Unexpected Effect of Electrode Architecture on High-Performance

Sep 10, 2018 - In the past years, considerable efforts have been devoted to the deliberate synthesis of nanosulfur in various hosts with sophisticated...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 33269−33275

Unexpected Effect of Electrode Architecture on High-Performance Lithium−Sulfur Batteries Peitao Xiao,† Lixia Sun,‡ Dankui Liao,*,‡ Phillips O. Agboola,§ Imran Shakir,*,∥ and Yuxi Xu*,†

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†

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China ‡ Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China § Mechanical Engineering Department, College of Applied Engineering, King Saud University (Al Muzahimiyah Branch), Riyadh 11421, Saudi Arabia ∥ Sustainable Energy Technologies Center, College of Engineering Center, King Saud University, Riyadh 11421, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: In the past years, considerable efforts have been devoted to the deliberate synthesis of nanosulfur in various hosts with sophisticated structures to improve the performance of lithium−sulfur batteries (LSBs) and reveal the structure−property relationship. It is taken for granted that these elaborate sulfur nanostructures are well maintained in the ultimate electrode after the traditional mixing and coating method. Herein, we, for the first time, reveal the unexpected sulfur structure deterioration in nanosulfur/ graphene composites during the electrode preparation using the traditional method because of the long-term neglected dissolution−recrystallization effect of sulfur in solvents. Consequently, compared with binder-free threedimensional graphene/sulfur electrodes, the milled graphene/sulfur electrodes exhibit much worse electrochemical performance. On the basis of this, we further propose a facile and universal graphene oxide-assisted assembly method to avoid the dissolution−recrystallization of sulfur, by which binderfree three-dimensional ethylenediamine-functionalized graphene/sulfur (3DEFGS) electrodes have been successfully prepared. The 3DEFGS electrodes with a high areal sulfur loading of ∼6 mg cm−2 exhibit an ultrahigh initial capacity of 1394 mA h g−1 at 0.1 C, an excellent rate performance with a capacity of 796 mA h g−1 at 4 C, and superior long-term cycling stability (885 mA h g−1 after 500 cycles at 1 C), which are among the best performances achieved by all reported LSB cathodes with high areal sulfur loadings. KEYWORDS: lithium−sulfur battery, processing methods, structure evolution, graphene-assisted assembly, binder-free electrode capacity and long-term cycling stability.3,4 Generally, highly conductive carbonaceous materials, such as carbon black, graphene, and carbon nanotubes, have been widely adopted to boost the conductivity of electrodes.5−8 Meanwhile, deliberately designed nonpolar porous materials, with different pore sizes and volumes,9,10 and polar hosts,11 such as heteroatomdoped materials,5,12,13 metal oxides or sulfides,14−18 Mxene,19 and metal−organic frameworks,20 have been used to alleviate the notorious shuttling effect of the LiPSs through physical or/ and chemical adsorption. Moreover, decreasing the size of the sulfur particles intrinsically leads to a higher specific area and provides sufficient contacting sites for sulfur with the electrolyte (significantly shortening the ion and electron

1. INTRODUCTION With the rapid development of renewable energy (solar, wind, tides, and so on) and increasing concerns about environmental issues, it is of paramount importance to develop efficiently advanced energy storage systems. Because of the low cost and environmental benignity of sulfur and ultrahigh theoretical energy density [2500 W h kg−1, more than five times that of commercial lithium-ion batteries (LIBs)], a tremendous number of studies have been conducted to design and fabricate high-performance lithium−sulfur batteries (LSBs).1,2 However, the practical application of LSBs has been severely hindered by the following issues: the volume variation during electrochemical process, self-discharge, especially the ultralow conductivity of sulfur and Li2S or Li2S2 (discharging products), and the solubility of lithium polysulfides (LiPSs) (Li2Sn, 4 ≤ n ≤ 8) and their shuttling effect between electrodes during the discharge−charge process, which dramatically compromise the © 2018 American Chemical Society

Received: July 15, 2018 Accepted: September 10, 2018 Published: September 10, 2018 33269

DOI: 10.1021/acsami.8b11883 ACS Appl. Mater. Interfaces 2018, 10, 33269−33275

Research Article

ACS Applied Materials & Interfaces

larger through dissolution−recrystallization during the traditional method. As a result, compared with milled graphene/ sulfur (MGS) electrodes, binder-free 3DGS electrodes show superior electrochemical performance. On the basis of this discovery, we further propose a facile and general graphene oxide (GO)-assisted assembly method to fabricate binder-free electrodes for LSBs, avoiding the effect of the solvent on the structure of sulfur, by which ethylenediamine-functionalized graphene/sulfur (EFGS) powder was successfully assembled into a free-standing three-dimensional EFGS (3DEFGS) hydrogel, which can be directly used as a binder-free electrode after freeze-drying and pressing (Figure 1b). The 3DEFGS electrodes with a high areal sulfur loading of ∼6 mg cm−2 can deliver a high capacity (1394 mA h g−1 at 0.1 C) along with excellent rate performance (796 mA h g−1 at 4 C) and superior long-term cycling stability (885 mA h g−1 after 500 cycles at 1 C), which is among the best performances achieved in all reported LSB cathodes with high areal sulfur loadings. This GO-assisted assembly can be applied to the assembly of other powdery active materials into binder-free electrodes as a universal method.

diffusion distance), conducting agents (highly improving the conductivity of the electrode), and the LiPS trapping agents (efficiently restricting the shuttling of LiPSs).21 Therefore, substantial studies have been conducted to load sulfur nanostructures on the abovementioned host materials, in which in situ growth of sulfur and melting infiltration are widely used, to produce sophisticated nanosulfur composites with improved electrochemical performance.22,23 As is well known, the deliberately synthesized nanosulfur composites should be processed into electrodes before electrochemical testing and practical use, and it is usually completed by the traditional method which is generally used in LIBs, that is, active materials are mixed with conducting agents and binders (polyvinylidene fluoride, PVDF) in N-methyl pyrrolidone (NMP) to obtain a homogenous slurry, followed by coating it on aluminum foil, and drying at a specific temperature.24 It is taken for granted that these elaborate sulfur nanostructures are well maintained in the ultimate electrode after the traditional mixing and coating method. However, the physicochemical property of sulfur in solvents like NMP is totally different from the inorganic active materials for LIBs, which is unfortunately neglected in all previous studies, and thus the influence of structure evolution of sulfur during electrode fabrication on the electrochemical performance has so far received little attention. To the best of our knowledge, there is no report on the effect of the processing methods on the structure evolution of sulfur and the electrochemical performance of the corresponding LSB electrodes, which, however, is of great importance to the development of highperformance LSBs. Herein, we report for the first time that the pristine structure of homogenous nanosulfur in graphene/sulfur composites can unexpectedly change significantly during the electrode preparation using the traditional method (mixing and coating) because of the long neglected dissolution−recrystallization effect of sulfur in solvents such as NMP. As schematically shown in Figure 1a, the self-assembly process has little effect on the particle size of sulfur in binder-free three-dimensional graphene/sulfur (3DGS) electrodes because sulfur hardly dissolves in water, whereas because of a certain solubility of sulfur in NMP, some sulfur nanoparticles become obviously

2. EXPERIMENTAL SECTION 2.1. Synthesis of GO. Natural graphite powder (325 mesh, Aladdin, 99.95% metal basis) was used to synthesize GO via the modified Hummers’ method. The chemical structure and size distribution of GO are characterized as shown in Figure S1. All the other chemicals were purchased from Adamas-beta and used without purification. 2.2. Synthesis of 3DGS. First, 15 mg of GO and 260 mg of sodium thiosulfate (Na2S2O3) were dissolved in 15 mL deionized water successively, and then mixed under magnetic stirring for 15 min. Diluted hydrochloric acid (HCl, 22 mL) aqueous solution (0.1 M) was added slowly into the abovementioned solution, followed by magnetic stirring for 1 h. Then, the suspension was heated at 95 °C for 1.5 h to reduce the GO after the addition of 375 μL of sodium ascorbate solution (1 M) to get the 3DGS hydrogel. Finally, the 3DGS aerogel was obtained by freeze-drying the 3DGS hydrogel for 24 h. 2.3. Synthesis of MGS. For the MGS samples used for the scanning electron microscopy (SEM) characterization, 30 mg of 3DGS and 200 μL of NMP were mixed and milled for 0.5 h to get the MGS slurry. Then, one part of the MGS slurry was vacuum-dried at room temperature, while the other part was dried at 60 °C overnight. For the MGS used as electrodes, 3DGS and PVDF binder (in a mass ratio of 9:1) were milled in NMP to form a homogeneous slurry. The slurry was coated onto aluminum foils and dried at 60 °C overnight. 2.4. Synthesis of 3DEFGS. First, 45 mL of GO suspension (1 mg mL−1) and 60 μL of EDA were mixed and heated at 75 °C for 6 h to synthesize EDA-functionalized graphene (EFG). Second, 2340 mg of sodium thiosulfate (Na2S2O3) was dissolved in the abovementioned solution, followed by mixing under magnetic stirring for 15 min. Diluted HCl aqueous solution (190 mL, 0.1 M) was added slowly into the abovementioned solution, followed by stirring for 1 h to obtain EFGS solution. The EFGS composite was obtained by filtration and rinsed with deionized water several times, followed by thermal treatment at 155 °C for 20 h. Third, the EFGS composite was redispersed into deionized water, and then 10 mL of GO (1 mg mL−1) and 500 μL of sodium ascorbate solution (1 M) were added successively under stirring, followed by heating at 95 °C for 1.5 h to get the 3DEFGS hydrogel. Finally, the 3DEFGS aerogel was obtained by freeze-drying the 3DEFGS hydrogel for 24 h. 2.5. Synthesis of MEFGS. For the MEFGS samples used for SEM characterization, 30 mg of EFGS and 200 μL of NMP were mixed and milled for 0.5 h to get an MEFGS slurry. Then, one part of the MEFGS slurry was vacuum-dried at room temperature, while the other part was dried at 60 °C overnight. For the MEFGS used as

Figure 1. Schematic illustration of the electrode fabricating process: (a) 3DGS and MGS electrodes and (b) 3DEFGS and MEFGS electrodes. 33270

DOI: 10.1021/acsami.8b11883 ACS Appl. Mater. Interfaces 2018, 10, 33269−33275

Research Article

ACS Applied Materials & Interfaces electrodes, EFGS, Super P, and PVDF binder (in a mass ratio of 7:2:1) were milled in NMP to form a homogeneous slurry. The slurry was spread onto aluminum foils and dried at 60 °C overnight. 2.6. Characterizations. The morphology characterizations were performed on the field emission scanning electron microscope (Zeiss Ultra-55) and transmission electron microscope (FEI Tecnai G2 20 TWIN, 200 kV). PANalytical X’Pert PRO X-ray diffraction (XRD, Cu Kα) was adopted to perform the XRD analysis from 10° to 60°. Pyris 1 TGA was used to carry out the thermogravimetric analysis (TGA) tests at a heating rate of 10 °C/min in N2. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000C ESCA system. 2.7. Electrochemical Characterization. The free-standing and binder-free 3DGS and 3DEFGS electrodes, obtained by the compression of the 3DGS and 3DEFGS aerogels, were used directly as cathodes for LSBs. The areal sulfur loading was about 6 mg cm−2. The area of the electrode was ∼1.1 cm2 for all the electrodes. LSBs were assembled in an argon-filled glovebox with metallic lithium and Celgard 2400, as the anode and the separator, respectively. LiTFSi (1 M) in DOL/DMC (1:1 by volume ratio) with 0.1 M LiNO3 was used as the electrolyte. The galvanostatic experiments were tested on a battery testing system (LAND, Wuhan, China) in the voltage window of 1.7−2.8 V at different current densities. A CHI 760D electrochemical work station was used to perform electrochemical impedance spectroscopy (EIS) in the frequency range of 100 kHz to 0.01 Hz, and cyclic voltammetry (CV) curves were obtained at the scanning rate of 0.1 mV s−1 in the potential range of 1.5−3 V.

Figure 3. (a) XRD patterns of 3DGS and 3DEFGS, (b) C 1s and (c) S 2p XPS spectra of 3DGS, and (d) TGA curves of 3DGS and 3DEFGS.

The morphology evolution of sulfur of the MGS composite during the mixing and coating process was carefully investigated by SEM. As shown in Figure 4a1−a4, before mixing with NMP, there are no obvious sulfur particles on the surface of graphene, and the element mapping images show that the sulfur is homogeneously distributed on graphene. After milling the mixture of 3DGS and NMP in a weight/ volume ratio (mg/mL) of 150:1, which is typically used for many studies, for 0.5 h and following the evaporation of NMP at room temperature, some sulfur nanoparticles recrystallized into larger particles with a size of 1−2 μm. In addition, these sulfur particles are prone to accumulate at some area (Figure 4b1), which may result from the inclination of the crystal to grow on the existing nucleus.27 The element mapping images also clearly demonstrate the growth of sulfur during the mixing process (Figure 4b2−b4). To further understand the morphology evolution of sulfur during the traditional process, the obtained slurry of 3DGS and NMP composites was coated on aluminum foil and dried at 60 °C for 8 h after milling. As clearly shown in Figure 4c1−c4, some bulk sulfur, rather than particles, appears in the MGS composites. The size of the bulk sulfur in MGS can be even larger than 10 μm (Figure S5). To further confirm the structure evolution of sulfur, neat hollow sulfur nanoparticles are synthesized,28 and a similar phenomenon of structure evolution is observed during the traditional milling process in NMP (Figure S6). All of the results above show that the traditional mixing and coating method can distinctly increase the size of sulfur, which, consequently, has a remarkable effect on the electrochemical performance of the electrode for the LSBs. The electrochemical performances of 3DGS and MGS were thoroughly investigated to understand the influence of structure change on the electrochemical property. The two distinct discharging plateaus at about 2.3 and 2.1 V correspond to the formation of LiPSs from S8 and Li2S or Li2S2 from LiPSs, respectively; whereas the charging plateaus are ascribed to the formation of LiPSs and S8 (Figure 5a), which are in good agreement with the CV curves (Figure S7).29 However, the voltage gap value between the charge and discharge curves of the 3DGS electrode is 0.16 V, while that of MGS is about 0.23 V, which is induced by the slower electrochemical kinetics of MGS.30 The cycling performance at the current density of

3. RESULTS AND DISCUSSION We first investigated the structure evolution of sublimed sulfur in NMP and observed that the sulfur particles dissolved quickly in NMP at 60 °C and recrystallized into larger lamellae (Figure S2), and the solubility of sulfur increased with temperature (Figure S3). We then grew sulfur nanoparticles uniformly on GO to obtain the GO/S dispersion, as revealed by transmission electron microscopy (TEM) in Figure 2a, followed by

Figure 2. (a)TEM image of GO/S and (b) SEM image of 3DGS; the inset in (b) shows the photograph of the 3DGS aerogel.

the self-assembly of GO/S into 3DGS through chemical reduction of GO (Figure 2b). The element mapping images of 3DGS also show that sulfur homogeneously anchored on the graphene (Figure S4), which is consistent with the result of TEM. The detailed composition and structures of 3DGS were further investigated by XRD, XPS, and TGA. Three dominant peaks at 23.4°, 26.0°, and 27.7° in the XRD of 3DGS, as shown in Figure 3a, are assigned to (222), (026), and (040) planes, which are in good accordance with the results for pure sulfur.25 Meanwhile, the peak at 285.6 eV, ascribed to C−S bonds, in the C 1s XPS spectrum in Figure 3b not only reveals the successful incorporation of sulfur in the 3DGS but also confirms the covalent bonding between sulfur and graphene, which is consistent with the S 2p XPS peaks in Figure 3c.26 The sulfur content in 3DGS is about 65.8%, according to the results of the TGA in Figure 3d. 33271

DOI: 10.1021/acsami.8b11883 ACS Appl. Mater. Interfaces 2018, 10, 33269−33275

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Figure 4. (a1) SEM images of 3DGS and the corresponding element mapping of (a2) C, (a3) S, and (a4) O; (b1) SEM images of MGS after evaporation of NMP at room temperature and the corresponding element mapping of (b2) C, (b3) S, and (b4) O; (c1) SEM images of MGS after evaporation of NMP at 60 °C and the corresponding element mapping of (c2) C, (c3) S, and (c4) O.

h g−1. In addition, when the current density returns to 0.1 C, the capacity of 3DGS recovers to 1220 mA h g−1 (Figures 5c and S8). Long-term cycling stability of 3DGS and MGS was also evaluated at the current density of 1 C. After 500 discharge−charge cycles, the 3DGS electrode can still deliver a reversible capacity of 703 mA h g−1, with a remarkable capacity retention of 90.7%; that is, the capacity loss is only 0.019% per cycle. In contrast, the MGS electrode only delivers 260 mA h g−1 after 500 cycles at 1 C (Figure 5d). Compared with MGS, the smaller sulfur in 3DGS has an intrinsically higher specific area, providing more contacting area between sulfur and the electrolyte or conducting agents, which can significantly enhance electrochemical kinetics by facilitating electron and ion transport, as illustrated by EIS in Figure 5e.31,32 Therefore, compared with the MGS electrodes, excellent electrochemical performance including high specific capacity, superior rate performance, and long-term cycling stability can be achieved in the 3DGS electrodes. It should be noted that, in many cases, the deliberately designed sulfur/host composites exist in the form of powders and can only be used to fabricate electrodes by traditional mixing and coating method. On the basis of the abovementioned results, a facile and universal method to prepare binder-free electrodes is of great importance to develop highperformance LSBs. To address this challenge, we proposed a GO-assisted assembly method to fabricate binder-free electrodes, which can be applied into various sulfur/host composites. To substantiate the feasibility of this method, we first grow sulfur on EFG, followed by thermal treatment at 155 °C to obtain the EFGS powder. The TEM results show that sulfur is homogeneously distributed on EFG (Figures 6a, and S9). By GO-assisted assembly, the 3DEFGS aerogel can be obtained (Figures 6b and S10), which can be directly used as electrodes for LSBs. The XRD (Figure 3a) and XPS (Figure S11) results also demonstrate the successful incorporation of sulfur in 3DEFGS, and the sulfur content in 3DEFGS is about 67.3% (Figure 3d). The traditional electrode (MEFGS) was also fabricated by milling the EFGS powder with Super P and PVDF in NMP solvent. The morphology evolution of sulfur was again investigated by SEM during the traditional process. As shown in Figure 7a1, there are no obvious sulfur particles in EFGS, indicating the homogenous distribution of sulfur on EFG, in good

Figure 5. Electrochemical characterizations of 3DGS and MGS electrodes: (a) charge−discharge curves of the 3DGS and MGS electrodes, (b) cycling performance of the 3DGS and MGS electrodes at 0.1 C, (c) rate performance of the 3DGS and MGS electrodes, (d) cycling performance of the 3DGS and MGS electrodes at 1 C for 500 cycles, and (e) Nyquist plots for the 3DGS and MGS electrodes.

0.1 C (1 C = 1675 mA g−1) was also tested. As shown in Figure 5b, the initial discharge specific capacity of 3DGS for LSBs is about 1328 mA h g−1, while that of MGS is only 877 mA h g−1. The specific capacity of 3DGS can still retain 968 mA h g−1 after 100 discharge−charge cycles, with a capacity retention of 72.8%; while that of MGS is only 405 mA h g−1, which is only 46.1% of the initial capacity. We further studied the rate performance of the 3DGS and MGS electrodes. Reversible capacities of 3DGS are 1238, 1074, 896, 730, and 570 mA h g−1 when the current density increases from 0.1 to 2 C, which are much higher than those of MGS. Even at a high current density of 4 C, the 3DGS electrode still delivers a capacity of 418 mA h g−1, while that for MGS is only 12.2 mA 33272

DOI: 10.1021/acsami.8b11883 ACS Appl. Mater. Interfaces 2018, 10, 33269−33275

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a) TEM images of EFGS and (b) SEM images of 3DEFGS; the inset shows the photograph of the EFGS and 3DEFGS aerogels, respectively.

accordance with the element mapping images in Figure 7a2− a5. After milling the mixture of EFGS and NMP for about 0.5 h and evaporation of NMP at room temperature, some sulfur particles with a size of ∼2 μm clearly appear in some areas of MEFGS (Figure 7b1−b5). Plenty of sulfur particles with a size of ∼5 μm anchor on EFG after drying MEFGS at 60 °C for 8 h, as shown in Figure 7c1−c5. These SEM results of MEFGS and MGS evidently demonstrate the huge effect of NMP on the morphology of sulfur during the traditional fabrication process, implying the importance of the GO-assisted assembly method in preserving the morphology of sulfur nanoparticles in the electrodes of LSBs. Electrochemical performances of 3DEFGS and MEFGS with areal sulfur loading up to ∼6 mg cm−2 were studied to demonstrate the role of the GO-assisted assembly method in fabricating high-performance binder-free electrodes for LSBs. As shown in Figure 8a, compared with that of MEFGS, the lower gap value between the discharge and charge curves of 3DEGS indicates highly enhanced electrochemical kinetics, whereas both the discharge−charge curves of 3DEFGS and MEFGS show the typical shape for the sulfur electrode for LSBs. When tested at 0.1 C, the 3DEFGS electrode delivers an initial capacity of 1394 mA h g−1, while that of MEFGS is only 1063 mA h g−1. After 100 cycles, the capacity of the 3DEFGS electrode can still retain 970 mA h g−1 (Figure 8b). When the current densities increase from 0.1 to 0.2, 0.5, 1, and 2 C, the 3DEFGS electrodes deliver reversible capacities of 1252, 1118, 995, 963, and 944 mA h g−1, respectively, while those of MEFGS are only 703, 502, 411, 355, and 300 mA h g−1, respectively. Even at a high current density of 4 C, 3DEFGS can still exhibit a reversible capacity of 796 mA h g−1.

Figure 8. Electrochemical characterizations of the 3DEFGS and MEFGS electrodes with a high areal sulfur loading of ∼6 mg cm−2: (a) charge−discharge curves of 3DEFGS and MEFGS electrodes, (b) cycling performance of 3DEFGS and MEFGS electrodes at 0.1 C, (c) rate performance of 3DEFGS and MEFGS electrodes, (d) cycling performance of 3DEFGS and MEFGS electrodes at 1 C for 500 cycles, and (e) Nyquist plots for the 3DEFGS and MEFGS electrodes.

Moreover, the capacity of the 3DEFGS electrode recovers to 1198 mA h g−1 when current density is back to 0.1 C (Figures 8c and S12). More importantly, the 3DEFGS electrode exhibits excellent long-term cycling stability with a reversible capacity of 885 mA h g−1 after 500 cycles at 1 C. However, MEFGS only delivers a capacity of 333 mA h g−1 after 500 cycles at 1 C (Figure 8d). The results of the EIS in Figure 8e also imply the superior electrochemical kinetics, in accordance with the results of the CV tests. Meanwhile, the rate performance of 3DEFGS with higher mass loading was also tested, as shown in Figure S13. Such excellent performances of 3DEFGS make it among the best electrodes in all reported LSBs with high areal sulfur loadings (Table S1).23,33−48

Figure 7. (a1) SEM images of 3DEFGS, and the corresponding element mapping of (a2) C, (a3) S, (a4) O, and (a5) N; (b1) SEM images of MEFGS after evaporation of NMP at room temperature, and the corresponding element mapping of (b2) C, (b3) S, (b4) O, and (b5) N; (c1) SEM images of MEFGS after evaporation of NMP at 60 °C, and the corresponding element mapping of (c2) C, (c3) S, (c4) O, and (c5) N. 33273

DOI: 10.1021/acsami.8b11883 ACS Appl. Mater. Interfaces 2018, 10, 33269−33275

ACS Applied Materials & Interfaces Compared with the aggregated sulfur particles in MEFGS, sulfur is homogeneously distributed on the surface of EFG in MEFGS, resulting in sufficient contact areas between sulfur and EFG. The intimate contact between sulfur and the EFG network not only facilitates the ion and electron transport but also offers synergistic physical restriction and chemical confinement (because of polar heteroatoms such as O and N in EFG)12 of dissoluble intermediate LiPSs during the electrochemical process.

ACKNOWLEDGMENTS

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REFERENCES

(1) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S. Rechargeable Lithium-Sulfur Batteries. Chem. Rev. 2014, 114, 11751−11787. (2) Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F. Advances in lithium-sulfur batteries based on multifunctional cathodes and electrolytes. Nat. Energy 2016, 1, 16132. (3) Fang, R.; Zhao, S.; Sun, Z.; Wang, D.-W.; Cheng, H.-M.; Li, F. More Reliable Lithium-Sulfur Batteries: Status, Solutions and Prospects. Adv. Mater. 2017, 29, 1606823. (4) Manthiram, A.; Chung, S.-H.; Zu, C. Lithium-Sulfur Batteries: Progress and Prospects. Adv. Mater. 2015, 27, 1980−2006. (5) Xiao, P.; Bu, F.; Yang, G.; Zhang, Y.; Xu, Y. Integration of Graphene, Nano Sulfur, and Conducting Polymer into Compact, Flexible Lithium-Sulfur Battery Cathodes with Ultrahigh Volumetric Capacity and Superior Cycling Stability for Foldable Devices. Adv. Mater. 2017, 29, 1703324. (6) Chen, Y.; Lu, S.; Zhou, J.; Qin, W.; Wu, X. Synergistically Assembled Li2 S/FWNTs@Reduced Graphene Oxide Nanobundle Forest for Free-Standing High-Performance Li2 S Cathodes. Adv. Funct. Mater. 2017, 27, 1700987. (7) Lu, S.; Chen, Y.; Wu, X.; Wang, Z.; Lv, L.; Qin, W.; Jiang, L. Binder-free cathodes based on sulfur-carbon nanofibers composites for lithium-sulfur batteries. RSC Adv. 2014, 4, 18052−18054. (8) Lu, S.; Chen, Y.; Wu, X.; Wang, Z.; Li, Y. Three-Dimensional Sulfur/Graphene Multifunctional Hybrid Sponges for Lithium-Sulfur Batteries with Large Areal Mass Loading. Sci. Rep. 2014, 4, 4629. (9) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries. Nat. Mater. 2009, 8, 500−506. (10) He, Y.; Bai, S.; Chang, Z.; Li, Q.; Qiao, Y.; Zhou, H. Porous Hybrid Aerogels with Ultrahigh Sulfur Loading for Lithium-Sulfur Batteries. J. Mater. Chem. A 2018, 6, 9032−9040. (11) Sun, Z.; Zhang, J.; Yin, L.; Hu, G.; Fang, R.; Cheng, H.-M.; Li, F. Conductive Porous Vanadium Nitride/Graphene Composite as Chemical Anchor of Polysulfides for Lithium-Sulfur Batteries. Nat. Commun. 2017, 8, 14627. (12) Hou, T.-Z.; Chen, X.; Peng, H.-J.; Huang, J.-Q.; Li, B.-Q.; Zhang, Q.; Li, B. Design Principles for Heteroatom-Doped Nanocarbon to Achieve Strong Anchoring of Polysulfides for LithiumSulfur Batteries. Small 2016, 12, 3283−3291. (13) Carter, R.; Ejorh, D.; Share, K.; Cohn, A. P.; Douglas, A.; Muralidharan, N.; Tovar, T. M.; Pint, C. L. Surface Oxidized Mesoporous Carbons Derived from Porous Silicon as Dual Polysulfide Confinement and Anchoring Cathodes in Lithium Sulfur Batteries. J. Power Sources 2016, 330, 70−77. (14) Li, Z.; Zhang, J.; Lou, X. W. D. Hollow Carbon Nanofibers Filled with MnO2Nanosheets as Efficient Sulfur Hosts for LithiumSulfur Batteries. Angew. Chem., Int. Ed. 2015, 54, 12886−12890. (15) Lei, T.; Chen, W.; Huang, J.; Yan, C.; Sun, H.; Wang, C.; Zhang, W.; Li, Y.; Xiong, J. Multi-Functional Layered WS2Nanosheets for Enhancing the Performance of Lithium-Sulfur Batteries. Adv. Energy Mater. 2017, 7, 1601843. (16) Yuan, Z.; Peng, H.-J.; Hou, T.-Z.; Huang, J.-Q.; Chen, C.-M.; Wang, D.-W.; Cheng, X.-B.; Wei, F.; Zhang, Q. Powering Lithium-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11883. Morphological and structural characterizations and electrochemical characterization (PDF)

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We acknowledge the support by the National Natural Science Foundation of China (51673042), the Young Elite Scientist Sponsorship Program by CAST (2017QNRC001), and the Open Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2017K011). The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group project no, RGP-VPP-312.

4. CONCLUSIONS In conclusion, the unexpected sulfur morphology evolution in the nanosulfur composite during the electrode preparation using the traditional method was revealed for the first time, and the electrochemical performance of the traditional electrodes (MGS) and the binder-free electrodes (3DGS) of LSBs were thoroughly investigated. During the traditional mixing and coating process, part of nanosulfur dissolves in the NMP solvent and recrystallizes into larger sulfur particles. The large size of the sulfur particles in MGS reduces the contacting area between sulfur and the conducting agents/electrolyte, which slows down the electron and ion transportation and thus results in the inferior electrochemical performance of the MGS electrodes compared with the 3DGS electrodes. To avoid the dissolution−recrystallization process of sulfur during the traditional electrode fabrication process, we proposed a GOassisted assembly method as a facile and universal approach to prepare high-performance binder-free electrodes. As an example to support this concept, 3DEFGS was successfully fabricated via this method and directly used as electrodes for LSBs. The 3DEFGS with a high areal sulfur loading of ∼6 mg cm−2 demonstrate excellent electrochemical performance including an ultrahigh initial specific capacity of 1394 mA h g−1 at 0.1 C, excellent rate capability with a capacity of 796 mA h g−1 at 4 C, and superior long-term cycling stability with a capacity of 885 mA h g−1after 500 cycles at 1 C. We believe that this study not only provides a new understanding of structure evolution of sulfur during the traditional electrode fabrication process, but also develops a facile and universal method to prepare high-performance binder-free electrodes, which may pave the way for the practical application of LSBs in energy.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.L.). *E-mail: [email protected] (I.S.). *E-mail: [email protected] (Y.X.). ORCID

Yuxi Xu: 0000-0003-0318-8515 Notes

The authors declare no competing financial interest. 33274

DOI: 10.1021/acsami.8b11883 ACS Appl. Mater. Interfaces 2018, 10, 33269−33275

Research Article

ACS Applied Materials & Interfaces Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts. Nano Lett. 2016, 16, 519−527. (17) Carter, R.; Oakes, L.; Muralidharan, N.; Cohn, A. P.; Douglas, A.; Pint, C. L. Polysulfide Anchoring Mechanism Revealed by Atomic Layer Deposition of V2O5 and Sulfur-Filled Carbon Nanotubes for Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces 2017, 9, 7185− 7192. (18) Kim, K.; Kim, P. J.; Youngblood, J. P.; Pol, V. G. Surface Functionalization of Carbon Architecture with Nano-MnO2 for Effective Polysulfide Confinement in Lithium-Sulfur Batteries. ChemSusChem 2018, 11, 2375−2381. (19) Liang, X.; Garsuch, A.; Nazar, L. F. Sulfur Cathodes Based on Conductive MXene Nanosheets for High-Performance Lithium-Sulfur Batteries. Angew. Chem., Int. Ed. 2015, 54, 3907−3911. (20) Xiao, P.; Xu, Y. Recent Progress in Two-Dimensional Polymers for Energy Storage and Conversion: Design, Synthesis, and Applications. J. Mater. Chem. A 2018, DOI: 10.1039/c8ta02820f. (21) Xiao, P.; Bu, F.; Zhao, R.; Aly Aboud, M. F.; Shakir, I.; Xu, Y. Sub-5 nm Ultrasmall Metal-Organic Framework Nanocrystals for Highly Efficient Electrochemical Energy Storage. ACS Nano 2018, 12, 3947−3953. (22) Mao, Y.; Li, G.; Guo, Y.; Li, Z.; Liang, C.; Peng, X.; Lin, Z. Foldable Interpenetrated Metal-Organic Frameworks/Carbon Nanotubes Thin Film for Lithium-Sulfur Batteries. Nat. Commun. 2017, 8, 14628. (23) Cao, J.; Chen, C.; Zhao, Q.; Zhang, N.; Lu, Q.; Wang, X.; Niu, Z.; Chen, J. A Flexible Nanostructured Paper of a Reduced Graphene Oxide-Sulfur Composite for High-Performance Lithium-Sulfur Batteries with Unconventional Configurations. Adv. Mater. 2016, 28, 9629−9636. (24) Jiang, H.; Liu, X.-C.; Wu, Y.; Shu, Y.; Gong, X.; Ke, F.-S.; Deng, H. Metal-Organic Frameworks for High Charge-Discharge Rates in Lithium-Sulfur Batteries. Angew. Chem., Int. Ed. 2018, 57, 3916−3921. (25) Liu, Y.; Li, G.; Fu, J.; Chen, Z.; Peng, X. Strings of Porous Carbon Polyhedrons as Self-Standing Cathode Host for High-EnergyDensity Lithium-Sulfur Batteries. Angew. Chem. 2017, 129, 6272− 6276. (26) Zhou, G.; Paek, E.; Hwang, G. S.; Manthiram, A. Long-Life Li/ Polysulphide Batteries with High Sulphur Loading Enabled by Lightweight Three-Dimensional Nitrogen/Sulphur-Codoped Graphene Sponge. Nat. Commun. 2015, 6, 7760. (27) Li, J.; Xiao, P.; Li, H.; Zhang, Y.; Xue, F.; Luo, B.; Huang, S.; Shang, Y.; Wen, H.; de Claville Christiansen, J.; Yu, D.; Jiang, S. Crystalline Structures and Crystallization Behaviors of Poly(l-lactide) in Poly(l-lactide)/Graphene Nanosheet Composites. Polym. Chem. 2015, 6, 3988−4002. (28) Li, W.; Zhang, Q.; Zheng, G.; Seh, Z. W.; Yao, H.; Cui, Y. Understanding the Role of Different Conductive Polymers in Improving the Nanostructured Sulfur Cathode Performance. Nano Lett. 2013, 13, 5534−5540. (29) Zhong, Y.; Yin, L.; He, P.; Liu, W.; Wu, Z.; Wang, H. Surface Chemistry in Cobalt Phosphide-Stabilized Lithium-Sulfur Batteries. J. Am. Chem. Soc. 2018, 140, 1455−1459. (30) Chen, T.; Zhang, Z.; Cheng, B.; Chen, R.; Hu, Y.; Ma, L.; Zhu, G.; Liu, J.; Jin, Z. Self-Templated Formation of Interlaced Carbon Nanotubes Threaded Hollow Co3S4 Nanoboxes for High-Rate and Heat-Resistant Lithium-Sulfur Batteries. J. Am. Chem. Soc. 2017, 139, 12710−12715. (31) Deng, D.-R.; Xue, F.; Jia, Y.-J.; Ye, J.-C.; Bai, C.-D.; Zheng, M.S.; Dong, Q.-F. Co4N Nanosheet Assembled Mesoporous Sphere as a Matrix for Ultrahigh Sulfur Content Lithium-Sulfur Batteries. ACS Nano 2017, 11, 6031−6039. (32) Liu, F.; Xiao, Q.; Wu, H. B.; Sun, F.; Liu, X.; Li, F.; Le, Z.; Shen, L.; Wang, G.; Cai, M.; Lu, Y. Regenerative PolysulfideScavenging Layers Enabling Lithium-Sulfur Batteries with High Energy Density and Prolonged Cycling Life. ACS Nano 2017, 11, 2697−2705. (33) Zhao, Q.; Zhu, Q.; Miao, J.; Guan, Z.; Liu, H.; Chen, R.; An, Y.; Wu, F.; Xu, B. Three-Dimensional Carbon Current Collector

Promises Small Sulfur Molecule Cathode with High Areal Loading for Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces 2018, 10, 10882−10889. (34) Gueon, D.; Hwang, J. T.; Yang, S. B.; Cho, E.; Sohn, K.; Yang, D.-K.; Moon, J. H. Spherical Macroporous Carbon Nanotube Particles with Ultrahigh Sulfur Loading for Lithium-Sulfur Battery Cathodes. ACS Nano 2018, 12, 226−233. (35) Li, M.; Carter, R.; Douglas, A.; Oakes, L.; Pint, C. L. Sulfur Vapor-Infiltrated 3D Carbon Nanotube Foam for Binder-Free High Areal Capacity Lithium-Sulfur Battery Composite Cathodes. ACS Nano 2017, 11, 4877−4884. (36) Zhu, X.; Zhao, W.; Song, Y.; Li, Q.; Ding, F.; Sun, J.; Zhang, L.; Liu, Z. In Situ Assembly of 2D Conductive Vanadium Disulfide with Graphene as a High-Sulfur-Loading Host for Lithium-Sulfur Batteries. Adv. Energy Mater. 2018, 8, 1800201. (37) Luo, L.; Chung, S. H.; Yaghoobnejad Asl, H.; Manthiram, A. Long-Life Lithium-Sulfur Batteries with a Bifunctional Cathode Substrate Configured with Boron Carbide Nanowires. Adv. Mater. 2018, 1804149. (38) Pan, Z.-Z.; Lv, W.; He, Y.-B.; Zhao, Y.; Zhou, G.; Dong, L.; Niu, S.; Zhang, C.; Lyu, R.; Wang, C.; Shi, H.; Zhang, W.; Kang, F.; Nishihara, H.; Yang, Q.-H. A Nacre-Like Carbon Nanotube Sheet for High Performance Li-Polysulfide Batteries with High Sulfur Loading. Adv. Sci. 2018, 5, 1800384. (39) Shi, H.; Niu, S.; Lv, W.; Zhou, G.; Zhang, C.; Sun, Z.; Li, F.; Kang, F.; Yang, Q.-H. Easy fabrication of flexible and multilayer nanocarbon-based cathodes with a high unreal sulfur loading by electrostatic spraying for lithium-sulfur batteries. Carbon 2018, 138, 18−25. (40) Ren, W.; Ma, W.; Zhang, S.; Tang, B. Nitrogen-Doped Carbon Fiber Foam Enabled Sulfur Vapor Deposited Cathode for High Performance Lithium Sulfur Batteries. Chem. Eng. J. 2018, 341, 441− 449. (41) Chen, A.; Liu, W.; Hu, H.; Chen, T.; Ling, B.; Liu, K. Threedimensional TiO 2 -B nanotubes/carbon nanotubes intertwined network as sulfur hosts for high performance lithium−sulfur batteries. J. Power Sources 2018, 400, 23−30. (42) Carter, R.; Davis, B.; Oakes, L.; Maschmann, M. R.; Pint, C. L. A High Areal Capacity Lithium-Sulfur Battery Cathode Prepared by Site-Selective Vapor Infiltration of Hierarchical Carbon Nanotube Arrays. Nanoscale 2017, 9, 15018−15026. (43) Peng, H.-J.; Zhang, Z.-W.; Huang, J.-Q.; Zhang, G.; Xie, J.; Xu, W.-T.; Shi, J.-L.; Chen, X.; Cheng, X.-B.; Zhang, Q. A Cooperative Interface for Highly Efficient Lithium-Sulfur Batteries. Adv. Mater. 2016, 28, 9551−9558. (44) Chung, S.-H.; Chang, C.-H.; Manthiram, A. A core-shell electrode for dynamically and statically stable Li-S battery chemistry. Energy Environ. Sci. 2016, 9, 3188−3200. (45) Zhou, W.; Guo, B.; Gao, H.; Goodenough, J. B. Low-Cost Higher Loading of a Sulfur Cathode. Adv. Energy Mater. 2016, 6, 1502059. (46) Song, J.; Yu, Z.; Gordin, M. L.; Wang, D. Advanced Sulfur Cathode Enabled by Highly Crumpled Nitrogen-Doped Graphene Sheets for High-Energy-Density Lithium-Sulfur Batteries. Nano Lett. 2016, 16, 864−870. (47) Peng, H.-J.; Xu, W.-T.; Zhu, L.; Wang, D.-W.; Huang, J.-Q.; Cheng, X.-B.; Yuan, Z.; Wei, F.; Zhang, Q. 3D Carbonaceous Current Collectors: The Origin of Enhanced Cycling Stability for High-SulfurLoading Lithium-Sulfur Batteries. Adv. Funct. Mater. 2016, 26, 6351− 6358. (48) Liang, X.; Nazar, L. F. In Situ Reactive Assembly of Scalable Core-Shell Sulfur-MnO2 Composite Cathodes. ACS Nano 2016, 10, 4192−4198.

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DOI: 10.1021/acsami.8b11883 ACS Appl. Mater. Interfaces 2018, 10, 33269−33275