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Effective Dual Polysulfide Rejection by Tannic acid/Fe(III) Complex Coated Separator in Lithium-Sulfur Batteries Hong Zhang, Chun-Er Lin, Xuanhe Hu, Baoku Zhu, and Dingshan Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01189 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Materials & Interfaces

Effective Dual Polysulfide Rejection by Tannic acid/Fe(III) Complex Coated Separator in Lithium-Sulfur Batteries Hong Zhang, a Chuner Lin,b Xuanhe Hu,a Baoku Zhu,*b and Dingshan Yu*a a.

Key laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Key Laboratory of High Performance Polymer-Based Composites of Guangdong

Province, The Key Lab of Low-carbon Chem &Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China. b.

Key Laboratory of Macromolecule Synthesis and Functionalization, ERC of Membrane and Water Treatment, Department of Polymer Science and Engineering, Zhejiang

University, Hangzhou 310027, China.

ABSTRACT: The solubility behaviour of polysulfides in electrolyte solutions is a major bottleneck prior to the practical application of the lithium-sulfur battery. To address this issue, we fabricate a tannic acid/Fe(III) complex coated polypropylene (PP) separator (TA/Fe(III)-PP separator) via a simple, fast and green method. Benefiting from dual-confinement effects based on Lewis acid-base interactions between Fe(III) and polysulfides, as well as the dipole-dipole interactions between rich phenol groups and polysulfides, the migration of polysulfides was effectively suppressed. Meanwhile, the porous structure of the PP separator was not destroyed by additional coating layer. Thus, the TA/Fe(III)-PP separator can retain rapid lithium ion transport, eventually leading to a significant improvement in both the discharge capacity and rate performance of the corresponding lithium-sulfur cells. The cell with the TA/Fe(III)-PP separator presents a low capacity fade of 0.06 % every cycle over 1000 cycles at 2.0 C, along with a high coulombic efficiency of >97% over 300 cycles at 0.5 C. With respect to the one with the bare PP separator, the cell with the TA/Fe(III)-PP separator exhibits a 1.7-fold increase in the discharge capacity at 3.0 C. The proposed simple and economical approach shows great potential in constructing advanced separators to retard the shuttle effect of polysulfides for lithiumsulfur batteries. KEYWORDS: Lithium-sulfur battery, Shuttle effect, Tannic acid/Fe(III) complex, Lewis acid-base interactions, Lithium ion conductivity.

INTRODUCTION The ever-growing energy demand has driven the development of energy storage systems to attain a higher energy density at much lower cost. Due to its high theoretical energy density and the natural abundance of the elemental sulfur, lithium-sulfur (Li-S) batteries are 1-3 regarded as one of the most promising competitors. Nevertheless, prior to the realistic application of Li-S batteries, several critical issues should be addressed, including the shuttle effect of polysulfides, the serious volume expansion (76%) of sulfur, the poor ionic and electronic conductivities of sulfur, and the growth of lithium dendrites on the anode. To overcome the shuttle effect in Li-S batteries, the most common strategy is to confine sulfur in various carbon materials with defined geometries and pore sizes. Despite these encapsulation approaches could improve the electrochemical performance of Li-S batteries, the construction of these fantastic structures often requires sophisticated and expensive processing technologies, and thus not suitable for the large-scale manufacturing.4,5 Another promising approach is to design an ion-selective separator, which is permeable for lithium ions but impermeable for polysulfides, blocking the shuttle pathway of polysulfides. According to the previous studies, a series of functional separators with electrostatic effect, screen size effect or chemical adsorption effect have been designed for this purpose. For example, the functional separators with electronegative coating layers such as nafion,6.7 lithium perfluorinated sulfonyl dicyanomethide8 and poly (acrylic acid)9 have been adopted to reject polysulfide anions via electrostatic repulsion. To achieve a sizeselective transport, the separator with sub-nm pore size has been fabricated using polymers of intrinsic microporosity (PIM) and managed to suppress the migration of polysulfides.10 As for the chemisorption ability, it is mainly expressed by “lithiophilic” affinity or “sulfiphilic” affinity. In detail, the heteroatom dopants (such as N,

O, and S, etc.) and exposed metal (M) sites could attract polysulfides via Li-X bonds (hydrophilic-hydrophilic interactions) and S-M bonds (Lewis acid-base interactions), respectively.11,12 Although these separators exhibit blocking characteristics for polysulfide, it should be noted that the retarding separators inevitably raise the lithium ion transfer resistance and eventually compromise the cell performance. Therefore, it is essential to design a kind of highselective separators, being equipped with a low transfer resistance for lithium ions. Tannic acid (TA), one kind of plant polyphenols, contains a central glucose core surrounded by digalloyl ester groups, as displayed in Figure 1a. Taking advantage of the luxuriant phenolic hydroxyl groups (hydrophilic units) in TA, Li et al. prepared a TAcoated polypropylene (PP) separator for the lithium-ion battery,13 demonstrating improved electrochemical performance with respect to those using bare PP separators owing to the unaltered pore structure and enhanced electrolyte wettability of the TA modified separator. Compared to the aforementioned modification process of separators for Li-S batteries, the TA coating method is likely much simpler (one-step dip-coating), more cost-effective (plant polyphenol), more environmentally friendly (without organic solvent) and robust (via non-covalent and/or covalent interaction)14 for long-term battery operation. As far as we know, the coating process based on TA to decorate the polyolefin separators used in Li-S batteries, has never been studied. Herein, a novel TA/Fe(III) complex coated PP separator, for the first time, was designed and applied in the Li-S battery. As reported, TA can provide polydentate ligands to coordinate Fe(III) ions, forming polyphenol-metal complex spontaneously and implementing adhesion.15 Compared to the time-consuming coating process (as long as 24 h) using TA as the sole coating precursor, such TA/Fe(III) complex formation process could be completed instantaneously (less

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than 30 s). Meanwhile, as the pore structure of the PP separator does not change after the introduction of the thin coating layer, the modified separator can still afford a rapid migration channel for lithium ions. In particular, such TA/Fe(III) complex design could render dual confining effect based on dipole-dipole interactions from the abundant oxygen-containing groups in TA and Lewis acids-bases interactions from the transition metal ions (Fe(III)). Benefiting from the above merits, the as-assembled Li-S cells using the TA/Fe(III)-PP separators achieved a remarkably enhanced cell performance with respect to the ones using the bare PP separators. As a proof of concept, a simple physical mixture of active materials and conductive carbon materials was employed as the cathode in this work. Once elaborately designed or specialized cathodes are adopted, the cells assembled with the TA/Fe(III)-PP separators are expected to deliver much superior electrochemical performance. EXPERIMENTAL SECTION Preparation of TA/Fe(III)-PP separator. First, TA and Fe(NO3)3 aqueous solution (1 mg·mL-1) were respectively fabricated and subsequently mixed together. Afterwards, the mixture solution was deposited on the commercial PP separator (Celgard 2350) by the vacuum filtration method. Noteworthy, NO3- was specially chosen as counterion in this work. As demonstrated in the literature, the components with N-O bonds contribute to the formation of the stable “solid electrolyte interface (SEI)” film, and thus retard the redox reaction of polysulfides on the lithium anode.9 To remove the residual reactants, the modified separators were repeatedly rinsed with deionized (DI) water and subsequently dried at 60 °C for 24 h. To determine the areal loading for the TA/Fe(III) coating layer, the sum weight of 20 samples was measured using an Electronic Balance (CPA225D, Sartorius, Germany, precision: 0.01 mg). The average value of the areal loading is calculated to be 0.07 ± 0.004 mg·cm-2. The thickness of the coating layer was approximately 15 nm obtained by ellipsometry (M-2000, J. A. Wollam, America). Such a low loading and thin thickness is beneficial to alleviate the polarization and enhance the cell performance.16-18 To investigate the role of TA, we particularly prepared the TA coated PP separator without Fe(III) for comparison. The bare PP separators were directly immersed in the TA aqueous solution of 0.5 mg·mL-1 for 24 h at ambient temperature, followed by repeated rinsing with DI water. The mass loading of TA on TA coated separators was nearly equal to that of TA/Fe(III) on TA/Fe(III)-PP separators.

Characterization. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) (Vecter 22 FTIR, Bruker Optics) was used to identify the chemical composition of the separators. Raman spectroscopy (Renishaw inVia) was measured with a He-Ne laser of 633 nm. The pore structure of the separators were observed using a field emission scanning electron microscope (SEM, Hitachi S4800). X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific ESCALAB 250. The energy-dispersive Xray (EDX) spectroscopy was attached to the JEM-2100F. Micromeritics AutoPore IV 9500 mercury intrusion porosimetry was conducted to acquire the size distribution of pores. Water contact

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angle test was conducted using an OCA20 contact angle goniometer (Dataphysics). The retention of polysulfide species was investigated by the permeation experiments using H-shaped devices. Cell assembly and electrochemical testing. The cathodes were produced by blending sulfur/Ketjen Black (70 wt%) composites, super P, and poly (vinylidene fluoride) in N-methyl-2-pyrrolidone. The ratio in weight was controlled to be 80:10:10. The slurry was subsequently deposited onto an Al foil and subsequently vacuumdried at 60 °C for 12 h. On average, the areal loading of sulfur was 2.2-2.5 mg·cm-2. The employed electrolyte is 1 M LiN(CF3SO2)2 (LiTFSI) in a mixed solvent containing dimethyl ether (DME)/1, 3dioxolane (DOL) (1:1 in volume) with the addition of 1wt% lithium nitrate (LiNO3). The added amount for the electrolyte in each cell is 10 µL eletrolyte/mg sulfur. The assembled cell was composed of a sulfurcontaining cathode and a lithium anode separated by our designed separator with the TA/Fe(III) coated side facing the cathode. All operations were completed in an Ar-filled glovebox, in which the contents for both O2 and H2O were < 1ppm. Cyclic voltammetry (CV) tests were performed on the Autolab electrochemical workstation at 0.1 mV·s-1. On the basis of the equation of σ=L/(Rb·A), the ionic conductivity (σ) of the separator was calculated. Herein, L, Rb and A represent thickness (cm), bulk resistance (ohm) and effective area (cm2), respectively. Rb was determined with the stainless steel (SS)/separator/SS cell. Electrochemical impedance spectroscopic (EIS) analysis was also done from 105 to 10-2 Hz. The cycling performance of the corresponding cell was tested from 1.7 to 2.8 V. Prior to tests, all the cells were equilibrated for 6 h and subsequently cycled at 0.2 C for 2 cycles to activate them.

RESULTS AND DISCUSSION Chemical composition of separators. As illustrated in Figure 1a, each Fe(III) can react with three galloyl groups in TA to yield a metalpolyphenol complex, while each TA molecule is capable to react with several Fe(III) centers to generate a cross-linked film.19 Owing to the versatile adhesion capability of TA, the obtained TA/Fe(III) complex is prone to generate a homogeneous coating layer on the PP separator. Due to the formed TA/Fe(III) complex on the surface, the modified separator shows a visible color change from white to dark blue, which is in accordance with the previous study.19 Meanwhile, the TA/Fe(III)-PP separator has no marked color change, even subjected to bending, wrinkling or soaking in the electrolyte process. It can speculate that the robust TA/Fe(III) layer is qualified to withstand the cell assembly process. To manifest the generation of the TA/Fe(III) complex, ATR-FTIR measurements were carried out to analyse the chemical composition of the surface of the TA/Fe(III) coated PP separators. As seen from Figure. S1, the spectrum of the modified separator presents several new peaks relative to the bare PP separator. The broad peaks located at 3100 and 3500 cm-1 represents the -OH stretching vibration due to the presence of polyphenols in TA, indicating the successful introduction of TA. As the ATR-FTIR patterns of ferric tannates are 2


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somewhat similar to those of the free tannins, Raman spectroscopy was further employed to determine whether the TA/Fe(III) complex was generated.20 As displayed in Figure 1b, the characteristic peak associated with the TA/Fe(III) complex appeared at 591 cm-1, corresponding to the bidentate chelation between Fe(III) ion and C3 oxygen in TA. 21,22 The XPS survey spectrum of the TA/Fe(III)-PP separator also revealed the existence of Fe (Fe 2p) and O (O 1s) (Figure 1c). In the Fe 2p spectrum (Figure 1d), two peaks located at ∼726 and ∼711 eV assign to Fe 2p1/2 and Fe 2p3/2, respectively. Such two distinct 2p peaks are separated by 15 eV, signifying the existence of the Fe(III) species.23,24 Furthermore, the high-resolution O1s spectrum shown in Figure 1e can be well fitted to two subpeaks at 533.0 and 531.2 eV, responding to Fe-OH and Fe-O respectively. The presence of Fe-O stems from the chelating structure generated by Fe(III) and phenolic hydroxyl groups in TA.25 Taken together, the above results testify that the TA/Fe(III) complex is indeed coated onto the surface of the PP separator.

Figure 2. (a) SEM images of the separators surface before and after 300 cycles (close to the S/C cathode); (b) Pore size distribution of the separators.

Contact angle measurements were performed to investigate the hydrophilicity property of the separators. As shown in Figure 3, the contact angle decreased from 117° to 88° after the TA/Fe(III) complex coating. This indicates that the modification improves the hydrophilicity of the PP separator, resulting from the existence of TA containing a wealth of oxygen groups. As demonstrated in the previous studies, separators with excellent hydrophilicity are prone to suppress the shuttle effect by adsorbing polysulfides on the surface.26-28 Moreover, the excellent hydrophilicity facilitates retaining liquid electrolyte within the separator, helping to reduce the transfer resistance of lithium ions and improve the cell performance, especially at high rates. 29

Figure 3. Contact angle of the separators.

Figure 1. (a) Schematic illustration of the fabrication of the TA/Fe(III) complex, digital picture of separators before and after wrinkling; (b) Raman spectroscopy of separators; (c) XPS spectrum of separators; (d) Fe 2p spectra; (e) O 1s spectra.

Morphology and Physical Properties of separators. The surface morphology for the separators was characterized by SEM. As illustrated in Figure 2a, the pristine PP separator exhibits a slit-like pore structure. After coating the TA/Fe(III) complex, no apparent change in pore structures is detected. This indicates that the additional coating process does not lead to obvious pore blocking and thus no additional impedance caused by the TA/Fe(III) coating layer is expected. Figure 2b gives the pore size distribution of the separators before and after modifying. The porosity of the separator is still as high as 71.1%, approaching to that of the bare PP separator (73.8 %). This signifies that the TA/Fe(III) coated PP separator can afford rapid migration channels for lithium ions.

In principle, by virtue of the dipole-dipole interactions between TA and polysulfides, together with the Lewis acid-base interactions between Fe(III) and polysulfides, the TA/Fe(III) coating layer would block the migration of polysulfides. The hypothesis was investigated by the permeation experiments using H-shaped devices. The polysulfides (0.1M L2S6) in DME were slowly added into the lefthand tube, while a blank DME was injected into the opposite tube. During the whole experimental process, the H-shaped devices were kept static to eliminate external influences on the polysulfide diffusion. As shown in Figure 4, polysulfides permeated through the bare PP separator, conspicuously entered the right-hand tube after 8 h and completely filled the blank electrolytes within 16 h. In comparison, no polysulfides passed through the TA/Fe(III)-PP separator within a period of 24 h. In general, a thicker coating could be more beneficial to reject the migration of polysulfides, but inevitably lower the ionic conductivity of the coated separators. In such a system, the TA/Fe(III)-PP separator is able to effectively retard the shuttle effect of polysulfides, even in the case of the ultralow areal loading. 3



Figure 4. Polysulfide diffusion tests of the separators after varying resting time

Electrochemical performance of Li-S cells. As displayed in Figure 5, CV profiles of all cells exhibit the typical redox characteristics with two cathodic peaks and an anodic peak in the reverse sweep. This agrees with the previous studies that cathodic peaks at 2.29-2.35 V and 1.97-2.02 V vs. Li+/Li corresponds to the conversion from sulfur to polysulfides and eventually to Li2S/Li2S2, respectively and the anodic peak indicates the converting process of sulfur.31 For the 2nd cycle, the CV curve of the cell with the TA/Fe(III)-PP separator shows minor changes, indicating that the TA/Fe(III) complex does not react with polysulfides within the applied voltage window.

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bare PP separator. As seen from Figure 6b, all discharge curves of the cells give two discharge plateaus assigned to the conversion from sulfur to polysulfides, coinciding well with the above CV profile. Besides, all discharge profiles are well preserved during the cycling process, further implying that the TA/Fe(III) complex is electrochemically inactive.32 In comparison with the cells with the bare PP separators, the discharge plateaus of those with the TA/Fe(III)-PP separators are all higher. This result indicates that the diffusion of polysulfides is significantly inhibited in the cell with the TA/Fe(III)-PP separator, since the reduced capacity at high plateau is considered as an indicator of the polysulfide shuttle effect.33 The cells were further subjected to prolonged cycling at 2.0 C and the corresponding results are shown in Figure 6c. After extensive 1000 cycles, the cell using the bare PP separator provides a discharge capacity lower than 30 mAh·g−1, suggesting the failure of the cell. In stark contrast, the discharge capacity of the battery using the TA/Fe(III)-PP separator is still as high as 174 mAh·g−1 after 1000 cycles, corresponding to a cyclic decay rate of 0.06 % per cycle.

Figure 6. Electrochemical tests of the cells. (a) The discharge capacity and the coulombic efficiency at 0.5 C (1.0 C = 1672 mA·g-1); (b) The cyclic curves at the initial, 100th and 300th cycles; (c) The discharge capacity at 2.0 C.

Figure 5. CV curves of cells with the separators.

Depending on the Rb value displayed in Figure S2, the ambient ion conductivity of the TA/Fe(III)-PP separator is calculated to be 1.1 mS·cm-1, higher than that of the original PP separator. Such improvement can be ascribed to its constant pore structure and superior electrolyte wettability. Higher ion conductivity for the separators is beneficial to improve the cell performance, especially the rate performance. The charge/discharge profiles and Coulombic efficiency of the cells were assessed at 0.5 C and plotted in Figure 6a. In this work, the discharge capacities and current densities are all solely based on the sulfur mass. With the alleviated shuttle effect, the cell with the TA/Fe(III)-PP separator exhibits a Coulombic efficiency of >97% after 300 cycles, while a Coulombic efficiency lower than 85% is observed for its counterpart with the bare PP separator. Moreover, the cell with the TA/Fe(III)-PP separator displays a discharge capacity of 394 mAh·g-1 in 300th cycle, almost 30% larger than that of the

To evaluate the rate performance, the cells were tested at various rates from 0.5 to 3.0 C. As shown in Figure 7, discharge capacities of 617 mAh·g-1, 517 mAh·g-1, 480 mAh·g-1 and 429 mAh·g-1 are detected for the cells with the TA/Fe(III)-PP separators at 0.5 C, 1.0 C, 2.0 C and 3.0 C, much higher than those of the cells with the bare PP separator at each rate. Especially at 3.0 C, the discharge capacity of 429 mAh·g-1 in the case of the TA/Fe(III)-PP separator is 1.7 times larger than that using the bare PP separator (159 mAh·g-1). Furthermore, when the current density was restored to 0.5 C, the discharge capacity of the battery using the TA/Fe(III)-PP separator was 481 mAh·g-1, superior to that with the bare PP separator (376 mAh·g-1). The superior stable operation and capacity of the cell with the TA/Fe(III)-PP separator can be attributed to the efficient capture of polysulfides, inhibiting the migration of the polysulfides to the anode. In addition to the abundant oxygen-containing groups, Fe(III) ions can also chemically anchor polysulfides via Lewis acid-base interactions, which will be proved in the following discussion. Furthermore, the TA/Fe(III)-PP separator with excellent electrolyte wettability could facilitate retaining liquid electrolyte within the 4


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separator, and thus reduces the transfer resistance of lithium ions and achieves superior rate performance.

Figure 7. Rate performance of the cells at various rates. (a) Cyclic curves of initial cycles; (b) The discharge capacity.

The electrochemical performance of the TA coated PP separators in Li-S cells was also displayed in Figure S3. As can be seen, benefiting from its enhanced hydrophilcity and unchanged pore structure, the ionic conductivity for the TA coated PP separator without Fe(III) was higher than the bare PP separator, up to 1.1 mS·cm-1. Moreover, relative to those with the bare PP separators, the cells using the TA coated PP separators display a superior cycling durability (Figure S3b) and rate performance (Figure S3c). These results verify the promotional role of TA in improving the cell performance. It is suggested that TA with rich polar groups alone can improve the electrolyte wettability and polysulfide adsorption of the separator, thereby resulting in an increased ionic conductivity and improved cell performance, as demonstrated in the previous study.34 However, the cell performance with the TA coated PP separators was inferior to that of the TA/Fe(III)-PP separators. This can be explained that the TA/Fe(III) coating could provide dual confinement effects based on dipole-dipole interactions and Lewis acid-base interactions, while there is only dipole-dipole interactions existed in the TA coating layer to suppress the shuttle effect of polysulfides. It should be pointed out that the current investigation mainly focuses on the role of the modified separator to suppress the shuttle effect. A simple physical mixture of sulfur and carbon material, rather than any elaborately designed or specialized composites, is employed as the cathode for a proof of concept in the present work. Therefore, the observed improvement in the electrochemical performance arises solely from the TA/Fe(III) coating layer in the modified separator. Once more advanced cathodes are adopted, Li-S cells assembled with the TA/Fe(III)-PP separators are promising to deliver much better electrochemical performance. In addition, as polysulfides were only adsorbed on the TA/Fe(III) layer without any electrochemical reactions, a proportion of pores of the TA/Fe(III)-PP separator may be clogged, which will compromise the ionic conductivity after a certain number of cycles. Further studies, including co-deposition of the TA/Fe(III) complex and conductive carbon/polymer materials, are currently underway to catalyze efficient interfacial reactions and further improve the cell performance.

in the glovebox and repeatedly washed with DME to remove the absorbed LiTFSI salt. As presented in Figure 8a, after 300 cycles, the separator surface (close to the S cathode) is much smoother relative to the surface of the cycled bare PP separator, indicating that the polysulfides are adsorbed and confined onto the surface of the TA/Fe(III)-PP separators during the cycling process. To verify the effect of NO3- on forming the stable SEI films on the anodes, the SEM images of the cycled anodes were also given in Figure S4. For comparison, we fabricated a new TA/Fe(III)* coated PP separator without NO3-, in which the Fe(III)* came from the FeCl3 aqueous solution, that is, Cl- was chosen as the counterion. Meanwhile, LiNO3-free electrolytes were adopted to eliminate the effect of NO3- in the electrolyte. As illustrated in Figure S4b, for the cell with the TA/Fe(III)* coated PP separator, there are obvious cracks and numerous lithium dendrites on Li anodes. In contrast, the lithium depositing/stripping process is much more reliable and reversible in that with the TA/Fe(III)-PP separator. As demonstrated in the previous studies, the different lithium deposition morphology might be caused by the stability of SEI, suggesting that NO3- can assist in forming stable SEI films.35 The interfacial resistances of the cells before and after 300 cycles were probed by EIS, as displayed in Figure 8b. As can be observed, the initial impedance of the battery using the TA/Fe(III)-PP separator is 38 Ω, lower than that in the case of the PP separator (52 Ω). This should be ascribed to the superior wettability of the TA/Fe(III)-PP separator, which could achieve higher storage capacity of electrolytes compared to the bare PP separator, facilitating rapid lithium ion transfer and thus leading to a lower interface impedance. After 300 cycles, the charge-transfer resistance (Rct) of the battery using the TA/Fe(III)-PP separator significantly drops to less than 50 % of that with the PP separator. The enlarged semicircle is attributed to the increased interfacial resistance at the lithium anode, originating from the uncontrolled polysulfides diffusion of the bare PP separator. XPS measurements were further carried out to reveal the subtle local interactions between the TA/Fe(III) coating layer and polysulfides. By comparing the Fe 2p spectra of the cycled TA/Fe(III)-PP separator with the pristine one, the Fe-S interaction can be validated (Figure 8c). After cycling, the Fe(III) binding energy changes from 711 to 714 eV. As TA/Fe(III) complex is electrochemically inactive, this change in binding energy testifies the interactions between polysulfides and Fe(III).34 The Lewis acidic Fe(III) center tends to coordinate with the polysulfide base, suppressing the migration of the polysulfides. EDX mapping images of the TA/Fe(III)-PP separator before and after cycling were given in Figure S6b. Clearly, elemental S evenly distributed throughout the TA/Fe(III)-PP separator after cycling, suggesting the effective polysulfide adsorption of the TA/Fe(III)-PP separator. Furthermore, the presence of elemental Fe before and after cycling (Figure S6a and Figure S6c) signifies that the uniform TA/Fe(III) layer could be robustly attached on the PP separator during the long-term cycling.

Postmortem analysis on cycled cells. The surface morphologies of the separators are observed before and after 300 cycles at 0.5 C. The separators retrieved from the cycled cells, which were disassembled 5



Figure 8. (a) SEM images of the surface of the separators (close to the cathode) before and after cycling; (b) Interfacial resistances of the separators; (c) XPS spectra of Fe 2p of the separators before and after cycling.

CONCLUSIONS In summary, a novel TA/Fe(III)-PP separator was designed and fabricated via a simple and fast coating method for the first time. Combined the Lewis acid-base interactions from Fe(III) with the dipole-dipole interactions from TA, the TA/Fe(III)-PP separator displayed efficient capture of polysulfides. As a result of the efficient polysulfide rejection and low lithium ions transfer resistance, the corresponding cell exhibited improved performance in both discharge capacity and rate performance. It presents a good cycling stability with a capacity fade of 0.06% each cycle over 1000 cycles at 2.0 C and a coulombic efficiency >97 % over 300 cycles at 0.5 C. The discharge capacity of the cell assembled with the modified separator at 3.0 C remains 429 mAh·g-1, 1.7 times higher than the one with the bare PP separator. Considering the simple, fast, and cost-effective fabrication process, our proposed strategy opens up a new avenue for the construction of advanced separators for Li-S batteries and beyond. AUTHOR INFORMATION Corresponding Authors *Prof. Dingshan Yu Phone: (86)-020-84111395 E-mail: [email protected] *Prof. Baoku Zhu Phone: (86)-571-87953723 E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support from the Natural Science Foundation of China (No. 51573214) and the Science and Technology planning project of Guangzhou (No. 201704020008). REFERENCES (1) Yin, Y. X.; Xin, S.; Guo, Y. G.; Wan, L. J., Lithium-Sulfur Batteries: Electrochemistry, Materials, and Prospects. Angew. Chem. Int. Ed. 2013, 52, 13186-13200. (2) Peng, H. J.; Huang, J. Q.; Cheng, X. B.; Zhang, Q., Review on High-Loading and High-Energy Lithium-Sulfur Batteries. Adv. Energy Mater. 2017, 7, 1700260-1700314. (3) Yuan, Z.; Peng, H. J.; Hou, T. Z.; Huang, J. Q.; Chen, C. M.; Wang, D. W.; Cheng, X. B.; Wei, F.; Zhang, Q., Powering LithiumSulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts, Nano Lett. 2016, 16, 519-527. (4) Seh, Z. W.; Li, W. Y.; Cha, J. J.; Zheng, G. Y.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y., Sulphur-TiO2 yolk-shell

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Table of Contents Graphic

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FeIII Complex

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