Covalently Grafted PolysulfurâGraphene Nanocomposites for

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Covalently-grafted Polysulfur-graphene Nanocomposites for Ultrahigh Sulfur-loading Lithium-polysulfur Batteries Chi-Hao Chang, and Arumugam Manthiram ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01031 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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ACS Energy Letters

Covalently-grafted Polysulfur-graphene Nanocomposites for Ultrahigh Sulfur-loading Lithium-polysulfur Batteries Chi-Hao Chang and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute The University of Texas at Austin, Austin, TX 78712, USA Corresponding Author E-mail: [email protected] (Arumugam Manthiram*) Tel: +1-512-471-1791; fax: +1-512-471-7681.

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ABSTRACT

Although lithium-polysulfur (Li-polyS) batteries employing organic polymeric sulfur as a cathode material outperforms the lithium-sulfur (Li-S) battery system, the relatively low sulfur loading (< 2 mg cm-2) in the current Li-polyS batteries compromises the areal capacity, constraining their practicality. We present here a new cathode active-material (a covalentlygrafted polysulfur-graphene nanocomposite (polySGN)) for ultrahigh-loading Li-polyS batteries. The new cathode active-material polySGN offers several advantages: (i) the well-dispersed graphene sheets offer highly electrically conductive pathways for electrons to travel within the polySGN matrix; (ii) the intermediate organosulfide moieties alleviate the irreversible sulfide deposition on electrodes; and (iii) the in-situ formed coating layer on the cathode-side surface of the polymeric separator further reduces polysulfide migration. The Li-polyS batteries employing polySGN as the cathode active-material accomplish the highest sulfur loading (up to 10.5 mg cm-2) and the highest areal capacity (~ 12 mA h cm-2) reported thus far in the literature.


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Elemental sulfur is one of the most abundant elements on the earth’s crust and a major by-product (> 60 million tons per year) of the petroleum refinery process (the hydrodesulfurization process) in the petroleum industry.1–3 Recognizing the wide availability of sulfur resources, sulfur has been established as a key player in the polymer science/engineering industry.4,5 Furthermore, due to the ability of sulfur to offer an extremely high theoretical capacity of 1,672 mA h g-1 at low cost, it is a target as a cathode active-material for the nextgeneration energy storage devices—the lithium-sulfur (Li-S) batteries.6,7 However, a few fundamental challenges limit the practicality of Li-S batteries.6 First, the poor ionic and electrical conductivities of sulfur and the end-discharge product Li2S lead to low electrochemical utilization. Moreover, the intermediate lithium polysulfide chains (LiPS, Li2Sx, 4 ≤ x ≤ 8) with small molecular sizes (1.0 – 1.8 nm in length) formed in the electrolyte during cycling readily migrate through the slit pores of the polymeric separator, leading to massive active-material loss and rapid capacity degradation.8,9 Electroactive sulfur-abundant polymers (polyS, up to 90 wt.% sulfur) are an emerging cathode active-material, which are synthesized through a diradical polymerization at high temperatures (> 185 oC).1,10 The sulfur-rich polymers with a high amount of sulfur are found to be effective in suppressing the irreversible sulfide deposition on the electrodes.10 As a result, lithium batteries utilizing polyS as the active material (Li-polyS battery) display better electrochemical performance as compared to Li-S batteries.1,10 However, the low electrical conductivity of polyS often limits the sulfur loading (< 2 mg cm-2) in the cathode of Li-polyS batteries. Even though much efforts have been dedicated to altering the polymeric backbones in polyS, not much attention has been paid in boosting the sulfur loading in Li-polyS batteries,


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leading to relatively lower areal capacity (< 3 mA h cm-2).10–21 Therefore, Li-polyS batteries still have limited practical applications as compared to the current Li-ion batteries. Graphene sheets, because of very high electrical conductivity (7200 S m-1), are commonly incorporated into polymer matrices as conductive nanofillers to boost the electrical conductivity.22–24 With this perspective, we present here a new electroactive polymer/graphene nanocomposite (a covalently-grafted polysulfur-graphene nanocomposite (polySGN)) in which highly conductive graphene sheets are introduced into the polyS matrix to enhance the electrical conductivity of polyS. PolySGNs with the increased conductivity are utilized, for the first time, as a cathode active-material to realize ultrahigh sulfur-loading Li-polyS batteries. The cells employing polySGNs as the active material with an ultrahigh-sulfur-loading (up to 10.5 mg cm-2) show a high areal capacity of ca. 12 mA h cm-2 and a stable cycling capability for 100 cycles. Despite the potential of graphene sheets to improve the electrical conductivity, pristine graphene itself possesses incompatible surface characteristics with polymer chains, leading to a severe aggregation and inhomogeneuous distribution within the polymer matrix.23,25 This poor miscibility weakens the merits of graphene sheets in polymer/graphene nanocomposites.23,25 Therefore, vinyl group-functionalized reduced graphene oxide (V-rGO) sheets were synthesized here to increase the dispersibility within the polyS matrix. The FT-IR spectrum of V-rGO shows the disappearance of the peaks at 3334 and 3222 cm-1 (–NH2 stretching) and an enhanced peak intensity at 1632 cm-1 (-C=O stretching), indicating that the vinyl groups are covalently bonded onto the graphene sheets (Figure S1). The synthesized V-rGO sheets thus possess abundant terminal vinyl functional groups that can then undergo in-situ diradical copolymerization with sulfur and 1,3-diisopropenylbenzene (DIB) through a facile inverse vulcanization at > 185 oC, as shown in Figure 1a. The resulting products are chemically stable polySGNs with a black color


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(inset in Figure 1a). The 1H NMR spectra of the polySGNs are similar to that of reported polyS (Figure S2).1,19 In Figure 1b, 1H NMR spectra of the polySGNs confirm the presence of aromatic protons ( = 6.8 – 7.8 ppm) and methyl protons ( = 1.0 – 2.2 ppm). The resonance peaks ( = 2.9 – 3.4 ppm), on the other hand, correspond to methylene moieties in DIB and VrGO, which are covalently grafted onto polymeric sulfur comonomer units.1,19 The assynthesized polySGNs were directly dip-coated onto a sheet of carbon paper. The carbon paper exhibiting intertwined morphology functionalize as a current collector to host a large amount of polySGNs (Figure S3). The scanning electron microscopy (SEM) image of the prepared cathodes show that the polySGNs are able to homogeneously fill the open spaces and possess a close contact with the carbon current collector (Figure 1c).

Figure 1. (a) Synthesis scheme of polySGNs (the photo of synthesized polySGNs is shown in the inset), (b)1H-NMR spectrum and chemical structure of polySGNs, and (c) SEM images and elemental mapping results of an uncycled polySGN cathode.


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The electrochemical behaviors of the cells employing polyS and polySGNs as cathodeactive material (polyS cathodes and polySGN cathodes) were investigated by various electrochemical analyses. The electrochemical impedance spectroscopy (EIS) of the freshly assembled cells shows that the cells utilizing the polySGN cathode exhibit lower internal resistance of ~ 24 Ω as compared to that of the cells using the polyS cathode (~ 46 Ω) (Figure S4). The lower resistance is consistent with the discharge/charge voltage profiles (Figure 2a), showing reduced polarization (∆E) from 180 to 138 mV. The decrease in the internal resistance is attributed to the well-dispersed highly conductive V-rGO sheets which form infinite and percolated electrical network throughout the insulating polyS matrix.22–24 This facilitates the transport of electrons within the polyS matrix and benefits the redox ability during cycling due to the enhanced electrical conductivity. Additionally, the similar discharge/charge profiles indicate that the cells employing the polyS cathode and the polySGN cathode exhibit similar redox reactions. Based on the previous reports on polyS, we propose a possible redox reaction of polySGNs as shown in Figure S5.10 The upper discharge plateau at ~ 2.38 V corresponds to the formation of organosulfide units, small LiPS chains, as well as covalently bonded LiPS-graphene sheets (LiPS-c-GS). Continuous discharge to the lower plateau at ~ 2.07 V results in the generation of organosulfide species with shortened oligosulfur moieties, insoluble sulfide mixtures (e.g., Li2S2/Li2S), and LiPS-c-GS with oligosulfur moieties.1,10


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Figure 2 (a) Voltage profiles and (b) electrochemical performances of the cells employing the polySGN cathodes and the polyS cathodes (sulfur loading: 3.7 mg cm-2). Cells utilizing the polySGN cathodes with ultrahigh sulfur loading: (c) specific capacity and (d) areal capacity. Comparative examination of the cell parameters of the polySGN cathodes with the reported LipolyS batteries: (e) specific capacity and (f) areal capacity (all published work of Li-polyS batteries are shown in the inset).


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The homogeneous incorporation of conductive V-rGO sheets into polyS matrix effectively increases the discharge capacity from 632 to 978 mA h g-1 at C/5 rate (Figure 2a – b). With similar sulfur loading and the same cell configuration, the improved electrochemical utilization from ~ 38% to ~60% is attributed to the infinite conductive pathways throughout the polyS matrix, which are composed of well-dispersed highly conductive V-rGO sheets. Moreover, the cells employing the polySGN cathodes exhibit a more promising capacity-retention rate of 67% after 100 cycles as compared to a capacity-retention rate of 46% for the cells employing the polyS cathodes. The long-term cyclability of the cells employing the polySGN cathodes is also presented in Figure S6, showing a low capacity fade rate of 0.18 % per cycle for 300 cycles. Benefiting from the more advantageous dynamic electrochemical properties of polySGNs, the polySGN cathodes first demonstrate Li-polyS batteries with an ultrahigh sulfur loading up to 10.5 mg cm-2 (sulfur content in the whole cathode: ~ 70 wt.%). Despite such a high sulfur loading/content, the cells employing the polySGN cathodes offer high initial discharge capacities (electrochemical utilization) of 1135 (68%), 992 (59%), 827 (49%) mA h g-1, respectively, at C/20, C/10, and C/5 rates (Figure 2c and Figure S7). Furthermore, the cells maintain excellent cycling performance for 100 cycles at various cycling rates. The reversible discharge capacities after 100 cycles are 760, 717, and 642 mA h g-1, respectively, at C/20, C/10, and C/5 rates. Considering of such ultrahigh sulfur loading, the cells show an excellent capacityretention rate of ~72% after 100 cycles. The high discharge capacities and ultrahigh sulfur loadings correspond to high initial areal capacities of 11.9, 10.4, and 8.7 mA h cm-2 and high reversible areal capacities of 8.0, 7.5, and 6.7 mA h cm-2 after 100 cycles, respectively, at C/20, C/10, and C/5 rates (Figure 2d). These values are much higher than that of the current Li-ion batteries (~ 4 mA h cm-2). The cells also exhibit high areal energy density (~ 25 mW h cm -2)


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when using a discharge voltage of 2.1 V (Figure S8). The electrochemical performance is able to be further improved by means of advanced cell configurations with the coated separators, as shown in Figure S9. The superior electrochemical performance of the cells employing the polySGN cathodes with an ultrahigh sulfur loading is compared with those relevant reports in the literature in Figure 2e – f. The comparisons illustrate that the polySGN-cathodes offer the best electrochemical performance thus far in terms of the highest active-material loading (more than ten times higher than the average sulfur loading) and the highest areal capacity (approximately 12 times higher than the average value). The high sulfur loading and high areal discharge capacity in the polySGN cathode significantly promote the practical utilization required of the Li-polyS battery system. The superior electrochemical performance of polySGNs, other than the greatly enhanced electrical conductivity, is attributed to the inhibited formation of inactive sulfide deposition on the electrodes. The polySGN cathodes after cycling remain intertwined and as porous structures without thick solid sulfide aggregates on the surface as shown in Figure S10 and S111. According to the previous reports, this is the so-called “plasticization” effect.10 During cycling, the presence of more soluble organosulfide moieties within insoluble sulfide phases prevents the irreversible deposition of these end-discharge solid Li2S2/Li2S mixtures on the electrode surface, moderating the fast capacity fade. Figure 3a presents the SEM images of the cathode-side surface of the cycled polymeric separators from the cells utilizing the polySGN cathodes. The surface facing toward the polySGN cathode reveals a uniform coating layer attached onto the polymeric separator. The cycled separator was washed with acetone and ethanol for several times to dissolve most of sulfide species. There is evidence of LiPS-c-GS flakes (diameter ~ 5 – 10 µm) attached onto the


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cathode-side surface of the polymeric separator as shown in the inset in Figure 3a. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) pattern of the coating layer on the cycled separator ensure the existence of LiPS-c-GS with a high degree of crystallinity (Figure 3b and Figure S12). The surface facing the Li-metal anode, on the other hand, remains as a porous structure (Figure S13). Based on the observations, we are able to conclude that the LiPS-c-GS is coated in-situ onto the separator. During cycling, some LiPS-cGS with covalently attached LiPS species is dissolved in the electrolyte and detached from the cathode. Although the intermediate LiPS-c-GS tends to migrate from the cathode region to the anode region because of the concentration difference between the two electrode regions, the much larger LiPS-c-GS (5 – 10 µm) is effectively screened by the polymeric separator (slit pores: < 1 µm).26 The screened LiPS-c-GS then aggregates on the cathode-side surface of the polymeric separator and form an in-situ coated conductive layer. On the contrary, the cathodeside of the cycled polymeric separators obtained from the cells utilizing the polyS cathodes show clear porous structures (Figure S14). The cathode-side of the cycled separator in the regular Li-S cell, as a reference, is shown in Figure S14b. Thick and huge solid sulfides randomly aggregate onto the cathode-side surface of the polymeric separator and block the slit pores.


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Figure 3 (a) SEM images of the cycled separator (the washed sample is shown in the inset). (b) TEM images of LiPS-c-GS retrieved from the cycled separator in the cells employing the polySGN cathodes (the inset shows the SAED pattern). Surface examinations of the cycled polysulfide traps: (c) polySGN cathode and (d) polyS cathode. (e) Proposed mechanisms of insitu formation of the conductive coating layer and the suppressed LiPS migration.


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The conductive coating layer comprised of LiPS-c-GS is able to suppress the migration of LiPS. The LiPS-trap cells (Figure S15) reveal the reduced LiPS migration in the cells employing the polySGN cathodes. The retrieved LiPS trap shows strong carbon signal but limited sulfur signal on the surface facing the cathode region (Figure 3c). The cycled LiPS trap obtained from cells employing the polyS cathodes, however, exhibits stronger sulfur signal (Figure 3d). The reduced sulfur signal on the LiPS trap employing the polySGN cathode indicates that the LiPS migration is retarded due to the in-situ formed conductive layer. This also results in excellent dynamic LiPS-retention capability in the cells employing the polySGN cathodes (Figure 3e). Moreover, the in-situ formed conductive coating layer is able to effectively reutilize/reactivate the trapped active material during cycling.27 This explains the superior electrochemical performance of the cells employing the polySGN cathodes as compared to the cells utilizing the polyS cathodes. In summary, we presented the synthesis of a new electroactive polymer-graphene nanocomposite, polySGNs, through a facile inverse vulcanization process at 185 oC. The graphene sheets homogeneously dispersed in the polyS matrix offer infinite electrical pathways, thereby improving the redox reactions and increasing the electrochemical utilization. The more soluble intermediates, organosulfide units, functionalize as “plasticizers” to mitigate the irreversible sulfide deposition on the electrodes. Furthermore, the generated LiPS-c-GS is sizeselectively netted by the polymeric separator due to their large structure. The sieved LiPS-c-GS is then deposited onto the cathode-side surface of the polymeric separator to form a conductive coating layer, further reducing the loss of active material, benefitting the electrochemical performance and cyclability. Thus, polySGNs enable the Li-polyS cells to achieve ultrahigh sulfur loading, high areal capacity, and superior cycling stability. The polySGN cathodes with


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high sulfur loading/content successfully enable a high areal capacity Li-polyS battery. This work offers a viable approach to effectively utilize the abundant sulfur resources from the petroleum industry for high energy-density Li-polyS batteries.

ASSOCIATED CONTENT Supporting Information. Experimental sections, scheme of V-rGO synthesis, XRD, NMR spectrum of polyS, EIS, TEM images, SEM images of cycled separators/cathodes, long-term cycling performance, and mechanism are presented in Supporting Information. AUTHOR INFORMATION E-mail: [email protected] (A. Manthiram) Tel: +1-512-471-1791; fax: +1-512-471-7681. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy (EERE), under Award Number DE-EE0007218. REFERENCES (1)

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