High-performance lithium sulfur batteries enabled by

Energy Storage Materials 16 (2019) 194–202

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Energy Storage Materials journal homepage: www.elsevier.com/locate/ensm

High-performance lithium sulfur batteries enabled by a synergy between sulfur and carbon nanotubes

MARK

Amir Abdul Razzaqa,b, Yuanzhou Yaoa,b, Rahim Shaha,b, Pengwei Qia,b, Lixiao Miaoc, ⁎ ⁎ Muzi Chend, Xiaohui Zhaoa,b, , Yang Penga,b, Zhao Denga,b, a

Soochow Institute for Energy and Materials Innovations, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Soochow University, Suzhou 215006, China c Sound Group Institute of New Energy, Beijing 101102, China d Analysis and Testing Center, Soochow University, Suzhou 215123, China b

A R T I C L E I N F O

A BS T RAC T

Keywords: Electrospinning Binder-free Synergistic effects Carbon nanotubes ex-situ XPS Lithium sulfur batteries

The urgent demand on high performance energy storage devices makes lithium sulfur batteries with a high energy density up to 2600 Wh kg−1 extremely attractive. However, the low capacity reversibility and poor rate capability still pose a significant hurdle on their real-world applications. Here, a freestanding thin-film composite containing sulfurized polyacrylonitrile with conductive backbone of carbon nanotubes has been fabricated by an electrospinning method followed by vulcanization, and employed as the binder-free cathode for lithium sulfur batteries without any aid of current collectors. A synergic effect from sulfur and carbon nanotubes, when co-spun together, has been discovered on promoting the electrochemical performance of the cathodes by simultaneously creating material porosity and conductive pathway. The optimized composite fibers made from a ternary precursor solution containing 20% carbon nanotubes present the best performance, delivering a high initial discharge capacity of 1610 mAh g−1 at 0.2C and outstanding cycle stability of 1106 mAh g−1 at 1C over 500 cycles. It is anticipated that the porous composite nanofibers and the multivariant fabrication methodology reported here can be extended to more energy storage applications, particularly for flexible lithium sulfur batteries.

1. Introduction The ever-increasing requirement of high energy and power density from consumer electronics, automobiles and power grids calls for nextgeneration energy storage devices beyond current lithium ion (Li-ion) technologies (< 300 Wh kg−1) [1,2]. The lithium sulfur (Li-S) battery, owing to its excellent gravimetric energy density of 2600 Wh kg−1, is one of the most promising candidates [3,4]. Moreover, sulfur has high natural abundance and low toxicity, bringing in extra economic and environmental benefits [5]. However, the practical implementation of Li-S batteries is still hindered as of today, suffering from severe selfdischarge, low capacity reversibility, and short life-span. These problems are mainly related to the insulating nature of both sulfur and its discharge products (Li2S2, Li2S), the shuttling of polysulfide intermediates between electrodes, the volume expansion of sulfur, as well as the corrosion of metallic lithium surfaces [6]. To address these issues, many approaches have been explored aiming to improve the electrochemical rechargeability of the sulfur

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cathodes. One strategy proven to be useful is the impregnation of sulfur into porous and conductive carbon matrix, forming nanostructured composites [7–9]. The suitable porosity not only provides confined spaces to inhibit the outward diffusion of polysulfides, but also helps to accommodate the volumetric expansion of sulfur during cycling [10]. In this context, numerous carbon materials have been endeavored, including porous carbon particles [11,12], carbon nanotubes [13,14], carbon nanofibers [15–17], graphene [18,19], and so on. Although in many of these studies the specific capacity of Li-S cells can be promoted to > 1000 mAh g−1, problems such as fast capacity drop in the first few cycles and short-term electrochemical stability still remain unsolved, mainly due to the poor physical and chemical interaction between sulfur and the carbon hosts, resulting in low utilization of active mass [20,21]. Alternatively, pyrolysis of polyacrylonitrile (PAN) with sulfur to make the sulfur/PAN (SPAN) composite provides another promising strategy by chemically anchoring sulfur within the polymer matrix [22,23]. Since its first introduction by Wang et al., SPAN has attracted

Corresponding authors at: Soochow Institute for Energy and Materials Innovations, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China. E-mail addresses: [email protected] (X. Zhao), [email protected] (Z. Deng).

https://doi.org/10.1016/j.ensm.2018.05.006 Received 6 March 2018; Received in revised form 4 May 2018; Accepted 4 May 2018 Available online 05 May 2018 2405-8297/ © 2018 Published by Elsevier B.V.


A. Abdul Razzaq et al.

obtain flexible films with fine nanofibers. The temperature and humidity were controlled at 30 °C and 35%, respectively. The sulfurization process was done in a tube furnace under a nitrogen atmosphere by placing extra sulfur power at tenfold of the as-fabricated electrospun film in mass, ensuring a complete vulcanization. The annealing temperature was initially hold at 155 °C for 1 h and then further ramped up to 400 °C for 6 h at an increasing rate of 2 °C min−1. After naturally cooling down to room temperature, the freestanding composite cathodes were finally obtained. Various concentrations of CNT were deployed in the electrospinning solution of sulfur and PAN, and the fabricated nanofibers are designated as SPAN-CNT#, where # is the mass ratio of CNT to PAN. For comparison, the same electrospun samples containing sulfur prior to vulcanization are designated as SPAN-CNT#. For control studies, binary S-PAN and PAN-CNT nanofibers were also electrospun and further underwent vulcanization, and accordingly these samples are designated as SPAN, PAN-CNT20/S, respectively. For clarification, refer to Table S1 for a complete nomenclature of samples involved in the current study.

considerable research interests because of its superior electrochemical stability over repeated cycles [24–26]. Many carbonaceous materials, such as MWCNT [27], graphene [28,29], highly conductive porous carbons (Ketjenblack) [30] etc., have been incorporated into the polymer matrix to further boost its electrochemical performance. Mechanism studies indicated that sulfur is covalently bonded to the polymer matrix in the molecular or atomic level during the cyclization of PAN under thermal treatment at 300–450 °C [31,32]. Unlike the elemental sulfur, SPAN composites can work in the commonly used carbonate electrolytes for Li-ion batteries, avoiding the dissolution of long chain lithium polysulfides in ether-based electrolytes [33–35]. What's more, the polymer-based SPAN is light-weighted with good mechanical strength, particularly promising for applications in flexible devices [36]. Nevertheless, despite the aforementioned advantages, SPAN has an intrinsic limit of low sulfur content (~40%), which severely plagued their practical applications. Embedding elemental sulfur in porous carbon prior to the vulcanization with PAN is an effective way to increase the sulfur content, but typically at the sacrifice of the specific capacity [30,37]. Another parallel measure that has been sought to improve the overall sulfur content in Li-S batteries is the construction of freestanding electrodes without binder and current collector, which compose a significant mass fraction of traditional slurry-casting electrodes [36,38]. To effectively compensate for the relatively low mass loading of active materials in SPAN-based electrodes, herein a freestanding nanofibrous SPAN-CNT composite is synthesized by the electrospinning method to serve as binder-free cathodes for Li-S batteries without involving current collectors. A facile procedure is employed with the ternary sulfur, PAN, and CNT precursors first co-spun into nanofibers, followed by the vulcanization in the additional sulfur atmosphere at raised temperature. It is the first time that sulfur has been directly electrospun with PAN and CNT to synthesize SPAN-CNT composites, of which sulfur and CNT are found to exert a synergy in creating thin and porous nanofibers upon thermal annealing, which in turn greatly promote the electrochemical properties of the as-prepared SPAN-CNT composite. Moreover, the interconnected fibrous structure and the CNT network provide highly conductive pathways for fast Li+ ion diffusion and electronic charge transfer. As a result, this unique ternary SPAN-CNT cathode offers highly stabilized discharge capacity of 1314 mAh g-1 for over 250 cycles at 0.5C with a Coulombic efficiency close to 100%, showing great perspective for applications in high performance Li-S batteries.

2.3. Characterization and electrochemical tests The surface morphology of the prepared composites was determined by field emission scanning electron microscope (FE-SEM, Hitachi SU8010) and the structural characterization of the nanofibers was performed by high resolution transmission electron microscope (TEM, FEI Tecnai G20 200 kV). The chemical state of sulfur, carbon, nitrogen, and lithium in the composite nanofibers were probed by Xray photoelectron spectroscopy (XPS, ESCALAB 250Xi) by using a monochromatic Al Kα (1486.6 eV) X-ray source. X-ray diffraction (XRD, Bruker D8 Advance) was used to determine the crystalline structure of both electrospun and sulfurized nanofiber films, and Raman spectra were performed on a LabRAM HR Evolution Spectrometer. Fourier Transform Infrared Spectroscopy (FT-IR, Thermo ScientificTM NicoletTM iS50) was used to determine the functional groups present on materials, and elemental analysis (vario Micro cube) was done to obtain the sulfur content in the composite nanofibers. Brunauer-Emmett-Teller method (BET, Micromeritics Tristar 3000) was applied to test the porosity of films and the Barrett-Joyner-Halenda (BJH) method was adopted to determine the specific surface area and pore structure of the composites. The electric conductivity of the composite was measured by the four-probe method (Suzhou Jingge Electronics ST2263). The binder-free composite films were punched into circular disks with a diameter of 13 mm and directly used as cathodes without any current collector. The sulfur loading in the composites were fixed at 0.9–1.1 mg cm−1 for all electrodes to assure a fair comparison of electrochemical properties. Lithium metal disks with a diameter of 16 mm as the anode and Celgard® 2500 as the separator were assembled together with the cathodes in the 2032 coin cells. 60 µl of 1 M lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (1:1:1 by volume) was used as the electrolyte. All cells were assembled in an argon-filled glovebox with both H2O and O2 content below 0.1 ppm. Cyclic voltammetry (CV) at a scan rate of 0.05 mV s−1 and electrochemical impedance spectroscopy (EIS) within a frequency range from 10 mHz to 1 MHz at an amplitude of 5 mV were determined by an electrochemical workstation (CHI660E). The galvanostatic charge/discharge measurements were monitored using the LANHE battery testing system (CT2001A, Wuhan LAND electronics Co., China) within a voltage range of 1–3 V.

2. Experimental section 2.1. Materials Sublimation sulfur (100-mesh particles) and PAN (Mw = 150,000 g mo1-1) were purchased from Sigma-Aldrich. N,N-dimethylformaldehyde (DMF) was purchased from Titan Scientific Co., Ltd., China. CNT with a diameter of 7–11 nm and ~1 μm in length was acquired from Cnano Technology, China. All chemicals were used as received. 2.2. Fabrication of binder-free SPAN-CNT cathodes The electrospinning precursor solutions containing 1.5 g of PAN and either sulfur or CNT or both in different mass ratios in DMF were prepared by the high-speed ball milling method for 2 h, in which the mass ratio of sulfur to PAN was fixed as 3:7 and that of CNT to PAN varied among 5, 10, 20, and 30%. The viscous solution thus obtained was loaded into a plastic syringe and with a stainless-steel nozzle connecting to high DC voltage and subjected to electrospinning. A high voltage of 20 kV, a constant flow rate of 0.03 ml min−1, a N22 nozzle, a distance from nozzle to collector of 13 cm, and a collector rotation speed of 700 rpm were used as the electrospinning parameters to

3. Results and discussions The freestanding SPAN-CNT composites were fabricated by electrospinning with ternary precursors of sulfur, PAN, and CNT together, followed by further vulcanization with extra sulfur to obtain films of 195



Fig. 1. a) Schematic illustration and b) photographs of the flexible SPAN-CNT20 films. FE-SEM images of c-e) electrospun and f-h) sulfurized SPAN, PAN-CNT20/S, and SPAN-CNT20, respectively.

superior flexibility (Fig. 1a-b). Fig. 1c-h show the morphology of SPAN, PAN-CNT20/S, SPAN-CNT20 nanofibers before and after vulcanization taken by FE-SEM. No distinctive change of the nanofibrous structure was observed for all composites after the vulcanization process at 400 °C including SPAN-CNT5–30 (Fig. S1), and the obtained nonwovens still exhibit great flexibility and mechanical strength (Fig. 1b). The composite nanofibers have an average fiber diameter of 310 ± 18, 880 ± 45, and 450 ± 27 nm for SPAN, PANCNT20/S, and SPAN-CNT20, respectively (Fig. S2a-c). Adding 5–10% of CNT to the electrospinning precursor dramatically inflated the nanofiber, resulting in significantly enlarged diameter when compared to SPAN fibers. As more CNT was added, the diameter of SPAN-CNT fibers decreased inversely (Fig. S2c-f). Therefore, with the same electrospinning parameters the morphology and diameter of the electrospun nanofibers are highly dependent on their composition, which can be further ascribed to changes from viscosity and surface tension [39]. SPAN nanofibers without CNT exhibit a smooth surface (Fig. 1f), which is significantly roughened by the addition of CNT (Fig. 1g-h). A growing number of pores on the nanofiber surface are visible when increasing the CNT addition from 5% to 30% (Fig. S1). The microstructure of nanofibers with different composition was further examined by TEM as shown in Fig. 2. While all composites with CNT exhibit porosity to different extent, no visible pores are observed on SPAN nanofibers, contradictory to the well-known fact that sulfur undergoes sublimation at high temperature (400 °C). It is likely that sulfur sublimation is significantly prohibited by the thick polymer matrix of SPAN, where most of sulfur reacts with PAN inside the nanofibers, leaving no visible voids in TEM images. As soon as CNT is added into the nanofibers, the polymer matrix is no longer densely packed (Fig. S3), creating channels for sulfur evaporation in addition to the formation of SPAN matrix. As shown in Fig. 2a-f, the porosity of

SPAN-CNT nanofibers is increasing with the addition of CNT, whereas their diameter decreases inversely. This in turn dramatically increases the fraction of porous space inside the nanofibers. Notably, in SPANCNT30 (Fig. 2f) more aggregated CNT clusters are visible, owing to the excess addition of CNT. Among all examined CNT concentrations, SPAN-CNT20 (Fig. 2e) has the best balance of porosity and CNT alignment, with well-dispersed CNT stretched by the high tensile force of electrospinning to form a conductive network [27]. EDX mapping further revealed the uniform distribution of sulfur within the polymer matrix of SPAN-CNT20 (Fig. 2g-j). Evidenced by these microscopic observations, it is therefore proposed that a synergy between sulfur and CNT is critical for obtaining the thin and porous structure of nanofibers, since the lack of either one (as in SPAN and PANCNT20/S) is unable to create such porosity. To further quantify the porosity of the composite fibers and study its origin, BET analysis, together with TEM, was used to characterize the as-spun composites and their vulcanized counterparts. For the vulcanized samples of SPAN, PAN-CNT20/S and SPAN-CNT20, a general increasing trend can be seen for the specific surface area and pore volume values (Table S2). BET analysis of the as-spun samples without vulcanization shows the highest surface area and pore volume on PAN-CNT20 samples without preloaded sulfur, followed by S-PANCNT20 and S-PAN, and all values are larger than their vulcanized counterparts. This observation clearly tells that the vulcanization with extra sulfur indeed clog the mesopores of the electrospun fibers through the expandable SPAN formation, and further verifies the view that the larger pores observed on SPAN-CNT20 by TEM is resulted from sublimation of sulfur preloaded into the electrospinning precursor and facilitated by its co-spinning with CNT. FT-IR and Raman spectra of the ternary composites before (S-PANCNT20) and after (SPAN-CNT20) vulcanization are shown in Fig. 3. In 196



Fig. 2. The TEM images of a) SPAN, b) PAN-CNT20/S, c) SPAN-CNT5, d) SPAN-CNT10, e) SPAN-CNT20, and f) SPAN-CNT30; g) STEM HADDF image of SPAN-CNT20 and its (h-j) EDX mapping of sulfur (yellow), carbon (red) and nitrogen (green), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the FT-IR spectrum of S-PAN-CNT20, the peak at 2239 cm−1 is assigned to the nitrile group (C≡N), whereas the two peaks at 2933 cm−1 and 1455 cm−1 are characteristic of aliphatic C-H stretching and bending, respectively (Fig. 3a), Upon vulcanization, these peaks are replaced by conjugated peaks for SPAN from 1200 cm−1 to 1500 cm−1, clearly indicating the formation of six-member aromatic ring-like structure with the presence of heteroatoms (Fig. 3b) [22,25,31,40,41]. In Raman spectra, pronounced peaks at 1590 cm−1 and 1330 cm−1 representing the graphitic G band and structurally defective D band are resulted from the presence of CNT (Fig. 3c), which after vulcanization is convoluted by carbonized products of SPAN (Fig. 3d) [42]. The characteristic peak for the chemical linking of C-S bond at 930 cm−1 confirms the successful synthesis of SPAN composite [30]. Further through elemental analysis, the mass content of sulfur was measured at ~42%, ~36%, and ~40% for SPAN, PAN-CNT20/S, and SPAN-CNT20, respectively (Table S3). It is apparent the preloading of sulfur in the electrospinning precursors enables a more complete SPAN transformation, in addition to its pore-generating function with CNT. The lowest sulfur content in PAN-CNT20/S is likely due to the diffusion-limited sulfurization of nanofibers, where the outer layer reacts with sulfur prior to the inner part, and might even blockade the sulfurization to later. In contrast, in SPAN and SPAN-CNT20 composites, the vulcanization is more complete with the existence of both internal pre-loaded sulfur and external sulfur vapor. The highest sulfur loading observed on SPAN could be a result of the less-significant sulfur sublimation when there is no presence of CNT in the nanofibers. The elemental chemical states of SPAN-CNT20 were further inspected by XPS. Prominent signals of S 2p, C 1s, and N 1s were detected from the XPS survey spectrum as shown in Fig. 4a. C 1s peak

can be deconvoluted into three species, corresponding to the C-C bond with sp2 hybridization (284.3 eV) [25,43], the covalent C-S bond formed upon sulfurization (285.2 eV) [44], and the sp2 C=N bond (286.7 eV) [43,45]. (Fig. 4b). The S 2p peak (Fig. 4c) consists of three doublets, of which the pink-colored doublet at 164.8 eV (S 2p1/2) and 163.6 eV (S 2p3/2) are resulted from the C-S/S-S bond [44,46]. The doublet at 163.0 eV and 161.7 eV can be assigned to short-chain oligomers of sulfur covalently linked to the PAN matrix, both confirming the formation of SPAN [44,47]. The third broad doublet in blue can be ascribed to sulfated species presented on the surface of nanofibers. N 1s spectrum for SPAN-CNT20 shows two peaks of pyridinic (C=N-C, 398.2 eV) and pyrrolic nitrogen atoms (C-NH-C, 400.0 eV), different from the typical C-C≡N bond at 399.0 eV in PAN [48] and proving the crosslinking of PAN during the sulfurization process (Fig. 4d) [43,49]. All these XPS spectra above fully support the successful chemical bonding of sulfur to the PAN matrix, forming the SPAN composite. Moreover, XRD analysis on the as-prepared electrospun films revealed the polymeric PAN peak at 17°, serial sharp peaks of crystalline orthorhombic sulfur, and graphitic peaks of CNT at 28° and 42° (Fig. S4a). After vulcanization at 400 °C, all peaks from PAN and sulfur disappear, suggesting the dehydrogenation of PAN and chemical-linking of sulfur with it to form heterocyclic SPAN (Fig. S4b) [50]. To evaluate the electron transportation of nanofibers with the incorporation of sulfur and CNT, measurements of electric conductivity were done using a four-probe tester for SPAN, PAN-CNT20/S and SPAN-CNT20 (Table S4). As expected, the electric conductivity of SPAN is so low that the four-probe method failed to give any readings. For PAN-CNT20/S, the conductivity is not homogeneous across the sample and the readings ranged from 1.2 × 10−4 to 1.6 × 10−3 S cm−1, depending on the testing location. In contrast, the SPAN-CNT20 197



Fig. 3. a,b) FT-IR and c,d) Raman spectra of electrospun and sulfurized SPAN-CNT20, respectively.

loading of sulfur and resonating with the conductivity measurements. All anodic scans have a single peak ~2.4 V for PAN-CNT20/S and SPAN-CNT20, indicating a full oxidation of Li2S2/Li2S back to the C-S or short-chain S-S species, as a result of enhanced charge transfer by CNT [44]. As for SPAN, the CV curves barely exhibit any anodic peaks (with a negligible one at ~1.9 V in the first cycle), indicating that the cathode is barely chargeable owing to its extremely low electric conductivity (Table S4). The charge/discharge profiles (Fig. S5) at different cycling status for SPAN-CNT20 show voltage plateaus that confirm to the CV profile (Fig. 5c). The rest discharge profiles up to 200 cycles present similar curves with a higher voltage plateau, indicating faster reaction kinetics and less electrode polarization upon activation [25]. Above the discharge curves, all charging profiles exhibit similar voltage plateau that echoes the CV curves. The cycle performance of SPAN, PANCNT20/S, and SPAN-CNT# (# = 5, 10, 20, 30) are compared to each other at 0.2C in a life-span of 200 cycles as shown in Fig. 5d and Fig. S6. Among all inspected samples, SPAN showed the lowest discharge capacity of 200 mAh g−1 sulfur as a result of its poor electric conductivity. In contrast, 5% of CNT addition greatly improved the discharge capacity of the cell, which underwent a long activation process of around 40 cycles and a drop of capacity seen after 100 cycles. By continually increasing the CNT addition, both the capacity and stability of the as-fabricated Li-S cells were further improved, with the highest initial discharge capacity of 1610 mAh g−1sulfur (644 mAh g−1composite) and 1400 mAh g−1 sulfur (560 mAh g−1composite) after 200 cycles achieved by SPAN-CNT20. This superior performance is likely resulted from the porous structure and conductive pathway in SPAN-CNT composites synergically contributed by the co-spinning of sulfur and CNT. On the other hand, PAN-CNT20/S only offered a discharge capacity of 999 mAh g−1sulfur (359 mAh g−1composite) after 200 cycles even with a

samples gave more consistent and higher conductivity readings with the average being (1.4 ± 0.4) × 10−2 S cm−1. The lack of homogeneity and conductivity for PAN-CNT20/S can be explained from the viewpoint of CNT exposure among the nanofibers, as evidenced by TEM characterizations (Fig. 2b, e and Fig. S3b, c). As seen from the TEM images, when compared to the porous SPAN-CNT20, nanofibers of PAN-CNT20/S are bolder with a relatively thicker surface layer of SPAN, under which most of CNT are encapsulated. In contrast, more CNT are exposed, or at least less encapsulated, on the thinner SPANCNT20 fibers with larger pores, resulting in a more homogenous and higher conductivity. Taking the fabricated freestanding nonwovens for binder-free cathodes, Li-S batteries without using current collectors were assembled with the metallic lithium used as anodes. Fig. 5a-c display the CV profiles of multiple redox reactions for SPAN, PAN-CNT20/S, and SPAN-CNT20, respectively. All three samples present two cathodic peaks in the first cycle, with those from SPAN located at 1.1 V and 1.5 V in a much lower voltage regime, resulting from its relatively poor electric conductivity without the conductive CNT backbone. In the following sequential cathodic scans, both peaks from the first cycle disappear, and a new peak emerges at ~1.3 V for SPAN and ~1.7 V for PAN-CNT20/S and SPAN-CNT20, indicating the formation of final reduction products of Li2S2/Li2S [36,51]. Notably, a broad reduction peak at 2.1 V is observed in PAN-CNT20/S after the first cycle, inferring the formation of long-chain polysulfides during the redox reaction [36]. This is likely due to the poorly bonded sulfur species when there is only external sulfur source. For both PAN-CNT20/S and SPAN-CNT20, the current of the rest cycles is comparative to that of the second scan, showing a good capacity reversibility. Particularly, the charge/discharge current density of SPAN-CNT20 is much higher, representing a faster kinetics of redox reactions even at a higher

198



Fig. 4. XPS spectra of a) survey, b) C 1s, c) S 2p, and d) N 1s for SPAN-CNT20, respectively.

of CPE1/R2 into CPE2/R3 is the best to fit the data (Fig. S9), and the resulted charge transfer resistance (R3) is three times more than that of the fresh cell (Table S5), owing to the formation of SEI layer during the activation process [52]. After 200 cycles the value of R3 drops close to the fresh one again. This indicates an improving electrochemical environment of the cell throughout the repeated redox reactions with little structural change of electrodes, resulting in a highly reversible capacity of SPAN-CNT20 over 200 cycles, which is in good agreement with the stable physical structure (Fig. S11) and cycle performance of the Li-S cell (Fig. 5d). In contrast, the unfinished semicycles observed for SPAN suggest an extremely high interfacial resistance even after 20 cycles (Fig. S7a). Among the three cells, PAN-CNT20/S had the lowest initial resistance due to its high CNT content and low sulfur content. However, after 20 cycles PAN-CNT20/S developed higher interfacial resistance than SPAN-CNT20 (Fig. S7b-c), possibly due to the formation of more long-chain polysufides as suggested by the aforementioned CV studies. The unfavored long-chain polysufides are harmful to the internal electrochemical environment, resulting in worse cycle performance [53]. Ex-situ XPS was performed to shed light on the lithiation mechanisms of sulfur in SPAN-CNT20 after the 1st discharge, 1st cycle, and 2nd cycle, respectively. After cycling, the coin cells were unfolded in the argon-filled glovebox. The composite electrodes were washed with mixed ethylene carbonate and diethyl carbonate (EC: DEC = 1: 1 by volume) solvents to remove any residual salts and organic compounds before a thorough drying. As discussed previously in Fig. 4c, The S 2p spectrum of freshly prepared SPAN-CNT20 confirms the presence of covalent C-S bond after vulcanization. After the 1st discharge, two doublets at 162.7, 161.6 eV and 161.3, 160.0 eV emerge and can be assigned to the intermediate Li2Sx (x < 4) and final reduction products

lower sulfur content, which signifies the benefit of electrospinning sulfur into the nanofibers. SPAN-CNT30 showed lower discharge capacity as well, possibly due to the less-conductive fiber network with aggregated CNT. The rate capability of SPAN, PAN-CNT20/S, and SPAN-CNT20 from 0.1C to 5C are also compared as shown in Fig. 5e. As expected, SPAN-CNT20 demonstrated the best rate capability up to 5C with a discharge capacity of 1000 mAh g−1sulfur, whereas PANCNT20/S obtained 800 mAh g−1sulfur at the same C-rate. Together with the results from SPAN, these rate capability tests highlight the role of CNT in providing a highly conductive pathway for fast charge transfer. It is worth to note that both capacity and cycle performance achieved by the SPAN-CNT20 in this study outperform many other relevant work on SPAN (Table S6), delivering a superior capacity retention of 1314 mAh g−1sulfur (526 mAh g−1composite) at 0.5C and 1106 mAh g−1sulfur (442 mAh g−1composite) at 1C after 250 and 500 cycles, respectively (Fig. 5g). Electrochemical Impedance Spectroscopy (EIS) on SPAN-CNT20 before and after cycling was conducted to study the evolution of the electrochemical environment inside the cell (Fig. 5f and Fig. S7). After the first cycle, the interfacial resistance of the cathode increases dramatically, owing to the activation of the composite nanofibers and the formation of solid electrolyte interface (SEI) layers, and then at the 200th cycle fell back to a lower value close to its fresh state. Further, with the Zview software, these EIS data are quantitatively fitted with an equivalent circuit composed of R1 + CPE1/R2 + CPE2/R3 + W (Fig. 5f, Fig. S8, S9 and S10). Where, R1 is the electrolyte resistance, CPE1/R2 is attributed to the electrode/electrolyte charge transfer interface impedance at high frequency (HF), CPE2/R3 is attributed to the charge transfer impedance at medium frequency (MF) and W is the Warburg impedance at low frequency (LF). Note that for the first cycle, merging 199



Fig. 5. Electrochemical performance of SPAN-CNT composites. a-c) CV curves of in SPAN, PAN-CNT20/S, and SPAN-CNT20, respectively; d) cyclic performance of the composites at 0.2C; e) rate capability of SPAN, PAN-CNT20/S, and SPAN-CNT20, respectively; f) EIS plot of SPAN-CNT20; and g) long-term cycle performance at 0.5 and 1C of SPAN-CNT20.

covalent bonds [58]. After the second complete cycle, Li+ from Li2Sx (x < 4) undergoes more complete reduction as evidenced by the symmetric metallic Li peak at 55.6 eV (Fig. 6f). Moreover, ex-situ FESEM study revealed no significant morphology change of SPAN-CNT20 after 200 cycles and no thick SEI layer observed on the surface of nanofibers, confirming the stable electrochemical environment of SPAN-CNT20 in the carbonate-based electrolyte (Fig. S11).

of Li2S2/Li2S, respectively (Fig. 6a) [54,55]. This is in agreement with the reported mechanism that a dense polymer matrix helps confine sulfur in the small oligomer state, which allows SPAN functioning in carbonate electrolytes [25]. The doublet colored in pink at 164.8 and 163.6 eV indicates that a small portion of C-S/S-S bond still remained, due to their strong covalent nature. Upon delithiation, the doublet of 161.30 and 160.0 eV disappears, and instead, a doublet at 162.3 and 163.5 eV can be observed after the 1st cycle, indicative of the reformation of C-S bonds (Fig. 6b) [7]. Similar deconvolution of the S 2p peak after the 2nd cycle is shown in Fig. 6c, with a higher peak ratio of the C-S/S-S bonds to covalently linked sulfur oligomers, indicating the reorganization of sulfur species within the composite electrode [56]. In all cycles of SPAN-CNT20, there is no evidence for the formation of long-chain polysulfides from the redox reactions, coinciding with the superior capacity retention upon repeated cycles. The chemical states of Li were also evaluated as shown in Fig. 6d-f. During the first discharge, peaks of Li-S (54.4 eV) and Li-N (55.1 eV) evolve as a result of the electron rich species bonded to Li ions (Fig. 6d) [2,57–59]. After the first cycle, the peak of Li-S remains while that of Li-N disappears, likely due to the abundant sulfur in the electrochemical environment as well as the partially reduced lithium (Fig. 6e). Note that the Li-S species are not reflected in the S 2p spectrum (Fig. 6b), probably masked by the chemical state of covalently linked sulfur oligomers with a close binding energy. This abnormal behavior of the 1st cycle is typical for SPAN based Li-S batteries due to the initial break-down of vast

4. Conclusion Freestanding thin-film composites of SPAN-CNT have been successfully fabricated via a facile electrospinning method by co-spinning the ternary precursors of sulfur, PAN, and CNT together, followed by vulcanization. On one side, the pre-added sulfur not only promotes the full sulfurization of PAN, but also helps create a porous structure within the nanofibers upon vaporization. On the other side, in addition to providing a conductive pathway, the added CNT also facilitates the escape of sulfur from the inside of nanofibers, leading to the nanoporous morphology. As a result, enhanced electrochemical performance has been achieved. When compared to the composites fabricated with binary electrospinning precursors, i.e. SPAN and PAN-CNT/S, SPANCNT nanofibers significantly outperformed, with the optimized mass ratio of CNT to PAN found at 20%. Impressively, the SPAN-CNT20 delivered an initial discharge capacity of 1610 mAh g−1 at 0.2C and retained 1400 mAh g−1 after 200 cycles. Good rate capability was 200



Fig. 6. Ex-situ XPS spectra of a-c) S 2p and d-f) Li 1s for SPAN-CNT20 after the 1st discharge, 1st charge, and 2nd cycle.

achieved with an excellent capacity retention of 1106 mAh g−1 at 1C for over 500 cycles. Ex-situ XPS studies revealed short-chain Li2Sx (x < 4) as the major reduction products of the redox reaction. Consequently, the facile fabrication process of conductive freestanding Li-S cathodes reported in this study should practically enhance the overall battery capacity by eliminating the metal current collectors, and both of the multi-variant material and method can be extended to other energy storage devices.

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