Suppressed Polysulfide Crossover in Li–S Batteries through a High

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Suppressed Polysulfide Crossover in Li−S Batteries through a High-Flux Graphene Oxide Membrane Supported on a Sulfur Cathode Mahdokht Shaibani,†,‡ Abozar Akbari,† Phillip Sheath,† Christopher D. Easton,‡ Parama Chakraborty Banerjee,† Kristina Konstas,‡ Armaghan Fakhfouri,† Marzieh Barghamadi,‡,§ Mustafa M. Musameh,‡ Adam S. Best,‡ Thomas Rüther,‡ Peter J. Mahon,§ Matthew R. Hill,*,‡,⊥ Anthony F. Hollenkamp,*,‡ and Mainak Majumder*,† †

Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3168, Australia ‡ CSIRO, Clayton, VIC 3168, Australia § Department of Chemistry and Biotechnology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia ⊥ Department of Chemical Engineering, Monash University, Clayton, VIC 3168, Australia S Supporting Information *

ABSTRACT: Utilization of permselective membranes holds tremendous promise for retention of the electrode-active material in electrochemical devices that suffer from electrode instability issues. In a rechargeable Li−S batterya strong contender to outperform the Li-ion technologymigration of lithium polysulfides from the sulfur cathode has been linked to rapid capacity fading and lower Coulombic efficiency. However, the current approaches for configuring Li−S cells with permselective membranes suffer from large ohmic polarization, resulting in low capacity and poor rate capability. To overcome these issues, we report the facile fabrication of a high-flux graphene oxide membrane directly onto the sulfur cathode by shear alignment of discotic nematic liquid crystals of graphene oxide (GO). In conjunction with a carbon-coated separator, the highly ordered structure of the thin (∼0.75 μm) membrane and its inherent surface charge retain a majority of the polysulfides, enabling the cells to deliver very high initial discharge capacities of 1063 and 1182 mAh g−1 electrode for electrodes with 70 and 80% sulfur content, respectively, at the practical 0.5 C rate. The very high sulfur utilization and impressive capacity retentions of the high sulfur content electrodes result in some of the highest performance metrics in the literature of Li−S (e.g., electrode capacity of 835 mAh g−1 electrode after 100 cycles at 0.5 C with a sulfur content of 80%). We show that the structural order of the shear-aligned GO membrane is key in maintaining good kinetics of the charge transfer processes in Li−S batteries. KEYWORDS: lithium−sulfur battery, shear-aligned graphene oxide membrane, polysulfide retention, high sulfur content

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The insulating nature of sulfur and its reduction products (Li2Sx), the high solubility of higher-order polysulfides (Li2Sx, 4 ≤ x ≤ 8) in the electrolyte, and large volumetric expansion of sulfur upon reduction are known to be major issues which lead to the low utilization of active material, rapid capacity fading, poor rate capability, and low Coulombic efficiency of Li−S cells.9,11−14 Among these drawbacks, the continuous diffusion/ migration of highly soluble polysulfides from the cathode through the separator to the lithium anode and vice versa is primarily responsible for rapid capacity fading of Li−S cells.15,16

dvances in battery technologies will impact a wide array of applications such as extending the transportation range of electric vehicles and enabling compact energy storage systems to use renewable sources efficiently. Such applications demand batteries with considerably higher specific energy than state-of-the-art lithium-ion devices which have storage capacities of around 240−300 Wh kg−1.1 Recently, lithium−sulfur (Li−S) battery chemistry has received much attention due to its theoretical specific capacity of 1675 mAh g−1 and theoretical specific energy of 2600 Wh kg−1.2−7 Moreover, elemental sulfur is inexpensive, abundant, and lightweight, which predicts both high-quality and competitive price points,8−10 and yet a number of challenges remain unresolved toward realizing the potential of Li−S batteries. © 2016 American Chemical Society

Received: May 18, 2016 Accepted: July 11, 2016 Published: July 11, 2016 7768

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Figure 1. Fabrication and characterization of the shear-aligned GO membrane on the sulfur cathode: (a) Schematic illustration of shearalignment processing of a nematic phase of GO to a membrane on the sulfur cathode. Processed false-color images of the (b) nematic GO and (c) shear-aligned GO membrane; regions with the same color have the same azimuth angles, as depicted by the slow axis orientation, showing the high degree of order and alignment imparted to graphene sheets upon applying shear to nematic GO. This is supported by the vector overlay with the corresponding polar histogram of the azimuth angles and the in-plane order parameter S for (d) nematic GO and (e) shearaligned GO membrane. (f,g) Scanning electron microscopy images of the cross section of the GO coating on the sulfur cathode, showing that an ultrathin membrane of submicron (∼0.75 μm) thickness has covered the sulfur cathode surface.

materials has been the most widely used strategy in the literature to address the key challenges of Li−S batteries. Conductive host materials such as various types of conducting polymers or carbon were shown to be more successful in overcoming the insulating nature of sulfur,17,20 while host materials with the ability to interact directly with polysulfides, including metal oxides,14 metal organic frameworks (MOFs),21

Strategies for retention of sulfur and its reduction products on the cathode side of the battery have been suggested by improvements in the sulfur cathode durability5,17 and by utilizing membrane separators.16,18,19 Efforts in this field have been particularly focused on tailoring the structure and composition of the sulfur cathode.18,19 Producing composite cathodes by confining sulfur particles within various host 7769

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the electrode by spreading the viscoelastic GO fluid by application of large shear forces. The small volume fraction of solids in the GO dispersion (1.67%) ensures the formation of a thin membrane, and the shearing force confers a high degree of in-plane orientational order of the GO sheets in the GO membranes. We note that this microstructural order has been observed to enhance water transport behavior through the organized and less tortuous channels formed by the interlayer galleries of GO.31,32 Additionally, the permselective GO membrane on the sulfur cathode allows the battery to benefit from an advantageous configuration in which the functional GO membrane is in direct contact with an electrically conducting carbon-coated separator to form a cathodic “subcell” for capturing and reducing polysulfide species that may escape the GO membrane in a manner similar to the conducting interlayer concept demonstrated earlier.5,33−36 This tandem strategy of a GO membrane and a conducting carboncoated separator enables much higher utilization of the active mass. Just as importantly, the submicron thick highly ordered GO membrane is shown to have negligible effect on the rate capability, which enables delivering high initial discharge capacities of 1063 and 1182 mAh g−1 electrode for electrodes with 70 and 80% sulfur content, respectively. In addition to high sulfur utilization, stable cycling behavior and high Coulombic efficiencies for sulfur cathodes with active material contents as high as 80% suggest that this approach may be used in practical Li−S cells. As a thin shear-aligned GO membrane excels at retaining this system’s capacity, it is anticipated that it may be applied generally to electrodes in other electrochemical systems where dissolution of active material in the electrolyte is a problem.

graphene oxide (GO),22,23 and functionalized carbon materials5 have been proven to promote capacity retention to higher levels. Although these studies have had some success in retaining the capacity, there are remaining challenges with the approach of composite cathodes. For achieving high total gravimetric capacities, the active material fraction (% S), load (mgs cm−2), and utilization (mAh gs−1) are crucial factors alongside the passive weight of the cell components.1 Evaluation of the state-of-the-art Li−S research shows that the active material fraction in composite cathodes is typically smaller than 60%, which is clearly lower than that of Li-ion cells (≈90%). Additionally, achieving high sulfur utilization especially at high C rates is difficult given that a high fraction of the electrode composition is devoted to a host material which is not very conductive in most cases.14,21−23 The separator, sitting between the positive and negative electrodes, assumes a critical role in liquid electrolyte energy storage devices beyond preventing physical contact of the electrodes. In general, the membrane separator should show sufficient wettability, porosity, chemical, mechanical, thermal, and dimensional stability while demonstrating high permeability for ion flow.24 However, for electrochemical systems where redox-active species are dissolved or dispersed in the electrolyte, such as redox flow batteries,25−27 fuel cells,28,29 and Li−S batteries,16,18 permselectivity of these membranes could be harnessed advantageously by retaining unreacted and unutilized electrode reaction products while allowing the easy passage of ionic species. More specifically, this approach holds tremendous promise for mitigating the polysulfide migration issue facing Li−S battery technology, but it has received much less attention.16,18 Arguably, integrating a permselective membrane could restrict the crossover of polysulfides more efficiently and to a greater extent. Helms et al. demonstrated the use of ∼10 μm thick membranes fabricated from polymers of intrinsic microporosity (PIMs) to dramatically decrease the migration of polysulfides.18 One other promising material that could serve effectively as permselective membrane is GO. This is built on the fact that GO is an excellent sulfur immobilizer arising from the plethora of oxygen functional groups on the surface,22,23,30 and that these oxygenated groups can repel the negatively charged polysulfides and resist their transport to the anode side, resulting in higher capacity retention. Taking advantage of this, Zhang et al. showed that a GO membrane separator fabricated by a vacuum filtration technique can be cycled at 0.1 C, starting at ≈1000 mAh gs−1 and retaining 70% capacity across 100 cycles.16 While being successful in retention of polysulfides to a degree (compared to cells without a GO membrane), low capacity and poor rate capability of the Li−S cells configured with a ∼4 μm thick tortuous structured GO membrane fabricated by a vacuum filtration technique16 suggest that the factors involved in successful use of GO membranes are yet to be fully elucidated. In particular, low thickness and fast mass transport properties of the separators are needed to reduce Ohmic polarization, which limits the use of a permselective membrane separator for achieving high-energy/power characteristics.24 Here, we demonstrate the fabrication and use of a submicron thick GO membrane coated directly onto the sulfur cathode by a blade coating technique;31 the GO membrane assists in retaining the polysulfides without unduly hindering ion transport. Typically, we use the discotic nematic liquidcrystalline phase of GO, and the GO membrane is formed on

RESULTS AND DISCUSSION Fabrication and Characterization of the ShearAligned GO Membrane on a Cathode. Graphene oxide with an initial concentration of ∼0.25 mg mL−1 was concentrated to ∼30 mg mL−1 using an innovative method developed previously31 (see Supporting Information). At 30 mg mL−1, GO has already surpassed the colloidal phase transitions from isotropic to nematic liquid-crystalline phase.37 The liquidcrystalline behavior allows the GO layers to flow and respond to macroscopic force-fields such as shear.31,38 Taking advantage of this, we readily fabricated a GO-coated cathode by a lab-scale doctor blade as a shear-alignment tool. The doctor blade has a rectangular outlet formed between the blade and the substrate, through which the movable blade spreads the GO dispersion on the sulfur cathode (Figure 1a). Subsequently, the resultant GO-coated cathode was dried at 50 °C under vacuum for 12 h to remove any water residues. The degree of order and alignment of graphene sheets upon coating the nematic phase dispersion of GO has been demonstrated by polarized light microscopy imaging. The LC-PolScope-Abrio (LPS) is a sensitive polarized light microscopy technique capable of quantifying the in-plane order within graphene-based membranes.31,39,40 Using the standard deviation of the azimuth distribution of the GO assemblies, the consistency of alignment is calculated as a scalar order parameter S, where S = 1 and S = 0 represent, respectively, a perfectly oriented and an isotropic system.31,39 Figure 1b,c shows processed false-color images of the nematic phase of GO and the shear-aligned GO membrane, where the hue represents the azimuth as depicted by the slow axis orientation. Regions with the same hue represent the same 7770

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Figure 2. (a) Schematic illustration of a Li−S cell configured with a GO-coated sulfur cathode facing a carbon-coated separator. (b) First charge−discharge profiles of GO-coated cathodes at various C rates. (c) SEM images of GO-coated cathode (all scale bars are 5 μm): (I) before cycling, (II) after 15 deep cycles at the end of discharging, and (III) at the end of charging. The presence of insoluble discharge products on the surface of the GO membrane at the end of discharging shows that the conductive carbon layer on the separator acts as an upper current collector for the escaped partially reduced polysulfides and assists with their further reduction to the final discharge products. The accumulated discharge products on the GO membrane at the end of the following period of charging are noticeably absent, which demonstrates the reversibility of the redox reactions at the interface of the GO membrane/conductive separator, showing the efficiency of our cathodic “subcell”. (d,e) Discharge capacity of GO-coated cathodes at various cycling rates showing the high rate capability and excellent capacity retention of our innovative cell configuration. (f) Long-term cycling performance of sulfur cathode at 1 C rate with and without the GO membrane.

GO membrane and the in-plane order parameter S. A uniform hue in Figure 1c and a S parameter calculated as ∼0.99 demonstrate the highly ordered structure of the GO membrane coated by blade coating of GO in its liquid-crystalline phase. This is as opposed to the membranes formed by vacuum

azimuth angle, showing the high in-plane order imparted to the shear-aligned GO membrane. This is supported by their slow axis vector representations in Figure 1d,e, showing the vector overlay together with the polar histogram of the azimuth angles taken from the same part of the nematic GO and shear-aligned 7771

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ACS Nano filtration of GO dispersions which possess an in-plane order parameter of S ∼ 0.3, representative of a membrane with disordered graphene layers.31,39 Scanning electron microscopy (SEM) images of the GO membrane on the sulfur cathode are shown in Figure 1f,g. SEM images of the cross section of the GO coating show that, further to its highly ordered structure, it is of submicron (∼0.75 μm) thickness and has covered the sulfur cathode surface. Due to the extreme surface roughness of the sulfur cathode, a uniform continuous coating could only be achieved at thicknesses around 0.70−0.80 μm (Figure S1, Supporting Information), which is still very thin compared to reports dealing with protective coatings and interlayers.16,33,35,41 The influence of the structural properties of the shear-aligned GO membrane on the cycling behavior of the Li−S battery will be discussed in the next sections. Electrochemical Performance of a GO-Coated Cathode. First, the ability of the GO membrane to retain polysulfides was evaluated by cycling typical sulfur cathodes prepared by a melt−diffusion method (NPC-35S);17 the performances of cathodes with higher fractions of sulfur are described in the next section. The configuration of a typical cell comprising a GO-coated sulfur electrode as the cathode, lithium metal as the anode, and a carbon-coated glass fiber mat as the separator is shown in Figure 2a. Applying the GO membrane on the sulfur cathode allows our configuration to benefit from having an additional carbon layer coated on the separator in conjunction with the cathode. We speculate that if the higher-order polysulfides escape the GO membrane they would be in contact with either the cathode surface or the carbon-coated separator, as a conducting interlayer,42 to maximize utilization of the active material. More information on the characteristics of microporous carbon used in this work is available in Figures S2 and S3 in the Supporting Information. A control cyclic voltammetry (CV) experiment on a cell assembled with a GO-coated aluminum foil cathode, carboncoated glass fiber separator, and lithium metal anode was conducted to test the electrochemical properties of the GO in the typical cycling conditions of a Li−S battery. It is shown in Figure S4 (Supporting Information) that only very small redox peaks were seen during cycling between 1.8 and 2.8 V, the typical voltage range of a Li−S battery, thereby demonstrating the high electrochemical stability of the additional layers in our configuration. A cyclic voltammogram of a Li−S cell, with a GO-coated sulfur cathode, at a scan rate of 0.1 mV s−1 is shown in Figure S5 in the Supporting Information. It reveals two well-defined reduction peaks assigned to the multistep reduction mechanism of elemental sulfur. Figure 2b presents typical examples of initial charge−discharge V−t profiles for our Li−S cells at different discharge rates between 2.8 and 1.8 V. The electrochemical reduction of elemental sulfur during discharge can be divided into two major stages in accordance with the two-plateau discharge profile in Figure 2b. The top, shorter, plateau at 2.2−2.4 V is related to the formation of higher-order polysulfides which are highly soluble in the electrolyte. Electrochemical reduction of sulfur is further completed in the long and flat plateau at potentials ∼2.1 V, where high-order polysulfides are reduced to Li2S2 and Li2S.11,43 Here, the cell cycled at 0.2 C exhibits a very high discharge capacity of 1616 mAh gs−1 (∼96% of the theoretical capacity of sulfur), which is a remarkable result for a moderate cycling rate. Several previous studies have been unsuccessful in achieving a first discharge

capacity close to the theoretical maximum value (1675 mAh g−1). This indicates that the higher-order polysulfides produced during the first step of the discharge process were almost completely reduced to Li2S. We attribute this to the oxygen functional groups of the GO membrane (Figures S6 and S7, Supporting Information), which retains highly soluble polysulfides within the cathode, assisted by the carbon-coated separator that allows for further utilization of the leaching polysulfides. This also prevents the GO membrane from being passivated by the insoluble and insulating discharge products bound to its surface. This beneficial aspect of our rational configuration is further explored by careful inspection of the GO membrane after cycling. Figure 2c-I shows an SEM image of the GO membrane on a sulfur cathode extracted from an uncycled cell showing the same features of the pristine GOcoated cathode in Figure S1a (Supporting Information). Changes in the GO membrane after 15 deep cycles (0.1 C) at the end of discharging and at the end of charging are shown in Figure 2c-II and 2c-III, respectively. Insoluble discharge products are clearly seen on the surface of the GO membrane in Figure 2c-II, validating that the dissolved polysulfides which may have escaped through the GO membrane could be further utilized at the interface of GO membrane/conductive separator. Vanishing of the accumulated discharge products on the GO membrane at the end of the following period of charging demonstrates the reversibility of the redox reactions at this interface (Figure 2c-III). Cycling at relatively higher rates of 0.5 and 1 C show initial discharge capacities of 1403 and 1170 mAh gs−1 corresponding to 84 and 70% sulfur utilization, respectively (Figure 2b). Not only are the initial discharge capacities high, but the capacity retentions are also outstanding (Figure 2d,e), which demonstrates the excellent performance of our Li−S cells. To further emphasize the effect of the GO membrane on the rate capability and capacity retention of the sulfur cathode, long-term (400 cycles) cycling performance of a GO-coated cathode at 1 C rate was compared to that of an uncoated control cathode (Figure 2f). As shown in Figure 2f, the discharge capacity of the GO-coated cathode increases gradually within the first 20 cycles until it reaches its maximum of ∼1100 mAh gs−1, with ∼750 mAh gs−1 being sustained after 400 cycles, which corresponds to an impressive 70% retention. The fact that it takes some cycles for the capacity to reach its maximum originates from a wetting issue as it does not happen while cycling at lower rates, where there is enough time for the electrolyte to diffuse through the GO layer and wet the sulfur cathode. This diffusion issue, particularly at high rate cycling (1 C and above), could be overcome by putting the cell on rest before cycling (Figure 2d and Supporting Information Figure S8). It should also be noted that charge−discharge testing on a control cell (GO membrane alone) at the same current density as a Li−S cell with a GO-coated cathode showed negligible capacity contribution from the GO membrane (Figure S9, Supporting Information). On the other hand, the uncoated sulfur cathode cycled in similar conditions and configuration shows an initial discharge capacity of 1300 mAh gs−1, which is 15% higher than the maximum capacity a GO-coated cathode could reach at this rate; however, the retention of this high initial capacity is only 32% over 400 cycles. Clearly, the conductive and porous carbon film on the separator is effective for promoting high utilization of active material. It is not very effective, however, in limiting the migration of polysulfides. This is where the addition of the thin GO membrane on the 7772

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Figure 3. Visual and analytical confirmation of polysulfide retention by the GO membrane: first and fifth cycle of a Li−S cell configured with a (a) shear-aligned GO-coated membrane and (b) bare membrane. Nyquist plots (c) before and (d) after cycling of Li−S cell configured with a bare, an ordered, and a disordered GO-coated separator. (e) Disassembled Li−S cells after 15 deep cycles at 0.1 C rate in the discharged state: (I) in the cell configuration with an uncoated cathode, both the separator and lithium anode exhibit a yellow appearance due to the presence of polysulfides; (II) in the cell configuration with a GO-coated cathode, both the separator and lithium anode exhibit no signs of discoloration due to the retention of the polysulfides by the GO membrane. (f) High-resolution X-ray photoelectron spectroscopy S 2p spectra of the uncoated and GO-coated cathodes extracted from disassembled Li−S cells after 15 deep cycles at 0.1 C rate in the discharged state; light blue region represents the higher-order polysulfide region. A maximum contribution of 1.5% for polysulfide species on the surface of the GOcoated cathode compared to 6.1% on the surface of the uncoated cathode verifies that the GO coating on the cathode decreases the leakage of polysulfides from the cathode to a large extent.

cathode inhibits the liberation of active material (polysulfides) into the electrolyte, thereby resulting in outstanding capacity retention. The average Coulombic efficiencythe ratio of discharge of a certain cycle to the charge of the preceding cycleof 99.75% for the GO-coated cathode is clearly a result of greatly suppressed shuttling of polysulfides. This is, to the best of our knowledge, the highest value reported in the literature of Li−S batteries and demonstrates that the properties of our cell are close to those required for a practical rechargeable battery. Additionally, we conducted comprehen-

sive cycling tests at lower rates. Figure S10 (Supporting Information) shows that after 400 cycles the cells can still maintain specific capacities of 1100 mAh gs−1 at 0.1 C, 985 mAh gs−1 at 0.2 C, and 885 mAh gs−1 at 0.5 C rates, providing one of the best performances demonstrated so far for a Li−S cell. Influence of GO and Its Structural Order on the Performance of Li−S Batteries. It is well-known16,44 that the negatively charged oxygen functional groups present on GO can repel the negatively charged polysulfide species back to the 7773

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process from the current collector to the reaction sites. The other time constant represents the charge transfer processes and is composed of a charge transfer resistance, Rct, and constant phase element, Qdl. Qdiff is the constant phase element, which represents the Li-ion diffusion process at the lowfrequency range.47 Incorporation of constant phase elements in the proposed EEC improved the agreement between the simulated and the experimental impedance data, which is justified by noting the distributed surface reactivity, roughness, electrode porosity, current, and potential distributions associated with the electrode.48 The fitting procedure, comparison of the simulated and the experimentally obtained impedance data, and the related error plots are elaborated in Figure S13 in the Supporting Information. Charge transfer resistance is one of the governing factors to control the electrochemical response and the capacity fading of the Li−S battery.44,47 Charge transfer resistance of Li−S cell can increase due to (i) excessive polysulfide dissolution and (ii) formation of irreversible insulating precipitates (Li2S and Li2S2), which forms a nonporous film on the cathode and hinders ion transport to it and can also restrict the path of ionic conduction in the separator. Prior to CV cycling, the charge transfer resistances of the cells containing the ordered and disordered GO separators were similar yet 3 times higher than that of the cell containing a bare separator. This is due to the fact that the dense uniform GO film blocks the penetration of the electrolyte into the macropores of the membrane. However, after five cycles, Rct of the cell containing the bare separator increased six times, which explains the increase in the total impedance (Figure 3d) and the decrease in the electrochemical response (Figures 3b) of this cell due to CV cycling. In fact, evolution of the sulfur cathode into a blocking interface due to CV cycling is evident in the Nyquist plot of a cell containing the bare separator (Figure 3d), which shows that the lowfrequency inclined line (as observed before CV cycling, Figure 3c) was replaced by a semicircle that overlapped with the medium- and the high-frequency semicircles after five cycles of CV. Rct of the cell containing the disordered GO separator increased two times than that prior to CV cycling. In disordered GO coatings, the graphene sheets have random orientation that leads to disordered channels with a broad range of pore sizes.31,39 The random orientation of graphene sheets results in increased tortuosity, mechanical roughness, and chaotic interconnectivity between graphene sheets, which significantly increases the flow resistance of these coatings. Thus, after five cycles of CV, generation of insoluble and insulating Li2S and Li2S2 precipitates on the sulfur cathode and the inherently high flow resistance of the disordered GO membrane result in significant reduction in the number density of electrochemically active reaction sites. This explains the increased impedance and decreased electrochemical response of the cell containing a disordered GO-coated separator. Conversely, little change in Rct was observed in the Li−S battery configured with the ordered shear-aligned GO separator (Table S1 in the Supporting Information). This is attributed to the highly ordered structure of graphene sheets that result in wellorganized precise channels, which significantly enhance electrolyte flow through them. The increased electrolyte flow through the ordered GO membrane maintains good charge transfer kinetics at the electrode/electrolyte interface and thereby facilitates reoxidation of Li2S2 and Li2S. Additionally, efficient reoxidation of insulating insoluble Li2S2 and Li2S will

cathode, acting as an effective shuttle inhibitor to the sulfur and polysulfides. However, the insertion of a GO membrane16 or any other permselective membrane18,44 would naturally bring extra resistance to the battery system, degrading the energy efficiency of the Li−S battery, particularly at high rates.44 Conversely here, we demonstrate that there is a negligible compromise between selectivity and permeability of the shearaligned GO membrane that we introduced in our Li−S cell, resulting in a significantly improved rate performance compared to that of the functional membranes reported to date.16,44−46 To better reveal the impact of the GO membrane and the importance of its structural order in the cycling performance of Li−S batteries, electrochemical and X-ray photoelectron spectroscopy (XPS) analysis were carried out for contrast. As a demonstration, we evaluated the electrochemical responses of Li−S cells assembled with different membrane separators: (a) a bare separator; (b) our highly ordered shearaligned GO-coated separator; and (c) a disordered31,39 GOcoated separator fabricated by a vacuum filtration technique. The detailed experimental procedure and analysis methods can be obtained in the Experimental Section and Figure S11 in the Supporting Information. Cyclic voltammetry plots (Figure 3a) of the Li−S cell consisting of shear-aligned GO-coated separator show little change in the current response after five cycles of CV at a relatively high scan rate of 0.14 mV s−1 (equivalent to 0.5 C rate in galvanostatic charge−discharge). On the other hand, the current response significantly decreased in cases of the Li−S cells containing the bare membrane and the disordered GOcoated membrane (Figures 3b and S11 in the Supporting Information). Electrochemical impedance spectroscopy was also performed on these Li−S cells before and after cycling to further investigate the influence of the GO membrane and its structural order on facilitating ion transport in the Li−S cells (Figure 3c,d). It is seen in Figure 3c that, before cycling, the impedance of the Li−S cells with GO membranes in their configuration was similar, irrespective of the order parameter of the graphene sheets (Sordered ≈ 0.99 and Sdisordered ≈ 0.3) and indeed higher than that of a cell configured with a bare separator. After cycling, the Nyquist plots of the Li−S cell configured with the ordered GO separator consisted of a depressed semicircle in the high-frequency region (100−1 kHz), a depressed semicircle in the medium-frequency range (1 kHz to 1 Hz), and a straight line with a finite slope in the lowfrequency region (1 Hz to 10 mHz). On the other hand, the high-frequency and medium-frequency semicircles merged together in the case of the cells comprising the bare separator and the disordered GO separator (Figure 3d). Moreover, the impedance of the cell with the ordered GO separator remained unchanged after five cycles of CV, whereas the impedances of the cells containing the bare and the disordered GO separators increased 2-fold due to cycling. In order to obtain a detailed mechanistic insight we have further analyzed the impedance data using an electrical equivalent circuit (EEC). The proposed EEC (Figure S12 in the Supporting Information) consists of an electrolyte resistance (Re), two time constants representing the highfrequency and medium-frequency depressed semicircles, and a constant phase element (Q) representing the low-frequency inclined line. The first time constant, consisting of a resistance, Rint, and a constant phase element, Qint, represents the interphase contact resistance and its related capacitance in the sulfur cathode, which simulates the electron conduction 7774

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polysulfide species. In other words, the maximum possible atomic contribution from polysulfide species is 6.1%. The GOcoated cathode, on the other hand, has 8.29% S with 81.86% of this being associated with LiTFSI, thus leaving a maximum contribution of 1.5% for polysulfide species. This quantitative analysis provides strong evidence of the success of the GO membrane in retaining the polysulfides on the cathode side of the cell. Survey data and component fitting of high-resolution S 2p spectra measured by XPS are presented in Tables S2 and S3 (Supporting Information). Additionally, we cycled electrodes in a LiNO3-free electrolyte to further verify the retarding diffusion of polysulfides in the presence of the GO membrane. It is seen in Figure S16 in the Supporting Information that the cell which benefited from the GO membrane exhibited much improved cycling stability, and the Coulombic efficiency stabilized at around 91%. Cathodes with High Sulfur Content. A key parameter of a practical sulfur cathode is the overall gravimetric capacity of the cathode and or cathodic system. In addition to good sulfur utilization (mAh gs−1), increasing the sulfur content of the cathode and minimizing the fraction of inactive materials (host, binder, conductive agent, and additional coatings) are also of crucial importance. Additionally, it is of great interest to the manufacturing industry to create high sulfur content electrodes with low-cost carbon and industrially adaptable techniques. While the approach of embedding sulfur in porous frameworks or functional hosts has proven to be very successful in several reports, the limitations in the fraction of sulfur in the composite cathode and complex multistep fabrication processes are a major limitation in their commercial adoption. On the other hand, in our design or similar designs,41,45 it is more feasible to attain a high sulfur content in the cathode as long as good utilization of sulfur and high Coulombic efficiency is achievable. To further demonstrate the benefit of our design and highlight the effect of active material content on the overall gravimetric capacity of the cell, we cycled sulfur cathodes with 70% sulfur content (S70). As can be seen in Figure 4, the stability of the S70 cathode (Figure 4b) is better than that of NPC-S35 (Figure 4a). This is attributed to the lower amount of carbon in S70 (20% as opposed to 55% in NPC-S35), which lowers the electrolyte uptake of the electrode and demands a smaller volume of electrolytes to be applied on it.4,49 The electrode capacity and the overall cathodic system capacity (considering the mass of everything on the cathode side of the cell) after 100 stable cycles at a practical rate of 0.5 C are 834 mAh g−1 electrode and 572 mAh g−1, respectively, which is significantly higher than that of the cell with the NPC-S35 cathode (Table S4 in the Supporting Information). We also cycled cathodes with 80% sulfur content, which is well above the average active material fraction in the literature for a Li−S battery and close to the active material fraction in commercialized Li-ion cells (≈90%).1 The electrode capacity and the overall cathodic system capacity after 100 cycles are 835 mAh g−1electrode and 577 mAh g−1, respectively, which provide some of the highest performance metrics in the literature for Li−S (Figure 4c).50 Table S4 in the Supporting Information shows a summary of the performance of several Li−S cells using either interlayers or functional hosts to tackle the issue of polysulfide dissolution. It is seen that the approach of using protective interlayers is more effective if the interlayers are lightweight,41,45 and the approach of using functional hosts is efficient when both the sulfur utilization and sulfur content are high.51,52

potentially inhibit the agglomeration of these insulating precipitates, which would facilitate retention of the kinetics of electron conduction from the reaction site to the current collector. This is further confirmed by the insignificant change in Rint (Table S1 in the Supporting Information) due to CV cycling in the case of the cell containing the ordered GO separator. On the other hand, Rint of the cells containing the bare separator and the disordered GO-coated separator increases 2-fold due to CV cycling (Table S1 in the Supporting Information). Recently, we demonstrated that structural order dramatically promotes the flux of a GO membrane,31 and herein, we show that this can be translated effectively to an improvement in the kinetics of the charge transfer processes and thereby the electrochemical response of Li−S cells configured with a GO membrane separator. Apart from the highly ordered structure of the GO membrane, which allows for fast mass transport through the membrane,31 the submicron thickness of the GO membrane is also a key factor that enables high rate cycling of the cathode. As can be seen in Figure S14 (Supporting Information), a thicker membrane adversely affects both the capacity and its retention. On the other hand, we noted that the thickness and mass of the conductive interlayer do not play a crucial role in the performance of the cell. The performance of the cell degraded only slightly when we decreased the areal loading and thickness of the carbon layer to as low as 0.24 mg cm−2 and 6 μm (Figure S15 in the Supporting Information). It is noteworthy that if we calculate the gravimetric capacity of the cell by considering everything in the cathodic side of the cell, then the overall gravimetric capacity of the composite cathodic system (mcathode + mGO+carbon) is higher in the case of cells with a lighter carbon interlayer. This highlights the importance of the lightweight coatings as has recently been demonstrated by Manthiram41 and Zhang.45 To obtain quantitative information on the success of the GO membrane in retaining the polysulfides on the cathode side of the battery, we disassembled cells containing GO-coated and uncoated cathodes after 15 deep cycles at 0.1 C rate in the discharged state. It should be noted that bare separators were used in this experiment to allow for exclusive analysis of the effect of the GO membrane in retaining the polysulfides. Figure 3e clearly shows that the separator and lithium anode used in the cell with the GO-coated cathode visibly remain as-received after cycling, compared to the separator and anode employed in the cell with the uncoated cathode, which exhibits a yellow appearance. To quantify the amount of polysulfides accumulated on the surface of the cycled cathodes in Figure 3e, XPS analysis was conducted on the unwashed samples (Figure 3f). The difference in the intensities of the peaks in the polysulfide region (159−164 eV) of the high-resolution S 2p spectra supports the observations made in Figure 3e. Since the samples are not washed, there is a significant sulfur contribution from the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) residues on the surface of the samples, that is, the intensity in the high binding energy region. Using the elemental quantification derived from the survey scans together with fitting of the highresolution S 2p spectra, the total concentration of LiTFSI compared with that of other S species, including polysulfides, can be calculated. A total atomic fraction of 11.14% sulfur is present on the surface of the uncoated cathode, where 45.28% of this is associated with LiTFSI. Therefore, 6.1% of the total S concentration on the uncoated cathode is associated with other S species, with the dominant contribution originating from 7775

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mAh g−1 electrode for S70 and S80, respectively. We attribute the outstanding high rate performance of our GO-coated sulfur electrodes to the submicron thickness and the high degree of alignment of graphene sheets upon coating GO in its liquidcrystalline form. Such a well-structured GO membrane offers the benefit of utilizing its favorable oxygen functional groups without lowering ion conductivity unduly. We also demonstrate that benefiting from our innovative cell configuration, high content sulfur cathodes deliver very high overall capacities of ∼835 mAh g−1 electrode after 100 cycles, a noticeable improvement to many other advanced composite cathodes. With the Coulombic efficiency of the whole cycling process within these 100 cycles being close to 99.30 and 98.5% for S70 and S80, respectively, this configuration of the cell should form the basis of a Li−S battery that will excel in a range of important practical applications. Along with improvement in performance metrics of the Li−S battery, our innovative configuration, which is compatible with the current battery fabrication process, is an industrially suitable and cost-effective solution that should help to bring this technology closer to commercial reality. Additionally, we expect that our strategy to coat a protective GO membrane on the electrode will cover general interest and finds immediate application in many other electrochemical energy storage devices suffering from similar electrode instability issues.

EXPERIMENTAL SECTION Preparation of Highly Concentrated GO Dispersions. Graphene oxide was synthesized using modified Hummers method.54 SP-1 grade 325 mesh graphite powder, sulfuric acid, potassium persulfate, phosphorus pentoxide, and potassium permanganate were used for the synthesis. The synthesized GO was exfoliated by sonication (UP-100 ultrasonic processor) in RO water for 1 h, followed by centrifugation to remove the unexfoliated GO. An Ocean Optics USB4000 UV−vis spectrometer was used to determine the GO concentrations by measuring the absorbance at 230 nm (using a quartz cuvette, Starna Cells Pty. Ltd. Australia). We used superabsorbent polymer hydrogel beads, which are strongly hydrophilic, to produce concentrated GO dispersions. Concentration of a GO dispersion occurs because of the ability of the hydrogel beads to absorb and retain water without the hydrogel beads dissolving in water or absorbing GO sheets. Typically, cross-linked polyacrylate copolymer-based hydrogel beads were used. These hydrogel beads can absorb water up to 90 times their weight. The time taken to concentrate a GO dispersion depends on the initial concentration, the desired concentration, and the mass of beads used. To avoid possible concentration polarization around the beads and to speed up the absorbent process, the container was mildly agitated by a magnetic stirrer. After the hydrogel beads were saturated with water, they were removed from the concentrated solution. Cell Fabrication and Electrochemical Performance Test. NPC-S35 cathode was fabricated based on the melt−diffusion method reported by Nazar’s group.17 Nanopowder carbon−sulfur composite (NPC-S powder) was prepared by mixing NPC and sulfur in a ratio of 50/50 wt % and heating at 155 °C for 10 h under N2. The composite cathode was prepared by mixing 70 wt % composite powder, 20 wt % Super P, and 10 wt % polyvinylidene fluoride (PVDF) in N-methyl-2pyrrolidinone (NMP) to form a homogeneous slurry. The slurry was then coated on a battery-grade carbon-coated Al foil and dried at room temperature for 48 h, followed by overnight drying at 40 °C under vacuum to remove all traces of the solvent. In this work, the sulfur mass loading in NPC-S35 cathodes falls typically in a range of 1−1.2 mg cm−2. The sulfur cathodes (S70 and S80) were prepared by mixing 70 wt % sulfur, 20 wt % carbon black, and 10 wt % PVDF in the case of S70 and 80 wt % sulfur, 15 wt % carbon black, and 5 wt % PVDF in the case of S80 in NMP to form homogeneous slurries. The slurries were then coated on Al foil and dried at room temperature for 6 h,

Figure 4. Discharge capacities (0.5 C) of (a) nanoporous carbon/ sulfur composite cathode with 35% sulfur content, (b) sulfur cathode with 70% sulfur content, and (c) sulfur cathode with 80% sulfur content, based on the mass of sulfur (black lines), total mass of the electrode (blue lines), and total mass of the electrode and additional interlayers on the cathode side of the cell.

A summary of the performance of Li−S cells with GO in their configuration is also presented in Table 5 in the Supporting Information. Although it is difficult to make a comparison with these studies due to the different composition of the cathodes, it is reasonable to conclude that the configuration in this work containing a submicron thin GO membrane imparted with microstructural order represents a very effective use of GO in Li−S cells and delivers metrics which are better than most approaches reported to date. It is anticipated that the combination of this facile strategy with promising sulfur cathodes reported very recently5,53 may be used in future Li−S batteries. This configuration appears to be particularly well-suited for targeting both high gravimetric and volumetric specific capacities considering the negligible contribution of the GO membrane on the weight and volume of the sulfur cathode.

CONCLUSION In summary, we have designed a GO-protected cathode for a highly stable Li−S battery with exceptional rate capability. We showed that the high degree of order and alignment imparted to graphene sheets upon applying shear to nematic GO results in a permselective membrane with minimum transfer resistance. Coupled with a carbon-coated separator, our protected cathodes deliver initial discharge capacities of 1063 and 1182 7776

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ACS Nano followed by overnight drying at 60 °C under vacuum. In this work, the sulfur mass loading in S70 and S80 cathodes falls typically in a range of 1−1.2 and 1.2−1.4 mg cm−2, respectively. The GO membrane was applied on the sulfur cathode by doctor blading of a 30 mg mL−1 GO dispersion followed by overnight drying at 50 °C under vacuum to remove all traces of water. The carbon-coated separators were prepared by two techniques: blade coating of a slurry of microporous carbon (90 wt %) and PVDF (10 wt %) on glass fiber and vacuum filtration of a suspension of 0.04 mg mL−1 microporous carbon in dimethylformamide through a PVDF membrane, followed by overnight drying at 100 °C under vacuum to remove all traces of the solvent. The electrolyte was 1.0 M LiTFSI/0.1 M LiNO3 in a mixed solvent of 1,3-dioxolane (DOl) and 1,2-dimethoxyethane (DME) (1:1, v/v). For the NPC-S35 cathode, 20 μL of electrolyte was used to wet the cathode, and for S70 and S80 cathodes, 15 μL of electrolyte was used. In cells with a glass fiber as the separator, 60 μL of electrolyte was used to wet the separator, and in cells with PVDF as the separator, 40 μL of electrolyte was applied on the separator. Cells were assembled in an argon-filled glovebox and galvanostatically discharged and charged using a Solartron 1470 cell test galvanostat at room temperature. Electrochemical Analysis To Examine the Effect of GO and the Organization of Graphene Sheets. In order to provide a general understanding on the role of GO in our Li−S batteries and also highlight the effect of the organization of graphene sheets on the charge transfer resistance, we prepared three cells with three different configurations (Figure S11 in the Supporting Information): (a) a cell assembled with a bare separator, (b) a cell assembled with a shearaligned (ordered) GO-coated separator made by blade coating, and (c) a cell assembled with a vacuum filtration made (disordered) GOcoated separator. It should be noted that as opposed to the batteries which we rationally assembled for cycling purposes (which had the GO coating directly on the cathode and the conductive interlayer on the separator), the batteries that we assembled for mechanistic analysis did not have the conductive interlayer to allow for exclusive analysis of the effect of the GO. Besides, for the sake of a fair comparison, we applied the GO coating on the separator and not on the cathode because it was not possible to directly apply a vacuum filtration made GO membrane on the cathode as a nonporous substrate. The shearaligned GO membrane was applied on a PVDF membrane by blade coating of a 30 mg mL−1 GO dispersion followed by overnight drying at 50 °C under vacuum to remove all traces of water. Disordered GO membrane was applied on the battery separator by vacuum filtration of a dilute suspension of GO in water through a PVDF membrane, followed by overnight drying at 50 °C under vacuum to remove all traces of water. Identical cathodes (1 mgs cm−2, 70% S) were used in all cells. Polarized Light Imaging. Microscopy was carried out using a Leica DM IRB microscope with a LPS Abrio imaging system from CRI, Inc. To prepare GO membranes for characterization by polarized light imaging, a highly concentrated GO dispersion of 30 mg mL−1 was coated on copper foil by doctor blading. The coated membrane with a thickness comparable to that of the membrane applied on the sulfur cathode was then transferred onto glass slide by etching the copper foil in a bath of 0.1 M ammonium persulfate. Scanning Electron Microscopy. For SEM studies, to monitor changes in the GO membrane after cycling, electrodes were washed with 1 mL of DOl/DME (1:1, v/v) after disassembly in an argon glovebox. It is believed that soluble polysulfides are washed away and only insoluble Li2S remains on the surface of the washed GO-coated cathodes. SEM was carried out on a FEI Nova NanoSEM 450 FEG SEM instrument. X-ray Photoelectron Spectroscopy. Unwashed samples for XPS were mounted onto samples holders within an argon glovebox and sealed in a vessel before loading into the load lock of the XPS instrument to minimize exposure to the atmosphere. XPS analysis was performed using an AXIS Ultra DLD spectrometer (Kratos Analytical Inc., Manchester, UK) with a monochromated Al Kα source at a power of 144 W (12 kV × 12 mA), a hemispherical analyzer operating in the fixed analyzer transmission mode and the standard aperture

(analysis area: 0.3 mm × 0.7 mm). The total pressure in the main vacuum chamber during analysis was typically between 10−9 and 10−8 mbar. Survey spectra were acquired at a pass energy of 160 eV. To obtain more detailed information about the chemical structure, oxidation states, etc., high-resolution spectra were recorded from individual peaks at 40 eV pass energy (yielding a typical peak width for polymers of 1.0 eV). Each specimen was analyzed at an emission angle of 0° as measured from the surface normal. Assuming typical values for the electron attenuation length of relevant photoelectrons, the XPS analysis depth (from which 95% of the detected signal originates) ranges between 5 and 10 nm for a flat surface. Data processing was performed using CasaXPS processing software version 2.3.15 (Casa Software Ltd., Teignmouth, UK). All elements present were identified from survey spectra. The atomic concentrations of the detected elements were calculated using integral peak intensities and the sensitivity factors supplied by the manufacturer. The accuracy associated with quantitative XPS is ca. 10−15%. Precision (i.e., reproducibility) depends on the signal/noise ratio but is usually much better than 5%. The latter is relevant when comparing similar samples. Determination of Specific Surface Area. Surface area and pore textural characteristics of the microporous carbon used in this work were obtained from the N2 adsorption isotherms for pressures up to 1 bar measured by a volumetric method using a Micromeritics ASAP 2420 instrument at 77 K (liquid nitrogen bath). Samples were evacuated and activated at 200 °C under dynamic vacuum at 10−6 Torr for 12 h to remove any residual solvent and measure the sample mass precisely. Gas adsorption measurements were performed using ultrahigh-purity nitrogen. Brunauer−Emmett−Teller surface area and pore size distribution data were calculated from the N2 adsorption isotherms based on the density functional theory model in the software provided within the Micromeritics ASAP 2420 instrument. Fourier Transform Infrared Spectroscopy. To evaluate the presence of functional groups in the GO membrane, FTIR spectra were recorded using an attenuated total reflectance Fourier transform infrared spectromerter (PerkinElmer, USA) in the range of 500−4000 cm−1 at an average of 32 scans with a resolution of 4 cm−1.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03285. SEM images of the coatings, characterization of carbon, XPS and FTIR data, control experiments on the GO membrane, and additional cycling data (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s): An Australian provisional patent 2014904644 on the synthesis of GO membrane has been filed.

ACKNOWLEDGMENTS The Monash team acknowledges financial support from the Australian Research Council through LP 140100959 and also from Ionic Industries Ltd. M.R.H. acknowledges FT130100345 for funding support. M.S. acknowledges Maryam Talebi Nia for the schematic in Figure 2a. REFERENCES (1) Hagen, M.; Hanselmann, D.; Ahlbrecht, K.; Maça, R.; Gerber, D.; Tübke, J. Lithium-Sulfur Cells: The Gap Between the State-of-the-Art 7777

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DOI: 10.1021/acsnano.6b03285 ACS Nano 2016, 10, 7768−7779