Propelling Polysulfide Conversion by Defect-Rich MoS2 Nanosheets

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Propelling Polysulfides Conversion by Defect-rich MoS2 Nanosheets for High-performance Lithium-sulfur Batteries Mengmeng Liu, Congcong Zhang, Junming Su, Xiang Chen, Tianye Ma, Tao Huang, and Aishui Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03011 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Materials & Interfaces

Propelling Polysulfides Conversion by Defect-rich MoS2 Nanosheets for Highperformance Lithium-sulfur Batteries

Mengmeng Liua, Congcong Zhanga, Junming Sua, Xiang Chena, Tianye Maa, Tao Huanga, Aishui Yua, b, * a

Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China

b

Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy

Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200438, China * Corresponding author: [email protected]

Abstract Lithium-sulfur (Li-S) batteries have tremendous energy density and are costeffective and environmentally compatible, thereby deemed one of the most promising secondary energy storage systems. However, Li-S batteries present sluggish polysulfide intermediate redox kinetics due to the unavoidable “shuttle effect”, thus hindering its industrialization and resulting in low sulfur utilization, rapid capacity fading, poor coulombic efficiency and anode corrosion. Herein, the present study updates a one-step hydrothermal method to synthesize a highly efficient sulfur host integrating threedimensional porous graphene aerogel (GA) with uniformly dispersed defect-rich MoS2 nanosheets (200-300 nm) (GA-DR-MoS2). The electrochemical studies reveal that these MoS2 nanosheets with abundant defects could provide the strong chemical adsorption for polysulfides, as well as act as an electrocatalyst to markedly accelerate polysulfide redox reactions during charge/discharge process. The resultant GA-DRMoS2 composites (70 wt% of sulfur loading) present a high initial discharge capacity of 1429 mAh g-1 at 0.2 C, an outstanding cycling stability with a low capacity decay rate of 0.085% per cycle over 500 cycles at 0.2 C, and a superior rate performance with improved capacity from 290 mAh g-1 to 581 mAh g-1 at 5 C. The presented strategy is effective in achieving high energy density Li-S batteries from the point of 1

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electrocatalysis and facilitating their practical application. Keywords: Cathode, Defect-rich MoS2, Electrocatalysis, Graphene aerogel, Lithium-sulfur batteries

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1. Introduction Rechargeable lithium-sulfur (Li-S) batteries have been deemed a promising alternative to traditional lithium-ion batteries due to their unparalleled theoretical specific energy (2600 Wh kg-1), environmental benignancy and low cost 1-4. However, practical Li-S battery application is impeded by many inherent problems such as: (1) insulativity of sulfur and solid discharge products (Li2S2 and Li2S); (2) apparent volumetric expansion (~80%) following discharge reactions; and (3) high lithium polysulfide (Li2S4–Li2S8, LiPSs) solubility, thereby generating the notorious “shuttle effect” 5, 6. Carbon materials, particularly graphene materials, have been widely applied in Li– S batteries as sulfur hosts and conductive frameworks on account of large surface area and high conductivity, which effectively solve insulation issues of sulfur and accommodate volume changes 7-9. Nevertheless, the “shuttle effect” more greatly limits the practical application of Li-S batteries compared to the above shortages as this generally results in low sulfur utilization, poor cycling stability, low rate capability and anode corrosion

10, 11.

Many methods have been proposed, such as physical or/and

chemical adsorption. However, these only focus on the superficial problem and do not directly address the dissolution of LiPS at its source 12-14. Li-S battery discharge follows a multistep solid-solid reaction (16Li + S8 → 8Li2S), which reduces cyclo-S8 to longchain LiPSs by Li+ firstly and further transform long-chain LiPSs to solid Li2S2/Li2S. Intermediate products, LiPSs, have high solubility in most of ether electrolytes, which can lead to the lose of electric contact between active materials and cathode surface 15. On the one hand, polysulfide flooding can be ascribed to thermodynamically inevitable diffusion. On the other hand, the accumulation of LiPSs in the electrolyte is imputed to slow polysulfide redox reactions. So enhancing the transformation rate from the captured polysulfides to solid products, and vice versa, is a promising technique to restrain the “shuttle effect”. Several research studies have recently reported that certain difunctional hosts present the strong chemical affinity towards polysulfides as well as their catalytic performance to accelerate translation of LiPSs in redox reactions, thereby 3



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suppressing the shuttling of LiPSs and improving Li-S battery electrochemical properties 16-18. Yang et al. reported α-Fe2O3 nanoparticles loaded on graphene facilitate the efficient conversion from LiPSs to solid products, further suppressing the “shuttle effect” 19. Zhang et al. implanted CoS2 to the carbon/sulfur cathode of Li–S batteries, of which the results indicated that CoS2-LiPSs interactions could accelerate LiPSs redox reactions, thereby significantly enhancing the battery performance

20.

Li et al.

synthesized a novel sulfur host, specifically a porous VN nanoribbon/graphene composite, of which the strong chemical affinity between VN and LiPSs enhanced the redox kinetics and restrained the “shuttling effect”

21.

At the same time, MoS2 as an

excellent electrocatalyst also is used in Li-S batteries. Lee et al. found the ability of MoS2-x/reduced graphene oxide (rGO) to catalyze the reactions of LiPSs, thereby improving battery property 22. Ling et al. developed a new sandwich-type MoS2/S/rGO hybird cathode to kinetically accelerate sulfur redox reactions and hence improve sulfur utilization and long cycle life 23. However, in above cases, these MoS2-based composite materials require a complicated and multi-step preparation process and high cost. Herein, the present study reports a facile one-step hydrothermal method to develop a three-dimensional (3D) graphene aerogel (GA) decorated with defect-rich MoS2 nanosheets (GA-DR-MoS2). The resultant GA-DR-MoS2 composites possess an interconnected porous structure stacked by the graphene sheets with uniformly dispersed MoS2 nanosheets (200-300 nm). Only by controlling the addition of the precursor in the process of preparation, plenty of defects in the MoS2 nanosheets can be prepared simply. Defect-rich MoS2 nanosheets exhibited partial cracking in their catalytically inert basal planes, thereby resulting in additional active edge-site exposure. Therefore, defect-rich MoS2 nanosheets not only exhibit chemical affinity for LiPSs, but they also encourage polysulfide conversion reaction kinetics throughout both charge and discharge processes, thus restraining the “shuttle effect.” As a consequent, the implementation of GA-DR-MoS2 composites as Li-S battery cathode materials present excellent electrochemical properties, such as an outstanding rate performance, an initial discharge specific capacity (1429 mAh g-1 and 762 mAh g-1 at 0.2 C and 2 C), 4


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an ultra-low capacity decay rate (0.085% per cycle at 0.2 C and 0.042% per cycle at 2 C over 500 cycles), and a coulombic efficiency approaching 100%. 2. Results and discussion

Figure 1. Schematic illustration of the GA-DR-MoS2/S composite fabrication process. Fig. 1 presents the synthesis of the GA-DR-MoS2/S composites. The GA-DR-MoS2 composites were first synthesized via a one-step hydrothermal method using (NH4)6Mo7O24·4H2O, an excess of thiourea and graphene oxide (GO) as the precursors. In the self-assembly process of GO sheets, abundant functional groups on their surfaces adsorb the MoS2 precursors to ensure they could attach to the 3D GA well without the exfoliation. At the same time, an excess of thiourea act as a reductant for Mo6+ to Mo4+ reaction on the one hand. On the other hand, they serve as an efficient additive for MoS2 nanosheets defect manufacturing 24. Primary nanocrystallites adsorb excessive thiourea on their surface, thus partially impeding oriented crystal growth and resulting in the formation of a defect-rich structure. Sulfur was then infiltrated into internal pores of the GA-DR-MoS2 composites by the liquid-phase chemical reaction (S2O32− + 2H+ → S↓ + H2SO3) and subsequent melting-diffusion process, forming the GA-DR-MoS2/S composites 25. 5



Figure 2. SEM images of GA-DR-MoS2 at a: (a) low-magnification and (b) highmagnification, respectively, wherein the inset in (a) presents the GA-DR-MoS2 6


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macroscopic morphology. (c) Low-magnification and (d) high-resolution TEM images of GA-DR-MoS2. (e) High-angle annular dark-field scanning TEM image of GA-DRMoS2/S with the corresponding elemental distribution of (f) Mo, (g) S, (h) C, and (i) O. (j) SEM image of GA-DR-MoS2/S at a low-magnification. (k) TEM image of GA-DRMoS2/S at a low-magnification.

As shown in Fig. 2a inset, the optical image of GA-DR-MoS2 composites indicates their macroscopic morphology present a highly porous cylinder. The scanning electron microscopy (SEM) images of the GA-DR-MoS2 composites reveal clearly the presence of countless graphene sheets stacked atop one another, thus forming a loosened interconnected structure with great numbers of micron-sized voids (Fig. 2a, b). The suggested structure agrees well with that of GA (Fig. S1a). The transmission electron microscopy (TEM) image of the GA-DR-MoS2 composites present uniformly and loosely attached dark flower-like nanosheets on the graphene sheets, as well as the absence of naked graphene sheets or free MoS2 nanosheets (Fig. 2c). The nanocrystals exhibited sizes of about 200-300 nm. In addition, the graphene edges can be clearly observed (arrows in Fig. 2c). The high-resolution GA-DR-MoS2 composites TEM image presents well-defined crystalline lattice fringe spacings of 0.22, 0.27 and 0.62 nm (Fig. 2d), which corresponds well with the interplanar spacing of hexagonal MoS2, specifically facets of (103), (100) and (002), respectively 26. Notably, the results indicate a novel defect-rich structure owing to the presence of many distortions and dislocations. The directions of individual (100) planes on the basal surface have a slight angular deviation compared to the others, demonstrating that the atomic arrangement is relatively disordered. Then due to this disordered atomic arrangement, he basal planes cracks and thus causes the increase of edge formations. Furthermore, the curled edge presents discontinuous (002) facets crystal fringes on account of the presence of abundant defects. However, the HRTEM image of the GA-DF-MoS2 composites do not display any defects (Fig. S1d). The high-angle annular dark-field scanning TEM image and elemental mapping of Mo, S, O and C of the GA-DR-MoS2 composites (Fig. 2e-i) 7



indicate the presence of attached uniform MoS2 nanosheets on the graphene base further. The presence of O could be ascribed to the insufficient reduction of GO, which is rich in oxygen-containing functional groups The GA-DR-MoS2/S composite SEM image (Fig. 2j) still indicates a compact 3D interconnected structure following S infusion. And the GA-DR-MoS2/S composite TEM image (Fig. 2k) does not present any bulk S or S agglomerates.

Figure 3. (a) The XRD pattern of S, MoS2, GA, GO, GA-DR-MoS2, GA-DF-MoS2, and GA-DR-MoS2/S. (b) Raman spectra of GA-DR-MoS2 and GA-DF-MoS2. (c) The atomic vibration direction of A1g and E12g vibrational modes of MoS2.

A broad peak and a weak peak appear at around 25° and 43° in the GA X-ray diffraction (XRD) patterns. Due to the reduction of GO during the hydrothermal process, a typical GO peak at 10° disappears (Fig. 3a) 27. Five noticeable diffraction peaks are observed in the GA-DR-MoS2 composite XRD patterns, of which three can be indexed to the (002), (100)/(101), and (110) facets of hexagonal MoS2 28. Notably, compared with the standard pattern, the (002) peak of the GA-DR-MoS2 composites slightly shift 8


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to lower 2 theta values, indicating an increase in the lattice parameter caused most likely by abundant defects 29. The other two peaks agree well with the characteristic peaks of GA, which suggest the addition of MoS2 has no effect on the degree of the reduction of GO. Numerous S8 characteristic peaks are observed in the XRD patterns of the GADR-MoS2/S composites following sulfur infusion, indicating that sulfur is successfully incorporated into the original materials. In the Raman spectra of GO, the D-band and G-band at 1344.9 and 1600.3 cm-1 respectively shifts to 1328.9 and 1585.7 cm-1 following the hydrothermal treatment, which well match with the value of pristine graphite (Fig. S2). In addition, the ratio of D-band intensity to G-band intensity increases from 0.96 (GO) to 1.02 (GA-DR-MoS2). Above results both confirm the successful reduction of GO 30. Furthermore, the GADR-MoS2 and GA-DF-MoS2 composites Raman spectra (Fig. 3b) both exhibit 3 distinct peaks located at 380, 405 and 450 cm-1, which correspond to the in-plane E12g, out-off plane A1g, and longitudinal acoustic phonon modes of MoS2, respectively 31, 32. The A1g and E12g modes are excited preferentially in the edge-terminated and terrace-terminated films, respectively. The atomic vibration directions of E12g and A1g modes are presented in Fig. 3c

33, 34.

The GA-DR-MoS2 composite Raman spectra presents a significantly

stronger A1g mode as compared to the E12g mode, indicating edge-terminated MoS2 occupies the main status in the GA-DR-MoS2 composites. The intensity ratio of A1g to E12g in GA-DR-MoS2 composites is higher than that in GA-DF-MoS2 composites. Therefore, the fabrication of MoS2 defects may increase the presence of exposed active edge-sites. The N2 adsorption-desorption isotherms of the GA-DR-MoS2 composites, GA-DFMoS2 composites and GA (Fig. S3) all exhibits IV-type curves with H3-type distinct hysteresis loops, which is typical in mesoporous structure. The specific GA-DR-MoS2 composites surface area is 341 m2 g-1, which is slightly lower than that of the GA (422 m2 g-1). Due to the introduction of the MoS2 sheets, a lower BET surface area is observed. The specific surface area of the GA-DF-MoS2 composites (352 m2 g-1) is closed to that of the GA-DR-MoS2 composites. The pore size distribution of the GA, 9



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GA-DR-MoS2, and GA-DF-MoS2 composites are all near 5-10 nm. The defect-rich MoS2 loading increased from 18 wt% to 46 wt%, whereas the surface area decreases from 383 m2 g-1 to 305 m2 g-1.

Figure 4. (a) The GA, GA-DR-MoS2 and GA-DF-MoS2 TG curves in flowing oxygen. (b) TG curves of S, GA/S, GA-DR-MoS2/S and GA-DF-MoS2/S in flowing nitrogen. Mo 3d and S 2s XPS curves of (c) GA-DR-MoS2 and (d) GA-DR-MoS2/S. S 2p XPS spectrum of (e) GA-DR-MoS2 and (f) GA-DR-MoS2/S. (g) Optical photograph of the Li2S6 solutions in DME/DOL with the immersion of GA, GA-DR-MoS2, and GA-DFMoS2. Optical photograph of visible cell (h) before discharge and after discharge with (i) GA-DR-MoS2/S, (j) GA-DF-MoS2/S, and (k) GA/S as the cathode.

The MoS2 content in the GA-DR-MoS2 and GA-DF-MoS2 composites were measured by thermogravimetric analysis (TG) (Fig. 4a). MoS2 is oxidized to MoO3 at about 450°C in the presence of oxygen and graphene was eliminate after 550°C

35.

According to the calculations, the MoS2 in the GA-DR-MoS2 and GA-DF-MoS2 composites exhibited a quality fraction of 31% and 29%. According to the TG results 10


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in nitrogen, the sulfur content is as high as 70 wt% in all of GA-DR-MoS2/S, GA-DFMoS2/S and GA/S composites. Clearly, this type of 3D porous framework has the ability to hold large amounts of S, thus allowing high areal capacity retention for improving the possibility of practical application. The other two GA-DR-MoS2-x composites exhibit MoS2 contents of 18 wt% and 46 wt%, respectively (Fig. S4). As a control, the sulfur content in the other GA-DR-MoS2-x/S composites is regulated to be 70 wt%. The chemical composition and bonding configuration analyses of the GA-DR-MoS2 and GA-DR-MoS2/S composite employ X-ray photoelectron spectroscopy (XPS) spectra. XPS full spectrum of GA-DR-MoS2/S composites (Fig. S5a) indicates the elements C, Mo, S and O present on the material surface. Compared with the C 1s spectrum of GO with two peaks, only sp2-hybridized C-C peaks exist in that of GADR-MoS2 and GA-DR-MoS2/S composites (Fig. S5b) and become much narrower and stronger, which suggests the successful reduction of the oxygen-containing functional groups 27. Fig. 4c and d respectively present the high resolution Mo 3d and S 2s XPS spectra of the GA-DR-MoS2 and GA-DR-MoS2/S composites. The GA-DR-MoS2 composites spectra present a fitted peak located at 226.5 eV, which corresponds to the S 2s of divalent sulfide ions (S2-). Two peaks are observed at 232.7 eV (Mo 3d3/2) and 230.1 eV (Mo 3d5/2), which can be assigned to Mo4+. In addition, the presence of two other peaks at 235.9 eV (Mo 3d5/2) and 232.9 eV (Mo 3d5/2) can be related back to Mo6+ (MoO3) due to partial oxidation of Mo4+. The GA-DR-MoS2/S composite XPS spectra present a new peak at 228.2 eV, which can be assigned to S 2s of elemental sulfur. Different GA-DR-MoS2 composites sulfur environments are identified into three components (Fig. 4e). The fitted binding energy of 168.6 eV corresponds to S 2p of SO42- and SO32-, which is produced following sulfur oxidation during the hydrothermal reaction. Two peaks at 164.6 eV (S 2p1/2) and 163.4 eV (S 2p3/2) can be assigned to S2-, and two other peaks at 163.2 eV (S 2p1/2) and 162.1 eV (S 2p3/2) is defined as S22- 36. The presence of S22- is because that the addition of the precursor of S is excessive. After introduction of sulfur in GA-DR-MoS2 composites, three similar components (S2-, S2211



and SO42-/SO32-) of sulfur environments can be identified, but a new peak representing simple substance S at 165 eV appears (Fig. 4f). Notably, both S2- and S22- binding energies of GA-DR-MoS2/S composites are higher than those in the GA-DR-MoS2 composites, indicating that the electron-deficient S atoms in the simple substance sulfur are able to accept electrons from S2-/S22- in MoS2, thus enhancing the affinity interaction between MoS2 and elemental S. To further demonstrate the defect-rich MoS2 nanosheet and polysulfide interactions, equivalent masses of GA as well as a series of GA-DR-MoS2 and GA-DF-MoS2 composites were individually added to a 1 mM Li2S6 solution. Material adsorption ability was evaluated based on visually detecting a color change in the resultant solution. A blank Li2S6 solution was used as a reference. About 12 h later, the solution containing the GA-DR-MoS2 composites become completely colorless, the GA-DF-MoS2 composites take second place, whereas the GA-containing solution remains yellow (Fig. 4g). The solution color lightened with increasing MoS2 quantities in the GA-DR-MoS2 composites, indicating that defect-rich MoS2 significantly affected the adsorption of LiPSs. Subsequently, the GA/S, GA-DR-MoS2/S and GA-DF-MoS2/S composites were used as the cathodes, and lithium metal was employed as the anode to form visible cells. After 20 cycles at 0.1 C, the order of cell yellow degree from dark to light is: GA > GADF-MoS2 > GA-DR-MoS2 (based on the same S loading in the pole piece) (Fig. 4h-k), indicating that defect-rich MoS2 nanosheets have the strongest electrocatalytic effect to accelerate LiPSs redox reactions during charge/discharge processes.

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Figure 5. (a) GA-DR-MoS2/S, (b) GA-DF-MoS2/S and (c) GA/S cathode CV curves at a scan rate of 0.1 mVs-1. (d) Second cycle of GA-DR-MoS2/S galvanostatic chargedischarge profiles at 0.2, 0.5, 1, 3, and 5 C. (e) Rate performance of the GA-DR-MoS2/S, GA-DF-MoS2/S and GA/S cathodes at different current densities. (f) Electrochemical impedance spectra of the GA/S, GA-DR-MoS2/S, and GA-DF-MoS2/S cathodes prior to the cycle. The insert of (f) represents the corresponding electrical equivalent circuit diagram. The present study investigated the electrochemical properties of the composites as Li-S battery cathode materials by assembling coin cells using lithium metal as the anode. The typical cyclic voltammetry (CV) profiles of the GA-DR-MoS2, GA-DF-MoS2 and 13



GA/S cathodes (0.1 mV s-1) all exhibits 2 anodic peaks and 2 cathodic peaks (Fig. 5ac). The GA-DR-MoS2 composite CV curves (2-10 cycles) during cathodic reduction process exhibits two representative reductive peaks at around 2.33 V (vs Li/Li+; E1) and 2.04 V (E2), which defines the sulfur reduction to long-chain LiPSs and short-chain Li2S2/Li2S formation, respectively

21.

Notably, compared with GA/S cathode, the E1

and E2 intensity present an obvious increase. indicating significant enhancements of reaction kinetics (S8 + Li+ + e− → Li2S) at this potential 19. In addition, the potentials increase to a higher voltage range following the addition of MoS2, which suggests the reactions are more available. The anodic segment exhibits 2 peaks at approximately 2.32 V (E3) and 2.41V (E4), which suggest the oxidation of Li2S2/Li2S to intermediate polysulfides as well as the transformation from LiPSs to S, respectively. The E3 and E4 peaks of GA-DR-MoS2/S were more prominent and intense than the two combined peaks in the GA/S cathode. When scan rate increases to 3 mV s-1, a similar regularity appears in the CV profiles, except that E3 and E4 merge into one peak (Fig S7). All of the presented results validate the applicability of MoS2 in accelerating the reaction kinetics of soluble LiPSs to insoluble Li2S2/Li2S or S and decreasing the polarization, thus significantly reduce the probability of “shuttle effect” in the electrolyte. Since the peaks of GA-DR-MoS2/S cathode are all higher than those of GA-DF-MoS2/S, suggesting abundant defects in MoS2 nanosheets participate in the polysulfide conversion and catalyze the redox reactions of polysulfide. Therefore, the GA-DRMoS2 composites more effectively suppresses the “shuttle effect.” After four cycles, the GA-DR-MoS2/S cathode curves almost fit each other, indicating good capacity retention and reversibility of cells. The charge-discharge profiles of the GA-DR-MoS2/S, GA-DF-MoS2/S and GA/S cathodes are composed of 2 charge plateaus (between 2.25 V and 2.38 V ) and two discharge plateaus (at about 2.37 V and 2.12 V ) at 0.2 C (Fig S8). The GA-DR-MoS2/S cathode exhibits least polarization and the longest plateaus flattering with the highest capacity in three cathodes, which is consistent with the CV results. The GA-DRMoS2/S cathode is able to deliver discharge capacities of 1429, 1078, 930, 700 and 581 14


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mAh g-1 at different rates of 0.2, 0.5, 1, 3, and 5 C, respectively (second cycle) (Fig. 5d). The rate performances of Li-S batteries with GA-DR-MoS2/S, GA-DF-MoS2/S and GA/S composites used as cathode all exhibits a gradual decrease following a current increase (Fig. 5e). GA-DR-MoS2/S, followed by GA-DF-MoS2/S, exhibits the highest capacity at different currents in all three of the composites. Even at 5 C, the capacity of GA-DR-MoS2/S cathode can maintain 581 mAh g-1, which is much superior to that of GA/S (290 mAh g-1). Furthermore, GA-DR-MoS2/S cathode recovers a capacity of 1190 mAh g-1after the current abruptly switched back to 0.1 C after 48 cycles, indicating that MoS2 can improve Li-S battery electrochemical stability. Specifically, defect-rich MoS2 is more effective than defect-free MoS2. The Nyquist plots of the GA-DR-MoS2/S, GA-DF-MoS2/S and GA/S cathode (Fig. 5f) in electrochemical impedance spectroscopy (EIS) measurement all have 2 parts. In the high-frequency region, the semicircle represents the interface charge transfer resistance (Rct, at the electrode-electrolyte interface). In the low-frequency region, the straight line associates with the mass transfer process. Due to fast charge transport, good electrolyte infiltration and rapid transformation of LiPSs, the GA-DR-MoS2/S cathode has the smallest Rct (35 Ω) among three samples, inosculating well with the best rate performance. The Rct of three cells all become larger after 30 cycles at 0.2 C (Fig. S9). However, GA-DR-MoS2/S cathode is still superior to the others.

15



Figure 6. Coulombic efficiency and cycling performance of GA/S, GA-DR-MoS2/S, and GA-DF-MoS2/S cathodes at (a) 0.2 C and (b) 2 C for 500 cycles. Fig. 6 exhibits the cycling performances of the GA-DR-MoS2/S, GA-DF-MoS2/S and GA/S cathode. Compared with GA-DF-MoS2/S (1420 and 649 mAh g-1) and GA/S (1230 and 308 mAh g-1) cathodes, the GA-DR-MoS2/S cathode displays the highest initial discharge capacity (1429 mAh g-1) and maintains the best reversible capacity (821 mAh g-1) after 500 cycles at 0.2 C with a coulombic efficiency approaching 100%. The capacity decay rate is calculated to be 0.085%, 0.109%, and 0.150% for the GADR-MoS2/S, GA-DF-MoS2/S and GA/S cathode over 500 cycles at 0.2 C, respectively. After an activated process (at 0.1 C), the GA-DR-MoS2/S cathode still maintains outstanding cycling performance including a high initial discharge capacity of 762 mAh g-1, a low capacity fading of 0.042% per cycle and a stable coulombic efficiency of 98.7% after 500 cycles as the current density increased to 2 C. All the composites loading MoS2 clearly exhibits better cycling performances compared with that of the GA/S at different currents. Furthermore, due to existence of defects in MoS2, GA-DR-MoS2/S exhibits more optimal cycling performance than GA-DF-MoS2/S cathode. 16


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For further comparison, the rate performances and the cycling performances of different GA-DR-MoS2/S cathodes are evaluated. As the sulfur content increases from 1.5 mg cm-2 to 3 mg cm-2, although the rate and cycling performances both decrease (Fig. S10), a rate capacity of 251 mAh g-1 is maintained at a high current of 5 C and 280 mAh g-1 at 2 C after 200 cycles. Different GA-DR-MoS2 composites are denoted as GA-DR-MoS2(y), in which y wt% represents MoS2 content. Among three cathodes with different defect-rich MoS2 contents, the GA-DR-MoS2(31)/S cathode exhibits the highest capacity at the same rates (Fig. S11) and the best cycling stability (Fig. S12), thereby confirming that the moderate addition of defect-rich MoS2 greatly enhances the capacity retention and cycling stability through the restriction of “shuttle effect” due to the chemical adsorption as well as the accelerated transformation of LiPSs. However, excessive additions lower the electrical conductivity and specific surface area of the materials, which are harmful to the Li-S battery electrochemical performance.

Figure 7. (a) Schematic illustration of the discharge process in (a) carbon/S cathode and (b) GA-DR-MoS2/S cathode. (c) S conversion during electrochemical reaction process on the graphene surface with defect-rich MoS2 nanosheets. The table in (c) presents the Li2S molecule and different MoS2 atomic site interactions.

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The redox reactions of LiPSs in the liquid phase connect the two solid phases of sulfur and discharge products (Li2S2 and Li2S) during the battery discharge process, which is the core of aprotic sulfur redox chemistry. In addition, the corresponding electron transport demands the participation of conductive substrates

20.

Pure

carbon/sulfur cathode follows rate-controlled polysulfide reduction and is dominated by polysulfide diffusion (Fig. 7a). In contrast, defect-rich MoS2-incorporated graphene/sulfur cathode follows accelerated polysulfide reduction and weakened polysulfide diffusion (Fig. 7b). Uniformly dispersed MoS2 nanosheets on the GA provide a large amount of polar surfaces for strong LiPS chemical anchoring effect (Fig. 7c). More importantly, abundant edge-sites with unsaturated dangling bonds exposed by defects in MoS2 demonstrate higher binding energy interacting with Li2S, thus effectively promote the redox reaction kinetics during charge/discharge process 22, 37, 38.

The synergistic effect of the above two aspects greatly suppresses the shuttling of

soluble polysulfides and improves the Li-S battery electrochemical performances. In addition, the presented 3D hierarchical graphene-based framework generates enhanced ion and electron transfer channels as well as produces enough space to accommodate S volume changes. 3. Conclusion In conclusion, the present study delivers a novel strategy for the preparation of a 3D conductive porous GA with controllable defect-rich MoS2 nanosheets by a facile one-step hydrothermal treatment. The GA-DR-MoS2 composites exhibit applicability as highly efficient sulfur hosts in Li-S batteries. The defect-rich MoS2 nanosheets provides abundant polar surfaces to exert chemical interactions with S/LiPSs. More importantly, the defect-rich MoS2 nanosheets exhibit the ability to serve as an electrocatalyst to significantly accelerate polysulfide redox kinetics during charge/discharge process owing to the enhanced exposure of active edge-sites, thus effectively suppressing the LiPSs “shuttle effect.” This composite material also presents physical sulfur confinement, high conductivity and expedited ion channels due to its interconnected mesoporous GA framework. As a result, the GA-DR-MoS2/S 18


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cathode shows superior electrochemical properties. Compared to the GA-DF-MoS2/S and GA cathodes, it exhibits better specific capacity and less capacity fading with a coulombic efficiency approaching 100%. The present study introduces defect-rich MoS2 in the Li-S battery cathode for the polysulfide chemical adsorption and new insight in the design of high rate and long cycle life cathode materials by enhancing soluble polysulfide transformation. Associated content Supporting Information Available: Reactant quantities; micromorphology images; nitrogen adsorption-desorption isotherms; pore size distributions; TG curves; XPS spectra; optical photo of Li2S6 solutions; electrochemical performances. Acknowledgments Financial support of the present study was from Science and Technology Commission of Shanghai Municipality, China [No.08DZ2270500]. Thank my boyfriend, Ye Yuan, for his contribution to this work, including the emotional support and actual assist. May you enjoy health, happiness and every success!

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Propelling Polysulfide Conversion by Defect-Rich MoS2 Nanosheets

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