A Facile Bottom-Up Approach to Construct Hybrid Flexible Cathode

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A Facile Bottom-up Approach to Construct Hybrid Flexible Cathode Scaffold for High Performance Lithium-Sulfur Batteries Arnab Ghosh, Revanasiddappa Manjunatha, Rajat kumar, and Sagar Mitra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11180 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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ACS Applied Materials & Interfaces

A Facile Bottom-up Approach to Construct Hybrid Flexible Cathode Scaffold for High Performance Lithium-Sulfur Batteries Arnab Ghosh1, Revanasiddappa Manjunatha2, Rajat Kumar2 and Sagar Mitra1* 1

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

Institute of Technology Bombay, Mumbai‒400076, India. 2

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore‒

560012, India.

KEYWORDS Bottom-up approach, flexible cathode, lithium-sulfur batteries, electrochemical performances, insitu Raman spectroscopy

ACS Paragon Plus Environment


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ABSTRACT Lithium-sulfur batteries mostly suffer from the low utilization of sulfur, poor cycle life, and low rate performances. The prime factors that affect the performance are enormous volume change of the electrode, soluble intermediate product formation, poor electronic and ionic conductivity of S and end discharge products (i.e., Li2S2 and Li2S). The attractive way to mitigate these challenges underlying in the fabrication of a sulfur nanocomposite electrode consisting of different nanoparticles with distinct properties of lithium storage capability, mechanical reinforcement and ionic as well as electronic conductivity leading to a mechanically robust and mixed conductive (ionic and electronic conductive) sulfur electrode. Herein, we report a novel bottom-up approach to synthesize a unique free-standing, flexible cathode scaffold made of porous reduced graphene oxide, nanosized sulfur, and Mn3O4 nanoparticles, and all are three-dimensionally interconnected to each other by hybrid polyaniline/sodium-alginate (PANI−SA) matrix to serve individual purposes. A capacity of 1098 mAh g‒1 is achieved against lithium after 200 cycles at a current rate of 2 A g‒1 with 97.6 % of initial capacity at a same current rate, suggesting the extreme stability and cycling performance of such electrode. Interestingly, with the higher current density of 5 A g‒1, the composite electrode exhibited an initial capacity of 1015 mA h g‒1 and retained 71 % of the original capacity after 500 cycles. The in-situ Raman study confirms the polysulfide absorption capability of Mn3O4. This work provides a new strategy to design a mechanically robust, mixed conductive nanocomposite electrode for high-performance lithium-sulfur batteries and strategy that can be used to develop flexible large power storage devices.


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INTRODUCTION The reduction of sulfur in the presence of any cations (for example- Li+, Na+, Mg2+, etc.) does not depend on solid-state ion transport; hence, the use of sulfur cathode may yield an active cathode reaction for the rechargeable batteries. Realization of a Li-S battery is of great interest among all because of the theoretical cell capacity could achieve up to 1165 mA h g−1 with an average voltage of 2.1 V as estimated based on the Gibbs formation energy. The theoretical specific energy is about 2447 Wh kg−1 (three to five times that of a commercial LiCoO2/graphite or NMC/Si batteries) for Li-S battery chemistry, making it a promising battery chemistry for large-scale (100−110 Wh) applications like grid storage that require both high energy density and low cost. However, several issues are hindering the practical application of Li-S batteries. The most serious issue associated with Li-S batteries is the formation of soluble higher ordered polysulfides at the cathode during the charge-discharge reaction. At the cathode, these higher ordered polysulfides are dissolved into the electrolyte and transport towards the lithium metal anode where they are chemically reduced to insulating metal sulfide (viz. Li2S2 or Li2S) along with lower ordered polysulfides. These lower ordered polysulfides can diffuse back to the cathode and re-oxidized to higher ordered polysulfides. This phenomenon is commonly known as “polysulfide shuttling” and results in loss of active material from cathode leading to a poor cell performance. Furthermore, during the shuttle phenomenon, some of the insoluble lower ordered polysulfides irreversibly deposit on both the electrode surfaces, blocking the flow of metal ions as well as electrons. Severe shuttle phenomena could permanently obstruct both ionic and electronic transport, leading to significant increase in cell impedance. Other significant drawbacks of metal-sulfur batteries are insulator like electronic conductivity of sulfur (5 × 10−16 S cm−1 at 25 oC and exhibit severe volume changes during redox process (~ 80% from S to Li2S



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in Li-S battery).1 Large volume change is expected to cause the pulverization of electrode material, which leads to weak interfacial contact between sulfur particles and the current collector, de-lamination of the electrode, the growth of unstable solid-electrolyte interface, etc. All of these issues synergistically deteriorate the overall performance of the batteries. Therefore, not only the improvement of electronic conductivity of sulfur or reduction of polysulfide dissolution into electrolyte is an optimal solution for high-performance metal-sulfur batteries; rather, pulverization issue also needs to be solved alongside. Several strategies have been used to address the aforementioned problems. Various types of porous materials viz. mesoporous carbon, porous hollow carbon, carbon nanotubes, carbon fiber cloth, etc.2−5 have been applied as scaffolds for the sulfur cathode. The introduction of active sulfur encapsulation by melt-diffusion strategy and physisorption of intermediate polysulfides into the void spaces of those porous carbonaceous materials exhibited a significant enhancement in active material utilization as well as the electrochemical performance of metal-sulfur batteries. Recently, porous metal oxide frameworks6,

7

have gained much more attraction as cathode

additives to trap the intermediate polysulfides through chemisorption along with physical encapsulation of sulfur into their pores. Conformal coating of sulfur particles by conductive polymers like polyaniline, poly (3, 4-ethylenedioxythiophene), polythiophene, etc.8−10 also showed remarkable improvement in the electrochemical performance. Nowadays, “inverse vulcanization” is a new and cost-effective strategy to prepare sulfur-containing polymers which displayed excellent electrochemical activity and their suitability as cathode material for Li-S batteries.11−13 However, neither of these class of materials (i.e., carbonaceous materials or metal oxide frameworks or sulfur copolymers) possesses enough mechanical rigidity and stiffness to buffer very well against the large volume changes of sulfur during Li+ uptake/removal.


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Therefore, the possibility of pulverization of electrode material persists and no strategy has satisfactorily solved all the major cathode related issues together. Furthermore, certain undetermined trade-offs have been observed in previously reported studies. Therefore, the rational design of flexible cathode scaffolds which maintains good interfacial contacts between all the building blocks are still in high demand for high-performance Li-S batteries with following characteristics: (1) enhancement in electronic conductivity throughout whole cathode scaffold; (2) high active material utilization; (3) achieve low rate of polysulfides dissolution; and (4) high mechanical strength to resist large volume change during repeated charge/discharge process. These characteristics can be achieved by incorporating different types of building components into a single cathode scaffold where each component is expected to play their specific role. In this report, we introduce a facile bottom-up approach to design a unique free-standing cathode scaffold, made of porous reduced graphene oxide, nanosized sulfur, and nanosized Mn3O4, and all are three-dimensionally interconnected by hybrid polyaniline/sodium-alginate (PANI−SA) matrix. Sulfur nanoparticles have been incorporated in the cathode scaffold by an insitu precipitation method, to greatly reduce the solid-state transport length for ion/electron diffusion and therefore to accelerate the ion/electron transfer kinetics.14 Moreover, the appropriate distribution of sulfur nanoparticles throughout the cathode scaffold effectively improve the contact with conductive agents and thus enhance the active material utilization. Porous reduced graphene oxide provides high electronic conductivity as well as physically confines the sulfur in its void spaces. It is well known that being highly polar in nature; lithium polysulfide intermediates can be trapped by any polar component like metal oxides, metal organic frameworks, etc. through “electrostatic” Lewis acid-base interactions.15−18 In this study,



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Mn3O4 nanoparticles have been used as a key component to absorb the soluble polysulfide intermediates. However, besides the poor conductivity of sulfur and rapid dissolution of polysulfide intermediates, another major problem related to the sulfur-based cathode is its pulverization during lithiation/de-lithiation reactions. We strongly believe that the pulverization issue can only be solved by choosing suitable binder along with reduced graphene oxide, which possesses inherent mechanical rigidity and stiffness to minimize the structural deformation of sulfur particles during charge/discharge. Moreover, it is well accepted that any weak or strong chemical interaction between the binder and active material could be a beneficiary in terms of active material utilization and cycling performance. It has been demonstrated that the polymer binders are containing –COOH or –OH groups can provide enough mechanical rigidity as well as effectively interact with the active material through H−bonding and hence are being considered as effective binders for conversion based materials associated with large volume change; such as Si, S, P, etc. For example, Wang and co-workers proposed that chemical interaction between cross-linked carboxymethyl cellulose-citric acid binder and phosphorous/CNT mixture is the key factor behind the improved electrochemical performance of phosphorous.19 Similarly, several studies revealed that advanced silicon-based anode could be obtained by using binder polymer containing –COOH and –OH groups.20−24 Recently, Bao et al. claimed that better electrochemical performance from sulfur-based cathode could be achieved by using sodiumalginate as a binder, instead of PVdF.25 Our group also proved that α-Fe2O3, a conversion based anode associated with huge volume expansion during lithiation, can exhibit high rate performance with interactive alginate binder.26 Moreover, the binders containing oxygenic functional groups (i.e., –COOH and –OH groups) can effectively transfer Li+ cations. The lone pair of electrons on oxygen atoms present in those oxygenic functional groups assist to form a


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coordination complex with metal cations. The metal complex cations can then move between coordination sites supported by the segmental motion of polymer matrix.27−31 Interestingly, the presence of nitrogen and oxygen containing functional groups (i.e., –OH, epoxy or –NH2 groups) in hydrazine reduced graphene oxide and the presence of –COOH/ –OH groups in sodiumalginate binder can also effectively anchor the intermediate polysulfides and supress their dissolution into electrolyte.32−35 Therefore, considering the mechanical rigidity, ionic conductivity and ability to form chemical bonding with the active material, one of the natural polysaccharides, sodium-alginate has been used as a binder in this study. The use of sodiumalginate as binder avoids “toxic” and “non-sustainable” N-Methyl-2-pyrrolidone (NMP) as a slurry solvent, making the synthesis approaches more environmental-friendly and sustainable. However, it is well-known that hybrid sodium-alginate/polyaniline matrix shows better electronic conductivity compare to pure sodium-alginate. Therefore, in addition, to secure an excellent electrical conductivity along with exceptional mechanical stiffness, a hybrid sodiumalginate/polyaniline (SA−PANI) binder was incorporated into the cathode scaffold. The hybrid sodium-alginate/polyaniline matrix was integrated into the cathode scaffold through in-situ polymerization of aniline in an aqueous dispersion of porous rGO/sulfur nanoparticles/Mn3O4 nanoparticles composite, using sodium-alginate as a doping agent. This unique synthesis approach results in a hybrid nanocomposite of porous rGO, nano-sulfur and Mn3O4 nanoparticles interconnected to each other by hybrid polyaniline/sodium-alginate conducting matrix. A freestanding porous cathode scaffold was obtained by vacuum filtration of the final hybrid product followed by vacuum drying at 80 oC. The sulfur loading in this free-standing cathode was 2.05 mg (cmelectrode)−2. The increasing demand for flexible electronic devices can be satisfied by



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introducing of such flexible cathode scaffold with the requisite amount of active material content in lithium-sulfur batteries.

EXPERIMENTAL SECTION Synthesis of porous reduced graphene oxide (rGO) foams.

Porous reduced graphene oxide

was prepared as reported by Z. Niu et al.36 Briefly, the graphene oxide (GO) was synthesized via modified Hummer’s method.37 The GO thin film was obtained by vacuum filtration of colloidal GO suspension using an AAO membrane as filter. The GO film was peeled off from AAO membrane. The freestanding GO film was then placed in an autoclave and hydrazine monohydrate (98%, Merck) was added to it. The autoclave was heated at 90 oC for 10 hours to obtain reduced graphene oxide (rGO) foam. Synthesis of Mn3O4 nanoparticles.

Monodisperse Mn3O4 nanoparticles were synthesized by

a simple solution based route as reported earlier.38 In typical reaction, 2 mol L−1 of KOH−C2H5OH (40 ml) mixture was slowly added to 2 mol L−1 solution of Mn(CH3COO)2.4H2O in ethanol while stirring. The resulting mixture was kept at elevated temperature of 60 °C for 24 hours with mild stirring. The white solution turned to black color, indicating the probable formation of Mn3O4 nanoparticles. After cooling the reaction mixture to room temperature, the particles were precipitated by adding ethanol. The product was separated by centrifuge and washed with ethanol for several times. The powder sample was collected and used after vacuum drying at 50 °C.


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Preparation of porous reduced graphene oxide/Mn3O4/S nanocomposite.

In brief, 10 mg

of Mn3O4 nanoparticles and 25 mg of porous rGO were disperse properly into deionized water by ultra-sonication for 1 hour. Then 200 mg of sublimed sulfur (Sigma Aldrich) was dissolved into 20 ml of anhydrous ethylene diamine (Sigma Aldrich) which was then added drop wise into the aqueous dispersion of porous rGO/Mn3O4 under vigorous stirring. After completing the addition of sulfur-EDA solution, the suspension was continuously stirred for overnight. Then the final nanocomposite was collected through centrifuge, repeat washing with deionized water and finally drying at 60 °C under vacuum for overnight. Afterwards, the composite was heated at 150 °C under argon atmosphere for 2 hours to ensure the uniform impregnation and distribution of sulfur. Preparation of three dimensionally interconnected free-standing cathode scaffold.

First,

350 mg of porous rGO/Mn3O4/S nanocomposite was well-dispersed in deionized water. Then 30 mg of sodium-alginate (Sigma Aldrich) was dissolved and 50 mg of aniline (≥ 99.5%, Sigma Aldrich) was added to the homogeneous solution. The resulting solution was cooled down to 0 °C. On stirring, a pre-cooled solution of ammonium persulfate (98%, Sigma Aldrich) to the solution and resulting solution was stirred for 30 min. The free-standing porous cathode was obtained by vacuum filtration followed by vacuum drying at 80 °C for overnight. The freestanding cathode is designated and further referred as PRGO/S/Mn3O4@PANI−SA. For a control experiment, rGO/Mn3O4/S nanocomposite without polyaniline/sodium-alginate coating was used as cathode material. The cathode was prepared by following slurry coating technique, using PVdF as binder instead of sodium-alginate. This composite material is designated here as PRGO/S/Mn3O4@PVdF.



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Materials Characterization.

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The morphology and structure characterization of the samples

were carried out through field emission gun scanning electron microscopy (FEG-SEM; JEOL7600F) and field emission gun transmission electron microscopy (FEG-TEM, JEOL-2100F). Elemental mapping of the final nanocomposite was carried out to visualize the distribution of different elements. Powder XRD patterns were recorded on Philips X’-pert diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å). Fourier transform infrared (FTIR) spectra of the final sample was recorded in the wavelength range of 400−4000 cm‒1 using a Thermo Scientific FTIR analyzer (NICOLET 8700). Raman characterization was carried out using an in Via Raman microscope (Renishaw).Thermogravimetric analysis of the electrode material was carried out on a TA instrument Q600 within the temperature range from room temperature to 600 oC under nitrogen atmosphere, at a heating rate of 5 oC min−1. Cell Fabrication and Electrochemical Measurements.

PRGO/S/Mn3O4@PANI−SA free-

standing film was directly used as a cathode. For PRGO/S/Mn3O4@PVdF, the cathode was prepared by simple slurry coating process using PVdF as binder (90 wt% rGO/Mn3O4/S + 10 wt% PVdF). In both the cases, electrodes/cathodes were cut into 12 mm circular disks with almost same sulfur loading of ~2.05 mg (cmelectrode)−2. Li-S cell configurations were fabricated in an argon filled glovebox (Mbraun) using CR2032 coin cells. For lithium cells, thin lithium foil pasted on stainless steel discs and used as counter as well as reference electrodes. Borosilicate glass microfiber filters (GF/D Whatman) was used as a separator soaked with 50 µL electrolyte comprised of 1 M lithium bis-(trifluoromethane)-sulfonamide (LiTFSI) and 0.1 M lithium nitrate (LiNO3) in 1:1 (v/v) mixture of 1, 2-dimethoxy ethane (DME) and 1, 3-dioxolane (DOL). The cyclic voltammetry (CV) experiment was performed by measuring i−V response at a scan rate of 0.1 mV s−1 between 1.2‒2.8 V using Bio-logic VMP-3 model. Galvanostatic charge-discharge


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tests were carried out using Arbin Instrument (BT2000, USA), at different current densities. The charge-discharge experiments were carried out within the potential range of 1.8−2.6 V, at different current densities. Electrochemical impedance spectroscopy (EIS) was performed using Bio-logic VMP-3 instrument within the frequency range of 0.1 Hz to 1 MHz at potentiostatic signal amplitude of ∆V = 5 mV. All the electrochemical characterizations were performed at a temperature of 20 oC. The specific capacities were calculated based on the mass of sulfur loading in the electrodes. In-situ Raman Analysis.

To investigate the polysulfides absorption ability of Mn3O4, an in-

situ Raman analysis was carried out using as prepared sulfur/carbon composite cathode containing the Mn3O4 (5 wt%). A cell for in-situ Raman analysis was fabricated in a quartz cuvette to record the Raman spectra. The experiment was performed between 100 cm-1 and 800 cm−1.39 The acquisition time was 120s per spectrum. In this analysis, the cell was first discharged from its OCV to 2.1 V(where the higher order polysulfides get used to formed) at very slow scan rate of 50 mA g−1 (∼ C/33.5 where C = theoretical capacity of sulfur = 1672 mA h g−1). After reaching to 2.1 V, the spectra were recorded at the interim of 15 min, 2 hours, 5 hours and 10 hours, respectively; and this duration, the cell potential was held constant through chronoamperometry technique. For a control and comparison, a similar experiment was carried out for sulfur/carbon composite cathode without Mn3O4. RESULTS AND DISCUSSION Synthesis and Structure Analysis.

Our synthesis approach to the porous self-supported

PRGO/S/Mn3O4@PANI−SA cathode was first to prepare the porous rGO/Mn3O4 binary nanocomposite, then to deposit sulfur nanoparticles uniformly on the binary nanocomposite, resulting

a

ternary

hybrid

nanocomposite.

Final

step


was

conformal

coating

of


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polyaniline/sodium-alginate as “conductive adhesive matrix” on the rGO/Mn3O4/sulfur ternary nanocomposite. The free-standing cathode was obtained by vacuum filtration of the final product followed by vacuum drying. The overall synthesis procedure is schematically described in Figure 1.

Figure 1. Schematic illustration of the synthesis of different building units and their composite PRGO/S/Mn3O4@ PANI–SA cathode.

The formation of rGO was confirmed by XRD characterization and laser Raman spectroscopy. X-ray diffraction pattern of rGO, recorded within a diffraction angle range of 10 to 70 degree in 2θ scale (Figure S1), exhibits the expected peaks at 23.86 and 42.88 positions. Raman spectra of the rGO (Figure S2) shows two prominent peaks at 1342 cm−1 and 1580 cm−1, which are corresponding to the D and G band, respectively. The D band originates from disorder-induced mode associated with structural defects, while the G band is corresponding to the first-order scattering of the E2g mode from the sp2 hybridized carbon domains. The intensity ratio of D and G band (i.e., ID/IG) is used to measure the disorder in graphitic materials. The increased value of ID/IG suggests the restoration of C=C bonds in rGO after escaping the oxygen containing functional groups from GO. However, in our case, the calculated value of ID/IG for rGO was 1.51, indicating the development of more C=C bonds after reduction. X-ray diffraction study was also conducted for bulk Mn3O4 to explore its structural properties. The XRD pattern of as


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prepared Mn3O4 sample (Figure S3) shows a high degree of crystallinity. All of the peaks matched well with Bragg diffraction of the standard hausmannite structure (JCPDS file no. 240734). Figure 2a represents the X-ray diffraction patterns of individual samples and their composite. Pristine sulfur showed typical X-ray diffraction peaks at 22o and 28o, indicating a well-crystallized orthorhombic structure.40 The XRD pattern of polyaniline/sodium-alginate (PANI−SA) exhibits one sharp peak at 2θ = 20.3o and a broad peak at 25.3o respectively. The peak at 2θ = 20.3o arises due to the layers of alternative polymeric chains formation;41 while the peak at 25.3o may be ascribed to the periodicity of parallel and perpendicular to the polymer chain.42 However, the final composite (i.e., rGO/sulfur/Mn3O4@ PANI−SA) shows almost similar XRD pattern with PANI−SA and no detectable diffraction peaks of elemental sulfur are observed, suggesting well dispersion and encapsulation of sulfur nanoparticles in the cathode framework.43 FTIR spectra was recorded to identify the individual compounds present in the final composite composed of porous rGO/sulfur/Mn3O4@PANI−SA (Figure 2b). The peaks at 630, 502 and 458 cm−1 respectively are attributable to the tetrahedral and octahedral Mn–O bands in Mn3O4. The peak at 778 cm−1 could be assigned to the out-of-plane vibration of the C– H mode.44,

45

The intense peak at 1120 cm−1 arises from −N ═ quinoid ═ N− stretching

vibration.46, 47 The peak at 1300 cm−1 arises from C−N stretching vibration; while the peaks at 1476 cm−1 and 1568 cm−1 are correspond to the characteristic C−C stretching of the benzenoid and quinoid rings, respectively. Finally, the peaks at 2930 cm−1 and 3440 cm−1 are ascribed to the N−H bond of polyaniline and O−H bond of sodium-alginate.48 The thermal behaviour of rGO/sulfur/Mn3O4@PANI−SA cathode is shown in Figure 2c. From TGA profile of rGO/sulfur/Mn3O4@ PANI−SA it can be observed that there is a weight loss around 60.7% at 450 oC. This 60.7% weight loss was arise from the thermal degradation of the components;



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particularly sulfur, sodium-alginate and polyaniline. However, calculation (based on the weight ratio and thermal degradation of pure components) showed the presence ∼ 56.2 wt% of sulfur in the final composite and the average sulfur loading in electrode was estimated to be 2.05 mg cm−2. From the TGA profile of rGO/sulfur/Mn3O4@ PANI−SA cathode, further slight weight loss was observed around 450−600

o

C temperature. This could be attributed to the

decomposition of sodium-alginate at the temperature range of 450−600 oC, forming sodium carbonate (Na2CO3) and sodium oxide (Na2O) as major degradation products. Furthermore, the mechanical property of the flexible cathode was investigated by nanoindentation technique. The thickness of the cathode film was estimated to be 35 µm. The experiment was carried out with 100 µN load and six indentation were performed. The average reduced Young’s modulus value for the cathode film was 1.15 (± 0.16) GPa indicating good mechanical flexibility and low stiffness.


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Figure 2. (a) X-ray diffraction patterns of individual Mn3O4, porous reduced graphene oxide (rGO), sulfur, polyaniline sodium alginate and their final composite cathode, (b) FTIR spectra of PRGO/S/Mn3O4@PANI–SA cathode, and (c) Thermogravimetric analysis of sulfur, polyaniline, sodium-alginate and PRGO/S/Mn3O4@PANI– SA cathode.



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Figure 3. (a, b) Top-view and cross-sectional SEM images of highly exfoliated and porous reduced graphene oxide (scale bar- 500 nm). (c) SEM image of sulfur nanoparticles in the size-range of 25–50 nm (scale bar- 50 nm). (d) SEM image of Mn3O4 nanocubes in the size-range of 8–10 nm (scale bar- 50 nm). (e) Low resolution SEM image of free standing, porous cathode scaffold (scale bar- 500 nm). (f) High resolution SEM image of free standing, porous cathode scaffold (scale bar- 50 nm). (g‒l) Elemental mapping of the selected area of PRGO/S/Mn3O4@PANI–SA cathode (scale bar-500 nm)

– (g) selected area, (h) carbon mapping, (i) sulfur mapping, (j) nitrogen mapping, (k)

manganese mapping, and (l) oxygen mapping.


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The microstructures of different samples and composites were characterized by FEG-SEM and FEG-TEM experiments. Figure 3a and b represent the top-view and cross-sectional SEM images of reduced graphene oxide (rGO), respectively. The top-view image of porous rGO (Figure 3a) exhibited a compact layer-by-layer structure, which is quite consistent with previous report.49 From Figure 3b, it can be observed that the rGO inherently owned well-defined 3D open porous structure after hydrothermal reduction of GO in presence of hydrazine. This well-defined 3D porous open structure of the rGO allows efficient and fast ion-diffusion.

Figure 4. (a) TEM image of crumpled reduced graphene oxide. (b) SAED pattern of reduced graphene oxide. Lowresolution TEM images of (c) sulfur nanoparticles, (d) Mn3O4 nanoparticles, and (e) PRGO/S/Mn3O4/PANI-SA composite and (f) High-resolution TEM image of PRGO/S/Mn3O4/PANI-SA composite.

Moreover, the 3D interconnected nature of the multi-layered rGO can provide continuous electron pathways. Figure 3c represents the average size of 25−50 nm for the sulfur nanoparticles obtained by following sulfur-amine chemistry-based method.50 The as synthesized Mn3O4 nanocubes was about 8−10 nm in size (Figure 3d). Figure 3e and f depict the low and high



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resolution images of as obtained porous cathode scaffold, respectively. From Figure 3e, it is noteworthy to mention that the different building blocks are three dimensionally interconnected by conducting, adhesive polyaniline/sodium-alginate nanofibers. This hybrid conducting adhesive matrix helps well to improve the mechanical stability of cathode during chargedischarge and provides continuous e– transport pathway. In addition, the available free πelectrons and well organized imine groups (coordinating sites) in polyaniline help to improve electronic as well as ionic conductivity throughout the cathode architecture. The distribution of different building units throughout the cathode scaffold was further probed by energy-dispersive X-ray (EDX) spectroscopy, as shown in Figure 3. EDX elemental mapping confirms the welldistribution of carbon, sulfur, nitrogen, manganese and oxygen throughout the cathode, suggesting porous rGO, sulfur nanoparticles and Mn3O4 nanocubes were well-distributed and finally well-coated by the polyaniline/sodium-alginate matrix. Further, FEG-TEM imaging provides more details about the individual cathode components and their composite. Figure 4a represents the TEM images of porous reduced graphene oxide, showing the crumpled sheet-like structure. The crumpled structure of rGO can significantly increase the power capability of the electrode. The SAED pattern of the porous rGO (Figure 4b) suggests the successful reduction of graphene oxide (GO). Figure 4c and d depict the TEM images of sulfur nanoparticles and Mn3O4 nanoparticles, respectively. The low-resolution TEM image of final cathode composite (Figure 4e) suggests that the sulfur and Mn3O4 nanoparticles were well-embedded onto the porous rGO surface and polyaniline/sodium-alginate matrix even after long period of sonication during sample preparation. The high-resolution TEM image (Figure 4f) reveals the interface between reduced graphene oxide sheet, sulfur and Mn3O4


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nanoparticles, indicating that the sulfur and Mn3O4 nanoparticles were closely attached to the surface of graphene sheet. Electrochemical Evaluation of Porous Free-Standing Cathode. In order to evaluate the robust electrochemical behaviours of porous hybrid rGO/sulfur/Mn3O4/polyaniline/sodium-alginate cathode (i.e., PRGO/S/Mn3O4@PANI−SA), cyclic voltammetry and galvanostatic chargedischarge experiments were performed using CR2032 coin cells with respect to lithium. Figure 5a demonstrates the cyclic voltammogram of PRGO/S/Mn3O4@PANI−SA cathode in the potential range of 1.2−2.8 V vs. Li at 0.1 mV s−1 scan rate. During the 1st cathodic scan, three prominent peaks were observed. According to the reaction mechanism of sulfur with lithium during discharge, the peak at 2.35 V is attributed to the reduction of elemental sulfur (S8) to higher order lithium polysulfides (Li2Sn; n = 6−8). The following peak at 2 V results from further reduction of higher order lithium polysulfides to Li2S2/Li2S. However, the third peak in the potential zone of 1.65−1.7 V appeared from the irreversible reduction of LiNO3 additive present in electrolyte. In the subsequent anodic scan, an intense oxidation peak at 2.42 V, related to the conversion of Li2S2/Li2S to S8 through the polysulfides intermediate (i.e., Li2S/Li2S2→ Li2Sn→ S8; n = 4−8) was observed. After the 1st cycle, both the peaks positions and area of CV curves remain almost identical, indicating the excellent reversible lithium-ion storage property of the PRGO/S/Mn3O4@PANI−SA cathode. Figure 5b describes the galvanostatic charge-discharge profiles of PRGO/S/Mn3O4@PANI−SA electrode at the current density of 200 mA g−1. Figure 5c represents the corresponding cycling performances of PRGO/S/Mn3O4@PANI−SA electrode as well as PRGO/S/Mn3O4@PVdF electrode at the current density of 200 mA g−1. PRGO/S/Mn3O4@PVdF electrode displayed a reversible specific capacity of 1284 mA h g‒1 in the 1st cycle and average capacity of 1045 mAh g‒1 and 980 mAh g‒1 after 100 cycles with a



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gradual decay in specific capacity. The capacity decay was noteworthy after 70 cycles for PRGO/S/Mn3O4@PVdF electrode.

Figure 5. (a) Cyclic voltammetry of PRGO/S/MO@ PANI−SA cathode at a scan rate of 20 µV s−1, within 1.2−2.8 V potential window. (b) Charge-discharge profiles of PRGO/S/MO@PANI−SA cathode at the current density of 200 mA g‒1 at 20 °C (c) Cycling performances comparison of PRGO/S/MO@PVdF and PRGO/S/MO@PANI−SA cathodes at 200 mA g−1. (d) Charge-discharge profiles of PRGO/S/MO@ PANI−SA electrode at different current densities. (e) Rate performances of PRGO/S/MO@PVdF and PRGO/S/MO@PANI−SA cathodes at different current densities. (f, g) Cycling performances of PRGO/S/MO@ PANI−SA cathode at 2 A g−1 and 5 A g−1, respectively, after initial cycle of activation at 200 mA g−1.


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Therefore, to improve the electrochemical stability of the device, an advanced cathode has been fabricated using polyaniline/sodium-alginate hybrid binder (i.e., PRGO/S/Mn3O4@ PANI−SA electrode). The PRGO/S/Mn3O4@PANI−SA electrode delivered an initial specific capacity of 1362 mA h g‒1 (corresponding to 81.4% active sulfur utilization based on the theoretical capacity of sulfur, 1672 mA h g‒1) and 1148 mA h g‒1 after 100 cycles with very good capacity retention (84.3 % of initial capacity). Moreover, it can be noticed that the specific capacity of the batteries was increased after replacing PRGO/S/Mn3O4@PVdF electrode with PRGO/S/Mn3O4 @PANI−SA electrode. The improved specific capacity and cycling stability of PRGO/S/Mn3O4 @PANI−SA electrode can be explained by the fact that apart from the other components in the cathode, hybrid polyaniline/sodium-alginate binder offers both the excellent electronic/ionic conductivity throughout the cathode and excellent mechanical rigidity, resulting an improvement in active material utilization in electrode. On other side, because of the insulating nature of PVdF, the resistivity of an electrode get increased, which caused low active material utilization. In addition, in spite of having excellent adhesive property, PVdF does not possess enough mechanical rigidity to overcome the pulverization issues related to conversion based electrode materials like: sulfur. This could be the reason behind the observation that after 70 cycles PRGO/S/Mn3O4 @PVdF cathode showed gradual capacity fading, especially after 70 cycles. The charge-discharge profiles of PRGO/S/Mn3O4@PANI−SA cathode at different current densities are

represented

in

Figure

5d.

Figure

5e

compares

the

rate

performances

of

PRGO/S/Mn3O4@PANI−SA and PRGO/S/Mn3O4@PVdF cathodes at various current densities. The Figure 5e clearly reveals that the PRGO/S/Mn3O4@PANI−SA cathode showed much better rate capability, especially at high current densities (i.e., 2 and 5 A g‒1) in comparison to the PRGO/S/Mn3O4@PVdF cathode. At 2 A g‒1 current rate, the respective specific capacity of



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PRGO/S/Mn3O4@PANI−SA electrode was 1112 mA h g‒1, which is about 85 % of the average specific capacity at 200 mA g−1. At 5 A g‒1, PRGO/S/Mn3O4@PANI−SA electrode showed the specific capacity of 1045 mA h g‒1 (i.e., 80 % of the average specific capacity at 200 mA g−1). Furthermore,

when

the

current

rate

was

reversed

to

200

mA

g−1,

the

PRGO/S/Mn3O4@PANI−SA electrode recovered its initial capacity after 25 cycles, suggesting an excellent rate capability and robustness of the electrode. To further interrogate the robustness of PRGO/S/Mn3O4@PANI−SA electrode, long term cycling experiments were carried out at 2 and 5 A g‒1, respectively, after a 1st cycle of activation at slow current rate (200 mA g‒1). At moderate current density of 2 A g‒1, the initial capacity obtained (1125 mA h g‒1) became lower than the corresponding value at 200 mA g‒1, but the cycling stability improved (Figure 5f). A capacity of 1098 mA h g‒1 was obtained after 200 cycles at 2 A g‒1 with a capacity retention of 97.6%, suggesting extremely stable cycling performance at moderate current density. To investigate the reason behind extremely stable cycling performance of PRGO/S/Mn3O4 @PANI−SA electrode at 2 A g‒1, electrochemical impedance spectroscopy was carried out before and after 200 cycles at 2 A g‒1 (Figure S4). The electrochemical impedance analysis was based on the equivalent circuit shown in the inset of Figure S4. However, when the chargedischarge rate was further raised to 5A g‒1, the PRGO/S/Mn3O4@PANI−SA electrode exhibited an initial capacity of 1015 mA h g‒1 and retained 71 % of the initial capacity (i.e., 722 mA h g‒1) at the end of 500 cycles (Figure 5g). It can be noticed that during cycling performance at 5 A g‒1 the rate of capacity fading decreases with time and the capacity become very stable after 300 cycles. The above results demonstrate that as prepared PRGO/S/Mn3O4@PANI−SA freestanding cathode exhibited impressive electrochemical performances in terms of specific


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capacity, rate capability and cyclability in lithium-sulfur batteries. We believe that these excellent electrochemical behaviours achieved by PRGO/S/Mn3O4@PANI−SA cathode were originated from the well-interconnected, well-controlled nanoarchitecture and the advantages involve in the novel bottom-up approach of using different building units with specific purpose. Firstly, the nanosized sulfur particles and their uniform distribution facilitates the efficient transport of ions into the deeper portion of the sulfur nanoparticles because of the short diffusion pathway as well as improve the active material utilization. Secondly, the porous reduced graphene oxide facilitates the electron transport to reach each part of the electrode and physically confine the active sulfur molecules in its void space. Thirdly, the Mn3O4 nanoparticles absorbed the intermediate polysulfides and restrict their dissolution into electrolyte. Fourthly, the in-situ formation of polyaniline/sodium-alginate matrix plays several roles: (i) functioning as binder/adhesive to hold the different building parts (i.e., sulfur, reduced graphene oxide and Mn3O4) together in the scaffold firmly, leading to a mechanically rigid self-supporting electrode; (ii) functioning as structural buffer to effectively accommodate the huge volume change caused by Li+ insertion/extraction during cycling; (iii) serving as continuous conductive matrix throughout the electrode to accelerate both ionic/electronic conductivity. Additionally, the microporous nature of PRGO/S/Mn3O4@PANI−SA cathode helps the electrolyte to get access each part of the electrode and thus further accelerate the Li+ ion transport.



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Figure 6. (a) In-situ Raman experimental set-up. (b) Raman spectra recorded for sulfur/carbon cathode containing no Mn3O4 recorded at 0 min. (black), after 15 min. (yellow), after 2 hours (green), after 5 hours (blue) and after 10 hours (red). (c) Raman spectra recorded for sulfur/carbon cathode containing 5 wt% Mn3O4 recorded at same time interim as previous.


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However, to realize the absorption ability of Mn3O4 metal oxide an in-situ/operando Raman experiment was performed. The elemental sulfur cathode with (i.e., 5% by wt.) and without metal oxide were used in this study as reference. Two different cells were prepared in two different quartz cuvettes dipping the cathode and lithium anode inside the electrolyte. The entire cell setup and experiment is schematically shown in Figure 6a. Both the cell were discharged to 2.1 V at very slow rate of 50 mA g−1 (∼ C/33.5) to get enough concentration of lithium polysulfides (i.e., Li2Sn; n = 4−8). After reaching the cell voltage at 2.1 V, the spectra were recorded at different times (viz. 15 min, 2 hours, 5 hours and 10 hours) of interim. Figure 6b represents the Raman spectra of control sulfur cathode containing no Mn3O4, while the Raman spectra of sulfur/Mn3O4 composite cathode is represented in Figure 6c. At the OCV, both the cells exhibited only one peak at 220 cm−1 which is related to elemental sulfur (S8). During discharging of the cells, the concentration of S8 gradually decreased and that of intermediate products gradually increased. After reaching to the potential, 2.1 V both the cells showed two strong peaks at 518 cm−1 and 738 cm−1 which could be attributed to the formation of intermediate S3• − and Sn2− (n = 4−8), respectively.51 It was observed, with time the areas under the peaks, related quantitatively with the amount of intermediate products (generated at 2.1 V), extensively suppressed for pure sulfur cathode (55.2% and 42.4% suppression of area under the peaks corresponding to S3• − and Sn2−, respectively, after 10 hours with respect to Raman spectra obtained after 15 min). For sulfur/metal oxide cathode, the suppression of areas under the peaks corresponding to S3• − and Sn2−, were comparatively less (32.4% and 26.8% suppression of area under the peaks corresponding to S3• − and Sn2−, respectively, after 10 h with respect to Raman spectra obtained after 15 min). These observations suggest that the dissolution of polysulfides was more restricted in sulfur/Mn3O4 cathode. However, the difference in intensities were very



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minimal with and without Mn3O4, which could be due to very small Raman laser incident area. Furthermore, owing to the presence of very minute amount of metal oxide additive in the electrode and low Raman activity, no characteristic peak was observed for Mn3O4. In order to visualize the polysulfides absorption ability of Mn3O4, an absorption experiment was performed taking Li2S6 as the representative polysulfide prepared by previously reported method.52 The deep brown coloured Li2S6 solution turned completely colourless after adding Mn3O4 followed by stirring about 30 min. which indicates the strong affinity of Mn3O4 towards the intermediate polysulfides (Figure S5). Moreover, the UV-Vis spectrum indicated that the peaks of deep brown coloured Li2S6 solution significantly decreased after adding Mn3O4 nanoparticles (Figure S6). Xray photoelectron spectroscopy (XPS) survey proved the existence of chemical interaction between Mn3O4 and Li2S6. In the Mn 2p XPS spectrum of bare Mn3O4 nanoparticles (Figure S7), the binding energies of Mn 2p1/2 and Mn 2p3/2 appeared at 639.9 eV and 651.6 eV, respectively, with a splitting width of 11.7 eV. The negative shift in the XPS binding energy of Mn 2p region with a reduced splitting width (11.3 eV) for Li2S6 absorbed Mn3O4 compared with the bare Mn3O4 nanoparticles can be attributed to the strong electronic interaction of negatively charged S62− moiety with positively charged Mn center in Mn3O4.

CONCLUSION In summary, we developed a facile bottom-up approach to fabricate a free-standing and flexible porous cathode scaffold consisting of reduced graphene oxide, sulfur nanoparticles and Mn3O4 nanoparticles, three-dimensionally interconnected by sodium alginate/polyaniline hybrid conducting matrix. In this unique cathode architecture, all the components played their respective roles. The synergistic effects of each component ensured the outstanding electrochemical


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performances in terms of high specific capacity, excellent rate capability and unprecedented cycling stability. The chemical interaction between Mn3O4 and metal polysulfides which has been well documented from in-situ Raman studies. It is believed that the current simple and low cost electrode design concept could open up the new avenues for industrial application of high energy density lithium-sulfur batteries.

ASSOCIATED CONTENT Supporting Information Available: XRD patterns of rGO and Mn3O4; Raman spectrum of GO and rGO; EIS spectra of PRGO/S/Mn3O4@ PANI−SA cathode (before and after cycling); camera images and UV-Vis characterization of Li2S6 solution (before and after addition of Mn3O4); XPS spectra of bare Mn3O4 and Li2S6 absorbed Mn3O4; cycling performance of PRGO/S/Mn3O4@ PANI−SA cathode with high sulfur loading.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] Tel: +91 22 2576 7849

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The authors (A. G. and S. M.) are thankful to the project DST/RCUK/SGES/2012/13 for providing instrumental and financial supports. The authors acknowledge the instrumental support provided by Sophisticated Analytical Instrument Facility (SAIF) and infrastructure facilities



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provided by Industrial Research and Consultancy Centre (IRCC) in IIT Bombay. The authors are thankful to Prof. S. Sampath for providing help in in-situ Raman measurement and analysis. The authors are also thankful to Prof. Krishna Jonnalagadda for fruitful discussion on electrode film flexibility study.

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