Toward Environmentally Friendly Lithium Sulfur Batteries: Probing the

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Towards environmentally-friendly lithium sulfur batteries: Probing the role of electrode design in MoS2-containing Li-S batteries with a green electrolyte Lei Wang, Alyson Abraham, Diana Marie Lutz, Calvin Quilty, Esther S. Takeuchi, Kenneth J. Takeuchi, and Amy C. Marschilok ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06141 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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ACS Sustainable Chemistry & Engineering

Towards environmentally-friendly lithium sulfur batteries: Probing the role of electrode design in MoS2-containing Li-S batteries with a green electrolyte Lei Wang1, Alyson Abraham1, Diana M. Lutz1, Calvin D. Quilty1, Esther S. Takeuchi1,2,3, Kenneth J. Takeuchi1,3, and Amy C. Marschilok1,2,3* 1Department

of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400

2Energy

and Photon Sciences Directorate, Interdisciplinary Sciences Building, Building 734, Brookhaven National Laboratory, Upton, NY 11973

3Department

of Materials Science and Chemical Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275 Corresponding author : [email protected]

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Abstract While lithium sulfur batteries (Li-S) hold promise as future high energy density low cost energy storage systems, barriers to implementation include low sulfur loading, limited cycle life, and the use of toxic electrolyte solvents. A comprehensive study of Li-S cells in the environmentally benign di(propylene glycol) dimethyl ether (DPGDME)-based electrolyte, using as-prepared MoS2 nanosheets derived from a facile aqueous microwave synthesis as polysulfide trapping agents, is reported herein for the first time. Conventional coated foil electrodes and binder-free electrodes (BFEs) with various structures are systematically generated and tested, to correlate electrode design with the resulting electrochemical behavior. Significantly improved Li-S electrochemistry is demonstrated through the synergy of MoS2 chemistry and binder free electrode engineering. In the coating configuration, the MoS2-containing cell evinced better rate performance and stable cyclability than cell without MoS2. In comparison with the coating counterparts, the BFE cells exhibited excellent cycle stability and superior rate capability (10fold capacties and energy density per electrode weight with 20% higher retention rate), despite 2X higher areal sulfur loading. The BFE cell improvement can be attributed to the synergistic effect of the i) interconnected macroporous structure of CNT interlayers, providing a conductive framework; and ii) the efficient polysulfide trapping by the MoS2 nanosheets. Keywords: MoS2 nanosheet, sulfur, Li-S battery, polysulfide, electrode design, green electrolyte Synopsis: Lithium sulfur batteries incorporating a novel environmentally benign electrolyte, with aqueous-derived MoS2 nanosheets and beneficial electrode engineering are reported herein.


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Introduction Rechargeable lithium sulfur (Li-S) batteries have garnered much interest due to both the high theoretical capacity of a sulfur cathode upon full electrochemical conversion (1672 mAh/g), with a theoretical gravimetric energy density of 2500 Wh kg-1 and a volumetric energy density of 2800 Wh L-1.1-2 Additionally, sulfur as the electroactive material is a highly abundant, non-toxic element, with a desirable operating voltage of 1.5 – 2.5V (vs. Li/Li+), thus making it a sustainable and cost effective candidate for the future of high energy density batteries.3 However, the continued development and commercialization of the Li-S battery has been limited by the complex problem of polysulfide dissolution, in which parasitic reactions during the (dis)charge process can reduce the useable capacity and shorten the cycle life of the battery.4-7 The inherently low electrical conductivity of sulfur (5 x 10-20 S cm-1) provides an additional challenge.8 To improve upon the inherently low electrical conductivity of sulfur, a conductive matrix, which can be a carbon source, conductive polymer or metal, is often introduced to the electrode. Various carbonaceous materials, including but not limited to multi-walled carbon nanotubes, have been demonstrated to effectively enhance the conductivity of the sulfur active material.9-12 To tackle the polysulfide dissolution problem, “trapping” of mobile polysulfide species by introduction of additives demonstrating affinity towards polysulfide ions has been explored.1314

A range of additives has been previously demonstrated, including carbon sources, such as

carbon nanospheres,15-16 Ketjenblack carbon,17 carbonized eggshell membrane,18 graphene sheets,19 graphene coated cotton-carbon,20-21 and carbon interlayers,22 as well as metal carbides,23 2D conductive MXene nanosheets,24 and metal chalcogenides.25-27 Recently, MoS2 has been investigated as an additive to sulfur cathodes,3, 28-30 as well as a coating on separators.31-32 Ab initio DFT studies indicated higher binding energies for Li2S8, Li2S6, Li2S4, Li2S2 and Li2S 3 ACS Paragon Plus Environment


species on the MoS2 surface (0.10, 0.22, 0.32, 0.65 and 0.87 eV, respectively), which relative to the corresponding binding energies on the hierarchical carbon spheres.33 The enhanced binding of the MoS2 substrate originates from the strong chemical interaction with the Li atoms in Li2Sn.33-34 In addition, simulations further confirmed selectivity of Li2Sn binding to MoS2, with an increased affinity for binding to edge sites relative to planar surfaces, referred to as “terrace sites”. Specifically, binding energy calculations showed lower binding energy of Li2Sn to the MoS2 terrace site (~ 0.87eV), relative to the Mo- and S-edge sites (4.48 and 2.70 eV, respectively). The MoS2 terrace site had a higher binding energy compared to graphene (0.29eV), but the edge sites demonstrated stronger affinity than the terrace sites, leading to a selectivity towards the edge site binding compared to the terrace sites, for the polysulfides.35 Therefore, generating MoS2 with greater edge site availability (i.e. nanosheets) could potentially increase its ability to trap polysulfides. Previous studies have generated MoS2 nanosheets by heating Mo and S containing precursors using a microwave reactor, which can generate MoS2 nanosheet in a reduced time frame compared to traditional hydrothermally or solvothermally conducted reactions. The microwave method has been shown to decrease the reaction time from 18 hours for hydrothermal preparation36 to 2 hours.37 Frequently, organic solvents are used including glycerol,38, ethylene glycol,39 and dimethyl formamide.28, 37 In addition, some reactions require the use of inert environment depending on the reaction conditions.38 Solvents of the liquid electrolyte play a critical role that impacts the electrochemistry of Li-S batteries. Ether-based electrolytes using solvents such as 1,3-dioxolane (DOL), 1,2dimethoxyethane (DME), tetra-(ethylene glycol) dimethyl ether (TEGDME), and tri(ethylene glycol) dimethyl ether (triglyme) have been confirmed to be more suitable for the Li-S batteries 4 ACS Paragon Plus Environment

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than their carbonate-based counterparts.40 The most-commonly used electrolytes in Li-S batteries invariably consist of ether-based mixtures so as to allow optimization against a range of key parameters (volatility, viscosity, conductivity, etc.).41 A DOL:DME mixture has been predominantly used in recent publications owing to the high lithium polysulfide (LiPS) solubility and fast LiPS reaction kinetics, offered by linear DME, accompanied by the stabilization of the Li metal surface with cyclic DOL. However, DME is considered to be a hazardous component due to its high flammability and toxicity, rendering the consideration of alternative electrolytes highly desirable.42 Ethylene oxide based glyme electrolytes have been popular alternatives to the DME based solvent, as they can solvate Li+ through oxygen-ion chelation and dissolve Li salts of high concentration. Glymes have been studied as a single solvent and in binary or ternary mixture for Li-S battery electrolytes, benefiting from their low flammability and minimal toxicity, as well as high oxidative stability when complexing with Li+.43-44 Glymes solvate the lithium salt quickly, but their relatively high viscosity results in the accumulation of LiPS in the cathode and therefore low sulfur utilization.42 To date, the glymes studied as electrolyte solvents for Li-S batteries have been ethylene oxide-based. Di(propylene glycol) dimethyl ether (DPGDME) is a colorless aprotic solvent with a mild, pleasant odor, which is not a hazardous air pollutant, has low toxicity, and offers an environmentally friendly alternative to solvents being phased out as a result of the Clean Air Act.43 It is worth noting that some of the ethylene glymes that are often used as solvents in previous Li-S battery studies, such as triethylene glycol dimethyl ether and diethylene glycol dimethyl ether, have been confirmed to have a tendency to cause health effects, including birth defects and blood toxicity, and are under restricted use (import, manufacture or processing) by the U.S. Environmental Protection Agency (EPA) in the U.S. marketplace since 2015 (EPA-HQ5 ACS Paragon Plus Environment


OPPT-2009-0767).45 On the contrary, DPGDME is known as a very versatile and environmentally friendly solvent, which is not listed as a hazardous air pollutant (HAP), and has been proven to be less toxic than those ethylene-based glycol ethers.46 To our best knowledge, there is only one pre-existing paper that studied Li-S electrochemistry in DPGDME electrolyte, which was published by our group last year, probing the structural change during the first (dis)charge cycle in the Li-S system with the presence of TiS2 as additive, using in situ XRD technique.42 However, the cycling stability and electrode configuration of the Li-S cells were not probed in that paper and are introduced here for the first time. In addition to solvents of the liquid electrolyte, electrode architecture design has been demonstrated to play an important role in the resulting electrochemistry of batteries. Previously, binder-free electrodes were noted to possess higher capacities and better cycling stability at all tested rates, as compared with electrodes prepared by a conventional coating method in a CuFe2O4 nano/sub-micron wires-carbon nanotube composite system,47 attributed to the elimination of the passive binder and current collector by anchoring the redox-active material on a conductive fibrous network.48 Binder-free sulfur/carbon composite electrodes prepared using a sulfur sublimation method yielded a sulfur loading of 1.1 mg/cm2, and demonstrated a reversible capacity >700 mAh/g, and a Coulombic efficiency >90% after 300 cycles at C/2 rate.49 Threedimensional (3D) CNTs/S composite binder-free electrode prepared by thermo-mechanical (at ~155°C) pressing of sulfur powder onto freestanding 3D CNTs demonstrated a maximum specific energy of ~1200 Wh/kg.50 A significant recent advance was a tandem cathode comprised of integrated layers of single-walled carbon nanotube (SWCNT)/carbon nanofiber (CNF) thin films and CNF/S thin films which exhibited a high initial areal capacity of 12.3 mAh/cm2 with stable cycling stability even with a high sulfur loading of up to 16 mg/cm2.20 It is worth noting 6 ACS Paragon Plus Environment

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however that all the aforementioned electrodes were tested in the toxic and flammable DME:DOL-based electrolyte. Herein, we present a comprehensive study of Li-S cathode design in the environmentally benign DPGDME electrolyte, using as-prepared MoS2 nanosheets derived from facile microwave synthesis as polysulfide trapping agents. This includes an environmentally benign, one-pot preparation of MoS2 nanosheets generated by the microwave method, using only water as solvent. Different electrode configurations, including conventional coating electrodes and binder-free electrodes (BFEs) with either mixture or layer motif are systematically generated and subsequently tested, including demonstration of the benefit of BFE electrode construction on capacity and capacity retention. By systematically studying the role of electrode design in a series of MoS2-containing Li-S batteries with a green electrolyte, this study furthers progress towards the future development of environmentally sustainable lithium sulfur batteries.

Experimental Synthesis of S-CNT composite. The S-CNT composite was synthesized using an method adapted from a previous report.51 Porous multiwalled carbon nanotubes (MWNTs) were heated in air at 550 ℃ for 30 min, followed by a rapid cooling to room temperature. The CNT were subsequently used as host materials for sulfur impregnation, where a S-CNT composite was generated using a facile process whereby sulfur powder was sonicated in ethanol before mixing with the MWNT. The final product was collected by vacuum filtration, and dried in vacuo. Synthesis of MoS2 nanosheets.



The MoS2 nanosheets were synthesized by modification of a previously reported method.37 Briefly, the MoS2 nanosheets reported herein were prepared in aqueous media using tetrathiomolybdate ((NH₄)₂MoS₄) as the reagent. A CEM Discover SP microwave reactor was used at a 200°C with 2 hours reaction time. The product was collected by centrifugation and washed with water and ethanol before drying in vacuo. Sample Characterization. X-ray diffraction (XRD) patterns of the as-prepared MoS2 nanosheets were acquired using a Rigaku Smartlab diffractometer with Cu Kα radiation. To confirm the sulfur loading in the S-CNT composite, thermogravimetric analysis (TGA) data were collected with a Q500 instrument over the temperature range of 30-500 ℃ under N2 atmosphere. The structure and morphology of S-CNT composite and the representative S-CNT-MoS2 coating and binder-free electrodes were probed using an analytical high-resolution JEOL 7600F scanning electron microscope (SEM), operating at 10 kV accelerating voltage. Lowmagnification transmission electron microscope (TEM) images were acquired using an accelerating voltage of 120 kV on a JEOL JEM-1400 instrument, with a 2048 x 2048 Gatan CCD Digital Camera. High resolution TEM characterization results, including data associated with morphology and selected area electron diffraction, were acquired with a JEOL JEM 2100F instrument, equipped with a field-emission electron gun operating at 200 kV and a highresolution pole-piece possessing a 0.19 nm point-to-point resolution. The camera length (L) was calibrated using an Au standard sample. Raman spectra of the CNTs before and after air treatment were acquired on a Horiba Scientific XploRA instrument with a 532nm laser, using a grating of 1,200 lines/mm. Wavelength and intensity calibrations were completed by using an internal silicon standard,


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based upon a reference peak at 520 cm−1. Raw spectra were subsequently processed and analyzed using the WiRE 4.1 software. Baselines were subtracted from all of the spectra, and the integrated spectral areas were normalized to an area of 1. Brunauer−Emmett−Teller (BET) surface area of the material was determined using a Quantachrome NOVA 4200e with N2 gas as the adsorbent. Raman spectroscopy was used to evaluate the crystallinity and thickness information of the as-prepared MoS2 on a Horiba Scientific XploRA instrument with a 532nm laser. Wavelength and intensity calibrations were completed by using an internal silicon standard, based upon a reference peak at 520 cm−1. UV-visible (UV-vis) spectra in a range of wavelengths spanning from 251 to 700 nm were gathered at high resolution using a Spectronic Genesys5 instrument on various solutions within sealed quartz cells, maintaining a 10 mm path length. Absorption spectra were obtained for Li2S8 with known concentrations ranging from 0.2-1mM in dry DPGDME solvent (30ppm H2O) to create a calibration curve. For the adsorption studies, a known amount of MoS2 was added to 1 mM Li2S8/DPGDME solution followed by vigorous stirring for 30 min and storage without stirring overnight.52 Polysulfide concentration was subsequently determined according to the Beer-Lambert law. Electrochemical Methods. Preparation of S-CNT electrodes The conventional CNT-S coating electrode was prepared on aluminum foil using a combination of 80% active material (CNT-S composite with sulfur loading of 60wt%), 10% Ketjen black carbon, and 10% polyvinylidene fluoride (PVdF) binder by weight. The CNT-SMoS2 coating electrode was prepared using 71.4% S-CNT composite, 8.6% MoS2, 10% Ketjen



black carbon, and 10% PVdF binder, so that the MoS2 was 20wt%, relative to the sulfur mass present. The CNT-S binder-free electrodes (BFEs) were prepared by the vacuum filtration processes. A small amount of pristine CNTs (10wt%) were filtered first to form uniform thin layer, which served as a trapping layer with mesopores to prevent loss of sulfur nanoparticles through the filter papers. The S-CNT composite (80wt%) prepared in section 2.1 was mixed with additional 10% pristine MWNT through sonication in ethanol prior to the filtration process. With respect to the MoS2-containing BFEs, two different electrode configurations were prepared. While each had the same mass ratios (8.6wt% MoS2, 71.4wt% CNT-S composite, and 10% pristine MWNTs), for the mixture structure (designated as CNT-S-MoS2-M), the MoS2, CNT-S composite, and pristine MWNTs were sonicated together before vacuum filtration as one monolith, whereas for the layer structure (designated as CNT-S-MoS2-L), the MoS2 was prepared as a separate layer distinct from the CNT based layer (comprised of S-CNT composite, and pristine MWNTs). For both CNT-S-MoS2-M and CNT-S-MoS2-L BFEs, a thin layer of pristine MWNTs (10wt%) was prepared separately, similar to the aforementioned S-CNT BFE. The CNT-S-MoS2-M and CNT-S-MoS2-L BFEs were assembled with the thin CNT layer facing the separator. Two control BFEs, which contained only CNTs or only CNT-MoS2, were generated using a similar vacuum filtration process in absence of the CNT-S composite. Electrochemical testing The cathodes prepared as noted above were used to assemble two-electrode stainlesssteel experimental-type coin cells with lithium foil anodes, Celgard 2500 separator, and an electrolyte containing 1.0M lithium bis(trifluoromethane)sulfonimide (LiTFSI) and 0.2M LiNO3 salts in DPGDME solvent.


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Cycling and rate capability tests were conducted at 30°C using a Maccor Battery Tester. Lithium/sulfur cells were discharged and charged in a voltage window between 2.8-1.7V. The areal loadings of sulfur were calculated to be 0.56 mg/cm2 and 1.19 mg/cm2, for the coating and BFE cells, respectively. A rate capability test with discharge and charge rates applied in the sequence of C/20, C/2, 1C, 2C and C/20 rate (1C=1670 mA/g) for 5 cycles each. All the capacities in the following discussion were calculated based on the weight of sulfur and on the total weight of cathode. To further assess the cycling stability, the various cells were cycled at C/5 for additional 75 cycles. Cyclic voltammetry (CV) data were collected using a two-electrode configuration wherein the reference and counter electrodes were both lithium metal. Voltage limits for the CV test were 1.7 V and 2.8 V at a scan rate of 0.1 mV/s. The same cells used for the CV measurements were further tested under a more vigorous cycling at C/2 rate for additional 200 cycles, corresponding to areal currents of 0.5 and 1.0 mA/cm2 for the coating and BFE cells, respectively. Electrochemical impedance spectroscopy (EIS) data were collected over a frequency range of 1MHz to 10 mHz with a 10 mV amplitude at 30 ℃. Analysis of the impedance measurements was conducted by using ZView software. Results and Discussion Structure of the as-prepared CNT-S composite, MoS2 nanosheets, and resulting electrodes The morphology and structure of the as-prepared CNT-S composites were probed by TEM. The oxidized CNTs (Figure 1A) measured 8.9 ± 2.0 nm in diameter and possessed hollow tubular structures with partially degraded side walls. The Raman spectra associated with both pristine and oxidized CNTs are presented in Figure S1†. Specifically, the intensity ratio of the D band located at 1343 cm-1 to the tangential mode (G band) localized at 1576 cm-1 (ID/IG) definitively enhanced after the air treatment process. Such increase in the ID/IG ratio can typically be attributed to expected distortion of the intrinsic conjugated sp2 carbon lattice, arising from the 11 ACS Paragon Plus Environment


presence of amorphous carbon and other symmetry-breaking oxygenated defects in the CNTs generated during the oxidation.53-54 Crystalline sulfur nanoparticles (Figure 1A and B) with an average diameter of 8.5 ± 0.9 nm, were distributed on the surface of the CNTs through a physisorption interaction initiated by sonication. Diffraction patterns associated with selected area shown in Figure 1A were depicted in Figure 1C, which can be indexed to the CNT(002), S(135), S(333), and CNT(020) lattice planes, with corresponding d-spacings of 3.57, 3.07, 2.57, and 2.16 Å, respectively. TGA analysis (Figure 1D) of the S-CNT composite confirmed a sulfur mass loading of 60wt%.


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Figure 1. TEM image(A), HRTEM image (B), SAED pattern (C), and TGA profile (D) of the as-prepared S-CNT composite. The SEM (Figure 2A) and TEM (Figure 2B) of the as-synthesized MoS2 nanosheets showed ‘crumpled paper’-shape structures with uniform thickness, measuring 2.7± 0.6 nm. SAED patterns (inset Figure 2B) indexed to the (100), (103), and (110) of polycrystalline 2H MoS2. Side-on HRTEM (Figure 2C) suggested that the thin nanosheets were indeed composed of ~4-5 MoS2 layers, with an interlayer spacing of 6.4 ± 0.6 Å. To further investigate the crystal structure of the as-prepared MoS2 nanosheets, XRD data were acquired (Figure 2D). Based on the calculation using Bragg’s law and position of the 002 peak, the interplanar (002) spacing for 13 ACS Paragon Plus Environment


the as-synthesized MoS2 (d002) was determined to be 6.19 Å, consistent with observed in the HRTEM image (Figure 2B). Raman spectra were collected for both the bulk MoS2 and the as-prepared MoS2 nanosheets (Figure 2E). Two distinct Raman modes (E12g and A1g bands) can be clearly identified for both materials, which correspond to in-plane and vertical plane vibrational modes of Mo–S bonds. The linewidth of Raman peaks is useful to qualitatively compare crystal quality where narrower peaks indicate better crystallographic order.55 The MoS2 nanosheets demonstrated wider bands than their bulk counterpart, consistent with the less crystalline nature of the nanosheets. An additional peak at 220 cm−1 was noted for the MoS2 nanosheet sample only, corresponding to scattering from longitudinal acoustic (LA) phonons at the M point of the Brillouin zone, which was previously reported to indicate increasing lattice disorder, i.e. defect density.55-56 Typically, the two Raman modes, E12g and A1g, exhibited a thickness dependence, with the frequency of the former up-shifting and that of the latter down-shifting with decreasing thickness.57 The A1g band down-shifted from the bulk MoS2 to the MoS2 nanosheets (Figure 2E), which confirmed that the nanosheet sample was thinner than the bulk MoS2. However, the E12g band also down-shifted in the nanosheet sample, which can be attributed to the water adsorbed on the surface of the exfoliated MoS2 nanosheets affecting the in-plane atomic bending vibration, related to the behavior of E12g mode.58 The as-synthesized MoS2 had a relatively high surface area, measuring 76.3 m2/g based on the BET results. To further evaluate the adsorption capability of the MoS2 nanosheets, UV-vis spectra (Figure 2F) were measured before and after the absorption of 1mM Li2S8 polysulfide in the DPGDME solvent. For ether-based solvents, the peaks in the 300−550 nm range are generally attributed to polysulfide dianions.52, 59-60 The peak located at 308 nm has been 14 ACS Paragon Plus Environment

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previously assigned to the S62- species, while the peak at 443 nm attributes towards the presence of S42- species.60-61 After the overnight absorption from the 5 mg MoS2 nanosheets, the peak located at 443 nm completely disappeared, indicating a strong trapping effect of the S42- species. In addition, a significant decrease was noted in the intensity of the peak at 308 nm associated with S62- species, and the resulting concentration of polysulfides was estimated based on the Lambert-Beer law using absorption coefficient obtained from the calibration curve shown in Figure S2†. Based on the calculation, the concentration of the polysulfide solution decreased from 1mM to 0.69 mM after the absorption. In addition, the disappearance of the color of the dissolved Li2S8 solution (Figure 2F inset) unambiguously indicated the strong affinity of MoS2 edge sites towards the polysulfides.

Figure 2. (A)SEM, (B) TEM and SEAD pattern (inset), (C) HRTEM and (D) XRD profile of the as-prepared MoS2 nanosheets. (E) Raman spectra of both bulk MoS2 and MoS2 nanosheets. (F) UV-vis spectra as well as photographs (inset) of the 1mM Li2S8/DPGDME solution before and after the absorption by MoS2 nanosheets. Three different electrode configurations were developed to systematically probe the role of electrode design on the resulting electrochemical behaviors of the MoS2-containing Li-S cells 15 ACS Paragon Plus Environment


in the environmentally-benign DPGDME electrolyte (Figure 3). Specifically, in the coating cell, the cathode had a a diameter of ½ inch and a thickness of 20 μm with Al current collector and PVdF binders, resulting in an areal sulfur loading of 0.56 mg/cm2. Both BFEs measured ~100μm in thickness with an areal sulfur loading of 1.19 mg/cm2, two-fold higher than that in the coating cells. A thin interlayer composed of interconnected pristine CNTs was used in the construction of both BFEs. The CNT layer was produced together with the S cathode layer through a facile one-step vacuum filtration process, rendering a closer interaction between the CNT layer and the active materials.

Figure 3. Schematic illustration of coating, BFE-mixture and BFE-layer cell configurations. The morphology and structure of the various electrodes were investigated using SEM (Figure 4). Overall, the coating electrodes (Figure 4A-B) showed higher degree of aggregations as compared with the BFEs (Figure 4C-E), where continuous and interconnected CNT network was noted. Optical images of the BFE are also provided (inset, Figure 4C). The overall diameter of the electrodes was ~15 mm. The MoS2 nanosheets were uniformly dispersed within the CNT 16 ACS Paragon Plus Environment

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matrix in the CNT-S-MoS2-M electrode (Figure 4D), while in the CNT-S-MoS2-L electrode, no observable MoS2 nanosheets can be detected in the surface-view SEM image, since they existed as an interlayer in this electrode (Figure 3 and Figure 4H inset). The overall thickness of these BFEs (Figure 4F-H) was ~0.10 mm, with the top CNT layer (Figure 4F-H) and the MoS2 interlayer (Figure 4H) measuring 10 and 8 μm, respectively.

Figure 4. Surface-view SEM images of the CNT-S coating (A), CNT-S-MoS2 coating (B), CNTS BFE (C), CNT-S-MoS2-M (D), and CNT-S-MoS2-M (E), as well as side-view SEM images of the CNT-S BFE (F), CNT-S-MoS2-M (G), and CNT-S-MoS2-M (H) electrodes. Electrochemical behaviors of the CNT-S coating electrodes and BFEs. Cyclic voltammetry. Cyclic voltammograms of five cell groups containing CNT-S coating (A), CNT-S-MoS2 coating (B), CNT-S BFE (C), CNT-S-MoS2-M BFE (D), and CNT-SMoS2-L BFE (E) as cathode materials were collected (Figure 5). In the cathodic scan, all cells presented two remarkable reduction peaks at about 1.95 V and 2.30 V attributed to the formation of long-chain (i.e., Li2S8, Li2S6, Li2S4) and short-chain (i.e., Li2S2, Li2S) polysulfides, respectively.62 The reverse oxidation took place through an electrochemical processes between 2.2 and 2.5 V, attributed to conversion of Li2S2 or Li2S into soluble lithium polysulfides and elemental sulfur.63 Interestingly, two distinct oxidation peaks at 2.3 V and 2.4 V were noted in all



the cells, which have been shown previously only in a few sulfur conductive matrix systems with high conductivity and low polarization.60 The slight overpotential in the initial cathodic and anodic sweep in the coating cells (Figure 5A-B) in our system can be attributed to the polarization caused by the phase transition from the conversion–dissolution–diffusion process of the sulfur and polysulfides.22, 64 It is noted that compared to the coating cells, the BFE cells (Figure 5C-E) showed more polarization in the first cycle. This can be potentially attributed to the insufficient wetting of the electrode in the initial cycle, since the BFE has an overall thickness of 100 μm, which was five times higher than that of the coating cell. In addition, the sulfur loading in the BFE was two times higher than that in the coating cell, which lead to a higher electrode/electrolyte interface resistance initially. However, after the first cycle, the overpotential of the BFE decreased, likely due to wetting of the BFE with electrolyte, and rearrangement of the migrating active material to electrochemically favorable positions,64 with a corresponding decrease in electrode/electrolyte interface resistance.9 In addition, it is also noted that in the first cycle the CNT-S-MoS2-M BFE cell demonstrated more distinct oxidation peaks and less overpotential than those in the CNT-S and CNT-S-L BFE counterparts, which might be attributed to the low polarization and better electrochemical kinetics, resulting from the uniform dispersion of MoS2 within the CNT-S matrix.65 Two small reduction peaks evinced at 1.8V and 2.1V in the MoS2-containing BFEs, likely originating from the lithiation of the MoS2 additives in the first cycle, as suggested by the voltage profiles of the CNT-MoS2 BFE control cells in Figure S4† in the supporting information. Both peaks disappeared in the following cycles, indicating the initial lithiation of MoS2 does not contribute to the capacities of the Li-S cells subsequently. CV profiles in the third and subsequent cycles for all the cells were well overlapped, suggesting less polarization and high reversibility in the following cycles. The CNT-S-MoS2-M BFE (Figure


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5D) demonstrated higher current and better reversibility than its layered counterparts (Figure 5E).

Figure 5. CV results of various cells for cycles 1–5 at a scan rate of 0.1 mV/s, using CNT-S coating (A), CNT-S-MoS2 coating (B), CNT-S BFE (C), CNT-S-MoS2-M BFE (D), and CNT-SMoS2-M BFE (E) as cathode materials. Rate capability. Rate capability measurements were carried out on the various electrodes at a series of current densities (Figure 6). A total of 25 cycles using sequential rates of C/20, C/2, 1C, 2C, and C/20 (1C=1670 mA/g) were tested, and the corresponding specific capacities were calculated based on both the mass of sulfur (Figure 6A-B) and the mass of total cathode, including the weight of the binders, carbon additives, and the Al current collectors (Figure 6CD). All five cell groups demonstrated a observable fade under the slow rate of C/20, since a slow cycling rate of Li–S cells is known to be more vulnerable to the loss of sulfides.25 In terms of the coating cells (Figure 6A), the CNT-S cell delivered 987 mAh/g, 432 mAh/g, 358 mAh/g, 293 mAh/g, and 483 mAh/g, while the MoS2-containing analogue exhibited 949 mAh/g, 549 mAh/g,



483 mAh/g, 406 mAh/g, and 606 mAh/g at C/20, C/2, 1C, 2C, C/20, in cycle 1, 6, 11, 16, and 21. The MoS2 additives clearly improved the rate capability of the CNT-S coating cell. In contrast, the CNT-S BFE cell delivered 1888 mAh/g, 1085 mAh/g, 870 mAh/g, 750 mAh/g, and 1297 mAh/g under the same testing condition, despite a two-fold higher areal sulfur loading and higher areal current being applied to the BFEs (Figure 6B). The excellent rate performance of the BFE cell is in part attributed to the good conductivity of CNT interlayer, which can increase the electronic conductivity of the cathode. Furthermore, the porous CNT layer can enhance the tight trapping of sulfur species, as well as serve as a reservoir of the liquid electrolyte in which the transfer of lithium ions to active material can be accelerated. Contrary to the coating cells, the MoS2 additives in the BFEs did not show any appreciable improvement in their capacities under C/20 to 1C rates. The mixture motif delivered higher capacities at all rates than the layer counterparts, and exceeded the CNT-S BFE in cycle 19 and 20 at the high rate of 2C (Figure 6B). The high initial capacities (more than theoretical capacity) at the C/20 rate in the CNT-S and CNT-S-MoS2-M BFEs can be ascribed to the additional capacities originating from the CNTs and MoS2 sheets, as displayed in Figure 6B. It is worth noting that, when calculated using the weight of the total cathode, all three BFE cells demonstrated c.a.10-fold capacties (Figure 6D) in comparison with those of the coating cells (Figure 6C) at all rates tested, suggesting a much better rate capability than the coated electrode cells, attributed to the elimination of the inert binder materials and the Al current collector. The incorporation of MoS2 as a trapping layers on separators has been extensively studied in Li-S batteries in a DOL/DME solvent. For instance, the Cui group recently reported Li-S batteries using carbon nanofiber (CNF) plus Li2S8 catholyte and a MoS2 deposited Celgard separator, which demonstrated reversible specific capacity of 1010 mAh/g and 600 mAh/g at 0.5 C and 2 C rate, respectively.66 20 ACS Paragon Plus Environment

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A similar MoS2 coated Celgard separator was used by the Tang group in a Li-S coating cell, which delivered discharge capacities of 1471, 1039, 770 and 550 mAh g−1 at 0.1, 0.2, 0.5, and 1 C, respectively.67 By contrast, using our DPGDME-based electrolyte, the as-prepared CNT-SMoS2-M BFE presented in the current work delivered a capacity of 1812, 1011, 804, and 657 mAh/g at C/20, C/2, 1C and 2C, respectively, suggesting that the novel environment-friendly electrolyte can yield comparable if not better electrochemical performance when compared with that reported previously in the DOL/DME based electrolyte. The corresponding gravimetric energy density calculated based on the weight of sulfur as well as total cathode are displayed in the supporting information as Figure S3†. Notably, even at a high rate of 2C, the CNT-S and CNT-S-MoS2 BFE cells still delivered a specific energy of 600 Wh/kgelectrode in cycle 20 (Figure S3D), while the CNT-S and CNT-S-MoS2 coating cells only demonstrated 75 Wh/kgelectrode and 40 Wh/kgelectrode, respectively.



Figure 6. Specific capacity versus cycle number calculated based on sulfur weight and the total cathode weight for the coating cells (A and C) and the BFEs (B and D) measured at 200, 400, 800, 1600, and 200 mA/gsulfur discharge/charge current density. Voltage profile. The galvanostatic charge−discharge profile of five electrodes measured at various rates under the voltage window of 1.7−2.8 V vs. Li+/Li in the aforementioned rate capability test, was plotted (Figure 7). A distinct two-stage voltage plateau was observed for all five cells. The first plateau appeared at approximately 2.4 V, corresponding to the reduction of elemental S8 to higher order polysulfides (Li2S8). The overall reaction associated with the voltage window from 2.8-2.1 V can be expressed as S8 + 4Li+ + 4e-→ 2Li2S4. The second plateau at 2.1 V can be assigned to the formation of insoluble Li2S2 and Li2S through association with additional lithium ions, i.e., 2Li2S4 + 12Li+ + 12e-→ 8Li2S. Theoretically, the capacity 22 ACS Paragon Plus Environment

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corresponding to the discharge of the second stage should be three times higher than the capacity of the first stage. Hence, the capacity ratio of the capacity in the second stage to that in the first stage can be used as an indicator to the trapping effect of polysulfides from the MoS2 nanosheets. However, the relatively sluggish solid-state diffusion in the second plateau as well as the dissolution loss of higher order polysulfides often leads to a capacity ratio less than 3.62 As summarized in Table S1, the ratio of CNT-S coating cell drastically reduced from 2.15 to 1.09 with an increasing (dis)charge rate, while the MoS2-containing coating cell demonstrated slightly higher ratios spanning from 2.18 to 1.17. The BFE cells consistently exhibited higher ratios than their coating counterparts, with the CNT-S-MoS2-M BFE yielding ratios from 2.53 to 1.50, representing the highest ratios amongst all five cells. These results above collectively confirmed i) MoS2 nanosheets effectively contributed to higher utilization of sulfur in both the coating and BFE cells; ii) the BFE cells lead to more efficient polysulfide trapping than the coating counterpart; iii) the mixture BFE cell configuration is more suitable for improved cell performance than the tandem layer motif. A small plateau at 1.9V was noticed in the CNT-SMoS2 BFE cells (Figure 7D and E), likely originating from the lithiation of MoS2 nanosheets and CNTs at this stage, consistent with the voltage profile of the CNT-MoS2 only BFE control cell (Figure S4†). To further delineate the capacity contribution from the CNT and MoS2 within the CNT-S-MoS2 BFE cells, the capacities of the CNT-only and CNT-MoS2 only BFEs at various (dis)charge rates, as well as the corresponding voltage profiles are provided in Figure S4 in the supporting information. In the CNT-only BFE control cell, an initial discharge capacity of 48 mAh/g was noted at C/20 rate (Figure S4A), which mainly corresponded to the plateau between 1.9-1.8V in the voltage profile displayed in Figure S4C. The capacities drastically decreased to 6 mAh/g at C/2 rate in cycle 6, and remained low for the subsequent cycles. Similarly, the CNT-



MoS2 only BFE control cell delivered a discharge capacity of 103 mAh/g at C/20 in the first cycle (Figure S4B), which was mainly originated from the lithiation of MoS2 at 1.9V, as suggested by the discharge profile in Figure S4D. The lithiation of MoS2 in the first cycle contributed additional capacity to the CNT-S-MoS2 BFE, which leads to an overall capacity higher than the theoretical value based on sulfur mass alone. Notably, the plateau disappeared starting from cycle 6 at C/2. The CNT-MoS2 only BFE control cell delivered 32, 25, 17, and 40 mAh/g, at C/2, 1C, 2C, and C/20, in cycles 6, 11, 16, and 21, respectively (Figure 6). Hence, the capacity contribution from the CNT and MoS2 to the CNT-S-MoS2 BFE cells is assumed to be negligible.

Figure 7. Voltage profiles of CNT-S coating (A), CNT-S-MoS2 coating (B), CNT-S BFE (C), CNT-S-MoS2-M BFE (D), and CNT-S-MoS2-M BFE (E) as cathode materials. Cycling behavior and specific capacities. The cycling behavior including the Coulombic efficiency based on the weight of sulfur in the coating and BFE cells was determined (Figure 8). 24 ACS Paragon Plus Environment

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The corresponding capacities calculated based on the weight of the total electrode were also determined (Figure S5†). The same cells tested for rate capability (25 cycles) were subsequently cycled at C/5 rate for additional 75 cycles. In cycle 26, the CNT-S and CNT-S-MoS2 coating cells (Figure 8A) delivered capacities of 308 mAh/g and 510 mAh/g, respectively. After 75 cycles, a capacity of 210 mAh/g and 399 mAh/g was maintained, for the two coating cells, corresponding to the retention rate of 68% and 78%, respectively. The Coulombic efficiency was around 100% and 99% for the CNT-S and CNT-S-MoS2 coating cell, respectively. Therefore, the MoS2-containing coating cell clearly exhibits enhanced capacity and improved stability as compared with the coating cell without MoS2. In terms of the BFE cells (Figure 8C), the CNT-S, CNT-S-MoS2-M, and CNT-S-MoS2-L cells delivered 1181 mAh/g and 1093 mAh/g, 972 mAh/g and 811 mAh/g, 547 mAh/g and 481 mAh/g, in cycle 26 and 100, corresponding to retention rates of 93%, 83%, and 88%, respectively, much higher than those of the coated electrode cells. The Coulombic efficiencies measured 100%, 99%, and 100% for the CNT-S, CNT-S-MoS2-M, and CNT-S-MoS2-L BFE cells, respectively. To further assess the cycling stability at higher rate, the same cells used for CV measurement were subsequently cycled at C/2 rate for additional 200 cycles, as displayed in Figure 8B and D, for the coating cells and BFE cells, respectively. In terms of the coating cells, the cell without MoS2 delivered 458 mAh/g in the first cycle and maintained a capacity of 221 mAh/g after 200 cycles, corresponding to a retention rate of 48%, and a Coulombic efficiency measuring 97%. By contrast, the cell containing MoS2 nanosheets delivered 585 and 277 mAh/g for cycles 1 and 200, respectively, suggesting a retention rate of 47% and Coulombic efficiency of 89%. Our data indicated that although the MoS2-containing coating cell delivered higher capacities at a C/2 rate, the retention rate and Coulombic efficiency were indeed lower than those 25 ACS Paragon Plus Environment


of the cell without MoS2. With respect to the BFE cells, the CNT-S, CNT-S-MoS2-M, and CNTS-MoS2-L cells delivered 704 and 437 mAh/g, 1088 and 694 mAh/g, 581 and 373 mAh/g, in cycles 1 and 200, corresponding to capacity retentions of 62%, 64%, and 64%, respectively. The Coulombic efficiency was ~100% for all three BFE electrode types under this condition. The remarkably improved cycling performance and Coulombic efficiency of the BFE cells may be attributed to the macroporous CNT interlayer, providing an electronic conductive network to suppress soluble long-chain polysulfides diffusing to the anode and enhance the active material utilization. It is worth noting the CNT-S-MoS2-M BFE demonstrated 1.5-fold higher capacities with similar capacity retention as compared to those of the CNT-S BFE cell at the higher rate (C/2), although the latter exhibited better rate capability (Figure 6B) and higher capacities at a slower cycling rate of C/5 (Figure 8C). These data indicate that the incorporation of MoS2 nanosheets was more beneficial and critical at higher discharge/charge rates. The outstanding high rate cycling performance of the CNT-S-MoS2-M BFE can be possibly attributed to the better trapping effect of the MoS2 nanosheets within this cell configuration, as suggested in Table S1. The CNTS-MoS2-L BFE cell consistently delivered lower capacities at various rates (Figure 6B) and during extended cycling (Figure 8C and D), which can be ascribed to the low electrical conductivity of the MoS2 interlayer, limiting the efficient redox reactions of the trapped soluble polysulfides, leading to the formation of huge aggregation of the irreversible discharge products on the MoS2 surface.


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Figure 8. Capacities and coulombic efficiency versus cycle numbers during extended cycling at C/5 rate and long-term cycling at C/2 rate for the coating cells (A and B), as well as BFE cells (C and D). Capacities were calculated based on sulfur weight in the electrode. The corresponding gravimetric energy density during the cycling were measured using the weight of sulfur, and the weight of total electrode, which were displayed in Figure 9 and Figure S6†, respectively. In terms of the coating cells, the CNT-S and CNT-MoS2 cells delivered 443 and 843 Wh/kgsulfur at cycle 100 under a C/5 rate, as well as 464 and 579 Wh/kgsulfur at cycle 200 under a C/2 rate. The MoS2 additives unambiguously improved the energy density of the resulting coated electrode cell. With respect to the BFE cells, the CNT-S, CNT-S-MoS2-M, and CNT-S-MoS2-L cells delivered 2281, 1681, and 996 Wh/kgsulfur in cycle 100 at C/5, as well as 914, 1435, and 777 Wh/kgsulfur in cycle 200 at C/2. Overall, the BFE configuration lead to a 2fold gravimetric energy density when compared to that of the coating counterpart. 27 ACS Paragon Plus Environment


When normalized to the total weight of electrode (Figure S6†), the CNT-S-MoS2-M BFE cell delivered a specific energy of 615 Wh/kgelectrode in cycle 200 at C/2, while the CNT-S-MoS2 coating cell only demonstrated 55 Wh/kgelectrode. In order to further understand the outstanding cycling performance of the BFE cells at a high rate of 2C, cells were disassembled after 200 cycles, and the separators were taken out of the cells. The photographs of the separators are shown in Figure S7† in the supporting information. In the center of the separators from the BFE cells, the white color of the pristine separator was retained without any yellow or brown (typical color of high order polysulfides) tinge, even after 200 cycles at C/2. By contrast, a yellowish brown color was over the entire separator area in the two coated electrode cells, consistent with a greater degree of dissolution and polysulfide shuttling,68 indicating the BFE cell configuration is indeed more efficient in terms of trapping the soluble polysulfides.


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Figure 9. Specific energy versus cycle numbers during extended cycling at C/5 rate and longterm cycling at C/2 rate for the coating cells (A and B), as well as BFE cells (C and D). Energy densities were calculated based on sulfur weight in the electrode. EIS. To further investigate the reaction kinetics, Nyquist plots of the coating and BFE cells before and after the rate capability test for 25 cycles were acquired (Figure 10), with the equivalent circuit used for fitting displayed as inset in Figure 10D. In the model, R1 is ohmic resistance of the cell, including the electrolyte and electrode resistances. CPE is a constant phase element. R2 at the higher frequency region is related to the solid-electrolyte interface (SEI) film formed during the charge–discharge process. R3 at the high frequency to medium is attributed to the interface charge-transfer resistance. The inclined line at the low-frequency region is the Warburg element Wo.69 The detailed fitting results of R1, R2, and R3 are summarized in Table 29 ACS Paragon Plus Environment


S2. Before cycling, the CNT-S coating cell had the highest R2 of 12.1 Ω amongst all five cell configurations, which can be attributed to the reaction between the soluble long-chain polysulfides formed during self-discharging process and the Li anode, forming solid short-chain lithium sulfides layers.68 The incorporation of MoS2 additives in the coating cell lead to decrease of R2 to 4.5 Ω, suggesting an efficient trapping of soluble polysulfides by MoS2. All three BFE cells demonstrated R2 around 1-2 Ω, indicating a tighter trapping in the BFE configurations. In terms of the charge transfer resistance, both coating cells showed similar values of 29 Ω, while the BFE cells exhibited smaller charge transfer impedance of 15.8, 22.7, and 17.7 Ω, for the CNT-S, CNT-S-MoS2-M, and CNT-S-MoS2-L cells (Figure 10B), in part due to the more conductive CNT interlayer, which in turn result in improved rate capability in comparison with that of the coating cells. After 25 cycles, it is noted that all the cells demonstrated a remarkably smaller charge transfer resistance except the CNT-S coating cell. The descending trend of Rct after cycling has been reported previously,22, 69 attributed to an increase of reactive contact area over time with continuous immersion in electrolyte. For the BFE cells, R2 remained relatively similar before and after cycling, reflecting the stability of the solid-state interphase. Meanwhile R2 increased while the charge-transfer resistance R3 decreased drastically for the CNT-S-MoS2 coating cell, possibly due to the reorganization of the passivation film and redistribution of the active material in this cell.


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Figure 10. Nyquist plot of the alternating-current impedance response of coating and BFE cells (A and C) before discharge and (B and D) after 25 cycles. Inset in Figure 9D was used as equivalent circuit.



Conclusion In this paper, MoS2 nanosheet-containing Li-S cathodes with three different electrode configurations were successfully developed and subsequently measured in an environmentally friendly DPGDME-based electrolyte for the first time. The DPGDME solvent used for the electrolyte in our present study is non-toxic, a significant advance for future high-performance Li–S batteries as next generation energy storage systems. In the coated electrode construction type, the cells containing MoS2 nanosheets as additives consistently demonstrated improved rate capability and better cycling stability than the cells without MoS2. The BFE cells exhibited significantly improved cycle stability with high specific energy density, and superior rate capability compared to the coated electrode cells, despite two fold higher areal sulfur loading and higher areal current applied to the BFE. The improvement can be attributed due to the synergistic effect of i) interconnected macroporous structure of CNT interlayers, providing a conductive framework to trap polysulfides in the cathode side, while increasing the accessibility of active material to the electrolyte and charge, thus enhancing the transfer of ions and electrons; and ii) the efficient polysulfide trapping by the MoS2 nanosheets. The MoS2 nanosheets were incorporated through either a direct mix with the active materials forming a heterostructure, or as a separate layer residing in between the CNT layer and the S cathode layer underneath. The mixed binder free electrode (BFE) construction (CNT-SMoS2-M BFE) demonstrated improved high rate cycling performance compared to the layered analogue (CNT-S-MoS2-L), delivering a capacity of 694 mAh/gsulfur and a specific energy of 1435 Wh/kgsulfur (600 Wh/kgelectrode) at C/2 rate after 200 cycles. The improvement is attributed to the better trapping effect of the MoS2 nanosheets and more efficient redox reactions within this cell configuration. These results should inform future development of high functioning


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lithium sulfur batteries, by synergistically harnessing the beneficial aspects of materials chemistry and electrode engineering.

Acknowledgements This work was funded by the U. S. Department of Energy Office of Energy Efficiency and Renewable Energy, under award DE-EE0008208. Experimental research characterizations were carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, an Office of Science User Facility, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. We would also like to acknowledge the Advanced Energy Research and Technology Center for access to the ThINC facility. E.S.T. acknowledges the William and Jane Knapp Chair in Energy and the Environment. Supporting Information Raman spectra; UV-vis spectra; rate capability results and voltage profiles for CNT-only BFE and CNT- MoS2 only BFE cells; capacities, Coulombic efficiencies, and specific energies versus cycle numbers during extended cycling at C/5 rate and long-term cycling at C/2 rate for coating and BFE cells reported as total electrode weight; photographs of the separators after 200 cycles at C/2 rate; Total capacity as well as ratio of capacity from 2nd plateau and 1st plateau at all rates; EIS circuit parameters of the coating and BFE cells, both before and after 25 cycles. References 1. Ji, X.; Nazar, L. F., Advances in Li–S Batteries. Journal of Materials Chemistry, 2010, 20, 9821 - 9826, DOI 10.1039/B925751A. 2. Yu, S.-H.; Feng, X.; Zhang, N.; Seok, J.; Abruña, H. D., Understanding Conversion-Type Electrodes for Lithium Rechargeable Batteries. Accounts of Chemical Research, 2018, 51, 273 281, DOI 10.1021/acs.accounts.7b00487. 33 ACS Paragon Plus Environment


3. Gu, X.; Lai, C., Recent Development of Metal Compound Applications in Lithium– Sulphur Batteries. Journal of Materials Research, 2017, 33, 16 - 31, DOI 10.1557/jmr.2017.282. 4. Mikhaylik, Y. V.; Akridge, J. R., Polysulfide Shuttle Study in the Li/S Battery System. Journal of the Electrochemical Society, 2004, 151, A1969 - A1976, DOI 10.1149/1.1806394. 5. Evers, S.; Nazar, L. F., New Approaches for High Energy Density Lithium–Sulfur Battery Cathodes. Accounts of Chemical Research, 2013, 46, 1135 - 1143, DOI 10.1021/ar3001348. 6. Safari, M.; Kwok, C. Y.; Nazar, L. F., Transport Properties of Polysulfide Species in Lithium–Sulfur Battery Electrolytes: Coupling of Experiment and Theory. ACS Central Science 2016, 2, 560 - 568, DOI 10.1021/acscentsci.6b00169. 7. Mistry, A. N.; Mukherjee, P. P., “Shuttle” in Polysulfide Shuttle: Friend or Foe? Journal of Physical Chemistry C, 2018, 122, 23845 - 23851, DOI 10.1021/acs.jpcc.8b06077. 8. Gu, X.; Lai, C.; Liu, F.; Yang, W.; Hou, Y.; Zhang, S., A Conductive Interwoven Bamboo Carbon Fiber Membrane for Li–S Batteries. Journal of Materials Chemistry A, 2015, 3, 9502 - 9509, DOI 10.1039/C5TA00681C. 9. Carbone, L.; Coneglian, T.; Gobet, M.; Munoz, S.; Devany, M.; Greenbaum, S.; Hassoun, J., A Simple Approach for Making a Viable, Safe, and High-Performances LithiumSulfur Battery. Journal of Power Sources, 2018, 377, 26 - 35, DOI 10.1016/j.jpowsour.2017.11.079. 10. Zhang, X.; Wang, Z.; Yao, L.; Mai, Y.; Liu, J.; Hua, X.; Wei, H., Synthesis of Core-Shell Covalent Organic Frameworks/Multi-Walled Carbon Nanotubes Nanocomposite and Application in Lithium-Sulfur Batteries. Materials Letters, 2018, 213, 143 - 147, DOI 10.1016/j.matlet.2017.11.002. 11. Kim, M. S.; Ma, L.; Choudhury, S.; Moganty, S. S.; Wei, S.; Archer, L. A., Fabricating Multifunctional Nanoparticle Membranes by a Fast Layer-by-Layer Langmuir–Blodgett Process: Application in Lithium–Sulfur Batteries. Journal of Materials Chemistry A, 2016, 4, 14709 14719, DOI 10.1039/C6TA06018H. 12. Ai, G.; Dai, Y.; Mao, W.; Zhao, H.; Fu, Y.; Song, X.; En, Y.; Battaglia, V. S.; Srinivasan, V.; Liu, G., Biomimetic Ant-Nest Electrode Structures for High Sulfur Ratio Lithium–Sulfur Batteries. Nano Letters, 2016, 16, 5365 - 5372, DOI 10.1021/acs.nanolett.6b01434. 13. Zhang, J.; Huang, H.; Bae, J.; Chung, S.-H.; Zhang, W.; Manthiram, A.; Yu, G., Nanostructured Host Materials for Trapping Sulfur in Rechargeable Li–S Batteries: Structure Design and Interfacial Chemistry. Small Methods, 2017, 2, 1700279 - N/A, DOI 10.1002/smtd.201700279. 14. Peng, L.; Zhu, Y.; Chen, D.; Ruoff, R. S.; Yu, G., Two-Dimensional Materials for Beyond-Lithium-Ion Batteries. Advanced Energy Materials, 2016, 6, 1600025 - N/A, DOI 10.1002/aenm.201600025. 34 ACS Paragon Plus Environment

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TOC figure

Synopsis Environmentally benign electrolyte and MoS2 from a facile aqueous microwave synthesis result in excellent cycle stability for lithium sulfur batteries.


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Toward Environmentally Friendly Lithium Sulfur Batteries: Probing the

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