Polymer Nanofibers with High

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Coaxial Three-Layered Carbon/Sulfur/Polymer Nanofibers with High Sulfur Content and High Utilization for Lithium-Sulfur Batteries Feng He, Jian Ye, Yuliang Cao, Lifen Xiao, Hanxi Yang, and Xinping Ai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00542 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017

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ACS Applied Materials & Interfaces

Coaxial Three-Layered Carbon/Sulfur/Polymer Nanofibers with High Sulfur Content and High Utilization for Lithium-Sulfur Batteries Feng He a, Jian Yea, Yuliang Caoa, Lifen Xiao*b, Hanxi Yanga, and Xinping Ai*a a

Hubei Key Lab. of Electrochemical Power Sources, College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China.

b

College of chemistry, Central China Normal University, Wuhan 430079, China.

KEYWORDS: sulfur cathode, conductive polymer, high sulfur loading, coaxial nanofibers, lithium-sulfur batteries.

ABSTRACT: Great progress has been made on the cyclability and material utilization in recent development of lithium–sulfur (Li–S) batteries; however, most of the sulfur electrodes reported so far have a considerable low loading of sulfur (60%), which causes a substantial decrease of energy density and is therefore difficult for battery applications. To deal with this issue, we fabricate a novel sulfur composite with a coaxial three-layered structure, in which sulfur is deposited on carbon fibers and coated with

poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT/PSS),

thus

enabling a high sulfur loading of 70.8 wt% without the expense of its electrochemical performance. Benefiting from the rigid conductive framework of carbon fibers and flexible buffering matrix of the polymer for blocking the diffusion loss of discharge 1 ACS Paragon Plus Environment


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intermediates, the as-fabricated composite electrode exhibits a high initial reversible capacity of 1272 mA h g−1 (based on the total mass of the composite), a stable cyclability with a retained capacity of 807 mA h g−1 after 200 cycles, and a high Coulombic efficiency of ~ 99 % upon extended cycling, offering a new selection for practical Li-S battery applications.

Introduction Increasing demand for efficient and economic electric energy storage system has intensively promoted the development of lithium-sulfur (Li-S) batteries as a high capacity alternative to current Li-ion batteries in the past decade.1−4 Li-S couple has a theoretical energy density of 2600 W h kg−1, at least four times higher than the present Li-ion batteries. In addition, sulfur as a cathode material has competitive advantages of low cost, natural abundance and environmental friendless, all of which make it ideal for large-scale application.5 However, realizing the Li-S chemistry has encountered several difficulties.6,7 First, the insulating nature of sulfur and its discharge products (Li2S2 and Li2S) prevents the full discharge of sulfur cathode and leads to a low electrochemical utilization. Second, the high solubility of lithium polysulfide (Li2Sn, 4≤n≤8) intermediates in organic electrolyte can cause the active material loss and shuttle effect, resulting in serious capacity fading and low Coulombic efficiency. Furthermore, the large volume expansion caused by the density change from sulfur (d = 2.03 g cm−3) to lithium sulfide (Li2S, d = 1.66 g cm−3) leads to structure collapse of the cathodes, which would in turn accelerate the degradation of battery performance. 2 ACS Paragon Plus Environment

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To address these problems, great efforts have been devoted to build nanostructured sulfur/carbon (S/C) composites by dispersing sulfur onto the conductive carbon matrixes with high surface area and abundant pore volume. Except the enhancement on the electrical conductivity and electrochemical activity of sulfur, the carbon matrixes can also significantly improve the cyclability of sulfur cathode due to their porous structures that has a certain adsorption ability to restrain the outward diffusion of polysulfides and thus to alleviate the shuttle behaviour. Recently, various porous carbon materials have been extensively investigated as sulfur hosts to develop S/C composites, such as carbon nanotube, nanofiber (CNFs),

17−21

porous hollow carbons,

22−25

8−16

carbon nanospheres

carbon

26−29

and

graphene.30−38 Among these carbons, one-dimensional CNFs has an unique advantage of the large length/diameter ratio, which favours to form a three-dimensional conductive network for fast conduction of both electrons and Li ions. Besides, the excellent mechanical strength of CNFs gives a strong tolerance to the volume expansion and benefits to sustain the structural integrity of sulfur cathodes during repeated charge and discharge. However, compared with other porous carbon materials, the most of CNFs only have low surface area and low pore volume, which limit their hold ability for polysulfides. Recently, various attempts have been made to keep the polysulfides within the cathode region by inserting an interlayer between the cathode and separator to capture the dissolved polysulfides,39,40 tailoring electrolytes to restrict the intermediate dissolution and to retard their mobility,41,42 adding safeguard additives 3 ACS Paragon Plus Environment


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to prevent the reduction of polysulfides on lithium anode, composites

with

conductive

polymer

to

inhibit

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43−45

the

and coating the C/S out-diffusion

of

polysulfides.46−57 Among these attempts, conductive polymer coatings are more attractive because of their physical and chemical dual confinements on polysulfide intermediates. As has been reported, the electron-rich functional groups comprising highly electronegative elements (N, O, S) on conductive polymers, such as nitriles, amines, thiophenes and pyrrolidones, are effective sites for grasping lithium polysulfides, which can greatly reduce the active material loss and enhance the cyclability of S/C composites.58 Besides, conductive polymer coatings can significantly improve the utilization of S/C composites without much sacrifice of sulfur loading, due to their high electric conductivity and good film-forming ability, enabling them to easily form thin and lightweight conductive coating layers. As revealed by previous study, introducing only 2 wt% of conductive polymer to create an onion-skin-like flexible membrane on the surface of S/C composites can lead to stable cycling.59 Benefiting these advantages, conductive polymers with functional groups have been widely used as coating layers for S and S/C materials. For example, Wen et al. have reported a simple PPY-coated CMK-8/S composite with a largely improved capacity of 1100 mA h g−1 and a significantly enhanced stability with a remained capacity of 860 mA h g−1 after 100 cycles.60 Yuan et al. also reported a PEDOT-coated sulfur electrode with a reversible capacity as high as 1061 mA h g−1 and a stable capacity of 638 mA h g−1 after 100 cycles.61 In light of the respective advantages of CNF scaffolds and conductive polymer 4 ACS Paragon Plus Environment

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coatings, in this work, we selected vapor-grown carbon fibers (CFs) as conductive matrix

and

poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)

(PEDOT/PSS) as coating material to fabricate a novel sulfur composite with a coaxial three-layered structure (CF/S/PEDOT), which achieves a high sulfur loading (70.8 wt%) and strong cyclability as a Li-S battery cathode. As illustrated in Scheme 1, sulfur is homogeneously distributed on the CFs surfaces through chemical deposition and then enwrapped by conductive polymer PEDOT/PSS, forming a coaxial sandwich-like structure. In this structure, cross-connected CFs serve as a 3D conductive framework to facilitate electrons and ions transport in electrode, while outer polymer coating as flexible buffer and barrier to accommodate the volume change of sulfur interlayer during cycling and to suppress the outward diffusion of polysulfide intermediates through both physical confinement and chemical binding, thus stabilizing the structure and cyclability of sulfur cathode. As a result, the thus-fabricated composite exhibits a high reversible capacity of 1272 mA h g−1 (based on the total mass of the composite), a stable cyclability with a retained capacity of 807 mA h g−1 after 200 cycles, and a high Coulombic efficiency of ~ 99 % upon extended cycling, showing a greatly improved performance.

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Scheme 1. Schematic illustration for the fabrication of CF/S/PEDOT composite.

Experimental Section Preparation of CF/S composite: In this work, we selected vapor-grown carbon fibers (VGCFs) as conductive matrix to prepare the CF/S composite. VGCF is a commercial product produced by Showa Denko in Japan with a length of 10 – 20µm, a diameter of 130 - 150 nm and a specific surface area of about 20 m2 g−1. 0.20 g of CFs (VGCF, Showa Denko, Japan) was well dispersed in 150 mL of distilled water with the aid of 0.02 g of cetyltrimethyl ammonium bromide (CTAB) and sonicating for 1 h. 0.60 g of elementary sulfur was added into a 50 mL of Na2S (40 mol L−1) solution with continuous stirring for 2h until sulfur was completely dissolved as the colour of the solution gradually changed from yellow into orange. Then the obtained Na2Sx solution and CFs suspension were mixed together and slowly added into a 100 mL of 2.0 mol L−1 formic acid solutions. After overnight stirring, the CF/S precipitation was filtered and washed with ultrapure water and ethanol for several times and dried at 60 oC under vacuum over 12 h. Preparation of CF/S/PEDOT composite: 0.30g of CF/S composite and 1.15 g of 6 ACS Paragon Plus Environment

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PEDOT-PSS solution (1.3 wt%, Sigma-Aldrich) were mixed first and then added into a 37.50 g of ethanol/water (1:4, w/w) solution under sonication for 4 h. Then the solvent was removed by rotary evaporation at 60 ºC. Finally, the CF/S/PEDOT composite was dried at 60 ºC in vacuum for use. Structural characterizations: The morphological and structural characteristics were performed with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The SEM and TEM images were taken with a SUPRA 55VP FESEM microscopy (Zeiss Sigma) operated at 15 kV and a Tecnai G2 (FEI, Netherland) microscopy at an accelerating voltage of 300 kV respectively. The TEM was coupled with an energy dispersive X-ray spectrometer (EDX) for determining the chemical composition of the samples. The XRD patterns were recorded with a Shimadzu XRD-6000 (Kyoto, Japan) diffractrometer by using a Cu Kα target at 40 kV and 30 mA with a scan rate of 4 º min−1. The sulfur contents in the composites were detected using a thermogravimetric analyzer (Diamond TG/DTA6300) under Ar atmosphere with a flow rate of 50 mL min−1 at a heating rate of 10 oC min−1 from RT to 500 oC. Electrochemical measurements: The sulfur electrode was prepared by mixing the composite, Super P carbon and PVDF binder in a weight ratio of 80:10:10 in NMP (N-methyl pyrrolidinone) to form uniform slurry, which was then pasted onto Al foil and dried at 60 oC under vacuum. The electrode film was cut into Φ=1 cm disks with a sulfur loading of about 2.0 mg cm−2. The electrolyte was 1 M of bis(trifluoromethane) sulfonamide lithium salt (LiTFSI) dissolved in 1,3-dioxolane 7 ACS Paragon Plus Environment


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and 1,2-dimethoxyethane (DOL/DME, 1:1 in volume) containing 0.1 M LiNO3. Lithium metal foil was used as counter electrode and Celgard 2300 microporous membrane was used as separator. The 2016 cells were assembled in an argon-filled glove box and the electrolyte amount of 20 µL was used for each cell. The galvanostatic charge and discharge tests were performed on a CT2001A Land Battery Testing System within a voltage range of 1.7–2.8 V. The specific capacity was calculated based on the total mass of the whole composite. Cyclic voltammetric measurements were performed on a CHI 660c electrochemical workstation at a scan rate of 0.1 mV s−1.

Results and Discussion In this work, we chose VGCFs as conductive matrix for sulfur mainly due to its ultra-high electrical conductivity, large length/diameter ratio (typically 20—50), excellent mechanical properties and commercial availability with a promise for large-scale application. Selection of PEDOT/PSS for sulfur coating was mainly based on the consideration that large PSS anions-doped PEDOT has a high conductivity and good film-forming ability to form a homogeneous and compact surface layer on sufur. Particularly, the heteratoms in the PEDOT molecules can grasp the polysulfide anions to make them confined in the microdomains of the cathode.58 Fig. 1 a-c compares the SEM images of the pristine CFs, CF/S and CF/S/PEDOT composites. It reveals that after sulfur deposition and subsequent PEDOT coating, 8 ACS Paragon Plus Environment

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the two composite materials still maintain the original nanofiber morphology. This indicates that sulfur generated through the disproportionation reaction of Sx2− with H+ ion is completely sandwiched between the CFs and PEDOT layers. Besides, the surfaces of CF/S and CF/S/PEDOT become relatively rough due to the small sulfur microcrystallines.

Figure 1. SEM images of pristine CFs (a), CF/S (b) and (c) CF/S/PEDOT composites

The TEM images of the CFs, CF/S and CF/S/PEDOT composites are displayed in Fig. 2 a-c. As observed, the pristine CFs has a diameter of ~ 137 nm, which then grows to 160 nm after sulfur deposition and to 165 nm after subsequent PEDOT coating. This enlarged diameter indicates the effective deposition of sulfur on CFs substrate and further coating of PEDOT on sulfur layer. Fig. S1 compares the diameter distribution of pristine CFs and CF/S/PEDOT composite fibers. As can be seen, the diameters of pristine CFs mainly distribute in the range of 135 – 140 nm and after sulfur deposition and subsequent PEDOT coating, their diameter enlarged to 165 – 170 nm. Fig. 2c clearly shows a uniform PEDOT coating layer on the surface of composite fiber with a thickness of ~ 6 nm. Apparently, such a thin surface layer occupies a small part in overall weight and volume of CF/S composite 9 ACS Paragon Plus Environment


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but contributes a high sulfur-loading of the cathode material. To further characterize their structure features, the cross-section of the CF/S/PEDOT composite fibers was viewed by TEM. As shown in Fig. S2, the composite fibers exhibts a three-layered coaxial structure. Fig. 2 d-e and Fig. S3 show the EDX mapping of the CF/S/PEDOT and CF/S composites, respectively. The images illustrate even distribution of C and S elements along the nanofiber. These results from SEM and TEM characterizations demonstrate that the CF/S/PEDOT composite have a three-layered coaxial structure with sulfur interlayer sandwiched between inner carbon wall and outer PEDOT coating.

Figure 2. TEM images of pristine CFs (a), CF/S (b) and (c) CF/S/PEDOT composites; The corresponding elemental mapping of carbon (d) and (e) sulfur for CF/S/PEDOT nanofibers.

The XRD patterns of the elementary S, CF/S and CF/S/PEDOT composites are 10 ACS Paragon Plus Environment

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given in Fig. 3a. As can be observed, the elementary S exhibits intense diffraction peaks, indicating a highly crystalline bulk state. The CF/S composite shows distinctly weakened patterns, suggesting a well distributed state of S. The CF/S/PEDOT composite shows further weakened diffraction signal due to the coating of PEDOT on the S particles. The S contents in the CF/S and CF/S/PEDOT composites were determined by TGA under N2 atmosphere from RT to 500 oC at a heating rate of 10 oC min−1. As shown in Fig. 3b, the S contents are 75.2 and 70.8 wt% in the CF/S and CF/S/PEDOT composites, respectively. Besides, it can be found that the onset temperature of weight loss appears delayed for 30 oC after PEDOT coating, indicating a depressed subliming of S.

Figure. 3. a. XRD patterns of the elementary S, CF/S, and CF/S/PEDOT composites; b. TGA curves of the CF/S and CF/S/PEDOT composite under N2 atmosphere from room temperature to 500 oC at a heating rate of 10 oC min−1.

The electrochemical performances of the CF/S and CF/S/PEDOT composite 11 ACS Paragon Plus Environment


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electrodes are compared in Fig. 4. Fig. 4a shows the CV curves recorded from the both composite electrodes. During the cathodic scan, the CF/S electrode shows two reduction peaks at 2.26 and 2.05 V, corresponding to the conversion of S8 into soluble lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) and the subsequent conversion of lithium polysulfides into insoluble Li2S/Li2S2, respectively. In the reversed anodic scan, only one sharp oxidation peak is observed at 2.55 V, reflecting the conversion of insoluble Li2S/Li2S into high-order soluble lithium polysulfides. The CV curve for the CF/S/PEDOT electrode presents the same electrochemical reaction route, except the first reduction peak shifts positively to 2.32 V, and the oxidation peak shifts negatively to 2.44 V, implying an improved electrochemical reversibility. Fig. 4b compares the first discharge-charge curves of the CF/S and CF/S/PEDOT composite electrodes at 0.1 C. Both of the curves display two typical discharge plateaus attributed to the first step reduction of S8 to the high-order lithium polysulfides (Li2Sn, 4≤n≤8) and then to the short-order lithium polysulfides. The first charge/discharge plateaus of the CF/S/PEDOT electrode are slightly lower/higher than those of the CF/S electrode, which agree well with the CV behaviour in Fig. 4a. The higher discharge potential of the CF/S/PEDOT electrode demonstrates that the surface PEDOT coating benefits to improve the electrochemical activity of sulfur. Accordingly, the CF/S/PEDOT electrode displays considerably increased discharge/charge capacities of 1348/1272 mAh g−1 (based on the total weight of the composite) in comparison with the CF/S electrode (1176/1159 mAh g−1). Also, it can be found that the increase of CF/S/PEDOT 12 ACS Paragon Plus Environment

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electrodes in discharge capacity mainly arises from the extension of their high-voltage platform at ~ 2.3 V and their low discharge platform at ~ 2.1 V almost has no contribution to the capacity increase. This phenomenon can be explained by the fact that the first step reduction of S8 to soluble Li2Sn involves a conversion process from solid phase to liquid phase and the subsequent reduction of soluble Li 2Sn into insoluble Li2S/Li2S2 corresponds to a conversion from liquid phase to solid phase. The reaction rate of the former is determined by the electron transfer process and the reaction rate of the latter is controlled by the diffusion process. Therefore, the conductive PEDOT coating can considerably accelerate the electrochemical conversion of S8 into high-order Li2Sn but has no effect on their subsequent conversion. The cycling stability of the CF/S and CF/S/PEDOT composite electrodes is presented in Fig. 4c. As can be seen, after an initial capacity loss, the discharge capacities of the CF/S/PEDOT and CF/S electrodes decline to 1180 and 1020 mAh g−1 at the 2nd cycle, respectively and then keep quite stable at this level. After 50 cycles at 0.1C, the discharge capacity of the CF/S/PEDOT electrode still remained at 982 mAh g−1, while the CF/S electrode shows a fast decay with a retained capacity of only 644 mAh g−1. To prove the role of three layer coaxial structure in improving the electrode performance, a control experiment has been done by directly adding the CF/S composite into PEDOT-PSS solution to obtain a mixture and then testing the charge-discharge performance of the mixture electrode in Li-S cells. As shown in Fig. S4, the mixture electrode can deliver a discharge capacity of 13 ACS Paragon Plus Environment


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890 mAh g−1 at the 2nd cycle. After 50 cycles at 0.1C, the discharge capacity rapidly declined to 627 mAh g−1, showing a fast decay. These comparisons indicate that the PEDOT coating layer plays a vital role in improving the cyclability of CF/S composite.9 This effect may arise from the mechanism that the coated PEDOT network serve as a blocking layer to impose not only physical confinement, but also chemical anchoring sites for the polysulfides,58,62 thus effectively suppressing the diffusion loss of the active material and enhancing the cyclability of the composite cathode.

Figure 4. Electrochemical performances of the CF/S and CF/S/PEDOT cathodes: a. cyclic voltammograms at the second cycle in the voltage range of 1.5–3 V (vs. Li + /Li) at a scan rate of 0.1 mV s−1; b. the first discharge/charge profiles at 0.1C; c. cycling performance and Coulombic efficiency curves at 0.1 C, the insert shows the extended 14 ACS Paragon Plus Environment

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cycling performance of the CF/S/PEDOT electrode from 50–200 cycles; d. rate capability at various rates from 0.1C to 1.0 C. The insert in Fig. 4c gives the extended cycling performance of the CF/S/PEDOT electrode from 50–200 cycles. As displayed, the composite still retains a quite high capacity of 807 mAh g−1 after 200 cycles, demonstrating a strong long-term cyclability. Besides the higher discharge capacity and cycling stability, the CF/S/PEDOT electrode also shows higher columbic efficiency in the duration of cycling. After 30 cycles, the columbic efficiency reaches up to 99%.

Fig. 4b

shows that the rate capacity of the CF/S/PEDOT electrode is 840, 690, 520, and 455 mAh g−1 at 0.2, 0.4, 0.8 and 1 C respectively. As the current rate returns to 0.1 C, the capacity recovers to 950 mAh g−1 immediately.

As a comparison, the CF/S

electrode delivers 810, 640, 480, and 400 mAh g−1 at 0.2, 0.4, 0.8 and 1 C, respectively, and recovers to a capacity of 644 mAh g−1 as the rate is set back to 0.1 C. This comparison indicates that the CF/S/PEDOT composite electrode has much better rate capability than the CF/S electrode. In addition, as listed in Table S1, the CF/S/PEDOT composite as-fabricated in this work also exhibits certain advantages in specific capacity and cycling stability comapred to the sulfur electrodes reported in literatures.63−65 To investigate the influence of sulfur loading on the electrochemical performance of the CF/S/PEDOT electrode, three kinds of high sulfur loading electrodes were prepared and cycled in Li-S cells at 0.1C. As shown in Fig. S5, all the electrodes can exhibit stable cycling performance, although their discharge capacity are 15 ACS Paragon Plus Environment


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significantly decreased with the sulfur loading increase. As the sulfur loading is increased from 2 mg cm -2 to 3, 4 and 5 mg cm-2, the stable capacities of the electrodes declined from 1180 mA h g−1 to 670, 600 and 480 mA h g−1, respectively. The above-presented results demonstrate that the fibrous CF/S/PEDOT composite with a high sulfur loading can exhibit not only high material utilization and specific capacity, but also strong cyclability and high Columbic efficiency as a Li-S battery cathode. Such excellent cathode behaviors should arise from the combined advantages of CFs fibers and PEDOT polymer. Previous study has demonstrated that larger and connected porosity can significantly improve the performance of the sulfur electrode.66 As illustrated in Fig. 5, CFs fibers as highly conductive scaffold with large length/diameter ratio are easy to form a 3D network for the composite cathode, which can not only facilitate the electron and ion transport for sulfur reactions, but also provide sufficient voids for accommodating the volume change of cathode during cycling, thus ensuring a high utilization and structural stability of the CF/S/PEDOT composite. In addition, the surface coating layer of PEDOT serves as a flexible buffer and barrier not only capable to restrain the outward diffusion of polysulfides, but also to buffer the mechanical stress, thus enabling high columbic efficiencies and good cycling stability. Furthermore, the highly conductive PEDOT film provides active surface and reaction sites for recharge of the diffused polysulfides, thus further enhancing the reversible capacity, material utilization and cyclability. The role of PEDOT layer in stabilizing the fibrous structure and the cycling performance of the composite can be visualized 16 ACS Paragon Plus Environment

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from the SEM images of the both deeply cycled CF/S and CF/S/PEDOT electrodes. As shown in Fig. S6, severe agglomeration of S can be clearly observed on the fibrous surface of CF/S electrode (Fig. S1a). Nevertheless, the morphology of the CF/S/PEDOT electrode (Fig. S1b) is similar to the fresh sample, indicating that S is tightly fixed in-between the CFs fiber and PEDOT coating layers during repeated cycling and that PEDOT layers impose an effective confinement on the polysulfides to make them resided within the microdomains of the composite. This was further evidenced by the TEM image of CF/S/PEDOT electrode after 50 cycles. As shown in

Fig.

S7,

the

CF/S/PEDOT

fibers

can

maintain

the

original coaxial three-layered structure after cycling only with their diameters enlarged from ~ 170 nm to ~ 230 nm, which is most likely caused by the changes of sulfur interlayer from a dense state to a loose state at repeated charge and discharge.

Figure 5. Illustration for the working mechanism of the CF/S/PEDOT cathode.

CONCLUSIONS In summary, we have fabricated a high sulfur loading CF/S/PEDOT composite for Li-S batteries by chemically depositing sulfur on CFs fibers and subsequently 17 ACS Paragon Plus Environment


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coating sulfur with PEDOT polymer to form a sandwich-like structure, where the inner CFs fiber acts as a structurally stable conductive backbone to disperse sulfur and to transport electrons for the charge-discharge reactions, meanwhile the outer PEDOT layer serves as a flexible barrier to prevent the outward diffusion of polysulfide intermediates, thus ensuring a high specific capacity and a strong cyclability. The experimental results demonstrate that the as-fabricated composite electrode can deliver a high initial charge capacity of 1272 mAh g−1 at 0.1 C; after 200 cycles, the electrode can still retain a quite high capacity of 807 mAh g−1, showing a great promising for battery application. More significantly, the synthetic method for preparing the CF/S/PEDOT composite is facile, low-cost and easily adopted for large-scale manufacture. Also, the structural design proposed in this work can be extended to a wide range of redox-active materials for developing cycling-stable electrode materials.

ASSOCIATED CONTENT

Supporting Information Available: Diameter distribution of pristine CFs and CF/S/PEDOT composite fibers, cross-sectional-view TEM image of the CF/S/PEDOT composite fibers, elemental mapping for CF/S nanofibers, cycling performance of the mixture electrode of CF/S and PEDOT at 0.1 C, cycling performance of the high sulfur loading CF/S/PEDOT electrodes at 0.1 C, SEM and TEM images of the CF/S and CF/S/PEDOT composite electrodes after 50 cycles. This material is available free of charge via the Internet at http://pubs.acs.org. 18 ACS Paragon Plus Environment

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Phone: +86-27-68754526.

ACKNOWLEDGMENT

We thank the financial supports from the National Science and Technology Major Project for New Energy Vehicles (No. 2016YFB0100200) and National Science Foundation of China (No. 21273090).

.

REFERENCES

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(9) Jia-jia, C.; Xin, J.; Qiu-jie, S.; Chong, W.; Qian, Z.; Ming-sen, Z.; Quan-feng, D. The Preparation of Nano-Sulfur/MWCNTs and Its Electrochemical Performance. Electrochim. Acta 2010, 55 (27), 8062-8066.

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(11) Yin,

L.;

Wang,

J.;

Yang,

J.;

Nuli,

Y.

A

Novel

Pyrolyzed

Polyacrylonitrile-Sulfur@MWCNT Composite Cathode Material for High-Rate 20 ACS Paragon Plus Environment

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(36) Chen, Y.; Lu, S.; Zhou, J.; Wu, X.; Qin, W.; Ogoke, O.; Wu, G. 3d Graphene Framework Supported Li2S Coated with Ultra-Thin Al2O3 Films: Binder-Free 24 ACS Paragon Plus Environment

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(46) Lim, S.; Thankamony, R. L.; Yim, T.; Chu, H.; Kim, Y. J.; Mun, J.; Kim, T. H. Surface Modification of Sulfur Electrodes by Chemically Anchored Cross-Linked Polymer Coating for Lithium–Sulfur Batteries. ACS Appl. Mater. Interfaces 2015, 7 (3), 1401-1405.

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(51) Lee, J.; Choi, W. Surface Modification of Sulfur Cathodes with Pedot:Pss Conducting Polymer in Lithium-Sulfur Batteries. J. Electrochem. Soc. 2015, 162 (6), A935-A939.

(52) Wei, W.; Li, G. C.; QingWang; Li, G. R.; Ye, S. H.; Gao, X. P. Sulfur-Polypyrrole/Graphene Multi-Composites as Cathode for Lithium-Sulfur Battery. J. Electrochem. Soc. 2013, 160 (6), A805-A810.

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(55) Huang, J.-Q.; Zhang, Q.; Zhang, S.-M.; Liu, X.-F.; Zhu, W.; Qian, W.-Z.; Wei, F. Aligned Sulfur-Coated Carbon Nanotubes with a Polyethylene Glycol Barrier at One 27 ACS Paragon Plus Environment


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(64) Li, W. Y.; Zheng, G. Y.; Yang, Y.; Seh, Z. W.; Liu, N.; Cui, Y. High-Performance Hollow Sulfur Nanostructured Battery Cathode through a Scalable, Room Temperature, One-Step, Bottom-up Approach. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (18), 7148-7153.

(65) Li, W. Y.; Zhang, Q. F.; Zheng, G. Y.; Seh, Z. W.; Yao, H. B.; Cui, Y. Understanding the Role of Different Conductive Polymers in Improving the Nanostructured Sulfur Cathode Performance. Nano Lett. 2013, 13 (11), 5534-5540.

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Polymer Nanofibers with High

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