Toward More Reliable Lithium–Sulfur Batteries: An All-Graphene

Toward More Reliable Lithium−Sulfur Batteries: An All-Graphene Cathode Structure Ruopian Fang, Shiyong Zhao, Songfeng Pei, Xitang Qian, Peng-Xiang Hou, Hui-Ming Cheng, Chang Liu,* and Feng Li* Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China S Supporting Information *

ABSTRACT: Lithium−sulfur (Li−S) batteries are attracting increasing interest due to their high theoretical specific energy density, low cost, and eco-friendliness. However, most reports of the high gravimetric specific capacity and long cyclic life are not practically reliable because of their low areal specific capacity derived from the low areal sulfur loading and low sulfur content. Here, we fabricated a highly porous graphene with high pore volume of 3.51 cm3 g−1 as the sulfur host, enabling a high sulfur content of 80 wt %, and based on this, we further proposed an allgraphene structure for the sulfur cathode with highly conductive graphene as the current collector and partially oxygenated graphene as a polysulfide-adsorption layer. This cathode structural design enables a 5 mg cm−2 sulfur-loaded cathode showing both high initial gravimetric specific capacity (1500 mAh g−1) and areal specific capacity (7.5 mAh cm−2), together with excellent cycling stability for 400 cycles, indicating great promise for more reliable lithium−sulfur batteries. KEYWORDS: lithium, sulfur, batteries, graphene, high sulfur loading electrodes with micro/mesoporous carbons,11,18 carbon nanotubes,12,19 graphene,9,10,13,14 and carbon fibers.16,17 On the other hand, carbon materials also show great promise for use as a bifunctional polysulfide-adsorption layer between the sulfur electrode and separator. This proves to be effective in enhancing the conductivity of the sulfur electrode and alleviating the migration of polysulfides to the anode, leading to significantly improved utilization and cycle life of the active materials.15,20−22 Recently, considering the nonpolar nature of carbon materials which lead to poor affinity with polar polysulfides,23 functionalized carbon materials with the ability to chemically absorb polysulfides, such as nitrogen-doped porous carbon19,24−26 and graphene oxide,10,27,28 have gained much attention. Chemical bonding between polysulfides and carbon materials can enable a uniform distribution of active materials to be produced and guarantee good electrical contact between the active materials and the conductive surface of the electrode, leading to better sulfur utilization and longer cycle life.23,28,29 As a result, many reported functionalized sulfur/ carbon composites result in a great increase in specific capacity and cycling stability compared with conventional carbon/sulfur

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he research and development of high-energy energy storage systems has seen growing interest because of the increasing demand for powering portable electronics and electric vehicles, and storing renewable energy on a large scale.1 Lithium−sulfur (Li−S) battery is an attractive and promising candidate among emerging secondary battery technologies because of the high theoretical specific capacity (1675 mAh g−1) and energy density (2567 Wh kg−1) of sulfur.2−6 In addition, sulfur is inexpensive, abundant, and environment-friendly. Despite these attractive advantages, this electrochemical system suffers from several inherent problems, including the low conductivity of sulfur and solid reduction products (Li2S2 and Li2S), severe side reactions between highly soluble intermediate polysulfides (Li2Sn, 3 ≤ n ≤ 8) and the lithium anode, and a large volumetric expansion (∼80%) from sulfur to Li2S.6 These problems lead to low specific capacity, fast capacity decay, poor rate performance and low Coulombic efficiency.5,6 To overcome the obstacles described above, a variety of strategies has been explored, including the development of sulfur composite cathodes,2−4 new electrolytes7 and protective coatings on the anode.8 On the cathode side, conductive carbon has received considerable attention due to its tunable pore structure and high electrical conductivity, which simultaneously improve the cathode conductivity and confine dissolved polysulfides within the cathode.2−4,9−18 Efforts have been devoted to the development of various carbon-based sulfur © 2016 American Chemical Society

Received: June 17, 2016 Accepted: August 18, 2016 Published: August 18, 2016 8676

DOI: 10.1021/acsnano.6b04019 ACS Nano 2016, 10, 8676−8682

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Figure 1. Schematic of the all-graphene structural design of the sulfur cathode.

Figure 2. (a) Typical SEM image of HPG. (b) Nitrogen adsorption−desorption isotherms and (c) pore size distribution of the HPG. (d) Raman spectrum of the HPG. (e) TGA curves of the S/HPG and pure sulfur from room temperature to 600 °C in argon. (f) XRD patterns of the S/HPG, HPG and pure sulfur.

electrical conductivity, high specific surface area, large pore volume and good structural stability.3,32,33 It can also have a variety of characteristics and properties due to its different preparation methods, enabling it to be used in different components of Li−S batteries.3 Here, we report the facile synthesis of highly porous graphene (HPG) with a pore volume of up to 3.51 cm3 g−1, allowing a sulfur content of at least 80 wt %, which further enables a high sulfur loading of 5 mg cm−2. From the perspective of integrated structural design of the cathode, we further proposed an all-graphene configuration, in which highly conductive graphene (HCG) with an almost perfect structure was employed as the current collector, and partially oxygenated graphene (POG) with a moderate number of oxygen-containing functional groups was used as a polysulfide-adsorption layer, as illustrated in Figure 1. Benefiting from the all-graphene structural design together with the collaboration of three graphene materials, the asprepared sulfur cathode shows ultrahigh sulfur utilization with a high initial specific capacity of 1500 mAh g−1 and excellent cycling stability with a high areal capacity of 4.2 mAh cm−2 even after 400 cycles, demonstrating a more reliable high-performance Li−S battery.

composites, where sulfur is usually physically confined inside the carbon host.10,19,24,27,28 Recently, taking the practical application of Li−S batteries into account, much attention has been raised to the development of sulfur cathodes with high areal specific capacity, and it has been estimated that, to outperform state-of-the-art Li-ion batteries (∼4 mAh cm−2), the areal sulfur loading need to be increased to ∼5 mg cm−2.16,17,26,30,31 However, the actual sulfur loading in most previously reported electrodes is significantly lower than this value (usually less than 2 mg cm−2) because of unsatisfied sulfur weight ratio in the composite electrode materials. For porous carbon materials, sulfur is accommodated in their pores, maintaining intimate electrical contact with the conductive carbon framework. A high pore volume is particularly required to achieve a high sulfur content in the sulfur/carbon composite. It can be calculated that, taking the ∼80% volumetric expansion from sulfur to Li2S into account, a high pore volume of 3.48 cm3 g−1 is needed for a sulfur/porous carbon composite to have a 80 wt % sulfur content (Table S1). In this regard, it is very important to develop a high-pore-volume sulfur host that not only allows a high sulfur content and a high areal sulfur loading, but also enables high sulfur utilization and good cycling stability, to make the Li−S battery more reliable. Graphene has been regarded as one of the most promising conductive matrixes for Li−S batteries due to its superior

RESULTS AND DISCUSSION HPG was synthesized through thermal exfoliation of graphite oxide prepared by an improved Hummers’ method,34 and its 8677

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adsorption−desorption isotherms of the POG (Figure S1) show a BET specific surface area of 200 m2 g−1, which is beneficial for lithium ion transport. Figure 4c shows the surface morphology of the POG polysulfide-adsorption layer. It is worth noting that the POG was reduced at a temperature of 1000 °C, which ensures good electrical conductivity. The square resistance of the POG layer was measured by the fourpoint probe method to be only 25 Ω/□, which facilitates fast electron transport. In addition, the areal density of the POG interlayer is only 0.2 mg cm−2 (the thickness is about 3 μm, as shown in Figure 1), much lighter than the sulfur loading in the cathode (about 5 mg cm−2), ensuring a high energy density. An SEM image of the surface of the HCG current collector is shown in Figure 4d. The as-prepared current collector shows a high conductivity with a low square resistance of 3 Ω/□. On the one hand, the HCG current collector enables improved adhesion to the sulfur to it due to its surface roughness and thus lowers the impedance and polarization of the Li−S battery. On the other hand, it contributes to the improved practical energy density of the cell because of its lightweight (12 μm in thickness, 1 mg cm−2 in areal density).15 A series of electrochemical measurements was carried out to evaluate the effectiveness of the all-graphene sulfur cathode using metallic lithium as an anode. Considering practical applications, electrodes with a sulfur loading of about 5 mg cm−2 were used here for the following investigation of electrochemical performance, and the areal loading density of HPG on the cathode was calculated to be 1.25 mg cm−2 (considering 80 wt % sulfur weight ratio in the S/HPG composite). Electrochemical impedance spectroscopy (EIS) plots before cycling and after 10 and 20 cycles are shown in Figure 5a. The single depressed semicircles in the high-tomedium frequency region can be ascribed to the charge-transfer resistance, which represents the electrode reaction kinetics.38 It can be seen that, after cycling, the value of the charge-transfer resistance shows a dramatic decrease, revealing good electrolyte infiltration into the cathode and fast charge transport due to this integrated electrode. Moreover, the charge-transfer resistance shows almost no change after 10 and 20 cycles, indicating the high stability of the electrode. Figure 5b shows the cyclic voltammetry (CV) curves of the all-graphene sulfur cathode recorded at 0.1 mv s−1. Two cathodic peaks are observed, that at the higher potential (I) corresponds to the conversion of sulfur (S8) to soluble long-chain polysulfides (Li2Sn, 4 ≤ n ≤ 8) and that at the lower potential (II) corresponds to the subsequent formation of insoluble Li2S2/ Li2S.2,4 Theoretically, the II/I area ratio is expected to be 3, assuming all the polysulfides are reduced to Li2S. From Figure 5b, this ratio was calculated to be 2.6, indicating that most polysulfides are converted into insoluble Li2S2/Li2S. Indeed, it is very challenging to completely convert all the long-chain polysulfides to the final discharge product Li2S because of the sluggish reaction kinetics in II which involves conversion reactions between solid Li2S2/Li2S.4 When scanning back to 2.8 V, the overlapping anodic peak (III) centered at ∼2.35 V corresponds to the delithiation of Li2S to Li2Sx and eventually to elemental sulfur. We evaluated the area of III in Figure 5b, and the cathodic (I + II) /anodic (III) area ratio was calculated to be ∼99%, indicating a high efficiency. The inset in Figure 5b shows the CV profiles for five cycles, and the cathodic and anodic peaks remain almost constant for the recorded five cycles, demonstrating good reversibility and stability. The rate capability of the all-graphene sulfur cathode was investigated

scanning electron microscope (SEM) image in Figure 2a shows a typical wrinkled and corrugated laminar structure. Nitrogen adsorption/desorption isotherms together with pore size distribution of the HPG are shown in Figure 2b,c. Brunauer− Emmett−Teller (BET) specific surface area is as high as 771 m2 g−1 with a high pore volume of 3.51 cm3 g−1, and a broad pore size distribution ranging from 1 to 60 nm. Figure 2d shows the Raman spectrum of the HPG, and two typical Raman bands of carbon can be observed, a D band at 1336 cm−1 arising from defects and disordered carbon, and a G band at 1593 cm−1 due to the stretching mode of the C−C bonds of graphitic carbon.35 The relatively higher intensity of the D band than that of the G band indicates a decrease in the average size of sp2 domains of the HPG.36 The large pore volume of HPG enables the accommodation of a high amount of sulfur, achieving a high sulfur content of 80 wt % in the S/HPG composite prepared by melt diffusion, as shown in the thermogravimetric analysis (TGA) curves in Figure 2e. X-ray diffraction (XRD) patterns of the S/HPG, HPG, and pure sulfur are shown in Figure 2f. The diffraction peaks for crystallized sulfur in the S/HPG can still be observed with decreased intensity compared to pure sulfur. Transmission electron microscope (TEM), scanning transmission electron microscope (STEM), and SEM images of the S/HPG are shown in Figure 3a, b, and c, respectively, and show

Figure 3. (a) TEM image and (b) STEM image of the S/HPG. (c) SEM image of the S/HPG and (d) the corresponding elemental map of sulfur.

a wavy, curving 2D morphology with no sulfur agglomerates. The corresponding elemental map of sulfur in Figure 3d further confirms a homogeneous distribution of sulfur within the laminar structure formed by stacking ultrathin graphene sheets. The POG was prepared by the thermal reduction of graphene oxide, which contains a moderate number of oxygen-containing functional groups.27 The C 1s XPS spectrum of the POG in Figure 4a reveals the presence of CO (286.3 eV), CO (287.9 eV) and OCO (289.0 eV) groups, and the oxygen content was shown to be 9 wt %. The carbonyl and ether groups such as furans, pyrans, and pyrenes37 on the POG sheets (inset in Figure 4a) are expected to chemically bind with sulfur species, contributing to well-localized polysulfides in the cathode side. Figure 4b shows an SEM image of the POG, revealing a layered, crumpled structure, and the nitrogen 8678

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Figure 4. (a) C 1s XPS spectrum of the POG (the inset shows the structural illustration of POG sheets with carbonyls and ether rings). (b) SEM image of the POG. (c) SEM image of the surface of the POG adsorption layer. (d) SEM image of the surface of the HCG current collector (the inset shows the cross-sectional SEM image).

and high Coulombic efficiency at a high sulfur loading. This excellent electrochemical performance demonstrates the advantages of this rational design for advanced Li−S batteries.

under various current densities, as shown in Figure 5c. The cathode exhibits a high initial discharge capacity of 1500 mAh g−1 at 0.34 A g−1, reaching 90% of the theoretical capacity and indicating ultrahigh sulfur utilization. When the current density was increased to 0.5, 0.84, 1.34, and 1.68 A g−1, the cathode was able to deliver discharge capacities of 1244, 1168, 1089, and 1039 mAh g−1, respectively. Figure 5d gives the corresponding charge/discharge profiles, and two obvious stable charge/ discharge plateaus are retained even at high current densities, revealing high electrical conductivity and improved charge transfer kinetics through the cathode. The long-term cycling stability of the integrated electrode was further evaluated at 0.34 A g−1 for up to 400 cycles, as shown in Figure 5e. The cell retains a reversible discharge capacity of 841 mAh g−1 after 400 cycles, and the average Coulombic efficiency was calculated to be above 99.5%, demonstrating excellent cycling stability and high reversibility. The inset in Figure 5e further shows the corresponding areal capacity. Remarkably, the all-graphene sulfur electrode delivers a high initial areal capacity of 7.5 mAh cm−2 with an areal capacity of 4.2 mAh cm−2 after 400 cycles, higher than that for commercially available Li-ion batteries (around 4 mAh cm−2).31 SEM images of the S/HPG electrode before and after cycling (Figure S2) show that the electrode still has a porous surface (Figure S2b) without large sulfur agglomerates, similar to the pristine morphology (Figure S2a), which indicates a high reversibility of the cathode. Figure S3 further shows the SEM images of the POG interlayer and the HCG current collector after cycling, it can be seen that porous structures are retained with no obvious passivation layer, indicative of high structural stabilities of both POG and HCG. The integrated electrode therefore demonstrates high sulfur utilization, high rate capability, excellent cycling stability,

CONCLUSIONS In summary, we have demonstrated the fabrication of HPG, a high-pore-volume sulfur host, which contains a high sulfur content up to 80 wt %. An all-graphene cathode structure was further proposed, with HCG and POG serving as the current collector and the polysulfide-adsorption layer, respectively. HCG contributes to improved practical energy density due to its lightweight, and POG functions to chemically bind with the migrating polysulfides to alleviate the shuttling effect. As a result, the all-graphene sulfur cathode with a high sulfur loading of 5 mg cm−2 demonstrates high capacities (including both the gravimetric specific capacity and areal capacity) and stable cycling performance even after 400 cycles, demonstrating possible solutions toward more reliable Li−S batteries. METHODS Preparation of Highly Porous Graphene (HPG) and S/ HPG Composite. HPG was prepared by the thermal exfoliation of graphite oxide. The graphite oxide was synthesized by oxidation of graphite using an improved Hummers’ method.34 The graphite oxide was thermally exfoliated at 300 °C for 5 min in air and subsequently heated to 1000 °C under an argon atmosphere at a heating rate of 2 °C/min, where it remained for 3 h with to obtain the HPG sample. As-prepared HPG and sulfur powder (99.5%, Alfa Aesar) in a mass ratio of 1:4 were ground together for 30 min. 8679

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Figure 5. (a) Electrochemical impedance spectra of the all-graphene sulfur cathode before cycling and after 10 and 20 cycles (the inset is the magnified plot for the black rectangle region). (b) CV profiles of the all-graphene sulfur cathode recorded at 0.1 mV s−1 (the inset is the CV profiles for 5 cycles). (c) Rate capability and (d) corresponding charge/discharge profiles of the all-graphene sulfur cathode at different current densities. (e) Cycling stability and Coulombic efficiency of the all-graphene sulfur cathode at 0.34 A g−1 (the inset is the corresponding areal capacity).

directly used in coin cells. The areal density of the POG layer was ∼0.2 mg cm−2. Preparation of the Highly Conductive Graphene (HCG) and HCG Current Collector. The HCG was prepared by an intercalation−exfoliation technique (Chinese patent No. CN201110282370.5) and was purchased from Deyang Carbonene Technology Ltd. The HCG current collector was fabricated by a vacuum-filtration and peeling-off process according to a method previously reported by our group.15 The areal density of the HCG current collector was ∼1 mg cm−2. Materials Characterization. The morphology and structure of the samples were characterized using a scanning electron microscope (SEM, FEI Nova NanoSEM 430, 15 kV) and a transmission electron microscope (TEM, Tecnai F20). XPS analysis was performed using an ESCALAB 250 instrument with Al Kα radiation (15 kV, 150 W) under a

The mixture was then transferred to a sealed stainless steel vessel and heated to 155 °C for 15 h to obtain the S/HPG. Preparation of the Partially Oxygenated Graphene (POG) and POG Adsorption Interlayer. A graphene oxide (GO) colloidal suspension was prepared according to a method previously reported by our group.39 The GO suspension was freeze-dried to obtain GO powder, which was thermally reduced by heating to 1000 °C at a rate of 2 °C/min under an argon atmosphere, where it remained for 2 h to obtain the POG powder. The POG adsorption interlayer was prepared by a vacuum-filtration process. Typically, the POG powder was dispersed in ethanol to form a suspension, followed by ultrasonication for 20 min. The suspension was then vacuum filtered onto a commercial polymer separator (Celgard 2400), followed by air drying at 60 °C for 24 h to obtain a separator coated with the POG adsorption interlayer, and this was 8680

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ACS Nano pressure of 4 × 10−8 Pa. X-ray diffraction (XRD) patterns were recorded on a Rigaku diffractometer using Cu Kα radiation. Nitrogen adsorption/desorption characteristics were determined using a Micromeritics ASAP 2020 instrument at 77 K. Before the measurements, the sample powder was degassed at 200 °C until a pressure of 2 mmHg was reached. Raman measurements were performed using a Jobin Yvon Lab RAM HR800 instrument with a 632.8 nm He−Ne laser. TGA was performed with a NETZSCH STA 449 C thermo-balance in air with a heating rate of 10 °C min−1 from room temperature to 600 °C. The square resistance of the POG interlayer and HCG current collector were measured using a standard four pointprobe resistivity measurement system (RTS-9, Guangzhou, China). Five measurements were taken at different positions on each sample, and the average value was calculated and used. Electrochemical Measurements. A well-mixed slurry (85 wt % S/HPG composite, 5 wt % Super P as conductive additive and 10 wt % polyvinylidene fluoride as binder in N-methyl-2pyrolidone) was coated onto the as-prepared HCG current collector followed by drying under vacuum at 60 °C for 24 h. The areal sulfur mass loading was 4.8−5.2 mg cm−2. The S/ HPG electrode was shaped into a circular pellet with a diameter of 12 mm, and the photograph of the as-prepared electrode is present in Figure S4, which shows good mechanical stability with desirable flexibility. The separator coated with the POG adsorption interlayer was directly placed on the S/HPG/HCG electrode to complete the formation of the integrated electrode. Then, 2025-type stainless steel coin cells were assembled inside an Ar-filled glovebox with lithium metal foil as the anode. The electrolyte was prepared by dissolving lithium bis-trifluoromethanesulphonylimide (LITFSI, 99%, Acros Organics,1 M) and lithium nitrate (LiNO3, 99.9%, Alfa Asea, 0.2 M) in 1,2dimethoxyethane (DME, 99.5%, Alfa Asea) and 1,3-dioxolane (DOL, 99.5%, Alfa Asea) (1:1 ratio, by volume). The electrolyte ammount added in each cell is 60 μL. A LAND galvanostatic charge−discharge instrument was used to perform electrochemical measurements. The charge/discharge voltage range was 1.7−2.8 V. The CV test and the electrochemical impedance spectroscopy (EIS) measurements were made using a VSP-300 multichannel workstation. The frequency range of the EIS measurements was 100 kHz to 10 mHz with an AC voltage amplitude of 5 mV at an open-circuit potential.

ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (2014CB932402), NSFC (Nos. 51521091, 51525206, 51172239, 51372253, 51272051, and U1401243), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA01020304, XDA09010104), the Key Research Program of the Chinese Academy of Sciences (Grant No. KGZD-EW-T06), and the CAS/SAFEA International Partnership Program for Creative Research Teams. REFERENCES (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2011, 11, 19−29. (2) Liang, J.; Sun, Z. H.; Li, F.; Cheng, H. M. Carbon Materials for Li-S Batteries: Functional Evolution and Performance Improvement. Energy Storage Mater. 2016, 2, 76−106. (3) Yu, M.; Li, R.; Wu, M.; Shi, G. Graphene Materials for LithiumSulfur Batteries. Energy Storage Mater. 2015, 1, 51−73. (4) Wang, D. W.; Zeng, Q. C.; Zhou, G. M.; Yin, L. C.; Li, F.; Cheng, H. M.; Gentle, I. R.; Lu, G. Q. M. Carbon−Sulfur Composites for Li− S Batteries: Status and Prospects. J. Mater. Chem. A 2013, 1, 9382− 9394. (5) Zhang, S. S. Liquid Electrolyte Lithium/Sulfur Battery: Fundamental Chemistry, Problems, and Solutions. J. Power Sources 2013, 231, 153−162. (6) Manthiram, A.; Fu, Y. Z.; Su, Y. S. Challenges and Prospects of Lithium-Sulfur Batteries. Acc. Chem. Res. 2013, 46, 1125−1134. (7) Nair, J. R.; Bella, F.; Angulakshmi, N.; Stephan, A. M.; Gerbaldi, C. Nanocellulose-Laden Composite Polymer Electrolytes for High Performing Lithium−Sulphur Batteries. Energy Storage Mater. 2016, 3, 69−76. (8) Cao, R.; Xu, W.; Lv, D.; Xiao, J.; Zhang, J.-G. Anodes for Rechargeable Lithium-Sulfur Batteries. Adv. Energy Mater. 2015, 5, 1402273−1402295. (9) Chen, R.; Zhao, T.; Lu, J.; Wu, F.; Li, L.; Chen, J.; Tan, G.; Ye, Y.; Amine, K. Graphene-Based Three-Dimensional Hierarchical Sandwich-type Architecture for High-Performance Li/S Batteries. Nano Lett. 2013, 13, 4642−4649. (10) Li, N.; Zheng, M.; Lu, H.; Hu, Z.; Shen, C.; Chang, X.; Ji, G.; Cao, J.; Shi, Y. High-Rate Lithium-Sulfur Batteries Promoted by Reduced Graphene Oxide Coating. Chem. Commun. 2012, 48, 4106− 4108. (11) Wang, D. W.; Zhou, G.; Li, F.; Wu, K. H.; Lu, G. Q.; Cheng, H. M.; Gentle, I. R. A Microporous-Mesoporous Carbon with Graphitic Structure for a High-Rate Stable Sulfur Cathode in Carbonate SolventBased Li-S Batteries. Phys. Chem. Chem. Phys. 2012, 14, 8703−8710. (12) Zhou, G.; Wang, D.-W.; Li, F.; Hou, P.-X.; Yin, L.; Liu, C.; Lu, G. Q.; Gentle, I. R.; Cheng, H.-M. A Flexible Nanostructured Sulphur−Carbon Nanotube Cathode with High Rate Performance for Li-S Batteries. Energy Environ. Sci. 2012, 5, 8901−8906. (13) Zhou, G. M.; Yin, L. C.; Wang, D. W.; Li, L.; Pei, S. F.; Gentle, I. R.; Li, F.; Cheng, H. M. Fibrous Hybrid of Graphene and Sulfur Nanocrystals for High-Performance Lithium Sulfur Batteries. ACS Nano 2013, 7, 5367−5375. (14) Wang, H.; Yang, Y.; Liang, Y.; Robinson, J. T.; Li, Y.; Jackson, A.; Cui, Y.; Dai, H. Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium-Sulfur Battery Cathode Material with High Capacity and Cycling Stability. Nano Lett. 2011, 11, 2644−2647. (15) Zhou, G. M.; Pei, S. F.; Li, L.; Wang, D. W.; Wang, S. G.; Huang, K.; Yin, L. C.; Li, F.; Cheng, H. M. A Graphene-Pure-Sulfur Sandwich Structure for Ultrafast, Long-Life Lithium-Sulfur Batteries. Adv. Mater. 2014, 26, 625−631. (16) Miao, L. X.; Wang, W. K.; Yuan, K. G.; Yang, Y. S.; Wang, A. B. A Lithium-Sulfur Cathode with High Sulfur Loading and High

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04019. Relationship between the pore volume of porous carbon and the theoretical content of sulfur; nitrogen adsorption−desorption isotherm of the POG; SEM images of the S/HPG electrode before and after 50 charge/discharge cycles. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (C. Liu): [email protected]. *E-mail (F. Li): fl[email protected]. Author Contributions

(R.F. and S.Z.) These authors contributed equally to this work. Notes

The authors declare no competing financial interest. 8681

DOI: 10.1021/acsnano.6b04019 ACS Nano 2016, 10, 8676−8682

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DOI: 10.1021/acsnano.6b04019 ACS Nano 2016, 10, 8676−8682