Sulfur Copolymer: A New Cathode Structure for Room-Temperature

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Sulfur Co-polymer a New Cathode Structure for Room Temperature Sodium-Sulfur Batteries Arnab Ghosh, Swapnil Shukla, Monisha Monisha, Ajit Kumar, Bimlesh Lochab, and Sagar Mitra ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00714 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Sulfur

Co-polymer

a

New

Cathode

Structure

for

Room

Temperature Sodium-Sulfur Batteries Arnab Ghosh,† Swapnil Shukla,‡ Monisha Monisha,‡ Ajit Kumar,† Bimlesh Lochab*‡ and Sagar Mitra*†

†

Electrochemical Energy Laboratory, Department of Energy Science and Engineering, Indian

Institute of Technology Bombay, Mumbai 400076, India ‡

Materials Chemistry Laboratory, Department of Chemistry, School of Natural Sciences, Shiv

Nadar University, Gautam Buddha Nagar, Uttar Pradesh 201314, India

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ABSTRACT High energy electrochemical storage containing earth abundant materials could be a choice of future battery development. Recent research dimensions indicated the possibility of room temperature sodium-ion sulfur chemistry for large storage including smart grids. Here, we report a room-temperature sodium-sulfur battery cathode that will address the native downsides of sodium–sulfur battery, such as polysulfide shuttling and low electrical conductivity of elemental sulfur. In this report, we use a sustainable route which ensures a large sulfur confinement (i.e., ~ 90 wt %) in cathode structure. The sulfur-embedded polymer is realized via thermal ring-opening polymerization of benzoxazine in presence of elemental sulfur (CS-90) and later composite with reduced graphene oxide (rGO). The resulting CS-90 allows a homogeneous distribution of sulfur due to in-situ formation of the polymer backbone and allow maximum utilization of sulfur. This unique electrode structure bestows CS90-rGO with an excellent Coulombic efficiency (99%) and healthy cycle life.

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Metal-sulfur batteries appeared as an attractive energy storage options, owing to its high cell voltage and impressive specific capacity to supplement the current growing energy demands. In past, relatively much attention is paid to the designing of sulfur cathodes to entrap the benefits of its high theoretical capacity specific capacity (1672 mA h g‒1), cost viability (US$ 150-200 per ton) and environmental compatibility.1,

2

However, the future sustainability of metal sulfur

batteries depends on relative abundance of the anode material. The limited and localized natural abundance of lithium (0.02 g kg‒1 in earth crust3) may turmoil the practical and large scale application of Li–S batteries. As a viable replacement, the next element in the first group of periodic table to lithium i.e., sodium appeared as a better and cheaper raw material due to its high natural abundance (27 g kg‒1 of earth crust and 11 g L‒1 in seawater), low-cost, world-wide easier availability, high redox potential (–2.71 V vs S.H.E.) for its exploration in metal sulfur batteries. Traditional Na–S batteries employ sodium ß-alumina (NaAl11O17) as the electrolyte, which produces free Na+ at >300 oC. However, the higher operating temperature and incorporation of ß-alumina solid electrolyte impose high operational cost, safety concerns and 3 ACS Paragon Plus Environment

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maintenance issues which limits their widespread application.4, 5 The efforts to develop RT Na–S batteries employing liquid organic electrolytes with porous separators have been reported since 2006.6–9 However, a plethora of conventional drawbacks viz. dissolution of higher order polysulfides (Na2Sn, 4≤ n ≤ 8), shuttle effects, irreversible lower order polysulfides (Na2Sn, 1≤ n ≤ 2) deposition on electrode, intensive leaching of active material from cathode have considerably plagued the development in the arena of RT Na–S batteries.10 Earlier lessons secured to resolve such issues in Li–S batteries are also worth implementing in Na–S batteries. In this respect, cathode materials based on physical encapsulation of sulfur and higher order polysulfides into porous and hetero-atom doped carbonaceous materials, metal organic frameworks are explored for RT Na–S batteries.6–13 Recently, a shrewd approach is being employed to overcome the dissolution of higher order polysulfides in Na–S batteries, where the formation soluble higher order polysulfides is completely prevented through restricting the number of sulfur atoms between 2–4 in a single void space of porous carbonaceous materials.14 Nevertheless, nano-confined low atomic sulfur species, essentially leads to deterioration of nominal voltage as well as overall energy density of the cells. Moreover, in physical encapsulation technique, requirement of microporous matrix limits sulfur mass loadings to 50–60 wt %.15−19 Higher loadings (> 70 wt %) are necessary and desirable to reach high gravimetric energy density. In addition, controlled physical encapsulation strategy is time-consuming, expensive, and demands advanced engineering protocols. In another approach, the dissolution of sodium polysulfides can be controlled by incorporating an effective barrier in the form of either solid electrolyte or separator which permits selective movement/entry across the two electrodes. For example, PVdF/sodium triflate solid electrolyte was incorporated as ion-conducting membrane, which preferentially allows Na+ ion passage while restricting the diffusion of


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dissolved polysulfides.8 Similarly, in order to suppress polysulfide shuttling, sodiated Nafion (Na-Nafion) has been used as an ion-selective separator in different studies.20, 21 The approaches of incorporating an ion-selective membrane between sulfur cathode and sodium anode has been validated to enhance the overall performances of RT Na–S batteries. However, application and utility of so designed Na–S batteries may be limited due to higher cost of constituent materials and sophisticated fabrication strategy and, life time of battery, which may affect the sustainability and impart challenges for their commercialization. Therefore, a shift is underway for high performing RT Na-S batteries with higher economic viability and sustainability. Recently our group demonstrated the utility of naturally occurring phenolic compound present in agro-waste, cardanol, as an effective renewable building block to covalently link with sulfur via solvent free inverse vulcanization route.22−24 In this endeavour, a sulfur-rich copolymer (CS90) derived from cardanol based benzoxazine (Ca) and elemental sulfur have been incorporated as a cathodic entity assisting sodium as anode is worth exploring to design a sustainable RT Na–S battery. However, the CS90 copolymer is expected to inhibit the electron percolation into the bulk of active material. Therefore, a rational nanocomposite of CS90 with reduced graphene oxide (rGO) was prepared where latter was synthesized through chemical reduction of graphene oxide (GO) using another naturally occurring phenolic compound, gallic acid.25 The rGO so obtained exhibited high surface area as well as good dispersion in organic solvents because of the stabilizing effect of the reducing agent, gallic acid. This allows dispersion of rGO throughout the bulk of sulfur copolymer, creating a percolation network for electrons. The CS90-rGO nanocomposite exhibits an excellent electrochemical performance in RT Na–S battery. Herein, we address the aforementioned issues related to sulfur-based batteries, using a sustainable approach with covalently confining sulfur at


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high loadings which is an essential and practical requirement for a greener and large scale battery applications. The synthetic methodology for synthesizing sulfur copolymer and reduced graphene oxide as cathodic entities (Figure 1) follows our reported strategies.22–24 The feasibility of this strategy was aided by the low viscosity of Ca monomer along with a unique molecular structure. Apart from low viscosity and high reactivity, prompt miscibility of Ca in molten sulfur was also found to assist the copolymerization to form CS90 copolymer. It is expected that the presence of several reactive sites in Ca monomer is effective in chemically linking to various polysulphane chains during melt polymerisation. However, the possibility of co-existence of traces of elemental sulfur in CS90 copolymer matrix cannot be ruled out. The probable chemical structure of CS90 copolymer is illustrated in Figure 1b. The incorporation of sulfur (S8) in copolymer and occurrence of ring opening polymerization (ROP) of Ca to form CS90 was confirmed by appearance of Raman peaks at 150, 220 and 472 cm–1 due to S–S bending and stretching vibration (Figure S1) and absence of peaks at 750–770 cm–1 due to ring breathing mode of benzoxazine ring.22–24,

26, 27

However, in

comparison to Ca, FTIR spectra (Figure S2) of CS90 copolymer showed the appearance of a new peak at 658 cm–1 due to C–S bond confirming the copolymerization reaction. The occurrence of ROP in Ca monomer was suggested by absence of benzoxazine ring C–O–C stretching and outof-plane bending vibrations at 1240, 1030 and 950 cm–1, respectively. The 1H-NMR spectra of Ca and CS90 copolymer (Figure S3) indicated the absence of characteristic oxazine signals at 4.57 ppm and 5.37 ppm respectively. The existence of C–S bonds was confirmed by appearance of new signals at 2.9–3.4 ppm and at 4.1– 4.5 ppm corresponding CH2–CH2–S and N–CH2–S signals formed due to free radical addition and ROP 6 ACS Paragon Plus Environment

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reactions, respectively. The free radical addition and substitution reaction with polysulphane radical was indicated by OBERS (Opening of Benzoxazine Ring by sulfur radicals) and thiol-ene reaction. XPS measurement (Figure 2a and b) used to assess the chemical nature and composition of the CS90. The survey scans spectrum shows that the material consists of C, S, N and O. The C1s spectrum shows a broad peak at 285.84 eV. The deconvoluted spectrum of C1s showed three peaks at 285.16, 285.78, and 286.43 eV, while former two are attributed to the aromatic sp2 and aliphatic C–C bond in the copolymer.30 The peak at 286.43 eV is ascribed to the C–S bond supporting the copolymerization Ca and sulfur.31 To illustrate the chemical nature of sulfur in the copolymer, deconvolution of S2p spectra showed a doublet of S2p3/2 and S2p1/2 with an energy separation value of 1.2 eV and intensity ratio of 2:1. The binding energy of S2p3/2 supports the presence of C–S bond.32 An existence of covalently bound sulfur in CS90 copolymer was further confirmed by optical microscopy studies. The drop casted sample of CS90 copolymer and a physical blend of Ca:S (10:90 w/w) in the same ratio was analyzed HSM studies (Figure 2c and d). An existence of homogenous phase was observed in case of CS90 as compared to the physical blend which showed isolated localized melt domains, reiterating the role of inverse vulcanization in the formation of a covalently linked copolymer. Differential scanning calorimetry (DSC) studies of the copolymer showed absence of curing exotherm (exothermic peak at 263 oC and an onset of curing at 242 oC) due to ROP in Ca as reported in our previous papers28,

29, 33

suggesting absence of Ca impurity in CS90. This

observation is in concurrence with the NMR studies, where the signals associated with the benzoxazine rings were not observed post copolymerization with elemental sulfur. The absence of elemental sulfur in CS90 was indicated by absence of melting endotherms at 109 oC and 119 o

C due to orthorhombic and monoclinic phases (Figure S4). The CS90 copolymer showed a


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melting temperature at 104 oC and 116 oC suggesting existence of different crystallite size. The crystallite size was calculated using Scherrer’s equation from PXRD (Figure S5) pattern for elemental sulfur and CS90 as 65 nm and 57 nm respectively. A smaller crystallite size of sulfur domains in CS90 polymer matrix as compared to elemental sulfur accounts to a lower melting point, according to Gibbs Thomson effect. The cathodic material was prepared by incorporation of reduced graphene oxide rGO(S) as a conductive filler into the low-conducting CS90 copolymer to augment the performance of device. To this effect, the formation of rGO(S) and its characterization is shown in Figure S6. Reduced graphene oxide rGO(S) was prepared by using gallic acid, a naturally occurring lignin phenolic reductant to graphene oxide (GO) prepared by Hummer’s method (Figure S6a). The successful synthesis of rGO(S) was confirmed by UV-vis, IR and SAED (selected area electron diffraction) studies.23 In case of rGO(S), a red shift of the absorption peak from 229 to 270 nm indicates the revival of conjugation and successful reduction (Figure S6b). FTIR spectra of rGO(S) (Figure S6c) showed absence of peaks corresponding to oxidative functionalities such as hydroxyl, carboxyl, carbonyl, aromatic C=C, epoxy, ester stretching vibrations at 3100, 1718, 1615, 1350, 1220, 1050 cm–1, respectively. The SAED pattern of rGO(S) (Figure S6d) confirmed a set of six-fold symmetric diffraction points of a typical hexagonal configuration corresponding to the formation of crystalline states. The microstructures of CS90 copolymer and its composite with 2.5 wt% rGO(S) were well-understood by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. The SEM image of CS90 copolymer (Figure 3a) reveals that the average size of as synthesized CS90 copolymer was 10–15 µm. Figure 3b represents the sulfur mapping of CS90 copolymer, which indicates the uniform distribution of sulfur throughout copolymer. The 8 ACS Paragon Plus Environment

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corresponding energy dispersive X-ray (EDX) spectra is shown in Figure 3c. Elemental analysis confirmed the presence of 88.67 wt% sulfur in CS90 copolymer. Figure 3d and e exhibit the lowand high-resolution SEM images of CS90-rGO(S) composite. Both low- and high-resolution SEM images show that the CS90 copolymer was almost uniformly coated by rGO(S). The TEM analysis of CS90-rGO(S) composite also confirmed the uniform dispersion of rGO(S) throughout copolymer (Figure 3f). It is expected that the uniform dispersion of rGO(S) throughout CS90 copolymer (as observed in SEM and TEM images) could improve the electrical conductivity of CS90 copolymer. From the TEM image, it is also interesting to notice that the presence of voids in CS90-rGO(S) composite which could improve the electrolyte and electrode contact leading to higher utilization of active material and high specific capacity.34 However, to investigate the enhancement of electrical conductivity of the sulfur copolymer upon rGO(S) incorporation, two different pellets of CS90 copolymer and its composite with 2.5 wt% rGO(S) were prepared followed by electrical conductivity measurement using standard four-probe technique. The dc measurements revealed electrical conductivity values of 4.7 Х 10‒16 and 8.3 Х 10‒12 S m–1 for CS90 copolymer and CS90-rGO(S) composite, respectively. The enhancement of electrical conductivity of CS90-rGO(S) composite by 4 order of magnitude indicates the efficacy of rGO(S) as an excellent electronically conductive additive. Electrochemical performances of CS90-rGO(S) cathode:

Na-S cells were assembled to

evaluate the electrochemical performances of CS90-rGO(S) cathode at room temperature. Figure 4a illustrates the CV curve of CS90-rGO(S) cathode with respect to sodium anode within the potential window of 1.2‒2.8 V vs Na+/Na at 20 µV s−1 scan rate. The cyclic voltammetry curves for initial two cycles are shown in Figure 4a. During first cathodic scan, two prominent peaks at 2.22 and 1.6 V were observed, corresponding to the formation of higher order sodium


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polysulfides (Na2Sn, 4 ≤ n ≤ 8) and short-chain sodium sulfides (Na2Sn, 1≤ n ≤ 3), respectively.10 However, the peaks at 1.82 and 2.4 V in the subsequent anodic scan are ascribed to the transition of short-chain sodium sulfides to long-chain polysulfides and sulfur. It is interesting to observe that there is a shift in cathodic peak from first scan to the second scan in cyclic voltammetry. This observation could be attributed to the phenomenon proposed by Pyun et al.35 During first discharge CS90 copolymer reduces to higher order sodium polysulfides along with formation of higher order organosulfide units. During the progress of first discharge process, the higher order sodium polysulfides converts to lower order sodium polysulfides along with formation of the lower order organosulfur moieties. However, after completing the initial discharge, during recharging CS90 copolymer cannot recover its original structure. Therefore, after first cycle of discharge-charge, the CS90 cathode might behave as an elemental sulfur cathode and show similar electrochemical behaviour as S8. The First cycle charge-discharge profile of CS90-rGO(S) cathode with respect to sodium anode at the current density of 0.2 A g‒1 is represented in Figure 4b. The initial discharge profile exhibited a plateau at around 2.22 V which is in well-agreement with first reduction peak observed in the first cycle of CV curve. Figure 4c depicts the charge-discharge profiles of CS90rGO(S) cathode for the second and fifth cycles, at same current density of 0.2 A g‒1. The corresponding charge-discharge cycling performance of CS90-rGO(S) cathode at 0.2 A g‒1 is shown in Figure 4d. Figure 4d represents the comparative cycling performances of different cathode materials viz. elemental sulfur, CS90 copolymer and CS90-rGO(S) composite, respectively. An initial irreversible extra discharge capacity was observed for all the cathodes, which could be attributed to the decomposition of electrolyte on the surface of electrodes to form stable solid-electrolyte interface (SEI) layers. However, it is noteworthy to observe that the CS90


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copolymer exhibited better capacity and cycling stability compare to elemental sulfur. The elemental sulfur showed a reversible specific capacity of 498 mA h g‒1 at second cycle and after 50 cycles a specific capacity of 110 mA h g‒1 with a constant capacity decay of 1.56% per cycle was observed. On the contrary, CS90 copolymer cathode exhibited an initial reversible capacity of 542 mA h g‒1 at second cycle and 335 mA h g‒1 after 50 cycles with an average capacity fading rate, 0.76% per cycle. The better cycling stability of CS90 sulfur copolymer could be due to the chemical attachment active sulfur with organic moiety which might reduce the dissolution of active material into electrolyte and thus increase the active sulfur utilization. Moreover, the plasticizing effect of small organosulfides, which are generated along with lower order polysulfides, during deep discharge of sulfur copolymer,23,

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could be another beneficial

reason for better performances of CS90 copolymer. After generation, these organosulfur units (or, organosulfides) get dispersed into insoluble lower order polysulfide (i.e., Na2Sn, 1≤ n ≤ 3) phase, converting them into soluble phase and thus significantly suppress irreversible deposition of these lower order polysulfides.34 A composite of CS90 copolymer and conductive reduced graphene oxide (i.e., CS90-rGO(S) composite) was further prepared to extract better electrochemical performances from the copolymer. At the same current density of 0.2 A g‒1, CS90-rGO(S) composite delivered an as much as high reversible capacity of 650 mA h g‒1 at second cycle. In terms of reversible utilization of active material at second cycle, the CS90rGO(S) composite had a higher sulfur utilization of 38.87% (corresponding to the observed capacity of 650 mA h g‒1) in comparison to sulfur utilization by pristine CS90 copolymer (i.e., 32.42%; corresponding to the observed capacity of 542 mA h g‒1). The CS90-rGO(S) composite cathode retained 498 mA h g‒1 capacity after 50 cycles with a relatively low capacity decay of 0.47% per cycle and superior Coulombic efficiency approximately 100%. The long-term cycling


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performances of CS90 copolymer and CS90-rGO(S) composite cathodes at a moderately high current density of 1 A g−1 are represented in Figure 4e. As the current rate raised to 1 A g−1, the capacities of both CS90 copolymer and CS90-rGO(S) composite get reduced due to solid-state ion diffusion limitation. After 100 cycles, an average capacity of 180 and 285 mA h g−1 were observed for CS90 copolymer and CS90-rGO(S) composite. An improve rate performance of CS90 copolymer upon reduced graphene oxide addition can be observed from Figure 4f. CS90 copolymer exhibited an initial poor rate performance during the transition of current density from 0.2 to 0.5 A g−1. The observed capacity for CS90 copolymer became worse with increasing the subsequent current densities. However, the CS90-rGO(S) composite exhibited much better rate capability, especially at moderate current densities (i.e., 0.5 and 1 A g−1). The observed superior electrochemical performances of CS90-rGO(S) composite cathode in terms of enhanced specific capacity, improved rate capability and Coulombic efficiency is presumably caused by the uniform dispersion of 2.5 wt% rGO(S) throughout CS90 copolymer. Due to the high electronic conductivity and extremely high surface area, a substantially lower amount of rGO might create better percolation network throughout CS90 copolymer and thus enhance the overall active material utilization in cathode. To gain insight into the better electrochemical performances of CS90 copolymer, the elemental sulfur and CS90 cathodes after 50 cycles (at fully charged state) were characterized thoroughly. After extensive charge-discharge cycling, a dense phase of insoluble products can be observed on the surface of elemental sulfur cathode (Figure 5a). In contrast, no such extent of decomposition product aggregation was observed for CS90 cathode (Figure 5b). This dense insoluble product phase was characterized through XRD and XPS techniques. The XRD pattern of sulfur cathode after 50 charge-discharge cycles, at fully charged state, exhibits the appearance


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of two new peaks at 24.85o and 29.14o, respectively, which were not observed for pristine sulfur cathode before cycling (Figure S10). These two peaks could be assigned to the lower order sodium polysulfides (Na2Sn, 1≤ n ≤ 3).37 XPS analysis was also performed on the sulfur cathode to identify the chemical composition of dense product phase deposited on the electrode surface during cycling. The wide-scan XPS spectra of pure sulfur cathode after cycling shows the characteristic peaks of all expected elements- sulfur, carbon, sodium, and oxygen. Figure S11b displays the S 2p spectrum of sulfur cathode after cycling, at fully charged state. From Figure S11, a weak peak at the binding energy of 161.2‒162.8 and a relatively intense broad peak at the binding energy of 167.5‒170 eV, related to lower order sodium polysulfides (Na2Sn, 1≤ n ≤ 3) could be observed along with a characteristic doublet of elemental sulfur at 164.0 eV (S 2p3/2) and 165.2 eV (S 2p1/2). Therefore, it can be proposed that during cycling, an insoluble and insulating product phase of lower order sodium polysulfides, generated from deep discharge of sulfur, continuously deposit on the sulfur cathode which not only block the ionic as well as electronic transport, but also lead to irreversible loss of active material. However, the XRD and XPS studies of CS90 cathode after cycling (Figure S12 and S13) indicate no such extent of insulating product deposition on the surface of CS90 electrode during cycling. The effect of extent of deposition of lower order sodium polysulfides was also reflected by electrochemical impedance spectroscopy (Figure S14). An increase in cell impedance was observed for sulfur cathode after 50 cycles; while, the CS90 cathode shows comparatively lower cell impedance after cycling. The lower increase in cell impedance for CS90 cathode during cycling could be the reason behind the better capacity retention in comparison to elemental sulfur cathode. The beneficial effect of organic part in CS90 copolymer in terms of reduced dissolution of higher order sodium polysulfides can also be observed from ICP elemental analysis were


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performed to the solutions after washing the cell components (except cathode) that cycled for different charge-discharge cycles for both pure sulfur and CS90 cathodes and from SEM and EDX analysis of the surface of sodium metal anode placed towards CS90 cathode after cycling (Figure 5c, d). From Figure S7 and S9 it can be noticed that with the progressing of chargedischarge cycles of the cell, the dissolution of sodium polysulfides increases for CS90 cathode in less extend. However, after 20 cycles, the CS90 cathode showed lower dissolution of polysulfides and active material into electrolyte in comparison to elemental sulfur (Figure S8 and S9), indicating restriction of dissolution rate in CS90 copolymer through chemical confinement of sulfur. The SEM and EDX characterizations of sodium metal anode were performed without involving any washing step. Some cracks and irregular pores could be observed on the surface of sodium metal counter electrode after 50 cycles at slow rate (Figure 5c). The EDX analysis conducted over the total area shown in Figure 5c, a very less amount of sulfur was detected on anode surface (Figure 5d), indicating less deposition/aggregation of insoluble Na2S3/Na2S2 matrix on the surface of sodium anode during charge-discharge cycling, which is due to the plasticizing ability of lower order organosulfides. In conclusion, this work demonstrates a facile and sustainable approach of designing room-temperature sodium-sulfur battery cathode using two industrial wastes- sulfur and cardanol. The conductive material, rGO was also processed by a greener, sustainable route of gallic acid reduction and used as a conductive additive in electrode. The CS90 copolymer showed an improved cycling performance and rate capability over elemental sulfur cathode. The incorporation of rGO in CS90 copolymer cathode further enhances the active material utilization, improves the rate capability and Coulombic efficiency. The underlying origin of the robust electrochemical performances of CS90-rGO(S) composite in comparison to elemental sulfur is the


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plasticizing effect of small organosulfide units formed during deep discharge of sulfur copolymer and extremely high conductivity of rGO(S), resolving both major issues in RT Na–S batteries viz. irreversible deposition of insulating end-discharged products and low conductivity of active material. The superior electrochemical performances using low-cost raw materials with facile synthesis of cathode for RT Na–S is an attractive approach towards promoting sodium-ion based batteries at industrial scale.

ASSOCIATED CONTENTS Supporting Information Raman spectra of Ca, CS90 and elemental sulfur; FTIR spectra of Ca and CS90; 1H-NMR spectra of Ca and CS90; DSC scans of elemental sulfur and CS90; physical characterizations of graphite, graphene oxide and gallic acid reduced graphene oxide. AUTHOR INFORMATION *Corresponding Authors *E-mail: [email protected] Tel: +91 22 2576 7849 *E-mail: [email protected] Tel: +91 120 3819 100 NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors (A.G., A. K. and S.M.) are thankful to the “National Centre for Photovoltaics Research and Education (NCPRE)” Ministry of New and Renewable Energy, Govt. of India for instrumental support and the project DST/RCUK/SGES/2012/13 for providing financial support. 15 ACS Paragon Plus Environment

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The authors acknowledge the IRCC, IIT Bombay for infrastructure facilities. The authors (S.S., M. and B.L.) would like to acknowledge financial support from Shiv Nadar University and DST/BL/2014/15 and SCCPL®, India for providing cardanol. A.G. and S.S. equally contributed to this manuscript.


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Figure 1. Illustration of composite and copolymer synthesis: (a) strategies utilized to prepare the CS90-rGO(S) composite cathode; (b) probable chemical structure of copolymer by reaction of Ca monomer with sulfur.

Figure 2. Confirmation of covalent bonded sulfur in CS90 copolymer: (a, b) deconvoluted S2p and C1s spectra of CS90 copolymer; optical microscopy images of samples heated to a temperature of 130 oC (c) mixture of sulfur and C-a (physical blend in a ratio of 9:1), and (d) CS90 copolymer.


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Figure 3. (a) SEM image of CS90 copolymer; (b, c) distribution mapping of sulfur and EDX spectrum for the selected area in CS90 copolymer; the inset table in Figure 3c shows the percentage of the constituting elements present in copolymer based on CHNS (O) analysis; (d, e) low and high-magnification SEM images of CS90-rGO(S) composite; (f) TEM image of CS90rGO(S) composite.


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Figure 4. (a) cyclic voltammetry curves at the scan rate of 20 µV s−1, within the potential window of 1.2−2.8 V vs Na+/Na; (b) first cycle charge-discharge profile of at the current density of 0.2 A g‒1; (c) charge-discharge profiles of second and fifth cycles at 0.2 A g‒1; (d) cycling performances comparison between elemental sulfur (green), CS90 copolymer (blue), and CS90rGO(S) (red) cathodes at 0.2 A g−1 current density; (e) cycling performances comparison between CS90 copolymer (blue), and CS90-rGO(S) (red) cathodes at 1 A g−1; (f) rate performances of CS90 copolymer (blue), and CS90-rGO(S) (red) cathodes in terms of specific capacities obtained at different current densities of 0.2, 0.5, 1 and 2 A g−1, respectively.


ACS Energy Letters

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Figure 5. Post cycling investigation of the electrodes after 50 cycles at fully charged state: (a) SEM image of elemental sulfur cathode after 50 cycles; (b) SEM image of elemental CS90 copolymer cathode after 50 cycles; (c, d) SEM and EDS analysis of the surface of sodium metal placed towards CS90 copolymer cathode after 50 cycles.


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Sulfur Copolymer: A New Cathode Structure for Room-Temperature

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