A novel separator coated by carbon for achieving exceptional high performance lithium-sulfur batteries

Author’s Accepted Manuscript A Novel Separator Coated by Carbon for Achieving Exceptional High Performance LithiumSulfur Batteries Jiadeng Zhu, Yeqian Ge, David Kim, Yao Lu, Chen Chen, Mengjin Jiang, Xiangwu Zhang www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(15)00499-1 http://dx.doi.org/10.1016/j.nanoen.2015.12.022 NANOEN1077

To appear in: Nano Energy Received date: 29 September 2015 Revised date: 10 December 2015 Accepted date: 22 December 2015 Cite this article as: Jiadeng Zhu, Yeqian Ge, David Kim, Yao Lu, Chen Chen, Mengjin Jiang and Xiangwu Zhang, A Novel Separator Coated by Carbon for Achieving Exceptional High Performance Lithium-Sulfur Batteries, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.12.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Novel Separator Coated by Carbon for Achieving Exceptional High Performance Lithium-Sulfur Batteries

Jiadeng Zhua, Yeqian Gea, David Kimb, Yao Lua, Chen Chena, Mengjin Jianga, and Xiangwu Zhanga,*

a

Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USA b

Sceye, Inc., 1020 19th St NW, Washington, DC 20036, USA

Abstract Lithium-sulfur batteries have received intense attention because of their high theoretical capacity, low cost and environmental friendliness. However, low active material utilization and poor cycle life limit their practical applications. Here, we report a strategy to obtain high capacity with long cycle life and rapid charge rate by introducing a carbon coating on the separator. Excellent cycling performance with a high capacity 956 mAh g-1 after 200 cycles and outstanding high-rate response up to 4C are achieved, which are among the best reports so far. High electrochemical performance can be obtained even at a high sulfur loading of 3.37 mg cm-2. Such improved results could be ascribed to the conductive carbon coating, which not only reduces the cell 1

resistance but blocks the diffusion of soluble polysulfides avoiding shuttle effect during cycling. This study demonstrates a feasible, low cost and scalable approach to address the long-term cycling challenge for lithium-sulfur batteries.

Keywords: Lithium-sulfur batteries, Carbon coating, Separator, Rate capability, Sulfur loading

Introduction With the continuously increasing demand of the world energy consumption, energy storage systems are essential for the extensive development of portable electronic devices, electric vehicle, etc.[1-10]. Lithium-sulfur (Li-S) batteries are one of the prospective candidates in this regard because S has a high theoretical capacity of 1,675 mAh g-1, and is low cost and environmental friendliness. However, the practical applications of Li-S batteries are hindered by the low utilization of the active material, severe capacity fading, and low Coulombic efficiency. The poor utilization of active material results from the insulating nature of sulfur, which limits the electron transfer during the electrochemical reactions [11-15]. The fast capacity fading is due to the dissolution of lithium polysulfide intermediates (Li2Sx, 4 ≤ x ≤ 8) in the electrolyte and the low Coulombic efficiency is caused by the internal “shuttle reaction”[16-18]. Many efforts have been made to overcome the challenges faced by Li-S batteries. The main approaches are performing nanomaterials as conductive frameworks for sulfur cathodes to achieve high capacity and improve cycle life, including porous hollow carbon [19, 20], carbon

2

nanofiber [21, 22], carbon nanotubes [23], graphene oxide [24], graphene [25], yolk-shell TiO2 spheres [26], conductive polymers [27], etc. The active material utilization and cyclability are improved because these conductive frameworks are able to enhance the electrical conductivity of the cathode and minimize the loss of soluble polysulfide intermediates during the cycling. These sulfur cathodes with conductive frameworks are often accompanied with electrolyte additives, such as lithium nitrate [28], phosphorous pentasulfide [29] to increase the Coulombic efficiency. Unfortunately, those structural designs are complex and high cost, which limit the practicality of Li-S batteries. In addition, those cathode systems generally have a relatively low sulfur content and loading which can significantly reduce the energy density of the entire cell. Therefore, it is still urgent to discover a low cost and scalable approach to achieve the high performance for Li-S batteries with a high sulfur content and loading. Recently, Manthiram, et al. introduced a conductive interlayer to the Li-S battery system [30]. This conductive interlayers are able to supply fine contact with the cathode surface, offering electro pathways through the insulating S/Li2S and preventing the migrating polysulfide intermediates, and this new cell design has shown the promise to improve the performance of the Li-S battery [31, 32]. However, the thickness (hundreds of micrometers) and the mass (several milligrams) of the carbon interlayers are large, which results in a reduced energy density of the cell. Compared with the intensive studies on the electrode material designs, electrolyte additives and interlayers, there are only a few researchers working on separators, which are considered as a critical component in the battery system. Separators are typically made of insulating materials, such as polypropylene (PP) and polyethylene (PE), and have a porous structure that allows the transportation of ions. Recently, Celgard® PP membranes coated with graphene [33], multiwall 3

carbon nanotubes [34] and carbon nanoparticles [35] have been investigated for preventing the migration of polysulfides away from the cathode to the Li metal anode in Li-S batteries. With these coated separators, Li-S cells have shown improved cycling stability, however, the low sulfur content and loading, as well as the poor rate performance and high cost, still make them unsuitable for practical applications. Glass fiber (GF) membrane has received attention for use as a battery separator because it has a higher porosity (65%) compared to PP or PE membranes (40%), which could lead to higher electrolyte intake and consequently greater ionic conductivity when placed in an electrolyte, facilitating rapid ionic transportation [36, 37]. In this study, we present an effective approach to achieve a stable and high-performance Li-S battery with exceptional rate capability by simply using carbon-coated GF (CGF) as the separator. It can effectively decrease the cell resistance, resulting in an enhancement of active material utilization. The thin carbon coating used in this work is able to block the shuttling of soluble polysulfides in the electrolyte and makes them available to be reutilized even for prolonged charge/discharge cycling. This conductive carbon coating can be considered as a second current collector for accommodating the migrating active material from sulfur cathodes. Therefore, the originality of our work is related to the use of a simple coating method instead of any complex electrode structure design to achieve excellent electrochemical performance (e.g., high cycling stability and good rate capability simultaneously) for Li-S batteries. In addition, the cell configuration suggested by this work can be fabricated in large scale with low cost, further enhancing the practicality of Li-S batteries.

4

Experimental Section

Carbon-coated separator fabrication A super P (TIMCAL, Graphite & Carbon Ltd., C-65)/polyvinylidene fluoride (PVDF) slurry was prepared by mixing 70 wt% super P with 30 wt% PVDF in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) solvent, which was coated onto one side of a Glass Fiber mat (GF, GE healthcare) by the doctoral blade casting method. The carbon-coated separator was dried in a vacuum oven at 60 oC for 12 h. Material Characterizations The morphology of the samples was characterized using a filed-emission scanning electron microscopy (FESEM, Quanta 3D FEI, USA) with an energy dispersive X-ray spectroscopy (EDS) detector and a transmission electron microscopy (TEM, JEOL 2010F). X-ray Diffraction (XRD) patterns were recorded on a Rigaku D/Max 2400 (Japan) type X-ray spectrometer with Cu Kα radiation (λ=1.5418 Å). Electrode fabrication and cell assembly The sulfur electrodes were prepared by casting a slurry containing 70 wt% pure sulfur (SigmaAldrich), 20 wt% conductive additive (TIMCAL, Graphite & Carbon Ltd., C-65), and 10 wt% PVDF binder in NMP (Sigma-Aldrich) on the carbon-coated aluminum foil, followed by drying in a vacuum oven at 60 oC for 12 h. The areal loading of sulfur for the as-prepared electrodes ranged from 0.70 to 3.37 mg cm-2. To test the electrochemical properties, 2032 type coin cells were assembled using the sulfur electrodes, PGF/CGF separators, and Li metal (Sigma-Aldrich)

5

as

the

counter

electrode.

The

electrolyte

was

prepared

by

dissolving

1

M

bis(trifluoromethane)sulfonamide lithium (LiTFSI, Sigma-Aldrich) and 0.1 M lithium nitrite (99.99% trace metals basis, Sigma-Aldrich) in a mixture of 1,3-dioxolane (DOL, Sigma-Aldrich) and 1,2-dimethoxyethane (DME, Sigma-Aldrich) (1: 1 by volume). The amount of the electrolyte used in this work was 40 µL per 1 mg S. The cell assembly process was done in an argon-filled glove box and all capacity values were calculated based on the sulfur mass. Electrochemical measurement Electrochemical impedance spectroscopy data were obtained with a device from 100,000 to 1 Hz with an AC voltage amplitude of 5 mV. A programmable battery cycler (Arbin Instruments) was used to record discharge/charge profiles cycle data. The cyclic voltammetry (CV) measurements were carried out by using a Gamry reference 600 device with a scan rate of 0.1 mV s-1 in a potential range of 2.8 – 1.7 V vs. Li+/Li. Several coin cells were disassembled in the charged state for further analysis after cycling. Morphological characterization and elemental mapping of the Li electrodes before and after cycling were carried out with a FE-SEM equipped with an EDS. X-ray photoelectron spectroscopy (XPS, SPECS FlexMod, Germany) was performed to execute elemental analysis at room temperature by using a Kratos Analytical spectrometer and monochromatic Mg Kα X-ray source. The lithium electrodes were washed using DOL and dried in the argon-filled glove box. A pure lithium plate before cycling was used as a comparison. Polysulfide diffusion test Li2S8 was synthesized by adding Li and S into the electrolyte (in volumetric flask) with a proper molar ratio under an intensive stirring at 80 oC for 24 h. The concentration of Li2S8 in the 6

electrolyte was adjusted to 0.1 M. A vial with a hole on its cap combined with an O-ring and a beaker were used as the Li2S8 solution and blank electrolyte containers, respectively. The diffusion images were recorded by a camera.

Results and discussion The conductive carbon coating was applied onto one side of the GF separator (toward the cathode side). The main interaction mechanism of carbon coating in Li-S cell is that the conductive carbon coating can act as an interlayer to suppress the diffusion of the lithium polysulfides, which in turn minimizes the shuttle effect and enhances the active material utilization [30, 35]. In addition, this conductive carbon coating on the separator surface may also act as a current collector for low-conductivity sulfur and thus enhance the utilization of active materials, raising the specific capacity of the cell [30, 32, 35]. In this work, the conductive carbon coating was prepared by a facile slurry coating of carbon nanoparticles and polyvinylidene fluoride (PVDF) binder with an optimized weight ratio of 7:3 (Figure S1). The configuration of the cell is shown in Figure 1a. Here, we employ pure sulfur powder as the active material to fabricate conventional sulfur cathodes instead of using sulfur-carbon composite or applying conductive polymer surface modification to demonstrate that the observed improvement of electrochemical performance is solely contributed from the conductive carbon coating. Figure 1b shows the scanning electron microscope (SEM) image of the conductive carbon coating used in this study. It is seen that the nanosized carbon particles (C-65) are uniformly dispersed on the surface of GF separator, which is able to reduce the electrical resistance by providing sufficient surface contact with the sulfur cathode. The highly porous coating structure is beneficial for electrolyte penetration during cycling [30]. Transmission 7

electron microscope (TEM) was used to evaluate the size of the carbon particles in the coating. The particle size ranges from 30 to 50 nm, as shown in Figure 1c. The crystal nature of the carbon particles was identified by high-resolution TEM (Figure 1d), which is in good agreement with its XRD pattern (Figure 1e). The crystallized carbon structure generally provides good electrical conductivity, which is also helpful for improving the electrochemical performance of battery cells. To analyze the impact of the conductive carbon coating, impedance analysis was performed to compare the cells with pristine GF (PGF) and CGF before cycling (Figure S2). The diameter of depressed semicircle in the high-frequency region, regarded as the charge transfer resistance of the cell [30, 38], decreases from 58 Ω cm2 to 8 Ω cm2 after the introduction of the conductive carbon coating onto the separator surface. This conductive carbon coating can block the diffusion of the lithium polysulfides, avoiding the shuttle effect, enhancing the active material utilization [35]. The reduced charge transfer resistance is probably because the conductive carbon coating on the separator surface provides a conductive pathway, facilitating the charge transfer for surface reactions, and thus reduces the charge transfer resistance [35, 39]. Moreover, the charge transfer resistance remarkably decreases for the cell with CGF, indicating a weaker polarization for the Li-S cell with the addition of carbon coating layer [40]. Figure 2a shows the cyclic voltammetry (CV) of the cell with CGF for the first five cycles within a cutoff voltage window of 1.7-2.8 V at a scan rate of 0.1 mV s-1. The C rates specified in this study are based on the mass and theoretical capacity of sulfur (1 C = 1,675 mA g-1). Two cathodic peaks at around 2.3 and 2.0 V, and two overlapped anodic peaks at around 2.38 and 2.45 V are typical of Li-S cells without the carbon coating (Figure S3a) [41]. The cathodic peak in the first cycle appears at 2.2V, which is probably due to the over potential of the electrode, 8

caused by the morphology and position changes during the discharge. This cathodic peak shifts to a relatively stable value, 2.3 V, from the second cycle, which is due to the rearrangement of the migrating active material to electrochemically favorable positions [42-44]. The discharge/charge profiles in Figure 2b, exhibiting two discharge plateaus and two closely spaced charge plateaus, are consistent with the cyclic voltammogram plots. The upper discharge plateau (Region 1) represents the transformation of sulfur into long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8) and the lower discharge plateau (Region 2) represents the conversion of the semi-solid phase Li2S4 to solid phase low-order Li2S2, or even Li2S. During the cell charging, the two continuous charge plateaus at 2.38 and 2.45 V correspond to the oxidation reactions from lithium sulfides to polysulfides and finally to sulfur [19]. Additionally, the polarization potential (as shown in Figure 2b) of the cell with CGF is significantly reduced to 117 mV, only 40.6% of that of PGF (288 mV), probably due to its small cell impedance, which suggests a kinetically efficient reaction process with a small barrier [23]. It is important to note that the capacities of the cells with CGF and PGF for Region 1 are almost the same (around 410 mAh g-1), which indicates the sulfur could be fully reacted to form Li2S4 (the theoretical capacity of this reaction is 418 mAh g1

). Thus, the increased capacity for the cell with CGF is mainly contributed by the extended

Region 2. It is probably because the conductive carbon coating acts as a second current collector, therefore, the solid lithium sulfides tend to deposit on the surface of CGF, instead of being formed on the anode surface (as illustrated in Figure 2c), which enhances the utilization of active material. Figure 3a compares the cycling performance of Li-S cells with PGF and CGF separators, and it demonstrates that the carbon-coated GF allows the successful implementation of a pure sulfur cathode containing 70 wt% sulfur and leads to a high discharge capacity and stable cyclability. 9

After updating the PGF separator to the CGF, the initial discharge capacity (with sulfur utilization in parentheses) increases from 976 (58%) to 1352 mAh g-1 (81%) at 0.2C rate. The Coulombic efficiencies of the Li-S cells with PGF and CGF are 96.7% and 97.6%, respectively, even at the 200th cycle. Figure 3b shows the long-term cycling behavior of Li-S cells containing CGF at higher current densities. When the current density increases to 1C and 2C, the cell still delivers reversible capacities of 721 and 607 mAh g-1 at 200th cycle with high Coulombic efficiencies of 98.6% and 96.7%, respectively. The electrochemical performance of Li-S cells with PGF at higher current densities are also included in Supplementary Information for comparison (Figure S3b). It is apparent that the electrochemical performance of Li-S cells is improved at high C-rates by the introduction of a conductive coating on the separator surface. The capacities of the cell with PGF at 1C and 2C are only 44.8% and 31.1% of that of the cell with CGF, respectively, which further indicates the essentiality of the conductive carbon coating in this system. Rate capability is one of the critical parameters for batteries, especially for high power applications. The rate capability of the Li-S cells with PGF and CGF is evaluated by increasing the discharge/charge current density stepwise from 0.2C to 4C every 10 cycles. In Figure 4a, the first discharge capacity of the cell with CGF is as high as 1352 mAh g-1 (corresponding to 81% utilization of sulfur), which is 60% greater than that of the cell with PGF, demonstrating the effectiveness of the conductive carbon coating in enhancing the conductivity and active material utilization. The capacity becomes stable after the initial decay stage. As the current density increases, the capacity of the cell with CGF decreases slowly from the reversible capacity of 1151 mAh g-1 at 0.2C to 1008, 869, 683, 544 and 417 mAh g-1 at 0.5C, 1C, 2C, 3C, and 4C, respectively. Importantly, a high reversible capacity of 1025 mAh g-1 (89% of the initial 10

reversible capacity) is achieved when the current density is lowered back to 0.2C, indicating a highly reversible and efficient electrode enabled by the introduction of the conductive carbon coating. In contrast, the cathode assembled with PGF exhibits not only much lower capacity at the same discharge rate but also inferior capacity retention ratio of 0.8% (from 487 mAh g-1 at 0.2C to 4 mAh g-1 at 4C). Additionally, Figure 4b gives the comparison of the rate performance between the Li-S cell with CGF in this work and Li-S cells reported in literature [23, 26, 33, 35, 45-47]. Our Li-S cell with CGF exhibits a more stable high-rate capability as the current density increases, indicating the electrochemical performance of Li-S cells can be significantly improved even without the complex cathode structure design or modification. However, it should be noted that our rate capability is not the highest among all reported results [18, 48]. The high-rate capability should be improved, especially at 4C rate. Some possible approaches can be performed to further improve the high-rate capability, such as optimizing coating thickness, selecting more conductive carbon filler, introducing more effective polymer binder, modifying sulfur cathode structure, etc. [35, 49, 50]. Representative voltage profiles of the Li-S cell with CGF at different rates are shown in Figure S4a and all curves show highly reversible capacities with two feature plateaus. However, the discharge/charge plateaus at high current rates obviously shift or even disappear in the cell with PGF, which indicates their high polarization and slow redox reaction kinetics with inferior reversibility (Figure S4b). As can be seen in Figure S4c, the Li-S cell with PGF has significantly greater polarization potential than the cell with CGF, and the potential difference remarkably increases from 171 to 507 mV when the current density increases from 0.2 to 2C, indicating a kinetically efficient reaction process with a small barrier for the cell with CGF.

11

To further understand the function of the CGF, the Li-S cells were disassembled after 100 cycles (at a current density of 1C) in an argon-filled glove box. For comparison, the metallic lithium anode before cycling was first observed by SEM. It can be seen that the surface of metallic lithium before cycling is smooth, as shown in Figure 5a and b. However, after 100 cycles in the Li-S cell with PGF (Figure 5c and d), obvious corrosion damage can be observed on the lithium anode surface with some cracks and the deposition of complicated sulfur-containing chemicals, such as Li2S2 and Li2S, etc., which is due to the side reactions between soluble lithium polysulfides and metallic lithium during cycling. In contrast, a relatively smooth, less damaged surface was found on the lithium anode after cycling in the Li-S cell with CGF (Figure 5e and f). The improvement in lithium surface morphology is in good agreement with the Smapping analysis by EDS, as showing in Figure 5g and h. The S intensity on the Li surface of Li-S cell with PGF is much stronger than that of CGF, demonstrating more lithium sulfide compounds are deposited on the surface, which results in the loss of active materials and corrosion damage of the lithium anode. XPS was also performed to characterize the surfaces of lithium anodes from the Li-S cells with PGF and CGF after 100 cycles. As shown in Figure S5, the peaks at 167-171 eV and 160-165 eV for the Li anode from the cell with PGF can be attributed to LixSOy species, which are due to the reaction of the active material with the LiTFSI, and the discharge product Li2S/Li2S2, respectively [51-53]. Similar phenomenon is also found for the Li anode from the cell with CGF. However, it is noteworthy noting that the intensity of the peaks at 160-165 eV for the cell with CGF is significantly reduced, indicating less short-chain Li2S2 and Li2S are deposited on the Li metal surface with the introduction of the carbon coting layer on the separator. This result demonstrates the ability of CGF in reducing the diffusion of

12

polysulfides to the Li anode, which is critically important for enhancing the active material utilization and protecting the Li anode surface. Photographic images of the separator surfaces that face the lithium plate are also obtained after 100 cycles at a current density of 1C. The yellowish color on the surface of the PGF indicates that polysulfides could diffuse through the separator and react with the lithium anode during cycling (as shown in Figure S6a), leading to shuttling reaction and reduced active material utilization. However, the surface of the CGF remains relatively clean and free of yellowish color (Figure S6b), which means the polysulfide migration is well controlled by the conductive carbon coating. All the results demonstrate that the CGF acts as a strong absorbent and an effective barrier to limit the diffusion of polysulfide through the separator and onto the lithium anode during the discharge/charge process, which reduces the polysulfide shuttle phenomenon and lithium surface corrosion [54]. In this work, we have presented the electrochemical performance of traditional sulfur electrode using carbon-coated GF as a separator. A simple diffusion model was used to observe the polysulfide diffusion across the separator, as shown in Figure S7. The polysulfides were fully blocked by either PGF or CGF at the beginning. The electrolyte with PGF turned to yellow after 15 min, while the color of the electrolyte with CGF did not change. Even after 30 min diffusion, the electrolyte with CGF only showed slight change in color, displaying a good inhibiting effect of polysulfide diffusion due to the carbon coating, which further resulted in better electrochemical performance of the cell. Obviously, the high loading of active material is extremely important for practical applications. The surface loading used in the previous section was 0.7-1.0 mg cm-2. Here, we

13

investigated the cycling performance of electrodes with higher sulfur loadings of 2.13 and 3.37 mg cm-2, respectively (Figure S8). The cell with a sulfur loading of 2.13 mg cm-2 can still deliver a reversible capacity as high as 1218 mAh g-1 at a current density of 0.2C for the first cycle with a low decay of 0.4% per cycle over 30 cycles. When the sulfur loading increases to 3.37 mg cm-2, an excellent retention of 94% can be achieved over 30 cycles with a high reversible capacity of 815 mAh g-1. It should be noted that the electrochemical performance of the Li-S cell with CGF is probably related to many factors, such as the type and amount of electrolyte, the porosity and thickness of the GF, the loading and thickness of conductive carbon coating, etc. In addition, S particle size is also essential because the shorter Li+ diffusion lengths could be obtained by using smaller S particles, which further results in better rate capability. What’s more, on one hand, the large amount of electrolyte and thick carbon coating can cause decreased energy density of lithium-sulfur cells; on the other hand, the large amount of electrolyte and three dimensional current collector such as carbon coating will improve the electrochemical performance by reversible dissolution of lithium polysulfides [55]. Therefore, it is important to determine the appropriate electrolyte amount and carbon coating structure for achieving optimized overall performance. Further work is needed to reduce the carbon content and coating thickness so that the cathode capacity can be further improved.

Conclusions In summary, a carbon coating has been developed to significantly improve the cycle performance of the Li-S batteries. Benefiting from the favorable adsorption properties of the conductive carbon coating on the separator, the sulfur cathodes show high capacity with excellent rate 14

capacity and stable cyclability even with high sulfur content (70 wt%) and high sulfur loading (up to 3.37 mg cm-2). The conductive carbon coating has multiple roles in enhancing electrochemical performance of the Li-S batteries. Firstly, it provides sufficient contact with the cathode surface, offering a high active material utilization. Secondly, it inhibits the migration of polysulfide intermediates, avoiding shuttle reactions. What’s more, the improved cells involve only a simple adjustment of the cell configuration by introducing a carbon coating on the separator instead of complex carbon-sulfur composite structure design or sulfur-conductive polymer modification, enhancing the commercial viability of Li-S batteries.

Acknowledgements This work was supported by SCEYE S. A. (2015-0888). The authors appreciate the use of Analytical Instrumentation Facility (AIF) at North Carolina State University.

References [1] L. Qie, A. Manthiram, A facile layer-by-layer approach for high-areal-capacity sulfur cathodes. Adv. Mater. 27 (2015) 1694-1700. [2] A. Manhiram, Y. Fu, Y-S. Su, Challenges and prospects of lithium-sulfur batteries. Acc. Chem. Res. 46 (2013) 1125-1134. [3] Y. Ge, H. Jiang, J. Zhu, Y. Lu, C. Chen, Y. Hu, Y. Qiu, X. Zhang, High cyclability of carbon-coated TiO2 nanoparticles as anode for sodium-ion battery. Electrochim. Acta 157 (2015) 142-148. 15

[4] J. Zhu, C. Chen, Y. Lu, Y. Ge, H. Jiang, K. Fu, X. Zhang, Nitrogen-doped carbon nanofibers derived from polyacrylonitrile for use as anode material in sodium-ion batteries. Carbon 94 (2015) 189-195. [5] C. Chen, K. Fu, Y. Lu, J. Zhu, L. Xue, Y. Hu, X. Zhang, Use of a tin antimony alloy-filled porous carbon nanofiber composite as an anode in sodium-ion batteries. RSC Adv. 5 (2015) 30793-30800. [6] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemon, J. Liu, Electrochemical energy storage for green grid. Chem. Rev. 111 (2011) 3577-3613. [7] R. Elazari, G. Salitra, A. Garsuch, A. Panchenko, D. Aurbach, Sulfur-impregnated activated carbon fiber cloth as a binder-free cathode for rechargeable Li-S batteries. Adv. Mater. 23 (2011) 5641-5644. [8] S. Zhang, Z. Lin, L. Ji, Y. Li, G. Xu, L. Xue, S. Li, Y. Lu, O. Toprakci, X. Zhang, Cr-doped Li2MnSiO4/carbon composite nanofibers as high-energy cathodes for Li-ion batteries. J. Mater. Chem. 22 (2012) 14661-14666. [9] L. Xue, G. Xu, Y. Li, S. Li, K. Fu, Q. Shi, X. Zhang, Carbon-coated Si nanoparticles dispersed in carbon nanotube networks as anode material for lithium-ion batteries. ACS Appl. Mater. Inter. 5 (2013) 21-25. [10] B. Guo, Y. Li, Y. Yao, Z. Lin, L. Ji, G. Xu, Y. Liang, Q. Shi, X. Zhang, Electrospun Li4TiO5/C composites for lithium-ion batteries with high rate performance. Solid State Ionic 204-205 (2011) 61-65.

16

[11] S. Lu, Y. Cheng, X. Wu, X., J. Liu, Significantly improved long-cycle stability in high-rate Li-S batteries enabled by coaxial graphene wrapping over sulfur-coated carbon nanofibers. Nano Lett. 13 (2013) 2485-2489. [12] H.-J. Peng, J. Liang, L. Zhu, J.-Q. Huang, X.-B. Cheng, X. Guo, W. Ding, W. Zhu, Q. Zhang, Catalytic self-limited assembly at hard templates: a mesoscale approach to graphene nanoshells for lithium-sulfur batteries. ACS Nano 8 (2014) 11280-11289. [13] F. Wu, L. Shi, D. Mu, H. Xu, B. Wu, A hierarchical carbon fiber/sulfur composite as cathode material for Li-S Batteries. Carbon 86 (2015) 146-155. [14] J. Lim, J. Pyun, K. Char, Recent approaches for the direct use of elemental sulfur in the synthesis and processing of advanced materials. Angew. Chem. Int. Ed. 54 (2015) 2-12. [15] S. Evers, L.F. Nazar, New approaches for high energy density lithium-sulfur battery cathodes. Acc. Chem. Res. 46 (2013) 1135-1143. [16] X. Ji, L.F. Nazar, Advances in Li-S batteries. J. Mater. Chem. 20 (2010) 9821-9826. [17] Y.V. Mikhaylik, J.R. Akridge, Polysulfide shuttle study in the Li/S battery system. J. Electrochem. Soc. 151 (2004) A1969-A1976. [18] Y. Qiu, W. Li, W. Zhao, G. Li, Y. Hou, M. Liu, L. Zhou, F. Ye, H. Li, Z. Wei, S. Yang, W. Duan, Y. Ye, J. Guo, Y. Zhang, High-rate, ultralong cycle-life lithium/sulfur batteries enabled by nitrogen-doped graphene. Nano Lett. 14 (2014) 4821-4827. [19] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Porous hollow carbon@sulfur composites for high-power lithium-sulfur batteries. Angew. Chem. 123 (2011) 6026-6030. 17

[20] C. Liang, N.J. Dudney, J.Y. Howe, Hierarchically structured sulfur/carbon nanocomposite material for high-energy lithium battery. Chem. Mater. 21 (2009) 4724-4730. [21] G. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Hollow carbon nanofiber-encapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries. Nano Lett. 11 (2011) 4462-4467. [22] K. Fu, Y. Li, M. Dirican, C. Chen, Y. Lu, J. Zhu, Y. Li, L. Cao, P.D. Bradford, X. Zhang, Sulfur gradient-distributed CNF composite: a self-inhibiting cathode for binder-free lithiumsulfur batteries. Chem. Commun. 50 (2014) 10277-10280. [23] J.-Q. Huang, Q. Zhang, S.-M. Zhang, X.-F. Liu, W. Zhu, W.-Z. Qian, F. Wei, Aligned sulfur-coated carbon nanotubes with a polyethylene glycol barrier at one end for use as high efficiency sulfur cathode. Carbon 58 (2013) 99-106. [24] Z. Wang, Y. Dong, H. Li, Z. Zhao, H.B. Wu, C. Hao, S. Liu, J. Qiu, X.W. Lou, Enhancing lithium-sulfur battery performance by strongly binding the discharge products on aminofunctionalized reduced graphene oxide. Nat. Commun. 5 (2014). [25] J.-Q. Huang, X.-F. Liu, Q. Zhang, C.-M. Chen, M.-Q. Zhao, S.-M. Zhang, W. Zhu, W.-Z. Qian, F. Wei, Entrapment of sulfur in hierarchical porous graphene for lithium-sulfur batteries with high rate performance from -40 to 60 oC. Nano Energy 2 (2013) 314-321. [26] Z.W. Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.-C. Hsu, Y. Cui, SulfurTiO2 yolk shell nanoarchitecture with internal void space for long-cycle lithium-sulfur batteries. Nat. Commun. 4 (2013).

18

[27] Y. Yang, G. Yu, J.J. Cha, H. Wu, M. Vosgueritchian, Y. Yao, Z. Bao, Y. Cui, Improving the performance of lithium-sulfur batteries by conductive polymer coating. ACS Nano 5 (2011) 9187-9193. [28] S.S. Zhang, Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions. J. Power Sources 231 (2013) 153-162. [29] Z. Lin, Z. Liu, W. Fu, N. J. Dudney, C. Liang, Phosphorous pentasulfide as a novel additive for high-performance lithium-sulfur batteries. Adv. Funct. Mater. 23 (2013) 1064-1069. [30] Y.-S. Su, A. Manthiram, Lithium-sulfur batteries with a microporous carbon paper as a bifunctional interlayer. Nat. Commun. 3 (2012) 1166. [31] X. Gu, C. Lai, F. Liu, W. Yang, Y. Hou, S. Zhang, Conductive interwoven bamboo carbon fibers membrane for Li-S batteries. J. Mater. Chem. A 3 (2015) 9502-9509. [32] R. Singhal, S.-H. Chung, A. Manthiram, V. Kalra, A free-standing carbon nanofiber interlayer for high-performance lithium-sulfur batteries. J. Mater. Chem. A 3 (2015) 4530-4598. [33] G. Zhou, L. Li, D.-W. Wang, X.-Y. Shan, S. Pei, F. Li, H.-M. Cheng, A flexible sulfurgraphene-polypropylene separator integrated electrode for advanced Li-S batteries. Adv. Mater. 27 (2015) 641-647. [34] S.-H. Chung, A. Manthiram, High-performance Li-S batteries with an ultra-lightweight MWCNT-coated separator. J. Phys. Chem. Lett. 5 (2014) 1978-1983. [35] H. Yao, K. Yan, W. Li, G. Zheng, D. Kong, Z.W. Seh, V.K. Narasimhan, Z. Liang, Y. Cui, Improved lithium-sulfur batteries with a conductive coating on the separator to prevent the

19

accumulation of inactive S-related species at the cathode-separator interface. Energy Environ. Sci. 7 (2014) 3381-3390. [36] Y. Wang, H. Zhan, J. Hu, Y. Liang, S. Zeng, Wet-laid non-woven fabric for separator of lithium-ion battery. J. Power Sources 189 (2009) 616-619. [37] L. Wang, J. Liu, S. Haller, Y. Wang, Y. Xia, A scalable hybrid separator for a high performance lithium-sulfur batteries. Chem. Commun. 51 (2015) 6996-6999. [38] L. Yuan, X. Qiu, L. Chen, W. Zhu, New insight into the discharge process of sulfur cathode by electrochemical impedance spectroscopy. J. Power Sources 189 (2009) 127-132. [39] G. Zhou, E. Paek, G.S. Hwang, and A. Manthiram. Long-life Li/polysulphide batteries with high sulphur loading enabled by lightweight three-dimensional nitrogen/sulphur-codoped graphene sponge. Nat. Commun. 6 (2015). [40] G. Ma, Z. Wen, J. Jin, M. Wu, X. Wu, and J. Zhang. Enhanced cycle performance of Li–S battery with a polypyrrole functional interlayer. J. Power Sources 267 (2014) 542-546. [41] Y.-S. Su, A. Manthiram, A facile in situ sulfur deposition route to obtain carbon-wrapped sulfur composite cathodes for lithium-sulfur batteries. Electrochim. Acta 77 (2012) 272-278. [42] D. Zheng, X. Zhang, J. Wang, D. Qu, X. Yang, and D. Qu. Reduction mechanism of sulfur in lithium–sulfur battery: From elemental sulfur to polysulfide. J. Power Sources 301 (2016): 312-316. [43] H. Ye, Y.X. Yin, and Y.G. Guo. Insight into the loading temperature of sulfur on sulfur/carbon cathode in lithium-sulfur batteries. Electrochim. Acta 185(2015) 62-68.

20

[44] Y. Fu, Y.S. Su, and A. Manthiram. Highly reversible lithium/dissolved polysulfide batteries with carbon nanotube electrodes. Angew. Chem. 125(2013) 7068-7073. [45] S. Lu, Y. Cheng, X. Wu, J. Liu, Significantly improved long-cycle stability in high-rate Li-S batteries enabled by coaxial graphene wrapping over sulfur-coated carbon nanofibers. Nano Lett. 13 (2013) 2485-2489. [46] G. Zhou, L.-C. Yin, D.-W. Wang, L. Li, S. Pei, I.R. Gentle, F. Li, H.-M. Cheng, Fibrous hybrid of graphene and sulfur nanocrystals for high-performance lithium-sulfur batteries. ACS Nano 7 (2013) 5367-5375. [47] S. Zheng, F. Yi, Z. Li, Y. Zhu, Y. Xu, C. Luo, J. Yang, C. Wang, Copper-stabilized sulfurmicroporous carbon cathodes for Li-S batteries. Adv. Func. Mater. 24 (2014) 4156-4163. [48] J.Q. Huang, H.J. Peng, X.Y. Liu, J.Q. Nie, X.B. Cheng, Q. Zhang, and F. Wei. Flexible allcarbon interlinked nanoarchitectures as cathode scaffolds for high-rate lithium–sulfur batteries. J. Mater. Chem. A 2 (2014) 10869-10875. [49] K. Park, J.H. Cho, J.-H. Jang, B.-C. Yu, T. Andreah, K.M. Miller, C.J. Ellison, and J.B. Goodenough. Trapping lithium polysulfides of a Li–S battery by forming lithium bonds in a polymer matrix. Energy Environ. Sci. 8 (2015) 2389-2395. [50] G. Zhou, L. Li, C. Ma, S. Wang, Y. Shi, N. Koratkar, W. Ren, F. Li, and H.-M. Cheng. A graphene foam electrode with high sulfur loading for flexible and high energy Li-S batteries. Nano Energy 11 (2015) 356-365.

21

[51] S. Xiong, K. Xie, Y. Diao, and X. Hong. On the role of polysulfides for a stable solid electrolyte interphase on the lithium anode cycled in lithium–sulfur batteries. J. Power Sources 236 (2013) 181-187. [52] J. Zheng, M. Gu, H. Chen, P. Meduri, M.H. Engelhard, J.G Zhang, J. Liu, and J. Xiao. Ionic liquid-enhanced solid state electrolyte interface (SEI) for lithium–sulfur batteries. J. Mater. Chem. A 1 (2013) 8464-8470. [53] N. Azimi,, Z. Xue, I. Bloom, M.L. Gordin, D. Wang, T. Daniel, C. Takoudis, and Z. Zhang. Understanding the effect of a fluorinated ether on the performance of lithium–sulfur batteries. ACS Appl. Mater. Inter. 7(2015) 9169-9177. [54] G. Zhou, S. Pei, L. Li, D.-W. Wang, S. Wang, K. Huang, L.-C. Yin, F. Li, H.-M. Cheng, A graphene-pure-sulfur sandwich structure for ultrafast long-life lithium-sulfur batteries. Adv. Mater. 26 (2014) 625-631. [55] G.L. Duffitt, and P.G. Pickup. Enhanced ionic conductivity of polypyrrole due to incorporation of excess electrolyte during potential cycling. J. Chem. Soc., Faraday Trans. 88 (1992) 1417-1423.

22

Figure Captions Figure 1 (a) Schematic configuration of a Li-S cell with CGF as the separator. (b) SEM image of the surface of the CGF (the inset is its photograph). (c) Low-resolution and (d) high-resolution transmission electron microscope images of C-65. (e) XRD spectrum of C-65. Figure 2 (a) Cyclic voltammogram scans of the Li-S cell with CGF. (b) Discharge/charge profiles in the first cycle of Li-S cells with PGF and CGF at a current density of 0.2C. (c) Schematic of polysulfide diffusion in Li-S cells with PGF (left) and CGF (right) during discharge. Figure 3 (a) Cycling performance of Li-S cells with PGF and CGF at a current density of 0.2C. (b) Cycling performance of the Li-S cell with CGF at high rates of 1C and 2C. Figure 4 (a) Rate cyclability of Li-S cells with PGF and CGF up to 4C. (b) Comparison of rate capabilities of the Li-S cell with CGF in this work and previously reported Li-S cells. Figure 5 (a, b) SEM images of the surface view of pristine Li metallic plate before cycling at different magnifications. (c, d) SEM images of the surface view of Li metal anode from the Li-S cell with PGF after 100 cycles at different magnifications. (e, f) SEM images of the surface view of Li metal anode from the Li-S cell with CGF after 100 cycles at different magnifications. (g) Selemental mapping of Li metal anodes from Li-S cells with PGF (g) and CGF (h). The current density used for the 100 cycles was 1C.

23

Figures.

a

b Cathode Li2Sx Carbon Coated Separator

Anode

d

e Intensity (a.u.)

c

10

20

30

40

50

2 theta (o)

60

70

Figure 1 (a) Schematic configuration of a Li-S cell with CGF as the separator. (b) SEM image of the surface of the CGF (the inset is its photograph). (c) Low-resolution and (d) high-resolution transmission electron microscope images of C-65. (e) XRD spectrum of C-65.

24

0.004 1st 2nd 3rd 4th 5th

Current (A)

0.003 0.002

b

3.0

PGF

Voltage (V. vs. Li+/Li)

a

0.001 0.000

-0.001 -0.002

2.7 2.4 V = 117 mV

2.1 Region 1

1.8

1.8

2.0

2.2

2.4

2.6

1.5

2.8

Li2S2 Li2S

solid state

0

Voltage (V)

c

V = 288 mV

Region 2

S8 Li2S8 Li2S4 Li2S4

-0.003

1.6

CGF

200 400 600 800 1000 1200 1400 1600

Specific Capacity (mAh g-1)

PGF

CGF

Li2Sx

Li2Sx

Li2S2/Li2S

v

Cathode

Anode Cathode

Anode

Figure 2 (a) Cyclic voltammogram scans of the Li-S cell with CGF. (b) Discharge/charge profiles in the first cycle of Li-S cells with PGF and CGF at a current density of 0.2C. (c) Schematic of polysulfide diffusion in Li-S cells with PGF (left) and CGF (right) during discharge.

25

Specific Capacity (mAh g-1)

Coulombic Efficiency (%)

a

100

1200 80 800

60 40

400

PGF

CGF

0

20 0

0

25

50

75

100

125

150

175

200

Coulombic Efficiency (%)

b

Specific Capacity (mAh g-1)

Cycle Number 100 1200 80 800

60 40

400

2C

1C

0

20 0

0

25

50

75

100

Cycle Number

125

150

175

200

Figure 3 (a) Cycling performance of Li-S cells with PGF and CGF at a current density of 0.2C. (b) Cycling performance of the Li-S cell with CGF at high rates of 1C and 2C.

26

b

1400

uncoated separator coated separator

1200

this work ref. 23 ref. 26 ref. 33 ref. 35 ref. 45 ref. 46 ref. 47

1400

Specific Capacity (mAh g-1)

Specific Capacity (mAh g-1)

a

1200

1000

1000

0.2C 0.2C

800 0.5C

600

0.5C 1C

1C 2C

400

3C

2C

4C

200 0

3C

0

10 20 30 40 50 60 70 80 90 100 110

Cycle Number

800 600 400 200 0

0

1

2

3

4

5

6

Current Density (A g-1)

7

Figure 4 (a) Rate cyclability of Li-S cells with PGF and CGF up to 4C. (b) Comparison of rate capabilities of the Li-S cell with CGF in this work and previously reported Li-S cells.

27

Figure 5 (a, b) SEM images of the surface view of pristine Li metallic plate before cycling at different magnifications. (c, d) SEM images of the surface view of Li metal anode from the Li-S cell with PGF after 100 cycles at different magnifications. (e, f) SEM images of the surface view of Li metal anode from the Li-S cell with CGF after 100 cycles at different magnifications. (g) Selemental mapping of Li metal anodes from Li-S cells with PGF (g) and CGF (h). The current density used for the 100 cycles was 1C.

28

Authors’ personal portrait photos and biosketches

Jiadeng Zhu received his B.S. degree in Macromolecular Materials & Engineering from Soochow University, China and his M.S. degree in Textile Engineering from Chonbuk National University, Korea. He is currently pursuing his Ph.D. degree in Fiber and Polymer Science under the direction of Prof. Xiangwu Zhang at North Carolina State University. His research interest spans from materials synthesis, electrochemistry, and cell design to advanced characterizations with the goal of understanding the synthesis-structure-performance relationship in energy-related materials and their underlying reaction mechanisms. His current research focuses on lithiumsulfur batteries, lithium/sodium-ion batteries, thin film, polymer synthesis, and carbon-based materials’ applications.

Yeqian Ge is currently a doctoral student at Donghua University, China. She received her B.Sc. degree (2010) from the Department of Textile Engineering of Shaoxing University, China and then continued M. Sc. and Ph.D. joint study (2010-2015) in the Department of Textile Materials Science and Product Design at Donghua University. In 2013-2015, she worked as a visiting scholar for two years in Dr. Xiangwu Zhang’s lab in the Department of Textile Engineering, Chemistry and Science at North Carolina State University (NCSU). Her research focuses on nanostructured materials for advanced energy storage applications.

29

David Kim received his BSc in Chemistry and MSC in Occupational & Environmental Health from University of Toronto, and PhD in Environmental Science & Engineering from UNC Chapel Hill. He is Vice President, R&D, for Sceye. His research focuses on the development of technologies for improving global public health with specific focus on Sub-Saharan Africa. The sectors where these technologies are applied include Public Health, Food Security, and Energy.

Yao Lu received his Bachelor’s degree in Textile Engineering from Donghua University in 2011 and Master’s degree in Textile Engineering from North Carolina State University in 2012. He is currently pursuing his Ph.D. degree in Fiber and Polymer Science in the College of Textiles at North Carolina State University. His research mainly focuses on advanced nanofiber preparation methods and electrode materials for energy storage systems.

Chen Chen received his Bachelor’s degree from the College of Textiles at Donghua University of Shanghai in 2011 and Master of Science degree in Textile Engineering from the College of Textiles at North Carolina State University in 2013. He is currently pursuing his Ph.D. degree under the direction of Prof. Xiangwu Zhang in the College of Textiles at North Carolina State

30

University. His research interests mainly focus on nano-structured Na-ion battery anode materials, as well as electrospun nanofiber in the storage and conversion application in energy.

Mengjin Jiang received his Ph.D. degree in polymer science and engineering from Sichuan University, China in 2008. He has been an assistant and associate professor of polymer materials science at Sichuan University since 2008. Since 2014, he has been working with Prof. Xiangwu Zhang at NC State University as a visiting scholar. His current research interests include the preparation, characterization and application of gel polymer electrolyte, and the design of flexible compact energy storage device.

Xiangwu Zhang is a Professor in the Department of Textile Engineering, Chemistry, and Science in the College of Textiles at North Carolina State University. He is currently serving as the Associate Department Head and Director of Graduate Programs. Zhang earned a B.S. in Polymer Materials and Engineering in 1997 and a Ph.D. in Materials Science and Engineering in 2001, both from Zhejiang University, China. Zhang's research interests focus on nanostructured and multifunctional polymer, fiber, and textile materials with an emphasis on energy storage and conversion applications. His research encompasses both fundamental materials studies such as synthesis and physical characterization, as well as system design and fabrication.

31

Graphical abstract

Li2S2/Li2S

Specific Capacity (mAh g-1)

1400

CGF

1200 1000 0.2C 0.2C

800 0.5C

600

0.5C 1C 2C

400

3C

3C

2C

v

4C

200 0

Li2Sx

1C

PGF CGF

0

10 20 30 40 50 60 70 80 90 100 110

Cathode

Anode

Cycle Number

Highlights 

GF has been demonstrated as a novel separator for Li-S batteries.



Li-S cells with CGF have shown excellent cycling stability and rate capability.



The CGF separator significantly inhibits the migration of polysulfide intermediates.

32