Thin-Layered Molybdenum Disulfide Nanoparticles as an Effective

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Thin-Layered Molybdenum Disulfide Nanoparticles as an Effective Polysulfide Mediator in Lithium-Sulfur Batteries Pauline J. Han, Sheng-Heng Chung, and Arumugam Manthiram ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05397 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Applied Materials & Interfaces

Thin-Layered Molybdenum Disulfide Nanoparticles as an Effective Polysulfide Mediator in LithiumSulfur Batteries

Pauline Han, Sheng-Heng Chung, and Arumugam Manthiram*

AUTHOR ADDRESS

Ms. Pauline Han, Dr. Sheng-Heng Chung, Prof. Arumugam Manthiram

Materials Science and Engineering Program & Texas Materials Institute

The University of Texas at Austin, Austin, TX 78712, USA

*E-mail: [email protected] (Arumugam Manthiram)

KEYWORDS Lithium-sulfur batteries, molybdenum disulfide, electrochemical performance, coated separator, polysulfide barrier

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ABSTRACT Lithium-sulfur (Li-S) batteries are attractive as sulfur offers an order of magnitude higher charge-storage capacity than the currently used insertion-compound cathodes. However, their practical viability is hampered by low electrochemical stability and efficiency, which results from severe polysulfide (LiPS) shuttling during cycling. We present here thin-layered MoS2 nanoparticles (MoS2-NPs) synthesized through a one-pot method and coated onto a commercial polymeric separator (as MoS2-NP-coated separator) as an effective LiPS mediator, facilitated by the nanodimension, polar interactions, and the better edge-binding sites of the MoS2-NPs. The resulting MoS2-NPs have an interlayer spacing of 0.55 nm and are stacked with a few layers. At a sulfur loading of 4.0 mg cm-2, the Li-S cell with a MoS2-NPs-coated separator attains a peak discharge capacity of 983 mA h g-1, improving the electrochemical utilization of sulfur. The cell is able to maintain a high capacity of 525 mA h g-1 after 150 cycles at a C/5 rate. The MoS2-NPs are able to effectively anchor the LiPS species to their large S2- anions, enhancing the redox accessibility of sulfur cathodes and enabling better capacity retention.


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INTRODUCTION Lithium-sulfur (Li-S) batteries are attractive as next-generation energy storage systems due to their high specific energy density (2,600 W h kg-1), arising from a high theoretical capacity (1,672 mA h g-1) of sulfur.1,2 Also, sulfur is abundant and inexpensive. However, there still are a multitude of problems, which impinge upon extended cycling of Li-S batteries, inhibiting their practicality.1,3–6 During intermediate charge and discharge stages, lithium polysulfides (Li2Sx, 4 ≤ x ≤ 8, LiPSs) form and readily dissolve in the organic electrolytes currently used in the Li-S system. The dissolved LiPSs diffuse out from the cathode matrix and shuttle between the cathode and anode, which causes rapid capacity fade and low electrochemical efficiency.7,8 Furthermore, there is a large volume change (~ 80%) between sulfur and its end-discharge product Li2S, causing a loss of electrochemical contact and fast capacity degradation.9 Finally, S and Li2S are insulating in nature, leading to a low activematerial utilization. In order to address these issues, many efforts have been made to impede the irreversible LiPS relocation by hosting the active material within the cathode region with physical and conductive barriers.10 Being highly conductive, carbon additives are the most popular approach to increase the electron transport inside the Li-S cathode configuration. Furthermore, many carbon materials can provide frameworks that provide a good physical trapping barrier for LiPS species and when applied, can strategically improve the cyclability of the cathode. Thus, many carbon-trapping



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materials have been explored, including architectures such as carbon paper,11–14 carbon aerogels,15–19 and carbon-coated separators.20–22 Although such carbons are able to physically trap LiPSs with good material designs, carbon still has weak nonpolar interactions with polar LiPS species, limiting the ability to fully thwart the LiPS shuttling. Thus, further efforts have been aimed at applying or adding to the nonpolar carbon substrates polar materials, such as metal oxides,23–26 metal sulfides,27–29 and heteroatom-doped carbon frameworks.30,31 Recently, there has been great interest in transition-metal sulfides (TMS) with layered structures (MoS2, SnS2, WS2, etc).

23,29

TMSs have high electrical conductivity due to the highly covalent nature of the

bonding between the transition metal and sulfur, while the layers are bound together by weak Van der Waal forces.32 The benefit of layered TMSs in Li-S batteries is the host of accessible valence electrons, which are able to interact with the LiPSs, assisting the absorption and trapping.23,33,34 MoS2 in particular has been shown to exhibit strong interaction with LiPS species due to the large size of its own S2- anions, which readily bind to LiPSs.2,27 Widely regarded as a “super-material” due to its versatility, there have been a vast number of research developments focused on tuning the morphology of MoS2, including chemical vapor deposition (CVD), electrochemical synthesis, atomic vapor deposition, and chemical routes, which determine the structure of MoS2.34–36 The synthesis process of MoS2 is also highly sensitive to temperature, with temperature determining most of its intrinsic physical properties. Such processes, however, are time-consuming and not cost-effective for bulk applications in the battery field. Two-dimensional (2D) MoS2 and carbon-MoS2, such as layered


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sheet-like MoS2 or MoS2-CNT, coated separators have been applied to sulfur cathodes to improve the conductivity within the electrode and the electrochemical performance; however they involve either a lengthy, expensive fabrication process or include bulky carbons as a formation substrate.37,38 There has not been much attention focused on the morphology of MoS2 nanoparticles with cost-effective, facile, and scalable fabrication routes in regards to LiPS anchoring in Li-S battery cathodes. Here, we utilize a facile, low-temperature solvothermal route to synthesize MoS2-nanoparticles (MoS2-NPs) with increased edge-defect sites and inherent thinlayer spacings with an aim to improve the absorption of LiPSs through the MoS2-NPs and increase the electron transport and reversibility of Li-S battery cathodes. The multifunctionality of MoS2-NPs in boosting the Li-S battery performances is demonstrated by applying MoS2-NPs onto the Celgard separator as an electrochemical testing platform. The cells employing the MoS2-NP-coated separators exhibits enhanced electrochemical utilization of 983 mA h g-1 and a high capacity retention (RQ) of 53% after 150 cycles.

RESULTS AND DISCUSSION

The MoS2-NPs were readily synthesized by adding Mo(CO)6 and sulfur powder in a 1:2 mol ratio to a teflon-lined autoclave at 140 oC for 12 h. The corresponding reaction to yield black MoS2-NPs is Mo(CO)6 + 2S à MoS2 + 6CO

(1)



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This is a facile, low-temperature, scalable route to yield small near-spherical particles of MoS2 with thin-layer properties. Tap density experiments were performed to investigate the differences in compactness of each powder for the investigation of the coating materials’ mass and volume parameters, which are necessary in developing a lightweight coating. The synthesized MoS2-NPs (0.71 g ml-1) and commercial MoS2 with a large particle size and clusters (0.83 g ml-1, as a control) were found to have a higher tap density than Super P (0.19 g ml-1), indicating that these MoS2 particles have more advantageous properties than low surface-area carbons for lightweight and low volume loading. Focusing on the physical properties of the coating materials, MoS2-NPs and commercial MoS2, scanning electron microscopy (SEM) images were observed of the respective loose powders. Figure 1 shows the SEM images observed of the MoS2-NPs (Figure 1a) and commercial MoS2 (Figure 1b) loose powders, demonstrating the difference in particle sizes. The MoS2-NPs have a particle size of 150 - 200 nm, while the commercial MoS2 powders show large chucks with particle sizes of > 5 µm and with hexagonal platelets and stacking. The MoS2-NPs were then readily implemented into a lightweight coating onto the surface of a commercial Celgard separator to obtain a coated separator - a component readily utilized in batteries to analyze LiPS mediating ability and effectively decrease LiPS migration. The coating layer was made by first combining MoS2-NPs, Super P carbon, and Polyvinylidene fluoride (PVDF) binder in a 80 : 10 : 10 wt. ratio and tape-casting onto a propylene separator at an automatic traverse speed resulting in a total MoS2-NP-coated separator loading of 1.0 mg cm-2.


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Figure S1a demonstrates the smooth coating of the MoS2-NPs on the coated Celgard separators, which shows a uniform black MoS2-NPs coating. A Celgard separator coated with commercial MoS2 under the same parameters as MoS2-NP-coated separator was assembled as a control. The commercial MoS2 coating is smooth and is also tightly attached on the Celgard substrate, while the coating color of layer turns to dark grey/metallic-like (Figure S1b). We hypothesize that the color difference might result from the particle-size disparities between the MoS2-NPs and commercial MoS2. Figures S1c and S1d demonstrate the SEM observation of the MoS2-NPcoated separator and commercial MoS2-coated separator surface. The small particle size of MoS2-NPs allows them to have good contact with the conductive carbon and a homogenous distribution on the coated separator. In contrast, the larger particle sizes of the commercial MoS2 results in a poor blending with the small super P carbon. This contrast is seen in the energy dispersive spectroscopy (EDS) mapping pictured with the SEM images to display the elements present. Whereas the MoS2-NPs (Figure S1c) have an even distribution of the coating material, the separator coated with the commercial MoS2 shows a less homogeneously packed configuration (Figure S1d). The carbon distribution predominately fills in the areas not occupied by larger MoS2 particles. Figures 2a and 2b demonstrate the transmission electron microscopy (TEM) images of MoS2-NPs, which shows their nano-sized particle features. The MoS2 NPs can be observed in the TEM images to have rough edges with defects (Figure 2b, inset) and an average particle size of 150 - 200 nm with varying degrees of thicknesses ranging up to 4 interlayers. Other than the



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rough edge-defects, the nanoparticles are observed to be near-spherical with a small degree of crystallization in the HRTEM (Figure 2b). Upon closer inspection, there are visually bent interlayers scattered throughout the material. This confirms that the small particles have some curved interlayers upon synthesis, potentially aiding fast Li+ ion diffusion pathways. The d002 reflection is found to give an interlayer spacing of 0.55 nm. The prepared MoS2-NPs were confirmed with X-ray diffraction (XRD) analysis and compared with commercial MoS2 powders (Figure 2c). The XRD pattern of MoS2-NPs demonstrates peaks at 16.3o, 33.4o, and 59.0o, which correspond, respectively, to the (002), (100), and (110) reflections and compares with the XRD patterns of single-layer MoS2.39 The (002) peak shifts and is indicative of the stacking among MoS2, while the 100 and 110 peaks are reflective of the bulk 2H-MoS2. The XRD pattern of commercial MoS2 powder displays a sharper and more crystalline peaks (Figure 2c), comparing well with the data in JPCDS 371492.27,33. The interlayer spacing for the synthesized MoS2-NPs (d002 = 0.55 nm) is in agreement with the TEM results. The commercial MoS2 reveals a slightly larger interlayer spacing (d002 = 0.61 nm) in agreement with the TEM results (Figure S2). Both powders are in accordance with previous the reports of MoS2 in the hexagonal phase with the space group of P63/mmc (D46h).27,33 The hexagonal crystal phase consists of trigonal prisms with Mo at the center coordinated to six S atoms. The two layers of MoS2 in a hexagonal pattern is referred to as 2HMoS2.40 The bonding between the layers along the basal plane is Van der Waals-type with weak interlayer forces, which can facilitate Li+-ion transport.33,41 Therefore, the smaller interlayer


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spacings observed for MoS2-NPs and the advantageous smaller particle size are able to assist with effective ionic motion in the Li-S system. In order to gain insight into the thin-layer composition of the MoS2-NPs, Microraman spectroscopy was utilized (Figure 2d). As compared to the E12g (390.9 cm-1) and A1g (414.9 cm-1) Raman peaks of commercial MoS2, the infrared-active modes E12g (375.4 cm-1) and A1g (402.5 cm-1) of the synthesized MoS2-NPs are found to be red-shifted (Figure S3). The peak shifts of the E12g and A1g vibrations are due to the changes in the c lattice parameter.42 As the layer thickness of the MoS2 decreases from that of the bulk MoS2, the visible E12g and A1g peaks are red-shifted. Therefore, the results demonstrate a thin-layer interlayer spacing with < 4 layers in MoS2-NPs, consistent with the literature reports.35,40,42,43 X-ray photoemission spectroscopy (XPS) was utilized to further elucidate the structure and properties of MoS2-NPs (Figures 3a - 3c). The at. % ratio of Mo : S based on peak area is 1 : 2. The atomic ratio agrees with the XRD analysis and additionally confirms the composition of MoS2 in the synthesized powder. Figure 3b depicts the Mo 3d doublet peaks with a spin-orbital splitting to Mo 3d5/2 (229.8 eV) and Mo 3d3/2 (232.9 eV) states.44 The deconvoluted Mo 3d doublet peaks demonstrate both 2H and 1T character. 2H-MoS2 is the standard hexagonal MoS2 with semiconducting properties, while the 1T-MoS2 has tetragonal symmetry and metallic character.34 This indicates that the MoS2-NPs should demonstrate excellent electrical conductivity over commercial MoS2 (2H-MoS2) in sulfur cathodes based on its phase composition. The S 2p doublet peaks with spin-orbital splitting were deconvoluted to S 2p3/2



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(162.7 eV) and S 2p1/2 (163.8 eV), in accordance with the known S peaks of MoS2 (Figure 3c).45,46 To determine whether the increase in edges and edge-defect sites of the MoS2-NPs seen in the TEM images benefit the absorption ability and improve the binding sites for LiPS species, a series of LiPS absorption tests were performed with MoS2-NPs, commercial MoS2, and Super P in Li2S6 solution (Figure 4). In separate vessels (Figure 4a), 50 mg of the synthesized MoS2NPs and commercial MoS2 that were applied as the coating materials were each added to a Li2S6 solution prepared in the organic electrolyte (DOL : DME, 1 : 1 vol. ratio) as the testing LiPS absorbents. Super P carbon, the conductive carbon commonly used in cathode preparation and used here, was also prepared in a separate vessel. Super P has a spherical structure with low surface area and is used here to eliminate carbon as a LiPS binding element when alluding to the sulfur cathodes. A dark brown coloring of the clear organic electrolyte is indicative of the Li2S6 in solution as seen in Figure 4b. Therefore, as the LiPS absorbents take the LiPS species away from the original solution, there should be a visual confirmation of change from a darker to a lighter color. Within 4 h, visual confirmation of rapid LiPSs binding to MoS2-NPs is evidenced as the dark brown color fades and becomes light brown, while the color of the commercial MoS2 and Super P vessel remain unchanged (Figure 4b). Within 24 h, the MoS2-NPs vessel turns into a yellow color, indicating it has visibly secured a larger amount of LiPSs, while still no visible color change is observed in either the commercial MoS2 vessel or Super P vessel, demonstrating a lack of effective LiPS anchoring (Figure 4c). After 48 h, the LiPS solution containing MoS2-


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NPs has turned light yellow, indicating that a large amount of LiPS species from the solution has been absorbed onto the MoS2-NPs (Figure 4d), while the vessels containing commercial MoS2 and Super P carbon have a nominal color change, indicating that the LiPS species have largely remained in the solution with a poor binding affinity to either of them. These visual results confirm that well-designed structures with improved edge-binding sites for LiPS species to anchor onto are paramount for LiPS mediation. Recognizing the good LiPS binding affinity observed for the synthesized MoS2-NPs, electrochemical data were collected to study the capability of the material in a Li-S system. Coin cells were assembled with a sulfur cathode, MoS2-NPs or commercial MoS2-coated separator, and a lithium-metal anode (Figure 5a). The sulfur cathode was prepared with a traditional slurry method with a 70 wt.% sulfur and 15 wt.% each of Super P conductive carbon and PVDF binder under magnetic stirring overnight. The cathode was blade-casted onto aluminum (Al) foil as the current collector, dried in a vacuum oven, and cut into the resulting cathodes. The electrochemical impedance of fresh Li-S cells was observed with electrochemical impedance spectroscopy (EIS) measurements. Figure 5b shows the EIS plots of the cells fabricated with the MoS2-NP-coated separator (referred to as MoS2-NP cell) and commercial MoS2-coated separator (referred to as commercial MoS2 cell) and a control Li-S battery with a bare Celgard separator as a control (referred to as bare Celgard cell). The bulk resistivity of the MoS2-NP cell is significantly lower than both the bare Celgard cell and the commercial MoS2 cell. The MoS2-NP cell had a charge-transfer resistance (RCT) of 72.1 Ω, while the commercial MoS2 cell had a



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higher impedance RCT of 147.5 Ω, illustrating a 51% decrease in RCT with the MoS2-NPs coating. Both the cells with the MoS2-NPs and commercial MoS2 had a respective 85% and 70% decrease in RCT compared the cell with the bare Celgard, indicating a sizeable decrease in impedance with the addition of either MoS2. The substantial decrease in the impedance of the cell with MoS2NPs compared to that of commercial MoS2 can be attributed to (i) better MoS2 conductivity, (ii) smaller particle size with a more homogeneous distribution, and (iii) increased edge-defect sites for better LiPS binding. In order to evaluate the redox process, Figures 5c and 5d depict a side-by-side comparison of the cyclic voltammetry (CV) curves for the cathode assembled with the MoS2-NP cell and the commercial MoS2 cell. For the MoS2-NP cell, there is evidence of two sharp cathodic peaks at 2.3 V (indicative of the reduction of S8 to intermediate LiPSs) and 2.0 V (referring to the conversion of intermediate LiPSs to Li2S2). The same two peaks are broader and are shifted by 0.2 V for the commercial MoS2 cells, demonstrating the poor electrochemical kinetics, most likely attributed to the larger particle size. There is also a large voltage difference between the anodic and cathodic peaks for the commercial MoS2 cell, revealing further evidence of poor reversibility when using bulk crystalline MoS2 particles.47,48 When examining the reverse charge, there is evidence of two anodic peaks at 2.45 and 2.5 V for the first cycle, referring to the conversion into long-chain LiPSs and then to S8.49 These peaks are shifted to higher potentials (0.15 V) for the commercial MoS2 cell than the MoS2-NP cell and can be attributed to poor


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utilization and fast capacity fade of the cell. The larger current density for the MoS2-NP cell indicates superior active-material utilization compared to the commercial MoS2 cell. Microstructural analysis of the MoS2-NP-coated separator and the corresponding sulfur cathode used in the same testing cell after 150 cycles was observed with SEM and EDS for physical inspection of the materials after electrochemical cycling (Figure S4). The cycled sulfur cathode surface is visible in Figure S4a, and it demonstrates the typical defects arising from the volume expansion and LiPS diffusion. Likewise, in Figure S4b, the surface examination of the MoS2-NP-coated separator demonstrates good stability and consistency on the surface with no visible fractures or gaps causing loss of electrochemical contact. The EDS mapping of the MoS2NP-coated separator surface shows that the material is still homogeneously distributed across the sampled material without any aggregation or separation during cycling (Figure S4c). The good homogeneity of the elements on the MoS2-NP-coated separator is maintained after cycling, indicating that there remains good physical adhesion. The cross-sectional examination of the cycled MoS2-NP-coated separator (Figure S4d) demonstrates good adhesion and a lack of cracking or defects, retaining its good uniformity. In contrast, the cross-sectional examination of the commercial MoS2-coated separator presented in Figure S5 after cycling evidences large particles remaining in the rigid composite structure. The larger particle sizes give rise to uneven layering of the cross-section and large spaces within the depth of the material leading to loss of electrochemical contact and allow for the LiPS species to diffuse through.



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Electrochemical analysis was performed in order to elucidate the performance of the cells employing MoS2-NP-coated separators (Figure 6a). At a C/5 rate, the cell assembled with the MoS2-NP-coated separator is able to attain a peak discharge capacity of 983 mA h g-1 and maintain a high capacity of 525 mA h g-1 after 150 cycles. This corresponds to a high capacity retention rate (RQ) of 53% and an average capacity fading rate of 0.34% per cycle. Furthermore, at C/10 and C/20 rates, the cell employing the MoS2-NP-coated separator is able to attain good rate capabilities with high initial discharge capacities of, respectively, 1,182 and 1,388 mA h g-1. This high discharge capacity confirms that the MoS2-NPs facilitate good electron transport and LiPS mediation ability. In order to evaluate the electrochemical benefits provided by the particle size and thinlayer properties, electrochemical cycling analysis was also performed with the cells assembled with commercial MoS2-coated separators (Figure 6b). At a C/5 rate, the discharge capacity of the commercial MoS2 cell is reduced from an initial discharge capacity of 869 mA h g-1 to 620 mA h g-1 after 50 cycles, resulting in a lower RQ of 71%; while the MoS2-NP cell is able to attain a very high discharge capacity of 763 mA h g-1 and a high RQ of 78% at the same cycle number. This confirms that bulk crystalline particle sizes are unable to aid capacity retention. After 150 cycles, the commercial MoS2 cell maintains a capacity of 496 mA h g-1, reflecting a RQ of 57% and a faster capacity fade rate of 0.38% per cycle. At C/10 and C/20 rates, the cells with commercial MoS2 are able to attain peak capacities of respectively, 1,027 and 1,291 mA h g-1. Due to the better ionic conductivity of MoS2-NPs and the better edge-binding sites, the MoS2-NP cell is


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able to achieve better discharge capacities.50 At a C/20 rate, the redox reaction proceeds at a slower rate, and there is time for LiPS shuttling to occur. Due to the large particle sizes of commercial MoS2 that result in non-homogeneous separator coating, there is severe LiPS shuttling, leading to cell failure within the first 30 cycles. The electrochemical performance is detailed and compared in Table S1 with other MoS2-coated separators previously reported, with the MoS2-NP-cell demonstrating improved cyclability and cell performance. In accordance with the above data, the MoS2-NPs benefit Li-S batteries with their smaller interlayer spacing (0.55 nm), their smaller particle size (~ 200 nm), and their thin-layer properties (≤ 4 layers). These properties are consistent with benefitting Li+-ion diffusion and anchoring LiPSs at MoS2-NPs edge sites, as reflected by the superior electrochemical performance. The excess edge-binding sites and homogeneous distribution of particles allowed for less irreversible capacity loss. The better packing of the smaller-sized MoS2-NPs on the surface of the coated separator also results in a lower RCT of the Li-S cell in comparison to those of commercial MoS2 or layered sheet-like MoS2 cells. Additionally, the better conductivity observed in the EIS results is attributed to the 2H-MoS2/1T-MoS2 mixed phase properties and the small size of the synthesized MoS2-NPs. The fast ion/electron transport characteristics results in the high discharge capacities within the first few cycles of the Li-S cell utilizing the MoS2-NPs-coated separator demonstrating significant progress in Li-S cell design. Figures 6c – 6f plot the discharge/charge voltage profiles of the cells with the MoS2-NPand commercial MoS2-coated separators as a function of (i) cycling rates and (ii) cycle numbers



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at a C/5 rate. The charge/discharge profiles demonstrate two plateaus (2.3 and 2.1 V) in accordance with the reduction process as seen in the CV curves (Figures 5c and 5d). The voltage profiles for the peak discharge capacities of the cells with MoS2-NP (Figure 6c) and commercial MoS2 (Figure 6d) are compared at C/5, C/10, and C/20 rates. The discharge curves with MoS2NP demonstrates higher peak capacities and lower polarization than those with commercial MoS2 at each cycling rate (C/5, C/10, C/20), indicating better redox ability in the cell with MoS2NP and confirming the influence of particle size and thin-layer properties on cycling. The charge/discharge profiles are shown over a series of cycle numbers at a C/5 rate in Figures 6e and 6f. There is a large change in the second discharge peak capacity for the commercial MoS2 cell from the peak cycle to the latter cycles indicating poor electrochemical reutilization. The MoS2-NP cell is able to achieve better capacities through good active material retention from the MoS2-LiPS binding. To further investigate the size dependency of MoS2, the voltage profiles at C/10 and C/20 were investigated (Figure S6). The polarization (ΔE) is smaller for the Li-S cells employing MoS2-NPs than that employing commercial MoS2 at the peak capacities, indicative of lower internal resistance, faster charge transfer, and better conductivity in the MoS2-NPs cell (Figures S6a and S6b). The voltage profiles at C/10 and C/20 were also studied as a function of cycle number. Cells assembled with thin-layered MoS2-NP-coated separators demonstrate better reversibility and less active material loss per cycle (Figures S6c and S6d). The large difference observed in charge and discharge capacities in the commercial MoS2 cells as compared to the MoS2-NPs cells reveals poor reversibility (Figures S6e and S6f).


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The large particle size of the commercial MoS2 does not allow for sufficient LiPS binding or electrochemical contact with the cathode components resulting in severe LiPS shuttling and thus poor cyclability.

CONCLUSION

This study was aimed at understanding the electrochemical benefits of MoS2-NPs in Li-S batteries formed by reducing the MoS2 particle size and interlayer spacing (0.55 nm) through a facile chemically synthesis route. MoS2 is a relatively less expensive material and has good LiPS mediation properties due to its good binding affinity for LiPS species during cycling. The metallic conductivity from a combined 2H-MoS2 and 1T-MoS2 and their homogeneity with carbon in the coated separator are able to provide the Li-S cell with excellent electron transport. The smaller particle size of the MoS2-NPs also contains increased edge sites for LiPS interfacial interactions and LiPS retention during cycling. At a C/5 rate, the MoS2-NP-coated separator is able to attain a peak discharge capacity of 983 mA h g-1 and maintain a high capacity of 525 mA h g-1 after 150 cycles. The high binding affinity between the MoS2-NPs and LiPS species is readily observed in the electrochemical analysis, showing that a facile, chemically tailored route for synthesizing MoS2-NPs can greatly improve the reaction kinetics in Li-S cells.

METHODS



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Sample preparation: In order to synthesize MoS2-NPs, molybdenum hexacarbonyl (Mo(CO)6, Aldrich) and Sulfur powder (Aldrich) were first magnetically stirred (Mo : S = 1 : 2 mol. ratio) in o-xylene for 30 min. The solution was then transferred to a teflon-lined autoclave and maintained at 140 oC over 12 h. The black precipitate was filtered and washed three times each with water and acetone. The resulting powder was dried in a vacuum oven at 50 oC overnight. Electrochemical measurements: In order to examine the electrochemical properties of the MoS2-NPs in a Li-S cell, CR2032 coin cells were assembled with the MoS2-NPs as the coating material on the Celgard separator.10,51 To make the coated separator as an electrochemical testing platform, a slurry was constantly made up of MoS2 powders, polyvinylidene fluoride (PVDF, Aldrich), and super P carbon (TIMCAL, 80 : 10 : 10 in wt. ratio) in N-methyl pyrrolidone (NMP, Aldrich). The resulting slurry mixture was tape-casted onto a polypropylene membrane (Celgard 2500) and the coated separator was dried in a vacuum oven at 50 oC overnight. The MoS2-NPs synthesized by the abovementioned method were employed as the experimental samples and commercial MoS2 (Aldrich) was employed as the control. The MoS2-NPs and commercial MoS2 coatings were made by the same procedure and controlled to have the same thickness of ~ 25 µm. The MoS2-NP coated separator had a sheet conductivity of 1.1 x 10-3 S cm-1. Coin cells were assembled inside a glovebox under an argon environment. In order to make the electrodes, a slurry consisting of a 70 : 15 : 15 wt. ratio of sulfur powder, Super P carbon, and PVDF were combined in NMP and mechanically stirred overnight. The slurry was


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then blade-coated onto an aluminum foil current collector and dried in vacuum oven at 50 oC overnight. The electrolyte was prepared with 1.85 M lithium trifluoromethanesulfonate (LiCF3SO3, Aldrich) and 0.1 M lithium nitrate (LiNO3, Across Organics) in a 1:1 (vol. ratio) mixture of 1,3-dioxolane (DOL, Across Organics) and 1,2 -dimethoxyethane (DME, Acros Organics) solvent. To assemble the cells, sulfur cathodes were cut out and placed into the bottom cell cap. The sulfur loading was held at 4.0 mg cm-2. The MoS2-NP and commercial MoS2coated separators were placed with the coating facing the top of the sulfur cathode and followed by a lithium-chip (Aldrich) anode. The cell was closed with a nickel foam spacer and the top cell cap. Electrolyte was placed on either side of the coated separator. The cells were rested for 3 h before the electrochemical analysis. Electrochemical cycling experiments were carried out on a battery cycler (Arbin) at room temperature. The specific capacities were calculated based on the sulfur loading and its theoretical capacity (1,672 mA h g-1). Electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 1 MHz to 100 mHz and cyclic voltammetry (CV) profiles within a potential window of 1.7 – 2.8 V vs Li/Li+ at a scan rate of 0.055 mV S-1 were carried out with a VMP3 multichannel potentiostat (VMP3, Biologic Science Instruments).



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FIGURES

Figure 1. SEM microstructural examination of (a) MoS2-NPs and (b) commercial MoS2 loose powders.


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Figure 2. (a, b) TEM morphological characterization of MoS2-NPs. (c) XRD plots of MoS2-NP and commercial MoS2 powders. (d) Microraman analysis of MoS2-NP loose powders with the imbedded schematic detailing the MoS2 Raman active modes A1g and E12g.



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Figure 3. Chemical composition of the pristine MoS2-NPs powder: (a) survey and deconvoluted (b) Mo 3d and (c) S 2p peaks, demonstrating both the 1T-MoS2 and 2H-MoS2 characteristics.


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Figure 4. (a) Images of MoS2-NPs, commercial MoS2, and Super P carbon as loose powders (50 mg) in glass vessels. A series of Li2S6-LiPS absorption solutions as a function of time: (b) 4 h, (c) 24 h, and (d) 48 h, demonstrating the effect of binding LiPS species.



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Figure 5. (a) Schematic detailing the configuration of the Li-S cell. (b) Electrochemical impedance spectroscopy comparison of the cells before cycling. CV profiles of the Li-S cells assembled with (c) MoS2-NP-coated separator and (d) commercial MoS2-coated separator at a scan rate of 0.055 mV s-1.


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Figure 6. Electrochemical cycling plots of the cells with the (a) MoS2-NP-coated separator and (b) commercial MoS2-coated separator at C/5, C10, and C/20 cycling rates. Discharge/charge voltage profiles of cells assembled with (c) MoS2-NP -coated separator and (d) commercial MoS2-coated separator at C/5, C/10, and C/20 rates at their peak discharge capacities. Voltage profiles of cells assembled with (e) MoS2-NP-coated separator and (f) commercial MoS2-coated separator as a function of cycle number at a C/5 rate.



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ASSOCIATED CONTENT

Corresponding Author *E-mail: [email protected]

ORCID Arumugam Manthiram: 0000-0003-0237-9563

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by ExxonMobil through its membership in The University of Texas at Austin Energy Institute.


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SUPPORTING INFORMATION Supporting Information Available: Images of the MoS2-coated separators, illustrating the morphological analysis of sulfur cathodes after cycling, coated separators after cycling, and cross-sectional examination of the coated separators after cycling, and elemental mapping. TEM characterization of commercial MoS2 powders. Microraman spectroscopy of commercial MoS2 powder. Charge and discharge curves of MoS2(-NP, commercial) -coated separators at C/10 and C/20 rates. This material is available free of charge via the Internet at http://pubs.acs.org.



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Table of Contents Graphic and Synopsis


Thin-Layered Molybdenum Disulfide Nanoparticles as an Effective

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