Chemical Roles to

Dec 5, 2017 - correlates with short-chain LiPSs (e.g., Li2S2, Li2S), giving rise to a greenish color that is clearly evident in Figure 2b.33,47,48. Po...
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Modified Separator Performing Dual Physical/ Chemical Roles to Inhibit Polysulfide Shuttle Resulting in Ultrastable Li−S Batteries Syed Ali Abbas,†,‡,⊥ Jiang Ding,⊥,§ Sheng Hui Wu,¶ Jason Fang,¶ Karunakara Moorthy Boopathi,⊥ Anisha Mohapatra,†,‡,⊥ Li Wei Lee,∥ Pen-Cheng Wang,† Chien-Cheng Chang,§ and Chih Wei Chu*,⊥,# †

Department of Engineering and Systems Science, National Tsing Hua University, Hsinchu 30013, Taiwan Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, National Tsing Hua University, Hsinchu 30013, Taiwan ⊥ Research Center of Applied Sciences, Academia Sinica, Taipei 115, Taiwan § Institute of Applied Mechanics, National Taiwan University, 1 Sec. 4, Roosevelt Road, Taipei 106, Taiwan ¶ Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan ∥ Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan # College of Engineering, Chang Gung University, Taoyuan 33302, Taiwan ‡

S Supporting Information *

ABSTRACT: In this paper we describe a modified (AEG/CH) coated separator for Li−S batteries in which the shuttling phenomenon of the lithium polysulfides is restrained through two types of interactions: activated expanded graphite (AEG) flakes interacted physically with the lithium polysulfides, while chitosan (CH), used to bind the AEG flakes on the separator, interacted chemically through its abundance of amino and hydroxyl functional groups. Moreover, the AEG flakes facilitated ionic and electronic transfer during the redox reaction. Live H-cell discharging experiments revealed that the modified separator was effective at curbing polysulfide shuttling; moreover, X-ray photoelectron spectroscopy analysis of the cycled separator confirmed the presence of lithium polysulfides in the AEG/CH matrix. Using this dual functional interaction approach, the lifetime of the pure sulfur-based cathode was extended to 3000 cycles at 1Crate (1C = 1670 mA/g), decreasing the decay rate to 0.021% per cycle, a value that is among the best reported to date. A flexible battery based on this modified separator exhibited stable performance and could turn on multiple light-emitting diodes. Such modified membranes with good mechanical strength, high electronic conductivity, and anti-self-discharging shield appear to be a scalable solution for future high-energy battery systems. KEYWORDS: lithium−sulfur batteries, chitosan, separators, activated expanded graphite, lithium polysulfide shuttle

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benignancy, low cost, natural abundance, and a wide range of operating temperatures.4−9 Despite a discharge potential lower than that of other intercalated cathode materials, sulfur, when coupled with a lithium metal anode, can deliver a high theoretical energy density of 2600 W h kg−1, approximately 5 times higher than that of commercial LIB systems. Although sulfur has several advantages as a high-capacity cathode material, its commercialization has been hampered by

he development of rechargeable batteries possessing high versatility, sustainability, and capacity has been pursued thoroughly over the last two decades, primarily due to the rapid growth of portable electronic devices and electric vehicles.1,2 Because state-of-the-art lithium-ion batteries (LIBs) containing lithium metal oxide (i.e., LiCoO2, LiFePO4, LiNiMnCoO2) cathodes offer theoretical capacities of less than 300 mA h g−1, there remains much room for developing materials of high capacity for use as cathodes in rechargeable batteries.3 In this regard, lithium−sulfur batteries (LSBs) that feature a cathode based on sulfur are exciting candidates for use as a next-generation rechargeable batteries because of their high theoretical capacity (1675 mA h g −1 ), environmental © 2017 American Chemical Society

Received: September 12, 2017 Accepted: December 5, 2017 Published: December 5, 2017 12436

DOI: 10.1021/acsnano.7b06478 ACS Nano 2017, 11, 12436−12445

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Cite This: ACS Nano 2017, 11, 12436−12445

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Figure 1. (a, b) SEM images of AEG flakes. (c) Cross-sectional SEM image of the as-prepared AEG/CH-coated separator. (d) Schematic representation of the capturing of LiPS during the discharge mechanism of a Li−S cell incorporating the AEG/CH-coated separator.

separator, causing the shuttling phenomenon. Considering the limited effect of modified cathode designs because of the volume expansion suffered by sulfur, modification of the separator is an alternative strategy for curbing polysulfide shuttling in LSBs. Many approaches have been tested, including the application of various carbon coatings and ionic conductive polymers directly on top of the separator to interact either physically or chemically with the LiPSs, effectively confining them to the cathode side.14,31−35 Studies in the field of LSB suggest that meso- to microporous carbons can serve as a polysulfide trap; furthering this concept, we have activated expanded graphite flakes (EG) at 800 °C in the presence of KOH/polyvinylpyrrolidone (PVP) to induce micro- to mesoporous structures to effectively trap LiPSs, denoted as activated expanded graphite (AEG). Recently, chitosan (CH) has exhibited high performance as a binder for graphite anodes; due to its abundance of hydroxyl and amino functional groups, chitosan can display good chemical affinity toward LiPSs in LSBs.36,37 Accordingly, in this study we employed a hybrid strategy to mitigate LiPS shuttling by coating AEG flakes blended with chitosan on top of the separator (i.e., forming an AEG/CH-coated separator). This dual physical/chemical interaction approach has many attractions: (i) the simple processing of the AEG/CH-coated separator appears to be a practical solution for mitigating the polysulfide issue and could be readily transferrable and implementable on the industrial scale; (ii) the AEG/CHcoated separator can facilitate rapid electron and ionic transport during the redox reaction, resulting in a lowering of the overall impedance of the cell; (iii) the AEG/CH separator facilitates anti-self-discharging behavior, with LSBs incorporating the AEG/CH separator retaining their capacity much better than corresponding devices containing the pristine separator after multiple periods of rest; and (iv) the AEG/CH separator resulted in high initial capacity (1264 mA h g−1 at 1C) and

its rapid degradation of capacity and its short life span. The electrochemical activity is restrained by the insulating nature of sulfur (5 × 10−30 S cm−1 at 25 °C), while the poor cyclability and low Coulombic efficiency are mainly associated with loss of the active material. During the typical discharge of an LSB, solid sulfur (S8) is transformed into soluble lithium polysulfide (LiPS; Li2Sx; x = 4−8) intermediates that can diffuse through the electrolyte and deposit on top of the anode, a phenomenon commonly known as “shuttling”. Such shuttling of LiPS from the cathode to anode can lead to further reduction reactions on top of the anode (i.e., Li2S4 → 2Li2S2 → 4Li2S).10−13 These short-range LiPSs on top of the anode are mainly responsible for active sulfur loss and high cell impedance, thereby resulting in severe capacity degradation and low sulfur utilization.14 Various strategies have been proposed to solve these issues; the most common is to modify the cathode structure by confining sulfur into porous or hollow structures, such as those made of carbon (e.g., hollow carbon, mesoporous carbon, carbon nanotubes, graphene, graphene oxide).15−27 These modified designs can not only enhance the electronic conductivity of the sulfur cathode but also offer a physical barrier to stop polysulfide diffusion to the anode. Other strategies involve metal oxides as sulfur hosts, polymer coatings, stronger binders, and additives.12,14,28−30 Nonetheless, sulfur particles suffer from large volumetric expansion upon lithiation, limiting the effect of structural modifications during extended charging/discharging cycles.28 As a result, research emphasis has shifted from modification of the cathode to modification of the separator. A separator is a membrane used as an electronic insulator to prevent short-circuiting between the anode and cathode. The membranes used routinely in LIBs are normally porous polymers that have no influence on the transport of lithium ions. Nevertheless, during the typical discharge of an LSB, LiPSs can migrate to the anode through the pores of the 12437

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Figure 2. (a, b) Visual electrochemical tests performed in H-type glass cells and corresponding optical images recorded at various discharge points; sulfur (right) and lithium metal (left) were used as the cathode and anode, respectively, with the pristine separator employed in (a) and the AEG/CH-coated separator in (b).

the separator; moreover, the AEG flakes also functioned to filter out the LiPSs through physical inhibition and reutilize them by working as an upper current collector.14,39,42−44 For fabrication of the AEG/CH separator, an AEG/CH slurry [AEG (95%), CH (5%)] was thoroughly mixed in a ball milling machine, in the presence of water/2-propanol as a mixed diluent, and then coated on the cathode side of a pristine separator. Introduction of CH not only ensured excellent mechanical and binding strength of the AEG to the pristine separator but also resulted in chemical interactions with the LiPSs.36,37 CH is a linear polysaccharide featuring randomly distributed β-(1−4)-linked D-glucosamine and N-acetyl-Dglucosamine units; its abundant OH and NH2 functional groups can interact noncovalently with LiPS.45,46 Scanning electron microscopy (SEM) analysis of the AEG and corresponding cross-sectional image of the AEG/CH separator are shown in Figure 1a,b,c. The coated layer had a thickness of 22.5 μm, and the coating added a real mass loading of only ∼0.13 mg cm−2, corresponding to only 12.5% of the weight of the pristine separator. Figure 1d presents the schematic picture of the AEG/CH-coated separator utilized in the LSB and polysulfide capturing phenomenon. In situ electrochemical tests were conducted in an H-shaped glass cell to examine the effectiveness of the AEG/CH-coated

cycling stability of 3000 cycles at 1C, being achieved when using a simple ball-milled sulfur carbon black mixture cathode, without adding any additive or applying long melt diffusion processes.

RESULTS AND DISCUSSION To form an effective barrier on top of the membrane to stop LiPS shuttling, first nonexpanding graphite was treated at 800 °C for 10 min to form expanded graphite; nonexpanded graphite flakes generally start to expand at temperatures above 300 °C.34 The porous texture of the EG flakes was analyzed using a nitrogen physisorption isotherm (Figure S3a), suggesting a type III isotherm with an H3 hysteresis loop and having a low surface area of 23 m2 g−1.38 As a high surface area along with a meso-/microporous network plays a crucial role in trapping LiPSs, EG flakes were activated in the next step as AEG in the presence of KOH/PVP at 800 °C.38−41 A nitrogen physisorption isotherm for AEG indicates an increased surface area from 23 m2 g−1 to 368 m2 g−1 and a type II isotherm with an H4 hysteresis loop, signifying the presence of micro- and mesopores (Figure S3b). Moreover Table S1 compares the surface area and micro-/mesoporous volume of EG and AEG flakes. The micro-/mesoporous morphology of the AEG facilitated electrolyte infiltration and Li-ion transport through 12438

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Figure 3. (a) Tilted-angle SEM image and corresponding EDX mapping of cycled AEG/CH-coated separator. (b) High-resolution S 2p XP spectra of the cycled AEG/CH-coated separator. Black dashed line and red line correspond to the raw data and overall fitted data, respectively.

lithium−sulfur U-shaped glass cells. During the complete discharge in Figure 2a, the brownish solution from the cathode side tended to penetrate through the separator and reach the lithium metal anode, giving rise to the shuttling phenomenon; in addition, the solution’s color corresponded to higher-order LiPS, implying that some proportion of higher-order LiPS had not been reduced to short-chain LiPS, resulting in lithium metal degradation (higher-order LiPS reduced to lower-order LiPS on top of the anode), active material loss, and self-discharge.49,50 In contrast, the cell incorporating the AEG/CH-coated separator effectively suppressed the LiPS shuttling, with the color change from brown to green corresponding to the majority of higherorder LiPS having been reduced to lower-order species; these phenomena were also evident from the higher discharge time of the cell. To further investigate the presence of LiPS within the AEG/CH layer, a lithium−sulfur coin cell was disassembled in a glovebox after 1000 discharge/charge cycles. Figure 3a presents tilt-angle SEM images (cross-section and surface) of the AEG/

separator for inhibiting polysulfide shuttling at various discharge points (Figure 2). The H-shaped glass shell contained lithium foil on the left side and a sulfur electrode on the right side; they were separated by either a pristine or AEG/CHcoated separator (with the AEG/CH coating facing the cathode side); the glass cell was filled with the electrolyte. Figure 2a and b display the discharging profiles of the pristine and AEG/CHcoated separators, further divided into four stages. The discharging began at point 1 (marked at 3 V) and continued to point 2 (at 2.2 V) and point 3 (at 1.9 V), and finally the cells were fully discharged at point 4 (at 1.5 V). The second point at 2.2 V corresponds to the start of the upper discharge plateau of the LSB, where elemental sulfur is converted to long-chain soluble LiPSs (e.g., Li2S8, Li2S6), revealed by a deepening bright brown color in the electrolyte, whereas the third point at 1.9 V correlates with short-chain LiPSs (e.g., Li2S2, Li2S), giving rise to a greenish color that is clearly evident in Figure 2b.33,47,48 Point 4 at 1.5 V completes the discharge process in the 12439

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Figure 4. (a) Cycling performance of Li−S cells incorporating a pristine separator and an AEG/CH-coated separator at a current rate of 0.1 C (1C = 1670 mA g−1). (b) Continuous CV traces (scan rate: 0.1 mV s−1) of the Li−S cell incorporating the AEG/CH-coated separator. (c, d) Discharge/charge profiles of the 1st, 50th, 100th, 150th, and 200th cycles of Li−S cells incorporating the (c) AEG/CH-coated and (d) pristine separators. (e) Long-term cycling performance of a Li−S cell incorporating the AEG/CH separator at a current density of 1C.

2p3/2 component 2p3/2/2p1/2 doublet is quoted. The two 2p3/2 doublets centered at lower binding energy at 161.95 eV are associated with a terminal sulfur species, indicating the presence of higher-/lower-order lithium polysulfide compounds. However, the two 2p3/2 centered at a higher binding energy at 163.45 eV associated with a bridging sulfur indicate the presence of elemental sulfur.46,51,52

CH-coated separator; even after the prolonged cycling, the coated separator revealed no indication of critical damage to the coating. Moreover, corresponding energy dispersive X-ray (EDX) mapping revealed uniform distributions of carbon and sulfur signals. X-ray photoelectron spectroscopy (XPS) of the cycled separator was performed in the discharged state to confirm the presence of LiPS in the AEG/CH layer. In the high-resolution S 2p region spectrum (Figure 3b), only the S 12440

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Figure 5. (a−d) Photographs of (a) a flexible Li−S battery featuring a sulfur cathode, a lithium anode, and an AEG/CH-coated separator; (b) the flexible Li−S battery lighting up four parallel red LED circuits; and (c, d) the bending test of the flexible Li−S battery in (c) flat and (d) bent states. (e) Cycle performance of the flexible battery in its flat and bent states at 50 mA g−1.

reduction to long-chain PS) and 2.04 V vs Li/Li+ (II; long-chain polysulfide reduction to Li2S2/Li2S), while one anodic peak appeared near 2.43 V vs Li/Li+ (oxidation of Li2S2/Li2S back to sulfur). In all five cycles, the peak current and the integrated area of the anodic/cathodic peaks were maintained, revealing good reversibility for the electrochemical process. For the pristine separator, however, the CV data (Figure S13) revealed a gradual decay in the cathodic peak (ca. 2.03 V) associated with limited conversion of higher-order LiPS to lower-order LiPS, resulting from dissolution of the higher-order LiPS in the electrolyte, thereby decreasing the battery capacity. Figure 4c and d present galvanostatic charge/discharge voltage profiles of the pristine and AEG/CH-coated separators for the first, 50th, 100th, 150th, and 200th cycles. The voltage profiles for both sets of cells display similar voltage plateaus during the charge/ discharge process, related to the typical two-step redox reaction in an LSB. We attribute the first discharge plateau near 2.3 V to the conversion of sulfur (S8) to higher-order LiPS (Li2Sn; 4 < n ≤ 8), and the second near 2.1 V to the formation of lowerorder LiPS (Li2S2/Li2S). On the other hand, we detected one

To monitor the improvement in performance of the LSB incorporating the AEG/CH-coated separator, a simple pure sulfur/carbon black cathode was prepared by mixing elemental sulfur, carbon black, and poly(vinylidene fluoride) (PVDF) binder at 75:15:10 wt % in N-methylpyrrolidone (NMP). The sulfur cathode used in testing had a real sulfur loading of 1.6−2 mg cm−2. Figure 4a compares the cycling performance of the AEG/CH-coated and pristine separators at 0.1C. The cells containing the AEG/CH-coated separator had an initial discharge capacity of 1301 mA h g−1, much higher than that of the pristine separators (1011 mA h g−1). We attribute the rise in capacity in the case of the AEG/CH-coated separator to the presence of the conductive AEG sheets, thereby enhancing the utilization of the active material.39 After 200 cycles, the AEG/CH-coated separator outperformed the pristine separator, with capacities of 992 and 406 mA h g−1, respectively. Figure 4b displays the cyclic voltammetry (CV) performance of a cell employing the AEG/CH-coated separator, over the initial five cycles at a scan rate of 0.1 mV s−1. In the CV curves, two separated cathodic peaks appear near 2.3 V vs Li/Li+ (I; sulfur 12441

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together to form a battery core and congregated into aluminum−plastic film packages. The Experimental Methods section provides a detailed description of the assembly process. The resulting flexible battery was capable of lighting up four parallel red light emitting diodes for 1 min, as displayed in Figure 5a and b. Furthermore, a flexibility test of the same battery (Figure 5c and d) revealed the same potential difference under flat and bent conditions. In C-rate tests performed for the flexible battery at a low current density of 50 mA g−1, the initial capacity of 725 mA h g−1 under flat conditions decreased to 670 mA h g−1 in the bent state and then increased to 684 mA h g−1 upon returning to the flat state after one cycle. Hence, the AEG/CH-coated separator also displayed stable performance when applied in flexible battery systems.

distinct charge plateau in accordance with the CV curves. There was, however, a clear difference in voltage hysteresis between the two sets of LSBs. Batteries operated using the pristine separator exhibited polarization (ΔE) in the first cycle (230 mV) much higher than that when using the AEG/CH-coated separator (110 mV); similarly, after 200 cycles, the polarization observed in the AEG/CH-coated battery (205 mV) was much lower than that in the pristine separator (288 mV). Thus, modification of the pristine separator with AEG sheets and CH greatly enhanced the redox reaction kinetics of the system. The long-term cycling performance of Li−S cells incorporating the AEG/CH-coated separator was examined at 1C (Figure 4e). The cell exhibited a high initial discharge capacity of 1264 mA h g−1 (75.4% of theoretical capacity), good Coulombic efficiency (CE) of greater than 97%, and, even after 3000 cycles, a high reversible capacity of 498 mA h g−1, corresponding to a very low degradation rate of 0.021% per cycle, one of the best values reported for a modified separator (see Table S3). Furthermore, to showcase the cycling versatility of the Li−S cell incorporating the AEG/CH at various current densities, the cells were cycled at 0.25C and 0.5C for 2000 cycles (Figures S11 and S12). In both cases, we observed high sulfur utilization, a low decay rate, and a stable cycle life. Notably, the Li−S cell incorporating the AEG/CH-coated separator could display electrochemical performance much superior to that of the pristine one in the absence of LiNO3 in the electrolyte (cf. Figures S16a and b). The Li−S battery featuring the modified separator had an average CE (90%) and capacity retention (60%) higher than the cells featuring the pristine separator (67% and 15%, respectively) after 500 cycles at 0.5C. The detailed comparison of polysulfide-capturing ability of PVDF with CH and EG with AEG is shown in Figures S17 and S18. Electrochemical impedance spectroscopy (EIS) provided further insight into the electrochemical performance of the Li− S batteries incorporating the pristine and AEG/CH-coated separators (Figure S4a and b). The Nyquist plots for the Li−S batteries featuring the pristine and modified separators consisted of one depressed semicircle in the high- and medium-frequency range, associated with charge transfer resistance (Rct), and a higher frequency intercept (on the lefthand side) corresponding to the bulk resistance (Rs), including the electrode and electrolyte resistance.53,54 The EIS data recorded for the battery using the pristine separator revealed a high charge transfer resistance of 382 Ω for the fresh cell (Figure S4a); a great decrease in the value of Rct to 43 Ω occurred, however, after the first cycle, attributable to the loss of a large amount of active material through dissolution of the LiPSs in the electrolyte. Moreover, after 100 cycles, the value of Rct rose again (to 97 Ω), related to an increase in another semicircle in the medium-frequency range, which is attributed to nonconductive discharge products having deposited on top of the cathode.55 In comparison with the pristine separator, the Li−S battery incorporating the AEG/CH-coated separator exhibited much lower values of Rct of 42.2 Ω for the fresh cell and 34.5 Ω after the first cycle. Even after 100 cycles, the value of Rct remained low (32.59 Ω), presumably because the conductive AEG/CH-coated layer acted as a pseudo-upper current collector, lowering the effective resistance of the cathode through the highly insulating sulfur. Therefore, higher sulfur utilization could be achieved. To investigate the potential applications of the AEG/CHcoated separator in flexible Li−S batteries, a sulfur cathode, AEG/CH-coated separator, and lithium foil were laminated

CONCLUSION We have introduced a modified AEG/CH-coated separator that functions based on a dual (physical/chemical) interaction mechanism with LiPS. The AEG/CH-coated separator successfully retards polysulfide shuttling and offers upper contact to the sulfur cathode, thereby increasing the degree of sulfur utilization during the charging/discharging process. Batteries incorporating the AEG/CH-coated separator had an extremely low decay rate of 0.021% per cycle, resulting in an extraordinary cycle life of up to 3000 cycles based on pristine sulfur cathodes. Flexible batteries incorporating the AEG/CHcoated separator also exhibited great performance under various stress conditions, confirming the suitability of the design of the modified separator in flexible power source systems. Combined with the simple fabrication methodology, we believe that AEG/ CH-coated separators will be highly applicable for enhancing the performance of commercial Li−S batteries. Moreover, our study suggests a general approach of combining modified AEG/ CH-coated separators with rechargeable batteries, increasing the potential of modified membranes for use in advanced energy storage systems. EXPERIMENTAL METHODS Preparation of AEG/CH-Modified Separators. Nonexpanded graphite was first dried in a vacuum oven at 60 °C for 12 h. After drying, it was placed in a steel crucible and heated at 800 °C for 10 min in a muffle furnace. The expansion of graphite flakes occurred at temperatures above 300 °C. Activation of expanded graphite was carried out in accordance with the work reported before.38 A 0.5 g amount of expanded graphite was dispersed in 50 mL of 10 wt % PVP and sonicated for 10 h. After sonication 2.5 g of KOH was further added to the solution, and the resulting mixture was stirred and dried at 60 °C. The dried solid precipitate was put into a horizontal tube furnace at 800 °C in the presence of argon. The black material (AEG) obtained was thoroughly cleaned with 1 M HCl to wash away remaining KOH and then cleaned with DI water and further dried. CH (2.5 g, 2 wt %) was dissolved in a mixture of 98% deionized water and 2% acetic acid under constant magnetic stirring for 2 days. The resulting CH was mixed with AEG flakes in a 5:95 wt % ratio in a mixed diluent of 2-propanol and deionized water. The resulting blend was thoroughly mixed and ground in a ball miller for 12 h with the help of zirconia beads. The slurry was cast on one side of a Celgard 2500 membrane. The modified separator was dried in a vacuum oven at 40 °C for 24 h. To prepare the AEG/PVDF-coated separator, a ratio of PVDF and AEG flakes of 5:95 wt % was used in the NMP solvent. To prepare the EG/CH-coated separator EG and CH were mixed with a 5:95 wt % and dispersed and ball milled with a mixed diluent of 2propanol and deionized water. Characterization. The morphologies of the samples were examined, and energy dispersive X-ray spectroscopy was performed 12442

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ACS Nano using an FEI Nova 200 scanning electron microscope (2−10 kV). Xray photoelectron spectroscopy was performed using a PHI 5000 Versa Probe apparatus equipped with an Al Ka X-ray source (1486.6 eV). Nitrogen sorption experiments were performed using a Micromeritics ASAP 2020 system, and data analysis was performed using an ASAP 2020 surface area and porosity analyzer. Prior to measurement, the sample to be tested was degassed under dynamic vacuum at 300 °C for 24 h. The specific surface area was calculated at a relative pressure of 0.05−0.2, using the multipoint BET method. Powder X-ray diffraction patterns of the AEG were obtained using a Rigaku D/Max 2550 VB/PC X-ray diffractometer using Cu (Ka) radiation, with 2θ angles recorded from 10° to 70°. XPS of AEG was performed using a PHI 5000 Versa Probe equipped with an Al Ka Xray source (1486.6 eV). Preparation of the Li−S Cell. Pure sulfur was added to a mixture of carbon black (super p) and PVDF binder at a ratio of 75:15:10 wt %. NMP was used as a solvent, and the resulting mixture was placed in a ball miller machine and thoroughly mixed for 2 h to form a slurry, which was coated homogeneously on top of an Al foil current collector. The slurry was dried for 24 h at 50 °C under vacuum and then cut into circular disks (diameter: 12 mm). The typical loading of sulfur in the coin cell ranged from 1.6 to 2 mg cm−2. CR2032 coin cells were prepared using sulfur as the cathode; pristine Celgard 2500 and AEG/PVDF- and AEG/CH-modified separators; and lithium metal foil as the counter electrode. The electrolyte was 1.3 M bis(trifluoromethane)sulfonamide lithium (LiTFSI) in dimethoxyethane (DME) and dioxolane (DOL) (1:1, v/v) with 1 wt % LiNO3 as an additive. The coin cells were assembled and disassembled in an Arfilled glovebox. The quantity of electrolyte added to each coin cell was 35 μL. Electrochemical Measurements. The galvanostatic charge/ discharge performance of the cell was studied at room temperature using a battery tester (AcuTec Systems, Taiwan) with a voltage window of 2.8−1.5 V; prior to testing at various C-rates, every cell was activated at 0.05C for 2 cycles. EIS (frequency 0.01−100,000 Hz; amplitude: 5 mV) and cyclic voltammetry (1.5−3.0 V, scan rate: 0.1 mV s−1) were performed using an Autolab PGSTAT302N apparatus (Eco Chemie, The Netherlands). Assembly of the Flexible Battery. Flexible (soft packing) Li−S batteries containing pristine and modified separators were assembled in a glovebox. The compositions of cathode, separator, counter electrode, and electrolyte were similar to those mentioned above for the coin cells. Aluminum and nickel strips were attached as electrode tabs to the sides of the cathode and anode, respectively. The electrodes, separator, and lithium foil were laminated together to form the battery core and congregated into aluminum−plastic film packages. The electrolyte was injected into the package, followed by sealing of the battery under vacuum. The assembled flexible battery was rested for 10 h at room temperature to ensure permeation of the electrolyte into the electrodes and separator. Flexible batteries were charged/ discharged at 0.01C for 2 cycles prior to performing further tests. Visible Electrochemical Cell Test. In situ electrochemical tests were performed in an H-type glass cell to examine the diffusion of LiPS through the pristine and modified (AEG/CH) membranes during discharge. The whole test was performed in a glovebox to exclude the effects of oxygen and water. Pristine and modified membranes were placed in the middle of an H-type glass cell. Lithium foil was loaded onto Cu foil (2.84 mg cm−2), and a sulfur−carbon composite with an active material (S content: 75 wt %) was pasted onto the Al foil (active material on Al foil: 5.4 mg cm−2), serving as the anode and cathode, respectively. The electrolyte [DOL/DME, 1:1 (v/ v); 10 mL] containing 1.3 M LiTFSI was added into the glass cell. The H-type glass cell was tested in galvanostatic mode at a rate of 0.1C discharge (1C = 1600 mA g−1) at 25 °C within a voltage range from 1.5 to 3 V using a VMP3 multichannel potentiostat and EC-Lab control software (BioLogic, France). Preparation of a Li2S6 Solution. The preparation of Li2S6 was carried out by mixing appropriate ratios of sulfur powder (S8) and Li2S powder in a DOL/DME (1:1, v/v) solution for 24 h.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06478. Movie (AVI) Additional figures (PDF) Movie (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Karunakara Moorthy Boopathi: 0000-0003-2042-9595 Chih Wei Chu: 0000-0003-0979-1729 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by the Ministry of Science and Technology (MOST) of Taiwan (104-2221-E-001-014-MY3), the U.S. Airforce (AOARD), the Sinica-ITRI corporation project, Taiwan’s Deep Decarbonization Pathways toward a Sustainable Society, and the Career Development Award of Academia Sinica, Taiwan (103-CDA-M01). 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) Scrosati, B.; Hassoun, J.; Sun, Y.-K. Lithium-Ion Batteries. A Look into the Future. Energy Environ. Sci. 2011, 4, 3287−3295. (3) Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed. 2008, 47, 2930−2946. (4) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S. Rechargeable Lithium−Sulfur Batteries. Chem. Rev. 2014, 114, 11751−11787. (5) Nazar, L. F.; Cuisinier, M.; Pang, Q. Lithium-Sulfur Batteries. MRS Bull. 2014, 39, 436−442. (6) Bresser, D.; Passerini, S.; Scrosati, B. Recent Progress and Remaining Challenges in Sulfur-Based Lithium Secondary Batteries - A Review. Chem. Commun. 2013, 49, 10545−10562. (7) Xu, G.; Ding, B.; Pan, J.; Nie, P.; Shen, L.; Zhang, X. High Performance Lithium-Sulfur Batteries: Advances and Challenges. J. Mater. Chem. A 2014, 2, 12662−12676. (8) Yin, Y.-X.; Xin, S.; Guo, Y.-G.; Wan, L.-J. Lithium−Sulfur Batteries: Electrochemistry, Materials, and Prospects. Angew. Chem., Int. Ed. 2013, 52, 13186−13200. (9) Zhang, S. S. Liquid Electrolyte Lithium/Sulfur Battery: Fundamental Chemistry, Problems, and Solutions. J. Power Sources 2013, 231, 153−162. (10) Mikhaylik, Y. V.; Akridge, J. R. Polysulfide Shuttle Study in the Li/S Battery System. J. Electrochem. Soc. 2004, 151, A1969−A1976. (11) Agostini, M.; Xiong, S.; Matic, A.; Hassoun, J. Polysulfidecontaining Glyme-based Electrolytes for Lithium Sulfur Battery. Chem. Mater. 2015, 27, 4604−4611. (12) Wang, J.; Yao, Z.; Monroe, C. W.; Yang, J.; Nuli, Y. Carbonyl-βCyclodextrin as a Novel Binder for Sulfur Composite Cathodes in Rechargeable Lithium Batteries. Adv. Funct. Mater. 2013, 23, 1194− 1201. (13) Park, J.-W.; Yamauchi, K.; Takashima, E.; Tachikawa, N.; Ueno, K.; Dokko, K.; Watanabe, M. Solvent Effect of Room Temperature Ionic Liquids on Electrochemical Reactions in Lithium−Sulfur Batteries. J. Phys. Chem. C 2013, 117, 4431−4440. 12443

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