Nitrogen-doped hollow porous carbon nanotubes for high-sulfur loading Li–S batteries

Electrochimica Acta 324 (2019) 134849

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Nitrogen-doped hollow porous carbon nanotubes for high-sulfur loading LieS batteries Jiaona Shi a, Qi Kang b, Yan Mi c, Qingquan Xiao a, * a

Department of Electronic Science, College of Big Data and Information Engineering, Guizhou University, Guiyang, 550025, China Department of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai, 200240, China c Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi University for Nationalities, Nanning, 530006, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 June 2019 Received in revised form 3 August 2019 Accepted 8 September 2019 Available online 11 September 2019

Lithium-sulfur (LieS) batteries have attracted extensive attention in the past decades owing to their high theoretical energy density, low-cost and eco-friendliness. However, poorly cycling stability and rapidly decay caused by shuttle effects at high-sulfur loading condition restrain their practical applications. Focusing on these issues, N-doped hollow porous carbon nanotubes (N-HPCNT) as a self-supporting sulfur cathode is developed. The inherent conductivity and abundant pores of 3D continuous framework not only allow a high-sulfur content (67 wt%) and block the polysulfides via physical confinement, but also trap the polysulfides by the nitrogen heteroatoms (4.60 wt%) via strong chemisorption. Benefiting from the above-mentioned merits, the self-supporting S/N-HPCNT cathode with 3 mg cm
2 sulfur delivers high reversible capacity, super rate capability (1152 and 852.8 mAh g
1 at 0.2 and 3 C, respectively) and long cycling stability (717.9 mAh g
1 after 500 cycles at 1 C). Even under a higher sulfur loading (8 mg cm
2), it still exhibits a stable areal capacity of 7.72 mAh cm
2 (corresponding to 965.1 mAh g
1) after 60 cycles. This work provides a pathway for developing high performance LieS batteries. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Lithium-sulfur batteries N-doped hollow porous carbon nanotubes (N-HPCNT) Self-supporting High-sulfur loading Long-term cycling

1. Introduction To meet the ever-growing demands for portable electronic devices and electrical vehicles, low cost and eco-friendly lithiumsulfur (LieS) batteries as one of promising candidate have attracted considerable attention owing to their high theoretical energy density (2600 Wh Kg
1) [1e3]. Moreover, LieS batteries also own a high theoretical specific capacity (1675 mA g
1), which is one order of magnitude higher than that of traditional lithium-ion batteries (LIBs) [2,4]. However, several critical challenges including the lower electrical conductivity of sulfur and its discharged products (Li2S/ Li2S2), the low cycling stability induced by dissolution of intermediate polysulfides (Li2Sn, 4  n  8), and the large volumetric expansion (~80%) of sulfur during lithiation/delithiation processes also restrict their practical applications [2,5,6]. To overcome these drawbacks, strenuous efforts have been made to optimize the structure and composition of sulfur cathode, and mitigate the

* Corresponding author. E-mail address: [email protected] (Q. Xiao). https://doi.org/10.1016/j.electacta.2019.134849 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

“shuttling effect” through the sulfur-host materials over the past decade. Numerous multi-structured sulfur host materials, such as carbon materials, metal oxides/sulfides/nitrides, metal/covalent organic frameworks and polymer composite frameworks have been designed to alleviate the shuttle effect [7e21]. However, most of the reported hosts/sulfur composites possessed low sulfur content (<60 wt%) and low sulfur-loading (1e3 mg cm
2) or even lower, which make them fail to meet the requirements for practical application [4,22]. Thus, it is crucial to construct a high-areal capacity LieS battery (>4 mAh cm
2) that can be comparable to the state-of-the-art commercial LIBs, and its cathode not only has high sulfur loading (>3 mg cm
2) but also has a high content (>60 wt%). Recently, a few carbon-based/sulfur hybrids could reach both a high sulfur content and loading, but are limited by poor rate capability and short cycle life [23e26]. To address above-mentioned issues, many multi-architecture host materials have been developed to reduce the shuttle effect and enhance the electrochemical kinetics of sulfur. Among them, non-polar carbon materials with good conductivity, structures and morphologies diversity, thus are main option to explore the advanced LieS batteries host materials for sulfur-based cathodes

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[27]. Several carbon-based hosts such as zero-dimensional (0D), meso/microporous carbon, one-dimensional (1D) carbon nanotubes (CNTs)/carbon nanofibers (CNFs), two-dimensional (2D) carbon sheets/graphene and three-dimensional (3D) carbon nanocages/hollow carbon spheres have been developed to improve the electrochemical performance of the LieS batteries [28e41]. However, the weak chemisorption interactions between non-polar carbon materials and polar polysulfides species are not sufficient to alleviate the shuttle effect [33]. Thus, for carbon-based host materials, it is necessary to enhance the chemical interaction with polysulfides species by doping with heteroatoms, such as B, N, S and P doping [42e46]. In general, an ideal carbon host material should feature: (I) high specific area to ensure the expansion of sulfur species, (II) high electrical conductivity to ensure the rate capacity, (III) well-balanced pore framework for fast ion/electron transport and (IV) effective adsorption of polysulfides, especially at high current density with high-sulfur-loading electrode. Hence, referring to the aforementioned prerequisites for an ideal cathode material, it is great to explore and develop more efficient carbon host with strong chemisorption without sacrificing other outstanding properties. It has been proved that carbon with doping heteroatoms (e.g. B and N) could effectively modulate the electronic structure and enhance the chemical adsorption ability of polysulfides [42e44,47]. Therefore, this approach should be an optimal choice to enhance hosts’ chemical adsorption capacity since it does not change the intrinsic merits of the materials. As known, 1D carbon nanotubes/nanofibers own a long conductive network, excellent mechanical properties and can form a continuous conductive 3D network more easily, especially the ultralong nanotubes/fibers [32,48]. Currently, polar/N-doped hollow carbon nanotube hybrid materials, such as CoS2/NCNTF [49], S-NC@MoS2 [50] and NeH-CNTs [51] have attracted extensive attention to serve as the anode of LIBs and hosts in LieS batteries. Owing to the novel hollow structure, above-mentioned materials exhibited superior electrochemical performances. Hence, 3D hollow structured nitrogen-doped carbon nanotubes may be the promising one for carbon-based sulfur cathodes to fulfill all above requirements, at same time maximize the sulfur content and areal loading. In this work, we report the 3D N-doped hollow porous carbon nanotubes (N-HPCNT) with high specific surface area, high conductivity and porous microstructure, which is prepared by MnO2hard template method. In the proposed structure, the hollow carbon nanofibers not only perform conducting and connecting function, but also provide a certain physical entrapment for polysulfides. In addition, the N-atoms can effectively trap the soluble polysulfides by chemical interaction, which would work synergistically with the hollow porous structures to enhance the performance of the LieS batteries. As a result, the S/N-HPCNT cathodes exhibit a high rate capacity of 852.8 mAh g
1 (3 C) and ultra-stable cycling life at a high current density (717.9 mAh g
1 at 1 C after 500 cycles) with a high sulfur loading of 3 mg cm
2. Even the sulfur loading is increased to 8 mg cm
2, these cathodes still deliver a high areal capacity of 7.72 mAh cm
2 (corresponding to 965.1 mAh g
1) at 0.2C after cycling for 60 cycles. 2. Experimental section 2.1. Materials Tetraethoxysilane (TEOS), 1, 2-Ethylenediamine (EDA), and resorcinol were purchased from Alfa Aesar. MnSO4$H2O, KClO3, CH3COOK, CH3COOH and sulfur were supplied by Aladdin. Ethanol, hydrofluoric acid (HF), hydrochloric acid (HCl), ammonia (NH3$H2O), carbon disulfide (CS2) and formaldehyde were supplied by Sinopharm Chemical Reagent Co. All chemicals are analytical

grade and used without further purification. 2.2. Preparation of ultralong MnO2 nanowires The ultralong MnO2 nanowires were prepared via a hydrothermal method according to the previous work [52] with a little modification. In a typical procedure, 4 mmol of MnSO4$H2O, 7 mmol of CH3COOK and 7 mmol of KClO3 were dissolved in 60 mL of deionized water with the aid of ultrasonic. When it was dissolved absolutely and formed a clarified solution, 3.2 mL of CH3COOH was added into the above solution under stirring for 4 h. After that, the mixed solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, then put into an oven and keep at 160  C for 8 h. After the reaction system was naturally cooled to room temperature, the precipitates were collected by centrifugation and washed with deionized water for several times, and finally were dried at 70  C overnight. Finally, the ultralong MnO2 nanowires powder were obtained. 2.3. Preparation of core-shell structured MnO2@SiO2 and MnO2@SiO2@PB/SiO2 nanowires The MnO2@SiO2 and MnO2@SiO2@PB/SiO2 nanowires were synthesized according to the previous work with a little of modification [53]. In a typical procedure, 1.5 g MnO2 nanowires were firstly dispersed in a mixture of 120 mL of ethanol and 15 mL of DI water with the assistance of ultrasonic. After stirring for 10 min, 6 mL of ammonia and 4 mL of TEOS were added dropwise into the dispersion under vigorous stirring. After reaction for 6 h at 30  C, MnO2@SiO2 nanowires were collected and washed by centrifugation with DI water and ethanol for several times. Then, as-prepared MnO2@SiO2 nanowires were dispersed into 30 mL of DI water, and further diluted into a 90 mL of mixture solution (DI water (60 mL) and ethanol (30 mL)), followed by addition of 0.2 g of resorcinol, 0.3 mL of formaldehyde, 0.3 mL of EDA and 0.6 mL of TEOS under stirring at 30  C. After reaction for 24 h, the resultant MnO2@SiO2@PB/SiO2 nanowires membrane was collected by vacuum filtration and washed with DI water for several times and finally dried at 60  C for 4 h. Thus, a self-supporting MnO2@SiO2@PB/SiO2 film was prepared. 2.4. Preparation of N-doped hollow porous carbon nanotubes (NHPCNT) The self-supporting MnO2@SiO2@PB/SiO2 films were carbonized at 800  C for 4 h under an Ar atmosphere. After annealed, the samples are stirred and etched in 10% HF aqueous solution and 12 M HCl aqueous solution for 24 h and washed with DI water to remove the SiO2 and MnO2 template, respectively. Finally, N-HPCNT film was obtained after dried at 60  C for 24 h. 2.5. Preparation of S/N-HPCNT cathodes The S/N-HPCNT was prepared as follows. First, the as-prepared N-HPCNT film was cut into a circular shaped disk with a 12 mm of diameter. Then, the N-HPCNT disk was immersed into an S/CS2 (25 mg mL
1) solution for 2 h and dried at 45  C. After that, sulfurcontaining N-HPCNT disk was sealed in a glass bottle and heated at 155  C for 12 h. In order to remove extra sulfur on the surface of the N-HPCNT, the as-prepared sulfur-based films were further heated in an open glass bottle at 160  C for 30 min. Finally, the S/N-HPCNT samples were prepared and the sulfur content was determined to be 67 wt%.

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2.6. Characterization Scanning electron microscopy (SEM, Zeiss SIGMA at 15 kV), transmission electron microscopy (TEM, TECNAI F-30 at 300 kV), Xray diffraction (XRD, Bruker D2 Phaser at 40 kV and 40 mA), Raman spectroscopy (laser excitation 532 nm, IDSpec ARCTIC), Elemental Analyzer (Vario ELIII, Elementar Analysensyetem GmbH, Germany) and X-ray photoelectron spectroscopy (XPS, PHI QUANTUM 2000) were used to characterize the morphologies, structures, and components of the samples. N2 adsorption/desorption isotherms were extracted on TriStar II 3020 system. The sulfur content was measured by thermogravimetry analysis (Pyris Diamond TG-DTA) at a heating rate of 10  C min
1 under an Ar atmosphere. 2.7. Electrochemical measurements All of the electrochemical performance measurements are tested in CR-2032-type coin cells. The coin cells are assembled by using N-HPCNTs/S as the cathode, Celgard 2400 as separator, lithium foil as the counter electrode and 1 M LiTFSI in a mixed solution of dioxolane (DOL) and 1,2-dimethoxyethane (DME) (volume ratio 1:1) with 2 wt% LiNO3 as the electrolyte in an Ar-filled glove box (H2O, O2 < 0.1 ppm). The ratio of electrolyte to sulfur (E/S) is controlled as 20 mL mg
1. The areal sulfur loading mass is about 3e8 mg cm
2. The galvanostatic charge-discharge curves were tested on LAND CT2001A in a voltage window of 1.8e2.7 V (vs Li/Liþ). The cyclic voltammetry (CV) curves and electrochemical impendence spectroscopy (EIS) curves were tested on an electrochemical station (CHI 660E) at a scan rate of 0.1 mV s
1 and a frequency range from 0.01 Hz to 100 kHz, separately. 3. Results and discussion The detailed synthesis process of S/N-HPCNT is schematically illustrated in Fig. 1. First, the ultralong MnO2 nanowire was synthesized via hydrothermal method to act as a hard template. Then, a thin layer of SiO2 was coated on the surface of the MnO2 nano€ber method. After this process, a layer of wires (MnO2@SiO2) by Sto poly-benzoxazine (PB) was coated on MnO2@SiO2 nanowires surface, as well as another layer of SiO2 was coated on the surface of PB through a one-pot sol-gel method based on the intrinsic charge characteristics [53]. As we all know, the surface of SiO2 displays negative charge characteristics, meanwhile, PB shows positive charge nature. Hence, the MnO2@SiO2@PB/SiO2 was obtained and prepared as a self-supporting film by a vacuum filtration method. Finally, the self-supporting N-doped hollow porous carbon nanotubes (N-HPCNT) film was achieved after carbonization and twice acid treatment process.

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The typical scanning electron microscopy (SEM) image of MnO2 nanowires with an ultralong length are shown in Fig. 2a. The phase and crystallinity of MnO2 were studied by using the X-ray diffraction (XRD). As shown in Fig. S1 (Supporting Information), the conspicuous peaks can be indexed into the tetragonal a-MnO2 (JCPDS card, No.81e1947). And the ultralong MnO2 nanowires can form a self-supporting membrane easily by a vacuum filtration method, as shown in Fig. S2 (Supporting Information). After coating with silica, each MnO2 nanowires are encapsulated by a layer of SiO2 (MnO2@SiO2), and their structure are well maintained (Fig. 2b). The diameter of pristine MnO2 nanowires are ~100 nm, which increase to ~200 nm after SiO2 coating. Then, a layer of PB/ SiO2 is grown on the surface of MnO2@SiO2 nanowires through a one-pot sol-gel and formed a self-supporting MnO2@SiO2/PB/SiO2 film by vacuum filtration method (Fig. 2c). After carbonization and twice acid treatment processes, the self-supporting N-HPCNT films are obtained, as shown in Fig. 2d and e. The digital photographs of the self-supporting N-HPCNT films are shown in Fig. S3 (Supporting Information). Because the ultralong MnO2 nanowires can form a 3D networks easily (Fig. S2, Supporting Information), thus the NHPCNT film owns a 3D continuous conductive network and selfsupporting structure, providing the fast electrons and Liþ ions transport (Figs. S3a and S3b, Supporting Information). In our strategy, PB can act as a N-doped carbon precursor, and TEOSderived SiO2 can serve as a convenient pore-forming agent, which make the generated-carbon contain the N atoms and generate sufficient pores during follow-up treatment [53]. Thus, the self-supporting N-HPCNT with a large number mesopores was obtained, which is beneficial to accommodate a high-sulfur content and to provide efficient pathways of electrons and Liþ ions. In addition, the transmission electron microscopy (TEM) image further reveals the hollow structure of N-HPCNT (Fig. 2f). In comparison, hollow carbon nanotubes (HCNT) were prepared by modifying the preparation of N-HPCNT (as described in Supporting Information), and it also shows the hollow structure while without any N atoms doped (Figs. S4aec, Supporting Information). N2 adsoprtion/desorption isotherms exhibit a IV-type curve with a high nitrogen uptake, which indicates the existence of mespores (Fig. 3a). The Brunauer-Emmett-Teller (BET) surface areas of HCNT and N-HPCNT are 872.6 and 1892.7 m2 g
1, respectively. As shown in Fig. 3b, the average pore size distribution of HCNT and NHPCNT both are centered ~2.3 nm, which were calculated by Barrett-Joyner-Halenda (BJH) method. The total pore volume of HCNT is 0.6 cm3 g
1, due to the porous structure, the total pore volume of N-HPCNT is 1.7 cm3 g
1, which is superior than most previous reported data [4,11,53e55]. Based on these results, it proves that the N-HPCNT can provide more active sites for surface contact between liquid electrolyte and cathode. The component

Fig. 1. Schematic illustration of S/N-HPCNT hybrids.

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Fig. 2. SEM images of (a) MnO2, (b) MnO2@SiO2, (c) MnO2@SiO2@PB/SiO2, (d, e) N-HPCNT and (f) TEM image of N-HPCNT.

Fig. 3. (a) N2 adsorption and desorption isotherms; (b) Size distribution of HCNT and N-HPCNT; (c) XPS survey spectrum of N-HPCNT; (d) N 1s XPS spectrum.

and elements content of the N-HPCNT were measured by X-ray photoelectron spectroscopy (XPS). Because the PB is the N-enriched precuror, thus it can generate a N-doped carbon when it was annealed under an Ar atmosphere. The survey spectrum shows the C, O and N signals which reveals that the N-HPCNT is composed of these elements (Fig. 3c). For the N 1s spectrum (Fig. 3d), three peaks locate at 398.6, 400.8 and 403.5 eV corresponding to pyridinic-N, pyrrolic-N and quaternary-N in N-HPCNT, respectively. The C 1s XPS spectrum of the N-HPCNT has a major peak at 284.5 eV, attributing to the CeC/C]C bond, as well as other three peaks at 285.7, 287.1 and 289.1 eV, which can be assigned to CeO/CeN, C]O and OeC]O species, respectively (Fig. S5a, Supporting

Information) [50]. The presence of the CeN bonds demonstrating successful N doping in N-HPCNT. As depicted in Fig. S5b (Supporting Information), the O 1s spectra exhibit three peaks locate at 531.7, 532.8 and 533.8 eV, which can be ascribed to C]O, CeO and OH bonds [37]. Overall, the XPS spectra of the N-HPCNT in Fig. 3c confirm the presence of C, N and O elements. Based on DFT calculation and experiment results, all of these three nitrogen species are favorable to anchor lithium ploysulfides, leading to the improvement of the electrochemical performance [53,56e59]. The N contents of the N-HPCNT were measured by Elemental Analyzer, as shown in Table S1 (Supporting Information). According to the results, the corresponding N contents are about 4.60 wt% in the N-

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HPCNT. The high N content can provide more polar sites to anchor polysulfides intermediates. Such unique porous structure and high conductivity are conductive to achieving the high specific area and fast ion/electron charge tranfer, thus providing more accommodation interspace for sulfur and accelerate redox reaction. Moreover, the self-supporting N-HPCNT film with a 3D conductive cross-network, which becomes a promising host for high sulfur loading. The S/N-HPCNT cathodes were prepared by a simple melt-diffusion method (For details, see in experimental section). The actual sulfur content in S/N-HPCNT film was determined to be 67 wt% (Fig. 4a) by thermogravimetric analysis (TGA) under an Ar atmosphere. After sulfur loading, the morphology of S/N-HPCNT is similar to pristine N-HPCNT and no obvious sulfur particles can be observed on the surface of N-HPCNT (Fig. 4b). TEM observation shows that S/N-HPCNTs are not hollow (Fig. 4c), in marked contrast to pristine hollow-structured N-HPCNT (Fig. 2f), indicating the sulfur is mainly loaded inside the inner void of N-HPCNT and pores of N-HPCNT shells. As a contrast, the SEM images of S/HCNT show a lot of sulfur particles on the surface of HCNT due to its lower specific surface area than that of N-HPCNT (Fig. S6, Supporting Information). The XRD pattern of the S/NHPCNT composite confirms the presence of the sulfur. And it exhibits much weaked diffraction than sulfur and S/HCNT, indicating the sulfur was confined inside the pores of N-HPCNT (Fig. S7, Supporting Information). Raman spectroscopy was used to measure the structures of the pure sulfur, HNCT and S/HPCNT. As shown in Fig. S8 (Supporting Information), pure sulfur contains three characteristic peaks at 148, 213 and 477 cm
1 [60]. Not only the NHPCNT but also the S/N-HPCNT show two typical peaks of D and G bands at 1338 and 1574 cm
1, respectively [23,60]. The presence of such bands in the Raman spectra suggests the structure of these carbon materials. Additionally, the S/N-HPCNT shows a weaker peak than pure sulfur, which is agreement with the XRD results (Fig. S7, Supporting Information), suggesting that the sulfur is confined in the pores of N-HPCNT. The cross-sectional SEM image (Fig. 4d) of the self-supporting S/N-HPCNT film shows a packed structure with a thickness of ~80 mm. Moreover, the high-angle annular dark-field scanning image (HAADF-STEM) (Fig. 4e) reveals the solid structure of S/N-HPCNT. Energy-dispersive X-ray spectrocopy (EDX) elemental mappings further reveal the uniform

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distribution of N element in carbon host and the sulfur was homogeneous encapsulated by the host (Fig. 4feh). In conclusion, NHPCNT should be a promising cathode host for sulfur and can anchor polysulfides with it hollow structure and N-doped active sites. The interactions between hosts and polysulfides were confirmed by a static adsorption experiment. A solution of 0.05 M Li2S6 in mixed DOL/DME (volume ration 1:1) as a representative of lithium polysulfides, as-prepared host powder (N-HPCNT and HCNT) was immersed into the aforementioned solution. As evidence, after the Li2S6 solution mixed with N-HPCNT powder, the solution color was turned from yellow to colorless within 30 min (Fig. S9, Supporting Information), while the HCNT powder only decolored slightly, indicating the strong adsorption ability of NHPCNT. To evaluate the electrochemical performances of the selfsupporting S/N-HPCNT and S/HCNT cathodes, CR2032 coin cells were assembled with Li foils as the counter electrodes. Both the areal sulfur loading and sulfur content of the S/N-HPCNT and S/ HCNT were controlled to be 67 wt% and 3 mg cm
2, respectively. Fig. 5a shows the galvanostatic charge/discharge (GCD) profiles of S/N-HPCNT cathode at different current densities, and each profile from 0.2 to 3 C displays two typical discharge plateaus and one charge plateau, which corresponding to the reduction of S8 to longchain polysulfides (Li2Sn, 4  n  8), and the further reduction of long-chain polysulfides to short-chain polysulfides (Li2S2 and Li2S), respectively [1e3]. As shown in Fig. 5b, the initial discharge capacity of the S/N-HPCNT cathode reaches 1304.2 mAh g
1 at 0.1C, which is higher than the S/HCNT cathode (1146.8 mAh g
1, Fig. S10a, Supporting Information). Moreover, when the current density increases to 0.2, 0.5, 1.0, 1.5, 2.5 and 3.0C (the corresponding areal current density of 1.0, 2.5, 5.0, 7.5, 10.1, 12.6 and 15.1 mA cm
2, respectively), a high specific capacity of 1152, 1005.4, 960, 926.7, 902.3, 879.4 and 852.8 mAh g
1 was achieved, respectively. When the current is set back to 0.2C, the discharge capacity recovers to 1027 mAh g
1. In contrast, S/HCNT cathode exhibits 913.8, 888.5, 855.5, 768.9, 679.8, 446.2 and 307.8 mAh g
1 at current densities of 0.2, 0.5, 1.0, 1.5, 2.5 and 3.0C, respectively (Figs. S10a and S10b, Supporting Information). And when the current density was set back to 0.2C, the discharge capacity increases to 980.3 mAh g
1 due to the activation in cycling. It is much lower than the S/N-HPCNT cathode. In addition, the cyclic voltammetry (CV) profiles of S/N-

Fig. 4. (a) TG curve of S/N-HPCNT; (bec) SEM images of S/N-HPCNT; (d) Cross-section SEM image of S/N-HPCNT; (e) HAADF-STEM image and (feh) element mapping of S/N-HPCNT.

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Fig. 5. Electrochemical performances of the S/N-HPCNT cathodes with high areal sulfur loading. (a) Charge/discharge curves; (b) Rate performances, and (c) Long-term cycling capacities of the S/N-HPCNT cathodes with sulfur loading of 3 mg cm
2; (d) Charge/discharge curves and (e) cycling performance of the S/N-HPCNT cathodes with ultrahigh sulfur loading of 8 mg cm
2.

HPCNT cathode at a scan rate of 0.1 mV s
1, exhibit two cathodic peaks (i and ii) around 2.34 and 2.03 V, corresponding to the reduction from S8 to Li2S4-8 and further to Li2S2/Li2S (iii and iv process in Fig. S9a, Supporting Information). Subsequently, the two pairs of anodic peaks around 2.29 and 2.40 V are corresponded to the Li2S2/Li2S to Li2S4-8 and then to S8, which is in good agreement with its GCD curves (Fig. 5a). And the CV profiles without obvious change from 2nd and 3rd cycle (Fig. S11a, Supporting Information), indicating the reversible sulfur redox conversion of the selfsupporting S/N-HPCNT cathode [37]. Additionally, the electrochemical impendence spectroscopy (EIS) curves of S/N-HPCNT cathode shows a very lower charge-transfer resistance than S/ HCNT cathode (Fig. S11b, Supporting Information), which is conductive to rate performance. The super rate performances and their capacity recovery of S/N-HPCNT cathodes are attributed to the ultralong continuous 3D conductive and porous framework, and strong chemisorption of N-heteroatoms to polysulfides species [56e59,61e63]. To further evaluate the stability of the S/N-HPCNT cathode, the long-term cycling performance of this cathode is tested at a high rate of 1 C. After activated at 0.05C for two cycles, the initial discharge capacity of the S/N-HPCNT cathode reaches 900.2 mAh g
1, as shown in Fig. 5c. And it remains a high capacity of 829.5 mAh g
1 after 200 cycles, accounting for the 92.2% retention of its initial capacity. In contrast, the S/HCNT cathode shows an initial capacity of 817.5 mAh g
1 and decreases to 337.9 mAh g
1 after 200 cycles

(Fig. 5c), indicating irreversible cycling due to its low specific area and weak adsorption to lithium polysulfides. In addition, the S/NHPCNT cathode delivers high capacities of 785.5, 741.6 and 717.9 mAh g
1 after 300, 400 and 500 cycles, corresponding to capacity retentions of 87.3%, 82.4% and 80%, and accompanying with average decays of 0.042%, 0.044% and 0.041% per cycle, respectively. Moreover, the Coulombic efficiency of S/N-HPCNT cathode is above 99.5% during the 500 cycles, suggesting the S/NHPCNT can effectively suppress the shuttle effect of polysulfides (Fig. 5c). The excellent electrochemical rate and cycling performances of the S/N-HPCNT cathode should be attributed to the above-mentioned ultralong 3D continuous conductive structural superiority and high affinity of N-doped atoms to polysulfides. Furthermore, compared with the hollow-structured sulfur cathodes and other typical sulfur hosts in the literature (Table S2, Supporting Information), the S/N-HPCNT cathode also exhibits remarkable electrochemical performances [13,16,22,27,28,33e36,53e55,64]. In consideration of potential practical application of the S/NHPCNT cathode in LieS battery, a high-areal loading sulfur cathode also is prepared. The GCD curves of the S/N-HPCNT cathode with areal sulfur loading of 8 mg cm
2 at 0.2C (corresponding to 2.68 mA cm
2) are shown in Fig. 5d. The initial discharge capacity of S/N-HPCNT cathode can reach 8.30 mAh cm
2 (corresponding to 1037.6 mAh g
1), and after cycling at 0.2C for 30 and 60 cycles, still maintain ultrahigh discharge capacities of 7.93 and 7.72 mAh cm
2

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(corresponding to 991.5 and 965.1 mAh g
1, Fig. 5e), which are much higher than the value of state-of-art commercial cathode electrodes of LIBs. The capacity retention is calculated to be 93% of its 1st discharge capacity. The remarkable performances of the S/NHPCNT cathode with ultrahigh sulfur loading are ascribed to the ultralong 3D conductive network, strong polysulfides chemisorption and physical confinement characteristics of the N-HPCNT, which provide the fast electron and Liþ ions transport, and improve the utilization of sulfur. 4. Conclusions In summary, a 3D hollow-structured porous N-HPCNT has been designed and prepared by a facile hard-templating method. With the superior 3D hollow structure and high specific area, continuous conductive network, strong chemical affinity and physical confinement for polysulfides trapping, the S/N-HPCNT cathode can effectively improve the sulfur utilization at high sulfur loading and show excellent rate and cycling performances. More importantly, even under an ultrahigh areal sulfur loading of 8 mg cm
2, the S/NHPCNT cathode still achieves a high areal specific capacity of 7.72 mAh cm
2 (corresponding to 965.1 mAh g
1). The present work may bring new insight into the fabrication of 3D ultra-long conductive framework for advanced sulfur cathode for LieS batteries, and provides new opportunity for next-generation flexible/ wearable batteries. Acknowledgments This work is jointly supported by the National Natural Science Foundation of China (61264004), Foundation for Sci-tech Activities for the Overseas Chinese Returnees in Guizhou Province ([2018] 09) and High-level Innovative Talent Training Project in Guizhou Province ([2015] 4015). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134849. References [1] S. Evers, L.F. Nazar, New approaches for high energy density lithium-sulfur battery cathodes, Acc. Chem. Res. 46 (2013) 1135e1143. [2] A. Manthiram, Y.Z. Fu, S.H. Chung, C.X. Zu, Y.S. Su, Rechargeable lithium-sulfur batteries, Chem. Rev. 114 (2014) 11751e11787. [3] H.J. Peng, J.Q. Huang, X.B. Cheng, Q. Zhang, Review on high-loading and highenergy lithium-sulfur batteries, Adv. Energy Mater. 7 (2017) 1700260. [4] L.B. Ma, H.N. Lin, W.J. Zhang, P.Y. Zhao, G.Y. Zhu, Y. Hu, R.P. Chen, Z.X. Tie, J. Liu, Z. Jin, Nitrogen-doped carbon nanotube forests planted on cobalt nanoflowers as polysulfide mediator for ultralow self-discharge and high areal-capacity lithium-sulfur batteries, Nano Lett. 18 (2018) 7949e7954. [5] Y. Yang, G.Y. Zheng, Y. Cui, Nanostructured sulfur cathodes, Chem. Soc. Rev. 42 (2013) 3018e3032. [6] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Lithium-sulfur batteries: electrochemistry, materials, and prospects, Angew. Chem. Int. Ed. 52 (2013) 13186e13200. [7] X.L. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries, Nat. Mater. 8 (2009) 500e506. [8] Z.W. Seh, W.Y. Li, J.J. Cha, G.Y. Zheng, Y. Yang, M.T. McDowell, P.C. Hsu, Y. Cui, Sulphur-TiO2 yolk-shell nanoarchitecture with internal void space for longcycle lithium-sulphur batteries, Nat. Commun. 4 (2013) 1331. [9] X. Liang, L.F. Nazar, In situ reactive assembly of scalable core-shell sulfurMnO2 composite cathodes, ACS Nano 10 (2016) 4192e4198. [10] X.L. Wang, G. Li, J.D. Li, Y.N. Zhang, A. Wook, A.P. Yu, Z.W. Chen, Structural and chemical synergistic encapsulation of polysulfides enables ultralong-life lithium-sulfur batteries, Energy Environ. Sci. 9 (2016) 2533e2538. [11] T. Chen, L.B. Ma, B.R. Cheng, R.P. Chen, Y. Hu, G.Y. Zhu, Y.R. Wang, J. Liang, Z.X. Tie, J. Liu, Z. Jin, Metallic and polar Co9S8 inlaid carbon hollow nanopolyhedra as efficient polysulfide mediator for lithium-sulfur batteries, Nano Energy 38 (2017) 239e248. [12] X.H. Xu, A. Manthiram, Hollow cobalt sulfide polyhedra-enabled long-life, high areal-capacity lithium-sulfur batteries, Nano Energy 33 (2017) 124e129.

7

[13] L.B. Ma, W.J. Zhang, L. Wang, Y. Hu, G.Y. Zhu, Y.R. Wang, R.P. Chen, T. Chen, Z.X. Tie, J. Liu, Z. Jin, Strong capillarity, chemisorption, and electrocatalytic capability of crisscrossed nanostraws enabled flexible, high-rate, and longcycling lithium-sulfur batteries, ACS Nano 12 (2018) 4868e4876. [14] Z.M. Cui, C.X. Zu, W.D. Zhou, A. Manthiram, J.B. Goodenough, Mesoporous titanium nitride-enabled highly stable lithium-sulfur batteries, Adv. Mater. 28 (2016) 6926e6931. [15] D.R. Deng, F. Xue, Y.J. Jia, J.C. Ye, C.D. Bai, M.S. Zheng, Q.F. Dong, Co4N nanosheet assembled mesoporous sphere as a matrix for ultrahigh sulfur content lithium-sulfur batteries, ACS Nano 11 (2017) 6031e6039. [16] L.B. Ma, H. Yuan, W.J. Zhang, G.Y. Zhu, Y.R. Wang, Y. Hu, P.Y. Zhao, R.P. Chen, T. Chen, J. Liu, Z. Hu, Z. Jin, Porous-shell vanadium nitride nanobubbles with ultrahigh areal sulfur loading for high-capacity and long-life lithium-sulfur batteries, Nano Lett. 17 (2017) 7839e7846. [17] J.M. Zheng, J. Tian, D.X. Wu, M. Gu, W. Xu, C.M. Wang, F. Gao, M.H. Engelhard, J.G. Zhang, J. Liu, J. Xiao, Lewis acid-base interactions between polysulfides and metal organic framework in lithium sulfur batteries, Nano Lett. 14 (2014) 2345e2352. [18] Y.Y. Mao, G.R. Li, Y. Guo, Z.P. Li, C.D. Liang, X.S. Peng, Z. Lin, Foldable interpenetrated metal-organic frameworks/carbon nanotubes thin film for lithium-sulfur batteries, Nat. Commun. 8 (2017) 14628. [19] Z.A. Ghazi, L.Y. Zhu, H. Wang, A. Naeem, A.M. Khattak, B. Liang, N.A. Khan, Z.X. Wei, L.S. Li, Z.Y. Tang, Efficient polysulfide chemisorption in covalent organic frameworks for high-performance lithium-sulfur batteries, Adv. Energy Mater. 6 (2016) 1601250. [20] H.P. Liao, H.M. Ding, B.J. Li, X.P. Ai, C. Wang, Covalent-organic frameworks: potential host materials for sulfur impregnation in lithium-sulfur batteries, J. Mater. Chem. A 2 (2014) 8854e8858. [21] L. Hencz, H. Chen, H.Y. Ling, Y.Z. Wang, C. Lai, H.J. Zhao, S.Q. Zhang, Housing sulfur in polymer composite frameworks for Li-S batteries, Nano-Micro Lett. 11 (2019) 17. [22] X.J. Gao, X.F. Yang, M.S. Li, Q. Sun, J.N. Liang, J. Luo, J.W. Wang, W.H. Li, J.W. Liang, Y.L. Liu, S.Z. Wang, Y.F. Hu, Q.F. Xiao, R.Y. Li, T.K. Sham, X.L. Sun, Cobalt-doped SnS2 with dual active centers of synergistic absorption-catalysis effect for high-S loading Li-S batteries, Adv. Funct. Mater. 29 (2019) 1806724.  Caballero, J. Morales, J. Hassoun, A lithium[23] A. Benítez, D.D. Lecce, G.A. Elia, A. ion battery using a 3D-array nanostructured graphene-sulfur cathode and a silicon oxide-based anode, ChemSusChem 11 (2018) 1512e1520. [24] D. Gueon, J.T. Hwang, S.B. Yang, E. Cho, K. Sohn, D.K. Yang, J.H. Moon, Spherical macroporous carbon nanotube particles with ultrahigh sulfur loading for lithium-sulfur battery cathodes, ACS Nano 12 (2018) 226e233. [25] M. Li, Y.N. Zhang, Z.Y. Bai, W.W. Liu, T.C. Liu, J. Gim, G.P. Jiang, Y.F. Yuan, D. Luo, K. Feng, R.S. Yassar, X.L. Wang, Z.W. Chen, J. Lu, A lithium-sulfur battery using a 2D current collector architecture with a large-sized sulfur host operated under high areal loading and low E/S ratio, Adv. Mater. 30 (2018) 1804271. [26] J.L. Guo, S.P. Zhao, G.H. He, F.X. Zhang, Novel synergistic strategy for developing high-performance lithium sulfur batteries of large areal sulfur loading by SEI modified separator, ACS Appl. Energy Mater. 1 (2018) 932e940. [27] Z. Li, B.Y. Guan, J.T. Zhang, X.W. (David) Lou, A compact nanoconfined sulfur cathode for high-performance lithium-sulfur batteries, Joule 1 (2017) 576e587. [28] Z. Li, Y. Jiang, L.X. Yuan, Z.Q. Yi, C. Wu, Y. Liu, P. Strasser, Y.H. Huang, A highly ordered meso@microporous carbon-supported sulfur@smaller sulfur coreshell structured cathode for Li-S batteries, ACS Nano 8 (2014) 9295e9303. [29] Z. Li, L.X. Yuan, Z.Q. Yi, Y.M. Sun, Y. Liu, Y. Jiang, Y. Shen, Y. Xin, Z.L. Zhang, Y.H. Huang, Insight into the electrode mechanism in lithium-sulfur batteries with ordered microporous carbon confined sulfur as the cathode, Adv. Energy Mater. 4 (2014) 1301473. [30] J.C. Guo, Y.H. Xu, C.S. Wang, Sulfur-impregnated disordered carbon nanotubes cathode for lithium-sulfur batteries, Nano Lett. 11 (2011) 4288e4294. [31] Y. Zhao, W.L. Wu, J.X. Li, Z.C. Xu, L.H. Guan, Encapsulating MWNTs into hollow porous carbon nanotubes: a tube-in-tube carbon nanostructure for highperformance lithium-sulfur batteries, Adv. Mater. 26 (2014) 5113e5118. [32] F.P. Fang, K. Chen, L.C. Yin, Z.H. Sun, F. Li, H.M. Cheng, The regulating role of carbon nanotubes and graphene in lithium-ion and lithium-sulfur batteries, Adv. Mater. 31 (2019) 1800863. [33] Z. Li, J.T. Zhang, X.W. (David) Lou, Hollow carbon nanofibers filled with MnO2 nanosheets as efficient sulfur hosts for lithium-sulfur batteries, Angew. Chem. Int. Ed. 54 (2015) 12886e12890. [34] X.Q. Zhang, B. He, W.C. Li, A.H. Lu, Hollow carbon nanofibers with dynamic adjustable pore sizes and closed ends as hosts for high-rate lithium-sulfur battery cathodes, Nano Res 11 (2018) 1238e1246. [35] H.L. Wang, Y. Yang, Y.Y. Liang, J.T. Robinson, Y.G. Li, A. Jackson, Y. Cui, H.J. Dai, Graphene-wrapped sulfur particles as a rechargeable lithium-sulfur battery cathode material with high capacity and cycling stability, Nano Lett. 11 (2011) 2644e2647. [36] S. Feng, J.H. Song, C.Z. Zhu, Q.R. Shi, D. Liu, J.C. Li, D. Du, Q. Zhang, Y.H. Lin, Assembling carbon pores into carbon sheets: rational design of threedimensional carbon networks for a lithium-sulfur battery, ACS Appl. Mater. Interfaces 11 (2019) 5911e5918. [37] F. Pei, L.L. Lin, D.H. Ou, Z.M. Zheng, S.G. Mo, X.L. Fang, N.F. Zheng, Self-supporting sulfur cathodes enabled by two dimensional carbon yolk-shell nanosheets for high energy-density lithium-sulfur batteries, Nat. Commun. 8 (2017) 482. [38] F. Pei, L.L. Lin, A. Fu, S.G. Mo, D.H. Ou, X.L. Fang, N.F. Zheng, A two-dimensional

8

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

J. Shi et al. / Electrochimica Acta 324 (2019) 134849 porous carbon-modified separator for high-energy-density Li-S batteries, Joule 2 (2018) 323e336. Z.Y. Lyu, D. Xua, L.J. Yang, R.C. Che, R. Feng, J. Zhao, Y. Li, Q. Wu, X.Z. Wang, Z. Hu, Hierarchical carbon nanocages confining high-loading sulfur for highrate lithium-sulfur batteries, Nano Energy 12 (2015) 657e665. Q. Sun, B. He, X.Q. Zhang, A.H. Lu, Engineering of hollow core-shell interlinked carbon spheres for highly stable lithium-sulfur batteries, ACS Nano 9 (2015) 8504e8513. J.J. Song, C.Y. Zhang, X. Guo, J.Q. Zhang, L.Q. Luo, H. Liu, F.Y. Wang, G.X. Wang, Entrapping polysulfides by using ultrathin hollow carbon spherefunctionalized separators in high-rate lithium-sulfur batteries, J. Mater. Chem. A 6 (2018) 16610e16616. C.P. Yang, Y.X. Yin, H. Ye, K.C. Jiang, J. Zhang, Y.G. Guo, Insight into the effect of boron doping on sulfur/carbon cathode in lithium-sulfur batteries, ACS Appl. Mater. Interfaces 6 (2014) 8789e8795. T.Z. Hou, X. Chen, H.J. Peng, J.Q. Huang, B.Q. Li, Q. Zhang, B. Li, Design principles for heteroatom-doped nanocarbon to achieve strong anchoring of polysulfides for lithium-sulfur batteries, Small 12 (2016) 3283e3291. J.X. Song, M.L. Gordin, T. Xu, S.R. Chen, Z.X. Yu, H. Sohn, J. Lu, Y. Ren, Y.H. Duan, D.H. Wang, Strong lithium polysulfide chemisorption on electroactive sites of nitrogen-doped carbon composites for high-performance lithium-sulfur battery cathodes, Angew. Chem. Int. Ed. 54 (2015) 4325e4329. N. Li, F.Y. Gan, P. Wang, K.H. Chen, S.Y. Chen, X. He, In situ synthesis of 3D sulfur-doped graphene/sulfur as a cathode material for lithium-sulfur batteries, J. Alloy. Comp. 754 (2018) 64e71. M.Q. Guo, J.Q. Huang, X.Y. Kong, H.J. Peng, H. Shui, F.Y. Qian, L. Zhu, W.C. Zhu, Q. Zhang, Hydrothermal synthesis of porous phosphorus-doped carbon nanotubes and their use in the oxygen reduction reaction and lithium-sulfur batteries, N. Carbon Mater. 3 (2016) 352e362. L.J. Yang, S.J. Jiang, Y. Zhao, L. Zhu, S. Chen, X.Z. Wang, Q. Wu, J. Ma, Y.W. Ma, Z. Hu, Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction, Angew. Chem. Int. Ed. 50 (2011) 7132e7135. Y.S. Liao, C.M. Chen, D.G. Yin, Y. Cai, R.S. He, M. Zhang, Improved Naþ/Kþ storage properties of ReSe2-carbon nanofibers based on graphene modifications, Nano-Micro Lett. 11 (2019) 22. J.T. Zhang, L. Yu, X.W. (David) Lou, Embedding CoS2 nanoparticles in N-doped carbon nanotube hollow frameworks for enhanced lithium storage properties, Nano. Res. 10 (2017) 4298e4304. W. Yang, W. Yang, L.B. Dong, X.C. Gao, G.X. Wang, G.J. Shao, Enabling immobilization and conversion of polysulfides through a nitrogen-doped carbon nanotubes/ultrathin MoS2 nanosheet core-shell architecture for lithium-sulfur batteries, J. Mater. Chem. A 7 (2019) 13103e13112. L. Chen, C.X. Xu, L.M. Yang, M.J. Zhou, B.H. He, Z.G. Chen, Z. Li, M.T. Shi, Z.H. Hou, Y.F. Kuang, Nitrogen-doped holey carbon nanotubes: dual polysulfides trapping effect towards enhanced lithium-sulfur battery performance, Appl. Surf. Sci. 454 (2018) 284e292.

[52] B. Lan, L. Yu, T. Lin, G. Cheng, M. Sun, F. Ye, Q.F. Sun, J. He, Multifunctional freestanding membrane from the self-assembly of ultralong MnO2 nanowires, ACS Appl. Mater. Interfaces 5 (2013) 7458e7464. [53] F. Pei, T.H. An, J. Zang, X.J. Zhao, X.L. Fang, M.S. Zheng, Q.F. Dong, N.F. Zheng, From hollow carbon spheres to N-doped hollow porous carbon bowls: rational design of hollow carbon host for Li-S batteries, Adv. Energy Mater. 6 (2016) 1502539. [54] Y.T. Liu, D.D. Han, L. Wang, G.R. Li, S. Liu, X.P. Gao, NiCo2O4 nanofibers as carbon-free sulfur immobilizer to fabricate sulfur-based composite with high volumetric capacity for lithium-sulfur battery, Adv. Energy Mater. 9 (2019) 1803477. [55] M.F. Chen, Q. Lu, S.X. Jiang, C. Huang, X.Y. Wang, B. Wu, K.X. Xiang, Y.T. Wu, MnO2 nanosheets grown on the internal/external surface of N-doped hollow porous carbon nanospheres as the sulfur host of advanced lithium-sulfur batteries, Chem. Eng. J. 335 (2018) 831e842. [56] T.Z. Hou, W.T. Xu, X. Chen, H.J. Peng, J.Q. Huang, Q. Zhang, Lithium bond chemistry in lithium-sulfur batteries, Angew. Chem. Int. Ed. 56 (2017) 8178e8182. [57] Y.C. Qiu, W.F. Li, W. Zhao, G.Z. Li, Y. Hou, M.N. Liu, L.S. Zhou, F.M. Ye, H.F. Li, Z.H. Wei, S.H. Yang, W.H. Duan, Y.F. Ye, J.H. Guo, Y.G. Zhang, High-rate, ultralong cycle-life lithium/sulfur batteries enabled by nitrogen-doped graphene, Nano Lett. 14 (2014) 4821e4827. [58] J.J. Chen, R.M. Yuan, J.M. Feng, Q. Zhang, J.X. Huang, G. Fu, M.S. Zheng, B. Ren, Q.F. Dong, Conductive Lewis base matrix to recover the missing link of Li2S8 during the sulfur redox cycle in Li-S battery, Chem. Mater. 27 (2015) 2048e2055. [59] Q. Pang, J.T. Tang, H. Huang, X. Liang, C. Hart, K.C. Tam, L.F. Nazar, A nitrogen and sulfur dual-doped carbon derived from polyrhodanine@cellulose for advanced lithium-sulfur batteries, Adv. Mater. 27 (2015) 6021e6028. [60] G.X. Li, J.H. Sun, W.P. Hou, S.D. Jiang, Y. Huang, J.X. Geng, Three-dimensional porous carbon composites containing high sulfur nanoparticle content for high-performance lithium-sulfur batteries, Nat. Commun. 7 (2016) 10601. [61] J.S. Lee, W. Kim, J. Jang, A. Manthiram, Sulfur-embedded activated multichannel carbon nanofiber composites for long-life, high-rate lithium-sulfur batteries, Adv. Energy Mater. 7 (2017) 1601943. [62] L.L. Lin, F. Pei, J. Peng, A. Fu, J.Q. Cui, X.L. Fang, N.F. Zheng, Fiber network composed of interconnected yolk-shell carbon nanospheres for highperformance lithium-sulfur batteries, Nano Energy 54 (2018) 50e58. [63] R.P. Fang, G.X. Li, S.Y. Zhao, L.C. Yin, K. Du, P.X. Hou, S.G. Wang, H.M. Cheng, C. Liu, F. Li, Single-wall carbon nanotube network enabled ultrahigh sulfurcontent electrodes for high-performance lithium-sulfur batteries, Nano Energy 42 (2017) 205e214. [64] Z.Y. Wang, L. Wang, S. Liu, G.R. Li, X.P. Gao, Conductive CoOOH as carbon-free sulfur immobilizer to fabricate sulfur-based composite for lithium-sulfur battery, Adv. Funct. Mater. 29 (2019) 1901051.