High-Performance Li–S Batteries with an Ultra-lightweight MWCNT

May 21, 2014 - functions (i) as an upper current collector to facilitate electron transport and high active- material utilization and (ii) as a filter...
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High-Performance Li−S Batteries with an Ultra-lightweight MWCNTCoated Separator Sheng-Heng Chung and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: A bifunctional separator consisting of a layer of multiwall carbon nanotubes (MWCNTs) on the cathode-side of a Celgard polypropylene sheet has been investigated to overcome the challenges of Li−S cells. The conductive/porous MWCNT-coating functions (i) as an upper current collector to facilitate electron transport and high activematerial utilization and (ii) as a filter to intercept/absorb the migrating polysulfides and thereby suppress the polysulfide diffusion. Also, the access to the electrolyte through the porous network of MWCNT along with its fast electronic transport facilitates the reutilization of the trapped active material and superior long-term cyclability. The MWCNT-coating is lightweight (0.17 mg cm−2), yet allows the successful use of regular sulfur cathodes (high sulfur content of 70 wt %) with high discharge capacity, excellent rate performance, and long cycle life, demonstrating that the MWCNT-coated separator is a viable solution to practical Li−S batteries. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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cathodes and (ii) novel cell configurations.3,9,10 The composite cathodes focus on improving the electrical conductivity, cushioning the volume change, and localizing the migrating polysulfides by adding a micro/mesoporous conductive matrix or applying a surface treatment, e.g., porous nanostructured substrates3,10−19 or conductive polymer additives.3,20−24 However, the introduction of nanocomposites or polymer additives involves complex or unpractical manufacturing processes and often reduces the sulfur content in the cathode.5,7 As a result, there exists a trade-off between the cell performance and the feasibility of this approach.7,25 The incorporation of modified or novel cell components between the cathode and anode is another attractive strategy for achieving excellent cycling performance. These advanced cell architectures are designed to stabilize the sulfur, polysulfides, and liquid electrolyte within the cathode region;3,26−32 or to intercept the migrating polysulfides and prevent them from diffusing through the separator.3,32−37 Moreover, the applied flexible porous substrates channel off the severe volume variation from the active material during the charge/discharge process.27,29,34 Although cell configuration modification is a facile approach for realizing high-performance Li−S cells and utilizing regular sulfur cathodes, the added weight of the modified or extra components may result in new concerns of lowering the overall energy density.5 Furthermore, many of the novel cell components require a unique “free-standing

he increasing demand for high-capacity cathodes to power electronic devices and electric vehicles has sparked a concerted research effort toward a viable lithium−sulfur (Li−S) battery.1,2 Sulfur, one of the most abundant and inexpensive elements in nature, offers a high theoretical capacity of 1672 mA h g−1, which is an order of magnitude higher than those of the commercial transition-metal oxide cathodes.1−3 However, several technical challenges hinder the commercialization of Li−S batteries: (i) the insulating nature of sulfur and its discharge end products (Li2S2/Li2S), (ii) the huge volume change of active material during discharge/charge processes, and (iii) severe polysulfide (Li2Sx, 4 < x ≤ 8) diffusion.3−6 The insulating nature of sulfur and lithium sulfides leads to poor active material utilization and decrease in discharge capacity,3,4 limiting the application of the regular sulfur cathode or a composite cathode containing a reasonable sulfur content (e.g., > 60 wt %).7 Here, the regular sulfur cathode means that it contains only the normal proportions of conductive carbon and binder used during electrode fabrication.8 The second challenge involves a volume change of 79.2% during a full lithiation from S to Li2S, resulting in poor contacts between active material and the conductive substrate, and even raising safety problems.3,5 The third challenge, i.e., polysulfide diffusion, leads to rapid capacity fade during cycling. The polysulfide intermediates formed during cycling are highly soluble in the liquid electrolytes currently used in Li−S cells. The dissolved polysulfides freely diffuse through the separator and then shuttle between the anode and cathode, resulting in low discharge/charge efficiency and loss of active material.4−6 The mainstream approaches to overcome these scientific challenges include the application of (i) sulfur-based composite © 2014 American Chemical Society

Received: April 7, 2014 Accepted: May 21, 2014 Published: May 21, 2014 1978

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architecture” for their normal function, introducing additional raw materials and synthesis/processing concerns.26−30,32−37 To address the main scientific issues facing the Li−S technology without falling into the limitations imposed by many composite cathodes or novel cell architectures, we present here the feasibility and practicality of a bifunctional separator with an ultralightweight multiwall carbon nanotube (MWCNT)-coating on a commercial Celgard polypropylene sheet for Li−S cells. On the cathode-side of the separator, the MWCNT-coating functions (i) as a filter for obstructing and trapping the diffusing polysulfides within its porous conductive skeleton and (ii) as an upper current collector for enhancing the utilization of sulfur and ensuring the reactivation of the trapped active material. On the anode side, the commercial Celgard polypropylene serves its usual function as an electrically insulating membrane while remaining as a flexible and robust substrate for supporting the MWCNT-coating on the cathode side. As a result, the MWCNT-coated separator provides the Li−S cell with significant enhancements in cycling performance and commercial feasibility. The outstanding cell performance is evidenced by a high discharge capacity of 1324 mA h g−1, excellent rate performance from C/5 to 1C rates, and a low capacity fade rate of 0.14% per cycle for 300 cycles tested so far. The promising attributes of the MWCNT-coated separator is demonstrated by the successful application of the regular sulfur cathode with a high sulfur content of 70 wt %. The MWCNT-coated separator is fabricated by ultrasonically dispersing the MWCNTs in isopropyl alcohol (IPA), followed by a simple vacuum filtration of the suspension through the commercial Celgard separator without any additional additives or treatments (Figure S1, Supporting Information). The prepared MWCNT-coated separator is flexible and robust (Figure S2, Supporting Information) with excellent mechanical strength for ensuring its normal function in cells (Figure S2c, Supporting Information). After cycling, the coating layer of the cycled MWCNT-coated separators maintains good integrity and homogeneity (Figure S 2d, Supporting Information), consistent with the above statements and indicating that the robust and flexible MWCNT-coating can cushion the strain generated by the volume changes of the active material conversion. Most importantly, the weight of the MWCNTcoating is only 0.17 mg cm−2. As a reference, the weights of the Celgard separator and the active material are, respectively, 1.0 mg cm−2 and ∼2.0 mg cm−2. Thus, even if the weight of the MWCNT-coating is included in the calculation of the active material content, the cell utilizing the MWCNT-coated separator has a sulfur content of 65 wt %, achieving a reasonable sulfur loading7 that is higher than that in many highperformance Li−S cells. Figure 1a shows the schematic configuration of the cell with the MWCNT-coated separator. The MWCNT-coating side facing the regular sulfur cathode intercepts the diffusing polysulfides before they freely migrate through the polypropylene separator. As a result, the polysulfide species is stabilized within the cathode region of the cell (marked as red circulating arrows) and a stable electrochemical environment exists (marked as blue circulating arrows). Figure 1b shows the scanning electron microscopy (SEM) image and the corresponding energy-dispersive X-ray spectroscopy (EDX) elemental mapping of the MWCNT-coated separator. The inset shows the high magnification SEM inspection of the MWCNT-coating. The MWCNT-coating layer consists of interwoven, curved MWCNTs that are

Figure 1. (a) A schematic cell configuration of the Li−S cell employing the MWCNT-coated separator. SEM observation and elemental mapping: (b) MWCNT-coated separator, (c) cycled MWCNT-coated separator, (d) the separator side of the cycled MWCNT-coating, and (e) the broken surface of the cycled MWCNTcoated separator.

deposited as a bundled/porous filter on the Celgard separator. This porous filter with uneven surface is the key architectural element for blocking the free migration of polysulfides.32,33,37 The MWCNT-coating possesses a high surface area of 410.42 m2 g−1 with a total pore volume of 2.76 cm3 g−1 (0.18 cm3 g−1 1979

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The electrochemical analyses of the cell with the MWCNTcoated separator are summarized in Figure 2. Figure 2a shows

for micropore volume). Thus, the MWCNT-coated separator possesses not only abundant porous space for localizing the electrolyte containing dissolved polysulfides but also the microporous absorption sites for trapping the intercepted polysulfides. Moreover, its long-range porous network ensures charge transport and electrolyte immersion, which is necessary for reactivating the trapped active material.33,37 The suppressed polysulfide diffusion is evident in the cycled MWCNT-coated separator, displaying obvious morphological and elemental changes compared to the fresh one, as shown in Figure 1c and Figure S3 (high magnification of the marked region). The SEM inspection shows the obstructed active material, which was filtered out by the MWCNT-coating, and the corresponding EDX elemental mapping shows clear elemental sulfur signal (marked as red) distributed in the carbon matrix (marked as green), evidencing the excellent interception and absorption effects of the porous MWCNTcoating. As a result, the cycled MWCNT-coating that was painted off from the MWCNT-coated separator by a razor blade shows low surface area of 30.58 m2 g−1 with a low pore volume and micropore volume of, respectively, only 0.06 cm3 g−1 and 0.0085 cm3 g−1 (Figure S4). The decrease in the surface area and pore volume demonstrates that the porous space and microporous absorption sites of the MWCNT-coating are effectively utilized for absorbing and then trapping the migrating polysulfides. On the other hand, it is worth emphasizing that the uniform elemental sulfur signal (Figure 1c and Figure S3) shows no dense spots, and the elemental carbon signal remains strong and distinguishable. These phenomena indicate that there is no formation of severe nonconductive agglomerations on the MWCNT-coated separator. The reason for this may come from two possible mechanisms. First, the interwoven conductive MWCNTs successfully transfer electrons to reactivate the trapped active material during cycling, suppressing the formation of inactive precipitates.34−36 Second, the uneven and porous structure of MWCNT-coating is unfavorable for the formation of nonconductive agglomerations.27,34 It can also be seen that the elemental fluorine and oxygen signals are homogeneous with the carbon signals, implying good electrolyte immersion and penetration. Therefore, the electrochemically active materials are stabilized within the cathode region of the cell with intimate three-phase boundary involving the active material, the conductive network, and the electrolyte. Such an optimized electrochemical environment ensures efficient sulfur utilization and high reversibility. To understand the ability of the MWCNT-coated separator to physically inhibit polysulfide diffusion, it is beneficial to look at the morphology of the “separator side” of the cycled MWCNT-coating. In Figure 1d, the separator side of the MWCNT-coating retains its porous structure and shows no obvious polysulfide agglomerations, which is confirmed by the strong carbon signal and the weak sulfur signal in the corresponding elemental mapping. The weak sulfur signal may come from the LiCF3SO3 salt in the electrolyte. Furthermore, in Figure 1e and Figure S5 (low magnification SEM), the broken surface SEM of the cycled MWCNT-coated separator were obtained by scraping the MWCNT-coating from the Celgard polypropylene sheet. The scraped region shows almost no elemental sulfur signal on the surface of the Celgard separator. Therefore, we believe that the dissolved polysulfides are trapped within the MWCNT-coating and are not able to penetrate the separator to cause severe capacity fading.

Figure 2. Electrochemical measurements of Li−S cells employing the MWCNT-coated separator: (a) Discharge/charge curves at a C/5 rate; (b) cyclic voltammograms at a 0.1 mV s−1 scanning rate; (c) upper-plateau discharge capacities at various cycling rates.

the discharge/charge voltage profiles during the initial 20 cycles at a C/5 rate. The upper discharge plateau at 2.35 V and lower discharge plateau at 2.05 V represent, respectively, the reduction from sulfur to long-chain polysulfides and from long-chain polysulfides to Li2S2/Li2S.38,39 The initial discharge capacity is 1324 mA h g−1 with the sulfur utilization approaching 80%, facilitated by the MWCNT-coating that provides additional conductive pathways and increases the cell conductivity.36,37 The significantly smaller semicircle in the impedance analysis, as an electrochemical evidence, reconfirms that the MWCNT-coated separator increases the cell 1980

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conductivity by reducing the charge-transfer resistance of the cell by about 85% (Figure S6, Supporting Information).32,33 The excellent charge-transfer ability of the conductive MWCNT-coating enables a high reactivation of the trapped active material, inhibiting severe inactive agglomeration and ensuring the stable cyclability of the cell.3,40 Thus, in the subsequent cycles, the overlapping discharge curves show no severe shrinkage and decline in capacity.37 The two continuous charge plateaus at 2.25 and 2.40 V represent the reversible oxidation reaction from Li2S2/Li2S to Li2S8/S. The vertical voltage rise from 2.4 to 2.8 V at the end of charge indicates a complete charge process with limited polysulfide shuttling.35,41 At various cycling rates of C/2 and 1C, the cells utilizing the MWCNT-coated separator also possess overlapping discharge curves and charge curves (Figures S7 and S8, Supporting Information), demonstrating superior cycle stability and excellent rate performance.30,37,38 Figure 2b shows the cyclic voltammograms (CV) of the cell with the MWCNT-coated separator during the initial 20 cycles at a scanning rate of 0.1 mV s−1. The two cathodic peaks and overlapping anodic peaks are in agreement with the discharge/charge curves (Figure 2a), displaying the typical sulfur reduction/oxidation reactions of Li−S cells.39,42 After the initial cycle, the disappearance of the overpotential of the cathodic peak implies that the active material rearranges itself and migrates to electrochemically favorable positions.30,33 It can be visualized in Figure 2b that there is no decrease in peak intensity or a shift in potential during subsequent CV scans, confirming the high reversibility facilitated by the MWCNT-coated separator. The suppressed polysulfide diffusion and the high electrochemical reversibility facilitated by the MWCNT-coated separator can be analyzed by investigating the changes in the upper discharge plateaus and their corresponding discharge capacity (QH, theoretical capacity = 419 mA h g−1) during cycling because this region corresponds to the formation and existence of highly soluble polysulfides.4,6,31 First, in Figure 2a, S7, and S8, the completeness of the overlapping upper discharge plateaus during cycling provides evidence that the MWCNT polysulfide filter efficiently suppresses polysulfide diffusion and that severe active material loss has not occurred.33,35 Second, in Figure 2c, the initial QH at a C/5 rate is 414 mA h g−1 approaching 99% of the theoretical value, implying that the severe polysulfide diffusion has been suppressed. During cycling, the QH of the cells with the MWCNT-coated separator remains highly reversible at various cycling rates. However, the QH of the cell with the Celgard separator (marked in green) decreases to 53% of its original value after the initial cycle at a C/5 rate, exhibiting the typical capacity fade issue. The complete upper discharge plateau and stable QH throughout cycling demonstrate that the MWCNTcoating effectively alleviates the polysulfide diffusion and eliminates the loss of active material/capacity. The properties of the bifunctional MWCNT-coated separator allows successful implementation of a regular sulfur cathode containing 70 wt % sulfur and leads to high discharge capacities (sulfur utilization in parentheses) of 1324 (79%), 1107 (66%), and 1073 mA h g−1 (64%) at, respectively, C/5, C/2 and 1C rates, as shown in Figure 3a. The excellent rate capability allows the cells to remain stable under a range of cycling rates from C/5 to 1C. After 150 cycles, the reversible discharge capacities of the cells with the MWCNT-coated separator are 881, 809, and 798 mA h g−1 at, respectively, C/5, C/2, and 1C rates. The corresponding capacity fading rates at

Figure 3. Cell performance of the Li−S cells employing the MWCNTcoated separator: (a) cycle stability and rate performance and (b) long-term cycle life.

various cycling rates are only 0.19 ± 0.03% per cycle. For a comparison, the same regular sulfur cathode with the Celgard separator suffers from low capacity, severe capacity fade, and short cycle life. The excellent cycle stability achieved by the application of the MWCNT-coated separator arises from two mechanisms: (i) the soluble polysulfides are stabilized within the cathode region by the porous MWCNT-coating and (ii) the conductive MWCNT-coating facilitates the successive reutilization of the trapped active materials within the conductive bundled filter during subsequent cycles.32,35 These effects ensure no severe loss of active material and suppression of inactive agglomerations covering on the cycled regular sulfur cathode (Figures S9 and S10).32,37 Moreover, Figures S9 and S10 show the rearrangement of large sulfur agglomerations to uniformly dispersed active material that is well surrounded by the conductive carbon, which greatly enhances the cycling stability. With a high reversibility, the cell with the MWCNT-coated separator achieves long cycle life over 300 cycles with a reasonable capacity retention and high discharge/charge efficiency of >96% at a 1 C rate, as shown in Figure 3b. The capacity after 300 cycles is 621 mA h g−1 with a corresponding capacity fade rate of as low as 0.14% per cycle. Such long-term cycle stability results from an alleviation of the severe polysulfide diffusion and the reutilization of the trapped polysulfides within the conductive MWNCT-coating.33,34 In summary, the MWCNT-coated separator successfully integrates an ultralightweight bundled polysulfide filter with the polypropylene separator component in the cell and offers several enhancements. The bifunctional separator possesses the conductive/porous MWCNT-coating for obstructing the free polysulfide diffusion, reactivating the trapped active material, and stabilizing the electrochemical material within the cathode 1981

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(4) Yin, Y.-X.; Xin, S.; Guo, Y.-G.; Wan, L.-J. Lithium−Sulfur Batteries: Electrochemistry, Materials, and Prospects. Angew. Chem., Int. Ed. 2013, 52, 13186−13200. (5) Zhang, S. S. Liquid Electrolyte Lithium/Sulfur Battery: Fundamental Chemistry, Problems, and Solutions. J. Power Sources 2013, 231, 153−162. (6) Mikhaylik, Y. V.; Akridge, J. R. Polysulfide Shuttle Study in the Li−S Battery System. J. Electrochem. Soc. 2004, 151, A1969−A1976. (7) Zhang, S. S.; Read, J. A. A New Direction for the Performance Improvement of Rechargeable Lithium/sulfur Batteries. J. Power Sources 2012, 200, 77−82. (8) Shim, J.; Striebel, K. A.; Cairns, E. J. The Lithium/Sulfur Rechargeable Cell Effects of Electrode Composition and Solvent on Cell Performance. J. Electrochem. Soc. 2002, 149, A1321−A1325. (9) Ji, X.; Nazar, L. F. Advances in Li−S Batteries. J. Mater. Chem. 2010, 20, 9821−9826. (10) Evers, S.; Nazar, L. F. New Approaches for High Energy Density Lithium-Sulfur Battery Cathodes. Acc. Chem. Res. 2013, 46, 1135− 1143. (11) Lai, C.; Gao, X. P.; Zhang, B.; Yan, T. Y.; Zhou, Z. Synthesis and Electrochemical Performance of Sulfur/Highly Porous Carbon Composites. J. Phys. Chem. C 2009, 113, 4712−4716. (12) 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. (13) Evers, S.; Yim, T.; Nazar, L. F. Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li−S Battery. J. Phys. Chem. C 2012, 116, 19653−19658. (14) Xin, S.; Gu, L.; Zhao, N.-H.; Yin, Y.-X.; Zhou, L.-J; Guo, Y.-G.; Wan, L.-J. Smaller Sulfur Molecules Promise Better Lithium−Sulfur Batteries. J. Am. Chem. Soc. 2012, 134, 18510−18513. (15) Wang, L.; Zhao, Y.; Thomas, M. L.; Byon, H. R. In Situ Synthesis of Bipyramidal Sulfur with 3D Carbon Nanotube Framework for Lithium−Sulfur Batteries. Adv. Funct. Mater. 2014, 24, 2248− 2252. (16) Liu, J.; Yang, T.; Wang, D.-W.; Lu, G. Q.; Zhao, D.; Qiao, S. Z. A Facile Soft-Template Synthesis of Mesoporous Polymeric and Carbonaceous Nanospheres. Nat. Commun. 2013, 4, 2798. (17) Kim, H.; Lim, H.-D.; Kim, J.; Kang, K. Graphene for Advanced Li/S and Li/Air Batteries. J. Mater. Chem. A 2014, 2, 33−47. (18) Zhao, C.; Liu, L.; Zhao, H.; Krall, A.; Wen, Z.; Chen, J.; Hurley, P.; Jiang, J.; Li, Y. Sulfur-Infiltrated Porous Carbon Microspheres with Controllable Multi-Modal Pore Size Distribution for High Energy Lithium−Sulfur Batteries. Nanoscale 2014, 6, 882−888. (19) Wang, C.; Chen, H.; Dong, W.; Ge, J.; Lu, W.; Wu, X.; Guo, L.; Chen, L. Sulfur−Amine Chemistry-Based Synthesis of Multiwalled Carbon Nanotube−Sulfur Composites for High Performance Li−S Batteries. Chem. Commun. 2014, 50, 1202−1204. (20) Xiao, L.; Cao, Y.; Xiao, J.; Schwenzer, B.; Engelhard, M. H.; Saraf, L. V.; Nie, Z.; Exarhos, G. J.; Liu, J. A Soft Approach to Encapsulate Sulfur: Polyaniline Nanotubes for Lithium-Sulfur Batteries with Long Cycle Life. Adv. Mater. 2012, 24, 1176−1182. (21) Fu, Y. Z.; Manthiram, A. Orthorhombic Bipyramidal Sulfur Coated with Polypyrrole Nanolayers as a Cathode Material for Lithium−Sulfur Batteries. J. Phys. Chem. C 2012, 116, 8910−8915. (22) Lin, Z.; Liu, Z.; Fu, W.; Dudney, N. J.; Liang, C. Phosphorous Pentasulfide as a Novel Additive for High-Performance Lithium-Sulfur Batteries. Adv. Mater. 2013, 23, 1064−1069. (23) Yang, J.; Xie, J.; Zhou, X.; Zou, Y.; Tang, J.; Wang, S.; Chen, F.; Wang, L. Functionalized N-Doped Porous Carbon Nanofiber Webs for a Lithium−Sulfur Battery with High Capacity and Rate Performance. J. Phys. Chem. C 2014, 118, 1800−1807. (24) Song, J.; Xu, T.; Gordin, M. L.; Zhu, P.; Lv, D.; Jiang, Y.-B.; Chen, Y.; Duan, Y.; Wang, D. Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of HighAreal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium−Sulfur Batteries. Adv. Funct. Mater. 2014, 24, 1243−1250.

region of the cell. As a result, the sulfur cathode employing the MWCNT-coated separator displays a high initial discharge capacity of 1324 mA h g−1, excellent rate performance from C/ 5 to 1C rates, and superior long-term cycle stability over 300 cycles. In addition, the successful use of the regular sulfur cathode with a high sulfur content of 70 wt % narrows the gap between scientific research and commercial feasibility.

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EXPERIMENTAL METHODS Ultra-lightweight MWCNT-Coated Separator Preparation. The MWCNTs (0.025 g) (Nanolab, Inc.) were ultrasonically dispersed in 500 mL of isopropyl alcohol (IPA), vacuum filtered through a commercial Celgard 2500 separator, and dried at 50 °C for 24 h in an air-oven. The resultant MWCNTcoating (0.17 mg cm−2) forms a bundled nanotube layer attached to the Celgard separator. Electrochemical and Microstructural Analyses. Electrochemical measurements were conducted in CR2032-type coin cells with the regular sulfur cathode, MWCNT-coated separator, lithium foil anode, and nickel foam spacers. The regular sulfur cathode involves 70 wt % sulfur, 20 wt % Super P carbon, and 10 wt % polyvinylidene fluoride binder. The electrolyte contains 1.85 M LiCF3SO3 salt and 0.1 M LiNO3 salt in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1 volume ratio). Discharge/charge and cycling performances were evaluated with an Arbin battery cycler between 1.8 and 2.8 V. The cyclic voltammetry (CV) measurement was carried out by a universal potentiostat between 1.8 and 2.8 V at a 0.1 mV s−1 rate. The microstructural inspections of the MWCNT-coated separator, the cycled MWCNT-coated separator, and the cycled regular sulfur cathode were carried out with a field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray (EDX) spectrometers for elemental mapping.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental methods, morphological observation of the MWCNT-coated separator, BET analyses, electrochemical impedance analysis, electrochemical analyses at various cycling rates, and SEM inspection of the cycled cathode. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (512) 471-1791. Fax: (512) 471-7681. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award Number DE-SC0005397.

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