Nafion composite coated separator as potential

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Perm selective SPEEK/Nafion composite coated separator as potential polysulfide cross-over barrier layer for Li-S batteries Dasari Bosu Babu, Krishnan Giribabu, and Kannadka Ramesha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04888 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

Perm Selective SPEEK/Nafion Composite Coated Separator as Potential Polysulfide Cross-Over Barrier Layer for Li-S Batteries Dasari Bosu Babua,b, Krishnan Giribabua, Kannadka Ramesha a,b* a

CSIR-Central Electrochemical Research Institute-Chennai Unit, CSIR-Madras Complex, Taramani, Chennai 600113, India

b

Academy of Scientific and Innovative Research (AcSIR), CSIR-CECRI, Karaikudi 630003, India.

Abstract Minimizing the shuttle effect by constraining polysulfides to cathode compartment and activating passive layer between cathode and separator is highly important for improving the Li-S cell performance, coulombic efficiency and cycle life. Here, we report submicron thin coating of perm selective 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 by adding nafion improve the capacity value. Among different ratios of Nafion and SPEEK (25:75, 50:50 and 75:25), the composite with SPEEK: Nafion ratio 50:50 showed controlled shuttle effect with stable cell capacity of 600 mAhg-1 up to the 300 cycles. This modified separator with perm selective coatings not only reduces the polysulfide shuttle but also improves the wettability and interfacial contact which results in improving average cell potential and lithium diffusivity. It is demonstrated here that the combination of functional (ionomer coating on separator) and non-functional (extra cathode layer) physical barriers effectively suppress the polysulfide cross over and improve electrochemical performance of Li-S battery. The cell shows initial capacity of 1300 mAhg-1 and capacity retention of 650 mAhg-1 over 500 cycles with 6 mg/cm2 sulfur loading. Key words: Li-S battery, separator, interlayer, physical barrier, polymer, CNT paper.

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Introduction An efficient energy storage technology is need of the hour not only for alleviating from alarming issues such as depletion of fossil fuel reserves, global warming, etc., 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, due to its ultrahigh theoretical capacity and high energy density of 1672 mAhg-1 and 2500 Wh kg-1. The high copiousness of sulfur accompanied by its non-toxic and low price puts LiS battery research on the top priority in recent years to develop sustainable energy storage system. Though the Li-S battery possess theoretical advantages in terms of capacity and energy density, the commercial realization of 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). Re-oxidation will take place at cathode after migration from anode, this phenomenon is termed as shuttle effect and leads to continuous capacity fading, hence the poor cyclability and low coulombic efficiency in Li-S batteries. These polysulfides react with lithium anode to form irreversible side products on anode.7 Hence it is highly important to suppress the shuttle effect. To restrict the shuttle effect, researchers had employed several strategies by modifying cathode, separator, electrolyte or anode.

8,9

The effective way of trapping

polysulfide in 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 cathode or on the separator. The advantage of interlayer is that it physically mitigates polysulfide shuttle and helps in capacity retention.14 Among different interlayers conductive functionalized polymer 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 The functionalized polymer separator coatings like polydopamine (PDA),16 polyethylene glycol (PEG),17 polyacrylic acid (PAA),18 polyethylene oxide (PEO),19 polyvinylidene fluoride (PVDF),20 PEDOT: PSS,21 polyaniline (PANI),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 (PTFE) backbone that


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stabilizes the nafion in electrolyte and the perfluorinated side chain having SO3- group that selectively allow positive ions (lithium ions) and repel hopping of negative ions (sulfides) through coulombic interactions. Because of these added advantages, nafion find applications as a binder

25,26

, polymer separator27, and coating layer (cathode-side or separator-side)

28–30

in Li-S batteries. However, cost of nafion is very high when compared to conventional separators. Huang et.al optimized the nafion amount to 0.7 mg/cm2 (and 1µm thick) for 0.53 mg/cm2 loading of sulfur which is actually more than the weight of sulfur used. The amount of nafion additive used here may be on higher side compared to other separator modifiers31. Further this thick coating increases the cell resistance and decreases the energy density. However, the permeability of nafion membrane is relatively low

32

. So 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 sulfonated poly(ether ether ketone) (SPEEK) modified separator for Li-S battery. To our knowledge functional polymer SPEEK for separator modification is not reported so far.

PEEK is a thermo-stable 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 that can be represented as SPEEK. Similar to the nafion, SPEEK is also a functional separator and exhibits perm selectivity due to 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, physical/chemical/thermal stability. In addition to this, favorable microstructural features of SPEEK over nafion commanding its use in direct methanol fuel cell (DMFC). Here, we coated SPEEK on the separator with about 0.4mg/cm2 loading which is much lesser than conventional nafion coating (0.7 mg/cm2). It is observed that even lower amount (0.4mg) of SPEEK coating gives stable capacities. For comparison separator coated with nafion of similar amount is also prepared. Though it showed high capacity the retention was poor. So, we optimized coating composition by mixing both ionomers. Interestingly, coating composition containing 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 mg/cm2 to 6mg/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 DMF solvent which gives uniform distribution of sulfur on carbon.

2. Experimental section 2.1 Material preparation Modification of separator Celgard 2400 separator is taken as skeleton for ionomer coating. 1% of 40µƖ ionomer in DMF solvent is drop casted on the celgard membrane which results 0.4mg/cm2 of ionomer presence on the separator. This coated separator is vacuum dried at 70°C in 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. Table1: The representation of mixed ionomers. Representation in this SPEEK

Nafion percentage

article

percentage

SPEEK-100

100

0

SPEEK-75

75

25

SPEEK-50

50

50

SPEEK-25

25

75

SPEEK-0

0

100

Colloidal sulfur preparation 100 mg of sulfur powder (RANKEM chemicals, 99%) is added in 50ml of dimethylformamide (DMF) (SRL chemicals, 99.9%) and sonicated until the yellow color solid dissolves. This colorless solution is added into 500 ml distilled water. The solution transforms from colorless to white color indicating formation of colloidal sulfur (Figure1).


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MWCNT paper and Sulfur composite preparation MWCNTs and Sulfur (55:45 ratio) are taken in 50ml of Dimethylformamide (DMF) solvent and sonicated for 4h to make sure that sulfur dissolves. This solution is added to beaker containing 500 ml water, immediately solution color changes to white indicating colloidal sulfur formation. The colloidal sulfur is slowly absorbed by MWCNTs while stirring the solution for 2h. This S@MWCNT composite is vacuum filtered for obtaining S@CNT paper. In this solution method the colloidal sulfur gets adsorbed uniformly on MWCNT surface. For each cm2 area of the paper, 3 mg of sulfur is observed. This S@MWCNT paper is used as cathode. Here solvent not only assists in producing colloidal sulfur but also facilitates dispersion of carbon throughout the solution which will enhance uniform sulfur distribution. Schematic diagram of the synthesis procedure is shown in Figure1. The CNT-nano S composite will exhibit better electronic and Li reactivity and alleviate volume expansion during discharge-charge cycles. 3. Results and Discussions

3.1 Structural analysis The chemical structure of Nafion and SPEEK are shown in Figure 2 (a, b). Figure 3(a, b) shows SEM images of colloidal sulfur and MWCNTs. The SEM image of colloidal sulfur shows 50-70 nm sized spheres due to agglomeration of particles. But, in Figure S 1(c) composite of S@MWCNTs does not show these sulfur spheres, this is attributed to absorption of formed colloidal sulfur into the pores of CNTS. Figure 3(c-f) shows SEM images of pristine separator, nafion coated, in-plane and cross sectional image of SPEEK-50 and SPEEK-100 coated separators. The pristine celgard 2400 separator possesses inter connected 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, might be due to insufficient amount of nafion per unit area. The SEM image of SPEEK-25 (Figure S1a), SPEEK-50, SPEEK-75 (Figure S1b) and SPEEK-100 coated separator shows uniform ionomer coating with complete filling of separator pores. The cross sectional SEM image reveal 0.7 to 0.9 µm thick uniform coating of SPEEK on separator. For comparison SPEEK coated and non-coated (scratched) regions of the separator is shown in Figure 3h.



Powder XRD of sulfur, MWCNTs and S@MWCNTs are shown in Figure (S2). The S@MWCNTs composite shows 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 AFM technique. Figure 4 show 3D topographic AFM images of pristine, nafion, SPEEK-25, SPEEK-50, SPEEK- 75 and SPEEK-100 coated separators respectively. It is observed that the surface properties clearly change with ionomer coatings. In the conventional celgard separator (pristine) having micron range pores, AFM show high intense peaks as seen in Figure 4a. Nafion coated separator also shows similar AFM features, however 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 intensity of the peaks gradually decreasing 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, whereas the peak intensities reduced for SPEEK-50, 75, 100 coatings. This infers that with SPEEK-50, SPEEK-75 and SPEEK-100 coating, the separator pores are completely filled by ionomer. The roughness profile (Rdc) depends on porous structure of separator. The Rdc value for pristine, nafion, SPEEK-25, SPEEK-50, SPEEK- 75 and SPEEK-100 separators are 13.8nm, 2.52 nm, 2.28 nm 0.695 nm, 0.695 nm and 0.297 nm respectively also show similar trends. Interestingly the roughness of the separator related to the amount of SPEEK in the composite. It is evident in Figure 4g that with increase in SPEEK percentage the roughness of the surface is decreasing. The supplementary Figure S3 shows 2D and 3D images of these compositions at lower magnifications. The surface wettability property of polymer coated celgard separator is investigated by contact angle measurements. Figure 5 shows the static contact angle for the membranes by static sessile drop method. Pristine celgard separator is hydrophobic which has contact angle of 116° (Figure 5a) whereas SPEEK coated separator exhibits contact angle of 74° (Figure 5e) which shows 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) forms large clusters in the domains of polymer33 and hence increases wettability of the separator. This demonstrates that polymer coating can tune surface properties of celgard and can impart hydrophilic nature. It is observed that increasing


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the SPEEK content in the composite coating the contact angle decreases which points out the increase in wettability of the separator. 3.2 Electrochemical Analyses The electrochemical performance was evaluated for Li-S cell in a 2032 type coin cell, assembled with metallic lithium as a counter electrode, S@MWCNT composite as 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, Sigma-Aldrich) in a mixture of 1, 2-dimethoxyethane (DME, Sigma-Aldrich) and 1,3 Dioxalane (DOL, Sigma-Aldrich) in 1:1 V /V%. The improved electrochemical performance is observed when we replace pristine separator with ionomer coated separator. Cyclic voltammetry (CV) of functionalized polymer modified separators are shown in Figure 6. The cathodic scan has two peaks, first one around 2.3 V (peak-1) and next one 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. The peak-2 is the major peak in cathodic scan which will deliver significant capacity. This peak corresponds to polysulfides formed at higher voltage getting converted to end product Li2S2/ Li2S. During first anodic scan a single peak is observed at 2.4 V indicating that major amount of sulfur is reformed in the initial cycle. From second cycle onwards the voltage is shifted to lower values and peak split into two, the first peak is due to conversion of Li2S to polysulfides and the next peak is due to further conversion of polysulfides to elemental sulfur. Figure 6 show CVs of pristine, nafion coated, SPEEK-50 and SPEEK-100 coated separators respectively. The potential difference between the two intense peaks are 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 indicate high polarization and the slower electrochemical kinetics for SPEEK-100.

Discharge-Charge stability analysis The galvanostatic charge-discharge cycling curves for cells containing pristine (celgard) separator, SPEEK coated separator, nafion coated separator, and SPEEK-Nafion composite coated separators (SPEEK-25, SPEEK-50 and SPEEK-75) at 0.2C 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 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 is the conversion of higher order polysulfide to lower order insoluble polysulfides Li2S2/Li2S which is kinetically sluggish reaction which gives 75% capacity of the cell 34 Charging process also follows two plateaus from 2.2 to 2.4 V the oxidation reaction taking place and Li2S is oxidized to higher order polysulfide followed by formation of sulfur. It is observed that for the cell with SPEEK-100 coated separator, the upper discharge (higher order polysulfide) plateau is stable pointing out good reversibility of higher order polysulfides and suppressed polysulfide crossover. This may be attributed to complete filling the submicron pores of separator by SPEEK coating and also due to the charge repulsion between SPEEK and sulfide ions. The potential difference ∆E between discharge and charge curve (at discharge plateau 2.1 V) for nafion, SPEEK-100 and pristine separator are 0.16, 0.21, 0.23 V for the first cycle. From these values it is clear that the electrochemical polarization was found to be more for cells containing pristine and SPEEK-100 coated separator than the cell containing nafion coated separator and this higher polarization results in relatively lower conductivity leading to lower kinetics. The increase in polarization of SPEEK containing cell might be due to less ionic conductive property of SPEEK compared to nafion35–37. This can be explained on the basis of the chemical structure 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 while SPEEK supports ionic conductivity only through sulfonate groups. In the case of pristine separator such ion conducting functional groups are absent and it exhibits higher polarization. It is observed that the composite of SPEEK and nafion show lesser polarization. The cycling curves in Figure 7 indicate that ∆E is in the order SPEEK-75 > SPEEK-50 > SPEEK-25 which also supports the role of nafion in decreasing the chargedischarge polarization. Figure 7(c) shows the ∆E comparison plots for cells with pristine, SPEEK-100, SPEEK-25, SPEEK-50, SPEEK-75 and nafion coated separators for 100 cycles. The ∆E value of SPEEK coating decreases with increasing cycle numbers whereas for pristine and nafion coated separator it increases slightly. Figure 7 (d, f) show electrochemical impedance data for the fresh cells with pristine, nafion, SPEEK-100 and SPEEK-25, SPEEK50, SPEEK-75 separators respectively. It is evident that charge transfer resistance has decreased with the presence of modified separator. The improved conductivity is due to the


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sulfonated groups present in ionomer coating that increases the lithium ion transport. Nafion coating shows better Rct value than SPEEK-100, this ratifies that nafion coating improves electrode/electrolyte interface conductivity. For 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 show slightly higher charge transfer resistance than SPEEK-50 possibly due to the extrinsic factors such as nonhomogeneous coating of SPEEK-25 on the separator surface compared to SPEEK-50. Battery performance analysis Figure 8 (a, b) compares cycling stability of Li-S cells containing pristine separator and nafion, SPEEK-100, and SPEEK-Nafion composite coated separators (SPEEK-25, SPEEK50 and SPEEK-75). The performance was studied at current density of 0.2 C. A capacity of 800 mAhg-1 is observed in the initial cycle with the pristine separator, this indicates 50% of sulfur utilization whereas in the case of SPEEK, nafion coated separators a capacity value of 1000, 1200 mAhg-1 can be observed which corresponds to 60% and 70% sulfur utilization respectively. The high initial capacity for cells with SPEEK and nafion coated separators was mainly credited to good ionic conductivity of the polymers. Though cell containing pristine separator exhibited capacity of 800 mAhg-1 at first cycle after 300 cycles it could only retain specific capacity of 350 mAhg-1. The poor capacity retention might be due to the shuttle effect where higher order polysulfides which are in the range of 1 to 1.8 nm for Li2Sx (4 < x < 8)38,39 can easily pass to anode compartment

through the micro pores of the pristine

separator. Whereas, the cell made using nafion coated separator showed capacity of 1200 mAhg-1 initially but 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 insufficient amount of nafion to cover the separator pores completely, which may lead to the diffusion of polysulfides. This crossover phenomena 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 cell with SPEEK-100 coated separator an initial

capacity of 1000 mAhg-1 is observed but degrades to 600 mAhg-1 within 20 cycles. However from that point onwards capacity stabilizes and degradation is very minimum (0.18%). After 300 cycles the cell shows a capacity value of 550 mAhg-1. The high capacity retention for the case of SPEEK is attributed to the interconnected sulfonated group which increases polysulfides repulsion along with increased lithium ion conductivity. The uniform coating and complete filling of separator pores by SPEEK can further inhibit polysulfide shuttle.



From observed results, it can be perceived that the sulfur crossover pathway might be different for nafion and SPEEK. In nafion, large non-ionic perfluorinated backbone and small ionic sulfonated ions are present and in polar solvents these sulfonated groups aggregate to form ionic domains. Due to size difference there will be nano separation between ionic and non-ionic 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 nafion, the rigid backbone structure and low concentration of ionic/non-ionic groups do not facilitate sulfur crossover. 40,41 As cells with nafion exhibited high capacity with poor retention whereas SPEEK demonstrated reasonable capacity with good retention, we are further interested to investigate electrochemical property of their composite mixtures. In this scenario we prepared different ratios of i.e. SPEEK: Nafion i.e. (25: 75), (50:50) and (75:25) as shown in Table 1. As expected due to the synergistic effect the composite ionomer coating exhibited better performance and cycling stability compared to the end members. For example, SPEEK-75 containing cell shows slightly higher capacity than SPEEK-100 (capacity is 660 mAhg-1) and with good capacity retention over 100 cycles. Similarly SPEEK-50 cell shows a capacity of 605 mAhg-1 with a cycling stability for long number cycles. SPEEK-25 cell showed large capacity however exhibited poor retention as expected. So we selected SPEEK-50 as the optimum ionomer composite for further investigations. Figure 8 (c, d) shows the improved in coulombic efficiency when celgard separator is coated with SPEEK-50 composite. The high coulombic efficiency represent 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 important factor as any increment in cell voltage will increase the specific energy (Specific energy = Specific capacity x Average voltage). The average cell potential V is calculated as V= Energy density/Capacity. So we have calculated average discharge voltage of Li-S cells constructed using aforementioned coated/uncoated separators which is depicted in Figure 8e. Improvement in average cell voltage is observed for Li-S cells with composite ionomer coated separators. Among all modified separators, SPEEK-50 exhibited higher average cell voltage with less decay up to 300 cycles. The lowest average voltage observed is for the cell with SPEEK-100 coated separator due to high polarization between charge and discharge. The cells constructed using SPEEK-100 and SPEEK-50 coated separators showed about 100mV difference in their


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average voltage, emphasizing that composite ionomer coating is advantageous in terms of specific energy. 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 2.4-2.1 V. Qhps obtained for cells using various ionomer coated separators are shown in Figure 8f. Among all, the cell with SPEEK-50 coated separator shows high Qhps values and the cell with pristine separator shows minimum Qhps values. The capacity retention ratio is calculated as (Qhps) (Qhps)

initial cycle

final cycle

/

x 100. The Qhps retention of SPEEK coated separator is 75% after

300cycles. The capacity retention ratios of other coated/uncoated separators SPEEK-25, 50, 75,100 and pristine 65%, 75%, 65%, 82%, 50% respectively after 300 cycles (capacity retention are listed after 25 cycles for above mention separators). The Qhps retention studies prove that polysulfide crossover is controlled in ionomer coated separators than commercial pristine separator. High shielding of polysulfides is possible with 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-type cell is filled with 0.5 M Li2S6 dissolved in the solution of 1M LiTFSI electrolyte in 1:1 v% of DME : DOL. 0.25 M LiNO3 was added as additive. The other side of the tube contains only 1M LiTFSI in 1:1 v% of DME : DOL with 0.25 M LiNO3 additive. As expected when pristine polypropylene separator is used the poly sulfide cross over is unavoidable due to 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 48h indicating considerable polysulfide crossover. Interestingly, ionomer coated separator is effectively minimizing the polysulfide cross over, as we observe only slight coloration even after 48 h. To further enhance the Li-S cell cycling performance we have investigated effect of placing 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 mg/cm2 to 6mg/cm2. Figure 10b shows 2nd, 25th, 50th, 75th and 100th charge-discharge cycles. It is worth to mention that relatively small ∆E is observed here compared to the cell with single cathode layer indicating improved conductivity. The cell shows initial capacity of 1300 mAhg-1 and maintained 850 mAhg-1 capacity after 100 cycles, which is superior to 700 mAhg-1 capacity observed for single layer cathode. The high capacity and capacity retention can be ascribed to the cumulative effect, viz. the presence of 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 shield polysulfide cross over from cathode to anode. The resultant cell shows good capacity retention for 500 cycles (Figure 10c). The rate capability of cells with one and two cathode layers are compared in Figure 10d. The cells are cycled initially at lower rates 0.1C, 0.2C and 0.3C followed by cycling at higher rates of 1C, 2C, 3C and 4C then decreasing cycling rate from 4C to 0.2C over 40 cycles. It can be seen that relatively stable capacities of 1160, 1000, 950, 810, 725, 650, 480 and 880 mAhg-1 are observed for the cells with two cathode layers and 1123, 792, 745, 691, 549, 395, 296, 750 mAhg-1 observed for cell with one cathode layer respectively at 0.1C, 0.2C, 0.3C,1C, 2C, 3C, 4C and 0.2C rates. One of the main limitation of Li-S system is coming from the usage of Li metal anode. The major drawback of using Li as anode is short cycle life, may be few hundred cycles. Being highly electronegative and highly reactive, Li spontaneously react with organic solvents causing electrolyte degradation. Similarly it also responsible for lithium dendrite formation which results in cell shortening. Li anode can also result in 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 two to three 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 this was possible due to the SPEEK-Nafion polymer coating on the separator which not only reduces the shuttle effect but also prevent lithium dendrite growth through the separator. Besides, when unmodified celgard separator was used, the coulombic efficiency was continuously decreasing and reached a value of 82 % within 300 cycles. Whereas, 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 WhKg-1, practical achievable energy


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density is five to six times lower due to other active and inactive components. The overall practical energy density calculated in the present work is around 400 WhKg-1 which is similar to the existing commercial prototypes43,44. Ex-situ XPS: XPS analysis of pristine, discharged and charged samples are shown in Figure 11. The pristine sample has characteristic binding energies of 164.2 eV and 165.4eV with the spin orbital splitting energy of 1.2 eV which can be assigned to S 2p. The discharge sample shows different chemical environment with respect to elemental sulfur signaling existence of sulfur in various oxidation states. The spectrum can be deconvoluted into two regions, one at lower binding energy region where terminal (162.7 eV) and bridging sulfur atoms (164 eV) of Li2Sn (38) polysulfides observed. Here, small intensity of the bridging sulfur indicates Li2S2/L2S formation and confirming the absence of long chain polysulfides. This indicates complete conversion of S8 to Li2S2 during discharge process. Second set of peaks at 169.45 eV and 167.61 eV are 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. Figure11 (b’) shows the deconvoluted XPS spectra of lithium in the discharged state. The peaks at 55.7 eV, 56.15 eV assigned to Li2Sn and LiF products.45 Figure 11 (c), (c’) shows XPS of the charged sample showing shifting back of binding energies to elemental sulfur (164.2 eV, 165.3 eV) and lithium demonstrating complete conversion of Li2S2 to S8 during the charging.

Conclusion In summary, S@MWCNT cathode is prepared by absorption of colloidal sulfur on to multiwalled carbon nanotubes. The high perm selectivity was realized by coating thin layer of Nafion-SPEEK composite on cathode side of the polypropylene separator which improves capacity and cycling stability of Li-S battery. The sulfonated functional groups present on the ionomer selectively allow lithium ion diffusion while repelling sulfide ions. The composite ionomer impart good conductivity and cycling stability to cell due to the synergistic effect of parent ionomers. It is observed that the modified separator retards shuttle effect by preventing polysulfide cross-over resulting in stable cell capacity of 600 mAhg-1 up to the 300 cycles.



The cell capacity is further enhanced by placing extra cathode layer which acts as physical barrier for polysulfide diffusion. This also enhances energy density by doubling the overall sulfur loading (3 to 6 mg/cm2). It is suggested here that the combination of non-functional (extra cathode layer) and functional (Nafion-SPEEK composite ionomer coating) physical barriers effectively suppress the polysulfide cross over and improve the performance of Li-S battery.

Author information Corresponding Authors *E-mail: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgements Bosu Babu Dasari thanks CSIR for CSIR-UGC fellowship (23/12/2012(ii) EU-V). Krishnan Giribabu thank SERB for NPDF fellowship (PDF/2016/000163). The authors thank the Central Instrumentation Facility, CECRI, Karaikudi, CSIR, India, for providing characterization facilities. Supporting information SEM images of SPEEK25, 75 coated separator. XRD of sulfur, MWCNT and MWCNT@ Sulfur composite. AFM 2D and 3D- morphological images of Pristine, Nafion, SPEEK-100 coated

separator. Structural and electrochemical characterization. Discharge-charge profile of Li-S cell with pristine, SPEEK-100, nafion, SPEEK-25, SPEEK-50, SPEEK-75 ionomer coated separators.


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Figure Captions: Figure 1: Synthesis process of collidal S@MWCNT composite paper.

Figure 2: The chemical structure of (a) Nafion (b) SPEEK.

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

Figure 4: AFM 3D- morphological images of coated separators. (a) pristine separator (b) nafion coated (c) SPEEK-25 coated (d) SPEEK-50 coated (e) SPEEK-75 coated and (f) SPEEK-100 coated separator. (g) roughness profile for above mentioned ionomer coated separators.

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

Figure 6: Cyclic voltammogram of Li-S cell constructed using various coated separators. (a) pristine separator (b) nafion coated (c) SPEEK-50 coated and (d) SPEEK-100 coated separator.

Figure 7: Comparison of (a, b) charge-discharge cycles of cells with pristine, SPEEK, nafion and SPEEK-25,50,75 coated separators, (c) The average potential Vs cycle number of cells with pristine, SPEEK, nafion and SPEEK-25,50,75 coated separators. (d, e) EIS of the fresh cells with above mentioned various ionomer coated separators. Inset to (d) & (e) shows


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enlarged plot of the semicircle region, (f) comparison of polarization of cells with SPEEK100 and SPEEK-50 coated separators.

Figure 8: Performance of Li-S cell with various ionomer coated separators at 0.2C rate. (a, b) comparison of cycling performance and coulombic efficiencies of cells using pristine, nafion, SPEEK-100 coated separators, (c, d) comparison of cycling performance and coulombic efficiencies of SPEEK-25, SPEEK-50 and SPEEK-75 coated separator, (e) average voltage of pristine, SPEEK, nafion and SPEEK-25,50,75 coated separator, (f) analysis of Qhps of various ionomer coated separators employed in Li-S cell. Figure 9: Visual demonstration of polysulfide crossover in H-Type cell with (a - c) celgard separator after 0h, 24h and 48h (a’ – c’) SPEEK- 50 coated separator after 0h, 24h and 48h.

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

Figure 11: X-ray photoelectron spectra (XPS) of (a) S spectra of pristine sample, (b & b’) S and Li spectra of discharged sample, (c, c’) S and Li spectra of charged sample.



Figures:

Figure 1: Synthesis process of collidal S@MWCNT composite paper.


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(a)

(b)

Figure 2: The chemical structure of (a) Nafion (b) SPEEK



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


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Figure 4: AFM 3D- morphological images of coated separators. (a) pristine separator (b) nafion coated (c) SPEEK-25 coated (d) SPEEK-50 coated (e) SPEEK-75 coated and (f) SPEEK-100 coated separator. (g) roughness profile for above mentioned ionomer coated separators.



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


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Figure 6: Cyclic voltammogram of Li-S cell constructed using various coated separators. (a) pristine separator (b) nafion coated (c) SPEEK-50 coated and (d) SPEEK-100 coated separator.



Figure 7: Comparison of (a, b) charge-discharge cycles of cells with pristine, SPEEK, nafion and SPEEK-25,50,75 coated separators, (c) The average potential Vs cycle number of cells with pristine, SPEEK, nafion and SPEEK-25,50,75 coated separators. (d, e) EIS of the fresh cells with above mentioned various ionomer coated separators. Inset to (d) & (e) shows enlarged plot of the semicircle region, (f) comparison of polarization of cells with SPEEK100 and SPEEK-50 coated separators.


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



Figure 9: Visual demonstration of polysulfide crossover in H-Type cell with (a - c) celgard separator after 0h, 24h and 48h (a’ – c’) SPEEK- 50 coated separator after 0h, 24h and 48h.


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



Figure 11: X-ray photoelectron spectra (XPS) of (a) S spectra of pristine sample, (b & b’) S and Li spectra of discharged sample, (c, c’) S and Li spectra of charged sample.


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Graphical abstract:


Nafion composite coated separator as potential

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