Flame-retardant and thermal-stable separator trapping polysulfides for lithium-sulfur battery

Journal Pre-proof Flame-retardant and thermal-stable separator trapping polysulfides for lithium-sulfur battery Yuanyuan Li, Jiawen Zhang, Cuifang Zhou, Min Ling, Jianguo Lu, Yang Hou, Qinghua Zhang, Qinggang He, Xiaoli Zhan, Fengqiu Chen PII:

S0925-8388(20)30560-0

DOI:

https://doi.org/10.1016/j.jallcom.2020.154197

Reference:

JALCOM 154197

To appear in:

Journal of Alloys and Compounds

Received Date: 17 October 2019 Revised Date:

12 January 2020

Accepted Date: 4 February 2020

Please cite this article as: Y. Li, J. Zhang, C. Zhou, M. Ling, J. Lu, Y. Hou, Q. Zhang, Q. He, X. Zhan, F. Chen, Flame-retardant and thermal-stable separator trapping polysulfides for lithium-sulfur battery, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154197. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Credit Author Statement Yuanyuan Li: Conceptualization, Methodology , Investigation, Reviewing and Editing, Jiawen Zhang: Investigation Cuifang Zhou: Investigation Min Ling: Investigation Jianguo Lu: Investigation Yang Hou: Investigation Qinghua Zhang: Supervision, Reviewing and Editing, Qinggang He: Supervision Xiaoli Zhan: Supervision Fengqiu Chen: Supervision

Abstract Graphics:

Flame-retardant and thermal-stable separator trapping polysulfides for lithium-sulfur battery Yuanyuan Lia, Jiawen Zhanga, Cuifang Zhoua, Min Linga, Jianguo Lub,c, Yang Houa,b, Qinghua Zhanga,b*, Qinggang Hea,b, Xiaoli Zhana,b, Fengqiu Chena,b *

Qinghua Zhang, [email protected]

a

College of Chemical and Biological Engineering, Zhejiang University,

Hangzhou, 310027, China b

Ningbo Research Institute, Zhejiang University, 315100, China

c

State Key Laboratory of Silicon Materials, School of Materials Science and

Engineering, Zhejiang University, 310027, China

Abstract The shuttle effects of polysulfides and abominable safety problems restrict the practical application of lithium-sulfur batteries. Herein, we report a polyimide-based Janus separator by a simple filtration method introducing MoO3 nanoparticles and MWCNTs-COOH. Using this separator, the lithium-sulfur battery delivers a reversible capacity of 1274 mAh g-1 at 0.2 C and a capacity 637 mAh g-1 at 5 C. Moreover, the separator is flame-retardant and thermal-stable without obvious shrinkage even at 300

. Experimental observations reveal that uniformly

dispersed MoO3 nanoparticles and MWCNTs-COOH as a pore structure regulator can obviously decrease the pore diameter and porosity of electrospun PI skeleton for intercepting polysulfides. The synergistic effect between MWCNTs-COOH, MoO3 and polyimide fibers enables the faster kinetics and more efficient conversion of polysulfides. Keywords: Polyimide, Thermal-stable separator, MoO3, Li-S batteries

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1. Introduction With the high theoretical energy density of 2600 Wh kg-1 and the abundance of sulfur, lithium-sulfur (Li-S) batteries are considered as the most potential candidates for replacing lithium-ion batteries (LIBs) [1, 3]. However, there remain problems in Li-S batteries, especially flammability and thermal stability, on account of the existence of lithium metal, sulfur, organic electrolytes, and organic polyolefin separator. In terms of separator, there have been some works for improving the high-temperature performance of Li-S batteries, such as introducing ammonium polyphosphate (APP) into the polyacrylonitrile (PAN) [4] and cellulose-based membrane [5]. However, the safety problem of Li-S batteries is not fully appreciated due to insufficient demonstration so far [4]. Therefore, more efforts should be devoted to clarify and address the safety issues for accelerating their practical applications. The multifunctional separator with improved safety and electrochemical performance can be a progress in this direction. Electrospun polyimide (PI) exhibits high porosity, excellent thermal-stability (>200

)

and inherent nonflammability [6], which are of interest to academia and industry for LIB separators. In particular, electrospun PI in Li-S battery is also helpful for the nonflammability, and thermal stability. The abundant nitrogen-, oxygen- containing functional groups of PI benefit trapping polysulfides [7]. However, it is noteworthy that the electrospinning PI separator has big pore structure which could have a bad influence on alleviating shuttle effects, even it could improve the wettability for electrolytes [8]. Taking these merits and drawbacks into consideration, we predicted that introducing small sized materials by simple filtration method is a good way to regulate the pore structure.

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In addition, the shuttle effects of polysulfides still remain a tough challenge in the application of Li-S batteries [9-11]. Over the past few years, considerable efforts have been devoted to solving those problems [12-16]. Among them, introducing an appropriate interlayer is considered as a much simpler method in practical production [17, 18]. Carbon materials such as graphene and carbon nanotubes have been widely applied in Li-S batteries to increase the electrical conductivity of separator coating materials [19]. However the capacity decays rapidly in the later cycles, because it is difficult to effectively trap polysulfides by the physical van der Waals forces between the nonpolar surface of carbon materials and polysulfides. Recently, polar metal materials, for example metal oxides (SnO2 [20], TiO2 [21], MnO2 [22]), metal-organic frameworks (MOFs) [23], metal sulfides [13, 24] and transition metal phosphides [25] have been known to trap polysulfides through stronger chemical interaction with polysulfides instead of simple physical adsorption. Theoretical calculation of the bond energy between oxides and polysulfides has demonstrated that MoO3 is superior to most oxides [26]. The metal materials could further improve the rate performance and cycling life for Li-S batteries by combining with carbonaceous materials. In this work, a polyimide-based Janus separator is first prepared by vacuum filtrating MoO3 nanoparticles and carboxyl acid functionalized multiwall carbon nanotubes (MWCNTs-COOH) on the polyimide (PI) electrospinning nanofibers. The achieved MoO3/MWCNTs-COOH functionalized PI membrane (MC/PI) dramatically improves rate behavior (1274 mAh g-1 at 0.2 C and 637 mAh g-1 at 5 C) and safety performance (without obvious shrinkage at 300

) of Li-S batteries. Experimental results demonstrate that the

well-dispersed MoO3 nanoparticles and MWCNTs-COOH could regulate the pore structure of

3

polyimide skeleton by decreasing the pore diameter from 2.87 µm to 0.27 µm for alleviating “shuttle effect”. The conductive MWCNTs-COOH could also provide abundant electron pathways and physically trap polysulfides. Meanwhile, polar MoO3 nanoparticles act as an immobilizer and a catalyst, chemically interacting with polysulfides and increasing the redox reaction kinetics of the polysulfides.

2. Experimental 2.1 Materials 4,4'-oxybisbenzenamine (ODA, Sigma-Aldrich), pyromellitic dianhydride (PMDA, Sigma-Aldrich), N-dimethylfromamide (DMF, Aladdin Co. Ltd., China), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Sinopharm Chemical Reagent Co., Ltd, China), sulfur powder (S, 99.5%, Aladdin Co. Ltd., China), N-methyl-2-pyrrolidone (NMP, Aladdin), 1,3-dioxolane (DOL, Sigma-Aldrich), 1,2-dimethoxyethane (DME, Sigma-Aldrich), super P carbon black (C-65, Shenzhen Kejing Star Technology Co., China) and polyvinylidene fluoride (PVDF, Taiyuan Lizhiyuan Battery Company, China) were used as received. 2.2. Fabrication of MoO3 and MWCNTs-COOH The MoO3 was prepared through a simple process. Ammonium molybdate tetrahydrate was placed in Muffle furnace at 800 °C for 2 h with a heating rate of 2 °C min-1 to obtain massive MoO3. The MWCNTs-COOH were prepared through the pretreatment of pristine MWCNTs. The 500 mg pristine MWCNTs were dispersed in 300 mL mixed acid (Concentrated sulfuric acid (98%) and nitric acid (60%) = 3:1) with heating for 2 h in 70

, followed by

washing with deionized water. Finally MoO3 and MWCNTs-COOH dispersed in water-ethanol

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mixed solution (1:1, v/v) with ultrasonication for 30 min to obtain MoO3/MWCNTs-COOH suspension. 2.3. Fabrication of MC/PI separator The precursor of polyimide, poly(amid acid) (PAA) was synthesized as reported previously [27]. To be specific, PMDA and ODA were dissolved in DMF with an equivalent molar ratio stirring for 12 h constantly, thus the pristine PAA solution with a concentration of 18% was obtained. Electrospinning was carried out by using the optimized condition with flow rate of 0.96 mL h-1, voltage of 17 KV and tip-collector distance of 20 cm. The PAA nanofiber membranes were collected on aluminum foil with thicknesses of 50 µm-80 µm. Then the MoO3/MWCNTs-COOH suspension was filtered through PAA nanofiber membrane, following dried for 6 h in oven. The membranes were thermally imidized at a stage heating: heating up at a rate of 5 °C min-1 to 100, 200, 300 °C, followed by a constant temperature at each stage for 1 h. Finally the MoO3/MWCNTs-COOH functionalized PI membrane (MC/PI) was obtained. MWCNTs-COOH functionalized PI membrane (CNT/PI) and pristine PI were also prepared following the similar process. 2.4 Characterizations 2.4.1. Membrane structure characterization The morphologies of the functionalized separators were observed by scanning electron microscopy (SEM, Phenom ProX, The Netherlands; Utral 55, Germany). The crystal structure of MoO3/MWCNTs-COOH were determined by PANalytical x-ray diffractometer (Cu Kα radiation, λ=1.5406 Å) in a 2-theta range from 10° to 80°. FT-IR measurements were analyzed by a Nicolet 5700 spectrometer. Contact angle was measured on a SDC-100 optical

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contact-angle goniometer (SINDIN Co., Ltd., China) at room temperature. The pore structure was

measured

with

a mercury intrusion porosimeter

(AutoPore IV 9510, Micromeritics, USA). Liquid electrolyte uptakes were measured by soaking membranes in the liquid electrolyte 1 M LiTFSI in 1:1 (v/v) 1, 2-dimethoxyethane (DME) and 1, 3-dioxacyclopentane (DOL) with 1% LiNO3 for 2 h and calculated by following formula: Electrolyte uptake(%) = (W － W0)/ W0 × 100% where W0 and W are the weights of the dry and wet membranes. The thermal behavior of membranes was analyzed by differential scanning calorimetry (DSC-7, Perkin-Elmer Co., USA) at a heating rate of 10 °C min−1 under N2 flow. Thermogravimetry (TGA Q500) was performed at a heating rate of 10 °C min−1 under N2 flow.

2.4.2. Electrochemical measurements Sulfur cathode preparation and Li-S cells assembly: Coin-type (CR2025) cells were assembled in an argon-filled glove box. The cathodes were prepared by the slurry-coating method. Sulfur was introduced into super P host by a melt-diffusion method with a weight rate of 8:2. The slurry mixed by S/C powder, super P carbon black and poly(vinylidene fluoride) (PVDF) binder (8.75:0.25:1) was laminated on the carbon-coated aluminum foil followed by drying at 60 °C under vacuum overnight. The final sulfur loading in each cathode was 1-1.2 mg. The electrolyte was composed of 1 M LiTFSI in 1:1 (v/v) 1, 2-dimethoxyethane (DME) and 1, 3-dioxacyclopentane (DOL) with 1% LiNO3. The sulfur/electrolyte ratio is about 15 µL mg-1. Lithium metal foil was used as the counter electrode. Polysulfides on anode and membrane

6

were analyzed by XPS (Thermo Scientific, USA) with an Al Kα X-ray source. The galvanostatic cycling performance of batteries was conducted on the NEWARE CT-4008 Battery Cycler (Shenzhen, China) in a voltage range of 1.7-2.8V. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) performance were characterized on a CHI660e electrochemical workstation with a scan rate of 0.1 mV s-1 in the potential range of 1.7-2.8 V and frequency range from 0.1 Hz to 100 kHz respectively. The ion conductivity of separators was measured with AC impedance method between 105 and 10−2 Hz via CHI660e electrochemical workstation. The ionic conductivity was calculated from the following equation: σ = d/(RbS) where σ is the ion conductivity, d is the thickness of the membranes, S is the active area of the stainless steel and Rb is the bulk resistance.

2.4.3. Preparation of 0.007 M Li2S6 solution The sulfur powder and Li2S (5:1 by mole ratio) were dissolved in 20 mL DOL and DME solution (1:1 by volume) in an Ar-filled glove box. The 0.15 M Li2S6 solution was obtained by keeping stirring for 8 h at 80

and then diluting it to 0.007 M to obtain the bright yellow

lithium polysulfide solution.

3. Results and discussion The poly(amid acid) (PAA) was prepared by a typical method and the FTIR was confirmed as shown in Fig. S1a. The peaks from 2500 cm-1 to 3100 cm-1 and 3258 cm-1 correspond to the vibration of O-H bonds and N-H bonds, respectively. While after imidization,

7

these characteristic absorption peaks disappear (Fig. S1b). Meanwhile the characteristic absorption peaks of PI at 1775 cm-1 and 1728 cm-1 are attributed to the vibration of CO bonds, while the peak at 1375 cm-1 is attributed to the vibration of C-N bonds in the imide ring. Those results indicate the PI has been successfully synthesized. The reflection peaks of MoO3 in Fig. S2 are consistent with orthorhombic MoO3 (PDF Card No. 05-0508), which confirms that we have prepared pure MoO3.

Fig. 1. SEM images of (a) PI pristine nanofibers and (b) surface of MC/PI; (c) corresponding EDS elemental mapping of molybdenum and oxygen; SEM images of (d) interface between MoO3/MWCNTs-COOH layer and PI fibers layer and (e) cross-section of MC/PI membrane. The morphology of PI pristine nanofibers is shown by SEM in Fig. 1a. The diameter of nanofibers is about 200 nm. After the filtration, the MoO3 and MWCNTs-COOH uniformly distribute on the PI membrane (Fig. 1b). EDS mapping further demonstrates a homogeneous distribution of MoO3 and MWCNTs-COOH (Fig. 1c). The well-dispersed MoO3 and MWCNTs-COOH could regulate the pore structure of PI fibers, suppressing polysulfide 8

migration in Li-S batteries. According to the interface between MoO3/MWCNTs-COOH layer and PI fibers layer (Fig. 1d), it is obvious that the MWCNTs-COOH are attached to PI fibers, which is helpful to decrease the pore diameter and porosity of PI fibers. The thickness of the MoO3/MWCNTs-COOH layer is about 14 µm (Fig. 1e), corresponding loading agents weight of 0.48 mg/cm2.

Fig. 2. (a) Contact angles of PP and MC/PI, and (b) photographs of PP, PI, and CNT/PI and MC/PI separators (from left to right) after thermal exposure at different temperatures for 1 h. The great wettability of membrane can decrease cell resistance which is good for electronic performance. Fig. 2a shows the contact angles of different separators. The electrolyte forms a bead with the contact angle of 44° after dropping electrolyte on PP separator for 0.5 s. In contrast, the MC/PI Janus membrane can absorb the electrolyte easily with a contact angle of 7.1° [28]. This is ascribed to the highly porous structure of PI fibers which enhances the affinity of electrolyte and the transport of Li ion. The liquid electrolyte uptake and Li-ion conductivity tests of separators are measured as shown in Table S2. The highest electrolyte uptake of 820% is achieved by PI separator, which is mainly attributed to the porous nature of PI membrane.

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When the MoO3 particles and MWCNTs-COOH are introduced. The pore structure turn smaller. Therefore, MC/PI and CNT/PI separator have lower electrolyte uptake. The MC/PI battery also shows the highest ionic conductivity of 2.50 mS cm-1. When MoO3 was introduced, the space charge layer formed by the interaction between the particles and liquid electrolyte, improving the conduction of Li ion. The thermal dimensional stability of membrane is an important part in battery safety [29]. The thermal shrinkage test was investigated by observing the dimension changes of different membranes after storage at 150 exposure to 150

, 190

and 300

for 1 h. PP separator shrank severely after

(Fig. 2b). In contrast, all PI-based membranes maintained their shapes

without significant shrinkage even up to 300 . This result suggests that PI-based separators all have excellent thermal dimensional stability even at extreme temperature environments, which is vital to practical production. The DSC and TG tests are conducted to further confirm the thermal stability of polyimide-based separator. As shown in Fig.S3, no significant peaks were observed in the PI membrane over the entire temperature range. However, the endothermic peak at about 170

is found in the PP membrane, which correlated to the typical melting point

of polymer. Although the introduction of MoO3 and MWCNTs-COOH barely affected the degradation temperature of PI membrane, it still maintain high degradation temperature.

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Fig. 3. Photograph about flame-retardant properties of PP, PI, and CNT/PI and MC/PI separators.

Combustion behaviors of the MC/PI Janus membranes were presented through burning test in Fig. 3. When PP was ignited, it instantaneously shrank and engulfed in flame. On the contrary, the PI fibers just slightly shrank, and the fire was self-extinguished as soon as the lighter removed. This is ascribed to the flame-retardant ability of PI. The MC/PI Janus membrane showed almost no shrinkage, which performed better than PI pristine film. This character of separator in batteries is significant for safety, but which is usually overlooked.

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Fig. 4. (a) Rate performance of different separators; (b) galvanostatic change/discharge profiles of PP, PI, CNT/PI and MC/PI cells; cyclic voltammogram scans of Li-S batteries with (c) MC/PI separator and (d) PP separator. The electrochemical tests of batteries with different membranes were conducted for investigating the electrochemical performance. The rate performance of different separators are shown in Fig. 4a. Compared with different batteries, the battery with the MC/PI membrane, denoted as MC/PI battery, exhibits the best rate performance. At different current densities of 0.2 C, 0.5 C, 1 C and 2 C, the battery with the MC/PI separator delivers reversible capacities of 1274 mAh g-1, 1117 mAh g-1, 981 mAh g-1 and 845 mAh g-1, respectively. Even at high current density of 5 C, the MC/PI battery can still obtain high capacity of 637 mAh g-1. Moreover, the most of the reversible capacity recovers when the current density abruptly switches back to 0.2 C. This result implies the excellent polysulfide redox reaction kinetics of MC/PI membrane and good reversibility, even compared with the previous report [30] which delivered more MoO3@CNT. The charge/discharge profile of MC/PI battery shows the smallest polarization with the longest and flattest voltage platform (Fig. 4b.). Specifically, the voltage hysteresis (∆E) 12

between the charge plateau and discharge plateau of MC/PI battery is 175.8 mV , which is smaller than the voltage hysteresis of CNT/PI battery, PI battery and PP battery with 190 mV, 277.1 mV and 205.2 mV, respectively. This means the best chemical reversibility of MC/PI separator in the discharge/charge process. The interconnected conductive network of the CNT/PI membrane has great electrical conductivity and can provide abundant electron pathways, which contributes to low polarization [31]. When the MoO3 is introduced, the voltage gap further decreases. This is attributed to both high conductivity of MWCNTs-COOH and the chemical absorption of MoO3. The MoO3 particles can effectively adsorb lithium polysulfide through the Mo-O-Mo bonds on MoO3. The Mo-O-Mo bonds turn to be Mo-O-S bonds during the interaction between MoO3 and sulfur species [30]. Subsequently, the advantages of this MC/PI separator is revealed by cyclic voltammetry (CV) tests. Fig. 4c shows the CV curves of the MC/PI separator for the first four cycles at a scan rate of 0.1 mV s-1. The two typical reduction peaks of all cells are at about 2.31 V and 2.04 V, corresponding to the conversion of S8 to long chain soluble polysulfides (Li2Sx, 4 ≤ x ≤ 8) and intermediate to insoluble Li2S2 and Li2S [32]. While the overlapped oxidation peaks of MC/PI battery at around 2.44 V derives from the reverse process of Li2S2/Li2S to polysulfides and finally to S8. The oxidation peak of PP battery at around 2.34 V is associated with the formation of polysulfides (Li2Sx, 4 ≤ x ≤ 8). This process continues until polysulfides are completely converted into elemental sulfur at 2.45 V [33]. Obviously, the fourth-cycle CV curve is well overlapped with the first cycle curve, demonstrating the excellent stability and reversibility of the electrochemical process of the MC/PI battery. While the results of other kinds batteries are not as good as MC/PI battery (Fig. 4d and Fig. S4), which indicates that the MC/PI layer can

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enhance reaction kinetics better.

Fig. 5. Cycling performance of PP, PI, CNT/PI and MC/PI batteries. The galvanostatic change/discharge tests at 0.2 C (based on the mass of sulfur in the cell, 1 C=1675 mAh g-1) are conducted and illustrated in Fig. 5. As expected, the battery with MC/PI composite delivers the largest discharge capacity of 1602.3 mAh g-1 at initial cycle, and retains the superior reversible capacity of 905.5 mAh g-1 after 100 cycles. In comparison, the capacity of CNT/PI membrane, PI membrane and PP pristine membrane are 772.1 mAh g-1, 406.7 mAh g-1 and 520.8 mAh g-1 after 100 cycles, respectively. And those batteries also have lower coulombic efficiency. The discharge capacity of PI battery is smaller than the pristine PP battery, which is attributed to the higher porosity and the larger pore diameter of electrospinning PI fibers leading to easier diffusion of polysulfides. After MoO3 and MWCNTs-COOH are introduced, the pore structure of membrane turns better. The synergistic effect between MWCNTs-COOH, MoO3 and PI fibers make it more efficient to trap polysulfides. After 300 cycles, the battery of MC/PI still has the highest specific discharge capacity of 611.1 mAh g-1. This figure is higher by 30.8%, 91.6% and 34.5% than battery of CNT/PI (467.1 mAh g-1), pristine PI (318.9 mAh g-1) and pristine PP (454.1 mAh g-1), respectively. The MC/PI separator was further coupled with sulfur-loading electrode (5.5 mg cm−2). Fig S5 shows stable cycling 14

performance with a capacity above 800 mAh g−1 after 100 cycles at 0.2 C, implying excellent cycling stability of MC/PI battery. Table 1 Average pore diameter and porosity of PP, PI, CNT/PI and MC/PI separators. Sample

PP

PI

CNT/PI

MC/PI

Average Pore Diameter (µm)

0.10

2.87

1.58

0.27

Porosity (%)

55.37

86.53

80.83

74.50

The mercury intrusion porosimetry is used to explain the electrochemical influence of different separators’ pore structure. The table 1 shows the average pore diameter and the porosity of PP, PI, CNT/PI and MC/PI. The large pores composed of fibers are the main cause of poor electrochemical performance, even though the abundant acylamino groups of PI have a reversible adsorption function to dissoluble polar polysulfide. The PI fibers have average pore diameter of 2.87 µm and porosity of 86.53%, which is much larger than the pore diameter of commercial PP separator (0.10 µm and 55.37%). After introducing MoO3 and MWCNTs-COOH, the pore diameter and porosity of MC/PI separator turns to 0.27 µm and 74.50%, significantly smaller than PI fibers. This is because the MWCNTs-COOH can stick to the PI fibers, which largely decreases the pore diameter and porosity of PI fiber. Moreover the MoO3 nanoparticles uniformly distributed in MWCNTs-COOH can further decrease the pore diameter of membrane. The dense MWCNTs-COOH and MoO3 barrier on the PI fibers can effectively intercept the dissolved polysulfide and confine them in the cathode side.

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Fig. 6. The electrochemical impedance spectra (EIS) curves of different batteries before cycling (a) and after cycling (b); (c) CV curves of the MC/PI battery at different scan rates; (d) digital image of the color change for Li2S6 solution before and after socking in MoO3 and MWCNTs-COOH powder. The small impedance and great electrochemical stability is a key factor for high performance Li-S batteries. The electrochemical impedance spectra (EIS) curves of different batteries are shown in Fig. 6a and b. There is one semicircle in the high to medium frequency region representing the charge transfer resistance (Rct) [34]. Before the cycling test, the Rct value of MC/PI membrane shows 46.16 Ω, which is much lower than CNT/PI (161.80 Ω), PI (63.86 Ω), and PP (368.80 Ω). This is ascribed to the fast charge transfer in the MC/PI separator. After 200 cycles at 0.5 C, the Rct of MC/PI battery is much lower than the initial Rct (6.44 Ω). This is mainly attributed to the enhanced electron and Li ion transport, and the uniform redistribution of insulating sulfur. Meanwhile, Compared with these cells (Table S1), the 16

battery assembled with MC/PI provides the smallest Rct, which is due to the great interconnection of MWCNTs-COOH and PI fibers and the rapid Li ion diffusion. This is consistent with the rate performance result discussed above. The CV tests were also performed to evaluate the electrocatalytic behavior, especially the rate capability of different separators, as illustrated in Fig. 6c and Fig. S6a-c. The lithium ion diffusion characteristics of the batteries are discussed by the slopes of the CV curves at different scanning rates of 0.1, 0.2, 0.3, 0.4, 0.5 mV s-1. As shown in Fig. S6d-f, the peak currents have a linear response with the square root of scan rates. The slopes of peaks (I), peaks (II) and peaks (III) of MC/PI battery are higher than those of other counterparts, which suggests the highest lithium-ion diffusion coefficient. This finding demonstrates that the synergistic effect between MoO3 particles, MWCNTs-COOH and PI fibers could effectively improve the lithiation kinetics, especially the conversion of soluble Li2Sx (4 ≤ x ≤ 8) to Li2S/Li2S2. To further verify the strong anchoring effect of the MC/PI to polysulfides, 20 mg MoO3/MWCNTs-COOH powder was added to 0.007 M lithium polysulfide solution. As shown in Fig. 6d, after 3 h, the solution became colorless, indicating the strong interaction between polysulfides and MoO3/MWCNTs-COOH. The XPS measurements of the surface of the cycled anode (at 0.2 C for 300 cycle) were examined as shown in Fig. S7 (a). The sulfur 2p spectra exhibits two sulfur states at 163.0 and 161.8 eV, which can be assigned to bridging (SB0) and terminal (ST−1) sulfur atoms in polysulfide anions, respectively [34, 36]. The peaks of MC/PI are considerably weaker than that of PP, which confirmed that the soluble long-chain polysulfides are reversibly captured and released by the MC/PI separator.

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Fig. 7. SEM images of the PP, PI, CNT/PI and MC/PI separators after 300 cycles. Furthermore, the different cells were disassembled after 300 cycles at 0.2 C. The cycled separators facing Li metal anode are observed by SEM images (Fig. 7). The surface of the pristine PP is covered with a dense layer containing some flakes. The fibers of PI and CNT/PI separators also are covered with a dense layer, and some pores of PI separator are even plugged. Those kinds of morphologies may be related with the deposition and aggregation of insulating sulfur species [37]. In contrast, the MC/PI separator still maintains excellent fibers and porous structure with just small quantity of solid materials, demonstrating that the polysulfides have already been intercepted largely by MC/PI separator due to the synergistic effect of MoO3, MWCNTs-COOH and PI fibers. The corresponding EDS mapping of the cycled MC/PI and PP separators is shown in Fig.S8. There is less sulfur (2.55 wt%) on the MC/PI membrane compared with PP separator, which further demonstrates that the deposition of the sulfur containing species ie suppressed on the MC/PI separator. These results clearly demonstrated the effectiveness of the separator in inhibition of polysulfide shuttling. The XPS measurements of MC/PI and PP separators surface facing anode also were conducted. In Fig. S7 (b), the peak of PP at 164.0 eV is assigned to elemental sulfur [35, 38], which indicates the elemental sulfur is formed on PP separator. This would lead to the irreversible capacity loss. While this peak is 18

inexistence on the surface of MC/PI. Additionally, the weaker peaks of MC/PI further indicates a better polysulfides trapping ability of MC/PI separator.

Fig. 8. The cycling performance of the batteries with PP, PI, CNT/PI and MC/PI separators during the disruption period. Self-discharge property is an important criterion evaluating the capacity retention of Li-S batteries [39]. It is well known that the soluble polysulfides across separator can migrate to anode, react with the lithium metal and transform into insoluble Li2S2 and Li2S. This leads to the irreversible discharge capacity and the reduction in the battery voltage [40]. Different batteries were investigated to explain the efficiency of MC/PI separator for preventing the self-discharge of Li-S battery. Specifically, Li-S batteries are first cycling for 10 cycles at 0.3 C, resting for 48 h, then continue cycling for 40 cycles. After 48 h rest, the discharge capacities loss (∆) compared with the last cycle are 121.1 mAh g-1, 127.9 mAh g-1, 168.5 mAh g-1, 191.5 mAh g-1 for the batteries of MC/PI, CNT/PI, PI and PP separators, respectively (Fig. 8). The 19

capacity loss rate value of the batteries with MC/PI, CNT/PI, PI and PP separators are 9.6%, 12.2%, 36.6% and 35.0%, respectively. The better anti-self-discharge behavior of MC/PI separator indicates that the synergistic effect between MWCNTs-COOH, MoO3 and PI fibers. The Fig. S9d shows that the battery with MC/PI separator can retain voltage as high as 2.3781 V after 48 h rest, which is better than the batteries with CNT/PI, PI and PP separators (Fig. S9a-c). This result further demonstrates the high polysulfides anchoring capacity of MC/PI separator.

4. Conclusion In conclusion, we developed a polyimide-based Janus separator by filtrating MWCNTs-COOH and MoO3 nanoparticles on PI fibers. The achieved MC/PI membrane shows excellent flame-retardant ability and thermal stability without significant shrinkage even at 300

. In addition, the lithium-sulfur battery delivers better rate performance with a reversible

capacity of 1274 mAh g-1 at 0.2 C and a capacity 637 mAh g-1 at 5 C. The experiments reveal that MWCNTs-COOH and MoO3 regulate pore structure of PI fibers by decreasing the pore diameter and porosity of PI skeleton for suppressing the migration of polysulfides. The synergistic effect between MWCNTs-COOH, MoO3 and PI fibers enhances the physical and chemical interaction with polysulfides. For future development, our work provides a new perspective to deal with the polysulfides shuttling and safety issues in Li-S batteries simultaneously.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No.

20

21978258, 21776249 and 21676248).

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Highlight Polyimide-based

Janus

separator

possesses

good nonflammability,

excellent

thermal-stability. The MoO3/MWCNTs-COOH could regulate the pore structure of membrane for trapping polysulfides. Polyimide-based Janus separator facilitate electron and ion transfer, and mitigate the shuttle effect.

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: