Research Article www.acsami.org
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,*,∥ and 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
ACS Appl. Mater. Interfaces 2016.8:28566-28573. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/25/19. For personal use only.
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S Supporting Information *
ABSTRACT: Lithium-sulfur batteries can potentially be used as a chemical power source because of their high energy density. However, the sulfur cathode has several shortcomings, including fast capacity attenuation, poor electrochemical activity, and low Coulombic efficiency. Herein, multi-walled carbon nanotubes (CNTs), graphene oxide (GO), and manganese dioxide are introduced to the sulfur cathode. A MnO2/GO/CNTs-S composite with a unique three-dimensional (3D) architecture was synthesized by a one-pot chemical method and heat treatment approach. In this structure, the innermost CNTs work as a conducting additive and backbone to form a conducting network. The MnO2/GO nanosheets anchored on the sidewalls of CNTs have a dual-efficient absorption capability for polysulfide intermediates as well as afford adequate space for sulfur loading. The outmost nanosized sulfur particles are well-distributed on the surface of the MnO2/ GO nanosheets and provide a short transmission path for Li+ and the electrons. 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 capacities are up to 1500, 1300, 1150, 1048, and 960 mAh g−1 at discharging rates of 0.05, 0.1, 0.2, 0.5, and 1 C, respectively. Moreover, the composite cathode shows a good cycle performance: the specific capacity remains at 963.5 mAh g−1 at 0.2 C after 100 cycles when the area density of sulfur is 2.8 mg cm−2. KEYWORDS: polysulfide adsorption, MnO2, graphene oxide, carbon nanotube, lithium sulfur
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To solve the above problems of the cathode in the Li-S battery, considerable efforts have been made to reduce the resistance of sulfur-based cathodes and inhibit the dissolution and shuttling of polysulfide, which have focused on optimization of the electrolyte,12−15 synthesis of sulfurconductive polymer hybrids, and preparation of sulfur-carbon composites using porous carbon material as the matrix.16−19 It is worth mentioning that carbon-based materials such as carbon nanotubes,20 carbon nanofibers,21 carbon spheres,22 mirco/ mesoporous carbons,7,23 and graphene sheets24−27 have been proven to be very effective in optimizing the properties of sulfur by increasing the electrical conductivity and providing space for alleviating volume changes. However, such carbon-sulfur material architectures can only partially retain polysulfides by
ithium-sulfur batteries are representative of next-generation high-energy density rechargeable batteries. When calculated based on the redox reaction of S and lithium sulfide (Li2S), the specific capacity and energy density of the Li-S battery reach 1675 mAhg−1 and 2600 Wh kg−1, respectively, which are higher than those of commercial Li-ion batteries. In addition, the sulfur cathode has many merits, including abundant resources, low cost, and low pollution.1−4 However, Li-S batteries still have some defects in their 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 the charging and discharging process, leading to damage on the structure of the electrode;8 and the shuttle reaction caused by dissolution of polysulfide into the liquid electrolyte, which gives rise to low Coulombic efficiency.9−11 © 2016 American Chemical Society
Received: April 11, 2016 Accepted: July 29, 2016 Published: July 29, 2016 28566
DOI: 10.1021/acsami.6b04270 ACS Appl. Mater. Interfaces 2016, 8, 28566−28573
Research Article
ACS Applied Materials & Interfaces physical adsorption because the chemical interactions between the nonpolar carbons and polar polysulfides are rather weak.28,29 To further decrease the capacity fade rate, carbon interlayers30−33 and surface modifications of the 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 heterogeneous atom-doped carbon-based materials have strong chemisorption for polysulfides and improve the cycle stability of the Li-S battery.35−39 For example, the N-doped nanocarbon is liable to form an electron-modified interface which adsorbs polar polysulfides and ameliorates the deposition and recharging of Li2S.40 In addition, research on electrolytes (additive or new solvent)41,42 and new structures of electrodes43,44 to inhibit migration of polysulfide has also been carried out and achieved good results. For example, Wang’s group developed a novel sandwichstructured cathode using two flexible porous carbon membranes, which was proven to effectively 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 that of 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 the Li-S battery electrode. Enlightened by these ideas, researchers have developed MnO2@HCF-S55 and CNT- NiFe2O4-S56 hybrids, which increased the specific capacity and rate performance. Generally, compared with that of polar metal oxide/sulfide, GO has not only excellent adsorption for polysulfide but also high specific surface area and flexibility, which effectively provide a carrier of sulfur and ease volume expansion.46 To date, research wherein a polar metal oxide and GO, both having good polysulfide absorption together with highly conductive CNTs, are simultaneously introduced to a sulfur cathode has not been reported. In this study, we developed a hybrid cathode material with a synergistic function for a Li-S battery. CNTs, GO, and MnO2 are employed in the cathode simultaneously. As illustrated in Figure 1, the MnO2/GO/CNTs composite with three-dimensional (3D) structure is first synthesized by a one-pot chemical method, and then sulfur nanoparticles are induced and uniformly distributed in the matrix of MnO2/GO/CNTs by a 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 dualefficient absorption capability for polysulfide intermediates.46,52The outmost nanosized S attached 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.
Figure 1. Schematic illustration of the synthesis and discharge process of the 3D-structured MnO2/GO/CNTs-S composite.
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RESULTS AND DISCUSSION Raman spectroscopy was used to explore the vibrational properties of the molecules. Figure 2a shows the comparison of
Figure 2. (a) Raman spectra of pristine CNTs and MnO2/GO/CNTs. (b) X-ray diffraction patterns of CNTs, MnO2/GO/CNTs, and the MnO2/GO/CNTs-S composite.
Raman spectra between CNTs and MnO2/GO/CNTs. The signals at 1350 and 1600 cm−1 are the characteristic peaks of graphite, and the intensity ratio of the D to G band (ID/IG) is usually employed to estimate the chemical structure and degree of graphitization. The increase in ID/IG reveals that more defects existed in the MnO2/GO/CNTs compared to CNTs, which was caused by chemical oxidation. The results are consistent with the reported results.58 Furthermore, the peaks at 575 and 650 cm−1 are due to Mn−O vibration and are recognized as characteristic of MnO2.58 The XRD patterns of three samples are shown in Figure 2b. For the CNTs, the peak at approximately 26° is 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 421317), respectively. The characteristic peak at 11° of GO can 28567
DOI: 10.1021/acsami.6b04270 ACS Appl. Mater. Interfaces 2016, 8, 28566−28573
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maintained the morphology of the MnO2/GO/CNTs scaffold. In addition, sulfur nanoparticles with sizes of 20 nm were uniformly dispersed and formed a nanolayer on the surface of MnO2/GO/CNTs. The BET surface area of the MnO2/GO/ CNTs-S composite is still up to 42 m2 g−1, which is in favor of contacting with the electrolyte and increasing the capacity. To identify the distribution of elements in the MnO2/GO/ CNTs-S hybrid architecture, energy-dispersive X-ray spectroscopy (EDS) mapping under SEM mode was used, as shown in Figure 4. Both Mn and O were observed within the structure,
be distinguished from the broad peak on the XRD pattern of the MnO2/GO/CNTs, although its intensity is relatively weak. For the MnO2/GO/CNTs-S composite, the sharp diffraction peaks at 2θ = 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 thermogravimetric (TG) analysis (Figure S1), and the sulfur content in the composite was further confirmed using elemental analysis (Table S1). Scanning electron microscopy (SEM) and transmission electron microscopy (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
Figure 4. (a) SEM image of the MnO2/GO/CNTs-S composite and (b) the corresponding EDS mapping of the distribution in the MnO2/ GO/CNTs-S composite.
suggesting the oxides 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 results in Figure 3d. The 3D-structured MnO2 /GO/CNTs composite was integrated into the lithium-sulfur battery to explore its role on the charge−discharge properties. The charge−discharge curves of the MnO2/GO/CNTs-S composite were evaluated by coin-cell-type batteries at different current densities, and the capacity was calculated using the S mass. Figure 5a shows
Figure 3. (a) SEM and (b) TEM image of the MnO2/GO/CNT composite. (c) SEM and (d) TEM image of the MnO2/GO/CNTs-S composite.
that numerous 2D ultrathin petal-like sheets connect tightly onto the CNTs. The lateral size and thickness of the nanosheets are ∼250 and 5 nm, respectively. It is also can be observed that GO nanosheets attach to both CNT backbones and MnO2 (Figure S2). To fully explore the distributing state of GO in the MnO2/ GO/CNTs composites, the MnO2 sheets were first stripped off. The method and results have been reported by our coworkers:59 the sample was mixed with plenty of H2O2 and HCl. As can be seen in Figure S3, the Raman spectroscopy results show that almost all of the MnO2 in the sample was removed. In addition, it can be clearly observed that the GO nanosheets collapse and scatter around the CNTs due to the removal of MnO2, indicating that the 2D nanosheets 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 m2 g−1, which provides not only adequate space for accommodating sulfur and Li2Sx (x = 1 or 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 a representative feature of birnessite-type MnO2 and consistent with XRD analysis.52 Figures 3c and d display TEM images of the 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
Figure 5. (a) Typical charge−discharge curves for MnO2/GO/CNTsS at various currents from 0.05 C to 1 C and (b) comparison of the rate performance of MnO2/GO/CNTs-S and CNT-S.
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 the Li-S battery. The two plateaus are ascribed to the cyclic S8 and soluble lithium polysulfides (Li2Sx, 4 ≤ x < 8) and further reduction to insoluble Li2S2 and Li2S.7 While only one plateau appeared in charging curves, it is the result of lithium polysulfides and lithium sulfide converted back into elemental sulfur. Moreover, from the charge−discharge curves, it can been seen that there is 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 S4−2 in the electrolyte reached the maximum, which increased the viscosity 28568
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Research Article
ACS Applied Materials & Interfaces and reduced the conductivity of the electrolyte. Then, with progressive discharging, S4−2 is gradually reduced to insoluble Li2S2 and Li2S. The viscosity of the electrolyte decreases and the conductivity increases, resulting in a higher voltage. In the charging curve, the voltage also displays a tiny drop at the start due to the chemical oxidization of Li2S and Li2S2 by higherorder 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 theoretical capacity. When the current densities are increased from 0.1 to 0.2 to 0.5 C, the sample electrode exhibits capacity of 1300, 1150, and 1048 mAh g−1, respectively. Even when 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 the current was enhanced, indicating highly efficient kinetics of the MnO2/GO/CNTs-S composite. The high specific capacity at different current densities can be attributed to fine nanoparticles of S well-distributed in the MnO2/GO/CNTs scaffold, in which the nanosize 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 0.05 to 1 C, as shown in Figure 5b. Compared with that of the CNT-S composite cathode, the MnO2/GO/CNTs-S composite exhibited a higher capacity, even more than 300 mAh g−1 at the discharging rate of 1 C, which can be ascribed to the special microstructure of the MnO2/GO/CNTs-S composite. First, the conductive CNT 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 nanosized S particles anchored onto the nanosheets of the MnO2/GO/CNTs increase the electrochemically active surface area, which are beneficial to the infiltration of electrolytes, ion diffusion, and electron transfer. It should be also noted that the MnO2/GO/CNTs-S composite cathode presented a much better reversibility at the same discharge rate compared with that of the CNT-S composite cathode. The high specific surface area and porous 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 the S cathode due to the good dual adsorption of polysulfide. It should be pointed out that, although the rate capabilities seem to be not as good as other reported research,7,28,29,48,52,53,56 the material in our work possessed higher sulfur loading (80 wt %), and our cathode also displayed a higher sulfur content (64 wt %) and an areal density of 2.8 mg cm−2. Further, the mass ratio of electrolyte to sulfur was 2.5/1. The above parameters are closer to those of the practical application. To clearly identify the influence of MnO2/GO sheets in the 3D MnO2/GO/CNTs composite on the battery cycle, 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 that of the MnO2/GO/CNTs-S and MnO2/CNTs-S composite cathodes, the CNT-S composite cathode shows 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 cycle. Moreover, the
Figure 6. (a) Comparison of cycling stability and Coulombic efficiency of CNT-S, MnO2/CNTs-S, and MnO2/GO/CNTs-S composites at 0.2 C. (b) Nyquist plots of the CNT-S, MnO2/CNTs-S, and MnO2/ GO/CNTs-S electrodes before cycling.
CNT-S composite cathode showed the worst Coulombic efficiency among the three samples. While the 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 the CNT-S composite cathode but still worse than that of the MnO2/GO/CNTs-S composite cathode. The MnO2/GO/ CNTs-S composite cathode has an average capacity delay of only 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 in cycle life than the CNT-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 or MnO2 still have a Coulombic efficiency much higher than that of the CNT-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 barriers.36 While the MnO2/GO/ CNTs-S composite cathode showed the best electrochemical performance among the samples, whether LiNO3 was added 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-S battery. In the meantime, 3D nanoarchitecture of the MnO2/GO/CNTs composite is also responsible for the improved performance. Moreover, the impedance spectra are displayed in Figure 6b. It is interesting to find that introduction of MnO2/GO does not obviously increase the impedance. 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 the MnO2/GO/CNTs composite turned completely colorless after standing for 2 h, indicating that the MnO2/GO/CNTs composite possesses strong adsorption of Li2S6. The solution containing MnO2/CNTs was slightly colored, confirming that the MnO2/CNTs composite also has certain adsorption capability for Li2S6, but it is lower than that of MnO2/GO/CNTs composite. The CNT solution has only a 28569
DOI: 10.1021/acsami.6b04270 ACS Appl. Mater. Interfaces 2016, 8, 28566−28573
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ACS Applied Materials & Interfaces
cooperation among components of the composite. First, the innermost CNTs work as a conducting additive and a backbone to form a conducting network and matrix. Second, the MnO2/ GO nanosheets anchored on the surface of CNTs can be used as a highly efficient polysulfide adsorbent to suppress the shuttling effect during the charge−discharge process as well as increase the electrical conductivity. Third, the MnO2/GO/ CNTs composite, with its 3D structure, affords plenty of areas for sulfur deposition and adequate space for accommodating volume changes during the electrochemical process. Finally, the outmost nanoscale sulfur particles, which are well-distributed on the MnO2/GO nanosheets, can be absorbed when reduced to polysulfide, and the fine sulfur shortens the transmission length for Li+ and electrons.
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CONCLUSIONS A novel MnO2/GO/CNTs-S composite with 3D architecture has been synthesized and employed as a cathode material for a 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 adsorbents and can suppress the shuttling effect. The nanosized sulfur particles uniformly distributed on the surface of MnO2/GO/CNTs facilitate fast diffusion of Li+ and electrons. The MnO2/GO/CNTs composite, with its 3D structure, affords adequate space for accommodating volume changes during the electrochemical process. At the same time, the synergistic effects of components in the composite are responsible for the enhanced performance. Moreover, the MnO2/GO/CNTs-S composite is easily synthesized on a large scale and may be a promising candidate for commercial application.
Figure 7. (a) Visual photo of different host materials soaked in polysulfide solution for 2 h. (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/CNT composites. (d) High-resolution XPS S 2p spectra of Li2S6 and Li2S6-MnO2/GO/CNT composites.
little change in color compared with that of the blank Li2S6 solution, suggesting that CNTs have weak interactions with Li2S6. The above results show that the MnO2/GO/CNTs composite has the strongest adsorption capability for Li2S6 which can attributed to the high specific surface area and the typical 3D morphology. This excellent dual-efficient polysulfide absorption capability is beneficial to improving the cycling and Coulombic efficiency. In addition, the excellent absorption ability of polysulfide of the MnO2/GO/CNTs composite was also proven by the 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 the MnO2/GO/CNTs composite compared to those in the solution without the MnO2/GO/ CNTs composite. 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 with MnO2/GO nanosheets very well. X-ray photoelectron spectroscopy (XPS) was employed to detect the chemical bonds between Li2S6 and the MnO2/GO/CNTs composite, and the results are shown in Figures 7c and d. Compared with those of pristine Li2S6, the Li 1s spectra of the Li2S6-MnO2/GO/CNTs composite showed a 0.45 eV red shift, indicating a strong bond between Li2S6 and the MnO2/GO/CNTs composite, which is consist with another report.51 The binding peaks at 164 eV of the S 2p spectra manifest the existence of Li2S6 attached on the surface of the MnO2/GO/CNTs composite after absorption. The spectra of Li 2S6 bound to the MnO2/GO/CNTs composite are different from those of pristine Li2S6, which suggests that the bonding of Li2S6 and MnO2/GO/CNTs may be chemically forced, possibly through electron transfer from polysulfide to the electropositive manganese and/or active groups on GO. Based on the above discussions, the improvements in electrochemical performance with the MnO2/GO/CNTs-S composite can be ascribed to its superiority of structure and
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EXPERIMENTAL SECTION
Synthesis of the MnO2/GO/CNTs Composite. Sublimed sulfur (sulfur), hydrogen peroxide (H2O2), nitric acid (HNO3), potassium permanganate (KMnO4), and sulfuric acid (H2SO4) were all purchased from Guoyao Company. Commercial multi-walled CNTs were obtained from Shenzhen Nano Co. Ltd. The MnO2/GO/CNTs composite was prepared by a modified onepot synthesis as follows:58 1 g of CNTs was added into 100 mL of deionized water with stirring for 10 min at room temperature (25 °C). One milliliter of concentrated H2SO4 and 4 mL of concentrated HNO3 were added to the above solution with an additional 120 min of stirring at 70 °C. After that, the mixed solution was quickly cooled to 0 °C with the help of ice bags, and then 5 mL of concentrated H2SO4 was poured into the mixed solution and further stirred for 10 min. Subsequently, the mixture was heated at 55 °C, and then 3 g of KMnO4 was added by being constantly stirred for 120 min. Then, the above solution was stirred for 10 min at 90 °C. After vacuum filtration, the precipitated MnO2/GO/CNT material was washed repeatedly and dried at 60 °C for 12 h. Synthesis of the MnO2/GO/CNTs-S Composite. Sulfur and the MnO2/GO/CNT material with a 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 °C for 240 and 20 min, respectively. After that, the obtained composite was dispersed into a H2O2 solution and stirred for 2 h with the purpose of removing partial MnO2. After being filtered 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 possessed the same S and MnO2 content as the MnO2/GO/CNTs-S composite and was prepared in the same way by the low-temperature heating method. The sample of GO/CNTs-S was synthesized by heating GO/CNTs and S together at 155 °C for 4 h under Ar protection. Another 28570
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reference S/CNTs sample without GO and MnO2 was prepared by grinding S and CNTs together in an agate mortar and then heating by the same procedure. The sample for UV−vis was prepared as follows: In the glovebox filled with argon, a 0.6 M Li2S6 solution was first prepared by employing certain stoichiometric amounts of sulfur and lithium powder dissolved in DOL mixed with DME in a volumetric ratio of 1/ 1. Then, an amount of the 0.6 M Li2S6 solution was removed and diluted into 0.05 M using DOL and DME (1/1). After that, 0.5 g of the MnO2/GO/CNTs composite was poured into 20 mL of the Li2S6 solution and allowed to stand for 2 h. Then, the upper parent solution was used as the testing sample. Material Characterization. The X-ray diffraction (XRD) measurements of the samples were performed on a Rigaku D/MAX2600pc diffractometer (Japan) using Cu Kα radiation (λ = 1.54056 Å) from 10 to 80°. Raman tests were done using a confocal micro-Raman spectrometer (Renishaw Invia Reflex). The surface morphology of the samples was observed by SEM (S4800 Hitachi) and TEM (Tecnai G2F20). The component ratio of the 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 (Micromeritics ASAP 2020M). UV−vis spectrometry was carried on a Lambda 950 UV−visible spectrophotometer (PerkinElmer). Electrochemical 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 °C. The average sulfur loading in the 14 mm circular disks was about 2.8 mg cm−2. CR-2025-type coin cells were assembled in a glovebox filled with argon. Lithium metal was employed as the anode, and the electrolyte was 0.8 mol L−1 LiTFSI/DOL + DME (1/1 by volume) with 1 wt % LiNO3 additives. Galvanostatic curves and cycle performance data for the cells were collected by BT2011C (Land). Electrochemical impendence spectroscopy (EIS) was measured by a Solartron 1287 instrument.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04270.
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Research Article
TG curves, TEM and HR-TEM images, Raman shifts, cycling stability curves, UV−vis curves, and elemental analysis data (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
Y.L. and D.X.Y. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the Science and Technology Talent Program of Shanghai (Grant 15QB1402000), the Natural Science Foundation of China (Grant 21373137), the Scientific Research Projects (Grant 14JC1491800), and the Engineering Center Ability Enhancement of the Shanghai Committee for Science and Technology (Grant 15DZ2282000). 28571
DOI: 10.1021/acsami.6b04270 ACS Appl. Mater. Interfaces 2016, 8, 28566−28573
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