DOI: 10.1002/chem.201400385
Communication
& Lithium–Sulfur Batteries
Dual Protection of Sulfur by Carbon Nanospheres and Graphene Sheets for Lithium–Sulfur Batteries Bei Wang,[a] Yanfen Wen,[a] Delai Ye,[a] Hua Yu,[a] Bing Sun,[b] Guoxiu Wang,[b] Denisa Hulicova-Jurcakova,[a] and Lianzhou Wang*[a] Abstract: Well-confined elemental sulfur was implanted into a stacked block of carbon nanospheres and graphene sheets through a simple solution process to create a new type of composite cathode material for lithium–sulfur batteries. Transmission electron microscopy and elemental mapping analysis confirm that the as-prepared composite material consists of graphene-wrapped carbon nanospheres with sulfur uniformly distributed in between, where the carbon nanospheres act as the sulfur carriers. With this structural design, the graphene contributes to direct coverage of sulfur to inhibit the mobility of polysulfides, whereas the carbon nanospheres undertake the role of carrying the sulfur into the carbon network. This composite achieves a high loading of sulfur (64.2 wt %) and gives a stable electrochemical performance with a maximum discharge capacity of 1394 mAh g1 at a current rate of 0.1 C as well as excellent rate capability at 1 C and 2 C. The improved electrochemical properties of this composite material are attributed to the dual functions of the carbon components, which effectively restrain the sulfur inside the carbon nano-network for use in lithium–sulfur rechargeable batteries.
The worldwide energy crisis has raised many concerns as fossil-based energy sources are gradually depleted and has raised awareness of related environmental issues such as global warming and climate change.[1, 2] Reliable and sustainable energy sources are therefore in huge demand to supply renewable energy and pave the way for future electric vehicles.[3] Lithium-ion rechargeable batteries are one of the most promising products of the past two decades; however, their relatively low energy and power densities have limited their use as they do not satisfy the current critical criteria.[4] [a] Dr. B. Wang, Y. Wen, D. Ye, Dr. H. Yu, Dr. D. Hulicova-Jurcakova, Prof. L. Wang Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology The University of Queensland, St. Lucia, Brisbane, QLD 4072 (Australia) E-mail: [email protected] [b] Dr. B. Sun, Prof. G. Wang Centre for Clean Energy Technology and School of Chemistry and Forensic Science, University of Technology Sydney Broadway, Sydney, NSW 2007 (Australia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201400385. Chem. Eur. J. 2014, 20, 5224 – 5230
Recently, research on lithium–sulfur batteries has been significantly increasing as sulfur is capable of delivering an ultrahigh theoretical capacity of 1685 mAh g1 upon full reduction of sulfur to lithium sulfide (S8 + 16 Li$8 Li2S), surpassing all the conventional cathode materials in lithium-ion batteries.[5] In addition to their low cost and wide availability, sulfur-based cathodes are viable to be commercialized in powerful lithium rechargeable batteries. However, current challenges remain in the insulating nature of sulfur and its dramatic loss in the form of polysulfide during electrochemical charge and discharge processes (also known as the “shuttle issue”). There have been numerous investigations into improving sulfur cathodes by using conducting carbon-based components to enable its electrochemical activities and restrain sulfur within the electrode. These include the use of mesoporous carbon (CMK-3[6] and microporous–mesoporous carbon[7]), graphene nanosheets,[8–12] conducting polymers,[13, 14] porous hollow carbon,[15] doubleshelled hollow carbon spheres,[16] and carbon nanotubes[17] as additives to enhance the conductivity of the lithium–sulfur cells and simultaneously prevent diffusion of any polysulfide ions formed. This strategy has been widely considered to make insulating sulfur active in electrochemical reactions. Alternatively, modifications to the cell configuration have been implemented, for example, by introducing an additional free-standing multiwalled carbon nanotube[18] or microporous carbon paper[19] as a functional interlayer to localize dissolved polysulfide, and thus to improve the active material utilization and capacity retention of lithium–sulfur cells. Another attempt comes from Guo’s research group, who have synthesized small sulfur molecules consisting of short-chained S2–4 crystallines,[20] which can eliminate the transition process between S8 and S42 and effectively diminish the shuttle issue. Up to now, significant progress has been made in improving the specific capacity, sulfur utilization, as well as the cycling retention of lithium–sulfur batteries. However, most of the proposed solutions to address the shuttle issue require complicated synthetic processes and restricted environment, and are not easy to follow or repeat.[11, 18–20] In addition, the sulfur content in some reports was too low (< 50 wt %) to achieve a practical energy density for the sulfur cathode,[7, 10] or the yielded capacities were only moderate even with a high proportion of sulfur.[8] To simplify the synthetic processes, use mild conditions, and to make the sulfur-based electroactive composite easily reproducible, it is critically important to develop reliable and facile approaches to the design of lithium–sulfur cathode materials. Herein, we report a new type of sandwiched carbon
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Communication network, formed by simply combining carbon nanospheres and graphene sheets through a scalable and low-cost synthetic route. Our key idea is to accommodate and protect sulfur from severe loss during electrochemical activities by utilizing wrapped graphene sheets to restrain sulfur, which is initially coated onto carbon nanospheres. The resultant composite material possesses a high weight percentage (64.2 wt %) of sulfur thanks to the assistance of the carbon nanospheres, which are readily redispersible in ethanol and thus able to transport sulfur into the interlayers of graphene sheets. Unlike most techniques involving the initial establishment of a sulfur-confined carbon network followed by the imbibition of sulfur, or the production of sulfur together with the build-up of the carbon domain, our method deals directly with precoated elemental sulfur in ethanol solution. This allows, for the first time to our knowledge, easy handling and also brings opportunities for future low-cost treatment of sulfur-coated carbon nanospheres with various other conducting candidates for sulfurbased cathode materials. When the prepared composite was applied as the lithium–sulfur cathode, the highest specific discharge capacity of 1394 mAh g1 was obtained at a current rate of 0.1 C (1 C = 1685 mAh g1), and the cycling stability was well maintained for up to 100 cycles. This composite was also able to deliver specific capacities of 746 and 604 mAh g1 at 1 C and 2 C, respectively. The individual role and function of each carbon component in the composite material are discussed as well. The schematic diagram illustrating the synthetic strategy is shown in Figure 1. Carbon nanospheres (CS) prepared by the reported hydrothermal method are solid carbonaceous material with hydroxyl groups (OH) on their surface.[21, 22] We found that these carbon nanospheres can act as sulfur carriers to achieve high sulfur loading through a simple thermal treatment owing to the low melting point and recrystallization process of sulfur. The treated carbon nanospheres were coated with a large proportion of sulfur (S@CS), and their further dispersion in ethanol is enabled by the surface hydroxyl groups. By dis-
persing sulfur-coated carbon nanospheres into graphene sheets through a reduction reaction with hydrazine, a packed 3D carbon network (G@S@CS) is formed and sulfur is consequently confined inside the block (Figure 5 and Figure 6). Such morphology is expected to limit the diffusion of polysulfide during the charge/discharge process and further enhance the performance and lifetime of the lithium–sulfur cells as shown hereafter. X-ray diffraction (XRD) patterns of pure sulfur, CS, S@CS, and G@S@CS materials are shown in Figure 2 a. All the sulfur-containing samples present typical diffraction lines of crystalline sulfur (JCPDS card No. 08-0247) as indexed. However, as shown
Figure 2. (a) XRD patterns and (b) Raman spectra of pure sulfur, CS, S@CS, and G@S@CS. I = intensity; k = Raman shift.
Figure 1. Schematic diagram of the formation of the G@S@CS nanocomposite. Chem. Eur. J. 2014, 20, 5224 – 5230
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in the XRD patterns of S@CS and G@S@CS, the intensity of the diffraction peaks gradually decreases and the peaks become less resolved and broader; this is due to the reduced particle size of sulfur (Figure 6) and the introduction of carbon nanospheres and graphene sheets. Furthermore, CS exhibits a broad XRD peak at 238, indicating the formation of a carbonaceous material with very poor crystallinity. It is also noteworthy that no signs of carbon nanospheres or graphene sheets were identified in the XRD patterns of S@CS and G@S@CS as these carbon components were homogenously incorporated with sulfur crystallines and graphene was highly exfoliated in the G@S@CS composite.[8, 23] 5225
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Communication Figure 2 b shows the Raman spectra of crystalline sulfur, CS, S@CS, and G@S@CS. It can be seen that for pure sulfur, the Raman spectrum shows various peaks, which correspond to the vibrational modes of sulfur, in the low wave number region. All the other spectra of carbon-containing samples present two Raman peaks at around 1360 and 1596 cm1. The former peak represents the in-plane vibration of disordered amorphous carbon, whereas the latter represents crystalline graphitic carbon.[24] These identified peak features are in good agreement with those in previous reports for carbon nanospheres.[21, 25] In the spectrum of G@S@CS, two Raman peaks are located at similar positions (1343 and 1584 cm1) and they are marked as the D band and G band; these are characteristics peaks associated with broken symmetry and the E2g vibrational mode, respectively, in graphene-related materials.[26] The intensity ratio of the D band to the G band (ID/IG) of G@S@CS is quite high and much larger than those in the spectra of CS and S@CS. This indicates the broken symmetry of an ordered carbon domain in the G@S@CS composite, further confirming the presence of a decreased sp2 carbon region and the exfoliation of graphene sheets.[27] There are no characteristic bands of sulfur in the spectra of S@CS and G@S@CS owing to the phonon confinement effect,[28] as sulfur crystallines are homogeneously distributed in the carbon matrix (Figure 5 and Figure 6). The XRD patterns combined with Raman spectra prove the co-existence of carbon nanospheres and graphene sheets in the G@S@CS composite. Figure 3 shows the X-ray photoelectron spectrometry (XPS) spectra of pure sulfur, CS, S@CS, and G@S@CS. The presence of sulfur in S@CS and G@S@CS can be confirmed through the identification of S 2s and S 2p3/2 binding, as indicated in the
Figure 4. TGA curves of pure sulfur, S@CS, and G@S@CS measured from room temperature to 700 8C in air. Wp = weight percentage; T = temperature.
the FTIR measurements (see the Supporting Information, Figure S1), which detect weak absorptions of hydroxyl functional groups. Thermogravimetric analysis (TGA) curves for pure sulfur, S@CS, and G@S@CS are shown in Figure 4. The pure sulfur sample was used as a reference to determine the sulfur content in the S@CS and G@S@CS composite materials. In the TGA curve of pure sulfur, the weight loss from ~ 150 to 300 8C can only be ascribed to the consumption of sulfur in air. Therefore, it can be determined that the S@CS sample has a very high initial loading of sulfur (82.7 wt %). On the other hand, the sulfur content of G@S@CS was reduced to 64.2 wt % with graphene wrapping, as confirmed by the TGA result of the G@S@CS sample. Figure 5 presents the field-emission (FE) SEM images of CS and G@S@CS at low magnifications. The prepared CS sample consists of uniform carbon spheres with a diameter of approxi-
Figure 3. XPS spectra of pure sulfur, CS, S@CS, and G@S@CS. C = counts; E = binding energy.
reference XPS spectrum of pure sulfur. No other forms of sulfur-related binding can be found in the spectra of these composites as sulfur is only present in the elemental state. In addition, CS exhibits two strong peaks, which are assigned to C 1s and O 1s binding, suggesting a dominant carbonaceous material with O-enriched functionalities, such as surface hydroxyl groups, an observation that is in good agreement with Chem. Eur. J. 2014, 20, 5224 – 5230
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Figure 5. FE-SEM images of (a) CS and (b) G@S@CS.
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Communication mately 250 nm. These spheres are in the form of densely packed aggregates (Figure 5 a). In Figure 5 b, it can be seen that the nanosized carbon spheres are well isolated and homogeneously trapped by wavy and soft graphene sheets, a fact that provides intimate contacts between the graphene sheets and carbon nanospheres. There is no aggregation of sulfur crystallines observable in Figure 5 b, implying a homogeneous sulfur deposition within the sandwiched carbon network over the examined region. It should also be noted that owing to the high energy of the electron beam focused on a small selected area, sulfur becomes quite unstable in the pure elemental state and in the S@CS composite (82.7 wt % of sulfur) and will quickly sublime and disappear. Therefore, additional morphological features were investigated by the TEM examinations shown in Figure 6 for pure sulfur, CS, S@CS, and G@S@CS. It
ure 5 b. No isolated sulfur aggregation can be located, suggesting successful incorporation of S@CS between graphene layers through our solution process and also revealing homogenous sulfur distribution in the G@S@CS composite. To further prove the presence of sulfur in the G@S@CS composite and examine its distribution, SEM mapping analysis was performed to obtain detailed elemental distribution information. In a selected region, as shown in Figure 7 a, CS are uni-
Figure 7. (a) SEM image of a selected region of the G@S@CS nanocomposite and the corresponding elemental maps of (b) carbon, (c) sulfur, and (d) carbon and sulfur.
Figure 6. TEM images of (a) pure sulfur, (b) CS, (c) S@CS, and (d) G@S@CS.
can be seen from Figure 6 a that pure sulfur particles are densely aggregated and their size ranges from a few hundred nanometers to a few micrometers. Furthermore, CS is found to consist of nanosized spheres with a uniform diameter of around 250 nm (Figure 6 b), consistent with the FE-SEM observations. For S@CS, the surface of the spherical carbonaceous units becomes rougher after sulfur coating, compared with untreated CS, and there are also very small amounts of deposited material visible surrounding the spheres, material which is likely to be loosened sulfur crystallines from the surface of CS (Figure 6 c). These sulfur crystallines form tiny aggregates and become noticeable under microscope analysis. Nevertheless, no large aggregation of sulfur can be found and this demonstrates the uniform coating of sulfur on CS. Figure 6 d shows the typical structure of the G@S@CS composite with graphene sheets wrapping S@CS, similar to the FE-SEM images of FigChem. Eur. J. 2014, 20, 5224 – 5230
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formly distributed between graphene layers, in good agreement with the FE-SEM and TEM observations. The carbon and sulfur elemental maps (Figure 7 b and c, respectively) prove that both carbon and sulfur exist densely in this region, and Figure 7 d shows a mixed elemental map of both carbon and sulfur, indicating a uniform distribution of sulfur in the carbon matrix. The elemental mapping results are direct evidence that confirms the successful incorporation of sulfur into the carbon network constructed by carbon nanospheres and graphene sheets in our G@S@CS composite material. Cyclic voltammetry (CV) curves of the G@S@CS electrode for the 1st, 2nd, and 5th cycles are presented in Figure 8. It should be noted that all the CV curves show typical cathodic peaks at 2.3–2.4 and 2.0–2.1 V regions, corresponding to the reduction processes from elemental sulfur to long-chain polysulfide and from long-chain polysulfide to short-chain sulfide. At around 2.5–2.7 V, characteristic anodic peaks are found, revealing a reverse oxidation stage from sulfide to elemental sulfur.[5] The CV curve in the 1st cycle exhibits broad peaks, which are likely to be related to the activation of the cell and the gradual enhancement of the sulfur utilization. Furthermore, from the 2nd cycle onwards, the CV peaks do not shift significantly, suggesting highly reversible redox reactions as the CV test continues. Figure 9 shows the charge/discharge profiles of the G@S@CS electrode at a current rate of 0.1 C. Two typical reduction plateaux were identified at 2.4–2.1 V and 2.1 V in the discharge profile, in accordance with the two cathodic peaks in the CV
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Figure 8. CV curves for the G@S@CS electrode at a sweep rate of 0.1 mV s1 in the voltage range of 1--3 V vs. Li/Li + . I = current; E = voltage.
Figure 10. Long-term cycling performance of the G@S@CS and S@CS electrodes at a current rate of 0.1 C for 100 cycles. Cs = specific capacity; N = cycle number (1 C = 1685 mAh g1).
Figure 9. Charge/discharge profiles of the G@S@CS electrode at a current rate of 0.1 C for the 1st, 2nd, and 5th cycles. E = voltage; Cs = specific capacity (1 C = 1685 mAh g1).
curves (Figure 8). The discharge capacity reached its highest value of 1394 mAh g1 in the 1st cycle. Upon oxidation, the plateau at 2.4 V represents the transformation of polysulfide to sulfur.[5] Notably, obvious discharge and charge capacity variations are observed between the 1st and 2nd cycles. This suggests a sulfur loss occurring after the 1st cycle. It is very likely that there are still some gaps or uncovered voids and space present in the carbon matrix, allowing polysulfide to dissolve and be transported, thereby causing capacity loss, despite the fact that the graphene sheets and carbon nanospheres restrain polysulfides during the electrochemical activities. Nevertheless, from the 2nd cycle onwards, the capacity loss is fairly limited and the cycling performance becomes stabilized owing to the protection of both the carbon nanospheres and graphene sheets. The shuttle issue of lithium–sulfur cells is consequently diminished by using the G@S@CS electrode, but not completely eliminated. The prolonged cycling performance of the G@S@CS and S@CS electrodes in terms of specific discharge capacity versus cycle number is shown in Figure 10. The maximum discharge capacity of G@S@CS was 1394 mAh g1 in the 1st cycle, folChem. Eur. J. 2014, 20, 5224 – 5230
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lowed by a reduced capacity of 1165 mAh g1 in the 2nd cycle. Afterwards, the discharge capacities became stable for up to 100 cycles and a final capacity of 815 mAh g1 was retained. On the contrary, the S@CS electrode showed a very low initial capacity of 317 mAh g1 in the 1st cycle, and the largest capacity of 375 mAh g1 in the 2nd cycle. The capacity of S@CS then decreased dramatically and only 65 mAh g1 was retained after 100 cycles. The inferior performance of S@CS implies low sulfur utilization and severe sulfur loss. It can be concluded that with the protection of graphene on S@CS, the sulfur utilization and electrochemical properties of G@S@CS are significantly improved and the designed structural arrangement plays a crucial role to confine sulfur on the electrode and maintain its vital role to the cyclic life of the battery. The rate performance of the G@S@CS electrode was evaluated at different current rates as shown in Figure 11. The capacities obtained at the elevated current rates were slightly decreased compared with those at lower rates: they could reach as high as 746 and 604 mAh g1 at 1 and 2 C, respectively, with minor capacity decays. When the current rate dropped to 0.1 C, a reasonable capacity of 818 mAh g1 could still be main-
Figure 11. Step-by-step rate performance of the G@S@CS electrode at current rates of 0.1, 0.2, 0.5, 1 and 2 C. Cs = specific capacity; N = cycle number (1 C = 1685 mAh g1).
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Communication tained, revealing a durable cycleability and the well-preserved structure of the cycled G@S@CS electrode. To identify the morphology changes of the S@CS and G@S@CS electrodes, a postmortem FE-SEM analysis was performed on these electrodes and compared with the FE-SEM analysis performed before the cyclic tests, as shown in Figure 12. Fresh electrodes were stored for direct examinations,
Figure 12. Postmortem FE-SEM images of the S@CS electrode (a) before cycling and (b) after 100 cycles; and the G@S@CS electrode (c) before cycling and (d) after 100 cycles.
up a highly feasible route to further process sulfur in solution with an extremely high loading. Moreover, the majority of the coverage and trapping of sulfur should be credited to the graphene wrapping, which protects the formed polysulfide within a well-built carbon network and provides intimate contact with sulfur as a highly conducting medium. Also, the soft and flexible graphene sheets can buffer the side effects associated with volume expansion of the active sulfur within the battery cell during charge/discharge processes to achieve a durable service life. Nanosized carbon nanospheres with surface hydroxyl groups (OH) are reported, for the first time, to carry a high weight percentage (82.7 wt %) of precoated sulfur and form a composite material upon further solution-based incorporation with graphene (containing 64.2 wt % of sulfur) as a novel lithium–sulfur battery cathode. The prepared G@S@CS composite had a homogenous sulfur distribution within the 3D carbon architecture and exhibited the highest specific capacity of 1394 mAh g1 at a current rate of 0.1 C, sustainable cycling durability for up to 100 cycles, and satisfactory rate capability. The excellent overall electrochemical properties were achieved owing to contributions of both the carbon nanospheres and the graphene sheets, which substantially restrict the diffusion and loss of active sulfur and improve the electrode durability.
Experimental Section Synthesis of carbon nanospheres (CS)
whereas the cycled electrodes were disassembled from the coin cells and washed with acetone and dried prior to the microscopy analysis. It can be seen that both the fresh electrodes of S@CS and G@S@CS exhibit integral microstructure with fairly smooth surfaces as shown in Figure 12 a and c. However, after 100 continuous charge and discharge cycles, the microstructure of the S@CS electrode was found to be severely damaged (Figure 12 b). There are large holes and voids present and the surface of the electrode is no longer flat. This could result from the dramatic loss of active sulfur owing to the lack of protection and the volume expansion of elemental sulfur during the electrochemical reactions. For comparison, the morphological characteristic of the G@S@CS electrode after 100 cycles is presented in Figure 12 d. Only slight cracking of the surface can be seen, indicating that the entire electrode microstructure was well conserved throughout the cyclic test. It is believed that the preservation of the G@S@CS electrode can be attributed to the graphene-wrapping effect, which not only improves sulfur utilization to give large specific capacities, but also effectively restricts sulfur from diffusion and alleviates the volume expansion of sulfur between the flexible graphene layers. In summary, the improved performance of lithium–sulfur cathodes consisting of trapped sulfur in a domain of carbon nanospheres and graphene sheets is reported. The carbon nanospheres and graphene sheets provide dual protection to conserve sulfur in the resultant composite material. We found that carbon nanospheres with surface hydroxyl groups open Chem. Eur. J. 2014, 20, 5224 – 5230
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Glucose (4 g) was dissolved in de-ionized (DI, 40 mL) water and transferred to a Teflon-lined autoclave. The autoclave was sealed and heated at 180 8C in an oven for 20 h. The dark-brown product was collected, washed with DI water three times, and dried at 60 8C.
Synthesis of graphene oxide (GO) The preparation of graphene oxide was derived from the Hummers’ method.[29] Briefly, graphite (1 g) was added to H2SO4 (25 mL, 98 %) in an ice bath, with sequent addition of KMnO4 (4 g) and NaNO3 (0.5 g). The mixture was stirred for 3 h and then diluted to 150 mL with DI water. Afterwards, H2O2 (60 mL, 5 %) was introduced into the mixture, which turned pale yellowish in color. The product (GO) was repeatedly washed with DI water to neutral pH and finally dried at 60 8C.
Synthesis of sulfur-coated carbon nanospheres (S@CS) The CS were sintered at 400 8C for 1 h under argon protection prior to the incorporation of sulfur to increase their conductivity. In a typical process, CS (200 mg) was mixed with elemental sulfur (1 g) in a mortar and then the mixture was transferred and heated in an oven at 150 8C for 6 h. A black product (S@CS) was obtained and stored for further use.
Synthesis of S@CS with graphene wrapping (G@S@CS) S@CS powder (100 mg) was dispersed with an ultrasonic probe (Branson Digital Sonifier, S-450D) in ethanol (100 mL) cooled with an ice bath. The ice bath is to protect the solution from overheating, which may melt the sulfur. To the homogeneous dispersion,
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Communication an aqueous solution of GO (20 mL of 2 mg mL1) was gradually added, followed by the addition of hydrazine hydrate (80 mL) as a reducing agent. The mixture was stirred for 24 h at room temperature, then filtered and washed with ethanol and DI water to obtain the final product (G@S@CS).
Keywords: carbon nanospheres · graphene · high capacity · lithium–sulfur batteries · sulfur
Materials characterizations and electrochemical properties X-ray diffraction (XRD) patterns of sulfur, CS, S@CS, and G@S@CS were obtained on a Bruker D8 Advance powder X-ray diffractometer. Raman Spectroscopy was recorded on an Alpha 300 R Confocal Raman Microscope (WiTec, Germany) with a laser source of 532 nm. The spectra were obtained with an exposure time of 5 s for 30 accumulations from 100 to 2000 cm1. The X-ray photoelectron spectrometry (XPS) analysis was conducted on a Kratos Axis Ultra X-ray Photoelectron Spectrometer, which uses AlKa (1253.6 eV) X-ray. Thermogravimetric analysis (TGA) was utilized to determine the sulfur content of the composite materials. Test conditions were set to a ramping rate of 10 8C min1 from room temperature to 700 8C in air on a Mettler Toledo TGA/DSC Thermogravimetric Analyzer. Field-emission scanning electron microscopy (FESEM) images were captured on a JEOL 7001 facility. Transmission electron microscopy (TEM) examinations were performed on a JEOL 1010 Transmission Electron Microscope to analyze the morphological structure of the samples. Elemental maps of carbon and sulfur of the as-prepared G@S@CS nanocomposite were acquired on an Environmental Scanning Electron Microscope (FEI Quanta 200) by utilizing the energy dispersive X-ray (EDX) mapping technique. The electrochemical properties of the as-prepared G@S@CS material were evaluated in assembled CR2032 coin cells, fabricated inside an argon-filled glovebox (MBraun, Germany). Electrodes were prepared by pasting a mixture slurry of G@S@CS (80 wt %), acetylene black (10 wt %), and polyvinylidene fluoride (10 wt %) in the presence of N-methyl-2-pyrrolidone, on aluminum substrates with subsequent heating at 80 8C for 12 h under vacuum. Metallic lithium foils were used as the negative electrode and the electrolyte was prepared by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI, 1 m) in 1,2-dimethoxyethane and 1,3-dioxolane (1:1 v/v) with additional LiNO3 (0.1 m). The cyclic voltammetry (CV) results were obtained on an electrochemistry workstation (CHI660E) at a scan rate of 0.1 mV s1 in the voltage range of 1–3 V against Li/ Li + . The galvanostatic charge/discharge capacities were recorded on a battery tester (LAND, 5 V, 10 mA) at 0.1 C (1 C = 1685 mA g1) for 100 cycles. Stepwise rate performance was examined at elevated current rates of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C on the same battery tester.
Acknowledgements Financial support from the Australian Research Council (through its DPs and Centre’s grant) is gratefully acknowledged. Dr. Elena Taran is greatly appreciated for the Raman Spectroscopy analysis, which was performed at the Queensland Node of the Australian National Fabrication Facility
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(ANFF), a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers.
[1] D. Lindley, Nature 2010, 463, 18 – 20. [2] Q. Schiermeier, J. Tollefson, T. Scully, A. Witze, O. Morton, Nature 2008, 454, 816 – 823. [3] P. G. Bruce, B. Scrosati, J. M. Tarascon, Angew. Chem. 2008, 120, 2972 – 2989; Angew. Chem. Int. Ed. 2008, 47, 2930 – 2946. [4] G. X. Wang, H. Liu, J. Liu, S. Z. Qiao, G. Q. M. Lu, P. Munroe, H. Ahn, Adv. Mater. 2010, 22, 4944 – 4948. [5] X. Ji, L. F. Nazar, J. Mater. Chem. 2010, 20, 9821 – 9826. [6] X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500 – 506. [7] D. W. Wang, G. M. Zhou, F. Li, K. H. Wu, G. Q. Lu, H. M. Cheng, I. R. Gentle, Phys. Chem. Chem. Phys. 2012, 14, 8703 – 8710. [8] S. Evers, L. F. Nazar, Chem. Commun. 2012, 48, 1233 – 1235. [9] N. Li, M. Zheng, H. Lu, Z. Hu, C. Shen, X. Chang, G. Ji, J. Cao, Y. Shi, Chem. Commun. 2012, 48, 4106 – 4108. [10] B. Wang, K. Li, D. Su, H. Ahn, G. Wang, Chem. Asian J. 2012, 7, 1637 – 1643. [11] H. L. Wang, Y. Yang, Y. Y. Liang, J. T. Robinson, Y. G. Li, A. Jackson, Y. Cui, H. J. Dai, Nano Lett. 2011, 11, 2644 – 2647. [12] F. F. Zhang, X. B. Zhang, Y. H. Dong, L. M. Wang, J. Mater. Chem. 2012, 22, 11452 – 11454. [13] Y. Yang, G. Yu, J. J. Cha, H. Wu, M. Vosgueritchian, Y. Yao, Z. Bao, Y. Cui, ACS Nano 2011, 5, 9187 – 9193. [14] L. C. Yin, J. L. Wang, F. J. Lin, J. Yang, Y. Nuli, Energy Environ. Sci. 2012, 5, 6966 – 6972. [15] N. Jayaprakash, J. Shen, S. S. Moganty, A. Corona, L. A. Archer, Angew. Chem. 2011, 123, 6026 – 6030; Angew. Chem. Int. Ed. 2011, 50, 5904 – 5908. [16] C. Zhang, H. B. Wu, C. Yuan, Z. Guo, X. W. Lou, Angew. Chem. 2012, 124, 9730 – 9733; Angew. Chem. Int. Ed. 2012, 51, 9592 – 9595. [17] G. M. Zhou, D. W. Wang, F. Li, P. X. Hou, L. C. Yin, C. Liu, G. Q. Lu, I. R. Gentle, H. M. Cheng, Energy Environ. Sci. 2012, 5, 8901 – 8906. [18] Y.-S. Su, A. Manthiram, Chem. Commun. 2012, 48, 8817 – 8819. [19] Y.-S. Su, A. Manthiram, Nat. Commun. 2012, 3, 1166. [20] S. Xin, L. Gu, N.-H. Zhao, Y.-X. Yin, L.-J. Zhou, Y.-G. Guo, L.-J. Wan, J. Am. Chem. Soc. 2012, 134, 18510 – 18513. [21] X. M. Sun, Y. D. Li, Angew. Chem. 2004, 116, 607 – 611; Angew. Chem. Int. Ed. 2004, 43, 597 – 601. [22] X. M. Sun, J. F. Liu, Y. D. Li, Chem. Eur. J. 2006, 12, 2039 – 2047. [23] L. C. Yin, J. L. Wang, X. L. Yu, C. W. Monroe, Y. NuLi, J. Yang, Chem. Commun. 2012, 48, 7868 – 7870. [24] A. Ilie, C. Durkan, W. I. Milne, M. E. Welland, Phys. Rev. B 2002, 66, 045412. [25] X. M. Sun, Y. D. Li, Langmuir 2005, 21, 6019 – 6024. [26] M. J. Allen, V. C. Tung, R. B. Kaner, Chem. Rev. 2010, 110, 132 – 145. [27] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen, R. S. Ruoff, Carbon 2007, 45, 1558 – 1565. [28] J. Zuo, C. Y. Xu, Y. P. Liu, Y. T. Qian, Nanostruct. Mater. 1998, 10, 1331 – 1335. [29] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339 – 1339. Received: January 30, 2014 Published online on April 1, 2014
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& Lithium–Sulfur Batteries
Dual Protection of Sulfur by Carbon Nanospheres and Graphene Sheets for Lithium–Sulfur Batteries Bei Wang,[a] Yanfen Wen,[a] Delai Ye,[a] Hua Yu,[a] Bing Sun,[b] Guoxiu Wang,[b] Denisa Hulicova-Jurcakova,[a] and Lianzhou Wang*[a] Abstract: Well-confined elemental sulfur was implanted into a stacked block of carbon nanospheres and graphene sheets through a simple solution process to create a new type of composite cathode material for lithium–sulfur batteries. Transmission electron microscopy and elemental mapping analysis confirm that the as-prepared composite material consists of graphene-wrapped carbon nanospheres with sulfur uniformly distributed in between, where the carbon nanospheres act as the sulfur carriers. With this structural design, the graphene contributes to direct coverage of sulfur to inhibit the mobility of polysulfides, whereas the carbon nanospheres undertake the role of carrying the sulfur into the carbon network. This composite achieves a high loading of sulfur (64.2 wt %) and gives a stable electrochemical performance with a maximum discharge capacity of 1394 mAh g1 at a current rate of 0.1 C as well as excellent rate capability at 1 C and 2 C. The improved electrochemical properties of this composite material are attributed to the dual functions of the carbon components, which effectively restrain the sulfur inside the carbon nano-network for use in lithium–sulfur rechargeable batteries.
The worldwide energy crisis has raised many concerns as fossil-based energy sources are gradually depleted and has raised awareness of related environmental issues such as global warming and climate change.[1, 2] Reliable and sustainable energy sources are therefore in huge demand to supply renewable energy and pave the way for future electric vehicles.[3] Lithium-ion rechargeable batteries are one of the most promising products of the past two decades; however, their relatively low energy and power densities have limited their use as they do not satisfy the current critical criteria.[4] [a] Dr. B. Wang, Y. Wen, D. Ye, Dr. H. Yu, Dr. D. Hulicova-Jurcakova, Prof. L. Wang Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology The University of Queensland, St. Lucia, Brisbane, QLD 4072 (Australia) E-mail: [email protected] [b] Dr. B. Sun, Prof. G. Wang Centre for Clean Energy Technology and School of Chemistry and Forensic Science, University of Technology Sydney Broadway, Sydney, NSW 2007 (Australia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201400385. Chem. Eur. J. 2014, 20, 5224 – 5230
Recently, research on lithium–sulfur batteries has been significantly increasing as sulfur is capable of delivering an ultrahigh theoretical capacity of 1685 mAh g1 upon full reduction of sulfur to lithium sulfide (S8 + 16 Li$8 Li2S), surpassing all the conventional cathode materials in lithium-ion batteries.[5] In addition to their low cost and wide availability, sulfur-based cathodes are viable to be commercialized in powerful lithium rechargeable batteries. However, current challenges remain in the insulating nature of sulfur and its dramatic loss in the form of polysulfide during electrochemical charge and discharge processes (also known as the “shuttle issue”). There have been numerous investigations into improving sulfur cathodes by using conducting carbon-based components to enable its electrochemical activities and restrain sulfur within the electrode. These include the use of mesoporous carbon (CMK-3[6] and microporous–mesoporous carbon[7]), graphene nanosheets,[8–12] conducting polymers,[13, 14] porous hollow carbon,[15] doubleshelled hollow carbon spheres,[16] and carbon nanotubes[17] as additives to enhance the conductivity of the lithium–sulfur cells and simultaneously prevent diffusion of any polysulfide ions formed. This strategy has been widely considered to make insulating sulfur active in electrochemical reactions. Alternatively, modifications to the cell configuration have been implemented, for example, by introducing an additional free-standing multiwalled carbon nanotube[18] or microporous carbon paper[19] as a functional interlayer to localize dissolved polysulfide, and thus to improve the active material utilization and capacity retention of lithium–sulfur cells. Another attempt comes from Guo’s research group, who have synthesized small sulfur molecules consisting of short-chained S2–4 crystallines,[20] which can eliminate the transition process between S8 and S42 and effectively diminish the shuttle issue. Up to now, significant progress has been made in improving the specific capacity, sulfur utilization, as well as the cycling retention of lithium–sulfur batteries. However, most of the proposed solutions to address the shuttle issue require complicated synthetic processes and restricted environment, and are not easy to follow or repeat.[11, 18–20] In addition, the sulfur content in some reports was too low (< 50 wt %) to achieve a practical energy density for the sulfur cathode,[7, 10] or the yielded capacities were only moderate even with a high proportion of sulfur.[8] To simplify the synthetic processes, use mild conditions, and to make the sulfur-based electroactive composite easily reproducible, it is critically important to develop reliable and facile approaches to the design of lithium–sulfur cathode materials. Herein, we report a new type of sandwiched carbon
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Communication network, formed by simply combining carbon nanospheres and graphene sheets through a scalable and low-cost synthetic route. Our key idea is to accommodate and protect sulfur from severe loss during electrochemical activities by utilizing wrapped graphene sheets to restrain sulfur, which is initially coated onto carbon nanospheres. The resultant composite material possesses a high weight percentage (64.2 wt %) of sulfur thanks to the assistance of the carbon nanospheres, which are readily redispersible in ethanol and thus able to transport sulfur into the interlayers of graphene sheets. Unlike most techniques involving the initial establishment of a sulfur-confined carbon network followed by the imbibition of sulfur, or the production of sulfur together with the build-up of the carbon domain, our method deals directly with precoated elemental sulfur in ethanol solution. This allows, for the first time to our knowledge, easy handling and also brings opportunities for future low-cost treatment of sulfur-coated carbon nanospheres with various other conducting candidates for sulfurbased cathode materials. When the prepared composite was applied as the lithium–sulfur cathode, the highest specific discharge capacity of 1394 mAh g1 was obtained at a current rate of 0.1 C (1 C = 1685 mAh g1), and the cycling stability was well maintained for up to 100 cycles. This composite was also able to deliver specific capacities of 746 and 604 mAh g1 at 1 C and 2 C, respectively. The individual role and function of each carbon component in the composite material are discussed as well. The schematic diagram illustrating the synthetic strategy is shown in Figure 1. Carbon nanospheres (CS) prepared by the reported hydrothermal method are solid carbonaceous material with hydroxyl groups (OH) on their surface.[21, 22] We found that these carbon nanospheres can act as sulfur carriers to achieve high sulfur loading through a simple thermal treatment owing to the low melting point and recrystallization process of sulfur. The treated carbon nanospheres were coated with a large proportion of sulfur (S@CS), and their further dispersion in ethanol is enabled by the surface hydroxyl groups. By dis-
persing sulfur-coated carbon nanospheres into graphene sheets through a reduction reaction with hydrazine, a packed 3D carbon network (G@S@CS) is formed and sulfur is consequently confined inside the block (Figure 5 and Figure 6). Such morphology is expected to limit the diffusion of polysulfide during the charge/discharge process and further enhance the performance and lifetime of the lithium–sulfur cells as shown hereafter. X-ray diffraction (XRD) patterns of pure sulfur, CS, S@CS, and G@S@CS materials are shown in Figure 2 a. All the sulfur-containing samples present typical diffraction lines of crystalline sulfur (JCPDS card No. 08-0247) as indexed. However, as shown
Figure 2. (a) XRD patterns and (b) Raman spectra of pure sulfur, CS, S@CS, and G@S@CS. I = intensity; k = Raman shift.
Figure 1. Schematic diagram of the formation of the G@S@CS nanocomposite. Chem. Eur. J. 2014, 20, 5224 – 5230
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in the XRD patterns of S@CS and G@S@CS, the intensity of the diffraction peaks gradually decreases and the peaks become less resolved and broader; this is due to the reduced particle size of sulfur (Figure 6) and the introduction of carbon nanospheres and graphene sheets. Furthermore, CS exhibits a broad XRD peak at 238, indicating the formation of a carbonaceous material with very poor crystallinity. It is also noteworthy that no signs of carbon nanospheres or graphene sheets were identified in the XRD patterns of S@CS and G@S@CS as these carbon components were homogenously incorporated with sulfur crystallines and graphene was highly exfoliated in the G@S@CS composite.[8, 23] 5225
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Communication Figure 2 b shows the Raman spectra of crystalline sulfur, CS, S@CS, and G@S@CS. It can be seen that for pure sulfur, the Raman spectrum shows various peaks, which correspond to the vibrational modes of sulfur, in the low wave number region. All the other spectra of carbon-containing samples present two Raman peaks at around 1360 and 1596 cm1. The former peak represents the in-plane vibration of disordered amorphous carbon, whereas the latter represents crystalline graphitic carbon.[24] These identified peak features are in good agreement with those in previous reports for carbon nanospheres.[21, 25] In the spectrum of G@S@CS, two Raman peaks are located at similar positions (1343 and 1584 cm1) and they are marked as the D band and G band; these are characteristics peaks associated with broken symmetry and the E2g vibrational mode, respectively, in graphene-related materials.[26] The intensity ratio of the D band to the G band (ID/IG) of G@S@CS is quite high and much larger than those in the spectra of CS and S@CS. This indicates the broken symmetry of an ordered carbon domain in the G@S@CS composite, further confirming the presence of a decreased sp2 carbon region and the exfoliation of graphene sheets.[27] There are no characteristic bands of sulfur in the spectra of S@CS and G@S@CS owing to the phonon confinement effect,[28] as sulfur crystallines are homogeneously distributed in the carbon matrix (Figure 5 and Figure 6). The XRD patterns combined with Raman spectra prove the co-existence of carbon nanospheres and graphene sheets in the G@S@CS composite. Figure 3 shows the X-ray photoelectron spectrometry (XPS) spectra of pure sulfur, CS, S@CS, and G@S@CS. The presence of sulfur in S@CS and G@S@CS can be confirmed through the identification of S 2s and S 2p3/2 binding, as indicated in the
Figure 4. TGA curves of pure sulfur, S@CS, and G@S@CS measured from room temperature to 700 8C in air. Wp = weight percentage; T = temperature.
the FTIR measurements (see the Supporting Information, Figure S1), which detect weak absorptions of hydroxyl functional groups. Thermogravimetric analysis (TGA) curves for pure sulfur, S@CS, and G@S@CS are shown in Figure 4. The pure sulfur sample was used as a reference to determine the sulfur content in the S@CS and G@S@CS composite materials. In the TGA curve of pure sulfur, the weight loss from ~ 150 to 300 8C can only be ascribed to the consumption of sulfur in air. Therefore, it can be determined that the S@CS sample has a very high initial loading of sulfur (82.7 wt %). On the other hand, the sulfur content of G@S@CS was reduced to 64.2 wt % with graphene wrapping, as confirmed by the TGA result of the G@S@CS sample. Figure 5 presents the field-emission (FE) SEM images of CS and G@S@CS at low magnifications. The prepared CS sample consists of uniform carbon spheres with a diameter of approxi-
Figure 3. XPS spectra of pure sulfur, CS, S@CS, and G@S@CS. C = counts; E = binding energy.
reference XPS spectrum of pure sulfur. No other forms of sulfur-related binding can be found in the spectra of these composites as sulfur is only present in the elemental state. In addition, CS exhibits two strong peaks, which are assigned to C 1s and O 1s binding, suggesting a dominant carbonaceous material with O-enriched functionalities, such as surface hydroxyl groups, an observation that is in good agreement with Chem. Eur. J. 2014, 20, 5224 – 5230
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Figure 5. FE-SEM images of (a) CS and (b) G@S@CS.
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Communication mately 250 nm. These spheres are in the form of densely packed aggregates (Figure 5 a). In Figure 5 b, it can be seen that the nanosized carbon spheres are well isolated and homogeneously trapped by wavy and soft graphene sheets, a fact that provides intimate contacts between the graphene sheets and carbon nanospheres. There is no aggregation of sulfur crystallines observable in Figure 5 b, implying a homogeneous sulfur deposition within the sandwiched carbon network over the examined region. It should also be noted that owing to the high energy of the electron beam focused on a small selected area, sulfur becomes quite unstable in the pure elemental state and in the S@CS composite (82.7 wt % of sulfur) and will quickly sublime and disappear. Therefore, additional morphological features were investigated by the TEM examinations shown in Figure 6 for pure sulfur, CS, S@CS, and G@S@CS. It
ure 5 b. No isolated sulfur aggregation can be located, suggesting successful incorporation of S@CS between graphene layers through our solution process and also revealing homogenous sulfur distribution in the G@S@CS composite. To further prove the presence of sulfur in the G@S@CS composite and examine its distribution, SEM mapping analysis was performed to obtain detailed elemental distribution information. In a selected region, as shown in Figure 7 a, CS are uni-
Figure 7. (a) SEM image of a selected region of the G@S@CS nanocomposite and the corresponding elemental maps of (b) carbon, (c) sulfur, and (d) carbon and sulfur.
Figure 6. TEM images of (a) pure sulfur, (b) CS, (c) S@CS, and (d) G@S@CS.
can be seen from Figure 6 a that pure sulfur particles are densely aggregated and their size ranges from a few hundred nanometers to a few micrometers. Furthermore, CS is found to consist of nanosized spheres with a uniform diameter of around 250 nm (Figure 6 b), consistent with the FE-SEM observations. For S@CS, the surface of the spherical carbonaceous units becomes rougher after sulfur coating, compared with untreated CS, and there are also very small amounts of deposited material visible surrounding the spheres, material which is likely to be loosened sulfur crystallines from the surface of CS (Figure 6 c). These sulfur crystallines form tiny aggregates and become noticeable under microscope analysis. Nevertheless, no large aggregation of sulfur can be found and this demonstrates the uniform coating of sulfur on CS. Figure 6 d shows the typical structure of the G@S@CS composite with graphene sheets wrapping S@CS, similar to the FE-SEM images of FigChem. Eur. J. 2014, 20, 5224 – 5230
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formly distributed between graphene layers, in good agreement with the FE-SEM and TEM observations. The carbon and sulfur elemental maps (Figure 7 b and c, respectively) prove that both carbon and sulfur exist densely in this region, and Figure 7 d shows a mixed elemental map of both carbon and sulfur, indicating a uniform distribution of sulfur in the carbon matrix. The elemental mapping results are direct evidence that confirms the successful incorporation of sulfur into the carbon network constructed by carbon nanospheres and graphene sheets in our G@S@CS composite material. Cyclic voltammetry (CV) curves of the G@S@CS electrode for the 1st, 2nd, and 5th cycles are presented in Figure 8. It should be noted that all the CV curves show typical cathodic peaks at 2.3–2.4 and 2.0–2.1 V regions, corresponding to the reduction processes from elemental sulfur to long-chain polysulfide and from long-chain polysulfide to short-chain sulfide. At around 2.5–2.7 V, characteristic anodic peaks are found, revealing a reverse oxidation stage from sulfide to elemental sulfur.[5] The CV curve in the 1st cycle exhibits broad peaks, which are likely to be related to the activation of the cell and the gradual enhancement of the sulfur utilization. Furthermore, from the 2nd cycle onwards, the CV peaks do not shift significantly, suggesting highly reversible redox reactions as the CV test continues. Figure 9 shows the charge/discharge profiles of the G@S@CS electrode at a current rate of 0.1 C. Two typical reduction plateaux were identified at 2.4–2.1 V and 2.1 V in the discharge profile, in accordance with the two cathodic peaks in the CV
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Figure 8. CV curves for the G@S@CS electrode at a sweep rate of 0.1 mV s1 in the voltage range of 1--3 V vs. Li/Li + . I = current; E = voltage.
Figure 10. Long-term cycling performance of the G@S@CS and S@CS electrodes at a current rate of 0.1 C for 100 cycles. Cs = specific capacity; N = cycle number (1 C = 1685 mAh g1).
Figure 9. Charge/discharge profiles of the G@S@CS electrode at a current rate of 0.1 C for the 1st, 2nd, and 5th cycles. E = voltage; Cs = specific capacity (1 C = 1685 mAh g1).
curves (Figure 8). The discharge capacity reached its highest value of 1394 mAh g1 in the 1st cycle. Upon oxidation, the plateau at 2.4 V represents the transformation of polysulfide to sulfur.[5] Notably, obvious discharge and charge capacity variations are observed between the 1st and 2nd cycles. This suggests a sulfur loss occurring after the 1st cycle. It is very likely that there are still some gaps or uncovered voids and space present in the carbon matrix, allowing polysulfide to dissolve and be transported, thereby causing capacity loss, despite the fact that the graphene sheets and carbon nanospheres restrain polysulfides during the electrochemical activities. Nevertheless, from the 2nd cycle onwards, the capacity loss is fairly limited and the cycling performance becomes stabilized owing to the protection of both the carbon nanospheres and graphene sheets. The shuttle issue of lithium–sulfur cells is consequently diminished by using the G@S@CS electrode, but not completely eliminated. The prolonged cycling performance of the G@S@CS and S@CS electrodes in terms of specific discharge capacity versus cycle number is shown in Figure 10. The maximum discharge capacity of G@S@CS was 1394 mAh g1 in the 1st cycle, folChem. Eur. J. 2014, 20, 5224 – 5230
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lowed by a reduced capacity of 1165 mAh g1 in the 2nd cycle. Afterwards, the discharge capacities became stable for up to 100 cycles and a final capacity of 815 mAh g1 was retained. On the contrary, the S@CS electrode showed a very low initial capacity of 317 mAh g1 in the 1st cycle, and the largest capacity of 375 mAh g1 in the 2nd cycle. The capacity of S@CS then decreased dramatically and only 65 mAh g1 was retained after 100 cycles. The inferior performance of S@CS implies low sulfur utilization and severe sulfur loss. It can be concluded that with the protection of graphene on S@CS, the sulfur utilization and electrochemical properties of G@S@CS are significantly improved and the designed structural arrangement plays a crucial role to confine sulfur on the electrode and maintain its vital role to the cyclic life of the battery. The rate performance of the G@S@CS electrode was evaluated at different current rates as shown in Figure 11. The capacities obtained at the elevated current rates were slightly decreased compared with those at lower rates: they could reach as high as 746 and 604 mAh g1 at 1 and 2 C, respectively, with minor capacity decays. When the current rate dropped to 0.1 C, a reasonable capacity of 818 mAh g1 could still be main-
Figure 11. Step-by-step rate performance of the G@S@CS electrode at current rates of 0.1, 0.2, 0.5, 1 and 2 C. Cs = specific capacity; N = cycle number (1 C = 1685 mAh g1).
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Communication tained, revealing a durable cycleability and the well-preserved structure of the cycled G@S@CS electrode. To identify the morphology changes of the S@CS and G@S@CS electrodes, a postmortem FE-SEM analysis was performed on these electrodes and compared with the FE-SEM analysis performed before the cyclic tests, as shown in Figure 12. Fresh electrodes were stored for direct examinations,
Figure 12. Postmortem FE-SEM images of the S@CS electrode (a) before cycling and (b) after 100 cycles; and the G@S@CS electrode (c) before cycling and (d) after 100 cycles.
up a highly feasible route to further process sulfur in solution with an extremely high loading. Moreover, the majority of the coverage and trapping of sulfur should be credited to the graphene wrapping, which protects the formed polysulfide within a well-built carbon network and provides intimate contact with sulfur as a highly conducting medium. Also, the soft and flexible graphene sheets can buffer the side effects associated with volume expansion of the active sulfur within the battery cell during charge/discharge processes to achieve a durable service life. Nanosized carbon nanospheres with surface hydroxyl groups (OH) are reported, for the first time, to carry a high weight percentage (82.7 wt %) of precoated sulfur and form a composite material upon further solution-based incorporation with graphene (containing 64.2 wt % of sulfur) as a novel lithium–sulfur battery cathode. The prepared G@S@CS composite had a homogenous sulfur distribution within the 3D carbon architecture and exhibited the highest specific capacity of 1394 mAh g1 at a current rate of 0.1 C, sustainable cycling durability for up to 100 cycles, and satisfactory rate capability. The excellent overall electrochemical properties were achieved owing to contributions of both the carbon nanospheres and the graphene sheets, which substantially restrict the diffusion and loss of active sulfur and improve the electrode durability.
Experimental Section Synthesis of carbon nanospheres (CS)
whereas the cycled electrodes were disassembled from the coin cells and washed with acetone and dried prior to the microscopy analysis. It can be seen that both the fresh electrodes of S@CS and G@S@CS exhibit integral microstructure with fairly smooth surfaces as shown in Figure 12 a and c. However, after 100 continuous charge and discharge cycles, the microstructure of the S@CS electrode was found to be severely damaged (Figure 12 b). There are large holes and voids present and the surface of the electrode is no longer flat. This could result from the dramatic loss of active sulfur owing to the lack of protection and the volume expansion of elemental sulfur during the electrochemical reactions. For comparison, the morphological characteristic of the G@S@CS electrode after 100 cycles is presented in Figure 12 d. Only slight cracking of the surface can be seen, indicating that the entire electrode microstructure was well conserved throughout the cyclic test. It is believed that the preservation of the G@S@CS electrode can be attributed to the graphene-wrapping effect, which not only improves sulfur utilization to give large specific capacities, but also effectively restricts sulfur from diffusion and alleviates the volume expansion of sulfur between the flexible graphene layers. In summary, the improved performance of lithium–sulfur cathodes consisting of trapped sulfur in a domain of carbon nanospheres and graphene sheets is reported. The carbon nanospheres and graphene sheets provide dual protection to conserve sulfur in the resultant composite material. We found that carbon nanospheres with surface hydroxyl groups open Chem. Eur. J. 2014, 20, 5224 – 5230
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Glucose (4 g) was dissolved in de-ionized (DI, 40 mL) water and transferred to a Teflon-lined autoclave. The autoclave was sealed and heated at 180 8C in an oven for 20 h. The dark-brown product was collected, washed with DI water three times, and dried at 60 8C.
Synthesis of graphene oxide (GO) The preparation of graphene oxide was derived from the Hummers’ method.[29] Briefly, graphite (1 g) was added to H2SO4 (25 mL, 98 %) in an ice bath, with sequent addition of KMnO4 (4 g) and NaNO3 (0.5 g). The mixture was stirred for 3 h and then diluted to 150 mL with DI water. Afterwards, H2O2 (60 mL, 5 %) was introduced into the mixture, which turned pale yellowish in color. The product (GO) was repeatedly washed with DI water to neutral pH and finally dried at 60 8C.
Synthesis of sulfur-coated carbon nanospheres (S@CS) The CS were sintered at 400 8C for 1 h under argon protection prior to the incorporation of sulfur to increase their conductivity. In a typical process, CS (200 mg) was mixed with elemental sulfur (1 g) in a mortar and then the mixture was transferred and heated in an oven at 150 8C for 6 h. A black product (S@CS) was obtained and stored for further use.
Synthesis of S@CS with graphene wrapping (G@S@CS) S@CS powder (100 mg) was dispersed with an ultrasonic probe (Branson Digital Sonifier, S-450D) in ethanol (100 mL) cooled with an ice bath. The ice bath is to protect the solution from overheating, which may melt the sulfur. To the homogeneous dispersion,
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Communication an aqueous solution of GO (20 mL of 2 mg mL1) was gradually added, followed by the addition of hydrazine hydrate (80 mL) as a reducing agent. The mixture was stirred for 24 h at room temperature, then filtered and washed with ethanol and DI water to obtain the final product (G@S@CS).
Keywords: carbon nanospheres · graphene · high capacity · lithium–sulfur batteries · sulfur
Materials characterizations and electrochemical properties X-ray diffraction (XRD) patterns of sulfur, CS, S@CS, and G@S@CS were obtained on a Bruker D8 Advance powder X-ray diffractometer. Raman Spectroscopy was recorded on an Alpha 300 R Confocal Raman Microscope (WiTec, Germany) with a laser source of 532 nm. The spectra were obtained with an exposure time of 5 s for 30 accumulations from 100 to 2000 cm1. The X-ray photoelectron spectrometry (XPS) analysis was conducted on a Kratos Axis Ultra X-ray Photoelectron Spectrometer, which uses AlKa (1253.6 eV) X-ray. Thermogravimetric analysis (TGA) was utilized to determine the sulfur content of the composite materials. Test conditions were set to a ramping rate of 10 8C min1 from room temperature to 700 8C in air on a Mettler Toledo TGA/DSC Thermogravimetric Analyzer. Field-emission scanning electron microscopy (FESEM) images were captured on a JEOL 7001 facility. Transmission electron microscopy (TEM) examinations were performed on a JEOL 1010 Transmission Electron Microscope to analyze the morphological structure of the samples. Elemental maps of carbon and sulfur of the as-prepared G@S@CS nanocomposite were acquired on an Environmental Scanning Electron Microscope (FEI Quanta 200) by utilizing the energy dispersive X-ray (EDX) mapping technique. The electrochemical properties of the as-prepared G@S@CS material were evaluated in assembled CR2032 coin cells, fabricated inside an argon-filled glovebox (MBraun, Germany). Electrodes were prepared by pasting a mixture slurry of G@S@CS (80 wt %), acetylene black (10 wt %), and polyvinylidene fluoride (10 wt %) in the presence of N-methyl-2-pyrrolidone, on aluminum substrates with subsequent heating at 80 8C for 12 h under vacuum. Metallic lithium foils were used as the negative electrode and the electrolyte was prepared by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI, 1 m) in 1,2-dimethoxyethane and 1,3-dioxolane (1:1 v/v) with additional LiNO3 (0.1 m). The cyclic voltammetry (CV) results were obtained on an electrochemistry workstation (CHI660E) at a scan rate of 0.1 mV s1 in the voltage range of 1–3 V against Li/ Li + . The galvanostatic charge/discharge capacities were recorded on a battery tester (LAND, 5 V, 10 mA) at 0.1 C (1 C = 1685 mA g1) for 100 cycles. Stepwise rate performance was examined at elevated current rates of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C on the same battery tester.
Acknowledgements Financial support from the Australian Research Council (through its DPs and Centre’s grant) is gratefully acknowledged. Dr. Elena Taran is greatly appreciated for the Raman Spectroscopy analysis, which was performed at the Queensland Node of the Australian National Fabrication Facility
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(ANFF), a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers.
[1] D. Lindley, Nature 2010, 463, 18 – 20. [2] Q. Schiermeier, J. Tollefson, T. Scully, A. Witze, O. Morton, Nature 2008, 454, 816 – 823. [3] P. G. Bruce, B. Scrosati, J. M. Tarascon, Angew. Chem. 2008, 120, 2972 – 2989; Angew. Chem. Int. Ed. 2008, 47, 2930 – 2946. [4] G. X. Wang, H. Liu, J. Liu, S. Z. Qiao, G. Q. M. Lu, P. Munroe, H. Ahn, Adv. Mater. 2010, 22, 4944 – 4948. [5] X. Ji, L. F. Nazar, J. Mater. Chem. 2010, 20, 9821 – 9826. [6] X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500 – 506. [7] D. W. Wang, G. M. Zhou, F. Li, K. H. Wu, G. Q. Lu, H. M. Cheng, I. R. Gentle, Phys. Chem. Chem. Phys. 2012, 14, 8703 – 8710. [8] S. Evers, L. F. Nazar, Chem. Commun. 2012, 48, 1233 – 1235. [9] N. Li, M. Zheng, H. Lu, Z. Hu, C. Shen, X. Chang, G. Ji, J. Cao, Y. Shi, Chem. Commun. 2012, 48, 4106 – 4108. [10] B. Wang, K. Li, D. Su, H. Ahn, G. Wang, Chem. Asian J. 2012, 7, 1637 – 1643. [11] H. L. Wang, Y. Yang, Y. Y. Liang, J. T. Robinson, Y. G. Li, A. Jackson, Y. Cui, H. J. Dai, Nano Lett. 2011, 11, 2644 – 2647. [12] F. F. Zhang, X. B. Zhang, Y. H. Dong, L. M. Wang, J. Mater. Chem. 2012, 22, 11452 – 11454. [13] Y. Yang, G. Yu, J. J. Cha, H. Wu, M. Vosgueritchian, Y. Yao, Z. Bao, Y. Cui, ACS Nano 2011, 5, 9187 – 9193. [14] L. C. Yin, J. L. Wang, F. J. Lin, J. Yang, Y. Nuli, Energy Environ. Sci. 2012, 5, 6966 – 6972. [15] N. Jayaprakash, J. Shen, S. S. Moganty, A. Corona, L. A. Archer, Angew. Chem. 2011, 123, 6026 – 6030; Angew. Chem. Int. Ed. 2011, 50, 5904 – 5908. [16] C. Zhang, H. B. Wu, C. Yuan, Z. Guo, X. W. Lou, Angew. Chem. 2012, 124, 9730 – 9733; Angew. Chem. Int. Ed. 2012, 51, 9592 – 9595. [17] G. M. Zhou, D. W. Wang, F. Li, P. X. Hou, L. C. Yin, C. Liu, G. Q. Lu, I. R. Gentle, H. M. Cheng, Energy Environ. Sci. 2012, 5, 8901 – 8906. [18] Y.-S. Su, A. Manthiram, Chem. Commun. 2012, 48, 8817 – 8819. [19] Y.-S. Su, A. Manthiram, Nat. Commun. 2012, 3, 1166. [20] S. Xin, L. Gu, N.-H. Zhao, Y.-X. Yin, L.-J. Zhou, Y.-G. Guo, L.-J. Wan, J. Am. Chem. Soc. 2012, 134, 18510 – 18513. [21] X. M. Sun, Y. D. Li, Angew. Chem. 2004, 116, 607 – 611; Angew. Chem. Int. Ed. 2004, 43, 597 – 601. [22] X. M. Sun, J. F. Liu, Y. D. Li, Chem. Eur. J. 2006, 12, 2039 – 2047. [23] L. C. Yin, J. L. Wang, X. L. Yu, C. W. Monroe, Y. NuLi, J. Yang, Chem. Commun. 2012, 48, 7868 – 7870. [24] A. Ilie, C. Durkan, W. I. Milne, M. E. Welland, Phys. Rev. B 2002, 66, 045412. [25] X. M. Sun, Y. D. Li, Langmuir 2005, 21, 6019 – 6024. [26] M. J. Allen, V. C. Tung, R. B. Kaner, Chem. Rev. 2010, 110, 132 – 145. [27] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen, R. S. Ruoff, Carbon 2007, 45, 1558 – 1565. [28] J. Zuo, C. Y. Xu, Y. P. Liu, Y. T. Qian, Nanostruct. Mater. 1998, 10, 1331 – 1335. [29] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339 – 1339. Received: January 30, 2014 Published online on April 1, 2014
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