Silica-templated hierarchically porous carbon modified separators for lithium–sulfur batteries with superior cycling stabilities

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Silica-templated hierarchically porous carbon modified separators for lithium–sulfur batteries with superior cycling stabilities Changhoon Choi, Dong-Wan Kim * School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, 02841, South Korea

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Hierarchically porous-carbon-modified glass fiber separators are fabricated. � The textural property of porous carbon is regulated by using different sized SiO2. � The efficient polysulfide trapping and high capacity are achieved for Li–S batteries. � Superior cycle stability is achieved by design of more hierarchal pore distribution.

A R T I C L E I N F O

A B S T R A C T

Keywords: Li–S battery Separator modification Polysulfide shuttle effect Porous carbon

Lithium–sulfur batteries (LSBs) have attracted considerable attention for use in next-generation rechargeable storage devices owing to their high theoretical capacities (1675 mA h g 1) and natural abundance of sulfur. However, the commercialization of LSBs is hindered by the polysulfide shuttle effect and unstable cycling per­ formances of the conventional cell configurations. As the separator is a crucial component of the cell assembly, separator modification is considered an effective approach to the fabrication of a high-performance LSB without the use of a sophisticated cathode. In this study, hierarchically porous carbons are used for the fabrication of multi-functional glass fiber (GF) separators as upper current collectors and polysulfide trapping materials. An optimized porous carbon (denoted as MC-SM) is fabricated by tuning the porosity properties such as the Bru­ nauer–Emmett–Teller surface area and pore distribution. The MC-SM-coated GF separator provides excellent discharge capacity of 1019 mA h g 1 and Columbic efficiency (~100%) at a current density of 0.2C after 150 cycles. Even at high current rates, the cell with the fabricated porous carbon can deliver considerable reversible capacities of 700 mA h g 1 at 1C and 591 mA h g 1 at 2C after 500 cycles.

1. Introduction The recent development of electric vehicles and portable electric devices has led to an increasing demand for rechargeable lithium-ion batteries (LIBs). The conventional LIBs cannot provide high-

performance energy storage devices owing to the low theoretical ca­ pacities of the active electrode materials [1]. To overcome this issue, various studies on post-LIB technologies, such as lithium–sulfur batteries (LSBs) [2,3], lithium–oxygen batteries [4,5], and aqueous batteries [6] have been carried out. In particular, the LSB provides high theoretical

* Corresponding author. E-mail address: [email protected] (D.-W. Kim). https://doi.org/10.1016/j.jpowsour.2019.227462 Received 10 September 2019; Received in revised form 26 October 2019; Accepted 13 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Changhoon Choi, Dong-Wan Kim, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227462

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specific capacity of 1675 mA h g 1 and theoretical energy density of 2600 W h kg 1, and thus is regarded as the most promising post-LIB candidate. In addition, several advantages of elemental sulfur, such as its natural abundance, environmental benignity, low cost, and non­ toxicity, are also beneficial for its use as a preferable cathode material for next-generation energy storage devices [7]. Despite the advantages, the commercialization of LSBs has been severely impeded by several drawbacks of the electrode materials and Li–S battery system. First, the inherent electronically insulating struc­ ture of sulfur (σ ¼ 5 � 10 30 S m 1 at 25 � C) and its reduced discharge products (Li2S/Li2S2) lead to a large internal resistance and inefficient active material utilization [8]. Second, the dissolution and shuttle effect of soluble lithium polysulfides (Li2Sy, 4 � y � 8) in organic electrolytes during the multi-step electro–chemical process cause various adverse effects including a rapid capacity fading, low Coulombic efficiency, irreversible loss of active materials, and lithium anode corrosion [9–11]. The shuttle effect of easily soluble long-chain polysulfides is likely the most challenging drawback affecting the overall operation of an LSB. Third, the very large volume expansion (~80%) during the electro­ –chemical conversion reaction between sulfur (2.03 g cm 3) and Li2S (1.66 g cm 3) can lead to a severe pulverization of the active materials, followed by an unstable electro–chemical contact within the cathode and fast capacity reduction [12,13]. Over the past few decades, numerous studies have been carried out to overcome the above drawbacks and improve the electro–chemical properties of LSBs. The mainstream approaches have focused on the fabrication of well-designed sulfur cathode materials for the encapsu­ lation/immobilization of active materials by introducing suitable host materials such as porous carbon [14,15], hollow carbon [16,17], carbon nanofibers [18,19], and conductive polymers [20]. Such approaches not only improve the electrical conductivity of the cathode and reduce the migration of lithium polysulfides by trapping them during the cycling, but also accommodate the volume change between the sulfur and Li2S. Despite the progress in the electro–chemical performances of LSBs, such strategies commonly involve elaborate and costly syntheses, limited sulfur loading masses in the cathodes, and consumption and/or pro­ duction of toxic chemicals, which hinder the industrial-scale Li–S bat­ tery production [21]. Furthermore, to the best of our knowledge, the composite requirements for the commercialization of reliable Li–S bat­ teries including a high loading mass of sulfur (�70 wt%, �5 mg cm 2), stably high capacity (�800 mA h g 1, 6 mA h cm 2), optimized electrolyte-to-sulfur ratio, and scaled-up electrode size (�1 cm2) have not been simultaneously achieved by a large number of reported LSBs [22]. Hence, in-depth studies on a scalable and low-cost approach are required to achieve a high-performance high-sulfur-loading LSB. To overcome the inherent limitations in the engineering of sulfur cathode materials, further progress on the modification of the cell configuration beyond the cathode side is needed. As an essential component of an LSB, the porous separator is an electrical insulator and maintains a large Li-ion transport between the electrodes [23]. How­ ever, the widely used conventional polymer separators such as poly­ propylene (PP) [24,25] and polyethylene (PE) [26] cannot inhibit the shuttle effect. In this regard, since Manthiram et al. have reported super-P-carbon-coated functional separators for the regulation of the lithium polysulfide diffusion without the use of a sophisticated cathode [27], the research trend on high-performance LSBs has changed with a large increase in the number of publications related to the fabrication of multi-functional separators [28,29]. Various conductive carbon mate­ rials including graphene [30], graphene oxide [31], carbon nanotubes [32], and various types of porous carbon [33,34] have been actively investigated as upper current collectors and polysulfide diffusion bar­ riers in LSBs. With these carbon-based-material-coated separators, the modifica­ tion of the PP and PE separators has been regarded as a promising approach to improve the electro–chemical properties of LSBs. Further studies have been carried out on materials with stronger chemical

interactions such as metal oxides/sulphides [35] and heteroatom (N, P, etc.)-doped carbon [36,37] to enhance the polysulfide trapping effect of multi-functional separators. However, inorganic chemical-interaction materials generally need to be combined with carbon-based materials [38,39]. Their complex and/or unfavorable synthetic processes for large-scale production can restrict their practical applications [40]. In this respect, practical synthesis methods for useful polysulfide trapping materials and suitable coating processes for their large-scale fabrications are required. The glass fiber (GF) separator has attracted attention as an alterative separator owing to its higher porosity (~65%), which can lead to a higher ionic conductivity and rapid ionic transport by the large elec­ trolyte intake in a rechargeable battery system [41,42]. In this study, we developed effective designs of silica-templated hierarchically porous carbon modified multi-functional GF separators through a simple vac­ uum filtration process. In addition, a series of meso–macroporous car­ bon types was fabricated through a simple colloidal silica nanocasting route by using phenol as a precursor. The porosity of the obtained car­ bon could be hierarchically designed through the selection of suitable colloidal silica, which inspired us to focus on the influence of the porosity on the LSB with simply designed pure sulfur electrodes. The silica-templated hierarchically porous carbon coated GF separator suc­ cessfully served as both polysulfide trapping material and upper current collector. In addition, the LSBs with the separators designed in this study exhibited stable and improved electro–chemical performances including outstanding rate capabilities, which indicate an improved electro­ –chemical utilization of sulfur and reactivation of the trapped active materials in the fabricated porous-carbon-coated layer. Moreover, the overall fabrication process, including the synthesis of the porous carbon and fabrication of the carbon-modified GF separator, can be carried out on a large scale, which indicates that our multi-functional separator has considerable potential for practical applications in LSBs. 2. Experimental 2.1. Preparation of silica-templated hierarchically porous carbon The synthesis of the hierarchically porous carbon was carried out by using a simple silica-templated nanocasting route based on previous reports [43,44]. In a typical synthesis, 78 mmol of phenol (99.0%, Samchun Chemicals) and 185 mmol of formaldehyde (35 wt%, Samchun Chemicals) were dissolved in 50 mL of a 0.2-M NaOH solution (98.0% bead, Samchun Chemicals) under magnetic stirring at 70 � C for 40 min. Subsequently, 50 g of Ludox SM-30 (average particle size of 7 nm, 30-wt % SiO2 suspension in H2O, Aldrich) was added to the above solution through continuous stirring at 70 � C for another 30 min. The mixture was then transferred to a sealed bottle and heated in a mineral oil bath for 72 h. The obtained hydrogel was completely dried at 90 � C in a convection oven for 24 h, and then pulverized with a mortar and pestle. The pulverized powder was carbonized at 800 � C for 3 h at a heating rate of 5 � C min 1 under an argon atmosphere. The SiO2 of the carbonized product was fully etched by using 200 mL of a 2-M NaOH solution at 80 � C for 48 h. Subsequently, repetitive washing with deionized water (DW) was carried out until the pH became constant. Finally, the hier­ archically porous carbon, denoted as MC-SM, was obtained by freeze-drying the washed product. In addition, another product, denoted as MC-TM, was prepared by using the above procedure, but with 30 g of Ludox TM-50 (average particle size of 22 nm, 50-wt% SiO2 suspension in H2O, Aldrich) as a colloidal silica precursor and additional 20 mL of DW. 2.2. Fabrication of hierarchically porous carbon coated GF separators The hierarchically porous carbon (MC-SM and MC-TM) synthesized in this study was coated onto one side of a commercially available porous GF separator (Whatman, GF/F) through a facile vacuum filtra­ tion process. Subsequently, 50 mg of the prepared porous carbon was 2

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mixed with 3 mL of polyethylene glycol (PEG, average molecular weight of 300, Samchun Chemicals) and 2 mL of isopropyl alcohol (IPA) by using a mortar and pestle. The mixed slurry was diluted by adding 8 mL of an additional IPA and stirred for 12 h. Subsequently, 3 mL of the mixed porous carbon/PEG solution was dispersed in 300 mL of IPA by ultrasonication for 20 min. The well-dispersed hierarchically porous carbon suspension was then vacuum-filtered on a GF separator. The fabricated porous-carbon-coated GF separator was dried at 70 � C for 24 h in a convection oven and punched into circular disks for a coin cell assembly. The optimized thickness of the coating layer was approxi­ mately 50 μm, which could be adjusted by controlling the amount of the mixed porous carbon/PEG solution.

out by using a Raman Fourier-transform infrared spectrometer (HORIBA Jobin Yvon, LabRAM ARAMIIS IR2). The carbon, oxygen, and hydrogen contents were measured by an elemental analysis (EA, Elementar, VarioMicro Cube). The Brunauer–Emmett–Teller (BET) specific surface areas and Barrett–Joyner–Halenda (BJH) pore size distributions of the prod­ ucts were analyzed by using an N2 adsorption–desorption process at 77 K with a BELSORP-max instrument (MicrotracBEL Corporation). For a polysulfide trapping test, a 1-M Li2S8 solution was prepared by dis­ solving pure sulfur and Li2S (99.9%, Alfa Aesar) in a molar ratio of 7:1 in a mixture of DME and DOL (volume ratio of 1:1) at 60 � C and stirring for 24 h in an argon-filled glove box, which yielded a dark-brown solution. 3. Results and discussion

2.3. Fabrication of simple pure sulfur electrodes

In this study, an effectively customized separator for a highperformance LSB was designed by employing suitable polysulfide trap­ ping materials and commercialized GF separator. The GF membrane has recently emerged as an alternative separator of the LSB owing to its highly porous microstructure, large liquid electrolyte uptake, outstanding thermal stability, and enhanced electro–chemical proper­ ties in a Li–S cell, compared to those of the conventional PP/PE sepa­ rators [45,46]. In addition, the merits of the GF contribute to an even vacuum filtration at a moderate speed, including the decompression and drying steps. An overall schematic of the fabrication of the silica-templated hier­ archically porous carbon modified GF separator and Li–S cell configu­ ration with/without the prepared porous carbon (MC-SM and MC-TM) coating are shown in Fig. 1. To induce the formation of a larger-gradient pore distribution of the synthesized carbon, we used a small-particle aggregated colloidal silica (Ludox SM-30, ~7 nm, Fig. S1a) as the main product (MC-SM) and compared it to a control (MC-TM) consisting of a large-particle relatively stable colloidal silica (Ludux TM-50, ~22 nm, Fig. S1b). Through the sol–gel process and drying in a convection oven, a reddish-brown polymerized hydrogel was formed in a con­ stricted volume (~50%) by using the mixed precursor solution. As shown in the middle of Fig. 1, the surface of the carbonized powder (MCSM-SiO2 and MC-TM-SiO2) prior to the SiO2 etching was densely deco­ rated with SiO2 nanoparticles (NPs), mainly owing to the utilization of a large amount of SiO2 NPs, which could induce pulverization through SiO2 boundaries. As indicated by the digital image of the punched separator in Fig. S2, the porous carbon coating formed by using the welldispersed porous carbon suspension exhibited the form of a dense layer without fissures on the surface. The improved thermal stability and wettability of the GF separator were demonstrated by comparison to a PP separator fabricated in a previous study [47]. The MC-SM-coated GF separator retained the unwavering properties in the thermal shrinkage (Fig. S3) and wettability (dropping of electrolyte of 1 M of LiTFSI in DOL/DME with 0.2 M of LiNO3 on the separator, recorded in Videos S1 and S2) tests. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.227462. Further, we compare the operation mechanism of the MC-SM-coated separator to that of a pristine separator by using the Li–S cell configu­ ration shown in Fig. 1. Soluble lithium polysulfides shuttle through the pristine separator during the cycling, which leads to adverse effects on the Li–S cell, such as the deposition of Li2S/Li2S2 on the Li anode and gradual loss of active materials. In contrast, the MC-SM-coated separator can effectively trap polysulfides on the cathode side by using the dense coated layer, which can impede the considerable decrease in Li–S cell performance during a long-term cycling. The formation of a typical porous morphology on the surface of MCSM is demonstrated through SEM images and corresponding EDS maps before and after the SiO2 etching, as shown in Fig. 2a and b. The EDS analysis of MC-SM-SiO2 shows that the signals of silicon and oxygen are predominant, considering the filling of voids with carbon, whereas after the SiO2 etching (MC-SM), the elemental carbon is distributed uniformly

Simple pure sulfur electrodes were fabricated by casting a slurry consisting of 75 wt% of pure sulfur (99.5%, 325 mesh, Alfa Aesar), 15 wt% of a conductive additive (Ketjen black EC-600JD, Mitsubishi Chemical), and 15 wt% of a polyvinylidene fluoride (Kynar 2801) binder in N-methyl-2-pyrrolidinone (99.5%, Sigma-Aldrich) onto an aluminum foil by using a doctor blade and following drying at 70 � C for 12 h in a convection oven. The active material slurry was prepared by mixing the above three components for 20 min by using a mortar and pestle without the conventional melt diffusion or carbon hosts. For a routine electro­ –chemical evaluation, the cast electrode was punched into circular disks to assemble the coin cells. The sulfur loadings of the as-fabricated electrodes were in the range of 1.1–1.4 mg cm 2. 2.4. Li–S cell assembly and electro–chemical measurement The CR2032-type coin cells were assembled by using simple pure sulfur electrodes, hierarchically porous carbon modified GF separators (MC-SM or MC-TM), and lithium foil anodes (99.9%, Alfa Aesar) in an argon-filled glove box. The electrolyte was prepared by using 1 M of lithium bis(trifluoromethane) sulfonimide (LiTFSI, 99.95%, SigmaAldrich) in a mixed solvent of 1,3-dioxolane (DOL, 99.8%, SigmaAldrich) and 1,2-dimethoxyethane (DME, 99.5%, Sigma-Aldrich) (vol­ ume ratio of 1:1), including 0.2 M of LiNO3 (Sigma-Aldrich) as a cosalt. The porous-carbon-coated GF separator was placed between the pure sulfur cathode and Li anode with the coated layer facing the sulfur cathode. For comparison, a Li–S coin cell with a GF separator was also fabricated. The experimental parameters in the electro–chemical mea­ surement were fixed to evaluate the Li–S cell performance accurately with/without the synthesized porous carbon coating. The amount of electrolyte used for each cell was fixed at 85 μL per 1 mg of sulfur to sufficiently infiltrate the active materials of the cathode during the fabrication. In addition, the assembled Li–S cells were equally allowed to rest for 20 min at room temperature before the electro–chemical mea­ surement. A galvanostatic charge–discharge test, C-rate test, and cyclic voltammetry (CV, scan rate of 0.2 mV s 1) were carried out in a voltage range of 1.7–2.8 V by using an automatic battery cycler (4300 K Desktop, MACCOR). All galvanostatic capacities of the Li–S cells were calculated based on the sulfur loading masses. 2.5. Characterization The porous morphologies of the products were characterized by using field-emission scanning electron microscopy (SEM, Hitachi, SU70) with an energy-dispersive X-ray spectroscopy (EDS) detector and transmission electron microscopy (TEM, JEOL, JEM-2100). Cross-sec­ tion images of the hierarchically porous carbon modified GF separators were acquired by using a normal SE microscope (COXEM, CX-200) to identify the coating thicknesses of the synthesized porous carbons. X-ray diffraction (XRD) patterns of the powdered products were measured by using an X-ray diffractometer (Rigaku, Ultima III) with Cu Kα radiation (λ ¼ 1.5406 Å) in a 2θ range of 10–60� . A spectral analysis was carried 3

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Fig. 1. Schematic of the fabrication of the silica-templated hierarchically porous carbon modified GF separator and Li–S cell configuration with or without a coating layer of the prepared porous carbon.

Fig. 2. Surface SEM images and corresponding EDS maps of (a) MC-SM-SiO2 and (b) MC-SM, (c) XRD patterns of MC-SM and MC-SM-SiO2, and (d) Raman spec­ troscopy (inset: EA) of MC-SM.

in the selected SEM image. This shows the high efficiency of the SiO2 etching by using a 2-M NaOH solution, which provided the porous morphology of the product. Although the signal of silicon almost dis­ appeared, oxygen was evenly detected in the EDS map of MC-SM, which indirectly demonstrates the presence of oxygen-containing functional groups on the surface of MC-SM. The XRD patterns of MC-SM-SiO2 and

MC-SM were analyzed to confirm the removal of numerous silica NPs through the etching, as shown in Fig. 2c. The XRD patterns of MC-SMSiO2 can be indexed to cristobalite SiO2 (International Center for Diffraction Data (ICDD) PDF#76–0935) [48,49]. The SiO2 peaks were not observed in the XRD pattern of MC-SM, consistent with the EDS analysis. In addition, two broad diffraction peaks at 2θ of ~24 and ~44� 4

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are observed in the XRD pattern of MC-SM, which can be assigned to the (002) and (100) planes of the hexagonal graphite, respectively. This indicates that MC-SM is a typical amorphous carbon with a non­ crystalline structure [50,51]. Raman spectroscopy and EA were employed to further investigate the structure of MC-SM. As shown in Fig. 2d, the Raman spectrum of MCSM shows two well-known peaks, at 1350 and 1600 cm 1, which correspond to the D and G bands, respectively. The D band is attributed to structural defects, edge defects, and dangling sp3 band of disordered carbon [52], whereas the G band originates from the E2g vibration mode of graphitic sp2-bonded carbon [53]. These two bands indicate the successful conversion of the polymerized hydrogel into the porous car­ bon upon the carbonization and etching, consistent with the EDS and

XRD analyses. The D-to-G band-integrated intensity ratio (ID/IG) is commonly calculated to evaluate the crystallinity of a carbon material [54,55]. ID/IG of MC-SM was 1.74, which indicates a highly disordered carbon, consistent with the XRD pattern (Fig. 2c). In addition, the EA of MC-SM (carbon: 74.93 wt%; oxygen: 9.45 wt%; hydrogen: 1.59 wt%) indicates the functionalization of MC-SM with oxygen-containing func­ tional groups, in agreement with the EDS analysis, as summarized in the inset table in Fig. 2d. The total elemental content calculated by the EA was smaller than 100 wt%, owing to residual silicon-based amorphous inorganic impurities in MC-SM. This phenomenon was commonly observed in previous studies on porous carbon materials synthesized by chemical etching [56–58]. The abundantly porous structure of carbon is one of the key

Fig. 3. High-magnification SEM images of (a) MC-SM and (b) MC-TM and low- and high-magnification TEM images of (c, e) MC-SM and (d, f) MC-TM, respectively. 5

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requirements for the fabrication of high-quality functional carboncoated separators because it can accommodate the sulfur and poly­ sulfides while facilitating the electrolyte infiltration [59]. To further study the impact of the hierarchically porous structure of the synthe­ sized carbon to achieve an outstanding polysulfide trapping and eval­ uate the enhanced electro–chemical properties, two types of porous carbon with different sizes of the colloidal silica NPs were fabricated (MC-SM and MC-TM). The morphologies of the fabricated porous car­ bons were characterized by using SEM and TEM, as shown in Fig. 3. The high-magnification SEM and low-magnification TEM images in Fig. 3a–d shows that all products exhibited typical disordered porous micro­ structures with uniformly distributed pores having various sizes. Notably, the rough and porous surfaces of all products reflect the se­ lective chemical etching for evenly distributed silica NPs. MC-TM exhibited a relatively uniform pore size distribution based on the NP size (~22 nm) of the utilized colloidal silica (Ludox TM-50), whereas MC-SM exhibited a hierarchical pore size distribution in the range of several nanometers to tens of nanometers, as shown in Fig. 3e and f. To further analyze the pore structures of MC-SM and MC-TM, the textural properties of the products were assessed by the N2 absorp­ tion–desorption isotherms, as summarized in Fig. 4 and Table 1. MC-SM and MC-TM exhibited similar isotherm shapes, combinations of type-II and -IV isotherms, including the shape of the H1-type hysteresis loop, whereas MC-SM-SiO2 and MC-TM-SiO2 exhibited type-IV isotherms, according to the International Union of Pure and Applied Chemistry (IUPAC) classification. Thus, the pore structures of MC-SM and MC-TM were reconstructed through the chemical etching. The H1-type hyster­ esis loops at relatively high pressures (p/p0 � 0.6) are typical

Table 1 Specific BET surface areas and total pore volumes of MC-SM-SiO2, MC-TM-SiO2, MC-SM, and MC-TM (p p0 1 ¼ 0.990). Sample

SBET (m2 g 1)

Vtot (cm3 g

MC-SM-SiO2 MC-TM-SiO2 MC-SM MC-TM

152.49 143.49 1058.3 582.23

0.1974 0.2366 3.6682 2.3323

1

)

characteristics of mesoporous materials associated with the agglomer­ ates of approximately homogeneous SiO2 NPs [43,60]. In addition, the long tails at p/p0 of approximately 1.0 in Fig. 4a indicate that MC-SM and MC-TM exhibited wide porous networks and abundant macro­ porous structures [34]. The specific BET surface area and total pore volume of MC-SM were 1058.3 m2 g 1 and 3.6682 cm3 g 1, whereas those of MC-TM (582.23 m2 g 1 and 2.3323 cm3 g 1, respectively) were smaller. In addition, the BJH pore size distribution of MC-SM shows the existence of numerous mesopores and macropores having sizes of a few nanometers to ~200 nm and hierarchical pore distribution, compared to MC-TM (Fig. 4c). Their open macroporous structures with mesopores are beneficial to maximize the reutilized sulfur loading by providing more reactive positions of the trapped sulfur and facilitating the electrolyte infiltration [61]. Moreover, porous carbon materials with small meso­ pores have the unique advantage of trapping soluble polysulfides, considering the largest chain length (~2 nm) of Li2S8, and thus enhance the electro–chemical properties of the LSBs [62]. The polysulfide trapping properties of the as-fabricated porous-car­ bon-coated separators have key roles in the operations of the LSBs. The

Fig. 4. (a, b) N2 adsorption–desorption isotherms and (c, d) BJH pore size distributions of MC-TM and MC-SM before and after the SiO2 etching, respectively. 6

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permeabilities of the polysulfides through the three types of separator (pristine, MC-TM, and MC-SM) were analyzed to visually demonstrate the effect of each prepared carbon type on the suppression of the poly­ sulfide shuttling, as shown in Fig. 5. Small vials containing 1-M Li2S8 solutions in DOL/DME (volume ratio: 1:1) were separated by using separators prepared by using a blank DOL/DME solvent. After a small vial with a pristine separator was placed in a blank DOL/DME solvent, the polysulfide solution started to rapidly diffuse out of the small vial and the color of the solution in the outer vial changed to reddish brown after 16 h, as shown in Fig. 5a. In the case of MC-SM, the color of the DOL/DME solution in the outer vial exhibited the smallest change after 16 h, which implies that MC-SM effectively blocked the polysulfide diffusion across the separator. In contrast, in the case of MC-TM, the color of DOL/DME gradually changed to light brown, compared to the outer vial with the pristine GF. This demonstrates that the hierarchical porous structure of the carbon material effectively suppressed the pol­ ysulfide shuttling effect. Thus, the electro–chemical performances of Li–S cells could be improved by employing the MC-SM- and MC-TMcoated GF separators. Its favorable properties including the hierarchical pore distribution with a high porosity, oxygen-containing functional groups, and sup­ pression of the polysulfide shuttling suggest that the obtained porous carbon could simultaneously serve as a polysulfide trapping material and upper current collector in the LSB. In the advanced Li–S cell as­ semblies considered in this study, we matched the coating thicknesses of MC-SM and MC-TM (50 μm) (Fig. S4) for an accurate electro–chemical measurement. To identify the electro–chemical kinetic processes in the Li–S cells with the hierarchical MC-SM and MC-TM, the dis­ charge–charge profiles at a current density of 0.2C (1C ¼ 1675 mA h g 1) during the ten initial cycles were investigated, as presented in Fig. 6a–c. All discharge–charge profiles of the cells with and without the porous carbon coatings exhibited two reduction plateaus (I and II) and two broad oxidation plateaus (IV and V), associated to the well-known two-step sulfur redox reactions. The upper reduction plateau I in­ dicates the rapid kinetic conversion of elemental sulfur (S8) into highorder soluble polysulfides, whereas the lower reduction plateau II is attributed to the strong reduction of soluble polysulfides into the insoluble Li2S2 and Li2S [2,63]. During the charging, the two continuous oxidation plateaus IV and V originated from the reverse reaction of Li2S/Li2S into Li2S4 and oxidation of Li2S4 into Li2S8, and eventually into sulfur [33]. Notably, the first discharge capacities of the cells with the prepared porous-carbon-coated separators exceed the theoretical discharge capacity of pure sulfur, whereas their first charge capacities are considerably smaller than the discharge capacities, as shown in Fig. 6a and b. The irreversible discharge capacity related to the addi­ tional reduction plateau III below 1.85 V is attributed to the capacitive energy storage of the porous carbon in the fabricated carbon-coated separator, typically observed in previous studies on interlayers and modified separators with porous carbons for LSBs [55,64]. The sloping plateau III gradually disappeared during the initial five cycles, as indi­ cated by the yellow box in Fig. 6a and b. The analysis of the potential difference (i.e., polarization, ΔE) be­ tween the charge and discharge curves at a depth of discharge of 50% reveals the advantages of the configurations of the MC-SM- and MC-TM-

coated separators, as shown in Fig. 6a–c. The cell assembled with the pristine GF exhibited shrunk reduction/oxidation plateaus and consid­ erably increased polarization, from 0.54 V (ΔE1) to 0.74 V (ΔE), after the ten cycles owing to the loss of active materials and formation of nonconductive Li2S2/Li2S agglomerates on the surface of the Li anode [34,65]. Despite the use of equivalent additive carbon/sulfur ratios in the pure sulfur cathodes in all cells, ΔE of the cell with MC-SM was significantly reduced to 0.18 V after the ten cycles, only 24% of that of the cell with the pristine GF. A similar trend was observed for the cell with MC-TM, as shown in Fig. 6b. In particular, for the cell with MC-SM, the oxidation plateau IV initially decreased and ΔE decreased, from 0.27 to 0.18 V, during the first ten cycles, which indicates that the active materials in the modified separator gradually rearranged to position at electro–chemically favorable sites [42]. These results indicate that the modifications of the GF separator with MC-SM and MC-TM are equally beneficial to improve the reversibility and redox reaction kinetics of the LSB. CV curves were measured at 0.2 mV s 1 for the first 20 cycles to confirm the enhanced electro–chemical kinetics of the cell with MC-SM, as presented in Fig. 6d. A broad anodic peak and two main cathodic peaks consistent with oxidation/reduction plateaus I–V in the dis­ charge–charge profiles were observed. The CV curves related to the irreversible discharge capacity of MC-SM were overlapped with one of the typical cathodic peaks below 1.85 V in several initial cycles, and then gradually vanished with the increase in number of cycles. After ten cycles, almost overlapped cathodic and anodic peaks were observed with a typical curve shape of a successfully fabricated Li–S cell [33,66], which indicates a high cycling stability and superior reversibility of the cell with MC-SM. The obvious shifts of the oxidation/reduction peaks toward reduced polarizations during the initial cycles are a clear evi­ dence of the stabilization, as mentioned in the analysis of the dis­ charge–charge profiles [67]. In addition, the separation between the two cathodic peaks I and II is attributed to the high polysulfide trapping ability of MC-SM [68]. For a quantitative evaluation of the polysulfide trapping and redox abilities of the fabricated separators, we analyzed the behaviors of the upper discharge plateau capacities (QH) and lower discharge plateau capacities (QL) of the cells at a current density of 0.2C, as summarized in Fig. 7 [27,62]. QH corresponds to a rapid solid-to-liquid phase transition of sulfur to polysulfide with a theoretical capacity of 419 mA h g 1. In contrast, QL corresponds to a slow conversion reaction of polysulfide into the insoluble Li2S2/Li2S with a theoretical capacity of 1256 mA h g 1 [69]. For a reasonable analysis of QH and QL, we used the first modified discharge capacities of the cells with MC-SM and MC-TM to minimize the influences of the irreversible capacities of the porous carbons. The initial QL and QH were calculated as the utilization rates with respect to the theoretical capacities. As shown in Fig. 7a and b, the cells with MC-SM and MC-TM exhibited high initial QH values of 97.9 and 88.8% and QH retention rates (RQH) of 79.3 and 84.9% after 100 cycles, respectively. Similarly, the modified GF separators with MC-SM and MC-TM exhibited high initial QL and QL retention rates (RQL). Despite the high QH of the cell with the pristine GF, which exhibited low RQL of 44.5% and RQH of 37.2% after 100 cycles, the high initial QH and RQH indicate an effective polysulfide trapping [70], whereas the high

Fig. 5. Digital images in the polysulfide diffusion test of the (a) pristine GF, (b) MC-TM-coated GF, and (c) MC-SM-coated GF. 7

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Fig. 6. Discharge–charge profiles in the first ten cycles of the cells with (a) MC-SM, (b) MC-TM, and (c) pristine GF at a current density of 0.2C, and (d) CV curves of MC-SM (scan rate of 0.2 mV s 1).

Fig. 7. Upper/lower discharge plateau capacities of the cells with (a) MC-SM, (b) MC-TM, and (c) pristine GF for 100 cycles at a current density of 0.2C.

initial QL and RQL demonstrate an exhaustive reduction of the trapped polysulfides and outstanding electro–chemical reversibility [32]. The obtained results for MC-SM and MC-TM are remarkable as QH and QL

considerably decreased during the initial stabilizations of the cells. Fig. 8a demonstrates that the introductions of the MC-SM and MCTM coatings on the surfaces of the GFs significantly improved the 8

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capacities of 700 mA h g 1 at 1C and 591 mA h g 1 at 2C with a high reversibility, whereas the cell with MC-TM exhibited a lower capacity of 453 mA h g 1 at 2C. The comparison to similar previous studies on carbon-based polysulfide trapping materials shows the high electro­ –chemical performances of the cell with the proposed hierarchically porous carbon, achieved even without heteroatom doping or melt diffusion applied to the cathode, as summarized in Table S1. Moreover, according to the initial cycles, the cell with MC-SM exhibited a higher polysulfide trapping ability than that of the cell with MC-TM. This suggests that the effective design of a more hierarchal pore distribution is crucial for the fabrication of a high-performance LSB.

cycling performances of the cells at a current density of 0.2C. After 150 cycles, the reversible discharge capacities of MC-SM and MC-TM approached 1019 and 1022 mA h g 1 with Coulombic efficiencies of approximately 100%, corresponding to capacity retentions of 74.1% (0.17% fading per cycle) and 77.1% (0.148% fading per cycle), based on the modified first discharge capacities, respectively. In contrast, a considerable initial capacity decay and low cycling performance (442 mA h g 1, 39.8% after 150 cycles) were observed for the cell with the pristine GF under the same measurement conditions. The C-rate performances and long-term cycling performances at 1C and 2C were measured to demonstrate the advantages of the more hi­ erarchical porous structure, as shown in Fig. 8b and c. In the cyclings at 0.1, 0.2, 0.5, 1.0, and 2.0C, the cell with MC-SM exhibited high reversible capacities of 1,220, 1,152, 1,084, 977, and 804 mA h g 1, respectively. Even at a very high current density of 5.0C, a discharge capacity of 398 mA h g 1 was achieved, which signifies a high rate capability of the cell with MC-SM. The cell with MC-TM exhibited a different trend of the reversible capacities at high current densities (�1C). Typically, lower discharge capacities of 655 and 200 mA h g 1 at 2.0C and 5.0C were measured in the C-rate test of MC-TM, respectively. Thus, the more hierarchical porous structure of MC-SM is advantageous for the fast discharge/charge cycling of the LSB, mainly owing to the abundant macropores of MC-SM, which efficiently favor the Li-ion transfer during the fast cycling, and increased total pore volume of MC-SM with numerous mesopores [71]. The superiority of MC-SM at a high current density was also demonstrated in the long-term cycling performance test, as shown in Fig. 8c. All cells maintained high Columbic efficiencies of approximately 100% after the initial stabiliza­ tion cycles. After 500 cycles, the cell with MC-SM exhibited remarkable

4. Conclusion The simple silica-templated nanocasting route was employed to achieve outstanding electro–chemical performances of the LSB with the hierarchically porous carbon as an effective polysulfide trapping mate­ rial coated onto one side of the pristine GF separator. The porosity of the product was designed based on the sizes of the colloidal SiO2 NPs. The main product, MC-SM, having an optimized hierarchical porous struc­ ture not only reduced the polarization of the cell, but also suppressed the polysulfide shuttling effect during the discharge–charge cycling. Thus, owing to the use of the MC-SM-coated GF separator as the current col­ lector and polysulfide trapping layer, the pure sulfur cathode achieved an outstanding cycling performance with a high capacity retention (1019 mA h g 1 after 150 cycles) and remarkable C-rate performance. In particular, we demonstrated the importance of the use of the porous carbon with a well-designed porosity as an effective modification of the separator by comparing the long-term cycling performances achieved by the two porous carbons having different surface areas, total pore

Fig. 8. (a) Cycling performances at a current density of 0.2C and (b) C-rate performances of the cells with different separators, and (c) long-term cycling perfor­ mances of the cells with MC-SM and MC-TM at high current densities (including the first cycle at 0.1C). 9

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volumes, and pore size distributions at a high current density. This study provides a valuable strategy for the fabrication of multi-functional separators for high-performance LSBs and paves the way for the commercialization of reliable LSBs in the near future.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT (2019R1A2B5B02070203), and by Creative Materials Discovery Pro­ gram through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2018M3D1A1058744). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227462. References [1] N. Nitta, F.X. Wu, J.T. Lee, G. Yushin, Li-ion battery materials: present and future, Mater. Today 18 (2015) 252–264. [2] A. Manthiram, Y.Z. Fu, S.H. Chung, C.X. Zu, Y.S. Su, Rechargeable lithium–sulfur batteries, Chem. Rev. 114 (2014) 11751–11787. [3] M.Y. Wang, X.H. Xia, Y. Zhong, J.B. Wu, R.C. Xu, Z.J. Yao, D.H. Wang, W.J. Tang, X.L. Wang, J.P. Tu, Porous carbon hosts for lithium–sulfur batteries, Chem. Eur J. 25 (2019) 3710–3725. [4] A. Kraytsberg, Y. Ein-Eli, Review on Li–air batteries – opportunities, limitations and perspective, J. Power Sources 196 (2011) 886–893. [5] H.P. Guo, W.B. Luo, J. Chen, S.L. Chou, H.K. Liu, J.Z. Wang, Review of electrolytes in nonaqueous lithium–oxygen batteries, Adv. Sustain. Syst. 2 (2018), 1700183. [6] Y.G. Wang, J. Yi, Y.Y. Xia, Recent progress in aqueous lithium-ion batteries, Adv. Energy Mater. 2 (2012) 830–840. [7] A. Manthiram, Y.Z. Fu, Y.S. Su, Challenges and prospects of lithium–sulfur batteries, Accounts Chem. Res. 46 (2013) 1125–1134. [8] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Li–O2 and Li–S batteries with high energy storage, Nat. Mater. 11 (2012) 19–29. [9] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Lithium–sulfur batteries: electrochemistry, materials, and prospects, Angew. Chem. Int. Ed. 52 (2013) 13186–13200. [10] X.L. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon–sulphur cathode for lithium–sulphur batteries, Nat. Mater. 8 (2009) 500–506. [11] A.F. Hofmann, D.N. Fronczek, W.G. Bessler, Mechanistic modeling of polysulfide shuttle and capacity loss in lithium–sulfur batteries, J. Power Sources 259 (2014) 300–310. [12] G.M. Zhou, S.F. Pei, L. Li, D.W. Wang, S.G. Wang, K. Huang, L.C. Yin, F. Li, H. M. Cheng, A graphene–pure-sulfur sandwich structure for ultrafast, long-life lithium–sulfur batteries, Adv. Mater. 26 (2014) 625–631. [13] J.X. Song, Z.X. Yu, T. Xu, S.R. Chen, H. Sohn, M. Regula, D.H. Wang, Flexible freestanding sandwich-structured sulfur cathode with superior performance for lithium–sulfur batteries, J. Mater. Chem. 2 (2014) 8623–8627. [14] J.X. Song, T. Xu, M.L. Gordin, P.Y. Zhu, D.P. Lv, Y.B. Jiang, Y.S. Chen, Y.H. Duan, D.H. Wang, Nitrogen-doped mesoporous carbon promoted chemical adsorption of sulfur and fabrication of high-areal-capacity sulfur cathode with exceptional cycling stability for lithium–sulfur batteries, Adv. Funct. Mater. 24 (2014) 1243–1250. [15] H.J. Peng, J.Q. Huang, M.Q. Zhao, Q. Zhang, X.B. Cheng, X.Y. Liu, W.Z. Qian, F. Wei, Nanoarchitectured graphene/CNT@porous carbon with extraordinary electrical conductivity and interconnected micro/mesopores for lithium–sulfur batteries, Adv. Funct. Mater. 24 (2014) 2772–2781. [16] C.F. Zhang, H.B. Wu, C.Z. Yuan, Z.P. Guo, X.W. Lou, Confining sulfur in doubleshelled hollow carbon spheres for lithium–sulfur batteries, Angew. Chem. Int. Ed. 51 (2012) 9592–9595. [17] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Porous hollow carbon@sulfur composites for high-power lithium–sulfur batteries, Angew. Chem. Int. Ed. 50 (2011) 5904–5908. [18] G.Y. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Hollow carbon nanofiberencapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries, Nano Lett. 11 (2011) 4462–4467. [19] K. Fu, Y.P. Li, M. Dirican, C. Chen, Y. Lu, J.D. Zhu, Y. Li, L.Y. Cao, P.D. Bradford, X. W. Zhang, Sulfur gradient-distributed CNF composite: a self-inhibiting cathode for binder-free lithium–sulfur batteries, Chem. Commun. 50 (2014) 10277–10280.

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