Nafion Composite-Coated

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Permselective SPEEK/Nafion Composite-Coated Separator as a Potential Polysulfide Crossover Barrier Layer for Li−S Batteries Dasari Bosu Babu,†,‡ Krishnan Giribabu,† and Kannadka Ramesha*,†,‡ †

CSIR-Central Electrochemical Research Institute-Chennai Unit, CSIR-Madras Complex, Taramani, Chennai 600113, India Academy of Scientific and Innovative Research (AcSIR), CSIR-CECRI, Karaikudi 630003, India

‡

S Supporting Information *

ABSTRACT: Minimizing the shuttle effect by constraining polysulfides to the cathode compartment and activating the passive layer between cathode and separator are highly important for improving the Li−S cell performance, Coulombic efficiency, and cycle life. Here, we report a submicron thin coating of permselective sulfonated poly(ether ether ketone) (SPEEK) composite layer on the separator that would reduce polysulfide crossover, imparting a significant improvement in cycle life. It is observed that SPEEK increases the stability, and adding Nafion improves the capacity value. Among different ratios of Nafion and SPEEK (25:75, 50:50, and 75:25), the composite with a SPEEK/Nafion ratio of 50:50 showed a controlled shuttle effect with a stable cell capacity of 600 mA h g−1 up to 300 cycles. This modified separator with permselective coatings not only reduces the polysulfide shuttle but also improves the wettability and interfacial contact, which results in an improvement in average cell potential and lithium diffusivity. It is demonstrated here that the combination of functional (ionomer coating on separator) and nonfunctional (extra cathode layer) physical barriers effectively suppresses the polysulfide crossover and improves the electrochemical performance of Li−S batteries. The cell shows an initial capacity of 1300 mA h g−1 and a capacity retention of 650 mA h g−1 over 500 cycles with a 6 mg/cm2 sulfur loading. KEYWORDS: Li−S battery, separator, interlayer, physical barrier, polymer, CNT paper lithium anode to form irreversible side products on the anode.7 Hence, it is highly important to suppress the shuttle effect. To restrict the shuttle effect, researchers had employed several strategies by modifying the cathode, separator, electrolyte, or anode.8,9 The effective way of trapping polysulfides in the cathode compartment is by employing a physical barrier between cathode and separator.10−13 The barrier can be an independent interlayer or a coating either on the cathode or on the separator. The advantage of the interlayer is that it physically mitigates polysulfide shuttle and helps in capacity retention.14 Among different interlayers, conductive functionalized polymers are attractive as they can simultaneously increase the conductivity of interface along with mitigating the polysulfide crossover due to the presence of functional groups.15 Functionalized polymer separator coatings such as polydopamine,16 polyethylene glycol,17 polyacrylic acid,18 polyethylene oxide,19 polyvinylidene fluoride,20 PEDOT:PSS,21 polyaniline,22 polyacrylonitrile (PAN),23 and Nafion24 are introduced. Among these, Nafion got immense attention as it imparted high capacity with good retention, which is attributed to the polytetrafluoroethylene backbone that stabilizes the Nafion in electrolyte and the perfluorinated side chain having a SO3− group that selectively allows positive ions (lithium ions)

1. INTRODUCTION An efficient energy storage technology is need of the hour not only for alleviating alarming issues, such as depletion of fossil fuel reserves and global warming, but also to meet the increasing demand for powering electronic gadgets, electric vehicles, and grid applications.1−3 This energy crisis necessitated researchers to search for new technologies beyond lithium ion chemistry that can provide an efficient energy storage along with some cost advantage.4−6 In recent years, Li− S battery chemistry has received considerable attention because of its ultrahigh theoretical capacity and high energy density of 1672 mA h g−1 and 2500 W h kg−1, respectively. The high copiousness of sulfur accompanied by its nontoxicity and low price puts Li−S battery research on the top priority in recent years to develop sustainable energy storage systems. Though the Li−S battery possesses theoretical advantages in terms of capacity and energy density, the commercial realization of a Li− S battery system is limited because of the inherent low conductivity of sulfur and its discharge product Li2S, large volume expansion (∼79%) on cycling, and dissolution of higher order polysulfide (Li2Sx; 4 ≤ x ≤ 8) intermediates in the electrolyte. These higher order sulfides are reduced to lower order sulfides at the anode (Li2S2/Li2S). Reoxidation will take place at the cathode after migration from the anode; this phenomenon is termed shuttle effect and leads to continuous capacity fading and hence poor cyclability and low Coulombic efficiency in Li−S batteries. These polysulfides react with © XXXX American Chemical Society

Received: March 26, 2018 Accepted: May 14, 2018 Published: May 14, 2018 A

DOI: 10.1021/acsami.8b04888 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces and repels hopping of negative ions (sulfides) through Coulombic interactions. Because of these added advantages, Nafion finds applications as a binder,25,26 polymer separator,27 and coating layer (cathode-side or separator-side)28−30 in Li−S batteries. However, the cost of Nafion is very high when compared to those of conventional separators. Huang et al. optimized the Nafion amount to 0.7 mg/cm2 (and 1 μm thick) for 0.53 mg/cm2 loading of sulfur, which is actually more than the weight of sulfur used. The amount of Nafion additives used here may be on a higher side compared to other separator modifiers.31 Further, this thick coating increases the cell resistance and decreases the energy density. However, the permeability of the Nafion membrane is relatively low.32 Therefore, there is quest for developing a novel ion-selective but highly permeable membrane that can increase both energy density and power density of lithium−sulfur batteries. In this study, we used a sulfonated poly(ether ether ketone) (SPEEK)-modified separator for the Li−S battery. To our knowledge, the functional polymer SPEEK for separator modification is not reported so far. PEEK is a thermostable polymer which has 1,4-disubstituted phenyl groups which are separated by ether and carbonyl linkages. PEEK alone is proton-nonconductive; the conductivity can be increased by sulfonating the PEEK, which can be represented as SPEEK. Similar to Nafion, SPEEK is also a functional separator and exhibits perm selectivity because of the presence of −SO3− ions, which is suitable for inhibiting the polysulfide shuttle effect in Li−S batteries. However the advantages of SPEEK over Nafion are its low cost and physical/chemical/thermal stability. In addition to this, favorable microstructural features of SPEEK over Nafion command its use in direct methanol fuel cells. Here, we coated SPEEK on the separator with about 0.4 mg/ cm2 loading, which is much lesser than conventional Nafion coating (0.7 mg/cm2). It is observed that even a lower amount (0.4 mg) of SPEEK coating gives stable capacities. For comparison, a separator coated with Nafion of similar amount is also prepared. Though it showed high capacity, the retention was poor. Therefore, we optimized coating composition by mixing both ionomers. Interestingly, coating composition containing an equal percentage of two ionomers gives better and stable capacities. The capacity and cycle life can be further enhanced by placing another cathode layer in the cathode compartment. This additional cathode layer not only doubles the sulfur loading (from 3 to 6 mg/cm2) but also increases capacity retention by acting as a physical barrier for polysulfides. In addition to this, a simple and cost-effective synthesis approach for colloidal sulfur preparation is adopted using the dimethylformamide (DMF) solvent, which gives uniform distribution of sulfur on carbon.

Table 1. Representation of Mixed Ionomers representation in this article

SPEEK percentage

Nafion percentage

SPEEK-100 SPEEK-75 SPEEK-50 SPEEK-25 SPEEK-0

100 75 50 25 0

0 25 50 75 100

chemicals, 99.9%) and sonicated until the yellow color solid dissolves. This colorless solution is added into 500 mL of distilled water. The solution transforms from colorless to white color, indicating the formation of colloidal sulfur (Figure 1).

Figure 1. Synthesis process of colloidal S@MWCNT composite paper. 2.3. Multiwalled Carbon Nanotube Paper and Sulfur Composite Preparation. Multiwalled carbon nanotubes (MWCNTs) and sulfur (55:45 ratio) are taken in 50 mL of DMF solvent and sonicated for 4 h to make sure that sulfur dissolves. This solution is added to a beaker containing 500 mL of water; immediately, the solution color changes to white, indicating colloidal sulfur formation. The colloidal sulfur is slowly absorbed by MWCNTs while stirring the solution for 2 h. This S@MWCNT composite is vacuum-filtered for obtaining S@CNT paper. In this solution method, the colloidal sulfur gets adsorbed uniformly on the MWCNT surface. For each centimeter square area of the paper, 3 mg of sulfur is observed. This S@MWCNT paper is used as a cathode. Here, the solvent not only assists in producing colloidal sulfur but also facilitates dispersion of carbon throughout the solution, which will enhance uniform sulfur distribution. A schematic diagram of the synthesis procedure is shown in Figure 1. The CNT-nanoS composite will exhibit better electronic and Li reactivity and alleviate volume expansion during discharge−charge cycles.

3. RESULTS AND DISCUSSION 3.1. Structural Analysis. The chemical structures of Nafion and SPEEK are shown in Figure 2a,b. Figure 3a,b shows the scanning electron microscopy (SEM) images of

2. EXPERIMENTAL SECTION 2.1. Material Preparation. 2.1.1. Modification of the Separator. Celgard 2400 separator is taken as a skeleton for ionomer coating. One percent of 40 μL of ionomer in the DMF solvent is drop-casted on the Celgard membrane, which results in 0.4 mg/cm2 ionomer presence on the separator. This coated separator is vacuum-dried at 70 °C in a vacuum oven overnight. After drying, a thin layer of ionomer coating is observed on the surface of pristine separator. Nafion−SPEEK composite ionomers are prepared by mixing Nafion and SPEEK in different ratios and represented as SPEEK-75 (SPEEK 75%−Nafion 25%), SPEEK-50 (SPEEK 50%−Nafion 50%), and SPEEK-25 (SPEEK 25%−Nafion 75%). The representation is shown in Table 1. 2.2. Colloidal Sulfur Preparation. Sulfur powder (100 mg, RANKEM chemicals, 99%) is added in 50 mL of DMF (SRL

Figure 2. Chemical structures of (a) Nafion and (b) SPEEK. B


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Figure 3. SEM images of (a) colloidal sulfur, (b) MWCNTs, (c) pristine separator, (d) Nafion-coated separator, and (e) SPEEK-50-coated separator. (f) In-plane SEM images of SPEEK-100-coated separator, (g) cross-sectional view of SPEEK-100-coated separator, and (h) SPEEK ionomer-coated and noncoated (scratched) regions of the separator.

Figure 4. Three-dimensional AFM morphological images of coated separators. (a) Pristine, (b) Nafion-coated, (c) SPEEK-25-coated, (d) SPEEK50-coated, (e) SPEEK-75-coated, and (f) SPEEK-100-coated separators. (g) Roughness profile for the abovementioned ionomer-coated separators.

Powder X-ray diffraction (XRD) patterns of sulfur, MWCNTs, and S@MWCNTs are shown in Figure S2. The S@MWCNT composite shows the presence of sulfur on CNTs. Also, broadening of the XRD peaks in the composite phase might be due to the decreased particle size of sulfur. The surface morphology and surface roughness of modified separators are studied by using the atomic force microscopy (AFM) technique. Figure 4 shows the 3D topographic AFM images of pristine and Nafion-, SPEEK-25-, SPEEK-50-, SPEEK-75-, and SPEEK-100-coated separators. It is observed that the surface properties clearly change with ionomer coatings. In the conventional Celgard separator (pristine) having micron range pores, AFM shows high intense peaks, as seen in Figure 4a. The Nafion-coated separator also shows similar AFM features, but a with lesser peak intensity (Figure 4b). This might be due to the thin coating of Nafion which may not be able to sufficiently cover the surface. It is observed that the intensity of the peaks gradually decrease with increasing SPEEK percentage in the composite. In the case of SPEEK-25, small and broad peaks indicate that the separator surface and pores are not completely covered with the composite ionomer,

colloidal sulfur and MWCNTs. The SEM image of colloidal sulfur shows 50−70 nm-sized spheres due to the agglomeration of particles. However, in Figure S1c, the composite of S@ MWCNTs does not show these sulfur spheres, which is attributed to the absorption of formed colloidal sulfur into the pores of CNTs. Figure 3c−f shows the SEM images of pristine and Nafion-coated separators, in-plane, and cross-sectional images of SPEEK-50- and SPEEK-100-coated separators. The pristine Celgard 2400 separator possesses interconnected submicron pores in the range of 100−300 nm. Interestingly, in the case of Nafion-coated separator, some regions with uneven coating are observed in Figure 3d, which might be due to insufficient amount of Nafion per unit area. The SEM images of SPEEK-25- (Figure S1a), SPEEK-50-, SPEEK-75- (Figure S1b), and SPEEK-100-coated separators show a uniform ionomer coating with complete filling of separator pores. The cross-sectional SEM image reveals 0.7−0.9 μm thick uniform coating of SPEEK on the separator. For comparison, SPEEKcoated and noncoated (scratched) regions of the separator are shown in Figure 3h. C


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we replace the pristine separator with the ionomer-coated separator. Cyclic voltammetry (CV) curves of functionalized polymermodified separators are shown in Figure 6. The cathodic scan

whereas the peak intensities reduced for SPEEK-50, SPEEK-75, and SPEEK-100 coatings. This infers that with SPEEK-50, SPEEK-75, and SPEEK-100 coatings, the separator pores are completely filled by the ionomer. The roughness profile (Rdc) depends on the porous structure of the separator. The Rdc values for pristine, Nafion, SPEEK-25, SPEEK-50, SPEEK-75, and SPEEK-100 separators are 13.8, 2.52, 2.28, 0.695, 0.695,, and 0.297 nm, respectively, and also show similar trends. Interestingly, the roughness of the separator is related to the amount of SPEEK in the composite. It is evident in Figure 4g that with an increase in SPEEK percentage, the roughness of the surface is decreasing. Supporting Information Figure S3 shows the 2D and 3D images of these compositions at lower magnifications. The surface wettability property of the polymer-coated Celgard separator is investigated by contact angle measurements. Figure 5 shows the static contact angle for the

Figure 6. Cyclic voltammogram of Li−S cell constructed using various coated separators. (a) Pristine, (b) Nafion-coated, (c) SPEEK-50coated, and (d) SPEEK-100-coated separators.

has two peaks, first one at around 2.3 V (peak-1) and second one at around 2.1 V (peak-2). Peak-1 is due to the conversion of cyclic sulfur (S8) to polysulfides [Li2Sx (4 ≤ x ≤ 8)], which can dissolve in the electrolyte. Peak-2 is the major peak in the cathodic scan, which will deliver a significant capacity. This peak corresponds to polysulfides formed at higher voltages getting converted to the end product Li2S2/Li2S. During the first anodic scan, a single peak is observed at 2.4 V, indicating that a major amount of sulfur is reformed in the initial cycle. From the second cycle onward, the voltage is shifted to lower values and the peak splits into two: the first peak is due to the conversion of Li2S to polysulfides and the next peak is due to further conversion of polysulfides to elemental sulfur. Figure 6 shows the CV curves of pristine and Nafion-, SPEEK-50-, and SPEEK-100-coated separators. The potential difference between the two intense peaks is calculated. It is observed that the peak separation is more for SPEEK-100 (0.3173 V) compared to other Nafion- and SPEEK-50-coated membranes, which indicates the high polarization and slower electrochemical kinetics for SPEEK-100. 3.2.1. Discharge−Charge Stability Analysis. The galvanostatic charge−discharge cycling curves for cells containing pristine (Celgard) separator, SPEEK-coated separator, Nafioncoated separator, and SPEEK−Nafion composite-coated separators (SPEEK-25, SPEEK-50, and SPEEK-75) at a 0.2 C rate are shown in Figure S4. During the discharge process, we could identify two plateaus, first one with upper plateau voltage at 2.35 V with nearly 25% of theoretical capacity.34 This plateau represents the conversion of sulfur to higher order polysulfides, which are highly soluble in electrolyte, and the corresponding process is kinetically rapid. Other plateau at 2.05 V represents the conversion of higher order polysulfide to lower order insoluble polysulfides Li2S2/Li2S, which is a kinetically sluggish reaction, which gives 75% capacity of the cell.34 The charging

Figure 5. Contact angle measurement of the coated separators. (a) Pristine, (b) Nafion-coated, (c) SPEEK-25-coated, (d) SPEEK-50coated, (e) SPEEK-75-coated, and (f) SPEEK-100-coated separator.

membranes by the static sessile drop method. Pristine Celgard separator is hydrophobic, which has a contact angle of 116° (Figure 5a), whereas the SPEEK-coated separator exhibits a contact angle of 74° (Figure 5e), which shows the high wettability of the separator. The increase in wettability of the separator is due to the −SO3H groups present on the polymer. Polar solvent molecules absorbed by the hydrophilic groups (−SO3H) form large clusters in the domains of polymer33 and hence increase the wettability of the separator. This demonstrates that polymer coating can tune the surface properties of Celgard and can impart hydrophilic nature. It is observed that increasing the SPEEK content in the composite coating decreases the contact angle, which points out the increase in the wettability of the separator. 3.2. Electrochemical Analyses. The electrochemical performance was evaluated for Li−S cells in a 2032 type coin cell, assembled with metallic lithium as a counter electrode, S@ MWCNT composite as a cathode material, and ionomer-coated separator. The fabricated Li−S cell can be represented as follows: S@MWCNT/separator/Li. The electrolyte is 1 M bis(trifluoromethane)sulfonamide lithium salt (LiTFSI, SigmaAldrich) in a mixture of 1,2-dimethoxyethane (DME, SigmaAldrich) and 1,3-dioxolane (DOL, Sigma-Aldrich) in 1:1 v/v %. An improved electrochemical performance is observed when D


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Figure 7. Comparison of (a,b) charge−discharge cycles of cells with pristine; SPEEK-; Nafion-; and SPEEK-25-, SPEEK-50-, and SPEEK-75-coated separators. (c) Average potential vs cycle number of cells with pristine; SPEEK-; Nafion-; and SPEEK-25-, SPEEK-50-, and SPEEK-75-coated separators. (d,e) electrochemical impedance spectroscopy spectra of the fresh cells with abovementioned various ionomer-coated separators. The insets of (d,e) show the enlarged plots of the semicircle region. (f) Comparison of polarization of cells with SPEEK-100- and SPEEK-50-coated separators.

Figure 8. Performance of Li−S cell with various ionomer-coated separators at a 0.2 C rate. (a,b) Comparison of cycling performance and Coulombic efficiencies of cells using pristine, Nafion-, and SPEEK-100-coated separators, (c,d) comparison of cycling performance and Coulombic efficiencies of SPEEK-25-, SPEEK-50-, and SPEEK-75-coated separators, (e) average voltage of pristine; SPEEK-; Nafion-; and SPEEK-25-, SPEEK-50-, and SPEEK-75-coated separators, and (f) analysis of Qhps of various ionomer-coated separators employed in Li−S cell.

containing Nafion-coated separator, and this higher polarization results in relatively lower conductivity leading to lower kinetics. The increase in polarization of the SPEEK-containing cell might be due to less ionic conductive property of SPEEK compared to Nafion.35−37 This can be explained on the basis of the chemical structures of modified separators. The high-ionic conductivity pathway of Nafion was imparted by the presence of perfluorovinyl ether groups and sulfonate groups in its chemical structure, whereas SPEEK supports ionic conductivity only through sulfonate groups. In the case of the pristine separator, such ion-conducting functional groups are absent and it exhibits higher polarization. It is observed that the composite of SPEEK and Nafion shows lesser polarization. The cycling curves in Figure 7 indicate that ΔE is in the order SPEEK-75 > SPEEK-50 > SPEEK-25, which also supports the role of Nafion in decreasing the charge−discharge polarization. Figure 7c

process also follows two plateaus from 2.2 to 2.4 V: the oxidation reaction takes place and Li2S is oxidized to higher order polysulfide followed by the formation of sulfur. It is observed that for the cell with the SPEEK-100-coated separator, the upper discharge (higher order polysulfide) plateau is stable, pointing out the good reversibility of higher order polysulfides and suppressed polysulfide crossover. This may be attributed to complete filling of the submicron pores of the separator by SPEEK coating and also due to the charge repulsion between SPEEK and sulfide ions. The potential differences ΔE between discharge and charge curves (at discharge plateau 2.1 V) for Nafion, SPEEK-100, and pristine separators are 0.16, 0.21, and 0.23 V for the first cycle, respectively. From these values, it is clear that the electrochemical polarization was found to be more for cells containing pristine and SPEEK-100 coated separators than for the cell E


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ACS Applied Materials & Interfaces shows the ΔE comparison plots for cells with pristine and SPEEK-100-, SPEEK-25-, SPEEK-50-, SPEEK-75-, and Nafioncoated separators for 100 cycles. The ΔE value of SPEEK coating decreases with increasing cycle numbers, whereas for pristine and Nafion-coated separators, it increases slightly. Figure 7d,e shows the electrochemical impedance data for the fresh cells with pristine, Nafion, SPEEK-100, SPEEK-25, SPEEK-50, and SPEEK-75 separators. It is evident that the charge-transfer resistance has decreased with the presence of the modified separator. The improved conductivity is due to the sulfonated groups present in ionomer coating, which increases the lithium ion transport. Nafion coating shows a better Rct value than SPEEK-100; this ratifies that Nafion coating improves the electrode/electrolyte interface conductivity. For the composite ionomer-coated separators, the Rct values are between those of Nafion and SPEEK and the improved conductivity might be due to good wettability and electrolyte retention ability. SPEEK-25 shows a slightly higher chargetransfer resistance than SPEEK-50, possibly because of the extrinsic factors such as non-homogeneous coating of SPEEK25 on the separator surface compared to SPEEK-50. 3.2.2. Battery Performance Analysis. Figure 8a,b compares the cycling stability of Li−S cells containing pristine separator and Nafion-, SPEEK-100-, and SPEEK−Nafion compositecoated separators (SPEEK-25, SPEEK-50, and SPEEK-75). The performance was studied at a current density of 0.2 C. A capacity of 800 mA h g−1 is observed in the initial cycle with the pristine separator, which indicates 50% of sulfur utilization, whereas in the case of SPEEK- and Nafion-coated separators, capacity values of 1000 and 1200 mA h g−1 can be observed, which correspond to 60 and 70% sulfur utilization, respectively. The high initial capacity for cells with SPEEK- and Nafioncoated separators was mainly credited to the good ionic conductivity of the polymers. Though the cell containing the pristine separator exhibited a capacity of 800 mA h g−1 in the 1st cycle, after 300 cycles, it could only retain a specific capacity of 350 mA h g−1. The poor capacity retention might be due to the shuttle effect, where higher order polysulfides which are in the range of 1−1.8 nm for Li2Sx (4 < x < 8)38,39 can easily pass to the anode compartment through the micropores of the pristine separator. Whereas the cell made using the Nafioncoated separator showed a capacity of 1200 mA h g−1 initially, the capacity retention was only 50% after 150 cycles. This observation is consistent with the reported values.31 The degradation in capacity might be attributed to an insufficient amount of Nafion to cover the separator pores completely, which may lead to the diffusion of polysulfides. This crossover phenomenon of polysulfides may also be due to the flexibility of aliphatic chains, as the methanol crossover with Nafion is a serious issue in methanol fuel cells.40 In the case of the cell with the SPEEK-100-coated separator, an initial capacity of 1000 mA h g−1 is observed but degrades to 600 mA h g−1 within 20 cycles. However, from that point onward, the capacity stabilizes and degradation is very minimum (0.18%). After 300 cycles, the cell shows a capacity value of 550 mA h g−1. The high capacity retention for the case of SPEEK is attributed to the interconnected sulfonated groups, which increase polysulfide repulsion along with increased lithium ion conductivity. The uniform coating and complete filling of separator pores by SPEEK can further inhibit polysulfide shuttle. From the observed results, it can be perceived that the sulfur crossover pathway might be different for Nafion and SPEEK. In Nafion, a large nonionic perfluorinated backbone and small ionic

sulfonated ions are present, and in polar solvents, these sulfonated groups aggregate to form ionic domains. Because of size difference, there will be nanoseparation between ionic and nonionic ends, which, to some extent, can allow passage of smaller polar groups (sulfides).41 On the other hand, the microstructure of SPEEK is distinctly different from that of Nafion; the rigid backbone structure and low concentration of ionic/nonionic groups do not facilitate sulfur crossover.40,41 As cells with Nafion exhibited high capacity with poor retention, whereas SPEEK demonstrated a reasonable capacity with good retention, we are further interested to investigate the electrochemical property of their composite mixtures. In this scenario, we prepared different ratios of SPEEK/Nafion (i.e., 25:75, 50:50, and 75:25), as shown in Table 1. As expected, because of the synergistic effect, the composite ionomer coating exhibited better performance and cycling stability compared to the end members. For example, the SPEEK-75-containing cell shows a slightly higher capacity than SPEEK-100 (the capacity is 660 mA h g−1) and good capacity retention over 100 cycles. Similarly, the SPEEK-50 cell shows a capacity of 605 mA h g−1 with a cycling stability for long number cycles. The SPEEK-25 cell showed a large capacity but exhibited poor retention as expected. Therefore, we selected SPEEK-50 as the optimum ionomer composite for further investigations. Figure 8c,d shows the improvement in Coulombic efficiency when Celgard separator is coated with the SPEEK-50 composite. The high Coulombic efficiency represents a high ionic selectivity and low capacity loss. The observed high Coulombic efficiency is due to the perm selectivity of the ionomer composite, resulting in low capacity loss. The average voltage delivered by a cell is an important factor as any increment in cell voltage will increase the specific energy (specific energy = specific capacity × average voltage). The average cell potential V is calculated as V = energy density/ capacity. Therefore, we have calculated the average discharge voltage of Li−S cells constructed using aforementioned coated/ uncoated separators, which is depicted in Figure 8e. An improvement in average cell voltage is observed for Li−S cells with composite ionomer-coated separators. Among all modified separators, SPEEK-50 exhibited a higher average cell voltage with less decay up to 300 cycles. The lowest average voltage observed is for the cell with the SPEEK-100-coated separator because of high polarization between charge and discharge. The cells constructed using SPEEK-100- and SPEEK-50-coated separators showed about 100 mV difference in their average voltage, emphasizing that composite ionomer coating is advantageous in terms of specific energy. 3.2.3. Polysulfide Retention Studies. The polysulfide rejection capability of coated separators is analyzed by extracting the capacity contribution corresponding to the formation of higher order polysulfides (Qhps). This basically corresponds to the discharge capacity between 2.4 and 2.1 V. Qhps values obtained for cells using various ionomer-coated separators are shown in Figure 8f. Among all, the cell with the SPEEK-50-coated separator shows high Qhps values and the cell with the pristine separator shows the minimum Qhps values. The capacity retention ratio is calculated as (Qhps)final cycle/(Qhps)initial cycle × 100. The Qhps retention of the SPEEK-coated separator is 75% after 300 cycles. The capacity retention ratios of other coated/uncoated separators SPEEK-25, SPEEK-50, SPEEK-75, and SPEEK-100 and pristine separator are 65, 75, 65, 82, and 50%, respectively, after 300 cycles (capacity retention is listed after 25 cycles for the F


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ACS Applied Materials & Interfaces abovementioned separators). The Qhps retention studies prove that polysulfide crossover is controlled in ionomer-coated separators than the commercial pristine separator. High shielding of polysulfides is possible with the SPEEK-coated separator (82% after 300 cycles). As discussed before, this high polysulfide rejection capability of SPEEK coating can be attributed to the microstructure and polysulfide rejection groups present in the ionomer. The composite ionomer SPEEK-50-coated separator also shows good Qhps retention of about 75% at the end of 300 cycles. The visual demonstration of polysulfide crossover through the pristine Celgard separator while its mitigation when coated with SPEEK-50 is shown in Figure 9. The left side of the H-

Figure 10. (a) Cell construction depiction with pristine (left) and modified separator (right), (b) discharge−charge profile of two-layer sulfur cathode at 0.2 C, and (c) long cycling performance of pristine, single-layer, and double-layer cathode for Li−S batteries. (d) Rate performance of single- and double-layer sulfur cathodes.

and two cathode layers are compared in Figure 10d. The cells are cycled initially at lower rates 0.1, 0.2,, and 0.3 C followed by cycling at higher rates of 1, 2, 3,, and 4 C and then decreasing the cycling rate from 4 to 0.2 C over 40 cycles. It can be seen that relatively stable capacities of 1160, 1000, 950, 810, 725, 650, 480, and 880 mA h g−1 are observed for the cells with two cathode layers and 1123, 792, 745, 691, 549, 395, 296, and 750 mA h g−1 are observed for cell with one cathode layer, respectively, at 0.1, 0.2, 0.3, 1, 2, 3, 4,, and 0.2 C rates. One of the main limitations of a Li−S system is coming from the usage of Li metal anode. The major drawback of using Li as an anode is its short cycle life, which may be few hundred cycles. Being highly electronegative and highly reactive, Li spontaneously reacts with organic solvents, causing electrolyte degradation. Similarly, it is also responsible for lithium dendrite formation, which results in cell shortening. A Li anode can also result in a lower Coulombic efficiency in view of the polysulfide crossover to anode side and getting reduced and deposited there. In this regard, lithiated Si-based anodes have shown 2−3 times longer cycle life than lithium anode.42 Interestingly, in this work, though we have used metal lithium anode, the cell could cycle more than 500 cycles. We believe that this was possible due to the SPEEK−Nafion polymer coating on the separator, which not only reduces the shuttle effect but also prevents lithium dendrite growth through the separator. Besides, when an unmodified Celgard separator was used, the Coulombic efficiency was continuously decreasing and reached a value of 82% within 300 cycles. On the other hand, when the SPEEK-50-coated separator is used, the Coulombic efficiency remains greater than 99%, even after 500 cycles, showing the positive effect of the separator coating in preventing the polysulfide shuttle effect. Though the theoretical energy density of Li−S batteries is 2567 W h kg−1, practically achievable energy density is 5−6 times lower because of other active and inactive components. The overall practical energy density calculated in the present work is around 400 W h kg−1, which is similar to the existing commercial prototypes.43,44 3.2.4. Ex Situ X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis of pristine, discharged, and charged samples are shown in Figure 11. The pristine sample has characteristic binding energies of 164.2 and

Figure 9. Visual demonstration of polysulfide crossover in H-type cell with (a−c) Celgard separator after 0, 24,, and 48 h and (a′−c′) SPEEK-50-coated separator after 0, 24,, and 48 h.

type cell is filled with 0.5 M Li2S6 dissolved in the solution of 1 M LiTFSI electrolyte in 1:1 v % of DME/DOL. LiNO3 (0.25 M) was added as an additive. The other side of the tube contains only 1 M LiTFSI in 1:1 v % of DME/DOL with 0.25 M LiNO3 additive. As expected, when the pristine polypropylene separator is used, the polysulfide crossover is unavoidable because of the porous nature of the separator, and as a result, within 24 h, yellow color is seen in the right compartment. The equilibrium with polysulfide is reached within 48 h, indicating a considerable polysulfide crossover. Interestingly, the ionomer-coated separator effectively minimizes the polysulfide crossover, as we observe only slight coloration, even after 48 h. To further enhance the Li−S cell cycling performance, we have investigated the effect of placing a second cathode layer between ionomer composite separator (SPEEK-50) and first cathode layer, as shown in Figure 10. This extra S@CNT layer doubles the sulfur loading from 3 to 6 mg/cm2. Figure 10b shows 2nd, 25th, 50th, 75th, and 100th charge−discharge cycles. It is worth mentioning that relatively small ΔE is observed here compared to the cell with a single cathode layer, indicating an improved conductivity. The cell shows an initial capacity of 1300 mA h g−1 and maintained 850 mA h g−1 capacity after 100 cycles, which is superior to 700 mA h g−1 capacity observed for a single layer cathode. The high capacity and capacity retention can be ascribed to the cumulative effect, viz. the presence of a second cathode layer (carbon layer) physically restraining polysulfide dissolution while the functional layer (ionomer-coated separator) chemically mitigating polysulfide crossover. Combination of both interlayers effectively shields polysulfide crossover from the cathode to the anode. The resultant cell shows good capacity retention for 500 cycles (Figure 10c). The rate capabilities of cells with one G


Research Article


physical barriers effectively suppresses the polysulfide crossover and improve the performance of Li−S batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04888. SEM images of SPEEK-25- and SPEEK-75-coated separator; XRD of sulfur, MWCNT, and MWCNT@ sulfur composite; 2D and 3D AFM morphological images of pristine and Nafion- and SPEEK-100-coated separators; structural and electrochemical characterization; and discharge−charge profile of Li−S cell with pristine and SPEEK-100-, Nafion-, SPEEK-25-, SPEEK50-, and SPEEK-75 ionomer-coated separators (PDF)

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Figure 11. XPS spectra of (a) S spectra of pristine sample, (b,b′) S and Li spectra of discharged sample, and (c,c′) S and Li spectra of charged sample.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

165.4 eV with a spin orbital splitting energy of 1.2 eV, which can be assigned to S 2p. The discharge sample shows different chemical environments with respect to elemental sulfur, signaling the existence of sulfur in various oxidation states. The spectrum can be deconvoluted into two regions, one at the lower binding energy region where terminal (162.7 eV) and bridging sulfur atoms (164 eV) of Li2Sn (3 < n > 8) polysulfides observed. Here, a small intensity of the bridging sulfur indicates Li2S2/L2S formation and confirms the absence of long-chain polysulfides. This indicates complete conversion of S8 to Li2S2 during discharge process. The second set of peaks at 169.45 and 167.61 eV is attributed to sulfur present in the electrolyte salt LiTFSI.45,46 Along with these peaks, an unusual peak at 170.7 eV is observed, which can be assigned to S1V presumed to be formed due to the reduction of S in LiTFSI salt during the discharge process. Figure 11b′ shows the deconvoluted XPS spectra of lithium in the discharged state. The peaks at 55.7 and 56.15 eV are assigned to Li2Sn and LiF products, respectively.45 Figure 11c,c′ shows the XPS spectra of the charged sample showing shifting back of binding energies to elemental sulfur (164.2 and 165.3 eV) and lithium, demonstrating complete conversion of Li2S2 to S8 during charging.

Dasari Bosu Babu: 0000-0001-8816-2984 Krishnan Giribabu: 0000-0002-2821-1673 Kannadka Ramesha: 0000-0003-3089-0406 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.B.B. thanks CSIR for CSIR-UGC fellowship (23/12/2012(ii) EU-V). K.G. thanks SERB for NPDF fellowship (PDF/2016/ 000163). The authors thank the Central Instrumentation Facility, CECRI, Karaikudi, CSIR, India, for providing characterization facilities.

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REFERENCES

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4. CONCLUSIONS In summary, the S@MWCNT cathode is prepared by absorption of colloidal sulfur on to MWCNTs. The high perm selectivity was realized by coating a thin layer of Nafion− SPEEK composite on the cathode side of the polypropylene separator, which improves the capacity and cycling stability of Li−S batteries. The sulfonated functional groups present on the ionomer selectively allow lithium ion diffusion while repelling sulfide ions. The composite ionomer imparts good conductivity and cycling stability to the cell because of the synergistic effect of parent ionomers. It is observed that the modified separator retards shuttle effect by preventing polysulfide crossover, resulting in a stable cell capacity of 600 mA h g−1 up to 300 cycles. The cell capacity is further enhanced by placing an extra cathode layer, which acts as a physical barrier for polysulfide diffusion. This also enhances the energy density by doubling the overall sulfur loading (3−6 mg/cm2). It is suggested here that the combination of nonfunctional (extra cathode layer) and functional (Nafion−SPEEK composite ionomer coating) H


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