Sandwich-type porous carbon/sulfur/polyaniline composite as cathode material for high-performance lithium–sulfur batteries

Abstract

Sandwich-type porous carbon/sulfur/polyaniline (SPC–S–PANI) composite with active sulfur nanoparticles confined within porous carbon is prepared. As a cathode material for Li–S batteries, the SPC–S–PANI composite with over 60 wt% sulfur content delivers high reversible capacity up to 1335 mA h g⁻¹ for the first cycle and 834 mA h g⁻¹ maintained over 100 cycles at 0.1C with an high coulombic efficiency of 96.5%. The high performance is attributed to the rationally designed hierarchical structure, which resulted in increased electrical conductivity, and hampered the dissolution of lithium polysulfide and provided a large pore volume for sulfur impregnation. Based on these merits, this sandwich-type porous carbon/polyaniline sulfur cathode shows the great potential for application in high-performance lithium–sulfur batteries.

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1 Introduction

Rechargeable lithium–sulfur (Li–S) batteries have attracted significant attention due to their high theoretical gravimetric capacity of 1675 mA h g⁻¹ and energy density of 2600 W h kg⁻¹ (with a metallic Li anode).^1,2 From a practical perspective, sulfur is abundant on the earth, low-cost and environmentally benign compared with other traditional lithium metal oxide cathode materials, which makes the Li–S battery one of the most competitive candidates for next generation electrochemical energy storage. Unfortunately, although significant progress has been made, the practical application of Li–S batteries is still hampered by some major challenges: (1) sulfur has a very low conductivity of 5 × 10⁻³⁰ S cm⁻¹;^3,4 (2) the volume expands as large as 80% for sulfur fully converted to Li₂S;^5,6 (3) shuttle effect in the batteries leads to a severe degradation of cycle life. Such huge volumetric change of sulfur during the charge/discharge process leads to the fading capacity as a result of the pulverization of cathode electrode. The shuttle effect enables the dissolution of high-order polysulfide which further diffuses freely to the anodes and will be reduced to insoluble low-order polysulfide by the lithium metal. The low-order polysulfide will return to the cathode in a converse manner.^7,8

Enormous efforts have been dedicated to address these issues. For example, carbon/sulfur materials (e.g. ordered mesoporous carbon,^9–11 hollow carbon sphere,^12–14 and carbon nanotubes^15–19/nanofibers^20,21), polymer/sulfur composites^22–25 and metal organic frameworks^26–28 are used as a host to accommodate sulfur, all of which are in an attempt to improve conductivity and reduce the dissolution of lithium polysulfide. Among these materials, carbon materials with specific nanostructure have received much attention because of their optimized structure, high conductivity and electrochemical stability. Zhao et al. synthesized a carbon nanomaterial with tube-in-tube structure to impregnate sulfur, showing good cycling stability and excellent rate performance.²⁹ Li et al. have prepared a pomegranate-like material with encapsulation of sulfur in conductive carbon shells, which leads to a high utilization of active material.³⁰ Jung et al. encapsulated sulfur in the core of the hierarchical porous carbon by ultrahigh speed spays pyrolysis to prevent the fatal dissolution of the lithium polysulfide into the electrolyte.¹¹ These studies represent a substantial progress in the sulfur electrode materials design. It can be concluded that the electrode combined with highly conductive nanomaterial is necessary in designing hybrid materials with high performance. In addition to carbon nano-materials, conducting polymers open another possibilities for improving cycling life in the Li–S batteries due to their easy preparation and scale-up, mechanical structure, self-healing, and good electrical conductivity. Polyaniline is an interesting conducting polymer because it contains large amount of nitrogen functional groups and can be efficient to trap polysulfide lithium. However, polyaniline suffers from the limited electrical conductivity and large volumetric change which hinder its application in lithium sulfur batteries.^31–33 Also proper structure can prevent the intermediate product polysulfide dissolution, such as hollow sphere structure, sandwich structure and so on.^34–36 For example, Li et al. have prepared a sulfur–polyaniline–graphene nanoribbon composite with excellent cycling stability;³⁷ Gao et al. synthesized core–shell sulfur-wrapped polyaniline nanofiber composite present good cycling stability and excellent rate performance.³⁸ Therefore, it is essential to design special structure with porous carbon and conductive polymer composite materials for further decreasing of the specific capacity loss caused by the dissolution of polysulfide.

Herein, we presented the rational design and synthesis of a sandwich-type porous carbon/polyaniline sulfur composite by sulfur melt-diffusion and in situ polymerization. The PANI coated material displays remarkably improved cycling performances with the porous carbon materials, which indicates that the coated method can efficiently restrict the dissolution of polysulfide. Such sandwich-type porous carbon/polyaniline composite with micro-pore carbon core for active sulfur nanoparticles anchoring and intimate contacting the graphene as the electronic conductive layer as well as the sandwich-type matrix. The coating polyaniline acts as a network to limit the dissolution of polysulfide species and the accommodation of volume change during the charge/discharge. As expected, the as-prepared sulfur cathode shows a high loading and utilization of sulfur, moreover, it exhibits excellent cycling stability, compared with non-polyaniline coating electrodes.

2 Experimental section

2.1 Synthesis of sandwich-type porous carbon–sulfur–polyaniline (SPC–S–PANI)

The sandwich porous carbon sulfur (SPC–S) composite was prepared by incorporating the sublimed sulfur (Sigma-Aldrich) into the sandwich-type porous carbon via the melt-diffusion method. The synthesis details of sandwich-type porous carbon (SPC) were shown in ESI.† Typically, the SPC and sulfur (35 : 65 wt/wt) were mixed by grinding before heated to 155 °C for 15 h in argon filled vessel. The composite is named as SPC–S. Additionally, SPC–S–PANI was synthesized via in situ polymerization of aniline (AN) monomers in the presence of SPC–S. The dried SPC–S composite (100 mg) was milled and put into deionized water with violent stirring for 20 min to form a uniform suspension. Then, aniline dissolved in 1 M HCl (the weight ratio of SPC–S : AN is 2 : 1) was slowly added to the suspension, followed by stirring in an ice bath for 2 h. Simultaneously, the oxidant ((NH₄)₂S₂O₈) dissolved in 1 M HCl was added into the above solution with a mole ratio to aniline of 1 : 1. The stirring was conducted in an ice bath for 12 h. The prepared composite was filtered and washed with DI water several times to remove salts and impurities. After that, the products were dried at 60 °C in a drying oven for 12 h.

2.2 Materials characterization

The structure of as-prepared products were characterized by X-ray powder diffraction (XRD) patterns, recorded on a Philip X, Pert Pro MPP X-ray powder diffract meter with Cu K_α radiation (λ = 1.54 Å) at the scan rate of 3 deg min⁻¹ with a scan range from 10° to 70°. Materials morphology was characterized using a JEOL JEM-1010 transmission electron microscope (TEM) at 200 kV and scanning electron microscopy (SEM, JEOL JSM-6700 SEM). Thermo-gravimetric analysis (TGA) was conducted on a STA449C-QMS403C instrument from room temperature to 800 °C at a speed of 10 °C min⁻¹ in N₂.

2.3 Electrochemical measurements

The electrochemical measurements were carried out with 2025 coin cells assembled in an argon-filled glove box with lithium metal as the anode. The cathode was prepared by spreading a mixture of 80 wt% active materials, 10 wt% conductivity agent (BP2000) and 10 wt% binders (PVDF) onto Al foil current collector. The electrode was separated by Celgard2400. We used 80 μL of the electrolyte to assemble each cell, which is consisted of 1 M LiN(CF₃SO₂)₂ (LiTFSI) in a 1 : 1 v/v mixture of 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME) containing LiNO₃ (1 wt%). We used 80 μL of the electrolyte to assemble each cell. The electrolyte was purchased from Fosai New Materials Co., Ltd. (Su Zhou, China).

The discharge/charge performance of the cells were tested with a constant current rate of 0.1C (1C = 1675 mA g⁻¹) at a cut-off potential of 1.4–3.0 V under room temperature by Land battery test systems (LAND-V34, Land Electronic Co, Ltd, Wuhan). All the specific capacities are calculated based on the mass of sulfur. Sulfur contents in the composite accounting were 66 wt% and 60 wt% in SPC–S and SPC–S–PANI, respectively, with sulfur mass loading of 0.78–0.93 mg cm⁻² in the electrode. Cyclic voltammetry (CV) testing and electrochemical impedance spectroscopy (EIS) of the battery were performed on an electrochemistry working station CHI660.

3 Results and discussion

The strategy for fabrication of SPC–S–PANI is illustrated in Fig. 1. The hierarchical sandwich porous carbon (SPC) materials were synthesized through hydrothermal and chemical activation, and the microstructure of the SPC was characterized and shown in ESI Fig. S2.† The BET surface area for SPC was calculated to be 1612 m² g⁻¹. After incorporating the pure sulfur into the porous carbon via melt-diffusion method, the sulfur nanoparticles is well impregnated in the pores of SPC. Then in situ polymerization and deposition strategy were adopted for coating with polyaniline. Thus the hierarchical porous sandwich carbon with polyaniline coating was obtained. The SPC with high specific surface area (1612 m² g⁻¹) can ensure high sulfur loading without the reduction of electronic conductivity of SPC. Fig. 2 shows the microstructures of the SPC–S and SPC–S–PANI composite.

Fig. 1 Schematic representation of the as-prepared materials.

Fig. 2 (a) and (b) scanning electron microscopy (SEM) image of SPC–S. (c) and (d) SEM images of SPC–S–PANI. (e) and (f) TEM images of SPC–S and SPC–S–PANI, respectively.

For SPC–S composite, it is clearly observed that no sulfur particle is present in the surface in the SPC nano-sheets, as shown in Fig. 2a and b. For SPC–S–PANI composite, after in situ polymerization of polyaniline, it is clearly observed that the SPC–S–PANI sample becomes thick and rough in contrast to SPC–S, as shown in Fig. 2c and d. This indicates that the thin PANI layer was covered on the surface of SPC–S nano-sheets, which is also illustrated by TEM images in Fig. 2e and f. It is clearly observed that the surface of two composites exhibit different morphology, but SPC–S–PANI sample presents a rough surface as compared to SPC–S.

The energy dispersive X-ray spectroscopic (EDS) measurement of SPC–S–PANI was characterized as shown in Fig. 3a, it is found that C, N and S were dominant in the as-prepared composite and S is uniformly distributed in the matrix. The contents of sulfur in SPC–S and SPC–S–PANI were measured through thermo-gravimetric analysis (TGA) in nitrogen atmosphere. As shown in Fig. 3b, it indicates that sulfur loading in the SPC–S is about 66 wt% and SPC–S–PANI is about 60 wt% respectively. X-ray diffraction (XRD) patterns of the pure sulfur powder, pure SPC, SPC–S and SPC–S–PANI are presented in Fig. 3c. The pure sulfur exists in a crystalline state with an orthorhombic structure (JCPDS card no. 08-0248) in SPC–S and SPC–S–PANI. In addition, the SPC matrix shows a very weak and extremely broad (002) peak in the 20–30° range caused by the amorphous carbon covering the graphene nano-sheets. Also we calculated the crystallite size with Debye Scherrer formula, and the crystallite size is about 3–6 nm in accord with the SPC pore size in Fig. S2.† The Debye Scherer formula: D = Kλ/β cos θ, where λ denotes the wavelength of X-ray (λ = 1.5418 Å), θ is the diffraction angle of the corresponding peak, β is half-peak width and K equals to 0.89. It suggested that the sulfur is impregnated into the pores of carbon materials.

Fig. 3 (a) EDS mapping images of SPC–S–PANI and (b) thermo-gravimetric curves of SPC–S–PANI and SPC–S materials in N₂ with a heating rate of 10 K min⁻¹ (c) XRD patterns of the sublimed sulfur powder, pure SPC, SPC–S and SPC–S–PANI.

Fig. 4a and c show the cyclic voltammetry (CV) curves which were carried out between 1.4 and 3.0 V at a sweep rate of 0.1 mV s⁻¹ for the first, second and third cycles. The two main reduction peaks around 2.2 and 1.9 V are observed during the positive scan, which was due to the multiple reduction of sulfur in the presence of metal Li as anode. The peak at 2.2 V is attributed to the reduction of the elemental sulfur and the electrolyte to form lithium polysulfide (Li₂S_x, 4 < x < 8). And the other peak at 1.9 V is ascribed to the decomposition of the polysulfide's chain in lithium polysulfide to produce insoluble lithium sulfides (Li₂S₂ or Li₂S).

Fig. 4 CV curves of (a) SPC–S–PANI and (c) SPC–S electrodes. And the discharge/charge voltage of (b) SPC–S–PANI and (d) SPC–S electrodes at 0.1C.

These two changes correspond to the two discharge plateaus in the discharge/charge curves (Fig. 4b and d). In the oxidation process, a main peak located at about 2.6 V can be observed, which is attributed to the oxidation of Li₂S and Li₂S₂ to Li₂S₈ into long polysulfide or sulfur. In the second cycle, both the CV peak and the areas almost remain unchanged, indicating relatively good reversibility. The discharge/charge curve of the SPC–S–PANI at 0.1C is shown in the Fig. 4b. Notably, the cell could deliver a specific capacity of ∼1335 mA h g⁻¹ at the initial discharge process with a long discharge plateau, which is nearly 79.8% of the theoretical capacity of S; while the specific capacity of SPC–S is ∼1316 mA h g⁻¹, with the nearly 78.5% of the theoretical capacity of S (Fig. 4d). It is demonstrated that the sandwich porous carbon can act as a sulfur loading matrix and effectively trap the sulfur nanoparticles, maximizing the utilization of cathode.

To gain further insight into the improvement, the electrochemical impedance spectra (EIS) of freshly prepared cells were investigated at the open circuit potential from 1 MHz to 0.01 Hz. As shown in Fig. 5a, Nyquist plots of the two electrodes are composed of a semicircle at high-frequency region and a short inclined line in the low frequency region. The small semicircles in the high-frequency region could reflect the constant charge transfer resistance and a short inclined line in the low frequency could be attributed to the diffusion within the cathode. Obviously, the SPC–S–PANI cell has a smaller diameter than SPC–S, suggesting a lower charge transfer resistance. This is an evidence of the improved conductivity resulted from the PANI coating, as discussed before.²¹ It is well known that the improved conductivity of the electrode materials can enhance the rate performance. The rate capacity (Fig. 5b) of the SPC–S–PANI and SPC–S was measured at different current rate in the potential range of 1.4–3.0 V for cooperation. Notably, the SPC–S–PANI delivered outstanding rate performance with specific capacities of 1337, 1062, 911 and 586 mA h g⁻¹ at 0.05C, 0.1C, 0.5C and 1C (where 1C = 1675 mA g⁻¹), superior than SPC–S electrode which only delivered a discharge capacity of 1236 mA h g⁻¹ at 0.05C, 956 mA h g⁻¹ at 0.1C, 791 mA h g⁻¹ at 0.5C and 512 mA h g⁻¹ at 1C. The excellent capacity retention is attributed to the synergetic effect of polyaniline and hierarchical sandwich porous carbon; because both the SPC and PANI have good conductivity and outstanding adsorption properties to effectively entrap polysulfide during cycling. This good trapping capability of the SPC and PANI coating is also reflected by the excellent cycling performance and coulombic efficiency. As shown in Fig. 5c, the SPC–S–PANI electrode delivers initial discharge capacities of 1335 mA h g⁻¹ at 0.1C and 833 mA h g⁻¹ over 100 cycles at 0.1C with enhanced coulombic efficiency of 96.5%. By contrast, SPC–S exhibits initial discharge capacities up to 1317 mA h g⁻¹ and still maintains 594 mA h g⁻¹ over 100 cycles with enhanced coulombic efficiency of 93.5%. The excellent cycle performance of the SPC–S–PANI is attributed to the polyaniline coating on the surface of hierarchical porous nano-sheets carbon. After in situ polymerization, the layer of PANI on the surface of SPC–S can effectively mitigate the dissolution of polysulfide in the electrolyte. Also, the rough and elastic surface area of PANI not only can alleviate the sulfur volume expansion, but also improve the materials conductivity and facilitate better contact with the electrolyte. In fact, the sandwich-type porous layers itself not only acts as an excellent electronic conductivity matrix, but also serves as a container to encapsulate sulfur. The PANI contains many N functional group that can efficient prevent the dissolution of intermediates products. The N functional group can afford strong affinity to the polysulfide intermediates of Li ion during cycling test.^39–41 Also the covered PANI layer can act as a polysulfide reservoir and prevent the polysulfide from diffusion out of the cathode, which is effective to diminish the shuttle effect and to significantly improve the cycling the stability. Moreover, the excellent resiliency of the PANI layer accommodates the sulfur volume variation.

Fig. 5 (a) Nyquist plots of SPC–S–PANI and SPC–S cathodes in the frequency range of 0.01 Hz to 100 kHz. (b) Rate capabilities of SPC–S–PANI and SPC–S electrodes at various current rates. (c) Cycling performance of SPC–S–PANI and SPC–S electrode at 0.1C.

4 Conclusion

In conclusion, for the first time, we rationally designed and synthesized a sandwich-type porous carbon/polyaniline composite with active sulfur nanoparticles confined within porous carbon as cathode material for Li–S batteries. The as-prepared SPC–S–PANI composite with over 60 wt% sulfur content delivers high reversible capacity up to 1335 mA h g⁻¹ at first cycle and 834 mA h g⁻¹ over 100 cycles at 0.1C with high coulombic efficiency of 96.5%. The micro porous in the carbon and the sandwich structure can efficiently load the sulfur and improve the electron conductivity which leads to an outstanding electrochemical performance. It reveals good performance compared with SPC–S, because the coating polyaniline acts as a network to limit the dissolution of polysulfide species and the accommodation of volume change during charge/discharge. Based on the above merits, the as-prepared materials show excellent performance as cathode material for Li–S batteries. Our study provides a new strategy for the design and synthesis of high performance cathode material for Li–S batteries.

Acknowledgements

This work was supported by the One Hundred Talents Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China (no. 51342009), the Natural Science Foundation of Fujian Province (no. 2014J05027) and Science and Technology Planning Project of Fujian Province, Grant (no. 2014H2008), the National Natural Science Foundation of China (no. 21501173).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23943a

‡ These authors contributed equally to this work.

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