Spider-Web-Inspired Nanocomposite-Modified Separator: Structural

Jan 9, 2019 - Spider-Web-Inspired Nanocomposite-Modified Separator: Structural and Chemical Cooperativity Inhibiting the Shuttle Effect in Li–S Batter...
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Spider-Web-Inspired NanocompositeModified Separator: Structural and Chemical Cooperativity Inhibiting the Shuttle Effect in Li−S Batteries ACS Nano Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.

Daliang Fang,†,‡ Yanlei Wang,† Xizheng Liu,§ Jia Yu,† Cheng Qian,∥ Shimou Chen,*,† Xi Wang,⊥,# and Suojiang Zhang*,† †

Beijing Key Laboratory of Ionic Liquid Clean Process, CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ∥ Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University, Yangling 712100, China ⊥ Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, Beijing 100044, China # Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China S Supporting Information *

ABSTRACT: Despite their high theoretical capacity density (1675 mAh g−1), the application of Li−S batteries has been seriously hindered by the shuttle effect of polysulfides. Here, inspired by the working principle of natural spider webs, we synthesized a spider-web-like nanocomposite in which many hollow mesoporous silica (mSiO2) nanospheres/Co nanoparticles were threaded by interconnected nitrogen-doped carbon nanotubes (NCNTs). Then the nanocomposite (denoted as Co/mSiO2−NCNTs) was coated on the commercial separator by a simple infiltration to mitigate the above issue. The intimate combination of three-dimensional conductive networks (NCNTs) with abundant polysulfide adsorbent sites (SiO2 and N)/polysulfide conversion catalysts (Co and Co−Nx species) allows the Co/mSiO2−NCNTs coating layer to not only effectively capture polysulfides via both physical confinement and chemical bonding but also accelerate the redox kinetics of polysulfides significantly. Furthermore, the combination of ex situ experiment and theoretical calculation demonstrates that the reversible adsorption/desorption of polysulfides on mSiO2 nanospheres benefits uniform deposition of Li2S2/Li2S on the conductive networks, which contributes to long-term cycling stability. As a result, Li−S batteries with Co/mSiO2−NCNTs-coated separators exhibited both excellent cycling stability and rate performance. KEYWORDS: ZIF-67, spider web, nanocomposite, modified separator, Li−S batteries

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ithium−sulfur (Li−S) batteries have been treated as a promising next-generation energy storage system because of their high theoretical capacity, low cost, and nontoxicity. However, their application has been seriously © XXXX American Chemical Society

Received: October 1, 2018 Accepted: January 9, 2019 Published: January 9, 2019 A

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Scheme 1. Schematic Illustration of the (a) Synthetic Process of Co/mSiO2−NCNTs and (b) Li−S Battery Employing a Co/ mSiO2−NCNTs-Coated Separator

problem. Actually, the intrinsic slow reaction kinetics of polysulfides is also another key factor which causes the shuttle effect. It has been discovered that catalytic metal (e.g., Ni, Pt, and Co)19−21 and conductive compounds (e.g., CoS2, MoS2, and TiN)22−24 can accelerate conversion of polysulfides through catalytic effect. Recent works have demonstrated that using these materials to make interlayers can accelerate the redox kinetics of polysulfides, thus improving the electrochemical properties of Li−S batteries.25−27 Nevertheless, the adsorption abilities of these catalysts are usually inferior to nonconductive polar inorganics.27 Therefore, it is still a big challenge to design an ideal functional interlayer to effectively anchor and reutilize polysulfides. For such an interlayer, it should simultaneously have the following advantages: (1) strong physical and chemical interaction with polysulfides to effectively capture polysulfides: (2) fast electron and lithium ion pathways to reutilize anchored polysulfides; (3) abundant catalytic active sites to promote the redox kinetics of polysulfides. Here, as shown in Scheme 1a, we in situ synthesized a nanocomposite through pyrolysis of ZIF-67 encapsulated by mesoporous silica (ZIF-67@mSiO2), in which hollow mSiO2 nanospheres/cobalt (Co) nanoparticles were threaded by many interconnected nitrogen-doped carbon nanotubes (NCNTs). As natural spiders can effectively catch insects by their sticky webs with special structures, this spider-web-like nanocomposite (denoted as Co/mSiO2−NCNTs) can effectively capture insect-like polysulfides via both physical confinement and chemical bonding (Scheme 1b). The physical confinement is due to its spider-web-like structure with abundant mesopores, which also benefits permeation of electrolyte and transfer of lithium ions. The strong chemical bonding derives from the strong interaction between polysulfides with the mSiO2 and N heteroatom on the “web”, and the three-dimensional conductive networks formed by interconnected NCNTs can also provide

hindered by the low conductivity of sulfur and Li2S and the dissolution of long-chain polysulfides in electrolyte during repeated discharge/charge processes. Among these issues, dissolution of polysulfides in the electrolyte is the most fatal one, causing fast capacity decay and a low Coulombic efficiency. The polysulfides dissolved in the electrolyte diffuse to be reduced by the Li anode and return to be oxidized by the cathode, which is known as the “shuttle effect” of polysulfides. Coating functional materials on a commercial polypropylene (PP) separator as polysulfide-blocking interlayers has been proven to be a facile and low-cost approach to inhibit the shuttle effect. Among these functional materials, carbonaceous materials (including porous carbon,1,2 graphene/graphene oxide,3,4 carbon nanotubes/nanofibers,5,6 and their hybrids7) have attracted tremendous attention. These carbonaceous materials can not only suppress the shuttle effect of polysulfides by physical barrier but also reutilize the polysulfides dissolved in the electrolyte as the upper collector. However, due to the weak interaction between nonpolar carbonaceous materials and polar polysulfides, the shuttle effect of polysulfides cannot be effectively suppressed by only physical confinement.8 Although heteroatom (e.g., O, B, N, S) doping can strengthen the interaction between carbonaceous materials with polysulfides, this interaction is still not strong enough.9−12 Recently, it was found that some nonconductive polar inorganics (e.g., Al2O3, SiO2, TiO2) can effectively adsorb polysulfides due to their strong chemical interaction with polysulfide.13−15 However, the conductivity of these inorganics is usually poor, compromising the reutilization of adsorbed polysulfides. Therefore, the combination of these inorganics with carbon-based materials to fabricate functional interlayers on separators can integrate the merits of each other.16−18 However, the only cooperation of physical barrier with chemical bonding still cannot completely solve the shuttle effect B

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Figure 1. (a) FESEM image. (b) TEM image. (c,d) Magnified TEM images. (e,f) High-resolution TEM images of Co/mSiO2−NCNTs. SEM images of Co/mSiO2−NCNTs-coated PP separator: (g,h) top suface (the inset in (g) shows a digital photo) and (i) cross section.

elemental mapping images of the resulting ZIF-67@mSiO2 further confirm successful coating (Figure S3). The final product Co/mSiO2−NCNTs was obtained by a simple pyrolysis of ZIF-67@mSiO2 at 700 °C under Ar/H2 mixed with DMF vapor. Figure 1a−c shows typical field-emission scanning electron microscopy (FESEM) and TEM images of Co/ mSiO2−NCNTs. Co/mSiO2−NCNTs was composed of hollow mSiO2 nanospheres which were threaded by a large number of NCNTs. The inside of the hollow nanosphere contained many 5−15 nm Co nanoparticles (nanospheres) in Figure 1d, which is also confirmed by the element mappings (Figure S4). Based on N2 sorption isotherms (Figure S5a), Co/mSiO2−NCNTs had a Brunauer−Emmett−Teller (BET) surface area of 293.1 m2 g−1 and a wide pore-size distribution of 1−160 nm, centered at 30 and 70 nm (Figure S5b). Figure 1e shows that a single CNT grew out of the thin wall (around 10 nm) of the hollow mSiO2 nanosphere, confirming that the hollow mSiO2 nanosphere is indeed threaded by these NCNTs. The high-resolution TEM image of the tip of the CNT confirms the existence of Co nanoparticles encapsulated in graphitic carbon shells (Figure 1f), demonstrating catalyzation of Co in forming CNTs.30 The interplane distances of the outside shell and the inner nanoparticle were ∼0.34 and ∼0.20 nm, which corresponded to the C (002) plane and the Co (111) plane, respectively.31 The XRD patterns of Co/mSiO2−NCNTs also demonstrate the presence of Co nanoparticles and CNTs (Figure S5c). Many defects can be identified on the walls of CNTs in Figure 1f, which is also proven by the Raman spectrum of Co/mSiO2−

fast electron multipathways to reutilize the anchored polysulfides. Furthermore, abundant Co nanoparticles and Co−Nx species on the “web” can accelerate the redox kinetics of these polysulfides by a catalytic effect. Co/mSiO2−NCNTs was coated on the PP separators by simple infiltration and then used in the Li−S batteries. Owing to structural and chemical cooperativity, the Co/mSiO2−NCNTs interlayer can be expected to efficiently trap and reutilize the polysulfides dissolved in the electrolyte. Indeed, compared with the PP separators, Co/mSiO2−NCNTs-coated separators improve the cycling and rate performance of Li−S batteries significantly. We further carried out a detailed electrochemical test, spectroscopic/microscopic analysis, and theoretical calculations to reveal the mechanism of improved properties.

RESULTS AND DISCUSSION The synthesis process of Co/mSiO2−NCNTs is illustrated in Scheme 1a. In the first step, ZIF-67 was synthesized as reported previously. X-ray diffraction (XRD) patterns of the products (Figure S1) matched well with that of previous works.28 As expected, as-prepared ZIF-67 nanoparticles exhibited a typical dodecahedral shape with an average size of around 200 nm (Figure S2a,b). Then the nanoparticles were coated with a mSiO2 layer through the hydrolysis of tetraethyl orthosilicate with cetyltrimethylammonium bromide as a pore-directing template.29 After being coated with mSiO2, the surface of ZIF-67 become obviously rougher, as shown in Figure S2c,d. The scanning transmission electron microscopy (TEM) image and corresponding energy-dispersive X-ray spectroscopy (EDS) C

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Figure 2. Electrochemical properties of Li−S batteries with pristine separator, (Co/NCNTs@mSiO2)/CNTs-coated separator, and Co/ mSiO2−NCNTs-coated separator: (a) initial discharge/charge curves at 0.1C; (b) cycling performances at 0.1C; (c) rate performances at different current densities; (d) discharge/charge curves of the Li−S battery with Co/mSiO2−NCNTs-coated separator at different current densities; (e) long cycle performance of the Li−S battery with Co/mSiO2−NCNTs-coated separator at 1C.

NCNTs in Figure S5d (The ID/IG ratio is 1.22). Such highly disordered carbon may derive from nitrogen doping into CNTs. To better understand the role of DMF vapor in the heating process, ZIF-67@mSiO2 pyrolyzed without DMF vapor was also prepared. In this condition, ZIF-67@mSiO2 turned into Co nanoparticles and NCNTs encapsulated in hollow mSiO2 nanospheres (denoted as Co/NCNTs@mSiO2) as shown in Figure S6. The XRD patterns and Raman spectrum of Co/ NCNTs@mSiO2 further confirms the existence of Co and NCNTs (Figure S7a,b). The NCNT content of Co/NCNTs@ mSiO2 was only 11.67% (Figure S7c), which was much lower than ∼71% of Co/mSiO2−NCNTs (Figure S7d). Therefore, we can infer that DMF provides carbon and nitrogen sources for the growth of NCNTs under Co catalyzation. Once the limited space of hollow mSiO2 nanospheres cannot accommodate so many NCNTs, some NCNTs would get out through the mesopores of hollow mSiO2 to form Co/mSiO2−NCNTs. The full X-ray photoelectron spectroscopy (XPS) of Co/ mSiO2−NCNTs in Figure S8a proved the existence of C, N, Si,

O, and Co elements. The three peaks in the C 1s spectrum (Figure S8b) were at 284.8, 285.7, and 289.6 eV, corresponding to CC, C−N, and −COO components, respectively. In addition, the N 1s spectrum (Figure S8c) can be fitted into three types of N species: pyridinic N (398.6 eV), pyrrolic N (400.2 eV), and graphitic N (401.3 eV). Nitrogen doping can strengthen the interaction between CNTs with the polysulfides.32 Moreover, the Co 2p3/2 spectrum displayed in Figure S8d showed two prominent peaks, including metallic Co at 778.3 eV and Co−N at 779.9 eV. It is believed that both metallic Co and Co−Nx species are effective catalysts for polysulfides.33,21 The hollow SiO2 nanospheres can effectively absorb the polysulfides due to both physical confinement and chemical binding.14,34 Therefore, the unique structure and components of Co/mSiO 2−NCNTs has the following advantages: (1) Abundant mesopores and polysulfide adsorbent sites (SiO2 and N) can endow Co/mSiO2−NCNTs to effectively capture polysulfides through physical and chemical interactions. (2) The interwoven NCNTs provide fast electron transportation/ D

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discharge plateau at around 2.3 V is assigned to the conversion of cyclo-S8 to long-chain polysulfides (LixS2, x = 4−8), and the lower discharge plateau (∼2.1 V) corresponds to further reduction of long-chain polysulfides to lithium sulfides (Li2S2/ Li2S). The two successive charge potential plateaus at about 2.3 and 2.4 V are attributed to the oxidations of polysulfides to S8.4 The battery with the (Co/NCNTs@mSiO2)/CNTs-coated separator had a voltage hysteresis (ΔE) lower than that of the pristine separator, which indicates that the (Co/NCNTs@ mSiO2)/CNTs layer can promote the electrochemical reactions of polysulfides, thus increasing the utilization of sulfur materials. When the Co/mSiO2−NCNTs layer was applied, ΔE was further reduced. This may result from a better conductive connection between Co/mSiO2 and CNTs in Co/mSiO2− NCNTs than that in (Co/NCNTs@mSiO2)/CNTs, which is also confirmed by electrochemical impedance spectroscopy (EIS) results where the battery with the Co/mSiO2−NCNTscoated separator showed the lowest charge-transfer impedance (Figure S14a). Figure 2b exhibits the cycling performances of batteries with three kinds of separators at 0.1C. The Li−S battery with Co/mSiO2−NCNTs-coated separator revealed the most stable cycling performance. Its initial discharge capacity was 1294 mAh g−1. After 50 cycles, the capacity decreased to 1067 mAh g−1 with the capacity retention of 82.4%. The capacity retentions of batteries with a (Co/NCNTs@mSiO2)/CNTs separator and pristine separator after 50 cycles were 63.5% (824 mAh g−1) and 38.5% (432 mAh g−1), respectively. The cycling stability of the Li−S battery with a Co/mSiO2−NCNTs-coated separator was further tested by CV test (Figure S14b). The typical cathodic peaks and anodic peaks were in agreement with the discharge/charge curves. Notably, after the first two cycles, the following curves overlapped, suggesting high reversibility and stability. As a contrast, the CV curves of the Li−S battery with pristine separator cannot completely overlap during the first five cycles (Figure S14c). The areal mass of Co/mSiO2− NCNTs on the separator is also a key factor that influences the electrochemical performance of batteries. After 50 cycles at 0.1C, the discharge capacities of the batteries with Co/mSiO2− NCNTs interlayers of 0.1, 0.25, and 0.5 mg cm−2 decayed to 681, 1064, and 975 mAh g−1, respectively (Figure S14d). When the areal mass of Co/mSiO2−NCNTs interlayers is too low, the polysulfides cannot be completely blocked. When the areal mass of Co/mSiO2−NCNTs interlayers is too high, lithium ion diffusion would be negatively influenced and the energy density would also decrease. Therefore, the areal mass of Co/mSiO2− NCNTs interlayers was optimized at 0.25 mg cm−2. The rate performances of batteries with the three kinds of separators are illustrated in Figure 2c. Compared with the batteries based on pristine and (Co/NCNTs@mSiO2)/CNTscoated separators, the battery with a Co/mSiO2−NCNTscoated separator delivers superior rate performance. With an increasing current density, the reversible capacities slightly decreased from 1262 to 1193, 1075, 1012, and 915 mAh g−1 at rates of 0.1, 0.2, 0.5, 1, and 2C. Even at a high current density of 5C, the reversible capacity of 552 mAh g−1 can still be maintained. When the current density reverted to 0.1C, the original capacity was almost retrieved, suggesting excellent capacity reversibility. Figure 2d shows the discharge/charge curves of the battery with the Co/mSiO2−NCNTs-coated separator at different current densities. As the current density increased, ΔE becomes larger and the corresponding capacities decreased. It should be noted that, although the (Co/NCNTs@ mSiO2)/CNTs interlayer has almost the same constituents as

lithium ion paths to reutilize polysulfides. (3) Many catalysts (Co and Co−Nx) can accelerate the conversion of polysulfides. Hence, Co/mSiO2−NCNTs was expected to be an ideal material for making functional interlayers to improve electrochemical properties. Co/mSiO2−NCNTs was coated on the single side of a commercial PP separator through vacuum filtration. Figure 1g reveals that a large number of Co@mSiO2 nanospheres were well distributed in the interlaced NCNTs. The magnified SEM images (Figures 1h and S9) clearly show that these Co@mSiO2 nanospheres were threaded in conductive networks formed by NCNTs, not just mixed with NCNTs as (Co/NCNTs@ mSiO2)/CNTs shown in Figure S10. The Co/mSiO2−NCNTs interlayer was uniformly coated on the PP separator, and the thickness was about 11.3 μm (Figure 1i). Due to the strong adhesion of NCNTs to the PP separator, the Co/mSiO2− NCNTs-coated PP separator without any binder was still integrated after being cut into a wafer (Figure S11a). For comparison, the coating layer of the (Co/NCNTs@mSiO2)/ CNTs-coated PP separator fell off a little along the edge (Figure S11b). The situation was much worse for the Co/NCNTs@ mSiO2-coated PP separator (Figure S11c). The ability of the Co/mSiO2−NCNTs-decorated separator to prohibit the diffusion of polysulfides was tested by an H-type visualized glass cell (Figure S12). The left chamber was filled with 0.1 M Li2S6 in DOL/DME solvent (a typical polysulfide), and the right chamber was filled with pure DOL/DME solvent. The two chambers were separated by the pristine separator (top row) or the Co/mSiO2−NCNTs-decorated separator (bottom row). The pure DOL/DME solvent turned from colorless to dark brown after 24 h, demonstrating the rapid diffusion of polysulfides through the pristine separator. In contrast, little color change happened in the right chamber of the glass cell with the Co/mSiO2−NCNTs-decorated separator, revealing that the Co/mSiO2−NCNTs-decorated separator can effectively inhibit the diffusion of polysulfides. The lithium ion diffusion coefficients of the battery with pristine and Co/mSiO2− NCNTs-coated separators were investigated by cyclic voltammetry (CV) tests with different sweep rates (Figure S13). The cathodic peaks at 2.3−2.2 and 2.0−1.9 V were assigned as peaks A and B, and the anodic peak at 2.4−2.5 V was peak C (Figure S13a,c). Based on the linear fitting of the peak currents (Figure S13b,d), the lithium ion diffusion coefficients were calculated according to the Randles−Sevick equation35 (Table S1). It is interesting that the diffusion coefficients at peaks A, B, and C for the Co/mSiO2−NCNTs-coated separator were all almost twice that of the pristine separator. Instead of blocking the lithium ion transfer, the Co/mSiO2−NCNTs interlayer actually promoted the process significantly. This is due to abundant mesopores of the Co/mSiO2−NCNTs interlayer, which improves electrolyte wettability and reduces impedance of lithium ion diffusion. The above results confirm the great potential of the Co/mSiO2− NCNTs-coated separator in suppressing the shuttle effect of polysulfides. To monitor the practical effect of the Co/mSiO2−NCNTscoated separator on the electrochemical properties, the cycle and rate performances of batteries with a Co/mSiO2−NCNTscoated separator were examined. Meanwhile, performances corresponding to (Co/NCNTs@mSiO2)/CNTs-coated separator and pristine separator were also compared. The initial discharge/charge curves of batteries based on different separators are shown in Figure 2a. The two discharge potential plateaus are due to the multistep reduction of sulfur. The upper E

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Figure 3. (a,b) SEM images of the Co/mSiO2−NCNTs interlayer after 100 cycles. (c) Corresponding elemental mapping images. (d) TEM image, (e) magnified TEM image, and (f) high-resolution TEM image of cycled Co/mSiO2−NCNTs. (g) Corresponding element mapping. (h) Comparison of Co 2p XPS spectrum of the Co/mSiO2−NCNTs interlayer before and after cycling. (i) S 2p XPS spectrum of the cycled Co/ mSiO2−NCNTs interlayer.

the Co/mSiO2−NCNTs interlayer, both cycling stability and rate performance of batteries with (Co/NCNTs@mSiO2)/ CNTs-coated separators were inferior to those with Co/ mSiO2−NCNTs. This confirms that the better conductive connection between CNTs and Co/mSiO2 in Co/mSiO2− NCNTs than that in (Co/NCNTs@mSiO2)/CNTs leads to more effective capture and reutilization of the polysulfides. The long-term cyclic performance of Li−S batteries with Co/ mSiO2−NCNTs-coated separator was further evaluated at 1C (Figure 2e). After 250 cycles, the battery retained a discharge capacity of 774 mAh g−1 with a capacity retention of 77% and a low capacity decay rate of 0.09% per cycle, demonstrating an excellent cycling stability. Overall, the electrochemical performance of the Li−S battery with a Co/mSiO2−NCNTs-coated separator can be comparable with many state-of-the-art reported works related with carbon-based separators (Table S2). To meet the practical demand, it is of importance to achieve high mass loading of sulfur in Li−S batteries. Figure S15 shows that, under a high sulfur loading of about 5.76 mg cm−2, the battery exhibits an initial discharge capacity of 932 mAh g−1 (5.37 mAh cm−2) with a capacity retention of 87% after 35 cycles at 0.1C, proving the potential of a Co/mSiO2−NCNTs-coated separator for future practical use. To further reveal the mechanism of the Co/mSiO2−NCNTs interlayer in improving the electrochemical properties of Li−S batteries, we disassembled the batteries with/without a Co/ mSiO2−NCNTs interlayer after 100 cycles (at a discharged state of 1.8 V) and then soaked their electrodes in DOL/DME solution. As shown in Figure S16, the solution in the left glass vessel is much clearer than that in the right one, revealing that

most polysulfides in electrolyte were absorbed by the Co/ mSiO2−NCNTs interlayer. The surface of the lithium anode from the cycled battery with a Co/mSiO2−NCNTs interlayer is obviously smoother than that without a Co/mSiO2−NCNTs interlayer (Figure S17), demonstrating that the Co/mSiO2− NCNTs interlayer can effectively prevent polysulfides from crossing the separator to react with the lithium anode. SEM images showed that the Co/mSiO2−NCNTs coating can keep the structure stable during cycles, and the active materials immobilized by the Co/mSiO2−NCNTs coating can be obviously observed, according to Figure 3a,b. The strong and even-dispersed sulfur signals on the coated layer in Figure 3c evidence that the migrating polysulfides are certainly trapped in the Co/mSiO2−NCNTs coating. We further carried out TEM to study the detailed morphology of cycled Co/mSiO2− NCNTs. As shown in Figure 3d, the cycled Co/mSiO2− NCNTs still maintained its original structure very well, except for some polysulfides absorbed on its surface, consistent with the SEM results. The perfect structure stability is mainly caused by the following two reasons: (1) the Co/mSiO2−NCNTs interlayer does not participate in the lithiation reaction at the potential range between 1.8 and 2.8 V, which is demonstrated by the Figure S18; (2) many voids in the Co/mSiO2−NCNTs contribute to accommodate volume change of polysulfides during repeated lithiation/dislithiation. The magnified TEM image in Figure 3e shows some nanoparticles with a size about 2−3 nm appearing in the polysulfides adsorbed on the NCNTs. The interplane distances of these nanoparticles are 0.23 and 0.32, corresponding to the LiF (111) plane and Li2S (111) plane,36 respectively (Figure 3f). The LiF nanoparticles may F

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Figure 4. (a) SEM image, (b) TEM image, and (c) corresponding element mapping images of the cycled Co/mSiO2−NCNTs interlayer at a discharge state of 2.1 V. (d) SEM image, (e) TEM image, and (f) corresponding element mapping images of the cycled Co/mSiO2−NCNTs interlayer at a discharge state of 1.8 V. (g) Atomic structure of Li+, [TFSI]−, DOL, DME, Li2S4, and a half of the symmetrical simulation model in the present work, where green, yellow, gray, and cyan are used to represent Li+, Li2S4, DOL, and [TFSI]−, respectively. (i) Schematic diagram showing the mechanism of the Co/mSiO2−NCNTs in trapping and reutilizing polysulfides during the discharge process.

polysulfides, thus enhancing the electrochemical properties of Li−S batteries. To further understand the adsorption and conversion processes of polysulfides on Co/mSiO2−NCNTs interlayers, distribution of sulfur moieties within Co/mSiO2−NCNTs interlayers at different discharge states was investigated by SEM and TEM. Figure 4a exhibits the SEM image of Co/ mSiO2−NCNTs extracted from a cycled cell discharged to 2.1 V. At this potential, S8 is completely converted into soluble polysulfides (Li2Sx, x = 4−8).38 The mSiO2 nanospheres were covered by a polymer-like layer (considered as polysulfides), and the surface of nearby NCNTs was relatively clear. The corresponding TEM image (Figure 4b) also shows that mSiO2 nanospheres were completely covered by polysulfides, whereas only tiny polysulfides were unevenly adsorbed on the exposed NCNTs, which is further proven by the overlap of S and Si element mappings in Figure 4c. Figure 4d exhibits the SEM image of Co/mSiO2−NCNTs obtained from another cycled cell, which was discharged to 1.8 V. At this potential, soluble polysulfides would completely turn into solid Li2S2/Li2S.38 In this case, interestingly, the surface of the mSiO2 nanosphere was relatively clear, whereas the nearby NCNTs were covered by a polymer-like layer (considered as Li2S2/Li2S), which was also proven by the corresponding TEM image (Figure 4e), in which NCNTs were almost totally covered by Li2S2/Li2S. The element mappings in Figure 4f exhibit that the distribution of S was much wider than that of Si, revealing that most Li2S2/Li2S was adsorbed on the NCNTs rather than the mSiO2 nanospheres.

derive from the decomposition of remaining LiTFSI in the polysulfides under electron beam irradiation,37 and the Li2S nanoparticles result from the reduction of long-chain polysulfides into Li2S2/Li2S. The selected area electronic diffraction (SAED) patterns of cycled Co/mSiO2−NCNTs also confirm the existence of Li2S and LiF nanoparticles (Figure S19). Figure 3e shows no existence of Li2S nanoparticles on the surface of mSiO2 nanoparticles, which is further proved by the EDS element mapping images of the cycled Co/mSiO2−NCNTs in Figure S20. Although S was evenly dispersed on the cycled Co/ mSiO2−NCNTs, nearly no S signals were found on the surface of the mSiO2 nanosphere (Figure 3g). This may be due to the insulation of SiO2, in which polysulfides adsorbed on the hollow mSiO2 nanospheres need to migrate to nearby NCNTs to be further reduced into Li2S2/Li2S.14,34 The interaction between polysulfides and Co/mSiO2−NCNTs was investigated by XPS. Figure 3h,i shows the Co 2p and S 2p XPS spectra of cycled Co/ mSiO2−NCNTs interlayers, respectively. The slightly negative shift of the Co−N bond and the appearance of the Co−S bond can be assigned to the chemical interaction between Co and polysulfides. The peak at about 162.7 eV in the spectrum of S 2p can be attributed to S−C binding, which indicates a chemical bond between polysulfides and NCNTs. Moreover, the interaction between SiO2 and polysulfides can be verified by the peak at about 167.3 eV, which is considered as S−O bonding. The above results prove that the Co/mSiO2−NCNTs interlayer can effectively anchor and reutilize the dissolved G

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Figure 5. (a) Digital images of Li2S6 solutions before and after contact with mSiO2−NCNTs, Co/mSiO2−CNTs, and Co/mSiO2−NCNTs composites. (b) CVs at the scan rate of 10 mV s−1 and (c) EIS of Li2S6 symmetric cells employing mSiO2−NCNTs, Co/mSiO2−CNTs, and Co/ mSiO2−NCNTs as the identical electrodes. (d) Rate performances of Li−S batteries with mSiO2−NCNTs, Co/mSiO2−CNTs, and Co/mSiO2− NCNTs-coated separators. (e) Schematic illustration of dual-catalyst Co−Nx species of Co/mSiO2−NCNTs promoting adsorption and the redox reaction of polysulfides during charge and discharge.

Moreover, to study the roles of Co and N atoms in polysulfide conversion, we prepared mSiO2−NCNTs and Co/mSiO2− CNTs after removing Co and N under certain conditions. We first investigated the polysulfide adsorption abilities of Co/ mSiO2−NCNTs, mSiO2−NCNTs, and Co/mSiO2−CNTs in Figure 5a. First, 0.01 M Li2S6 dissolved in DOL/DME (1:1) solution was used as a typical polysulfide. Then the same mass (3.0 mg) of the above three materials was added to 10 mL of the solution. After 1.0 h, although both Co/mSiO2−CNTs and Co/ mSiO2−NCNTs decolored the Li2S6 solution, the latter did more completely than the former. As a contrast, mSiO2− NCNTs powders only changed the solution color from yellow into light yellow. Consequently, the adsorption ability of the three materials can be as follows: mSiO2−NCNTs < Co/ mSiO2−CNTs < Co/mSiO2−NCNTs. The existence of both Co and N contributes to the strong chemical interaction and good affinity with polysulfides, which is believed to promote the redox kinetics of polysulfides.33 To validate the view, we designed Li2S6 symmetric cells by using the three materials as two identical electrodes. As shown in Figure 5b, compared with mSiO2−NCNTs and Co/mSiO2−CNTs symmetric cells, the Co/mSiO2−NCNTs exhibited the highest current densities, which means the fastest conversion of polysulfides on the electrolyte/electrode surface.39 The redox reaction of polysulfides was negatively influenced by removing Co or N of Co/ mSiO2−NCNTs, which demonstrates the synergistic catalytic effect of Co and N in conversion of polysulfides. Electrochemical impedance spectra of these symmetric cells also revealed that the Co/mSiO2−NCNTs possessed the lowest charge-transfer impedance (Figure 5c), which reflects that both Co and N can contribute to reduce the interfacial charge-transfer impedance. Therefore, we infer that Li−S batteries with a Co/mSiO2− NCNTs coating separator should have a rate performance superior to those with Co/mSiO2−CNTs or mSiO2−NCNTs coating separators. This conjecture is confirmed by Figure 5d.

To understand the above phenomenon, as shown in Figure 4g, we carried out molecular dynamics simulations to study the distribution of Li2S4 as the concentration (c) of Li2S4. In Figure 4h, when the c is low (0.08 mol/L), the density of Li2S4 near aSiO2 is lower than that in the liquid region, implying that Li2S4 would mainly exist in the latter. However, when c increases, a density peak of Li2S4 would emerge near the a-SiO2, showing that it would aggregate near the solid surface. When c increases continually, the intensity of the peak would be further enhanced and the position of the peak would be closer to the substrate, revealing that more Li2S4 molecules are adsorbed on the a-SiO2. The molecular dynamics simulations demonstrate that when the concentration of polysulfides in the electrolyte is high, these polysulfides tend to be adsorbed on the mSiO2 nanospheres. As most polysulfides converted into solid Li2S2/Li2S and the concentration of polysulfides in electrolyte decreased to a low value, the polysulfides adsorded on the mSiO2 nanospheres would desorb and return back to electrolyte. Therefore, we infer the mechanism of Co/mSiO2−NCNTs in trapping and reusing the polysulfides, as illustrated in Figure 4i. At the beginning of the discharge process, the liquid long-chain polysulfides (Li2Sx, x = 4−8) would be preferably adsorbed on these active sites (Co, N, and SiO2). Compared with pure carbon, these sites have stronger chemical interaction with the polysulfides. As the discharge process continues, the polysulfides attached on Co or N atoms are gradually reduced into Li2S2/ Li2S. Near the end of the discharge process (the concentration (c) of polysulfides in electrolyte decreased to a low value), the polysulfides anchored on the hollow mSiO2 nanospheres would desorb and transfer to nearby pure carbon to be further reduced into Li2S2/Li2S. Therefore, the reversible adsorption/desorption ability of mSiO2 for polysulfides not only prevents the diffusion of polysulfides but also benefits an even deposition of Li2S2/Li2S on the conductive surface of Co/mSiO2−NCNTs, both of which are crucial for long-cycling stability. H

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filtered through the commercial PP separator (Celgard 2400). The resulting Co/mSiO2−NCNTs-coated separator was dried at 50 °C for 12 h under vacuum and then punched into circular disks with a diameter of 16 mm. The weight loading of the Co/mSiO2−NCNTs coating was about 0.25 mg cm−2. Separators coated with (Co/ NCNTs@mSiO2)/CNTs, Co/NCNTs@mSiO2, mSiO2−NCNTs, and Co/mSiO2−CNTs were also prepared by the same method. Particularly, in (Co/NCNTs@mSiO2)/CNTs coating, the weight ratio of Co/NCNTs@mSiO2 to CNTs is 1:2, indicating that the mass proportion of CNTs in (Co/NCNTs@mSiO2)/CNTs coating is almost the same as that in Co/mSiO2−NCNTs. Characterizations. The XRD patterns of the products were determined by a Bruker D8 Focus X-ray diffractometer equipped with a Ni-filtered Cu Kα radiation (λ = 0.15406 nm) source. Raman spectra were tested by LabRAM HR800 (Horiba Jobin-Yvon) using a 514 nm Ar ion laser. The morphology and structure of the products were characterized by a scanning electron microscope (JEOL JSM-7001F) and a transmission electron microscope (JEOL JEM-2100F). Elemental mappings were obtained using EDS attached to the JEM-2100 and JSM-7001F. The BET-specific surface areas of products were collected by a Quadrasorb SIMP analyzer at liquid-nitrogen temperature. Thermogravimetric analysis was collected using a STA7200RV apparatus (Hitachi High-Tech). XPS measurements were carried out by a Thermo Fisher Scientific ESCALAB 250Xi instrument. Electrochemical Measurements. The Super P/S cathode was prepared by mixing sulfur, Super P and poly(vinylidene fluoride) (PVDF) binder at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP). The resulting homogeneous slurry was coated on aluminum foil, followed by drying at 80 °C for 12 h. Coin 2025 cells were assembled in an Ar-filled glovebox by using Super P/S cathode, modified or pristine PP separators, and Li foil anodes. The electrolyte was 1.0 M LiTFSI in DOL/DME (1:1 by volume) with 1 wt % LiNO3. The areal loading of sulfur in the Super P/S cathode is about 1.15 mg cm−2, and a high loading of about 5.76 mg cm−2 was also evaluated. The ratio of electrolyte to sulfur (E/S) was 10 mL g−1. The galvanostatic charge−discharge measurements were tested on a Neware battery testing system. The cyclic voltammetry and electrochemical impedance spectroscopy were measured by an Autolab (PGSTAT302N) electrochemical workstation. Symmetrical Cell Assembly and Measurements. The electrodes for symmetrical cells were fabricated without the presence of active material sulfur. Each electrode material (Co/mSiO2−NCNTs, Co/ mSiO2−CNTs, and mSiO2−NCNTs) and PVDF binder at a weight ratio of 4:1 was dispersed in NMP. The resultant slurry was coated on aluminum foil. After thorough drying, the electrode disks with a diameter of 14.0 mm were punched out of slurry-coated aluminum foils. Those disks were used as identical working and counter electrodes, and 40 μL of electrolyte containing 0.1 mol L−1 Li2S6 and 1 mol L−1 LiTFSI dissolved in DOL/DME (v/v = 1/1) was added as electrolyte. CV measurements and EIS tests of the symmetrical cells were conducted on an Autolab (PGSTAT302N) electrochemical workstation. Details of Molecular Dynamics Simulations. All the molecular dynamics simulations in this work were completed using a large-scale atomic/molecular massively parallel simulator.40 The time step for integrating Newtonian equations of motion was 0.5 fs. To simulate the LiTFSI + DOL/DME + Li2S4 liquids in the amorphous silica layer, we built a model as shown in Figure 4g, where the substrate was amorphous silica with a thickness of 2.1 nm, and the cross section of the system was 6.3 × 6.4 nm2. In the simulations, the LiTFSI + DOL/DME + Li2S4 solution with a concentration of 1.0 M lithium salt was considered, and for the case c (L2S4) = 0.08 M, the system consisted of 1761 DOL molecules, 1188 DME molecules, 216 Li+, 216 TFSI−, and 20 Li2S4 molecules. The periodic boundary condition was adopted along three directions. The OPLS-AA potential41 was employed for the LiTFSI, DOL, and DME molecules. The modified optimized Tersoff potential42,43 was employed to describe the covalent interaction between silicon and oxygen atoms in amorphous silica. The interactions between amorphous silica, Li+, TFSI−, DOL/DME, and Li2S4 include two parts: van der Waals interactions and electrostatic terms. The former one was described using the Lennard-Jones potential 4ε[(σ/r)12

Therefore, as shown in Figure 5e, the presence of Co and N can endow the Co/mSiO2−NCNTs good affinity with polysulfides through their strong chemical interactions with polysulfides, which inhibits the diffusion of polysulfides and allows the conductive surface of Co/mSiO2−NCNTs to adsorb more polysulfides. The intimate contact between the conductive surface and polysulfides benefits the interfacial charge transfer, thus promoting the conversion of polysulfides during charge and discharge processes.

CONCLUSIONS In summary, inspired by the natural spider web, we have designed and synthesized a nanocomposite, Co/mSiO2− NCNTs, composed of the hollow mSiO2 nanospheres/Co nanoparticles threaded by interwoven NCNTs. Through a simple filtration, Co/mSiO2−NCNTs can be firmly coated on the commercial separator. Owing to possessing abundant polysulfide adsorbent sites (SiO2 and N) and polysulfide conversion catalysts (Co and Co−Nx species), both of which are evenly dispersed in the 3D conductive frameworks formed by NCNTs, the Co/mSiO2−NCNTs interlayer can inhibit the “shuttle effect” by effectively trapping and reutilizing the polysulfides. Moreover, the combination of theoretical calculation and ex situ experiment demonstrates that the reversible adsorption/desorption of polysulfides on mSiO2 nanospheres benefits even deposition of Li2S2/Li2S on the conductive surface of Co/mSiO2−NCNTs during the discharge process. As a result, the Li−S batteries with Co/mSiO2−NCNTs-coated separators exhibited excellent cycling stability even at a high loading of 5.76 mg cm−2. At a high current density of 5C, the reversible capacity still reached 552 mAh g−1. We believe that this work will provide a design concept to fabricate functional separators for the application of Li−S batteries. METHODS Synthesis of ZIF-67. Co(NO3)2·6H2O (5 mmol) and 2methylimidazole (40 mmol) were dissolved in 100 mL of methanol. The two solutions were mixed at room temperature under magnetic stirring for 30 min and kept for 24 h. The solid ZIF-67 product was collected by centrifugation and washed with methanol three times, followed by vacuum drying at 80 °C for 12 h. Synthesis of ZIF-67@mSiO2. ZIF-67 (200 mg) was dispersed in 200 mL of ethanol, and then 100 mg of cetyltrimethylammonium bromide and 3 mL of aqueous ammonia solution (28%) were added. The solution was stirred at room temperature for 1 h. Next, 1 mL of tetraethyl orthosilicate (TEOS) was slowly injected to the solution, and the resulting dispersion was stirred for 4 h for complete hydrolysis of TEOS. The ZIF-67@mSiO2 core−shell nanoparticles were separated by centrifugation and washed with ethanol three times. Synthesis of Co/mSiO2−NCNTs, Co/NCNTs@mSiO2, and Co/ mSiO2−CNTs. Co/mSiO2−NCNTs were obtained by pyrolysis of ZIF-67@mSiO2 at 700 °C for 2 h under Ar/H2 (90%/10% in volume ratio). During the pyrolysis process, the Ar/H2 flow was allowed to bubble through pure DMF solution. The as-prepared black powder products were obtained by slow cooling to room temperature. Co/ NCNTs@mSiO2 was obtained by the same process without bubbling through pure DMF solution. Co/mSiO2−CNTs were obtained by the same process except bubbling through pure ethanol solution. Synthesis of mSiO2−NCNTs. Co/mSiO2−NCNTs powders were added in 0.1 M HCl solution for 12 h to remove mostly Co and then washed by deionized water for several times until the pH value of the solution reached 7.0. The final product was obtained by drying the treated powders at 80 °C overnight. Fabrication of Different Modified Separators. Ten milligrams of Co/mSiO2−NCNTs was dispersed into ethanol by sonication for 1 h. Then a certain volume of the as-prepared dispersion was vacuum I

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ACS Nano − (σ/r)6] at an interatomic distance r, where ε is the depth of the potential well and σ is the finite distance at which the potential is zero. The Lorentz−Berthelot mixing rules were used to model the parameters, which are truncated at 1.2 nm. The latter, long-range Coulombic interaction is computed using the particle−particle− particle−mesh algorithm.44 The system was first relaxed at a temperature of 300 K and 1.0 bar using a Berendsen thermostat45 for 5 ns. Then the system was relaxed in the NVT ensemble for 2 ns to archive to an equilibrated state. After the equilibrium was archived, the additional 1 ns simulation was run to analyze the Li2S4 number density distribution.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b07491. SEM, TEM, XRD, nitrogen adsorption, and additional electrochemical data (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yanlei Wang: 0000-0002-2214-8781 Shimou Chen: 0000-0002-2533-4010 Xi Wang: 0000-0003-3910-9575 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Key Projects for Fundamental Research and Development of China (No. 2016YFB0100104), National Natural Science Foundation of China (No. 91534109), Beijing Municipal Science and Technology Project (D171100005617001), Beijing Natural Science Foundation (2184124). REFERENCES (1) Pei, F.; Lin, L. L.; Fu, A.; Mo, S. G.; Ou, D. H.; Fang, X. L.; Zheng, N. F. A Two-Dimensional Porous Carbon-Modified Separator for High-Energy-Density Li-S Batteries. Joule 2018, 2, 323−336. (2) Zhang, Z. A.; Wang, G. C.; Lai, Y. Q.; Li, J.; Zhang, Z. Y.; Chen, W. Nitrogen-Doped Porous Hollow Carbon Sphere-Decorated Separators for Advanced Lithium-Sulfur Batteries. J. Power Sources 2015, 300, 157−163. (3) Peng, H.-J.; Wang, D. W.; Huang, J. Q.; Cheng, X. B.; Yuan, Z.; Wei, F.; Zhang, Q. Janus Separator of Polypropylene-Supported Cellular Graphene Framework for Sulfur Cathodes with High Utilization in Lithium−Sulfur Batteries. Adv. Sci. 2016, 3, 1500268. (4) Huang, J. Q.; Zhuang, T. Z.; Zhang, Q.; Peng, H.-J.; Chen, C. M.; Wei, F. Permselective Graphene Oxide Membrane for Highly Stable and Anti-Self-Discharge Lithium Sulfur Batteries. ACS Nano 2015, 9, 3002−3011. (5) Pang, Y.; Wei, J. S.; Wang, Y. G.; Xia, Y. Y. Synergetic Protective Effect of the Ultralight MWCNTs/NCQDs Modified Separator for Highly Stable Lithium−Sulfur Batteries. Adv. Energy Mater. 2018, 8, 1702288. (6) Chung, S.-H.; Han, P. L.; Singhal, R.; Kalra, V.; Manthiram, A. Electrochemically Stable Rechargeable Lithium−Sulfur Batteries with a Microporous Carbon Nanofiber Filter for Polysulfide. Adv. Energy Mater. 2015, 5, 1500738. (7) Gnana kumar, G.; Chung, S.-H.; Raj kumar, T. R.; Manthiram, A. Three-Dimensional Graphene−Carbon Nanotube−Ni Hierarchical J

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