Room-Temperature, Ambient-Pressure Chemical Synthesis of Amine

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Room-Temperature, Ambient-Pressure Chemical Synthesis of Amine-Functionalized Hierarchical CarbonSulfur Composites for Lithium-Sulfur Battery Cathodes Changju Chae, Jinmin Kim, Ju Young Kim, Seulgi Ji, Sun Sook Lee, Yongku Kang, Youngmin Choi, Jungdon Suk, and Sunho Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19181 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Room-Temperature, Ambient-Pressure Chemical Synthesis of Amine-Functionalized

Hierarchical

Carbon-Sulfur

Composites for Lithium-Sulfur Battery Cathodes

Changju Chae,a,‡ Jinmin Kim,a,‡ Ju Young Kim,a Seulgi Ji,a Sun Sook Lee,a Yongku Kang,a,b Youngmin Choi,a,b Jungdon Suk,a,b,* Sunho Jeonga,b,*

a

Division of Advanced Materials, Korea Research Institute of Chemical Technology

(KRICT), 19 Sinseongno, Yuseong-gu, Daejeon 305-600, Korea.

b

Department of Chemical Convergence Materials, Korea University of Science and

Technology (UST), 217 Gajeongno, Yuseong-gu, Daejeon 305-350, Korea

KEYWORDS: Synthesis, Amine, Sulfur, Composite, Cathode

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ABSTRACT Recently, the achievement of newly designed carbon-sulfur composite materials has attracted a tremendous amount of attention as high-performance cathode materials for lithium-sulfur batteries. To date, sulfur materials have been generally synthesized by a sublimation technique in sealed containers. This is a well-developed technique for the synthesizing of well-ordered sulfur materials, but it is limited when used to scale up synthetic procedures for practical

applications.

In

this

study,

we

suggest

an

easily

scalable,

room-

temperature/ambient-pressure chemical pathway for the synthesis of highly functioning cathode materials using electrostatically assembled, amine-terminated carbon materials. It is demonstrated that stable cycling performance outcomes are achievable with a capacity of 727 mAhg-1 at a current density of 1 C with good cycling stability by a virtue of the characteristic chemical/physical properties (a high conductivity for efficient charge conduction and the presence of a number of amine groups that can interact with sulfur atoms during electrochemical reactions) of composite materials. The critical roles of conductive carbon moieties and amine functional groups inside composite materials are clarified with combinatorial analyses by X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy.

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INTRODUCTION In the past decade, the rechargeable lithium-sulfur (Li-S) battery, based on the reversible oxidization-reduction reaction between sulfur and lithium, has drawn significant interest given the steady increase in the demand for clean and efficient energy storage devices.1,2 Compared to commercialized lithium-ion batteries, Li-S batteries possess a characteristic advantage in storing an electrical energy (2.6 kWh/kg) owing to the high theoretical specific capacity of 1672 mAhg-1 of sulfur itself, which is used as a cathode material.3 This value is nearly five times higher than those of conventional transition-metaloxide- and phosphate-based cathodes used in lithium-ion batteries.4 However, there are critical issues that should be resolved for practical applications of Li-S batteries, including the low conductivity of the sulfur material, the dissolution of lithium polysulfides (LiPSs) in organic electrolytes, and the volume expansion of sulfur during the discharge/charge process.1-4 These are critically associated with a poor cycle life, a limited specific capacity and low energy efficiency. To address these issues, various chemical/physical methodologies have been suggested, including (i) confining the sulfur spatially inside conductive, stress-releasing carbon nanostructures,5,6 (ii) introducing N-doped7-11 and/or chemically functionalized12 conductive carbonaceous materials, and (iii) surrounding the sulfur with non-conductive oxide materials13,14 and moderately conductive inorganic layers15. In particular, carbonaceous materials (including carbon nanotubes, graphene derivatives and calcined carbon materials) with a variety of structural factors have been demonstrated to facilitate sulfur-based cathode 3



composite materials with characteristic performance capabilities, including excellent cycle lifetimes, high specific capacity and high rate capability.16-18 As found in many previous studies, the general methodology when synthesizing such highly functioning sulfur-carbon hybrid materials is based on a subsequent heating/cooling process at a confined volume in order to evaporate and condensate the sulfur materials on top of the carbon materials or inside nano-architectured carbon structures.5-8,14-17 This is a well-defined technique of synthesizing high-quality, nanostructured sulfur materials that are indispensable when attempting to reduce the volume expansion and shorten the electrochemical reaction pathway. However, the amounts of evaporated sulfur atoms are predominantly determined by the vapor pressure in the confined container at the given temperature.19 This implies that when the reaction chamber is enlarged greatly for the mass production of complexly designed composite materials, the vapor pressure should be optimized overall with a uniform vapor flux in the sealed reaction chamber by adjusting the sublimation temperature depending on the volume of the chamber. In contrast, a chemical synthetic methodology in which composite materials are obtainable as products by designated chemical reactions is easily scalable without a limitation when scaling up a reaction batch as long as the synthesis temperature does not exceed the sublimation temperature (approximately 80 oC) of the extremely volatile sulfur. As another requisite, composite materials synthesized at low temperatures should not undergo subsequent annealing processes, which are inevitable for converting pristine carbonaceous materials into less defective, conductive materials. In this study, we design a room-temperature, ambient-pressure chemical synthetic method by which to obtain multi-stacked, amine-functionalized carbon-sulfur composite materials. By an aqueous chemical conversion from sodium thiosulfate (Na2S2O3), sulfur 4


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layers are deposited on pre-formed two-dimensional carbon templates, which are synthesized through the electrostatic interaction between negatively charged graphene oxides and positively charged amine-terminated multi walled carbon nanotubes. All chemical reactions are carried out at a room temperature without sealing the reaction batch, with no subsequent heat-treatment process. The resulting carbon-sulfur composite materials can resolve the critical issues associated with Li-S battery cathodes given their characteristic advantages of (i) the capability to capture sulfur atoms chemically by amine-sulfur interactions, (ii) the highly conductive nature of pristine graphene-oxide-based carbon templates by virtue of the uniformly distributed carbon nanotubes, (iii) good structural flexibility to accommodate mechanical stress related to volume expansions, (iv) a facile chemical synthesis without the use of sealed reactors, and (v) a green chemical pathway which uses deionized water as a solvent to synthesize the 2D carbon templates and the final carbon-sulfur composites. In particular, it is revealed that characteristic three-dimensional hierarchical carbon-sulfur composites consisting of non-destructive amine-functionalized carbon nanotubes and polyethyleneimine can allow for stable cycling performance with a capacity of 730 mAhg-1 at a current density of 1 C and with good cycling stability during 400 cycles.

EXPERIMENTAL METHODS Synthesis of amine-functionalized multi-walled carbon nanotubes (NH2MWNTs). Perylene-3,4,9,10-tetracarboxylic dianhydride (PTD, 97%), methylene chloride (99.5%, Samchun), triethylamine (99%, Samchun), and ethylenediamine (EDA, ≥99%, Sigma Aldrich) were used without further purification processing. Perylene-3,4,9,10-tetracarboxylic dianhydride (PTD) was kept overnight in a vacuum at 200 oC. A mixture consisting of 1.4 g 5



of MWNT (97%, length: ~10 µm, Applied Carbon Nano Co. Ltd.), 0.35 g of PTD, 350 mL of methylene chloride, 70 mL of triethylamine, and 14 mL of ethylenediamine was sonicated for 1 h and stirred vigorously for 24 h. The mixture was then centrifuged, and the obtained NH2MWNT samples were washed with methanol, methylene chloride and methanol in that order by centrifugation. The NH2MWNT samples were then dried overnight under a vacuum. Synthesis of amine-functionalized carbon-sulfur composite materials. 0.15 g of NH2MWNTs were dispersed in 72 ml of deionized water (DI water) and GOs were added at a weight ratio of NH2MWNT:GO=450:1. The mixed GO/NH2MWNT precipitates thus obtained by centrifugation separation were mixed with a polyethyleneimine aqueous solution in which 0.9 g of PEI was dissolved. The polyethyleneimine solution (PEI, MW ~1,300) was purchased from Aldrich and was used without further purification processing. After stirring for 1 hr, the GO/NH2MWNT-PEI products were washed with DI water via centrifugation separation. To synthesize the sulfur layer, the obtained precipitates were dispersed in 89 ml of DI water where 1.73 g of sodium thiosulfate (Sigma Aldrich, 99%) is dissolved, followed by adding 93 ml of 0.52 M HCl aqueous solution. After completing the 1 hr reaction at room temperature, the GO/NH2MWNT-PEI-S products were washed twice with DI water by centrifugation separation, and the final products of GO/NH2MWNT-PEI-S-PEI were obtained using a PEI-aqueous solution with a method identical to that described above. Another GO/NH2MWNT assembly was synthesized at a weight ratio of NH2MWNT:GO=900:1 and was then mixed with the final products and separated by a centrifugation method. The fully stacked composite materials were then dried at 70 oC in a vacuum oven and kept in air. Note that all synthetic procedures were carried out at room temperature in air without the use of sealed reactors. 6


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Cell fabrication. The cathodes were prepared as follows: First, the aminefunctionalized carbon-sulfur composite powders were mixed with super P and polyvinylidene fluoride (PVDF, Kynar) to the following ratio: carbon-sulfur composite powders : Super P : PVDF = 70 : 15 : 15 by weight, to which was added 1-methyl-2-pyrrolidinone (NMP, 99.5%, Sigma Aldrich Chemical) to form a homogeneous slurry. This slurry was then pasted onto aluminum current collectors (20-mm-thick Al foil, Hohsen, Japan) using the doctor blade method. The pasted electrodes were dried in a vacuum oven at 60 oC for 12 hrs and then punched to obtain a circular shape. The sulfur loading was 1.5~2 mg/cm2 and the ratio of electrolyte to sulfur was ~45. The electrodes were used as the cathodes of 2032-type coin cells assembled in an Ar-filled glovebox (MOTEC). The dimension of cathode electrodes was 16 mm in diameter. Li foil (thickness: 0.3 mm, Honjo Metal) was used as the anode material, and a polyethylene membrane was used as a separator. 0.11 ml of electrolyte contained 1 M LiTFSI in a mixed solvent of DOL/DME (1:1, vol %), with 0.2 M LiNO3 as an additive. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data were obtained with a Bio-Logic SAS-VMP3 device. The cyclic voltammograms were recorded in a voltage window of 1.8−2.8 V at a scan rate of 0.05 mV s−1. The EIS measurements were recorded from 1 MHz to 100 mHz with an AC voltage amplitude of 10mV at the open-circuit voltage. The cells were cycled at room temperature with battery testing equipment (TOSCAT3100U, Toyo Inc.). Characterization. The morphologies of the composite materials were observed by scanning electron microscopy (SEM, JSM-6700, JEOL). The crystal structures were analyzed using an X-ray diffractometer (XRD, D/MAX-2200V, Rigaku), and a chemical structural analysis was conducted by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher 7



Scientific). The thermal decomposition behaviors of the composite materials were monitored by means of a thermal gravimetric analysis (TGA, SDT2960, TA Instruments) in an inert atmosphere. To measure the conductivity of the carbon materials, 4 ml of carbon suspensions at a concentration of 2 mg/ml were vacuum-filtrated on filter papers (AnodiscTM 47, pore size: 0.2 µm, diameter: 47mm, WhatmanTM). The resulting carbon samples were then dried at room temperature in an ambient condition for 20 hrs. The sheet resistance of the samples was measured with a four-point probe (FPP-HS8, Dasol Eng.). The dimension of samples was 35 mm in diameter, and the thickness of samples was 0.1~0.4 mm.

RESULTS AND DISCUSSION Synthesis of amine-functionalized, hierarchical composite materials. Scheme 1 depicts the experimental procedures used to synthesize the multi-stacked, aminefunctionalized carbon-sulfur composite materials. The surfaces of the multi-walled carbon nanotubes (MWNTs) are functionalized with amine-terminated molecules. N,N’-di(2aminoethyl)-perylene-3,4,9,10-tetracarboxylic diimide (AE-PTDI) was synthesized by reacting perylene-3,4,9,10-tetracarboxylic dianhydride (PTD) with ethylenediamine, which was immobilized on the surfaces of the MWNTs via a strong π-π stacking interaction.20 The amine-functionalized MWNTs (NH2MWNTs) were assembled electrostatically with graphene oxides in an aqueous medium.21 The neutral amine groups are converted into positively charged moieties by adjusting the pH of the aqueous medium, and the graphene oxides with intrinsic surface defects are negatively charged in an aqueous medium. Polyethyleneimine (PEI), which is a typical molecule consisting of numerous N-H chemical groups, was then adsorbed onto the surfaces of pre-formed carbon assemblies to ensure complete coverage of 8


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the amine group, followed by sulfur chemical synthesis and the subsequent additional adsorption of PEI. Finally, the resulting amine-functionalized carbon assemblies are stacked onto each other through chemical interactions between polar dipoles residing on the surfaces of the carbon-sulfur assemblies, allowing for three-dimensionally interconnected, fully amine-interactive sulfur cathode materials. Figure 1a shows the Fourier transform infrared spectroscopy (FT-IR) results for the NH2MWNTs and the PTD used as a pristine reactant. When the PTD is transformed into AEPTDI, ethylenediamine (EDA) is incorporated into the PTD by replacing the oxygen in the PTD with the nitrogen of the amine group in the EDA. In the spectrum of NH2MWNTs, the peaks at 1440, 1340-1249 and 1157 cm-1 are attributable to the C-H bending vibration, C-N stretching vibration and N-H bending vibration of AE-PTDI, respectively, while the characteristic C-O-C peak of PTD, observable at 1020 cm-1, was distinct.22 This indicates that the AE-PTDI was successfully adsorbed on the surfaces of MWNTs. The surface functionalization of MWNTs was also confirmed with sedimentation tests (Figure 1b). The pristine MWNTs are dispersible in non-coordinating solvents such as toluene and dichlorobenzene owing to their hydrophobicity. When a proper surfactant is not used, they cannot be dispersed in a polar solvent such as DI water or alcohol, forming precipitates consisting of highly entangled agglomerates. However, the surfaces of NH2MWNTs are hydrophilic owing to the characteristic nature of the surficial amine groups, resulting in the opposite case of NH2MWNTs becoming well dispersed in DI water but not in toluene. In order to determine the capability of forming electrostatic assemblies, we measured the pH-dependent surface charges of NH2MWNTs and GOs dispersed in an aqueous medium (Figure 1c). Owing to the presence of intrinsic oxygen-containing defects that are generated 9



during graphene oxide synthesis procedures, the GOs have a negative surface charge of approximately -33 ~ -21 mV regardless of the pH range (from 3 to 11). For the NH2MWNTs, they become positively charged with values of 37 ~ 44 mV due to a deprotonation reaction in an acidic environment and lose their positive surface charge after a protonation reaction in a basic environment at a pH above 8.3. This implies that both NH2MWNTs and GOs are capable of being assembled through electrostatic interactions between them by simply mixing them in an aqueous solution with a neutral pH. The pH of the as-prepared deionized water (DI water) should be 7. However, an instantaneous dissolution of carbon dioxide from air results in the spontaneous formation of a weak acid, H2CO3; this turns the pH of the DI water slightly acidic, which is appropriate to realize the positive charge in NH2MWNTs. As shown in Figure 1d, long NH2MWNTs are packed closely on the surfaces of graphene oxides without a spatial void between neighboring NH2MWNTs. All GO/NH2MWNT assemblies are connected spatially by an intra-entanglement between carbon nanotubes with a high surfaceto-volume ratio. The formation of a well-structured assembly was also confirmed by the electrical properties (Figure 1e). To measure the electrical properties, carbon paper samples were prepared using a vacuum filtration method. In fact, pristine graphene oxides could not be used as a conductive template skeleton. The numerous existing defects should be eliminated by converting them into the relatively conductive reduced one. The conductivity of pristine GO paper was out of the measurement range (< 0.025 S/cm), while that of thermally reduced (at 400 oC) GO paper was measured and found to be 0.9 S/cm, comparable to the value of 0.6 S/cm of hydrazine-treated GO paper. In contrast, the MWNTs can be used as a conductive moiety without a thermal post-treatment owing to their relatively defect-free carbon 10


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structures, as confirmed by the conductivity of 22 S/cm of the pristine MWNT paper; however, a one-dimensional morphology of carbon nanotubes is not appropriate to accommodate a sufficient amount of sulfur material on top of them. Notably, for our GO/NH2MWNT paper samples, the conductivity of 1.4 S/cm was measured without the use of any additional thermal/chemical treatments. This indicates that the GO and NH2MWNT play individual roles in the assemblies; the pristine GOs provide two-dimensional template skeletons and do not act as a conductive framework, and the electrostatically incorporated MWNTs compensate for the insulating property of the pristine GOs, providing highly networked, conductive structures. As a control sample, MWNTs were treated with sodium dodecyl sulfate (SDS), which is used commonly as a small-molecule surfactant to disperse MWNTs in water. The SDS-MWNT paper samples exhibited conductivity of 14 S/cm, slightly higher than that of the NH2MWNT paper sample. This indicates that the discrepancy in the conductivity between the MWNT and GO/NH2MWNT paper samples is attributable to the presence of relatively bulky AE-PTDI molecules adsorbed on the surfaces of the NH2MWNTs. However, it should be noted that the conductivity of the GO/NH2MWNT paper sample is still higher than those of thermally and chemically treated GO paper samples. To synthesize the cathode materials, sulfur was deposited via wet-chemical synthesis on the internal surfaces of pre-formed GO/NH2MWNT assemblies. Subsequently, in order to ensure surface functionalization with the amine groups, the GO/NH2MWNT assemblies were treated with polyethyleneimine (PEI) as a chemical moiety with a sufficient number of amine groups in an aqueous solution. As shown in Figure 2a, octatomic sulfur was grown without any by-products. It appeared that the sulfur layer was uniformly formed on the internal surfaces of the assemblies overall without the formation of individual sulfur particles, as 11



heterogeneous nucleation was manipulated completely to restrict the formation of free sulfur atoms (Figure 2b). In the elemental mapping results, it was observed that the sulfur element was placed within the morphological boundaries of individual assemblies, indicative of preferential deposition along the surfaces of the carbon assemblies. Carbon, oxygen and nitrogen were detectable in all assemblies due to the presence of NH2MWNT stemming from the individual assemblies. Another role of NH2MWNT is to allow for facile sulfur nucleation on the surfaces of the carbon assemblies. As shown in Figure S1, the aqueous chemical synthesis process used in this study is not possible on the surfaces of pristine graphene oxide sheets. Upon completion of the sulfur synthesis step in the presence of GOs, the precipitates which separated after centrifugation were composed of two layers; the top layer consists of graphene oxides, and sulfur particles exist in the bottom layer. This indicates that a heterogeneous nucleation process is not favorable predominantly on the surfaces of defective graphene oxides. Thus, it can be presumed that the sulfur layers formed in our study are mostly interacted with underlying amine groups and not rather with other functional groups of GOs. As another control experiment, the GOs were modified to have surface amine groups by adsorbing solely PEI molecules using a previously reported electrostatic assembly technique.23 After the sulfur synthesis procedure, it was noted that uniform sulfur-carbon composite materials are obtainable with the evolution of a highly crystallized sulfur phase (Figures S2 and S3). This characteristic role as a chemical binding site of the amine group during a synthesis step is distinctive evidence which supports the possibility of sulfur-amine chemical interaction, which is necessary to realize a long-term cycle life in Li-S battery cathodes. Electrochemical performances of amine-functionalized hierarchical composite 12


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materials. The cycling performances of sulfur-carbon composite materials were studied at a current density of 0.1 C (Figure 3a). In order to clarify the importance of the amine functional groups and conductive NH2MWNTs in the composite materials, two control cells were prepared using GO-PEI-S and GO-PEI-S-PEI composite materials. The fully stacked composite material in this case was the GO/NH2MWNT-PEI-S-PEI composite material covered with additional GO/NH2MWNT assemblies to ensure complete electrical conduction within the composite materials. According to thermogravimetric analysis (TGA) results in a nitrogen atmosphere (Figure S4), the amount of deposited sulfur was measured and found to be 48 wt% in the fully stacked composite material. It is believed that a sulfur content can be increased further by designing the new light-weight carbon templates that possess both of efficient electrical-conduction pathways and abundant surficial amine groups. The first discharge-charge process was carried out at a current density of 0.1 C to stabilize the electrochemical reaction, and the cycling data was recorded after the second discharge-charge process. The initial measured discharge capacities were 365, 416 and 853 mAhg-1 and the discharge capacities after 400 cycles were maintained with values of 270, 385 and 545 mAhg1

for cells employing the GO-PEI-S, GO-PEI-S-PEI and fully stacked composite materials,

respectively. In an ideal discharge process, cyclo-S8 is reduced, forming high-order LiPSs and lower order counterparts with distinctively different discharge plateaus at 2.3 and 2.1 V, respectively. However, the high-order lithium polysulfides (LiPSs) tend to dissolve in an organic electrolyte as the repeated discharge-charge process proceeds. When the voltage profiles after 400 cycles are compared for the GO-PEI-S and GO-PEI-S-PEI cells, the more prolonged discharge plateau at 2.3 V is clearly observable for the case of the GO-PEI-S-PEI cell (Figure 3b). This implies that stronger sandwich-structured surface passivation is crucial

13



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to suppress the dissolution of high-order LiPSs, indicative of the effectiveness of amine functional groups in stabilizing chemically the sulfur atoms. Interestingly,

the

fully

stacked

composite

materials

prepared

from

the

GO/NH2MWNT assemblies exhibited a more prolonged discharge plateau at 2.3 V. Both the AE-PTDI molecule-based amine-functionalization step and the electrostatic assembly of NH2MWNTs on graphene oxides allow for the formation of more complete template skeletons on which the sulfur atoms become anchored firmly after chemical synthesis at room temperature, suggesting the superiority of AE-PTDI-treated NH2MWNTs over PEI in creating surficial amine groups. Note that PEI is the most common molecule used for incorporating amine-terminated moieties in a variety of applications, including organic solar cells,24 thin-film transistors,25 stretchable elastomers26 and lithium-ion battery anodes.23 The improved discharge capacity is also attributable to the improved electrical conductivity of the fully stacked composite material due to the use of highly conductive GO/NH2MWNT assemblies rather than pristine GO sheets, as more sulfur atoms can take part in electrochemical discharge-charge chemical reactions. This is in line with the well-known fact that the discharge capacity would be improved greatly simply by adding carbon nanotubes to the anode materials of lithium ion batteries.27 The importance of electrical conductivity is confirmed by the cycling performance of the cell utilizing the fully stacked composite materials synthesized on GO sheets (other than GO/NH2MWNT assemblies); a limited initial discharge capacity was measured with a value of 562 mAhg-1 (Figure S5). The molecular weight of PEI in the composite materials also has another critical impact on the electrochemical performance capabilities. When PEI with a molecular weight of 1,300 is replaced with PEI with a molecular weight of 750,000, the initial discharge capacity is 14


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reduced from 562 to 393 mAhg-1 (Figure S6a). The bulky interfacial polymer layer would affect adversely the conductivity of the resulting composite materials, hampering charge conduction inside a heterogeneous couple of synthesized sulfur and conductive carbon layers. The degrees of crystallinity of the sulfur loaded into both composite materials are nearly identical (Figure S6b). These facts suggest that the sophisticated design of the highly conductive template skeleton capable of capturing sulfur atoms is the most crucial factor when attempting to exploit high-performance sulfur-carbon composite cathode materials for Li-S batteries. Analyses on electrochemical stability of LiPSs captured chemically in composite materials. Figure 4a shows the X-ray photoelectron spectroscopy (XPS) S 2p spectrum of the pristine composite material, which did not undergo any electrochemical reactions. It has been reported that amine groups can interact firmly with elemental sulfur, shifting negatively the binding energy of the elemental sulfur.28,29 Previous studies in other research fields have also suggested that sulfur-amine bonds can be formed in amine-containing organic molecules via the donation of an electron from the amine group to elemental sulfur.29-31 The sub-peaks positioned at 163.9 and 165.1 eV with a split of 1.2 eV are attributed to the free elemental sulfur, supported by the fact that the areal ratio for both sub-peaks is ~2. The sub-peaks located at 163.4 and 164.6 eV are associated with the elemental sulfur which interacted with the amine-functionalized composite materials.28,29 It is considered that the sub-peak observed at a high binding energy of approximately 168 eV results from the formation of sulfate during the aqueous chemical synthesis step. This is one of the reasons why a discharge capacity slightly lower than the theoretical value for sulfur is obtained with our composite material. It is expected that the optimization of more delicate synthetic conditions would improve the 15



electrochemical performance further. When the areas of the sub-peaks of the free elemental sulfur and the interacted elemental sulfur are denoted as areas (I) and (II), respectively, the ratio of area(I)/[area(I)+area(II)] is calculated to be 0.57. This indicates that half of the sulfur loaded in the composite materials interacts on the internal surfaces of the composite materials, which corresponds well to the fact that the dissolution of LiPSs is prohibited to some extent, as confirmed in the cycling data and voltage profiles. The XPS Li 1s spectra for the composite material after the discharge process are shown in Figure 4b. The sub-peaks positioned at 53.1 and 52.3 eV are attributable to Li-S and Li-N bonds, respectively,32 indicative of a chemical interaction between the amine group and the LiPSs. The negative shift in the binding energy associated with the Li-N bond is due to the chemical interaction between the less electronegative Li and the more electronegative nitrogen atoms. The cyclic voltammetry (CV) profiles of the fully stacked composite materials are shown in Figure 5a. There are two characteristic reduction peaks that correspond to the reduction of elemental sulfur to high-order (Li2Sx, 4 ≤ x≤ 8) polysulfides at 2.3 V and the formation of Li2S2/Li2S at 2.1 V. Two oxidation reaction peaks are observed in the subsequent anodic scan. The first peak at 2.30 V is associated with the formation of Li2Sn (2 ≤ n≤ 8) and the second peak at 2.37 V is attributable to the formation of elemental sulfur. It should be noted that more distinctively distinguishable oxidation peaks are observable in this case compared to those in previously reported results. This indicates that more complete oxidation of high-order Li2Sx to S8 takes place in our composite material due to the effective adsorption of Li2Sx on the amine-functionalized surface.16 Reactive reversibility and good cycling stability are also observable, as all redox peaks remain sharp and constant upon cycling. Another interesting feature is that the two oxidation peaks are positioned at a low voltage of 16


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around 2.6 V, which is associated with low polarization and inner resistance due to the incorporation of a highly conductive intra-stacked carbon structure.33,34 The capability of capturing LiPSs is clearly confirmed when immersing the NH2MWNTs in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (1:1)-based LiPS solutions. As shown in Figure 5b, after a sufficient amount of NH2MWNTs was added to the electrolyte solution, NH2MWNTs precipitated and the color of the supernatant solution turned much lighter due to the facile adsorption of LiPSs onto the surfaces of the NH2MWNTs. Electrochemical impedance spectroscopy (EIS) (Figure 5c) was utilized to confirm the electrochemical stability of the fully stacked composite before and after chargingdischarging for 30 cycles. The Randle-type equivalent circuit model is depicted in Figure S7. The Nyquist plots display two semicircular loops from a high to a medium frequency region and a sloped line in the low-frequency region. The first semicircle at a high frequency is considered to stem from lithium ion diffusion through the surface layers and the second one at a medium frequency region is ascribed to the charge transfer resistance.35 After the first cycles, the cell with the fully stacked composite material showed stable semicircles, indicating that our intra-stacked carbon composite cathode suppresses the migration of polysulfides and the irreversible agglomeration/deposition of insulating polysulfides on the surfaces of the composite materials. Cell performances at high current densities. Figure 6a shows the cycling performance of a cell employing the fully stacked composite material at a current density of 1 C. It is clearly observable that a capacity as high as 730 mAhg-1 is well maintained even at a high current density compared to the result at a current density of 0.1 C. Notably, as shown in Figure 6b, the voltage profiles during 100 cycles were not altered at all, providing evidence 17



of the excellent electrochemical stability of the polysulfides which form during the repeated charge-discharge process. This is attributable to the chemical contribution of the amine groups residing in the composite materials that interact firmly with sulfur, as proven by earlier XPS, C-V and EIS analyses. Figure 6c shows the rate performance results for cells with the fully stacked composite and the GO-PEI-S composite electrodes. It is shown that the complete carbon-sulfur composite material, which possesses intra-electrical conduction pathways and stronger chemically anchoring sites for sulfur, exhibits much better rate capability and higher capacity levels. The capacity levels were measured and found to be 825, 728, 671, 644, 604 and 406 mAhg-1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5C, respectively. As shown in the voltage profiles at various current densities ranging from 0.1 to 2 C for the cell employing the fully stacked composite material (Figure 6d), the representative electrochemical reactions were well preserved even at a high current density, with the evolution of the characteristic plateaus. A unique advantage of our stacked-structured composite material is the capacity to endow efficient charge conduction between sulfur and carbon compartments along with the chemical passivation of sulfur, even when synthesized simply under ambient conditions without annealing to improve the electrical properties of carbon materials. In order to clarify the advantage of the wet-chemical strategy suggested in this study, we compared the rate performances of previously reported, synthesized cathode materials with the results for our cells. As shown in Figure 7, the cell based on the fully stacked composite material exhibited rate performance capabilities comparable to those of the best cells (ref. #36-38) reported thus far. However, those previously reported cathodes were prepared by the ex-situ polymerization of conductive polymers on the surfaces of pre-synthesized hollow sulfur spheres36,37 or by the 18


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infiltration of a sulfur-carbon disulfide (CS2) solution in pre-formed (annealed at 900 oC) three-dimensional porous carbon structures.38 These methods are based on the structural confinement of sulfur inside hollow and porous conductive frameworks accompanied by delicate synthesis procedures. In contrast, the composite materials developed here do not need structural template materials; rather, the sulfur layers were synthesized easily on top of chemically designed conductive hybrids. Other than our study, the rate capability outcomes of cathode materials (ref. #39-41) that have been prepared by conformal-coating methods using various non-structured conductive materials have been limited. CONCLUSIONS In summary, we have designed highly functioning hierarchical carbon-sulfur composite materials by synthesizing sulfur layers on the basis of electrostatically driven GO/NH2MWNT assemblies. All of the chemical synthesis procedures here were carried out in ambient conditions without additional annealing processes. It was revealed that this roomtemperature, ambient-pressure chemical synthesis is achievable with the use of AE-PTDItreated MWNTs as amine-functionalized carbon nanotubes. The characteristic hierarchically stacked carbon-sulfur composite materials showed an initial discharge capacity of 853 mAhg1

at a current density of 0.1 C and a discharge capacity after 100 cycles of 690 mAhg-1 at a

current density of 1 C. It was clarified that such high performance is attributable to the highly conductive nature of the AE-PTDI-treated MWNTs and to the chemical contribution of the amine groups, interacting with sulfur atoms, in the resulting composite materials.

19



Scheme 1. Schematics showing the synthetic procedures of amine-functionalized sulfurcarbon composite materials

20


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Figure 1. (a) FT-IR spectra of the PTD and NH2MWNT samples; (b) photographs of pristine MWNTs and NH2MWNTs dispersed in DI water and toluene; (c) variations of the zeta potential for NH2MWNT and GOs dispersed in DI water; (d) SEM images of the GO/NH2MWNT assemblies synthesized in this study; (e) variations of the conductivities for carbon papers consisting of MWNTs, SDS-MWNTs, NH2MWNTs, GO/NH2MWNT assemblies, T-RGOs (thermally reduced graphene oxides) and C-RGOs (chemically reduced graphene oxides).

21



Figure 2. (a) X-ray diffraction result and (b) SEM image and elemental mapping data for the GO/NH2MWNT-PEI-S composite materials. 22


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Page 23 of 35

100

-1

Capacity (mAhg )

a 1200

GO-PEI-S 90 GO-PEI-S-PEI Fully-stacked composite 80

900 600

70 300 0

60 0

100

200

50 400

300

Cycle Number 3.2 GO-PEI-S GO-PEI-S-PEI Fully-stacked composite

+

Voltage (V vs Li/Li )

b

Coulombic Efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.8

2.4

2.0

1.6

after 400 cycles 0

200

400

600 -1

Capacity (mAhg )

23



Figure 3. (a) Cycling performances and (b) voltage profiles after 400 cycles for electrodes employing GO-PEI-S, GO-PEI-S-PEI and fully stacked composite materials. The current density was 0.1 C and voltage range was between 1.7 and 2.6 V.

Intensity (a.u.)

a

168

164

160

Binding Energy (eV)

b Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

58

56

54

52

50

Binding Energy (eV) Figure 4. (a) S 2p XPS spectra of pristine sulfur-carbon composite materials and (b) Li 1s XPS spectra of sulfur-carbon composite materials after the discharge process

24


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25



Figure 5. (a) CV profiles of fully stacked composite materials, (b) photographs showing LiPS solutions prepared without and with NH2MWNTs, and (c) EIS response of the cell with the anode consisting of the fully stacked composite materials during repeated discharging processes

800 80

Discharge Charge Coulombic Efficiency

400

60

Current Density: 1C

0

0

20

40

60

80

100

b

2.8

+

-1

100


1200

Capacity (mAhg )

a

Coulombic Efficiency (%)

nd

2.4

2 th 5 th 10 th 20 th 50 th 100

2.0

1.6

0

d

1200

-1

Fully-assembled composite GO-PEI-S 0.1C 0.2C

800

0.5C 1C

0

0

10

2C

20

900

1200

2.8

+

0.1C

5C

400

600

Capacity (mAhg )


c

300

-1

Cycle Number

Capacity (mAhg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

2.0

1.6

30

0.1C 0.2C 0.5C 1C 2C

2.4

0

300

600

900

1200

-1

Cycle Number

Capacity (mAhg )

Figure 6. (a) Cycling performance and (b) voltage profiles at a current density of 1 C for electrodes employing fully stacked composite materials, (c) rate performances of electrodes employing GO-PEI-S and the fully stacked composite materials, and (d) voltage profiles at different current densities for electrodes employing the fully stacked composite materials.

26


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized Capacity

Page 27 of 35

100

this study ref.33 ref.34 ref.35 ref.36 ref.37 ref.38

80 60 40 20 0

0

2

4

6

Current Density (C-rate) Figure 7. Rate performance comparison with previously reported, chemically synthesized sulfur-carbon composite cathodes for Li-S batteries.

27



ASSOCIATED CONTENT Supporting Information. Chemical formulas of PTD and AE-PTDI, SEM images/XRD

results/elemental mapping data/TGA results for synthesized composite materials, and cycling performances of electrodes employing composite materials synthesized using PEI with different molecular weights. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (J. Suk) *E-mail: [email protected] (S. Jeong)

Author Contributions ‡

C. Chae and J. Kim contributed equally.

ACKNOWLEDGMENT This research was supported by Global Research Laboratory Program of the National Research Foundation (NRF) funded by Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015K1A1A2029679), and partially supported by the Nano·Material Technology Development Program through the National Research 28


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Foundation of Korea funded by the Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015M3A7B4050306). This work was financially supported by the R&D Convergence Program of the National Research Council of Science and Technology (CAP-15-02-KBSI), Republic of Korea.

29



Page 30 of 35

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Sources 2015, 295, 182-189.

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600 70 300

60 0

100

200

300

50 400

Cycle Number

Fully-assembled composite GO-PEI-S

-1

Capacity (mAhg )

GO-PEI-S 90 GO-PEI-S-PEI Fully-stacked composite 80

900

0

1200

100

-1

Capacity (mAhg )

1200

Coulombic Efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1C 0.2C

800

0.5C 1C

5C

400

0

0.1C 2C

0

10

20

Cycle Number

35


30

Room-Temperature, Ambient-Pressure Chemical Synthesis of Amine

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