Dual-Conductive, Heterolayered Battery

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Polysulfide-Breathing/Dual-Conductive, Heterolayered Battery Separator Membranes Based on 0D/1D Mingled Nanomaterial Composite Mats Jeong-Hoon Kim, Gwan Yeong Jung, Yong-Hyeok Lee, JungHwan Kim, Sun-Young Lee, Sang Kyu Kwak, and Sang-Young Lee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04830 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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Nano Letters

Polysulfide-Breathing/Dual-Conductive, Heterolayered Battery Separator Membranes Based on 0D/1D Mingled Nanomaterial Composite Mats

Jeong-Hoon Kim§1, Gwan Yeong Jung§1, Yong-Hyeok Lee§1, Jung-Hwan Kim1, Sun-Young Lee2*, Sang Kyu Kwak1*, and Sang-Young Lee1* 1

Department of Energy Engineering, School of Energy and Chemical Engineering

Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea 2

§

Department of Forest Products, Korea Forest Research Institute, Seoul 02455, Korea

These authors contributed equally to this work.

E-mail: [email protected], [email protected] (S. K. Kwak); [email protected] (S. -Y. Lee)

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ABSTRACT Facile/sustainable utilization of sulfur active materials is an ultimate challenge in highperformance lithium-sulfur (Li-S) batteries. Here, as a membrane-driven approach to address this issue, we demonstrate a new class of polysulfide-breathing (capable of reversibly adsorbing and desorbing polysulfides)/dual (electron and ion)-conductive, heterolayered battery separator membranes (denoted as “MEC-AA separators”) based on 0D (nanoparticles)/1D (nanofibers) composite mats. The MEC-AA separator is fabricated through an in-series, concurrent electrospraying/electrospinning process. The top layer of the MEC-AA separator comprises close-packed mesoporous MCM-41 nanoparticles spatially besieged by multi-walled carbon nanotubes (MWNT)-wrapped polyetherimide (PEI) nanofibers. The MCM-41 in the top layer shows reversible adsorption/desorption of polysulfides and the MWNT-wrapped PEI nanofibers act as a dual-conductive upper current collector. Preferential deposition of the MWNTs along the PEI nanofibers and dispersion state of the separator components are elucidated theoretically using computational methods. The support layer, which consists of densely-packed Al2O3 nanoparticles and polyacrylonitrile nanofibers, serves as a mechanically/thermally stable and polysulfide-capturing porous membrane. The unique structure and multifunctionality of the MEC-AA separator allow for substantial improvements in redox reaction kinetics and cycling performance of Li-S cells far beyond those achievable with conventional polyolefin separators. The heterolayered nanomat-based membrane strategy opens a new route toward electrochemically-active/permselective advanced battery separators.

KEYWORDS: Heterolayered Separator Membranes, Li-S Batteries, Polysulfides, Reversible Adsorption/Desorption, Dual-Conduction

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Enormous concerns about the depletion of fossil fuels and environmental pollution have inspired the relentless pursuit of eco-friendly clean energy solutions. Rechargeable power sources with reliable electrochemical performances are regarded as an essential prerequisite to ensure progress in this green-energy era1,2. Lithium-ion rechargeable batteries have occupied a predominant position in portable electronics and are currently desired to expand their application fields to electric vehicles and grid-scale stationary energy storage systems. However, the ongoing surge in demand for high-energy density/low-cost batteries has spurred the search for new alternative power sources3-5. Among the various energy storage systems reported to date, lithium-sulfur (Li-S) batteries have garnered considerable attention as a promising electrochemical system to address the aforementioned issues, because of their high theoretical capacity (1672 mAh g−1), natural abundance, and environmental benignity of sulfur active materials6-8. Despite the appealing characteristics of Li-S batteries, several technical problems remain unsolved and has hindered their practical application. The foremost challenges9-12 include low electronic conductivity of sulfur (S8)/lithium polysulfides (Li2Sx), structural instability of sulfur cathodes, unstable Li metal-electrolyte interface, and shuttle effect of polysulfides. Numerous approaches have been undertaken to overcome these problems of Li-S batteries, with a particular focus on synthesis and engineering of new sulfur cathodes and electrolytes7-18. In addition, carbon substance-based conductive interlayers19-24, which are positioned between sulfur cathodes and separators, have been recently investigated as so-called “functional upper current collectors”. They promoted electrochemical reaction kinetics of sulfur cathodes and contributed to a better utilization of sulfur active materials, in addition to their microporous structure-driven capturing of dissolved polysulfides. Another noteworthy approach is the use of polysulfide-capturing metal oxides as an additive in sulfur cathodes or 3


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separator membranes26,31. However, most of the metal oxides investigated so far have relied on surface related adsorption (specifically, driven by chemisorption process), which means that the polysulfide adsorption amount is highly dependent on their surface area. Furthermore, adsorption onto the metal oxides is not reversible (i.e., the adsorbed polysulfides are not allowed to return to their initial states), eventually resulting in the loss of cell capacity during charge/discharge cycling26,27. On the basis of an in-depth analysis of the previous studies, we proposed that key requirements for an ideal interlayer are as follows: i) high electronic conduction, ii) highly porous structure (that will be filled with liquid electrolyte) for facile ionic transport, iii) high specific surface area, and iv) reversible adsorption/desorption of polysulfides. In addition, to minimize the interlayer thickness, monolithic integration of the interlayer with a separator membrane (acting as a support layer) is recommended7,10,31. Here, we demonstrate a new class of polysulfide-breathing (i.e., capable of reversibly adsorbing and desorbing polysulfides)/dual (electron and ion)-conductive, heterolayerd battery separators based on 0D (nanoparticles)/1D (nanofibers) composite mats as a facile and effective membrane strategy to enable high-performance Li-S batteries. The top layer of the heterolayered battery separators comprises close-packed mesoporous MCM-41 nanoparticles spatially besieged by multi-walled carbon nanotube (MWNT)-wrapped polyetherimide (PEI) nanofibers. Nazar et al.28 recently reported a new sulfur-carbon composite cathode material that contains a mesoporous silica additive of SBA-15, in which the SBA-15 reversibly adsorbed and desorbed polysulfides as a kind of reservoir. Intrigued by this unusual functionality of SBA-15, we employ sphere-shaped MCM-41 nanoparticles as a major component of the top layer to allow for reversible adsorption/desorption of polysulfides. Meanwhile, the MWNTs

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are preferentially deposited along the PEI nanofibers, leading to the formation of a dual (electron/ion)-conductive upper current collector. The support layer, which is composed of densely-packed Al2O3 nanoparticles and polyacrylonitrile (PAN) nanofibers, exhibits superiority in polysulfide-trapping capability, mechanical/thermal stability, and ion conduction, compared to conventional polyolefin battery separators. The Al2O3 nanoparticles are chosen to act as a chemical trap for polysulfides that are generated from sulfur cathodes and MCM-41. The interstitial voids formed between the Al2O3 nanoparticles offer ion-conducting pathways. The PAN nanofibers serve as a mechanical skeleton in the support layer. Driven by the aforementioned unique structure and exceptional multifunctionality, the resulting heterolayered separators (i.e., MCM-41/(PEI/MWNT) top layers on PAN/Al2O3 support layers, referred to as “MEC-AA separators”) provide substantial improvements in electrochemical performance of Li-S batteries, which lies far beyond that currently attainable with conventional polyolefin separators. To fabricate the MEC-AA separators with the heterolayered composite nanomats, we used an in-series, concurrent electrospraying/electrospinning process (Scheme 1). Specifically, the support layer was prepared via concurrent electrospraying (for Al2O3)/electrospinning (for PAN), leading to the close-packed Al2O3 nanoparticles/PAN nanofibers composite mat. On top of the support layer, the top layer was fabricated using the same concurrent electrospraying (for MCM-41/MWNT)/electrospinning (for PEI) technique. The rational control of thermodynamic affinity between the components allowed for the dense packing of hi nanoparticles and also preferential wrapping of MWNTs along the PEI nanofibers. Both the support and the top layers have interstitial voids between the Al2O3 (or MCM-41) nanoparticles. Importantly, the interstitial voids offer ion-conducting channels in the MEC-AA separator after being filled with liquid electrolyte. Meanwhile, the MWNT-wrapped PEI nanofibers act as a 5


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kind of upper current collector, which promotes utilization of sulfur active materials. The spherical MCM-41, similar to the fiber-shaped SBA-1528-30, is anticipated to exhibit reversible voltage-dependent adsorption/desorption of polysulfides during charge/discharge cycling. The top layer is designed to exhibit dual (electron/ion) conduction and reversible adsorption/desorption of polysulfides. To better understand the unusual multifunctionality of the top layer, we performed a model study using a conventional polyethylene (PE) separator (thickness = 20 µm) as a support layer. First, the MWNTs were electrosprayed on top of the PE separator without the assistance of polymeric components. The MWNTs were densely deposited onto the PE separator in the form of a top layer (Supporting Information Figure S1). However, when the separator was crumpled, considerable amounts of the MWNTs were detached from the PE separator, revealing poor structural integrity of the MWNT top layer. To resolve this problem, we mixed the MWNTs with polymeric nanofibers in the top layer. Two polymers with different polarities (here, PAN and PEI) were chosen as examples. The PAN (or PEI), together with the electrosprayed MWNTs, was concurrently electrospun on the PE separator (Figure 1a). Both the PEI/MWNT and PAN/MWNT top layers maintained structural stability even after being crumpled (insets of Figure 1b and c), underlying the advantageous role of polymeric nanofibers as an essential component of the top layers. A notable difference between the two top layers was their morphology. The scanning electron microscopy (SEM) image of the PEI/MWNT top layer showed that the MWNTs were preferentially deposited along the PEI nanofibers, resulting in the development of a porous structure (Figure 1b). By comparison, in the case of the PAN/MWNT top layer, the spaces between the PAN nanofibers were plugged with the MWNTs (Figure 1c). The porous structure of the PEI/MWNT top layer was confirmed by the lower Gurley value (333 [sec 100 cc-1]) of the PEI/MWNT-coated PE separator compared to that (8326 [sec 100 cc-1]) of the 6


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PAN/MWNT-coated PE separator, where the lower Gurley value indicates higher air permeability (i.e., a highly porous structure)33,34. Meanwhile, from the TGA measurements (Supporting Information Figure S2), the composition ratio of the PEI/MWNT top layer was estimated to be 54/46 (w/w). This morphological difference was elucidated theoretically using molecular dynamics (MD) simulation, with a focus on intermolecular interaction energy between MWNTs and polymers (i.e., PEI or PAN) (see “Modeling and simulation details” in the Supporting Information). Figure 1d shows a comparison of intermolecular interaction energy between MWNTs and polymers. The PEI/MWNT system exhibited stronger binding affinity than the PAN/MWNT one. This result was attributed to intermolecular π-π stack interactions induced by the alignment of aromatic rings of PEI on outermost surface of MWNTs (Supporting Information Figure S3). Therefore, the MWNTs are speculated to be preferentially deposited onto the PEI nanofibers rather than being independently mixed with the PEI nanofibers. In addition to the electron-conductive PEI/MWNTs, another major component of the top layer was the MCM-41 nanoparticles. A number of metal oxides, which are capable of adsorbing polysulfides, have been explored as sulfur cathode additives35,36 to capture polysulfides. The effect of these additives is known to be strongly influenced by their adsorption surface area. Therefore, after the whole surface of the additives is covered with polysulfides, they no longer exhibit polysulfide-capturing capability. Given the previous results reported for fiber-shaped SBA-15 which showed reversible adsorption/desorption of polysulfides as a function of cell voltage28, we introduced spherical MCM-41 as a separator component to satisfy the aforementioned requirements. The MCM-41 nanoparticles were synthesized via a sol-gel method37,38 (Supporting Information Figure S4a) using tetraethyl orthosilicate (TEOS) and hexadecyltrimethylammonium bromide 7


(also known as

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cetyltrimethylammonium bromide, CTAB). Figure 2a shows that the synthesized MCM-41 nanoparticles were spherical, with a uniform size (≈ 100 nm). The structural characteristics of the mesoporous MCM-41 were determined in detail. From the N2 adsorption-desorption isotherms (Supporting Information Figure S4b), the Brunauer-Emmett-Teller (BET) surface area and pore volume of the MCM-41 were estimated to be 1078 m2 g−1 and 1.30 cm3 g−1, respectively. The pore size distribution obtained from the Barrett-Joyner-Halenda (BJH) analysis (inset of Figure S4b) presents a sharp peak at 2.54 nm, indicating the formation of well-defined uniform mesopores. The transmission electron microscopy (TEM) image (Supporting Information Figure S4c) shows the presence of regularly distributed, hexagonal arrays of uniform porous channels, which are a peculiar structural feature of MCM-4139. The MCM-41 nanoparticles, due to their large surface area and mesoporous structure described above, are anticipated to effectively adsorb polysulfides. We theoretically estimated the adsorption amounts of lithium polysulfides on the MCM-41 and Al2O3 using grand canonical Monte Carlo (GCMC) simulations at a fixed pressure of 4 bar (i.e., 1 bar each for Li2Sx, x = 1, 4, 6, and 8, see “Modeling and simulation details” in the Supporting Information). The adsorption amounts were normalized by surface area that was determined using Connolly surface method40 for exact comparison. The lithium polysulfides were mostly adsorbed in the mesopores of the MCM-41 (Figure 2b), compared to adsorption exclusively on the surface of the Al2O3. Figure 2c shows the adsorption amount per unit surface area for MCM-41 and Al2O3. In overall, the amounts of polysulfides adsorbed onto the MCM-41 were much larger than those adsorbed onto the Al2O3 due to its highly developed mesoporous structure. Meanwhile, the amount of polysulfides adsorbed onto the MCM-41 tended to decrease with increasing polysulfide size.

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This theoretical consideration was verified by experimentally investigating polysulfide adsorption onto the MCM-41. The MCM-41 and Al2O3 nanoparticles were soaked in an electrolyte solution (0.1 M Li2S8 in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) = 1/1 (v/v)27,41), and the amount of adsorbed polysulfides was quantitatively measured using inductively coupled plasma mass spectrometry (ICP-MS) analysis. The MCM-41 exhibited superior adsorption capability (concentration of adsorbed polysulfides ≈ 16,450 ppm) over the Al2O3 (≈ 1,850 ppm) (Figure 2d), which appeared consistent with the aforementioned theoretical calculations. To fabricate the MEC (i.e., MCM-41/(PEI/MWNT)) top layer on the PE separator, we prepared an MCM-41 (2.0 wt%)/MWNT (0.5 wt%) suspension using a mixture of water/isopropyl alcohol (IPA) = 70/30 (w/w) as a solvent (Supporting Information Figure S6a). A further increase in the MCM-41 content to 5.0 wt% resulted in unwanted agglomeration of the particles (Supporting Information Figure S6b) and an abrupt increase in viscosity (Supporting Information Figure S6c), which were not favorable for the electrospraying process. The as-prepared MCM-41/MWNT suspension and the PEI solution were electrosprayed and electrospun onto the PE separator, respectively. The thickness of the resulting composite separator was 35 µm (top layer = 15 µm and PE separator (as a support layer) = 20 µm). From the TGA measurement (Supporting Information Figure S7), the composition ratio of the top layer was estimated to be MCM-41/PEI/MWNT = 27/39/34 (w/w/w). Figure 2e shows that the MCM-41 nanoparticles were close-packed preferentially between the MWNT-wrapped PEI nanofibers rather than being randomly mixed with the MWNT and PEI nanofibers. This morphology can be explained by considering the affinity difference between the MCM-41/MWNT and PEI/MWNT systems. From MD simulations, the intermolecular binding energy (-1712.3 kcal mol-1) of the PEI/MWNT system was calculated 9


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to be more than 5 times larger than that (-246.0 kcal mol-1) of the MCM-41/MWNT system (Supporting Information Figure S8). The remarkably higher binding energy of the PEI/MWNT system mainly results from the π-π stack intermolecular interaction between the PEI and MWNTs. Meanwhile, the porous structure of the MCM-41 is speculated to additionally weaken the intermolecular interactions between the MCM-41 and MWNTs due to non-interactive pore space. Consequently, the MCM-41 nanoparticles are allowed to exist independently with the MWNT-wrapped PEI nanofibers, thus creating well-reticulated interstitial voids between the components.

After

being

filled

with

the

liquid

electrolyte

(1M

LiTFSI

in

DOL/DME/TEGDMA = 1/1/1 (v/v/v)), the interstitial voids act as ion-conducting channels, thereby enabling the MCM-41/(PEI/MWNT)-coated PE separator to provide satisfactory ionic conductivity (1.2 mS cm-1). In addition to this facile ionic transport, the electronic networks formed by the MWNT-wrapped PEI nanofibers were maintained in the presence of the MCM41. The electronic conductivity (1.8 S cm-1) of the MCM-41/(PEI/MWNT) top layer was slightly different from that (3.0 S cm-1) of the PEI/MWNT top layer. We investigated effect of the modified PE separators on charge/discharge behaviors of Li-S cells. To electrochemically activate the cells, 1 M LiTFSI in DOL/DME/TEGDME (1/1/1 (v/v/v)) with 0.2 M LiNO3 additive was used as a liquid electrolyte. The coin-type cells were charged/discharged at a charge/discharge current density of 0.1 C/0.1 C (0.1 mA cm-2) between 1.5 and 2.8 V. Two typical charge/discharge plateaus42-44 were observed for all of the cells (Figure 2f). The PEI/MWNT-coated PE (denoted by “EC-PE”) separator exhibited a larger charge/discharge capacity than the pristine PE separator, verifying the beneficial role of the PEI/MWNT top layer as a conductive upper current collector. The conductive carbonaceous interlayer is known to promote efficient utilization of active materials and to boost electrochemical reaction kinetics7,10,19. The MCM-41/(PEI/MWNT)-coated PE (referred to as 10


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“MEC-PE”) separator showed the largest charge/discharge capacity among the separators examined, demonstrating the MCM-41-assisted additional improvement of cell performance. The aforementioned results were further verified by cyclic voltammetry (CV) profiles of Li-S cells assembled with PE, EC-PE, and MEC-PE separators (Supporting Information Figure S9). The overlapped oxidation peaks at 2.4-2.6 V (corresponding to electrochemical transformation

of

low-order

lithium

polysulfides

to

long-chain

lithium

polysulfides/sulfur7,10,11,48) and two reduction peaks at approximately 2.00 V and 2.35 V (ascribed to the reverse electrochemical transformation of polysulfides) were observed in the anodic and cathodic scans, respectively. The MEC-PE separator exhibited sharper and larger anodic/cathodic peaks than the other separators, which was well consistent with the charge/discharge profiles shown in Figure 2f. The similar CV results and discussion were observed in the previous studies48,49 that reported the effect of chemically modified conductive separators on the redox reaction activity. To explore the MCM-41-enabled adsorption/desorption of polysulfides in the MEC-PE separator, the Li-S cell was disassembled before (i.e., 2.15 V)/after full discharge (i.e., 1.50 V). The separator was collected at the corresponding voltages and subjected to structural analysis. The MEC-PE separator taken at the discharge voltage of 2.15 V showed a larger amount of adsorbed polysulfides than the one collected at 1.50 V (Supporting Information Figure S10a). This ICP-MS result was confirmed by the morphology (Supporting Information Figure S10b and c) of the MEC top layer adjacent to the sulfur cathode. The dense and porous structures were observed at the discharge voltages of 2.15 V and 1.50 V, respectively, reflecting the adsorption (at 2.15 V) and desorption (at 1.50 V) of the polysulfides. These results exhibit that the MCM-41 nanoparticles in the MEC top layer allow for the cell voltage-dependent adsorption/desorption of polysulfides. 11


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Based on this comprehensive understanding of the MCM-41/(PEI/MWNT) (i.e., MEC) top layer, the MEC-AA separator was fabricated using the in-series (AA support layer  MEC top layer), concurrent electrospraying/electrospinning processes (Scheme 1). Figure 3a shows that the support layer (thickness ≈ 30 µm) consisted of electrosprayed, close-packed Al2O3 nanoparticles and electrospun PAN nanofibers, and was seamlessly integrated with the MEC top layer (~ 15 µm). The support (white) and top (black) layers showed different colors (insets of Figure 3a), underlying the unique structure (i.e., heterolayered composite nanomats) of the MEC-AA separator (total weight = 3.87 mg cm-2: MEC top layer = 1.84 mg cm-2, AA support layer = 2.03 mg cm-2). The tensile properties of the MEC-AA separator and also the AA support layer were measured using a universal testing machine (Supporting Information Figure S11). No significant difference between the MEC-AA separator and AA support layer was observed, indicating the important role of the AA support layer in the tensile properties. Prior to conducting an in-depth characterization of the resultant MEC-AA separator, we investigated the basic membrane properties of the Al2O3/PAN support layer. From the TGA measurement, the composition ratio of the support layer was Al2O3/PAN = 83/17 (w/w) (Supporting Information Figure S12). The highly-reticulated interstitial voids in the support layer allowed the facile ion transport (after being filled with the liquid electrolyte), leading to the higher ionic conductivity (2.39 mS cm-1) than that (0.97 mS cm-1) of the PE separator (Supporting Information Figure S13a). The ICP-MS analysis showed that the support layer presented the larger adsorption of polysulfides than the PE separator (Supporting Information Figure S13b), indicating the potential capability of Al2O3 nanoparticles as a polysulfidecapturing agent. We explored MCM-41 as an alternative to Al2O3 in the support layer. The MCM41/PAN separator membrane was fabricated using the same technique employed for the 12


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Al2O3/PAN support layer. The MCM-41 nanoparticles were densely packed with being spatially besieged by the PAN nanofibers (Supporting Information Figure S14a), which appeared similar to the morphology of the Al2O3/PAN support layer. The Li-S cell containing the MCM-41/PAN separator membrane was prepared and then cycled between 1.5 and 2.8 V at a charge/discharge current density of 0.1 C/0.1 C. During the 1st discharge, the cell showed two typical discharge plateaus and a discharge capacity of 958 mAh g-1 (Supporting Information Figure S14b). However, subsequent charging led to a low initial coulombic efficiency (~ 51%), signifying the occurrence of a redox shuttle phenomenon7,11,13. As a result, a considerable decrease in the discharge capacity and coulombic efficiency was observed for the 2nd cycle. This behavior can be explained by the MCM-41-driven adsorption/desorption of the polysulfides (Supporting Information Figure S10). As the discharge reaction proceeded (i.e., the cell voltage decreased), the MCM-41 in the MCM-41/PAN separator membrane released the polysulfides that were adsorbed by the MCM-41 at higher discharge voltages. The polysulfides desorbed from the MCM-41 may provoke an unwanted shuttle effect and prompt deposition of insoluble polysulfides onto the electrodes. This result reveals that the simple use of MCM-41 in battery separators is not a good approach, which thus forces us to develop the MEC-AA separator as an effective way to address this issue. To better understand this unusual charge/discharge behavior, the adsorption energies of the polysulfides on MCM-41 and Al2O3 were investigated theoretically using DFT calculations (Supporting Information Figures S15-S17 and Table S1). The adsorption energy (∆Eads) was deconvoluted into three energy contributions45: i) structural change of the polysulfides (∆ELi2Sx), ii) reconstruction of the host structure of MCM-41 and Al2O3 (∆Erecon), and iii) bond formation (∆Ebond) (see “Energy calculations” for calculation details in the Supporting Information). The adsorption (∆Eads) and bond formation (∆Ebond) energies can be used as indicators for the 13


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adsorption and desorption of polysulfides, respectively. Although the ∆Eads of Al2O3 was similar to that of MCM-41, the ∆Ebond of Al2O3, which was accompanied by large structural changes in the polysulfides (∆ELi2Sx) and host structure (∆Erecon), was estimated to be much higher than that of MCM-41. This theoretical calculation demonstrates that, after the polysulfides adsorb onto the Al2O3 surface, the stronger bond energy (∆Ebond) makes their desorption difficult. In comparison, the polysulfides are expected to easily desorb from MCM41. These results guided us to design the architecture and components of the MEC-AA separator with the purpose of maximizing the adsorption/desorption effect of MCM-41. Specifically, the MCM-41 nanoparticles should exist as a component of the top layer while the Al2O3 nanoparticles (capable of capturing polysulfides) should be incorporated into the support layer. The membrane properties of the MEC-AA separator were characterized in detail. Driven by the polar components and well-established interstitial voids, the MEC-AA separator presented a higher electrolyte-immersion height than the PE separator (Supporting Information Figure S18a), indicating better electrolyte wettability. The Al2O3/PAN support layer, in combination with the MCM-41/(PEI/MWNT) top layer, enabled the MEC-AA separator to show exceptional thermal tolerance (i.e., no thermal shrinkage at 150 °C/0.5 h, Supporting Information Figure S18b). Neither structural disintegration nor detachment of the components was observed for the MEC-AA separator after the crumpling/uncrumpling and also the tape test (using commercial 3M Scotch® tape) (Supporting Information Figure S18c). The polysulfide-capturing capability of the MEC-AA and MEC-PE separators was examined (Supporting Information Figure S19). The comparison with the results of the AA and PE separators (Supporting Information Figure S13b) shows that the MEC top layer plays a crucial role in the polysulfide adsorption. 14


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The effect of the MEC-AA separator on the electrochemical performance of Li-S cells was investigated, with a particular focus on the PEI/MWNT (acting as a dual (electron/ion)conductive upper current collector), MCM-41-assisted adsorption/desorption of polysulfides, and Al2O3-driven capturing of polysulfides. Figure 3b compares the discharge rate capability of the separators examined herein, where the discharge current densities varied from 0.2 to 1.0 C at a fixed charge current density of 0.2 C under a voltage range of 1.5 - 2.8 V. The MEC-AA separator showed the highest discharge capacities than the other separators over a wide range of discharge current densities, demonstrating the synergistic effects of the dual-conductive MEC top layer and ion-conductive AA support layer on redox reaction kinetics. Notably, the superior discharge rate capability of the MEC-AA separator compared to that of the MEC-PE separator underscores the importance of a porous structure in the support layers (i.e., the AA support layer is more favorable for ionic transport than the PE support layer). We investigated the cycling performance of the cells, wherein the cells were cycled between 1.5 and 2.8 V at a charge/discharge current density of 0.5 C/0.5 C. The MEC-AA separator showed the largest charge/discharge capacities with cycling than the MEC-PE and pristine PE separators (Supporting Information Figure S20). Moreover, the cell polarization was significantly alleviated at the MEC-AA separator (i.e., the lowest charge and highest discharge plateau potentials) in comparison to the results of the MEC-PE and pristine PE separators. Figure 3c presents the capacity retention of the separators as a function of cycle number. The MEC-AA separator showed the highest initial discharge capacity and also decent capacity retention after 150 cycles. In the case of the MEC-PE separator, the initial discharge capacity was higher than that of the PE separator, however, its capacity retention deteriorated with incresing cycle number. This result reveals that the MCM-41 nanoparticles in the MEC top layer released the previously trapped polysulfides at the end of discharge (i.e., 1.50 V) and 15


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the released polysulfides easily migrated to the lithium metal anode via the chemically inert PE separator, giving rise to the shuttle effect and the deposition of insoluble polysulfides on the electrodes. The aforementioned difference in the cycling performances between the MEC-AA and MEC-PE separators demonstrates the beneficial function of Al2O3 (in the support layer) as a polysulfide-capturing agent to prevent migration of the polysulfides (desorbed from the MCM41) toward the lithium metal anode. The local confinement of the polysulfides between the sulfur cathode and MEC-AA separator, in combination with the conductive PEI/MWNT nanofibers in the top layer, could promote efficient utilization of sulfur active materials, eventually leading to the superior cycling performance. The exceptional multifunctionality of the MEC-AA separator is conceptually illustrated in Scheme 1. To better elucidate these advantageous effects of the MEC-AA separator on the cycling performance, a post-mortem analysis of the Li-S cells assembed with the MEC-AA separator (vs. PE separator) was conducted. After the cycling test (100 cycles at 0.5 C/0.5 C), the cells were disassembled and then their major components including the sulfur cathode, lithium metal anode and separator were characterized. Figure 4a shows that the porous structure of the sulfur cathode assembled with the MEC-AA separator was well preserved after 100 cycles. In comparison, for the cell with the PE separator (inset of Figure 4a), the cathode surface was seriously contaminated with the dense resistive layer, which is believed to be non-active sulfur species22,31,45 that resulted in low electrochemical utilization of the sulfur active materials. In addition to the sulfur cathodes, the separators and lithium metal anodes were analyzed. Figure 4b shows that the top layer (adjacent to the sulfur cathode) of the MEC-AA separator was covered with sulfur-containing agglomerates, which was confirmed by the EDS mapping image (inset of Figure 4b). This result demonstrates that the MEC top layer effectively 16


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captured the polysulfide species during the cycling. By contrast, no appreciable change in the porous morphology of the PE separator was observed (Figure 4c), with the exception of sparsely populated clogged pores. To further verify this structural difference in the separator surfaces, an ex-situ Raman analysis was conducted (Figure 4d). The MEC-AA separator showed the three characteristic peaks (assigned to elemental sulfur46) at 152, 218, and 470 cm−1 while no detectable level of peaks were observed in the spectra of the PE separator. In addition, X-ray photo-electron spectroscopy (XPS) analysis (Supporting Information Figure S21), which was performed after the outermost layer (thickness = 20 nm) of the MEC-41/(PEI/MWNT) surface was etched using ion sputtering, exhibited three S 2p doublets47,48, indicating capture of long-chain polysulfide compounds by the MEC-AA separator. The surface of the separators positioned toward the lithium metal anode was examined using XPS analysis (Figure 4e). Both separators showed characteristic peaks at 162.2 and 164.0 eV, which are ascribed to the Li2S and S-S bonds50, respectively. The MEC-AA separator provided the weaker peak intensity at 167.0 eV (associated with lithium sulfate (LixSOy)) than the PE separator. The formation of insulating LixSOy compounds is known to be irreversible in Li-S cells, thus exerting harmful influence on cycling performance47,51,52. This result shows that the MEC-AA separator suppressed the migration of polysulfides toward the lithium metal anode. The absolute amount of the polysulfides deposited on the lithium metal anode after the cycling test (100 cycles) was quantitatively analyzed using ICP-MS technique. Figure 4f shows that a substantially lower sulfur content was detected on the lithium metal anode assembled with the MEC-AA separator, confirming the separator-enabled trapping of polysulfides. In summary, we presented the multifunctional, heterolayered battery separator membrane (MEC-AA separator) based on 0D (nanoparticles)/1D (nanofibers) composite mat as a new membrane-driven strategy to enable high-performance Li-S batteries. The MEC-AA 17


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separator was fabricated using the in-series, concurrent electrospraying/electrospinning process. The top layer of the MEC-AA separator was composed of the close-packed mesoporous MCM-41 nanoparticles that were spatially besieged by the MWNT-wrapped PEI nanofibers. The preferential deposition of the MWNTs along the PEI nanofibers and the dispersion state of the major separator components were elucidated using the theoretical calculations. In the top layer, the MCM-41 exhibited the adsorption/desorption of polysulfides as a function of cell voltage and the MWNT-wrapped PEI nanofibers acted as a dualconductive upper current collector, leading to the superiority in the utilization of sulfur active materials and the capacity retention with cycling. The support layer, consisting of the denselypacked Al2O3 nanoparticles and PAN nanofibers, enabled the facile ion transport and also prevented the crossover of polysulfides toward the lithium metal anode as a mechanically/thermally stable, polysulfide-capturing porous membrane. Owing to the structural uniqueness and exceptional multifunctionality, the MEC-AA separator remarkably improved the redox reaction kinetics and cycling performance far beyond those achievable with conventional polyolefin separators. We envision that the heterolayered separators featuring the elaborately designed 0D/1D composite nanomats hold great promise as a versatile and effective membrane platform technology for next-generation energy storage systems (including Li-S, metal-air, and flow batteries) which are in urgent need of electrochemicallyactive/permselective advanced separator membranes.

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FIGURE CAPTIONS

Scheme 1. Schematic of the fabrication of MEC-AA separators and their multifunctionality. The MEC-AA separators, which feature the multifunctional (i.e., adsorption/desorption of polysulfides, dual (electron/ion)-conduction,

and

superior

mechanical/thermal stability) heterolayer structure based on 0D (nanoparticles)/1D (nanofibers) composite

mats,

were

fabricated

electrospraying/electrospinning

process.

through The

the

support

in-series,

layer

concurrent

(close-packed

Al2O3

nanoparticle/PAN nanofiber composite mat) was prepared via the concurrent electrospraying (for Al2O3) and electrospinning (for PAN). On top of the support layer, the top layer (densely packed MCM-41 nanoparticles spatially besieged by MWNT-wrapped PEI nanofibers) was fabricated using the same concurrent electrospraying (for MCM-41/MWNT) and electrospinning (for PEI).

Figure 1. Fabrication and structural analysis of the PEI (or PAN)/MWNT top layers on PE separators. (a) Schematic of the electrospun PEI (or PAN)/electrosprayed MWNT top layer on the PE separator. (b) SEM image of the PEI/MWNT top layer. Inset is a photograph of the PEI/MWNT-coated PE separator after crumpling test. (c) SEM image of the PAN/MWNT top layer. Inset is a photograph of the PAN/MWNT-coated PE separator after crumpling test. (d) Theoretical comparison of the intermolecular interaction energy (calculated via MD simulation) between MWNT and polymers (i.e., PEI or PAN).

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Figure 2. Fabrication and structural/electrochemical analysis of the MCM41/(PEI/MWNT) top layers on PE separators. (a) SEM image of the mesoporous MCM-41 nanoparticles synthesized via a sol-gel method. (b) Snapshots (calculated through GCMC simulation at a fixed pressure of 4 bar) depicting the binding configurations of polysulfides in MCM-41 and Al2O3. Silicon, oxygen, hydrogen, and aluminum atoms are colored as orange, red, white and pink, respectively. Blue, cyan, orange, and green dots represent Li2S, Li2S4, Li2S6, and Li2S8, respectively. (c) Theoretically calculated adsorption amounts of polysulfides per unit surface area in MCM-41 and Al2O3. (d) Amount of polysulfide anions (experimentally measured using the ICP analysis) trapped by the Al2O3 and MCM-41 particles. (e) SEM image of the MCM-41/(PEI/MWNT) (i.e., MEC) top layer. Inset is high-magnification SEM image of top layer. (f) Charge/discharge profiles of the Li-S cells assembled with the pristine PE, ECPE, and MEC-PE separators. The cells were charged/discharged at a charge/discharge current density of 0.1 C/0.1 C between 1.5 and 2.8 V.

Figure 3. Fabrication and structural/electrochemical analysis of the MEC (i.e., MCM41/(PEI/MWNT))-AA (i.e., Al2O3/PAN) separators. (a) SEM images of the MEC-AA separator: (left, surface view) AA support layer, (middle, cross-sectional view) MEC-AA separator, and (right, surface view) MEC top layer. Insets are photographs of the AA support layer (white color) and MEC top layer (black color), respectively. (b) Effect of (pristine PE, MEC-PE, and MEC-AA) separators on discharge rate capability of the Li-S cells, in which the discharge current densities varied from 0.2 to 1.0 C at a fixed charge current density of 0.2 C under the voltage range of 1.5 - 2.8 V. (c) Effect of (pristine PE, MEC-PE, and MEC-AA) separators on cycling performance (charge/discharge current density = 0.5 C/0.5 C) of Li-S cells. 20


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Figure 4. Post-mortem analysis (after 100 cycles) of the Li-S cells assembled with MECAA separator (vs. PE separator). (a) SEM image of the sulfur cathode (surface) assembled with the MEC-AA separator. Inset is a SEM image of the sulfur cathode (surface) assembled with the PE separator. (b) SEM image of the top layer (adjacent to the sulfur cathode) of the MEC-AA separator. Inset is an EDS mapping image. (c) SEM image of the PE separator. Inset is an EDS mapping image. (d) Ex-situ Raman spectra of the MEC-AA and PE separators. (e) XPS S 2p spectra of the MEC-AA and PE separators (surface) positioned toward the lithium metal anodes. (f) Amount of polysulfides deposited on the lithium metal anodes.

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ASSOCIATED CONTENT Supporting Information. Details of methods and supplementary results. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental procedures, calculation and simulation details, additional structural/membrane properties of MEC-AA separators and electrochemical performance of cells.

AUTHOR INFORMATION Corresponding Authors *E-mail: (S. K. K.) [email protected]; (S.-Y.L.) [email protected] Author contribution J.-H.K., G.Y. J. and Y.-H.L. contributed equally. J.-H.K., G.Y.J., Y.-H.L., S.K.K. and S.-Y.L. participated in conceiving and designing the project. J.-H.K., G.Y.J. and Y.-H.L. contributed to preparing the samples and conducted data analysis. J.-H.K. assisted the electrochemical analysis. G.Y.J. designed and conducted DFT calculations, MD simulations and GCMC simulations. S.K.K. and S.-Y.L. coordinated and supervised the overall project. All authors discussed the results and participated in manuscript preparation. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2015R1A2A1A01003474) and also Wearable Platform Materials Technology Center (2016R1A5A1009926) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning. This work was also supported by the Korea Forest Research Institute (FP 22


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0400-2016-01), Industrial Technology Innovation Program (20152020104730) funded by the Ministry of Trade, Industry & Energy, and Batteries R&D of LG Chem. S.K.K. acknowledges the financial support from NRF-2014R1A5A1009799 and computational resources from UNIST-HPC and KISTI-PLSI.

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Table of Contents Graphic Polysulfide-Breathing/Dual-Conductive, Heterolayered Battery Separator Membranes Based on 0D/1D Mingled Nanomaterial Composite Mats

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Scheme 1

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Dual-Conductive, Heterolayered Battery

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