Fluorinated, Sulfur-Rich, Covalent Triazine Frameworks for Enhanced

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Fluorinated, sulfur-rich, covalent triazine frameworks for enhanced confinement of polysulfides in lithium-sulfur batteries Fei Xu, Shuhao Yang, Guangshen Jiang, Qian Ye, Bingqing Wei, and Hongqiang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10991 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Applied Materials & Interfaces

Fluorinated, sulfur-rich, covalent triazine frameworks for enhanced confinement of polysulfides in lithium-sulfur batteries Fei Xu, * † Shuhao Yang, † Guangshen Jiang, † Qian Ye, † Bingqing Wei, † ,‡ Hongqiang Wang * † † State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi'an, 710072, P. R. China. ‡ Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, United States.

KEYWORDS Fluorinated, Trimerization, Porous organic polymers, Covalent triazine frameworks, Polysulfides confinement, Lithium-sulfur batteries

ABSTRACT

Lithium-sulfur battery represents a promising class of energy storage technology owing to its high theoretical energy density and low cost. However, the insulating nature, shuttling of soluble

ACS Paragon Plus Environment

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polysulfides and volumetric expansion of sulfur electrodes seriously give rise to the rapid capacity fading and low utilization. In this work, these issues are significantly alleviated by both physically and chemically restricting sulfur species in fluorinated porous triazine-based frameworks (FCTF-S). One-step trimerization of perfluorinated aromatic nitriles monomer with elemental sulfur allows the simultaneous formation of fluorinated triazine-based frameworks, covalent attachment of sulfur and its homogeneous distribution within the pores. The incorporation of electronegative fluorine in frameworks provides a strong anchoring effect to suppress the dissolution and accelerate the conversion of polysulfides. Together with covalent chemical binding and physical nanopore-confinement effect, the FCTF-S demonstrates superior electrochemical performances, as compared with the sulfur-rich covalent triazine-based framework without fluorine (CTF-S) and porous carbon delivering only physical confinement. Our approach demonstrates the potential of regulating lithium-sulfur battery performances at molecularly scale promoted by the porous organic polymers with flexible design.

INTRODUCTION Advanced energy storage technologies are essential to the future society and sustainable economy. Lithium-sulfur (Li-S) batteries deliver fivefold higher energy density than conventional lithium-ion batteries by taking advantage of the high theoretical specific capacity of sulfur (1675 mAh g-1).1-4 Besides, the low cost, natural abundance, and environmental benignity of sulfur together make Li-S batteries competitive as a kind of next-generation energy storage devices. In spite of the significant potential, several major problems have to be overcome before Li-S battery can find its widespread practical realization.5-7 Sulfur electrode works on the redox


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transformation between cyclic S8 and lithium sulfide accompanied by large volume changes, during which high soluble linear polysulfides intermediates are produced. The intrinsic insulation of sulfur and lithium sulfide leads to poor utilization of the active material; the large volume expansion (80%) by lithiation of sulfur during discharge gives rise to mechanical damage of cathode materials; particularly important, the loss of sulfur from the electrode into the electrolyte by dissolution of polysulfides with so-called shuttling effect causes serious degradation of cycle stability and worse coulombic efficiency. Much effort has been devoted to solving these problems in the past several years. One of efficient strategies is to develop composite cathode materials by physical encapsulation of sulfur into conductive porous hosts that are capable of immobilizing polysulfides and improving conductivity.8-13 Exciting progress has been made recently by using various carbon hosts such as micro/mesoporous carbons, carbon nanotubes/nanofibers, hollow carbon spheres, carbon aerogels and graphene.

10, 14-25

While enhanced sulfur utilization and better electrochemical

performance have been achieved using carbon hosts, a gradual decrease in capacity is still observed with prolonged cycling. This is likely due to that the nonpolar property of carbons is insufficient and kinetically unfavorable in immobilizing the polar polysulfides by physical adsorption/confinement alone.26 Accordingly, efforts have been made to modify the carbon hosts with heteroatom doping or surface functionalities,27-28 so as to intensify specific interactions between sulfur species and carbon hosts. However, the restraining efficiency is still not satisfying due to the inherently low doping or modification ratio in carbons, giving rise to limited active sites available for chemisorption. In this context, constructing strong chemical bonding with sulfur species seems essential for chemical trapping of polysulfides. Organic materials bearing rich functional groups show their


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advantages in this respect. For example, the covalent chemical bonding with elemental sulfur was used for producing sulfur-containing polymers via inverse vulcanization strategy,29 ringopening polymerization of sulfur along the thiol surfaces of trithiocyanuric acid,30 and also in cross-linked sulfur-polyaniline composite with interconnected disulfide bond.31 Benefiting from the strong covalent chemical interaction, the dissolution of polysulfides is effectively suppressed.29, 32 Nevertheless, these sulfur-rich polymers generally exhibit poor conductivity and lack micro-/mesopores for physical encapsulation, precluding the accomplishment of high utilization of active materials, good cycling and improved rate performance. Therefore, it would be highly desirable to develop novel, alternative cathode host materials that enable strong chemical trapping of polysulfides, while maintaining good physical confinement in a similar fashion like that in carbons. Porous organic polymers (POPs), including conjugated microporous polymers and covalent organic frameworks, are a class of porous materials that combine the predesignable polymeric frameworks and permanent nanopores.33-40 They offer a high flexibility for the molecular design of both skeletons and nanopores. In principle, there would be a great opportunity to rationally design POPs as hosts for sulfur, in consideration of complementary utilization of predesignable skeletons for strong chemical trapping of sulfur species and tailored nanopores for physical adsorption of polysulfides. Some POPs were employed to serve as sulfur hosts, while most of them showed moderate restriction efficiency owing to the lack of strong chemical interactions,4146

and very few works witnessed enhanced interactions with sulfur species by covalent bonding

or dual chemical adsorption of Li+ and Sx2-.47-48 However, the rational design of POP hosts with suitable pore environment for efficiently sequestering the soluble polysulfides still remains to be


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well explored. Meanwhile, it is also highly demanding to unravel the relationship of structureproperty between the POP hosts and cell performances at a molecular level.

a (1) 160 oC Sulfur

(2) 400 oC

b

Figure 1. (a) Schematic synthesis of FCTF-S composite by one-step sulfur-assisted cyclotrimerization of aromatic nitriles, (b) chemical structures of FCTF-S and CTF-S from cyclotrimerization with sulfur. Herein we present the fluorinated sulfur-rich covalent triazine frameworks (FCTF-S) as efficient sulfur immobilizer for Li-S batteries by combining its physical and chemical confinement effects. The FCTF-S was synthesized by one-step sulfur-assisted trimerization of perfluorinated aromatic nitriles, resulting in the simultaneous formation of triazine-based frameworks, covalent attachment of sulfur and its homogeneous distribution within the pores (Figure 1). The well-designed structure of FCTF-S has several features: 1) the abundant nanopores and covalent chemical binding of sulfur could help to physically and chemically trap


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sulfur and thus suppress the diffusive loss of soluble polysulfides; 2) the incorporation of fluorine species with polar characteristic in FCTF-S would further facilitates its strong interactions with polar polysulfides; 3) the semiconducting property of triazine based frameworks would be beneficial for the conductivity, compared to the most organic materials for Li-S batteries. Owing to these structural features, FCTF-S demonstrated a high capacity of 1296 mAh g-1 at 0.1 C, stable cycling performance of 833 mAh g-1 after 150 cycles at 0.5 C, as well as improved high-rate capability, superior to CTF-S without fluorine.

EXPERIMENTAL SECTION A description of chemicals, synthetic procedures, material characterizations, and cell fabrication and electrochemical measurements of Li-S batteries is provided in the Supporting Information.

RESULTS AND DISCUSSION Sulfur-rich covalent triazine based frameworks were synthesized by the sulfur-assisted cyclotrimerization of aromatic nitriles (Figure 1). To introduce the fluorine in the frameworks, perfluorinated aromatic nitrile was used as monomers. A stepwise heating at 160 °C and then at 400 °C was applied to first dissolve the sulfur and then initiate the ring opening polymerization of sulfur into polymeric sulfur, accompanied by the trimerization of the nitrile.47 In such a process, the formation of fluorinated triazine-based frameworks, the covalent attachment by sulfur polymerization/insertion, and the nanoconfinement of sulfur within the micropores, can be simultaneously accomplished. A notable color change from khaki to black was observed (Supporting Information, Figure S1), indicating the trimerization and partial graphitization of the


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framework. The occurrence of trimerization reaction was evidenced by the significant attenuation of the otherwise intense carbonitrile band at 2230 cm-1 found in monomers; while the presence of the characteristic absorption bands at about 1500 and 1320 cm-1 indicates the formation of triazine rings in FTIR (Figure 2a).49 Such triazine based frameworks formation can be also validated by C1s XPS spectra in both FCTF-S and CTF-S (Figure 2c). The lowest peak of 284.6 eV was associated with the C=C of the aryl rings in frameworks, whereas the peak at 286.9 eV corresponds to C in triazine N=C-N rings, in accordance with the result from FTIR.

Figure 2. (a) Fourier transform infrared spectroscopy (FTIR) of FCTF-S, CTF-S, and terephthalonitrile monomer, (b) F1s XPS spectrum of FCTF-S, (c) C1s and (d) S2p spectra of FCTF-S and CTF-S. For the sulfur, both

covalent attachment in frameworks owing to the

sulfur

polymerization/insertion and nanoconfinement within the micropores were accomplished in the


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preparation process. The covalent attachment in the frameworks can be revealed by the XPS spectra of S2p and C1s (Figure 2c, d). The high-resolution S2p XPS spectra can be deconvoluted into three peaks at 164.8, 163.7, 163.1 eV for FCTF-S, and 164.8, 163.6, 161.7 eV for CTF-S, respectively (Figure 2d). The former two dominated peaks can be attributed to the S2p1/2 and S2p3/2 of S-S bonds in sulfur. The latter peak at low binding energy represents the C-S bond in both FCTF-S and CTF-S, indicating the existence of bonding interactions between the C atom and S atom.30, 32 Such covalent bonding interactions can be also confirmed by the presence of peak at 286.1 eV in C1s spectra, which can be assigned to the C-S bonding in both FCTF-S and CTF-S (Figure 2c). The incorporation of fluorine in FCTF-S was verified by the presence of C-F bond at 993 cm-1 in FTIR and F1s signal in survey XPS spectra of FCTF-S, which otherwise cannot be found in CTF-S (Figure 2a and Supporting Information, Figure S2). To investigate the chemical bonding nature, the F1s of FCTF-S could be deconvoluted into three peaks at 688.6, 687.4, and 685.6 eV, respectively (Figure 2b). The value at high binding energy of 688.6 eV attributes to the covalent C-F bond, while the peaks at 687.4 and 685.6 eV correspond to F atoms semi-ionically bound to the sp2 carbon.50-51 This result indicates that the ionic contribution to the C-F bonding is also noticeable owing to the high-temperature preparation process. Furthermore, an additional peak of 288.3 eV at higher binding energy was found in C1s spectrum of FCTF-S (Figure 2c), corresponding to the C atom in C-F bonding in FCTF-S.50 However, this peak was absent in CTF-S (Figure 2c). It is expected that the polar C-F bonding will increase the interactions with polar polysulfides, and their semi-ionically properties with positively charged frameworks also help to adsorb negatively charged polysulfides.


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a

b

c

d

50 nm

e

50 nm

50 nm

Figure 3. (a) Scanning electron microscope (SEM) and (b) transmission electron microscope (TEM) images of FCTF-S, (c) annular dark-field TEM image, and the corresponding elemental mappings of (d) sulfur and (e) carbon of the region indicated by the white square in (c). Nitrogen adsorption/desorption shows that the adsorbed amount is very low (Supporting Information, Figure S3), suggesting that the inherent pores of triazine based frameworks were fully occupied by elemental sulfur. Such physically confined sulfur can be removed by dynamic vacuum heating, judging from the color change from black to brown and the appearance of sulfur on the upper wall of the dynamic vacuum tube (Supporting Information, Figure S1c). The powder X-ray diffraction (PXRD) analysis shows that some crystalline peaks remained in the FCTF-S and CTF-S, consistent with that of crystalline elemental sulfur (Supporting Information, Figure S4). This result reveals that the physically trapped sulfur crystallizes inside the nanopores after the high-temperature treatment. The morphology of as-synthesized materials FCTF-S appears as well-defined nanoparticles (Figure 3a), in agreement with the TEM observation in Figure 3b, whereas CTF-S consists of seriously aggregated large particles with average sizes of 1


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μm (Supporting Information, Figure S5). TEM with elemental mapping was used to track the elemental distribution of sulfur in the FCTF-S (Figure 3c). The elemental mapping shows the spatial distributions of sulfur and carbon (Figure 3d and 3e), implying that the sulfur is homogeneously encapsulated in the nanopores of frameworks.

Figure 4. (a) CV profiles of the FCTF-S and CTF-S cathodes at a scan rate of 0.1 mV s-1 in the potential range from 1.7 to 2.8 V; (b) Galvanostatic discharge-charge profiles of the FCTF-S and CTF-S cathodes recorded at 0.5 C; (c) Cycling performance of the FCTF-S, CTF-S and porous carbon/S composite cathodes at 0.5 C for 150 cycles; (d) Nyquist plots of FCTF-S and CTF-S cathodes from 100 kHz to 10 mHz. It is speculated that such fluorinated, sulfur-rich based triazine frameworks will perform well for energy storage in Li-S batteries. The physical nanoconfinement from nanopores, chemical covalent binding with frameworks and polar and ionic characteristic of incorporated fluorine


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species will work co-operatively to restrain the polysulfide shuttling effect and increase the active sulfur utilization. Therefore, we carried out electrochemical performances by using FCTFS as cathode material for Li-S batteries. The sulfur content is around 51 wt%, as determined by repeated Energy Dispersive Spectrometer (EDS) line scans in SEM (51 wt%), TEM with elemental mapping (48.5 wt%, Figure 3d and Supporting Information, Figure S6) and thermogravimetric analysis (53 wt%, Supporting Information, Figure S7). The sulfur mass loading is around 0.7 mg cm-2, unless otherwise stated. For comparison, CTF-S and porous carbon/S were used as references. The cathode electrode was prepared by integrating active materials FCTF-S, conductive carbon Super P and binder fluoride polyvinylidene using the doctor-blade method. Cyclic voltammetry (CV) profiles were measured within a potential window of 1.7-2.8 V at a scan rate of 0.1 mV s-1 (Figure 4a), both showing two main cathodic peaks and two anodic peaks. The cathodic peak located at 2.31 V is assigned to the reduction of cyclic S8 to long-chain lithium polysulfide (Li2Sx, 4≤x≤8), whereas another peak appears at 2.03 V that reflects the further reduction via long-chain lithium polysulfide to short-chain polysulfides (Li2Sx, x≤3) and Li2S. In the subsequent anodic scan, two broad oxidation peaks at 2.23-2.43 V are observed, assignable to the conversion of Li2S2/Li2S to sulfur via the formation of the intermediate lithium polysulfides. Interestingly, the reduction peaks of FCTF-S cathode (2.31 and 2.03 V) appear at higher potentials than that of CTF-S without fluorine cathode (2.28 and 2.02 V), while the oxidation peaks appear at lower potentials (2.43 vs 2.46 V for FCTF-S and CTF-S, respectively). Such distinct positive shift in the reduction peaks (i.e. 34 and 8 mV) and negative shift in the oxidation peaks (i.e. 30 mV) of FCTF-S indicate a decrease in polarization and an accelerated kinetics for the polysulfide redox. Similar improvement of redox reaction kinetics was observed by several works using the precious metals or metal nitride/oxide


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(Pt,52 VN,53 Nb2O554) as electrocatalysts to facilitate the conversion of polysulfide back to soluble long-chain polysulfide. These results indicate that the incorporation of polar F atoms in the framework helps to boost the redox reaction kinetics of polysulfide owing to their strong interactions. Galvanostatic discharge-charge tests were further carried out at current rate of 0.5 C, as shown in Figure 4b. Two plateaus at 2.30 and 2.06 V were observed in the discharge curves, and a two charge plateaus between 2.24 and 2.40 V were present in the charge process, respectively, in agreement with the redox peaks (Figure 4a) observed in the CV measurements. The polarization, reflected by the gap between the oxidation and reduction plateaus potential, provides information on the redox reaction kinetics. Notably, FCTF-S shows a lower polarization than CTF-S (Figure 4b), further manifesting the promoted kinetics caused by the introduction of fluorine, similar to the result from CV. FCTF-S delivers an initial discharge capacity of 1131 mAh g-1 at 0.5 C, whereas CTF-S without polar C-F bond and porous carbon/S composite with only physical confinement show only a low capacity of 862 and 488 mAh g-1, respectively (Figure 4c). More significantly, the plateaus retain their shape with cycling, implying the stable structure of FCTFS for immobilizing sulfur species (Supporting Information, Figure S8). FCTF-S is able to maintain a stable cycling performance after 150 cycles with a capacity retention ratio of 73.7% and a fading rate of 0.17% per cycle (Figure 4c). In contrast, CTF-S fades at a higher rate of 0.30% with capacity retention of 55.5% after 150 cycles. Thus, the FCTF-S cathode could retain a much higher reversible capacity of 833 mAh g-1 than that of CTF-S cathode (478 mAh g-1) after 150 cycles. To show the potential of FCTF-S for practical applications, we increased the sulfur loading to 1.3 mg cm-2, and found that FCFT-S still demonstrated stable cycling performances at 0.5 C, in spite of a slight decrease in capacity (Supporting Information,


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Figure S9). At low rate of 0.1 C, capacities up to 1296 and 1243 mAh g-1 are achieved for FCTF-S and CTF-S at initial cycle, respectively. With further charge-discharging to 50 cycles, the difference between capacities becomes significant (Supporting Information, Figure S10) for FCTF-S and CTF-S. These results vividly show that the chemical and physical restraining effect together with polar and ionic C-F bond work effectively to improve sulfur utilization and mitigate the dissolution of polysulfides into the organic electrolyte. Owing to the low conductivity of the organic host materials, a large amount of conductive agent was used in the conventional cathode electrodes using POPs as hosts (around 30 wt%, Supporting Information, Table S1). However, the capacities of sulfur-rich triazine based frameworks, e.g., FCTF-S are still superior to most of previously reported POP hosts even with reduced amount of the conductive agent (10 wt%, Supporting Information, Table S1) in the electrode. This is thought to be attributed to the semiconductivity properties of the triazine based frameworks.55 The conductivity of FCTF-S and CTF-S was measured to be 2.26*10-4 and 2.02*10-4 S cm-1, which is ~21 orders of magnitude higher than sulfur (5*10-26 S cm-1) and is also greater than many insulating organic porous hosts. Electrochemical impedance spectra in Nyquist plots of the FCTF-S and CTF-S were also compared (Figure 4d). The charge transfer resistance of FCTF-S, determined by the semicircle at high-frequency region, is much smaller than that of the CTF-S. This can be explained by increased interfacial affinity between fluorinated triazine based frameworks with polar and ionic characteristics and polysulfides. Moreover, FCTF-S also delivered an excellent high-rate performance, as shown in Figure 5a. The electrode was cycled at high rate of 1 C for 100 cycles. The cell of FCTF-S was able to deliver a stable discharge capacity. In contrast, the CTF-S electrode exhibited lower discharge capacity under the same conditions. In additon, the coulombic efficiency is stable and close to


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98%. These results demonstrate the higher rate capabilities of FCTF-S due to the introduction of polar and ionic C-F bond, as compared with that of CTF-S. To verify the strong anchoring ability of FCTF-S for polysulfides, we compared the polysulfide adsorption ability of the pure frameworks FCTF and CTF, which were prepared by conventional ZnCl2-assitted ionothermal synthesis (Supporting Information, Experimental Section). Photos were taken after adding 5 mg of CTF or FCTF powders to glass vials containing Li2S6 solution (5 mmol L-1, 1.5 ml) for 1 h. As shown in Figure 5b, the digital image shows a visible decoloration of Li2S6 solution after the adsorption, whereas the solution containing CTF showed less decoloration. Furthermore, the cell was disassembled in Ar-filled glove box after discharging the cell to 2.08 V at the 5th cycle and keeping at this potential for 12 hours, followed by washing several times with 3 mL of 1,3-dioxolane to collect the polysulfide-containing solution. The polysulfides dissolved in the electrolyte solution were examined by UV-Vis adsorption spectra (Supporting Information, Figure S11). The results show that the amount of polysulfides is less for the FCTF-S battery, as compared with that of CTF-S, implying that the incorporation of F is helpful to minimize loss of polysulfides into the electrolyte. Such difference suggests strong adsorption of Li2S6 molecules to frameworks with fluorine functionalities, owing to polar and ionic bonding of C-F in FCTF-S.


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Figure 5. (a) Cycle stability of the FCTF-S and CTF-S cathodes and coulombic efficiency of FCTF-S at current rate of 1 C; (b) Digital picture of the fast adsorption of lithium polysulfides by FCTF and CTF frameworks.

CONCLUSIONS We have designed and prepared fluorinated, sulfur-rich covalent triazine based frameworks for Li-S batteries, by one-step trimerization of fluorinated aromatic nitriles monomer with elemental sulfur. This structure benefits from the advantages of (i) the physical confinement of polysulfides by nanopores of frameworks, (ii) chemical confinement of sulfur by covalent binding with frameworks, and (iii) a strong anchoring effect for polysulfides by highly electronegative fluorine atoms in frameworks to accelerate the polysulfide conversion. With this strategy, the FCTF-S electrode exhibited a high specific capacity of 1296 mAh g-1 at 0.1 C, stable cycling performance of 833 mAh g-1 after 150 cycles at 0.5 C, and improved high-rate capability, superior to CTF-S without fluorine and porous carbon/S composite. Our finding will provide an impetus for the utilization of these porous organic materials in a broad spectrum of applications for energy storage, especially by modulating the performances via molecularly design.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: Description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.


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Experimental section, digital pictures, XPS spectra, nitrogen adsorption/desorption isotherm, XRD patterns, SEM images, EDS spectra, thermogravimetric analysis curve, galvanostatic discharge-charge curves, cycling performance and coulombic efficiency, UV-vis absorption spectra, and summary of the electrode composition and capacity of representative porous organic polymers host cathodes. (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. * E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Projects of NSFC (51702262, 51472204, 51672225), the Natural Science Foundation of Shaanxi Province (2017JQ5003, 2017JM5028), the Fundamental Research Funds for the Central Universities (3102017OQD057, G2017KY0002), the Project of Young Talent Fund of University Association for Science and Technology in Shaanxi, China (20160103), Creative Research Foundation of Science and Technology on Thermostructural Composite Materials Laboratory (6142911030512), the Key laboratory of Polymeric Composite & Functional Materials of Ministry of Education (PCFM201602), the Program of Introducing


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Talents of Discipline to Universities (B08040). Prof. Wang acknowledges as well the financial support of the 1000 Youth Talent Program of China.

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Fluorinated, Sulfur-Rich, Covalent Triazine Frameworks for Enhanced

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