Spider-Web-Inspired Nanocomposite Modified Separator: Structural

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Spider-Web-Inspired Nanocomposite Modified Separator: Structural and Chemical Cooperativity Inhibiting the Shuttle Effect in Li-S Batteries Daliang Fang, Yanlei Wang, Xizheng Liu, Jia Yu, Cheng Qian, Shimou Chen, Xi Wang, and Suojiang Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07491 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Nano

Spider-Web-Inspired

Nanocomposite

Modified

Separator: Structural and Chemical Cooperativity Inhibiting the Shuttle Effect in Li-S Batteries Daliang Fang,ab Yanlei Wang,a Xizheng Liu,c Jia Yu,a Cheng Qian,d Shimou Chen,*a Xi Wang,ef and Suojiang Zhang*a aBeijing

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. bUniversity

cTianjin

of Chinese Academy of Sciences, Beijing 100049, China.

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. dKey

Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry

of Education, Northwest A&F University, Yangling 712100, China. eKey

Laboratory of Luminescence and Optical Information, Ministry of Education, School of

Science, Beijing Jiaotong University, Beijing 100044, China.

ACS Paragon Plus Environment

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fTianjin

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Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry

Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering , Tianjin 300072, China

ABSTRACT： Despite of 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 web，we synthesized a spider-web-like nanocomposite, in which lots of hollow mesoporous-silica (mSiO2) nanospheres/Co nanoparticles were threaded by the interconnected nitrogen doping carbon nanotubes (NCNTs). Then the nanocomposite (donated 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 polysulfides adsorbent sites (SiO2 and N)/polysulfides conversion catalysts (Co and Co-Nx species) endows 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 even 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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Lithium sulfur (Li-S) batteries have been treated as the promising next-generation energy storage system because of their high theoretical capacity, low cost and nontoxicity. However, their application has been seriously hindered by the low conductivity of sulfur (S) and Li2S and the dissolution of long-chain polysulfides in the 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 Li anode and return to be oxidized by cathode, which is known as the “shuttle effect” of polysulfides. Coating functional materials on the commercial polypropylene (PP) separator as polysulfidesblocking interlayers has been proved 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/nanofibers5,6 and their hybrids7) have attracted tremendous attentions. 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 heteroatoms (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 has been found that some non-conductive polar inorganics (e.g., Al2O3, SiO2, TiO2) can effectively adsorb polysulfides due to their strong chemical interaction with polysulfide.13-15 But the conductivity of these inorganics is usually poor, compromising the reutilization of adsorbed polysulfides. Therefore, the combination of these inorganics with the


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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 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 non-conductive 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 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 ZIF67

encapsulated

by

mesoporous-silica

(ZIF-67@mSiO2),

in

which

hollow

mSiO2

nanospheres/Cobalt (Co) nanoparticles were threaded by many interconnected nitrogen doping carbon nanotubes (NCNTs). As natural spiders can effectively catch insects by their sticky webs with special structures, this spider-web-like nanocomposite (donated 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


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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 fast electron multi-pathways 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 catalytic effect. Co/mSiO2-NCNTs was coated on the PP separators by a simple infiltration and then used in the Li-S batteries. Owing to structural and chemical cooperativity, 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 LiS batteries significantly. We further carried detail electrochemical test, spectroscopic/microscopic analysis and theoretical calculation 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 previous works reported. 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 mSiO2 layer through the hydrolysis of tetraethyl orthosilicate with cetyltrimethylammonium bromide as a pore directing template.29 After coated with mSiO2, the surface of ZIF-67 become obviously rougher as shown in Figure S2c,d. The scanning TEM image and corresponding EDS elemental mapping images of the resulting ZIF67@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 ℃ under Ar/H2 mixed with DMF


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vapor. Figure 1a-c show typical field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (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 exhibits 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 inter-plane distances of the outside shell and the inner nanoparticle were ~ 0.34 nm and ~0.20 nm, which corresponded to the C (002) plane and the Co (111) plane, respectively.31 The X-ray diffraction (XRD) patterns of Co/mSiO2-NCNTs also demonstrates 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 proved by the Raman spectrum of Co/mSiO2-NCNTs in Figure S5d (The ID/IG ratio is 1.22). Such highly disordered carbon may derives from nitrogen doping into CNTs. To better understand the role of DMF vapor in the heating process, ZIF-67@mSiO2 pyrolysed without DMF vapor was also prepared. In this condition, ZIF-67@mSiO2 turned into Co nanoparticles and NCNTs encapsulated in hollow mSiO2 nanospheres (donated 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 NCNTs content of Co/NCNTs@mSiO2 was only 11.67% (Figure S7c), which was much lower than ~71%


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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 can not 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 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. Besides, 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 Besides, 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/mSiO2-NCNTs has the following advantages: 1) The 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 provides fast electron transportation/lithium ion paths to reutilize polysulfides. 3) Lots of catalysts (Co and CoNx) 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. Then Co/mSiO2-NCNTs was coated on 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 (Figure 1h and S9) clearly show


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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. Co/mSiO2-NCNTs interlayer was uniformly coated on the PP separator and the thickness was about 11.3 um (Figure 1i). Due to the strong adhesion of NCNTs to the PP separator, Co/mSiO2-NCNTs coated PP separator without any binder still maintained integrated after cutting into a wafer (Figure S11a). For comparison, the coating layer of (Co/NCNTs@mSiO2)/CNTs coated PP separator fell off a little along the edge (Figure S11b). The situation was much worse for Co/NCNTs@mSiO2 coated PP separator (Figure S11c). The ability of Co/mSiO2-NCNTs decorated separator in prohibiting 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), while the right chamber 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 CV tests with different sweep rates (Figure S13). The cathodic peaks at 2.3-2.2 V and 2.0-1.9 V were assigned as Peaks A and B, and the anodic peak at 2.4-2.5 V as 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 Peak A, B and C for Co/mSiO2-NCNTs coated separator were all almost


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twice than that of Pristine separator. Instead of blocking the lithium ion transfer, the Co/mSiO2NCNTs interlayer actually promoted the process significantly. This is due to abundant mesopores of Co/mSiO2-NCNTs interlayer，which improves electrolyte wettability and reduces impedance of lithium ion diffusion. The above results confirm the great potential of Co/mSiO2-NCNTs coated separator in suppressing the shuttle effect of polysulfides. To monitor the practical effect of the Co/mSiO2-NCNTs coated separator on the electrochemical properties, the cycle and rate performances of batteries with Co/mSiO2-NCNTs coated 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 were shown in Figure 2a. The two discharge potential plateaus are due to the multistep reduction of sulfur. The upper 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). While, 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

(Co/NCNTs@mSiO2)/CNTs coated separator had a lower voltage hysteresis (ΔE) than the one with 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, the ΔE was further reduced. This may result from the 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-NCNTs coated separator showed the lowest charge-transfer impedance (Figure S14a). Figure 2b exhibits the cycling performances


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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%. While, the capacity retentions of batteries with (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 Li-S battery with Co/mSiO2-NCNTs coated separator was further tested by cyclic voltammetry (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 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). It was because that when the areal mass of Co/mSiO2-NCNTs interlayer is too low, the polysulfides cannot be completely blocked. When the areal mass of Co/mSiO2-NCNTs interlayer 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 interlayer 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)/CNTs coated separator, the battery with Co/mSiO2-NCNTs coated 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


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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 Co/mSiO2-NCNTs coated separator at different current densities. As the current density increased, the ΔE become larger and the corresponding capacities decreased. It should be noted that, although (Co/NCNTs@mSiO2)/CNTs interlayer has the almost same constituents as 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/mSiO2NCNTs 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 battery with Co/mSiO2-NCNTs coated separator was further evaluated at 1 C (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 Li-S battery with 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 the Li-S batteries. Figure S15 exhibits 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 Co/mSiO2-NCNTs coated separator in the future practical use. To further reveal the mechanism of Co/mSiO2-NCNTs interlayer in improving the electrochemical properties of Li-S batteries, we disassembled the batteries with/without


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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. Besides, the surface of lithium anode from cycled battery with Co/mSiO2-NCNTs interlayer is obviously smoother than that without Co/mSiO2NCNTs interlayer (Figure S17), demonstrating that Co/mSiO2-NCNTs interlayer can effectively prevent polysulfides from crossing separator to react with lithium anode. SEM images showed that the Co/mSiO2-NCNTs coating can keep structure stable during cycles and the active materials immobilized by the Co/mSiO2-NCNTs coating can be obviously observed, according to Figure 3a, b. And the strong and even-dispersed sulfur signals on the coated layer in Figure 3c evidences that the migrating polysulfides are certainly trapped in the Co/mSiO2-NCNTs coating. We further carried 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 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 1.8-2.8 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 appeared in the polysulfides adsorbed on the NCNTs. The inter-plane 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 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


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polysulfides into Li2S2/Li2S. The selected area electronic diffraction (SAED) patterns of cycled Co/mSiO2-NCNTs also confirms the existence of Li2S and LiF nanoparticles (Figure S19). Figure 3e shows no existence of Li2S nanoparticles on the surface of mSiO2 nanoparticle, which is further proved by the EDS element mapping images of the cycled Co/mSiO2-NCNTs in Figure S20. Although S element 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, 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/mSiO2NCNTs was investigated by XPS spectrum. Figure 3h, i show the Co 2p and S 2p XPS spectra of cycled Co/mSiO2-NCNTs interlayer, respectively. The slightly negative shift of Co-N bond and the appearance of 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 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 polysulfides, thus enhancing the electrochemical properties of Li-S batteries. To further understand the adsorption and conversion processes of polysulfides on Co/mSiO2NCNTs 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 Obviously, the mSiO2 nanospheres were covered by a polymer-like layer (considered as polysulfides), while the surface


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of nearby NCNTs was relatively clear. The corresponding TEM image (Figure 4b) also shows that mSiO2 nanospheres were completely covered by polysulfides, while, only tiny polysulfides were unevenly adsorbed on the exposed NCNTs, which is further proved 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 mSiO2 nanosphere was relatively clear, while the nearby NCNTs were covered by a polymerlike layer (considered as Li2S2/Li2S), which was also proved 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 element was much wider than that of Si element, revealing that most Li2S2/Li2S was adsorbed on the NCNTs rather than the mSiO2 nanospheres. To understand the above phenomenon, as shown in Figure 4g, we carried the 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 a-SiO2 is lower than that in the liquid region, implying that Li2S4 would mainly exist in the latter. However, when the c increases, a density peak of Li2S4 would emerge near the a-SiO2, showing that it would aggregate near the solid surface. When the c increases continually, the intensity of peak would be further enhanced and the position of peak would be more closer to the substrate, revealing that more Li2S4 molecules are adsorbed on the a-SiO2. The molecular dynamics simulations demonstrate that when the concentration (c) of polysulfides in electrolyte is high, these polysulfides tend to be adsorbed on the mSiO2 nanospheres. While, as most polysulfides converted into solid Li2S2/Li2S and the concentration (c) of polysulfides in electrolyte decreased to a low value, the polysulfides adsorded on the mSiO2 nanospheres would desorb and return back to electrolyte.


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Therefore, we infer the mechanism of Co/mSiO2-NCNTs in trapping and reusing the polysulfides as illustrated in Figure 4i. At the beginning of discharge process, the liquid longpolysulfides (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. Moreover, to study the roles of Co and N atoms in polysulfides conversion, we prepared mSiO2NCNTs and Co/mSiO2-CNTs after removing Co and N under certain conditions. We first investigated the polysulfides adsorption abilities of Co/mSiO2-NCNTs,

mSiO2-NCNTs and

Co/mSiO2-CNTs in Figure 5a. 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 were added to 10 ml of the solution, respectively. After 1.0 h, although both Co/mSiO2-CNTs and Co/mSiO2NCNTs 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


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three materials as two identical electrodes respectively. 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 (EIS) 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 Co/mSiO2-NCNTs coating separator should have superior rate performance than those with Co/mSiO2-CNTs or mSiO2-NCNTs coating separators. This conjecture is confirmed by Figure 5d. Therefore, as shown in Figure 5e, the presence of Co and N can endows 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 and, thus, promoting the conversion of polysulfides during charge and discharge process. 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 polysulfides adsorbent sites (SiO2 and N) and polysulfides conversion catalysts (Co and Co-Nx species), both of which are evenly dispersed in the 3D conductive frameworks formed by NCNTs, Co/mSiO2-NCNTs


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interlayer can inhibit “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 discharge process. As a result, the Li-S batteries with Co/mSiO2-NCNTs coated separator exhibited excellent cycling stability even at a high loading of 5.76 mg cm-2. At a high current density of 5 C, the reversible capacity still reached 552 mAh g-1. We believe that this work will peovide a design concept to fabricate functional separators for the application of Li-S batteries. METHODS Synthesis of ZIF-67: Co (NO3)2 ·6H2O (5mmol) and 2-methylimidazole (40mmol) were dissolved in 100 mL methanol, respectively. 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 for 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 ethanol, and then 100 mg cetyltrimethylammonium bromide (CTAB) and 3 ml ammonia aqueous solution (28%) were added. And the solution was stirred at room temperature for 1 h. 1 ml 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 was separated by centrifugation and washed with ethanol for three times. Synthesis of Co/mSiO2-NCNTs, Co/NCNTs@mSiO2 and Co/mSiO2-CNTs: Co/mSiO2-NCNTs was obtained by pyrolysis of ZIF-67@mSiO2 at 700℃ for 2 h under Ar/H2 (90%/10% in volume


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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 was 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 solution reached 7.0. The final product was obtained by drying the treated powders in 80 ℃ for overnight. Fabrication of different modified separators: 10 mg Co/mSiO2-NCNTs was dispersed into ethanol by sonication for 1h. Then a certain volume of the as-prepared dispersion was vacuum filtered through the commercial PP separator (Celgard 2400). The resulting Co/mSiO2-NCNTs coated separator was dried at 50 ℃ for 12h under vacuum and then punched into circular disks with diameter of 16 mm. The weight loading of Co/mSiO2-NCNTs coating is 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, making that the mass proportion of CNTs in (Co/NCNTs@mSiO2)/CNTs coating is almost the same as that in Co/mSiO2-NCNTs. Characterizations: The X-ray diffraction (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


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electron microscope (SEM, JEOL JSM-7001F) and a transmission electron microscope (TEM, JEOL JEM-2100F). Elemental mappings were obtained using energy dispersive X-ray spectrometries (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. Thermo gravimetric (TG) analysis was collected using a STA7200RV apparatus (Hitachi High-Tech). XPS measures 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 foils and followed by drying at 80 ℃ for 12 h. Coin 2025 cells were assembled in Ar-filled glovebox by using Super P/S cathode, modified or pristine PP separators, 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 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 materials (Co/mSiO2NCNTs, Co/mSiO2-CNTs and mSiO2-NCNTs ) and PVDF binder at a weight ratio of 4:1 were dispersed in NMP. The resultant slurry was coated on aluminum foils. 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 electrolyte containing 0.1 mol L-1 Li2S6 and 1 mol L-1 LiTFSI dissolved in DOL/DME (v/v = 1/1) was added


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as electrolyte. CV measurements and Electrochemical impedance spectroscopy (EIS) tests of the symmetrical cells were were conducted on an Autolab (PGSTAT302N) electrochemical workstation. Details of Molecular Dynamics Simulations: All the molecular dynamics (MD) simulations in this work were completed using a large-scale atomic/molecular massively parallel simulator (LAMMPS).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 which thickness was 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 - (σ/r)6] at an interatomic distance r, where ε is the depth of 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 later one, long-range coulombic interaction, is computed using the particle-particle-particle-mesh (PPPM) 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 is relaxed in the NVT ensemble for 2 ns to archive to an equilibrated state.


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After the equilibrium was archived, the additional 1 ns simulation was run to analyze the Li2S4 number density distribution.

ASSOCIATED CONTENT Supporting information SEM, TEM, XRD, nitrogen adsorption and additional electrochemical data.

AUTHOR INFORMATION

Corresponding Author

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

ACKNOWLEDGMENT 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) and Henan province science and technology cooperation project (172106000061).


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(38) Conder, J.; Bouchet, R.; Trabesinger, S.; Marino, C.; Gubler, L.; Villevieille, C. Direct Observation of Lithium Polysulfides in Lithium–Sulfur Batteries Using Operando X-ray Diffraction. Nat. Energy 2017, 2, 17069. (39) Zhong, Y. R.; Yin, L. C.; He, P.; Liu, W.; Wu, Z. S.; Wang, H. L. Surface Chemistry in Cobalt Phosphide-Stabilized Lithium−Sulfur Batteries. J. Am. Chem. Soc. 2018, 140, 1455−1459. (40) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1-19. (41) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225-11236. (42) Munetoh, S.; Motooka, T.; Moriguchi, K.; Shintani, A. Interatomic Potential for Si–O Systems Using Tersoff Parameterization. Comp. Mater. Sci. 2007, 39, 334-339. (43) Wang, Y.; Song, Z.; Xu, Z. Mechanistic Transition of Heat Conduction in Two-Dimensional Solids: A Study of Silica Bilayers. Phys. Rev. B 2015, 92, 245427. (44) Hockney, R. W.; Eastwood, J. W. Computer Simulation Using Particles; Taylor and Francis: New York, 1988; pp 564. (45) Berendsen, H. J. C.; Postma, J. P. M.; Van Gunsteren, W. F.; DiNola, A.; Haak, J. R. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684-3690.


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Figures and Captions:

Scheme 1. a) Schematic illustration of the synthetic process of Co/mSiO2-NCNTs. b) Schematic illustration of Li-S battery employing Co/mSiO2-NCNTs coated separator.


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ACS Nano

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 Figure 1g shows a digital photo) and i) cross section.


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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.


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ACS Nano

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.


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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) The atomic structure of Li+, [TFSI]-, DOL, DME, Li2S4, and a half of the symmetrical simulation model in the present work, where green, yellow, gray, cyan color 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 discharge process.


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ACS Nano

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/mSiO2CNTs 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.

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Spider-Web-Inspired Nanocomposite Modified Separator: Structural

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