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Highly Fluoro-Substituted Covalent Organic Framework and Its Application in Lithium-Sulfur Battery Degao Wang, Nuo Li, Yiming Hu, Shun Wan, Min Song, Guipeng Yu, Yinghua Jin, Weifeng Wei, Kai Han, Gui-Chao Kuang, and Wei Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14213 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 16, 2018

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ACS Applied Materials & Interfaces

Highly Fluoro-Substituted Covalent Organic Framework and Its Application in LithiumSulfur Battery De-Gao Wang†, Nuo Li‡, Yiming Hu , Shun Wan∥, Min Song†, Guipeng Yu‡, Yinghua §

† ‡ † Jin , Weifeng Wei , Kai Han *, Gui-Chao Kuang *, Wei Zhang * §

†

,

,§,

§,

State Key Laboratory of Power metallurgy, Central South University, Changsha,

Hunan 410083, P. R. China. ‡

Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry

and Chemical Engineering, Central South University, Changsha, Hunan 410083, P. R. China.

Department of Chemistry and Biochemistry, University of Colorado, Boulder,

§

Colorado 80309, United States.

∥

NCO Technologies LLC, Concord, North Carolina 28027, United States

KEYWORDS: Fluoro-Substituted, Covalent Organic Frameworks, Li-Sulfur Battery, Porous Polymers, Sulfur Confinement

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ABSTRACT: We report the preparation of a highly fluoro-substituted crystalline covalent organic framework (COF) and its application as a cathode material in lithium sulfur battery (LSB) upon sulfur confinement. A sulfur-functionalized COF with high sulfur content (60 wt%) was obtained through physisorption of elemental sulfur and subsequent S Ar reaction of sulfur with aromatic fluorides on the COF backbone. After N

such physical and chemical confinement of sulfur through post-functionalization approach, the COF material still shows some structural order, allowing us to investigate the structure-property relationship of such COF materials in LSB application. We compared the electrochemical performances of the two cathode materials prepared from a crystalline COF and its amorphous counterpart and studied the important factors that affect battery capacity, reaction kinetics, and cycling stability.

INTRODUCTION Lithium-sulfur batteries (LSBs) have shown great potential as next-generation energy storage device due to their high energy densities, low cost, and environmental friendliness. However, one critical bottleneck problem is their 1-6

poor cycle stabilities and short lifespan. The dissolution of lithium polysulfide intermediates formed during cycling, which shuttle between anode and cathode, has been recognized as a main contributing factor of this problem, causing the loss of active sulfur content and also self-discharge even in the resting state.

5, 7-11

Various porous materials have been developed as hosts to chemically or physically trap polysulfides into the pores and suppress their dissolution and 2 ACS Paragon Plus Environment

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migration.

12-14

Some of these examples include mesoporous carbons,

15-22

porous carbon, carbon nanotubes, 23

organic frameworks (MOFs),

32,33

24-27

hierarchical

nanobox, porous organic polymers, 28

29-31

metal

and covalent organic frameworks (COFs).

34-38

Among them, MOFs and COFs are of particular interest, since their pore structures and sizes can be controlled at molecular level through reticular synthesis using judiciously selected small building blocks. Compared to COFs, MOFs have more possible topologies due to a variety of coordination geometries around metal centers. However, COFs are composed of solely light elements and strong covalent linkages, thus they can have higher thermal and chemical stability, and lower density.

39-44

Additionally, various functional groups can be

easily introduced to target specific materials functions (e.g. electrode in LSBs). For example, boroxine moieties have been successfully introduced in the COFs to enhance the interactions between the polysulfides (PSs) and pore surfaces and improve the cycling stability.

19, 26, 45, 46

However, in most reported cases, polysulfides

were only physically confined within the pores through relatively weak interactions. Only a couple of examples have been reported to chemically attach sulfur to polymer frameworks. In these reported cases, direct one-pot polymerization of monomers in the presence of elemental sulfur was applied, resulting in amorphous polymers with ill-defined pore structures.

30, 31

To the best

of our knowledge, there have been no reports of structurally ordered COF materials with covalently bound sulfur. Since the materials porosity and pore structures would directly affect the sulfur confinement, ion transportation, 3 ACS Paragon Plus Environment


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polysulfide shuttling, and thus battery performance, herein we developed a COF with covalently-linked sulfur and well-defined pore structures as a cathode material for LSB. For the first time, we explored the structure-property relationship of such COF-based cathode materials in lithium-sulfur battery by comparing with the amorphous analogue. EXPERIMENTAL SECTIONS Materials and methods All commercially available reagents and solvents were used as received without further purification, unless otherwise noted. 4,4',4''-(1,3,5-triazine-2,4,6triyl)trianiline (2) was synthesized following the published procedures. H NMR 47 1

spectra were taken on Inova 400 and Inova 500 spectrometers. Solid-state cross polarization magic angle spinning (CP/MAS) C NMR spectra were recorded on 13

an AVANCE III 400 MHz spectrometer produced by Bruker. The FT-IR spectra of starting materials and as synthesized COF-F, COF-F-S, POP-F, and POP-FS were obtained from Agilent Technologies Cary 630 FT-IR. Thermogravimetric analyses (TGA) were performed on a thermogravimetric/ differential thermal analyzer by heating the samples at 10 C min to 700 C under nitrogen o

-1

o

atmosphere. The morphologies of COF-F, COF-F-S, POP-F, and POP-F-S were obtained by a transmission electron microscopy (TEM) were recorded on a JEOL JEM-2100F microscope. Scanning electron microscopy (SEM) images were acquired in a FEI SIRION200 microscopy with an accelerating voltage of 10 kV. All the samples were coated with gold before test. The powder wide angle 4 ACS Paragon Plus Environment

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X-ray diffraction pattern (PXRD) were recorded on an X Pert-Pro MPD diffractometer (Netherlands PANalytical) with a Cu Kα X-ray source (λ = 1.540598 Å). Raman spectroscopy was acquired using XploRA PLUS HORIBA Raman Imaging spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALab MKII X-ray photoelectron spectrometer using Mg-Kα radiation. Nitrogen sorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 sorption analyzer. Brunauer-Emmett-Teller specific surface area, pore size distribution and pore volume were estimated based on the absorption-desorption isotherms. The samples were heated at 120 C and kept at o

this temperature for at least 20 hours under vacuum for activation. Ultra-high purity grade (99.999 % purity) N were used for all free space corrections and 2

measurements. For the gas adsorption measurement, the temperatures were controlled by using a refrigerated bath of liquid N (77 K). 2

Electrochemical measurements The cathodes were prepared by a slurry coating method with a doctor blade. The cathode slurry was prepared by mixing 70 wt % COF-F-S or POP-F-S, 20 wt % commercial carbon black and 10 wt % poly-(tetrafluoroethylene) (PVDF) in N-methyl-2-pyrrolidone (NMP) solution. The slurry was stirred at room temperature for 24 h. Then, the slurry was coated onto the surface of aluminum foil and dried at 60 C for 12 h. CR2016-type coin cells were assembled in an Aro

filled glove box, in which water and oxygen contents were less than 1 ppm. The prepared cathodes were punched into 12 mm diameter discs and were used as a 5 ACS Paragon Plus Environment


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cathode. Before the cells were assembled, the prepared cathodes were dried at 60 o

C for overnight. Lithium foils punched to 16 mm diameter discs were used as

anodes. PP membranes were used as separator. The electrolyte was 1.0 M lithium bistrifluoromethanesulphonylimide (LiTFSI) in 1,3-dioxolane (DOL) and 1,2dimeth-oxyethane (DME) (1:1 by volume) with 1 wt % LiNO additive. The Li3

S cells were galvanostatically discharged and charged over a potential range of 1.7-2.8 V (versus Li/Li ) on a LAND CT2005A testing system. The current +

density set for tests was referred to the mass of sulfur in the cathode and was varied from 0.1 to 2C, where 1C = 1675 mAh g . The CV curves were measured -1

using the CHI660E electrochemical workstation with a scan rate of 0.1 mV s in -1

the voltage range of 1.7-2.8 V. Electrochemical impedance spectroscopy (EIS) results were collected on a CHI 660E electrochemical workstation from 10 to 10 5

2

-

Hz with applied amplitude of 5 mV.

Experimental procedures (1) Synthesis of COF-F: We followed the procedure reported by Lu et. al. To a 48

glass tube (5 mL, the outer diameter is 10 mm and the inner diameter is 8 mm) was added 2,3,5,6-tetrafluoroterephthalaldehyde 1 (15 mg, 0.075 mmol) 4,4',4''(1,3,5-triazine-2,4,6-triyl)trianiline 2 (18 mg, 0.05 mmol), 3 M acetic acid (0.1 mL), dioxane (1.8 mL), and mesitylene (0.2 mL). The tube was flash frozen at 77 K in liquid nitrogen and evacuated to the internal pressure of ~100 mTorr, and then sealed under an open flame. The mixture was first warmed to room temperature and then the temperature was slowly raised to 120 C over 2 h. The o


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reaction was kept at 120 C for 3 days and cooled to room temperature over 12 h. o

The orange precipitate was filtered under vacuum, washed with a large amount of acetone and CH Cl , and dried under vacuum to yield the fluorinated COF 2

2

(COF-F, 24 mg, 74%). (2) Synthesis of POP-F: To a round bottom flask (25 mL) was added 2,3,5,6tetrafluoroterephthalaldehyde 1 (15 mg, 0.075 mmol), 4,4',4''-(1,3,5-triazine2,4,6-triyl)trianiline 2 (18 mg, 0.05 mmol), acetic acid (3 M, 0.1 mL), dioxane (1.8 mL), and mesitylene (0.2 mL). The flask was degassed by several freezepump-thaw cycles, and then kept reflux with stirring for 3 days. After cooling to room temperature, the orange precipitate was collected by vacuum filtration, washed with a large amount of acetone and CH Cl , and dried under vacuum to 2

2

yield the amorphous polymer (POP-F, 26 mg, 77%). (3) Synthesis COF-F-S and POP-F-S: Both COF-F-S and POP-F-S were prepared using similar reported procedure. Take COF-F-S as an example, COF30

F (30 mg) and elemental sulfur (90 mg) were mixed in a glass tube (10 mL, the outer diameter is 10 mm and the inner diameter is 8 mm) with the polymer/sulfur weight ratio of 1:3. The tube was degassed and sealed under an open flame. The mixture was heated to 160 C for 15 h and at 350 C for another 15 h. After cooling o

o

to room temperature, the product was obtained as black powder COF-F-S.

RESULTS AND DISCUSSION



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A sulfur-containing COF with some structural order was prepared through stepwise post-functionalization of crystalline COF. We intentionally introduced high content of aromatic fluoride substituents on the backbone of the COF, which can undergo S Ar reaction with sulfur (Figure 1). Multi-fluoro substituted N

2,3,5,6-tetrafluoroterephthalaldehyde (1) was selected as a monomer to enhance the reactivity of S Ar reaction between sulfur and aromatic fluoride and N

maximize the covalent attachment of sulfur. We also included triazine moieties in monomer 2, which can interact with lithium ions and stabilize lithium polysulfide. Under optimized reaction conditions, condensation of monomer 1 and 2 through Schiff base chemistry afforded a highly ordered crystalline COFF with triazine and tetrafluoro benzene moieties (Figure 1a). To study the effect of crystallinity on the materials performance in LSBs, an amorphous polymer analog with the same chemical composition (POP-F) was also prepared from the same monomers. COF-F displayed deep red colour whereas POP-F was yellow (Figure 1a). The physical properties of the polymers were characterized by FTIR spectroscopy, C solid state NMR spectroscopy, thermogravimetric analysis 13

(TGA), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FT-IR spectrum of COF-F shows the typical imine bond absorption at 1603 cm with negligible -NH stretch band around 3500-3300 cm and C=O stretch -1

-1

2

band around 1704 cm , indicating the high conversion of amine and aldehyde -1

groups to imines (Figure S1). Furthermore, C solid state NMR spectrum shows 13


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imine carbon resonance at 160 ppm with the absence of aldehyde peak at >190 ppm, confirming the successful formation of imine bonds (Figure S2).

49

Thermogravimetric analysis (TGA) shows that COF-F and POP-F are thermally stable, only losing 5% weight at 400 C (Figure S3). o

The crystallinity of the above polymers was investigated by PXRD measurements, which clearly show that COF-F is a crystalline material with a long-range order (Figure 1b). The diffraction pattern of COF-F exhibits a set of well-resolved peaks centered at 2θ = 2.90, 4.96, 5.74, 7.56, 9.84 and 26.21 , o

which are attributed to (100), (110), (200), (210), (220) and (001) facets, respectively. The simulation and Pawley refinement of crystalline COF-F was conducted using Materials Studio to obtain possible molecular packing model.

50

We simulated the model in hexagonal units according to those reported in COF5 and COF-LZU-1. After energy minimization, the simulated PXRD pattern 39

51

well matched the experimental profile. The lattice parameters were determined to be a = b = 35.18 Å and c = 3.39 Å. The simulated results demonstrated that the COF-F was organized in AA stacking mode with the interlayer distance of 3.39 Å (Table S1). The theoretical pore size was calculated to be 2.8 nm (Figure 1c). By contrast, POP-F is an amorphous powder as shown in the PXRD pattern, which displays a big halo in the low-angle region (Figure S4). Consistently, TEM image of COF-F clearly suggests crystallinity and order, whereas no obvious structural order was observed in TEM image of POP-F (Figure 3). SEM images



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of COF-F and POP-F show sponge-like macroscopic morphologies for both polymers. The porous properties of COF-F and POP-F were evaluated by nitrogen adsorption-desorption measurement at 77 K. Both samples show type IV adsorption isotherm character (Figure 2), typical of permanent mesoporous materials.

Crystalline

sample

COF-F

shows

significantly

higher

Brunauer−Emmett−Teller (BET) surface areas compared to POP-F (1532 m g 2

-1

vs. 979 m g ). The pore size distributions were measured by nonlocal density 2

-1

functional theory. Similar pore size of 2.8 nm was observed for both porous polymers (Table S2). The sulfur confinement was achieved by reacting elemental sulfur with porous COF-F and POP-F (Figure 2a). In a typical procedure, a mixture of elemental 30

sulfur and polymer in 3:1 weight ratio was heated at 160 C for 15 h to ensure o

sufficient impregnation of molten linear sulfur chains in the polymer pores. Subsequent heating at 350 C for another 15 h assists S Ar reaction between o

N

aromatic fluoride and sulfur to covalently link sulfur to the polymer backbone. After such physical and chemical confinement, sulfur-containing polymers, COF-F-S and POP-F-S were obtained as a black powder from COF-F and POP-F, respectively. Elemental analysis of COF-F-S shows 61 wt% sulfur content, which is consistent with the TGA result (Figure S3a), showing a mass loss of 60 wt% in the range of 160 to 375 C. By contrast, only 20 wt% weight o

loss was observed in the range of 160 to 375 C for POP-F-S in TGA graph, o


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although elemental analysis shows 62 wt% of sulfur content. These results show that POP-F-S has better thermal stability and the loss of sulfur in POP-F-S is more restricted, which might also explain the better cycling stability of Li/POPF-S battery cell as discussed later. After sulfur-confinement, both COF-F-S and POP-F-S become non-porous showing BET surface area of about 40 m g , 2

-1

supporting high sulfur encapsulation in the pores (Table S2). SEM images of 31

COF-F-S and POP-F-S show more-filled morphologies compared to those of porous COF-F and POP-F (Figure 3a, b and Figure 3 d, e). Elemental mapping showed homogeneous distributions of sulfur and carbon (Figure 3c, f) in both COF-F-S and POP-F-S. Although we only observed a halo band in PXRD patterns of COF-F-S (Figure S5), TEM image of COF-F-S clearly shows crystallinity, suggesting the structural order of COF-F is maintained to some extent (Figure 3g, h). As expected, POP-F-S is amorphous according to both PXRD patterns (Figure S5) and TEM images (Figure 3i, j). COF-F-S and POP-F-S were further characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (Figure 4 and S6). For COF-F-S, the C 1s XPS band could be deconvoluted into four peaks at 286.2, 285.7, 284.9 and 284.2 eV, which are ascribed to the carbons from C-F, N-C=N, C-S and C=C bonds, respectively. The four peaks at 165.1 and 163.8 eV, 165.7 and 164.7 eV 30

deconvoluted from S 2p XPS band are assigned to the sulfur from S-S and C-S bonds respectively. In addition, the F 1s band could be deconvoluted into three 30

peaks at 689.4, 687.5 and 686.0 eV, the first of which is attributed to F atoms of 11 ACS Paragon Plus Environment


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the C-F bond and the latter two are attributed to F atoms semi-ionically bound to the sp carbon. 2

52, 53

Similar spectra were observed for POP-F-S (Figure 4c and 4d).

However, semi-ionic level of COF-F-S was higher than that of POP-F-S (Table S4). The presence of C-F bonds indicates that only portion of fluorine atoms 54

underwent S Ar substitution after high temperature reaction in both polymers. N

We also acquired Raman spectra of COF-F-S and POP-F-S. However, due to the presence of the strong vibration bands of aromatic rings 1000-1600 cm , and -1

several broad peaks in the range of 550-750 cm where a weak-to-medium band -l

of the aromatic C-S stretching vibration and aromatic C-H deformation vibration bands are expected, we were unable to obtain solid evidence supporting the formation of C-S bonds or the absence of C-F bonds based on the Raman data (Figure S6). The electrochemical performances of COF-F-S and POP-F-S were then evaluated after fabrication as cathode materials in 2016-type coin cells (Li/COFF-S and Li/POP-F-S). The cathode was fabricated by mixing 70 wt % COF-FS or POP-F-S, 20 wt% commercial carbon black and 10 wt% poly(tetrafluoroethylene) (PVDF) in N-methyl-2-pyrrolidone (NMP) solution and casting the slurry onto the surface of aluminium foil through doctor blade method. The areal sulfur loading in COF-F-S cathode was determined to be 0.5 mg cm , which is typical for porous organic polymer-based LSB cathode -2

materials. The coin cell was assembled using lithium foil as an anode and 19

lithium bistrifluoromethanesulphonylimide (LiTFSI, 1.0 M) in 1,3-dioxolane 12 ACS Paragon Plus Environment

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(DOL) and 1,2-dimethoxyethane (DME) (1:1 by volume) with 1 wt % LiNO

3

additive as the electrolyte. Cyclic voltammetry curves of Li/COF-F-S cell show two main reduction peaks at 2.29 and 2.05 V in the first cathodic scan, which can be attributed to the typical reduction of elemental sulfur to high order polysulfides Li S and the 2

n

formation of Li S /Li S from soluble polysulfides species (Figure 5a), 2

2

2

respectively. In the following anodic scan, one main oxidation peak appeared 55

with a small shoulder, which correspond to the oxidation of Li S /Li S to 2

2

2

polysulfides and sulfur. Although the CV curves of Li/POP-F-S show similar redox peaks (Figure S7a), lower current was observed compared to the case of Li/COF-F-S, suggesting that COF-F-S material is electrochemically more active, likely owing to the higher sulfur content and enhanced lithium ion transport arising from more ordered pore structures. Interestingly, rapid current decay was observed for Li/COF-F-S in the first three cycles of CV measurements, whereas the current was relatively stable in the CV graphs of Li/POP-F-S. This result suggests that the migration of active sulfur by dissolution from COF-F-S cathode is likely facilitated by the ordered pore structures, resulting in the faster deposition of lithium sulphide, increase of the cell resistance, and eventually the current drop. Thus there might be a trade-off in electrode systems based on porous materials: the existence of well-ordered pore channels would facilitate lithium ion transport, but at the same time, it would also allow easier migration of polysulfides intermediates. 13 ACS Paragon Plus Environment


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The fresh cell assembled with COF-F-S cathode exhibited relatively low charge transfer resistance of about 50 Ω according to the EIS plots (Figure 5b). The rate performance of the cells was evaluated by galvanostatic dischargecharge experiments. A short plateau at about 2.3 V and a long flat plateau at around 2.1 V were clearly observed from the discharge profiles of Li/COF-F-S at all the current densities (Figure 5c), which correspond to the step oxidation of sulfur to polysulfides/sulfides and is consistent with the CV peaks in Figure 5a. With the increase of current rate from 0.1 to 0.2, 0.5, 1 and 2C, the voltage gap between the charge and discharge curves increased, mainly due to the electrochemical polarization. Similar charge/discharge profiles were observed for Li/POP-F-S. However, as the current rate increase, the capacity differences between Li/COF-F-S and Li/POP-F-S are more pronounced. As shown in Figure 5d, although Li/POP-F-S shows similar capacity retention at low rates as Li/COF-F-S (0.1-0.2 C), considerably decreased capacity retentions were observed for Li/POP-F-S at high rates (0.5-2 C). These results suggest the cathode composite made of crystalline sample COF-F-S exhibits faster reaction kinetics compared to the amorphous POP-F-S likely due to its higher sulfur loading and the presence of ordered pore channels that can facilitate the ion transport. The cycling stabilities of the cells were also evaluated to further compare the battery performances of the COF-F-S and POP-F-S cathodes. Although the content of sulfur in the cathode electrodes was determined to be only 0.5 mg·cm

-


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2

, Li/COF-F-S shows high initial discharge capacity of 1120 mAh g at 0.1 C -1

(Figure 5e), which rapidly decreased to 962 mAh g at the third cycle, consistent -1

with our previous observation in the CV measurement. The rapid decay of the discharge capacity in the first several cycles is presumably due to the migration of un-trapped loosely bound polysulfide in the highly porous COF-F-S. Further migration of physically trapped or covalently bound polysulfide is much slower, thus slow gradual decay to 645 mAh g over 100 cycles (0.04% drop per cycle) -1

was observed after the initial rapid capacity drop. By contrast, Li/POP-F-S shows significantly lower initial discharge capacity of 782 mAh g , which -1

gradually faded to 525 mAh g after 100 cycles at 0.1 C (Figure S7). These results -1

indicate that high porosity and ordered COF-F-S cathode materials provide high discharge capacities, however, with slightly lower cycling stabilities. Long term cycling stabilities were also tested at higher rate of 1 C (Figure 5f). Rapid initial capacity drop was also observed for COF-F-S electrode in the first five cycles. It was then stabilized and slowly faded to 257 mAh g after 1000 cycles at 1 C -1

(0.045 % decay each cycle from 5 to 1000 cycle). Li/POP-F-S shows less th

th

severe initial capacity drop and delivered capacity retention of 156 mAh g after -1

1000 cycles. These are consistent with our previous observations, showing that highly porous COF-F-S cathode composite with higher sulfur loading (physically and covalently bound) has better electrochemical activity and faster reaction kinetics, however, with inferior cycling stability. The presence of porechannels and well-defined 2D structures facilitates ion transport and electron 15 ACS Paragon Plus Environment


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conductivity, however, simultaneously also promotes polysulfide migration. Although the initial capacities are high, long term reversible capacities of both Li/COF-F-S and Li/POP-F-S cells are low, which is typically observed for electrodes composed of porous polymers with low inherent conductivity.

29-31

In

addition, the repetitive dissolution and shuttling of polysulfides cause deposition of reactive species on both electrodes and thus increase of impedance, irreversible loss of active materials, and low reversible capacity. For both Li/COF-F-S and Li/POP-F-S cells, the gradual increase of Coulombic efficiency (the ratio of discharge and charge capacity) was observed. During the initial cycling, solid-electrolyte interphase (SEI) layers form on the surface of active materials, leading to increased cell resistance, rapid capacity fade, and low Coulombic efficiency. As cycle number increases, SEI layers become thicker and a fully-developed stable SEI forms, thus the capacity fading slows down and the Coulombic efficiency increases. CONCLUSIONS We synthesized highly fluoro-substituted crystalline COF as a precursor to prepare sulfur-containing COF cathode materials. For the first time, we obtained covalently sulfur-linked COF with some structural order through postfunctionalization approach, namely initial physisorption of elemental sulfur followed by S Ar reaction between sulfur and aromatic fluoride groups on the N

backbone of crystalline COF. We further studied the performance of resulting COF-F-S as cathode materials in LSBs and compared its properties with those 16 ACS Paragon Plus Environment

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of the amorphous POP-F-S analog prepared. Due to the higher porosity, the crystalline COF-F provided higher sulfur loading (60 wt%) and more structural order after sulfur confinement compared to the amorphous POP-F. COF-F-S cathode with more ordered pore structures exhibit higher electrochemical activity as well as faster kinetics, whereas amorphous POP-F-S cathode shows slightly higher cycling stability. Our study clearly shows the advantages and disadvantages of using a crystalline 2D COF with well-defined pore channels as a host for sulfur confinement in LSBs: the advantages are the high porosity thus possible high sulfur loading, efficient ion transport thus fast reaction kinetics, and increased electrical conductivity due to the 2D planar sheet structure; the possible disadvantage might be the promoted migration of polysulfides through 1D pore channel causing faster initial capacity decay. Our work provides important information on the design and synthesis of COF-based electrode materials and their structure-property relationship, and would shed light on the future development of high-performance LSBs.



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Figure 1. (a) Syntheses of COF-F and POP-F. Reagents and conditions:1,4dioxane/mesitylene = 9:1 (v/v), acetic acid, 120 C, 3 days. Glass tube for COFo

F, yield 74%; round bottle flask for POP-F, yield 77%. The insets are the pictures of porous polymers. (b) Observed PXRD pattern (black), the Pawley-refined pattern (red), the difference plot (blue), and the simulated pattern (purple) for the eclipsed model. (c) Top and side views of the energy-minimized models of COFF (C, gray; N, blue; O, green; H atoms omitted for clarity).


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Figure 2. (a) Synthesis of COF-F-S and POP-F-S. Conditions: S , 160 C, 15 h, 8

o

then 350 C, 15 h. (b) N absorption-desorption isotherms and (c) pore size o

2

distribution of COF-F, POP-F, COF-F-S and POP-F-S.



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Figure 3. SEM images of COF-F (a), COF-F-S (b), POP-F (d) and POP-F-S (e); TEM images of COF-F (g), COF-F-S (h), POP-F (i), and POP-F-S (j); Scanning transmission electron microscopy (STEM) elemental mapping images of COF-F-S (c) and POP-F-S (f).


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Figure 4. (a) C1s and (b) S2p XPS spectra of COF-F-S and POP-F-S; F1s spectra of (c) COF-F-S and (d) POP-F-S.



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Figure 5. Electrochemical performance of the cells: (a) Cyclic voltammetry curves of Li/COF-F-S cell at a scan rate of 0.1 mV s-1 in the voltage range of 1.7-2.8 V; (b) Electrochemical impedance spectroscopy plot of the fresh 22 ACS Paragon Plus Environment

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Li/COF-F-S cell; (c) Galvanostatic charge-discharge profiles of Li/COF-F-S cell at different rates; (d) Rate performances of Li/COF-F-S and Li/POP-F-S cells from 0.1C to 2C; (e) Cycling performances of Li/COF-F-S and Li/POP-FS cells at rate of 0.1C in 100 cycles; (f) Long-term cycling performance of Li/COF-F-S and Li/POP-F-S cells at 1C. ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge: FT-IR and solid NMR spectra, TGA graphs, XRD patterns, porous properties, Raman spectra, XPS and elemental analyses of polymers and electrochemical performance of POP-F-S (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G. Kuang) *E-mail: [email protected] (K. Han) *E-mail: [email protected] (W. Zhang)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

NOTE The authors declare no competing financial interest. 23 ACS Paragon Plus Environment


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ACKNOWLEDGMENT This work is partially supported by the National Natural Science Foundation of China (21204047, 21706292, and 21674129). We appreciate Prof. He-Lou Xie from Xiangtan University for XRD and TGA measurements. G.-C. Kuang thanks the China Scholarship Council for financial support. K. Han thanks the support from the Hunan Provincial Science and Technology Plan Project (No. 2017TP1001). W. Wei thanks the National Key Research and Development Program of China (Grant No. 2018YFB010403). G. P. Yu thanks the Innovation Mover Program of Central South University (2018CX046). W. Zhang thanks the National Science Foundation for financial support (NSF, CBET-1605528). ABBREVIATIONS LSB: Lithium Sulfur Battery COFs: Covalent Organic Frameworks REFERENCES (1) Manthiram, A.; Fu, Y.; Su, Y.-S. Challenges and Prospects of Lithium-Sulfur Batteries. Acc. Chem. Res. 2013, 46, 1125-1134.

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