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PEO-Linked MoS2-Graphene Nanocomposites with 2D Polar-Nonpolar Amphoteric Surfaces as Sulfur Hosts for High-Performance Li-S Batteries Fugen Sun, Hao Tang, Bo Zhang, Xiaomin Li, Chuanqiang Yin, Zhihao Yue, Lang Zhou, Yongsheng Li, and Jianlin Shi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03306 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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ACS Sustainable Chemistry & Engineering

PEO-Linked MoS2-Graphene Nanocomposites with 2D Polar-Nonpolar Amphoteric Surfaces as Sulfur Hosts for High-Performance Li-S Batteries Fugen Sun,† Hao Tang,† Bo Zhang,† Xiaomin Li,† Chuanqiang Yin,† Zhihao Yue, † Lang Zhou,† Yongsheng Li*, ‡ and Jianlin Shi‡ †

Institute of Photovoltaics, Nanchang University, 999 Qianhu Road, Nanchang 330031, China

‡

Lab of Low-Dimensional Materials Chemistry, School of Materials Science and Engineering,

East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China * Corresponding author. E-mail: [email protected]

ABSTRACT: Layered MoS2-graphene nanocomposites with 2D polar-nonpolar amphoteric surfaces, which were used to confine sulfur for Li-S batteries, have been successfully fabricated through the assembly of polar MoS2 layers and nonopolar graphene with PEO cross-linking. Benefiting from the high conductivity of graphene and the strong chemical bonding between polar MoS2 and polysulfides, the MoS2-graphene composites not only ensure unimpeded electrical conducting to the insulating sulfur, but also effectively entrap polysulfides wherein. The ex-situ study further reveals that the MoS2-graphene composites enable spatially regulated Li2S deposition by the preferential deposition of solid Li2S product onto the polar MoS2 layers, making a large amount of fast electron transport paths exposed on graphene for further sulfur

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reduction. Therefore, the obtained MoS2-sulfur-graphene nanocomposites present excellent rate performance and cycling stability with high reversible capacities of 895 mAh g-1 at 0.2 C after 100 cycles and 524 mAh g-1 at 1 C after 200 cycles. These encouraging results suggest that the layer-structured sulfur cathode materials through the rational integration of 2D polar and nonpolar amphoteric surfaces would be a promising strategy for enhancing the electrochemical performances of advanced Li-S batteries.

KEYWORDS: Polar-nonpolar surfaces, Layered structures, Li2S deposition, Sulfur cathodes, LiS batteries

INTRODUCTION The widespread application of portable application of portable electronic devices and electric vehicles require the development of next-generation batteries with higher capacity and energy density.1, 2 Among all rechargeable batteries, lithium-sulfur (Li-S) batteries have been considered to be one of the most promising candidates due to their high theoretical specific capacity (1675 mAh g-1) and high specific energy (2600 Wh kg-1) at a moderate voltage of 2.2 V vs. Li/Li+.3, 4 In addition, sulfur also has other advantages, such as its cost-effectiveness and environmentalfriendliness.5,

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However, despite of these advantages, Li-S batteries are still not ready for

commercialization due to the following problems that remain unsolved: (i) poor cycle lifetime of sulfur cathodes due to the dissolution of the polysulfide intermediates,7,

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(ii) low sulfur

utilization due to the insulating nature of sulfur species,9, 10 and (iii) low coulombic efficiency and severe capacity deterioration due to the uncontrolled random deposition of the solid discharge product Li2S.11, 12


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To tackle the above problems, several technological solutions have been attempted.13,

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Improvements have been achieved by coupling the sulfur phase with conductive carbon materials, such as porous carbons,15, 16 carbon nanotubes17, 18 and graphene.19, 20 In this way, the conductive carbon frameworks confined sulfur within their pores or interlayers and generated essential electrical contact to the insulating sulfur.21, 22 Meanwhile, kinetic inhibition to diffusion within the frameworks and the physical sorption properties of the carbon both aided in trapping the polysulfides.23 Compared to zero-dimensional (0D) and one-dimensional (1D) carbon counterparts, two-dimensional (2D) graphene provides ultrahigh surface area, superior electrical conductivity, ultrathin thickness and good mechanical ﬂexibility, thus offering significant advantages for improving the sulfur utilization of Li-S batteries.24 However, due to the non-polar nature of C-C bond, graphene is less efficient in trapping polar polysulfides by weak physical confinement over long-term cycling, leading to limited improvement on cycle stability of sulfur cathodes. In comparison, polar adsorbents, such as oxides (MoO2,25 MnO2,26 Ti4O727) and sulfides (TiS2,28 CoS229) are highly effective in binding with polar polysulfides by relatively strong chemical adsorption, which are favorable for keeping polysulfides within the cathodes. Nazar et al.30 reported that Ti4O7 nanoparticles with polar O-Ti-O units have a strong ability to on-site adsorb polysulﬁdes, thus remarkably improve the cycle durability of sulfur cathodes. To further enhance the electrical conductivity of these polar adsorbents for elevating the sulfur redox kinetics, the hybrid structures of polar adsorbents with conductive carbon materials, such as La2O3/mesoporous carbons31 and tin-doped indium oxide/carbon nanofibers,32 have been employed as promising sulfur hosts, which has been a major focus of recent studies.33-36 Although improvements have been obtained, these polar-nonpolar hybrid hosts still suffer from


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the much lower electron conductivity of polar adsorbents than carbon materials. The chemically functionalized polar surface would inevitably result in the increased barrier for electron transfer at the sulfur/matrix interface, leading to significant polarization and capacity fading. Therefore, it still remains a great challenge for rationally integrating of polar and nonpolar surfaces of the polar-nonpolar hybrid hosts to solve all the technical problems of sulfur cathodes for highperformance Li-S batteries. Despite much efforts have been made to confine polysulfides in most hosts of previously reported sulfur cathodes, relatively less attentions have been paid on dealing with the uncontrolled random deposition of the solid Li2S, which will lead to the formation of large and electrochemically inactive agglomerates covering the sulfur/Li+/e- three-phase contact sites of electrochemical redox on the surface of hosts. Herein, we present the rational synthesis of a 2D layer-structured and multi-functional host composed of the polar MoS2 layer and the highly conductive graphene for the high-performance Li-S battery. Sulfur was uniformly intercalated between the MoS2 and graphene layer expanded by the PEO chains, namely the polar-nonpolar amphoteric interlayer served as a nano-reactor for sulfur redox reaction. Thanks to the synergistic effects of the high conductivity of graphene and the strong chemical bonding of polar MoS2 layer to polysulfides, polysulfide loss is significantly prohibited. More importantly, the MoS2 layer enables the spatially regulated deposition of insulating Li2S product through the preferential adsorption of Li2S to MoS2 layer, making a large number of reaction sites with fast electron transport exposed on graphene for further sulfur reduction. As a result, the MoS2-sulfurgraphene nanocomposites exhibit excellent cycling stability and rate performance. These encouraging results suggest that the design of 2D layer-structured and multi-functional hosts


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with the polar and nonpolar amphoteric surfaces would be a promising strategy to elevate the electrochemical performance of sulfur cathode materials for advanced Li-S batteries.

EXPERIMENTAL SECTION Preparation of MoS2-graphene nanocomposites The MoS2-graphene nanocomposite (MoS2-G) were synthesized through the layer-by-layer assembly cross-linked by polyethylene oxide (PEO) ligands from the exfoliated MoS2 and graphene oxides (GO) layer and subsequent hydrazine reduction. Typically, 0.15 g bulk MoS2 was dispersed in 5 mL n-butyllithium (n-BuLi, 1.6 M in hexane) and kept under magnetic agitation in argon atmosphere for a week. After fully lithiation, the mixture was filtrated and washed with hexane in argon atmosphere. Then, all the obainted LiMoS2 were rapidly dispersed into 200 mL aqueous solution containing 0.4 g GO, 0.03 g PEO (Mw = 100, 000) and 0.001 g CuCl2 ·2H2O. The resulting dispersion was sonicated for 2 h and consecutively stirred for 24 h. Next, 10 mL hydrazine hydrate (85%) was added to the above dispersion to react at 80 oC for 12 h with consecutive agitation. The MoS2-G composites were obtained by filtration and dried at 100 °C. Preparation of MoS2-sulfur-graphene nanocomposites The MoS2-sulfur-graphene nanocomposite (MoS2-S-G) were prepared following a conventional melt-diffusion strategy. In a typical synthesis procedure, 1 g sublimed sulfur and 0.5 g asprepared MoS2-G were mixed homogeneously. The mixture was degassed in a vessel and then sealed under vacuum. The melt infiltration was further carried out in the vacuum-sealed vessel at 155 °C for 10 h, and then the MoS2-S-G was obtained. For comparison, the conventional sulfurgraphene (S-G) nanocomposites were also prepared by incorporating the similar weight percent of sulfur into graphene.


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Material characterization The morphologies of the samples were observed under SEM (JEOL 7100F) and TEM (JEOL 2100F). The SEM mapping was performed under scanning electron microscopy (FEI Q-300). The XRD patterns of the samples were acquired on a Rigaku D/max 2550 diffractometer using Cu Kα radiation (λ = 1.5406 Å). The FTIR analysis of the samples was carried out using BRUKER-VERTEX 70 with high-sensitive detector MCT. Nitrogen adsorption/desorption isotherms were measured at 77 K with a Quadrasorb SI analyzer. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area. The total pore volume was calculated using a single point at relative pressure of 0.985. The pore size distributions were derived from desorption branch by using the Barrett-Joyner-Halenda (BJH) model. Thermogravimetric analysis (TA Instrument Q600 Analyzer) of samples was carried out in a nitrogen or air flow. The samples were heated to 600 °C with a rate of 10 °C min-1. The surface chemistry of the samples was analyzed using an Axis Ultra DLD X-ray photoelectron spectroscopy. The X-ray source was operated at 15 kV and 10 mA. The working pressure was less than 2 × 10-8 Torr (1 Torr = 133.3 Pa). The C 1s, S 2s, S 2p and Mo 3d XPS spectra were measured

at

a

step

size

of

0.1

eV.

The

binding

energies

were

calibrated

taking C 1s as a standard with a measured typical value of 284.6 eV. The S 2s, S 2p and Mo 3d XPS signals were fitted with mixed Lorentzian-Gaussian curves, and a Shirley function was used to subtract the background using a XPS peak processing software. Electrochemical tests The MoS2-S-G and S-G samples were slurry-cast onto a aluminium current collector. Typically, 80 wt.% MoS2-S-G, 10 wt% carbon black (Super P Conductive Carbon Black) and 10 wt% PVDF were mixed with N-methyl-2-pyrrolidone (NMP). The slurries were coated on aluminium


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current collectors and dried at 60 oC overnight. The active material loading is about 2 mg cm-2. Electrochemical tests of these electrode materials were performed using coin cells with the sulfur composite cathode and lithium metal as the counter electrode. The electrolyte was 1M bis(trifluoromethane) sulfonamide lithium salt (LiTFSI) dissolved in a mixture of 1.3-dioxolane (DOL) and dimethoxymethane (DME) (1:1 by volume). The separator was a microporous membrane (Celgard 2400). The cell was assembled in an argon filled glove box. The galvanostatic charge-discharge test and cyclic voltammetry measurements (CV) were conducted using an Arbin battery cycler (Arbin, BT2000, USA). All capacity values were calculated on the basis of sulfur mass. Electrochemical impedance spectroscopy (EIS) was performed with an electrochemical working station PCI4/300 (Gamry Instrument, Warminster, PA, USA). The sinusoidal excitation voltage applied to the coin cells was 5 mV, with frequency range from 100 kHz to 0.01 Hz. All the electrochemical tests were performed at room temperature.

RESULTS AND DISCUSSION


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Figure 1. Schematic illustration for the synthesis of MoS2-S-G layered structures (a) and the structural advantages of MoS2-S-G over S-G on trapping polysulfides and accommodating the volumetric change of sulfur during cycling (b).

The MoS2-graphene layered heterostructure (MoS2-G) was fabricated through the cross-linking assembly of polyethylene oxide (PEO) ligands with the exfoliated MoS2 and graphene oxide (GO) layer, and subsequent reduction process (Figure 1a). Briefly, liquid exfoliated MoS2, GO and PEO solution were homogenously mixed, followed by the addition of CuCl2. PEO chains were chemically binding to the MoS2 and GO layer by the coordination with Cu2+ ions, resulting in the confirmal assembly of interleaved structure. To improve the electrical conductivity of the MoS2-GO composite, GO was hydrazine-reduced into graphene. The detailed evolution of surface groups in the obtained MoS2-G sample was determined by the FTIR spectra, as shown in Figure S1. The elemental sulfur was then intercalated into the MoS2-G host through the facile melt diffusion method and occupied the interlayer space enlarged by the PEO chains between MoS2 and graphene layer. Thus, the 2D layered nanostructures composed of polar MoS2 layers, sulfur and highly conductive graphene were obtained. In this way, graphene and PEO chains benefit the electronic and ionic conductivity, respectively. Meanwhile, PEO chains could also act as the buffers for accommodating the volumetric expansion of sulfur during discharge processes. Moreover, benefiting from the efficient chemical bonding of MoS2 layer with polysulfides, the dissolution of polysulfides into the organic electrolyte is effectively mitigated during chargedischarge cycling (Figure 1b).


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Figure 2. (a) SEM, (b) SEM elemental mapping, (c) TEM and (d) HRTEM images of MoS2-G. (e) Schematic model of the MoS2-G structure at different different orientations. (f) SEM images of MoS2-S-G.


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The 2D nanoflake structure of the MoS2-G composite can be observed in SEM and TEM images. As shown in Figure 2a, the MoS2-G sample exhibits a curved graphene-like morphology with ultra-thin flaky appearance. SEM elemental mapping images (Figure 2b) reveal that MoS2 is homogeneously dispersed in the composite. TEM image in Figure 2c shows that the MoS2 layer is well supported on the surface of graphene, further confirming the layer-by-layer structure of the MoS2-G sample (Figure 2e). Moreover, HRTEM image (Figure 2d) reveals that the MoS2 nanosheets are in the form of 1-3 monolayers with an interlayer distance of 0.62 nm due to the efficient inhibition on the restacking of MoS2 layers by graphene layers. In the overlapped nanohybrid, the PEO chains sandwiched between MoS2 and graphene layers significantly enlarge the interlayer space, which could accommodate a large amount of sulfur wherein. As shown in Figure 2f, no sulfur agglomerates can be observed on the surfaces of MoS2-G hosts after sulfur loading. The N2 adsorption-desorption results in Figure S2 further demonstrate the successful incorporation of sulfur into the MoS2-G frameworks. As graphene and sulfur are both hydrophobic, sulfur will naturally affiliate the graphene. Such an close contact not only ensures the uniform dispersion of sulfur within the interlayer of MoS2-G host, but also favors the easy electron transportation between sulfur and graphene for sulfur redox.


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Figure 3. (a) XRD patterns of graphene, MoS2-G and MoS2-S-G. (b) TG curve of MoS2-S-G in N2 flow. The interleaved heterostructure of the MoS2-G and MoS2-S-G composites were further confirmed by XRD analysis. As shown in Figure 3a, the MoS2-G composite presents both MoS2 and graphene diffraction peaks with weak peak intensities, suggesting that the self-stacking of MoS2 and graphene has been greatly inhibited in the interleaved nanostructues. Additionally, two new broad peaks, centered at 6o and 12o (marked by #1 and #2, respectively), are found for the MoS2-G composite. The d-spacings of peak #1 and #2 are calculated to be 1.42 nm and 0.70 nm using the Bragg equation, respectively, which are different from the d(002)-spacing of MoS2 (0.61 nm) and graphene (0.34 nm). It is thus deduced that the peak #2 is possibly attributed to the PEO-enlarged interlayer spacing between the MoS2 and graphene layer, and the peak #1 with the twice d-spacing of peak #2 is assigned to the distance between the adjacent MoS2 layers. The layer-by-layer assembly between MoS2 and graphene in the MoS2-G sample could be also inferred from the small-angle XRD result in Figure S3. Thermogravimetric analysis (TGA) in Figure S4 reveals that the contents of MoS2 and PEO in the MoS2-G composite are approximately 24 wt% and 4 wt%, respectively. After sulfur incorporation into the MoS2-G host, peaks #1 and #2 disappeared. This is probably owing to the loss of the scattering contrast on sulfur imbibition,37 further confirming the filling of sulfur into the MoS2-G interlayer. Weak XRD diffraction peaks corresponding to crystalline sulfur can be identified in the MoS2-S-G composite, indicating that the nanosized crystalline sulfur is confined in between the MoS2-G interlayer. TGA analysis further shows that the content of elemental sulfur in MoS2-S-G is as high as 62 wt% after considering the mass loss of elemental sulfur and PEO in the TGA measurement (Figure 3b).


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Figure 4. High-resolution S 2p XPS spectra of MoS2-G (a) and MoS2-S-G (b). High-resolution S 2s and Mo 3d XPS spectra of MoS2-G (c) and MoS2-S-G (d).

The chemical bonding natures in the MoS2-G and MoS2-S-G composites were further investigated by high-resolution XPS. As shown in Figure 4a, two well-resolved S 2p1/2 and 2p3/2 peaks located at ∼163.2 eV and 162.0 eV with a spin-orbit splitting of 1.2 eV can be assigned to the divalent sulfide ions (S2-) of MoS2, and the satellite peak at 168.9 eV is corresponding to the Cu-S bonding originated from the coordination between MoS2 and Cu2+ ion in the MoS2-G composite. After sulfur loading, in addition to the S-Mo bonding in MoS2 and S-Cu bonding in the S-Cu coordination complex, a strong S 2p doublet peak at ∼165.1/163.9 eV, attributed to the S-S bonding in sulfur molecules, is present. The presence of sulfur molecule XPS spectra and the similarity of MoS2 XPS features in the MoS2-G composite before and after sulfur loading (Figure 4a-b) further confirm the intercalation of sulfur molecules into the interlayers of the MoS2-G composites during the sulfur melt diffusion process.


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To demonstrate the structural advantages of sulfur intercalated into the layered MoS2-G hosts with 2D polar-nonpolar amphoteric surfaces as cathode materials, the conventional sulfurgraphene (S-G) composites were also prepared by incorporating the similar weight percentage of sulfur with 2D nonpolar graphene for comparison (Figure S5). The electrochemical behaviors of the MoS2-S-G and S-G samples were studied by cyclic voltammetry (CV), galvanostatic chargedischarge testing, electrochemical impedance spectroscopy (EIS) and ex-situ XPS analysis.

Figure 5. (a) Initial charge-discharge curves of MoS2-S-G and S-G. (b) Discharge capacity ratios between at low- and high-voltage plateaus of MoS2-S-G and S-G at different cycles.

The initial charge-discharge profiles of the MoS2-S-G and S-G samples at 0.2 C are shown in Figure 5a. The S-G composite exhibits a clear overcharge capacity of ca. 100 mAh g-1, which is a typical feature of the polysulfide shuttling phenomenon. When sulfur is sandwiched within the polar-nonpolar interlayer, the overcharge capacity of MoS2-S-G disappears. Therefore, the polarnonpolar interlayers could serve as highly effective immobilizers to anchor polysulfide anions and prevent their outward diffusion. Besides, two voltage plateaus are observed for both the MoS2-S-G and S-G samples during the discharge process, which agree well with the two


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apparent reduction peaks in the CV curves (Figure S6). Considering the negligible electrochemial activity of MoS2 in the voltage range of 1.5-3.0 V (Figure S7-9) ,38, 39 the highvoltage plateaus at 2.0-2.4 V are corresponding to the reduction of S to soluble polysulfides (Li2Sx, 4 ≤ x ≤ 8), and the low-voltage plateaus at 1.5-2.0 V are assigned to the phase transformation from Li2S4 to insoluble Li2S. Both MoS2-S-G and S-G samples show similar specific capacities of 390 mAh g-1 (theoretically 419 mAh g-1) at the high-voltage plateaus, indicating that both samples have similarly high conversion efficiency from S to soluble Li2S4. Impressively, the MoS2-S-G composite demonstrates a much larger capacity enhancement at the low-voltage plateau of solid-state Li2S deposition than that of S-G composite, resulting in a significantly enhanced initial discharge capacity of 1231 mAh g-1, in comparison to that (1080 mAh g-1) of S-G composite. In order to better understand the Li2S deposition efficiency, the discharge capacity ratios between at low- voltage plateaus and at high-voltage plateaus (Qlow/Qhigh) has been calculated. As shown in Figure 5b, the MoS2-S-G composite offers significantly higher Qlow/Qhigh ratios than that of the S-G composite, confirming that the MoS2-S-G composite is more capable of reducing Li2S4 into the final Li2S product than the S-G couterpart. These results suggest that the 2D polarnonpolar interlayer structures of MoS2-G hosts could effectively adsorb polysulfides through strong chemical binding and spatially regulate the solid-state Li2S deposition with enhanced deposition efficiency, and as a result, sulfur is utilized much more efficiently in the MoS2-S-G than in conventional S-G composite.


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Figure 6. (a) Depth of the initial discharge curve of MoS2-S-G. Schematic illustration of (b) the uncontrolled solid Li2S deposition in the S-G composite and (c) the spatially regulated solid Li2S deposition in the MoS2-S-G composite. High-resolution S 2p XPS spectra of the MoS2-S-G composite at varied depthes of discharging (DOD): (d) 45% DOD at d point in Figure 6a; (e) 75% DOD at e point in Figure 6a; 100% DOD at f point in Figure 6a. Inset in Figure 6a is the schematic illustration of sulfur species distribution during discharging.

The spatial regulation of solid-state Li2S deposition in the MoS2-S-G composites with the 2D polar-nonpolar amphoteric surfaces, by the preferential chemisorption of sulfur species to polar MoS2 layers, were further confirmed by the ex-situ XPS analysis of discharge products at varied


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depthes of discharging (DOD). When the Li-S cells were discharged to 45% DOD (indicated by the d point in Figure 6a), in addition to the S 2p peaks in MoS2 and terminal (ST)/bridging sulfur (SB) of polysulfides, a doublet peak at the binding energy of ∼160.9/162.1 eV corresponding to Li2S was found, which could be attributed to the formation of solid Li2S product (Figure 6d). The Li2S peak position at 45% DOD is similar to that of the chemically adsorbed Li2S on the polar TiO2 surface as reported in the previous work,40 suggesting that the solid Li2S is preferentially deposited on the polar MoS2 layer at 45% DOD. When the cells were further discharged to 75% and 100% DOD (indicated by the e and f points in Figure 6a), the solid Li2S continuously formed and deposited, as shown in the inset of Figure 6a. After fully covering the MoS2 surface, the solid Li2S suspended and began to deposit on the graphene surface, which can be known from the presence of another doublet S 2p spectra corresponding to Li2S deposited on the nonpolar graphene surface with much lower binding energy than that on the polar MoS2 surface (Figure 6e-f). As sulfur reduction will take place more easily on the contact sites of sulfur/Li+/e- triple phases than at others, thus the spatially regulated preferential-deposition of insulating Li2S product on the polar MoS2 layer will enable a large number of reaction sites with fast electron transport exposed on graphene, thus providing significant enhancements in the discharging capacity and kinetics for sulfur reduction (Figure 6b-c).


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Figure 7. (a) Cycling performances and (b) Coulombic efficiencies of MoS2-S-G and S-G at 0.2 C. (c) Rate performances of MoS2-S-G and S-G at different current densities. (d) EIS of MoS2-SG and S-G before the first cycle and after the 20th cycle. (e) Long-term cycling performance of MoS2-S-G at 0.5 C and 1 C.


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Figure 7a compares the cyclic performances of the MoS2-S-G and S-G samples at 0.2 C. The S-G sample delivers a considerable discharge capacity of 664 mAh g-1 after 100 cycles, which is probably due to the enhanced electric conductivity and physical confinement of polysulfides within the graphene frameworks. On the basis of these, the MoS2-G interlayers demonstrate further suppressed shuttle effect and spatially regulated solid Li2S deposition, resulting in much enhanced cycling performances. As a result, The MoS2-S-G layered structure exhibits better electrochemical performances with a higher reversible capacity of 895 mAh g-1 after 100 cycles and a higher coulombic efficiency of 97.6% than the S-G counterpart (Figure 7b). Moreover, the MoS2-S-G sample presents better cycling responses to continuously varying current densities, as shown in Figure 7c. At the maximum discharging rate of 5 C (8.5 A g-1), the MoS2-S-G delivers a reversible capacity of 449 mAh g-1, much higher than that of S-G. After entended cycling for 200 cycles at 0.5 C and 1 C, the relatively high reversible capacities of 655 and 524 mAh g-1 have been retained for the MoS2-S-G sample (Figure 7e). The electrochemical performances of the MoS2-S-G sample are among the best series of the graphene-based sulfur cathode materials, as listed in Table S1. Such an excellent kinetic behavior of the MoS2-S-G composite is further supported by their much lower charge-transfer resistances and diffusion impedances than those of the S-G composite after 20 cycles from the EIS results (Figure 7d). These results further confirm that the layered MoS2-sulfur-graphene 2D structure demonstrates greatly enhanced kinetics of sulfur cathodes by exposing a large number of fast electron transport paths for sulfur reduction, exhibiting the attractive prospects of the spatial regulation strategy of solid Li2S deposition in the polar-nonpolar amphoteric layered structures for advanced Li-S batteries.


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CONCLUSION In conclusion, a 2D polar-nonpolar layered structure has been designed and constructed through the PEO cross-linked assembly with the polar MoS2 layer and nonpolar graphene layer, which was then employed as a multi-functional host to sandwich sulfur for the Li-S battery. The layered MoS2-sulfur-graphene structure enables greatly promoted electric and ionic transport of sulfur species and effective confinement of polysulfides via strong chemical bonding in the composite cathodes, and meanwhile, PEO acts as the buffer chains for accommodating the volumetric expansion of sulfur during discharging. Resultantly, such a unique 2D polar-nonpolar layered structure demonstrates spatially regulated deposition of insulating Li2S discharging product, and the excellent kinetics of accompanying sulfur reduction. Consequently, the layered MoS2-sulfurgraphene structure delivers excellent reversibility, capacity stability, and rate capability at up to 5 C. It is expected that the present sulfur intercalated into the layered MoS2-G hosts with 2D polarnonpolar amphoteric surfaces will be a highly promising sulfur cathode candidate, which may open up the prospect of constructing more efficient polar-nonpolar amphoteric nanostructures for high-energy Li-S batteries. ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: FTIR spectras of GO and MoS2-G, N2 adsorption-desorption isotherms and BJH pore size distributions of MoS2-G and MoS2-S-G, Small-angle XRD pattern of MoS2-G, TG curves of MoS2-G and PEO in air flow, TG curve of S-G in N2 flow, CV curves of MoS2-S-G and S-G at 0.1 mV s-1, The charge-discharge curves of MoS2-G at 320 mA g-1 between 0-3.0 V, The initial charge-discharge curves and cycling capacity of MoS2-S-G and MoS2 at 0.2 C bettween


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Graphic Table of Contents

Layer-structured MoS2-sulfur-graphene cathode materials with 2D polar-nonpolar amphoteric surfaces have been fabricated, which provide excellent cycle stability and rate performances for Li-S rechargeable batteries.


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Graphene Nanocomposites with 2D Polar ... - ACS Publications

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