Covalent Confinement of Sulfur Copolymers onto Graphene Sheets

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Covalent Confinement of Sulfur Copolymers onto Graphene Sheets Affords Ultrastable Lithium–Sulfur Batteries with Fast Cathode Kinetics Junpeng Ma, Jingbiao Fan, Shang Chen, Xinyue Yang, Kwun Nam Hui, Hongwen Zhang, Christopher W. Bielawski, and Jianxin Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00214 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Materials & Interfaces

Covalent Confinement of Sulfur Copolymers onto Graphene Sheets Affords Ultrastable Lithium−Sulfur Batteries with Fast Cathode Kinetics

Junpeng Ma,†,# Jingbiao Fan,# Shang Chen,# Xinyue Yang,# Kwun Nam Hui,‡ Hongwen Zhang,† Christopher W. Bielawski,§,¶ and Jianxin Geng*,#

†

Experimental Teaching Center, School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu Province 213100, China #

College of Energy, State Key Laboratory of Organic−Inorganic Composites, Beijing University of

Chemical Technology, 15 North Third Ring East Road, Chaoyang District, Beijing 100029, China ‡

Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau 999078, China

§

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea

¶

Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea * E-mail: [email protected]

KEYWORDS: sulfur copolymers, graphene, covalent binding, cathode kineties, lithium−sulfur batteries

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ABSTRACT: Lithium−sulfur (Li−S) batteries have received significant attention due to the high theoretical specific capacity of sulfur (1675 mA h g−1). However, the practical applications are often handicapped by sluggish electrochemical kinetics and the “shuttle effect” of electrochemical intermediate polysulfides. Herein, we propose an in-situ copolymerization strategy for covalently confining a sulfur-containing copolymer onto reduced graphene oxide (RGO) to overcome the aforementioned challenges. The copolymerization was performed by heating elemental sulfur and isopropenylphenyl-functionalized RGO to afford a sulfur-containing copolymer, i.e., RGO-g-poly(S-rIDBI), which is featured with a high sulfur content and uniform distribution of the poly(S-r-IDBI) on RGO sheets. The covalent confinement of poly(S-r-IDBI) onto RGO sheets not only enhances the Li+ diffusion coefficients by nearly one order of magnitude, but also improves the mechanical properties of the cathodes and suppresses the shuttle effect of polysulfides. As a result, the RGO-g-poly(S-r-IDBI) cathode exhibits an enhanced sulfur utilization rate (10% higher than elemental sulfur cathode at 0.1 C), an improved rate capacity (688 mA h g−1 for RGO-g-poly(S-r-IDBI) cathode vs. 400 mA h g−1 for elemental sulfur cathode at 1 C), and high cycling stability (a capacity decay of 0.021% per cycle, less than one tenth of that measured for elemental sulfur cathode).


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INTRODUCTION High-density and stable energy storage systems are urgently needed to satisfy the increasing market demands due to the developments of electric vehicles and portable electronic devices. Lithium−sulfur (Li−S) batteries have drawn considerable attention in both academic and industrial communities for a variety of reasons, including the high theoretical energy density of sulfur (2600 Wh kg−1), the high abundancy of sulfur, low cost, and good environmental compatibility.1-4 Despite these attractive advantages, the practical uses of Li−S batteries are handicapped by a number of deleterious drawbacks in the electrochemical processes. For example, the high solubilities of polysulfides (Li2Sn, 4 ≤ n ≤ 8) result in the so-called ‘shuttle effect’ as they diffuse between the cathode and anode. The poor electrical conductivities of sulfur and the discharged products (Li2S2 and Li2S) result in sluggish redox kinetics. Physical pulverization of sulfur-based cathode is often experienced due to significant volume changes that can exceed 80% as calculated from the densities of sulfur and the discharged products. Electrode pulverization results in the formation of isolated domains of electroactive material that effectively becomes inaccessible in subsequent charge/discharge cycles. Collectively, while the shuttle effect and cathode pulverization result in deteriorated cycling stabilities, the sluggish electrochemical redox kinetics leads to poor rate capabilities.5-8 Following Nazar’s pioneering work that utilized ordered mesoporous carbons to encapsulate sulfur,2 a variety of materials including hierarchical porous carbons,5, 9-10 carbon nanotubes/fibers,11-13 graphene/graphene oxide (GO),14-17 conducting polymers,18-21 MXenes,22-23 and transition metal oxides/sulfides24 have been extensively investigated to address the issues described above. Moreover, some of these materials, such as transition metal particles (e.g., Pt and Ni)25-26 and transition metal sulfides (e.g., MoS2 and ZnS),27-29 were found to catalyze battery electrochemistry. Although these approaches offer promising solutions to the fundamental problems associated with Li−S batteries, the 3 ACS Paragon Plus Environment


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practical demands of contemporary batteries have not yet to be satisfied, primarily because the physical confinement and chemical absorption that are used to prepare the sulfur-based electroactive materials are inadequate to suppress the shuttle effect. In addition, the use of transition metal compounds may also lead to increased costs and/or relatively heavy devices.30-31 Sulfur-containing polymers have recently been shown to effectively suppress the shuttle effect of polysulfides by covalently bonding the sulfur components to polymeric frameworks.32-34 We recently demonstrated that forming covalent C−S bonds between sulfur nanoparticles and carbon frameworks enhances the long-term cycling stability of the Li−S batteries prepared using these materials as cathodes.5,

35-36

Sulfur-containing polymers can be synthesized either by copolymerizing elemental

sulfur with vinylic monomers using “inverse vulcanization” methodology37-40 or by ring-forming reactions between the nitrile groups of poly(acrylonitrile) and elemental sulfur.41-42 However, despite these advances, the intrinsic insulating nature of sulfur-containing polymers remains challenging, and thereafter leading to a poor rate capacity of the Li−S batteries.43-46 Although a few efforts were attempted to graft sulfur-containing polymers or sulfur chains to graphene sheets,47-51 the fundamental issues faced by sulfur-containing polymers as cathode materials for Li−S batteries were not completely resolved. In this context, along with the structural design of cathode materials, the mechanical and electrochemical stabilities and the electrochemical redox kinetics in the electrochemical processes should be addressed. We hypothesized that GO may serve as an ideal substrate for the design of sulfur-based cathode materials due to its diverse functionalities coupled with the high electrical conductivity and large specific surface area of its reduced form (RGO). Herein, we report an in-situ copolymerization strategy for covalently grafting a copolymer of sulfur and 3-isopropenyl-α,α-dimethylbenzyl isocyanate (IDBI) (designated as poly(S-r-IDBI)) onto RGO sheets (designated as RGO-g-poly(S-r-IDBI)) (see Figure 1) 4 ACS Paragon Plus Environment

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as cathode for Li−S batteries. Compared with the reports that used carbon nanotubes as frameworks,11-12, 36

the composites of this research offered the following features: (1) an electrically-conductive

framework based on RGO, (2) high grafting efficiencies due to the large quantity of functional groups on GO (the precursor to RGO), and (3) a uniform distribution of the poly(S-r-IDBI) on RGO sheets due to the large specific surface area of RGO. In return, the covalently grafted poly(S-r-IDBI) enhanced the compatibility of the RGO sheets with the electrolyte. As a result, the cathodes fabricated from RGO-gpoly(S-r-IDBI) were found to exhibit high charge transport properties (e.g., Li+ diffusion coefficients were increased by nearly one order of magnitude when compared to the analogues prepared using elemental sulfur), a suppressed shuttle effect, and good mechanical properties. Moreover, the cathodes fabricated from RGO-g-poly(S-r-IDBI) offered an enhanced sulfur utilization rate (10% higher than elemental sulfur cathode at 0.1 C) and an increased rate capacity (688 mA h g−1 vs. 400 mA h g−1 for elemental sulfur cathode at 1 C). In particular, our RGO-g-poly(S-r-IDBI)-based cathodes exhibited outstanding long-term cycling stability as a small capacity decay of 0.021% per charge/discharge cycle was measured over 500 cycles at 1 C (less than one tenth of that recorded for elemental sulfur cathode).

RESULTS The synthetic strategy used to prepare RGO-g-poly(S-r-IDBI) is illustrated in Figure 1a. GO sheets were first grafted with isopropenylphenyl groups through the reactions of the highly reactive −NCO group of IDBI with the hydroxyl and carboxylic acid groups on GO sheets.52-53 The successful functionalization of GO sheets was approved by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. While the functionalized GO sheets were found to be thicker than the starting material (Figure S1), the number of grafted isopropenylphenyl groups was estimated from XPS data to be one per 14 basal carbon atoms (Figure S2). Next, the functionalized GO was reduced using 5 ACS Paragon Plus Environment


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hydrazine and standard protocols.54 The resulting product (i.e., RGO-g-IDBI) displayed XPS and FT-IR signals that were characteristic of the proposed structure and consistent with the formation of covalent linkages between the isopropenylphenyl groups and the RGO sheets (Figures S2 and S3).

Figure 1. An overview of the covalent confinement methodology and the advantages realized therefrom. (a) Synthesis of RGO-g-IDBI and RGO-g-poly(S-r-IDBI). (b) Optical images recorded for RGO, elemental sulfur, poly(S-r-IDBI), and RGO-g-poly(S-r-IDBI) each dispersed in dimethoxymethane (1 mg mL−1). (c) An illustration of a Li−S battery containing RGO-g-poly(S-r-IDBI) as a cathode material.


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Copolymerization of elemental sulfur and RGO-g-IDBI was achieved by heating their mixture at 175 C for 30 min. Thermal cleavage of the S8 rings should generate sulfur radicals that initiated the copolymerization reaction with the isopropenylphenyl groups in a random fashion to afford a composite (designated as RGO-g-poly(S-r-IDBI)). As a control experiment, the random copolymerization of sulfur and IDBI (with the corresponding product designated as poly(S-r-IDBI)) was also performed at the same ratio of sulfur to IDBI used to prepare RGO-g-poly(S-r-IDBI). The covalent grafting of poly(S-rIDBI) to RGO sheets effectively altered the properties of the composite produced. Compared with poly(S-r-IDBI), RGO-g-poly(S-r-IDBI) was more flexible (Figure S4). Moreover, the surface properties of the RGO sheets changed as RGO-g-poly(S-r-IDBI) was found to be dispersible in the solvents of electrolyte, whereas the dispersibilities of RGO and elemental sulfur were poor (Figure 1b). These findings suggested that cathodes based on RGO-g-poly(S-r-IDBI) should be compatible with organic electrolytes and thus exhibit good charge transport properties.



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Figure 2. Thermal property measurements. (a−c) DSC thermograms recorded for (a) elemental sulfur, (b) RGO-g-poly(S-r-IDBI), and (c) poly(S-r-IDBI). (d) TGA curves recorded for elemental sulfur, RGO-g-poly(S-r-IDBI), poly(S-r-IDBI), and RGO-g-IDBI (indicated).

The thermal properties of the polymeric materials were first investigated by differential scanning calorimetry (DSC). Analysis of elemental sulfur revealed sharp peaks at 48.3 and 117.1 C that were attributed to crystallization and melting, respectively (Figure 2a). In contrast, due to the random lengths of sulfur chains in the sulfur-containing polymer grafted to RGO,39 the crystallization and melting of the sulfur component in RGO-g-poly(S-r-IDBI) composite were found to occur in relatively wide temperature ranges that centered at ca. 43 and 108 C, respectively (Figure 2b). Likewise, the shifts of the measured crystallization and melting temperatures (Tc and Tm) were attributed to copolymer formation and thermal effects associated with the thermally conductive RGO sheets.39,

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The DSC 8

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thermogram recorded for poly(S-r-IDBI) showed a glass transition at ca. 64 C (Figure 2c), consistent with linear-type polymer chains. The melting peak observed at 113.4 C could be ascribed to the segment rearrangement of long sulfur chains in poly(S-r-IDBI).39 For comparison, a glass transition was not identified in the thermogram recorded for RGO-g-poly(S-r-IDBI) due to branched-type structures of the composite, which resulted from the isopropenylphenyl groups immobilized on the surfaces of the RGO sheets. As will be described in more detail below, this was a key feature of RGO-g-poly(S-r-IDBI) as it was found to facilitate charge transport and to enhance polysulfide confinement during charge/discharge cycling. Next, the thermal stabilities of the polymeric materials were studied by TGA (Figure 2d). While elemental sulfur underwent sublimation from 140 to 275 C, higher temperatures (ca. 34 C higher than elemental sulfur) were required to observe significant weight loss for poly(S-r-IDBI), presumably because the constituent sulfur atoms were covalently bonded to a polymer network.34, 55 By contrast, TGA data recorded for RGO-g-poly(S-r-IDBI) revealed that the material experienced weight loss at a lower temperature than that of poly(S-r-IDBI). This finding is ascribed to the thermally conductive property of RGO, which facilitates the thermal transport in RGO-g-poly(S-r-IDBI) during TGA test.5, 5659

The sulfur content of RGO-g-poly(S-r-IDBI) was estimated to be 62.3 wt% from the TGA data.



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Figure 3. XRD patterns recorded for elemental sulfur, RGO-g-poly(S-r-IDBI), and poly(S-r-IDBI) (indicated).

The crystallographic structures of the products were analyzed by XRD (Figure 3). Elemental sulfur exhibited characteristic peaks between 10−60° corresponding to the orthorhombic fddd phase, with the three strongest peaks at 23.2, 25.9 and 27.9° assigned to the (222), (026) and (040) planes, respectively.5 Poly(S-r-IDBI) exhibited no characteristic Bragg reflections, consistent with an amorphous feature due to the copolymerization of sulfur and IDBI.34 In contrast, a very weak (222) Bragg reflection was detected in the XRD pattern recorded for RGO-g-poly(S-r-IDBI), reflecting low crystalline feature of sulfur in the composite. The finding was consistent with the DSC data collected for RGO-g-poly(S-rIDBI) wherein melting phenomenon was detected.


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Figure 4. Molecular structure characterization. (a) FT-IR spectra recorded for elemental sulfur, RGO-gpoly(S-r-IDBI), and poly(S-r-IDBI) (indicated). (b) Raman spectra recorded for RGO-g-IDBI, RGO-gpoly(S-r-IDBI), and poly(S-r-IDBI) (indicated). (c) A XPS survey spectrum, (d) a C 1s XPS spectrum, and (e) an S 2p XPS spectrum recorded for RGO-g-poly(S-r-IDBI).

To elucidate the molecular structure of RGO-g-poly(S-r-IDBI), a variety of spectroscopic methods were employed. The FT-IR spectrum recorded for elemental sulfur showed a S−S vibration signal at ca. 471 cm−1 (Figure 4a). However, upon copolymerization, a new signal appeared at ca. 671 cm−1, which was attributed to the stretching vibration of C−S bonds in poly(S-r-IDBI) and RGO-g-poly(S-r-IDBI). The presence of C−S bonds further supported that the formation of sulfur-containing copolymers via copolymerization of sulfur either with IDBI molecules or with the isopropenylbenzyl groups grafted on RGO sheets. The covalent attachment of poly(S-r-IDBI) chains to the RGO sheets was indisputable in the RGO-g-poly(S-r-IDBI) composite since the requisite isopropenylphenyl groups were covalently 11 ACS Paragon Plus Environment


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grafted to RGO sheets in RGO-g-IDBI. In Figure 4b are displayed the Raman spectra recorded for RGO-g-IDBI, RGO-g-poly(S-r-IDBI), and poly(S-r-IDBI). The RGO-g-IDBI exhibited two characteristic peaks at 1352 and 1578 cm−1, which corresponded to the D and G bands, respectively. The D band was ascribed to defects and edges of the RGO sheets, whereas the G band was originated from the conjugated domains of sp2 hybridized carbons. For comparison, the RGO-g-poly(S-r-IDBI) exhibited a G band at 1591 cm−1, which was 13 cm−1 high-frequency shifted with respect to the value measured for RGO-g-IDBI due to the electron transfer from RGO to the more electronegative sulfur atoms.60-62 In control test, there was only one signal detected at ca. 1344 cm−1 corresponding to the symmetric stretching vibration of isocyanate group in the Raman spectrum recorded for poly(S-r-IDBI), consistent with the feature signal at 1370 cm−1 in FT-IR spectrum.63 Further evidence for the presence of C−S bonds in the RGO-g-poly(S-r-IDBI) was provided by XPS measurement. A XPS survey spectrum revealed the presence of C, S, O, and N (Figure 4c), consistent with the proposed molecular structure of the composite (Figure 1a). In Figure 4d is shown the C 1s XPS spectrum recorded for RGO-g-poly(S-r-IDBI). Upon deconvolution, a number of characteristic peaks were found at 284.8, 285.5, 286.7, and 288.9 eV, corresponding to C−C/C=C, C−N/C−S, C−O, and N−C=O/N−COO species, respectively. In Figure 4e is shown the S 2p XPS spectrum of RGO-g-poly(S-r-IDBI). The two dominated peaks at 163.7 and 164.9 eV with an energy separation of 1.2 eV were ascribed to the S 2p3/2 and S 2p1/2 doublet, respectively. The binding energy of the S 2p3/2 peak (163.7 eV) was found to be lower than that of elemental sulfur (164.0 eV), supporting the presence of C−S bonds in RGO-g-poly(S-r-IDBI).5,

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The signals at 167.7 and 168.9 eV were

ascribed to the S 2p3/2 and 2p1/2 doublet of sulfate, which may stem from the partial oxidization of sulfur during the copolymerization and the following processing operations.


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Figure 5. Electrochemical characterization of the Li−S batteries prepared from RGO-g-poly(S-r-IDBI) and S/RGO-g-IDBI as cathodes. (a) CV data. (b) Charge/discharge profiles recorded at different current densities. (c) Rate capability data. (d) Cycling performance data. (e) Charge/discharge profiles recorded at 0.2 C over 100 cycles. (f) EIS plots. (g) The plots of Ip as function of 1/2. (h) Long-term cycling tests at 1 C over 500 cycles.

The electrochemical performance of RGO-g-poly(S-r-IDBI) as cathode for Li−S batteries was examined using coin-type batteries. Poly(S-r-IDBI) and S/RGO-g-IDBI as control samples were also used for the preparation of batteries. Preliminary electrochemical measurements indicated that the batteries prepared from poly(S-r-IDBI) exhibited low electrochemical stability due to its high solubility 13 ACS Paragon Plus Environment


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in electrolyte.39 The result may be attributed to the monofunctionality of IDBI which can be expected to afford polymers with low crosslinking degrees. Indeed, multifunctional monomers are typically required to prepare polymeric networks.37-39 Therefore, subsequent attention was directed toward studying the other two cathodes in more details. Cyclic voltammetry (CV) was independently performed on the cathodes prepared from RGO-gpoly(S-r-IDBI) or S/RGO-g-IDBI at 0.1 mV s−1, and the data indicated that the two cathodes exhibited the same electrochemical conversions (Figure 5a). In the cathodic scan, two reduction peaks were observed and assigned to the multistep reductions of sulfur, i.e., the conversion of S8 to lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) and further conversion to Li2S2 and Li2S, respectively. In the subsequent anodic scan, two oxidation peaks were detected and assigned to the oxidation of Li2S/Li2S2 to Li2Sn (n>2) and eventually to S8, respectively.9 However, close inspection of the data revealed that the reduction/oxidation signals measured for the RGO-g-poly(S-r-IDBI) cathode appeared sharper than those recorded for the S/RGO-g-IDBI cathode and occurred ahead of those recorded for S/RGO-g-IDBI cathode.64 The relative efficient electrochemical processes displayed by the former were also evident in the measurement of the charge/discharge profiles recorded at various current densities (Figure 5b). The two plateaus at 2.1 and 2.3 V, as observed at 0.1 C in the discharge process, corresponded to the reduction peaks measured using CV, and the plateau at 2.3 V in the charge process was consistent with the oxidation peak. The potential gap (E) between the charge and discharge plateaus reflects the degree of polarization.64 With the increase of current density, in contrast to the rapid increase of E recorded for S/RGO-g-IDBI cathode, the E observed for RGO-g-poly(S-r-IDBI) cathode was less, reflecting the relatively fast and consistent electrochemical behaviors in RGO-g-poly(S-r-IDBI) cathode. The enhanced cathode kinetics in RGO-g-poly(S-r-IDBI) cathode could be ascribed to the following factors: (1) the covalent bonding between poly(S-r-IDBI) and RGO sheets facilitates the electron transfer during 14 ACS Paragon Plus Environment

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the electrochemical processes. (2) The improved compatibility of RGO-g-poly(S-r-IDBI) with electrolyte is beneficial to the charge transport in RGO-g-poly(S-r-IDBI) cathode. Rate capability was tested to measure the performance that a battery could deliver at different charge/discharge currents. When cycled at 0.1, 0.2, 0.5, and 1 C, the RGO-g-poly(S-r-IDBI) cathode delivered high discharge capacities of 1069, 872, 775, and 688 mA h g−1, respectively (Figure 5c). These values were comparable to those reported in the reference.43, 55, 65-66 And the discharge capacity was restored as the current density was reset back to 0.5, 0.2, and 0.1 C. In contrast, the discharge capacities measured for the S/RGO-g-IDBI cathode decreased sharply from 922 to 400 mA h g−1 as the rate was increased from 0.1 C to 1 C. The cathode also exhibited poor reversibility as the current density was reset back to 0.1 C. Collectively, these data indicated that the RGO-g-poly(S-r-IDBI) cathode exhibited a better rate capability and reversibility than the S/RGO-g-IDBI cathode. Considering the molecular structure of RGO-g-poly(S-r-IDBI), its improved rate capability was ascribed to the covalent grafting of poly(S-r-IDBI) to RGO as well as the good compatibility of RGO-g-poly(S-r-IDBI) with electrolyte, thus facilitating the charge transport in the cathode. The cycling performances of the RGO-g-poly(S-r-IDBI) and S/RGO-g-IDBI cathodes were measured at 0.2 C (Figure 5d). Benefiting from the covalent bonding between the poly(S-r-IDBI) and the RGO sheets, the RGO-g-poly(S-r-IDBI) cathode exhibited a high capacity retention rate of 82.3%, for which the discharge capacity was initially measured to be 868 mA h g−1 and maintained at 714 mA h g−1 after 100 cycles. In contrast, the S/RGO-g-IDBI cathode showed initial discharge capacities of 836 mA h g−1 and 496 mA h g−1 after 100 cycles, corresponding to a retention rate of only 59.3%. Figure 5e shows the charge/discharge curves recorded for the RGO-g-poly(S-r-IDBI) and S/RGO-g-IDBI cathodes during the cycling tests. Inspection of the data revealed that the E values measured for the RGO-g-poly(S-r-IDBI) cathode remained constant during the cycling tests, whereas the potential gaps 15 ACS Paragon Plus Environment


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measured for the S/RGO-g-IDBI cathode increased with cycling. In addition, the overpotentials observed before the charge plateaus displayed by the RGO-g-poly(S-r-IDBI) cathode were less than those observed for the S/RGO-g-IDBI cathode (see magnified view in Figure S5). These findings discovered the fundamental mechanism for the RGO-g-poly(S-r-IDBI) cathode to be stable during cycling. To obtain deeper insight into the electrochemical process of the RGO-g-poly(S-r-IDBI) and S/RGO-g-IDBI cathodes, electrochemical impedance spectroscopy (EIS) data were collected (Figure 5f). While the diameters of the semicircles shown in the high frequency portion of the Nyquist plots are related to the charge transfer resistance (Rct) at the electrode/electrolyte interface, the slopes of the straight lines at low frequency reflect the mass (Li+) diffusion in the cathodes. The Rct values were estimated to be 42.9 and 65.2  for RGO-g-poly(S-r-IDBI) and S/RGO-g-IDBI cathodes, respectively, using an established equivalent circuit (Figure S6). The lower Rct value supported a favorable charge transfer process at the interface between the RGO-g-poly(S-r-IDBI) cathode and the electrolyte. Meanwhile, the greater slopes of the straight line in the low frequency region suggested faster Li+ diffusion in RGO-g-poly(S-r-IDBI) cathode. These findings could be attributed to the following factors: (1) the covalent linkages between poly(S-r-IDBI) and RGO strongly bound the two components together so that the electrons occurred during the redox processes were readily transported between poly(S-r-IDBI) and the conductive RGO framework; (2) the excellent compatibility of RGO-g-poly(S-rIDBI) with electrolyte facilitated ion diffusion in the cathode. Using the Randles−Sevcik equation (1) and the CV data obtained at various scan rates (Figure S7), the Li+ diffusion coefficients (DLi+) were calculated.34, 67 Figure 5g shows that the peak current (Ip) of the two cathodic peaks (C1 and C2) and the anodic peak (A) changed linearly as function of the square root of scan rate (1/2) and that the slopes of the lines fitted for the RGO-g-poly(S-r-IDBI) cathode were 16 ACS Paragon Plus Environment

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greater than those of the S/RGO-g-IDBI cathode. Thus, the Li+ diffusion coefficient (DLi+) values corresponding to the redox reactions of RGO-g-poly(S-r-IDBI) should be greater than those of S/RGOg-IDBI. Indeed, the DLi+ values were calculated to be 3.6  10−9, 8.0  10−9, and 2.0  10−8 cm2 s−1 for the C1, C2, and A reactions, respectively, for the former electrode. The values were nearly one order of magnitude greater than those calculated for the latter (DLi+ = 6.8  10−10, 7.6  10−10, and 2.4  10−9 cm2 s−1). The results quantitatively showed that cathodes prepared from RGO-g-poly(S-r-IDBI) exhibited faster redox kinetics than analogues prepared from S/RGO-g-IDBI.

1/2

𝐼P = 2.69 × 105 𝑛3/2 𝐴𝐷Li+ 𝐶Li+ 1/2

(1)

Where, n represents the number of electrons involved in the electrochemical reaction (n = 2 for Li−S batteries); A represents the electrode area; CLi+ is the concentration of Li+ in electrolyte. Finally, the long-term cycling stability of RGO-g-poly(S-r-IDBI) cathode was measured at 1 C. At such a relatively high current density, the cells prepared from RGO-g-poly(S-r-IDBI) exhibited gradually increased capacity during the initial 50 cycles due to activation processes. Benefiting from the covalent confinement of poly(S-r-IDBI) to RGO sheets, the RGO-g-poly(S-r-IDBI) cathode exhibited an enhanced long-term cycling stability compared with that of the S/RGO-g-IDBI cathode (Figure 5h). The discharge capacity of RGO-g-poly(S-r-IDBI) cathode decreased from initial value of 634 mA h g−1 to 568 mA h g−1 after 500 cycles, whereas the discharge capacity of S/RGO-g-IDBI cathode decreased from 594 mA h g−1 to 292 mA h g−1 after 200 cycles. The capacity decay of the RGO-g-poly(S-r-IDBI) cathode was calculated to be only 0.021% per cycle over 500 cycles, which is less than one tenth of the capacity decay calculated for the S/RGO-g-IDBI cathode (0.254%). Such capacity retention is competitive with values reported in the literature (Figure S8). 17 ACS Paragon Plus Environment


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DISCUSSION

Figure 6. Morphology and elemental distribution analyses of the RGO-g-poly(S-r-IDBI) composite. (a) A TEM image collected for RGO-g-poly(S-r-IDBI) and the corresponding EDS elemental maps: (b) carbon, (c) sulfur, and (d) nitrogen.

As discussed above, the excellent electrochemical performances displayed by the RGO-g-poly(S-rIDBI) cathodes, including high Li+ diffusion coefficients, low polarization, and stable long-term cycling, can be ascribed to the covalent confinement of poly(S-r-IDBI) to the RGO sheets. To further support this hypothesis, the distribution of sulfur on the RGO sheets was investigated using TEM and EDS mapping. As evidenced by the TEM image recorded for RGO-g-poly(S-r-IDBI) (Figure 6a), a layer of a polymer-like material was found to be uniformly distributed over the surfaces of the RGO sheets and no sulfur particles were observed, even under a high magnification (Figure S9). EDS mapping of sulfur confirmed the uniform distribution of poly(S-r-IDBI) on the RGO sheets (Figure 6c). The efficient and uniform grafting of poly(S-r-IDBI) on the RGO sheets could be attributed to the high reactivity between 18 ACS Paragon Plus Environment

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the isocyanate groups of IDBI and the functional groups present on GO. Indeed, the former appeared to be uniformly distributed on the RGO sheets, as evidenced from the EDS mapping of nitrogen (Figure 6d). As stated above, the uniform distribution of poly(S-r-IDBI) on the RGO sheets is expected to facilitate liquid electrolyte access to sulfur cathode as well as sulfur utilization. In addition, covalently grafting the poly(S-r-IDBI) to the RGO should not only shorten the distance between the sulfur fragments and the RGO scaffold, thus enhancing electron transport at cathode interfaces, but also improve the cycling stability of the corresponding Li−S batteries by quenching the shuttle effect.



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Figure 7. In-situ and ex-situ characterization of RGO-g-poly(S-r-IDBI) cathode. (a) In-situ observation of the discharge process of a transparent beaker cell from 2.7 V to 1.8 V. (b) SEM images recorded for 20 ACS Paragon Plus Environment

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RGO-g-poly(S-r-IDBI) and S/RGO-g-IDBI electrodes before and after 500 charge/discharge cycles. (c) FT-IR spectra recorded for RGO-g-poly(S-r-IDBI) before and after 500 charge/discharge cycles. The batteries were disassembled at charged state.

To directly show that the unique structural features of RGO-g-poly(S-r-IDBI) influences the cycling stability, a transparent beaker cell was assembled and discharged from 2.7 to 1.8 V (Figure 7a). During this process, the electrolyte was found to maintain transparent and colorless, indicating that the polysulfides were confined inside the cathode due to the covalent grafting of the poly(S-r-IDBI) to RGO sheets. In contrast, the electrolyte of the cell prepared using the S/RGO-g-IDBI cathode gradually turned to a yellow-green color, indicative of the release of polysulfides during the discharge process. Comparison of the above observations provided visual support for the decisive role of the covalent bonds between poly(S-r-IDBI) and the RGO sheets in suppressing the shuttle effect. As a result, the Li−S batteries prepared using the RGO-g-poly(S-r-IDBI) cathode exhibited an enhanced long-term cycling stability. Coin-type Li−S batteries were disassembled and analyzed after the long-term cycling test (i.e., 500 charge/discharge cycles at 1 C). In contrast to the yellow color of the separator of the Li−S battery prepared using the S/RGO-g-IDBI cathode, the separator retrieved from the cell prepared using the RGO-g-poly(S-r-IDBI) cathode almost remained colorless (Figure S10). The cathode films were further studied using SEM (Figure 7b). While there were no obvious changes in morphologies for RGO-gpoly(S-r-IDBI) cathode before and after 500 charge/discharge cycles, significant cracks were evident in the S/RGO-g-IDBI cathode after 500 charge/discharge cycles, due to the volume changes of the cathode during the charge/discharge cycles. The different changes in morphology of these two cathodes can be ascribed to the molecular structures of the constituent cathode materials. The RGO-g-poly(S-r-IDBI) 21 ACS Paragon Plus Environment


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composite synergistically combines the flexibility of poly(S-r-IDBI) with the high mechanical strength of RGO.9,

39, 68

These characteristics effectively accommodate the volume changes experienced and

prevent the pulverization of the cathode during the charge/discharge cycles. Finally, a FT-IR spectrum recorded for the RGO-g-poly(S-r-IDBI) material subjected to 500 charge/discharge cycles revealed the signals at ca. 471 and 671 cm−1 (Figure 7c), which were assigned to the S−S and C−S vibrational modes, respectively, and in agreement with data collected before cycling. This finding reflects the high stability of the RGO-g-poly(S-r-IDBI) composite during the charge/discharge cycles and provides a molecularlevel support for the good cycling stability of the Li−S batteries.

CONCLUSIONS We report a novel strategy that is facile and allows covalent grafting of sulfur-containing copolymers to graphene sheets with a high efficiency and homogenous distribution. This strategy is fulfilled by an in-situ copolymerization of elemental sulfur and isopropenylphenyl functionalized RGO. The covalent bonding between the poly(S-r-IDBI) and RGO sheets is strong, and affords a mechanically robust material that exhibits fast charge transport kinetics during charge/discharge cycles. The Li+ diffusion coefficients calculated from the cathodic/anodic processes for RGO-g-poly(S-r-IDBI) cathode were nearly one order of magnitude higher than those obtained for elemental sulfur cathode. As a result, the RGO-g-poly(S-r-IDBI) cathode in Li−S batteries exhibits an enhanced sulfur utilization rate (10% higher than that of elemental sulfur cathode), an elevated rate capacity (688 mA h g−1 RGO-g-poly(S-rIDBI) cathode vs. 400 mA h g−1 for elemental sulfur cathode at 1 C), and an excellent capacity retention (89.6% over 500 cycles at 1 C). The results described herein offer new insight into controlling cathode kinetics and cycling stability of cathode materials using sulfur-containing copolymers. The methodology and the concepts presented are expected to expedite the preparation of sulfur-containing composites for 22 ACS Paragon Plus Environment

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use in Li–S batteries as well as other areas of contemporary interest, including energy storage applications, rubber vulcanization, sulfur utilization, and the basis for new classes of functional materials.

METHODS Materials. GO was synthesized by following a modified Hummer’s method.69 N, Ndimethylformamide (DMF) (AR grade) was dried over CaH2. IDBI (>95%) was purchased from TCI Co., Ltd. Hydrazine hydrate (>98%) was purchased from Aladdin (Shanghai) Co., Ltd. Sublimed sulfur (>99%) was purchased from Xilong Chemical Co., Ltd. All other reagents and solvents were purchased from Sinopharm Chemical Co., Ltd. Preparation of RGO-g-IDBI composites. GO-g-IDBI was synthesized via the condensation reaction between GO sheets and IDBI. In a typical reaction, GO (400 mg) was first dispersed in anhydrous DMF (80 mL) by sonication for 1 h. Next, IDBI (2 mL, 10 mmol) was added to the GO suspension under the protection of argon, followed by stirring the mixture at 50 °C for 24 h. After diluting with toluene, the GO-g-IDBI was isolated by centrifugation. The product was then dispersed in acetone, centrifuged, and collected (3), and finally washed with DI water. Hydrazine hydrate was used to reduce GO-g-IDBI. A suspension of GO-g-IDBI in DI water (1.5 mg mL−1, 300 mL) was charged with hydrazine hydrate (1 mL), and the resulting mixture was heated to reflux for 24 h. Finally, the RGO-g-IDBI was isolated by vacuum filtration, and then washed with DI water and ethanol. Preparation of RGO-g-poly(S-r-IDBI) and poly(S-r-IDBI). Sulfur was mixed with RGO-gIDBI to obtain S/RGO-g-IDBI by a solution process. First, RGO-g-IDBI (200 mg) and sulfur (350 mg) were dispersed in CS2 (20 mL) with the aid of sonication for 3 h. After the solvent evaporated, the resulting product (S/RGO-g-IDBI) was dried in a vacuum oven set to 50 °C. Second, the S/RGO-g-IDBI 23 ACS Paragon Plus Environment


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mixture was heated at 175 °C for 30 min, which afforded a black powder. As a control experiment, IDBI and sulfur were mixed at the same ratio as that used to prepare RGO-g-poly(S-r-IDBI) and heated in a similar manner to afford poly(S-r-IDBI) as an orange powder. Material characterization techniques. DSC data were obtained using a DSC Q2000 at a heating rate of 10 °C min−1 under an atmosphere of N2. TGA measurements were carried out using a TGA Q50 at a heating rate of 10 °C min−1 under an atmosphere of N2. XRD data were collected on a Bruker D8 Focus diffractometer using an incident wavelength of 0.154 nm (Cu K radiation) and a Lynx-Eye detector. FT-IR spectra were recorded on an Excalibur 3100 spectrometer with a resolution of 0.2 cm −1 using KBr pellets. Raman spectra were recorded on a Horiba LabRAM HR Evolution Raman spectrometer at an excitation wavelength of 532 nm. XPS spectra were performed on an ESCALAB 250Xi X-ray photoelectron spectrometer microprobe using monochromated Al Kα radiation of 1486.7 eV. SEM data were recorded on a field-emission SEM (JEOL-6701F). TEM and elemental mappings of the samples were carried out using a JEOL JEM-2100F microscope equipped with an EDS system operated under an accelerating voltage at 200 kV. Electrochemical measurements. CR2025 coin cells using RGO-g-poly(S-r-IDBI), poly(S-r-IDBI), or S/RGO-g-IDBI as cathode materials were fabricated for the electrochemical measurements. The cathode materials were first mixed with conductive carbon and a poly(vinylidene fluoride) (PVDF) binder at a mass ratio of 8 : 1 : 1 using a mortar and pestle, and then dispersed in N-methylpyrrolidone (NMP) to obtain a slurry. The slurry was then blade casted onto carbon-coated Al foils using a doctor blade (cathode material loading = 1.0 − 1.5 mg cm−2). Coin cells were assembled in an argon-filled glove box using a porous membrane (Celgard 3501) as the separator and lithium foil as the counter/reference electrode. A lithium bis(trifluoromethane) sulfonylimide solution (1.0 M) containing LiNO3 as an additive (2 wt%) in a mixture of 1,3-dioxolane/dimethoxymethane (1:1 v/v) was used as 24 ACS Paragon Plus Environment

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the electrolyte. Galvanostatic discharge/charge measurements were performed on a LAND system over a voltage range of 1.8−2.7 V (vs. Li+/Li). CV and EIS measurements were performed on a CHI 660E electrochemical workstation. EIS analysis was performed with an amplitude of 10 mV over a frequency range of 100 kHz to 10 mHz.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns of GO, GO-g-IDBI, and RGO-g-IDBI; XPS spectra of GO, GO-g-IDBI, and RGO-gIDBI; FT-IR spectra of IDBI, GO, GO-g-IDBI, and RGO-g-IDBI; Optical images of elemental sulfur, poly(S-r-IDBI), and RGO-g-poly(S-r-IDBI); A magnified view of the initial parts of the charge plateaus of RGO-g-poly(S-r-IDBI) and S/RGO-g-IDBI cathodes; The equivalent circuit used for fitting the EIS curves; CV curves of RGO-g-poly(S-r-IDBI) cathode and S/RGO-g-IDBI cathode obtained at various scan rates from 0.2 to 0.5 mV s−1; Comparison of the decay rates of elemental sulfur cathodes, sulfur copolymer cathodes, and this work; A TEM image of RGO-gpoly(S-r-IDBI) recorded at a relatively high magnification; Optical images of the cathodes and membranes of the Li−S batteries disassembled after 500 cycles at 1 C (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Christopher W. Bielawski: 0000-0002-0520-1982 25 ACS Paragon Plus Environment


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Jianxin Geng: 0000-0003-0428-4621 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51773211), the National High Level Talents Special Support Plan of China, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, and Beijing Municipal Science & Technology Commission. CWB acknowledges the Institute for Basic Science (IBS-R019-D1) as well as the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea for their support.

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Covalent Confinement of Sulfur Copolymers onto Graphene Sheets

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