Controlled Synthesis of Sulfur-rich Polymeric Selenium Sulfides as

Aug 9, 2018 - High-energy lithium/sulfur (Li/S) batteries still suffer from unsatisfactory cycle life and poor rate-capability caused by the polysulfi...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 29565−29573

Controlled Synthesis of Sulfur-Rich Polymeric Selenium Sulfides as Promising Electrode Materials for Long-Life, High-Rate Lithium Metal Batteries Panpan Dong,†,‡ Kee Sung Han,§ Jung-In Lee,† Xiahui Zhang,†,‡ Younghwan Cha,†,‡ and Min-Kyu Song*,†,‡

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†

School of Mechanical and Materials Engineering and ‡Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164, United States § Materials Sciences, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: High-energy lithium/sulfur (Li/S) batteries still suffer from unsatisfactory cycle life and poor rate capability caused by the polysulfides shuttle and insulating nature of S cathodes. Here, we report our findings in the controlled synthesis of selenium (Se)containing S-rich co-polymers of various compositions as novel cathode materials through a facile inverse vulcanization of S with selenium disulfide (SeS2) and 1,3diisopropenylbenzene (DIB) as co-monomers. Nuclear magnetic resonance and X-ray photoelectron spectroscopy results show that divinyl functional groups of DIB were chemically cross-linked with S/SeS2 chain radicals through a ring-opening polymerization. The newly formed bonds of C−S, C−Se, and S−Se in novel S−SeS2−DIB copolymers effectively alleviate the shuttle effects of polysulfides/polyselenides. Furthermore, various electrochemical techniques confirm the positive roles of Secontaining co-polymers in enhancing the electrode reaction kinetics and the formation of stable solid electrolyte interphase layer with low charge-transfer resistance, leading to improved high-rate performances. The as-synthesized co-polymer was then infiltrated into well-interconnected, porous nanocarbon networks (Ketjenblack EC600JD, KB600) to provide effective paths for the fast electron transport. Due to the synergistic combination of chemical and physical confinement of the reaction intermediates during cycling, good reversibility for 500 cycles with a low decay rate of 0.0549% per cycle was achieved at 1000 mA g−1. These encouraging results suggest that the combination of chemical incorporation of SeS2 into S-rich co-polymer and the physical confinement of carbon networks is a promising strategy for advancing Li/S batteries and their viability for practical applications. KEYWORDS: sulfur-rich co-polymers, selenium sulfides, inverse vulcanization, sulfur cathodes, lithium batteries

1. INTRODUCTION Lithium/sulfur (Li/S) batteries have been widely studied for high-energy applications during the past decades owing to the high theoretical specific capacity (1675 mAh g−1), low cost, natural abundance, and environmentally benign nature of sulfur.1 Despite these compelling advantages, sulfur cathodes still face limitations, such as poor electrical conductivity and dissolution/diffusion back and forth (known as “shuttle effect”) of reaction intermediate “polysulfides” in organic electrolytes. As a result, Li/S batteries suffer from low Coulombic efficiency (CE), short cycle life, and poor rate capability.2,3 Many efforts have been made to address these problems.4−8 One of the most common strategies is to adopt porous carbon hosts to confine sulfur by the melt-infusion methods.9−12 However, the dissolution of polysulfides still occurs during long-term cycling of Li/S batteries because of the insufficient physical confinement of polysulfides by carbonaceous materials with a high surface area. Therefore, many researches are focused on the combining effect of both physical and chemical interactions in Li/S batteries.13,14 © 2018 American Chemical Society

Recently, polymeric sulfur has been explored as one kind of electroactive materials for energy-storage systems. Pyun and co-workers first demonstrated the synthesis of sulfur-rich polymers by directly using elemental sulfur and 1,3diisopropenylbenzene (DIB) as co-monomers through the inverse vulcanization method, which showed promise for chemical confinement of polysulfides within polymer matrix.15 The prepared co-polymers showed a high initial specific capacity of ∼1100 mAh g−1 at 167.2 mA g−1, but limited rate performance (∼400 mAh g−1 at 3344 mA g−1) was observed, which can be associated with the low conductivity of these sulfur-rich co-polymers. In this respect, researchers have developed polymeric sulfur/carbon hybrids to enhance the electrochemical utilization of sulfur in co-polymers.16−21 Selenium (Se), which is a congener of sulfur and has a theoretical specific capacity of 675 mAh g−1, has been reported Received: May 31, 2018 Accepted: August 9, 2018 Published: August 9, 2018 29565

DOI: 10.1021/acsami.8b09062 ACS Appl. Mater. Interfaces 2018, 10, 29565−29573

Research Article

ACS Applied Materials & Interfaces as a new cathode material for lithium batteries by Amine and co-workers.22 Compared to sulfur, Se exhibits much higher electrical conductivity (Se: 1 × 10−3 S m−1 vs S: 5 × 10−28 S m−1) and comparable theoretical volumetric capacity (Se: 3240 mAh cm−3 vs S: 3467 mAh cm−3).23 It has also been reported that Se promotes higher electrochemical utilization than sulfur.24−29 On the basis of these findings, combined S− Se cathodes have been recently investigated for high-energy lithium batteries.30−35 For example, selenium sulfides, such as Se2S5, confined in a micro/mesoporous carbon was utilized as a novel cathode for Li batteries, demonstrating a high initial capacity of ∼1600 mAh g−1 at a current density of 138.9 mA g−1.36 However, fast capacity fading to 345.5 mAh g−1 after 50 cycles was observed. Similarly, Li et al. synthesized S0.94Se0.06/ C composites using the ball-milling followed by the heat treatment, which showed a high capacity of 910 mAh g−1 after 500 cycles at 1000 mA g−1.37 However, the S/Se solid solutions still suffered from the shuttling problems of polysulfides during cycling. Sun et al. designed heterocyclic S−Se molecules with an optimal mole ratio between Se and S (Se2S6) within nitrogen-doped mesoporous carbons, which showed good cycling performance (883 mAh g−1 after 100 cycles).38 Although these improvements in the specific capacity and cycling performance of Li batteries are promising, the shuttle effects of polysulfides and polyselenides during the long-term charge/discharge process are still unavoidable owing to the insufficient physical confinement by the carbon host alone. In terms of chemically binding selenium in sulfur materials, Boyd et al. copolymerized the crystalline S−Se compounds with DIB to obtain organically modified polymers for IR optics applications.39 The thermal stability and IR spectroscopy of the poly(S95Se5-r-DIB) and poly(S90Se10-r-DIB) terpolymer materials were investigated in their work. Similar poly(S-r-Se-rDIB) polymer materials with ultrahigh refractive indices were also recently reported by Anderson et al. for IR imaging applications.40 Moreover, Liu et al. have successfully introduced selenium into cross-linked sulfur polymers to increase the electrical conductivity by using selenium disulfide (SeS2), thus enhancing the performance of the photovoltaic applications.41 Nevertheless, the application of S−Se co-polymers for lithium storage is seldom reported. Recently, Gomez et al.42 prepared hybrid S−Se−DIB co-polymers via the inverse vulcanization as cathodes for lithium batteries and demonstrated an enhanced capacity of 550 mAh g−1 at 1622 mA g−1 and promising cycling performance up to 100 cycles. In their study, metallic Se was used in the synthesis of co-polymers. On the basis of Raman spectroscopy and liquid-state nuclear magnetic resonance (NMR) (77Se NMR and 1H NMR), it was suggested that “Se atoms were not bonded to the organic cross-linker, but were homogeneously dispersed in the sulfur matrix”. The use of solution NMR, however, made the analysis of molecular structure rather limited because S-rich copolymers are soluble only when large amount (e.g., 50 wt % in their study) of DIB is used. The fundamental understanding of the relationship between the processing, molecular structure of Se-containing polymeric sulfur, and their electrochemical performance still remains a challenge. Here, we report our findings in the controlled synthesis and characterization of polymeric selenium sulfides with novel molecular structure as promising electrode materials for advanced Li/S batteries. In contrast to the previous report,42

Se could be successfully cross-linked with S and DIB when sulfur-rich polymeric selenium sulfides (S−SeS2−DIB) were synthesized by the inverse vulcanization method directly using elemental S as a solvent, nontoxic SeS2 as the selenium source, and DIB as a co-monomer. While the metallic Se is expensive and toxic, cheaper and nontoxic SeS2 would be ideal as a precursor for scalable and environment-friendly processing of Se-containing materials. Solid-state 13C cross-polarization magic angle spinning (CP/MAS) NMR and X-ray photoelectron spectroscopy (XPS) directly confirmed the formation of C−S, C−Se, and S−Se bonds in S−SeS2−DIB co-polymers. It was also observed that the presence of Se in co-polymer could enhance the electrode reaction kinetics and achieve better electrochemical utilization of active materials. The effective chemical confinement of Se-containing co-polymers as well as physical confinement from conductive carbon networks provide effective paths for the fast electron transport and the formation of conductive solid electrolyte interphase (SEI) layer during cycling. Our results suggest a promising path for the development of advanced Li/S batteries with long cycle life and good rate capability. Additionally, the controllable and facile synthesis route of polymeric selenium sulfides with tunable properties may open up new opportunities for various applications.

2. RESULTS AND DISCUSSION 2.1. Preparation of Pure S−DIB and S−SeS2−DIB Copolymers. Figure 1 depicts the preparation scheme of polymeric selenium sulfides via the modified inverse vulcanization method. First, a glass vial with elemental S inside was heated at 155 °C in an oil bath to obtain liquid S, then SeS2

Figure 1. Synthesis route of polymeric selenium sulfides via inverse vulcanization method. 29566

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Figure 2. (a) Solution 13C NMR of DIB and (b) 13C cross-polarization magic angle spinning (CP/MAS) NMR measurement data of S−DIB and S−SeS2−DIB co-polymers, the asterisk denotes spinning side band; the proposed molecular structures of (c) S−DIB and (d) S−SeS2−DIB copolymers. For S−SeS2−DIB structure, each Se can be randomly replaced by S.

(112.4 ppm, C2) and the peak located at δ = 122.8 ppm (C3) disappeared in the spectra of S−DIB and S−SeS2−DIB co-polymers as shown in Figure 2b, suggesting the successful copolymerization. In addition, the presence of peaks located between 40 and 60 ppm of PSS100-0 as well as PSS90-10 copolymers further confirmed the copolymerization of DIB and sulfur units.15 Importantly, resonances at δ = 19.8 ppm were only observed in S−SeS2−DIB co-polymers, indicating the possible formation of C−Se bonds during the copolymerization process. The corresponding proposed molecular structures of S−DIB and S−SeS2−DIB co-polymers are shown in Figure 2c,d. In terms of S−SeS2−DIB co-polymers, each Se atom of the molecular structure can be randomly replaced by S atom. Further evidence of the presence of C−S in S−DIB and C− Se in S−SeS2−DIB co-polymers was obtained from the highresolution S 2p and Se 3d XPS spectra in Figure 3. As demonstrated in Figure 3a, the peak at ∼161.0 eV in the S 2p spectrum corresponded well with the C−S bond in PSS100-0 co-polymer, which was also observed in previous reports.18,30 Meanwhile, the C−S bond was also observed in S 2p spectrum of PSS90-10 co-polymer, mainly resulting from the copolymerization of S and DIB. However, the peak intensity of the C−S bond in PSS90-10 was lower than that of PSS100-0, which can be attributed to the formation of S−Se as well as C− Se bonds during copolymerization of S, SeS2, and DIB. As shown in Figure 3c, the observed peaks of C−Se (ca. 59.8 eV), S−Se (ca. 56.8 eV), and Se−Se (ca. 54.6 eV & 56.1 eV) of PSS90-10 clearly supported the presence of newly formed

was added slowly under vigorous stirring to form selenium sulfides. Then, the selenium sulfides mixture was heated up to 180 °C, followed by addition of 10 wt % DIB monomer (based on the total amount of S plus SeS2) under vigorous stirring. After 10 min copolymerization process, the vial was cooled down to room temperature. The glassy-like product was collected and ground into fine powder. The specific feeding composition and element information of various co-polymers prepared in this work are shown in Tables S1 and S2, respectively. The S−SeS2−DIB co-polymers with desired S/ SeS2 mass ratios are denoted as PSS(100 − x) − x, where x is the weight percentage of SeS2 in S/SeS2. For instance, the PSS90-10 co-polymer shows the weight ratio of S and SeS2 as 90:10 (wt %/wt %). As a control, S−DIB co-polymer was synthesized using the same method without introducing SeS2 and denoted as PSS100-0. The colors of various raw materials (S, SeS2) and co-polymers are shown in Figure S1. It was observed that SeS2 could not be fully dissolved in S matrix with the high content of >20 wt % in the current synthesis condition. When the content of SeS2 is larger than 20 wt % (based on the total weight of SeS2 and S), it is difficult to get uniform mixing due to the increased viscosity of the mixture and possibly the saturated solubility of SeS2 in S matrix. 2.2. Characterization of Pure S−DIB and S−SeS2−DIB Co-polymers. As shown in Figure 2, solution 13C NMR and 13 C CP/MAS NMR spectra were collected to demonstrate the molecular structure of DIB monomer and co-polymers, respectively. The characteristic methylene peak of DIB 29567

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Figure 3. High-resolution S 2p XPS spectra of (a) PSS100-0 and (b) PSS90-10; (c) Se 3d XPS spectra of PSS90-10. Indium shot was used as the background.

Figure 4. (a) TGA analysis and (b) XRD patterns of PSS100-0, PSS90-10, PSS100-0@KB600, and PSS90-10@KB600.

chemical bonds between the C, S, and Se atoms in S−SeS2− DIB co-polymer.43 The elemental surface compositions of S− DIB and S−SeS2−DIB co-polymers based on atomic content of elements by XPS analysis are also summarized in Table S3. 2.3. Preparation of S−DIB@KB600 and S−SeS2−DIB@ KB600 Hybrids. The as-synthesized co-polymer was infiltrated into a porous carbon host (Ketjenblack EC600JD, KB600) via the simple melt-diffusion method at 160 °C for 12 h to further improve the electronic conductivity of the copolymers and provide effective physical confinements. As shown in Figure S2, the PSS100-0 co-polymer is well covered with KB600 nanoparticles. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping analyses of PSS100-0@KB600 and PSS90-10@KB600 confirmed uniform distributions of S

and S/Se within the composites, respectively (Figure S3). The contents of the active materials in PSS100-0@KB600 and PSS90-10@KB600 were measured to be 55.6 and 57.7 wt %, respectively (Figures 4a and S4a). The X-ray diffraction (XRD) patterns of the co-polymers and co-polymer@KB600 composites are shown in Figure 4b. The characteristic peaks of S and SeS2 in PSS100-0 and PSS90-10 co-polymers became broadened/weakened, and some of them even disappeared when compared with elemental S and SeS2 (Figure S4b), as a result of the copolymerization of S, SeS2 with DIB.44 The relative ratio of polymerized/unpolymerized S in the final copolymers was further analyzed by using the differential scanning calorimetry (DSC) and XRD measurements (Figures S5 and S6). After heat treatment, the peak intensities of both 29568

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Figure 5. Electrochemical performance of PSS100-0, PSS90-10, PSS100-0@KB600, and PSS90-10@KB600 hybrids. (a) The first cycle CV curves at 0.02 mV s−1 and (b) constant-current cycling performances of pure PSS100-0 and PSS90-10 co-polymers; (c) initial voltage profiles of pure PSS90-10 and PSS90-10@KB600 hybrid; (d) the second cycle CV curves at 0.02 mV s−1, (e) initial charge−discharge profiles, and (f) rate capabilities of PSS100-0@KB600 and PSS90-10@KB600 hybrids at different current densities.

cyclic SenS8−n molecules.38 Most notably, PSS90-10 shows lower oxidation potential and narrower peak shape than that of PSS100-0 co-polymer, indicating that the presence of Se in copolymers can significantly enhance the electrode kinetics particularly for electrochemical oxidation reactions. The cycling performances at a higher current density (500 mA g−1) up to 100 cycles of PSS100-0 and PSS90-10 cathodes are shown in Figure 5b. It was reported that the improved battery performance for S−DIB co-polymers resulted from the organosulfur DIB units, which can be acted as “plasticizers” in the insoluble discharge products phase, suppressing the irreversible deposition of these discharge products to hinder capacity losses.21 With the presence of Se in the co-polymers, it is believed that the organosulfoselenide DIB products could play a similar role as the plasticizer during charge and discharge, which can lead to improved cycling performance with the better utilization of active materials at a given rate. In addition, PSS90-10 shows noticeably higher Coulombic efficiency than that of PSS100-0 co-polymer, suggesting improved cycling reversibility during charge−discharge process with the chemical confinement of S−SeS2−DIB cathode. In addition, the morphology changes of lithium foils paired with PSS90-10 and PSS100-0 cathodes after 100 cycles were investigated, as shown in Figure S7. The cycled lithium anodes were washed 2−3 times by using 1,3-dioxolane (DOL) solvent to remove the residue salts before scanning electron microscopy (SEM) observation. While the surface of fresh lithium foil is smooth, the cycled lithium foils with co-polymer cathodes are covered with surface layer. Moreover, the lithium anode paired with PSS90-10 cathode shows more uniform and dense layer than that with PSS100-0 cathode (without Se),

PSS100-0@KB600 and PSS90-10@KB600 were further reduced, which indicates the effective physical confinement of PSS100-0 and PSS90-10 co-polymers within the KB600 network. 2.4. Electrochemical Properties of S−DIB, S−SeS2− DIB, and S−SeS2−DIB@KB600 Hybrids. The electrochemical behaviors of the pure S−DIB and S−SeS2−DIB copolymer without KB600 were evaluated as cathodes for Li batteries and presented in Figure 5a,b. The initial cyclic voltammetry (CV) data of PSS100-0 and PSS90-10 copolymers were obtained at a scan rate of 0.02 mV s−1 under the voltage range of 1.8−2.8 V versus Li/Li+ (Figure 5a). Both the PSS100-0 and PSS90-10 show two main reduction peaks and two oxidation peaks. The first peak of PSS100-0 at around 2.3 V can be ascribed to the reduction of elemental S to longchain polysulfides and S−DIB co-polymer to higher order organosulfur DIB units at this potential range.19,21 Additionally, further reduction of the short-chain polysulfides and oligosulfur DIB units to Li2S2/Li2S and fully discharged organosulfur DIB units, respectively, occurred at a lower voltage of 2.03−2.05 V. Similarly, the first peak at 2.3 V of the PSS90-10 can be attributed to the reduction of selenium sulfides to high-order polysulfides/polyselenides and S−SeS2− DIB co-polymer to high-order organosulfoselenide DIB units and shortened oligosulfoselenide units. Further reduction resulted in the formation of fully discharged polysulfoselenides and organosulfoselenide DIB products at 2.01−2.03 V. In the following anodic scan, two broad oxidation peaks at 2.30−2.50 V for PSS100-0 and 2.28−2.45 V for PSS90-10 were consistent with the conversion of short-chain polysulfides/polyselenides to long-chain and low-order polysulfoselenides to high-order polysulfoselenides, which has also been reported for hetero29569

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Figure 6. (a) Long-term cycling of PSS90-10@KB600 under 1000 mA g−1 for 500 cycles, the cell was tested under 100 mA g−1 for the first five cycles for activation process; SEM images of PSS90-10@KB600 cathode (b) before cycling and (c) after 500 cycles.

shown in Figure S8, addition of all cathode samples rendered the Li2S6 solution light yellow after 24 h, indicating good adsorption and confinement of polysulfides. With the presence of selenium and KB600, interestingly, PSS90-10 (sample 3) and PSS90-10@KB600 (sample 5) showed much stronger chemical/physical confinement capability of polysulfides, compared to those of PSS100-0 and PSS100-0@KB600. Moreover, such results are corroborated by the visual observation on the color change of separators from the cycled cells with pure S, PSS100-0, PSS90-10, and PSS90-10@KB600 cathodes, as shown in Figure S9. The separators from PSS9010 and PSS90-10@KB600 cells showed the least color change (to yellow) than those from pure S and PSS100-0 cells, indicating the effective confinement and suppression of the dissolution and diffusion of polysulfides/polyselenides with the presence of chemical bonds from selenium and physical confinement from conductive carbon network. In addition, the insets show the corresponding cycled cathodes, which exhibit more stable structures of PSS90-10 and PSS90-10@KB600 compared to pure S cathode. The rate capabilities of PSS100-0@KB600 and PSS90-10@ KB600 at various current densities ranging from 100 to 2000 mA g−1 are presented in Figure 5f. Other S−SeS2−DIB@ KB600 hybrids with different contents of SeS2 are shown in Figure S10. The PSS90-10@KB600 exhibited the highest rate capability of 585 mAh g−1 among all of the S−SeS2−DIB@ KB600 hybrids even under the maximum current density of 2000 mA g−1, which was a significant improvement compared to the performance (400−450 mAh g−1) of polymeric sulfur (without Se) prepared with the same amount of DIB in the literature.21 As the current density was decreased back to 100 mA g−1, the reversible specific capacity of the PSS90-10@

indicating the positive effects of the Se-containing co-polymer in effectively protecting the lithium metal anode. To further enhance the electronic conductivity of S−SeS2− DIB co-polymer and provide effective paths for the fast electron transport at high rates, KB600 was utilized as a conductive network. As shown in Figure 5c, PSS90-10@KB600 cathode delivers much higher specific capacity (1027 mAh g−1) and less overpotential than those of pure PSS90-10 co-polymer (814 mAh g−1) with the introduction of KB600. Meanwhile, the PSS90-10@KB600 cathode shows better electrochemical behaviors than PSS100-0@KB600 based on the shape of CV curves (Figure 5d). Importantly, the reduction peak with the lower overpotential of PSS90-10@KB600 was much sharper than those of PSS100-0@KB600 due to the enhanced electrochemical reaction kinetics with the S−SeS2−DIB copolymer. Figure 5e presents initial charge−discharge profiles of PSS100-0@KB600 and PSS90-10@KB600 hybrids at a current density of 100 mA g−1. In good agreement with the CV curves, two typical discharge plateaus were observed for both hybrids. The higher initial capacity as well as the lower overpotential of PSS90-10@KB600 (1218 mAh g−1; PSS100-0@KB600: 1002 mAh g−1) can be ascribed to the enhanced electrochemical utilization of S−SeS2−DIB co-polymers. To further confirm the effective chemical/physical confinement of reaction intermediates by co-polymers and copolymer@KB600 composites, 0.5 M Li2S6 in DOL/1,2dimethoxyethane (DME) (1:1, v/v) solution was prepared by mixing sulfur and lithium sulfide at the stoichiometric ratio. Since the co-polymers in this work were sulfur rich, Li2S6 was used as the representative reaction intermediate. An equivalent amount of active materials (co-polymers and co-polymer@ KB600) was added to Li2S6 solution and rested for 24 h. As 29570

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ACS Applied Materials & Interfaces KB600 cathode was recovered to ∼812 mAh g−1, whereas the PSS100-0@KB600 cathode showed noticeably lower capacity (below 750 mAh g−1). The enhanced rate performance and improved reversibility/efficiency of the PSS90-10@KB600 hybrid can be attributed to the enhanced kinetics as well as the synergistic combination of physical porous KB600 network and chemical co-polymer matrix confinement of the reaction intermediates during cycling. The electrochemical impedance spectroscopy (EIS) results of PSS100-0@KB600 and PSS90-10@KB600 electrodes before/after different cycles at 500 mA g−1 are shown in Figure S11. Compared with PSS100-0@KB600 without Se, much lower charge-transfer resistances were observed in PSS90-10@KB600 hybrid both before and after the first cycling. PSS90-10@KB600 cathode shows much smaller charge-transfer resistance than that of PSS100-0@KB600 even after 50 and 100 cycles, indicating that more stable SEI layer with higher conductivity has been achieved with the presence of selenium according to the previous report,45 which is consistent well with the improved rate capability of PSS9010@KB600 cathode (Figure 5f). To further investigate the effect of the novel molecular structure of S−SeS2−DIB@KB600 hybrids on long-term cycling performance, a new cell based on the PSS90-10@ KB600 composite was assembled and cycled for 500 cycles under a constant current of 1000 mA g−1. The cell was first activated at 100 mA g−1 for five cycles. The PSS90-10@KB600 cathode exhibited a very stable cycling performance for up to 500 cycles with a decay rate of 0.0549% per cycle with a high CE of over 98.2%, as demonstrated in Figure 6. The SEM images of the PSS90-10@KB600 cathodes taken before and after 500 cycles clearly showed good structural stability without the appearance of cracks and large agglomerates. These results further attest that the chemical bonds of C−S, C−Se, and S− Se as well as enhanced electrode kinetics resulting from the introduction of SeS2 into S-rich co-polymer can lead to stable cycling performance and good rate capability of Li metal batteries.

cycling. This work provides new insights on the design and properties of polymeric cathode materials for advanced Li metal batteries. Furthermore, the synthesis strategy demonstrated in this study can be broadly applied to the scalable processing of sulfur-selenium-based materials with tunable properties, suggesting a substantial potential in improving the performance of various energy systems.

4. EXPERIMENTAL SECTION 4.1. Materials. Elemental S (99.5%), SeS2 (97%), lithium sulfide (99.9%, metals basis), and LiNO3 (99.999%) were purchased from Alfa-Aesar. 1,3-Diisopropenylbenzene (DIB, >97%, stabilized with 4tert-butylcatechol) was from TCI America. Poly(vinylidene fluoride) (PVDF), super P 45, and lithium metal (99.9%, 15.6 mm diameter × 0.45 mm thickness) were supplied by MTI. Ketjenblack EC600JD (KB600) was provided by Lion Specialty Chemicals Co., LTD. Bis(trifluoromethane) sulfonamide lithium salt (LiTFSI, 99.95%), 1,3dioxolane (DOL), and 1,2-dimethoxyethane (DME) were from Sigma-Aldrich. All solvents were utilized as received. 4.2. Preparation of S−DIB and S−SeS2−DIB Co-polymer. The S−DIB (denoted as PSS100-0) was synthesized through a previously reported method.15 First, elemental S powder was heated to 180 °C in a thermostat oil bath to obtain an orange molten phase. Then, 10 wt % DIB was directly added into the molten sulfur with a pipette to form a homogeneous mixture under rapid agitation for 10 min, followed by cooling down to room temperature. The S−SeS2− DIB co-polymers with various S/SeS2 weight ratios, denoted as PSS(100 − x) − x, where x is the percentage of SeS2 in the S/SeS2 mixture, were synthesized using the modified inverse vulcanization method. The concentration of DIB was controlled at 10 wt % based on the total weight of S or S plus SeS2 for all samples. For example, to synthesize PSS90-10 co-polymer, 0.1 g of selenium disulfide powder was dissolved in molten sulfur (0.9 g) under 155 °C to obtain a uniform mixture of orange color. After that 10 wt % of DIB (ca. 120 μL) was added dropwise into the mixture at 180 °C under vigorous stirring for 10 min to prepare the PSS90-10 co-polymer. The prepared co-polymer was then collected using a metal spatula and ground into a fine powder. 4.3. Preparation of S−DIB@KB600 and S−SeS2−DIB@KB600 Hybrids. The S−DIB co-polymer, i.e., PSS100-0, was thoroughly mixed with KB600 at a weight ratio of 70:30 to yield a black mixture. The mixture was then put into a tube furnace (MTI, OTF-1200X) and heated at 160 °C for 12 h under Ar atmosphere. The PSS100-0@ KB600 composites were collected into a glass vial and placed in a vacuum oven (MTI Corporation) for 24 h. PSS95-5@KB600, PSS9010@KB600, and PSS85-15@KB600 hybrids were prepared using the similar method. 4.4. Material Characterization. Scanning electron microscope (SEM) images and elemental mapping were obtained by using a FEI SEM Quanta 200 F (Field Emission Instruments) equipped with an energy-dispersive X-ray spectroscopy (EDS) attachment. The element analysis was performed to confirm the precise contents of S and Se in the final co-polymers. TGA was conducted using a thermogravimetric analyzer (Discovery TGA, TA Instruments). The sample was placed in a platinum pan and heated from room temperature to 600 °C at a rate of 10 °C min−1 under a nitrogen purge of 10 mL min−1. Differential scanning calorimetry (DSC) analysis was carried out using a Mettler Toledo DSC under a nitrogen atmosphere. The samples were heated from 25 to 300 °C at a rate of 10 °C min−1. The amount of the samples was controlled as ∼10 mg. XRD was performed with a Rigaku Miniflex 600. The tube was operated at 40 kV accelerating voltage and 15 mA current. Liquid-state 13C NMR of DIB was obtained on a 600 MHz NMR spectrometer (Agilent) equipped with a 5 mm liquid probe at ambient temperature (∼25 °C), and solid-state 13C cross-polarization magic angle spinning (CP/ MAS) NMR spectra were recorded on a 600 MHz solid-state NMR spectrometer (Bruker, Germany) with a 3.2 mm MAS probe at a spinning speed of 20 kHz at 25 °C. XPS analysis was performed on a Kratos AXIS-165 multitechnique electron spectrometer system with

3. CONCLUSIONS In summary, we report the sulfur-rich polymeric selenium sulfides as advanced cathode materials for Li metal batteries. By using a ring-opening copolymerization, SeS2 was successfully introduced to the S−DIB system to obtain the S−SeS2− DIB co-polymers with various S/SeS2 ratios. Solid-state 13C CP/MAS NMR and XPS results confirmed the formation of C−S, C−Se, and S−Se bonds from S−SeS2−DIB copolymerization. The presence of Se in S-rich co-polymer not only improved the electrochemical utilization with higher CE, but also enhanced the electrode reaction kinetics and led to the formation of more stable solid electrolyte interphase layer with much lower charge-transfer resistance. Porous carbon host KB600 was also employed to further enhance the physical confinement effect and provide paths for the effective electron transport at high rates. Among all co-polymer electrodes investigated in this study, S−SeS2−DIB@KB600 hybrid with an optimal S/SeS2 feed mass ratio of 9:1 exhibited the highest initial specific capacity (1218 mAh g−1 at 100 mA g−1) and superior rate capability (585 mAh g−1 at 2000 mA g−1). Importantly, a high overall CE of over 98.2% with a low decay rate of 0.0549% per cycle was obtained during 500 cycles at 1000 mA g−1 due to the synergistic combination of chemical and physical confinements of the reaction intermediates during 29571

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monochromatized Al Kα radiation (12 kV, 144 W) under a base pressure of 10−8 Pa. 4.5. Electrochemical Measurements. The S−SeS2−DIB@ KB600 hybrid was mixed with super-P 45 and PVDF binder to form the slurry at the weight ratio of 7:2:1. The slurries of the cathodes were coated onto an aluminum foil (MTI) and dried overnight under vacuum at 60 °C. The resulting electrode was then punched into disks with an area of 1.13 cm2. The active material loading of the individual cell was approximately 0.8 mg cm−2. S− DIB@KB600 electrode was fabricated in the same way. Pure S, PSS100-0, and PSS90-10 cathodes (without KB600) were made with the ratio of active material, super-P 45, and PVDF binder as 49:41:10 for comparison. All specific capacities reported in this study were calculated based on the mass of the co-polymer. The electrochemical characterization was carried out using a CR2032-type coin cell (Wellcos Corporation) with Celgard 2400 membrane as separators and a lithium metal as an anode. The electrolyte was prepared by dissolving 1 M LiTFSI in a mixed solvent (1:1 volume ratio) of DOL and DME with 0.2 M LiNO3 additive. The galvanostatic charge and discharge measurements were carried out under various current densities at a voltage range from 1.8 to 2.8 V (vs Li/Li+) using a battery cycler (BTS4008, NEWARE). CV and EIS measurements were performed on a potentiostat (VSP 200, Bio-Logic) system. CV tests were performed at a scan rate of 0.02 mV s−1 from 1.8 to 2.8 V. EIS data were collected in a frequency range from 5 MHz to 10 mHz with an AC oscillation amplitude of 10 mV.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b09062. Digital images of selenium sulfides; S−DIB, and S− SeS2−DIB co-polymers; TGA and XRD of raw materials like S, SeS2, DIB, and KB600; DSC measurements of elemental S, S−SeS2, and co-polymers; SEM images and EDS mappings of PSS100-0@KB600 and PSS90-10@ KB600; electrochemical properties of S−DIB and S− SeS2−DIB co-polymers as cathodes for Li metal batteries (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kee Sung Han: 0000-0002-3535-1818 Xiahui Zhang: 0000-0002-9122-7598 Min-Kyu Song: 0000-0002-8998-3853 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a start-up fund of Prof. M.-K.S. at Washington State University. The authors would like to acknowledge the Franceschi Microscopy & Image Center at Washington State University for SEM measurements and the kind help of Dr. Louis Scudiero for XPS measurements. The authors acknowledge Yuehong Zhang and Lang Huang for their kind help on TGA measurements. P.D. acknowledges the China Scholarship Council for the financial support. The NMR measurements were performed at the Environmental Molecular Sciences Laboratory (EMSL), a national user scientific facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). 29572

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