Carbon Nanotube Film as

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Free-standing porous carbon nanofiber/carbon nanotube film as sulfur immobilizer with high-areal-capacity for lithium-sulfur battery Ye-Zheng Zhang, Ze Zhang, Sheng Liu, Guoran Li, and Xue-Ping Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00190 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

Free-standing

porous

carbon

nanofiber/carbon

nanotube film as sulfur immobilizer with high-arealcapacity for lithium-sulfur battery Ye-Zheng Zhang,‡ Ze Zhang,‡ Sheng Liu, Guo-Ran Li,* and Xue-Ping Gao* Institute of New Energy Material Chemistry, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China. E-mail: [email protected]; [email protected] ‡Ye-Zheng Zhang and Ze Zhang contribute equally to this work. Keywords: lithium-sulfur battery; porous carbon nanofibers; carbon nanotubes; free-standing film; high-areal-capacity.

Abstract

Low sulfur utilization and poor cycle life of the sulfur cathode with high sulfur loadings remain a great challenge for lithium-sulfur (Li-S) battery. Herein, the free-standing carbon film consisting of porous carbon nanofibers (PCNFs) and carbon nanotubes (CNTs) is successfully fabricated by electrospinning technology. The PCNF/CNT film with three-dimensional (3D) and interconnected structure is promising for the uniformity of the high-loading sulfur, good penetration of electrolyte and reliable accommodation of volumetric expansion of the sulfur cathode. In addition, the abundant N/O-doped elements in PCNF/CNT film are helpful to chemically trap soluble polysulfides in the charge-discharge processes. Consequently, the

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obtained monolayer S/PCNF/CNT film as the cathode shows the high specific capacity, excellent cycle stability and rate stability with the sulfur loading of 3.9 mg cm−2. Moreover, the high-arealcapacity of 13.5 mAh cm−2 is obtained for the cathode by stacking three S/PCNF/CNT layers with the high sulfur loading of 12 mg cm−2. The stacking-layered cathode with high sulfur loading provides the excellent cycle stability, which is beneficial to fabricate high-energy-density Li-S battery in future.

1. Introduction The great demands for high-energy-density rechargeable batteries have aroused extensive interests in exploring new cathode materials with high specific capacity. Elemental sulfur is largely investigated as one of the promising cathode candidates, owing to its high theoretical capacity of 1675 mAh g−1, as well as the cost-effectiveness and environmental friendliness.1,2 When combined with lithium-metal anode, lithium-sulfur (Li-S) battery owns the extremely high theoretical energy density of 2600 Wh kg−1.3,4 Nevertheless, the commercialization of Li-S battery is plagued by several intrinsic problems, including low electronic/ionic conductivity of both sulfur and final product Li2S, large volumetric expansion, high solubility and shuttle effect of intermediate polysulfides. These issues bring about low sulfur utilization, rapid capacity fading and low coulombic efficiency.5-8 To deal with the aforementioned problems, significant approaches are explored via confining sulfur within various nanostructured carbon substrates, including meso/micro-porous carbons,9,10 hollow carbon spheres11,12 and tube-in-tube carbon.13 These carbon materials with hierarchical pore structure are beneficial for building conductive network to facilitate the electron transfer, buffer the volume expansion and mitigate polysulfide dissolution. Apart from the physical adsorption of sulfur species in alternative pores, the chemical confinement of polysulfide offered by functionalized carbon (such as heteroatom-


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doped carbon14-16 and graphene oxide17,18) is more effective for improving cycling stability of corresponding sulfur/carbon composites. Although the noteworthy progress is achieved in terms of the cycle stability, it should be particularly pointed out that the long cycling performance is usually obtained via low sulfur areal loading in cathode (< 2.0 mg cm−2),19,20 which is unsatisfied for fabricating the practical Li-S battery with high energy density. Thus, the challenge is to obtain sulfur-based cathode with the high sulfur areal loading, as well as the high sulfur utilization and good cycle performance. In particular, some inactive materials are usually added into the sulfur cathode in the conventional slurry-coating method, including polymer binder and conductive agent with a total mass fraction of ~20–30wt%, which definitely undermine the sulfur utilization, and practical capacity of the sulfur cathode.21-23 Therefore, the free-standing conductive carbon materials are increasingly introduced to fabricate the sulfur-based cathode without extra binders and conductive agents for increasing the sulfur areal loading without undermining the sulfur utilization.24-31 Moreover, the improvement in cycle performance of the cathode can be obtained owing to the restrained electrochemical corrosion since those flexible carbon films act as 3D carbon current collectors instead of Al foil.32 Recently, electrospun-derived porous carbon nanofiber (PCNF) films are used as sulfur host owing to their adjustable surface area, surface chemistry and porous structure.33-35 The PCNF films with rational design offer 3D conductive network for an effective electron transfer, considerable micro/mesopores for hosting polysulfides, and interfiber space for buffering volume expansion. However, it is noted that the electronic conductivity of PCNF is inferior as compared to that of carbon nanotubes (CNTs). As a response, CNTs can be interwoven with each other to form free-standing films with good electronic conductivity and robust mechanical properties for



the high-performance flexible sulfur-based cathode.36-38 The shortcomings is that the sulfur areal loading and sulfur utilization are unsatisfied due to the lack of porous structure for the S/CNTbased cathode. Naturally, to take a full advantage of both PCNFs and CNTs, free-standing PCNF/CNT hybrid film is reported as flexible sulfur immobilizer with an improved sulfur utilization and outstanding rate performance.39,40 In this work, the free-standing and flexible sulfur cathode (denoted as S/PCNF/CNT) by impregnating sulfur into the electrospun-derived PCNF/CNT film is investigated for Li-S battery. In particular, monolayer and multilayer S/PCNF/CNT films are compared to ensure the high sulfur loading and high areal capacity. As expected, the trilayer S/PCNF/CNT film presents the promising cycle stability and high areal capacity as ascribed to 3D interconnected conductive network and abundant N/O-doped elements in PCNF/CNT film, which can facilitate electron transport, store active material, and trap soluble polysulfide intermediates.

2. Experimental Section 2.1 Preparation of SiO2/polyacrylonitrile (PAN)/CNT film The SiO2/PAN/CNT film was manufactured via an electrospinning method. Typically, SiO2 nanoparticles (0.45 g) and multi-walled CNTs were dispersed in a mixed solvent of dimethyl formamide (DMF, 11.5 g) and tetrahydrofuran (THF, 2.4 g) by ultrasonic treatment for 2 h. Then, PAN (MW = 150000, 1.2 g) powder was added slowly into the aforementioned solution to form a viscous precursor by magnetically stirring for 12 h. The well-mixed SiO2/PAN/CNT suspension was sucked into a 20 mL syringe with blunt-ended 22-G needle and connected to a 25 kV work voltage for electrospinning. The spinneret-to-collector distance and flow rate were kept


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constant at 20 cm and 0.01 mL min−1, respectively. After electrospinning, the SiO2/PAN/CNT film was dried for 6 h at 60 °C to remove the residual solvents. 2.2 Formation of the free-standing PCNF/CNT film The as-collected electrospun SiO2/PAN/CNT film was stabilized in air atmosphere at 250 °C for 3 h with a rate of 1 °C min−1, which is necessary to keep the fiber form during a subsequent high temperature heat treatment. The stabilized fiber film was subsequently carbonized by heating at 800 °C for 2 h under argon-flow. SiO2 nanoparticles were etched by immersing the film in hydrofluoric acid (HF, 20 wt.%) overnight. After washing with deionized water and ethanol several times, the obtained free-standing PCNF/CNT film was dried at 60 °C overnight. As a control, PCNF film was also obtained via the same process without the addition of CNT. 2.3 Preparation and characterization of the free-standing sulfur cathode The as-prepared PCNF/CNT and PCNF film were cut into small pieces (4 cm × 6 cm). For the sulfur impregnation, the prepared S/CS2 solution (50 mg mL−1) was titrated drop-wise on the films by using a pipette, respectively. After evaporating the solvent in an airing chamber, the obtained mat was heat treated at 155 °C for 6 h to obtain flexible S/PCNF/CNT and S/PCNF cathode films. X-ray diffractometer (Rigaku Mini FlexII) was used to collect X-ray diffraction (XRD) patterns of the samples. Sulfur content was determined by thermogravimetric apparatus (Mettler-Toledo, TGA/DSC1) with flowing argon. Surface chemistry of the free-standing carbon material was studied by X-ray photoelectron spectra (XPS, Therm Fisher Scientific Escalab 250Xi) with Al Kα (1486.6 eV) radiation. The specific surface properties and pore size distribution were performed by Brunauer-Emmett-Teller (BET) measurement (JW-BK122). The



morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy TEM (FEI, Tecnai F20). 2.4 Electrochemical measurement The free-standing S/PCNF/CNT and S/PCNF film were punched into small circular films with a diameter of 10 mm, and was directly used as the cathode. The areal mass loading of sulfur for each as-obtained cathode is controlled at 3.9 ± 0.2 mg cm−2. The 2032-coin cells were assembled inside an Ar-filled glove-box with the metallic Li as anode, Celgard 2300 as separator, 1.0 M LiTFSI and 0.2 M LiNO3 in mixed solvents of DOL/DME (1:1 by volume) as electrolyte. The proportion of electrolyte to sulfur is about 20 µL mg−1. Further, the S/PCNF/CNT films are stacked by using different pieces of the single-layer film cathode to obtain various sulfur loading of 7.7 mg cm-2 (bilayer) and 12.0 mg cm-2 (trilayer). For good penetration of electrolyte, the amount of electrolyte was controlled into each layer film cathode during the coin-cell assembly. The electrochemical properties of the free-standing film cathodes were tested on battery test instruments (LAND CT2001A, JinNuo) in the operating voltage range of 1.7–2.8 V (vs. Li+/Li). Cyclic voltammetry (CV) measurement was conducted in the same voltage range at a scan rate of 0.05 mV s−1 for initial 5 cycles by using an electrochemical workstation (CHI 600A, ChenHua).

3. Results and discussion The fabrication procedure of the free-standing S/PCNF/CNT film is illustrated in Fig. 1. At the beginning, the mixture of PAN, CNTs and SiO2 nanoparticles in DMF/THF solvent is electrospun into nanofibers via an electrospinning method. Then, the collected fiber films are


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undergone several treatments including stabilization, carbonization and SiO2-etching to obtain the freestanding PCNF/CNT film. As seen in Fig. 1a, the obtained PCNF/CNT film shows remarkable robustness and flexibility. Finally, the film is served as the matrix for sulfur loading to form the freestanding cathode, denoted as S/PCNF/CNT. As a blank, S/PCNF is also prepared in the same process with the absence of CNTs. The S/PCNF and S/PCNF/CNT films are punched into smaller ones to directly act as the cathode to assemble Li-S coin cells.

Fig. 1 Schematic illustration of the preparation process of the free-standing PCNF/CNT film prepared by electrospun (a) and the free-standing S/PCNF/CNT film (b). The structure characteristics of the obtained films are shown in Fig. 2. Nitrogen adsorptiondesorption isotherms together with pore size distribution are collected by BET measurement to quantitatively analyze the pore structure of the freestanding films (Fig. 2a and b). The PCNF/CNT film possesses the high specific surface area of 1020 m2 g−1 and the large pore volume of 1.66 cm3 g−1, which are attributed to the pyrolysis of PAN and the pore-creating of SiO2 nanoparticles. After the impregnation of sulfur, the surface area and pore volume of the S/PCNF/CNT film are sharply decreased to 208 m2 g−1 and 0.34 cm3 g−1, respectively. It can be inferred that sulfur is almost penetrated into the meso/micro-pores of the PCNF/CNT film. As for XRD patterns (Fig. 2c), a broadened peak at around 24° can be observed in the PCNF/CNT



film, which is typically assigned to the reflection of the (002) plane of the graphitic carbon.20 In the case of the S/PCNF/CNT film, the characteristic peaks related to monoclinic sulfur are detected, rather than a typical orthorhombic phase. It implies that the orthorhombic-monoclinic phase transition of sulfur occurs during the cooling down of the molten sulfur in the PCNF/CNT substrate, similar to the results

reported previously.41-43 It is also confirmed by the weak

diffraction intensity that sulfur is mainly loaded into the pores of the PCNF/CNT film. In addition, TG curves are shown in Fig. 2d to measure the sulfur content in the S/PCNF and S/PCNF/CNT films. Two steps of the mass loss of sulfur are observed. It can be easily understood that sulfur in the large pores of films can be rapidly evaporated at 170–250 °C, while sulfur in small pores are evaporated at higher temperature owing to the strong confinement of sulfur by small pores.44 Thus, the sulfur content are calculated to be about 60 and 62 wt.% for the S/PCNF and S/PCNF/CNT films, respectively.

Fig. 2 (a) N2 adsorption-desorption isotherms and (b) pore size distribution of the PCNF/CNT and S/PCNF/CNT films; (c) XRD patterns of the pure sulfur, PCNF/CNT and S/PCNF/CNT films; (d) TG curves of the S/PCNF and S/PCNF/CNT films.


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Fig. 3 SEM images of (a and b) the PCNF/CNT film, and (c) S/PCNF/CNT film; (d) Crosssectional image of the S/PCNF/CNT film. The morphology and structure of the obtained films are characterized as shown in Fig. 3. The PCNF/CNT film appears as tangled and interlaced millimeter-long nanofibers with various diameters, resulting in 3D interconnected network configuration. Here, SiO2 nanoparticles are used as pore-creating agent of the polymer fibers since due to the adhesion effect of SiO2 with the polar cyanogroup in the PAN chains,45,46 thus leading to a good dispersion of SiO2 nanoparticles along the polymer fibers (Fig. S1a). The porous structure of the substrate can be formed after etching SiO2 nanoparticles by hydrofluoric acid. As for the PCNF film (Fig. S1b), some broken nanofibers can be clearly observed, caused by the heat shrinkage of nanofibers or the removal of SiO2. Obviously, the introduction of CNTs into PCNF can avoid the serious structure damage during the preparation process of the film. After the impregnation of sulfur, there are no noticeable changes in morphology of the S/PCNF/CNT film (Fig. 3c). Noticeably, no agglomerated sulfur is found on the surface of nanofibers, suggesting that sulfur diffuses into the pores of PCNF completely. The thickness of the S/PCNF/CNT film is about 340 µm (Fig.



3d). In addition, the film contains amounts of interfiber spaces (Fig. 3c), which are helpful for the penetration of electrolyte, and the accommodation of volumetric expansion. The microstructure is further investigated by TEM images (Fig. 4). Clearly, CNTs are embedded in the nanofibers (Fig. 4a), and the diameter of PCNF/CNT (Fig. 4b) and S/PCNF/CNT (Fig. 4c) is about 80 and 105 nm, respectively. It is further verified from elemental mappings in the selected-region of the S/PCNF/CNT film (Fig. 4e–h) that sulfur is evenly distributed, as well as the existence of abundant doping elements of N and O, which could chemically confine soluble polysulfides. Here, the uniform distribution of sulfur can be ensured by the good infiltration of S/CS2 solution in the PCNF/CNT film due to the high specific surface area and porous structure. Moreover, there are no lager aggregates of sulfur can be observed on the smooth surface of the CNF/CNT composite, further confirming the loading homogeneity of sulfur in the porous CNF/CNT composite.

Fig. 4 TEM images of (a and b) the PCNF/CNT film and (c) S/PCNF/CNT film; (d) STEM image recorded by the high angle annular dark field detector of the S/PCNF/CNT film and corresponding elemental mapping of (e) C, (f) N, (g) O and (h) S. To deeply understand the surface chemical composition of the PCNF/CNT film, X-ray photoelectron spectroscopy (XPS) is performed (Fig. 5) and the full range spectrum reveals the


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existence of C, O and N elements. The C1s spectra in Fig. 5b confirm the existing of graphitic CC (284.8 eV), C-OH (285.5 eV) and C≡N (286.7 eV).47 The N 1s spectra could be fitted into three typical peaks (Fig. 5c), including pyridnic N (398.4 eV), pyrrolic N (400.1 eV) and graphitic N (401.1 eV) with a total content of 4.3 at.%.15 Previous researches indicate that nitrogen atom can improve the electrical conductivity of the carbon substrates and offer effective chemical adsorption sites for polysulfides.48,49 Besides, a certain oxygen (4.7 at.%) as oxygencontaining groups like C-O (531.1 eV), C=O/N-O (533.0 eV) is brought in the nanofibers during peroxidation, which is reported to improve the mechanical strength of carbon nanofibers and confine the dissolution of polysulfides.50,51 Therefore, the free-standing and N/O dual-doped PCNF/CNT film is obtained, which is believed to be a promising matrix with high sulfur loading for Li-S battery. (b)

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Fig. 5 XPS spectra: (a) full range, (b) C 1s, (c) N 1s and (d) O 1s core levels of the PCNF/CNT film.



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Fig. 6 (a) CVs at the scan rate of 0.05 mV s−1 and (b) initial three discharge/charge curves of the S/PCNF/CNT film at 0.6 mA cm−2 between 1.7 and 2.8 V. To get an insight to the electrochemical performance of the freestanding S/PCNF/CNT film, the film is punched into small pieces with the diameter of 10 mm to act as the cathode directly, where neither conducting additive nor insulating binder are needed. The proportion of electrolyte to sulfur is about 20 µL mg−1. Fig. 6a shows the cyclic voltammograms (CVs) at the scan rate of 0.05 mV s−1 using the S/PCNF/CNT film with the sulfur loading of 3.9 mg cm−2. During the first cathodic scan, apart from two peaks at 2.28 and 2.04 V of the typical multi-step reduction of sulfur,15 there is a third reduction peak at 1.75 V assigned to the lithiation of sulfur in the micropores.44 It is notable that the cathodic peak at the low potential becomes weak in the 2nd cycle. This is mainly attributed to the dissolution and loss of sulfur species from the micropores since the chain solvent (DME) can easily penetrate into the micropores in the PCNF/CNT film. During the following cycles, both the potential and current of CV peaks show a small change, suggesting the relatively good electrochemical reversibility of the free-standing S/PCNF/CNT film. The charge/discharge curves of the free-standing S/PCNF/CNT film as cathode at 0.6 mA cm−2 (0.1 C) are shown in Fig. 6b. All the charge/discharge curves are consistent with the potential characteristics in CVs. In particular, the sloped platform at ~1.9 V in the initial discharge curve is related to the lithiation of sulfur in the micropores, contributing high discharge


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capacity of ~300 mAh g−1 since the abundant micropores in the PCNF/CNT film (Fig. 2b). However, a large shrinkage appears in the following cycles due to the the dissolution and loss of sulfur species from the micropores of the PCNF/CNT substrate, in line with the weakening of the cathodic peak at the low potential in CVs. (b) 1400 100

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Fig. 7 (a) Cycle performance at 0.6 mA cm-2 and (b) high-rate capability of the S/PCNF and S/PCNF/CNT films, (c) cycle stability of S/PCNF/CNT film at high rates of 0.5 C (3 mA cm-2) and 2 C (12 mA cm-2). The high initial capacity of 1321.7 mA h g−1 is obtained for the S/PCNF/CNT film, larger than that (1128 mA h g−1) for the S/PCNF film (Fig. 7a), demonstrating that the introduction of CNTs into the PCNF film can improve the sulfur utilization of the cathode. Along with the following test, the S/PCNF/CNT film remains the large capacity of 859.8 mA h g−1 after 100 cycles, showing the good capacity retention of ∼80% after the stable cycle (2th cycle). In meantime, the S/PCNF/CNT film exhibits the high coulombic efficiency (96.8–100%) during



cycling, which is probably due to the strong chemical adsorption of soluble polysulfides by doping N/O elements, as well as the LiNO3 additive. Of course, the coulombic efficiency of over 100% is indeed observed after 80 cycles due to the shuttle effect. It means that such strong chemical adsorption could be undermined to a certain extent duing long cycling.7 The high rate capability of two cathodes with the sulfur loading of 3.9 mg cm−2 are further evaluated. As demonstrated in Fig. 7b, the free-standing S/PCNF/CNT film shows the improved high rate capability as compared with the S/PCNF film. In particular, the S/PCNF/CNT film presents the large reversible capacity of 550 mA h g−1 at 2 C rate, far above that of the S/PCNF film (390 mA h g−1). The reversible capacity of 857.2 mA h g−1 can be resumed for the S/PCNF/CNT film, when the rate is recovered from 2 C to 0.1 C rate, indicating the excellent mechanical stability of the free-standing film. Moreover, the cycle stability of the S/PCNF/CNT film cathode at high current densitiy are investigated (Fig. 7c). The reversible discharge capacity of 798.5 mAh g−1 can be obtained at 0.5 C rate (3 mA cm-2) in the 2nd cycle, and the capacity fading rate is calculated as 0.051% per cycle within 200 cycles. More impressively, the capacity fading rate is low as 0.035% per cycle within 500 cycles at 2 C rate (12 mA cm-2), suggesting superior cycle stability of S/PCNF/CNT film cathode at high rate. The enhanced rate capability of S/PCNF/CNT film should be ascribed to the excellent conductivity of the sp2-hybridized CNT and 3D interconnected network.39,52 The improved cycle stability is mainly attributed to the strong interaction of intermediate polysulfides with doped N/O heteroatoms in the PCNF, which was demonstrated previously in N/O doped graphene-CNT composite by the visual inspection and UV–vis adsorption spectra.15 Considering potential application in Li-S full-cell, it is necessary to investigate the electrochemical performances of the cathode with much higher sulfur loading. The free-standing


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film cathode without a metallic current collector makes it possible to multiply sulfur loading by stacking the single-layer electrode together. The sulfur loading of the cathode can be greatly increased from 3.9 mg cm−2 (monolayer) to 7.7 mg cm−2 (bilayer) or 12.0 mg cm−2 (trilayer). As depicted in Fig. S2, the similar multi-step discharge/charge plateau is still maintained in spite of multiplying the sulfur loading. The good electrochemical reversibility of the thick cathode may benefit from the fast transport of electrons and good accessibility of electrolyte in the PCNF/CNT film. As shown in Fig. 8, the cathodes with various layer films deliver the large initial discharge capacities of 5.2 mAh cm−2 (1321.7 mAh g-1), 9.1 mAh cm−2 (1183.7 mAh g-1) and 13.5 mAh cm−2 (1125.8 mAh g-1) at the current density of 0.6 mA cm−2, respectively. Remarkably, the cathode with the high sulfur loading of 12.0 mg cm−2 provides the areal discharge capacity of 10.8 mAh cm−2 after 50 cycles, far higher than that of the commercial oxide cathode in Li-ion batteries (4 mAh cm−2). (a)

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Fig. 8 (a) Areal capacity and (b) specific capacity of the S/PCNF/CNT film cathodes with different sulfur loading by stacking different layers during cycling at the current density of 0.6 mA cm−2. To further understand the cycle stability of the S/PCNF/CNT film, the cell is disassembled after 100 cycles at the current density of 0.6 mA cm−2, and SEM observation is subsequently



conducted as shown in Fig. 9. The 3D interlaced nanofibers fibrous architecture is well maintained without serious damage, confirming the excellent structure stability of the S/PCNF/CNT film. Moreover, sulfur is uniformly distributed in the freestanding film without obvious sulfur-rich area as shown in the elemental mapping, implying that the PCNF/CNT film not only serves as the good electrochemical reactor but also the reliable captor of active materials.

Fig. 9 (a) SEM image of the S/PCNF/CNT film after 100 cycles, and corresponding mapping of (b) overlapped all elements, (c) S, (d) C, (e) N, and (f) O element. The superior electrochemical performance of the free-standing S/PCNF/CNT film as the cathode can be ascribed to as following: 1) the incorporation of CNTs effectively enhances the structural stability and electronic conductivity, and significantly improves the sulfur utilization and high rate performance of the cathode; 2) 3D crosslinked PCNFs serve as the structure framework of the cathode, and the long-range conductive carbon matrix to ensure highly efficient ion and electron transportation pathways, and the large interspaces between PCNFs could serve as reservoirs for electrolyte and accommodation of volume expansion; 3) the highly N/O dual-doped porous structure of the PCNF can facilitate trapping intermediate polysulfide within the cathode due to the strong physical/chemical interaction; 4) the free-standing film cathode, without metal current collector Al foil, extra conducting agent and binder, further


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improves the conductivity of the whole cathode, as well as avoids the corrosion of Al foil and the ineffectiveness of conducting agent and binder.

4. Conclusions In summary, the free-standing S/PCNF/CNT film is investigated by infiltrating sulfur into the electrospun-derived PCNF/CNT film as the cathode for Li-S battery. The PCNF/CNT film shows a 3D interconnected construction with good conductivity, mechanical strength and flexibility, which are beneficial for loading sulfur, storing electrolytes and accommodating volume expansion. In addition, the abundant doping N/O elements can help to confine soluble polysulfides chemically. As a result, when used as the cathode directly, the S/PCNF/CNT film with the sulfur loading of 3.9 mg cm−2 exhibits the high discharge capacity of 1321.7 mA h g−1 ( 5.15 mA h cm−2), as well as the excellent cycle stability and high rate capability. Furthermore, the high areal-capacity of 10.8 mA h cm−2 after 50 cycles is obtained with the high sulfur loading of 12.0 mg cm−2 by stacking three layer S/PCNF/CNT films. The obtained free-standing films with excellent cycle stability and high sulfur loading are promising cathodes for Li-S battery.

Supporting Information Available. SEM image of the PCNF film, the initial three charge-discharge curves of double-layer and triple-layer S/PCN-CNT film electrodes are included.

Acknowledgments Financial support from the New Energy Project for Electric Vehicles in National Key Research and Development Program (2016YFB0100200), and NSFC (21573114, 51502145, and 21421001) of China are gratefully acknowledged.



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Carbon Nanotube Film as

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