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Materials Today Energy 16 (2020) 100380

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Materials Today Energy

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Interface engineering by atomically thin layer tungsten disulfidecatalyst for high performance LieS battery

Mei Er Pam a, b, 1, Shaozhuan Huang a, 1, Shuang Fan a, Dechao Geng a, Dezhi Kong a,Song Chen a, Meng Ding a, Lu Guo a, Lay Kee Ang a, b, Hui Ying Yang a, *

a Pillar of Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, Singapore, 487372, Singaporeb Science and Math Cluster, Singapore University of Technology and Design, 8 Somapah Road, Singapore, 487372, Singapore

a r t i c l e i n f o

Article history:Received 20 November 2019Received in revised form21 December 2019Accepted 22 December 2019Available online xxx

Keywords:Tungsten disulfideLithium sulfur batteryInterlayerAtomically thin layerStructural engineering

* Corresponding author.E-mail address: yanghuiying@sutd.edu.sg (H.Y. Yan

1 These authors contributed equally.

https://doi.org/10.1016/j.mtener.2019.1003802468-6069/© 2020 Elsevier Ltd. All rights reserved.

a b s t r a c t

Owing to high aspect ratio of edge sites and superior catalytic activity, atomically thin transition metaldichalcogenides (TMDCs) show great promise to tailor the electrolyte/electrode interface properties forhigh performance lithium-sulfur battery (LieS battery). However, the TMDCs that engineer the electrode/electrolyte interface are usually produced through chemical hydrothermal methods, which show lowcrystallinity and thick multilayer structure. Herein, a highly crystalline and atomically thin tungstendisulfides on carbon cloth (WS2@CC) was developed via chemical vapor deposition (CVD) and served asan effective electrode/electrolyte interface for LieS battery. Our results demonstrate that the atomicallythin WS2 with high crystal quality and abundant edges sites can effectively accelerates the redox kineticsof sulfur/lithium polysulfides and regulates the precipitation/decomposition of insoluble Li2S. Moreimportantly, it was revealed that the hierarchical flower-stacked WS2 with excessive exposed catalyticedges shows extremely strong polysulfide adsorption, which causes the sulfur species aggregation andpassivation on the WS2@CC surface, thus resulting in deformed rate performance and poor cyclingstability as compared to the few-layer WS2@CC. Our work provides a new insight into the structuralengineering of TMDCs by CVD for LieS battery, and suggests the importance of rational chemisorptionand catalysis of the interface to realize the high-performance LieS battery.

© 2020 Elsevier Ltd. All rights reserved.

1. Introduction

Advances in rechargeable batteries technology for sustainableand clean energy applications such as electric vehicles, portableelectronic devices and smart grid storage remain challenging due tothe limited theoretical energy density and high cost of the currentlithium ion batteries (LIBs) technology [1e3]. Lithium-sulfur (LieS)batteries have emerged as a promising alternative to resolve theissue of renewable energy intermittency due to their low cost, highenergy density (2600 Wh kg�1), and environmental friendless[1,4e6]. However, the implementation of LieS battery for practicalapplications has been impeded by the following intrinsic issues: (a)the poor electronic conductivity of sulfur and its discharge products(Li2S/Li2S2) that results in sluggish redox kinetics and low sulfurutilization [7e9], (b) the diffusion of intermediate lithium

g).

polysulfide species during cycling causes shuttle effect and para-sitic reaction to lithium anode, resulting in a rapid capacity decayand short cycle life [2,9,10], and (c) the large volumetric changes ofthe electrode, which leads to the cathode electrode pulverizationthat significantly affects its cycling stability [11].

Considerable efforts have been made to solve the above-mentioned issues, such as cathode host material design, electrolytemodifications, and etc [12e14]. Although great progress has beenmade, however, in some cases, these strategies cannot realize highsulfur loading, stable cycling, fast redox transformation simulta-neously. Recently, engineering the interface between electrolyteand separator by inserting a functional interlayer such as carboncloth, carbon nanotube coated separator [15,16], has been devel-oped for the absorption of soluble polysulfide and reuse of theabsorbed active material. However, the diffusion of polysulfides isprone to the precipitation of polysulfides on electrode/separatorinterfaces, resulting in the passivation of active surface, aggregationof solid polysulfide on the interlayer [17,18]. Notably, the precipi-tation of insoluble polysulfides is highly relied on the surface

M.E. Pam et al. / Materials Today Energy 16 (2020) 1003802

chemistry where sulfur redox reactions occur. Designing an effec-tive collaborative sulfur/polysulfide physicochemical trans-formation interface between electrolyte and electrode with highelectrical conductivity, strong sulfur/sulfides chemisorption, supe-rior electrocatalysis and intense catalytic reactive sites simulta-neously is essential for improving the sulfur utilization, andrealizing stable and high-rate LieS batteries. In searching for anefficient collaborative sulfur/polysulfide physicochemical trans-formation interface to suppress the polysulfide shuttle effect andregulate the solid sulfur species deposition, two-dimensional (2D)transition metal dichalcogenides (TMDCs) are attractive owing totheir high catalytic reactive surface, superior chemical stability, andlow cost [19e23]. The catalytic activity of this type of materialshighly depends on the number of exposed edge sites that exhibitsunique chemical and electronic structure compared to their basalplanes [5,24]. It has been proven that atomically thin layeredstructures with high aspect ratio of edge sites and high effectivecatalytic surface is an effective catalytic interfacial candidate forsulfur redox reaction [5]. So far, intensive efforts have been done toimprove the aspect ratio of active catalytic sites in TMDCs throughnanostructure engineering [5,25e28]. However, these TMDCs areusually obtained through chemical hydrothermal method, whichshow unsatisfactory catalytic effect due to their low crystallinityand stacked multilayer structure [29,30]. Besides, the intrinsic de-fects in the crystal structure such as sulfur vacancy induce poorstructural stability to restrain the lamella pulverization and activesite invalidation, thus resulting in poor electrochemical perfor-mance. Chemical vapor deposition (CVD) is a promising, highlycontrollable method to synthesize high crystallinity and atomicallythin layer 2D materials. Up to date, several types of TMDCs havebeen prepared via CVD to construct the catalytic active sites andaccelerate the sulfur redox conversion, however, the large-scaleproduction of ultrathin TMDCs as well as the potential applicationof TMDCs as catalytic interface are rarely reported.

In this contribution, a conductive and catalytic interface con-structed by atomically thin layer WS2 catalysts on conductivesupport matrix is proposed to effectively inhibit the polysulfidesshuttle effect, facilitate the polysulfides redox transformation andregulate the Li2S deposition. A low-pressure CVD method wasemployed to synthesize the atomically thin WS2 on carbon cloth(WS2@CC) on a large scale to imitate the electrocatalysis-drivenprocess for achieving the commercial demand of the batteryapplication. By tuning the precursor quantity, growth period, andgrowth times, different morphologies of the WS2 nanostructuressuch as few-layer planar flakes or layer-by-layer grown hierarchicalflower-stacked structure were obtained. In addition, this strategyalso demonstrates the substrate-independent growth mechanism,suggesting a versatile growth route for WS2 nanostructures withdiverse substrate. Compared with the multilayer flower-stackedWS2, the atomically thin layer WS2@CC provides a more effectiveinterface with strong chemisorption, high reactive catalytic sitesand high electrical conductivity for promoting the redox conver-sion kinetics of sulfur intermediates and eliminating the poly-sulfide shuttle effects. Therefore, the cell with atomically thin layerWS2@CC interlayer delivers a high initial specific capacity of 1198mAh g�1 at 0.2 C, excellent rate capability (830 mAh g�1 at 2 C) andlong cycle life at 0.5 C (0.07% capacity decay per cycle). Our worksinspire the exploration of large-scale production of atomically thinlayer TMDCs by CVD method, which opens a new avenue fordeveloping efficient energy storage applications.

2. Results and discussions

Chemical vapor deposition (CVD) is a widely used technique forcontrollable synthesis of atomically thin layer transition metal

dichalcogenides (TMDCs). In our CVD synthesis, atomically thinWS2 grows onto carbon cloth (WS2@CC), which serves as a multi-functional interface among electrolyte/electrode for high-performance lithium-sulfur (LieS) battery. In order to synthesizeatomically thin WS2@CC, a modified low-pressure CVD approachwas applied. Prior to the growth process, the carbon cloth substratewas first treated with nitric acid to enhance its hydrophilicity andhence to facilitate the growth of atomically thinWS2 nanosheets. Asillustrated in Fig. 1a, a treated carbon cloth was located above thetungsten trioxide (WO3) powder in a parallel geometry with a5 mm gap. This configuration allows the uniform growth ofatomically thin WS2 nanosheets onto the carbon cloth. 15 sccm H2,a reducing agent for WO3, was used to promote the reduction ofWO3 during the WS2 synthesis. The growth of atomically thin WS2flakes usually takes place in a wide temperature range of850e1000 �C. Here, we employed a high growth temperature of1000 �C and low pressure of 10 torr throughout the experiment toensure high evaporation rate of tungsten sources. The synthesisprocess is illustrated in Fig. 1b.

To understand the synthesis mechanism of WS2 nanostructures,different growth conditions, such as substrate, growth period,precursor quantity and growth times have been studied. Firstly, wesynthesized the WS2 nanostructures onto carbon cloth and sap-phire concurrently as shown in the scanning electron microscopy(SEM) images (Fig. 2aec). As shown in Fig. 2a, abundant epitaxialgrown few-layer WS2 nanostructure of random shape were grownuniformly on the carbon cloth at a growth temperature of 1000 �Cwith a growth period of 1 h. The high magnification SEM image inFig. 2b shows that some underlying buffer layers uniformly grow onthe carbon cloth surface, and the secondary planar WS2 nucleatesand grows on the buffer layers. The presence of few-layer WS2crystals on carbon cloth is confirmed by Raman and photo-luminescence (PL) characterization respectively, as shown inFigs. S1a and S1b. The relative peak intensity ratio of the E2g to A1gmodes reflects the atomically thin WS2 layer. Interestingly, thisstrategy also demonstrates the similar growth mechanism of few-layer WS2 nanostructures on the typical growth substrates suchas sapphire. As shown in Fig. 2c and d, the SEM and atomic forcemicroscopy (AFM) images show the formation of few-layer stackedWS2 and monolayer WS2 on sapphire. The line scan of the edgeillustrates that the thickness of the basal plane is 0.8 nm, indicatesthe initial formation of monolayer WS2 basal plane on the sub-strate. The high-resolution transmission electron microscopy(TEM) image of WS2 nanosheets (Fig. S2) reveals clear lattice fringewith a d-spacing of 0.27 nm, attributed to the (100) plane of hex-agonal WS2, indicates that the as-grown WS2 nanosheets are wellcrystallized.

Different growth conditions have been further studied byvarying the growth period, precursor quantity and growth time. Itis worth to note that the growth period, precursor quantity andgrowth time significantly affect the overall thickness, nucleationsite and morphology of the as-grown WS2 crystals. As shown inFigs. S3 and S4, increasing the precursor quantity and growthperiod respectively, they will enhance the epitaxial nucleation, andpromote the formation of dense WS2 nanosheets. Through con-ducting WS2 growth under a continuous supply of tungsten sourceby multiple growth time, we could observe that high densityflower-stacked WS2 nanostructures by a layer-by-layer growth areformed. To shed light on the detailed synthesis process of WS2 oncarbon cloth, the evolution of WS2 nanostructures was monitoredby multiple growth times and synthesized concurrently on thesapphire and carbon cloth. As aforementioned, during the firstgrowth (Fig. 2aed), planar few-layer WS2 flakes randomly form onthe carbon cloth. When tungsten sources continue to supply bymultiple growth, initial planar WS2 growth eventually alters to

Fig. 2. Morphology investigation of atomically thin tungsten disulfides on carbon cloth (WS2@CC) after different growth time of CVD. (a, b) SEM images of few-layer WS2@CCsynthesized after one-time growth of CVD. (c, d) SEM and AFM images of few-layer WS2 on sapphire substrate under the concurrent growth of WS2@CC. (e, f) SEM images ofhierarchical flower-stacked WS2@CC synthesized after multiple growth of CVD. (g, h) SEM and AFM images of hierarchical flower-stacked WS2 on sapphire substrate under theconcurrent growth of hierarchical flower-stacked WS2@CC.

Fig. 1. Synthesis processes of WS2 on carbon cloth (WS2@CC). (a) Illustration of chemical vapor deposition (CVD) process for synthesis of WS2@CC. (b) Schematic diagram of thegrowth of WS2@CC under continuous supply of tungsten source and limited supply of tungsten source. Under limited supply of tungsten source, atomically thin layer WS2 crystalswith random shape were randomly grown on carbon cloth, whereas thicker and uniform hierarchical flower-stacked WS2 crystals were obtained under a continuous supply oftungsten source.

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upward growth, results in layer-by-layer growth flower-stackedWS2 crystals (Fig. 2e and f). The transition growth mode directlydemonstrates by the coexistence of 2DWS2 planar flakes and three-dimensional (3D) hierarchical-oriented flower-stacked in SEM im-ages. The AFM height images in Fig. 2d and h correspond to the firstgrowth and multiple growth of WS2, also further show the

transition growth mode of WS2 from isolated planar monolayerflakes to stacked planar flakes and finally to layer-by-layer hierar-chical-oriented flower-stacked nanosheets. As shown in Fig. S5a,the Raman spectrum shows the two typical characteristic peaks ofWS2 at 355 cm�1 (E2g mode) and at 420 cm�1 (A1g mode) forflower-stacked WS2@CC. Besides, the presence of a direct band gap

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PL peak indicates the few-layer nature of WS2 and further confirmsthe layer-by-layer growth mechanism. Fig. S5b shows the obtainedsamples exhibit the typical X-ray diffraction (XRD) pattern of WS2crystal phase. SharpWS2 crystal XRD peaks are detected, indicatinglarge crystalline domain, high crystal quality and high purity for theCVD obtained samples. The elementary mappings of carbon, sulfurand tungsten further confirm the structure of WS2@CC composite(See Fig. S6). Overall, the results demonstrate that the morphologyof the as-grown WS2 nanostructures is highly affected by thecontinuity precursor source supply through controlling the pre-cursor quantity, growth period and the growth times. By optimizingthe synthesis process, direct control over the density andmorphology can be ultimately achieved, suggesting the highcontrollability and versatility of this CVD method for practicalapplications.

To reveal the function mechanism of atomically thin WS2nanosheets as sulfur/polysulfide physicochemical transformationinterfaces in LieS batteries, cyclic voltammetry (CV) in symmetriccells was further measured with identical pristine few-layerWS2@CC (flower-stacked WS2@CC or CC) interlayer working andreference electrodes in a 1/3 M Li2S6 electrolyte within �1 to 1 Vvoltage window (See Fig. 3a). The few-layer and flower-stackedWS2@CC electrodes exhibit two distinct reduction peaksat �0.19, �0.50 and �0.07, 0.41 V, respectively and two prominentoxidation peaks at 0.19, 0.50 and 0.07, 0.41 V, respectively whereasCC electrodes without WS2 nanosheets shows broad redox featuresat 0.18, 0.6, �0.18 and 0.6 V. The two reduction peaks ascribe to thereduction of Li2S6 to Li2S/Li2S2 on the working electrode, while thetwo oxidation peaks correspond to reconstitution of Li2S6 by theoxidation of Li2S/Li2S2 on the working electrode [31]. Therefore, thepair of cathodic and anodic peaks are paired redox reaction of thesymmetric cell. In addition, it is worth to note that redox pair withsharp and narrow peak width exhibits an excellent electrochemicalreversibility and facile polysulfide conversion [32,33]. The observednarrower and sharper redox peaks of the few-layer and flower-stacked WS2@CC electrodes than CC electrodes further reveal theenhanced electrochemical reversibility and reaction kineticsenabled by atomically thin layer WS2, suggesting WS2@CC as aneffective catalyst for polysulfide conversion.

To further verify the polysulfide adsorption ability of atomicallythin layer WS2, a polysulfide adsorption test was performed with3 mL DOL/DME electrolyte including 2/3 mmol Li2S6 (Fig. 3b). Afterovernight stirring, the Li2S6 solutionwith few-layerWS2@CC shows

Fig. 3. (a) Cyclic voltammetry (CV) of symmetric cells with identical electrodes of carbon clowith 3 ml of 1/3 M Li2S6 catholyte in 2 ml of 1:1 DOL/DME mixture after midnight stirring. (cabsorption and few-layer WS2@CC after discharging.

more pale color than that of pristine CC, confirming the strongpolysulfide adsorption by atomically thin layer WS2. The results areconsistent with previous reported studies on the sulfophilic abilityby layered transition metal dichalcogenides [5]. X-ray photoelec-tron spectroscopy (XPS) was then employed to probe the nature ofthe intrinsic chemical interaction between adsorbed species andWS2 nanosheets. Fig. 3c shows the fitting spectra of the high res-olution XPS of S 2p peaks corresponding to pristine WS2@CCinterlayer, few-layer WS2@CC interlayer after Li2S6 absorption testand few-layer WS2@CC interlayer after one cycling. The S 2p core-level spectrum of pristine WS2@CC interlayer shows the typicalpeaks at 162.8 and 164.0 eV, which are ascribed to the S2� state inWS2. However, the S 2p spectra of the few-layer WS2@CC interlayerafter Li2S6 absorption display additional four predominant peaks,corresponding to the terminal (ST�1) sulfur and bridging sulfuratoms of Li2S6, thiosulfate, and polythiosulfate complexes, respec-tively. The thiosulfate complex peaks at 167.2 and 168.4 eV shouldarise from the salt-solvent redox reaction between Li2S6 and WS2,while the polythiosulfate complex formed at 169.1 and 170.2 eV,which is originated from further reaction between long chain pol-ysulfide and thiosulfate [5,31]. The redox reaction of Li2S6 to thio-sulfate and polythiosulfate is accompanied by an increase in the Woxidation, as shown in Fig. S7. We attribute this upshift of Woxidation to a strong interaction of exposed W site with sur-rounding sulfur ligand. Interestingly, after the 1st cycling, the pol-ysulfides are nearly absent and a strong signal at 169.4 eV can beobserved, which is from the Bis(trifluorometane) sulfonamidelithium salt (LiTFSI) in the electrolyte [18]. The results suggest thatalmost all the polysulfide species have dissolved and depositedback into cathode. Therefore, in the presence of electric field, pol-ysulfide conversion most likely occurs at the catalytic active edgesites on the atomically-thin WS2 catalyst surface.

To further investigate the function mechanism of WS2 nano-flakes of different layers and morphologies on the electrochemicalperformance, four different interlayers: pristine separator, bare CC,few-layer WS2@CC and flower-stacked WS2@CC were designed forLieS battery (Fig. S8). In the electrochemical study, the cathodematerial was prepared by infusing the commercial sulfur intoketjenblack powder with a high sulfur content of 70% (Fig. S9). Tostudy the role of atomically thin WS2 in catalytic decomposition ofLi2S, galvanostatic discharge/charge measurements (Fig. 4aec andS10) were conducted at a constant current rate of 0.1 C(1 C ¼ 1675 mA g�1). The CC interlayer without WS2 catalyst

th (CC) and WS2@CC at scan rate of 0.5 mV s�1. (b) Adsorption tests of CC and WS2@CC) High resolution XPS spectra of S2p for pristine WS2@CC, few-layer WS2@CC after Li2S6

M.E. Pam et al. / Materials Today Energy 16 (2020) 100380 5

exhibits high potential barrier at about 2.32 V in the initial chargingprocess, suggesting a sluggish activation process with high chargetransfer resistance (Fig. 4a). After introducing the few-layer WS2,the cell demonstrates a voltage jump about 2.30 V during theactivation process because of the semiconducting nature of WS2[34]. While the charge plateaus after the voltage jump indicate thephase conversion reaction from Li2S to intermediate Li-polysulfidesand finally to sulfur (see Fig. 4b) [34]. Compared with the otherinterlayers, the few-layer WS2@CC interlayer exhibits the highestdischarge plateaus at 2.34 and 2.08 V, and two lowest charge pla-teaus between 2.3 and 2.4 V, respectively (Fig. 4aec and S11). Inaddition, few-layer WS2@CC interlayer shows flatter and longerplateaus with a lower polarization. A similar charging phenomenonis observed for the flower-stacked WS2@CC interlayer with aninitial voltage jump at 2.32 V but with a broader activation plateaus,indicating a sluggish activation process with high internal resis-tance induced by high density nanosheet WS2 catalyst. Overall, thelower potential barrier and charge-discharge polarization of few-layer WS2@CC interlayer indicates the effective catalytic effect offew-layer WS2 towards polysulfide redox kinetics, Li2S nucleationand deposition, which considerably enhances the sulfur utilizationand energy conversion efficiency in the cell [34]. The obtained re-sults are consistent with the CV measurements (see Fig. 4d), wherethe few-layer WS2@CC interlayer shows the narrowest redox peaksseparation (oxidation peaks at 2.41 and 2.41 V, reduction peaks at2.34 and 2.02 V) among all the cells. The narrow redox peaksseparation of the few-layer WS2@CC interlayer indicates the lowlithiation/delithiation polarization and accelerated polysulfideredox kinetics by atomically thin layerWS2. Furthermore, the redox

Fig. 4. Electrochemical performances of LieS cells with and without WS2@CC functionalWS2@CC and flower-stacked WS2@CC interlayer cell, respectively. (d) Cyclic voltammetryWS2@CC and flower-stacked WS2@CC interlayer, respectively. (e) Long term cycling of thecycling of the few-layer and flower-stacked WS2@CC interlayer cells at the rate of 0.5 C aft

peak shifts of few-layer WS2@CC interlayer cell over the four CVinitial cycles are negligible, further suggesting good electro-chemical stability (see Fig. S11).

The proposed atomically thin layer active polysulfide catalystsinterface also enables the stable cycling performance of LieS bat-teries. Fig. 4e presents the cycling performance of the cells withseveral interlayers. The few-layer WS2@CC interlayer delivers amuch higher initial discharge/charge capacity of 1198/1169 mAhg�1 at 0.2 C and nearly 100% coulombic efficiency over 100 cycles,accompanied by a good stability with a capacity retention of 86.71%for 100 discharge/charge cycles. While for the flower-stackedWS2@CC interlayer, a lower initial discharge/charge capacity of998/960 mAh g�1 and capacity retention of 78.03% are observed.The worse cycling performance of flower-stacked WS2@CC inter-layer may be due to the extremely strong polysulfides absorptionand physicochemical transformation by high density atomicallythin layer WS2 that lead to the active surface passivation byaggregated insoluble sulfur species, and thus to the low sulfurutilization. In contrast, the cells with only PP separator or CCinterlayer exhibit much lower initial discharge/charge capacities of691/689 and 1157/1027, respectively and showcapacity retention of61.5% and 69.81% respectively after 100 cycles. The results implythat the few-layer WS2@CC interface enables high sulfur utilizationand cyclability due to the effective electrocatalytic activity andphysicochemical adsorption of the atomically thin WS2.

To further elucidate the role of atomically thin layer WS2@CC onaccelerating sulfur redox kinetics, the rate capability of the fourcells were then evaluated at different rates from 0.1 to 2 C (seeFig. 4f). The few-layer WS2@CC interlayer shows high average

interlayer. (aec). Galvanostatic discharge-charge profile at 0.1 C of the CC, few-layer(CV) curve at 0.1 mV s�1 of the typical cell without CC interlayer, with CC, few-layercells at the rate of 0.2 C. (f) Rate capabilities of the cells at various rate. (g) Long termer rate performance measurement. (h) Nyquist plots of the cells after four CV cycles.

M.E. Pam et al. / Materials Today Energy 16 (2020) 1003806

capacities of 1366, 1219, 1117, 981 mAh g�1 at 0.1, 0.2, 0.5, and 1 Crespectively, much better than the other three cell configurations.Even at a higher rate of 2.0 C, a high reversible capacity of793mA h g�1 can be achieved. And the capacity is able to recover toa high value of 1226mAh g�1 when the rate returns to 0.1 C, furthershowing the excellent redox stability of the few-layer WS2@CCinterlayer. Such outstanding rate capability is superior to most ofother transition metal sulfide-based materials reported previouslyfor LieS battery, as shown in Fig. S12. It is worth to note that theflower-stacked WS2@CC interlayer delivers a high discharge ca-pacity of 1280 mAh g�1 at 0.1 C, but a lower discharge capacity of1065, 969, 852, and 698 mAh g�1 at 0.2, 0.5, 1 and 2 C, respectively,suggesting the slower reaction kinetics and poorer stability of theflower-stacked WS2@CC interlayer cell at higher current rate. Incontrast, the typical cells with and without CC interlayer showlower average discharge capacities and poorer stability under thesame conditions. The superb rate performance of the few-layerWS2@CC is attributed to the enhanced sulfur redox kinetics byhigh aspect ratio of catalytic active edge sites of few-layer WS2@CCinterface among electrolyte/electrode. Moreover, the discharge/charge profiles at various rates of the few-layer WS2@CC show twotypical discharge plateaus and long charge plateaus (Fig. S13),further implying that the few-layer WS2@CC interlayer functionswell at high current rate, and the redox conversion between sulfurand Li2S at various rate are highly reversible. Long term cyclingperformance after rate performance measurement was furtherinvestigated at a rate of 0.5 C (Fig. 4g). Remarkably, the few-layerWS2 interlayer still exhibits a high capacity of 870 mAh g�1 after300 cycles, showing a high capacity retention of 77.7% (0.07% ca-pacity decay per cycle), in contrast to the flower-stacked WS2interlayer cell with a lower capacity of 513 mAh g�1 and capacityretention of 60.42% (0.13% capacity decay per cycle) after 300 cy-cles. In short, the results imply that although the cell with WS2catalyst on CC interlayer can enhance the polysulfide redox kineticsand increase the sulfur utilization, but interlayer with overmuchcatalyst may induce a large amount of Li2S precipitation on theinterlayer, which in turn causes the passivation of interlayer andloss of electronic contact with S cathode, leading to low sulfurutilization [18].

To further investigate the electrochemical kinetics of the few-layer and flower-stacked WS2@CC interlayer cell, the electro-chemical impedance spectroscopy (EIS) measurement for few-layerand flower-stacked WS2@CC interlayer before and after CV cycleswere performed (Figure S14, 4h and S15). Overall, the cell with few-layer WS2 interlayer exhibits a much smaller charge-transferresistance than the bare cell before and after cycling, suggestingthat the internal resistance of the LieS battery is reduced byintroducing few-layer WS2 catalyst. The Nyquist plots of the pris-tine flower-stacked WS2@CC interlayer shows higher chargetransfer resistance (45 U) than that of the few-layer WS2@CCinterlayer (17U), which can be explained by the increase of internalresistance of the cell by high-density atomically thin layer WS2catalysts (Fig. S14). After four cycles, the few-layer WS2@CC inter-layer cell still exhibits a lower charge transfer resistance (14 U) asthat of flower-stacked WS2 interlayer cell, while an additionalresistance attributed by the solid-state of the accumulated lithiumsulfide can be observed with the flower-stackedWS2@CC interlayercell (Fig. 4h and S15) [35]. The cycling performance of sulfur cath-ode with a higher sulfur loading were further compared by usingfew-layer WS@CC functional interlayer as shown in Fig. S16. Thecell with high sulfur loading of 2.4 mg cm�2 delivers initial ca-pacities of 1362 mAh g�1 at 0.1 C and an approximate capacity of856 mAh g�1 was retained after 100 cycles at 0.5 C, correspondingto a capacity retention of 89.22%. It implies that the functionalWS2@CC interlayer can enable a high sulfur loading cathode with

stable cycling performance and is highly potential for high energydensity LieS battery.

The catalytic ability of WS2 is determined by the polysulfideaccessibility to the active catalytic sites, while the density andmorphology of the catalyst, is of paramount significance to affectthe catalytic ability and thus the electrochemical stability [5,24]. Tofurther elucidate the effects of catalyst morphology and density ofthe interface on the electrochemical performance, ex-situ SEMimaging have been conducted on the few-layer WS2@CC interlayerand flower-stacked WS2@CC interlayer after 100 discharge-chargecycles. Fig. 5aed shows the ex-situ SEM images of few-layer andflower-stacked WS2@CC interlayer before and after washing withethanol. Ethanol was employed to remove liquid polysulfides onthe interlayer surface. No obvious solid precipitates are observed inthe ethanol with few layer WS2 @CC (see Fig. S17 and 60 min).Besides, it is clear that few-layer WS2@CC interlayer surface beforewashing is almost smooth with few aggregated solid sulfur species(Fig. 5a). After removing the surface polysulfide species, the few-layer WS2@CC shows the same few-layer WS2 flakes morphologyas the pristine one (Fig. 5b). In contrast, the cycled flower-stackedWS2@CC interlayer shows serious solid sulfur species aggregationon the CC (Fig. 5c). After washed with ethanol, the flower-stackedWS2 shows corrugated layered structures, where restack can beclearly observed (Fig. 5d). These results indicate the extremelystrong sulfur/sulfides absorption and chemical transformation byhierarchical flower-stacked WS2, causing nucleation and aggrega-tion of solid sulfur/polysulfides on the interface. With continuousphysicochemical transformation reaction on the interlayer surface,the severe aggregation of solid polysulfide intermediates andcorrugated WS2 layer could depress the mass transportation andinduce loss of electronic contact with sulfur cathode and formationof a “dead sulfur” layer on the interface, resulting in low sulfurutilization [18]. This will further reduce the charge transfer effi-ciency and result in poor cycling stability and lower capacities at ahigher current density. In contrast, the almost smooth surface offew-layer WS2@CC interlayer and XPS analysis after the 1st cycleindicates that almost most of the sulfur species have dissolved andreacted back into the catholyte. The results further confirm theeffective sulfur/Li-polysulfide physicochemical transformationinterface by few-layer WS2 towards polysulfide conversion.

According to our experimental results, the enhanced electro-chemical performance of the few-layer WS2@CC interlayer cell canbe readily explained as followed (See Fig. 5e). Firstly, the inherentsulfophilic property of WS2 leads to superb Li-polysulfides absorp-tion on its surface and hence regulating the polysulfides diffusion.Secondly, the intrinsic electrocatalytic properties of WS2 reduce thesulfur redox reaction barrier, promote the reversible conversion ofactive sulfur and hence improve the specific retention capacity ofthe cell. Thirdly, the high aspect ratio of catalytic active edge sites ofthe CVD-grown atomically thin layer WS2 allows large accessiblecatalytic active surface and hence promote the efficient polysulfideredox conversion during electrochemical cycling and regulate theprecipitation of insoluble sulfur species. However, the overmuchcatalytic edge sites of hierarchical flower-stacked WS2 exhibitextremely strong sulfur/polysulfide absorption and high sulfurconversion rate, causing the severe precipitation of sulfur species oninterlayer, and raising the sulfur redox conversion barrier (SeeFig. 5f). The continuous precipitation of the solid sulfur species willresult in corrugated and stacked WS2 nanosheets, loss of electroniccontact with the sulfur cathode and lead to deformed rate perfor-mance and poor cycling stability as compared to the cell with few-layer WS2 @CC. Therefore, designing the 2D CVD-based active ma-terials with controllable density and morphology is important todesign an optimized physicochemical interface with high conduc-tivity, superb electrocatalytic and strong chemisorption.

Fig. 5. (a, b) Ex-situ SEM images of the few-layer WS2@CC after 100 cycles before and after washing with ethanol. (c, d) Ex-situ SEM images of the flower-stacked WS2@CC after 100cycles before and after washing with ethanol. (e, f) Schematic illustration of the lithium polysulfides redox reaction and Li2S2 nucleation of the few-layer and flower-stacked WS2interlayer cells.

M.E. Pam et al. / Materials Today Energy 16 (2020) 100380 7

3. Conclusion

In summary, we have developed a novel strategy for controllablesynthesis of large-area and atomically thin WS2 nanosheets oncarbon cloth via a low-pressure CVD method, which can be used asa multifunctional interlayer for LieS battery. The formation mech-anism of atomically thin WS2 nanosheet on carbon cloth is inves-tigated. Benefiting from the layer-by-layer growth mechanism byour growth process, the obtained few-layer WS2@CC with abun-dant atomically thin fresh edges shows strong anchoring effect forpolysulfides, high conductivity, and excellent catalytic properties.As a result, the few-layer WS2@CC interlayer cell exhibits excellentrate capability, and good cycling stability. Our results further revealthat hierarchical flower-stacked WS2@CC with abundant exposedcatalytic edges shows extremely strong polysulfide adsorption andhigh conversion rate to low order solid species, which leads to se-vere precipitation of sulfur species on interlayer and results in lowelectrochemical stability and poor rate capability. Our work dem-onstrates CVD as a good alternative for large-scale atomically thinlayer TMDCs on other substrate and open up an effective way ofutilizing TMDCs nanosheets on conductive matrix for energy stor-age applications.

Author contribution

Mei Er Pam: Conceptualization, Methodology, Experiment;Shaozhuan Huang: Data analyzing, Writing- Original draft prepa-ration; Shuang Fan: Visualization, Investigation. Dechao Geng:Writing- Reviewing and Analysing; Dezhi Kong: Experiments;

Meng Ding: Experiments, Investigation. Lu Guo: Experiments,Investigation; Lay Kee Ang: Computation, Validation.: Hui YingYang: Supervision, Writing- Reviewing and Editing.

Declaration of competing interest

The authors declare no competing financial interest.

Acknowledgement

This work is funded by Singapore Ministry of Education Aca-demic Research Fund Tier 2 (MOE2018-T2-2-178) and SingaporeUniversity of Technology and Design International Design Center(IDC) grant.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.mtener.2019.100380.

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