MoS2-Coated N-doped Mesoporous Carbon Spherical Composite

Oct 19, 2018 - poor cycle stability of the LSBs, and serious security ... Hexadecyl trimethylammonium bromide (CTAB, 2.28 g) and NaSal (1.512g) were a...
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Cite This: ACS Sustainable Chem. Eng. 2018, 6, 16828−16837

MoS2‑Coated N‑doped Mesoporous Carbon Spherical Composite Cathode and CNT/Chitosan Modified Separator for Advanced Lithium Sulfur Batteries Shouxin Jiang,† Manfang Chen,† Xianyou Wang,*,† Zhenyu Wu,† Peng Zeng,† Cheng Huang,† and Ying Wang*,‡

ACS Sustainable Chem. Eng. 2018.6:16828-16837. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/29/19. For personal use only.

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National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan University, Hunan 411105, China ‡ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514, United States

ABSTRACT: The polysulfide shuttle effect is one of the most important problems hindering the commercial application of lithium−sulfur batteries (LSBs). In order to solve the above problem and promote LSBs commercialization, herein a holistic design strategy on the molybdenum disulfide-coated nitrogen-doped mesoporous carbon sphere/sulfur (NMCS@MoS2/S) composite cathode and carbon nanotube/chitosan modified separator (CNT/CH) is proposed. In the holistic design, the NMCS@MoS2 plays a role in the host of sulfur and the lithium polysulfides (LiPSs) adsorbent; the CNT/CH modified separator also has an inestimable role in promoting lithium ion transport and chemical adsorption of LiPSs. The results show that the LSBs with the NMCS@MoS2/S-CNT/CH release a high reversible capacity of 827 mAh g−1 with a high capacity retention of 92.4% at 0.5 C after 200 cycles. The delicate design exhibits apparently excellent electrochemical performance and provides an exciting strategy for solving the shuttle effect of LiPSs and boosting industrialization of LSBs. KEYWORDS: Lithium sulfur batteries, Polysulfides, Holistic design, Molybdenum disulfide, Chitosan

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problems.9−11 For solving the above-mentioned problems, the combination of sulfur and carbonaceous materials has been regarded as a common way because of the good conductivity of carbon materials. Nazar and co-workers first proposed the highly ordered mesoporous carbon (CMK-3) as the host materials of sulfur, and the resultant LSBs showed good electrochemistry performances.12 Enlightened by this work, carbon materials as the host materials of sulfur have arisen one after another, such as biomaterial carbon,13−15 hollow carbon spheres,16,17 carbon nanotubes,18−20 and graphene.21,22 Although this research has achieved some good progress, there is not a strong binding ability between the nonpolar carbon materials and the polar LiPSs by physical adsorption. The carbon/sulfur composite cathode still suffered from poor cycle stability over long cycles.23 Lately, polar materials are of

INTRODUCTION With the rapid development of portable electronic devices and electric vehicles, the requirement of advanced energy storage systems with low cost and high energy density has significantly increased.1−4 However, due to the low energy density of traditional lithium-ion batteries, the current technology of lithium ion batteries cannot meet the requirements of the development of the times. Lithium−sulfur batteries (LSBs) are considered promising candidates for next-generation energy storage equipment, on account of their high theoretical specific capacity (1675 mAh g−1) and energy density (2600 Wh kg−1).5−7 Simultaneously, elemental sulfur is low-cost, environmentally friendly, and abundant in nature.8 Nevertheless, the applications of LSBs are limited by three main issues: (1) poor conductivity of the elemental sulfur and the discharge products (Li2S and Li2S2), (2) large volumetric expansion (80%) from sulfur to Li2S, and (3) the “shuttle effect” produced by soluble LiPSs (Li2Sn, 4 ≤ n ≤ 8), which lead to a loss of the sulfur, poor cycle stability of the LSBs, and serious security © 2018 American Chemical Society

Received: August 21, 2018 Revised: September 29, 2018 Published: October 19, 2018 16828

DOI: 10.1021/acssuschemeng.8b04157 ACS Sustainable Chem. Eng. 2018, 6, 16828−16837

Research Article

ACS Sustainable Chemistry & Engineering

Synthesis of NMCS@MoS2. First, NMCS (50 mg) was distributed in deionized water (50 mL) under ultrasonication for 30 min to obtain a uniform black solution, and then sodium molybdate dihydrate (400 mg) and thiourea (400 mg) were ultrasonically dispersed in the above solution for 30 min and transferred to a 100 mL Teflon lined stainless steel autoclave and reacted at 200 °C for 24 h in a high temperature blast oven. The products were washed with deionized water and ethanol and then dried under a vacuum at 80 °C for 12 h. Finally, to obtain NMCS@MoS2, the product was heated at 800 °C for 6 h to improve the crystallinity in a tube furnace under an argon atmosphere to obtain NMCS@MoS2. Synthesis of NMCS@MoS2/S and NMCS/S. NMCS@MoS2 and sulfur were mixed at a mass ratio of 1:3, then added to a mixed solution of CS2 and alcohol with vigorous stirring. It was placed overnight in a fume hood for evaporation of CS2 and alcohol. The material was transferred to a sealed vial and held at 160 °C for 12 h under an argon atmosphere, followed by a treatment at 250 °C for 90 min to remove excess sulfur from the surface of the material. This gave the NMCS@MoS2/S composite, and the NMCS/S composite was prepared in the same manner. Synthesis of Li2S6 Solution. Lithium sulfide (Li2S) and elemental sulfur were mixed at a molar ratio of 1:5 and added to a suitable THF solution with vigorous stirring at 25 °C in an argon glovebox. Materials Characterization. The structure, morphology, composition information, and optical properties of these materials were probed by scanning electron microscopy (SEM, TM4000, Japan), transmission electron microscopy (TEM, JEOL, JEM-2100F), Raman spectroscopy (LabRAM HR800, 532 nm excitation), X-ray diffraction (XRD, Model D8-Advance, Germany), and X-ray photoelectron spectroscopy (XPS, K-Alpha+, America). The sulfur and MoS2 contents of the samples were researched on a Series Q500 instrument (TGA Instruments, USA). Brunauer−Emmett−Teller (BET) was mined to analyze the specific surface area and the holey structure. Electrochemical Measurements. The cathode electrodes for the LSBs were made by modulating a slurry of 70 wt % active materials (NMCS@MoS2/S or NMCS/S), 20 wt % Super P, and 10 wt % PVDF in NMP on the current collector and dried at 60 °C for 12 h before use. The active material mass loading was about 1.2 mg cm−2. The anode was a lithium wafer. A CNT/CH, CNT, or CH modified separator was employed as a separator. A total of 1 mol L−1 LiTFSI and 0.1 M LiNO3 were dissolved in a mixture of 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME; 1:1, v/v) as an electrolyte. LSBs were characterized at different constant current rates between 1.8 and 2.8 V on a Neware tester (BTS-XWJ-6.44S-00052 Neware, Shenzhen, China) at a petty voltage window of 1.8−2.8 V. Cyclic voltammetry (CV) and EIS measurements were tested with an electrochemical workstation (CHI660e, Chenhua, China). The CV examinations were examined between 1.8 and 2.8 V at a scan rate of 0.2 mV S−1. The frequency range of EIS was from 10−2 to 105 Hz with an amplitude of 5 mV. All the tests were conducted at 25 °C.

wide concern because of their strong chemical interaction with LiPSs. For instance, nitrogen−sulfur codoped carbon materials lead to a significant increase in the chemical adsorption capacity of LiPSs.24−26 Metal oxides,27,28 metal sulfides,29−31 metal nitrides,32−34 and metal carbides35,36 are also used as the hosts of sulfur, because their adsorption capacity for LiPSs is stronger than that of carbon materials. To raise the performance of LSBs to a new height, various interlayer and modified separators are introduced into the LSBs.37−42 Although the technology of LSBs has made great progress, so far, researchers have not proposed effective methods to provide high specific capacity and good stability capacity with high sulfur loading. At present, almost all of the works only focus on heightening the loading of sulfur without regard to the holistic design of the LSBs with the design of the cathode and modified separator. Herein, we propose a holistic design of high-performance LSBs, including a high-polarity molybdenum disulfide-coated nitrogen-doped mesoporous carbon sphere (NMCS@MoS2) as a host material of sulfur and a carbon nanotube/chitosan modified separator as a separator. In this holistic design, the high polarity MoS2 ameliorates the electrochemical reaction kinetics and promotes the adsorption ability for LiPSs. In addition, the CNT/CH heightens cycle stability and rate performance by the improvement of the Li+ ion transfer and adsorption capacity for LiPSs. Therefore, the proposed LSBs with NMCS@MoS2/S-CNT/CH deliver a high initial specific capacity of 847 mAh g−1 with a slight capacity attenuation of 0.08% per cycle after 500 cycles, showing excellent long-term cycle stability at 1 C.

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EXPERIMENTAL SECTION

Composite Synthesis. Synthesis of CNT/CH, CNT and CH Modified Separators. Chitosan (1 g) was distributed in a 2 wt % acetic acid solution under vigorous stirring for 24 h. The prepared chitosan dispersion was mixed up with CNT at a ratio of 5:95 wt % in a diluent of 1:1 (V/V) isopropanol/deionized water. Then, the mixture was transferred to the agate ball mill and ball milled for 10 h under 400 rpm using agate beads. The separator was coated with the slurry and dried at 40 °C overnight in a vacuum. To obtain the CNT/ CH modified separator, the coated separator was cut into a small circle with a diameter of 19 mm. The CNT and CH modified separators were prepared by the same method. Synthesis of Walnut-like Mesoporous Silica Nanosphere (WMSN). Triethylamine (TEA, 0.41 g) was scattered in deionized water (150 mL) with magnetic stirring at 70 °C for 1 h. Hexadecyl trimethylammonium bromide (CTAB, 2.28 g) and NaSal (1.512g) were added into the above solution with further stirring for 1 h. Then, tetraethoxysilane (TEOS, 24 mL) was added drop by drop under magnetic stirring for 12 h. The solution was washed in deionized water to remove the impurity. To obtain the WMSNs, the material was dried in an oven at 60 °C for 12 h. Synthesis of NMCS. The prepared WMSN (120 mg) was scattered in a buffer solution (90 mL, pH = 8.5) under ultrasonication for 1 h. Then, dopamine (180 mg) was scattered in the above solution and kept stirring for 1 day at room temperature. The product was obtained by filtration and washed several times with ethanol and deionized water. After being dried at 50 °C for 12 h, the product was transferred to a corundum crucible and annealed in a tube furnace at 800 °C for 3 h under an argon atmosphere with a heating rate of 3 °C min−1. Subsequently, the products after carbonization were transferred to a Teflon beaker, and hydrofluoric acid (HF, 5 wt %) was used to remove WMSN by stirring vigorously in the fume hood for 12 h. In order to remove the redundant HF, the products were washed with deionized water. The NMCS was gained after being dried at 60 °C in a vacuum for 12 h.

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RESULTS AND DISCUSSIONS The schematic diagram of the synthesis process of the NMCS@MoS2/S sample is shown in Figure 1a. First, the WMSN is prepared according to the reported method.43 After coating with polydopamine, the composite is heated at 800 °C for 3 h and processed in a 5% HF solution to remove the WMSN. The NMCS, having a uniform diameter of about 250 nm, is produced. Subsequently, NMCS@MoS2 is made through a simple solvothermal method and heated at 800 °C for 6 h in an argon atmosphere to improve the crystallinity. Finally, sulfur is infiltrated into the NMCS@MoS2 host. The schematic diagram of the adsorbed polysulfide by chitosan is shown in Figure 1b. The chitosan uniformly covers the surface of the carbon nanotubes after ball milling. The polar functional groups of hydroxyl and amino on the surface of the chitosan can tightly adsorb the LiPSs dissolved in the electrolyte. 16829

DOI: 10.1021/acssuschemeng.8b04157 ACS Sustainable Chem. Eng. 2018, 6, 16828−16837

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a conclusion can clearly be drawn that the prepared WMCS reflect a unique dendritic structure and uniform particle size of around 300 nm. NMCS shows a small particle size of about 250 nm in Figure 2c and d. At the same time, from the TEM images of Figure 2g, NMCS has abundant pore structures, which will help to increase sulfur loading. In addition, there is a distinct graphitization layer at the edge of NMCS. As shown in Figure 2e,f,i,k, the MoS2 nanosheets are evenly coated on NMCS. The HRTEM images of NMCS@MoS2 are shown in Figure 2j; the layer spacing is approximately 0.65 nm, which corresponds to the (002) lattice plane of MoS2. As shown in Figure 2l−o, elemental mapping images demonstrate the uniform distribution of Mo, S, C, and N in NMCS@MoS2. A digital photo of the prepared CNT/CH modified separator is shown in Figure 3a, the surface of the CNT/CH modified separator is smooth. After being bent several times, as shown in the illustration of Figure 3b, the material is attached firmly to the separator without any shedding or peeling, which shows good mechanical stability. From the SEM image of Figure 3c, the surface of the CNT/CH modified separator is smooth and compact. The cross-section SEM image of the CNT/CH is shown in Figure 3d, it can be clearly seen that the coated layer of the CNT/CH modified separator has a uniform thickness of 5 μm. In addition, the areal mass of the CNT/CH modified separator is lightweight (0.35 mg cm−2). N 2 adsorption/desorption isotherms of the NMCS, NMCS@MoS2, and NMCS@MoS2/S samples are confirmed and displayed in Figure 4a−c, where typical type-IV isotherms could be observed for all three samples, indicating a large number of mesopores in the composites. The corresponding Brunauer−Emmett−Teller (BET) specific surface areas of the

Figure 1. (a) Schematic diagram of the synthesis step of NMCS@ MoS2/S sample. (b) Schematic diagram adsorption of polysulfide by chitosan.

In order to study the morphology and nanostructure of asprepared composites, the representative SEM and TEM images are illustrated in Figure 2. As demonstrated in Figure 2a and b,

Figure 2. SEM images of (a, b) WMCS, (c, d) NMCS, (e, f) NMCS@MoS2, (g) TEM image of NMCS, (h) HRTEM image of NMCS, (i, k) TEM images of NMCS@MoS2, (j) HRTEM image of NMCS@MoS2, and elemental mapping images of (l) carbon, (m) nitrogen, (n) molybdenum, (o) sulfur. 16830

DOI: 10.1021/acssuschemeng.8b04157 ACS Sustainable Chem. Eng. 2018, 6, 16828−16837

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standard card (JCPSD no. 08-0247) as shown in Figure 5b. The TGA method is used to measure the sulfur content of the NMCS/S and NMCS@MoS2/S composite in a N2 flow under a heating rate of 10 °C min−1. As shown in Figure 5c, sulfur contents of NMCS/S and NMCS@MoS2/S composites are about 76 and 78 wt %, respectively. At the same time, in order to confirm the carbon content in the NMCS@MoS2, the NMCS@MoS2 composite is tested using the TGA method in an airflow under a heating rate of 10 °C min−1. The weight loss of 38 wt % of the NMCS@MoS2 corresponds to the conversion of MoS2 to MoO3 and the combustion of NMCS. The MoS2 content of the NMCS@MoS2 is calculated to be about 69 wt %.44 Raman spectra of the NMCS@MoS2 in Figure 5d exhibits two distinct peaks at around 80.8 and 407.8 cm−1 that are described as E12g and A1g, which are the vibration modes of MoS2. Simultaneously, the characteristic D band (1352 cm−1) and G band (1583 cm−1) are observed in NMCS and NMCS@MoS2. The ID/IG ratios of NMCS and NMCS@MoS2 are 0.86 and 1.01, respectively. Compared with NMCS, the ID/IG ratio of NMCS@MoS2 material increases, indicating that the reduction process of MoS2 changes the structure of NMCS and contains more structural defects,45 which is consistent with XRD results. In the XPS spectrum of NMCS of Figure 5e, it can be clearly seen that only three peaks at 284.1, 401.1, and 533.1 eV are revealed, corresponding to C 1s, N 1s, and O 1s, respectively. Differently, seven peaks can be clearly seen at 532.1, 413.0, 401.1, 281.1, 229.1, 162.1, and 37.1 eV in the XPS spectrum of NMCS@MoS2, which corresponds to O 1s, Mo 3p, N 1s, C 1s, Mo 3d, S 2p, and Mo 4p, respectively. Meanwhile, the atomic ratio of S to Mo is about 0.5, which is consistent with the stoichiometric ratio of MoS2. As shown in Figure 5f, the three C 1s peaks at 284.8, 286.1, and 289.4 eV are ascribed to C−C/CC, C−O/C−N, and CO/O−CO, respectively. Three obvious peaks of N 1s at 398.3, 400.9, and 403.5 eV are attributed to the pyridinic, pyrrolic, and graphitic N in Figure 5g. The Mo 3d spectra in Figure 5h displays two peaks at 229.7 and 232.9 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively, owing to the Mo4+ of MoS2. The peak located at 227.1 eV is corresponding to S 2s of S2−. As shown in Figure 5i, two apparent peaks at 162.5 and 163.7 eV in the S 2p spectrum are described as S 2p3/2 and S 2p1/2, respectively. All of these chemical compositions and valence states will help to improve the electrochemical performance of LSBs. A more intuitive method to detect the interaction of NMCS, NMCS@MoS2, and CNT/CH with Li2S6 is shown in Figure 6. First, 10 mg of the NMCS, NMCS@MoS2, and CNT/CH samples are separately added to the prepared Li2S6 solution. Unsurprisingly, when NMCS is added to the Li2S6 solution, the color becomes slightly lighter, indicating weak binding ability to LiPSs. Conversely, when NMCS@MoS2 and CNT/CH are added to the prepared Li2S6 solution, the yellow-colored Li2S6 solution rapidly becomes nearly colorless, indicating the strong binding ability of NMCS@MoS2 and CNT/CH to LiPSs. Meanwhile, in order to further analyze the Li2S6 concentration in the solution, UV adsorption spectroscopy is used. Compared with the Li2S6 solution, the NMCS is added to the Li2S6 solution, and the S62− characteristic peaks decreased slightly. Conversely, with the addition of NMCS@MoS2 and CNT/CH, the characteristic peaks of S62− are drastically reduced, indicating that most of the Li2S6 in the solution is absorbed by NMCS@MoS2 and CNT/CH, which proves the

Figure 3. Digital photo of (a) CNT/CH modified separator, (b) CNT/CH modified separator after being folded several times, (c) SEM image of the surface of CNT/CH modified separator, (d) cross section of CNT/CH modified separator.

NMCS, NMCS@MoS2, and NMCS@MoS2/S are 1198.1, 323.5, and 136.1 m2 g−1, respectively. MoS2 coating on NMCS results in a smaller specific surface area. The smaller one of the NMCS@MoS2/S is owed to the infiltration of sulfur to the NMCS@MoS2. As shown in Figure 4d−f and based on the BJH calculation, the pore sizes of NMCS, NMCS@MoS2, and NMCS@MoS2/S samples are 7.53, 13.02, and 8.97 nm, respectively. NMCS@MoS2 shows a large specific surface area and rich pore structure, which is beneficial to the infiltration of elemental sulfur, lithium ion transport, electrolyte infiltration, and physical confinement of LiPSs. The increase of the pore size for NMCS@MoS2 is due to the sheet-structured MoS2 coated on the surface of NMCS, while the decrease of the pore size for NMCS@MoS2/S is attributed to infiltration of sulfur. The pore volumes of the NMCS and NMCS@MoS2 and NMCS@MoS2/S are 2.52, 1.14, and 0.32 cm3 g−1, respectively. Obviously, the specific surface area and pore volume in the NMCS@MoS2/S material were significantly decreased, indicating that sulfur penetrated into the pore structure of the material. The crystal structures and phase information on NMCS, NMCS@MoS2, NMCS@S, and NMCS@MoS2/S are investigated using X-ray diffraction (XRD, XPS, and Raman spectrum). Figure 5a demonstrates the XRD pattern of the NMCS@MoS2 and the peaks can correspond to the 2H-MoS2 phase (JCPDS no. 37-1492). The diffraction peak positions of NMCS/S and NMCS@MoS2/S are in full accord with the 16831

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Figure 4. N2 adsorption/desorption isotherms and pore size distributions of NMCS (a, d), NMCS@MoS2 (b, e), and NMCS@MoS2/S (c, f).

high capacity because of the high conductivity of CNT and the strong chemistry adsorption of MoS2 and chitosan with LiPSs. The galvanostatic charge−discharge test of the LSBs with NMCS/S, NMCS@MoS2/S, and NMCS@MoS2/S-CNT/ CH is conducted at 0.1 C and shown in Figure 7d−f. The LSBs with NMCS/S and NMCS@MoS2/S deliver an original capacity of 1211 and 1302 mAh g−1 with potential polarization of 205 and 188 mV. The LSBs with NMCS@MoS2/S−CNT/ CH demonstrate a higher original capacity of 1375 mAh g−1 and lower potential polarization of 163 mV, which illustrates faster redox reaction kinetics for the LSBs with NMCS@ MoS2/S-CNT/CH. LSBs with NMCS/S, NMCS@MoS2/S, and NMCS/SCNT/CH deliver initial capacities of 780, 891, and 890 mAh g−1 with a fast capacity decay of 0.3%, 0.18%, and 0.2% per cycle after 200 cycles at 0.5 C as shown in Figure 8a. The rapid capacity decay of LSBs with NMCS/S is mainly due to the dissolution and migration of LiPSs and subsequent deposition of Li2S2 and Li2S on the surface of the lithium anode during the successive charge/discharge process, which leads to safety issues. The polar MoS2 and CNT/CH play a certain role in

strong adsorption capacity of NMCS@MoS2 and CNT/CH to LiPSs. To evaluate the effect of NMCS@MoS2−CNT/CH on the performance of the LSBs, CV and a galvanostatic charge− discharge test of the LSBs are performed with NMCS/S, NMCS@MoS2/S, and NMCS@MoS2/S-CNT/CH between 1.8 and 2.8 V. Figure 7a−c reveal the first to fifth CV profiles of LSBs with NMCS/S, NMCS@MoS2/S, and NMCS@ MoS2/S-CNT/CH. Two reduction peaks at about 2.4 and 2.0 V indicate that S8 is reduced to long-chain LiPSs (Li2Sn, 4 ≤ n ≤ 8) and that conversion from long-chain LiPSs to Li2S2/Li2S occurs, while the one oxidation peak is put down to the oxidation of Li2S2/Li2S to S8. As shown in Figure 7c, the high coincidence of the CV curves of the LSBs with NMCS@ MoS2/S-CNT/CH from the first to fifth cycles implies that the electrode holds excellent reversibility and low polarization. From the first to fifth cycles, in contrast to the LSBs with NMCS/S and NMCS@MoS2/S, the LSBs with NMCS@ MoS2/S-CNT/CH show better curve overlap and higher peak currents, which indicates that LSBs with NMCS@MoS2/S− CNT/CH possess excellent cycle stability, fast kinetics, and a 16832

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Figure 5. (a) XRD patterns of NMCS and NMCS@MoS2, (b) XRD patterns of NMCS/S and NMCS@MoS2/S, (c) TGA traces, sulfur contents of NMCS/S and NMCS@MoS2 in N2, carbon contents of NMCS@MoS2 in air, (d) Raman spectras of NMCS and NMCS@MoS2, (e) XPS measure spectrums of NMCS and NMCS@MoS2, (f) C 1s, (g) N 1s, (h) Mo 3d, (i) S 2p.

same time, in order to compare the effects of CNT and CH on the performances of LSBs, LSBs with NMCS@MoS2/S-CNT and NMCS@MoS2/S-CH are tested at 0.5 C. The LSBs with NMCS@MoS2/S-CNT deliver a higher discharge capacity than the LSBs with NMCS@MoS2/S-CH before 100 cycles because the conductivity of CNT is higher than that with CH. On the other hand, the capacity decay of the LSBs with NMCS@MoS2/S-CH (0.13%) is lower than that of LSBs with NMCS@MoS2/S-CNT (0.15%); the good cycle performance can be attributed to the inhibition of the shuttle effect of LiPSs by chitosan. The rate performances of the LSBs with NMCS/ S, NMCS@MoS2/S, and NMCS@MoS2/S-CNT/CH at the current rate ranging from 0.1 to 1 C are revealed in Figure 8b. The reversible discharge capacities of the LSBs with the NMCS@MoS2/S-CNT/CH are 1250, 1134, 904, and 843 mAh g−1 at 0.1, 0.2, 0.5, and 1 C, respectively. And the specific capacity was 1210 mAh g−1 when the current density was back to 0.1 C. It is nearly close to the original capacity and higher than the LSBs with NMCS/S and NMCS@MoS 2 /S. Simultaneously, the rate performance of LSBs with NMCS@ MoS2/S is much better than with NMCS/S, which is owed to the strong affinity between MoS2 and LiPSs. As shown in Figure 8d, the LSBs with NMCS@MoS2/S-CNT/CH are

Figure 6. UV/vis adsorption spectras and adsorption ability tests of NMCS, NMCS@MoS2, and CNT/CH with Li2S6.

inhibiting the “shuttle effect” of LiPSs, and the corresponding electrochemical performance is significantly improved. The LSBs with NMCS@MoS2/S-CNT/CH present more stable cycle performances conversely. The LSBs with NMCS@MoS2/ S-CNT/CH deliver a higher initial capacity of 893 mAh g−1 with a very low capacity decay of 0.04% after 200 cycles. At the 16833

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Figure 7. CV and galvanostatic charge−discharge profiles of NMCS/S (a, d), NMCS@MoS2/S (b, e), and NMCS@MoS2/S-CNT/CH (c, f).

Rct and Rs of the LSBs with NMCS@MoS2/S are smaller than those of the LSBs with NMCS/S, which indicates that interface properties are improved by the introduction of MoS2. Furthermore, the Rct and Rs of the LSBs with NMCS@ MoS2/S-CNT/CH are smaller than the LSBs with NMCS@ MoS2/S because the CNT/CH has a large number of active sites of chitosan and good electronic conductivity of CNT. Therefore, athe bove test results make it clear that the holistic design of NMCS@MoS2/S and CNT/CH plays a vital role in inhibiting the “shuttle effect” of LiPSs and improving the electrochemical performance of LSBs.

tested to investigate the long-term cycle stability at a high rate of 1 C for 500 cycles. The LSBs with NMCS@MoS2/S-CNT/ CH deliver a high initial specific capacity of 847 mAh g−1 with a slight capacity attenuation of 0.08% per cycle after 500 cycles, demonstrating outstanding cycle stability over a long cycle life. The high areal active material mass loading is a general strategy to improve the energy density of LSBs. LSBs with NMCS@MoS2/S-CNT/CH have a high areal active material mass loading of about 4.2 mg cm−2 and can deliver a high peak capacity of 690 mAh g−1 and high capacity retention of 89.2 % after 200 cycles at 0.5 C. In addition, the electrochemical performance of LSBs with NMCS@MoS2/S-CNT/CH compared with other similar research work is listed in Table 1.46−51 Evidently, this work shows outstanding electrochemical performance, good longterm cycle stability and high areal active material mass loading. EIS spectras of the LSBs with NMCS/S, NMCS@MoS2/S, and NMCS@MoS2/S-CNT/CH are presented in Figure 8c. The equivalent circuit mold in the inset of Figure 8c is employed to analyze and explain the impedance spectra, in which Rs is the electrolyte resistance, Rct is associated with charge transfer, and CPE is equivalent to the capacitor. Warburg impedance (W) represents the slope of the line. The

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CONCLUSION In summary, a holistic design with the NMCS@MoS2/S cathode and CNT/Chitosan modified separator is proposed to inhibit the “shuttle effect” of LiPSs in LSBs. As a host material equipped with highly conductive carbon and highly polar MoS2, NMCS@MoS2 not only availably adsorbs LiPSs but also significantly improves the electrochemical reaction kinetics. The CNT/CH modified separator plays a dual-functional role in the LSBs and further enhances LSBs’ electrochemical performances because the CNT layer of the CNT/CH modified separator as a highly conductive layer can boost 16834

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Figure 8. (a) Cycle stabilities of LSBs with NMCS/S, NMCS@MoS2/S, NMCS/S-CNT/CH, NMCS@MoS2/S-CNT, NMCS@MoS2/S-CH, and NMCS@MoS2/S-CNT/CH at 0.5 C, (b) rate performances of LSBs with NMCS/S, NMCS@MoS2/S, and NMCS@MoS2/S-CNT/CH; (c) Nyquist plots of LSBs with NMCS/S, NMCS@MoS2/S, and NMCS@MoS2/S-CNT/CH, (d) long-term cycling performance of NMCS@MoS2/SCNT/CH at 1 C, (e) cycle stability of LSBs with NMCS@MoS2/S-CNT/CH with high areal active material mass loading of approximately 4.2 mg cm−2 at 0.5 C.

CNT/CH deliver a reversible specific capacity of 501 mAh g−1 with a slight capacity attenuation of 0.08% per cycle after 500 cycles. Therefore, the holistic design strategy for LSBs can fully enhance sulfur loading, restrain the LiPSs shuttle effect, and

not only Li+ ion transfer but also sulfur utilization, and chitosan can further effectively reinforce LiPSs’ adsorption capacity by way of a large amount of amino and hydroxyl groups in the chitosan molecule. The LSBs with the NMCS@MoS2/S16835

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ACS Sustainable Chemistry & Engineering Table 1. Comparison of the Results with Other Literature sample

areal active material mass loading (mg cm−2)

NMCS@MoS2/S-CNT/ CH

Ti3C2/S−Ti3C2/PP S/Co3O4 MoS2-rGO/S S/NiS@C-HS S@MPC-DHSs NSF-S

1.2

0.5 C

200

894

826

this work

1.2 4.0 0.8−1.0 0.8 0.85 2.6 0.6−0.9 4.0

1C 0.5 C 0.5 C 0.2 C 1C 0.5 C 0.5 C 0.6 C

500 200 200 200 300 300 1000 150

847 690 about 820 1164 872 723 800 not mentioned

501 616 about 400 644 480 695 400 414

46 47 48 49 50 51

Corresponding Authors

*Tel.: +86 731 58293377. E-mail: [email protected]. *Tel.: +1 984-234-8489. E-mail: [email protected]. ORCID

Xianyou Wang: 0000-0001-8888-6405 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

ref

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AUTHOR INFORMATION

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reversible capacity (mAh g−1)

cycles

increase cycle stability, which are necessary to meet the requirements of high energy density and long-term stability of large-scale commercialization of LSBs.

■

initial capacity (mAh g−1)

rates

This work is supported financially by National Key Research and Development Program of China (2018YFB0104204), the National Natural Science Foundation of China (No. 51272221), the Hunan Provincial Innovation Foundation for Postgraduate (CX2017B292), and the Key Project of Strategic New Industry of Hunan Province (No. 2016GK4005 and 2016GK4030).

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DOI: 10.1021/acssuschemeng.8b04157 ACS Sustainable Chem. Eng. 2018, 6, 16828−16837