedgraphene layers or between two sulfur molecules. This providesa good idea to exfoliate graphite layers assisted with sulfur bysticking on the surface and edges of graphite, which act in asimilar manner to Scotch tape in micromechanical exfoliationprocesses. Meanwhile, the shear-force-dominated grindingcould provide mechanical forces to exfoliate graphite andannihilate the van der Waals forces between S8 molecules.19

With these considerations, we here design a simple but effectiveand versatile strategy for high-yield and mass production ofhigh-quality graphene–sulfur composites and freestandinggraphene sheets by planetary milling of edge-opened graphite(EG) assisted with sulfur.

In our approach, shear-force-dominated grinding simulatesmicromechanical exfoliation and sulfur is a moderately stickymaterial for the graphite plane, like a Scotch tape. The graphenelm (f 4 cm, 25 mm thickness) possesses a high electricconductivity (1820 S cm�1) and Hall mobility (200 cm2 V�1 s�1),an unprecedented result for graphene lms, superior to thereported edge-carboxylated graphene via dry ice ball milling(1214 S cm�1)20 and graphite (�300 S cm�1).21 More impor-tantly, this approach enabled us to obtain uniform sulfurmolecules anchored onto graphene sheets. The unique struc-ture containing 73 wt.% sulfur exhibits excellent lithiumstorage properties including an excellent cycle stability at highcurrent densities (615 mA h g�1 aer 100 cycles at 1 C andcorresponding average Coulombic efficiency of over 96%) and ahigh rate performance (570 mA h g�1 @ 2 C aer 10 cycles). Theexcellent overall electrochemical behavior is better than almostall other reported carbon–sulfur materials.1,2,4,11,13,16,22

2 Results and discussion2.1 Evidence of exfoliation to graphene

Elemental sulfur (S8) becomes “Scotch tape” due to its uniqueelectronic structure and interaction model with graphene. Themolecule of S8 is a zigzag ring of eight sulfur atoms with fourplanar S atoms above the other four. The S8molecule has a quitestrong interaction between the lone pairs of the S 3pz

2 electrons

This journal is ª The Royal Society of Chemistry 2013


Fig. 2 (a) Schematic illustration of the evolving process from graphite to gra-phene. (b) Mass production of graphene–sulfur composites prepared by ballmilling graphite flakes with sulfur.

Paper Energy & Environmental Science

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and the antibonding conjugated p* states of graphene, aspictorially summarized in Fig. 1a. The electronegativity can beused to roughly index the orbital polarization interaction. Thestrong interaction prefers two elements with a similar electro-negativity value, which can be proven by the wettability experi-ments of sulfur on various substrates (SiO2, halides, graphenepaper, etc.). The contact angle of the liquid sulfur dropletdecreased from 67.5� (NaF), 40.1� (NaCl), 34.4� (NaBr), 23.8�

(NaI), 57.2� (SiO2), to 4.3� (graphene), as shown in Fig. 1b. Theplot of the dependence of the contact angle on the electroneg-ativity difference (|ci � cS|, where i stands for nonmetalelements of the substrate) is shown in Fig. 1c and the contactangle decreases as |ci � cS| decreases. A smaller contact anglemeans that the surface free energies of the two materials arecloser and the interfacial attraction is stronger. The electro-negativity of sulfur (2.58) is very close to carbon (2.55), as listedin Table S1 in the ESI,† and elemental sulfur has good wetta-bility with graphene due to their strong attractive interaction.Moreover, a theoretical investigation claried the strong inter-action of the sulfur valence (3s, 3p) states with the graphiteinterlayer states and superconductivity behavior was experi-mentally found in the composite.23,24 A recent investigation alsofound that a single-layer sulfur superlattice is imbedded in thegraphene matrix,25 which uncovers one among many interac-tion manners between S and graphene. Therefore, elementalsulfur may act like Scotch tape during the ball milling exfolia-tion of graphite.

We then envision the scenario depicted in Fig. 2. Graphiteakes were rst made into chemically modied graphite (CMG)by the chemical intercalation of H2SO4 and HNO3, with oxida-tion of carbon atoms likely occurring at the edge and defectsites of graphite.26,27 We exfoliated CMG to graphene sheets bybrief heating at 800 �C for 60 s to form edge-opened graphite(EG) followed by simple ball milling for 6 h. The mass free-standing graphene sheets were obtained aer the removal ofsulfur.

Fig. 1 (a) Schematic illustration of the electron orbit for graphene and sulfur. (b)Wetting properties of sulfur on different substrates. (c) The plot of the depen-dence of the contact angle on the electronegativity difference, |ci � cS|, where istands for nonmetal elements of the substrate.


The degree of exfoliation to graphenewas characterized usingRaman spectroscopy, high resolution transmission electronmicroscopy (HRTEM), selected area electron diffraction (SAED),atomic force microscopy (AFM) and X-ray photoemission spec-troscopy (XPS). Raman spectroscopy is a powerful nondestruc-tive tool to evaluate the thickness and quality, as shown inFig. 3d. The graphene sheets via ball milling EG with sulfur for 6h have a G band at 1584 cm�1 and a 2D band at 2685 cm�1. Thefull-width at half-height maximum (FWHM) of the 2D band isabout 60 cm�1 and the intensity ratio I2D/IG is�0.8. These resultsindicate that they are bi- or tri-layer graphene sheets.28,29 The Dpeak at�1340 cm�1 is derived from some defects or edges of thegraphene, similar tomany reported results.28,30The layer numberand size of the graphene sheets were further conrmed byHRTEM images of the folded edges and AFM.When ball millingfor 6 h, a few-layered graphene (<5 layers) is produced, as shownin Fig. 3 and Fig. S1 (ESI),† which is consistent with the Ramanspectroscopy results. A representative AFM image in Fig. 3cshows that the graphene sheets that are cast ona siliconwafer areat,with thicknesses in the rangeof 0.5–1.7nmcorresponding tolayer numbers of less than ve. By analyzing a large number ofHRTEM and AFM images and paying close attention to theuniformity of theake edges andheight prole, we estimated thepercentage of few-layered graphene (<10 layers) to be 95% withthe ake-thickness statistics shown in Fig. S2 (ESI).† For thestatistical analysis, approximately 90% of the graphene sheetshave sizes in the range of 5–30 mm.The SAEDpattern reveals thatthe high crystalline graphitic structure on its basal plane can bepreserved during the grinding process (inset of Fig. 3a). In fact,when highly anisotropic graphite is processed by shear-force-dominated planetary mills, the high crystalline graphitic struc-ture on its basal plane can be preserved, while amorphouscarbon oen results frommilling with shock type ball impacts.31

Aer ball milling for 3 h, the number of graphene layersincreases to six (Fig. S3, ESI†), indicating that the graphene layernumber was regulated by the ball milling time.



Fig. 3 (a) A representative TEM image of the sulfur-assisted graphene andcorresponding SAED pattern. The bar of the inset is 5 1/nm. (b) HRTEM image ofthe folded edge of the freestanding graphene by ball milling for 6 h (G-6h),acquired from the black square in (a). (c) AFM image of a graphene sheet (G-6h)with a height profile (black curve) taken along thewhite line. (d) Raman spectra ofedge-opened graphite (EG) and as-prepared graphene sheets. (e) High resolutionXPS spectra of C 1s acquired from GO, CMG and EG fitted with Gaussian–Lor-entzian waveforms. (f) The thermal stability and resistance to oxidation of the as-made graphene sheets confirmed by TGA. We carried out the experiments afterremoving sulfur. Fig. 4 (a) High resolution XPS spectra of C 1s acquired from freestanding gra-

phene by ball milling for 6 h (G-6h) and the graphene–sulfur composite (G–S)fitted with Gaussian–Lorentzian waveforms. (b) Valence band XPS spectra of puresulfur and the G–S composite. (c) TGA curves of graphene, sulfur and the G–Scomposites.


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XPS was also used to monitor changes in the structural andchemical composition of the samples during the exfoliationprocess. XPS survey spectra (Fig. S4, ESI†) of the CMG show apronounced C 1s peak at 284.8 eV with small amounts of O(18.2 at.%) arising from acid-oxidation, and the small tails inthe C 1s peak at the higher binding energy region correspond toedge hydroxide (285.9 eV) and weak plane epoxide (287.1 eV), asshown in Fig. 3e. In contrast, the O 1s peak intensity of GOobtained from the Hummer's method increased signicantly (Ocontent 29.6 at.%) and two pronounced peaks from the C–O–Cand O]C–O bonds appeared in the C 1s spectrum (Fig. 3e). Theformer epoxide is the main cause of the plane structural defects.Compared with GO, weakly-oxidized CMG preserves the highcrystalline graphitic structure on its basal plane. The C 1s peakof EG became a fairly symmetric band centered at 284.8 eV asthe heat treatment repaired defective graphite layers by ther-mally desorbing covalently attached species. The XPS spectrumof the S-assisted graphene (G-6h) is shown in Fig. 4a. The C 1speak is symmetrically centered at 284.8 eV with FWHM 0.65 eVto reveal the perfect C sp2 lattice, similar to the characteristic


signals of graphene.32 The small bump within the hydroxideregion (�286.1 eV) arises from the adsorbed oxygen (1.1 at.%)during the ball milling process.

2.2 Characterization of graphene

The as-prepared graphene (G-6h) displays excellent thermalstability under a N2 atmosphere, as shown in Fig. S5 (ESI).†Surprisingly, the graphene also displays a much higher resis-tance to oxidation at a temperature of 650 �C with 5%mass loss,while that of commercial carbon black is 470 �C, as conrmedby the thermogravimetric analysis (TGA) shown in Fig. 3f.Therefore, the substantially higher resistance to oxidation ofgraphene in combination with its higher electron conductivity,mentioned below, due to the perfect sp2-hybridized honeycombstructure may open up a new avenue for fuel cell applications aswell as in other catalytic reactions.33



Table 1 Comparison of the electrical conductivity (s) and electro-chemicalproperties for different graphene samples

Sample sa (S cm�1)Mobility(cm2 v�1 s�1)

Capacityat 0.1b C(mA h g-1) Capacity 100th

S–G 1820 200 1502 615@1 CrGO4 0.316 — 1000 [email protected] CCO2–G

20 1214 — — —rGO <20034 <1240 — —N-GOc <91041 <40042 — —

a From the graphene lms. b 1 C ¼ 1675 mA g�1. c N-GO: non-grapheneoxide.


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The electrical characteristics of the graphene hold the keyfor their future applications. In order to evaluate the electricalconductivity, we fabricated graphene lms by vacuum ltrationof colloidal dispersions of graphene sheets through an Anodiscmembrane lter, which is similar to GO paper.34 The paper-likegraphene lm shows good exibility and can be bent, stretchedand twisted without breaking (Fig. S6, ESI†). Our graphenelm, with 25 mm thickness, has an excellent electricalconductivity of 242 S cm�1, better than the reported chemicallyexfoliated graphene (<200 S cm�1, see Table 1).18,35 In order toreduce the contact resistance between graphene sheets, thegraphene lm was compressed at 10 MPa by a hydraulic press.As expected, compressing the lm led to a further abruptincrease in the electrical conductivity up to 1820 S cm�1, whichis superior to that of the 300 MPa-pressed graphene lm (1214S cm�1).20 The carrier mobility estimated from a four-pointprobe setup with respect to the electric conductivity is �200cm2 V�1 s�1 at 300 K. The outstanding electrical conductivitysuggests again that the graphene obtained here contains fewdefects, in accordance with the above Raman spectroscopy,SAED and XPS analyses.

Fig. 5 SEM image (a), STEM bright field image (b) and the correspondingelemental mapping for S (c) reveal a homogeneous sulfur coating on the gra-phene sheets. (d) Raman spectrum of the graphene–sulfur composite.

2.3 Exfoliated mechanism of graphene

The proposed exfoliation of graphite seems to be a fairly goodmethod for low-cost and efficient mass production of graphene.In order to uncover the underlying mechanism, we systemati-cally investigated the entire process. Two keys elements of themethod include edge opening of graphite and the selection ofthe ball-milling assisted agent. Edge opening with a highersurface area enables ball milling to efficiently exfoliate graphite.We used a mild oxidization of H2SO4–HNO3 to introduce someadditional groups (e.g., –OH, O]C–O) onto the edges of thegraphite akes to form CMG, as evidenced by the XPS in Fig. 3e.The removal of the edge-attached hydroxide groups uponthermal treatment (60 s at 800 �C) effectively opens up the edgesof the CMG to form edge-opened graphite (EG), which wasconrmed by the HRTEM at the edges of the folded EG sheets.The HRTEM images of EG shown in Fig. S7 in the ESI† clearlyshow cleavage along the basal plane.

The above discussion and derivation of the interfacialattraction between the S–G composite is supported bydifferent experimental results, such as XPS and TGA. The C 1s


peak of the ball-milled graphene–sulfur composite (G–S, Ocontent 0.8 at.%) consists of sp2 C and C–S atoms (Fig. 4a),which is in good agreement with S-doped graphene.36 This isstrong evidence of an intimate interaction between sulfur andgraphene. The C–S bond peak vanished in the graphene (G-6h)aer removing the sulfur. More to the point, the valence bandin the XPS (Fig. 4b) of G–S exhibits a signicant intensity inthe range of 0.3–2.9 eV compared with the pure sulfur, whichis probably due to the increased density of states. The posi-tions of the valence band maximum relative to the Fermi levelwere evaluated by taking the onset of the valence bandemission and were found to be 0.4 and 2.3 eV for the G–S andpure sulfur, respectively. These results indicate that hybrid-ization between graphene and sulfur can increase the localcharge density37 and therefore trigger a strong adsorbingability to anchor sulfur atoms. The intimate interactionbetween sulfur and graphene is also conrmed by TGA(Fig. 4c). The weight loss in this analysis is due to the evap-oration of S. The evaporation rates of two graphene–sulfurcomposites with different S contents were much lower thanbare sulfur, which can be attributed to the attraction of S andgraphene.

In our G–S composite, the layer of sulfur uniformly dispersedon the graphene surface without bulk sulfur was observed fromthe scanning electron microscopy (SEM) image (Fig. 5a), STEMbright eld images (Fig. 5b and S8, ESI†) and the correspondingelemental mapping of sulfur (Fig. 5c). The existence of sulfurwas further conrmed by energy dispersive spectrometer (EDS)analysis in Fig. S9 (ESI).† The three sharp peaks (centered at155, 220 and 475 cm�1) in the Raman spectrum shown inFig. 5d are the characteristic signals of S8 species.38 No diffrac-tion rings or spots from crystalline elemental sulfur are shownin the SAED pattern (Fig. S8d, ESI†), indicating S8 moleculesrooting on the graphene without long-range ordering, similar tosingle-layer molecule dispersion.




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2.4 Electrochemical performance of the graphene–sulfurcomposites

The unique structure of the graphene–sulfur composites canimprove the overall electrochemical performance when it isused as a cathode material for Li–S batteries. Compared withthe above mentioned r-GO and other carbon materials, thegraphene prepared in our work exhibits extremely high qualityand electrical conductivity, which should be desirable andsignicant for sulfur cathode materials. Moreover, hybridiza-tion between the graphene layer and sulfur can increase thelocal charge density37 and therefore trigger a strong adsorbingability to anchor sulfur atoms. Consequently, grapheneprovides an ideal model for intimate interactions with sulfur toanchor the active materials and to effectively prevent thesubsequently formed polysuldes from dissolving in the elec-trolyte during cycling. The interaction renders the sulfur elec-trically conductive and also supports the sulfur atoms as theyswell and shrink during each cycling. This intimate interactionfavors the fast transport of electrons and lithium ions duringthe redox process of sulfur, leading to low electrochemicalpolarization.

Fig. 6a shows typical rst cycle discharge and charge voltageproles for the graphene–sulfur composites containing 73 wt.%sulfur at various rates, which conrms the same pattern ofdischarge and charge plateaus even at very high current rates.The discharge curves exhibited typical two-plateau behavior of asulfur cathode, corresponding to the formation of long-chainpolysuldes at 2.3 V and short-chain Li2S2 and Li2S at 2.1 V.2,11,22

Moreover, the second plateau is very at, suggesting a uniformdeposition of Li2S with little kinetic barriers.10 The ratecapability behavior of the graphene–sulfur composites atdifferent rates is shown in Fig. 6b and c. The specic dischargecapacity of around 1053 mA h g�1 is obtained at a rate of 0.1 C(1 C¼ 1675mA g�1) aer 10 cycles. This value is 835 and 665mAh g�1 at 0.5 C and 1 C, respectively. At a rate of 2 C, the specic

Fig. 6 Electrochemical characterization of graphene–sulfur composite cathodescontaining 73 wt.% sulfur. (a) The first cycle charge–discharge voltage profilesand (b) rate capability at various rates. (c) Galvanostatic discharge–charge profilesat the rate of 1C for the graphene–sulfur composite. (d) Cycling performance andcorresponding Coulombic efficiency for charge–discharge at a rate of 1 C.


discharge capacity is as high as 570 mA h g�1. The reversibilityis demonstrated by the fact that the capacity of 748 mA h g�1 isreached again aer 40 cycles when the rate is returned to 0.5 C.The cycling behavior of the graphene–sulfur composites isconsidered in greater detail in Fig. 6d. Aer the initial capacityloss, the graphene–sulfur composite material shows very highcapacity retention upon cycling. The specic capacity staysabove 615 mA h g�1 aer 100 cycles at 1 C, representing goodcycle stability. The average Coulombic efficiency of the gra-phene–sulfur composite is computed to be 96.1%, indicatingreliable stability. The overall electrochemical propertiesmentioned above are much better than bare sulfur (Fig. S10,ESI†). Our graphene–sulfur composite material exhibits a highcapacity and good cycling stability even with a higher sulfurcontent of 82 wt.%. It still exhibits a good cycling performanceof 530 mA h g�1 aer 100 cycles at 1 C and an excellent ratecapability behaviour of 480 mA h g�1 at 2 C aer 40 cycles atvarious rates, as shown in Fig. S11–13 in the ESI.†

The excellent overall electrochemical properties achieved615 mA h g�1 aer 100 cycles at 1 C, a Coulombic efficiency ofover 96% and 570 mA h g�1 at 2 C, superior to the reportedgraphene–sulfur composite materials (see Table 1), andbelonging to the best series of carbon-based sulfur cathodematerials (Table S2, ESI†). For conventional preparations ofcarbon-based sulfur cathodes, elemental sulfur is placed intothe high surface area carbon species by using thermal evapo-ration, sulfur solution, etc., which cannot promise rapid chargetransfer and prevent sulfur being dissolved into the electrolyte.In contrast, our G–S composites possess a strong interfacialattraction between sulfur and graphene with (i) a short contactdistance from the S atoms to the graphene sheet and (ii) aperfect C 2pz-conjugated hexagonal network. The ball-milled S8molecules prefer to be parallel on the surface of the graphenesheets. Such an arrangement of the sulfur atoms ensures thateach S atom can retain constant attraction during the insertionand extraction of lithium. The unique structure withoutstanding electrical conductivity also provides a short andrapid transport pathway for both electrons and Li ions to ach-ieve high capacity at high current densities. In addition, theplanar structure also provides a large space to accommodate thesignicant volume changes for sulfur during cycling and a largeconductive surface area for depositing insulating Li2S2 and Li2S,in order to preserve the morphology of the electrodes.39

3 Conclusions

In summary, a new composite structure of high performancesulfur–graphene is proposed and a new sulfur assisted exfolia-tion of graphite was developed for the mass production ofgraphene. The elemental sulfur, due to the large attractionbetween sulfur and graphene, plays the same role as Scotch tapeto assist with micromechanical exfoliation during shear-force-dominated grinding. The strong interfacial attraction between Sand graphene is attributed to a similar value in the electro-negativities of two elements, whose similar surface free energieswere proven by the wettability experiments. The obtainedgraphene sheets with outstanding electron conductivity




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(1820 S cm�1) provide an ideal model to homogenously disperseand anchor sulfur and remedy the insulating property of sulfur.Such a unique structure exhibits excellent overall electro-chemical behavior (615 mA h g�1 at 1 C aer 100 cycles and 570mA h g�1 at 2 C) for high power lithium–sulfur batteries. Themethodology developed in this study is scalable and compatiblewith industrial manufacturing approaches.

4 Experimental section4.1 Graphene preparation

Graphite akes (�200 mm) were rst infused into mixed acidwith H2SO4–HNO3 ¼ 1 : 1 for 4 h to obtain CMG. Here, nitricacid serves as an oxidizer and sulfuric acid is an intercalant.Secondly, rapid heating of the CMG to 800 �C caused the violentformation of gaseous species from the edge and intercalant ofCMG to open the edge. There is a visible, dramatic volumeexpansion of CMG aer the edge opening. The ball-millingexperiments were carried out in a planetary mill which mainlyexerts shear forces on the materials. Typically, 50 g of EG and150 g of sulfur powder were placed into an stainless steel pot (f15 cm) containing stainless steel balls 5 mm in diameter. Thecapsule was then agitated with 500 rpm for 3–6 h to obtaingraphene–sulfur composites. The resultant products weredispersed in a CS2 solution with stirring to remove sulfur andobtain freestanding graphene sheets.

4.2 Graphene lms fabrication

The as-prepared graphene was dispersed in deionized water andthe concentration was about 0.01 g L�1. Aer bath sonicationfor 20 min, the graphene suspension was ltered to form agraphene/lter membrane. The thickness was controlled byadjusting the volume and/or the concentration of the graphenesuspension.

4.3 Electrochemical characterization

The charge and discharge capacities were measured with coincells in which a lithium metal foil was used as the counterelectrode. The electrolyte was 1.0 M lithium bis(tri-uoromethanesulfonyl)imide in 1,3-dioxolane and 1,2-dime-thoxyethane (volume ratio 1 : 1). The active material (90 wt%)and polyvinylidene uoride binder (10 wt%) were homoge-neously mixed in a NMP solvent with magnetic stirring. Aerstirring for 3.5 h, the slurry was coated uniformly on analuminum foil. Finally, the electrode was dried under vacuum at100 �C for 20 h. Cell assembly was carried out in an argon-lledglove box. The coin cells were cycled under different currentdensities between cutoff voltages of 3.0 V and 1.5 V on aCT2001A cell test instrument (LAND Electronic Co.) at roomtemperature.

4.4 Characterizations

A Hitachi S-4800 FESEM operating at 1 kV was used to investi-gate the morphologies. Raman spectra were collected on aThermal Dispersive Spectrometer using a laser with an excita-tion wavelength of 532 nm at a laser power of 10 mW. TEM were


conducted using a JEOL 2100F microscope operating at 200 kV.XPS experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg–Ka radiation (hn ¼1253.6 eV). The sheet resistances and electron mobility of thewafer-sized graphene lms were measured by the four-probevan der Pauw method with an Accent HL5500. The TGAmeasurements were carried out in owing O2 : N2¼ 1 : 4 (in air)or under a N2 atmosphere by STA 409 PC. The samples wereheated up to 1000 �C at a rate of 2 �C min�1.

Acknowledgements

This work is nancially supported by the Graphene Project ofCAS (Grant no. KGZD-EW-303), the NSF of China (Grant no.51125006, 91122034, 51121064, 21203234, 51202274, 20901083,11274328) and the Science and Technology Commission ofShanghai (Grant no. 12JC1409000).

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scotch-tape-like exfoliation of graphite assisted with elemental sulfur and graphene–sulfur...

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