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Self-assembly of polyethylene glycol-grafted carbon nanotube/sulfur composite with nest-like structure for high performance lithium-sulfur batteries Han Li, Liping Sun, and Gengchao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12496 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016

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ACS Applied Materials & Interfaces

Self-assembly of polyethylene glycol-grafted carbon nanotube/sulfur composite with nest-like structure for high performance lithium-sulfur batteries Han Li, Liping Sun, Gengchao Wang ∗

Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, P.R.China

* Corresponding author. Tel: +86-21-64253527. E-mail address: [email protected] (Gengchao Wang)

ACS Paragon Plus Environment

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ABSTRACT The novel polyethylene glycol-grafted multi-walled carbon nanotube/sulfur (PEG-CNT/S) composite cathodes with nest-like structure are fabricated through a facile combination process of liquid phase deposition and self-assembly, which consist of the active material core of sulfur particle and the conductive shell of PEG-CNT network. The unique architecture not only provides a short and rapid charge transfer pathway to improve the reaction kinetics, but also alleviates the volume expansion of sulfur during lithiation and minimizes the diffusion of intermediate polysulfides. Such an encouraging electrochemical environment ensures the excellent rate capability and high cycle stability. As a result, the as-prepared PEG-CNT/S composite with sulfur content of 75.9 wt% delivers an initial discharge capacity of 1191 mAh g-1 and 897 mAh g-1 after 200 cycles at 0.2 C with an average coulombic efficiency of 99.5%. Even at a high rate of 2 C, an appreciable capacity of 723 mAh g-1 can still be obtained.

KEYWORDS: lithium-sulfur batteries, carbon nanotube, polyethylene glycol, nest-like structure, self-assembly


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1. INTRODUCTION To meet the continuously increasing demand of portable electronics, electric vehicles and renewable energy storage, the development of efficient energy storage systems with high energy density and long cycle life has attracted much attention worldwide.1-3 Among the various kinds of alternative rechargeable batteries, lithium-sulfur (Li-S) batteries have drawn an increasing interest due to their potential applications for next-generation high-energy power systems.4-6 As the cathode candidate, sulfur can deliver a high theoretical specific capacity of 1675 mAh g-1 with the advantages of natural abundance, low cost and non-toxic.7-10 Despite the appealing benefits, the practical application of Li-S batteries is still plagued by the insulating nature of sulfur and the final discharge product (Li2S and Li2S2), the dissolution of polysulfides along with “shuttle effect”, and the volume expansion of the cathode during the lithiation process.11-14 As a result, Li-S batteries suffer from the low utilization of active material, poor rate performance and limited cycle life.15-18 In order to overcome the intractable drawbacks, extensive research have been put forward by modifying the cathodes with the introduction of carbon materials,19-28 polymers29-32 or metal oxides.33-36 In particular, the carbon materials as the most effective hosts serve not only as the conductive agents to facilitate the electron transport of the composites and enhance the utilization of sulfur, but also as the reservoirs to restrict the diffusion of polysulfides and improve the reversibility of the cathodes. Among them, carbon nanotubes (CNTs) exhibit unique advantages, including excellent electronic conductivity, large aspect ratio, good electrochemical stability and mechanical properties.37-39 The CNTs in the cathodes can create the perfect


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three-dimensional conductive framework to reinforce the structural stability and promise high-rate charge transfer. Moreover, the surface of functionalized CNTs with oxygen groups can provide adsorption sites to confine the S species and chemically bind with the highly polar polysulfides to improve the cycling stability.40-42 However, the CNT/sulfur composites often contact with the electrolyte directly, leading to inevitable loss of the active materials and the redox shuttle effect during long cycling. An effective strategy to further stabilize polysulfides and improve the cyclability has proved to be the surface coating on the carbon/sulfur composites. Polymers,43-47 graphene48-51 and carbon spheres52-56 are often treated as the coating layers to form core-shell or yolk-shell structures, which always function as a conductive pathway

and provide efficient physical

confinement or chemical bonding to trap the soluble polysulfides during cycling. Moreover, these unique structures can relieve the large volumetric expansion and reduce the pulverization and destruction of the cathodes during cycling.57-59 However, the preparation and coating of the carbon/sulfur composites often tend to be carried stepwise and time-consuming. These processes are much complicated, which are not suitable for large-scale production. Herein, we develop a facile and simple method to fabricate the polyethylene glycol-grafted multi-walled carbon nanotube/sulfur (PEG-CNT/S) composites with nest-like structure. Easily-dispersed PEG-CNT is prepared by the esterification reaction between the carboxylated MWCNTs and PEG.60,61 The PEG-CNT/S composites are further synthesized through a combination process of liquid phase deposition and self-assembly. The formation process of the PEG-CNT/S composites is illustrated in Figure 1. The conductive CNT framework can provide a


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fast charge transport pathway and remarkably enhance the reaction kinetics of the active material, which is beneficial to improve the rate capability and the utilization of sulfur. Moreover, the nest-like structure also plays a role of coating layer, which can accommodate the large volumetric expansion of sulfur during cycling and suppress the diffusion of polysulfides. Besides, the grafted PEG not only facilitate the lithium ion transport, but also provide physical barrier and chemical gradient to immobilize the polysulfide species.62-65 This hydrophilic component can introduce a surface chemical gradient that preferentially solubilizes the polysulfides in relation to the electrolyte.66,67 As a result, the as-prepared PEG-CNT/S composites exhibit excellent electrochemical performance, especially the high-rate capability and long-term cycle stability.


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Figure 1. Schematic diagram of the formation of the PEG-CNT/S composite.

2. EXPERIMENTAL SECTION 2.1 Preparation of PEG-CNT The multi-walled carbon nanotubes (diameter of 10-20 nm, Chengdu Organic Chemicals Co. Ltd.) were dispersed in the mixture of concentrated H2SO4/HNO3 (3/1, v/v) under sonication for 10 min to make the homogeneous suspension. Then it was maintained at 60 oC under continuously stirring for 6 h, followed by filtering and washing with deionized water until the pH was around 7.0. The precipitate was dried at 70 °C for 24 h to obtain the carboxylated MWCNTs (aCNTs). 0.5 g aCNTs, 0.05 g PEG-6000 and 10 mL concentrated H2SO4 were dispersed in 200 mL deionized water. The mixture was kept at 60 oC under constant stirring for 18 h to ensure the complete esterification between aCNTs and PEG. The resulting product was filtered and washed with deionized water for several times. After drying at 70 °C, the polyethylene glycol-grafted multi-walled carbon nanotubes (PEG-CNT) were obtained. 2.2 Preparation of PEG-CNT/S composites The PEG-CNT/S composites were synthesized by the combination process of liquid phase deposition and self-assembly. Typical procedure was as follows: 0.5 g PEG-CNT and 10 mL concentrated H2SO4 were added into 200 mL deionized water, followed by sonication and stirring for 2 h to obtain a well-dispersed suspension. Then 200 mL Na2S2O3 aqueous solution was added dropwise to the above suspension with continuous stirring for 2 h. The product was


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filtered and washed several times with deionized water to isolate the precipitate. Finally, it was dried at 60 oC under vacuum for 24 h to obtain the PEG-CNT/S composite. The sulfur loading of the composites was regulated by changing the amount of Na2S2O3. The content of elemental sulfur for three composites is determined by thermal gravimetric analysis (Figure S1): P-CNT/S-1 (67.8 wt%), P-CNT/S-2 (75.9 wt%) and P-CNT/S-3 (82.3 wt%). The mass proportion of PEG in PEG-CNT is 4.6%. For comparison, the PEG/S and aCNT/S composites were synthesized following the similar procedure as that of P-CNT/S-2, but without the presence of aCNT and PEG, respectively. The PEG/aCNT/S composite was also prepared by adding Na2S2O3 aqueous solution into the mixture of PEG and aCNT as the same component condition to P-CNT/S-2 without esterification reaction. 2.3 Characterization The morphologies of the samples were characterized by the field-emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-1400). The elemental mapping analysis was carried out by using an energy dispersive spectrometer (EDS, QUANTAX 400-30). Fourier transform infrared spectra (FTIR) were obtained with a Nicolet 6700 spectrometer using KBr pellets. The sulfur content of the composites was ascertained by a TGA/SDTA/851E analyzer under N2 flow at a heating rate of 10 o

C min-1. Raman spectra were recorded with Renishaw inVia+Reflex using a 50 mW He-Ne laser

operated at 514 nm. X-ray diffraction (XRD) patterns were performed in a Rigaku D/Max 2550 VB/PC X-ray diffractometer using Cu (Kα) radiation with the 2θ-angle recorded from 3-70°.


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2.4 Cells assemble and electrochemical measurements The cathode was prepared by mixing the synthesized materials, conductive agent (Super P) and the binder (LA132) in a weight ratio of 80:10:10 with deionized water/ethanol as the dispersant. After homogeneous mixing, the slurry was coated on aluminum foil current collector and dried under vacuum at 60 oC for 24 h. Subsequently, the electrodes were cut into disks with a diameter of 12 mm. The mass loading of sulfur was about 1.2-1.3 mg cm-2. The CR2032 coin-type cells were fabricated in an argon atmosphere glove box by using lithium metal foil as the counter electrode and the polypropylene membrane (Celgard 2400) as the separator. The electrolyte was 1 M bis-(trifluoromethane)sulfonimide lithium (LiTFSI) in dimethoxyethane (DME) and dioxolane (DOL) (1/1, v/v) with 1% LiNO3 as an additive. The galvanostatic charge/discharge test was carried out on a LAND CT2001A battery tester between 1.5 and 3.0 V (versus Li/Li+). The cyclic voltammetry (CV) (scan rate: 0.1 mV s-1; cut-off voltage: 1.5-3.0 V) and electrochemical impedance measurements (frequency range: 0.01-100000 Hz; amplitude: 5 mV) were conducted on a CHI660D electrochemical workstation.

3. RESULTS AND DISCUSSION In order to improve the dispersion of CNT in aqueous solution, the water-soluble polymer PEG is grafted onto its surface. The reaction route is shown in Figure S2. The FTIR spectra are performed to confirm the presence of PEG after the esterification reaction (Figure 2a). For the spectrum of aCNT, the band at 1718 cm-1 is assigned to the C=O stretching vibration, which indicates the introduction of carboxyl groups after the mixture acid treatment. In the case of


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PEG-CNT, the characteristic band at 2890 cm-1 corresponding to -CH2- bond vibration can be observed. Moreover, the band corresponding to C=O stretching vibration is shifted to 1730 cm-1. 60,61

Above results indicate that PEG is successfully grafted onto the surface of aCNT. Besides,

the decomposition of the PEG chains in PEG-CNT shows significant shift to the higher temperature than that of pure PEG in Figure S1, which also implys the formation of ester bonds owing to the intimate interaction. To evaluate the dispersion of crude CNT, aCNT and PEG-CNT in aqueous solution, the TEM analysis is employed. As shown in Figure 2b, the crude CNT tends to agglomerate and intertwine with each other, derived from the poor hydrophilia and wettability. After acidizing treatment, the carboxylated aCNT displays significantly loose and scattered architecture (Figure 2c). The subsequent graft reaction further make the PEG-CNT more decentralized with less mutual entanglement, indicating its excellent dispersion in aqueous solution (Figure 2d).


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Figure 2. (a) FTIR spectra of PEG, aCNT and PEG-CNT. TEM images of (b) crude CNT, (c) aCNT and (d) PEG-CNT (all the scale bars: 0.5 µm). The morphologies of PEG-CNT and PEG-CNT/S composites with different sulfur contents are characterized by FE-SEM analysis. As shown in Figure 3a, the PEG-CNT are entangled and interconnected. However, the PEG-CNT/S composites have the different morphologies after the incorporation of sulfur. The P-CNT/S-1 composite with lower sulfur content (Figure 3b) exhibits an aggregation structure stacked by the particles with 1-3 µm due to the mutual entanglement of


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large numbers of PEG-CNT. As the sulfur content is increased, the P-CNT/S-2 composite (Figure 3c) forms more uniform particulate structure with homogenous diameters of 1-2 µm. However, the morphology of P-CNT/S-3 (Figure 3d) shows some irregular clumps with larger size along with the sulfur content continuing to be increased. This difference may be ascribed to the large accumulation of sulfur particles or the formation of bulk sulfur in the reaction process due to the lack of PEG-CNT.

Figure 3. FE-SEM images of (a) PEG-CNT and PEG-CNT/S composites with different sulfur contents: (b) P-CNT/S-1, (c) P-CNT/S-2, (d) P-CNT/S-3 (all the scale bars: 10 µm). The high magnification FE-SEM and TEM images further reveal that the P-CNT/S-2


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composite exhibits a nest-like structure (Figure 4). It is clearly found that the granular P-CNT/S-2 composite is made of the sulfur core and the PEG-CNT shell. As a result, the PEG-CNT shell layer can not only accommodate the volumetric strain during the expansion of sulfur but also act as an immobilizer to relieve the loss of active materials. Furthermore, numerous intertwined PEG-CNT connecting the composite particles can be detected as shown in Figure 4a and 4c, which can play the role of conductive network to facilitate the charge transportation and improve the utilization of sulfur. Besides, the energy dispersive spectroscopic (EDS) mapping analysis of the P-CNT/S-2 sample indicates that elemental sulfur is distributed along with PEG-CNT network in particulate form, which is consistent with the above FE-SEM and TEM observation (Figure S3).


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Figure 4. (a) Low (scale bars: 2 µm) and (b) high magnification FE-SEM images of P-CNT/S-2 composite (scale bars: 2 µm). (c) Low (scale bars: 2 µm) and (d) high magnification TEM images of P-CNT/S-2 composite (scale bars: 0.5 µm). The Raman spectra of sulfur, PEG-CNT and P-CNT/S-2 are shown in Figure 5a. Similar to that of pure sulfur, the P-CNT/S-2 composite shows three sharp peaks at 153, 218, 472 cm-1, which are assigned to the characteristic signals of S8 species.68 This further confirms the presence of sulfur after the liquid phase deposition. Moreover, both of PEG-CNT and P-CNT/S-2 display two peaks at 1355 and 1584 cm-1 corresponding to D band (due to the disordered carbon) and G band (due to the graphitic carbon),69 respectively. The intensity ratio of D band and G band (ID/IG)


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calculated from the area is 0.96 in P-CNT/S-2, similar to that of pure PEG-CNT (0.97), which indicates that PEG-CNT has no obvious structural change after the introduction of sulfur. The XRD patterns have also been conducted to identify the structure of sulfur in the PEG-CNT/S composites. As shown in Figure 5b, elemental sulfur exhibits several sharp and strong peaks from 15o to 60o, indicating a well-defined crystal structure. The PEG-CNT displays two typical peaks at 26.0o and 42.6o, corresponding to the diffraction of the (002) and (100) plane of the carbon material,70 respectively. After the formation of the PEG-CNT/S composites, the characteristic peaks related to crystal sulfur can still be detected. And the intensity of these peaks are strengthened with the increase of sulfur loading. Besides, no other peaks can be observed in the composites, implying that the as-prepared samples do not contain any crystalline impurity. Furthermore, the TG analysis shows that the temperature of 5% weight loss (Td5) in the PEG-CNT/S composites increases about 25 oC compared with that of pristine sulfur (Figure S1). Such a result is probably due to the retarding effect of the PEG-CNT coating layer and the existence of intimate interaction between sulfur and PEG-CNT, indicating that the introduction of PEG-CNT triggers a strong confinement effect for improving the stability of sulfur.


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Figure 5. (a) Raman spectra of sulfur, PEG-CNT and P-CNT/S-2. (b) XRD patterns of sulfur, PEG-CNT, P-CNT/S-1, P-CNT/S-2 and P-CNT/S-3. To further understand the formation mechanism of the PEG-CNT/S composites with nest-like structure, various contrast materials, including PEG/S, aCNT/S and PEG/aCNT/S composite are investigated by FE-SEM analysis (Figure 6). The FE-SEM image of PEG/S shows that the as-prepared samples have irregular shapes and different sizes, indicating that the PEG could not controllably manipulate the morphologies of the resulting S particles (Figure 6a). For the aCNT/S composite, only a small amount of granular structures can be found in the aggregation of the amorphous sulfur and aCNT bundles as shown in Figure 6b (indicated by the white arrows). This may be ascribed to the poor dispersion of aCNT in the reaction system, limiting the generation of the coating layer for sulfur particles. Figure 6c shows that PEG/aCNT/S has a similar structural configuration as P-CNT/S-2, whereas much more amorphous aCNT bundles can still be detected (indicated by the red arrows). This may be due to that the addition of PEG can improve the dispersion of aCNT, but the effect is inferior to the adoption of PEG-CNT directly.


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Figure 6. FE-SEM images of (a) PEG/S, (b) aCNT/S and (c) PEG/aCNT/S composite (all the scale bars: 10 µm).


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Based on the above analysis, a schematic description for the formation of the PEG-CNT/S composite is shown in Figure 1. Resulting from the strong interaction between the sulfur molecule and the carbon surface,71,72 the elemental sulfur and CNT are easily combined with each other to generate a composite in aqueous solution by self-assembly. As the formation of sulfur particles, much more free PEG-CNT with perfect dispersibility are inclined to intertwine and coat on the surface of the sulfur particles to form the nest-like structure instead of amorphous aggregation under the driving force based on the pz-π* electron interaction between the sulfur and the CNT.71 At the same time, they serve as the template to further control the growth and suppress the reunion of the sulfur particles, leading to the enhancement of the regularity. In order to evaluate the structural benefits of the PEG-CNT/S composites for improving the cathodic performance, a series of electrochemical measurements are investigated. The typical CV curves of the PEG-CNT/S samples as the cathodes in the potential window of 1.5-3.0 V at a scan rate of 0.1 mV s-1 are shown in Figure 7. Two pairs of redox peaks can be observed in both curves of P-CNT/S-1 (Figure 7a) and P-CNT/S-2 (Figure 7b). The remarkable reduction peaks at 2.30 V and 2.03 V during the cathodic scan are associated with the conversion of elemental sulfur to soluble lithium polysulfides (Li2Sn, 4

Sulfur

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