Synergistically Enhanced Polysulfide Chemisorption Using a Flexible

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Synergistically Enhanced Polysulfide Chemisorption Using a Flexible Hybrid Separator with N and S Dual-Doped Mesoporous Carbon Coating for Advanced Lithium–Sulfur Batteries Juan Balach, Harish Kumar Singh, Selina Gomoll, Tony Jaumann, Markus Klose, Steffen Oswald, Manuel Richter, Jurgen Eckert, and Lars Giebeler ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03642 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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ACS Applied Materials & Interfaces

Synergistically Enhanced Polysulfide Chemisorption Using a Flexible Hybrid Separator with N and S Dual-Doped

Mesoporous

Carbon

Coating

for

Advanced Lithium–Sulfur Batteries Juan Balach,*,a Harish K. Singh,a Selina Gomoll,a Tony Jaumann,a Markus Klose,a,b Steffen Oswald,a Manuel Richter,a,c Jürgen Eckerta,b,§ and Lars Giebelera,b

a

Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V., P.O. Box 270 116,

D-01171, Dresden, Germany. b

Technische Universität Dresden, Institut für Werkstoffwissenschaft, Helmholtzstraße 7, D-

01069 Dresden, Germany. c

Technische Universität Dresden, Dresden Center for Computational Materials Science (DCMS),

D-01069 Dresden. §

present address: Erich Schmid Institute of Materials Science, Austrian Academy of Sciences

and Department of Materials Physics, Montanuniversität Leoben, Jahnstr. 12, A-8700 Leoben, Austria. *Corresponding Author: Phone: +49 351 4659 693, e-mail address: [email protected]

ACS Paragon Plus Environment

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KEYWORDS: hybrid separator, nitrogen/sulfur co-doped, mesoporous carbon coating, polysulfide adsorption, DFT calculation, lithium-sulfur battery ABSTRACT: Due to the outstanding high theoretical specific energy density of 2600 Wh kg−1, the lithium–sulfur (Li–S) battery is regarded as a promising candidate for post lithium-ion battery systems eligible to meet the forthcoming market requirements. However, its commercialization on large scale is thwarted by fast capacity fading caused by the Achilles' heel of Li–S systems: the polysulfide shuttle. Here, we merge the physical features of carbon-coated separators and the unique chemical properties of N and S co-doped mesoporous carbon to create a functional hybrid separator with superior polysulfide affinity and electrochemical benefits. DFT calculations revealed that carbon materials with N and S co-doping possess a strong binding energy to high-order polysulfide species, which is essential to keep the active material in the cathode side. As a result of the synergistic effect of N, S dual-doping, an advanced Li–S cell with high specific capacity and ultralow capacity degradation of 0.041 % per cycle is achieved. Pushing our simple-designed and scalable cathode to a highly increased sulfur loading of 5.4 mg cm–2, the Li–S cell with the functional hybrid separator can deliver a remarkable areal capacity of 5.9 mAh cm–2, which is highly favorable for practical applications.


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INTRODUCTION Significant forward leaps in electrified transportation—hybrid-electric vehicles (HEVs) and battery-electric vehicles (BEVs)—and smart-grid systems can only ensue from the development of new rechargeable power sources with high energy/power densities.1,

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Despite their wide

application in many types of hand-held electronic devices, current lithium-ion batteries consisting of lithium transition metal oxide cathodes, bear some notable drawbacks such as highcosts, serious safety issues and critical limitations in specific energy densities—up to 400 Wh kg−1 at cell level under ideal conditions—that may restrict their further adoption in large-scale power source systems.1, 3, 4 Among various emerging energy storage systems, lithium–sulfur (Li– S) batteries have attracted tremendous attention as the prime candidate for the next-generation of high-energy secondary batteries due to its remarkable high theoretical specific energy density of 2600 Wh kg−1, skilled to meet the forthcoming market requirements.4, 5 Furthermore, the high natural abundance, low cost and environmental harmlessness of elemental sulfur as the active cathode material is an additional plus for extensive manufacturing applications.5 However, Li–S batteries have not been commercialized on large scale yet owing to various challenging problems. One of these issues related to the low active material utilization, which is caused by the insulating nature of pure sulfur and its discharging products (Li2S/Li2S2), have been partially overcome by infiltrating molten sulfur into conductive porous scaffolds (e.g. micro and/or mesoporous carbons,6-11 carbon nanostructures,12-16 MXene,17 metal oxides,18 conductive polymers19,

20

) to form composite cathodes. This approach takes advantage of the inherent

porosity of the conductive host material to accommodate the large volume expansion of sulfur


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during lithiation (≈80 %), stabilizing the cathode structure upon cycling. However, the so-called "shuttle effect" is the real Achilles' heel in Li–S battery systems, which involves the dissolution of high-order lithium polysulfide (LiPS) intermediates (Li2Sx, 4 ≤ x ≤ 8) into commonly used electrolytes, allowing their escape from the host cathode material and further undesirable migration to the anode side, triggering lithium anode contamination, active material loss, rapid capacity fading and short service life of the cell. The use of lithium salts and electrolyte additives have been adopted to prevent parasitic reactions between the metallic lithium anode and the soluble LiPSs.21-24 However, despite considerable improvement of the reversibility of the cells, these approaches still cannot solve the shuttle phenomenon and the active material loss. Recently, novel Li–S cell configurations with multi-functional interlayers/hybrid separators have gained remarkable attention due to their peculiar advantages to restrict the LiPS shuttle, reactivate dead sulfur-containing species and reduce side reactions on the anode side, resulting in a greatly improved performance.25 In addition, the re-configuration of the cell considerable reduces production costs by simplifying the electrode preparation.26,

27

Among others,

polypropylene-supported mesoporous carbon hybrid separators, preferably with large pore volume, are an interesting platform to trap and immobilize a relatively high amount of soluble LiPS species due to the extended electrochemical active surface area.26,

28

Furthermore, the

polysulfide-trapping ability of the modified separator can be improved by employing an N-doped porous carbon-based layer, which enhances the chemical affinity between the carbon-coating and the migrating LiPS intermediates.27, 29-32 The strengthening of the chemical interaction between carbon host and LiPS is crucial to rein the active material in the cathode region. Very recently, sulfur-infiltrated carbon scaffolds with nitrogen and sulfur dual-doping have shown stronger LiPS chemisorption in comparison with


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undoped and single N-doped carbons.33-36 Theoretical calculations revealed that adjacent thionic S/pyridinic N (or thionic S/pyrrolic N) functionalities work in cooperation to bind at least small Li2S/Li2S2 molecules, which is considered to be the main reason of the improved cyclability and cell performance.33, 34 In this contribution, we merge the physical features of carbon-coated separators and the unique chemical properties of N/S-co-doped mesoporous carbon to create a functional hybrid separator with superior LiPS affinity and electrochemical benefits. To the best of our knowledge, the modification of commercial polypropylene separator with N and S dual-doped mesoporous carbons has not been reported so far. The integration of an N-allylthiourea-derived N/S-co-doped mesoporous carbon barrier on one side of a commercial polypropylene separator greatly increases the affinity to LiPS species, improving their confinement in the cathode side. As a result, Li–S cells employing an N/S-co-doped mesoporous carbon-coated separator exhibit lower internal cell resistance, higher initial discharge capacity and better capacity retention compared to cells using N single-doped or undoped carbon-coated separators. Furthermore, our experimental results are well-supported by density functional theory (DFT) calculations which were particularly performed to shed light on the binding affinity between N-doped or N/S-codoped carbons and high-order LiPS intermediates. EXPERIMENTAL SECTION Chemicals. N-allylthiourea (ATU, 98 wt.%), N-allylurea (AU, 95 wt.%) Ludox HS-40 (40 wt.%, 12 nm SiO2 nanoparticles), elemental sulfur (S, 99.98 wt.%), lithium sulfide (Li2S, 99.98 wt.%), acacia-derived gum arabic (GA), poly(tetrafluoroethylene) (PTFE), N-methyl-2pyrrolidone (NMP, 99 wt.%), 1,3-dioxolane (DOL, 99.8 wt.%, anhydrous) and 1,2dimethoxyethane (DME, 99.5 wt.%, anhydrous) were purchased from Aldrich. Super P carbon


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(SPC), lithium bis(trifluoromethylsulfonyl)imide salt (LiN(CF3SO2)2, Li-TFSI, 99.9 wt.%), polyvinylidene difluoride (PVDF, Solef 21216) and hydrofluoric acid (HF, 40 wt.%) as well as lithium nitrate (LiNO3, >99.995 wt.%) were purchased from TIMCAL, BASF, Solvay and Merck, respectively. All chemicals were used as received except LiTFSI and LiNO3 which were dried at 100 °C under vacuum prior to use. Synthesis of MPC, NMPC and NSMPC. Nitrogen and sulfur co-doped mesoporous carbon (NSMPC) was synthesized by a facile SiO2-templated casting route, based on previous reports.37, 38

In a typically preparation, 4.0 g of ATU was dissolved in 6.16 ml of deionized water under

stirring at 80 °C for 10 min. Then, 3.84 ml of Ludox HS-40 (2 g of SiO2) was added to the solution prepared above and stirring was continued for another 20 min. After drying at 60 °C for 48 h, the ATU-coated SiO2 nanoparticles mixture was carbonized at 800 ºC for 1 h under Argon atmosphere. Then, the SiO2 was etched using 20 wt.% HF solution and the product was repeatedly washed with deionized water. N-doped mesoporous carbon (NMPC) was prepared following the above-mentioned procedure but using AU as carbon precursor and nitrogen source. Undoped mesoporous carbon (MPC) with similar characteristics compared to NSMPC was synthesized according to our previous report.38 In this case, a resorcinol-formaldehyde polymer was used as a carbon precursor and the mass ratio of SiO2 to resorcinol was 2. Hybrid separator preparation. The commercially available microporous polypropylene separator (Celgard 2500) was simply modified by coating of a shaker-milled carbon slurry, containing 87.5 wt.% of carbon material and 12.5 wt.% of PVDF binder in NMP, on one side of the Celgard 2500. A doctor blade was used for this process. The carbon-layered separators were


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dried at 50 °C for 20 h and then cut into circular disks of 16 mm diameter. The mass loading of the carbon on the separator was 0.5–0.6 mg cm–2. Simple-designed pure sulfur cathode preparation. Simple-designed, pure sulfur cathodes consisting of S, SPC together with GA binder39 were produced by shaker-milling in deionized water. The homogeneous slurries were coated onto Al foil by a doctor blade. The cathodes were dried at 60 ºC for 20 h and then punched into circular disks of 12 mm diameter. In this work, cathodes with S:SPC:GA weight ratios of 50:40:10 and 70:20:10 were used for cell set-ups with the routine and the hybrid separators, respectively. The areal loading of sulfur on the positive electrodes is typically as high as 2.1–2.3 mg cm–2. To evaluate the cell performance under highareal sulfur loading, a cathode with a S:SPC:PTFE weight ratio of 55:35:10 and an areal sulfur loading of 5.4 mg cm–2 was prepared by grinding of the powder components in a mortar and subsequently rolled out at ≈100 °C to finally obtain a mechanically stable cathode foil with a thickness of ≈85 µm. Electrochemical testing. Coin cells (CR2025) were assembled in an Ar-filled glove box. Lithium foil (Chempur, 13 mm diameter, 250 µm thick) was used as both counter and reference electrode. The electrolyte consisted of 1 M LiTFSI and 0.25 M LiNO3 additive in DOL:DME (1:1 v/v). The electrolyte without LiNO3 additive was prepared using the same solvent composition and conducting salt concentration as in presence of the LiNO3 salt. We used a commercial microporous polypropylene membrane (16 mm diameter, 25 µm thick, Celgard 2500) as routine separator. The cells with the hybrid separators were assembled with the carboncoating oriented towards the cathode. The amount of electrolyte used in the cells with the routine separator and the hybrid separators containing low/high-areal sulfur loadings is, respectively, 12, 18 and 12 µL mgS–1.


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Cells were galvanostatically discharged and charged between the cut-off potentials of 1.8–2.6 V at 25 °C using a BaSyTec Cell Test System (CTS). Electrochemical impedance spectroscopy (EIS) measurements were conducted with a VMP3 potentiostat (Bio-logic) between 200 KHz and 100 mHz using an AC voltage amplitude of 5 mV. Prior to long-term cycling at 0.5 C, the cells were pre-cycled at 0.1 C twice (1 C = 1672 mA g–1). The calculation of specific discharge capacities is based on the mass of elemental sulfur. Characterization. A Zeiss Gemini 1530 scanning electron microscope (SEM) was used to examine the NSMPC carbon and the routine/hybrid separators. Energy-dispersive X-ray spectroscopy (EDXS) was performed on the SEM with a Bruker XFlash 6 detector. Highresolution imaging of the mesoporous carbons were taken with a FEI Tecnai F30 transmission electron microscope (TEM) having a field emission gun (FEG) at 300 kV acceleration voltage. Nitrogen physisorption measurements were carried out at 196 °C using a Quantachrome Quadrasorb SI instrument and the data analysis was performed using the Quantachrome QuadraWin software (Version 5.05). Prior to physisorption measurements, the carbon samples were degassed under vacuum at 150 °C for 20 h. The specific surface area was calculated by the multi-point Brunauer–Emmett–Teller (BET) method at 0.05 ≤ p p0–1 ≤ 0.2 while the total pore volume was determined at a relative pressure of ≈0.97. The pore size distribution was obtained by the Quenched Solid Density Functional Theory (QSDFT) equilibrium model. The reported micropore volume is the cumulative pore volume at a diameter of 2 nm obtained from the QSDFT method. X-ray photoelectron spectroscopy (XPS) was performed with a PHI 5600 CI spectrometer (Physical Electronics) equipped with a hemispherical analyzer. A pass energy of 29 eV and Al Kα1 radiation (1.4867 keV at 350 W) with a neutralizer for charge compensation was used. The samples were transferred from an Ar-filled glove box using a special transfer chamber.


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All spectra were calibrated to C-H (284.8 eV) in the C 1s level as reference. A Gaussian function and basic linear background subtraction was used for peak fitting. Ultraviolet-visible (UV-Vis) absorption spectroscopy was conducted to analyze the relative concentration for the LiPS adsorption of MPC, NMPC and NSMPC. For such purpose, inside an Ar-filled glove box, 1 ml of 0.1 M Li2S6 in DOL:DME (1:1 v/v) solution29 was added to vials containing 10 mg of the carbon samples. The sealed vials were shaken for 6 h. In order to decant the carbon material, the vials were kept steady for 14 h. Then, the supernatant Li2S6 solutions were collected, diluted 15 times and analyzed by UV-Vis spectroscopy (SPECORD 250 UV-Vis spectrophotometer, Analytik Jena) at 415 nm. 0.1 M Li2S6 solution was used as a control. The adsorption of NMPC and NSMPC were normalized to the MPC adsorption. For SEM analysis of the lithium anode surfaces after 100 discharge–charge cycles at 0.5 C, the cells were disassembled and the remaining electrolyte was quickly removed by washing the lithium anode with a DOL:DME (1:1 v/v) solution inside a glove box. DFT calculations. The density functional theory calculations were performed with the allelectron full-potential local-orbital (FPLO) code, version 14.00-47,40 using the scalar relativistic mode and free boundary conditions (molecular mode). The exchange-correlation functional was approximated with the Perdew-Burke-Ernzerhof (PBE) parameterized form of the generalized gradient approximation (GGA).41 To design N-doped and N/S-co-doped graphene edges, we considered graphene flakes with 38 to 47 C atoms in various configurations. Dangling C bonds were passivated by H atoms. N and S atoms were doped at or close to the flake edges in configurations as considered earlier in Ref. 33. Geometry optimization was carried out using space-group symmetry P1. For each configuration of LiPSs interacting with doped graphene, two typical initial geometries were chosen, except for


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the case of Li2S6 interacting with N/S-co-doped graphene, where only one initial geometry was considered due to restrictions in computational resources. Thereby, two sets of calculations were run for each initial geometry. In the first set, geometry optimization was performed by fixing the geometry of the doped graphene flakes and only the LiPS structure was allowed to relax. In the second set, structure optimization was implemented by relaxing the full geometry. For some structures, the restricted optimization resulted in isomers with lower energy than those found in the full optimization. In the results section, the respective lowest-energy isomer structure is given. We observed that the binding energies of the other isomers are typically 0.1–0.2 eV lower than those presented. The interaction between LiPSs and DOL/DME was studied by full geometry relaxation only. A broadening temperature of 1000 K was used in the initial relaxations, but reduced to 100 K finally. The predicted binding energies were determined by using a broadening temperature of T = 100 K. RESULTS AND DISCUSSION Nitrogen and sulfur dual-doped mesoporous carbon (NSMPC) was prepared by carbonization of 1-allyl-2-thiourea (ATU) using SiO2 nanoparticles as hard template. The N/S-containing carbon precursor was firstly deposited onto the SiO2 nanoparticles via the dissolutionimpregnation-precipitation of ATU in the presence of the SiO2. With this route a uniform ATU coating is formed onto the SiO2 template which served as a sole carbon precursor and acts as an N and S source. Further carbonization of the silica/ATU solid mixture and subsequent removal of the silica template resulted in N/S-co-doped porous carbon with well-defined mesoporosity and large pore volume. For comparison, undoped mesoporous carbon (MPC) and N-doped mesoporous carbon (NMPC) were prepared following the above-mentioned procedure but using,


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respectively, resorcinol-formaldehyde resin and N-allylurea as carbon precursors (Further details in Experimental Section). The physical properties of MPC, NMPC and NSMPC were characterized by nitrogen physisorption measurements (Figure 1). As shown in Figure 1a, the three samples display type IV isotherms with well-pronounced hysteresis loops at relatively high pressures (p p0–1 > 0.6), which is characteristic for mesoporous materials.42 The isotherms also exhibit a steep increase in the nitrogen uptake at low relative pressures (p p0–1 < 0.1), indicating the presence of micropores. The specific surface areas and the total pore volumes were determined to be as high as 780 m2 g– 1

and 1.95 cm3 g–1, respectively. Further details are summarized in Table S1. The pore size

distributions calculated by the Quenched Solid Density Functional Theory (QSDFT) equilibrium model (Figure 1b) reveal the presence of micropores and mesopores with sizes around 1.3 and 12.4 nm, respectively (Table S1). The low-magnification TEM image in Figure 1c shows the typical morphology of the NSMPC which exhibits a homogeneously distributed sphere-like porous network, while the high-magnification TEM image (inset in Figure 1c) further confirms the presence of mesopores and is in good agreement with the SiO2 template size (Ø ≈ 12 nm). The predominant porosity and the large pore volume of the three mesoporous carbon materials are essential to physically contain a large amount of soluble LiPS intermediates as well as to facilitate lithium ion diffusion.26,

28, 38

In accordance with the energy-dispersive X-ray

spectrometry (EDXS) elemental mapping obtained from the SEM image in Figure 1d, N and S heteroatoms are uniformly incorporated within NSMPC, demonstrating a reasonable use of ATU as sole carbon precursor to effectively realize the homogeneous N and S co-doping of the carbon matrix.


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Figure 1. (a) Nitrogen physisorption isotherms and (b) the corresponding pore size distributions of the MPC, NMPC and NSMPC. (c) TEM and (d) SEM images of the pristine NSMPC and (e) the corresponding EDXS mapping obtained from (d).

To get a more detailed information about the surface chemical composition and the chemical state of the elements that exist within MPC, NMPC and NSMPC, X-ray photoelectron spectroscopy (XPS) analyses were conducted (Figure 2). As shown in Figure 2a, the survey spectrum of MPC reveals the presence of only C 1s and O 1s signals at approximately 285 and 532 eV, respectively. In comparison, the survey spectrum of NMPC shows an additional peak at around 400 eV corresponding to the N 1s signal, while the survey spectrum of NSMPC displays N 1s, S 2s and S 2p signals at, respectively, 399, 229 and 165 eV, confirming the successful


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incorporation of both N and S atoms within the NSMPC structure. The N content in NMPC is determined to be 9.9 wt.% (N/C atomic ratio of 10.3 %). Further analysis of the high-resolution N 1s spectrum reveals four different chemical states of the nitrogen atoms into NMPC with binding energy maxima centered at 398.3, 400.4, 401.8 and 404.4 eV, which are assigned to pyridinic N (38.7 %), pyrrolic N (38.9 %), quaternary N (12.9 %) and oxidized N (9.5 %) (Figure 2b). These nitrogen-containing functional groups can chemically adsorb LiPS species via interaction with pyridinic N and pyrrolic N atoms.27, 29, 43-45 In the case of the NSMPC the N and S contents are calculated to be, respectively, 8.9 and 3.7 wt.% (N/C and S/C atomic ratios of 9.2 % and 1.7 %). The N 1s spectrum of NSMPC is fitted by the same four components like the NMPC spectrum, with 40.0, 42.2, 9.5 and 8.3 % of pyridinic N, pyrrolic N, quaternary N and oxidized N, respectively (Figure 2d). Furthermore, the high-resolution S 2p spectrum in Figure 2c suggests the presence of two classified sulfur species: thiophene (73.2 %) is evidenced by two binding energy maxima at 163.9 eV and 165.1 eV originating from the spin-orbit coupling of the electrons in the p-orbital. The S 2p doublets at a binding energy of 167.9 eV are assigned to oxidized sulfur species (26.8 %). This dual-doping in NSMPC is crucial to chemically adsorb soluble LiPS species as determined by LiPS adsorption experiments (Figure 2e and Figure S1). Relative LiPS adsorptivity studies revealed that NSMPC is capable to adsorb about 148 % and 27 % more LiPSs than MPC and NMPC, respectively. Compared with the undoped or single Ndoped mesoporous carbons, the superior ability of NSMPC to adsorb LiPSs may be attributed to the synergetic effect of the adjacent thionic S/pyridinic N (or thionic S/pyrrolic N) groups which enhances the chemical interaction with LiPS species.33,

34

The superior LiPS affinity of the

NSMPC could highly enhance the hybrid separator abilities to intercept-immobilize-reactivate migrating LiPS species and thus improve cell performance and lifespan.


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Figure 2. (a) XP spectra of the surface chemical composition of MPC, NMPC and NSMPC. (b) N 1s XP spectrum of the NMPC. (c) S 2p XP spectrum and (d) N 1s XP spectrum of the NSMPC. (e) Relative LiPS adsorptivity of MPC, NMPC and NSMPC and the respective photograph of LiPS solutions after adsorption for 20 h (diluted in DOL:DME ≈15 times).

To prepare the hybrid separators (HS), a carbon slurry containing 87.5 wt.% mesoporous carbon and 12.5 wt.% PVDF, dispersed in NMP by shaker-milling for 60 min., was coated onto one side of the commercial separator. The photographs in Figure 3a show a flexible NSMPC-HS which exhibits a homogeneous distribution and very stable adhesion of the porous carbon to the surface of the Celgard separator with noticeable structural integrity even after folding and


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crumpling. The thickness of the carbon-coating of the hybrid separators is around 29 µm (Figure 3b and Figure S2), while the mass loading added to the separator is only 0.5–0.6 mg cm–2 (≈ 50 % in weight of the separator). The high surface area, large pore volume and N/S dual-doping of the NSMPC-coating are expected to promote the physical confinement and chemical anchoring of soluble LiPSs in the functional mesoporous cavities, reactivating lithium (poly)sulfide species and improving the electrochemical performance of Li–S batteries (Figure 3c).

Figure 3. (a) Photographs of the mechanically stable NSMPC-HS. (b) Cross-sectional SEM image of the NSMPC-HS. (c) Schematic configuration of the Li–S cell with a NSMPC-HS.

To evaluate the electrochemical performance of the Li–S cells with routine and hybrid separators, various electrochemical experiments were performed (Figure 4). The resistances of the fresh Li–S cells with routine and the hybrid separators were determined by electrochemical impedance spectroscopy (EIS) measurements, as shown in Figure 4a. The Nyquist plots for these cells present a solution resistance at high frequency, a semicircle in the medium frequency range attributed to the charge transfer resistance (RCT) and a straight line at low frequency as a result of solid state diffusion resistance.38 The cell with the NSMPC-HS presents the lowest RCT (51 Ω)


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which is almost four times lower than the RCT of the Li–S cell without carbon coating of the separator (188 Ω), while the cells with MPC-HS and NMPC-HS also show low RCT (81 and 63 Ω, respectively). This notable reduction of the cathode resistance for those cells with hybrid separators is the result of the high electrical conductivity of the carbon-coatings, which play the role of pseudo-current collectors, lessening the internal impedance of the cells.26, 46 Furthermore, it seems that the dual-doping of the NSMPC-HS facilitates the charge transport through the interface of the cathode and the carbon on the separator compared with both NMPC and undoped MPC coatings, implying lowest internal cell resistance for NSMPC-HS.34 Figure 4b shows the comparative discharge/charge voltage profiles corresponding to the second cycle at 0.2 C (1 C = 1672 mA g–1) for Li–S cells with routine and hybrid separators. During the discharge process, all cells display two main reductions plateaus at ≈2.30 and ≈2.08 V corresponding to the characteristic two-step sulfur reduction reactions.38, 47 The discharge plateau at higher potential is assigned to the reduction of octet sulfur (S8) to soluble long-chain LiPSs (Li2Sx, 4 ≤ x ≤ 8) and the discharge plateau at lower potential is related to the further reduction of long-chain LiPSs to short-chain LiPSs (Li2Sx, 1 < x < 4) and Li2S.48 The overlapping plateaus at ≈2.26 and ≈2.38 V are based on the reversible oxidation reactions of short-chain LiPSs to long-chain LiPSs and elemental sulfur, respectively. Further analysis reveals that, despite using similar carbon/sulfur ratios for all cells, the Li–S cell with NSMPC-HS shows larger voltage plateaus and lower polarization (∆E) at 50 % depth of discharge (DOD) than those cells with NMPC-HS, MPC-HS and the routine separator (Figure 4b).26,

49

This result confirms the observations from EIS

measurements since the lowest resistance was observed with NSMPC-HS separators and thus the polarization (∆E) of the NSMPC-HS is lowest at comparable current rates. As expected, the voltage polarization becomes more pronounced at higher current rates (Figure 4c and Figure S3)


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highlighting enhanced redox reaction kinetics with NSMPC-HS. The cycle performance of the Li–S cells with routine and hybrid separators was evaluated by galvanostatic cycling experiments at a current rate of 0.2 C. As shown in Figure 4d, the cell containing the NSMPC-HS displays the higher initial discharge capacity—1267 mAh g–1 in contrast to 1256, 1073 and 739 mAh g–1 for those cells with NMPC-HS, MPC-HS and routine separator, respectively—and the better capacity retention after 100 cycles with a value of 889 mAh g–1 in comparison with the 796, 693 and 387 mAh g–1 retained for those cells with, respectively, NMPC-HS, MPC-HS and routine separator. This demonstrates that the NSMPC-coating highly improves the active material utilization by the early and effective trapping of soluble LiPS species into its conductive mesoporous framework. Furthermore, all cells with the hybrid separators exhibit a Coulombic efficiency (CE) close to 100 %, while the conventional Li–S cell reveals a lower CE of approximately 97 %. Furthermore, the utilization of the NSMPC-HS also shows improved specific capacity and CE even in the absence of the LiNO3 additive in the electrolyte (Figure S4), evidencing the effectiveness of the chemical interaction to limit the LiPS shuttle. Note that the coating layer of the NSMPC-HS remains well-adhered to the Celgard membrane after cycling (Figure S5) without visible agglomerations of nonconductive sulfur-based species (Figure S6), demonstrating excellent mechanical strength to accommodate the sulfur volume changes over cycling and thus maintain the good integrity of the coating to further reactivate the migrating active material. Such improvement of the sulfur utilization is also well reflected by the evaluation of the longterm cycling stability conducted at 0.5 C for Li–S cells with MPC-HS, NMPC-HS and NSMPCHS (Figure 4e). The first cycle at 0.5 C for the cell with NSMPC-HS delivers a discharge capacity of 934 mAh g−1 and it retains 740 mAh g−1 after 500 cycles, corresponding to an


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ultralow capacity degradation of 0.041 % per cycle. This cycling performance is much better than that of the cells with MPC-HS and NMPC-HS which display retained capacities (capacity degradation per cycle in parentheses) of, respectively, 481 mAh g−1 (0.099 %) and 600 mAh g−1 (0.076 %) after 500 cycles. It is worth mentioning that the pristine NSMPC contributes only little capacity probably because of the double layer capacitance or the reversible reaction of lithium with the functional groups, delivering a negligible reversible capacity of around 16 mAh g−1 at a C-rate of 0.2 (Figure S7). In addition, the limited diffusion of LiPS intermediates to the anode side and the improved active material utilization by using hybrid separators result in a somewhat indirect protection of the lithium anode by reducing side reactions, as demonstrated by ex-situ lithium anode surface analysis (Further information in Figure S8).26, 28, 33


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Figure 4. (a) Comparative EIS spectra (fresh cells), (b) galvanostatic discharge/charge voltage profiles (2nd cycle at 0.2 C) and (c) comparative analysis of the potential difference (∆E) between the charge/discharge plateaus at different current densities of Li–S cells with routine separator, MPC-HS, NMPC-HS and NSMPC-HS (d) Cycling performance at C-rate of 0.2 and (e) long-term measurement at a C-rate of 0.5 C for Li–S cells with different hybrid separators.


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In order to improve the energy density of Li–S cells and to evaluate the advantages of using a NSMPC-HS under practical conditions, the loading of active sulfur on the electrode was substantially increased. Due to the serious problem in structural integrity of high-loading sulfur cathodes typical prepared by slurry coating process onto Al current collectors, a mechanically stable sulfur-carbon cathode foil with S:SPC:PTFE weight ratio of 55:35:10 and areal sulfur loading of 5.4 mg cm–2 was produced (Figure S9).50 Figure S10, shows the discharge/charge voltage profiles of the cell with a NSMPC-HS acquired between 1.8–2.6 V at C-rates of 0.1, 0.2 and 0.5. As expected, the higher areal sulfur loading in the cathode increases the internal resistance of the cell, causing the increases of the ∆E and the decreases of the specific discharge capacity, compared to the cell set-up with lower mass of sulfur on the electrode and a hybrid separator (Figure 4b). However, the cell with high mass loading of sulfur still reveals a good reversibility and active material utilization in comparison to the cell containing a routine separator (Figure 4d). Furthermore, despite the use of a non-infiltrated sulfur cathode, the highsulfur-loading cell with a NSMPC-HS demonstrates a notable cycling stability with a remarkably high areal capacity of 5.9, 4.4 and 2.9 mAh cm–2 at 0.1, 0.2 and 0.5 C, respectively (Figure 5). The high areal capacity delivered by the cell is not only one of the highest reported for this type of Li-S cell so far27-29 but also exceeds the areal capacities of typical Li-ion batteries and conventional Li–S cells with toilsome-manufactured but expensive sulfur-carbon composite cathodes (Figure 5).44, 51-53


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Figure 5. Combined cycling performance of the Li–S cell with a NSMPC-HS and a conventional sulfur/carbon mixture as cathode with an areal sulfur loading of 5.4 mgS cm–2 cycled at current rates of 0.1, 0.2 and 0.5 C.

To better understand the mechanism by which N and S dopants can promote chemical interactions to sulfur-based species, DFT calculations were performed particularly for N-doped and N/S-co-doped graphene flakes interacting with high-order LiPS intermediates. Recently, Zhou et al.33 conducted a series of DFT calculations using Li2S molecule formed at the end of the discharge—as simplified model to study its interaction with N/S-co-doped carbon host materials. While they found strong binding affinities for Li2S in the vicinity of adjacent thionic S/pyridinic N (or thionic S/pyrrolic N) functionalities, the high polarization of this small molecule may overestimate the affinity of bigger LiPS species to the carbon scaffolds. We think that an accurate study by using high-order LiPSs could be more appropriate to our cell configuration because the main role of the hybrid separator takes place in the carbon-coating/cathode interface during the initial stage of the discharge, where the generation and diffusion of dissolved LiPSs to the anode side are more significant. To verify our experimental observations we performed DFT


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calculations to systematically investigate the interaction behavior of high-order LiPSs (Li2S4 and Li2S6) with most commonly used electrolyte (DOL and DME) and doped graphene flakes (Ndoped and N/S-co-doped). From these calculations, we predict the most likely adsorption configuration of LiPSs with doped graphene flakes. Li2S8 was not considered in the present investigation, because a recent study shows that Li2S8 is kinetically unstable.45 The structures of LiPSs, DOL, and DME were optimized. To benchmark our calculations, we compared the LiPSs bond lengths and found them to be in good agreement with recently reported DFT values (Figure S11a and S11b),45 the mean deviation between both sets of calculated bond length being below 0.02 Å. First, we examined the interaction of LiPSs with the most commonly used electrolytes DOL and DME (Figure S11c and S11d). We found that the LiPSs bind mainly through a Li-O bond with the DOL and DME electrolyte molecules (Figure S12). The predicted binding energies of Li2S4 and Li2S6 with DOL (DME) are 0.80 (0.69) eV and 0.82 (0.70) eV, respectively. Based on previously reported binding energy values of Li2S4 and Li2S6 with graphene (0.75 eV and 0.87 eV, respectively),45 we noticed that the binding energy of Li2S6 with graphene is larger than with the considered electrolytic solvent molecules. For Li2S4, the binding with graphene is somewhat stronger (weaker) than with DME (DOL). These results suggest that at least a part of the high-order LiPSs will remain in a dissolved state instead of binding with graphene. Next, we determined the interaction of LiPSs with three configurations of N-doped graphene: pyrrolic N, pyridinic N (Figure 6a–d), and quarternary N (figure not shown). Note that the optimized configurations shown in Figure 6 represent fragment versions around the heteroatoms, while the complete configurations obtained in the calculations are shown in Figure S13–S15. The optimized structures of N-doped graphene flakes with LiPS are buckled for all four cases. We


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found that both Li2S4 and Li2S6 are stronger bound to pyrrolic N (≈1.3 eV) than to pyridinic N (≈0.85 eV)(Figure 6a–d and Table S2). To check the influence of additional S-doping, we optimized the geometry of LiPSs interacting with six doping configurations: thionic S/pyrrolic N, thionic S/pyridinic N, thiophenic S/pyrrolic N, and thiophenic S/pyridinic N (Figure 6e-l; Figure S14 and Figure S15) as well as thionic S/quarternary N and thiophenic S/ quarternary N (Figure S16). The predicted binding energy of Li2S4 with N/S-co-doped graphene flakes is highest for the thionic S close to pyrrolic N configuration (1.50 eV, Figure 6e). Additionally, Li2S4 exhibits strong binding with thionic S/pyridinic N (1.42 eV, Figure 6f) and thiophenic S/pyrrolic N (1.36 eV, Figure 6i). On the other hand, Li2S6 exhibits the strongest binding interaction with thiophenic S/pyrrolic N functionalities (1.46 eV, Figure 6k). Moreover, Li2S6 also shows appreciable binding with thionic S nearby pyrrolic N (1.42 eV, Figure 6g) and pyridinic N (1.34 eV, Figure 6h). According to our calculations, quaternary N (with or without additional S-doping) also could interact with high-order LiPSs (Figure S16; results without S-doping not shown). However, these interactions are weaker than the binding between high-order LiPSs with DOL or DME (Figure S12 and Table S2). For all investigated cases, the binding energies of Li2S4 and Li2S6 with different electrolytes or doping sites at the edge of a graphene flake agree with each other within 0.1 eV. This mutual agreement suggests that the existence of isomers with essentially higher binding energy is not likely. Comparing the binding energies between the cases of pure N-doping and of N/S-co-doping, see Table S2, we find that co-doping yields a significant enhancement of the binding energy (by more than about 0.1 eV) in five of the eight cases depicted in Figure 6. These five cases include


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three cases with pyrrolic N and two with pyridinic N. A modest enhancement of the binding energy by co-doping (by 0.03–0.05 eV) is found in the three remaining cases. For quarternary N (configurations not shown), co-doping by S (Figure S16) yields an enhancement of the binding energies by up to about 0.3 eV. Overall our DFT results clearly illustrate that the binding energy of LiPSs is significantly higher for N/S-co-doped graphene flakes than for N-doped graphene configurations, which is consistent with our experimental observations.


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Figure 6. DFT optimized structures and binding energies representing the most stable Li2S4 and Li2S6 binding configurations after relaxation of doped graphene flake structures. Li2S4 binding to a) pyrrolic N, b) pyridinic N, e) thionic S/pyrrolic N, f) thionic S/pyridinic N, i) thiophenic S/pyrrolic N and j) thiophenic S/pyridinic N. Li2S6 binding to c) pyrrolic N, d) pyridinic N, g) thionic S/pyrrolic N, h) thionic S/pyridinic N, k) thiophenic S/pyrrolic N, and l) thiophenic S/pyridinic N. Gray, blue, purple, yellow, and white balls represent C, N, Li, S and H atoms, respectively.


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All things considered, we attribute the enhanced performance of the Li–S cells with a NSMPC-HS to the exceptional features of the N/S dual-doped carbon-coating which provides a synergistic effect to strongly interact with LiPS species, immobilizing them in the cathode side for further reactivation and, thereby, magnifying the distinctive properties of this type of advanced separators.

CONCLUSIONS Throughout this work, we have demonstrated that the integration of an N-allylthiourea-derived N and S dual-doped mesoporous carbon barrier as a coating on a commercial polypropylene separator greatly increases the affinity to LiPS intermediates and keeps them on the cathode side. LiPS adsorption experiments and DFT calculations demonstrated that the N/S-co-doped mesoporous carbon possess a stronger LiPS chemisorption over N-doped and undoped mesoporous carbons. As a result of (i) the synergistic effect of N/S dual-doping to effectively bond LiPS species, (ii) the large electrochemically active surface area and (iii) the good conductivity of the NSMPC, the advanced Li–S cell with a functional NSMPC-coated separator demonstrates a superior high initial capacity of 1267 mAh g–1 at a current rate of 0.2 C and ultralow capacity degradation of 0.041 % per cycle for over 500 cycles at a C-rate of 0.5. In addition, the Li–S cell with the functional hybrid separator and a simple-designed pure cathode with an increased areal sulfur loading of 5.4 mg cm–2 can deliver a remarkable areal capacity of 5.9 mAh cm–2. The findings of this work would be valuable for the rational design of multifunctional hybrid separators with a critical role on the commercialization of high-performance Li–S batteries.


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ASSOCIATED CONTENT Supporting Information. Comparative textural properties and polysulfide adsorptivity of MPC, NMPC and NSMPC. SEM images and EDX spectra of MPC-HS, NMPC-HS and cycled Li anodes. Photograph and SEM images of the cycled routine separator and the cycled NSMPC-HS. Discharge/charge voltage profiles of cells with MPC-HS, NMPC-HS, NSMPC-HS and high S loading cathode. CV and cycling performance of NSMPC/PVDF cathode. Optimized configurations of Li2S4 and Li2S6 interacting with DOL, DME and doped graphenes. Supporting information for this article is given via a link at the end of the document. AUTHOR INFORMATION Corresponding Author: [email protected] (J. Balach) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conflict of Interest The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank A. Voß, A. Voidel and R. Buckan for their valuable technical support. We gratefully acknowledge financial support from the German Federal Ministry of Education and


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Research (BMBF) through the Excellent Battery – WING center “Batteries - Mobility in Saxony” (Grant Nos. 03X4637B and 03X4637C).

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Synergistically Enhanced Polysulfide Chemisorption Using a Flexible

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