High-content of sulfur uniformly embedded in

Journal of

Materials Chemistry A PAPER

Cite this: J. Mater. Chem. A, 2017, 5, 5905

High-content of sulfur uniformly embedded in mesoporous carbon: a new electrodeposition synthesis and an outstanding lithium–sulfur battery cathode† Liyuan Zhang,ab Hui Huang,a Yang Xia,a Chu Liang,a Wenkui Zhang,*a Jianmin Luo,a Yongping Gan,a Jun Zhang,a Xinyong Taoa and Hong Jin Fan*b We report a novel synthesis of a mesoporous carbon–sulfur composite (MCSC) by electrodeposition of sulfur into the mesopores via a self-limiting process. With the merits of a high sulfur content (77%), uniform distribution and strong C–S bonds achieved by this method, the as-prepared materials demonstrate excellent performance in lithium–sulfur batteries. The electrode delivers high specific

Received 11th January 2017 Accepted 23rd February 2017

capacities at various C-rates (e.g. about 1160 and 590 mA h g1 at current densities of 0.1 and 2.0 A g1, respectively) and remarkable capacity retention, remaining above 857 mA h g1 after 200 cycles at

DOI: 10.1039/c7ta00328e

a high rate of 0.5 A g1. This electrochemical strategy is industrially viable and scalable, and thus may

rsc.li/materials-a

pave a new way to push the commercialization of Li–S batteries.

Introduction High-energy-density rechargeable batteries have recently attracted great interest because of the urgent demand for energy storage in portable electronics, electric vehicles and grid-scale stationary storage systems. Among the investigated rechargeable batteries, lithium–sulfur (Li–S) batteries are particularly attractive for energy storage owing to their high theoretical energy density of 2600 W h kg1, abundance, low cost and environmental benignity.1–7 Nevertheless, the practical energy density and cycling stability of Li–S batteries are limited by the low electrical conductivity and large volume changes of the sulfur cathode. During the cycles, the higher-order lithium polysuldes (Li2Sn 4 # n # 8) are highly soluble in the electrolyte and would spread broadly through the whole cell, leading to the shuttle effect and low available capacity of active materials, which are regarded as the main reasons for the low coulombic efficiency and fast capacity fading in Li–S batteries.8,9 To overcome these problems, considerable efforts have been made to impregnate sulfur into carbon hosts10,11 or conducting polymers,12,13 which can not only suppress the dissolution of intermediates but also improve the conductivity of the electrode. Among them, mesoporous carbons (MCs)14–16 offer a

College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China. E-mail: [email protected]

b

School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental devices, current versus time curves of the electrodeposition process and XPS, TEM, SEM and electrochemical data of comparison samples. See DOI: 10.1039/c7ta00328e

This journal is © The Royal Society of Chemistry 2017

promising prospects for sulfur accommodation owing to their high specic surface area, ultrathin thickness, unique conductivity and superior structural properties. It has been demonstrated that the electrochemical properties of mesoporous carbon/sulfur composites (MCSCs) are closely related to the sulfur content, the distribution and the binding state of sulfur with carbon hosts. To date, various sulfur infusion methods have been developed including melt adsorption,17 vapor phase infusion,18,19 solvent evaporation,20,21 sulfur precipitation22 and deposition in aqueous solutions.23,24 However, these sulfur conning methods through architectural effects are greatly limited by the structure and surface chemistry properties of carbon hosts, and generally cannot ensure uniform distribution of sulfur in the composites. Residual sulfur particles would remain within the carbon framework without being well encapsulated, which would dissolve in the electrolyte and suffer from the shuttle problem.25 Moreover, sublimed sulfur, which is usually employed as the sulfur source in most methods, can hardly enter into the pores and channels of MC because of its larger crystal size. Thus, it still remains a great challenge to develop a facile and controllable sulfur loading method for the fabrication of high-performance sulfur composite cathodes. The ideal conguration of MCSCs is to have a uniform dispersion and high sulfur content, complete sulfur enclosure in a conned but accessible space, and strong sulfur-host affinity to achieve high capacity and excellent capacity retention. In our recent work, we presented a novel synthesis of a graphene–sulfur composite by electrolytic exfoliation of graphite coupled with in situ sulfur electrodeposition.6 It is found that sulfur can be deposited uniformly into graphene

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Journal of Materials Chemistry A

nanosheets. As compared to the above methods, the electrodeposition of sulfur shows exclusive advantages. As shown in Fig. 1, a sulfur electrodeposition reaction via electron/ion transfer at electrode/electrolyte interfaces can provide strong sulfur chemical bonding with carbon hosts. Furthermore, this method can allow the incorporation of sulfur into the channels and possibly even the interior of mesopores due to the good penetration of the electrolyte, thus achieving a highly uniform deposition of sulfur into the hosts. Meanwhile, excessive deposition of sulfur will be avoided based on the unique selflimiting mechanism. Because sulfur is both electronically and ionically insulating, once a thin sulfur layer covers the surface of carbon hosts, further electrodeposition reaction will be largely impeded. Owing to the superior advantages, the as-formed MCSC cathodes exhibit a remarkable electrochemical performance. We believe that this novel electrodeposition strategy for sulfur accommodation would inspire researchers to develop other advanced sulfur composite cathode materials.

Experimental Sample preparation MC samples used in this work were prepared by using an ARmesophase pitch as the carbon precursor and SBA-15 as the template.14 The pore diameter of the SBA-15 template is about 7.64 nm, with a specic surface area of 256 m2 g1 and a total pore volume of 1.21 cm3 g1. A mixture of 1.125 g pitch and 1.5 g template was heated at 300 C for 7 h and then at 900 C for 2 h in a tube furnace under an argon atmosphere. Aer naturally

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cooling to room temperature, the template was etched in 10% HF solution under magnetic stirring and the resultant material was washed with deionized water and dried in an oven. Sulfur was inltrated into MC via a facile electrodeposition method (experimental device, ESI Fig. S1†). The experiment was performed in a three-electrode system using a Pt ake (2 cm 2 cm) as the working electrode, a Pt wire as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode, respectively. The electrolyte (acidic media) was obtained by mixing 24 g sulfuric acid (98%), 1.5 g KOH, 1.0 M sulfourea and 500 mL deionized water. Sulfourea can be electro-oxidized to (SCN2H3)2 in acidic solution.26 (SCN2H3)2 is an unstable product in acidic solution and irreversibly decomposes to elemental sulfur and cyanamide.27 In addition, water reacts with cyanamide to give urea (refer to https://en.wikipedia.org/ wiki/Cyanamide). The electrolyte was based on Su's work.28 The graphite rod electrode will be exfoliated in this low pH electrolyte by applying a DC bias on the graphite electrode and the H2SO4 itself also results in strong oxidation of graphite. The electrolyte was amended in our last work because the graphite exfoliated too quickly and sulfur cannot be deposited on the surface of carbon in time.6 Thus, owing to the exfoliation reaction and oxidation of carbon materials, higher pH will be better for the structural stability of carbon materials when electrodeposited. Moreover, as shown in the last paper, the hydrogen evolution started at above 1.0 V (in the same acidic solution without sulfourea). In a typical experiment, MC powders were dispersed in the electrolyte under magnetic stirring (concentration of MC:

Fig. 1 Schematic illustration of the electrodeposition synthesis of the MCSC, in which high-content of sulfur is uniformly embedded within the mesoporous carbon host.

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4.7 mg cm3). The electrodeposition was conducted at a constant potential of 1.0 V. During the electrodeposition process, sulfourea molecules can inltrate sufficiently into the channels of MC, and are in situ converted into sulfur particles once MC comes into contact with the Pt ake current collector. Aer reaction for 5 hours, the MCSC product was collected by centrifugation, and rinsed with deionized water and dried at 60 C overnight. Finally, the MCSC product was treated at 200 C for 30 min to remove residual sulfur, which was deposited on the surface. Three MCSC samples were prepared at a potential of 0.6 (MCSC-0.6), 1.0 (MCSC-1.0) and 1.4 V (MCSC-1.4) vs. the SCE, respectively. For comparison, pure sulfur is also prepared by a similar electrodeposition method in the same electrolyte solution without the addition of MC powders. MC-pure sulfur (MC-S) composites were prepared by a heat-diffusion method at 155 C for 6 hours, and then maintained at 200 C in a vacuum for 30 min to remove the sulfur on the surface. The ratio of pure sulfur and MCs was 4 : 1. Materials characterization The morphology and structure of the samples were characterized by X-ray diffraction (XRD, X'Pert Pro diffractometer with Cu K radiation, l ¼ 0.15418 nm), scanning electron microscopy (SEM, Hitachi S 4700), and transmission electron microscopy (TEM, FEI, Tecnai G2 F30) equipped with an energy dispersive spectroscopy (EDS) detector. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 system with a monochromatic Al-Ka (1486.6 eV) X-ray source. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA, Q500, TA Instrument Corporation) in a ow of argon with a heating rate of 10 C min1 from room temperature to 600 C. Nitrogen adsorption– desorption isotherms were determined by the Brunauer– Emmett–Teller (BET) test using an ASAP 2020 (Micromeritics Instruments) surface area and pore analyzer. Electrochemical measurements Electrochemical performances of pure sulfur, MC-S and MCSC samples were evaluated by using a CR2025-type coin cell. The electrodes were comprised of an 80 wt% active material, 10 wt% Super-P and a 10 wt% polyvinylidene uoride (PVDF) binder with N-methyl-2-pyrrolidinone (NMP) as the dispersant. The slurry was pasted on an aluminum foil, and dried at 60 C for 12 h to remove the residual solvent. The loading mass of the active material in the electrode sheet is about 0.8 mg cm2. The amount of electrolyte used in the cells is about 10–20 mL cm2. The coin cells were assembled in an argon-lled glove box, using a lithium foil as the counter electrode and a Celgard membrane as the separator. The electrolyte solution was 1 M LiN(CF3SO2)2 (LiTFSI; 99.95% trace metals basis) and 1 wt% lithium nitrate (LiNO3) dissolved in a mixture of 1,3-dioxolane (DOL) and dimethoxymethane (DME) (1 : 1 by volume). Cyclic voltammetry (CV) measurements were performed on a CHI650B electrochemical workstation (Shanghai Chenhua, China) at a scan rate of 0.1 mV s1. Galvanostatic charge/discharge experiments were conducted on a Neware battery test system



at various current densities from 0.1 to 2 A g1 in the potential range of 1.9–2.6 V. All the electrochemical performance measurements were obtained at ambient temperature.

Results and discussion The linear sweep voltammetry curves of the graphite electrode in the electrolytes with or without the addition of sulfourea are shown in Fig. 2a. During scanning in the electrolyte containing sulfourea, two oxidation peaks are observed at around 0.5 and 1.4 V, which correspond to the electrochemical oxidation of sulfourea into sulfur. The detailed mechanism is not understood yet and could be due to the complex electrode reaction processes. Fig. 2b presents the XRD patterns of the assynthesized MCSC samples. As a comparison, pure sulfur, MC and MC-pure sulfur (MC-S) samples prepared by the heatdiffusion method are also measured. A broad diffraction peak of MC at around 25 reveals the amorphous nature of the carbon. The sharp and strong diffraction peaks of pure sulfur match well with the standard card of orthorhombic phase sulfur (JCPDS no. 74-1465), denoting the characteristics of a crystalline state. With regard to the three MCSC and MC-S samples, sharp crystalline sulfur diffraction peaks can be detected. No other peaks can be observed, indicating that all the samples do not contain any crystalline impurity. TGA was performed to determine the sulfur content in the samples. As determined from the TGA curves presented in Fig. 2c, the sulfur content of the MCSC-1.4, MCSC-1.0, MCSC0.6 and MC-S samples is estimated to be about 80, 77, 72 and 75 wt%, respectively. The loading amount is higher than most of the reported mesoporous carbon–sulfur composites obtained by other methods,29 indicating that a high sulfur content can be achieved by this facile electrodeposition method. The BET data have been added as shown in Fig. S2.† The BET surface area is about 766 m2 g1. The pore volume of MC is 1.532 cm3 g1. According to the volume density of sulfur, the theoretical sulfur loading in the pore of MC is 3.62 g sulfur per gram of MC (the sulfur content can reach about 78%). In addition, the selflimiting mechanism can be explained by the current change of MCSC-1.0 with time during the electrodeposition process, as shown in Fig. S3.† The Pt ake collector is cleaned every 1000 seconds, ensuring that the current is not inuenced by the deposition of sulfur on the Pt ake. In the rst 3000 seconds (about 1 h), the current stabilizes at about 0.35 A. From 3000– 11 000 seconds (about 3 h), the current is found to decrease with the deposition time because of the excessive formation of insulating sulfur on the surface of MC. Aer 11 000 seconds, the current becomes very low, indicating that the deposition of sulfur is almost self-terminated. So, in this study, the deposition time of 5 h is considered enough for the synthesis of MCSC samples. In order to demonstrate the interaction of sulfur and mesoporous carbon, XPS is employed to characterize the chemical state of sulfur in the composites. Fig. 3a presents the full survey scans of the samples. The XPS spectra of MCSC and MC-S composites show two obvious characteristic peaks of carbon at 285 eV (C 1s) and of sulfur at 164 eV (S 2p), indicating the J. Mater. Chem. A, 2017, 5, 5905–5911 | 5907


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Fig. 2 (a) Linear sweep voltammetry curves of the graphite electrode in the electrolytes with (solid line) and without the addition of sulfourea (dashed line) at a scan rate of 20 mV s1. (b) XRD patterns and (c) TGA curves of pure sulfur, MC, MC-S and MCSC samples with three different sulfur contents.

presence of sulfur in the composites.30,31 Fig. 3c and d and S4a and b† show the C 1s and S 2p XPS spectra of the MCSC samples. The C 1s spectra display a characteristic peak of the sp2 carbon (C]C) at 284.6 eV. Other peaks centered at 285.3, 285.7, 287.3 and 289.3 eV can be assigned to the C–H bond, C–S bond, and carbonyl (C]O) and carboxyl (HO–C]O) groups, respectively.30,32–34 In the S 2p spectra, various sulfur bonds can be observed in Fig. 3d and S4e and f.† The peaks at 164.0 and 165.2 eV are assigned to S–S species.30,31 The peaks at 163.7 and 164.9 eV correspond to C–S species, indicating that sulfur chemically bonds to MC.35 In addition, a weak broad peak in the range of 167–170 eV is attributed to the sulfate species from the remaining precursor during electrodeposition reactions or from the oxidized sulfur in air.34 Compared with the C 1s and S 2p spectra of the MC-S composite in Fig. S4c and g,† the MCSC-1.0 and MCSC-0.6 samples show stronger peaks of C–S binding, suggesting that the electrodeposition method can provide strong chemical interactions between sulfur and the carbon host. This interaction may have the adsorbing ability to anchor S atoms and prevent the polysuldes from dissolving in electrolyte during cycling. In addition, sulfate species and carboxyl

Fig. 3 (a) XPS full survey scans of samples. (b) Mass percentage of surface atoms. (c and d) XPS C 1s, and S 2p spectra of MCSC-1.0. XPS data for the other samples are provided in the ESI.†

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groups are detected in the MCSC-1.4 sample. The oxygen species may originate from the electrolysis of water occurring over 1.2 V vs. the SCE. Fig. 3b shows the surface C/S ratio of samples calculated from the XPS results. It is found that the surface content of sulfur in the samples is much lower than the value determined by the TGA results. This indicates that sulfur was uniformly deposited into inner pores of MC. Fig. 4 and S5† show TEM and SEM images of the as-formed materials, exhibiting the ordered structure of MCs. The pore depth estimated from Fig. 4c and d is about 0.7–1 mm. The dark ripples of MC indicate a typical characteristic of the graphite layer structure, and thus favor effective immobilization of sulfur.36 Fig. 4e further reveals that MC consisted of nanotube bundles. The diameter of the channels is about 8 nm, which corresponds to the pore diameter of the SBA-15 template. The nanosheets of MC are perpendicular to the long axis of pores. Moreover, Fig. 4f clearly displays the porous structure characteristics. Fig. 4a and b shows an interesting cocoon-like morphology with connected rods. To verify the dispersion of sulfur in the MCSC and MC-S composite, we carried out EDS mapping/imaging tests. As shown in Fig. 4g and h and S5,† there is no distinguishable composition difference among the MC samples. The elemental concentration mappings of C and S of the area outlined by the red frame as shown in Fig. 4g and S5† further corroborate that sulfur is mainly intercalated into the graphite interlayers of MC, with no signicant fraction on the external surface. However, sulfur is not well distributed in the framework of the MC-S composite prepared by the heatdiffusion method. Meanwhile, some bulk S aggregations can be found in the composite from the TEM image shown in Fig. 4h. The initial lithiation/delithiation behaviors of the MCSC, MC-S and pure sulfur cathodes are characterized by CV at a scan rate of 0.1 mV s1. As shown in Fig. 5a and S6,† two pairs of welldened redox peaks are clearly observed in the CV curves, revealing the multiple reaction mechanism between sulfur and lithium. The rst reduction peak at 2.3 V is assigned to a fast kinetic process, involving the open ring reduction of cyclic S8 to soluble lithium polysulde (Li2Sn) (4 # n # 8), while the second reduction peak at 2.0 V is ascribed to the decomposition of the polysulde chain in lithium polysulde to produce insoluble


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Electrochemical properties of the MCSC-1.0 cathode (some data for the other control samples are provided in the ESI†) (a) CV curves of the MCSC-1.0 cathode in the potential range of 1.9–2.6 V at 0.1 mV s1. (b) Rate performance of MCSC, MC-S and pure sulfur cathodes. (c) Long cycling performance of MCSC, MC-S and pure sulfur cathodes at 0.5 A g1 in the potential range of 1.9–2.6 V.

Fig. 5

(a–f) SEM and TEM images of MC show the regular porous structure. TEM images of (g) MCSC-1.0 and (h) the MC-S composite with EDS carbon and sulfur mapping of the indicated region. The sulfur is uniformly embedded in the MC by electrodeposition, but nonuniform in the MC-S sample obtained by a conventional heat diffusion method.

Fig. 4

lithium sulde (Li2S2 and Li2S). When the potential scan is reversed, the MCSC samples exhibit two well-separated oxidation peaks at around 2.3 and 2.4 V, respectively.9,37–40 However, the MC-S and pure sulfur electrodes show a broad oxidation peak centered at 2.4 V along with a shoulder peak. The separation of the oxidation peak on sulfur electrodes has also been observed previously in highly conductive hosts. The CV result implies that the conversion of lithium sulde and polysulde into elemental sulfur proceeds more easily on the MCSC samples relative to the MC-S and pure sulfur cathodes. The improved kinetics are attributed to the ideal conguration of carbon–sulfur composites derived from the electrodeposition reactions, which minimizes the barrier of electron transfer and


Li-ion migration. As compared to the traditional thermal method, electrodeposition can achieve highly uniform deposition of sulfur into the carbon hosts because sulfourea molecules in the electrolyte can sufficiently penetrate into the pores or channels of mesoporous carbon and possibly even the layers of nanosheets. Moreover, this method via electron/ion transfer at electrode/electrolyte interfaces can ensure intimate contact of sulfur particles with the nanosheets, which ensures high conductivity and good electrochemical reaction kinetics. As summarized by Wan et al.,9 long-chain lithium polysuldes (Li2Sn) (4 # n # 8) can easily react with or dissolve into the organic solvent of the electrolyte, leading to an irreversible capacity fading of the cathode. Moreover, Manthiram et al.40 reported an applicable way in conning the predominant electrochemical reactions between insoluble Li2S and Li2S4 through a simple control of the charge–discharge potential range, leading to long cycle life (over 500 cycles) with an excellent capacity retention (>99%). Li2S4 has a much lower solubility than other long-chain polysulde intermediates, which can effectively reduce the severe dissolution behavior of active materials.41 Therefore, we set a potential range of 1.9–2.35 V to prevent further oxidation of Li2S4 to Li2S6 or Li2S8 species. As shown in Fig. S7a,† only a pair of redox peaks appears in the CV curves, which is obviously different from Fig. 5a. From the 2nd to the 10th cycle, the shape and position of the cathodic and anodic peaks remain almost constant, showing excellent electrochemical reaction reversibility and stability between Li2S and Li2S4. The initial charge/discharge curves of the MCSC, MC-S and pure sulfur cathodes at 0.1 A g1 are presented in Fig. S8.† The discharge voltage prole of each material is indicative of a typical Li–S battery with two voltage plateaus, which are in good agreement with the CV results revealing a two-step reaction of sulfur with lithium. By comparison, the MCSC cathodes show a much higher specic capacity than the MC-S and pure sulfur cathodes. The discharge capacity at the

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rst cycle of the MCSC-1.4, MCSC-1.0 and MCSC-0.6 cathodes is about 634.6, 1105.2 and 1017.7 mA h g1, respectively, while, the rst discharge capacity of the MC-S and pure sulfur cathodes is only 551.3 and 529.6 mA h g1. The results show that MCSC-1.0 and MCSC-0.6 can effectively protect sulfur from dissolution because of their strong C–S bonds. Fig. 5b exhibits the rate capability of MCSC, MC-S and pure sulfur cathodes at different current densities from 0.1 to 2 A g1. Among the evaluated cathodes, the MCSC-1.0 sample shows the best rate performance. The specic capacity is about 1160, 1070, 860, 750 and 590 mA h g1 at current densities of 0.1, 0.2, 0.5, 1.0 and 2.0 A g1, respectively. When the current density returns back to 0.1 A g1, the capacity can be recovered to its original value, suggesting the reliability and stability of the MCSC interfacial structure as constructed by the electrodeposition method. Fig. 5c shows the cycling performance of MCSC, MC-S and pure sulfur electrodes at 0.5 A g1 in the potential range of 1.9–2.6 V. The MCSC-1.0 cathode also exhibits the best cycling stability with enhanced capacity retention. The discharge capacity of the 11th cycle is 851 mA h g1. Aer 200 cycles, the capacity almost remains unchanged. In contrast, the discharge capacity of the MC-S cathode monotonically drops from 452 to

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232 mA h g1 aer 200 cycles. The enhanced cyclability of the electrodeposited composites can be attributed to good chemical interaction between MC and S. In addition, the long-term cycling performance of the MCSC-1.0 cathode at 0.5 A g1 in the potential range of 1.9–2.35 V is presented in Fig. S7b.† As compared to Fig. 5c, about 70% of the capacity is maintained when the potential upper limit is changed from 2.6 to 2.35 V, however, the cycling stability has been improved remarkably.41 Table S1† shows the comparison of typical carbon/sulfur materials obtained from the reported methods. Compared with other methods, such as the thermal diffusion method, the electrodeposition method can embed sulfur into small pores and save time and energy. Fig. 6 shows the SEM images of the samples before and aer 200 cycles at 0.5 A g1. As displayed by SEM images in Fig. 6a, the MCSC-1.0 sample shows a good rod-like morphology with no obvious bulk sulfur aggregation, indicating that sulfur has been fully trapped on the surface of MC by electrodeposition reactions. Aer 200 cycles, the MCSC-1.0 cathode still displays a homogeneous particle morphology and the rod-like structure is well preserved (Fig. 6b). In contrast, large agglomerates of sulfur are formed on the surface of the MC-S sample by the heatdiffusion method (Fig. 6c). Aer 200 cycles, the sulfur agglomeration of the MC-S composite becomes severe (Fig. 6d). The SEM results further show the advantage of the electrodeposition method, increasing the chemical interaction during repeated cycling, which accordingly enhances the cycling stability and rate capability.

Conclusions In summary, we have successfully synthesized mesoporous carbon sulfur composites with uniform distribution and a high content of sulfur via a simple electrodeposition method. The asprepared MCSC cathodes deliver much higher initial specic capacity and more stable capacity retention compared with their counterparts prepared by the heat-diffusion method. The outstanding electrochemical properties of the MCSC cathode with a sulfur content of 77% can be exclusively attributed to the merits of the highly uniform distribution of sulfur and strong chemical binding between the MC and sulfur layer achieved by this method. This self-limiting deposition technique can be applied to many other mesoporous carbons widely reported in recent literature. Also, such a methodology is industrially viable and can be practically employed for commercial application of Li–S batteries.

Acknowledgements

SEM images of the samples. (a) and (b): MCSC-1.0, before and after 200 cycles at 0.5 A g1; (c) and (d): the MC-S composite, before and after 200 cycles at 0.5 A g1. Fig. 6

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The authors thank nancial support from the National Nature Science Foundation of China (Grant no. 21403196, 51572240 and 51677170), Natural Science Foundation of Zhejiang Province (Grant no. LY13E020002, LY15B030003, LY16E070004 and LY17E020010), Science and Technology Department of Zhejiang Province (Grant no.2016C33009, 2016C31012) and Ford University Research Program.


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