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Lightweight Reduced Graphene Oxide@MoS2 Interlayer as Polysulfide Barrier for High-Performance Lithium-Sulfur Batteries Lei Tan, Xinhai Li, Zhixing Wang, Huajun Guo, and Jiexi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18645 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Lightweight Reduced Graphene Oxide@MoS2 Interlayer as Polysulfide Barrier for High-Performance Lithium-Sulfur Batteries Lei Tan, Xinhai Li*, Zhixing Wang, Huajun Guo, Jiexi Wang School of Metallurgy and Environment, Central South University, Changsha 410083, China Email: [email protected] (X. Li) Abstract: The further development of lithium-sulfur (Li-S) batteries is limited by the fact that the soluble polysulfide leads to the shuttle effect, thereby reducing the cycle stability and cycle life of the batteries. To address this issue, here a thin and lightweight (8 µm and 0.24 mg cm-2) reduced graphene oxide@MoS2 (rGO@MoS2) interlayer between the cathode and the commercial separator is developed as polysulfide barrier. The rGO plays the role as polysulfide physical barrier and an additional current collector, while the MoS2 have a high chemical adsorption for polysulfide. The experiments demonstrate that the Li-S cell constructed with rGO@MoS2 coated separator shows a high reversible capacity of 1122 mAh g-1 at 0.2 C, a low capacity fading rate of 0.116% for 500 cycles at 1 C and an outstanding rate performance (615 mAh g-1 at 2 C). Such interlayer is expected to achieve lithium-sulfur battery applications because of its excellent electrochemical performance and simple synthesis process. Key words: lithium-sulfur batteries, polysulfide, reduced graphene oxide, MoS2, interlayer 1. Introduction

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As a new type of renewable energy storage system, rechargeable battery is an extremely effective response measure for the gradual depletion of traditional energy, as well as the increasing environmental pollution1-3. Lithium-sulfur (Li-S) battery is one of the most promising next generation batteries owing to its high specific capacity of 1675 mAh g-1 and high specific energy density of 2600 Wh kg-14-6, which are higher than the traditional lithium-ion batteries7-12 and sodium-ion batteties13. Moreover, sulfur is abundant in the nature, low price and environmental friendly. However, there are several obstacles for Li-S battery widely used: (1) the volume expansion (≈80%) caused by the density difference of sulfur (2.03 g cm-3) and the discharge product Li2S (1.66 g cm-3)， leading to structural collapse and poor electrochemical stability14. (2) electrical insulation of the element sulfur and the solid product Li2S 15-16, making it difficult to achieve electrochemical theoretical values. (3) the soluble intermediate product polysulfide (Li2Sx, 4≤ x ≤8) migration between the cathode and the anode (shuttle effect), resulting in self-discharge and short cycle life17-19. Enormous strategies have been proposed to overcome these drawbacks of Li-S batteries, especially the shuttle effect. One of the most effective technique is to limit the sulfur in the cathode by designing a hollow or porous structure. Carbon-based materials5, 20-24, conductive polymer25, as well as oxide and sulfide14, 26-27 are used as sulfur host and polysulfide trap to suppress the shutter effect and enhance the electrochemical performance for Li-S batteries. However, the complex and costly synthetic processes of the composite cathode are not conducive to practical


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applications. In contrast, due to the simple and low cost of synthesis process, developing an interlayer between the commercial separator and sulfur cathode is an another promising approach to promote the utilization of sulfur and impede polysulfide migration. Up to now, the interlayer such as conductive carbon black28, porous carbon29-31, carbon nanotubes32, graphene oxide (GO)33 and reduced graphene oxide (rGO)34-35 have been worked as a physical barrier for absorption polysulfide. In recent woks, oxide and sulfide (Al2O336, SiO237, V2O538, MoS239, etc.) have also been used for coating layer because of their strong binding energy with polysulfide. However, these two kind of interlayer can hardly meet both of the high electrical conductivity and strong chemisorption to polysulfide. Moreover, the density of metal oxide/sulfide is very high, resulting in large mass increased of the interlayer, which cause a reduction in the specific capacity of the battery. The combination of metal compounds and carbon materials may be a more desirable solution. Here, we develop a rGO@MoS2 interlayer on the pristine commercial separator (Celgard 2400) to inhibit the shuttle effect and improve the electrochemical performance for Li-S batteries, of which the rGO sheet is used as a polysulfide physical barrier and an electronical conductive network, and the MoS2 plays the role of chemical adsorption of polysulfide. In addition, MoS2 can catalyze the reaction of long-chain polysulfide to short-chain polysulfide and act as a lithium conductor

39-40

. In consequence, the

high active material utilization and stable cycle performance of Li-S batteries are achieved. 2. Experimental section



2.1. Preparation of GO：：GO was prepared based on improved Hummers method, in brief, 5 g nitrate of potash (KNO3) was dissolved in 200 mL sulfuric acid (H2SO4, 95-98%,), 5.0 g crystalline flake graphite was added to the solution and stirred for 0.5 h under 2 ℃. and then 20.0 g potassium permanganate (KMnO4) was added to the mixture slowly, followed by mechanical stirring for 2 h and ensure that the reaction temperature does not exceed 2 ℃. After that, the mixture was stirred for 2 h at 35 ℃ and diluted with 400 mL deionized water, followed by mechanical stirring for 1 h at 95 ℃. Then, the mixture was further diluted with 200 mL deionized water. 30% hydrogen peroxide (H2O2, 30 mL) was then added to the mixture and the color of the solution was turned from dark green to bright yellow. The product was centrifuged and washed with 1-2% HCl solution and deionized water for several times to remove the metal ions and acid. Finally, GO was obtained by Freeze-dried for 48 h. 2.2. Synthesis of rGO@MoS2, rGO and MoS2: rGO@MoS2 was synthesis through one step hydrothermal method. 0.12 g GO, 0.12 g Sodium Molybdate Dihydrate (Na2MoO4·2H2O) and 0.24g Sulfourea (NH2CSNH2) were dispersed in 120 ml deionized water with magnetic stirred for 6 h, followed by ultrasonic for 1 h. The solution was transferred into a 200 mL Teflon-lined autoclave, maintained at 200 °C for 24 h by hydrothermal treatment. The product was washed with deionized water several times and then freeze dried for 48 h to obtain the rGO@MoS2. The rGO was synthesized from GO solution and the MoS2 was synthesized without using GO. The synthesis process was the same as rGO@MoS2. 2.3. Fabrication of the rGO@MoS2 and rGO coated separator: rGO@MoS2


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composite (3 mg) was dispersed in ethanol (30 ml) and sonicated for 0.5 h to form a uniform mixture, and the rGO@MoS2 was coated on the circular separator with a diameter of 40 mm by vacuum-filtration method, the area mass of the rGO@MoS2 layer was about 0.24 mg cm-2. The rGO coated separator was fabricated by the same process. 2.4. Electrochemical characterization: The cathode slurry was made of 70% sulfur (≥99.98%, Sigma Aldrich), 20% carbon black and 10% PVDF in NMP, spread onto aluminium foil by a doctor blade and then dried 12 h at 60 oC under vacuum. The weight of sulfur in the cathode was 1.8-2.0 mg cm-2. The electrolyte contains a mixture of 1,3-dioxolane (DOL) and 1,2-dime-thoxyethane (DME) (1:1 v/v), with 1 M bis (trifluoromethane) sulfonamide lithium salt (LiTFSI) and 0.1M LiNO3 dissolved in it. The electrolyte for each cell is ~50 µl. Lithium sheet was used as anode electrode. The pristine separator and rGO@MoS2 coated separator was used in assembling the 2025 cells. The CHI600E electrochemical measurement system was used for cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS) measurement. CV was obtained at the scan rate of 0.1 mV s-1 in the potential window from 1.7 V to 2.8 V. The EIS were recorded in the frequency range between 0.01 Hz and 100 kHz. The cells were charge/discharge in the potential range from 1.7 V to 2.8 V under NEWARE BTS 4000. All experiments were carried out and analyzed at room temperature. 2.5.

Materials

characterizations:

The

crystal

structure

of

the

samples were examined by X-ray diffraction (XRD, Rigaku, Rint-2000) with Cu Kα



radiation at the range from 5° to 80°. The morphology and structure of the samples were observed by scanning electron microscope (SEM, JEOL, JSM-5612LV) and transmission electron microscopy (TEM, Tecnai G12, 200 kV). The elemental distributions in the rGO@MoS2 composite were investigated by energy dispersive spectrometer (EDS). Raman (Renishaw) and ultraviolet spectrophotometer (U-4100) were utilized to analyzed the chemical bond of the samples. The weight content of MoS2 in the rGO@MoS2 composite was characterized by thermel gravimetric analysis (TGA) with a heating rate of 10 ℃ min-1 in oxygen. 3. Results and discussion The schematic of Li-S cell with rGO@MoS2 coated separator is shown in Fig.1a. the rGO@MoS2 interlayer facing the pure sulfur cathode, acts as the polysulfide barrier, and further reduces the shuttle effect due to the strong interactions between MoS2 particles and polysulfide. Meanwhile, with the excellent lithium ion and electronic conductivity, the rGO@MoS2 interlayer contributes to the transmission of lithium ions and function as an alternative current collector to collect and supply electrons, which ensures the rate performance and long cycle stability of Li-S battery. Fig. 1b shows the SEM image of rGO@MoS2 composite. It is obvious that the MoS2 particles are embedded in the graphene sheet uniformly, which proves that the two materials are well compounded and are favorable for the polysulfide adsorption. Therefore, a high sulfur utilization and an effective active material reutilization can be obtained. The cross-sectional SEM image (Fig. 1c) and photographs (Fig. S1a) of the rGO@MoS2 coated separator further show that the separator combines well with


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the rGO@MoS2. In addition, the rGO@MoS2 interlayer with a thickness of ~8 µm and a mass loading of ~0.24 mg cm2 covers the pores of the separator and can effectively inhibit polysulfide diffusion. For comparison, the SEM image of rGO interlayer with the same mass loading is shown in Fig. S1(b, c)

Fig. 1. (a) The schematic Li-S cell with rGO@MoS2 coated separator, (b) SEM image of rGO@MoS2 composite, (c) Cross-sectional SEM image of rGO@MoS2 coated separator

Fig. S2a shows the XRD patterns of GO, rGO, rGO@MoS2, MoS2, respectively. The



typical diffraction peak of GO at 11° disappears and shifts to a broad and weak peak at around 25° (002) for rGO, confirming that GO is reduced to graphene during the hydrothermal process, and restored to an ordered crystal structure. The rGO@MoS2 sample contains the characteristic diffraction peaks of rGO and MoS2, validating that the rGO@MoS2 composite was well synthesized. Fig. S2b presents the Raman spectra of GO, rGO and rGO@MoS2, and all of them show two characteristic peaks: The G band (1588 cm-1) and the D band (1345 cm-1), which corresponds to the hexagonal structure and the defects or disorders in the carbon matrix41-42, respectively. Compared with GO and rGO, the ID/IG ratio of rGO@MoS2 increases, indicating that the reduction process with the presence of MoS2 alters the structure of rGO with more structural defects, which is in accordance with the XRD results. The TEM image of the rGO@MoS2 composite is shown in Fig. 2a. The MoS2 particles with a size of about 200 nm are deposited from MoS2 nanoflakes and evenly dispersed on the surface of graphene, which is consistent with SEM image. Fig. 2b exhibits the typical HRTEM image of MoS2 lamellar structure and the interlayer distance is about 0.63 nm, which corresponds to the (002) lattice plane of MoS243. The corresponding elemental mappings of C, S and Mo (Fig. 2c) is in conformity with the TEM structure and further demonstrates the successful synthesis of the rGO@MoS2 composite. The MoS2 content in the rGO@MoS2 composite is based on the thermal gravimetric analysis. As is shown in Fig. S3, when heated to 700 ℃ in oxygen，the mass of rGO, MoS2 and rGO@MoS2 remains 2.6%, 80.3% and 20.8%, respectively. The content of MoS2 in the rGO@MoS2 composite can be


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calculated by the following equation, in which W represents the residual mass ratio of the sample and X represents the proportion of the material in the composite. The results are shown in Table S1, there is about 26% MoS2 in the rGO@MoS2 composite. 1 @

(1)

Fig. 2. (a) TEM image of rGO@MoS2 composite, (b) HRTEM images of MoS2, (c) Bright-field TEM image and elemental mappings of C, S and Mo.

The polysulfide adsorption capability of MoS2 can be clearly observed by Fig. 3, 10 mg MoS2 nanoflakes is added to 200 µL Li2S6 solution (0.05 M) in 5 mL DOL/DME (1:1 by volume), the glass jar was placed in the glove box for 12 hours after fully vibrating. It can be seen that the color of the solution fades obviously and become nearly colorless (insert photograph) compare to the original solution. In addition, the



Li2S6 concentration in the solution is further analyzed by Ultraviolet Absorption Spectrum. It is notable that the characteristic peak of S62- at 260 nm44-45 becomes weaker after adsorbed by MoS2, suggesting that most of the Li2S6 are absorbed in the MoS2 solid phase, which proves the obvious adsorption ability of MoS2 for polysulfide

Fig. 3. Ultraviolet Absorption Spectrum and Photograph (insert) of Li2S6 solution before and after adsorbed by MoS2,

In order to assess the advantages of rGO@MoS2 coated separator cells in electrochemical performance. The cyclic voltammograms (CV) and galvanostatic


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charge-discharge test of the cells with pristine separator and rGO@MoS2 coated separator are carried out at a potential range of 1.7-2.8 V. Fig. 4(a, b) show the first five cycles CV profiles with the scan rate of 0.1 mV s-1, both of the two samples present two typical peaks at 2.05 and 2.30 V in cathodic scan, representing the multi-step electrochemical reaction process from sulfur element to Li2S. The first peak corresponds to the conversion of the element sulfur to the soluble long-chain polysulfide (Li2Sx, 4≤x≤8), while the second peak represents the reduction of short-chain polysulfide to insoluble Li2S2 or Li2S. The peak at 2.35 V in the anodic scan is related to the transformation of Li2S2 or Li2S to long-chain polysulfide and further reduced to S846. With scanning number increases, the cell with rGO@MoS2 coated separator shows better curve coincidence and higher peak currents than pristine separator. This means that the cell with rGO@MoS2 coated separator has excellent cycling stability, fast kinetics and high capacity, owing to the high conductivity of graphene and the strong adsorption of MoS2 to polysulfide. Fig.4(c, d) show the galvanostatic charge-discharge profiles of pristine separator and rGO@MoS2 coated separator at a current density of 0.2 C (0.62 mA cm-2, 1 C = 1675 mAh g-1), there are two discharge plateaus and one charge plateau, which are coincide with the cyclic voltammetry (CV) profiles. While the rGO@MoS2 coated separator displays higher and flatter discharge plateaus and lower charge plateau, what followed is the smaller potential polarization (∆E), obtained from the 1th cycle at 50% depth of discharge (DOD), which demonstrate a faster redox reaction kinetics for those cells with rGO@MoS2 coated separator.



Fig. 4. CV and galvanostatic charge-discharge profiles of (a, c) pristine separator and (b, d) rGO@MoS2 coated separator.

Cyclic performance under a current density of 0.2 C further demonstrates the superiority of rGO@MoS2 interlayer. As shown in Fig. 5a, the cell using rGO@MoS2 coated separator deliver an initial specific capacity of 1121 mAh g-1, after 200 cycles, the reversible capacity retains 671 mAh g-1 with a 60% capacity retention. Nevertheless, the initial specific capacities of the cells with rGO coated separator and pristine separator are 1098 and 774 mAh g-1, with the residual capacities of 491 and 399 mAh g-1 after 200 cycles, corresponding to 45% and 52% capacity retention, respectively. The results indicate that the rGO interlayer can increase the capacity of


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the cell but has few effect on the cycling stability, because of its high electron conductivity but only physical blocking ability on polysulfide. On the other hand, under the synergistic effect of high conductivity of rGO and strong polysulfide adsorption of MoS2, the cell with rGO@MoS2 interlayer inhibits the best cycle performance. The rate performance of Li-S cells at the current rate form 0.1-2 C is presented in Fig. 5b, the reversible discharge capacities of the cell with rGO@MoS2 coated separator are 1184, 1005, 830, 745, 615 mAh g-1 at 0.1 ,0.2, 0.5, 1, 2 C, respectively, with the current density back to 0.1 C, the capacity also rose to 903 mAh g-1, which are higher than those with rGO coated separator and the pristine separator. The capacity advantage is extremely obvious in the high rate (1 C and 2 C), while the capacity of rGO coated separator is close to the pristine one. The corresponding Galvanostatic charge-discharge profile of rGO@MoS2 coated separator showed in Fig. S4, it exhibits the characteristic charge-discharge plateaus at each current density, suggesting a good reaction kinetics. These are owing to the excellent electrical conductivity of rGO and high lithium conductivity and strong affinity for polysulfide of MoS2, providing electron and lithium ion transport channels and improving the utilization of active materials. The conductivity of fresh cells using different separator are evaluated by electrochemical impedance spectroscopy (EIS) measurement. As shown in Fig. 5c, the intercept on the real axis at high frequency refers to the interface resistance (Rs) of the electrolyte and electrode, the diameter of the semicircle represents the charge transfer resistance (Rct)47. Based on the fitting curves. The Rs of



rGO@MoS2 coated separator (4.3 Ω) is less than rGO coated separator (5.1 Ω) and is much smaller than the pristine one (8.7 Ω), which may be attributed to the good contact between rGO@MoS2 and the electrolyte, and the increased lithium ion transport capacity by MoS2. The cells using the interlayer has much lower value of Rct (≈29 Ω) compared to pristine separator (53.3 Ω), resulting from the high electronical conductivity of rGO. In short, the cell with rGO@MoS2 interlayer possesses a high lithium ion and electrical conductivity, thus shows better rate performance and lower resistance. Besides, the long cycle stability test of the Li-S cell using rGO@MoS2 coated separator at 1 C is shown in Fig. 5d. An initial reversible discharge capacity of 877 mAh g-1 is delivered. And after 500 cycles, the remainder discharge capacity of 368 mAh g-1 is obtained with a coulombic efficiency above 95% and a low average capacity fading of 0.116%, indicating an outstanding cycle stability under long cycle life. The rGO@MoS2 interlayer plays a key role in achieving the high performance of the Li-S cell, Herein, the lamellar structure rGO block the pore of the separator and obstruct the migration of polysulfide, which is further chemically adsorbed by MoS2. Therefore, it is effective to alleviate the shuttle effect and improve the cycle life of the battery. In order to study the feasibility of the rGO@MoS2 coated separator in practical utilization. The electrochemical performance of the Li-S battery with higher sulfur in the cathode is carried out. The areal sulfur loading of the cathode is increased to 3.64 mg cm-2, which is about twice as high as before, while the weight of rGO@MoS2


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coating is maintained at 0.24 mg cm-2. As shown in the Fig. S5, the cell delivers an initial discharge capacity of 945 mAh g-1 at 0.2 C (1.38 mA cm-2), which is approximately 15.7% lower compared to the cell lower areal sulfur loading. As a result, the areal capacity~ 3.44 mAh cm-2 is obtained.

Fig. 5. Electrochemical performance of lithium–sulfur cells with pristine separator, rGO coated separator and rGO@MoS2 coated separator. (a) Cycling stability at 0.2 C



(b) rate performance and (c) Nyquist plots. (d) Long cycling performance of the cell with rGO@MoS2 coated separator at 1 C.

To better understand the benefits of the rGO@MoS2 interlayer, the cycled Li anode and the rGO@MoS2 composite are further investigated. As shown in Fig. 6a, For the conventional Li-S cell, the surface of the lithium anode becomes rough, with irregular shape and size, and appears apparent cracks, suffering from the continuous chemical reaction of lithium metal and polysulfide in the anode side. Conversely, with rGO@MoS2 coated separator, the lithium maintains a smoother and more complete surface without obvious cracks, profiting from the strong adsorption of rGO@MoS2 to polysulfide and prevent it from reaching to the anode region through the separator, and thus reduce the side reaction between lithium metal and polysulfide. The rGO@MoS2 coated separator is washed several times with DOL/DME (1:1 v/v) solution and placed in glove box overnight to remove the DOL/DME solution. The SEM image of rGO@MoS2 composite after 200 cycles at 0.2 C is presented in Fig. S6. It shows a uniform structure with the MoS2 particles encapsulated in the rGO sheets, which is similar to the structure before cycle. In order to further confirm the polysulfide ability of rGO@MoS2 composite, the S 2p XPS spectra of the cycled rGO@ MoS2 composite is analyzed (Fig. S7). The 160.96 eV peaks could be attributed to the sulfur from MoS2, while the very prominent peak at 162.26 eV usually assigned to S22-, which suggests the presence of L2S2. The peak at 163.67 eV is characteristic of S-S bond of element sulfur48. The other two peaks at166.42 and


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167.76 eV are due to the presence of SOx2-, which were likely result from the oxidation of sulfur during the hydrothermal reaction and the remaining lithium salt (LiTFSI)49.

Fig. 6. Surface SEM image of Li anode after 200 cycles at 0.2 C: (a) pristine separator and (b) rGO@MoS2 coated separator

4. Conclusion In summary, the rGO@MoS2 composite synthesized by one step hydrothermal method is successfully coated on the commercial separator via vacuum-filtration technology. As a consequence of the MoS2 nanoflakes embedded in the rGO layers, the thin and lightweight rGO@MoS2 interlayer meets both high electrochemical conductivity and rapid lithium ion transport capability. The diffusion of polysulfide is efficiency prohibited by the combination of physical block and chemical adsorption, meanwhile, the side effects of lithium metal and polysulfide are suppressed. As a result, with 70% pure sulfur in the cathode, the Li-S cells of rGO@MoS2 coated separator exhibit high capacity, good cycle stability, excellent rate performance and



long cycle life. It is an ideal strategy to accelerate the commercialization of Li-S batteries. Acknowledgments This work was supported by the National Science and Technology Support Program of China (No. 2015BAB06B00) and the strategic emerging industry science and technology research project of Hunan Province, China (No. 2016GK4029). Supporting Information Available Photographs of the rGO@MoS2 coated separator; SEM image, XRD patterns, TGA curves and Raman spectra of the samples; Galvanostatic charge-discharge profiles of the Li-S cell with rGO@MoS2 coated separator at different current densities; Cycling performance of the Li-S cell with rGO@MoS2 coated separator for high sulfur loading at 0.2 C; SEM image and S2p XPS spectra of cycled rGO@MoS2 composite. References (1) Zou, B.; Peng, F.; Wan, N.; Wilson, J. G.; Xiong, Y., Sulfur dioxide exposure and environmental justice: a multi–scale and source–specific perspective. Atmos. Pollut. Res. 2014, 5 (3), 491-499. (2) Yuan, B.; Wu, X.; Chen, Y.; Huang, J.; Luo, H.; Deng, S., Adsorption of CO2, CH4, and N2 on ordered mesoporous carbon: approach for greenhouse gases capture and biogas upgrading. Environ. Sci. Technol. 2013, 47 (10), 5474-5480. (3) Yu, J. G.; Yu, L. Y.; Yang, H.; Liu, Q.; Chen, X. H.; Jiang, X. Y.; Chen, X. Q.; Jiao, F. P., Graphene nanosheets as novel adsorbents in adsorption, preconcentration and removal of gases, organic compounds and metal ions. Sci. Total. Environ. 2015, 502,


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