Performance Enhancement and Mechanistic Studies of Room

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Performance Enhancement and Mechanistic Studies of RoomTemperature Sodium-Sulfur Batteries with a Carbon-Coated Functional Nafion Separator and a Na2S/Activated Carbon Nanofiber Cathode Xingwen Yu, and Arumugam Manthiram Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04588 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 20, 2016

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Chemistry of Materials

Performance Enhancement and Mechanistic Studies of RoomTemperature Sodium-Sulfur Batteries with a Carbon-Coated Functional Nafion Separator and a Na2S/Activated Carbon Nanofiber Cathode Xingwen Yu and Arumugam Manthiram* Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX78712, USA ABSTRACT: Operation of sodium-sulfur batteries at room temperature has been proposed and studied for about a decade, but polysulfide-shuttle through the traditional battery separator and low-utilization of the sulfur cathode have commonly been the major challenges. Also, due to the highly active nature of the sodium metal, the conventional room temperature sodium-sulfur (RT Na-S) battery concept with the sodium-metal anode and elemental sulfur cathode imposes serious safety concerns. To overcome the above difficulties, we present here a RT Na-S system with an advanced membrane-electrodeassembly (MEA) comprising a carbon-coated, pre-sodiated Nafion membrane (Na-Nafion), and a sodium sulfide (Na2S) cathode. The Na-Nafion membrane provides a facile Na+-ion conductive path and serves as a cation-selective shield to prevent the migration of the polysulfides to the anode. The carbon coating on the Na-Nafion plays an upper-current-collector role and thereby improves the electrochemical utilization of the active Na2S. Employing Na2S as the cathode provides a pathway to develop the RT Na-S batteries with sodium-metal-free anodes. The RT Na-S battery with the above MEA exhibits remarkably enhanced capacity and cyclability in contrast to the Na-S batteries with the conventional electrolyteseparator configuration. Mechanistic studies reveal that the suppression of polysulfide migration through the Na-Nafion is due to size and electronic effects.

1. INTRODUCTION Li-ion batteries have dominated the rechargeable battery market for more than 20 years 1,2 This is attributed to their unrivalled energy density relative to that for the other well-developed rechargeable battery systems based on the NiOOH– metal hydride, nickel-cadmium and/or lead-acid chemistries.3,4 However, the ongoing and future demands to efficiently utilize the energy from renewable sources (non-fossil fuel energies) require grid-scale reversible battery systems that can be built with abundant, environmentally compatible, high-energy density electrode materials.5,6 With this perspective, the traditional lithium-ion batteries pose serious concerns due to the limitation in the lithium resources and the intrinsically low capacity of the currently used transitionmetal-oxide cathodes.7 The need for affordable, stationary energy storage systems has triggered intensive research on alternative battery systems.8,9 Electrochemical energy storage with the Na- and S- chemistries is gaining more and more attention as a promising lowcost technology for stationary energy storage applications, due to the intrinsically high capacities of the elemental sodium and sulfur electrodes and their abundant resources.7,10 Actually, use of the electrochemical couples of sodium/sulfur for energy storage application has a 50-year history. Molten sodium-sulfur batteries operating at high temperatures (> 300 °C) have been proposed and developed in the 1960s with an

intention for stationary energy-storage applications.9 However, operation of the molten-electrodes and the use of ceramic electrolytes induce a series of technical challenges in terms of safety, materials cost, high-temperature sealing, system maintenance, reliability etc., which limit their applications.12 As a result, the battery systems with low-temperature Na-S chemistry are recently drawing a significant attention as a promising alternative to support the smart grid system.13,14 The concept of RT Na-S batteries has been proposed and validated for about 10 years since 2006.15 However, the RT Na-S battery system is currently facing technical obstacles.15-30 Like its analogous Li-S battery system, a critical challenge is the so called “polysulfide shuttle” behavior. During charge/discharge of the cell, the polysulfide intermediates produced at the cathode readily dissolve into the organic solvents. They have a tendency to diffuse through the battery separator to the anode and react with the sodium-metal anode. The above un-expected process not only induces the loss of the active sulfur material in the cathode but also deteriorates the sodiummetal anode, thereby resulting in performance degradation of the cell during prolonged cycling.15-30 Recently, two approaches have been investigated for using Nafion as a cationselective material to inhibit the sodium polysulfide shuttling. One approach was to coat the porous polypropylene separator with Nafion to form a composite non-porous membrane.24 Another approach was that we recently reported the applica-

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tion of a pre-sodiated Nafion thin film as a polymer electrolyte.30 Both of these approaches have been validated to improve the cyclability of the RT Na-S batteries. We have also previously studied the possibility of using a lithiated Nafion membrane in the Li-S battery systems with a lithium polysulfide as the starting sulfur cathode.31 The lithiated Nafion membrane showed a reasonable Li+-ion conductivity. Furthermore, we noticed that the sodiation was even more facile than the lithiation of Nafion membrane. The Na+-ion conductivity of the sodiated Nafion membrane is higher than the Li+-ion conductivity of the lithiated Nafion membrane.30,31 However, in spite of the Nafion membrane showing significant effect for suppression of the polysulfide shuttle, the previous Nafionmembrane approach did not improve the cycling performances of the Na-S cells much due to the lack of a facile cell configuration. A carbon-based interlayer in our previous study improved the utilization of the sulfur cathode, but the cell showed poor pre-conditioning ability.30 In addition, the mechanism regarding the polysulfide retention by the Nafion membrane is still unclear. To address the above concerns, we report here a new cathode structure design by coating a thin layer of high-surface-area, nanostructured carbon onto the sodiated Nafion membrane to improve both the electrochemical utilization of the sulfur cathode and the pre-conditioning ability of the cell. A mechanistic analysis of the ion-selective behavior at the Nafion-electrode interface is performed to provide insights into the polysulfide-retention mechanism. In most of the RT Na-S battery studies, the cathodes have been fabricated with various types of carbon-sulfur composite materials. As a unique approach, a few studies have focused on using the dissolved sodium polysulfide as the starting cathode material to assemble the batteries in an intermediate state of charge/discharge.27,28,30 These approaches with either the elemental sulfur or the dissolved polysulfide as starting materials require a sodium-metal anode to provide the sodium source for cycling the cell. However, due to the highly active nature of the sodium metal, the approaches with metallic sodium as the anode impose serious safety concerns. Therefore, we recently investigated the possibility of fabricating Na-S batteries in a full discharge state by using sodium sulfide (Na2S) as the starting cathode material to provide the sodium source.29 In such a case, development of the RT Na-S batteries would allow to use other types of anodes, such as carbon-based, siliconbased, and metal oxide-based anode materials. Similar approach of using Li2S as the starting active cathode material has also been previously studied in the lithium-sulfur battery system.32,33 However, with a traditional porous separator (glass fiber), the RT Na-S batteries we previously studied with the Na2S-multiwall carbon nanotube (Na2S/MWCNT) fabric cathode showed limited cycle life.29 Therefore, in this study, the Na2S cathode is integrated with the ion-selective Nafion membrane with the advanced structure described above. The MWCNT material we previously used for fabricating the Na2S/MWCNT electrode possesses high-surface-area and a self-weaving behaviour, which can easily facilitate the formation of a free-standing paper electrode.28,29,34 However, the high cost of MWCNT demerits the low-cost advantage of the Na-S battery system. Accordingly, in this study, we employ an inexpensive carbon nanofiber (CNF) material, which also has the self-weaving property to form free-standing paper electrodes, as the matrix for the active Na2S cathode. In order to improve the surface area of the electrode, the CNF paper was activated before using for cell assembly. The activated carbon

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nanofiber material with high surface area is termed as AC-CNF in this paper.

2. EXPERIMENTAL SECTION Preparation of electrolyte. The liquid electrolyte for coincell assembly and for sodiation of Nafion-membrane was prepared by dissolving proper amounts of NaClO4 (Sigma Aldrich, > 99.0%) and NaNO3 (Sigma Aldrich, > 99.0%) into a tetraglyme (TEGDME, Acros Organics, > 99.0%) solvent to render a solution comprising 1.5 M NaClO4 and 0.2 M NaNO3 (both the NaClO4 and NaNO3 are soluble at the above concentration level). The resulting solution is termed as blank electrolyte. Fabrication of carbon nanofiber (CNF) electrodes. The CNF fabric electrodes were prepared with a self-weaving process similar to our previous method and without any binder material.31 Typically, 190 mg of CNF powder (Pyrograf Products Inc.) was dispersed into a mixture of 300 mL of deionized (DI) water and 25 mL of isopropyl alcohol (IPA). The mixture was sonicated for 10 - 15 min and was further stirred for 2 h. Then, the resulting slurry was poured onto a piece of filter paper under vacuum. The paper electrode formed on top of the filter paper was washed in sequences with DI water, ethanol, and acetone. Afterwards, the electrode was dried for 24 h at 50 °C under vacuum. Finally, the CNF paper was removed from the filter paper and was cut as circular disks with a diameter of 1.2 cm and a mass ~ 3.4 mg. Activation of the CNF fabric electrodes and the raw CNF powder. Either the CNF powder or the CNF electrodes were activated with a CO2 activation process similar to that in our previous study.31 Typically, the raw CNF powder or the prepared electrodes were heated to 875 oC under a CO2 environment and maintained at that temperature for 2 h. This activation process normally reduces 2/3 of the weight of the CNF electrodes (e.g., from ~ 3.4 mg to ~ 1.13 mg for a disk electrode with a 1.13 cm2 area. i.e., the areal weight was reduced from 3.0 mg cm-2 to 1.0 mg cm-2). The thickness of the electrodes was ~ 0.15 mm. The electrical conductivity of the raw CNF was ~ 105 – 106 S. m-1 at room temperature (provided by the supplier). There was no obvious conductivity loss after the activation of the material. For the cells prepared with the CNF pre-coated Nafion membrane, the weight of the AC-CNF paper electrodes were adjusted by using less amount of the starting CNF powder materials to obtain the CNF paper electrodes with the mass density of ~ 0.8 mg cm-2 after CO2-activation. Sodiation and coating of Nafion membrane. Sodiation of the Nafion membranes (Nafion 212, Fuel Cell Stores) was performed in a glove box under an argon (Ar) atmosphere by soaking the membranes in the blank liquid electrolyte for 7 days. After taking the membranes out of the electrolyte bath, the residual liquid electrolyte on the surface of the Nafion membrane was removed with kimwipes® paper. Then the membrane was ready for coating with the AC-CNF material. The sodiated Nafion membrane is termed as Na-Nafion in this paper.


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The slurry for coating the Na-Nafion membrane was prepared by stirring the mixture of the activated CNF powder (6.0 mg) and TEGDME solvent (1.0 mL) for 24 h. The slurry was then deposited onto the Na-Nafion with a net AC-CNF loading of ~ 0.2 mg cm-2 and a thickness of ~ 0.03 mm. Preparation of the Na2S/AC-CNF cathode. The Na2S/ACCNF slurry was prepared by mixing 122.0 mg of Na2S powder, (99.5%, Acros Organics), 7.0 mg of AC-CNF powder, and 1 mL of TEGDME solvent homogeneously with magnetic stirring. Then, the obtained slurry was dispersed into the AC-CNF electrode. The above experiments were conducted inside an argon-filled glove box, since the Na2S electrode was sensitive to moisture in the air.35 Assembly of coin cells. The Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cells were fabricated in a coin-cell configuration (CR2032). The cell assembly was performed in a glove box under Ar atmosphere. 20 µL of the Na2S/AC-CNF slurry was first dispersed into the AC-CNF fabric electrode (~ 66 wt.% Na2S in the Na2S/CNF electrode including the extra CNF coating on the Na-Nafion. The Na2S loading was ~ 2.2 mg cm-2). Then, a piece of pre-coated Na-Nafion was laid onto the ACCNF cathode with the coated side facing the cathode. 40 µL of the blank electrolyte was dropped on top of the Nafion membrane, and a prepared Na-anode (a piece of Na-foil punched into a disk shape) was laid on top of the Nafion membrane. The cell was finally sealed inside the glove box. A coin cell assembled in the above manner consisted of ~ 2.5 mg Na2S and ~ 60 µL liquid electrolyte. The CNF-interlayered cells were assembled in a different way by an insertion of a piece of CNF paper between the Nafion membrane and the cathode. Cell performance testing and electrochemical experiments. Battery performance was assessed with a BT 2000 Arbin® instrument. The assembled coin cells were cycled at various C-rates (C/10, C/5, and C/3, which respectively, correspond to 167.5 mA g-1 sulfur, 335.0 mA g-1 sulfur, and 558.3 mA g-1 sulfur. From the point of view of current density per area of the electrode, the above values correspond, respectively, to 0.15, 0.30, and 0.49 mA cm-2). The charge/discharge capacities of the batteries are normalized on the basis of the net weight of sulfur. The mass of sulfur material was determined from the total amount of Na2S in the electrode, i.e., mass of sulfur = (mass of Na2S) * (32/78) where 32 and 78 refer, respectively, to the formula weights of S and Na2S. Cyclic voltammetry (CV) experiments were performed with an electrochemical instrument (VoltaLab® PGZ402) at a scan rate of 0.1 mV s-1. The impedance/conductivity measurements of the Na-Nafion were conducted on a Solartron® 1287 instrument. The frequency was selected from 106 Hz to 10-1 Hz. Material characterization. Scanning electron microscope (SEM) images of the electrodes and the membranes were obtained with a FEI Quanta 650 instrument. The corresponding elemental-mapping experiments were performed with an energy dispersive X-ray spectroscopy (EDS) associated with the FEI Quanta 650 SEM. Collection of the samples for SEM and EDS experiments was performed in the glove box as well, by disassembling the cycled cells under Ar-atmosphere. The attenuated total reflection - Fourier transform infrared (ATRFTIR) analyses were conducted on a Nicolet IS5 FTIR spectrometer. The XRD patterns were obtained with a Philips X-

ray diffractometer equipped with CuKα radiation in a step of 0.02°.

3. RESULTS AND DISCUSSION Battery structure/configuration and material characterization. The battery configuration for this study is schematized in Figure 1a, in which a pre-coated Na-Nafion serves both as a polymer electrolyte and as a separator. The sulfur-resource cathode, Na2S/AC-CNF, is prepared by dispersing the Na2S powder into the network of AC-CNF matrix. This battery system is termed as Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell in this study. The CNF material used for coating the Na-Nafion is the same as that for preparing the cathode matrix, which was pre-activated before using as described in the experimental section. Figures 1b and 1c show the scanning electron microscope (SEM) image of the AC-CNF paper electrode, from which we can see that the CNF (the fiber diameter is ~ 100 – 200 nm, Figure 1c) interweaves as a fabric with the interspaces in micron-size. In comparison with the CNF electrodes before activation (provided in Figure S1), the CO2-activated CNF does not show significant morphology or structural change according to the SEM images. However, according to the Brunauer–Emmett–Teller (BET) measurements,36 both the BET surface area and the pore volume of the CNF materials had been significantly improved after the CO2activation process (Figure S2). Therefore, although the raw CNF material has a low BET surface (~40 m2 g-1) than the MWCNT material (~ 700 m2 g-1) we previously used for the RT Na-S study28,29, after an activation process, the activated CNF paper electrode could provide a BET surface (714 m2 g-1) comparable to that of the MWCNT. The enhancement in the surface area is important for the operation of the sulfur cathodes.37-39 Formation of the Na2S/AC-CNF cathode is schematized in Figure S3. A slurry consisting of Na2S powder, activated CNF powders, and tetraglyme (TEGDME) solvent was added into a piece of AC-CNF paper electrode. The resulting electrode was characterized with X-ray diffraction (XRD). As reflected by the XRD patterns in Figure 1d, a prominent peak of Na2S is present in the Na2S/AC-CNF electrode in contrast to that for the pristine AC-CNF electrode. Sodiation of the Nafion membrane was performed in an organic solvent medium (see the experimental section). Figure 1e shows the ATR-FTIR spectra of the pristine Nafion versus Na-Nafion. It can be seen that upon sodiation, the peak at 1450 cm-1 became less prominent. There also appeared a new peak at about 1640 cm-1. The above two features indicate the exchange of H+-ion with Na+-ion.24,40 According to related research in the literature, the vanishing of the peak at ~ 924 cm1 indicates the replacement of H+ within the -SO H group by 3 other alkali-metal cations.40-44 Also, upon sodiation, the symmetric stretching vibrations of the S-O bond remarkably shifts in the Nafion (from 1050 cm-1 to 1080 cm-1), which is in agreement with the previous studies regarding the H-form and the salt-form (alkali-metal salt) of Nafion membranes.45-48 According to our previous SEM/EDS studies, the Na+-ions are homogeneously distributed in the Na-Nafion.30 Noticeably, there was a size expansion (by ~ 1.2 times) upon saturation of the membrane with the NaClO4/TEGDME electrolyte (as detailed in the experimental section). An electrochemical impedance spectroscopic (EIS) analysis (Figure S4) reveals that



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structure and the homogenous distribution of the Na2S particles in the AC-CNF network. After the Na2S particles are activated upon a full charge, the subsequent discharge as well as the following charge-discharge profiles show clearly two voltage plateaus, similar to the characteristics of the Na-S batteries with the conventional elemental sulfur cathodes (Figure S5).

Figure 1. (a) Schematic of a Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell. (b, c) SEM image of the AC-CNF paper electrode. (d) X-ray diffraction (XRD) patterns of the freshly made Na2S/AC-CNF electrode versus the AC-CNF paper. (e) ATR-FTIR spectra of the fresh and Na-Nafion membranes. the Na+-ion conductivity of the Na-Nafion membrane is between 2 × 10-5 and 3 × 10-5 S. cm-1. Electrochemical cycling performances. Procedures for coating the Na-Nafion and for the battery assembly are given in the experimental section. The as assembled Na ǁ NaNafion/AC–CNF coating ǁ Na2S/AC-CNF cell with the Na2S cathode was in a fully discharged state, so it needed to be first charged. It has commonly been recognized with the lithiumsulfur battery system that the initial charge of a Li2S electrode needs to overcome an energy barrier for the nucleation of lithium polysulfides. As a result, the first charge profile usually exhibits a huge overpotential. This feature was proposed to be due to the large particle size (micron scale), high electrical resistivity, and low lithium-ion diffusivity in Li2S.32,33 Such a phenomenon has also been observed in our previous study with the Na2S as the active cathode material29 and in this study as well. Figure 2a presents the charge-discharge profile of the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell for the first three cycles. At a C/5 charge rate, it took ~ 8.5 h to complete the first charge (Figure 2a), which is ~ 1.7 times longer than expected (theoretically it would take 5 h to charge up the cell to obtain the theoretical capacity of the Na2S active material). Like Li2S, the slow first-charge of the Na2S/CNF cathode is supposed to be attributed to the low electrical conductivity and the low ionic (Na+) diffusivity in the pristine Na2S material. However, as seen in Figure 2a, the first charge of this cell did not show significant high overpotential in contrast to the voltage profiles of the Na-S batteries with a sulfur-carbon composite cathode.28 (Figure S5), indicating the facile electrode

Figure 2b shows the cyclic voltammograms of the Na ǁ NaNafion/AC–CNF coating ǁ Na2S/AC-CNF cell. The potential was initially scanned from open-circuit voltage (OCV, ~ 1.8 V) to 3.0 V in a positive direction followed by five cyclic sweeps between 3.0 and 1.2 V (the 1st , 2nd, and the 5th cycles are shown here as representatives) at the scan rate of 0.1 mV s-1. The voltage range selected for the CV experiments is the working voltage domain of the RT Na-S cells cycled with reasonable C-rates. Two obvious cathodic peaks, as well as two anodic peaks are observed. As those for the conventional Na-S cells with the elemental sulfur cathode (Figure S6), the oxidation and the reduction waves in the CVs correspond to the reversible transitions between elemental sulfur (e.g., S8), the longchain Na-polysulfides (e.g., Na2Sn, 4 < n < 8), and the shortchain Na-polysulfides (e.g., Na2S2) or sodium sulfide (Na2S).19,26 The outstandingly high-current peak for the first anode scan (the black curve in Figure 2b) is supposed to be attributed to the sluggish kinetics of the pristine Na2S/ACCNF cathode, which is consistent with the results in Figure 2a. Figure 2c shows representative charge/discharge profiles at the 5th cycle of the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cells at various C-rates. The discharge capacities shown in this figure are determined on the basis of the net sulfur within the Na2S cathode, which are ~ 800 mAhg-1, ~ 680 mAhg-1, and ~ 640 mAhg-1, respectively, at C/10, C/5, and C/3 rates. At the C/10 and C/5 rates, the voltage profiles exhibit two plateaus during both charge and discharge processes. In contrast, at C/3 rate, the second discharge plateau is relatively not so obvious, possibly due to the sluggish kinetics of the transition of the Na2S2 and/or Na2S. Use of the pre(AC-CNF)-coated Na-Nafion provides a great advantage over the traditional porous Celgard membrane in the RT Na-S batteries with the Na2S active cathode material. The pre-coating approach for the Na-Nafion (with the ACCNF material) provides a further improvement in the cell performances over our previously developed carbon-interlayer approach (this will be separately discussed later). Figure 2d and e compare the long-term cycling performances of a series of Na-Na2S cells for five cases by employing (1) a Celgard separator; (2) an AC-CNF coated Celgard separator; (3) a NaNafion; (4) a CNF-paper-interlayered Na-Nafion and (5) an AC-CNF coated Na-Nafion as the electrolyte/separator. The resulting cells are, respectively, termed as (1) Na ǁ Celgard ǁ Na2S/AC-CNF cell, (2) Na ǁ Celgard/AC-CNF coating ǁ Na2S/AC-CNF cell, (3) Na ǁ Na-Nafion ǁ Na2S/AC-CNF cell, (4) Na ǁ Na-Nafion/CNF-interlayer ǁ Na2S/AC-CNF cell, and (5) Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell. Compared to the cells with the Celgard separator, the cells assembled with the Na-Nafion showed significantly enhanced cyclability either with or without an AC-CNF coating on the membrane. The cells with the porous Celgard separator (integrated with the liquid electrolyte) show a relatively higher discharge capacity in the first few cycles than those with the Na-Nafion, which is possibly due to the high ionic diffusivity in the liquid


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Figure 2. (a) Charge-discharge profile of the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell for the first three cycles at C/5 cycling rate. (b) Cyclic voltammograms of the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell (0.1 mV s-1 scan rate). (c) Voltage versus capacity profiles of the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cells at various C rates. (d) Discharge capacities and (e) Coulombic efficiencies as a function of cycle number of a Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell, a Na ǁ Na-Nafion/CNF interlayer ǁ Na2S/AC-CNF cell, a Na ǁ Na-Nafion ǁ Na2S/AC-CNF cell, a Na ǁ Celard/AC-CNF coating ǁ Na2S/AC-CNF cell, and a Na ǁ Celgard ǁ Na2S/AC-CNF cell at the same C-rate (C/5). (f) Nyquist plots of the electrochemical impedance spectroscopies (EIS) for the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell after various cycles at C/5 rate. electrolyte. However, the capacity degradation of the Celgardseparator cells is much faster, which is majorly due to the “polysulfide shuttle” behavior. On the other hand, the cells with the Na-Nafion show significantly lower capacity fade than those with the Celgard separator throughout the cycling. Significant improvements in the cycling performances (both the discharge capacity and the cycle life) of the Na-S batteries on employing an AC-CNF coating onto the membranes (either to the Celgard separator or to the Na-Nafion) are also observed in Figure 2d. Combined use of the AC-CNF coating and the Na-Nafion provides the best cycling stability and the discharge capacity.

cycling condition (Figure 2d). This can also be reflected by the decrease in the impedance of the cell during the first few cycles (Figure 2f). The impedance of the cell becomes relatively stable between 5 and 40 cycles, and then starts to increase again. Correspondingly, after about 45 cycles, the discharge capacity of the cell starts to fade in a continuous manor. However, the cells with the Na-Nafion maintains a relatively stable Coulombic efficiency (in comparison to the Celgard-cells) even in case of the progressive capacity degradation after 45 cycles (Figure 2e), indicating the superiority of the Nafion membrane for suppressing the sodium polysulfide migration. This will be discussed further later.

In our previous study30 with a CNF interlayer inserted between the Nafion membrane and the Na2S6 cathode, the cells generally showed relatively low discharge capacities in the initial cycles. Then, the discharge capacity continuously increased in the first 10 - 15 cycles before leveling off. Such a prolongedconditioning behavior could be due to slow building up of a facile ionic interface between the Nafion membrane and the interlayer, as well as the relatively poor ionic diffusivity in the untreated CNF interlayer. To overcome the slow pre-conditioning issue, two approaches were used in this study. First, the CNF material was pre-activated as described in the experimental section. Second, the activated CNF material was precoated onto the Na-Nafion. The RT Na-S cells developed with this advanced membrane-electrode-assembly (MEA) approach shows significantly improved pre-conditioning ability, as compared with the CNF-interlayer approach in Figure 2d. However, it still takes a few cycles (< 5 cycles) for the Na ǁ NaNafion/AC–CNF coating ǁ Na2S/AC-CNF cell to reach its best

Although lithium is currently the primary anode material to integrate with the sulfur cathode chemistry to render a highcapacity Li-S battery system, sodium also shows strong competitive potential. Based on the cycling performance shown in Figure 2d, the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/ACCNF cell can yield a 790 Wh kg-1 energy density on the basis of the active electrode materials of sulfur and sodium. Although this energy density number is not comparable to that of the state-of-the-art battery system with the lithium-sulfur chemistry, the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/ACCNF cell provides a significant advantage in terms of the energy-cost, since the price of sodium is only 4 % of that of lithium. The energy cost and the energy density for the Li-S system and the Na-S system in this study are compared in Figure S7 (the calculations are derived on the basis of lithium, sodium, and sulfur, not including the other non-active cell components). The relatively lower energy-cost of the RT Na-S batteries make this energy storage system competitive for grid-scale



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power system applications. There may be a concern regarding the conductivity of the Na-Nafion, which could limit the current density to be applied to the small-scale cell systems. However, the overall impedance of the cell is expected to be compensated with the large-area membrane electrode assembly (large ratio of the area to the thickness of the membrane electrode assembly). Moreover, there are possibilities to further reduce the thickness of the Na-Nafion, which is expected to lower the impedance of the cell system as well. However, more efforts are needed to optimize the above engineering/technical issues towards the scale-up of this battery concept. Insight into the mechanisms regarding the suppression of sodium polysulfide permeation through the NaNafion. With a simple visualization experiment, we have previously verified the advantage of the Na-Nafion over the porous Celgard for preventing the sodium polysulfide permeation.30 Morphologies of the Na-Nafion and the Celgard are compared in Figure S8 (SEM images). The micron-scale pores of the Celgard would definitely not able to prevent the permeation of the soluble sodium polysulfides. In contrast, the morphology of the Na-Nafion is much denser (in contrast to the Celgard), which is a desired characteristic for prevention the migration-through of the sodium polysulfide species from a “macro-structure” point of view. As a co-polymer of tetrafluoroethylene and perfluoro-vinylether (generally can be expressed as the form of -(CF2CF2)m(CF2CF(OCF2CF(CF3)OCF2-CF2SO3H))n), the Nafion membrane has been commonly used in the low-temperature hydrogen- and/or liquid-fed fuel cells. When turning the H+-ion to the Na+-ion form, it has been proved that the Nafion membrane was able to transfer Na+ ions.24,30 From a “micro-structure” point of view, the Nafion membrane contains a hydrophobic region consisting of a poly-(tetrafluoroethylene) backbone and a hydrophilic ion-cluster regions (~ 40–50 Å in size, as illustrated in Figure 3a) with the -SO3- as fixed-charge sites and transferable cations (e.g., Na+ in the Na-Nafion) as counter-ions. These hydrophilic clusters are connected to each other through hydrophilic channels with the diameters of 10 – 20 Å (Figure 3a).49-52 On the other hand, the maximum molecular sizes (Van der Waals diameter) of the sodium polysulfide species can be estimated from the ionic radii of sulfur and sodium, the length of the S-S bond, and the length of the Na-S bond, as summarized in Table S1 (The relevant data for these estimations are from wikipedia). These estimations are also based on an assumption that the sodium polysulfide species exist as monomer units with a straight-chain structure. However, in the organic solvents, the structures of the polysulfide species are really complicated.53-55 There exist bond-length variations and bond angles in the monomer polysulfide molecules, as illustrated in Figure 3b. It may also form polysulfide clusters (e.g., dimer units as illustrated in Figure 3b). Furthermore, dissolving in the organic solvents, the size of the solvated polysulfide species under the solvation effect is supposed to be larger and varies with the different solvents. Therefore, the actual molecular size of the polysulfide species is difficult to be precisely calculated. However, based on the estimation data shown in Table S1 and the above possible structure-optimization, clusterization, solvation cases, the Van der Waals size of the polysulfides would be on the same order of magnitude (0.5 – 5.0 nm) as the “pore” size of the hydrophilic ion-cluster regions of

Figure 3. (a) Architecture of Nafion membrane. (b) Molecular schematics of monomeric Na2S6, Na2S8 and dimer Na2S4 units. The sulfur and sodium are respectively represented by yellow and red spheres. (c) Schematic of the ionic-selectivity of the Nafion membrane by ionic interactions at the hydrophilic pores of the membrane.

the Nafion membrane. Therefore, from the “size effect” point of view, there are significant obstacles for the polysulfide species to migrate through the hydrophilic pores of the Nafion membrane. In addition to the above “structure/size effect”, it has commonly been realized that there exist a series of ionization and dissociation equilibria (equations 1 and 2) of the sodium polysulfides in the electrolyte system.53,56,57 in such cases, the de-


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sodiated polysulfide ions (Sn2-) or the dissociated free radicals (Sn/2 . -) are actually in the negative charge states. On the other hand, as an excellent cation-exchange membrane, the -SO3- at the hydrophilic “pore” surface of the Nafion membrane offers a negatively charged environment, providing the ion-selectivity property by ionic interactions (as illustrated in Figure 3c). The counter Na+ ions with positive charge would freely transfer through the narrow hydrophilic “pores”. While the negatively charged polysulfide species would not be able to migrate through the Nafion membrane due to the “charge repulsion” effect from the negatively charged environment at the hydrophilic “pores.” (Figure 3c). Na2Sn → 2Na+ + Sn2Sn2- → 2 Sn/2·-

SEM and EDS (elemental mapping) analyses of the cycled ACCNF coating on the Na-Nafion (the cells were terminated in discharge state after 100 cycles) are presented in Figure 4f-j. The elements sulfur and carbon are from the active electrode of the CNF coating. The sodium and chloride are from the salt of the electrolyte. As expected, the discharge products are significantly more in the cathode than that in the AC-CNF coating (Figure 4a and 4f). The EDS analysis results also support the above expectation (Figure 4b, 4c and Figure 4g, 4h). These results suggests that most of the sulfur active materials are maintained in the AC-CNF cathode, but there are some active materials moved into the AC-CNF coating due to the diffusion of the sodium polysulfide species.

(1)

(2)

Post-mortem analyses of the cycled Na ǁ Na-Nafion/AC– CNF coating ǁ Na2S/AC-CNF cells. In order to obtain an insight into the capacity-fade mechanism, the components (the activated CNF cathode, the sodium anode, the Na-Nafion, and the AC-CNF coating on the Na-Nafion) of a cycled (after 100 cycles) Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell were analyzed. SEM images of the cycled AC-CNF electrode (after 100 cycles, terminated in discharge state) is presented in Figure 4a. A significant amount of discharged products and sodium salts have been observed at the interspaces and/or on the surface of the CNF fibers (in comparison to Figure 1b and 1c). Distribution of either the sulfur or the sodium salt (NaClO4) are homogenous in the cycled AC-CNF cathode, as reflected by the elemental mappings obtained with EDS (Figure 4b-e). However, the structure of the activated CNF electrode did not obviously change after cycling, suggesting a stable characteristic of the activated CNF matrix for the electrochemical transition of the sulfur, sodium sulfide, and sodium polysulfide species.

As the important intermediates, the sodium polysulfides are involved in the electrochemical processes during cycling the RT Na-S batteries. As soluble species in the electrolyte, the sodium polysulfides have the tendency to diffuse into the ACCNF coating (as shown in Figure 1a) due to the existing concentration gradient of the polysulfides between the AC-CNF matrix and the AC-CNF coating. In addition to the concentration gradient, the electric field between the sodium anode and the sulfur/sulfide cathode may also impact the diffusion-direction of the polysulfide species. As we described in the last section, once the polysulfide species forms during the charge or discharge process, they may undergo both the ionization and/or the dissociation processes in the organic electrolyte. The de-sodiated polysulfide ions (Sn2-) or the dissociated free radicals (Sn/2·-) are generally in the negative charge states. Therefore, during the charge and discharge of the cell, the polysulfides may have a tendency to diffuse into or move out of the AC-CNF coating due to the electric field built between the anode and the cathode. Anyway, from the structure point of view, the 3-D network of the AC-CNF coating can provide additional help to retain the soluble polysulfide species. Importantly, the AC-CNF coating on the Na-Nafion is electrically conductive and can serve as a secondary/upper current collector to sufficiently reuse the diffused polysulfide species. As a

Figure 4. (a) SEM image of the cycled (after 100 cycles) Na2S/AC-CNF electrode. (b) carbon, (c) sulfur, (d) sodium, and (e) chlorine elemental mappings of the cycled (after 100 cycles) Na2S/AC-CNF electrode obtained with EDS. (f) SEM image of the AC-CNF coating (on the Na-Nafion) after 100 cycles. (g) carbon, (h) sulfur, (i) sodium, and (j) chlorine elemental mappings of the cycled AC-CNF coating (on the Na-Nafion) after 100 cycles.



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result, some trapped sodium polysulfide species in the ACCNF coating can also be involved in the electrochemical reactions during the charge-discharge of the cells. Although the AC-CNF coating can alleviate the “polysulfide shuttling” behavior and enhance the utilization of the active sulfur materials (as shown in Figure 2d), it is not able to provide a “flawless” approach to completely prevent the diffusion of the polysulfide species. Due to the thickness limitation of the AC-CNF coating, the polysulfides may eventually diffuse to the coating/separator interface if the cells are operated for prolonged cycles. The polysulfide species in this case can still penetrate the pores of the separator if the cells were prepared with the conventional Celgard. Therefore, the cell with the AC-CNF coated Celgard separator showed a limited cycle life (Figure 2d). As discussed in the last section, a Nafion membrane is able to provide an effective “shield” for preventing the “polysulfide shuttle,” and thereby improves the cycle life of the cells (Figure 2d). Surface morphologies (SEM images) of cycled sodium anodes (upon 100 cycles) are compared with that of the fresh sodium metal, as presented in Figure 5. Obviously rough surfaces are observed for the cycled anodes relative to the pristine sodium sample (Figure 5a, b, and c). The roughness is even severe for the anode sample taken out of the cell with the Celgard membrane. As we previously discussed,30 the surface degradation of the sodium anode is supposed to be the major concern for the capacity degradation of the RT Na-S cells. It is also possible to form Na dendrite at the anode during cycling the cell.

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However, this issue may possibly be alleviated with high-concentration electrolyte according to a recent report.58 Detailed understanding of the effects from the anode-degradation will be performed at a later time. Corresponding EDS analyses (Figure 5d, e, f, and g) is in agreement with the SEM results. The higher sulfur signal on the Na anode in the Celgard-cell (relative to the anode taken out of the Nafion-cell) implies the relatively severe diffusion of the polysulfides. The SEM image, elemental (sodium and sulfur) mappings obtained with EDS for the Na anode taken from the cycled Na ǁ Na-Nafion ǁ Na2S/AC-CNF cell (after 100 cycles) are displayed in Figure S9. In comparison to the Nafion membrane, the effect from the AC-CNF coating is relatively less towards preventing the polysulfide shuttling. Therefore, the sulfur signal on the Na surface from the Na ǁ Na-Nafion ǁ Na2S/AC-CNF cell is quite similar to that from the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell. The Nafion membrane maintained a good shape after electrochemical cycling of the cell (as shown with a picture in Figure S10), implying its stability in the NaClO4/TEGDME electrolyte and the sodium polysulfides environment under the electrochemical cycling conditions. Figure 6a-c show the SEM/EDS analyses of the cycled Nafion membrane. The Na-ions homogeneously distributed in the cycled membrane (Figure 6b), indicating a facile Na+-ion diffusion path is maintained during cycling of the cell. This is further supported by the ATR-FTIR analyses results, as shown in Figure 6d in which the spectra of the cycled Nafion membrane shows almost the same characteristics as the freshly Na-Nafion (Figure 1e).

Figure 5. (a) SEM image of a pristine Na anode. (b) SEM image of a cycled (after 100 cycles) Na anode in the Na ǁ Na-Nafion/AC– CNF coating ǁ Na2S/AC-CNF cell. (c) SEM image of a Na anode in the Na ǁ Celgard/AC-CNF coating ǁ Na2S/AC-CNF cell after 50 cycles. (d) Sodium and (e) sulfur elemental mappings of the Na anode in the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell after 100 cycles obtained with EDS. (f) Sodium and (g) sulfur elemental mappings of the Na anode in the Na ǁ Celgard/ACCNF coating ǁ Na2S/AC-CNF cell after 50 cycles obtained with EDS.


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Figure 6. (a) SEM cross-section image of the cycled (after 100 cycles) Nafion membrane from the Na ǁ Na-Nafion/AC– CNF coating ǁ Na2S/AC-CNF cell. (b) Sodium and (c) fluorine elemental mappings of the cycled Nafion membrane in the cross-section area as marked in (a), obtained with EDS. (d) ATR-FTIR spectrum of the above cycled Nafion membrane. As seen in Figure 2d, in spite of the significant improvement in the cyclability of the RT Na-S cells by the pre-coated NaNafion approach, the discharge capacity of the Na ǁ NaNafion/AC–CNF coating ǁ Na2S/AC-CNF cell still continuously decrease during long-term electrochemical cycling. From the above post-mortem analyses, both the AC-CNF cathode matrix and the AC-CNF coating on the Nafion membrane maintained their structure during cycling of the cell. The Nafion membrane showed good compatibility with the electrolyte and maintained its sodiation characteristics as well. However, the sodium anode showed significant surfacedegradation after cycling, which may be majorly responsible for the capacity fade of the Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF battery upon prolonged cycling. However, the decrease in discharge capacity may possibly be caused by multiple factors, which need to be sorted out in the future. Besides the suppression in the polysulfide shuttle, protection of the sodium-metal anode would be another important research area. In addition, the size expansion of the sulfur atom upon sodiation, diffusion of the polysulfide out of the carbon matrix, consumption of the liquid electrolyte etc. can also result in a loss of the discharge capacity of the cell. As a result, many efforts need to be made to make the RT Na-S battery technology catch up its predecessor, the Li-S system.

with Na2S cathode will allow the integration of sodium-free anodes for the development of RT Na-S batteries, which can eliminate the safety concerns of using highly active sodium metal as the anode. The pre-activated CNF paper electrode has an enhanced surface area and a remarkably advanced structure for strategically reuse the diffused sodium polysulfides. The Na-Nafion can provide a facile Na+-ion conductive environment to sustain the electrochemical cycling of the cell. The most important role of the Nafion membrane is that it significantly suppresses the sodium polysulfide migration from the cathode to the anode through a “structure effect” and an “electronic effect” at the < 5 nm hydrophilic pores (or solution area) with the negatively charged environment of the Nafion membrane, demonstrating significant superiority to the porous separators. Application of a thin AC-CNF coating to the NaNafion (to the cathode side) can further alleviate the polysulfide diffusion and provide an additional secondary current collector to electrochemically reuse the polysulfides. Integration of the AC-CNF coating to the Na-Nafion further enhances the utilization of the sulfur active material of the battery. The Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell exhibits a reasonable energy density, but offers a superior advantage in energy-cost due to the inexpensiveness of both the sodium anode and the sulfur cathode.

ASSOCIATED CONTENT Supporting Information. Estimated maximum Van der Waals diameter of the sodium polysulfide species, scanning electron microscopy picture of an un-activated carbon nanofiber (CNF) fabric cathode, Brunauer–Emmett–Teller analyses of the pristine versus CO2-activated CNF paper electrodes, schematic of the formation of Na2S/AC-CNF cathode, electrochemical impedance spectroscopy of a Na-Nafion, typical voltage-versus-time profiles of a Na-S cell assembled with an elemental sulfur cathode, cyclic voltammogram (at a scan rate of 0.1 mV s-1) of a Na-S cell prepared with the traditional sulfur electrode, comparison of the practical energy density and energy cost of the Li-S and Na-S battery systems, SEM image of a porous Celgard separator and a Nafion membrane, a picture of a piece of cycled (after 100 cycles) Nafion membrane taken out from a Na ǁ Na-Nafion/AC–CNF coating ǁ Na2S/AC-CNF cell after removing the AC-CNF coating. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Phone: (512) 471-1791. Fax: 512-471-7681. Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0005397.

4. CONCLUSIONS In this study, we demonstrated a room-temperature Na-S battery system with a carbon-coated Na-Nafion membrane as the cation-exchange polymer electrolyte and an advanced cathode prepared by dispersing the sodium sulfide (Na2S) powder into the free-standing activated carbon nanofiber (AC-CNF) fabric. Assembly of the RT Na-S batteries in a discharged state

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