Selenium Edge as a Selective Anchoring Site for LithiumâSulfur

Subscriber access provided by UNIVERSITY OF TECHNOLOGY SYDNEY

Energy, Environmental, and Catalysis Applications

Selenium Edge as Selective Anchoring Site for LithiumSulfur Batteries with MoSe2/ Graphene-based Cathodes Hoilun Wong, Xuewu Ou, Minghao Zhuang, Zhenjing Liu, Md Delowar Hossain, Yuting Cai, hongwei liu, Hwanbin Lee, Caizhuang Wang, and Zhengtang Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03246 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Selenium Edge as Selective Anchoring Site for Lithium-Sulfur Batteries with MoSe2/ Graphene-based Cathodes

Hoilun Wong1, Xuewu Ou1, Minghao Zhuang1, Zhenjing Liu1, Md Delowar Hossain1, Yuting Cai1, Hongwei Liu1, Hwanbin Lee1, Cai-Zhuang Wang2, Zhengtang Luo1*

1 Department

of Chemical and Biological Engineering, William Mong Institute of Nano Science

and Technology and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction,

The Hong Kong University of Science and

Technology, Clear Water Bay, Kowloon, Hong Kong 2

Ames Laboratory, US Department of Energy, Ames, IA 50011, USA

*E-mail: [email protected] (Z.L.)

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract For lithium-sulfur batteries (LSBs), the dissolution of lithium polysulfide and the consequent “shuttle effect” remains a major obstacle for their practical applications. In this study, we designed a new cathode material comprising of MoSe2/ graphene to selectively adsorb polysulfides on the selenium edges, and thus to mitigate their dissolution. More specifically, few layered MoSe2 was firstly grown on nitrogen-doped reduced graphene oxide (N-rGO) using the chemical vapor deposition (CVD) method, and then infiltrated with sulfur as the cathodes for LSBs. An initial capacity of 1028 mAh g-1 was achieved for S/MoSe2/N-rGO at 0.2 C, higher than 981 and 405.1 mAh g-1, of pure graphene and sulfur respectively, along with enhanced cycling durability and rate capability. Moreover, the density functional theory (DFT) simulation, in addition with the experimental adsorption test, X-ray photoelectron spectroscopy (XPS) analysis and transmission electron microscopy (TEM) technique all reveal the dual roles that MoSe2 plays in improving the performance of LSBs, by functioning as the binding sites for lithium polysulfides and the platform that enables a fast Li-ion diffusion by reducing its diffusion barrier. The reported finding suggests that the transition metal selenides could be an efficient alternative material as the cathode material for LSBs.


Page 2 of 30

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction The increasing demand for energies due to the development of electrical vehicle, portable electronics and grid scale energy storage has initiated the development of lithium-sulfur batteries (LSBs), owing to its high theoretical specific capacity of 1675mAh g-1 and energy density of 2600 Wh kg-1.1-3

In addition to its natural

abundance, sulfur, as the main active material in LSBs, are low cost and environmental friendly.4,5 However, because of the insulating nature of sulfur (5×10-30 S/cm at 25 oC) and lithium sulfide6 , along with large volume change (over 80%) and the “shuttle effect” induced by irreversible dissolution and migration of lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8), LSB suffers from low sulfur utilization and severe loss of active materials, leading to a short life-cycle and low coulombic efficiency.7-10 In response to these challenges, intensive research has been carried out, including the incorporation of sulfur with highly conductive carbon materials11-16 or polymers.17-19 Despite of these significant progress, the cyclical performance of LSBs remained poor due to the limited interactions between non-polar carbon materials or polymers with polysulfides.

More recently, transition metal oxides, TiO220,21, MnO222,23 and Co3O424 and sulfides, WS225, ReS226, CoS227 and VS228 have been proven to have effective binding capabilities to Li2Sn.The strong chemical affinity is provided by the unsaturated dangling bond of heteroatom (oxygen and sulfur) , and thus greatly alleviating their dissolution in the electrolyte, and ultimately contributes to a stable and prolonged battery cycling life. Among them, 2D materials, owing to their large surface area, are believed to be a promising adsorption material for Li2Sn. In particular,

the edge of

CVD-grown WS2 and MoS2 nanosheets were shown to be the preferential adsorption sites for lithium polysulfide.29 Upon discharge, a uniform deposition of lithium 3 ACS Paragon Plus Environment


polysulfide was observed at the edge of WS2 flake. The electrode exhibited a specific capacity of 590 mAh g-1 at 0.5 C over 350 cycles, indicating good cycling stability. However, transition metal oxides and sulfides normally own a relatively large band gap, unfavorable for charge transfer. Previously, our group has demonstrated that highdensity perforated molybdenum diselenide (MoSe2) on polymer functionalized graphene plays a critical role in enhancing the catalytic performance in hydrogen evolution reaction.30 To our best knowledge, there has been no established study on investigating the role of transition metal selenides in LSBs.

Based on this, in this paper, a similar method has been adopted to firstly synthesize the MoSe2 nanocrystals on the thin layer of nitrogen-doped graphene (MoSe2/N-rGO) using the CVD method, and then composited with sulfur nanoparticles as the cathode material for LSBs. This MoSe2/N-rGO composite has the appealing advantages of large surface area, highly conductive and effective Li2Sn adsorption sites. The electrode of MoSe2/N-rGO/S enables an initial discharge capacity of 1310 mAh g-1 and 1028 mAh g-1 at 0.05 C and 0.2 C, respectively, with significantly improved cycling stability compared to the pure sulfur (S) and graphene (N-rGO/S) electrodes. Combining the experimental study with the density functional theory (DFT) calculation, we found that MoSe2, especially the selenium edge sites, possess a strong affinity to Li2Sn. Moreover, lithium diffusion on the surface of MoSe2 was found to be faster than that of graphene surface, pinpointing the role of MoSe2 in improving the cell performance of LSBs.

Experimental Section Chemicals: Graphite (Grafguard with the average particle size of 350 mm) was expanded by microwave before use. Potassium persulfate (K2S2O8), Phosphorus pentoxide (P2O5), Concentrated Sulfuric acid (98%), KMnO4, H2O2, (NH4)6Mo7O24,


Page 4 of 30

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


selenium powder (99.99%), dopamine hydrochloride, Tris (hydroxymethyl)aminomethane, sulfur powder and N-methyl-2-pyrrolidone (NMP) were all purchased from Sigma-Aldrich with analytical grade and used without further purification. Carbon black and poly(vinylidene difluoride) (PVDF) were from Lizhiyuan battery Corp. and 1 M lithium bistrifluoromethanesulfonylimide (LiTFSI) in 1:1 (v/v) mixture solution of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with 1% LiNO3 as the electrolyte was purchased from Duoduo chemicals Corp.. Throughout the whole experiment, deionized (DI) water was used.

Preparation of GO: The GO was prepared by a modified Hummers method.31,32 In a typical synthesis, 30mL of concentrated sulfuric acid is added to a flask with 1g of graphite, 5g of K2S2O8 and 5g of P2O5 and then reacted at 90oC for 4.5h. After that, the product was obtained via a 0.2 micro Nylon Millipore filter and sequentially washed with excess deionized water until the pH reaching 5.5. After drying the sample at 60oC for 3h, 150mL of concentrated sulfuric acid and 30g of KMnO4 were added slowly into a 250mL flask at 0oC and then reacted at 35oC for 4h. After the reaction, 1L of deionized water was added into the mixture and stirred for 2h. Finally, 50 mL of 30% H2O2 was added into the solution slowly until the color of solution becoming bright yellow. The solution was then stirred for another 2h. After the stirring, the solution mixture was settled for 24h. The supernatant was then discarded while the product was washed by 1L of 10% hydrochloric acid and excess deionized water until the pH reach ~7.

Synthesis of MoSe2/N-rGO and N-rGO: 15mg of the as-prepared GO and 200mg of dopamine were dispersed into 100ml of deionized water and stirred for 30 minutes. Then, 100mg of Tris-HCl was added into the above solution and followed by vigorous stirring at room temperature for 24h. After polymerization, the polydopamine coated 5 ACS Paragon Plus Environment


N-rGO was then separated from the dispersing solution by centrifugation and washed with deionized water which was re-dispersed into deionized water for later use.

For the growth of MoSe2 on N-rGO, it was synthesized according to our previous work.30 0.075mmol of (NH4)6Mo7O24 was added into the re-dispersed solution followed by freeze drying. The dried sample was firstly annealed in CVD furnace at 300oC for 1h with 100 sccm Ar and became MoO3/N-rGO, which was then mixed with 20mg Se powder and undergo selenization at 750oC for 1h with the carrier gas of 50 sccm Ar and 10 sccm H2. As for the composite of N-rGO, it was prepared following the same procedure without the part introducing the molybdenum precursor into the re-dispersed solution.

Preparation of MoSe2/N-rGO/S and N-rGO/S: Sulfur is infiltrated into the composites of MoSe2/N-rGO and N-rGO by melt-diffusion method. 60% of elemental sulfur was firstly mixed well with the composites. The mixture was then heated at 155oC in a sealed container for 24 h with argon protection. Both electrode composites were prepared in the same procedures to ensure a fair comparison in the electrochemical performance.

Material characterizations: The morphology and the distribution of the material were observed by Transmission Electron Microscopy (TEM, JEOL 2010F) with the EDS element mapping. Powder X-ray diffraction (XRD, PW1830) with the use of Cu Karadiation (λ=1.5406 Å) was applied to characterize the crystal structure at a scanning rate of 4.25o/min in the 2θ range from 5o to 90o. The content of MoSe2 and Sulfur in the material composite was determined by Thermogravimetric analysis (TGA, TA Q-50) at a heating rate of 5oC /min from room temperature to 800oC in air and 400oC in 6 ACS Paragon Plus Environment

Page 6 of 30

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nitrogen atmosphere, respectively. X-ray photoelectron spectroscopy (XPS, PHI 5600) was carried out to analyze the material composition and its elemental valence states.

Electrode preparation and electrochemical measurements: To prepare the slurry for electrode fabrication, 70 wt% of active material (MoSe2/N-rGO/S), 20 wt% of conductive carbon black (super P) and 10 wt% of binder (PVDF in NMP solution) were mixed uniformly and coated on aluminum foil by Doctor Blading. The areal sulfur loading of each electrode was kept at the same value of 1.1 mg/cm2. The electrode composite was dried in the oven at 60 oC overnight and punched into circular shape with a diameter of 1.2 cm for coin cell assembling. CR2025-type coin cells were assembled in an argon gas filled glovebox with a lithium metal foil as anode, a Celgard 2325 membrane as the separator and the electrolyte composed of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in the mixture of dimethyl ether (DME) and 1,3-dioxolane (DOL) in a volume ratio of 1:1 with 1 wt% lithium nitrate (LiNO3).

For the electrochemical test, the galvanostatic discharge/charge and the cycling performance were examined by LAND CT2001A battery tester at different scan rates with the voltage window of 2.8-1.8 V. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy were carried out on an electrochemical workstation (CHI 760E). For CV test, it was conducted at a scan rate of 0.1 mV/s with the same potential window of 2.8-1.8 V. EIS was performed at constant amplitude of 5mV in the frequency range from 100 kHz to 0.01 Hz.

Preparation of lithium polysulfide, Li2S6 for adsorption test: Li2S6 was prepared by dissolving the stoichiometric ratio of Li2S and elemental S into DME/DOL in a volume ratio of 1:1. The mixture was stirred at 80oC until all the powder is dissolved. 7 ACS Paragon Plus Environment


For the adsorption test, 30 mg of carbon black, N-rGO and MoSe2/N-rGO were immersed into 2mL of 0.005M Li2S6 solution, respectively. After the interaction, digital photo and ultraviolet-visible spectroscopy were taken to analyze the adsorption ability of the three materials.

XPS analysis on the interactions between Li2S6 and MoSe2/N-rGO: The XPS samples were obtained from the post adsorption test samples by centrifuge. The precipitate was collected and washed to remove redundant Li2S6. The washed sample was dried and mounted on the XPS holder readily for the XPS analysis.

Density functional theory (DFT) calculations: The DFT calculation is carried out by Vienna Ab Initio Simulation Package (VASP).33,34 The generalized gradient approximation (GGA) exchange-correlation function of Perdew-Burke-Ernzerhof (PBE) is used,35 and the ion-electron interaction is described by projector augment wave (PAW) pseudopotential. In addition, we adopt cut-off energy of 500 eV for plane-wave basis set throughout all the simulations. To investigate the diffusion property of Li on graphene and MoSe2, the climbing-image nudged elastic band (CI-NEB) method is applied.36 In order to determine the diffusion path, firstly, we have calculated the binding energy of Li atom at different sites on graphene and MoSe2 as shown in Figure S10. Secondly, we choose two nearest most stable adsorption site as the initial and final state during the CI-NEB method. In this study of lithium ion diffusion, some appropriate assumptions were made, such as, negligible effects on defects and impurities of graphene, lithium ions transport on sulfur and Li2S nanoparticles and between the adjacent layers of MoSe2.37,38 To study the adsorption of Li2Sn graphene and MoSe2, the structure of Li2Sn (n=1,2,4,6,8) clusters are constructed according to previous report and minimized for further calculation,39 and a 6*6 graphene (14.82 8 ACS Paragon Plus Environment

Page 8 of 30

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Å*14.82 Å) and a 5*5 MoSe2 (16.59 Å*16.59 Å) supercell are created respectively as the adsorption substrates. The vacuum space in the z direction is set to be 20 Å in order to minimize the effect from the images. The Brillouin zone is sampled by a k-points of 7*7*1 generated automatically using the Monkhorst-Pack’s technique. For Li2Sn adsorption at the edge site of MoSe2, both the vacuum distance along y and z directions are larger than 20 Å. For each Li2Sn, to obtain the accurate binding energy, we have tried different initial configurations with Li2Sn parallel or vertical atop the substrate. The binding energy value is calculated by: 𝐸𝑏 = 𝐸𝑠𝑢𝑏 + 𝐸𝐿𝑖/𝐿𝑖2𝑆𝑛 ― 𝐸𝑠𝑢𝑏 + 𝐿𝑖/𝐿𝑖2𝑆𝑛, in which 𝐸𝑠𝑢𝑏 is the ground state energy of the adsorption substrate such as graphene and MoSe2, 𝐸𝐿𝑖/𝐿𝑖2𝑆𝑛 is the ground state energy of Li atom or Li2Sn, and 𝐸𝑠𝑢𝑏 + 𝐿𝑖2𝑆𝑛 is the total energy after Li atom or Li2Sn adsorbed on the substrate. For all simulation, the ions are allowed to relax until the residual force is less than 0.02 eV/Å. For binding energy calculation with van der Waals force, vdW-DF2 functional is used.40,41 The 3D visualization models were constructed using VESTA software.42

Result and Discussion The synthesis process of MoSe2/N-rGO is illustrated in Scheme 1. Firstly, graphene oxide (GO) was synthesized by the modified Hummers method, and then combined with dopamine (DA). Ascribed to the non-covalent 𝜋 ― 𝜋 interaction between GO and DA, DA molecules adhere onto the terrace of GO. After adjusting the pH to ~8.5 by adding the Tris-buffer, DA was in situ polymerized onto the GO surface. Here, the coating layer of polydopamine (PDA) on the GO assisted for the growth of MoSe2 in the subsequent process. More specifically, (NH4)6Mo7O24 was added into DA/GO dispersion utilizing the electrostatic interaction between the amine groups in polydopamine and (NH4)5Mo7O24-. Here, (NH4)5Mo7O24- was confined on the graphene oxide surface, which in turn allow the uniform formation of MoSe2 after the 9 ACS Paragon Plus Environment


selenization.

Subsequently,

after

freeze-drying,

the

Page 10 of 30

as-prepared

composite

((NH4)6Mo7O24/PDA-N-rGO) was ground together with selenium powder and then selenized through the CVD process at 750 oC to fabricate the MoSe2/N-rGO composite.

The morphology and structure of MoSe2 /N-rGO were characterized using the transmission electron microscopy (TEM). From Figure 1a, it is clearly observed that triangular-shaped single crystals of MoSe2 (Figure 1b) with the size ~ 10 to 60 nm were evenly distributed on N-rGO. The high resolution TEM image in Figure 1c displays the side-views of MoSe2 and showing the thickness of less than 10 layers MoSe2 were tightly anchored on the graphene surface. The interlayer distance of 0.646 nm corresponds to the (002) facet of MoSe2, while the inter-planar distance of 0.28 nm measured in Figure 1d was attributed to (100) plane of MoSe2, in agreement with the SAED pattern (Figure 1e), where the main facet of (100) of MoSe2 was indicated, also implying a well-defined single crystalline structure of MoSe2 was successfully synthesized.

The crystal structure of MoSe2/N-rGO was further analyzed by X-ray diffraction (XRD). As shown in Figure 2a, both the XRD patterns of MoSe2/N-rGO (red) and NrGO (blue) show a broad diffraction peak at 24.1o corresponding to (002) lattice plane of nitrogen doped graphene. The typical peaks at 13.7o, 31.6o, 38.1o, 56.1o and 83.6o observed in the red pattern can be indexed to the MoSe2 plane of (002), (100), (103), (110) and (118), respectively. In addition, X-ray photoelectron spectroscopy (XPS) was used to study the valence state of and the elemental stoichiometry MoSe2/N-rGO. As shown in Figure 2b, the full spectrum of MoSe2/N-rGO indicates the presence of Mo, Se, N and O elements in the hybrid which further verify the structure of MoSe2. High resolution 3d spectra of Mo and Se were displayed in Figure 2c and 2d. The binding 10 ACS Paragon Plus Environment

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energies located at 228.7 and 231.8 eV represent the Mo 3d5/2 and Mo 3d3/2 orbitals. In addition, the divided peaks at 54.2 and 55.0 eV are attributed to the orbital of Se 3d5/2 and Se 3d3/2, respectively, indicating +4 and -2 oxidation states of Mo and Se, thereby verifying the elemental composition of MoSe2.30,43 Moreover, the nitrogen doping of the material was confirmed under XPS analysis. Based on Figure 2e, three types of nitrogen doping were identified from the divided peaks at the binding energy of 398.6, 400.0 and 401.4 eV, corresponding to pyridinic N, pyrrolic N and graphitic N, respectively, which are reported to be effective to suppress the shuttle effect.44

To have sufficient sulfur utilization, materials with a high surface area and pore volume are critical. In the Brunauer-Emmett-Teller (BET) analysis, the specific surface area of MoSe2/ N-rGO was calculated to be 372.4 m2/g by using the standard multipoint measurement (Figure S2, Supporting Information). The nitrogen adsorption/ desorption isotherm (Figure S3a) and the Barrett-Joyner-Halenda (BJH) pore size distribution in Figure S3b indicates the presence of mesopores. The mesoporous material has an average pore size of 5.1 nm and cumulative pore volume of 0.77 cm3/g which serves as a conductive host to accommodate sulfur. Sulfur was then infiltrated into the composite of MoSe2/N-rGO and N-rGO using the conventional melt-diffusion method. Thermogravimetric analysis (TGA) test in Figure 2f has shown that the sulfur loading in the materials of MoSe2/ N-rGO/S and N-rGO/S is ~62%, comparable to previous reports45-47. In addition, the elemental mapping results in Figure S4 and S5 (Supporting Information) suggest a homogenous sulfur encapsulation in the composites.

To evaluate the battery performance, the galvanostatic charge-discharge profiles of MoSe2/N-rGO/S and N-rGO/S, in comparison with the control samples, were examined at 0.05 and 0.2C, within the potential window of 1.8-2.8V, as seen in Figure S6 and 11 ACS Paragon Plus Environment


Figure 3a, respectively. It is clearly shown that both the initial discharge curves of MoSe2/N-rGO/S and N-rGO/S consist of two flat plateaus, at 2.31 and 2.08V correspondingly to the reduction of S8 to long-chain lithium polysulfides and Li2S, respectively. In sharp contrast, the first plateau of S was significantly shorter and more importantly, its lower plateau nearly disappeared. This reveals the insulating nature of sulfur and the sever loss of long-chain polysulfides resulting in a low discharge capacity of 405.1 mAh g-1 that only accounted for 24.2% of the theoretical capacity (expected to be 1675 mAh g-1). In addition, the MoSe2/N-rGO/S cathode delivered an initial capacity of 1028 mAh g-1, which was slightly higher than that of N-rGO/S (981 mAh g-1). After 50 cycles, MoSe2/N-rGO/S remained the discharge capacity of 924 mAh g-1 and showed no change in hysteresis (i.e. the voltage gap between the oxidation (charge) and reduction (discharge) plateaus). This is, however, in contrast to the N-rGO/S composite, which suffered from a much faster capacity decay with an obviously larger hysteresis. Moreover, although the charge capacity of sulfur electrode remained the same, the discharge capacity decreased significantly, evidencing a low Coulombic efficiency. These results indicate the vital role of MoSe2, which inhabits the shuttle effect and accelerates the redox kinetics.

The batteries cycling performance was evaluated by charging and discharging at 0.2 C. In Figure 3b, the N-rGO/S electrode encountered a rapid capacity drop from 981 to 692 mAh g-1 over 100 cycles, corresponding to a capacity retention rate of only 70.6%. In comparison, MoSe2/ N-rGO/S electrode exhibited a more stable cycling with the retention rate as high as 86.3%, as well as 98% Coulombic efficiency. As shown in Figure S7, the structure of MoSe2 in the composite remains chemically stable after cycling. Figure 3c displays the rate capabilities of MoSe2/N-rGO/S, in comparison with N-rGO/S and S, by increasing the current densities from 0.1 C to 1 C every five cycles 12 ACS Paragon Plus Environment

Page 12 of 30

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and then switching back to 0.1 C. MoSe2/N-rGO/S batteries shows the discharge capacities of 1213, 1118, 877 and 632 mAh g-1 at 0.1 C, 0.2 C, 0.5 C and 1 C, respectively. When the current density was returned to 0.1 C, a reversible discharge capacity of 1198 mAh g-1 was resumed. At higher current rate of 2 C, Figure S8 shows that MoSe2/N-rGO/S achieves a stable cycling at around 425 mAh g-1 with two clear discharge plateaus in discharge/charge profile (Figure S8b) in the comparison with NrGO/S and S, which evidencing the good battery reversibility and stability.

Electrochemical impedance spectroscopy (EIS) was subsequently carried out to understand the charge transfer resistance. The Nyquist plots of three electrodes with the corresponding equivalent circuit are illustrated in Figure 3d. Parameter, Re represents the resistance of electrolyte and the cells components, while R1 is the resistance of charge transfer between the electrode surface and electrolyte, and CPE1 and W1 are the constant phase elements and Warburg resistance, respectively. The impedance data were determined based on the model fitting and displayed in Figure S9 (Supporting Information). The R1 of MoSe2/N-rGO/S was 65.6 Ω, significantly lower than the NrGO/S (193.1 Ω) and S (148.7 Ω), suggesting a faster electron pass. The Impedance of S is lower than that of N-rGO/S is because of the same sulfur mass for all cathodes, the pure S electrodes is thinner than the composite electrodes, which have sulfur loading of ~62%, (TGA results from Figure 2f). Therefore, the composite cathode are much thicker than S electrode, and lead to higher ion diffusional impedance. To further understand the role MoSe2 in improving the performance of LSBs, density functional theory (DFT) calculation was performed to investigate the ionic conductivity of MoSe2 and its binding capability with Li2Sn (n = 1,2,4,6,8). Firstly, to study the Li ion diffusion on the MoSe2 surface, climbing-image nudged elastic band (CI-NEB) method was adopted here (refer to the computation method in Supporting 13 ACS Paragon Plus Environment


Page 14 of 30

Information for more details). Figure 4a shows the energy profile of Li atom diffusion on MoSe2 and graphene surface. As displayed in the inset images, the diffusion path is from one atop Mo site to the nearest atop Mo site for MoSe2, and from one hexagonal center to the nearest hexagonal center site for graphene, respectively, both the most stable adsorption sites based on the Li atom adsorption result as shown in Figure S10 that has the highest Li atom binding energy at the position of atop Mo site and atop hexagonal center for MoSe2 and graphene, respectively. In Figure 4a, a single peak is observed in the energy profile of graphene corresponds to the Li atom atop C-C bridge, while the two peaks are seen for MoSe2, correspond to the adsorption site of Li atom atop Mo-Mo bridge. The lower Li atom diffusion energy barrier of 0.2374 eV on MoSe2 was observed, compared with that of 0.3104 eV on graphene surface, indicating the improved Li ion transport, and consequently better rate-capacity induced by MoSe2.

The binding energy value is calculated by: 𝐸𝑏 = 𝐸𝑠𝑢𝑏 + 𝐸𝐿𝑖2𝑆𝑛 ― 𝐸𝑠𝑢𝑏 + 𝐿𝑖2𝑆𝑛

Eq (1)

Where 𝐸𝑠𝑢𝑏 is the ground state energy of the substrate including graphene and MoSe2, 𝐸𝐿𝑖2𝑆𝑛 is the ground state energy of Li2Sn (n=1,2,4,6,8), and 𝐸𝑠𝑢𝑏 + 𝐿𝑖2𝑆𝑛 is the total energy of Li2Sn adsorbed on the substrates. From this equation, it can be concluded that a positive 𝐸𝑏 indicates the favorable adsorption of Li2Sn. Since previous works have reported that the edge sites of WS2 and MoS2 play an important role to adsorb Li2Sn in the electrochemical process owing to their dangling bond29,30,48, accordingly, we have constructed two types MoSe2 edge structure based on recently published work: 50% MoSe2 edge and 100% MoSe2 edge, as depicted in Figure S11.43

Figure 4b compares

the binding energy of MoSe2 and graphene to Li2Sn, and it was found that both the surface and edge sites of MoSe2 show a higher binding energy than that of graphene, 14 ACS Paragon Plus Environment

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


suggesting the favorable adsorption of Li2Sn on MoSe2. Especially, the 50% MoSe2 edge displays much a higher binding energy, which should be attributed to the more electronegative property of Se at the edge48. Figure 4c and 4d elaborate the configuration of MoSe2 before and after Li2S4 adsorption at the edge, in which the structure of MoSe2 is distorted as well as the structure of Li2S4, indicating the strong interaction between them. The adsorption configuration of Li2Sn (n=1,2,4,6,8) on graphene and MoSe2 are shown in Figure S12, and the corresponding binding energy is listed in Table S1. Additionally, as suggested by some report, van der Waals force (vdW) should be considered in order to calculate a more accurate binding energy for Li2Sn.39,49 Figure S13 presents the results by introducing the vdW-DF2 functional.40,41 Taking the vdW force, the obtained binding energies are increased for both graphene and MoSe2, but their binding energy difference is almost unchanged, implying that the vdW force provides a similar contribution to Li2Sn for graphene and MoSe2.

To verify the anchoring ability of MoSe2 to the Li2Sn, adsorption test was carried out by immersing the same amount of MoSe2/N-rGO and N-rGO into the Li2S6 solution. The Li2S6-interacted sample was subsequently examined by XRD and its morphology change was observed by TEM. As shown in Figure S14a, a significant color fading in sample (III) was observed compared with sample (I), suggesting the affinity of MoSe2 to the polar polysulfide. In contrast, the color in sample (II) remains the same, indicating that N-rGO has poor adsorption ability to Li2S6 owing to the non-polarity of graphene. From the UV-vis spectra, at the wavelength of 536 nm (determined in Figure S14b), the spectra (III) shows a higher transmittance in the comparison of (I) and (II) revealing a stronger adsorption power of MoSe2/N-rGO, in agreement with the simulation result.

The precipitate in sample (III) was further analyzed by XPS to probe the chemical 15 ACS Paragon Plus Environment


interaction between Li2Sn and MoSe2 by looking at its chemical state change. From the Mo 3d spectrum, the pristine sample in the lower panel of Figure 5a shows two divided peaks located at 228.7 and 231.8 eV representing the Mo 3d5/2 and Mo 3d3/2 orbitals, respectively. After the interaction with Li2S6, the two original peaks were obtained but centered at the slightly increased position. This was attributed to the unavoidable oxidation during the sample transfer. Interestingly, the intensity of two original peaks was significantly reduced and a new peak was found at the binding energy of 233.3 eV indicating a bond formation of Mo-S,50 possibly caused by the defects of selenium vacancy from the material synthesis. We did not calculate the adsorption energy of defective MoSe2 to the polysulfides because defect engineering is not the main scope in this work, where the defects are unintentionally created during the material synthesis. Comparing the two spectra in Figure 5b, both of them consist of two peaks of Se 3d5/2 and Se 3d3/2. However, the locations of the two peaks in the interacted sample (upper panel) were positively shifted to a higher binding energy due to the withdrawal of valence electrons from the negatively charged Se to Li. Moreover, an additional peak is observed at the binding energy of 55.9 eV corresponding to Li-Se bonding51, evidencing a strong chemical interaction between Se and Li. In addition, the morphology of MoSe2 had changed after the interaction with Li2S6. The darker regions in Figure 5c illustrate the attachments of Li2S6. The corresponding elemental mapping detected a very strong signal of sulfur, highly correlated to the signals of selenium and molybdenum, confirming the strong chemical interactions between MoSe2 and Li2S6.

As mentioned in the above DFT calculation, we observed that the Li atom diffusion on the MoSe2 surface is faster and easier in the comparison of graphene surface. In reality, we could determine the lithium-ion diffusion coefficient experimentally by conducting the CV scans at different rates from 0.1 to 0.3 mV s-1. With the use of the 16 ACS Paragon Plus Environment

Page 16 of 30

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


well-known Randles-Sevcik equation (Eq 2), lithium ion diffusion coefficient (D) can be deduced in the relationship of peak current (IP) and square root of scan rate (v1/2) displayed in Figure 5d.

𝐼𝑃 = 0.4463 𝑛𝐹𝐴𝐶 (

𝑛𝐹𝑣𝐷 1/2 ) 𝑅𝑇

Eq (2)

Where 𝐼𝑃 is the peak current, 𝑛 is the number of electrons transfer, 𝐹 is the Faraday constant, 𝐴 is the electrode area, C is the concentration of lithium ion in the electrolyte, D is the lithium ion diffusion coefficient, v is the scan rate, R is the gas constant and T represents temperature.

The peak currents at different scan rates were determined from the CV curves of MoSe2/N-rGO/S and N-rGO/S in Figure S15a and S15b, respectively. The anodic peak represents the redox conversion of (Li2S4-S8), while the first cathodic peak at the voltage, 2.1-2.3V belongs to the reaction of (Li2S8- Li2S4). After the linear fitting, Figure 5d concludes that MoSe2/N-rGO/S has higher lithium ion diffusion coefficient in both anodic and cathodic reactions implying a better redox kinetic in the battery. In contrast, N-rGO/S has a poor lithium ion conductivity indicates its weak polysulfide capture capability that allows the highly viscos Li2Sn to dissolve in the electrolyte, resulting in low ion diffusivity.

Conclusion In summary, we successfully prepared the laminar structured MoSe2/ N-rGO as the cathode material and studied its adsorption ability to the lithium polysulfides. We found that by using the Se edge of MoSe2 as anchoring sites for polysulfides, we mitigate the



shuttle effect in LSBs. After the sequential sulfur infiltration of a sulfur loading of 62%, MoSe2/ N-rGO/S delivered an initial discharge capacity as high as 1028 mAh g-1 at 0.2 C with 86.3% retention rate over 100 cycles in the cycling performance. Significant improvement in cycling stability and rate capability was observed in the comparison with N-rGO/S and Sulfur electrodes. DFT calculation results demonstrated that the energy barrier of lithium diffusion on the surface of MoSe2 is much lower compared to that on the graphene surface, while selenium edge has high binding energy to the lithium polysulfides. Adsorption test with subsequent XPS and TEM analysis confirmed the strong chemical interactions between MoSe2 and Li2Sn. The strategy in this work utilizes MoSe2 as anchoring cathode materials to suppress the shuttle effects of the polysulfides, and our results show an obvious advantages over previously published works, particularly in the improved initial capacity29,52 and cycling stability20,24,53. Our work provides valuable insight on the role of transition metal selenides that could be a new effective anchoring material for lithium-sulfur batteries.

Author Information Corresponding author * E-mail: [email protected] (Z.L.). ORCID Cai-Zhuang Wang: 0000-0002-0269-4785 Zhengtang Luo: 0000-0002-5134-9240 Notes The authors declare no competing financial interests.

Acknowledgements This project was supported by the Research Grant Council of Hong Kong SAR (Project 18 ACS Paragon Plus Environment

Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


number 16204815), NSFC-RGC Joint Research Scheme (N_HKUST607/17), the Innovation and Technology Commission (ITC-CNERC14SC01), the Guangzhou Science & Technology (Project 201704030134). We acknowledge Mr. Soumyadip Majumder for the valuable discussion on the XPS analysis. Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated.

Supporting Information The corresponding TEM, BET, TGA, XPS and other analysis results are included in the supporting information for publication.

References (1) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid Chemical reviews 2011, 111, 35773613. (2) Manthiram, A.; Fu, Y.; Chung, S. H.; Zu, C.; Su, Y. S. Rechargeable LithiumSulfur Batteries Chemical reviews 2014, 114, 11751-11787. (3) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage Nature materials 2011, 11, 19-29. (4) Manthiram, A.; Chung, S. H.; Zu, C. Lithium-Sulfur Batteries: Progress and Prospects Advanced materials 2015, 27, 1980-2006. (5) Seh, Z. W.; Sun, Y.; Zhang, Q.; Cui, Y. Designing High-Energy Lithium-Suflur Batteries Chemical Society reviews 2016, 45, 5605-5634. (6) Dean, J. A. Lange's Handbook of Chemistry; 3rd ed.; McGraw-Hill, 1985. (7) Ji, X.; Nazar, L. F. Advances in Li-S Batteries Journal of Materials Chemistry 2010, 20, 9821-9826. (8) Yin, Y. X.; Xin, S.; Guo, Y. G.; Wan, L. J. Lithium-Sulfur Batteries: Electrochemistry, Materials, and Prospects Angewandte Chemie 2013, 52, 1318613200. (9) EVERS, S.; NAZAR, L. F. New Approches for High Energy Density LithiumSulfur Battery Cathodes Accounts of Chemical Research 2012, 46, 1135-1143. (10) Ou, X.; Yu, Y.; Wu, R.; Tyagi, A.; Zhuang, M.; Ding, Y.; Abidi, I. H.; Wu, H.; Wang, F.; Luo, Z. Shuttle Suppression by Polymer-Sealed Graphene-Coated Polypropylene Separator ACS applied materials & interfaces 2018, 10, 5534-5542. 19 ACS Paragon Plus Environment


(11) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries Nature materials 2009, 8, 500-506. (12) Guo, J.; Xu, Y.; Wang, C. Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium-Sulfur Batteries Nano letters 2011, 11, 4288-4294. (13) Jayaprakash, N.; Shen, J.; Moganty, S. S.; Corona, A.; Archer, L. A. Porous Hollow Carbon@Sulfur Composites for High-Power Lithium-Sulfur Batteries Angewandte Chemie 2011, 50, 5904-5908. (14) Schuster, J.; He, G.; Mandlmeier, B.; Yim, T.; Lee, K. T.; Bein, T.; Nazar, L. F. Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for LithiumSulfur Batteries Angewandte Chemie 2012, 51, 3591-3595. (15) Zhang, J.; Yang, C. P.; Yin, Y. X.; Wan, L. J.; Guo, Y. G. Sulfur Encapsulated in Graphitic Carbon Nanocages for High-Rate and Long-Cycle Lithium-Sulfur Batteries Advanced materials 2016, 28, 9539-9544. (16) Fang, R.; Zhao, S.; Pei, S.; Qian, X.; Hou, P. X.; Cheng, H. M.; Liu, C.; Li, F. Toward More Reliable Lithium-Sulfur Batteries: An All-Graphene Cathode Structure ACS nano 2016, 10, 8676-8682. (17) Yang, Y.; Yu, G.; Cha, J. J.; Wu, H.; Vosgueritchian, M.; Yao, Y.; Bao, Z.; Cui, Y. Improving the Performance of Lithium–Sulfur Batteries by Conductive Polymer Coating ACS nano 2011, 5, 9187-9193. (18) Fu, Y.; Su, Y. S.; Manthiram, A. Sulfur-Polypyrrole Composite Cathodes for Lithium-Sulfur Batteries Journal of the Electrochemical Society 2012, 159, A1420A1424. (19) Xiao, L.; Cao, Y.; Xiao, J.; Schwenzer, B.; Engelhard, M. H.; Saraf, L. V.; Nie, Z.; Exarhos, G. J.; Liu, J. A Soft Approach to Encapsulate Sulfur: Polyaniline Nanotubes for Lithium-Sulfur Batteries with Long Cycle Life Advanced materials 2012, 24, 1176-1181. (20) Evers, S.; Yim, T.; Nazar, L. F. Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li–S Battery The Journal of Physical Chemistry C 2012, 116, 19653-19658. (21) Wei Seh, Z.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y. Sulphur-TiO2 Yolk-Shell Nanoarchitecture with Internal Void Space for Long-Cycle Lithium-Sulphur Batteries Nature communications 2013, 4, 1331. (22) Li, Z.; Zhang, J.; Lou, X. W. Hollow Carbon Nanofibers Filled with MnO2 Nanosheets as Efficient Sulfur Hosts for Lithium-Sulfur Batteries Angewandte Chemie 2015, 54, 12886-12890. (23) Zhang, J.; Shi, Y.; Ding, Y.; Zhang, W.; Yu, G. In Situ Reactive Synthesis of Polypyrrole-MnO2 Coaxial Nanotubes as Sulfur Hosts for High-Performance LithiumSulfur Battery Nano letters 2016, 16, 7276-7281. 20 ACS Paragon Plus Environment

Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Wang, H.; Zhou, T.; Li, D.; Gao, H.; Gao, G.; Du, A.; Liu, H.; Guo, Z. Ultrathin Cobaltosic Oxide Nanosheets as an Effective Sulfur Encapsulation Matrix with Strong Affinity Toward Polysulfides ACS applied materials & interfaces 2017, 9, 4320-4325. (25) Park, J.; Yu, B.-C.; Park, J. S.; Choi, J. W.; Kim, C.; Sung, Y.-E.; Goodenough, J. B. Tungsten Disulfide Catalysts Supported on a Carbon Cloth Interlayer for High Performance Li-S Battery Advanced Energy Materials 2017, 7, 1602567. (26) Gao, J.; Li, L.; Tan, J.; Sun, H.; Li, B.; Idrobo, J. C.; Singh, C. V.; Lu, T. M.; Koratkar, N. Vertically Oriented Arrays of ReS2 Nanosheets for Electrochemical Energy Storage and Electrocatalysis Nano letters 2016, 16, 3780-3787. (27) Yuan, Z.; Peng, H. J.; Hou, T. Z.; Huang, J. Q.; Chen, C. M.; Wang, D. W.; Cheng, X. B.; Wei, F.; Zhang, Q. Powering Lithium-Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts Nano letters 2016, 16, 519-527. (28) Cheng, Z.; Xiao, Z.; Pan, H.; Wang, S.; Wang, R. Elastic Sandwich-Type rGOVS2/S Composites with High Tap Density: Structural and Chemical Cooperativity Enabling Lithium-Sulfur Batteries with High Energy Density Advanced Energy Materials 2018, 8, 1702337. (29) Babu, G.; Masurkar, N.; Al Salem, H.; Arava, L. M. Transition Metal Dichalcogenide Atomic Layers for Lithium Polysulfides Electrocatalysis Journal of the American Chemical Society 2017, 139, 171-178. (30) Zhuang, M.; Ding, Y.; Ou, X.; Luo, Z. Polymer-Confined Growth of Perforated MoSe2 Single-Crystals on N-Doped Graphene Toward Enhanced Hydrogen Evolution Nanoscale 2017, 9, 4652-4659. (31) Hummers, W. S.; Offeman, R. E. Preparartion of Graphitic Oxide Journal of the American Chemical Society 1958, 80, 1339-1339. (32) Luo, Z.; Lu, Y.; Somers, L. A.; Johnson, A. T. C. High Yield Preparation of Macroscopic Graphene Oxide Membranes Journal of the American Chemical Society 2009, 131, 898-899. (33) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas Physical Review 1964, 136, B864. (34) Kresse, G.; Furthmüller, J. Efficient Iterative sSchemes for AB Initio Total-Energy Calculations Using a Plane-Wave Basis Set Phys. Rev. B 1996, 54, 11169-11186. (35) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation Physical Review B 1992, 46, 6671-6687. (36) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths The Journal of Chemical Physics 2000, 113, 9901-9904. 21 ACS Paragon Plus Environment


(37) Zhou, G.; Tian, H.; Jin, Y.; Tao, X.; Liu, B.; Zhang, R.; Seh, Z. W.; Zhuo, D.; Liu, Y.; Sun, J.; Zhao, J.; Zu, C.; Wu, D. S.; Zhang, Q.; Cui, Y. Catalytic oxidation of Li2S on the surface of metal sulfides for Li−S batteries Proceedings of the National Academy of Sciences of the United States of America 2017, 114, 840-845. (38) Tao, X.; Wang, J.; Liu, C.; Wang, H.; Yao, H.; Zheng, G.; Seh, Z. W.; Cai, Q.; Li, W.; Zhou, G.; Zu, C.; Cui, Y. Balancing surface adsorption and diffusion of lithiumpolysulfides on nonconductive oxides for lithium-sulfur battery design Nature communications 2016, 7, 11203. (39) Yin, L.-C.; Liang, J.; Zhou, G.-M.; Li, F.; Saito, R.; Cheng, H.-M. Understanding the Interactions Between Lithium Polysulfides and N-Doped Graphene Using Density Functional Theory Calculations Nano Energy 2016, 25, 203-210. (40) Klimes, J.; Bowler, D. R.; Michaelides, A. Chemical Accuracy for the Van Der Waals Density Functional Journal of physics. Condensed matter : an Institute of Physics journal 2010, 22, 022201. (41) Klimes, J.; Bowler, D. R.; Michaelides, A. Van Der Waals Density Functionals Applied to Solids Physical Review B 2011, 83, 5131. (42) Momma, K.; Izumi, F. VESTA: A Three-Dimensional Visualization System for Electronic and Structural Analysis J. Appl. Cryst. 2008, 41, 653-658. (43) Lu, J.; Bao, D. L.; Qian, K.; Zhang, S.; Chen, H.; Lin, X.; Du, S. X.; Gao, H. J. Identifying and Visualizing the Edge Terminations of Single-Layer MoSe2 Island Epitaxially Grown on Au(111) ACS nano 2017, 11, 1689-1695. (44) Qiu, Y.; Li, W.; Zhao, W.; Li, G.; Hou, Y.; Liu, M.; Zhou, L.; Ye, F.; Li, H.; Wei, Z.; Yang, S.; Duan, W.; Ye, Y.; Guo, J.; Zhang, Y. High-Rate, Ultralong Cycle-Life Lithium/Sulfur Batteries Enabled by Nitrogen-Doped Graphene Nano letters 2014, 14, 4821-4827. (45) Chen, L.; Feng, J.; Zhou, H.; Fu, C.; Wang, G.; Yang, L.; Xu, C.; Chen, Z.; Yang, W.; Kuang, Y. Hydrothermal Preparation of Nitrogen, Boron Co-Doped Curved Graphene Nanoribbons with High Dopant Amounts for High-Performance Lithium Sulfur Battery Cathodes Journal of Materials Chemistry A 2017, 5, 7403-7415. (46) Zhang, J.; Shi, Y.; Ding, Y.; Peng, L.; Zhang, W.; Yu, G. Conductive Molecular Framework Derived Li2S/N,P-Codoped Carbon Cathode for Advanced Lithium-Sulfur Batteries Advanced Energy Materials 2017, 7, 1602876. (47) Yao, S.; Cui, J.; Huang, J.-Q.; Lu, Z.; Deng, Y.; Chong, W. G.; Wu, J.; Ihsan Ul Haq, M.; Ciucci, F.; Kim, J.-K. Novel 2D Sb2S3 Nanosheet/CNT Coupling Layer for Exceptional Polysulfide Recycling Performance Advanced Energy Materials 2018, 8, 1800710. (48) Wang, H.; Zhang, Q.; Yao, H.; Liang, Z.; Lee, H. W.; Hsu, P. C.; Zheng, G.; Cui, Y. High Electrochemical Selectivity of Edge Versus Terrace Sites in Two-Dimensional 22 ACS Paragon Plus Environment

Page 22 of 30

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Layered MoS2 Materials Nano letters 2014, 14, 7138-7144. (49) Zhang, Q.; Wang, Y.; Seh, Z. W.; Fu, Z.; Zhang, R.; Cui, Y. Understanding the Anchoring Effect of Two-Dimensional Layered Materials for Lithium-Sulfur Batteries Nano letters 2015, 15, 3780-3786. (50) Turner, N. H. Estimates of Peak Areas and Relative Atomic Amounts from Wide‐Scan XPS Spectra Surface and Interface Analysis 1992, 18, 47-51. (51) Sha, L.; Gao, P.; Ren, X.; Chi, Q.; Chen, Y.; Yang, P. A Self-Repairing Cathode Material for Lithium-Selenium Batteries: Se-C Chemically Bonded SeleniumGraphene Composite Chemistry-A European Journal 2018, 24, 2151-2156. (52) Ghazi, Z. A.; He, X.; Khattak, A. M.; Khan, N. A.; Liang, B.; Iqbal, A.; Wang, J.; Sin, H.; Li, L.; Tang, Z. MoS2/Celgard Separator as Efficient Polysulfide Barrier for Long‐Life Lithium–Sulfur Batteries Advanced materials 2017, 29,1606817. (53) Guo, P.; Liu, D.; Liu, Z.; Shang, X.; Liu, Q.; He, D. Dual functional MoS2/graphene interlayer as an efficient polysulfide barrier for advanced lithiumsulfur batteries Electrochimica Acta 2017, 256, 28-36.



Scheme 1. Schematic diagram illustrating the synthesis of molybdenum diselenide on the nitrogen doped reduced graphene oxide (MoSe2/N-rGO).


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Transmission Electron Microscope (TEM) characterization of MoSe2 on nitrogen-doped graphene. (a) TEM image of MoSe2/N-rGO. (b) TEM image of a single MoSe2 nanocrystal. High resolution transmission electron microscope (HRTEM) image of (c) side view (d) top view of a MoSe2 flake. (e) SAED pattern of MoSe2.



Figure 2. Material structural and chemical characterization. (a) XRD pattern of MoSe2/N-rGO (red) and N-rGO (blue). (b) Full XPS spectrum of MoSe2/N-rGO. High resolution XPS spectra of (c) Mo 3d, (d) Se 3d and (e) N 1s and Mo 3p. (f) TGA curves of MoSe2/N-rGO/S, N-rGO/S and S (62% of sample weight lost was found in the samples of MoSe2/N-rGO/S and N-rGO/S by increasing the temperature from 25oC to 400oC at the heating rate of 5oC/min in N2.)


Page 26 of 30

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Electrochemical characterizations. (a) Charge-discharge profile, (b) Cycling stability at current rate of 0.2 C. (c) Rate performance of cells of MoSe2/NrGO/S, N-rGO/S and S from 0.1 C to 1 C. (d) Nyquist plots of the fresh cells of MoSe2/N-rGO/S, N-rGO/S and S.



Figure 4. DFT simulation on lithium atom diffusion properties, binding energy comparison and adsorption configuration. (a) Energy profile of Li atom diffusion on graphene and MoSe2 surface. (b) Binding energy of Li2Sn on graphene surface, MoSe2 surface, 100% MoSe2 edge and 50% MoSe2 edge. (c) Structure of 50% coverage MoSe2 edge. (d) Structure of Li2S4 adsorbed at 50% MoSe2 edge.


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Li2S6 interactions and lithium ion diffusion analysis. XPS spectra of (a) Mo 3d and (b) Se 3d before (lower panel) and after (upper panel) the interaction with Li2S6. (c) EDS mapping of the Li2S6-MoSe2/N-rGO. (d) Relationship plot of peak current (IP) vs. square root of scan rate (v0.5) for both cathodic and anodic reaction with the linear fitting.



Table of Contents Graphic


Page 30 of 30

Selenium Edge as a Selective Anchoring Site for LithiumâSulfur

Recommend Documents

Selenium Edge as a Selective Anchoring Site for LithiumâSulfur