The Effective Design of a Polysulfide-Trapped Separator at the

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The Effective Design of Polysulfides Trapped Separator at the Molecular Level for High-Energy-Density Li-S Batteries Chao-Ying Fan, Haiyan Yuan, Huan-huan Li, Hai-Feng Wang, Wenliang Li, Hai-Zhu Sun, Xing-Long Wu, and Jingping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04578 • Publication Date (Web): 10 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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ACS Applied Materials & Interfaces

The Effective Design of Polysulfides Trapped Separator at the Molecular Level for High-Energy-Density Li-S Batteries Chao-Ying Fan, Hai-Yan Yuan, Huan-Huan Li, Hai-Feng Wang, Wen-Liang Li, * Hai-Zhu Sun, * Xing-Long Wu, * Jing-Ping Zhang* †

Faculty of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun 130024, China.

KEYWORDS: functionalized separator, organic macromolecule, chemical adsorption, high energy density, Li-S batteries

ABSTRACT: In this work, the light-weight and scalable organic macromolecule, graphitic carbon nitride (g-C3N4) with enriched polysulfides adsorption sites of pyridinic-N, was introduced to achieve the effective functionalization of separator at the molecular level. This simple method overcomes the difficulty of low doping content as well as uncontrolled existence form of nitrogen heteroatom in the final product. Besides the conventional pyridinic-N-Li bond formed in the vacanies of g-C3N4, the C-S bond was interestingly observed between g-C3N4 and Li2S, which endowed g-C3N4 with the inherent adsorption capacity for polysulfides. In addition, the micro-sized g-C3N4 provided the coating layer with good mechanical strength to guarantee its restriction function for polysulfides during long cycling. As a result, an excellent reversible capacity of 840 mA h g-1 was retained at 0.5 C after 400 cycles for pure sulfur electrode, much better than that of the cell with innocent 1

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carbon-coated separator. Even at current density of 1 C, the cell still delivered a stable capacity of 732.7 mA h g-1 after 500 cycles. Moreover, when further increasing the sulfur loading to 5 mg cm-2, an excellent specific capacity of 1134.7 mA h g-1 was acquired with the stable cycle stability, ensuring a high areal capacity of 5.11 mA h cm-2. Besides the intrinsic adsorption ability for polysulfides, the g-C3N4 is non-toxic and mass-produced. Therefore, the scalable separator decorated with g-C3N4 and commercial sulfur cathode promise the high energy density for the practical application of Li-S batteries.

1. INTRODUCTION Lithium-sulfur (Li-S) batteries are deemed to be one of the most attractive energy storage systems because the cells can reach an unparalleled energy density of 2567 W h kg-1.1, 2

The practical application of Li-S batteries could be achieved if the drawbacks of fast

capacity fading and short lifespan were overcome.3, 4 Aimed at these scientific issues, most attention is concentrated on the ideal design of cathode matrix with high conductivity and strong immobilization for soluble lithium polysulfides (LiPSs).5-8 In fact, it is more desirable for commercial available sulfur cathode by simplifying the whole preparation process. Therefore, the modification of other components within the cell has recently triggered immensely interest.9,

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For example, the pioneer of inserting the interlayer between the

cathode and separator has been opened by Manthiram and co-workers to trap and reutilize the dissolved LiPSs, then accompanied by abundant related research.11-14 Nevertheless, the self-standing and flexible features are requisite for interlayer with unavoidably complex preparation process. Furthermore, the untight contact between interlayer and separator will increase the internal resistance of cell. Innovatively, the similar concept is direct applied to 2


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separator that can serve as the natural support for the barrier layer of LiPSs, enabling the compact contact between the coating layer and separator. Up to now, there have been a few reports about the modified separator, and that, the coating materials employed are mostly various porous carbon.9, 15-21 However, it has been highlighted that weak physical interaction of the hydrophobic and non-polar carbon with polar and ionic LiPSs cannot ensure long-term restriction of LiPSs. The negative conditions can be overcome by introducing the nitrogen heteroatoms to separator to chemically adsorb LiPSs.22-28 However, even though the complex procedure, the introduced nitrogen content is always low (less than 10 wt%). Therefore, the high surface area is still necessary for the material in order to take full advantage of the scarce active sites, which further results in the difficulty of practical application for Li-S batteries. Moreover, based on the density functional theory (DFT) calculation, pyridinic-N exhibits much stronger chemical interaction with LiPSs compared with other forms of nitrogen.29,30 However, it is commonly difficult to ensure the dominance of pyridinic-N within the final product along with the high-temperature calcination, which is the most conventional method to introduce the nitrogen-doping.31 The hydrophilic metal oxide with polar metal-O bond has also been introduced into the separator to chemically adsorb LiPSs, as reported by Huang and co-workers.32 However, the metal oxide is usually heavier than carbon materials and their derivatives, which increases the inactive material faction in the cell. In perspective, in this work, the organic macromolecule, which is light-weight and scalable containing abundant pyridinic-N active sites, is direct employed to functionalize the separator as the anchor of LiPSs at the molecular level.

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The graphitic carbon nitride (g-C3N4), which has a high N-doping level (theoretically up to 60 wt%) especially dominant pyridinic-N in the vacancies of graphitic plane, is selected as the representative alterative to design the LiPSs trapped separator at the molecular level. As confirmed by DFT calculation, g-C3N4 exhibits the stronger interaction with Li2S (-3.02 eV) than graphene-like carbon molecule (-0.69 eV). Besides four conventional N-Li bonds between Li2S and pyridinic-N in the vacancies of g-C3N4, the C-S bond is even surprisingly observed, synergistically contributing to the stronger immobilization of LiPSs. A certain content of commercial carbon black (CB) is blended with g-C3N4 (denoted as g-C-coated separator) to decrease the internal resistance of cell and reactivate the entrapped LiPSs during cycling. As a result, the cell with pure sulfur electrode delivered a reversible capacity of 840 mA h g-1 at 0.5 C (1 C=1675 mA g-1) after 400 cycles with a slow capacity decay rate of 0.07%. Even at high current density of 1 C, the excellent discharge capacity of 732.7 mA h g-1 was still maintained after 500 cycles. When further increasing the sulfur loading to 5 mg cm-2, ultra-high areal capacity of 5.11 mA h cm-2 was acquired along with the high capacity of 1134.7 mA h g-1, which guaranteed the high energy density of Li-S batteries. Besides the abundant adsorption sites of pyridinic-N, the g-C3N4 is light-weight, non-toxic and mass-produced for effective separator. Therefore, the commercial available sulfur cathode and our facial, scalable g-C-coated separator guarantee the practical application of Li-S batteries. 2. EXPERIMENTAL SECTION 2.1. The synthesis of pure sulfur cathode. The CB-S composite was synthesized by mixing the CB and sulfur powder (Aladdin, 99.95%) with a weight ratio of 3:6. The CB-S cathode 4


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was acquired by mixing 90 wt% CB-S composite and 10 wt% polyvinylidene fluoride (PVDF) in N-methylpyrrolidinone (NMP) solvent, and then, the slurry was uniformly spread onto Al foil and dried at 60 oC for 24 h. The sulfur loading on the cathode was about 1.7 mg cm-2. For the test of areal capacity in the high sulfur loading, the 5 mg cm-2 loaded pure sulfur electrode was also fabricated. 2.2. The preparation of the coated separator. The C-coated separator was fabricated by coating the CB slurry on one side of the commercial glass fiber separator (Whatman). The slurry was prepared by mixing CB (90 wt%) and PVDF (10 wt%) in NMP solvent. As for g-C-coated separator, the fabrication method was similar except the slurry was composed of g-C3N4, CB, and PVDF in NMP solvent. The mass ratio of g-C3N4: CB: PVDF was adjusted to 1:8:1, 2:7:1, 6:3:1, and 8:1:1 to obtain the best electrochemical performance. The g-C3N4 here was synthesized by the polymerization of melamine at the low temperature according to the literature.33 2.3. Material characterization. The powder X-ray diffraction (XRD) (Rigaku P/max 2200VPC) using Cu-K (λ=0.15406 nm) radiation was employed to testify the crystalline phase. The amount of sulfur in the cathode (containing the PVDF) was decided by thermogravimetric analysis (TGA) (Pyris Diamond TG/DTA, PerkinElmer) from room temperature to 600 oC in a nitrogen flow at a heating rate 10 oC min-1. Raman spectrum was employed to analyse the structure feature of outcome based on a Raman spectrometer (JY HR-800, HORIBA JOBIN YVON) with an excitation laser beam wavelength of 633 nm. Mechanical properties of the coated separators were measured by TA instrument (Dynamic Mechanical Analyzer, DMA) DMA Q800. The X-ray photoelectron microscopy (XPS) 5



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(ESCALAB 250, Thermo) was performed with Al K Alpha radiation and Energy Step Size of 1 eV. Fourier transform infrared spectroscopies (FT-IR) were obtained by an infrared spectrometer (Nicolet 6700-FTIR, Thermo-Scientific). The morphology, EDX line-scan analysis, and elemental mapping of the resulting products were characterized by scanning electron microscopy (SEM) (XL 30 ESEM-FEG, FEI Company). The UV-vis absorption spectrum of the solution was acquired by the ultraviolet and visible spectrophotometer (UV-2550, SHIMADZU). 2.4. Electrochemical measurement. The CR 2032 coin cells were assembled in an Ar-filled glove box with the g-C-coated or C-coated separator inserted between cathode and Li metal. The electrolyte was 1 M lithium bis(trifluoromethanesulfonyl)imide (CJC, 99%) in 1:1 (v/v) 1,2-dimethoxyethane (DME) (Sigma-Aldrich, 99.8%) and 1,3-dioxolane (Sigma-Aldrich, 99%) containing 0.5 M LiNO3 (Aladdin, 99%). The specific capacity was obtained based on the mass of sulfur. Galvanostatic tests were conducted in the potential range of 1.7-2.8 V vs. Li/Li+ at 25 oC using LAND CT2001A battery-testing instrument. The cyclic voltammetry (CV) results were retained by the electrochemical station (CHI750E) with the 2032 coin cell at different scan rates between a voltage interval of 1.5-3.0 V vs. Li/Li+. The electrochemical impedance spectroscopy (EIS) tests with coin cells at different cycles were carried out at open-circuit voltage in the frequency range of 105 Hz and 10 mHz with a perturbation amplitude of 5 mV. 2.5. Computational method. All calculations have been performed at the B3LYP/6-31+G (d, p) level of DFT using Gaussian 09 package. It was demonstrated that the B3LYP/6-31+G (d,

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p) method can be successfully applied to the system with Li2S adsorbed.26 The binding energy (E) between g-C3N4 and Li2S was defined as Equation (1): = − −

(1)

Ea and Eb is the energy of g-C3N4 and Li2S, respectively. And the Eab represents the energy of the dimer of g-C3N4 and Li2S. 3. RESULTS AND DISSCUSSION The DFT calculation is first conducted to evaluate the intrinsically chemical interaction between g-C3N4 and sulfur-containing species at the molecular level. The graphitic plane of g-C3N4 is constructed from ordered tri-s-triazine subunits connected by the tertiary amino groups with periodic vacancies.34,35 Therefore, Li2S and C18N27 molecules are chosen as the simplified models. The full optimization is performed with several possible initial configurations to find the global minimum of dimer. After optimization, two stable configurations are obtained with E of -1.08 and -3.02 eV, as displayed in Figure 1a and b. For the dimer of global minimum in Figure 1b, Li2S molecule tends to form four N-Li bonds flatly with pyridinic-N in the vacancies of C18N27 molecule. Moreover, unlike other reports with the only N-Li bond formed, the C-S bond is also interestingly observed in this calculation, contributing to the much stronger interaction. The Mulliken charges of the highlighted carbon atoms in Figure 1b are found to be very positive (about +0.9 e), which leads to the strong interaction for the negative S atom of Li2S. The average of N-Li bond length is 1.95 Å, and the C-S bond distance is 1.92 Å. Because of the strong bonding with Li2S, the plane of C18N27 molecule is distorted. Based on the fact that the N-Li and C-S bonds are formed between Li2S and C18N27 molecules, g-C3N4 is considered to possess the intrinsic 7



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adsorption ability for LiPSs at the molecular level. On the contrary, the interaction of Li2S and graphene-like carbon molecule is very weak with the E of only -0.69 eV (Figure 1c and d). To further determine how much Li2S molecules can be adsorbed on the surface of g-C3N4 layer, the Li2S molecules are located on one side of C18N27 one by one, and optimized without any constraint of symmetry or position. Because only one side of g-C3N4 layer is exposed and available for adsorption of Li2S molecules in practical application, another side is not taken into consideration. The configurations of the optimized complexes are displayed in Figure S1. When three Li2S molecules are put on C18N27 layer, several strong Li-N and S-C bonds form. While the fourth Li2S molecule is put onto the system, all Li2S molecules on the surface form a layered cluster and trend to leave the C18N27 layer. This phenomenon provides a theoretical insight for the circulation of Li2S molecules adsorption on and departure from the g-C3N4 layer. Additionally, the E of 3 Li2S molecules on C18N27 layer is calculated to be -13.67 eV, while the E of 4 Li2S molecules decreases to be -8.66 eV, which indicates the fourth Li2S molecule assists the other three Li2S molecules to leave the g-C3N4 layer. Therefore, per mole g-C3N4 subunits of C18N27 can effectively adsorb more than three times the Li2S molecules. The mechanism comparison of g-C-coated separator with C-coated one is schematically illustrated in Figure 2. LiPSs still shuttle through the C-coated separator after long cycles because of the weak physical adsorption of CB, which results in the corrosion of anode and the deposition of Li2S/Li2S2 on the lithium anode. By contrast, the g-C-coated separator can effectively intercept LiPSs on the cathode side by N-Li and C-S bonding even after long-term cycling. Moreover, the mixed conductive CB can effectively reduce the entrapped LiPSs as the final discharge product (Li2S2 and Li2S), improving the 8


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active materials utilization. Therefore, the cell with the effective LiPSs reservoir of separator designed with g-C3N4 should deserve the better cycle lifespan.

Figure 1 The optimized structures of Li2S on g-C3N4 (a and b for two conformations) and on graphene-like carbon molecule (c and d for one conformation on top view and side view, respectively). (e) The sealed vials of the Li2S4/DME solution (left) and after contact with the bulk g-C3N4 (right). (f) The UV-vis absorption spectra of the Li2S4/DME solution and the Li2S4 /DME solution with g-C3N4.

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Figure 2 The schematic illustration for the cell configuration and the function mechanism comparison of the g-C-coated separator with C-coated one for the LiPSs entrapment. The structure of g-C3N4 is characterized by XRD pattern in Figure S2a. Where the distinct diffraction peaks at 13.1o and 27.3o correspond to the (100) and (002) plane of g-C3N4. The thickly stacked sheets with many macropores on the surface are observed by SEM image in Figure S2b. The specific surface area and pore volume of g-C3N4 is only 7.95 m2 g-1 and 0.06 cm-3 g-1, as shown by the N2 adsorption-desorption isotherm and pore size distribution curve in Figure S2c and d, so the immobilization for LiPSs is mainly ascribed to the chemical bonding of g-C3N4 rather than the surface physical adsorption. As exhibited by DFT calculation, Li2S mainly binds with pyridinic-N in the vacancies of the plane, so the adsorption sites will be not seriously influenced in spite of the stack of g-C3N4 sheets. The intrinsic adsorption capacity of bulk g-C3N4 for LiPSs is experimentally confirmed by the visual discrimination, as shown in Figure 1e. The almost disappeared color of Li2S4 solution after the contact with g-C3N4 indicates the strong adsorption for LiPSs, which can be further 10


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illustrated by UV-vis spectra in Figure 1f. The peak at 410 nm that is ascribed to the S42largely weakens after adding g-C3N4 to the Li2S4 solution. The Raman spectrum of the g-C3N4-Li2S4 solid powder recovered from the g-C3N4 soaked in the Li2S4 solution some time is also compared with that of the pure g-C3N4. As shown in Figure S3, the new peak at about 982 cm-1 is found for g-C3N4-Li2S4, further suggesting the strong chemical interaction between g-C3N4 and LiPSs. The peak may be attributed to the C-S bond considering the fact that the peak of the C-S vibrational mode is located at 900-1300 cm-1 in the FT-IR spectrum. The peak position in Raman spectrum is identical to that in FT-IR spectrum for the same function group in the molecule. Because of the synergistic effect of g-C3N4 and CB, the optimum electrochemical performance is explored by changing the ratio of them on the separator. As displayed in Figure S4, the best cycle performance is acquired with the ratio of 6:3 (nitrogen content is up to about 15.3wt% based on the XPS result). The subsequent characterization and test are based on this optimum ratio. After the slurry coating, the surface morphology of g-C-coated separator is characterized. A uniform and porous layer is attached on the surface of pristine separator without any crack, as shown by the large-scale SEM image in Figure 3a. The inserted photo in Figure 3a also visibly exhibits the intact coating of separator. When giving more legible examination, the micro-size particles of g-C3N4 are densely surrounded by CB nanoparticles, as highlighted by the circles in Figure S5a. Figure S6a presents the cross-sectional SEM image of g-C-coated separator. The thickness of the coating layer is about 26 µm and the weight is about 1.5 mg cm-2. The tight adhesion of the dense barrier layer on the separator is observed, which guarantees the good flexibility and durability of the 11



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coated separator. It also favors to decrease the contact resistance between the coating layer and pristine separator. The morphology of pure C-coated separator is also provided for comparison (Figure 3c and S5b). The cracks are clearly discovered on the surface of separator, which is also macroscopic from the inserted photo in Figure 3c, resulting from the high surface energy of nano-sized CB. Where the micro-size fissure will destroy the structure stability of the coating layer and result in the loss of active material. From the cross-sectional SEM image in Figure S7a, the thickness of 42 µm for C-coated layer is much larger than that of g-C-coated layer (26 µm), further indicating the loose stacking of CB nanoparticles, which readily leads to the further destroy of coating layer during the cycling. Therefore, the micro-sized g-C3N4 contributes to the superior mechanical strength of g-C-coated separator to C-coated one, as intuitively reflected by the stress-strain curves in Figure S8. The Young modulus of 135 MPa for g-C-coated separator is much higher than that of C-coated separator (97.1 MPa). Moreover, the fracture stress (1.15 MPa) of g-C-coated separator is also two times higher than that of the counterpart (0.707 MPa). The reduced thickness of g-C-coated layer will relieve the adverse impact on the volume energy density of cell.

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Figure 3 The surface SEM images of (a) g-C-coated separator and (c) C-coated separator before cycle. The surface SEM images and the corresponding elemental mappings of (b) g-C-coated separator and (d) C-coated separator after 200 cycles (The fibers in Figure 3d come from the glass fiber separator). In order to vividly illustrate the effective restriction of g-C-coated separator for LiPSs, the separator is taken out from the disassembled cell after 200 cycles, rinsed with DME solvent, and then dried for SEM characterization. As shown from the front-view SEM image in Figure 3b, the more compact surface implies the effective adsorption and capture of migrating LiPSs during the electrochemical cycling. Furthermore, the corresponding elemental mapping also witnesses the existence of sulfur after cycles, which is consistently distributed with nitrogen and carbon signals all over the barrier layer, demonstrating the effective immobilization of LiPSs by the adsorption sites of pyridinic-N and subsequent redox by the adjacent CB. Even so, the carbon and nitrogen signal is still strong with the content of 61 at% and 7 at% after 200 cycles, ensuring the good electrochemical stability for 13



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the longer cycle. Moreover, the coating layer still stays complete after 200 cycles, ensuring the effectiveness of the coating layer for LiPSs trap, as shown by the inserted photo in Figure 3b. The thickness of g-C-coated barrier layer is increased to 36 µm after cycles, further indicating the effective interception of LiPSs (Figure S6b). As shown by the EDX line-scan and elemental mapping analysis in Figure S6c and d, the sulfur also exhibits the coincident dispersion with carbon and nitrogen along with the longitude of barrier layer. The C-coated separator can also intercept LiPSs, as shown by SEM image and elemental mapping in Figure 3d. The sulfur content of 13 at% on the separator is much lower than that of g-C-coated one (23 at%), indicating the seriously shuttle loss of LiPSs because of the weak physical adsorption, which is further confirmed by weak sulfur detection in the EDX line-scan and corresponding elemental mapping result in Figure S7c and d. Moreover, the C-coated separator is seriously destroyed after cycles, agreed with the discussion above, indicating the inferior mechanical strength of the coating layer with only nanoscale CB, as displayed by the inserted image in Figure 3d. The largely decreased thickness also identifies the loss of C-coated layer after cycles, as displayed by the cross-sectional SEM image in Figure S7b. To further highlight the interception of g-C-coated separator for LiPSs, the evolution of lithium anode surface after 200 cycles is explored, as shown by SEM images and elemental mapping in Figure 4. It can be seen that the lithium surface of cell with C-coated separator is much tougher than that of cell with g-C-coated separator because of the greater discharge products deposition on the surface, which can be clearly confirmed by the stronger sulfur signal in the elemental mapping. All above results give the perceptible evidence that bulk g-C3N4 can effectively adsorb LiPSs in the cathode region. In addition, the g-C3N4 can pledge the 14


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integration of the coating layer. These preponderances are responsible to the long-term cycle stability of the Li-S batteries.

Figure 4 The SEM images of the lithium-metal surface after 200 cycles for the cell with (a) g-C-coated separator and (c) C-coated separator. The elemental mapping of the lithium anode surface corresponding to SEM images for the cell with (b) g-C-coated separator and (d) C-coated separator. The cell with g-C-coated separator is assembled to evaluate the electrochemical performance of pure sulfur electrode, and C-coated separator is also tested as a comparison. First, pure sulfur electrode is carefully characterized, as shown in Figure S9. The XRD result demonstrates the sulfur exists in CB matrix in the high dispersed state. The uniform distribution of sulfur is also identified by SEM image and elemental mapping. The sulfur content in the cathode (containing the PVDF) is about 54 wt%, as obtained by the TGA result. The more intact coating of g-C-coated separator than C-coated one may disturb the Li+ transfer, which is extremely significant for cell performance especially at high current density. 15



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Therefore, it is crucial to first evaluate the lithium ion diffusion coefficient (DLi+) of cell with g-C-coated separator, which can be calculated by the Randles-Sevick equation relative to the CV test at different scan rates, as shown in Figure S10a and b.18 The DLi+ obtained by averaging DLi+ (A1) and DLi+ (A2) is 8.97*10-8 cm2 s-1, which is considerable with that of cell with C-coated separator (6.06*10-8 cm2 s-1, shown in Figure S10c and d), indicating the micro-sized g-C3N4 has no obvious effect for lithium ion diffusion. Because of the low electron conductivity of g-C3N4, the introduction of g-C-coated separator should have some influence on the internal resistance of cell, which can be evaluated by the EIS before cycle, as shown in Figure 5a. The Nyquist plots of cell consist of the single semicircle in the high-frequency region and an inclined line at the low-frequency area, which are related to the charge transfer resistance (Rct) and diffusion-controlled Warburg impedance (Wo).36 Predictably, the Rct of cell with g-C-coated separator (40 Ω) is slight higher than that of cell with C-coated separator (23 Ω) (Figure 5b). The long-term cycle performance of cell with g-C-coated separator is measured. The cell is first cycled at 0.2 C (1 C=1675 mA g-1) because the slow rate will allow more time for LiPSs migration. As shown in Figure 5c, the initial discharge capacities of cell with g-C-coated separator are slightly lower than that of cell with pure C-coated one because of the higher internal resistance. However, the cell with g-C-coated separator delivers the superior cycle retention with a reversible capacity of 773.2 mA h g-1 after 400 cycles, while it only reserves a capacity of 611 mA h g-1 for cell with C-coated separator, which has received the most extensive research in the separator modification. Therefore, the inserted layer modified by bulk g-C3N4 can preferably control the shuttling of LiPSs among the long-term 16


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cycling. The detail electrochemical behavior of the two cells is explored by charge-discharge profiles, as shown in Figure S11a and c. Both of cells exhibit the similar voltage plateaus relative to the typical two-step sulfur redox reaction. Therefore, the capacities of the cells are completely contributed by sulfur. The low polarization (∆E) change from 176 to 212 mV derived from the almost overlapping voltage plateaus for the cell with g-C-coated separator indicates limited LiPSs diffusion and excellent electrochemical reversibility, as displayed in Figure 5e. However, the cell with C-coated separator exhibits higher polarization. Moreover, the polarization difference of the two cells is more and more obvious with the increase of cycle number, further indicating the better stabilization of g-C-coated separator for active material to realize the longer lifespan of Li-S batteries. The CV curves of the two cells are measured over the potential range of 1.5 and 3.0 V at a scan rate of 0.1 mV s-1 (Figure S11b and d), where two well-defined cathodic peaks and two overlapped anodic peaks are displayed, accordance with the discharge-charge curves. The cycle performance of cell with g-C-coated separator at 0.5 C is also measured, as presented in Figure 5d. After 400 cycles, the outstanding reversible capacity of 840 mA h g-1 is still retained (the test is first conducted at 0.1 C for two cycles). Moreover, the cell maintains the excellent coulombic efficiency (CE) of almost 100% after long cycles. The excellent cycle capacity of 732.7 mA h g-1 is even maintained after 500 cycles when further increasing the current density to 1 C, as displayed in Figure 5g. These desirable results can be ascribed to the following reasons: (1) the abundant active adsorption sites of g-C3N4 with the formation of multiple N-Li and C-S bonds between pyridinic-N and Li2S; (2) the synergetic reactivation and resulting

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reutilization of trapped LiPSs by CB; (3) the integrated and intact coating layer of separator with good mechanical strength made by micro-sized g-C3N4 during the cycle.

Figure 5 The comparison of the electrochemical properties of both cells. The EIS plots of the cell with (a) g-C-coated separator and (b) C-coated separator at different cycles. (c) The cycle performance of the cells at 0.2 C (1 C=1675 mA g-1). (d) The cycle stability of the cells at 0.5 18


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C. (e) The polarization of the electrodes for 400 cycles at 0.2 C. (f) The self-discharge test at different interval time. (g) The long cycle performance at the high current density of 1 C. Besides the dynamically electrochemical stability, the static stability of Li-S batteries is also systematically explored because the self-discharge usually occurs along with the severe LiPSs diffusion during the rest. As shown in Figure 5f, the capacity retention of the second discharge capacity relative to the first charge capacity is steady during different interval time for the cell with g-C-coated separator even after long rest time of 72 h, indicating the acquirement of anti-self-discharge specialty. In addition, the stable CE along with the extension of rest time also confirms the decreased spontaneous reaction of high-order LiPSs with the lithium anode during the self-discharge resting for the cell with g-C-coated separator (see inserted image in Figure 5f). Therefore, because of the chemical confine of g-C3N4 for LiPSs, the cell with g-C-coated separator possesses the lower self-discharge than C-coated one after the long-term rest. The EIS measurements at different cycles are carried out to attain additional insight into the effect of g-C-coated separator. The EIS spectra recorded after different cycle exhibit another semicircle in the medium-frequency region, which is corresponding to the resistance of passivation film arising from the deposition of non-conductive discharge product on electrode surface.37 As displayed in Figure 5a, the negligible change of high-to-medium resistance has been made for cell with g-C-coated separator even after 200 cycles, demonstrating the effective entrapment and reutilization of LiPSs. The immobilization of LiPSs can mitigate lithium surface corrosion and reduce the deposition of short-chain LiPSs on the anode. The reutilization of LiPSs by the barrier layer can largely decrease irreversible 19



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agglomeration of discharge product on the cathode surface. On the contrary, the resistance of cell with C-coated separator has an obvious change along with cycling, as shown in Figure 5b. The high-to-medium resistance is smaller at initial 50 cycles, while this value sharply increases with the extension of cycle number and exceeds the resistance of cell with g-C-coated separator after 200 cycles. This further indicates g-C3N4 with abundant adsorption sites can effectively stabilize the electrochemical reaction of sulfur electrode. The XPS analysis after cycles is conducted for g-C-coated separator to experimentally investigate the chemical binding between LiPSs and g-C3N4. As illustrated by the survey spectrum in Figure 6a, the sulfur signal at 168 eV is detected after cycles, demonstrating the effective entrapment for active material, which can be further illustrated by high-resolution S 2p spectrum in Figure 6d. The peaks centered at 162 and 163.5 eV are associated with the Li-S and S-S bond of LiPSs, respectively. Based on the DFT discussion above, the peak at 162 eV may also be ascribed to C-S binding between g-C3N4 and LiPSs. In addition, the peaks at 166.2 and 168 eV represent the Li2SO3 and -NSO2CF3, which are the products of solid electrolyte interphase (SEI) film due to the degradation of electrolyte.38 After cycles, the C 1s peak (Figure 6b) representing the C-N bond (285.6 eV) shifts to slightly higher binding energy (286.1 eV), which can be ascribed to the polarization of electron away from C atom to electropositive Li of Li2S due to the strong chemical interaction between g-C3N4 and Li2S.39 Moreover, the C-S bond is also observed from the new C 1s peak at 284.8 eV. As for N 1s spectrum in Figure 6c, pyridinic-N predominates in N 1s spectrum, agreed well with the discussion above. The slight positive shift from 399.2 to 399.4 eV of the C-N bond also identifies the existence of chemical interaction between g-C3N4 and sulfur species. It seems 20


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that the N content obtained from XPS result (1.86 at%) is far lower than that of elemental mapping results (7 at%) after 200 cycles, which may be attributed to coverage of the surface adsorption sites of pyridinic-N along with the deposition of active sulfur, resulting in the difficulty of the signal detection from XPS. The existence of N-Li and C-S bonds between g-C3N4 and LiPSs is further confirmed by FT-IR spectra of g-C-coated separator before and after 200 cycles, as shown in Figure S12. The peak observed at about 1610 cm-1 after cycles is attributed to the Li-S vibrational mode, indicating the adsorption of separator for LiPSs. The peaks located between 1200-1600 cm-1, which is related to the vibrational modes of tri-s-triazine units of g-C3N4, exhibits a slight red shift after cycles, which can be ascribed to the chemical interaction with LiPSs. Moreover, the peak at around 3420 cm-1 corresponding to the N-H stretching mode shifts to the lower number after cycles, further demonstrating the chemical interaction between nitrogen active sites and LiPSs. Obviously, the new peaks appear after cycles at around 620 and 1050 cm-1, which are corresponding to the N-Li and C-S bonds, respectively.28,40 Therefore, the XPS result combining with the FT-IR spectra explicitly certify the chemical bonds formed between g-C3N4 and LiPSs.

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Figure 6 The XPS analysis of the g-C-coated separator surface before and after 200 cycles for the (a) survey spectrum, (b) C 1s, (c) N 1s, and (d)S 2p spectrum. In consideration of the practical application of Li-S batteries, the high areal capacity is very significant because it weakens the inactive material fraction, which will enhance the gravimetric/volume energy density of cell. The areal capacity can be calculated by multiplying the specific capacity (mA h g-1) of sulfur and sulfur loading (mg cm-2). Therefore, the high sulfur loading is necessary for high areal capacity. However, there is a deal between the sulfur loading and specific capacity because of the inferior active material utilization of cell with high sulfur content. The abundant sulfur with low conductivity will increase the electrical resistance of cell, resulting in the hindrance of electron transfer. The thick electrode causes the difficulty of electrolyte immersion, which slows ion transfer down. Moreover, the shuttle loss of LiPSs is more serious with higher content of sulfur, leading to the loss of active material and the polarization of electrode. The abundant lithiated products during 22


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cycling also block the electron and ion channel and decrease the accessibility of active material. All these reasons bring about the difficulty for cell with high sulfur loading to achieve good capacity and cycle stability. In this work, the electrode with high sulfur loading of 5 mg cm-2 is simply fabricated due to the micro-sized clusters of CB-S composite. The well-maintained plateaus of sulfur electrode with negligible polarization change during charge-discharge cycles demonstrate the favorable electron and ion transfer for the electrochemical reversibility, as shown in Figure 7a. There is also an activation process in the initial cycles, agreed with the result of sulfur loading of 1.7 mg cm-2. The excellent areal capacity of 5.11 mA h cm-2 is delivered along with the high specific capacity of 1134.7 mA h g-1 at current density of 0.1 C. Even after 40 cycles, the cell still acquires a stable capacity of 901.6 mA h g-1, ensuring a high areal capacity of 4.06 mA h cm-2, as displayed in Figure 7b. Therefore, the combine of scalable g-C-coated separator with commercial sulfur electrode promises the high energy density for practical application of Li-S batteries.

Figure 7 The charge-discharge curves (a) and the areal capacity retention (b) of the cell with g-C-coated separator with the high sulfur loading of 5 mg cm-2.

4. CONCLUSION In summary, the bulk g-C3N4 containing abundant pyridinic-N adsorption sites was introduced onto separator to acquire excellent long-life and high-areal-capacity performance 23



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for pure sulfur electrode compared with carbon material, which has been widely employed to decorate the separator. Besides four N-Li bonds, the C-S bond was also formed, resulting in the stronger chemical interaction of g-C3N4-Li2S system (-3.02 eV) than that of graphene-like carbon-Li2S system (-0.69 eV). In addition, the micro-sized g-C3N4 could provide the coating layer with good mechanical strength even after long-term cycling, which further promised the restrict function for LiPSs. As a result, the pure sulfur electrode maintained the excellent capacity retention with a discharge capacity of 840 mA h g-1 after 400 cycles at 0.5 C, while it was only 641 mA h g-1 for cell with C-coated separator. Even at high rate of 1 C, the discharge capacity of 732.7 mA h g-1 was still acquired after 500 cycles. Moreover, when further increasing the sulfur loading to 5 mg cm-2, the excellent areal capacity of 5.11 mA h cm-2 was acquired along with high specific capacity of 1134.7 mA h g-1. After 40 cycles, the cell still maintained a high areal capacity of 4.06 mA h cm-2. Besides the excellent dynamically electrochemical stability, the existence of g-C3N4 also endowed the cell with anti-self-discharge specialty. Therefore, the scalable g-C-coated separator and commercial operational sulfur electrode ensure the feasibility of practical application of Li-S batteries. This slight design of the separator at the molecular level by the scalable and light-weight organic macromolecule with abundant LiPSs adsorption sites is inspirational for the study of Li-S batteries and other satisfactory candidates are also being tried.

ASSOCIATED CONTENT Supporting Information The DFT calculation of g-C3N4 and different amounts of Li2S, the characterization of g-C3N4, the Raman spectra of g-C3N4 with and without Li2S4, the comparison of electrochemical 24


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performance with different ratio of g-C3N4 and CB, the high-resolution SEM images of g-C-coated separator and C-coated separator before cycle, the cross-sectional SEM images, EDX line-scan and elemental mapping of g-C-coated separator and C-coated separator before and after 200 cycles, the stress-strain curves of g-C-coated separator and C-coated separator, the characterization of pure sulfur electrode, the lithium ion diffusion measurement, the CV and charge-discharge curves of the cells with g-C-coated separator and C-coated separator, the FT-IR spectra of g-C-coated separator before and after 200 cycles. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *

E-mail:

[email protected];

[email protected];

[email protected];

[email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial supports from the NSFC (21573036, 21574018, and 21274017) and the Science Technology Program of Jilin Province (20150520027JH, 20140101087JC) are gratefully acknowledged. REFERENCES 25



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Table of contents (TOC)

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The Effective Design of a Polysulfide-Trapped Separator at the

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