Highly Ordered Mesoporous Sulfurized Polyacrylonitrile Cathode

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Highly Ordered Mesoporous Sulfurized Polyacrylonitrile Cathode Material for High-Rate Lithium Sulfur Batteries Ying Liu, Anupriya K. Haridas, Kwon-Koo Cho, Younki Lee, and Jou-Hyeon Ahn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06625 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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The Journal of Physical Chemistry

Highly Ordered Mesoporous Sulfurized Polyacrylonitrile Cathode Material for High-Rate Lithium Sulfur Batteries

Ying Liua, Anupriya K. Haridasb, Kwon-Koo Chob, Younki Leeb,*, Jou-Hyeon Ahna,b,*

a

Department of Chemical Engineering and Research Institute for Green Energy

Convergence Technology, Gyeongsang National University, 501 Jinju-daero, Jinju 52828, Republic of Korea b

Department of Materials Engineering and Convergence Technology and RIGET,

Gyeongsang National University, 501 Jinju-daero, Jinju 52828, Republic of Korea

*To whom correspondence should be addressed: Prof. J.H. Ahn: [email protected] (J.H. Ahn) Tel: +82-55-772-1784 Prof. Y. Lee: [email protected] (Y. Lee) Tel: +82-55-772-1688

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ABSTRACT

A highly ordered mesoporous sulfurized polyacrylonitrile (MSPAN) composite has been synthesized via in-situ polymerization of polyacrylonitrile (PAN) in an SBA-15 template followed by sulfurization. The synthesized composite possessed high sulfur utilization, high Coulombic efficiency, and excellent cycling stability as a cathode active material for high-rate lithium sulfur (Li-S) batteries. A highly ordered mesoporous structure was observed in the MSPAN composite from transmission electron microscopy. Excellent electrochemical and stable cycling performances of the MSPAN composite were obtained, especially at high C-rates. The capacity retention of the MSPAN cell was 755 mAh g-1 after 200 cycles at 1 C and 610 mAh g-1 after 900 cycles at 2 C. Even at a higher rate of 5 C, the composite showed reasonable capacity retention. The superior performance of MSPAN composite was attributed to its highly porous structure, which could effectively improve the wettability, accessibility, and absorption of electrolyte, facilitating rapid ion transfer in Li-S batteries. The electrochemical results demonstrate that the highly ordered mesoporous MSPAN composite is a promising cathode active material for advanced Li-S batteries.

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1. INTRODUCTION

Lithium ion batteries (LIBs) are traditional rechargeable power sources for commercial portable devices and electronics that are nearing their peak performance because of their energy density limitations that leave them unable to meet the enhanced energy demands of the present era. Currently, LIBs can hardly meet the requirements of high energy density, sufficient power density, and long service life for electric vehicles (HEV, PHEV, and BEV).1,2 Because of this, lithium sulfur (Li-S) batteries have received much attention as next generation power sources. Elemental sulfur is a promising cathode material that possesses not only a high theoretical capacity (1675 mAh g-1) and high energy density (2600 Wh kg-1) but also the advantages of low cost and environmental friendliness. However, the issues that arise from the insulating nature of sulfur, dissolved lithium polysulfide intermediates, and volume expansion of the electrodes during the lithiation process can lead to low utilization of the active material, severe capacity fading, and safety hazards.3,4 To overcome these limitations, many efforts have focused on the design of various structural matrices to encapsulate sulfur, such as microporous,5,6 mesoporous,7,8 hierarchical porous carbon composites,9-11 and porous metal-organic frameworks.12,13 Additionally, new electrode active materials have been developed to suppress the dissolution of the polysulfides and prolong the cycle life of lithium sulfur batteries. In 2003, Wang and co-workers reported a sulfurized polyacrylonitrile (SPAN) composite as the cathode in rechargeable lithium batteries with considerably good electrochemical properties for the first time.14 Henceforth, SPAN composite has attracted much attention as a cathode material for Li-S batteries because of its outstanding properties like high -3-



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sulfur utilization, high Coulombic efficiency, and excellent cycling stability (Table S1 in the Supporting Information). Nevertheless, some obvious shortcomings of SPAN composite have been highlighted, such as limited sulfur content in the composite (typically less than 50 wt%), poor rate capability, and the expansion/shrinkage of the SPAN cathode.15-17 To alleviate the aforementioned limitations, it is essential to design a good structure for SPAN composite. Tailored porous structures with optimal porosity and high specific surface area not only provide directed pathways for the transport of lithium ions but also enhance the electrical and ionic conductivities of the cathode. Therefore, a porous structure has become an ideal choice for improving the electrochemical performances of Li-S batteries.18 In this work, a new strategy was introduced to synthesize highly ordered mesoporous sulfurized PAN (referred to as MSPAN from here onwards) composite by the direct sulfurization of polyacrylonitrile (PAN) in an ordered mesoporous structured SBA-15 template. The detailed synthesis process is illustrated in Figure 1. MSPAN possessed a large surface area and highly ordered mesoporous structure, which could promote electronic and ionic transport and thereby enhanced the utilization of active material. In comparison with the non-porous SPAN (referred to as NSPAN from here onwards) composite, the electrochemical performances of MSPAN composite were remarkably higher, especially at high C-rates, as the highly porous structure improved the wettability, accessibility, and absorption of the electrolyte, facilitating rapid ion transfer in the electrode.19-21 The properties of the MSPAN composite were investigated in detail using X-ray diffraction (XRD), other techniques like FTIR and Raman spectroscopy, and cyclic voltammetry (CV).

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2. EXPERIMENTAL SECTION

2.1 Materials Poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol) (P123,

Mn

5800, Sigma-Aldrich), hydrochloric acid (HCl, 35.0-37.0%, Samchun Pure Chemical Co., Ltd.), hydrofluoric acid (HF, 48.0-51.0%, AvantorTM Performance Materials), tetraethyl orthosilicate (TEOS, 99.0%, Sigma-Aldrich), acrylonitrile (AN, 99+%, Sigma-Aldrich), azobisisobutyronitrile (AIBN, 98%, Junsei Chemical Co., Ltd.), sulfur (S, 99.5%, SigmaAldrich), polyacrylonitrile (PAN, average Mw. 150,000, Sigma-Aldrich,), and Nmethylpyrrolidone (NMP, 99.5%, Samchun Pure Chemical Co., Ltd.) were used as received. 2.2 Preparation of MSPAN and NSPAN composites SBA-15 was synthesized using a modified procedure from a previous report.22 P123 (4.0 g) was dissolved in 150 mL of 1.6 M HCl with stirring at 35 °C. An additional 8.5 g of TEOS was added into the resulting solution. After vigorous stirring for a short time, a milky mixture was obtained and then maintained under static conditions at 35 °C for 24 h. The mixture was placed in a furnace at 120 °C for aging, which was followed by filtration, drying, and then calcination at 550 °C for 6 h in air to give a white colored SBA-15 powder. The SBA-PAN composite was synthesized according to our previous work23 by the insitu polymerization of polyacrylonitrile (PAN) in the presence of an initiator (AIBN) with an SBA-15 template using the incipient wetness technique. To obtain the SBASPAN composite, the SBA-PAN composite was initially heated with elemental sulfur -5-



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using a weight ratio of 1:8 at 155 °C for 3 h followed by heating at 330 °C for 4 h and at 550 °C for 2 h under an argon atmosphere in a sealed glass tube. The MSPAN composite was obtained after removing the SBA-15 template by etching with HF, followed by washing and drying. For comparison, an NSPAN composite was synthesized by mixing PAN powder and elemental sulfur in the same ratio (1:8) followed by heating using similar conditions as the MSPAN composite. 2.3 Structural characterization Morphologies and internal structures were characterized by field emission scanning electron microscopy (FE-SEM, Philips XL30S FEG) and transmission electron microscopy (TEM, TF30ST-300 KV). The specific surface area and pore volume were obtained using Brunauer-Emmet-Teller analysis (BET, ASAP 2010). The characteristic peaks of MSPAN and NSPAN composites were measured with an FTIR spectrometer (SMART-APEX II ULTRA) and HR micro Raman spectrometer (LabRAM HR800 UV). The crystal structures of elemental sulfur and the NSPAN and MSPAN composites were measured using an X-ray diffractometer (D2 Phaser Bruker AXS). The sulfur contents in the synthesized composites were determined by elemental analysis (Flash 2000 Series, Thermo Fisher Scientific). 2.4 Electrochemical characterization For the cathode electrode, 80 wt% of active materials, 10 wt% of Super-P carbon black, and 10 wt% of poly(vinylidene fluoride) (PVDF) were homogeneously mixed in NMP. The resulting slurry was coated on the Al current collector and vacuum dried at 60 °C for 12 h. Subsequently, a 10 mm diameter circular disc was cut. MSPAN and NSPAN -6-


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electrodes with similar thickness (~75 µm) and the loading of active material (2.45 and 2.24 mg cm-2) were chosen as the cathode materials for the study. Stainless steel (SS) Swagelok® cells were assembled in an argon filled gloved box, using a Li anode, Celgard® 2400 separator, 1 M lithium hexafluorophosphate (LiPF6) salt in ethylene carbonate/diethylene carbonate (EC/DEC, volume ratio = 1:1) as the electrolyte, and the cathode. The cells were cycled in the voltage range of 1.0–3.0 V using a WBCS3000 battery cycler (WonA Tech. Co.). Cyclic voltammetry (CV) measurements were conducted on the SS Swagelok® cell at a scan rate of 0.05 mV s-1 between 1.0 V and 3.0 V. Electrochemical impedance spectra (EIS) were obtained in the frequency range of 100 mHz–2 MHz at an amplitude of 5 mV by using an impedance analyzer (IM6, Zahner).

3. RESULTS AND DISCUSSION

The surface morphologies of the composites were observed by FE-SEM. The FE-SEM images of SBA-15, SBA-PAN, SBA-SPAN and MSPAN composites presented in Figure 2a-2d show a striking similarity, implying that the SBA-15 shape had been replicated completely by MSPAN. Therefore, the template was finely filled with PAN, and the chemical reaction of PAN with sulfur was achieved. The MSPAN composite particles also possessed uniform size and less aggregation, which could be beneficial to accelerate the reaction rate with lithium ions. In contrast, NSPAN composite particles (Figure 2e) possessed an irregular shape and aggregation, which could slow down the cell kinetics and could probably lead to a poor rate capability. The internal structure of MSPAN and NSPAN composites is shown in more detail in the TEM images in Figure 3. A typical SBA-15 structure with a unique hexagonal pore -7-



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shape (Figure 3a) and highly ordered long parallel mesoporous channels in the onedimensional space (Figure 3b) was observed in the MSPAN composite, which can be especially beneficial for accelerated ion transfer and complete utilization of the active material in the composite. However, the NSPAN composite as shown in the TEM images in Figure 3c-3d did not possess a pore structure. In addition, the uniform distribution of sulfur in the interior of the MSPAN composite was confirmed by elemental mapping with energy dispersive X-rays (EDS) as shown in Figure 4a-4f. The N2 sorption isotherms and pore size distributions for SBA-15, MSPAN, and NSPAN composites are presented in Figure 5. SBA-15 had a typical type IV isotherm and H1 type hysteresis loop, indicating a uniformly ordered mesoporous structure. Compared to the as synthesized SBA-15, the specific surface area and pore volume of SBA-PAN (Figure S1 and Table S2) are greatly decreased, which implies the pores of SBA-15 were filled with PAN. Moreover, after sulfurization, the specific surface area, pore volume and pore size are almost unchanged, which implies sulfur and SBA-PAN are chemically linked rather than being a physical mixture. After the template was etched, the MSPAN composite provided type I and type IV isotherms and an H2 type hysteresis loop, implying the existence of a mesoporous structure. The specific surface areas of the NSPAN and MSPAN composites were found to be 12.2 and 423.8 m2 g-1, respectively, while the pore volumes were determined to be 0.09 and 0.33 cm3 g-1, respectively. The aforementioned results were in agreement with the TEM images in which the mesoporous channels were evident. Details of the specific surface area, pore volume, and average pore size in the synthesized composites are shown in Table 1. In Table 2, the variation in the C/H and C/N ratios clarified the process of denitrification, dehydrogenation, cyclization, and aromatization in the synthesis of the -8-


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MSPAN and NSPAN composites. After sulfurization, the C/H ratio increased, implying the C-H bond was replaced by C-C or C-S bonds due to the formation of a polyaromatic structure. The C/N ratio was greater than 3, implying that the denitrification process had occurred at the higher reaction temperature. A detailed chemical analysis of MSPAN and NSPAN composites is shown in the FTIR and Raman spectra in Figure 6. The FTIR spectra shown in Figure 6a, a characteristic band at 2243 cm-1 represented the stretching vibration of C≡N, and the absorption band at 1452 cm-1 corresponded to the CH2 bending in the FTIR spectrum of PAN. For both the NSPAN and MSPAN composites, after heat treatment with sulfur and PAN, the formation of a hexahydric-ring structure containing C=C and C=N bonds was evident from the bands at 1600–1200 cm-1 and 802 cm-1.24,25 Moreover, even though the peaks at 939, 670, and 516 cm-1 were weak, they depicted the characteristic stretching of the C-S and S-S bonds,25 which indicated the complete chemical transformations of PAN to NSPAN and MSPAN composites. From the Raman spectra shown in Figure 6b, the peaks at 309, 380, and 935 cm-1 could reasonably be assigned to the C-S bonds of the NSPAN and MSPAN composites since C-S bonds clearly cannot be observed in elemental sulfur. Two peaks were also related to the D mode sp3 and G mode sp2 carbon bonds at 1327 and 1540 cm-1, respectively.24 Thus, both NSPAN and MSPAN composites were confirmed to have similar chemical characteristics. Figure 7 shows XRD diffraction patterns of sulfur, NSPAN and MSPAN composites. Sulfur had the typical orthorhombic crystalline diffraction peaks; conversely, both NSPAN and MSPAN composites exhibited a distinct broad (002) peak at around 25.0º, indicating a graphite-like π-stacking of the dehydrogenated six membered ring layers, similar to disordered carbon.26-28 It seems reasonable that the short –Sx– chains were -9-



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covalently bonded to the cyclized and dehydrogenated PAN backbones through the C-S bonds.28 The XRD diffraction patterns of NSPAN and MSPAN composites had overlapping shapes, which confirms the chemical nature of both. The reaction mechanism of the MSPAN electrode is illustrated through the CV curve shown in Figure 8a. During the first discharge, the voltage hysteresis observed might be related to higher energy input due to cleavage of S-S bonds.25,29 The reaction processes of the MSPAN cell were apparently different from those of the traditional Li-S batteries. Generally, traditional Li-S batteries have two cathodic peaks at 2.0 and 2.3 V that correspond to the reduction of S8 to higher order polysulfides (Li2Sx, 4 ≤ x ≤ 8) and their further reduction to the lower order polysulfides (Li2S2/Li2S), respectively. However, the MSPAN cell had cathodic peaks at 1.7 and 2.0 V, implying that lithium polysulfides were not formed in the cell probably because active sulfur existed principally as S3 or S2 attached to the adjacent carbon backbone.29 Accordingly, the shuttle phenomenon was eliminated in the MSPAN cell. The charge-discharge curves of NSPAN and MSPAN composites with similar sulfur content were compared. The 1st and 10th cycles of NSPAN and MSPAN at 0.1 C-rate are depicted in Figure 8b. The NSPAN and MSPAN composites delivered high initial discharge capacities of 1863 and 1775 mAh g-1, which were higher than the theoretical capacity of sulfur. The π-conjugated pyridinic carbon probably played a key role in the observed extra capacity for the composites.29 The cycling performances of the NSPAN and MSPAN composites both showed high Coulombic efficiency and excellent cycling stability at 0.1 C for up to 30 cycles (Figure 8c). Apparently, both the NSPAN and MSPAN cells showed similar capacity retentions at the low C-rate (0.1 C). Nevertheless, at higher C-rates, the MSPAN electrode stands out because of its superior - 10 -


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electrochemical performance. At a high rate of 1 C, the MSPAN electrode delivered an initial capacity of 1330 mAh g-1, and notable cycling stability was maintained from second cycle up to 200 cycles (Figure 8d). For comparison, the NSPAN electrode was tested using same conditions. The discharge capacity of the NSPAN cell increased gradually till 75 cycles, which indicates that the NSPAN electrode might need more time for the electrolyte to penetrate the cathode owing to non-porous and agglomerated structure. However, the stable cycling performance was observed from 75 to 200 cycles, which suggests the achievement of complete electrolyte percolation network within the electrode. The phenomenon often has been observed at high C-rate.30,31 Figure 8e shows that the MSPAN electrode exhibited exceptionally excellent electrochemical performance and cycling stability with a capacity retention of around 610 mAh g-1 and Coulombic efficiency of around 100% even after 900 cycles at the 2 C-rate. Even at a higher rate of 5 C, the MSPAN electrode delivered a capacity of 350 mAh g-1 (Figure 9). However, NSPAN electrodes hardly exhibited any effective capacity at the prescribed high C-rate (Figure 9). These results have completely proven that the MSPAN composite possessed enhanced electrochemical performances even under the extreme test conditions of high C-rates because of the high porosity and good accessibility of the composite, which led to rapid ion transfer through the electrode. Additionally, the large surface area of the MSPAN composite could disperse the high current density through the cathode while guaranteeing a high utilization of the active material.32 The surface morphologies of the MSPAN and NSPAN cathodes in the fresh state and fully charged state after 200 cycles at 1 C were observed by FE-SEM as shown in Fig. 10. The MSPAN and NSPAN particles in the electrodes could be identified clearly (Figure 10a and 10c). Even after the long cycling, MSPAN and NSPAN particles also can be observed as shown in Figure 10b and 10d. The surface morphology of NSPAN cathode - 11 -



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had little changes, showing an agglomerated surface owing to the continuous chargedischarge processes. Nevertheless, the structural integrity of the MSPAN particles was maintained even after many cycles of the reversible reaction (Figure 10b), which clearly indicates the superior electrochemical performance of the MSPAN composite. EIS spectra of MSPAN and NSPAN were conducted at the fully charged state after different cycles (Figure 11). The Nyquist plot is composed of the bulk resistance of the electrolyte, charge transfer resistance, and Warburg impedance. The electrolyte resistances at high frequency showed no significant differences between the initial and 10th cycle of electrodes, implying almost no change in the viscosity of the liquid electrolyte because no soluble polysulfides were generated during the lithiation and delithiation process. Furthermore, the depressed semicircle in the middle frequency region depended on the charge transfer process. The charge transfer resistances of MSPAN was clearly much smaller than those of NSPAN at both the initial and 10th cycle, implying that the porous structure of the composite could reduce the charge transfer resistance, which would greatly enhance the ion transfer.

4. CONCLUSIONS

A novel highly ordered mesoporous structure of sulfurized polyacrylonitrile composite was designed and applied as the cathode active material for Li-S batteries. The fundamental chemical structures of the synthesized MSPAN composite were investigated in detail by FTIR, Raman, and XRD measurements. The excellent Coulombic efficiency, electrochemical performance, and cycling stability of the MSPAN cell obtained, particularly at high C-rates, were attributed to 1) the highly ordered mesoporous structure - 12 -


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which enhanced the wettability, accessibility, and absorption of the electrolyte, facilitating rapid ion transfer in the Li-S battery and to 2) the high specific surface area that could disperse the current density through the electrode while simultaneously guaranteeing a high utilization of the active materials. A good rate capability was attained, signifying that the polarization of cells was suppressed since the porous structure provided easily accessible diffusion pathways for ion transport. The superior results of the highly ordered mesoporous MSPAN composite at high C-rates demonstrate that it is a promising cathode active material for advanced Li-S batteries.

ACKNOWLEDGEMENTS

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (No. NRF-2017R1A4A1015711).

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[20] He, G.; Evers, S.; Liang, X., Cuisinier, M., Garsuch, A.; Nazar, L.F. Tailoring Porosity in Carbon Nanospheres for Lithium-Sulfur Battery Cathodes. ACS Nano 2013, 7, 10920-10930. [21] Zhu, J.D.; Chen, C.; Lu, Y.; Zang, J.; Jiang, M.J.; Kim, D.; Zhang, X.W. Highly Porous Polyacrylonitrile/Graphene Oxide Membrane Separator Exhibiting Excellent Anti-Self-Discharge Feature for High-Performance Lithium-Sulfur Batteries. Carbon 2016, 101, 272-280. [22] Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024-6036. [23] Liu, Y.; Zhao, X.; Chauhan, G.S.; Ahn, J.H. Nanostructured Nitrogen-doped Mesoporous Carbon Derived from Polyacrylonitrile for Advanced Lithium Sulfur Batteries. Appl. Surf. Sci. 2016, 380, 151-158. [24] Frey, M.; Zenn, R. K.; Warneke, S.; Müller, K.; Hintennach, A.; Dinnebier, R. E.; Buchmeiser,

M.

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Easily

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Fiber-based

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Poly(acrylonitrile) as Li/S Cathode Material: Correlating Electrochemical Performance with Morphology and Structure, ACS Energy Lett. 2017, 2, 595-604. [25] Zhang, S.S. Understanding of Sulfurized Polyacrylonitrile for Superior Performance Lithium/Sulfur Battery. Energies 2014, 7, 4588-4600. [26] Yu, X.G.; Xie, J.Y.; Yang, J.; Huang, H.J.; Wang, K.; Wen, Z.S. Lithium Storage in Conductive Sulfur-Containing Polymers. J. Electroanalytical Chem. 2004, 573, 121-128. - 16 -


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Table Caption

Table 1 Specific surface area, pore volume and average pore size of SBA-15, NSPAN and MSPAN composites. Table 2 Elemental analysis results of NSPAN and MSPAN composites.

Figure Captions

Figure 1. Illustration of the sequential fabrication steps for MSPAN composite. Figure 2. FE-SEM images of (a) SBA-15, (b) SBA-PAN, (c) SBA-SPAN, (d) MSPAN and (e) NSPAN composites. Figure 3. TEM images of (a) MSPAN composite in the direction of the pore axis and (b) perpendicular to the pore axis; (c-d) NSPAN composite. Figure 4. (a) STEM image, and (b-f) EDS mapping of MSPAN composite. Figure 5. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of SBA-15, MSPAN and NSPAN composites (inset: magnification of MSPAN and NSPAN composites). Figure 6. (a) FT-IR spectra of MSPAN, NSPAN composites and pure PAN and (b) Raman spectra of MSPAN, NSPAN composites and sulfur. Figure 7. XRD patterns of MSPAN, NSPAN composites and sulfur. Figure 8. (a) CV profile of MSPAN cell (scan rate: 0.05 mV s-1, 1.0-3.0 V); (b) chargedischarge profiles of NSPAN and MSPAN cells; (c) cycling performance of NSPAN and MSPAN cells at 0.1 C-rate; (d) cycling performance and coulombic efficiency of MSPAN and NSPAN cells at 1 C-rate; (e) cycling performance and coulombic efficiency - 18 -


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of MSPAN cell at 2 C-rate. Figure 9. Rate performances of MSPAN and NSPAN composites. Figure 10. FE-SE morphologies of (a) fresh MSPAN electrode, (b) MSPAN after 200 cycles, (c) fresh NSPAN electrode, (d) NSPAN after 200 cycles. Figure 11. The EIS spectra of Li-S cells with MSPAN and NSPAN composites (inset: magnification of NSPAN and MSPAN composites).

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Table 1 Specific surface area, pore volume and average pore size of SBA-15, NSPAN and MSPAN composites. Sample

Specific surface area (m2 g-1)

Pore volume (cm3 g-1)

Average pore size (nm)

SBA-15

719.0

1.34

6.9

NSPAN

12.2

0.09

23.0

MSPAN

423.8

0.33

3.4

Table 2 Elemental analysis results of NSPAN and MSPAN composites. C

N

H

S

Compound

(wt %)

(wt %)

(wt %)

(wt %)

PAN*

67.9

26.4

5.7

0

3.0

0.99

NSPAN

35.58

12.62

0.54

48.89

3.29

5.49

MSPAN

35.84

12.67

0.60

45.87

3.30

4.98

* based on theoretical calculation

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C:N

C:H

(Ratio of Element)

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Figure 1. Illustration of the sequential fabrication steps for MSPAN composite.

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Figure 2. FE-SEM images of (a) SBA-15, (b) SBA-PAN, (c) SBA-SPAN, (d) MSPAN and (e) NSPAN composites.

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Figure 3. TEM images of (a) MSPAN composite in the direction of the pore axis and (b) perpendicular to the pore axis; (c-d) NSPAN composite.

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Figure 4. (a) STEM image, and (b-f) EDS mapping of MSPAN composite.

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Figure 5. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of SBA-15, MSPAN and NSPAN composites (inset: magnification of MSPAN and NSPAN composites).

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Figure 6. (a) FT-IR spectra of MSPAN, NSPAN composites and pure PAN and (b) Raman spectra of MSPAN, NSPAN composites and sulfur.

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Figure 7. XRD patterns of MSPAN, NSPAN composites and sulfur.

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Figure 8. (a) CV profile of MSPAN cell (scan rate: 0.05 mV s-1, 1.0-3.0 V); (b) chargedischarge profiles of NSPAN and MSPAN cells; (c) cycling performance of NSPAN and MSPAN cells at 0.1 C-rate; (d) cycling performance and coulombic efficiency of MSPAN and NSPAN cells at 1 C-rate; (e) cycling performance and coulombic efficiency of MSPAN cell at 2 C-rate.

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Figure 9. Rate performances of MSPAN and NSPAN composites.

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Figure 10. FE-SEM morphologies of (a) fresh MSPAN electrode, (b) MSPAN after 200 cycles, (c) fresh NSPAN electrode, (d) NSPAN after 200 cycles.

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Figure 11. The EIS spectra of Li-S cells with MSPAN and NSPAN composites (inset: magnification of NSPAN and MSPAN composites).

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TOC Graphic

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Highly Ordered Mesoporous Sulfurized Polyacrylonitrile Cathode

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