Shuttle Suppression by Polymer-sealed Graphene-coated

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Shuttle Suppression by Polymer-sealed Graphene-coated Polypropylene Separator Xuewu Ou, Yanzi Yu, Ruizhe Wu, Abhishek Tyagi, Minghao Zhuang, Yao Ding, Irfan Haider Abidi, Heng-An Wu, Feng-Chao Wang, and Zhengtang Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17251 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Applied Materials & Interfaces

Shuttle Suppression by Polymer-sealed Graphene-coated Polypropylene Separator Xuewu Ou,1† Yanzi Yu,2† Ruizhe Wu,1 Abhishek Tyagi,1 Minghao Zhuang,1 Yao Ding,1 Irfan Haider Abidi,1 Hengan Wu,2 Fengchao Wang,*,2 Zhengtang Luo*,1 *Email: [email protected]; [email protected] †These authors contributed equally to this work.

1

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology,

Clear Water Bay, Kowloon, Hong Kong, China 2

CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics; CAS

Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui, China

Abstract “Shuttle effect” of lithium polysulfides (LiPS) leads to poor performance and short cycle life of Li-S battery, thus limiting their practical application. We demonstrate here that after coating polypropylene (PP) separator with a continuous monolayer graphene, the “shuttle effect” can be significantly suppressed by limiting the passage of long chain LiPS. The graphene/PP separator can be further modified by sealing the big holes or pores on graphene with in situ polymerized nylon-66, via an interfacial polymerization reaction between diamine and adipoyl chloride supplied by the aqueous and oil phase, respectively from each side of the membrane. With this engineered membrane, an initial specific capacity of 1128.4 mAh g-1 at 0.05 C is achieved after test in a coin cell, higher than that of 983.2 mAh g-1 with pristine PP, along with Coulombic efficiency increased from 96.0% to 99.9% and enhanced cycling durability. Molecular dynamics (MD) simulations attest that the nanopores with appropriate size and structure are effective in acting as a “sieve” to selectively allow only Li+ ions passing through but prevent LiPS from

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migrating to the anode, consequently alleviating the “shuttle effect”. Our method provides a facile solution towards mitigated “shuttle effect” and eventually contributes to high performance Li-S batteries. Keywords Shuttle effect, lithium polysulfides, graphene, interfacial polymerization, molecular simulation Introduction With the explosive development of vehicles and electronics, advanced energy storage devices are in increasing demand. However, because of its low power density and limited energy density, current lithium ion (Li-ion) battery can no longer meet future requirements. Among post Li-ion batteries, lithium sulfur (Li-S) battery, which utilizes sulfur as the cathode material, is one of the most promising candidates. Besides its large theoretical capacity (1675 mAh g-1), which is much higher than commercialized Li-ion battery (~160 mAh g-1),1 Li-S battery also possess the advantages such as resource abundance, low cost and environmental benignity offered by sulfur element. The research of Li-S battery can date back to the 1960s, but real breakthroughs were achieved only until recent years.2, 3 Despite these fundamental progress, full commercialization of Li-S battery still faces a few challenges, such as: (1) insulating property (10-30 S cm-1 at 25℃) and low ionic conductivity of sulfur;4 (2) severe dissolution and migration of lithium polysulfide intermediates ( , 4 ≤ ≤ 8 );5 (3) large electrode volume change (~80%) ascribed to density difference between S and Li2S.6 Among them, the migration of lithium polysulfides (LiPS) across separator during charge/discharge process, or “shuttle effect”, leads to sulfur loss and self-discharge problems, which consequently results in fast capacity fade and low


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Coulombic efficiency. Previously, apart from compositing carbon materials with sulfur to enhance the conductivity,2, 7, 8 in order to alleviate the “shuttle effect”, polar materials that have strong chemical affinity to LiPS have been adopted, including heteroatom (N, P, S) doped carbon materials, functional polymers, metal oxides/chalcogenides.9-12 In addition to cathode design, to mitigate the “shuttle effect”, an alternative approach is through conductive interlayer insertion or separator modification.13-16 Nevertheless, the requirements of high conductivity and large surface area for these additional layers are usually satisfied by their large thickness, which is unfavorable to Li+ ion transport and will also add extra weight to the battery. Graphene, a two dimensional (2D) material with atomic thickness, has attracted tremendous attention in electronics and energy storage since 2004.17 And graphene also exhibits great potential application in water desalination and ion sieving.18, 19 Inspired by these pioneering work, here we propose using monolayer graphene as an interlayer to coat a piece of Celgard trilayer polypropylene/polyethylene/polypropylene (PP/PE/PP) membrane (hereinafter referred to as PP membrane for convenience) and use it as separator in Li-S battery. Taking advantage of the significant size difference between Li+ ion and LiPS ( , 4 ≤ ≤ 8), we hypothesize that the nanopores with the appropriate size on graphene will be capable of selectively blocking LiPS while maintaining Li+ ion transport across the separator, thereby alleviating the “shuttle effect”.16, 20

With this graphene/PP membrane, we have observed significant improvement in both capacity and cycle life, and the performance can be boosted after mending the big pores or holes on graphene via interfacial polymerization method. We attribute this enhancement to the “blocking effect” of graphene nanopores, which acts as a “sieve” to impede the migration of dissolved LiPS across the separator. To clarify this mechanism, molecular dynamic simulations were carried out.



Our simulation results confirm that the selectivity of graphene is strongly dependent on its pore size, and we also reveal that the edge structure especially the charge characteristics plays vital role in ion/molecule translocation. Experimental Section Growth of continuous monolayer graphene. Copper foil used for graphene growth is purchased from Alfa Aesar (>99.8%, No. 13382). Before chemical vapor deposition (CVD) growth, copper foil was firstly polished in FeCl3/HCl etchant solution for ~10 s, and then rinsed with diluted hydrochloric acid (HCl) solution and deionized (DI) water for several times. Graphene was grown according to a well-established method.21 Briefly, load the treated copper foil into quartz tube and increase the temperature to 1050 °C within 30 mins under 350 sccm Ar and 15 sccm H2, and graphene will be gradually formed after introducing 15 sccm diluted methane (500 ppm methane diluted in argon). After 30 mins’ growth, continuous monolayer graphene was synthesized on copper foil. Graphene transfer onto PP membrane or transmission electron microscope (TEM) grid. The procedure of graphene transfer onto PP membrane (Celgard separator) was done according to a standard wet transfer process used in our group.22 After graphene growth, the side of copper foil covered by graphene was firstly spin coated a polymethylmethacrylate (PMMA) layer (MicroChem Corp. 495 PMMA A6) with thickness of ~300 nm (3000 r/min 40s), and then bake the copper foil on a hot plate at 180 ℃ for 2 mins to solidify PMMA layer. Subsequently, the copper foil was etched away in FeCl3/HCl (Sigma-Aldrich, 10 g FeCl3, 10 mL 37% HCl, and 200 mL DI water) etchant solution for 3 hours. After washing PMMA coated graphene with diluted HCl solution and DI water thoroughly to remove Fe3+ residues, a piece of


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commercialized PP membrane or TEM grid was used to scoop out the floating PMMA coated graphene, after which it was dried in the air. Finally, to ensure the totally removal the PMMA, the PMMA-coated graphene/PP was immersed in acetone (Sigma-Aldrich) at room temperature for ~10 minutes. Interfacial polymerization process. The interfacial polymerization process was performed in a Franz cell.19 The graphene coated PP was firstly placed and clamped into a Franz cell with the graphene side downward. Subsequently, the bottom part of the Franz cell was injected with 5 mg/ml hexamethylenediamine (HMDA) (Sigma-Aldrich) aqueous solution up to the membrane brim, while the topside was filled with 5 mg/ml adipoyl chloride (APC) (Sigma-Aldrich) dissolved in hexane. Kept for 2 h, the chamber at the upper part is then washed with hexane (Sigma-Aldrich) and ethanol (Sigma-Aldrich) for 3 times separately. The final membrane was obtained by rinsing with ethanol bath after being removed from the Franz cell. As control experiment, the interfacial polymerization was also conducted on the commercialized PP separator. Preparation of sulfur/carbon black (S/CB). 20 mg carbon black (Lizhiyuan battery Corp.) was dispersed into 30 ml N-Methyl-2-pyrrolidone (NMP, Sigma-Aldrich), together with 30 mg sulfur (Sigma-Aldrich) dissolution under sonication. Then 60 mL H2O was dropwise added with a speed of 6~10 drops per minute. After filtering and washing with DI water, S/CB was collected and dried at 60 ℃ overnight. Electrode preparation and electrochemical measurement. The electrode was prepared by mixing 80 wt% S/CB with 10 wt% carbon black and 10 wt% polyvinylidene fluoride (PVDF) (Lizhiyuan battery Corp.). The resulting slurry mixture was then coated onto aluminum foil



surface with a doctor blade as cathode. The loading of sulfur was ~1 mg/cm2. Then the cathode was assembled into 2025 type coin cell with lithium metal as anode, and 1 M lithium bistrifluoromethanesulfonylimide (LiTFSI) in 1:1 (v/v) mixture solution of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with 1% LiNO3 as electrolyte (Duoduo chemicals Corp.). The electrochemical test was carried out on an electrochemical workstation (CHI 660 and LAND battery test system). Galvanostatic charge/discharge test was conducted at the range from 1.8 V to 2.8 V and impedance spectroscopy was tested at the range from 100 kHz to 10 mHz with amplitude of 10 mV. Crystal structure and morphology characterization. All the characterization tests are done in Materials Characterization and Preparation Facility (MCPF) in HKUST. The Raman (Renishaw Raman RM3000 scope) spectrum was tested using a 514 nm excitation argon laser, and X-ray photoelectron spectroscopy (XPS, PHI 5600) was used to analyze chemical composition and elemental valence states of nylon-66. Surface morphology of PP and graphene-coated PP were characterized by scanning electron microscope (SEM, JEOL 7100), and the graphene quality was checked using TEM (JEOL 2010). The sulfur mass ratio was measured with thermal gravimetric analysis (TGA, SDT Q600). The O2 plasma cleaner (Harrick Plasma PDC-002) was used to treated graphene on TEM copper grid to introduce pores. Molecular simulations. To explore the transporting mechanism of Li2Sn(n = 4, 6, 8) in electrolyte solution through defect areas of graphene, we first set up a model of 1,2dimethoxyethane (DME) solution at both sides of graphene,23-25 and graphene with different number of carbon atoms defects were created. The charges added on lithium atoms, and sulfur atoms (connected with lithium atom) are e and -e. The charges of carbon atoms around the defective pores are 0e, +0.115e, and -0.2e,18 respectively, which corresponds to no-charge and


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two types of functional groups (-H and -OH) modified graphene nanopores. In practical case, these chemical modified nanopores can be realized by various methods such as plasma treatment and local oxidation process similar to graphene functionalization on edges.26,

27

Except the

carbon atoms around the defective hole, other carbon atoms of graphene are electroneutral. The charges of carbon atom, oxygen atom, and hydrogen atom are 0.28e, -0.56e and 0e.24 The number of C4H10O2 molecules was 75 at each side area of graphene, and a Li+ ion or Li2Sn (n = 4, 6, 8) molecule was placed in the left area before simulation. The long-range electrostatic interactions were computed using the particle-particle particle-mesh (PPPM) algorithm,28 with a convergence parameter of 10-4. The cutoff distance for these Lennard-Jones (LJ) interactions was set to be 10.0 Å. The bond and angle parameters of the molecules (graphene, C4H10O2 and Li2Sn) are obtained from OPLS-AA force field.29 The bond and angle parameters of Li and S are referred to the literature.25 The model was placed into a simulation box of 24.2 Å×22.2 Å×52 Å. The periodic boundary conditions were applied in the X and Y directions. All the simulations were performed in the NVT (constant number of molecules, volume, and temperature) ensemble. The temperature was set to 298 K using a Nosé-Hoover thermostat. During the simulation, the graphene was fixed, and a Li-ion or Li2Sn molecule (n = 4, 6, 8) was dragged along the Z direction of the model from the left side to the right side through the defective area of graphene, where Z=0 is the position of graphene, and the energy barriers for Li+ ion and Li2Sn molecules across the nanopores are calculated. More simulation details can be referred to the supplementary information.

Results and Discussion Fig. 1a elaborates the steps for the preparation of graphene/PP membrane. Firstly, high quality continuous monolayer graphene was synthesized on copper by a well-established CVD method



that routinely used in our laboratory.21 Secondly, a layer of PMMA was spin coated atop graphene, followed by etching the copper foil with 1:1 FeCl3/HCl mixture solution for 3 h. After washing with diluted HCl solution and DI water thoroughly for 3 times to remove Fe3+ residues, graphene was then loaded onto PP surface and dried in air overnight, in order to completely remove the water at the interface and achieve tight contact between graphene and PP membrane. Finally, graphene/PP separator was obtained by dissolving PMMA with acetone. The Raman spectra of graphene on copper foil is shown in Fig. 1b, in which only G peak, corresponding to doubly degenerate zone center E2g mode, and 2D peak, attributed to the second order of zoneboundary phonons, are observed, indicating high quality and monolayer nature of the synthesized graphene.30 Raman results of graphene on PP membrane is presented in Fig. 1c, in which no obvious D peak is observed, suggesting that graphene is intact after transfer, and inset photo of graphene/PP membrane reveals the high transparency of graphene.31 Fig. 1d and 1e compare the surface morphology of PP membrane before and after graphene coating, and it can be seen that after graphene transfer, the ordered pores in pristine PP is fully covered. In addition, in Fig. 1e, the framework of PP under graphene can be clearly observed, indicating the completely removal of PMMA.

TEM image shows only small amount of wrinkles and negligible

contaminates/residues on the graphene surface in Fig. 1f, attesting its high quality. Furthermore, the selected area electron diffraction (SAED) in Fig. 1g displays a single set of six-fold diffraction pattern, confirming the monolayer thickness of graphene.22 As mentioned above, one major challenge in Li-S battery is the “shuttle effect”,5 i.e. the dissolution of long chain LiPS, which can shuttle between the S-containing cathode and lithium anode. Moreover, when they diffuse to the anode surface, LiPS can cause side reactions and selfdischarge of the battery, which result in electrode passivation, Coulombic efficiency reduction


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and cycle life fade. The “shuttle effect” can be visualized from Fig. S1, in which electrolyte color turns from transparent to yellow as charge/discharge proceeding, evidencing the dissolution and diffusion of LiPS. To test the capability of our graphene/PP membrane in suppressing the crossover of LiPS, the permeation experiment was conducted with a “U” shape setup shown in Fig. 2a, in which a blank electrolyte and 0.1 M Li2S6 solution were separately injected into the left and right side of the tube with a piece of membrane sandwiched in the middle. The color change at the left side was recorded with a digital camera at different time. As a function of time (0h, 1h, 3h, 6h, 24h), it can be clearly noticed that the color at the left side becomes darker quickly with the use of PP membrane, revealing the easy translocation of Li2S6. By contrast, only limited color change is observed even after 24 h when graphene/PP membrane is used. Actually, with perfect single crystal monolayer graphene, all electrolyte ions/molecules are impermeable, ascribe to the only ~1.4 Å graphene hexagonal ring. The permeation of Li2S6, in fact, provides evidence that holes or pores indeed exist on CVD graphene. Fig. 2b displays a representative nanopore on graphene produced during CVD growth, and the pores characterized by TEM has size distribution of 2.45±0.68 nm as shown in the inset histogram. These nanopores are mainly defects introduced during the chemical vapor deposition process, which has also been reported previously,32 and other process, including transfer process, could also contribute to the formation of nanopores. In practice, the pore size on CVD graphene can be tuned for instance by plasma treatment,33 and after O2 plasma treating for 1 min, the pore size is increased to 3.76±0.82 nm as shown in Fig. S2. We then assembled the coin cell using the prepared graphene/PP membrane as separator for Li-S battery test. Briefly, the electrochemical measurements were carried out in a 2025-type coin cell with S/CB composite as cathode material, lithium metal as anode and 1 M lithium bis-



trifluoromethanesulfonylimide (LiTFSI) in 1:1 (v/v) mixture solution of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with 1% LiNO3 additive as electrolyte. More details about electrode preparation and electrochemical test have been illustrated in the experimental section. The morphology information and energy dispersive X-ray (EDX) results of S/CB are shown in Fig. S3, in which nanosized sulfur particles are found uniformly anchored on the conductive surface of CB network, and sulfur ratio in S/CB composite confirmed by TGA in Fig. S4 is 64.4%. Fig. 2c displays the initial galvanostatic charge/discharge cycle within the range of 1.8V~2.8V at 0.05 C. From these curves, a specific capacity of 1045.3 mAh g-1 is obtained using graphene/PP separator, as compared to 983.2 mAh g-1 with pristine PP, together with increased Coulombic efficiency from 96.0% to 99.9%. From the cycle test in Fig. 2d, graphene/PP separator also outperforms pristine PP when tested at 0.2 C, with improved capacity retention from 66.3% to 74.0% after 95 cycles. In detail, in Fig. 2d, the discharge capacity of the first two cycles at 0.05 C are 983.2 mAh g-1 and 891.1 mAh g-1 for PP, and 1045.3 mAh g-1 and 903.6 mAh g-1 for graphene/PP. The capacity of 95th cycles at 0.2 C are 499.2 mAh g-1 and 576.2 mAh g-1 for PP and graphene/PP, respectively. Compared with pristine PP, improved performance is realized with graphene/PP by suppressing the shuttling process of LiPS, but the still observed continuous capacity loss during cycling should be attributed to the irreversible deposition of Li2S or Li2S2 and the conductive network failure due to the volume change after cycling.34 The extended cycling test of 307 cycles at 0.2 C is shown in Fig. S5, and capacity retention of 55.6% and 62.0% are obtained for PP and graphene/PP separator, respectively. However, although the Coulombic efficiency has already reached up to 99.9% after graphene coating, from the permeation experiment in Fig. 2a, small amount of LiPS can still pass through the graphene/PP membrane.


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To better reduce the “shuttle effect”, an interfacial polymerization method was adopted here to fill the big pores or holes (see details in experimental section). Fig. 3a presents the schematic of interfacial polymerization of nylon-66 with monomers of HMDA and APC dissolved in water and hexane, respectively, and the configuration of Franz cell used is shown in Fig. S6. The interfacial polymerization utilizes the highly hydrophobic nature of the membrane, and before polymerization, the membrane restricts the aqueous phase but gets wetted by the oil phase, thus the aqueous/oil interface is formed at the bottom brim of the membrane in Fig. S6. As a result, the polymerization will be confined at the surface corresponding to the right side brim in Fig. 3a. As a proof of concept with the use of PP membrane, after interfacial polymerization, the ordered pores on PP surface are fully filled with nylon-66 but the PP framework is well maintained as shown in Fig. 3b, indicating that this technique is to selectively amend the pores on hydrophobic membrane surface. This approach thus allows us to seal the big pores or holes of graphene on PP but maintaining its monolayer nature. XPS results of the interfacial polymerization product are shown in Fig. S7. Unlike pristine PP in Fig. S7a, N and O are detected in Fig. S7b after polymerization, and the high resolution C(1s) and N(1s) further confirm the successful formation of nylon-66. For graphene/PP membrane, after interfacial polymerization, the membrane underneath graphene is intact as shown in Fig. 3c, in consistent with that expected. Additionally, as reported previously, this interfacial polymerization process will not affect the nanopores in small size.19 We have also conducted the permeation experiment with nylon-66 sealed PP and nylon-66 sealed graphene/PP membranes in Fig. S8, and it can be found that after interfacial polymerization, most of the LiPS is blocked by the nylon-66 sealed graphene/PP membrane. With this nylon-66 sealed graphene/PP separator, an enhanced capacity of 1128.4 mAh g-1 is achieved at 0.05 C when applied in Li-S battery as shown in Fig. 3d, higher than that of



graphene/PP and nylon-66 sealed PP membrane in the control experiments. The lower initial discharge plateau with nylon-66 sealed graphene/PP should be attributed to the increased LiPS concentration at the cathode side and decreased Li+ ion flux resulting from the enhanced blockage by this defect-sealed separator. Aside from that, a capacity retention of 798.6 mAh g-1 is retained after 20 cycles at 0.1 C as shown in Fig. 3e, which is higher than that of graphene/PP and the control experiment with nylon-66 sealed PP. The improved capacity and cycle stability is probably because that nylon-66 sealed graphene/PP separator slows down the diffusion of dissolved LiPS intermediates and confines them at the cathode side, which increases the utilization of LiPS during the charge/discharge test.35-37 Additionally, from the rate capacity results in Fig. 4a, graphene/PP and nylon-66 sealed graphene/PP display improved rate performance than pristine PP separator. Even at 0.5 C, from Fig. 4a, graphene/PP and nylon sealed graphene/PP show an initial capacity of 698.1 mAh g-1 and 690.6 mAh g-1, respectively, higher than that of pristine PP with 554.9 mAh g-1. And after 5 cycles, a capacity of 677.4 mAh g-1 and 628.0 mAh g-1 are maintained for graphene/PP and nylon sealed graphene/PP, respectively. It can be found that, under high charge/discharge rate (0.5 C and 1 C), nylon-66 sealed graphene/PP separator shows lower capacity than graphene/PP separator. The decreased rate capacity should be due to the reduced Li+ ion flux passing through the separator by the blockage of graphene after nylon-66 sealing.38 To further explain this, we have conducted the impedance spectroscopy test. In Fig. 4b, nylon-66 sealed graphene/PP separator shows slightly higher equivalent series resistance (RS) of 5.1 Ω than pristine PP (4.7 Ω) and graphene/PP (3.6 Ω), and slightly increased charge transfer resistance (RCT) compared with graphene/PP. To understand the role of graphene nanopore in mitigating the “shuttle effect”, we have conducted MD simulations using the LAMMPS package to explore the transporting mechanism


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of Li+ ion and Li2Sn (n=4,6,8) in electrolyte solution through defect areas of graphene.23-25 First, we set up a model with DME (C4H10O2) solvent at both sides of graphene, as shown in Fig. S9a, and a Li+ ion or Li2Sn (n=4,6,8) molecule with structures shown in Fig. S9b is placed at the left area before simulation. The graphene models are created with different number of carbon atoms defects, as presented in Fig. S9c, and the relative diameters of defects are listed in Tab. S1. The interactions for all the atomic species are modeled by LJ and Coulombic terms, and all the LJ potential parameters are listed in the Tab. S2. Unlike previous simulation simply studying the solvated structure of LiPS,39 the energy barriers for Li+ ion and Li2Sn (n=4,6,8) molecules across graphene nanopores with different sizes and various structures are calculated. More simulation details are elaborated in the supplementary information. Fig. 5a shows a typical potential energy profile for the translocation of Li+ ion through graphene nanopore (neutral) along Z direction. It can be found that the potential energy increases when Li+ ion gets close to the nanopore but decreases when it moves away from graphene, thus the translocation energy barrier can be obtained from the peak value for a specific nanopore size. The corresponding potential energy profiles for Li2Sn (n=4,6,8) are shown in Fig. S10. In this way, we have plotted energy barrier vs. pore size for Li+ and Li2Sn as compared in Fig. 5b, in which we observe that the crossover energy barriers decrease gradually with enlarging the pore size. Additionally, due to the size difference between Li+ ion and Li2Sn molecules, their crossover energy barriers also vary from each other. For instance, with pore size of 8.17 Å, the energy barrier for Li+ ion (~47.654 kcal/mol) is much smaller than that for Li2Sn (at the range from 118.069 kcal/mol to 216.972 kcal/mol), which means easier translocation of Li+ ion compared to Li2Sn molecules at the same pore size condition. Moreover, since the charge distributed at the



pore edge plays important role on the translocation of ions/molecules, nanopores with positive charge (to model H-terminated nanopore) and negative charge (to model OH-terminated nanopore) have also been investigated.18, 40 Because of the electrostatic interaction, we assume that the critical pore size that allows the Li+ ion translocation is different for negative, neutral and positive charged nanopores, and the negative charged nanopores can enhance the selectivity for Li+ ions and Li2Sn molecules translocation. Fig. 5c compares the energy barrier for Li+ ion across neutral, positively and negatively charged nanopores, and the lower energy barrier obtained with negatively charged nanopore can be explained by the attraction force between nanopore edge and Li+ ion. Here, we use “critical diameter” to define the minimal nanopore size that allows Li2Sn (n=4,6,8) molecules to pass through graphene without resistance. In other words, when the nanopore size is larger than “critical diameter”, Li2Sn will be able to freely across graphene. Fig. 5d summarizes the translocation critical diameter of Li2Sn molecules under different pore structures. For neutral nanopore, the critical diameter for Li2S4, Li2S6 and Li2S8 are 11.838 Å, 14.613 Å and 16.339 Å, respectively, close to the values obtained previously.39 In reality, the diffusion of Li+ ion through the nanopore is even easier as electric field is applied between cathode and anode. Approximately, the kinetic energy of Li+ ion can reach up to ~50 kcal/mol when using 2.2 V as the average operation voltage in Li-S battery.1 Conclusions In summary, we have demonstrated the use of a graphene-coated PP as separator for Li-S battery. After continuous monolayer CVD graphene was transferred onto PP membrane, it was used as separator to replace pristine PP and tested for Li-S battery. As a result, an improved initial capacity from 983.2 mAh g-1 to 1045.3 mAh g-1 at 0.05 C was achieved, together with enhanced


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Coulombic efficiency from 96.0% to 99.9% and cycling durability. We have also further realized higher capacity of 1128.4 mAh g-1 by sealing the large defect parts on graphene via an interfacial polymerization process. We attribute this enhancement to the “blocking effect” of graphene, which can selectively prevent the translocation of lithium polysulfides but maintain Li+ ion transport. The molecular dynamic simulation results show that the crossover of Li+ ion and Li2Sn (n=4,6,8) molecules translocation is not only dependent on the nanopore size but also effected by nanopore structure. For neutral nanopore, all of the lithium polysulfides are not allowed to cross the graphene membrane when the nanopore size is smaller than 11.838 Å. With negative charge, since the energy barrier for Li+ ion translocation will be lowered hence the selectivity will be enhanced, while positively charged nanopore will repulse Li+ ion thus unfavorable for the “blocking effect”. From experimental work and our simulation result, it can be concluded that graphene coated PP can be used as an efficient separator for Li-S battery, and our strategy shows obvious advantage than previous work through separator modification with additional layer of large thickness (about several micrometers), in which although improved capacity and cycling stability are achieved, their rate capacity is largely depressed.34, 39, 41 For instance, compared with previous work, although cycling durability was largely improved (capacity retention increased from 34.4% to 60% after 500 cycles at 1C), the capacity at initial 10 cycles were even lower than that of routine membrane (first discharge capacity decreased from 906 mAh g-1 to 781 mAh g-1), and the same phenomenon was also observed at 0.2 C,34 which should be due to the large thickness of the Nafion coated layer. However, in order to further improve the performance of Li-S battery, more modification of the graphene coated PP membrane needs to be realized by controlling the nanopore size, nanopore structure as well as nanopore density. At this moment, the charged status of graphene nanopores is not adjustable and more experimental work is



underway to modify the nanaopores with charged functional groups including oxygen plasma treatment and electron beam irradiation.33, 42, 43 In conclusion, graphene modified PP membrane demonstrates an advanced separator to improve Li-S battery performance, interfacial polymerization method offers a strategy to seal graphene defects and MD simulation results provides a direction to achieve the optimal selectivity by controlling the nanopores. Acknowledgement This project is supported by the Research Grant Council of Hong Kong SAR (Project Number 16204815), the Innovation and Technology Commission (ITC-CNERC14SC01 and ITS/267/15), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB22040402) and the National Natural Science Foundation of China (11525211, 11572307, 11472263). We appreciate support from Center for 1D/2D Quantum Materials, financial support from the Guangzhou Science & Technology Project (2016201604030023 and 201704030134). Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated. The MD simulations have been performed on the supercomputing system in the Supercomputing Center of University of Science and Technology of China. Supporting Information Supporting materials include additional experimental and molecular simulation results. References (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S batteries with high energy storage. Nat. Mater. 2012, 11, 19-29. (2) Ji, X.; Lee, K. T.; Nazar, L. F. A highly ordered nanostructured carbon-sulphur cathode for lithium–sulphur batteries. Nat. Mater. 2009, 8, 500-506. (3) Danuta, H.; Juliusz, U., Electric dry cells and storage batteries. In Google Patents: 1962. (4) Song, M. K.; Cairns, E. J.; Zhang, Y. Lithium/sulfur batteries with high specific energy: old challenges and new opportunities. Nanoscale 2013, 5, 2186-2204.


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Fig. 1. Schematic of graphene transfer and corresponding characterization. (a) Wet transfer process of CVD graphene onto PP membrane. (b) Raman spectra of CVD graphene grown on Cu foil. (c) Raman spectra of PP membrane with/without graphene coating. (d) SEM image of pristine PP membrane. (e) SEM image of monolayer graphene-coated PP membrane. (f) TEM image of CVD graphene. (g) Selected area electron diffraction (SAED) pattern of CVD graphene.


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Fig. 2. (a) Permeation test results with “U” shape setup of pristine PP membrane and graphene/PP membrane at different times. The arrow shows the diffusion direction of lithium polysulfides. (b) High resolution TEM of intrinsic nanopore on graphene. Inset figure shows the statistical pore size of intrinsic nanopores on graphene, and the average pore size is 2.45 nm with standard deviation of 0.68 nm. (c) Initial charge/discharge comparison using PP and graphene/PP at 0.05 C. (d) Cycle stability comparison. The first two and following cycles are tested at 0.05 C and 0.2 C, respectively.



Fig. 3. Sealed separator through interfacial polymerization and corresponding electrochemical tests. (a) Schematic of interfacial polymerization of nylon-66. (b) SEM image of PP after interfacial polymerization of nylon-66. (c) SEM image of graphene/PP after polymerization. (d) Charge/discharge comparison using nylon sealed PP, graphene/PP and nylon sealed graphene/PP at 0.05 C. (e) Cycle stability comparison. First cycle is tested at 0.05 C and the following cycles are tested at 0.1 C.


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Fig. 4. Electrochemical test results. (a) Rate capacity comparison using PP, graphene/PP and nylon-sealed graphene/PP separator. (b) Corresponding impedance spectroscopy results



Fig. 5. Molecular simulation results. (a) Potential energy profile for Li+ ion passing through neutral nanopore. (b) Comparison of energy barriers for Li+ ion and Li2Sn (n=4, 6, 8) molecules across neutral nanopore. (c) Energy barrier comparison of Li+ ion passing through neutral, positively and negatively charged nanopore, respectively. (d) Critical diameter comparison of Li+ ion and Li2Sn molecules passing through graphene with different pore structure.


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Table of Contents:


Shuttle Suppression by Polymer-sealed Graphene-coated

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