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A MnO2/Graphene-Oxide/Multi-walled Carbon NanotubesSulfur Composite with Dual-efficient Polysulfide Adsorption for Improving Lithium-Sulfur Batteries Yong Li, Daixin Ye, Wen Liu, Bin Shi, Rui Guo, Hongbin Zhao, Haijuan Pei, Jiaqiang Xu, and Jing Ying Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04270 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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ACS Applied Materials & Interfaces

A MnO2/Graphene-Oxide/Multi-walled Carbon Nanotubes-Sulfur Composite with Dual-efficient Polysulfide Adsorption for Improving Lithium-Sulfur Batteries Yong Li†‡, Daixin Ye†§, Wen Liu‡, Bin Shi‡, Rui Guo‡, Hongbin Zhao∥, Haijuan Pei‡, Jiaqiang Xu∥*, Jingying Xie‡*

‡

State Key Laboratory of Space Power Technology, Shanghai Institute of Space Power Sources, shanghai, 200245, China ∥Department of Chemistry, Institute of Sciences, Shanghai University, shanghai, 200444, China § Department of Chemistry and Molecular Biology, University of Gothenburg, S-41296, Gothenburg, Sweden * Address correspondence to [email protected] or [email protected]

ABSTRACT: Lithium-sulfur battery is a kind of potentially chemical power source because of its high energy density. However, the sulfur cathode has several shortcomings of fast capacity attenuation, poor electrochemical activity and low coulomb efficiency. Herein, muti-walled carbon nanotubes (CNTs), graphene oxide (GO) and manganese dioxide are introduced to sulfur cathode. MnO2/GO/CNTs-S with a unique three-dimension (3D) architecture is synthesized by one-pot chemical method and heat-treatment approach. In this structure, the innermost CNTs work as a conducting additive and further a backbone to form conducting network. The MnO2/GO nanosheets anchored on the sidewalls of CNTs act as dual-efficient absorption capability for polysulfide intermediates, as well as affording adequate space for loading of sulfur. The outmost nanosized sulfur particles well distributed on the surface of the MnO2/GO nanosheets provide short transmission path for Li+ and electron. The sulfur content in the MnO2/GO/CNTs-S composite is as high as 80 wt%, and the as-designed MnO2/GO/CNTs-S cathode displays excellent comprehensive performance. The initial specific capacity is up to 1500, 1300, 1150, 1048 and 960 mAh g-1 at the discharging rates of 0.05 C, 0.1 C, 0.2 C, 0.5 C and 1 C, respectively. Moreover, the composite cathode shows a good cycle performance: the specific capacity remains 963.5 mAh g-1 at 0.2 C after 100 cycles when the area density of sulfur is 2.8 mg cm-2. 1

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Keywords: polysulfide adsorption, MnO2, graphene-oxide, carbon nanotube, lithium sulfur Lithium-sulfur (Li-S) battery is the representative of next generation high-energy- density rechargeable batteries. By calculating based on the redox reaction of S and lithium sulfide (Li2S), the specific capacity and energy density of Li-S battery reach up to 1675 mAhg-1 and 2600 Wh Kg-1 respectively, which are higher than the commercial Li-ion batteries. In addition, the sulfur cathode has many merits of abundant resources, cheap and non-pollution. 1-4 However, the Li-S batteries still exist some defects in liquid organic electrolytes, which hinder the practical scale-up application. Up to now, the main challenges mainly focused on three aspects: the poor electrical conductivity of sulfur, Li2S and Li2S2 species, which results in poor performance of capacity and rate capability; 5-7 a large volumetric expansion during charging and discharging process, leading to damage on the structure of the electrode;8 shuttle reaction caused by dissolution of polysulfide into the liquid electrolyte, which give rise to low coulombic efficiency. 9-11 To solve the above problems of cathode in the Li-S battery, considerable efforts have been made to reduce the resistance of sulfur-based cathode and inhibit the dissolution and shuttling of polysulfide, which focus on optimization of the electrolyte,12-15 synthesis of sulfur-conductive polymer hybrids and preparation of sulfur- carbon composites by using porous carbon material as the matrixes.16-19 It is worth to mention that the carbon-based materials, like carbon nanotubes,20 carbon nanofibers,21 carbon spheres,22 mirco/mesoporous carbons7,23 and graphene sheets,24-27 have been proved to be very effective to optimize the properties of sulfur by way of increasing the electrical conductivity and providing space for alleviating volume changes. However, such carbon-sulfur material architectures can only partially retain polysulfides just by physical adsorption because the chemical interactions between the nonpolar carbons and the polar polysulfides are rather weak.28,29 To further decrease the capacity fade rate, carbon interlayers30-33 and surface modifications of the 2


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separator34 have been employed by creating a “polysulfide-trapping interface,” which makes polysulfide-transport

difficult

and

facilitates

cathode

active-material

stabilization.

Furthermore, some reports have shown that heterogenous atoms-doped carbon-based materials have strong chemisorption for polysulfides and improve the cycle stability of Li-S battery.35-39 For example, the N-doped nanocarbon is liable to form electron-modified interface, which adsorbs polar polysulfides and ameliorates the deposition and recharging of Li2S.40 In addition, the researches on electrolyte (additive or new solvent)41,42 and new structure of electrode43,44 to inhibit the migration of polysulfide have also been carried out and achieved good results. For example, Wang.D.H’s group has developed a novel sandwich-structured cathode by using two flexible porous carbon membranes, which has been proved to be effectively to capture the polysulfide.45 Recently, CoS2 and metallic oxides such as TiO2, Ti4O7, Mn2O3, ITO, MnO2 and MgO have been introduced into the sulfur electrode based on their strong chemisorption for lithium polysulfide, improving the cycling and coulombic efficiency of Li-S batteries.28,

46-54

But

compared with graphitic carbons, the metallic oxides/sulfides have much lower electrical conductivity, which may lead to poor rate performance and low specific capacity. Therefore, preparation of composite material possessing good electrical conductivity and remarkable absorption of polysulfides would be an optimized way to boost the comprehensive performance of Li-S battery elctrode. Inlighten by the ideas, MnO2@HCF/S55 and CNTNiFe2O4-S56 hybrid have been developed, which increased the specific capacity and rate performance. Generally, compared with

polar metal oxide/sulfide, the GO has not only

excellent adsorption for polysulfide, but also high specific surface area and flexibility, which effectively provide carrier of sulfur and ease the volume expansion.46 To date, the research in which polar metal oxide and GO with both well ploysulfide absorption together with high conductive CNTs were introduced to sulfur cathode simultaneously has not been reported.

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In this study, we developed a hybrid cathode material with synergistic function for Li-S battery. CNTs, GO and MnO2 are employed in the cathode simultaneously. As illustrated in Figure 1, MnO2/GO/CNTs composite with three-dimensional (3D) structure is first synthesized by a one-pot chemical method and then sulfur nano-particles are induced uniformly distributed in the matrix of MnO2/GO/CNTs by heat-treatment approach. In such a composite, the innermost one-dimensional (1D) CNTs can form a conductive frame for long-range electron transfer, mass diffusion and structural stability. The two-dimensional (2D) petal-like ultrathin MnO2/GO nanosheets anchored on the sidewalls of the inner CNTs provide adequate space for sulfur deposition and have dual-efficient absorption capability for polysulfide intermediates.46,52The outmost nanosized S attatched onto the surface of the MnO2/GO nanosheets is the active component for energy storage. Thus the 3D structured MnO2/GO/CNTs-S composite is expected to demonstrate excellent properties of reactive activity, rating peculiarity and cycling. RESULTS AND DISCUSSION Raman spectroscopy was used to explore the vibrational properties of molecule. Figure 2a shows the comparision of Raman spectras between CNTs and MnO2/GO/CNTs. The signals at 1350 and 1600 cm-1 are the characteristic peaks of graphite, and intensity ratio of D to G band (ID/IG) is usually employed to estimate the chemical structure and degree of graphitization. The increase of ID/IG reveals that more defects existed in the MnO2/GO/CNTs compared to CNTs, which was caused by the chemical oxidation. The results are consist with the reported results.58 Furthermore, the peaks at 575 and 650 cm-1 are due to Mn-O vibration and can be recognized as the characteristic of MnO2.58 The XRD patterns of three samples are shown in Figure 2b. For the CNTs, The peak at approximate 26° is the characteristic of graphite.58 While the peaks of the MnO2/GO/CNTs at around 12, 25, 37 and 66° correspond to the crystal planes at (001), (002), (111), (312) in birnessite-type MnO2 (JCPDS 42-1317), respectively. The characteristic peak at 11° of GO 4


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can be distinguished from the broad peak on the XRD pattern of the MnO2/GO/CNTs, although its intensity is relatively weak. For MnO2/GO/CNTs-S composite, the sharp diffractions peaks at 2Ɵ = 23.4° and 28.0° can be indexed to the orthorhombic phase (JCPDS 08-0247). The MnO2 content in the MnO2/GO/CNTs-S composite is about 11 wt% by TG analysis (Figure S1) and the sulfur content in the composite is further confirmed by using the elemental analysis (Table S1). SEM and TEM were both utilized to explore the morphology and microstructures of MnO2/GO/CNTs and MnO2/GO/CNTs-S composites. It was observed that nanosheets with curved petal-like morphology were attached to CNTs forming a woven network (Figure 3a). Figure 3b shows that numerous 2D ultra-thin petal-like sheets connect tightly onto the CNTs. The lateral size and the thickness of the nanosheets is ~ 250 nm and 5 nm, respectively. It is also can be observed that GO nanosheets attach to both CNTs backbones and MnO2 (Figure S2). In order to fully explore the distributing state of GO in the MnO2/GO/CNTs composite, the MnO2 sheets were stripped off first. the method and results have been reported by our co-workers59: the sample was mixed with plenty of H2O2 and HCl. From the Figure S3, the raman spectrosopy manifests that almost all of the MnO2 in the sample have been removed. In addition, it is obviously observed that the GO nanosheets collapse and scatter around the CNTs due to the removal of MnO2, indicating that the 2D nanaosheets in the MnO2/GO/CNTs are composed of GO and MnO2 which sustain each other. The MnO2/GO/CNTs material shows a BET surface area of ～156 m2g-1, which provide not only adequate space for accommodating sulfur and Li2Sx (x=1-2), but also enough reaction sites for bonding the polysulfide. Furthermore, the lattice spacing of about 0.7 nm can be clearly observed from the high-resolution TEM (Figure 3b, inset), which is the representative feature of birnessite-type MnO2 and consistent with XRD analysis.52 5



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Figure 3c, d display the TEM images of MnO2/GO/CNTs-S material. The 3D architecture of MnO2/GO/CNTs with petal-like sheets and large specific surface area facilitate the generation of S. After the S deposition process, the material maintained the morphology of MnO2/GO/CNTs scaffold. In addition, the sulfur nanoparticles with size of 20 nm were uniformly dispersed and formed a nanolayer on the surface of MnO2/GO/CNTs. The BET surface area of MnO2/GO/CNTs-S is still up to 42 m2g-1, which is in favor of contacting with the electrolyte and increasing the capacity. In order to identify the distribution of the elements in the MnO2/GO/CNTs-S hybrid architecture, EDS mapping under SEM mode was used, as shown in Figure 4. Both Mn and O were observed within the structure, suggesting the oxide of Mn and O indeed exist in the petal-like nanosheets. Mapping of S revealed that S is uniformly distributed on the surfaces of the composite, consistent with the result of Figure 3d. The 3D structured MnO2/GO/CNTs composite was integrated into lithium sulfur battery to explore its role on the property. The charge-discharge curves of the MnO2/GO/CNTs-S composite were evaluated by coin-cell typed batteries at different current densities, and the capacity were calculated by using S mass. Figure 5a shows characteristic voltage profiles of the sample. It can be seen that every discharging profile presents two plateaus, which are attributed to the multistep reaction mechanism of Li-S battery. And the two plateaus are ascribed to the cyclic S8 to soluble lithium polysulfides (Li2Sx, 4≤x＜8) and further reduction to insoluble Li2S2 and Li2S. 7 While only one plateau appeared in charging curves is the result of lithium polysuldes and lithium sulfide converted back into element sulfur. Moreover, from the charge-discharge curves, it can been seen that a short slope between two typical plateaus. In the discharging process, the voltage curves show a small drop between two platforms, which can be attributed to the polysulfide solubility and solution viscosity. At the end of the first discharging platform, the concentration of S42- in the electrolyte reached to maximum, 6


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which increased the viscosity and reduced the conductivity of the electrolyte. Then with progressive discharging, S42- is gradually reduced to insoluble Li2S2 and Li2S.The viscosity of electrolyte decrease and the conductivity increase, resulting in a higher voltage. In the charging curve, the voltage also displays a tiny drop at the start, owing to the chemical oxidization of Li2S and Li2S2 by higher order polysulfide forming in the charging process.57 The discharging specific capacity of MnO2/GO/CNTs-S reaches 1500 mAh g-1 at 0.05 C, nearly 90% of the theory capacity. When increasing the current densities from 0.1, 0.2 to 0.5 C, the sample eletrode exhibits capacity of 1300, 1150 and 1048 mAh g-1, respectively. Even discharged at 1 C, the cathode also can deliver 960 mAh g-1. In addition, from the charge-discharge curves, there was slight increase in polarization when enhancing the current, indicating highly efficient kinetics of MnO2/GO/CNTs-S composite. The high specific capacity at different current density can be attributed to fine nanoparticles of S well distributed in the MnO2/GO/CNTs scaffold, in which the nano-size particles increase the electrochemical reaction surface area and shorten the diffusion path for electrons and ions within sulfur. The rate capabilities of the MnO2/GO/CNTs-S cathode were further investigated by way of discharging the battery to 1.5 V at different current densities from 0.05C to 1C, as shown in Figure 5b. Compared with CNTs-S composite cathode, the MnO2/GO/CNTs-S exhibited a higher capacity, even more than 300 mAh g-1 at the discharging rate of 1 C, which can be abscribed to the special microstructure of the MnO2/GO/CNTs-S composite. First, the conductive CNTs skeleton connected with the petal-like ultrathin MnO2/GO nanosheets form a 3D conducting porous network and electrons can rapidly transport between sulfur and Al foil, which allow for a high degree of sulfur utilization and rapid electrochemical kinetics to enhance rate performance. Second, the nano-sized S particles anchored onto the nanosheets of the MnO2/GO/CNTs increase the electrochemically active surface area, which are beneficial to the infiltration of electrolyte, ion diffusion and electron transfer. It should be also noted that 7



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the MnO2/GO/CNTs-S composite cathode presented much better reversibility at the same discharge rate compared with CNTs-S composite cathode. The high specific surface area and pores network of MnO2/GO/CNTs afford adequate space for alleviating the volumetric changes caused by discharged products. In addition, MnO2/GO nanosheets attached to the CNTs also play a important part in improving the cycling of S cathode owing to the good dual-adsorption of polysulfide. It should be pointed out that although the rate capabilities seem to be not so good as other reported researches,7,28,29,48,52,53,56 the material in our work possessed a higher sulfur loading (80 wt%) and our cathode also displayed a higher sulfur content (64 wt%), an areal density of 2.8 mg cm-2. Besides, the mass ratio of electrolyte to sulfur was 2.5/1. The above parameters are more closer to the practical application. To clearly identify the influence of MnO2/GO sheets in the 3D MnO2/GO/CNTs composite on the cycle of battery, the cycling and coulombic efficiency about three samples at the rate of 0.2 C were studied and the results were shown in Figure 6a. Compared with MnO2/GO/CNTs-S and MnO2/CNTs-S composite cathodes, the CNTs-S composite cathode show less capacity and much faster capacity fading during the 100 cycles, only maintaining 54.3% of initial capacity and a capacity decay of ～0.457% per cycles. Moreover, the CNTs-S composite cathode shown the worst coulombic efficiency among the three samples. While MnO2/CNTs-S composite cathode exhibited an improved performance of coulombic efficiency and cycle life, only 0.239% was decayed per cycle over 100 cycles, which was better than that of CNTs-S composite cathode but still worse than that of MnO2/GO/CNTs-S composite cathode. The MnO2/GO/CNTs-S composite cathode only has an average capacity delay of 0.162% per cycle and displayed the highest coulombic efficiency among the three samples. Furthermore, the GO/CNTs-S electrode also exhibited a better performance of cycle life than CNTs-S composite (Figure S4a), indicating that GO sheets played a vital role in enhancing the cycling. Even in absence of LiNO3 in the electrolyte, the composites with GO 8


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or MnO2 still have much higher coulombic efficiency than CNTs-S composite (Figure S4b), further showing that GO or MnO2 has excellent adsorption performance for polysulfide. Thus, those improvements in performance are achieved by the GO or MnO2 nanosheets, which can trap polysulfide intermediates to form surface-bound intermediates and decrease the loss of the active sulfur by a chemistry method, different from the previous strategies for restricting polysulfides just

by physical barriers36 While MnO2/GO/CNTs-S composite cathode shown

the best electrochemical performance among the samples whether adding LiNO3 into the electrolyte or not, which may be attributed to MnO2/GO nanosheets possessing the synergetic double-effect absorption to polysulfides, indicating that MnO2/GO sheets in the 3D MnO2/GO/CNTs composite indeed play a key role for the improved Li-sulfur battery. In the meantime, 3D nanoarchitecture of MnO2/GO/CNTs composite is also responsible for the improved performances. Moreover, the impedance spectras are displayed in Figure 6b, it is interesting to find that the introduction of MnO2/GO does not obviously increase the impedance. In order to further explore the trapping capability of the MnO2/GO/CNTs composite, Lithium polysulfide adsorption experiments were performed. Li2S6 was used as the polysulfide representative and 0.6 M Li2S6 solution was made by use of a mount of Li2S6 dissolved in DOL mixed with DME in a volumetric ratio of 1:1. Then the same amount of each test sample was added into a fixed volume of Li2S6 solution. As shown in Figure 7a, the solution containing MnO2/GO/CNTs composite turned completely colorless after standing 2 hours, indicating that MnO2/GO/CNTs composite possesses strong adsorption of Li2S6. The solution containing MnO2/CNTs was slightly colored, conforming that the MnO2 /CNTs also has certain adsorption capability for Li2S6, but lower than that of MnO2/GO/CNTs composite. While the CNTs solution only has a little change of color compared with the blank Li2S6 solution, suggesting that CNTs have weak interaction with Li2S6. The above results show that MnO2/GO/CNTs composite has the strongest adsorption capability for Li2S6 which can 9



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attributed to the high specific surface area and the typical 3D morphology. This excellent dual-efficient polysulfide absorption capability is beneficial to the improvement the properties of cycling and coulombic efficiency. In addition, the excellent absorption ability of polysulfide of MnO2/GO/CNTs composite was also proved by UV-visible spectrum, and the results are shown in Figure S5. The intensities of typical characteristic absorption peaks of Li2S6 in the DOL/DME solution greatly decreased after adding MnO2/GO/CNTs compared to the solution without MnO2/GO/CNTs. The state and distribution of polysulfide absorbed by the MnO2/GO/CNTs composite were also studied, as shown in Figure 7b. The polysulfide particles were distributed on the surface of the MnO2/GO/CNTs composite and bonded interaction with MnO2/GO nanosheets very well. X-ray photoelectron spectroscopy (XPS) was employed to detect the chemical bonds between the Li2S6 and MnO2/GO/CNTs composite and the results are shown in the Figure 7c and d. Compared with pristine Li2S6, the Li 1s spectra of Li2S6-MnO2/GO/CNTs shown a 0.45-eV red shift, indicating a strong bonding between Li2S6 and MnO2/GO/CNTs composite, which is consist with other report.51 The binding peaks at 164 eV of S 2p spectra manifest the existence of Li2S6 attached on the surface of MnO2/GO/CNTs composite after absorption. While the spectrums of Li2S6 bound to MnO2/GO/CNTs are different from pristine Li2S6, which suggesting that the bonding of Li2S6 and MnO2/GO/CNTs maybe chemical force and possibly through electrons transfer from polysulfide to the electropositive manganese and/or active groups on the GO. Based on the above discussions, the improvements of the electrochemical performances with MnO2/GO/CNTs-S composite can be ascribed to superiority of structure and cooperation among components of the composite. Firstly, CNTs located in the innermost work as a conducting additive and a backbone to form conducting network and matrix. Secondly, the MnO2/GO nanosheets anchored on the surface of CNTs can be used as an highly efficient polysulfide adsorbent to suppress the shuttling effect during the charging-discharging process, 10


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as well as increasing the eletrical conductivity in the meanwhile. Thirdly, the MnO2/GO/CNTs composite with 3D structure affords plenty of areas for sulfur deposition and adequate space for accommodating the volume changes during the electrochemical process. Finally, the outmost nanoscale sulfur particles well distributed on the MnO2/GO nanosheets are liable to be absorbed when reduced to polysulfide, and the fine sulfur shorten the transmission length for Li+ and electron.

CONCLUSIONS A novel MnO2/GO/CNTs-S composite with 3D architecture has been synthesized and employed as cathode material for Li-S battery with improved performance. The innermost CNTs provide a conducting network, while the MnO2/GO nanosheets with high specific surface area are highly efficient polysulfide adsorbent and can suppress the shuttling effect. The nano-sized sulfur particles uniformly distributed on the surface of MnO2/GO/CNTs facilitate fast diffusion of Li+ and electron. The MnO2/GO/CNTs composite with 3D structure affords adequate space for accommodating the volume changes during the electrochemical process. At the same time, the synergistic effects of components in the composite are responsible for the enhanced performances. Moreover, the MnO2/GO/CNTs-S composite is easily synthesized on a large scale and may be a promising candidate for commercial application.

EXPERIMENTAL SECTION Synthesis of the composite MnO2/GO/CNTs: Sublimed sulfur (sulfur), Hydrogen peroxide (H2O2), Nitric- acid (HNO3), Potassium permanganate (KMnO4) and Sulfuric acid (H2SO4) were all purchased from Guoyao Company. Commercial multiwalled CNTs was obtained from Shenzhen Nano Co. Ltd.

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The MnO2/GO/CNTs composite was prepared by a modified one-pot synthesis as follows58: 1 g of CNTs was added into 100 mL deionized water with stirring for 10 min at room temperature (25 ℃). 1 mL of concentrated H2SO4 and 4 mL of concentrated HNO3 were added to the above solution with an additional 120 min of stirring at 70 ℃. After that, the mixed solution was quickly cooled at 0 ℃ with the help of ice bags, then 5mL of concentrated H2SO4 was poured into the mixed solution and further stirred 10 min. Subsequently, the mixture was heated at 55 ℃ and then 3 g of KMnO4 was added and constantly stirred 120 min. Then, the above solution tirring 10 min at 90 ℃. After vacuum filtration, the precipitated MnO2/GO/CNTs material was washed repeatedly and dried at 60 ℃ for 12 h. Synthesis of the composite MnO2/GO/CNTs-S: The sulfur and MnO2/GO/CNTs material with weight ratio of 10:1 were ground together in the agate mortar, and the mixture was placed into a glass tube under Ar protection and heated at 155 ℃ and 300 ℃ for 240 min and 20 min, respectively. After that, the obtained composite was dispersed into an amount of H2O2 solution and stirring 2 h with the purpose of removing partial MnO2. After filtration and dried,the final composite of MnO2/GO/CNTs-S was collected. As a comparison, MnO2 nanosheets were prepared by reducing GO with KMnO4.60 The MnO2/CNTs-S composite possessing the same S and MnO2 content with MnO2/GO/CNTs-S was prepared in the same way by low temperature heating method. The sample of GO/CNTs-S was synthesized by heating the GO/CNTs and S together at 155 ℃ for 4 h under Ar protection. Another reference S/MWCNT sample without GO and MnO2 was prepared by just grounding S and MWCNT together in an agate mortar and then heated by the same procedure. The sample for UV-VIS are prepared as follows: In the glove box filled with argon, 0.6 M Li2S6 solution was first prepared by employing a certain stoichiometric amounts of sulfur and lithium power dissolved in DOL mixed with DME in a volumetric ratio of 1:1. Then, take out a mount of 0.6 M Li2S6 solution and diluted into 0.05 M using DOL and DME (1:1). After 12


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that, 0.5 g MnO2/GO/CNTs was pour into the 20 mL of Li2S6 solution and standed 2 hours. Then, the upper parent solution was used the testing sample. Materials characterization: The X-ray diffraction (XRD) measurements of the samples were performed on a Rigaku D/MAX-2600pc diffractometer (Japan) using Cu Kα radiation(λ= 1.54056 Å) from 10◦ to 80◦. Raman test were by using confocal micro-reman spectrometer (Renishaw invia Reflex). The surface morphology of the samples were observed by SEM (S4800 Hitachi) and TEM (Tecnai G2F20). The component ratio of composite was detected by chemical elemental analysis (CHNS, Vario EL Cube, Elementar) and thermogravimetric analysis (STA PT1600). Specific surface areas measurements were performed by nitrogen adsorption with a physisorption analyzer (Micro-meritics ASAP 2020M). UV-VIS was carried on Lambda 950 UV-visible spectrophotometer (Perkin Elmer). Eletrochemical measurements:The as-prepared samples were mixed with acetylene black and La132 binder in a ratio of 8:1:1. The mixture was slurried onto aluminum foil. The electrode was dried for 12 h in a vacuum at 50 ℃. The average sulfur loading in the 14 mm circular disks is about 2.8 mg cm-2. CR-2025 type coin cells were assembled in a glove box filled with argon. The lithium metal was employed as anode and the electrolyte was 0.8 mol L-1 LiTFSI/DOL + DME (1:1 by volume) with 1 wt% LiNO3 additives. Galvanostatically curves and cycle performance datas for the cells were collected by BT2011C (Land). Electrochemical impendence spectroscopy (EIS) was measured by solartron 1287.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions †

Y. L. and D.X.Y. contributed equally. 13



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Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information Available. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors gratefully acknowledge the financial support by the Science and Technology Talent Program of Shanghai (no. 15QB1402000), the Natural Science Foundation of China (no. 21373137), the Scientific Research Projects (no. 14JC1491800) and Engineering Center Ability Enhancement (no. 15DZ2282000) of shanghai committee for science and technology.

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High

Performance


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Figure 1. Schematic illustration of the synthesis and discharge process of the three-dimention structured MnO2/GO/CNTs-S composite.

Figure 2. (a) Raman spectra of pristine CNTs and MnO2/GO/CNTs ; (b) XRD patterns of CNTs, MnO2/GO/CNTs and MnO2/GO/CNTs-S composite.

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Figure 3. (a) SEM and (b) TEM image of MnO2/GO/CNTs composite; (c) SEM and (d) TEM image of MnO2/GO/CNTs-S composite.

Figure 4. (a) SEM image of MnO2/GO/CNTs-S composite; (b) corresponding EDS mapping for distribution in MnO2/GO/CNTs-S composite.

Figure 5. (a) Typical charge-discharge curves for MnO2/GO/CNTs-S at various current from 0.05C to 1C ; (b) comparison of the rate performance of MnO2/GO/CNTs-S and CNTs-S.

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Figure 6. (a) Comparison of cycling stability and coulombic efficiency of CNTs-S,

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MnO2/

CNTs-S and MnO2/GO/CNTs-S composite at 0.2 C ; (b) Nyquist plots of the CNTs-S, MnO2/ CNTs-S and MnO2/GO/CNTs-S electrode before cycling.

Figure 7. (a) Visual photo of different host materials soaked in polyulfide solution two hours later ; (b) SEM image of the MnO2/GO/CNTs composite after adsorbing lithium polysulfide; (c) High-resolution XPS Li 1s spectra of Li2S6 and Li2S6- MnO2/GO/CNTs; (d) High-resolution XPS S 2p spectra of Li2S6 and Li2S6- MnO2/GO/CNTs.

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