In-situ catalytic growth carbon nanotubes from metal organic frameworks for high performance lithium-sulfur batteries

In-situ catalytic growth carbon nanotubes from metal organic frameworks for high performance lithium-sulfur batteries

Materials Today Energy 8 (2018) 134e142 Contents lists available at ScienceDirect Materials Today Energy journal homepage:

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Materials Today Energy 8 (2018) 134e142

Contents lists available at ScienceDirect

Materials Today Energy journal homepage:

In-situ catalytic growth carbon nanotubes from metal organic frameworks for high performance lithium-sulfur batteries Jinxing Zhao a, 1, Cui Liu a, 1, Heming Deng b, Shun Tang a, Chang Liu a, Shengrui Chen a, Jinglong Guo a, Qian Lan a, Yuxiao Li a, Yan Liu a, Miao Ye a, Honghao Liu a, Jiyuan Liang a, *, Yuan-Cheng Cao a, ** a

Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, School of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, PR China State Grid Electric Power Research Institute, Wuhan, 430074, PR China


a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 February 2018 Received in revised form 2 March 2018 Accepted 23 March 2018

The highly uniformed hybrid carbon material has been prepared by in-situ growth carbon nanotubes (CNTs) from metal organic frameworks, with the cobalt species as the catalyst for CNTs growth and dicyandiamide (DICY) as a sacrificial agent for the formation of graphitic carbon. The CNTs with extraordinary conductivity supply more electron transport to the cathode and make much more sense in enhancing the rate performance of the sulfur cathode. In the matrix, cobalt nanoparticles and heteroatom nitrogen can help in immobilizing sulfur species, leading to the improvement of polysulfide shutting. Moreover, the increased specific surface area and mesoporous channel enhance the sulfur loading and facilitate electrolyte diffusion. On integrating these fascinating benefits into one electrode material, as a result, the hybrid composite ([email protected]/S) cathode presents a high initial capacity of 1316.1 mAh/g at the current rate of 0.1C. Even at high current rate of 5C, a decent capacity of 620.7 mAh/g can still be achieved. The capacity of hybrids materials can be maintained at 970 mAh/g after 500 cycles at 0.2C, and a capacity retention of 79.8%, revealing it great potential for energy storage application. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes Metal organic frameworks Rate capability Cycle stability Lithium-sulfur battery

1. Introduction Lithium ion batteries (LIBs) have the well-balanced electrochemical performances of energy density, rate capability and cycling ability. However, the recent development of mobile electronics and transportations with high-performance powered by electrochemistry power source such as electric vehicles and hybrid electric vehicles require improving performances of the energy storage systems [1]. In particular, under continuous research and development, LIBs are approaching the theoretical limitation of electrode materials [2]. Increasing the energy density is one of the prior tasks for future advanced energy storage. Elemental sulfur, as a promising candidate for cathodes, offers a high theoretical specific capacity of 1675 mAh/g, and also is naturally abundant, lowcost, and produced in quantity. Rechargeable lithium-sulfur

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Liang), [email protected] (Y.-C. Cao). 1 These authors contributed equally to this work. 2468-6069/© 2018 Elsevier Ltd. All rights reserved.

(LieS) batteries are regarded as a promising candidate for nextgeneration electrical energy storage due to their high theoretical energy density of 2600 Wh/kg [3e5]. However, plenty of challenges now hinder the commercialization applications of LieS battery. At first, low electrochemical utilization of the sulfur cathodes because of the insulating nature of sulfur and its reduction compounds with lithium, the Li2S2/Li2S mixtures is obvious [6,7]. And the most serious predicament resulting from the dissolution and migration of polysulfide generates a formidable side effect such as the low capacity, fast capacity degradation and low coulombic efficiency [8e10]. Up to now, lots of excellent efforts about the cathode materials have been devoted to improve the sulfur utilization and reduce the dissolution of polysulfide intermediates. Carbon nanomaterials, such as three dimensions reduced graphene oxide [11], porous graphene [12e14], carbon nanorods [15], CNT [16], microporous carbon [17], hierarchical porous carbon [18] and biomass derived carbon [9,19e21], have been discovered as the potential candidates for high-sulfur loading. The characteristics of large surface area and porous structure of the carbon materials, make them potential as

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the host to load the sulfur in carbon and confine the polysulfide intermediates within the carbon frameworks [22e24]. Moreover, heteroatom doping and immobilize metal catalysts in the carbon materials are the useful method to further suppress the dissolution of polysulfide intermediates due to chemisorptions between polysulfide intermediates and host materials [25e28]. Porous carbon materials derived from metal organic frameworks (MOFs) with metal ions and organic ligands have abundant nanoscale cavities and open channels for small molecules to access. To date, MOFs are widely used as the sulfur host materials for LieS batteries [29]. For example, Li [30] developed a multifunctionalized composite derived from MOFs containing a cobalt and N-doped graphitic carbon matrix, in which the entrapping of polysulfides, the catalyzing for sulfur redox and an ideal electronic conductive network were successfully achieved synchronously, leading to a significant improvement in the LieS performance. However, MOF derived porous carbon materials often suffer from a low degree graphitization. Hence, the low conductivity will cause poor rate performance of LieS battery. CNTs possess classic one dimensional structure and exhibit a self-weaving behavior to construct an interwoven conductive network for fast transfer of electrons. Meanwhile, combining one dimensional CNTs and MOFs to form structural matrix becomes well-reasoned. Apart from making full use of the merits of the MOFs, the resultant open, porous, and conductive network in the composite matrix is also believed to transfer electrons rapidly, store sulfur species, and trap soluble polysulfide well. In fact, the key issue is how to obtain an effective combination between CNTs with MOFs experimentally to ensure the significant improvement of the electrochemical performance for sulfur cathode. Recently, Mao [31] reports a strategy of using foldable interpenetrated metal-organic frameworks/CNT thin film for lithiumesulfur batteries in which the CNT interpenetrate through the metal-organic frameworks crystal and interweave the electrode into a stratified structure to provide both conductivity and structural integrity. Nevertheless, the CNTs were not in-situ formed on the MOFs. The internal junction contact resistance between MOFs and CNTs would be inevitable arouse, and thus resulting in the reduction of rate performance. Furthermore, CNTs have been prepared by chemical vapor deposition (CVD) method [32]. However, the fabrication process is cumbersome. Therefore, it is highly desirable to develop a facile method to prepare [email protected] carbon materials hybrid nanostructures for LieS battery. Herein, the ZIF-67 as polyhedron template which consists of cobalt ions as the metal center and 2-methylimidazole as the Ncontaining ligands was prepared to obtain the heteroatom doping carbon materials. And dicyandiamide (DICY) was utilized to form CNTs on the as-prepared MOFs materials via in situ catalytic method. Based on the above discussion, CNTs possess exceptional long-range conductivity with unique one dimensional structure which could overcome the drawback of powder carbon material, low conductivity. Meanwhile, abundant nanotubes within nanoscale improve the specific surface area and provided more pores for sulfur powders accommodation. This kind of special hybrid nanostructure and abundant micropore/mesopore from CNTs and MOFs are beneficial to the rate performance improvement for LieS battery.


(Shanghai) and sublimed sulfur was purchased from Kermel Company. All the chemicals were used as received without any further purification. 2.2. Preparation of [email protected] hybrid composite In a typical assay, firstly, 8 mmol of Co(NO3)2$6H2O and 32 mmol of 2-Methylimidazole were dissolved in 100 mL of methanol to form darkled and colorless clear solution, respectively. Then the solutions were mixed together to get purple solution and kept stirring for 1 h at room temperature. After that, the solution was further incubated at room temperature for 24 h. Finally, the precipitates were collected by centrifugation, washed with methanol for 2 times and dried at 60  C for 12 h resulting in the purple ZIF-67 crystals. The as-prepared ZIF-67 crystals and various amount of dicyandiamide powder (10e30%, based on the total weight) were ground together and then thermally converted to [email protected] composites though carbonization under a N2 flow at 700  C for 2 h, with a heating rate of 5  C/min. For comparison purpose, CoeNeC powder was prepared by directly carbonized the ZIF-67 in absence of dicyandiamide at 700  C in a N2 atmosphere for 2 h. 2.3. Fabrication of carbon-sulfur composites Sulfur-loaded carbon composites were prepared by a conventional melt-diffusion strategy. Typically, sublimed sulfur powder was mixed with the as-prepared carbon materials with a mass ratio of 3:1 in an agate mortar and strongly ground for 20 min. Then the mixture was transformed into a vessel and sealed under vacuum. The vessel was heated to 155  C and held for 10 h. After cooling to the room temperature, carbon-sulfur composites were obtained. 2.4. Characterization The microstructure and morphology of the samples were investigated using field-emission scanning electron microscopy (SEM, Hitachi, S-4700). Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were obtained by using JEOL FE-2011 equipped with energy-dispersive X-ray spectroscopy (EDS) element-mapping functionality. X-ray powder diffraction (XRD) patterns were conducted on a Rigaku D/ MAX2500V using Cu Ka radiation. Raman spectra data were recorded with a Renishaw spectrometer using an Ar ion laser. Nitrogen adsorption isotherms were measured at 196  C with a Micromeritics ASAP 2020 analyzer. The specific surface area of the samples was obtained using Brunauer-Emmett-Teller (BET) method and the pore size distribution was derived from the desorption branch of the N2 isotherm using the Barrett-Joyner-Halenda (BJH) model. The surface chemical composition of the samples was tested by X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II spectrometer) using MgeK X-ray source (1253.6 eV). Thermogravimetric (TG) analysis was performed with a PerkinElmer (TA Instruments) up to a target temperature in a N2 atmosphere with a heating rate of 10  C/min. The electronic conductivity of the samples was measured by a four point probe method using a Keithley 2400 source meter.

2. Experimental

2.5. Electrochemical measurements

2.1. Materials

A mixture slurry of 70 wt% of the active materials (carbon-sulfur composites), 20 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder was fabricated using N-methyl-2pyrrolidone (NMP) as a solvent. The slurries were coated on commercial Al foil substrates by doctor blade method and dried under

2-Methylimidazole, Cobalt nitrate hexahydrate (Co(NO3)2$6H2O) and Dicyandiamide (DICY) were purchased from Aladdin Chemistry. Methanol was purchased from Macklin Biochemical Company


J. Zhao et al. / Materials Today Energy 8 (2018) 134e142

vacuum at 50  C for 12 h then punched into disks like with a diameter of 13 mm. The electrolyte was a solution of 1 mol lithium bis-(trifluoromethylsulfonyl)imide (LITFSI) and 1 wt% LiNO3 in dimethoxymethane (DME) and 1,3-dioxolane (1:1, V/V). About 30 mL electrolyte was added in each cell. The separator was Celgard 2400 polypropylene membrane. 2025-type coin cells were assembled in an argon-filled glove box with lithium foil as the counter electrode. CV measurements were carried out on an Autolab electrochemical working station using a voltage range from 1.5 to 3.0 V at a scan rate of 0.2 mV/s. The galvanostatic chargedischarge was performed in a voltage window of 1.5e2.8 V using on a multichannel battery testing system (Land, CT2001A). The electrochemical impedance measurements were conducted by applying an AC voltage of 5 mV in the frequency range from 102 Hze105 Hz. All the electrochemical tests were carried out at room temperature. 3. Results and discussion The synthesis process of [email protected] is schematically depicted in Fig. 1. First, ZIF-67 with rhombic dodecahedral morphology is synthesized at room temperature. A subsequent thermal treatment process is introduced to transform ZIF-67 into CoeNeC and [email protected] The samples are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results are shown in Fig. 2. The XRD pattern of the assynthesized ZIF-67 is shown in Fig. S1. All the diffraction peaks are attributed to the ZIF-67 and without any detectable impurity, suggesting ZIF-67 is successfully synthesized. Fig. 2a shows the SEM image of ZIF-67. As can be clearly seen, ZIF-67 particles have regular dodecahedral structure with micrometer size around 250 nm. The surface of ZIF-67 particle is smooth, indicating the single-crystal-like feature [33]. After carbonization ZIF-67 in argon, highly uniform polyhedral morphology CoeNeC composites are clearly observed in Fig. 2b. Compared with ZIF-67, the size of CoeNeC is reduced and the surface becomes coarse. XRD results show that the characteristic peaks of ZIF-67 are disappeared completely after carbonization, and three apparent diffraction peaks at 44.2 , 51.6 and 75.9 appear in the CoeNeC pattern (Fig. 3), which are consistent with the standard crystalline data of cobalt (JCPDS No. 15e0806). It turns out that the cobalt ions in the frameworks are reduced to metallic Co due to the in situ formed carbon during thermal pyrolysis [30]. As shown in Fig. 2c, carbon nanotubes are in-situ formed on the CoeNeC composites when the dicyandiamide (30%) is carbonized together in argon under 700  C. Actually, pure dicyandiamide can be decomposed completely without any residue under 700  C in the nitrogen atmosphere (Fig. S2). However, carbon nanotubes only can be obtained when pyrolysis of the mixture of DICY and CoeNeC. Henceforth point, it can be concluded that, in the process

of carbonization, dicyandiamide is served as a sacrificial agent for the formation of graphitic carbon, and metal cobalt is used as catalyst to catalyze the dicyandiamide for the growth of carbon nanotubes. This phenomenon also has been observed by former researches [34,35]. In addition, it would be very interesting to find that there is a small peak at around 26 in the [email protected] pattern, which is more remarkable than CoeNeC pattern, suggesting an improved graphitic crystallinity owing to the in-situ formed carbon nanotubes during carbonization. The [email protected] sample is further characterized by TEM. As shown in Fig. 2d, Co particles are implanted in the carbon matrices. It can be seen that the size distribution of Co nanoparticles is not regular. The larger Co nanoparticles, 50e80 nm, may be caused by the agglomeration of small Co nanoparticles at high temperature. Moreover, some CNTs around 5e10 nm on the edge of polygon carbon frameworks also could be observed. To observe CNT clearly, the zoomed-in TEM of [email protected] is shown in Fig. 2e. CNT with the length of 100 nm and diameter of 8 nm marked with red arrow can be seen. Catalyst, Co nanoparticle with diameter of 15 nm, also can be observed at the top of the CNTs. The high-resolution TEM (HRTEM) image in Fig. 2f shows that the nanoparticle with distinct lattice fringe of 0.205 nm could be accounted for the (111) lattice plane of metallic Co, and the Co nanoparticle is surrounded by a few layered graphitic carbon shells. It can be inferred that the CNT is multiwalled structure. Furthermore, the spacing between the graphitic layers is of 0.36 nm, which is slightly larger than that of graphite (0.34 nm), can be explained by the doping of nitrogen atoms in the graphitic matrix [36]. The graphitic layers are not continuous, suggesting that existence many structural defects in CNTs. Based on the above observation, the structure of CNTs grafted on the CoeNeC framework makes the carbon materials more conducive, and hence suitable for energy conversion applications. The amount of dicyandiamide has great influence on the morphology of the final product, [email protected] CoeNeC was pyrolyzed with different amount of dicyandiamide to investigate the morphology change. It can be seen that there were no obvious CNTs grew on the composites when 10% and 20% dicyandiamide were used (Fig. S3). Thus, in order to obtain excellent structure of [email protected], the amount of dicyandiamide is fixed at 30% in this work. For the synthesis of the [email protected]/S composite, the sulfur powder is melt and diffused into the sample at 155  C due to capillary force. After loading the sulfur, the [email protected]/S pattern shows the composite peaks which is in good agreement with the S and [email protected] pattern in Fig. 3, indicating that sulfur in the [email protected]/S is mainly crystalline. To evaluate the distribution of sulfur in the composite, the HAADF-STEM image and the corresponding elements distribution maps are present in Fig. 2g. It is noted that the [email protected]/S sample is composited by C, N, Co and S without any elements. N element is from the decomposition

Fig. 1. Schematic illustration for the synthesis of [email protected]

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Fig. 2. SEM images of (a) ZIF-67, (b) CoeNeC and (c) [email protected] carbonization at 700  C with 30% DICY. (dee) TEM images of [email protected] CoeNeC. (f) HRTEM image of [email protected] (g) The HAADF-STEM image and the corresponding EDX elemental maps of [email protected]/S.

Fig. 3. X-ray diffraction patterns of sulfur powder, CoeNeC, [email protected] and [email protected]/S.

of 2-methylimidazole and dicyandiamide. The results show a homogenous distribution of sulfur within the carbon skeleton, indicating sulfur successfully infused into the porous structure of carbon during the heating process. To examine the surface chemical composition of CoeNeC and [email protected], X-ray photoelectron spectroscopy (XPS) is conducted. As depicted in Fig. 4a and b, the survey spectra reveal that the both CoeNeC and [email protected] samples contain C, N, Co and O elements, and excluding any other impurities. The relative atomic concentration of chemical elements of [email protected] composite are C1s (87.87), Co2p (1.87), N1s (6.21) and O1s (4.05), as listed in Table S1. Compared with CoeNeC composite, the [email protected] composite possessed more carbon content, which attributed from the carbon nanotubes formation. High resolution spectra for the N1s region of CoeNeC and [email protected] are shown in Fig. S3a and Fig. 4c, respectively. In particular, from peak deconvolution, three characteristic peaks are fitted at 398.8, 400.8, and 402.1 eV, corresponding to pyridinic, pyrrolic and graphitic nitrogen, respectively [37]. The various kind N contents in the [email protected] are


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Fig. 4. XPS survey spectra of (a) CoeNeC (b) [email protected] CoeNeC. (c) N1s and (d) Co2p spectra of the as-prepared [email protected] composite.

determined to be 49.45% pyridinic, 25.28% pyrrolic and 25.27% graphitic, respectively. Obviously, the main N species in the [email protected] composite annealed at 700  C is pyridinic N, which is considered as effective N species for modifying the electron distribution and improving the affinity of insulative Li2S [38]. Furthermore, compared with the CoeNeC, the relative content of graphitic nitrogen is increased. It is undoubted that [email protected] would show higher conductivity, thus improve the rate capability of lithium-sulfur battery. Meanwhile, the Co 2p spectra can be deconvoluted into three peaks (Fig. 4d and Fig. S3b), which correspond to, respectively, metallic Co at 778.5 eV, Co 2p3/2 at 780.3 eV and Co 2p1/2 at 795.9 eV [30,39]. According to the literature [40e42], Co nanoparticles and nitrogen species in the sample not only can strongly bind with lithium polysulfides and restrict their diffusion but also can improve the redox reactions kinetic of polysulfides. To characterize the specific surface area and porous structure of [email protected] and CoeNeC samples, N2 adsorption and desorption isotherm measurements are employed and the corresponding results are depicted in Fig. 5. Evidently, both samples demonstrate a combined type I/IV sorption isotherms: a steep increase at low relative pressure and a small slope at the intermediate pressure with a distinctive hysteresis loop, implying the co-presence of both micropores and mesopores [43]. Compared with the CoeNeC, the hysteresis loop is larger, indicating much more mesopores existed in the composites. The specific BrunauereEmmetteTeller (BET)

surface areas of [email protected] and CoeNeC samples are calculated to be 496.3 and 350.1 m2/g, respectively. The increased specific surface area of [email protected] mainly comes from the contribution of CNTs. Fig. 5b demonstrates the corresponding pore-size distribution curves. The adsorption average pore widths of [email protected] and CoeNeC samples are 2.97 nm and 2.39 nm, respectively. Although their show the similar pore size, the pore volume of [email protected] is up to 0.38 cm3/g which is twice as high as that of CoeNeC (0.19 cm3/g). The enlarged pore volume of [email protected] favors high sulfur content accommodation. Hence, it can be confirmed that CNTs grew on the CoeNeC composite could improve the specific area and the mesoporous structure, and thus could supply more pores to load sulfur. The large surface area is also suit for entrapping the intermediate polysulfides. One thus expects excellent performance for the [email protected]/S. Raman spectroscopy is performed to identify the introduction of CNTs grafted on the CoeNeC surface. Fig. 6 shows the Raman spectra of [email protected] and CoeNeC. Two distinct peaks, D-band at 1355 cm1 and G-band at 1598 cm1, are obtained. The G-band corresponds to graphitization degree of carbon materials, but the D-band is generally associated with the disordered degree of carbon materials. Moreover, the integrated area ratio of D-band to Gband reflects the degree of graphitization of the carbon materials [44]. The integrated area ratio (ID/IG) of D band to G band of [email protected] sample is 1.68, lower to that (ID/IG ¼ 2.09) of CoeNeC sample, indicating that [email protected] sample possesses

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Fig. 5. Isotherms (a) and pore distributions (b) of CoeNeC and [email protected] samples.

Fig. 6. Raman spectra of [email protected] and CoeNeC composites.

higher conductivity owing to the introduction of CNTs. Besides, it is noted that the 2D band of [email protected] CoeNeC sample at 2700 cm1 is more obvious than CoeNeC sample [44]. In addition, the electrical conductivities of [email protected] and CoeNeC samples are calculated to be 4.5 S/cm and 1.99 S/cm, respectively, as listed in Table S2. It can be concluded that CNTs growth and nitrogen doping would be beneficent to improve the conductivity of the composites. With the grafted CNTs on carbon skeleton, nitrogen doping and metallic Co in the carbon matrix, the [email protected] special structures were expected to possess high electron-conductivity, robust structural stability, and high-efficiency electron/ion transport pathway, thus rendering them potential candidates for lithiumsulfur application. To further explore the benefits of rational design of [email protected] framework, we investigated the electrochemical behavior of [email protected]/S and CoeNeC/S electrodes. All the specific capacities calculated in this work are according to the weight of sulfur. According to the TG results in Fig. S2, the sulfur contents of the [email protected]/S and CoeNeC/S are 71.4% and 63.1%, respectively. The higher S content in [email protected] can be contributed to its high specific surface area, and more S can melt in the tube of CNTs. The sulfur loading of the electrode slice is about 2 mg/cm2. Cyclic voltammograms (CV) tests of the [email protected]/S cathodes were first measured at a scan rate of 0.2 mV/s (Fig. 7a). In the first cathodic scan process, two main peaks at 2.31 and 2.04 V are clearly seen. The peak at around 2.31 V is attributed to the typical multistep reduction process of sulfur from solid S8 to the

soluble long-chain polysulfides Li2Sn (4  n  8) [45]. The second peak at 2.04 V corresponds to the further reduction of polysulfides to insoluble short-chain Li2S2/Li2S [46]. Correspondingly, the two adjacent peaks in the subsequent anodic scan around 2.37 V and 2.39 V are derived from the converse oxidation process. The anodic peaks are slightly increased in the next two cycles, suffering polarization of the electrode materials. This change is due to the increased viscosity of the electrolyte induced by the dissolution of polysulfides during the first cycle [47]. The CV curves present no remarkable changes after the first cycle, indicating the excellent electrochemical stability of the [email protected]/S electrode. The assembled LieS batteries are also evaluated by the galvanostatic discharge-charge. Fig. 7b shows the galvanostatic chargedischarge profiles of [email protected]/S at various current rates (0.1, 0.2, 0.5, 1, 2 and 5 C). In detail, at low current rates, it can be noted that there are two plateaus (about 2.31 and 2.10 V) in the discharge curves, while one plateau platform (2.37 V) in the charge profiles. These voltage plateaus are in accordance to the potential of characteristic peaks in the CV curves. Moreover, the plateau at 2.1 V is flat, indicating a uniform deposition of Li2S and Li2S2. Compared with the CoeNeC/S cathode (Fig. S6), the specific discharge capacitates of [email protected]/S are larger at the same current rates, indicating much more capacities are generated. This may be attributed to the significantly larger specific surface area of [email protected] electrode, for loading larger amounts of sulfur powders. For further study, the rate capabilities of the electrodes at different current rates are compared in Fig. 8. At the rate of 0.1C, [email protected]/S electrode could deliver a high specific discharge capacity of 1316.1 mAh/g (1.0C ¼ 1675 mAh/g), suggesting an effective utilization of sulfur (79%). Moreover, the capacity is slightly faded in the subsequent cycles and can be stabilized at 1303.2 mAh/g after five cycles, and still 131.6 mAh/g higher than that of CoeNeC/S cathode. In the second cycle, the capacity fading is ascribed to the unavoidable dissolution of polysulfide into the electrolyte [48]. With the increase of current rate to 2 C and 5 C, a specific capacity of [email protected]/S is maintained at 748.8 and 674.4 mAh/g, respectively. When back to the rate of 0.1C, the specific capacity of [email protected]/S cathode still is recovered to 1160.4 mAh/g, which is 147.2 mAh/g more than the CoeNeC/S cathode. The specific capacity retention achieved by the [email protected]/S is 51.2% at 5 C taking that at 0.1 C as the basis, demonstrating excellent lithium storage reversibility. Unfortunately, it can be seen that the specific capacity of the CoeNeC/S cathode faded to 515.2 mAh/g as soon as the current rate increased to 5C, and the specific capacity retention is only 38.7%, indicative of poor rate performance as compared with the [email protected]/S cathode. The improved rate capability of the [email protected]/S


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Fig. 7. (a) CV curves of [email protected]/S electrode at the scan rate of 0.2 mV/s (b) the galvanostatic charge-discharge profiles of [email protected]/S electrode tested under different current densities.

Fig. 8. The rate capability of the CoeNeC/S and [email protected]/S composite electrodes from 0.1C to 5C.

cathode may be caused by the following reasons: (1) in-situ grown CNTs increase the electrical conductivity of the carbon-sulfur composite and decrease the charge transport distances; (2) the increased mesopores shorten the penetration route of electrolyte into the porous structure. The products of CoeNeC carbonized with 10 and 20 wt% DICY are also loaded with sulfur and tested their electrochemical performances. In Fig. S7, the specific discharge capacitates are about 100 mAh/g lower than that of the CoeNeC/S (using 30 wt% DICY) at the same current rates. This may be due to that the decomposition product of DICY is not sufficient to the formation of CNTs. When the amount of DICY is increased to 30%, much more CNTs can be grown from the surface of MOF. And the obtained hybrid materials have high specific surface area and conductivity. As a result, CoeNeC/S (using 30 wt% DICY) cathodes can exhibit excellent electrochemical performances. The cycling performance of the [email protected]/S cathode electrode is shown in Fig. 9. As for the CoeNeC/S composite electrode, after activation at 0.1C for two cycles, it could deliver an initial reversible capacity of 1174.6 mAh/g, and this capacity rapidly fades to 831.4 mAh/g after 300 cycles. Then the Coulombic efficiency declines quickly and the specific capacity was reduced to 716 mAh/ g, which is only retained 60% of the initial value. This is probably attributed to the serious shuttle effect. However, For the [email protected]/S composite cathode, it delivers an initial capacity of 1214.9 mAh/g after being activated at 0.1C for two cycles. After 500 cycles at 0.2C, [email protected]/S composite electrode still remains a high capacity of 970 mAh/g. The capacity retention is calculated to

Fig. 9. Cycling performance and coulombic efficiency of the CoeNeC/S and [email protected]/S composite electrodes at 0.2C.

be 79.8%, correspondence to an ultralow capacity fade rate of 0.0403% per cycle, exhibiting good stability. Furthermore, the Coulombic efficiency is maintained above 99.5% at 0.2C within 500 cycles, demonstrating that the sulfur shuttle effect was prevented effectively. After 500 cycles, as shown in Fig. S8, the CV peak positions do not vary seriously which also indicated the excellent electrochemical stability of the [email protected]/S electrode. Based on the above results, the ameliorative performances of [email protected]/S composite cathode may be ascribed to the in situ grown CNTs are favorable for the polysulfides entrapment and further restricting them dissolution. All in all, these impressive results suggest that the carbon nanotubes and mesoporous channels facilitated ionic diffusion and electronic transport and thus overcome the electronically insulating nature of sulfur to improve the CoeNeC/S cathode rate property. Furthermore, the opened 3D structure and high surface area of [email protected]/S facilitate electron transfer and polysulfide physical adsorption. Moreover, Co ultrafine particles show a strong interaction with sulfur species to prevent its diffusion and also promise as an electrocatalyst to convert long-chain polysulfides to short-chain ones and even to Li2S efficiently during discharging [30]. In addition, doped-nitrogen in CoeNeC composites can provide one pair of lone electrons for trapping polysulfide species through the strong binding of Li in Li2Sn with nitrogen atoms which can significantly improve the sulfur utilization and alleviate the shuttling of polysulfide [30,31]. It is also worth mentioning that the electrochemical performances of the synthesized CNT/CoeNeC/S cathode are superior to some

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Acknowledgements This work was supported by the Natural Science Fund of Hubei Province (2017CFB155), the National Natural Science Foundation of China (51703081), the Scientific Research Plan Project of Hubei Education Department (B2017269) and 4th Yellow Crane Talent Project of Wuhan City. Appendix A. Supplementary data Supplementary data related to this article can be found at References

Fig. 10. The Nyquist plots of the cells with as-prepared [email protected]/S and CoeNeC/S cathodes.

previous reported carbon-sulfur composites in the literature. A more detailed comparison is compiled in Table S3. To better understand the improved electrochemical performance in both rate performance and cycling performance, the electrodes are further investigated by electrochemical impedance spectroscopy (EIS). Fig. 10 shows the Nyquist plots recorded for the two electrodes. It can be seen that the curve compose of a semi-circle at high frequency and a line at low frequency. The high frequency semi-circle and low frequency line represent the charge-transfer resistance and electrolyte ions diffusion resistance, respectively. Generally speaking, the larger the semi-circle, the higher is the charge-transfer resistance, and the steeper the line, the lower is the ions diffusion resistance. It is obvious that the [email protected]/S electrode presented a much lower chargetransfer resistance than that of CoeNeC/S electrode, indicating the [email protected] possessed higher electrical conductivity. Recall that the CNTs in situ grafted on the carbon skeleton and thus improve the electric conductivity of the carbon-sulfur cathode. In addition, it is also found that the slope of [email protected]/S at low frequency is steeper than that of CoeNeC/S electrode. This may be due to its higher specific surface area and large amount of mesopores, which facilitate the fast electrolyte ions transportation. The EIS results are in good agreement with the results of rate capability and cycling performance, further revealing that [email protected]/S is an excellent architecture for lithium-sulfur applications. 4. Conclusions In summary, a well-designed in-situ grafted CNTs from MOF composite has been prepared to host sulfur. Due to the special hybrid configuration and abundant micropore/mesopore, the [email protected] composite not only ensures sufficient space to host sulfur but also efficiently improve the conductivity of the materials. Furthermore, the co-existence of Co nanoparticles and doped nitrogen in the [email protected] composite effectively confines the soluble polysulfides by chemical interaction. Owing to the above merits, [email protected]/S electrode delivers a superior rate performance and a reversible capacity of >950 mAh/g after 500 cycles. This kind of material is beneficial to promote the development of the LieS battery. It is also expected that the [email protected] may be applicable for other reversible electrochemical energy and conversion technologies, such as supercapacitor and fuel cell.

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