Functional double-layer membrane as separator for lithium-sulfur battery with strong catalytic conversion and excellent polysulfide-blocking

Journal Pre-proofs Functional Double-Layer Membrane as Separator for Lithium-Sulfur Battery with Strong Catalytic Conversion and Excellent Polysulfide-Blocking Nanping Deng, Yong Liu, Quanxiang Li, Jing Yan, Leitao Zhang, Liyuan Wang, Yaofang Zhang, Bowen Cheng, Weiwei Lei, Weimin Kang PII: DOI: Reference:

S1385-8947(19)32328-9 https://doi.org/10.1016/j.cej.2019.122918 CEJ 122918

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

20 June 2019 9 September 2019 20 September 2019

Please cite this article as: N. Deng, Y. Liu, Q. Li, J. Yan, L. Zhang, L. Wang, Y. Zhang, B. Cheng, W. Lei, W. Kang, Functional Double-Layer Membrane as Separator for Lithium-Sulfur Battery with Strong Catalytic Conversion and Excellent Polysulfide-Blocking, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.122918

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Functional Double-Layer Membrane as Separator for Lithium-Sulfur Battery with Strong Catalytic Conversion and Excellent PolysulfideBlocking Nanping Deng1, Yong Liu1,2, Quanxiang Li3, Jing Yan2, Leitao Zhang2, Liyuan Wang2, Yaofang Zhang2, Bowen Cheng1,2*, Weiwei Lei3*, Weimin Kang1,2* 1.State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China 2. School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China 3. Institute for Frontier Materials, Deakin University, Geelong and Waurn Ponds, Victoria 3216, Australia

Abstract Lithium-sulfur (Li-S) battery with promising specific capacity and high energy density has attracted wide attention in recent years. However, the rapid capacity fading of the batteries caused by the “shuttle effect” of soluble polysulfides restricted its applications. Here, a novel thermostable double-layer membrane based on F-doped poly-mphenyleneisophthalamide (PMIA) membrane and F-mangano-manganic oxide (Mn3O4) co-doped PMIA membrane is fabricated through electrospinning method. The F-PMIA membrane and F-Mn3O4-co-doped PMIA membrane in the double-layer membrane can act as the matrix to prepare gel polymer electrolyte (GPE). More importantly, the F-doped PMIA membrane in combination with F-Mn3O4-co-doped PMIA membrane, render the engineered functional separator with extraordinary high electrolyte uptake and eminent preserving liquid electrolyte, excellent thermal stability and shrinkage resistance, strong catalytic conversion, forceful chemisorption and physical blocking the “shuttle effect” of lithium polysulfides for Li-S battery. Based on these merits, the assembled batteries with the novel separator exhibited a high first-cycle discharge capacity with 1237.1 mAh g-1, excellent discharge capacity retention of 814.0 mAh g-1 and high Coulombic efficiency of 98.83% after 1000 cycles at the current density of 0.5 C rate. In addition, the obtained LiS battery with GPE presented a low interfacial resistance and high rate capacity (624.1 mA h g-1 at 2 C rate after 600 cycles). The reasons of these excellent electrochemical performances for the battery using the prepared membrane were ascribed to the suppressed “shuttle effect” through both the physical trapping of lithium polysulfides by the GPE based as matrix on F-doped PMIA membrane and chemical binding of intermediates by F-Mn3O4-co-doped membrane. The new separator will open up an effective avenue to enhance the capability and cyclability of Li-S batteries.

Keywords Lithium-sulfur battery; Double-layer composite membrane; F-doped and F-Mn3O4-co-doped nanofiber; Strong catalytic conversion, Excellent polysulfide-blocking; Outstanding cycling stability *Corresponding author. E-mail: [email protected] (W. M. Kang), [email protected] (W. W. Lei), [email protected](B.W.Cheng)

■ INTRODUCTION 1

Continuously increasing demand for energy has driven the development of energy-storage technologies to overstep the conventional lithium-ion batteries. The safe, low cost and environmentally benign materials, and easy preparation processes are also pursued to meet the requirement of manufacturing and global sustainability [1]. Simultaneously, Li-S battery has received wide concerns and attentions owing to its high theoretical energy density of 2600 Wh kg-1 and theoretical specific capacity of 1675 mAh g-1 [2]. In addition, elemental sulfur is resourceful, nontoxic and costeffective. However, the commercialization process of Li-S cell is hindered by several difficulties and challenges including insulating nature of elemental sulfur and low order lithium polysulfides, considerable volume change and serious “shuttle effect” caused by the dissolved polysulfide (PS) [3]. Recently, several approaches have been applied to address these challenges including the application of conductive additives or absorbers, and the design of new electrodes and innovation in cell configuration [4,5]. For effectively reducing the “shuttle effect” of PS, some novel sulfur host materials (such as nano-conductive porous carbon materials) or special structures of cathode materials (such as various bionic structures) based on the principle of “physical confinement” or “chemical adsorption” to PS were applied in Li-S cells. At the same time, the growth of lithium dendrites was inhibited by the nano crystallization of lithium metal or the stabilization of the solid electrolyte interface on the surface of lithium metal. In addition, separator, which affects the safety and electrochemical performances of the battery, is an important component of Li-S battery. Conventional polyolefin separators are extensively applied in Li-S batteries because of their low cost, proper mechanical strength, good chemical stability and electro-chemical stability. However, these polyolefin separators are incapable of effectively retarding “shuttle effect” of PS due to the large-pore framework and lack of strong chemisorption to PS. More seriously, the charge-discharge performances of these assembled Li-S batteries using polyolefin separators are extremely poor when they work in rather “bad” conditions such as high temperature or large current density leading to the short circuit and potential safety hazard even explosion. For effectively inhibiting the “shuttle effect” of PS and significantly improving the safety of the battery, many efforts in the battery separators have been made to solve these thorny problems. One approach is to develop novel polyolefin separators with functional surface coating including carbon modified separators [6-8], polymer modified separators [9-12], inorganic and metallic compounds substance modified separators [13-18] and their combination of these modified separators [19-20]. Another method is to explore alternative separators or some membranes based on the heat-resistant materials and ceramic fibers for Li-S batteries [21-27]. Similarly, poly-m-phenyleneisophthalamide (PMIA) fiber membrane has been reported by many groups regarded as the separator in lithium ion batteries due to the advantages of excellent thermal resistivity, chemical resistance, self-extinguishing characteristics, good mechanical

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properties and electrical insulation ability [28-29]. However, it is difficult for these pure heat-resistant membranes including pure PMIA membrane to obtain high electrolyte affinity or excellent electrolyte retention capacity. Nowadays, the preparation and application of gel polymer electrolyte (GPE) has been considered as one of the most promising methods to reduce the “shuttle effect” of PS and suppress the formation of lithium dendrite on the surface of lithium metal for Li-S batteries [30]. This is because that a flexible and stable passivation layer on the carbon-sulfur electrode can be formed through using GPE in Li-S cells. Meanwhile, GPE such as PVDF membrane [31], PEO membrane [32], PETEA membrane [33] and their composite membranes [34] also can effectively inhibit the PS diffusion because of the strong integrated electrolyte/electrode structure. Nowadays, many researches also have presented that the formed C-F chemical bonds with high bond energy and short bond length can lead to make some Fdoped membranes like PVDF membrane, PTFE membrane and other membranes including F element have high dielectric constant, excellent resistance to chemical degradation, strong electron withdrawing function and low surface energy, which are helpful to provide the prepared separators with outstanding affinity to electrolyte solutions and excellent gelation degree [35]. It is generally known that electrospinning, chemical self-assembly method or other similar methods are rather effective ways to fabricate nanofiber membrane for various batteries. Because of high specific surface area, numerous interconnected pores, these nanofiber membranes are advantageous to form gelation for the reasonable separators in the charge-discharge processes [36-42]. But up to now, few researches have been reported on thermostable gel polymer electrolyte based on the nanofiber membrane with excellent physical and chemical adsorption to PS for Li-S battery. Although our group have once prepared one-layer F-doped tree-like structural PMIA nanofiber membrane for the Li-S batteries through structure design to realize the solely physical adsorption to PS, the long cycle performance of the battery, especially at high current density, is still unsatisfactory due to rather non-uniform pore size distribution and no chemical adsorption and catalysis [43]. As early as 2000, Poizot et al. reported for the first time that transition metal oxide nanomaterials (MOx, x = Cr, Mn, Fe, Co, Ni, Cu and so on) can be used as anode materials for Li-ion batteries [44]. Afterwards, some researchers also applied various modified Mn-based materials as anodes for Li-ion batteries because of high theoretical specific capacity. Different from the layer-type structure of MnO2, Mn3O4 is a spinel ion structure with Mn2+(Mn3+)2O4, in which Mn2+ and Mn3+ ions are distributed at two different lattice positions and the special structure can form different chains bonded each other through strong Van der Waals' force (vdW, which is the term applied to describe a general attractive force among neutral atoms or molecules) interactions. The polar PS can potentially be trapped by the polar Mn3O4 based on the strong vdW interactions [45]. Meanwhile, the chemisorption and catalysis roles of MnOx have been reported in some literatures [46-50], which also have presented high performance Li-S batteries through applying the 3

MnOx doped cathode materials can be obtained. However, Mn3O4 doped separators and GPEs both have not been reported for improving the electrochemical performance of Li-S cells [51]. On the basis of the above considerations, we prepared a novel thermostable double-layer membrane including Fdoped PMIA membrane and F-Mn3O4-co-doped PMIA membrane by electrospinning technology for Li-S battery exhibiting excellent discharge capacity and outstanding cycling stability. The F-doped PMIA membrane and F-Mn3O4co-doped PMIA membrane endow the prepared separator with extraordinary high electrolyte uptake, good preserving liquid electrolyte and excellent thermal stability. Simultaneously, the doping F and Mn3O4 can endow the modified PMIA membranes with strongly chemical adsorption and effectively physical blocking to PS due to the Lewis base role of Mn3O4 with the applied electrolytes during battery cycling, reduced pore size based on the nanoparticle doping and gel forming, and the strong catalysis for promoting the oxidation-reduction reaction of the battery. The batteries with such a composite separator show an ultra-high rate performance and excellent cycling performance, leading to a great potential for use in next generation energy storage. ■ EXPERIMENTAL SECTION The detailed information about the preparation procedures of the double-layer membrane based on the F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane, materials characterizations, and cell assembly process and electrochemical performance measurements of the Li-S battery assembled with the separators are presented in the Supporting Information. ■ RESULTS AND DISCUSSION The phase structure of the as-prepared nanoparticles was characterized by X-ray diffraction (XRD) testing firstly. As shown in Fig. 1(a), the main diffraction peaks can be well indexed to hausmannite Mn3O4 phase (JCPDS No. 08-0382). The morphology and structure characterizations of the as-synthesized Mn3O4 were presented in Fig. 1(b~d). As shown in Transmission Electron Microscope (TEM) images (Fig. 1(b~c)), the particle size of the obtained sample was about 20 nm. The HRTEM images of the prepared sample were presented in Fig. 1(c~d). It displayed the distinct lattice fringes of 0.492 nm, 0.275 nm, 0.316 nm and 0.156 nm, which were in good agreement with the interplanar distances of (011), (103), (112) and (121) planes of Mn3O4 phase, respectively. The corresponding selected area electron diffraction (SAED) pattern (Fig. S1) displayed a set of well-defined diffraction spots, indicating the high-quality single crystalline nature of the prepared Mn3O4 nanoparticles. The enlarged HRTEM image of the rectangular area in panel and the simulated HRTEM image (defocus: 15 nm; thickness: 10 nm) of Mn3O4 along the [3-1-1] crystallographic axis direction was presented in Fig. 1(e). The perfect consistency of actual HRTEM image was simulated with HRTEM image (Fig. 1(f)) using the REW software. Meanwhile, the FFT pattern of Mn3O4 in panel and simulated SADE pattern 4

with electron beam direction parallel to the [3-1-1] zone were shown in Fig. 1(g~h). From Fig. 1(e~h), we can know that the perfect consistency of actual HRTEM image with simulated HRTEM image and the FFT pattern with simulated SADE pattern provides powerful evidence for the successful embedding of Mn3O4 nanoparticle into the electrospinning nanofiber [52]. The preparation processes of the double-layer membrane and the battery assembly flow chart as well as the action mechanism of the membrane were showed in Fig. 2. For the convenience of writing, the double-layer membrane based on F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane was abbreviated to F-Mn3O4-co-doped PMIA membrane. The morphology and structure characterizations of the F-doped and F-Mn3O4-co-doped PMIA membranes was investigated by SEM and TEM tests. Fig. 1(i~j) presented that some nanoparticles were uniformly dispersed on the surface and in the prepared F-Mn3O4-co-doped PMIA membrane. From Fig. S2, the F-Mn3O4-co-doped PMIA nanofibers with different proportion of Mn3O4 contents have a similar and small average fiber diameter when compared with pure PMIA nanofibers. The reasons of the fine fibers were mainly assigned to the following reasons: the fluoro chemical based emulsion uniformly distributed in the PMIA spinning solution can effectively put a brake on the entanglement of PMIA macromolecular chains because of the repulsive force of particles. So that the jet drafting became more adequate, leading to the decreased fiber diameter during electrospinning process [53-54]. Meanwhile, the additional inorganic Mn3O4 nanoparticles can improve the conductivity of spinning solution, which resulted in the availably enhanced charge density of jet and facilitated the jet splitting to form thin nanofibers. The TEM image (Fig. 1(k)) and elemental mappings by energy-filtered TEM (EFTEM) (Fig. 1(l~o)) with Fig. S3 all demonstrate the coexistence and uniform distribution of C, O, Mn and F elements in the prepared fiber, suggesting that the F-doped emulsion and Mn3O4 were uniformly dispersed in the prepared F-Mn3O4-co-doped PMIA membrane [55].

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Fig.1 Morphology and structure characterizations of the as-synthesized Mn3O4 and F-Mn3O4-co-doped PMIA membrane: The XRD spectra (a); TEM image (b); HRTEM images (c~d); The enlarged HRTEM image (e) of the rectangular area in simulated HRTEM image (f) (defocus, 15 nm; thickness, 10 nm) of Mn3O4 along the [3-1-1] crystallographic axis direction; The FFT pattern of Mn3O4 (g) in the simulated SADE pattern (h) with electron beam direction parallel to the [3-1-1] zone; SEM image (i) and TEM images (j, k) of nanofiber F-3.5% Mn3O4-co-doped PMIA membrane and corresponding STEM-EDS elemental mapping for l) Carbon, m) Oxygen, n) Manganese and o) Fluorine.

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Fig. 2 (a) The preparation process of the composite separator based on F-Mn3O4-co-doped PMIA membranes and battery assembly flow chart; (b) The action mechanism of Mn3O4 suppressing the “shuttle effect” of lithium polysulfide and strong catalytic conversion. The contact angle measurement is applied to assess the wettability of membranes to the liquid electrolyte (Fig. 3(a)). The liquid electrolyte wettability of F-doped membrane increased drastically with the addition of F contents through the changed contact angle from 43° to 9° (the value of Celgard 2340 was about 68°). Furthermore, the liquid electrolyte wettability of the F-Mn3O4-co-doped PMIA membrane further increased when compared with that of the Fdoped PMIA membrane, which suggested that the liquid electrolyte wettability of the prepared membrane was greatly improved when F and Mn3O4 were co-doped in the PMIA membrane [56]. The obtained results could be ascribed to the addition of F in electrospun membrane, which lowers the average diameter of the fibers and increases the specific surface area of the membrane [57-59]. More importantly, the Mn3O4 nanoparticles also can play a complexation role as a Lewis base with the liquid electrolytes especially in the cycle process, thus increasing the electrolyte wettability of the membrane [60-62]. Furthermore, the crystallinity degree of the PMIA membrane decreases after doping F and Mn3O4 as presented in Fig. S4, which is beneficial to further enhance the liquid electrolyte wettability of the F-Mn3O4-codoped PMIA membrane [63-64].

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Fig. 3 The photographs showing liquid electrolyte wetting behavior (contact angel (a)) of (1) commercial Celgard separator (2) PMIA membrane, (3) F-doped PMIA membrane, (4) F-2.5% Mn3O4-co-doped PMIA membrane, (5) F-3% Mn3O4-co-doped PMIA membrane and (6) F-3.5% Mn3O4-co-doped PMIA membrane; (b) Stress-strain curves of PMIA nanofiber membranes with different mass of Mn3O4; The atomic force microscope (AFM) images of (c) pristine Celgard 2340 membrane, (d) PMIA membrane, (e) F-doped PMIA membrane, (f) F-2.5% Mn3O4-co-doped PMIA membrane, (g) F-3% Mn3O4-co-doped PMIA membrane and (h) F-3.5% Mn3O4-co-doped PMIA membrane. As everyone knows that mechanical performance of electrospinning nanofibrous separator is a rather vital component for the cycle stability of various batteries [65]. The typical stress-strain curves of the PMIA membrane, the F-doped PMIA membrane and the F-Mn3O4-co-doped PMIA membrane with various Mn3O4 contents are presented in Fig. 3(b). After the addition of Mn3O4 nanoparticles, the strength of F-doped PMIA nanofiber increased significantly, 8

making the mechanical performance of nanofiber membrane much stronger. The breaking strength of F-doped PMIA membrane (8.607 MPa) was lower than that of F-2.5% Mn3O4-co-doped PMIA membrane (9.48 MPa). With the increase of Mn3O4 content from 2.5% to 3.5%, the tensile stress of the membrane enhanced from 9.48 MPa to 10.24 MPa. The improved mechanical properties are mainly attributed to the enhancement of interface between polymer and F/Mn3O4 and the increased surface friction between these fibers due to the adhesion of Mn3O4 nanoparticles [66]. Although, the tensile stress of F-PMIA and F-Mn3O4-co-doped PMIA membrane were slightly lower than that of Celgard 2340 membranes (Fig. S5), their stress all can meet the requirement of commercialized application. Meanwhile, Fig. 3(c~h) presents the real three-dimensional (3D) atomic force microscope (AFM) images of the membranes. The surface morphology changed on microscopic level after doping F and Mn3O4 and the roughness of F-doped PMIA and PMIA membrane increased significantly when comparing with that of the pristine PMIA membrane. The root means square roughness of the surface to pristine Celgard 2340, PMIA, F-doped PMIA, F-2.5% Mn3O4-co-doped PMIA, F-3% Mn3O4-co-doped PMIA and F-3.5% Mn3O4-co-doped PMIA membrane are 68, 95, 107, 121, 133 and 152 nm. This rough surface on microscopic level is extremely beneficial to improve the liquid electrolyte wettability of the membranes. Through the above measurements and comparison of membranes with different mass ratio of Mn3O4 contents, the F-3.5% Mn3O4-co-doped PMIA membrane was found to be optimal due to the fine and uniform distribution fiber diameter, outstanding mechanical properties, and excellent liquid electrolyte wettability. Fig.4 shows SEM images of PMIA (a), F-doped PMIA (b) and F-3.5% Mn3O4-doped PMIA (c) membrane after being immersed in the liquid electrolyte. It was obvious that the gelatinizing phenomenon occurred after F-doped PMIA membrane and F-3.5% Mn3O4-co-doped PMIA membrane were immersed in the electrolyte. And the pure PMIA membrane did not form the gelation and much lithium salt originating from electrolyte unevenly aggregated on the surface of the membrane. The colors of pristine F-doped PMIA membrane and F-3.5% Mn3O4-co-doped PMIA membrane were white and yellow, respectively, and they showed a jelly-like surface morphology different from pure PMIA membrane after soaking in liquid electrolyte, as shown in Fig. 4(e). The phenomenon shows the actual condition of the prepared membrane applied in the Li-S batteries during the charge-discharge process. And the fibers morphology in F-doped PMIA membrane and F-3% Mn3O4-co-doped PMIA membrane maintained the original surfaces when compared with the PMIA membrane from Fig. 4(a~c). The mechanism of the gelation can be explained according to the related theoretical researches of Saito’s group and there were two processes in the gel formation [67]: Firstly, the “incursion” of solution appeared from the outside to the holes of polymeric substance, and then the continuous solution soaked into the holes in the polymer chains to form a swollen gel; Secondly, the remarkable complexation between the fluorinated polymer and lithium salts in liquid electrolyte due to Lewis acid-base binding effect and the reduced 9

crystallinity because of the addition of the prepared Mn3O4 nanoparticles can improve the electrolyte adsorption and retention capacity, which was considerably conducive to form the gel formation for the membrane. The performances of absorbing and preserving liquid electrolyte were measured through evaluating the mass change of separator before and after soaking in the electrolyte. As shown in Fig. 4(d), the performance of the absorbing and preserving liquid electrolyte of F-doped PMIA membrane were significantly improved when compared to that of the Celgard 2340 membrane and pure PMIA membrane. And the addition of Mn3O4 nanoparticles made the ability of absorbing and preserving liquid electrolyte of membrane further enhanced. The increased electrolyte absorbing and preserving capability was mainly attributed to the low pore size, high surface area and strong Lewis base with the electrolytes. The obtained polymer with C-F chemical bonds including low surface energy made membrane excellent affined with electrolyte solutions [68]. On the one hand, the crystallinity of PMIA membrane decreased with adding the commercial fluoro-chemical based emulsion and Mn3O4 nanoparticles. Therefore, a great deal of electrolyte can be trapped into the amorphous region for these modified membranes. Furthermore, the interaction between membranes and electrolyte including lone pair electrons of F in the membrane can make coordination effect with Li ions in the electrolyte. Meanwhile, some functional groups in PMIA membrane such as O- or N- can promote the adsorption of electrolyte [69]. As well-known that the ionic conductivity can be enhanced when the ability of the electrolyte absorbing and preserving is outstanding. The excellent ionic conductivity is remarkably conducive to reduce the resistance and improve the electrochemical properties of the Li-S battery.

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Fig. 4 The SEM images of (a) PMIA membrane, (b) F-doped PMIA membrane and (c) F-3.5% Mn3O4-doped PMIA membrane after immersion in liquid electrolyte; (d) the liquid electrolyte retention capacity curves of different separators; (e) Photos of (e1) PMIA membrane, (e2) F-doped PMIA membrane and (e3) F-3.5% Mn3O4-doped PMIA membrane after immersion in liquid electrolyte.

The nitrogen (N2) adsorption-desorption isotherms and pore-size distribution curves of the prepared membranes are derived from the Brunauer-Emmett-Teller (BET) measurements as presented in Fig. 5. Fig.5 (a) presents the effect of fluoro-chemical based emulsion and Mn3O4 nanoparticles on the pore size distribution of PMIA nanofiber membranes. It was observed that the average aperture size of F-doped membrane decreases from 1.07 to 0.94 μm when compared with pure PMIA membrane. Furthermore, the average aperture size of F-Mn3O4-co-doped PMIA membrane 11

further reduced from 0.94 to 0.45 μm. The reduced average aperture size was advantageous to suppress the “shuttle effect” of lithium polysulfides and inhibit lithium dendrite during the charge-discharge process [70]. Fig. 5(b~d) illustrates the nitrogen adsorption-desorption isotherms and the pore size distribution for the pure PMIA membrane, Fdoped PMIA membrane and F-Mn3O4-co-doped PMIA membrane, respectively. Based on the principle about the International Union of Pure and Applied Chemistry (IUPAC) standards, the nitrogen gas isotherm of the prepared membrane (Fig.5 (insets)) showed a combination of Type I and Type IV isotherms, which meant that these membranes had a hierarchical porous structure. The pore size distributions of these samples showed that the size ranges of pores were mainly mesopore and micropore for these prepared membranes. The surface area and pore volume of the FMn3O4-co-doped PMIA membrane can reach as high as 31.792 m2 g-1 and 0.076 cm3 g-1, respectively. The surface area and pore volume of the PMIA membrane (7.429 m2 g-1 and 0.013 cm3 g-1) and F-doped PMIA membrane (22.159 m2 g-1 and 0.056 cm3 g-1) were smaller than that of the F-Mn3O4-co-doped PMIA membrane. In contrast, the pristine Celgard 2340 possessed slit nanopores with size of around several-hundred nanometers to allow the facile flux of electrolyte and lithium ions (Fig. S6(a)). However, it is difficult to hinder the “shuttle effect” of PS for the Celgard 2340 separator during the charge-discharge process. Fig. 5(e) shows the photographs of the F-Mn3O4-co-doped PMIA membrane, F-doped PMIA and pristine Celgard 2340 membranes after the thermal treatment at 80°C, 120°C, 160°C, 200°C and 240°C for 2 h. It is seen that the FMn3O4-co-doped PMIA membrane and F-doped PMIA membrane exhibited slight dimension shrinkage resistance as compared to pristine Celgard 2340 membrane by visual comparison. The excellent performance was mainly attributed to superior thermal resistance of PMIA itself as shown in Fig. S7. Meanwhile, the prepared the inorganic Mn3O4.nanoparticles can provide with the common properties of inorganic nanoparticles such as high thermal stability and outstanding resistance of thermal shrinkage [71].

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Fig. 5 (a) The average aperture size of PMIA membrane, F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane; Nitrogen adsorption-desorption isotherms of (b) PMIA membrane, (c) F-doped PMIA membrane and (d) FMn3O4-co-doped PMIA membrane (those corresponding insets are their pore size distributions); (e) the photographs of the composite separator based on the F-Mn3O4-co-doped PMIA membrane (e1), F-doped PMIA (e2) and pristine Celgard 2340 membranes (e3) after thermal treatment at 80°C, 120°C, 160°C, 200°C and 240°C for 2 h 13

The bulk resistances (R0) of the polymer electrolyte are shown in Fig. S9. The ionic conductivity values for polymer electrolytes with different separators were measured at room temperature by the AC impedance method. The bulk resistance, thickness, and ionic conductivity of the polymer electrolytes were listed in Table S1. The ionic conductivity of F-Mn3O4-co-doped PMIA membrane (2.19×10-3 S cm-1) was higher than that of F-doped PMIA membrane (1.59×10-3 S cm-1) and PMIA membrane (1.28×10-3 S cm-1), which were all higher than commercial Celgard separator (0.4×10-3 S cm-1) as shown in Fig. S6(b) for the practical application [72]. The reasons of increased conductivity were attributed to the higher porosity (Fig.S8) and the presence of C-F chemical bonds with low surface energy in the polymer membrane, which can endow the membrane excellent affinity with electrolyte solutions. Another reason was that more amorphous region in the nanofiber membrane can be obtained when the commercial fluorochemical based emulsion and Mn3O4 nanoparticles were added into the PMIA solution, which was beneficial to improve the absorption performance of liquid electrolyte. The typical Nyquist plots of the membranes before and after the charge-discharge cycle were shown in Fig. 6(a~b). The semicircle corresponds to the contact resistance and charge-transfer resistance, and the inclined line is ascribed to ion diffusion with these membranes. The F-doped PMIA and F-Mn3O4-co-doped PMIA membranes both presented a smaller semicircular diameter than that of the PMIA membrane, which illustrated that they exhibited a lower chargetransfer resistance than that of PMIA membrane. The obtained results reflected that the improved interfacial properties were obtained among the electrode, separator and the electrolyte due to the addition of fluoro-chemical based emulsion and Mn3O4 nanoparticles. From Fig. 6(a), the Warburg impedances of the F-Mn3O4-co-doped PMIA membrane and Fdoped PMIA membrane were smaller than that of the PMIA membrane, which was favorable to the transportation of lithium ions. The reduced Warburg impedance could be ascribed to the greatly improved liquid electrolyte wettability of membrane after doping F in the PMIA membrane. The charge-transfer resistance was fitted with a typical EIS equivalent circuit, as presented in Fig. 6(a) (inset). The charge-transfer resistance (Rct) values of the F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane were 120.56 Ω and 159.91 Ω, respectively, which were much lower than that of the PMIA membrane (269.56 Ω). Therefore, the F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane both had better reversible electrochemical properties than the PMIA membrane. The reduced chargetransfer resistance can be ascribed to the C-F chemical bonds with low surface energy in the polymer membrane, which endowed a good affinity with electrolyte solutions in the battery. In addition, the formation of the gel was also extremely beneficial to the transportation of ions [73]. The impedance spectroscopy was changed and there were two obvious semicircles in Fig. 6(b) after 600 cycles for these batteries. The increased semicircle in the high-frequency region was ascribed to the formation of Li2S and Li2S2 in 14

the cycle processes of the batteries. From Fig. 6(b), after 600 cycles, these membranes exhibited obviously decreased resistances when compared with the fresh cells. The reduced impedance was attributed to the rearrangement of the physically stable active material to occupy more electrochemically favorable positions [74]. It implied the closer contact and better coverage among the active material, separator and collector after 600 cycles. It was also obvious that the formed insoluble polysulfide species resistance in the F-doped PMIA membrane (81.03 Ω) were lower than those of the PMIA membrane (97.52 Ω) after 600 cycles from the Fig. Fig. 6(b) (inset). It indicated the excellent conductivity and outstanding reversible electrochemical performances for the F-doped PMIA membrane can be obtained in the Li-S batteries. Moreover, the addition of Mn3O4 in the membrane made the formed insoluble polysulfide species resistance of the F-Mn3O4-co-doped PMIA membrane further decreased to 51.76 Ω after 600 cycles. These results indicated that Mn3O4 was superior host or separator modification materials for the lithium-polysulfides reservoir. Furthermore, the impedance spectra results can coordinate well with the outstanding electrochemical performances of the high reversible specific capacity [75]. Fig. 6(c~e) presents the CV curves of the F-Mn3O4-co-doped PMIA membrane, F-doped PMIA membrane and PMIA membrane at various scan rates. All the assembled cells with the three membranes showed two representative cathodic peaks at ~2.00 and ~2.30 V and one anodic peak was observed at ~2.42 V in the initial charge-discharge potential profiles. The cathodic peaks correspond to formation of long-chain polysulfides (Li2Sn, 3≤n≤8) by S8 reduction and lithium sulfide (Li2S2 or Li2S) by short-chain polysulfide reduction. Whereas anodic peak was observed which exhibited the oxidation reaction of Li2S and Li2S2 to final oxidation products of elemental sulfur. In addition, the reduction peak current of F-Mn3O4-co-doped PMIA membrane was also higher than that of PMIA membrane and Fdoped PMIA membrane, indicating that S8 deposited on the F-Mn3O4-co-doped PMIA membrane was easily reduced to lithium sulfide. It is attributed to the gel polymer electrolyte based as matrix with the F-Mn3O4-co-doped PMIA membrane can provide more ions pathways, strong e catalytic effect to polysulfide of Mn3O4, improved polysulfide redox kinetics and enhanced electrochemically active sites for the Li-S battery. Therefore, the “shuttle effects” of polysulfide can be suppressed by both the physical and chemical trapping of polysulfides through F and Mn3O4 codoping, which can provide more electrons to the insulating discharge products. And, as shown in the cyclic voltammograms curves of the PMIA, F-doped PMIA and F-Mn3O4-co-doped PMIA membrane, two discharge plateaus at ~2.02 V and ~2.28 V can be observed, corresponding to the reduction of sulfur to polysulfides and then further to Li2S2/Li2S, respectively. The charge plateau at about 2.42 V is related to the oxidation of Li2S2/Li2S to lithium polysulfides and elemental sulfur. Meanwhile, Mn3O4 nanoparticles were possible to promote the effective conversion of polysulfide to active substances in the charge of battery. More importantly, the CV test was applied to investigate 15

electrode kinetics with respect to the lithium ion diffusion coefficient. The cathodic and anodic current peaks (IA, IB, IC) of all of the cells using these membranes have a linear relationship with the square root of scanning rates (Fig. 6(f~h)), indicative of the diffusion-limited process. Thus, the Randles-Sevcik equation can be used to describe the lithium diffusion process:

I p  (2.69 105 )  n1.5  S  DLi 0.5  CLi  v 0.5 (where IP is the peak current, n is the charge transfer

number (n=2 for Li-S cell), S is the geometric area of the active electrode, DLi+ is the lithium ion diffusion coefficient, CLi+ is the concentration of lithium ions in the cathode, and v is the potential scan rate). The slope of the curve (Ip/v0.5) represents the lithium ion diffusion rate when n, S, and CLi+ are unchanged. We can obviously seen that the cell with PMIA membrane presented the lowest lithium ion diffusivity, which mainly arises from the poor polysulfide adsorption and Li2S catalyzing conversion capability or deposition of a thick insulating layer on the electrodes. However, the cell using the F-doped PMIA membrane especially the F-Mn3O4-co-doped PMIA membrane presented much faster diffusion and better reaction kinetics when compared with the PMIA membrane, which indicate that the prepared FMn3O4-co-doped PMIA membrane enables highly efficient catalyzing conversion of sulfur redox during the process of charge-discharge [76-77]. Meanwhile, it was clear that the voltage profile showed the differences among the three membranes. The serious polarization phenomenon can be observed in the charge-discharge profiles of commercial Celgard separator, the PMIA membrane and the F-doped PMIA membrane due to their weak ability to suppress the shuttle effect of polysulfides as presented in Fig. S10. However, the double-layer composite membrane based on the F-doped PMIA membrane and FMn3O4-co-doped PMIA membrane showed the less differences between discharge plateaus and charge plateaus, indicating much less severe polarization. The result was also well confirmed by the much lower charge transfer resistance observed in impedance spectra of F-Mn3O4-co-doped PMIA membrane when compared to that of pristine PMIA membrane and only F-doped PMIA membrane, which indicated that the utilization rate of active substances can been significantly enhanced and the excellent kinetics of redox reaction during the charge-discharge process can be obtained. The lithiation voltage curves (Fig. S11) for the battery with F-Mn3O4-co-doped PMIA membrane showed a smaller plateau at about 2.30 V and a larger one at ~2.00 V, which have been assigned to the reduction of elemental sulfur to high molecular weight polysulfides Li2Sn (4≤n≤8) and the formation of Li2S2/Li2S from soluble polysulfide species, respectively. The delithiation voltage curves presented charge plateau at about 2.42 V corresponding to the oxidation of Li2S2/Li2S to lithium polysulfides and sulfur. The lithiation and delithiation processes were consistent with the above CV curve. And with the battery cycling, the charge-discharge plateau and discharge capacity were more and more stable in the 1000th cycles than the starting 200th cycle.

16

Fig. 6 The Nyquist plots of PMIA membrane, F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane before charge-discharge cycle (a) and after 600 cycles (b) within the frequency range of 100 kHz to 100 mHz, the equivalent circuit for the electrode interface (inset); The CV curves of the F-Mn3O4-co-doped PMIA membrane (c), Fdoped PMIA membrane (d) and PMIA membrane (e) at various scan rates; The anodic oxidation process (IA: Li2S2/Li2S→S8) (f), the second cathodic reduction process (IB: Li2Sx→Li2S2/Li2S) (g), the first cathodic reduction process (IB: Li2Sx→Li2S2/Li2S) (h) versus the square root of the scan rates. 17

Fig. 7(a) exhibited that the batteries with pure PMIA, F-doped PMIA and F-Mn3O4-co-doped PMIA membrane delivered an initial discharge capacity of 1220, 1231.5 and 1237.1 mAh g-1, respectively. The discharge capacity of the PMIA, F-doped PMIA and F-Mn3O4-co-doped PMIA membranes was retained at 305.6, 633.1 and 814.0 mAh g-1 after 1000 cycles with the Coulombic efficiency of 98.21%, 98. 64% and 98.83%, respectively. These results demonstrated that the discharge capacity of the battery with F-Mn3O4-co-doped PMIA membrane was considerably higher and more stable than that of the pure PMIA membrane and F-doped PMIA membrane. The more stable voltage plateaus and smaller capacity loss indicated a strong adsorption or captured ability of F-Mn3O4-co-doped PMIA membrane to inhibit PS diffusion when compared with the pure PMIA and F-doped PMIA membrane [78]. Meanwhile, Fig. S12~ Fig. S14 presented the the XPS, SEM and Raman test results of the cycled battery separators after cycling, their detailed introductions in the Supporting Information. All of results illustrated that a better electrochemical performance of the Li-S batteries can be obtained through using the F-Mn3O4-co-doped PMIA separator in the Li-S cell. To demonstrate the rate capabilities of these batteries assembled with the three membranes, the testing current density was changed from 0.5 to 2 C rates, as shown in Fig. 7(b). The average discharge capacities for the F-Mn3O4-codoped PMIA membrane at 0.5 C, 1 C, 2 C, 1 C and 0.5 C rate were 1083.11, 893.35, 794.90, 883.42 and 1049.51 mAh g-1, respectively. These results indicated the battery with the F-Mn3O4-co-doped PMIA membrane had a high chargedischarge capability, which was significantly higher than PMIA membrane and F-doped PMIA membrane. Furthermore, the battery assembled with the F-Mn3O4-co-doped PMIA membrane also demonstrated a superior cycling performance with the capacity retention ratio of 61.9% after 600 cycles even at high current rates of 2 C rate. The improved electrochemical properties can be attributed to the relatively large specific surface, high pore volume, broadly and uniformly porous distribution and low aperture size of the prepared membrane, which can effectively inhibit polysulfide diffusion. It further confirms that the presence of F and Mn3O4 helps the electrochemical oxidation of Li2S and the stable formation of the protecting film on the electrode surface, which can effectively suppress the migration of soluble polysulfide to the Li anode surface by chemical action [79].

18

Fig. 7 Battery cycling performance with (a) PMIA membrane, F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane at rate of 0.5 C; (b) The battery rate capability of the F-Mn3O4-co-doped PMIA, F-doped PMIA and PMIA membrane; (c) F-Mn3O4-co-doped PMIA membrane at rate of 2 C rate.

19

The H-type visible cell was also employed to study the immobilization of polysulfides by the F-Mn3O4-co-doped PMIA membrane. As shown in Fig. 8, one side of the cell was filled with the blank electrolyte and the other side was the electrolyte with 2 M Li2S6. Driven by the concentration gradient, the Li2S6 would diffuse from one side to the other side across the separator. In Fig. 8(a~d), the H-type cell was split by as-prepared F-Mn3O4-co-doped PMIA membrane. Clearly, no obvious color change in the left chamber can be observed even after standing for 48 h, which suggested that the F-Mn3O4-co-doped PMIA membrane could retard the polysulfides effectively from diffusing away. For comparison, the same measurement was carried out with common separator (Celgard 2340). The two sides separated with commercial Celgard separator almost had the similar yellow color after 48 h in the H-type cell shown in Fig. 8(e~h), implying the concentration balance of polysulfides between two sides. Compared with various modified separators for Li-S batteries, the batteries with the F-Mn3O4-co-doped PMIA membrane had entered a class of high initial discharge capacity and super-stable-capacity as shown in Table 1. The excellent electrochemical performances for the batteries with the separator can also be explained with the below reasons.

Fig. 8 Photographs recording the diffusion process across the separator. a~d, cell with the F-Mn3O4-co-doped PMIA membrane as separator and e~h, cell with commercial Celgard separator for different hours.

20

Table 1 the electrochemical performances of various membranes as the separators of Li-S batteries Various membranes content AB/MWCNTs modified Celgard separator MWCNT/NCQD-coated Celgard separator MoS2/PDDA hybrid in conjunction with PAA coated Celgard separator PAN-grafted PP separator Amine porous organic polymer/acetylene black-polypropylene PP/PMMA separator Nano-TiO2 decorated carbon coated Celgard separator Ni3(HITP)2-modified Celgard separator Ion-selective prussian-blue-modified Celgard separator Vertical Co9S8 hollow nanowall arrays grown on a Celgard separator MoO3 nanobelt catalytic layer coated Celgard separator MoS2/Celgard composite separator Metal-organic framework-based separator Co-N-C/rGO layer coated Celgard separator

Discharge current rate (Testing Condition)

Initial discharge capacity (mAh g-1)

Cycle numbers

Capacity fading rate per cycle (%)

Ref. [7]

Date

1C

1500

200

0.280

2018

0.5 C

1330.8

1000

0.050

[8]

2018

0.5 C

~1050

100

~0.232

[9]

2019

0.2 C

[10]

2019

~1600

300

0.165

cm–2

~1322

800

0.041

0.1 mA cm–2

1100.1

100

0.442

[12]

2019

0.2 C

1227

180

0.155

[13]

2018

0.5 C

1021.3

300

0.142

[14]

2019

0.2 C

1190

100

~0.285

[15]

2018

1C

986

1000

0.039

[16]

2018

0.5 C

1377

200

0.251

[17]

2018

0.5 C.

808

600

0.083

[18]

2017

0.5 C

1126

500

0.06

[19]

2016

0.5 C

865

500

0.058

[20]

2018

0.2 C

1031

200

0.120

[21]

2018

1.5 A g−1

901

500

0.016

[22]

2018

PVDF/PSSLi composite membrane

0.5 C

955

200

0.266

[23]

2019

Understanding glass fiber membrane A few-layered Ti3C2 nanosheet/glass fiber composite separator MXene debris modified eggshell membrane Carbon-coated glass fiber (GF) membrane GPE based on PVdF-HFP GPE based on PETEA crosslinked with divinyladipate (Ester) P (VDF-HFP)/PVP polymer membrane

0.2 C

1033

100

0.410

[24]

2016

0.3 C

820

100

0.120

[25]

2016

0.5 C

1185

250

0.104

[26]

2018

2C

1004

200

0.19

[27]

2016

0.1 C

895

100

0.056

[31]

2018

0.5 C

~610

300

0.100

[33]

2018

0.2 C

1188

50

0.420

[34]

2019

1C

790

400

0.049

[36]

2018

0.5 C

811.3

200

0.080

[37]

2019

0.2 C

1322

200

0.256

[38]

2017

0.2 C

987

100

0.395

[39]

2016

0.3 C

986.3

500

0.040

[40]

2016

0.1 C

1175

100

0.280

[41]

2019

0.2 C 0.5 C 2C

1322 1237.1 1009.5

100 1000 600

0.283 0.034 0.063

[42]

2018

PAA separator PVA-based separator modified with lithium sulfonate/carboxylate

Electrospun PAN@APP membrane Electrospun PI nanofiber membrane Double-layer rGO–PVDF/PVDF composite nanofiber membrane Electrospun PAN/GO nanofiber membrane separator PETEA-based GPE into a PMMA-based electrospun network Non-shrinkable, porous PAN/PES electrospun membrane Cellulose-based porous membrane Composite separator based on F-doped and F-Mn3O4-co-doped PMIA

0.2 mA

21

[11]

2019

This work

The action mechanism of fast ion transport channel as well as the physical blocking and chemical interactions between lithium polysulfides and the prepared membrane were shown in Fig. 9. As presented in Fig. 9(a~b), the FMn3O4-co-doped PMIA membrane can obtain fast ion transport channel when compared with the pure PMIA membrane. The unexceptionable ion transport of the as-fabricated F-Mn3O4-co-doped PMIA membrane was mainly attributed to the following reasons: Firstly, the excellent liquid electrolyte affinity originated from some functional groups (O- or N-) in the membrane can effectively improve the diffusion of lithium ions. Secondly, the finer fiber diameter, higher porosity and specific surface area of the as-fabricated F-Mn3O4-co-doped PMIA membrane were favourable to provide more site for the transportation of lithium ions. Thirdly, the crystallinity degree of the PMIA membrane decreased after doping F and Mn3O4, which was beneficial to further enhance the liquid electrolyte wettability of the F-Mn3O4-codoped PMIA membrane. As is well known, the high electrolyte absorbing and preserving ability tended to have fast ion transport pathway and high ionic conductivity. Meanwhile, the reasons for suppressing the “shuttle effect” mainly included that the average aperture size in the certain volume decreased when the average diameter of the fiber decreased, which can be regarded as a barrier to constrain PS within the cathode region. In addition, the nitrogen, oxygen and fluorine groups especially oxygen in ketone groups in the F-doped PMIA membrane showed strong binding energies to polysulfide (Li2S4, Li2S6 and Li2S8) as shown in Fig. 9(c~d), which was favorable to trap the soluble intermediates retarding the shuttle of polysulfides. More importantly, the doped Mn3O4 nanoparticles doping contributed to mitigate the shuttling problem by reacting with initially formed polysulfides to form surface-bound intermediates and provide strong catalytic conversions of lithium polysulfides. Therefore, the unique novel thermostable double-layer membrane based on F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane rendered the excellent electrochemical properties in a working battery.

22

Fig.9 The action mechanism of (a) discontinuous and slow ion transport pathway for pure PMIA membrane, (b) continuous and fast ion transport channel, and (c~d) physical blocking and chemical interactions between lithium polysulfides and the prepared membrane. ■ CONCLUSIONS In conclusion, we successfully developed a novel thermostable double-layer membrane based on F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane for Li-S batteries by the electrospinning technique. The F-doped PMIA membrane, in combination with F-Mn3O4-codoped PMIA membrane, endowed the functional separator with extraordinary high electrolyte uptake, preserving liquid electrolyte, thermal stability and forcefully physical and chemical adsorption to polysulfides. In addition, the F-doped PMIA membrane can be acted as a matrix to prepare gel polymer electrolyte. Therefore, the Li-S batteries assembled with the membrane exhibited a high first-cycle discharge capacity with 1237.1 mAh g-1, excellent cycling stability with outstanding capacity retention of 814.0 mAh g-1 and coulombic efficiency of 98.83% after 1000 cycles at the current density of 0.5 C rate. Furthermore, the battery assembled with the membrane also demonstrated superior cycling performances with the capacity retention ratio of 61.9% after 600 cycles even at high current rates of 2 C rate. This was ascribed to the suppressed “shuttle effect” of 23

polysulfides through both the physical trapping of polysulfides by the gel polymer electrolyte based on the F-doped PMIA membrane and chemical binding of intermediates on F and Mn3O4 in a working cell. Therefore, the thermostable double-layer membrane based on F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane with excellent electrochemical performance and high safety can be applied as a promising separator for next-generation high performance and safe Li-S battery. ■ ACKNOWLEDGMENTS The author would like to thank the National Natural Science Foundation of China (51673148, 51678411), China Postdoctoral Science Foundation Grant (2019M651047) and The Science and Technology Plans of Tianjin (No.17PTSYJC00040 and 18PTSYJC00180) for their financial support. ■ REFERENCES [1] H.-J. Peng, J.-Q. Huang, X.-B. Cheng, Q. Zhang, Review on high-loading and high-energy lithium-sulfur batteries, Adv. Energy Mate. 7 (2017) 1700260. [2] W. M. Kang, N. P. Deng, J. G. Ju, Q. X. Li, D. Y. Wu, X. M Ma. L. Li, M. Naebe, B. W. Cheng, A review of recent developments in rechargeable lithium-sulfur batteries, Nanoscale. 8 (2016) 16541-16588. [3] A. Fu, C. Z. Wang, F. Pei, J. Q. Cui, X. L. Fang, N. F. Zheng, Recent advances in hollow porous carbon materials for lithium-sulfur batteries, Small. 15 (2019) 1804786. [4] M. Rana, M. Li, X. Huang, B. Luo, I. Gentle, R. Knibbe, Recent advances in separators to mitigate technical challenges associated with re-chargeable lithium sulfur batteries, J. Mater. Chem. A. 7 (2019) 6596-6615. [5] X. D. Hong, J. Mei, L. Wen, Y. Y. Tong, A. J. Vasileff, L. Q. Wang, J. Liang, Z. Q. Sun, S. X. Dou, Nonlithium metal-sulfur batteries: steps toward a leap, Adv. Mater. 31 (2019) 1802822. [6] J.-H. Kim, Y.-H. Lee, S.-J. Cho, J.-G. Gwon, H.-J. Cho, M. Jang, S.-Y. Lee, S.-Y. Lee, Nanomat Li-S batteries based on all-fibrous cathode/separator assemblies and reinforced Li metal anodes: towards ultrahigh energy density and flexibility, Energy Environ. Sci. 12 (2019) 177-186. [7] W. Z. Tian, B. J. Xi, H. Z. Mao, J. H. Zhang, J. K. Feng, S. L. Xiong, Systematic exploration of the role of a modified layer on the separator in the electrochemistry of lithium-sulfur batteries, ACS Appl. Mater. Interfaces. 10 (2018) 30306-30313. [8] Y. Pang, J. S. Wei, Y. G. Wang, Y. Y. Xia, Synergetic protective effect of the ultralight MWCNTs/NCQDs modified separator for highly stable lithium-sulfur batteries, Adv. Energy Mater. 8 (2018) 1702288. [9] J. Y. Wu, H. X. Zeng, X. W. Li, X. Xiang, Y. G. Liao, Z. G. Xue, Y. S. Ye, X. L. Xie, Ultralight layer-by-layer selfassembled MoS2-Polymer modified separator for simultaneously trapping polysulfides and suppressing lithium dendrites, Adv. Energy Mater. 8 (2018) 1802430. 24

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HIGHLIGHTS

•A novel multifunctional double-layer membrane is successfully prepared. •Double-layer membrane contains F-doped PMIA and F-Mn3O4 -co-doped PMIA membrane.

•Double-layer membrane can suppress shuttle effect and produce catalytic conversion.

•The Li-S battery with the double-layer membrane shows stable long-term cycle performance.

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