Flexible Cathode Materials Enabled by a Multifunctional Covalent

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Flexible Cathode Materials Enabled by Multifunctional Covalent Organic Gel for Lithium-Sulfur Batteries with High Areal Capacities Hui Pan, Zhibin Cheng, Hong Zhong, Ruihu Wang, and Xiaoju Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21639 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Materials & Interfaces

Flexible Cathode Materials Enabled by Multifunctional Covalent Organic Gel for Lithium-Sulfur Batteries with High Areal Capacities Hui Pan,†,‡ Zhibin Cheng, ‡ Hong Zhong, ‡ Ruihu Wang‡ and Xiaoju Li,†* †

Fujian Key Laboratory of Polymer Materials, College of Chemical and Materials Science,

Fujian Normal University, Fuzhou, Fujian, 350007, China ‡

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure

of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China KEYWORDS: covalent organic gels, freestanding electrodes, high areal capacity, lithium-sulfur batteries, shuttle effect

ABSTRACT: Foldable lithium-sulfur (Li-S) batteries have captured considerable interest in advanced flexible energy storage systems. However, sulfur utilization, cycling stability and mechanical durability are still not satisfactory for flexible batteries with high sulfur loadings. Herein, we present one type of new freestanding electrode materials derived from thiourea-based covalent organic gel (COG). COG can accommodate high loading of carbon nanotubes (CNTs) and sulfur with the concomitant formation of an embedded conductive CNTs network. The unique performance of COG not only facilitates ion transfer and electrolyte infiltration, but also effectively confines polysulfides in the internal cavities.

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These advantages endow the freestanding CNT/S/COG electrodes with high reversible capacity, good rate performance, excellent cycling stability and superior structural integrity. CNT/S/COG with an ultrahigh sulfur loading of 12.6 mg cm-2 delivers a high discharging capacity of 13.7 mA h cm-2 (1097 mA h g-1) at 0.1 C, the capacity retention is as high as 83.9% after 100 cycles. Moreover, CNT/S/COG could be processed into foldable pouch cells. This study has demonstrated great potential of COG for the fabrication of advanced flexible energy storage devices with high-energy density and long cycling life.

INTRODUCTION The ever-increasing demands for wearable electronics and smart textiles have driven blooming development of advanced flexible energy storage systems.1,2 Lithium-sulfur (Li-S) batteries possess great potential to meet the rapidly expanding demand of flexible electronics due to their high theoretical energy density, cost effectiveness and environmental friendliness.3-5 However, the commercialization of Li-S batteries is still impeded by several challenges, such as the insulating nature of sulfur and solid-state discharging products, the notorious shuttle effect caused by the dissolution and migration of soluble lithium polysulfides (PSs), and large volume fluctuation of the active materials during cycling. These problems inevitably lead to low sulfur utilization, poor cycling stability, serious anode corrosion and even a series of safety problems.6-9 To address such issues, tremendous efforts have been devoted to tailor-made nanostructured carbon materials to accommodate sulfur species. It is generally acceptable that the heteroatom doping and/or surface functionalization of the carbon matrices could


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strengthen the affinity of PSs in the cathode, resulting in great improvement in specific capacity and cycling life.10-13 However, the areal sulfur loading in most of conventional slurry-coating electrodes is less than 2.0 mg cm-2, which is far below the acceptable value to satisfy the demands for high energy density. As one promising alternative, three-dimensional (3-D) freestanding carbon materials have recently been proposed for the electrodes with high sulfur loadings.14,15 Great achievements have been made through judicial selection of various carbon materials, such as carbon nanotubes (CNTs) paper,16 hollow carbon fiber foam,17 bucky paper,18 3-D aligned porous carbon19 and 3-D graphene framework,20 but these materials still face the challenges either the accessibility of the electrolytes to the internal sulfur or the maintenance of the foldability for the flexible electrodes with ultrahigh sulfur loadings. Moreover, the active sulfur species in most of freestanding electrodes are hosted in the cathode region through physical confinement, resulting in serious shuttle effect of PSs during long-term cycling.15 Therefore, it is highly desirable to develop new type of flexible 3-D materials with abundant binding sites for achieving high areal capacity and good cycling stability. Covalent organic gels (COGs) have attracted increasing attention recently owing to their stable structure, good flexibility and versatile synthetic methods.21-23 They are usually generated inductively as intermediate aggregates in the polymerization of organic building blocks with multiple reactive groups, which enables sulfur particles, conductive materials and electrolytes to accommodate in their inherent cavities, resulting in homogeneous distribution of inclusions, rapid mass transfer and free diffusion of chemical species.24 The formed linkage provides abundant active sites to trap PSs through strong chemical interaction, which



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effectively suppresses the shuttle effect of PSs even for the electrodes with ultrahigh sulfur loadings. Advantageously, COGs could be facilely processed into any sizes and shapes required for practical applications.25,26 Despite these encouraging advantages, to our best knowledge, the application of COGs in Li-S batteries is still unexplored. As a proof-of-concept study, herein, we report a flexible thiourea-based COG constructed by tetrakis(4-aminophenyl)methane (TAPM) and 1,4-phenylene diisothiocyanate (PDI). COG is capable of encapsulating large amounts of CNT/S during the gelation to form flexible freestanding CNT/S/COG electrodes. The CNT/S/COG electrodes with sulfur loadings from 4.5, 8.3 and even to 12.6 mg cm-2 show outstanding electrochemical performance in terms of high sulfur utilization and good cycling stability, the corresponding initial areal capacities at 0.1 C are as high as 5.76, 9.16 and 13.7 mA h cm-2, respectively. The electrodes are foldable and could be used as soft-packed cells.

EXPERIMENTAL SECTION Synthesis of COG. A mixture of tetrakis(4-aminophenyl)methane (TAPM, 117 mg. 0.308 mmol)

and

1,4-phenylene

diisothiocyanate

(PDI,

118

mg,

0.615

mmol)

in

N-methyl-2-pyrrolidone (NMP, 2.5 mL) were fully mixed together in a 6 mL vial. The translucence cylindrical gel was obtained after standing the bottle for 1 h. Synthesis of COG membrane. TAPM (9.37 mg, 0.025 mmol) and PDI (9.37 mg, 0.050 mmol) were dissolved in 200 μL NMP. The resultant solution was spread out on a glass, followed by covering with the other glass sheet. After thorough drying at 60 °C for 12 h, the flexible gel membrane could be easily peeled off from the glass substrate.


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Synthesis of CNT/S composite. The CNT/S composite was prepared following a melt-diffusion method. In a typical procedure, CNT and sulfur (high purity sulfur, 99.999% metal basis, Adamas Reagent) with a mass ratio of 3:7 were ground and dispersed in CS2 solution, the mixture was stirred at room temperature until CS2 was completely evaporated. The obtained mixture was heated at 160 °C for 12 h. The product was collected after cooling to room temperature to generate CNTs/S. The sulfur content in this work was determined to be 65 wt%. Synthesis of CNT/S/COG cathode. To prepare CNT/S/COG-4.5 cathode, a solution of TAPM (18.75 mg, 0.05 mmol) and PDI (18.75 mg, 0.10 mmol) in NMP (400 uL) was added to the mixture of CNTs/S (300 mg) and CNTs (37.5 mg). The resultant slurry was pasted onto a glass substrate, followed by covering with the other glass sheet. The flexible freestanding electrode could be easily peeled off from the glass substrate after thorough drying at 60 °C for 12 h. The free-standing electrode was punched into smaller disks with a diameter of 10 mm, which was directly used as a working electrode. The CNT/S/COG-8.3 and CNT/S/COG-12.6 cathodes were fabricated following the same procedures as that of CNT/S/COG-4.5 except increasing the electrode thickness to 270 um and 410 um, respectively. The weight ratio of CNT/S : CNT: COG was fixed to 8:1:1. The cathode (CNT/S/COG) contained 52 wt% of sulfur in the whole electrode. Synthesis of CNT/S/PVDF cathode. In a typical procedure, 80 wt% CNT/S, 10 wt% CNTs and 10 wt% polyvinylidene difluoride (PVDF) were mixed with 1-methyl-2-pyrrolidinone (NMP) to form a slurry. The slurry was pasted on aluminum foil and dried under vacuum at



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60 °C for 12 h, subsequent cutting into circular pieces generated the electrode with ≈4.5 mg cm-2 sulfur loading. Synthesis of Li2S6 solution and adsorption test. All samples were dried under vacuum at 60 ºC overnight before the adsorption test. Li2S6 was prepared by chemical reaction between sublimed sulfur and Li2S with a molar ratio of 5:1 in DOL/DME solution (1:1 by volume). The mixture was stirred at 70 ºC overnight under nitrogen atmosphere to produce a brownish-red Li2S6 solution. The polysulfide adsorption was examined through adding 90 mg of each sample (CNT, CNT/PVDF, and CNT/COG) into the Li2S6 solution (10 mM, 2 mL).

RESULTS AND DISCUSSION The thiourea-forming condensation reaction is an attractive atom-economic reaction for the construction of COGs because the reaction could be performed under mild conditions without releasing byproduct molecules that may be trapped inside the formed network. As shown in Figure 1a, the condensation reaction between TAPM and two equivalents of PDI in N-methyl-2-pyrrolidone (NMP) at ambient temperature generated light-yellow translucent thiourea-based

COG.

After

the

removal

of

entrapped

solvent

molecules,

the

Brunauer-Emmett-Teller (BET) specific area surface of COG-aerogel is 56 m2 g-1 (Figure S1). In the FTIR spectrum of COG (Figure 2a), the stretching bands of isothiocyanate in PDI at 2000-2200 cm-1 totally disappear, while the characteristic peaks of thiourea unit for N-C=S and N-H occur at 1500 and 3450 cm-1, respectively, indicating the successful coupling of TAPM and PDI. The characteristic peak at 174 ppm in solid-state 13C NMR corresonds to the carbon atom of thiourea unit, which further confirms the formation of COG (Figure 2b). X-ray diffraction (XRD) pattern of COG shows amorphous characteristic for the gel materials


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(Figure S4). Cyclic voltammogram (CV) and thermogravimetric analysis (TGA) measurements indicate that COG possesses good electrochemical and thermal stability (Figure S2, S3). Scanning electron microscopy (SEM) images reveal that COG has a sponge-like structure consisting of nanoscale interconnected particles (Figure S5). Notably, COG shows a superior strain behavior (Figure S6), it could withstand a strain ratio over 50% without detectable cracks, indicating the robust backbone of the thiourea-based COG, which provides great potentials to accommodate a large amount of sulfur in its inherent cavities.

Figure 1. (a) Schematic illustration for the synthesis of thiourea-based COG. (b) Schematic illustration for the fabrication of CNT/S/COG electrodes. (c-e) Photographs to show flexibility of the CNT/S/COG electrodes. (f) The punched slices from CNT/S/COG. The flexible translucent COG membrane was readily obtained when the mixture of TAPM and PDI with a molar ratio of 1:2 in NMP was pasted on a glass substrate and covered with the other glass sheet (Figure S7). The membrane exhibits good flexibility and favorable



mechanical durability, no obvious irreversible deformation is detected after it is folded repeatedly (Video S1). The COG membrane also possesses good stability and corrossion resistance in the electrolyte. After COG membrane was soaked in the electrolyte for more than three days, there is no obvious swelling and structure change (Figure S8). These characteristics make COG very suitable for fabricating flexible freestanding electrodes.

Figure 2. (a) FTIR spectra of TAPM, PDI, COG and CNT/S/COG. (b) Solid-state 13C NMR spectrum of COG-aerogel. (c) Cross-sectional SEM images for CNT/S/COG-4.5 and corresponding EDS elemental mapping in marked region by red dashed rectangle.

As expected, the addition of CNT/S containing 65 wt% sulfur (Figure S9 and Figure S10) into the mixture of TAPM and PDI in NMP has no detectable effects on the gelation process, black CNT/S/COG membrane could be easily peeled off from the glass substrate after thorough drying (Figure 1c-e). The membrane could be cut and punched into slices for direct


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use as working electrodes without metal current collectors (Figure 1f, Video S2). Attractively, the thickness of the electrodes could be adjusted through varying the loading amount of CNT/S in the process of COG formation. The resultant electrodes with sulfur areal loadings of 4.5 and 8.3 mg cm-2 are denoted as CNT/S/COG-4.5 and CNT/S/COG-8.3, respectively. Compared with conventional sulfur melt-diffusion technology or solution-based methods for the fabrication of freestanding electrodes,27,28 this in situ sulfur loading approach could guarantee homogeneous distribution of sulfur in the whole electrode, which is favorable for high sulfur utilization during electrochemical reaction. Cross-sectional SEM images of both CNT/S/COG-4.5 and CNT/S/COG-8.3 show a densely packed structure without any fracture, their thicknesses are ~145 and 270 um, respectively (Figure 2c and Figure S11). The energy dispersive X-ray spectroscopy (EDS) elemental mappings exhibit uniform distribution of carbon, sulfur and nitrogen in the electrodes, suggesting that CNT/S are uniformly encapsulated in the COG, which is further conformed by transmission electron microscopy (TEM) and corresponding high-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure S12). Strikingly, intimate interconnection of CNTs forms an embedded conductive network in the internal cavities of COG, which is beneficial for efficient ion/electron transport and electrolyte infiltration. In the XRD pattern of CNT/S/COG-4.5, the typical peaks of sulfur disappear owing to strong interaction with COG, further suggesting sulfur species could be homogeneously encapsulated in COG (Figure S4). The FTIR spectrum of CNT/S/COG-4.5 is similar to that of COG (Figure 2a), the characteristic peaks of thiourea group could be clearly identified, which shows the encapsulation of CNT/S has no appreciable effect on the structural



backbone of COG. The BET specific surface area of CNT/S/COG-4,5 is 13 m2 g-1 (Figure S1b). The CNT/S/COG electrodes exhibit remarkable flexibility and mechanical durability. As shown in Figure 1c-f and Video S2, CNT/S/COG-4.5 could endure vigorous bending, twist and roll without detectable variation in shape. The excellent flexibility is rare in reported electrodes with such high sulfur loading.29,30 As a comparison, CNT/S/PVDF with a sulfur loading of ~4.5 mg cm-2 was also prepared through traditional blade-coating technology with aluminum foil as a current collector and PVDF as a binder. As shown in Figure S13, there are a few minor cracks on the surface of the electrode. After arbitrary bending, both the detachment of the active materials from the aluminum foil and surface crack are observed. Moreover, CNT/S/PVDF is incapable of recovering to its original state. These observations show that thiourea-based COG could not only accommodate more active materials in its internal cavities with the maintenance of a robust structure, but also avoid the problems caused by the use of aluminum current collector.31 Thus, the COG-derived electrodes hold great promises for application in flexible wearable electronics.


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Figure 3. (a) CV profiles in the second cycle, (b) galvanostatic charge/discharge curves, (c) electrochemical impedance spectroscopy, (d) rate performance, (e) cycling performance and (f) corresponding areal capacity at 0.1 C for the CNT/S/COG-4.5, CNT/S/COG-8.3 and CNT/S/PVDF electrodes. (g) Cycling performance and (h) corresponding areal capacities at 0.1 C for the CNT/S/COG-12.6. (i) Color changes of Li2S6 solution before and after exposure to CNT, CNT/PVDF and CNT/COG. To investigate the electrochemical performance of the COG-based freestanding electrodes, CNT/S/COG-4.5, CNT/S/COG-8.3 and CNT/S/PVDF were assembled into CR2025 coin cells with metallic lithium as a counter anode. CV measurements were performed at a scan rate of 0.1 mV s-1 in the potential window from 1.5 to 3.0 V. As shown in Figure 3a, all electrodes show two typical cathodic peaks and one anodic peak after the second run, corresponding to reversible conversion between elemental sulfur and solid-state Li2S2/Li2S through PSs intermediates. The relative sharp reduction peaks of CNT/S/COG-4.5 at 2.30 and 2.01 V correspond to the reduction of elemental sulfur to soluble lithium PSs and their further



reduction to solid-state Li2S2/Li2S, respectively, while the anodic peak at 2.40 V is associated with coupled conversion from Li2S to PSs, and ultimately to elemental sulfur.32,33 CNT/S/COG-8.3 shows an obvious positive shift in the reduction peaks and a negative shift in the oxidation peak owing to increased electrode resistance and slower dynamics for electrolyte infiltration with the increment of sulfur loading. However, further shift and remarkable broadening in the peaks are observed in CNT/S/PVDF. In addition, CNT/S/PVDF shows lower current density and higher onset potential in each redox peak than CNT/S/COG. The galvanostatic charge/discharge profiles of these electrodes at 0.1 C consist of two well-defined discharge plateaus and one charging plateau (Figure 3b), which are in agreement with CV results. Apparently, CNT/S/PVDF shows higher polarization than CNT/S/COG. It should be mentioned that CNT/S/COG-8.3 has a high sulfur loading, but its polarization remains at a low level, which indicates that accelerated redox kinetics of PSs in the freestanding CNT/S/COG electrodes when compared with that of the slurry-coating CNT/S/PVDF electrode. Electrochemical impedance spectroscopy (EIS) was used to further confirm low internal resistance of CNT/S/COG. As shown in Figure 3c, the semicircle in the Nyquist plot of CNT/S/COG-8.3 is broader than that of CNT/S/COG-4.5 owing to the increment of sulfur loading, but they are dramatically decreased in comparison with that of CNT/S/PVDF. This is mainly attributed that the unique COG-based electrode offers homogeneous sulfur distribution, facile electrolyte infiltration and fast ion transport, thereby leading to low electrochemical resistance and high sulfur utilization.

The rate capabilities of the electrodes were examined by increasing charge/discharge current density every 5 cycles from 0.05 to 1 C. As shown in Figure 3d, the CNT/S/COG-4.5


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electrode delivers an initial discharge capacity of 1520 mA h g-1, corresponding to 90.7% of the theoretical capacity of sulfur, revealing that most of sulfur could be converted into lithium sulfides during the first discharge.34 When the rates are increased to 0.1, 0.2, 0.5 and 1 C, the electrode still delivers reversible capacities as high as 1258, 1086, 927 and 696 mA h g-1, respectively. After the current density is abruptly switched back to 0.1 C, the reversible capacity is recovered to 1190 mA h g-1, which is very close to the initial value. In comparison with CNT/S/COG-4.5, the discharging capacities and rate capabilities of CNT/S/COG-8.3 only slightly degrade at various rates, but a sharp decrease is displayed for CNT/S/PVDF. The cycling performance of these electrodes at 0.1 C was illustrated in Figure 3e. CNT/S/COG-4.5 delivers a high initial capacity of 1280 mA h g-1, which is almost two times higher than that of CNT/S/PVDF (670 mA h g-1). After 120 cycles, the reversible capacity of CNT/S/COG-4.5 is still above 1070 mA h g-1, which is much better than that in CNT/S/PVDF (253 mA h g-1). The average Coulombic efficiency of CNT/S/PVDF is calculated to be 93.2%, which is much lower than that of CNT/S/COG-4.5 (98.1%), these results reveal that irreversible sulfur loss and shuttle effect of PSs could be effectively inhibited by COG. As for CNT/S/COG-8.3, an initial discharge capacity of 1103 mA h g-1 is achieved, and the capacity in the 120th cycle is maintained at 896 mA h g-1. Their areal capacities are shown in Figure 3f. CNT/S/COG-4.5 shows an initial areal capacity of 5.76 mA h cm-2 at 0.1 C. This value in CNT/S/COG-8.3 is enhanced to 9.16 mA h cm-2, which is almost three times higher than that in CNT/S/PVDF (3.01 mA h cm-2). In order to further show superiority of COG in the freestanding electrodes, the CNT/S/COG-12.6 electrode with higher sulfur loading of 12.6 mg cm-2 was fabricated. As



shown in Figure 3g-h, the electrode delivers a high initial capacity of 13.7 mA h cm-2 (1087 mA h g-1) at 0.1 C (2.11 mA cm-2) after three cycles activation. A high capacity of 11.5 mA h cm-2 (913 mA h g-1) is achieved after 100 cycles. The capacity retention and average Coulombic efficiency are 83.9% and 96.9%, respectively, further demonstrating high sulfur utilization and excellent stability of the CNT/S/COG electrodes. Strikingly, the areal capacity and cycling stability of CNT/S/COG-12.6 are comparable with state-of-the-art those in recently reported high-sulfur-loading Li-S batteries without the use of interlayers14,15,35 (Table S1). More attractively, CNT/S/COG could be processed into a rectangle-shape pouch cell. The light-emitting diodes could be lighted up by the assembled pouch cell in various folding angles and recovered state (Figure 4a, Video S3), indicating excellent flexibility of the soft-packed pouch cell, which endows CNT/S/COG with great potential in flexible electronics.

Figure 4. (a) The photograph for soft-packed pouch cell in different folding angles to light up LED. (b) Visual experiments displaying the dissolution of PSs intermediates into electrolyte


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after discharging for CNT/S/COG-4.5 and CNT/S/COG-8.3 and CNT/S/PVDF. (c) Adsorption binding energies and (d) the optimized geometries of the adsorption configurations between 1,3-diphenylthiourea and Li2Sx in different lithiation stages. (e) Photographs for electrolyte droplets on COG, CNT/S/COG and CNT/S. The outstanding performance of free-standing CNT/S/COG electrode is mainly attributed to unique advantages of thiourea-based COG, which not only buffers volume change of high-loaded sulfur during charge and discharge, but also serves as an electrolyte reservoir to ensure the reaction of most sulfur. Meantime, the strong chemical interaction between thiourea group and PSs effectively sequester PSs in the internal cavities of COG. The strong affinity of CNT/S/COG toward PSs has been verified by visual adsorption experiments. As shown in Figure 3i, when the same amount of CNT, CNT/PVDF and CNT/COG were added to a fixed volume of Li2S6 solution in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME). CNT/COG changes the solution color from tawny to yellow, while the solution color has no obvious change for CNT and CNT/PVDF. The sharp color contrast indicates that outstanding adsorption ability of COG toward PSs, which is beneficial for tightly anchoring PSs in the cathode region.36-38 The effective PSs sequestration in CNT/S/COG is further illustrated by self-discharge measurement of optically transparent cells. After CNT/S/COG-4.5 and CNT/S/COG-8.3 were discharged from 3.0 to 1.8 V in the rate of 0.02 mV s-1, their electrolyte keeps colorless (Figure 4b), suggesting that CNT/S/COG could strongly adsorb PSs and prevent their dissolution into electrolyte during discharging.39 In contrast, the electrolyte of CNT/S/PVDF changes from colorless to bright yellow, indicating that PSs diffuse out of the cathode and are dissolved in the electrolyte. This visual experiment offers direct evidence of effective PSs trapping by COG.



To better understand the chemical interaction between COG and PSs, density functional theory (DFT) calculations were implemented using 1,3-diphenylthiourea as a model molecule (Figure 4c-d). The distances between sulfur atom of thiourea and lithium ion of Li2Sx species are in the range from 2.40 to 2.56 Å. The binding energies between 1,3-diphenylthiourea and Li2Sx are in the range from 106.39 to 125.58 kJ mol-1, which are much higher than that between Li2Sx and the common PVDF binder (80 kJ mol-1).40 High binding strength further explains promising cycling stability of freestanding CNT/S/COG electrodes. The formed soluble PSs during cycling could be effectively trapped by COG, thus greatly suppressing the shuttle effect of PSs. It is known that sufficient amount of electrolyte is especially required for a high sulfur-loaded electrode during the electrochemical reaction of PSs, the wetting properties of the electrolyte on active materials have important influences on electrochemical performance of a battery.41,42 The wettability of the electrode was studied by electrolyte contact angle measurement. Figure 4e shows the photographs of electrolyte droplets on COG, CNT/S/COG and CNT/S interface. CNT/S/COG provides a contact angle of 14°, which is very close to that in COG (11o), suggesting the encapsulation of CNT/S in COG has no significant effect for wetting ability and electrolyte infiltration. In sharp contrast, the electrolyte contact angle in CNT/S is increased to 68°. The better wetting on the CNT/S/COG interface further validates that CNT/S/COG serves as an excellent electrolyte reservoir owing to unique performance of COGs to entrap guest molecules in the internal network, which could not only facilitate ion transport and mass transfer in the whole electrode, but also could sequester


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the dissolved PSs in the cathode region. Therefore, high sulfur utilization and superior cycling stability of the CNT/S/COG electrodes are achieved.

Figure 5. (a) photograph of CNT/S/COG-4.5 cathode after 100 cycles at 0.1 C. (b) SEM images of the cycled CNT/S/COG-4.5 cathode. (c) Cross-sectional SEM images of the cycled CNT/S/COG-4.5 cathode and (d) corresponding EDS elemental analysis. (e) Schematic illustration of the discharge/charge process in the CNT/S/COG electrode. In order to further show the advantage of COG in Li-S batteries, the CNT/S/COG-4.5 cathode at discharged state after 120 cycles at 0.1 C was removed from the coin cell. The integrity of the cathode piece is well maintained, and no appreciable cracks and detects are observed after cycling (Figure 5a), which is probably attributed to the fact that COG could effectively buffer volume fluctuation in the process of charge and discharge. In TEM and SEM images of cycled CNT/S/COG-4.5 (Figure 5b, S14), there is no large Li2S agglomeration, indicating that CNT/S/COG-4.5 possesses relatively abundant positions for uniform precipitation of insoluble Li2S, which is mainly attributed to orderly arrangement of high density of thiourea group in the cathode, the strong affinity between thiourea and PSs



allows the active sulfur species to homogeneously reside in electrochemically favorable positions, which facilitates surface redox reaction of PSs and the retention of sulfur species in the cathode.43 The high-resolution Li 1s and N 1s XPS spectra further corroborate the interaction between COG and PSs during charge/discharge (Figure S15), which effectively suppresses the shuttle effect of PSs. The cross-sectional SEM image of cycled CNTs/S/COG-4.5 cathode (Figure 5c) demonstrates that the structure of CNT/S/COG-4.5 remains intact after cycling except the increment of depth caused by volume expansion of the discharged products. The corresponding EDS elemental mapping (Figure 5d) reveals that carbon, sulfur and nitrogen still uniformly distribute in the whole cathode. It should be mentioned that there is fluorine signal in EDS of the cycled CNT/S/COG-4.5 (Figure S16), which originates from bis(trifluoromethane)sulfonimide owing to concomitant infiltration together with the electrolyte into COG during cycling, further showing that COG could serve as an electrolyte reservoir, which is conducive for sulfur utilization and electrochemical redox reaction.44 Moreover, when the separators of cycled CNT/S/COG-4.5 and CNT/S/PVDF were immersed in DME/DOL, the solution changes its color from colorless to yellow for cycled CNT/S/PVDF, while the separator of cycled CNT/S/COG-4.5 make the solution remain transparent and colorless (Figure S17). UV-Vis analyses show the soaking solution of cycled CNT/S/PVDF separator exhibits the characteristic absorption bands of PSs in the range of 400-450 nm, while the peaks for cycled CNT/S/COG separator is negligible. The striking contrasts further suggest COG could effectively suppress the shuttle effect of PSs.


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The above-mentioned observations have unambiguously demonstrated the unique advantages of COG as host materials of sulfur (Figure 5e). Specifically, the intriguing electrochemical performance of the CNT/S/COG electrodes is mainly ascribed to the following factors: (1) COG could provide enough space for homogeneous distribution of a large amount of CNT/S in the internal cavities, which induces the formation of an embedded conductive CNTs network, thus increasing overall electrical conductivity and effective sulfur utilization even for the electrode with an ultrahigh sulfur loading. (2) COG not only serves as an electrolyte reservoir for efficient ion diffusion and mass transfer, but also could be used as a buffering/absorption reservoir of PSs to ensure the electrochemical reaction of the active materials in the internal cavities of COG. (3) The excellent mechanical performance and unique properties of COG could accommodate volumetric expansion of sulfur particles during cycling. The formed solid Li2S particles after discharging could be homogeneously located in the cavities of COG, which maintains the structural integrity of the electrode after cycling. (4) High density of thiourea group in COG provides abundant active sites to anchor sulfur/Li2S particles and trap soluble polysulfide, which effectively suppresses the shuttle effect of PSs, thus resulting in high cycling stability. (5) Outstanding flexibility and mechanical durability of COG enable CNT/S/COG to be processed into desirable shape and size for practical applications, which holds great promises for the fabrication of foldable Li-S batteries with high sulfur loadings.

CONCLUTIONS One type of new COG-derived flexible freestanding electrodes have been presented. COG serves as a buffering/absorption reservoir of well-localized sulfur species to ensure the



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electrochemical reaction in the internal cavities. The unique intrinsic property of COG favors rapid ionic transport and efficient electrolyte infiltration, the migration of PSs toward anode is significantly inhibited by strong chemical interaction between thiourea and PSs. These advantages of COG significantly facilitate mass transfer, sulfur utilization and PSs sequestration in the cathode region even for the electrode with an ultrahigh sulfur loading of 12.6 mg cm-2. Most importantly, conspicuous flexibility, mechanical durability and processability of CNT/S/COG allow for the fabrication of foldable soft-packed cells. Both simple synthesis process and outstanding performance of CNT/S/COG make it one of the most promising freestanding cathode materials in flexible Li-S batteries with high areal capacities. We believe that this proposed methodology based on COG could be applied in many other high-performance electrodes, such as lithium ion batteries and supercapacitors, by loading suitable electrochemical active materials. This will hold great promises for the development of various high-performance flexible energy storage systems.

ASSOCIATED CONTENT Supporting

Information.

Additional

morphological

and

structural

characteristic,

electrochemical performance of the material. This material is available free of charge via the Internet at http://pubs.acs.org. Video of the COG membrane (Video S1); Video of the freestanding electrodes (Video S2); Video of foldable pouch cell (Video S3).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]


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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21671039 and 21471151). Natural Science Foundation of Fujian Province (2015J01038), New Century Excellent Talents in Fujian Province University, State Key Laboratory of Structural Chemistry and Program for Innovative Research Team in Science and Technology in Fujian Province University (IRTSTFJ).

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Flexible Cathode Materials Enabled by a Multifunctional Covalent

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