A 3D Graphene-CNT-Ni Hierarchical Architecture as a Polysulfide

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A 3D Graphene-CNT-Ni Hierarchical Architecture as a Polysulfide Trap for Lithium-sulfur Batteries G Gnana Kumar, Sheng-Heng Chung, T. Raj Kumar, and Arumugam Manthiram ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06054 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Materials & Interfaces

A 3D Graphene-CNT-Ni Hierarchical Architecture as a Polysulfide Trap for Lithium-Sulfur Batteries G. Gnana Kumar,†,‡ Sheng-Heng Chung,‡ T. Raj Kumar,† and Arumugam Manthiram‡,* †

Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai-625021, Tamil Nadu, India. ‡

Materials Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA.

ABSTRACT: Despite their high energy density and affordable cost compared to lithium-ion (Li-ion) batteries, lithium-sulfur (Li-S) batteries still endure from slow reaction kinetics and capacity loss induced by the insulating sulfur and severe polysulfide diffusion. To address these issues, we report here nickel nanoparticles filled in vertically grown carbon nanotubes on graphene sheets (graphene-carbon nanotube-nickel composite (Gr-CNT-Ni)) that are coated onto a polypropylene separator as a polysulfide trap for the construction of high-loading sulfur cathodes. The hierarchical porous framework of Gr-CNT physically entraps and immobilizes the active material sulfur, while the strong chemical interaction with Ni nanoparticles in Gr-CNT-Ni inhibits polysulfide diffusion. The covalently interconnected electron conduction channels and carbon shell-confined metal active sites provide feasible paths for the continual regeneration of active material during the charge/discharge process. Benefitting from these novel morphological and structural features, the Li-S cell with the GrCNT-Ni as a polysulfide trap demonstrates high specific capacity and good cycle life. This work provides new avenues for synergistically combining the advantages of hierarchical porous carbon architectures and metal active sites for the development of high-performance cathodes for Li-S batteries.

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KEYWORDS:

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3D hierarchical architecture, charge transfer, confined metal

nanoparticles, lithium-sulfur batteries, tip growth

Corresponding author: *Arumugam Manthiram: E-mail: [email protected]; Tel: +1-512-471-1791


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INTRODUCTION The decarbonization of electrical systems demands the development of reliable and costefficient electrical energy storage systems with high energy densities.1-4 With advantageous electrochemical properties, such as high charge-storage capacity (1,675 mA h g-1) and high specific energy (2,600 W h kg-1), lithium-sulfur (Li-S) batteries offer great promise for electrical energy storage.5 However, Li-S batteries suffer from (i) low sulfur utilization, (ii) shuttling of polysulfides formed during cycling between the two electrodes, and (iii) the deleterious effect of dissolved polysulfides on the Li-metal anode.6 These issues limit the electrochemical stability and Coulombic efficiency of the cell during cycling and resting.7 These problems need to be overcome with approaches, such as advanced interlayer-integrated separators8,9 and innovative carbon/polymer composite separators,10-12 in order for the Li-S batteries to become practically viable for use in grid storage and electric vehicles.13 Recently, 3-dimensional (D) composites containing 2 D graphene and 1 D carbon nanotubes (CNTs) have emerged as a versatile host for sulfur cathodes owing to their micro and mesoporous architectures.14,15 The tortuous pores developed in the composite may facilitate a stable accommodation of sulfur, which would enhance the sulfur utilization rate and restrict polysulfide diffusion.16,17 Furthermore, the linkage of CNTs to the graphene sheets can improve electron transport by increasing the basal plane spacing between the graphene layers.18 Hence, graphene-CNT composites prepared by thermal reduction,19 ultrasonication,20 and layer-by-layer self-assembly21 techniques have been exploited as sulfur hosts for Li-S battery cathodes. However, the reported graphene-CNT composites could not provide promising Li-S battery performance and cycling stability owing to the non-bonding interaction between graphene and CNTs. Other constraints, including the processing complexity, low productivity, agglomerated morphologies, and impurities in graphene-CNTs have also limited their applications.19-21 3 ACS Paragon Plus Environment


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Fabrication of covalently-linked graphene-CNT composites can be achieved with a catalyst-assisted chemical vapor deposition (CVD) technique.22 The catalytic growth of CNTs involves the decomposition of carbonaceous gas molecules (e.g., acetylene (C2H2)) at the surface of metallic structures and the surface diffusion of carbon on liquid-like metals.23,24 The existence of metal structures in CNTs may facilitate strong interactions between the metal and sulfur, which can react with dissolved polysulfides and stabilize the electrochemical performance of the cell.25

Accordingly, we report here a rational

development of a graphene-CNT-nickel (Gr-CNT-Ni) composite coated onto a polypropylene separator (Gr-CNT-Ni-coated separator) as a promising polysulfide trap for Li-S batteries.

RESULTS AND DISCUSSION The synthesis process of nickel nanoparticles distributed reduced graphene oxide (rGO-Ni) involves the electrostatic interaction of Ni2+ ions with the negatively charged GO sheets and their simultaneous reduction with urea to form rGO-Ni (Figure 1). Transmission electron microscopy (TEM) images of rGO-Ni show that the rGO sheets are well-exfoliated into a thin layer, with uniformly distributed 26 nm sized spherical Ni nanoparticles on the surface (Figure 2a). The catalytic conversion of rGO-Ni via the CVD process-generated the organized framework of densely packed, vertically grown tubular structures of CNTs on the surfaces of the layered graphene sheets (Figure 1). The CNTs are a few micrometers in length and have a mean diameter of 36 nm (Figure 2b and c). The CNTs are composed of multiwalled, centrally hollow tubes with the thick graphitic walls (11 nm) and an inner nanotube diameter of 14 nm (Figure 2d-f). From Figure 2f, it is evident that the Ni nanoparticles are located on the tips of the hollow nanotubes, encapsulated by a carbon shell, indicating that the tip-growth is the mechanism of formation of the CNTs in this process.26 Under high temperatures (750 oC), the generated carbon atoms diffuse toward and dissolve uniformly on the Ni nanoparticles in rGO-Ni, forming a metal-carbon solid-state solution. After reaching 4 ACS Paragon Plus Environment

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supersaturation, the CNTs proceed to grow as tubular structures by a continuous deposition of carbon atoms. This nucleation step induces further carbon deposition and leads to the elevation of Ni nanoparticles at the exposed end of the CNTs. Meanwhile, the bases of the CNTs merge with the graphene substrate, at which graphene is generated from the thermal reduction of rGO.27 This tip-growth model can facilitate contact between graphene and CNTs, leading to the formation of hierarchical Gr-CNT-Ni. The polycrystalline structure of the carbon shell-confined Ni nanoparticles in Gr-CNT-Ni is identified by selected area electron diffraction (SAED) pattern (inset in Figure 2f). The energy dispersive X-ray spectra (EDS) of Gr-CNT-Ni (Figure S1a) and rGO-Ni (Figure S1b) depict the elemental composition of C:O:Ni, respectively, as 92.3:3.7:4.0 and 71.2:15.7:13.1 (at %). The reduced Ni content in Gr-CNT-Ni compared to that in rGO-Ni ensures the utilization of Ni nanoparticles from rGO-Ni for the catalytic growth of CNTs. The metallic Ni in the rGO-Ni composite exhibits a face-centered cubic (fcc) structure as evidenced by its characteristic X-ray diffraction (XRD) peaks (JCPDS 04-0850) (Figure 3a).23 Although the Gr-CNT-Ni diffraction pattern shows metallic Ni peaks, the intensities of these peaks are lower than those shown in rGO-Ni, indicating that the majority of the Ni nanoparticles were utilized for the catalytic growth of CNTs. Furthermore, the graphitic peak corresponding to the (002) plane is slightly shifted in the Gr-CNT-Ni pattern, indicating a reduced interlayer d-spacing of 0.334 nm (vs. 0.342 nm in rGO-Ni).28 A lower ID/IG ratio and a sharper 2D band of Gr-CNT-Ni compared to that of rGO-Ni indicates a highly ordered graphitic structure and the curing of structural disorders/defects with the growth of CNTs (Figure 3b).29 X-ray photoelectron spectroscopy (XPS) survey scan spectra of rGO-Ni and Gr-CNT-Ni (Figure 4a) show the characteristic peaks of Ni 2p, O 1s, and C 1s, respectively, at 856, 531, and 285 eV. The de-convoluted C 1s peak of Gr-CNT-Ni (Figure 4b) shows the predominant



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peaks of sp2 C=C and sp3 C-C bonded graphitic carbon and the lower intensity O-C=O, C=O, and C-O peaks of oxygenated carbon functional groups.30 The O 1s characteristic peaks relevant to the C-OH/C-O, C=O, and Ni-O are also found in the Gr-CNT-Ni spectrum (Figure 4c). The striking increase in the C/O atomic ratio in Gr-CNT-Ni (9.23) (Figure 4a-c) compared to that in rGO-Ni (4.65) (Figure 4a and S2a and b) corroborates the highly graphitized structure of Gr-CNT-Ni.17 The doublet signals found for Gr-CNT-Ni at 872.3 and 854.2 eV are consistent, respectively, with the Ni 2p1/2 and Ni 2p3/2 peaks31 (Figure 4d) and their intensities are relatively lower compared to those of rGO/Ni (Figure S2c). The composition of rGO-Ni and Gr-CNT-Ni composites was evaluated with thermogravimetric analysis (TGA) operated under air atmosphere (Figure S3). The weight loss observed for the prepared composites in the region of 100 - 250 oC is due to the removal of oxygen-labile functional groups.32 From the weight loss in the above region, the oxygen content in rGO-Ni and Gr-CNT-Ni was found, respectively, to be 10.6 and 1.6 wt.%.33 The carbon content in Gr-CNT-Ni was determined to be 83.1 wt.% as evidenced from its weight loss in the region of 585 - 760 oC, whereas rGO-Ni showed a carbon content of 49.2 wt.%. The Ni contents in rGO and Gr-CNT-Ni composites were estimated, respectively, to be 40.2 and 15.3 wt.%, which were evaluated from the residual weight of Ni under air atmosphere at 800 °C.28 The elimination of oxygen labile functional groups and the formation of CNTs in Gr-CNT-Ni promote the thermal stability of Gr-CNT-Ni compared to rGO-Ni. Nitrogen adsorption/desorption isotherms of Gr-CNT-Ni show type I and IV isotherms, consisting of the filling-behavior characteristics of, respectively, micropores at relative pressures (P/Po) of < 0.1 and mesopores at P/Po = 0.4 - 0.9 (Figure S4a). The Gr-CNT-Ni shows a total pore volume and specific surface area of, respectively, 0.30 cm3 g-1 and 116 m2 g-1, which are higher than those of dense and nonporous rGO-Ni (0.08 cm3 g-1 and 12 m2 g-1) (Table S1). The existence of micropores and mesopores in Gr-CNT-Ni is


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confirmed from the pore-size distribution (PSD) analysis, performed with an integrated micro-meso-macro-pore calculation (Figure S4b). The peak micropores in Gr-CNT-Ni have diameters of 0.62 and 1.24 nm, and the disordered mesopores show a broad size distribution of 2 - 36 nm with a peak value at 3.8 nm. The benefits of the hierarchical architecture of the prepared materials towards the electrochemical behavior were accomplished by coating them onto a conventional polypropylene separator as a polysulfide trap in Li-S cells. The cyclic voltammetry (CV) curves of the Li-S cell fabricated with the Gr-CNT-Ni-coated separator display well-defined two cathodic (C1 and C2) and anodic (A1 and A2) peaks, which are associated, respectively, with the two-step reduction of S to polysulfides and subsequent reduction to Li2S2/Li2S and their reverse oxidation processes.34 In comparison with a control cell fabricated with a rGO-Ni-coated separator, the positive and negative shifts observed, respectively, for the cathodic and anodic peaks of Gr-CNT-Ni, confirm the lower polarization and superior catalytic effect of Gr-CNT-Ni on the conversion between sulfur and Li2S2/Li2S (Figure 5a).35 The cell with the Gr-CNT-Ni-coated separator did not exhibit any obvious variation in peak intensity or potential over 10 CV cycles, confirming the good cycling stability and reversibility of the material (Figure 5b). The charge-discharge voltage profiles of the cell with the Gr-CNT-Ni-coated and rGO-Nicoated separators at different current rates are presented in Figure 6a-d. The obtained curves consist of two well-defined discharge (lower and upper) plateaus and two intimate charge plateaus that are relevant to the redox processes of sulfur-to-polysulfides and polysulfides-tosulfides.36 The cell with the Gr-CNT-Ni-coated separator delivers a high initial capacities of 963 and 849 mA h g-1, respectively, at C/10 (Figure 6a) and C/5 rates (Figure 6b). By comparison, the cell with the rGO-Ni-coated separator demonstrates a lower initial capacities



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of 578 and 549 mA h g-1, respectively, at C/10 (Figure 6c) and C/5 rates (Figure 6d) due to the poor active material utilization and the lack of a porous structure in rGO-Ni. The charge-discharge plateaus of the cell with the Gr-CNT-Ni-coated separator overlap well during successive cycles at the lower rate (C/10), confirming the low polarization, high stability, and enhanced electrochemical reversibility. At the end of the charging step, the voltage rises from 2.4 to 2.8 V, indicating the complete charging. The elongated upper discharge plateaus show that the entrapment and stabilization of polysulfides within the Gr-CNT-Ni cathode restrict the loss of active material. Upon complete reduction of the active material, improved electrochemical reversibility is achieved with the cell with the Gr-CNT-Ni-coated separator, as evidenced by the extended lower discharge plateaus. The 3D porous structure of Gr-CNT-Ni coating layer on the separator facilitates the accommodation and entrapment of diffusing polysulfides. It achieves uniform trapping and reutilization of the active material, which is beneficial for the efficient charge-discharge reactions during long-term cycling.37 The integration of conductive CNTs between the graphene layers via the C-C nodal connection between the graphite and the carbon shellconfined Ni nanoparticles contribute to improved electrical conductivity (30.4 S cm-1) compared to that of rGO-Ni (9.5 S cm-1). This linkage effectively lowers the contact resistance between the active material and the cathode architecture. Furthermore, the existence of Ni nanoparticles on the tips of CNTs enhances the chemical conversion between sulfur and polysulfides and the overall electrochemical performance.25 Contradictorily, the cell with the rGO-Ni-coated separator demonstrates shortened chargedischarge plateaus due to the relatively low conductivity of the dense and nonporous rGO-Ni sheets. The absence of porous features in rGO-Ni retards the entrapment of active material during cycling, causing a loss of active material and capacity fade.38


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Good cycling stability is observed for the cell with the Gr-CNT-Ni-coated separator, as evidenced by the capacity-retention values of 80 % and 82 % after cycling, respectively, at the rates of C/10 and C/5 for 100 cycles. The Coulombic efficiencies at the rates of C/10 and C/5 are remarkably stable, at over 98 % after 100 cycles (Figure 7a). The rapid electron transport properties and the stabilization of soluble polysulfides within the porous Gr-CNT-Ni coating layer are responsible for the consistent performance. Furthermore, the reutilization of the trapped active materials in the porous bundles during the cycling process prevents active material loss and inhibits the shuttle process.39 The passivation of the lithium counter electrode with the commonly used lithium nitrate (LiNO3) electrolyte additive also improves the cell efficiency.36,39 In contrast, the cell with the rGO-Ni-coated separator (Figure 7b) exhibits lower Coulombic efficiencies, (90 % at C/10 rate and 91 % at C/5 rate) under identical conditions. Owing to the direct exposure of Ni to the electrolyte, insulating layers may be formed at the interface of solid electrolyte and the rGO-Ni cathode, which could prevent reactivation of the trapped active material. XPS analysis was carried out to distinguish the types of sulfur and identify their chemical states on the surface of S-Gr-CNT-Ni and S-rGO-Ni coated separators after the cycling processes (Figure S5). The S 2p3/2 and S 2p1/2 peaks of S-Gr-CNT-Ni with a spin-orbit splitting of 1.5 eV evidence the formation of C-S bonds with Li2S products and the S-S bond of amorphous sulfur (Figure S5a).40 The existence of S 2p3/2 peaks at a lower binding energy (163.6 eV) reveals that the S8 molecules are confined within the mesopores of Gr-CNT-Ni. The additional satellite peak at a higher binding energy (168.3 eV) confirms the subsistence of smaller S2-4 molecules in the micropores of carbon framework.40 The existence of trace amounts of lower S2-4 molecules in S-rGO-Ni is evidenced by the weak and broadened satellite peak at 169 eV (Figure S5b), which is due to the limited micropores in rGO-Ni. The intensity of the peak corresponding to the S-S bond (165.1 eV for S 2p1/2) at S-Gr-CNT-Ni is



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lower than those of S-rGO-Ni, indicating a higher utilization of sulfur species and the effective conversion of sulfur species into Li2S products at S-Gr-CNT-Ni.41 The peak at 170.1 eV is ascribed to the SOx species (polythionate/thiosulfate), which may arise from the surface redox reaction between the polysulfides and metal and the consequent oxidation of polysulfides into SOx species.42 The highly intense peak corresponding to the SOx species at S-Gr-CNT-Ni than those of S-rGO-Ni ensures a considerable curtailing behavior of polysulfide diffusion42 at S-Gr-CNT-Ni. After the cycling process, the Ni 2p peaks of S-Gr-CNT-Ni (Figure S5c) and S-rGO-Ni (Figure S5d) are slightly shifted to higher binding energies, signifying the interaction between Ni nanoparticles and sulfur species. The existence of micro/mesopores and carbon shell confined Ni nanoparticles in Gr-CNT-Ni serves as an ideal barrier to restrain the dissolved polysulfides,43 facilitating an improvement in the electrochemical performance of Gr-CNT-Ni based separator in Li-S batteries.

CONCLUSION In summary, we demonstrate herein the rational development of covalently-linked grapheneCNTs, with carbon shell-encapsulated Ni metal at the tips of the CNTs (Gr-CNT-Ni composite), and their use as a promising polysulfide trap in Li-S batteries. The cell fabricated with the hierarchical porous Gr-CNT-Ni coated onto a polypropylene separator suppresses polysulfide migration and retains and reutilizes the active material within the cathode region due to the high tortuosity and mechanical strength of Gr-CNT-Ni. A comparison of the electrochemical performances of the cells fabricated with the Gr-CNT-Ni- and rGO-Nicoated separators provides a fundamental understanding of the positive effects of the porous structure, continuous electron conductive framework, and carbon shell-encapsulation of Ni on Li-S cell performance.


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EXPERIMENTAL SECTION Materials. Graphite Powder (< 20 µm, Sigma-Aldrich), potassium permanganate (KMnO4, 98.5 %, Merck), sulfuric acid (H2SO4, 98 %, Merck), nickel (II) acetate tetrahydrate (Ni(CH3COO)2.4H2O,

98

%, Sigma-Aldrich),

ethanol

(HPLC,

≥

99.8

%, Sigma-

Aldrich), urea (Co(NH2)2, 99 %, Sigma-Aldrich), 1,3-dioxolane (DOL, 99.8 %, SigmaAldrich), dimethoxy ethane (DME, 99.5 %, Sigma-Aldrich), isopropyl alcohol (IPA, ≥ 99.5 %, fisher scientific), sulfur powder (99.5%, Acros Organics), lithium sulfide powder (Li2S, 99.9 %, Alfa Aesar), lithium trifluoromethanesulfonate (LiCF3SO3, 99.95 %, SigmaAldrich), and lithium nitrate (LiNO3, 99 %, Acros Organics) were used as received. The sulfur and Li2S powders were mixed into a blank electrolyte to form the polysulfide catholyte, which was injected into the cathode side as the starting active material (see Preparation of LiS batteries for details). Preparation of rGO-Ni composite. Natural graphite powder was oxidized on the basis of modified Hummer’s method for the preparation of GO sheets.44 0.1 M urea was added drop-wise in to a mixture of 0.05 M Ni(CH3COO)2.4H2O and 1 mg mL-1 of GO dispersion with stirring at room temperature. A 1:1 mass ratio was kept between the GO and Ni(CH3COO)2.4H2O. Subsequently, the resultant rGO-Ni was recovered after centrifugation, washed with excessive amounts of ethanol and de-ionized water, and dried at 100 oC. Preparation of Gr-CNT-Ni. The preparation of Gr-CNT-Ni was achieved with a CVD method. A quartz boat containing rGO-Ni composite (100 mg) was located onto the center of a horizontal alumina tubular furnace. Initially, the temperature of the furnace was raised to 750 oC under nitrogen (N2) atmosphere at a gas flow and heating rates of, respectively, 200 mL min-1 and 5 oC min-1. Afterward, C2H2 gas with 99.9 % purity was introduced at the same temperature for 30 min with a 60 mL min-1 flow rate. Then, the system was cooled under nitrogen (N2) atmosphere and the resultant Gr-CNT-Ni was collected. The yield of 11 ACS Paragon Plus Environment


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Gr-CNT-Ni from the catalytic conversion of rGO-Ni via the CVD process was found to be 175 %. The conversion yield was calculated with the formula of ((mtot – mcat) / mcat)×100 %), where mtot and mcat are referred to, respectively, as the total mass of carbon product and the mass of the catalyst.14 Materials characterization. A transmission electron microscope (TEM; Tecnai 20 G2, FEI, 100 KeV) equipped with energy dispersive X-ray spectroscopy (EDS), field-emission scanning electron microscope (FE-SEM, Nova Nano SEM-430, FEI, 15 kV), X-ray diffractometer (XRD, X'Pert-PRO Difractometter with Cu-Kα radiation of λ-1.54Ao), Raman spectrometer (HORIBA, Jobin Yovon), and X-ray photoelectron spectrometer (XPS, Theta Probe-AR system, Thermo Fisher Scientific with monochromated Al Ka (1486.6 eV) as an X-ray source functioning at 15 kV) were employed for the characterization of the prepared materials. The surface area and porosity of Gr-CNT-Ni and rGO-Ni were evaluated with a gas sorption analyzer (AutoSorb iQ2, Quantachrome Instrument, analysis temperature at 77 K). Brunauer-Emmett-Teller (BET) method was exploited to analyze the specific surface area for the prepared materials. The pore-size distribution (PSD) was analyzed with the integration of three different models: the Horvath-Kawazoe (HK) method for microporosity analysis, density functional theory (DFT) for mesopore measurement, and Barrett-Joyner-Halenda (BJH) analysis for broad PSD screening. The microporosity analysis was performed using the t-plot method under the activated carbon model and supported by the Dubinin-Radushkevich (DR) / Dubinin-Astakhov (DA) and Saito-Foley (SF) models. The four-probe technique was used with an Agilent semiconductor parameter analyzer (Agilent 4156C) to evaluate the electrical conductivities of rGO-Ni and Gr-CNT-Ni at room temperature. TGA analyses were carried out with a Perkin Elmer TGA-7 instrument under an air atmosphere.


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Preparation of Li-S batteries. Li-S cells were assembled by using polysulfide catholyte as the starting active material. The polysulfide catholyte contained 1.0 M dissolved Li2S6 polysulfides in a blank electrolyte. The blank electrolyte contained 1.85 M LiCF3SO3 and 0.2 M LiNO3 in 1:1 v/v of DOL/DME. The Gr-CNT-Ni-coated separator was prepared by vacuum filtering a Gr-CNT-Ni suspension through a Celgard 2500 polypropylene separator and then drying at 50 °C in an air-oven for 24 h. The Gr-CNT-Ni suspension was prepared by dispersing 40 mg of Gr-CNT-Ni powder in 500 mL isopropyl alcohol (IPA) with the aid of a high-power ultrasonication. The weight of the resulting Gr-CNT-Ni coating layer was 0.2 mg cm-2. The Li-S cells were assembled with a commercially available carbon paper as an 1 cm2 current collector (4.0 mg cm-2, NanoTechLabs, Inc.), 40 μL of dissolved polysulfide catholyte, a Gr-CNT-Ni-coated polypropylene separator with the coating facing towards the cathode side, 45 μL of blank electrolyte, Li-foil anode, and a Ni-foam spacer located in a CR2032 coin cell. The Ni-foam spacer was located outside of the two electrodes and was not involved into the cell’s electrochemical reactions. The resulting cells possessed a high sulfur loading, sulfur content, and low electrolyte-to-sulfur ratio of, respectively, 7.68 mg cm-2, 64 wt. %, and 10 μL mg-1. Cell performance parameters were evaluated in regard to the overall mass loadings of materials used in the cathode region. A control cell was prepared with an rGO-Ni-coated polypropylene separator as a reference. Electrochemical measurements. The CVs of the fabricated cells were recorded on a CHI-660E electrochemical workstation at 0.1 mV s-1 with the potential window of 1.8 - 2.8 V (vs Li/Li+). The Li-S battery tests were performed for 1 h before the electrochemical analysis. The charge-discharge and cyclability data were obtained with an Arbin Instrument battery cycler. The assembled cells were initially charged to 2.8 V and subsequently cycled between 1.7 and 2.8 V. The electrochemical characteristics were evaluated at the current rates of C/10 and C/5. To normalize the



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data obtained, all of the assembled cells were first screened to have an open circuit voltage (OCV) of 2.2940 ± 0.0025 V.

ASSOCIATED CONTENT Supporting Information EDAX, core level XPS spectra of rGO-Ni, BET, TGA, XPS spectra of discharged state and table. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The electrochemical characterization work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0005397. The synthesis work was supported by the Science and Engineering Research Board, India, Major Project Grant No. EMR/2015/000912.


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Figure 1. Schematic of the preparation process involved in Gr-CNT-Ni.



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Figure 2. (a) TEM image of rGO-Ni, (b and c) FE-SEM images of Gr-CNT-Ni with the inset in (c) depicting the Ni nanoparticle filled in the tip of CNT, and (d-f) TEM images of Gr-CNT-Ni with the inset in (f) depicting SAED pattern.


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a

b

Figure 3. (a) XRD patterns and (b) Raman spectra of the prepared nanostructures.



a

b

c

d

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Figure 4. (a) XPS survey scan of the prepared nanostructures and the de-convoluted XPS spectra of (b) C 1s, (c) O 1s, and (d) Ni 2p of the Gr-CNT-Ni nanostructures.


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Figure 5. (a) CVs of the cells fabricated with Gr-CNT-Ni-coated separator (black curve) and rGO-Ni-coated separator (red curve) and (b) CVs of the cells fabricated with the Gr-CNT-Nicoated separator over 10 cycles at 0.1 mV s-1.



a

b

c

d

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Figure 6. Charge-discharge voltage profiles of the cells fabricated with the Gr-CNT-Nicoated separator at the rates of (a) C/10 and (b) C/5 and the rGO-Ni-coated separator at the rates of (c) C/10 and d) C/5 rates.


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a

b

Figure 7. Cell cyclability at the rates of C/10 and C/5 with (a) Gr-CNT-Ni-coated separators and (b) rGO-Ni-coated separators.



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A 3D Graphene-CNT-Ni Hierarchical Architecture as a Polysulfide

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