Conductive MOF Modified Separator for Mitigating the Shuttle Effect of

Feb 21, 2019 - Although there are plenty of merits for lithium–sulfur (Li–S) batteries, its undesired shuttle effect and its insulated nature are ...
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Conductive MOF Modified Separator for Mitigating the Shuttle Effect of Lithium–Sulfur Battery through a Filtration Method Huanhuan Chen, Yawen Xiao, Chen Chen, Jiayi Yang, Cong Gao, Yangshen Chen, Jiansheng Wu, Yu Shen, Weina Zhang, Sheng Li, Fengwei Huo, and Bing Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22564 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Conductive MOF Modified Separator for Mitigating the Shuttle Effect of Lithium–Sulfur Battery through a Filtration Method Huanhuan Chen, Yawen Xiao, Chen Chen, Jiayi Yang, Cong Gao, Yangshen Chen, Jiansheng Wu, Yu Shen, Weina Zhang, Sheng Li*, Fengwei Huo, Bing Zheng*

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R. China. KEYWORDS: Conductive MOF, Hydrophilic, Separator, Lithium–sulfur battery, Filtration

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ABSTRACT Although there are plenty of merits for lithium–sulfur (Li–S) batteries, their undesired shuttle effect and insulated nature are hindering the practical applications. Here, a conductive metal–organic framework (MOF) modified separator has been designed and fabricated through a facile filtration method to address the issues. Specifically, its intrinsic microporous structure, hydrophilic polar property and conductive feature could make it easy to contact with and trap polysulfides and boost the kinetics of electrochemical reactions. Both the physical and chemical properties of the as-prepared separator are beneficial to alleviating the shuttle effect and enhancing the rate capability. Accordingly, the electrochemical performance of the battery with MOF modified separator was significantly improved.

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INTRODUCTION Lithium–sulfur (Li–S) batteries are now receiving a lot of attention because they are high in theoretical specific capacity (1675 mAh g-1), low in cost and abundant in resources, etc.1–3 However, several insufficiencies would lead to poor cyclic stability and low rate performance of Li–S batteries, such as the insulating property and the volume expansion of sulfur. More importantly, shuttle effect, which is caused by the migration of soluble polysulfides from cathode to anode in Li–S batteries during charge/discharge processes, would severely hinder their practical applications.4–7 Numerous efforts have been made to address these issues. The corresponding strategies include but are not limited to 1) conductive cathode designs;8 2) physical confinement of sulfur with porous materials;9–11 3) new binders and electrolyte additives designs12– 15

and 4) preparation of novel selectively permeable separators.16–18 As a popular and

effective strategy to solve the shuttling issue, separator modifications can prevent the migration of soluble polysulfides while allow the transport of lithium ions. Moreover, the separator decorations can even interact with and adsorb polysulfides, improving the cyclability of Li–S batteries. Many functional separator decorations have been explored, such as polymer,19–21 metal oxides,22–23 layered double hydroxide (LDH)24 and covalent organic frameworks (COFs).25 MOFs have been paid much attention these years for their special properties.26–28 They are self-assembled by inorganic metal centers (metal ions or metal clusters) and organic ligands.29–30 The unique features involving high surface area, tailorable chemistry, highly crystalline and uniform morphologies make MOFs popular precursors to fabricate different materials including porous carbon, metal oxides, metal sulfides, etc.31–36 These derivatives have been widely investigated and have exhibited excellent performance in Li–S batteries. However, MOFs after treatments will lose many of their specialties such as the coordination environment of metal nodes.37 Considering the special physical and chemical properties of MOFs,38–40 their direct applications to Li– S batteries are proven to be feasible, and some researchers have recently reported its feasibility. For example, designing and synthesizing suitable MOFs as host materials is 3

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found to be effective in trapping the polysulfides and enhancing the performance of Li– S batteries.41–43 This is because the strong Lewis acid–base interactions between polysulfides and metal nodes in MOFs could confine the soluble sulfur species within MOF to decrease the shuttle effect significantly. Besides, MOFs were also used as separators to prevent the migration of the polysulfide ions while allow the Li+ transport (Table S1).44–50 By using HKUST-1 to construct a MOF@graphene oxide separator as an efficient ionic sieve, Zhou’s group managed to slow down the shuttling of polysulfides to the anode.48 In that work, the authors emphasized that the smaller size of the highly ordered micropores than soluble polysulfides is beneficial for trapping the polysulfides. Li’s group synthesized different kinds of MOFs: ZIF-7, ZIF-8, Y-FTZB, and HKUST-1 and revealed that the more densely packed the MOF particles, the better the performance of Li–S batteries.49 Notably, the aforementioned MOFs used for separators are all three-dimensional (3D) ones with poor electronic conductivities while two-dimensional (2D) conductive MOFs with high atoms utilization, low transport barriers and uniform one-dimensional (1D) channels51–52 are rarely used for Li–S batteries.50 More efforts in separator decoration studies, preparation method designs and mechanism exploring are still highly needed. Considering the advantages enjoyed by MOFs, herein, a modified separator was fabricated by using conductive 2D Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2) through a rapid and scalable filtration strategy onto the general polypropylene (PP) separator, as shown in Scheme 1. Ni3(HITP)2, a MOF as the main raw material, was synthesized by Ni2+ and HATP·6HCl (Figure S1a), and more details could be found in the experimental section. The modified separator could not only provide conductivity and uniform pore structure, but also offer hydrophilic property and polar-to-polar adsorption capability for polysulfides, mitigating the shuttling and thus enhancing the cycling stability. Furthermore, the conductive property of the modifications could boost the kinetics of the reactions, improving rate performances. Accordingly, the Li–S battery with such a 2D conductive MOF modified separator shows an advanced specific capacity (1220.1 mAh g-1 at 0.1 C) and rate performance (800.2 mAh g-1 at 2 C). This design is effective, and the filtration method is simple, 4

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rapid and scalable, which could be extended to other future separator modification strategies. EXPERIMENTAL SECTION Materials. All commercial chemicals were purchased without further purification unless otherwise mentioned, including nickel chloride hexahydrate (NiCl2·6H2O, ≥ 98%,

Xilong

Chemical

Co.,

Ltd),

2,3,6,7,10,11-hexaaminotriphenylene

hexahydrochloride (HATP·6HCl, > 95%, Shanghai Tensus Biotech), concentrated aqueous ammonia (NH4OH, 25 ~ 28%, Shanghai Lingfeng Chemical Reagent Co., Ltd), N-methyl pyrrolidone (NMP, ≥ 99%, Sinopharm Chemical Reagent Co., Ltd), ethanol (C2H5OH, > 99.5%, Sinopharm Chemical Reagent Co., Ltd), super P carbon black (Shaorui Chemical (Shanghai) Co., Ltd), sulfur (S, 99.999%, Admas–Beta), and polyvinylidene fluoride (PVDF, Solvay). Synthesis of Ni3(HITP)2. Ni3(HITP)2 was synthesized according to a reported work with minor modification.53 NiCl2·6H2O (80.8 mg, 1.36 mmol) and HATP·6HCl (121.8 mg, 0.91 mmol) were dissolved in 5 ml and 35 ml of water, respectively, which was followed by pouring the former metal salt solution into the latter solution to form a uniform mixture. Then, the mixture was transferred to a flask with 1.125 ml of concentrated NH4OH being added. After that, such mixture was stirred for 1 h under the condition of air bubbling at 60 °C, then the stirring was continued for another 2 h. The resulting black powder was obtained and washed with deionized water for three times, then the final product (Ni3(HITP)2) was collected and dried overnight in a vacuum oven at room temperature for further use. Preparation of the Ni3(HITP)2-Modified Separator. 9 mg of as-synthesized Ni3(HITP)2 and 1 mg of PVDF were dispersed in 18 ml of NMP and ultra-sonificated for 1 h to obtain the uniform dispersion. After that, the dispersion was filtered directly onto PP separator. A vacuum oven was employed to dry the resulting modified separator at 60 °C for 24 h. The loading of Ni3(HITP)2 is approximately 0.33 mg cm-2. Preparation of the Cathode. The sulfur cathode was prepared by mixing element sulfur and super P together with PVDF (in a 6:3:1 mass ratio). Then a certain amount of NMP was added and stirred for 12 h to form an evenly distributed slurry. Next, an 5

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aluminum (Al) foil was used to coat the lustrous and uniform slurry, which was then dried in a vacuum oven at 60 °C for another 12 h. Finally, the electrodes were cut into circular pieces with a diameter of 12.7 mm before assembling the cells. Characterizations. The structure of the prepared materials was measured by X-ray diffraction (XRD, Cu Kα radiation, λ=1.5418 Å). The morphology and the elemental mapping of the separator before and after cycling were characterized by scanning electron microscope (SEM, JEOL, JSM-7800F) and transmission electron microscopy (TEM, JEOL, JEM-2100F). The N2 adsorption–desorption isotherm was measured on an adsorption apparatus (Micromeritics ASAP 2020) at 77 K. The thermogravimetric analysis (TGA, METTLER TOLEDO TGA2) curve was obtained from thermo analyzer. The samples before and after discharging were measured by the Fourier transform infrared spectra (FTIR, FTIR-650). The status of Ni3(HITP)2 before and after cycling were characterized by X-ray photoelectron spectroscopy (XPS, thermo scientific escalate 250XI). Electrochemical Measurements. The CR2025 coin cells were assembled in an argonfilled glovebox. The lithium metal was worked as anode and a mixture of 1, 2dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1 volume ratio) along with 1 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and 0.2 M LiNO3 were served as electrolyte. The cycling and rate performance tests were performed on a battery tester (LAND, CT-2001A) at room temperature in the voltage range of 1.8–3.0 V. Additionally, the cyclic voltammetry (CV) at a scan rate of 0.1 mV s-1 with a voltage range of 1.8– 3.0 V and electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 100 kHz – 10 mHz were performed on a multichannel electrochemical workstation (BioLogic Science Instruments, VMP-300).

RESULTS AND DISCUSSION The crystal structure of the materials has been investigated firstly. XRD spectrum in Figure 1a reveals that the peaks of synthesized Ni3(HITP)2 and Ni3(HITP)2 modified on PP agreed well with those of the simulated MOF, suggesting the structure is well6

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maintained. In addition, this MOF displays a 2D-layer crystal structure featuring hexagonal pores and 1D channels with interconnected coordination of Ni2+ and HATP linkers54 (Figure 1b, Figure S1b). For the MOFs, the low-magnification SEM image reveals the aggregated Ni3(HITP)2 crystals (Figure S2a), whereas a thorn-like morphology is found by the high-magnification SEM image (Figure S2b), which is consistent with the TEM images showing the thorn-like morphology and rough surface (Figure S2c, S2d). More material characterizations have been studied by TGA and Brunauer–Emmett–Teller (BET). The TGA curve measured under O2 shows that the framework of Ni3(HITP)2 can remain stable until 350 °C (Figure S3a). Furthermore, N2 adsorption–desorption isotherm (Figure S3b) reveals that the surface area of Ni3(HITP)2 is 245.65 m2/g and the pore-size distribution of Ni3(HITP)2 demonstrates that most of the pores are about 1.3 nm wide, further proving its microporous structure. For the Ni3(HITP)2-modified separator characterizations, its cross section SEM image (Figure 1c) shows that Ni3(HITP)2 layer is stacked well on the PP surface with a thickness of 8 μm, only less than one-half of the PP separator. Accordingly, the topview SEM image of Ni3(HITP)2 layer on the separator (Figure 1d) also shows its thornlike morphology. Surface hydrophilicity of the separator plays a non-negligible role in the inhibition of polysulfides migration during the charging and discharging of Li–S batteries.21 As the polysulfides are polar and hydrophilic, the separators with good hydrophilicity can exhibit affinity to polysulfides. To investigate the hydrophilicity of the separator before and after modification, contact angle tests are performed. As shown in Figure 2a–2d, the contact angle of the PP separator is 114.4° while that of the modified one is 49.0°. The difference between the contact angles suggests that the modified separator becomes more hydrophilic with the designed MOF, making it easier to contact with polysulfides. As can be seen, on the one hand, there are many polar sites within these hexagonal pores along 1D channels of Ni3(HITP)2; on the other hand, Ni3(HITP)2 with uniform micropores is more effective in decreasing the permeation of polysulfides than the PP separator with macropores. Moreover, in Figure 2e and 2f, the polysulfides permeation ability is tested with PP 7

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separator and Ni3(HITP)2-modified separator, respectively. At first, the left parts of the H-type glass tubes are the Li2S6 solution with the same concentration, and the right is the pure DME solvent. Li2S6 can migrate through the PP separator to the right tube only after 10 minutes while the other is colorless with the modified separator. Along with the time increases, the fast diffusion of Li2S6 can be found that the color changes to dark brown after 12 h with the PP separator. In comparison, the modified one could significantly slow the permeation rate with only a slight change of the color, evidencing this strategy could suppress the polysulfides shuttle. In addition, it is found that the dark yellow Li2S6 solution changes to colorless after adding the Ni3(HITP)2 powder which demonstrates that the Ni3(HITP)2 is promising for adsorbing the polysulfides and the XRD pattern shows that MOF is stable after adsorbing (Figure S4). The elemental mapping of SEM for Ni3(HITP)2 before and after cycling is performed. Compared with the MOF sample (Figure S5), the one after cycling (Figure S6) shows the presence of sulfur element on it. XRD pattern of Ni3(HITP)2 before and after cycling are all consistent with the simulation, which illustrated the stability of Ni3(HITP)2 during the charging and discharging (Figure S7a). The FTIR spectrum of Ni3(HITP)2 after discharging shows two peaks at 1235 cm-1 and 1074 cm-1, which can be ascribed to asymmetric and symmetric stretching vibrations of O=S=O46 (Figure S7b). In order to further investigate whether Ni3(HITP)2 reacts with the polysulfides, XPS analysis of Ni3(HITP)2 before and after cycling is also performed. Both the survey XPS spectra show that four peaks centering at about 285, 400, 532 and 856 eV correspond to C 1s, N 1s, O 1s and Ni 2p, respectively (Figure S8).55 Compared with spectrum before cycling, the appearance of S 2s and S 2p peaks of Ni3(HITP)2 after cycling at 229.0 eV and 169.0 eV demonstrates the combination of Ni3(HITP)2 and polysulfides. Furthermore, the red curve shifting a little to lower energy densities (0.3 eV) reveals the interaction between Ni3(HITP)2 and polysulfides (Figure 3a).56 It is worth mentioning that there is no obvious change in the valance of nickel, because the Ni in MOF is electrochemically inactive during 1.8 V to 3.0 V voltage range. 38, 50 As to the XPS spectrum of S 2p, the fitting peaks at 163.4 eV and 164.7 eV represent the terminal sulfur (ST) and bridging sulfur (SB) atoms of polysulfides, respectively, while those at 8

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168.2 eV are caused by the thiosulfate produced from the oxidation of sulfur in polysulfides. The peak at 169.7 eV is attributed to polythionate, which is generated by the reaction between thiosulfate and polysulfides (Figure 3b).57 In short, the polysulfides can be anchored on the conductive MOFs, which provides a strong support to improve the electrochemical behavior of Li–S batteries. The electrochemical performances of Li–S batteries containing the PP separators and the Ni3(HITP)2-modified separators are compared and studied as follows. EIS and simulation equivalent circuits diagram of Li–S batteries are exhibited in Figure 4a, showing two curves consist of high-frequency and low-frequency regions. The presence of a semicircle in the high-frequency region indicates the charge–transfer resistance (Rct), and the presence of a slash in the low-frequency region expresses the Li+ diffusion process within the electrodes.58 The diameter of the semicircle suggests that the Li–S batteries with the modified separators have lower Rct than that of the PP separators, which is a result of the intrinsic conductivity of the Ni3(HITP)2.54 The related specific data are as shown in Table S2. Additionally, CV was measured to verify the reversibility of the batteries with the modified separators for the first four cycles (Figure S9). The curves exhibit typical Li–S redox characteristics featuring two cathodic peaks and one anodic peak with a shoulder peak. Two cathodic peaks at around 2.26 V and 2.02 V are assigned to a solid-to-liquid (from S8 to soluble Li2S4) phase transition and a liquid-tosolid (from soluble Li2S4 to insoluble Li2S2 or Li2S) phase transition, respectively. The broad anodic peak at around 2.38 V indicates the conversion of Li2S2 or Li2S to sulfur in the charging process.54 Both shape and position of the cathodic and anodic peaks can be well overlapped as the cycling continues, revealing good reaction reversibility of the batteries with Ni3(HITP)2-modified separators. The oxidation potential of the battery with modified separator is lower than that with PP separator but the reduction potential is higher (Figure 4b), which exhibiting the positive effect of Ni3(HITP)2 on reducing the polarization and facilitating the kinetic process in the Li–S batteries due to its good conductivity.59–60 The long-life cycling performance of batteries with different separators is studied at 0.5 C (1 C=1675 mAh g-1, values of specific capacity were obtained according to the mass 9

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of sulfur) for 300 cycles (Figure 5a). For the battery with the modified separator, it can deliver a specific capacity over 600 mAh g-1 after 200 cycles and 585.4 mAh g-1 even after 300 cycles, and the approximately 100% Coulombic efficiency confirms the reversible virtue of lithium anode. The rate performance of batteries with different separators are also tested in different current densities at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 0.1 C (Figure 5b). The modified separators have significantly better rate performances. For the battery with modified separator, the initial specific capacity of discharge at low current density of 0.1 C is 1220.1 mAh g-1. As the current density increases in the abovementioned manner, specific capacities of 1156.6 mAh g-1, 1021.3 mAh g-1, 863.7 mAh g-1, 800.2 mAh g-1 are obtained, respectively. Importantly, the specific capacity achieved 1008.0 mAh g-1 when reverted to 0.1 C. In contrast, the battery with PP separator initially delivers a specific capacity of 1007.1 mAh g-1 at 0.1 C, which decreases to 465.2 mAh g-1 when discharging at 2 C and only rises to 638.3 mAh g-1 when converted to 0.1 C. Correspondingly, the charge/discharge profiles of the batteries with the PP and modified separators at different current densities also illustrate the typical plateaus of Li–S batteries which are concordant with the CV plots (Figure 5c and 5d). The upper discharge plateau expresses the reduction of sulfur to polysulfides while the lower discharge plateau represents the transformation from polysulfides to Li2S2/Li2S. Correspondingly, the charge plateau indicates that Li2S2/Li2S is oxidized to Li2S8/S. It should be noted that the polarization between the charge and discharge curves of the battery with modified separator is 10 mV less than that with PP separator, which also proves the active impact of Ni3(HITP)2 on improving the electrochemical reaction dynamic of Li–S batteries. CONCLUSIONS In conclusion, we proposed a facile filtration method to prepare a decorated separator with 2D conductive Ni3(HITP)2 layer. The designed separator is effective in mitigating the shuttle effect and enhancing the rate capability of Li–S batteries. It could deliver an improved cycling capability, along with a specific capacity of 1220.1 mAh g-1 at 0.1 C and rate performance of 800.2 mAh g-1 at 2 C. The result is mainly attributed to the 10

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special physical and chemical properties of Ni3(HITP)2 modifications. It is expected that this decoration design strategy and the convenient filtration method could be extended and applied in future to other separator-modified strategies or even in other battery systems. ASSOCIATED CONTENT Supporting Information Supporting Information Available: more SEM and TEM images, TGA curve, XRD spectra, BET data, photographs of adsorption tests, SEM mapping, FTIR and XPS spectra, CV curves, and tables. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ORCID Bing Zheng: 0000-0002-4373-9345 Sheng Li: 0000-0003-1645-6865 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The project was supported by the National Natural Science Foundation (21504043, 51702155, 21604038, 21574065), Jiangsu Provincial Founds for Natural Science Foundation (BK20170975, BK20160975, BK20160993), the National Science Foundation for Distinguished Young Scholars (21625401).

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Layer Deposited TiO2 on a Bitrogen-Doped Graphene/Sulfur Electrode for High Performance Lithium–Sulfur Batteries. Energy Environ. Sci. 2016, 9, 1495–1503. (3) Chen, T.; Zhang, Z.; Cheng, B.; Chen, R.; Hu, Y.; Ma, L.; Zhu, G.; Liu, J.; Jin, Z. Self-Templated Formation of Interlaced Carbon Nanotubes Threaded Hollow Co3S4 Nanoboxes for High-Rate and Heat-Resistant Lithium–Sulfur Batteries. J. Am. Chem. Soc. 2017, 139, 12710–12715. (4) Xing, L.-B.; Xi, K.; Li, Q.; Su, Z.; Lai, C.; Zhao, X.; Kumar, R. V. Nitrogen, SulfurCodoped Graphene Sponge as Electroactive Carbon Interlayer for High-Energy and Power Lithium–Sulfur Batteries. J. Power Sources 2016, 303, 22–28. (5) Tan, G.; Xu, R.; Xing, Z.; Yuan, Y.; Lu, J.; Wen, J.; Liu, C.; Ma, L.; Zhan, C.; Liu, Q.; Wu, T.; Jian, Z.; Shahbazian-Yassar, R.; Ren, Y.; Miller, D. J.; Curtiss, L. A.; Ji, X.; Amine, K. Burning Lithium in CS2 for High-Performing Compact Li2S–Graphene Nanocapsules for Li–S Batteries. Nat. Energy 2017, 2, 17090–17099. (6) Wei Seh, Z.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y. Sulphur–TiO2 Yolk–Shell Nanoarchitecture with Internal Void Space for LongCycle Lithium–Sulphur Batteries. Nat. Commun. 2013, 4, 1331–1336. (7) Wang, L.; Han, Y.; Feng, X.; Zhou, J.; Qi, P.; Wang, B. Metal–Organic Frameworks for Energy Storage: Batteries and Supercapacitors. Coord. Chem. Rev. 2016, 307, 361– 381. (8) Yang, Y.; Yu, G.; Cha, J. J.; Wu, H.; Vosgueritchian, M.; Yao, Y.; Bao, Z.; Cui, Y. Improving the Performance of Lithium Sulfur Batteries by Conductive Polymer Coating. ACS Nano 2011, 5, 9187–9193. (9) Zhao, Z.; Wang, S.; Liang, R.; Li, Z.; Shi, Z.; Chen, G. Graphene-Wrapped Chromium-MOF(MIL-101)/ Sulfur Composite for Performance Improvement of HighRate Rechargeable Li–S Batteries. J. Mater. Chem. A 2014, 2, 13509–13512. (10) Mao, Y.; Li, G.; Guo, Y.; Li, Z.; Liang, C.; Peng, X.; Lin, Z. Foldable Interpenetrated Metal–Organic Frameworks/Carbon Nanotubes Thin Film for Lithium– Sulfur Batteries. Nat. Commun. 2017, 8, 14628–14635. (11) Zhang, J.; Li, Z.; Chen, Y.; Gao, S.; Lou, X. W. D. Nickel–Iron Layered Double Hydroxide Hollow Polyhedrons as a Superior Sulfur Host for Lithium–Sulfur Batteries. 12

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Ultra-Stable

Li–S

Batteries.

Energy

Storage

Mater.

2018,

DOI:

10.1016/j.ensm.2018.08.013. (60) Sun, F.; Wang, J.; Long, D.; Qiao, W.; Ling, L.; Lv, C.; Cai, R. A High-Rate Lithium–Sulfur Battery Assisted by Nitrogen-Enriched Mesoporous Carbons Decorated with Ultrafine La2O3 Nanoparticles. J. Mater. Chem. A 2013, 1, 13283– 13289.

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Scheme 1. Illustration of the preparation process of Ni3(HITP)2-modified separator by filtration and its assembly into Li–S battery.

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Figure 1. (a) XRD comparison pattern of synthesized Ni3(HITP)2 and Ni3(HITP)2; (b) Structure illustration of Ni3(HITP)2 with 1D channels (grey, blue and green spheres represent C, N and Ni atoms, respectively); (c) Cross section SEM image of Ni3(HITP)2-modified separator; (d) Top-view SEM image of Ni3(HITP)2 layer modified on the PP.

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Figure 2. Photographs of (a) white PP separator and (b) black Ni3(HITP)2-modified separator; Images of contact angles of (c) PP separator (114.4°) and (d) modified separator (49.0°). Permeation test of Li2S6 with (e) PP separator and (f) Modified separator at different time, respectively.

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Figure 3. (a) Ni 2p XPS spectrum of Ni3(HITP)2 before and after cycling; (b) S 2p XPS spectrum of Ni3(HITP)2 after cycling.

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Figure 4. (a) EIS of Li–S batteries with PP and Ni3(HITP)2-modified separators, the inset is the simulation equivalent circuits diagram; (b) CV scans of Li–S batteries with PP and Ni3(HITP)2-modified separators at a scan rate of 0.1 mV/s.

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Figure 5. Electrochemical performance of Li–S batteries: (a) Long-time cycling performance of Li–S batteries with PP and Ni3(HITP)2-modified separators for 300 cycles at a current density of 0.5 C; (b) Rate performance of Li–S batteries with PP and Ni3(HITP)2-modified separators at different current densities of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 0.1 C; (c) and (d) Charge/discharge curves of Li–S batteries with pp and Ni3(HITP)2-modified separators at different current densities of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, respectively.

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