Encapsulating sulfur into highly graphitized hollow carbon spheres as high performance cathode for lithium–sulfur batteries

Abstract

Encapsulating sulfur into a highly graphitized hollow carbon sphere (GHCS) is proposed as sulfur cathode for the first time. After annealing the amorphous hollow carbon sphere (HCS) at a high temperature of 2600 °C, the obtained GHCS shows polyhedral morphology and few layers graphene characteristic with extremely low oxygen content. When used as sulfur cathode, the S@GHCS composite delivers a high discharge capacity of ∼800 mA h g⁻¹ at 4C rate and high capacity retention of 93.7% after 240 cycles at 1C rate, demonstrating much better rate capability and cycling performance compared to those of S@HCS composite.

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1. Introduction

Lithium–sulfur (Li–S) batteries have drawn ever-increasing attention as one of the most promising alternative power sources for next generation electric vehicles, due to their high energy density (2600 W h kg⁻¹), low toxicity and the low cost of sulfur.^1–6 However, the practical applications of Li–S batteries still face big challenges that need to be addressed urgently. The extremely low electric conductivity of sulfur (5 × 10⁻³⁰ S cm⁻¹ at 25 °C), along with the shuttle effects of Li₂S_n (4 ≤ n < 8) in liquid electrolytes^7,8 and the large volume expansion during charge–discharge process^9,10 leads to the low rate capacity and short cycle life. To address these issues, combining sulfur with varied carbon materials, such as carbon nanotubes,^11,12 porous carbon,^13–22 graphene oxide or graphene,^23–30 carbon nanofibers^31–33 and hollow carbon spheres,^34–44 has been widely adopted in recent years.

In the past five years, encapsulating sulfur into hollow carbon nanostructures^34–44 have been intensively studied due to their good electrical conductivity and well confinement of sulfur/polysulfides in the hollow structures. These studies mainly focus on tailoring the porosity,³⁹ pore size,³⁵ shell layers^36,37 and element doping^35,40 of hollow carbon spheres to understand the effects of these characteristics on the electrochemical performance of these composite electrodes. For example, Jayaprakash et al. first impregnated sulfur into a mesoporous hollow carbon sphere via multiple vapor phase infusion process³⁴ and the obtained composite showed stable cycling performance for 100 cycles. Zhang et al. confined sulfur into double-shelled hollow carbon spheres as sulfur cathode³⁶ and it showed improved electrochemical performance with 690 mA h g⁻¹ retained after 100 cycles. Chen et al. prepared multi-shelled hollow carbon spheres as sulfur host,³⁷ the obtained nanocomposite delivered a high discharge capacity and excellent capacity retention due to the well confinement of polysulfides in the multi-shell structure. He et al. found that sulfur–porous carbon nanosphere could reach better cycling capacity and rate performance by tailoring the shell porosity of carbon sphere.³⁹ Zhou et al.^35,40 prepared nitrogen-doped hollow carbon nanospheres and tailored the pore size, they found the hollow carbon–sulfur composite with a pore size of 2.8 nm demonstrated best cycling stability with 88% capacity retention after 100 cycles.

However, these hollow carbon materials, which are annealed at low temperature less than 1300 °C, are usually amorphous, thus it cannot make full use of the high electric conductivity of the sp² hybridized carbon. Would the highly graphitized hollow carbon nanostructures be better sulfur host than the amorphous hollow carbon materials? Up to date, there has been no report to address this issue.

Herein, we proposed a highly graphitized hollow carbon sphere (GHCS) to encapsulate sulfur as high performance cathode for Li–S batteries. The GHCS was prepared by simply annealing the amorphous HCS at a high temperature of 2600 °C to create graphitization of the amorphous carbon shells, after that sulfur was infiltrated into the GHCS via an in situ solution deposition route to obtain S@GHCS composite. The highly graphitized carbon shells are able to provide fast electron transport and robust mechanical support and has extremely low oxygen species, moreover, the hollow structures are beneficial to load enough sulfur as well as confine the sulfur/polysulfides within the shells. These unique structural features enable much better rate performance and more stable cycle life of the S@GHCS composite compared with those of the S@HCS composite, which delivers a high discharge capacity of ∼800 mA h g⁻¹ at a high rate of 4C and a high capacity retention of 93.7% after 240 cycles at 1C rate.

2. Experimental section

Synthesis of HCS and GHCS

The HCS was prepared by a Stöber method.^45,46 First, a mixture of 9.6 ml ammonia aqueous solution (25 wt%), 144 ml ethanol and 96 ml DI water was stirred for 10 min, then 12 ml tetraethylorthosilicate (TEOS) and 48 ml ethanol were mixed well and added under magnetic stirring. Subsequently, 1.44 g resorcinol and 2.12 g formaldehyde solution (37%) was added to the solution and stirred for 24 h at room temperature followed by hydrothermally treated at 100 °C for 24 h in a sealed autoclave. The obtained brown powders were collected, dried and heated to 900 °C for 2 h under Ar atmosphere, followed by etching of 10 wt% HF solutions to obtain black HCS powders. The GHCS was obtained by annealing the HCS at a high temperature of 2600 °C under Ar protection in a graphitized furnace.

Preparation of S@HCS and S@GHCS

The S@GHCS nanocomposite was fabricated via an in situ solution method.⁴⁶ Briefly, 0.05 g S@GHCS was dispersed in an aqueous-ethanol solution and sonicated for 1 h, then 1.0 g Na₂S·9H₂O and 0.81 g Na₂SO₃ were dissolved in DI water and mixed with the above solution. Subsequently, 20 ml 1 M HCl solution and 6 mg PVP was added under magnetic stir for 2 h. Finally, the reaction solution was sonicated for 30 min and centrifuged with DI water, after drying at 65 °C for 12 h, the S@GHCS composite was obtained. The S@HCS nanocomposite was prepared through the same procedure, except replacing the GHCS with HCS. The sulfur contents in the composites could be calculated by the mass change before and after the preparation, which almost agrees with TG analysis (Fig. S4†). The S@HCS and S@GHCS are determined to be approximately 64 wt% and 62.5 wt%, respectively.

Materials characterization

The structure of the prepared samples were characterized by XRD (SIEMENS D-500) using Cu Kα irradiation. The micro-morphologies of the composites were studied using field emission scanning electron microscope (HITACHI S4800, Japan). TEM and STEM-EDS elemental mapping were tested with a FEI Tecnai 2100 instrument. The nitrogen adsorption–desorption analysis was measured at 77.3 K on a V-Sorb 2800 equipment. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an Axis Ultra (Kratos Analytical Ltd.) imaging photoelectron spectrometer using a monochromatized Al Kα anode.

The thermogravimetric analysis (TGA) was tested with a TGA-600 under N₂ atmosphere with a heating rate of 10 °C min⁻¹ to confirm the content of sulfur in the composite. The electric conductivity of the samples was measured using a four probe tester (RTS-8, Four Probes Tech.). Because the HCS and GHCS are nano-sized particles that are hard to be compressed as pellets, so we disperse the HCS or GHCS in 5% LA133 solution and paste in slide glass, then use the four probe tester to measure their electrical conductivities. It is noted that the electrical conductivity by this method is much lower than those using pellets.

Electrochemical characterizations

Electrochemical experiments were carried on using 2016 type coin-cells. The working electrodes were prepared by mixing sulfur cathode materials, Super P and LA133 aqueous binders (80 : 12 : 8 by weight) using isopropanol and water as the solvent. The slurry was stirred and pasted on Al foil and dried at 65 °C overnight. 0.5 M lithium bis-trifluoromethanesulfonylimide (LiTFSI) in a solvent of 1,3-dioxolane and 1,2-dimethoxyethane (DOL/DME, 1 : 1 by volume) with 0.5 M LiNO₃ (Fosai New Material Co., Ltd) was used as electrolyte, polypropylene membrane (Celgard Inc.) was used as the separators and lithium metal foil was used as the anode. Approximately 80 μl of electrolyte was used in the fabrication of the coin-cells. Galvanostatic charge–discharge tests were performed using a battery tester (LAND CT-2001A, Wuhan, China) in a potential range of 1.7–2.8 V (vs. Li⁺/Li) at room temperature at various current densities from 0.1C to 4C (1C = 1672 mA h g⁻¹). Cyclic voltammetry (CV) was measured with an electrochemical workstation (CHI 660C) between 1.7 and 2.8 V at a sweep rate of 0.1 mV s⁻¹. Electrochemical impedance (EIS) analyses were conducted using the same equipment from 100 mHz to 1 MHz.

3. Result and discussion

The resorcinol–formaldehyde@silica (RF@SiO₂) spheres were prepared by a Stöber method, which was previously reported.⁴⁵ Then the RF@SiO₂ spheres were annealed at 900 °C followed by HF acid etching to form amorphous HCSs. The amorphous HCSs were sintered at 2600 °C in a graphitized furnace to obtain the GHCSs, during the graphitization process, the amorphous carbon shells transformed into highly graphitized carbon shells. Finally, sulfur was encapsulated into the HCSs and GHCSs by an in situ liquid method⁴⁶ to obtain the S@HCS and S@GHCS nanocomposite, respectively. The synthesis procedures of the S@HCS and S@GHCS composites could be briefly illustrated in Fig. 1. The S@HCS composite was prepared as a control sample of the S@GHCS composite.

Fig. 1 Schematic diagram for the synthesis of the S@HCS and S@GHCS composites.

Fig. 2 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis of the HCSs and GHCSs. For the HCS, Fig. 2a shows the spherical shape with an average diameter of ∼150 nm, the TEM image (Fig. 2b) confirms the hollow structures of the HCS with a shell thickness of ∼11 nm, the HRTEM image (Fig. 2c) further confirms the amorphous carbon shell of the HCS. Fig. 2d shows the micro-morphology of the GHCS after high temperature graphitization, interestingly, the GHCS has a polyhedron morphology instead of the spherical shape, indicating the spherical shells of the HCS shrink and turn into straight during high temperature annealing. The average diameter of the GHCS is about 140 nm, which is smaller than that of the HCS. In Fig. 2e, the TEM image indicates the hollow structure with polygon figure of the GHCS, the thickness of the shell is measured as about 6 nm, which is much smaller than that of the HCS, indicating a shrinkage and densification process of the amorphous carbon shell. Compared with the disordered structure of the HCS shell in Fig. 2c, clear straight stripes can be observed in the HRTEM image in Fig. 2f, indicating the disordered carbon shells change into ordered graphite shells. The distance between the stripes is 0.332 nm, corresponding to the (002) plane of graphite structure. By counting the stripes number, we could conclude that the shell is consisted of 10–20 graphite layers, which is close to few-layers graphene with highly crystalline. The highly graphitized carbon shells with few graphite layers could provide rapid electronic transportation passway between sulfur and the GHCSs, which is highly important to achieve excellent rate performance and high sulfur utilization for Li–S batteries.

Fig. 2 Micromorphology of the HCSs and GHCSs: (a) SEM and (b) and (c) TEM images of the HCSs, (d) SEM and (e and f) TEM images of the GHCSs.

XRD and Raman spectra results further confirm the amorphous and graphitized nature of the HCS and GHCS, which are demonstrated in Fig. 3a and b, respectively. For the HCS, a broadened peak at 24° with low density appears in Fig. 3a, indicating an amorphous nature of the HCS. In contrast, the GHCS presents a sharp peak at 26.23°, showing the highly graphitized characteristic of the GHCS.⁴⁷ The graphitic content of the GHCS estimated from the XRD patterns is 62.8%, according to the following formula,⁴⁸

where d₀₀₂ is 0.3386 nm estimated from the XRD pattern. Fig. 3b shows the Raman spectroscopy of the HCS and GHCS. The HCS shows relatively larger intensity of D band near 1350 cm⁻¹ compared to those of the G band near 1575 cm⁻¹. After high temperature annealing, the D band intensity of the GHCS is reduced substantially and the G band intensity is increased sharply, moreover, the 2D band near 2670 cm⁻¹ is observed and has a higher intensity than that of the D band, indicating the formation of the few-layer graphene structure.⁴⁹ This implies the amorphous HCS is converted to highly graphitized GHCS with improved sp² hybridized carbon, which agrees well with the XRD results. X-ray photoelectron spectroscopy (XPS) was used to investigate the surface properties of the HCS and GHCS composites, as shown in Fig. S1.† Compared with the HCS, the oxygen peaks almost disappear from the survey spectra, indicating very low oxygen content in the GHCS. The element contents in the two carbon material are shown in Fig. S1,† the content of oxygen in GHCS is only 0.57 at%, much lower than that in the HCS (2.55 at%). The pore structures of the HCS and GHCS were further analyzed by low temperature nitrogen adsorption/desorption isotherm at 77 K, the results are shown in Fig. S2.† Fig. S2a and b† reveal that the pores <10 nm of the GHCS obviously decrease, and the distribution of macropores (80–150 nm) shift to a lower range (60–120 nm), indicating the shrinkage and densification of the amorphous carbon during graphitization process. Accompanied with the shrinkage and densification of HCS, the GHCS shows a lower specific surface area of 91.86 m² g⁻¹ and a smaller pore volume of 0.82 cm³ g⁻¹ compared to those of the HCS, which are 491.8 m² g⁻¹ and 2.75 cm³ g⁻¹, respectively. Considering the low sulfur content of the S@GHCS composite, the decreased specific surface area and pore volume of the GHCS will still be sufficient for sulfur infiltration into the pores of GHCS. The electrical conductivities of the HCS and GHCS, which were tested using a four probe tester method, are measured as 6.76 × 10⁻² and 1.52 × 10⁻¹ S cm⁻¹, respectively (Table S2†), showing much improved electrical conductivity after highly graphitization of the amorphous hollow carbon spheres.

Fig. 3 (a) XRD patterns and (b) Raman spectroscopy of the HCSs and GHCSs.

The SEM images of the obtained S@HCS and S@GHCS are shown in Fig. 4a and b. Both S@HCS and S@GHCS composites show similar morphology and structure compared to their pristine particles before sulfur encapsulating, no bulk sulfur can be found in the SEM images, demonstrating a homogenous distribution of nano-sulfur in the HCS and GHCS. However, the SEM image of the S@HCS nanocomposite (Fig. 4a) is obscure while the S@GHCS (Fig. 4b) shows clear spherical figures, indicating the enhanced electrical conductivity of the S@GHCS compared with that of the S@HCS. Fig. 4c shows the STEM-EDS elemental maps of the S@GHCS composite. The elemental mappings of carbon and sulfur demonstrate that sulfur was both attached on and confined within the GHCS shells. The micro- and meso-pores on the shells could make sulfide ions in the liquids adsorb on the surface and infiltrate inside the hollow shells of the GHCS and produce nano-sulfur at corresponding sites during chemical reaction, thus the obtained S@GHCS composite shows a homogenous micro-morphology. These results reveal that both high electrical conductivity and homogenous distribution of sulfur into hollow shells could be achieved using the GHCS as sulfur host. The sulfur in the S@HCS and S@GHCS both exist in a crystallized form according to the XRD patterns of the two samples (Fig. S3†), in agree with our previous report.⁴⁶ Thermo gravimetric analysis (TGA) was used to determine sulfur content in the sulfur–carbon composite, the results shown in Fig. S4† indicate that the contents of sulfur in the S@HCS and S@GHCS are approximately 64 wt% and 62.5 wt%, respectively.

Fig. 4 (a) SEM image of the S@HCS composite, (b) SEM image and (c–f) STEM-EDS elemental maps of the S@GHCS composite.

The typical cyclic voltammogram (CV) curves of the S@HCS and S@GHCS composite electrodes in different cycles are shown in Fig. 5. Both CV curves of the two electrodes show two well-defined cathodic and two anodic peaks, similar to previous reports.^29,46 However, for the S@HCS electrode, the intensity of the reduction peak at 2.02 V decreases gradually from the first cycle to the 10th cycle, whereas for the S@GHCS electrode, the redox peak at 2.02 V overlaps well for the initial, 2nd, 5th and 10th cycles. These results suggest the better stability and reversibility of the S@GHCS electrode compared to the S@HCS electrode, which is also conformed in the charge–discharge tests.

Fig. 5 Cyclic voltammogram (CV) curves at different cycles: (a) S@HCS and (b) S@GHCS composite.

The rate performance of the two electrodes under the galvanostatic mode in the range from 1.7 V to 2.8 V was tested from 0.1C to 4C (1C = 1672 mA h g⁻¹) rate, as shown in Fig. 6a. The discharge capacities of the S@GHCS composite electrode were stabilized at about 1100, 1010, 920, 870, 850 mA h g⁻¹ at 0.1, 0.2, 0.5, 1 and 2C rates, respectively. Even at a high rate of 4C, it showed a high discharge capacity of ∼800 mA h g⁻¹, and when the C rate was back to 0.1C, the electrode recovered the initial capacity, indicating both high rate performance and excellent reversibility of the S@GHCS composite electrode. Fig. 6b shows the charge–discharge profiles of the S@GHCS composite electrode at different C rates, these curves kept similar shapes with low overpotentials, two typical discharge plateaus can be observed even at a high rate of 4C. The rate capability of the S@GHCS was visibly better than that of the S@HCS composite electrode, probably due to the highly graphitized carbon shells which greatly enhanced the electrical conductivity of the S@GHCS composite.^34,47 Fig. S5† shows the EIS impedance of the fresh coin cells of the two electrodes, both curves show two semicircles: the high-frequency semi-circle should be assigned to the charge transfer process (R_ct) at the conductive matrix interface, the middle-to-low frequency semicircle corresponds to the liquid-to-solid transformation through insoluble Li₂S₂/Li₂S.^26,50 According to the fitting results (Table S1†), the R_ct of the S@GHCS composite electrode is about 5.74 Ω, much lower than that of the S@HCS composite electrode (14.48 Ω). The improved electrical conductivity is derived from the graphitization process: first, amorphous phase and defects in the HCS could decrease of the electric conductivity of the HCS, both increasing sp² hybridized carbon and reducing O content could effectively improve the conductivity of the carbon materials, moreover, the face-to-face contact between the GHCSs replaces the point-to-point contact between the HCSs, which also enhance the whole electrical conductivity of the highly graphitized GHCSs.

Fig. 6 (a) Rate performance of the S@HCS and S@GHCS electrodes, (b) charge–discharge profiles of the S@GHCS at different C rate, (c) cycling performance of the S@HCS and S@GHCS at 1C rate.

Fig. 6c shows the cycling stability of the S@HCS and S@GHCS composite electrodes at 1C rate. We could observe a slow capacity increase during the initial cycles, which should be ascribed to the slow activation process,^10,28 the nonpolar graphitized GHCS has a lower surface area and poorer wettability to the polar electrolyte, thus it takes longer time for the electrolyte to inflow the internal surface of the S@GHCS particles, at high current density, the charge or discharge process is rather short to allow good contact between sulfur and electrolyte, which may need several cycles to finish the activation process. The S@GHCS composite electrode showed an initial discharge capacity of 888 mA h g⁻¹ (Fig. S6†), then the discharge capacity slowly increased to 921 mA h g⁻¹ at the 50th cycle, afterwards, it showed very slow capacity decay and a reversible capacity of 832.6 mA h g⁻¹ was still maintained after 240th cycles, with a high capacity retention of 93.7%. In contrast, the S@HCS composite electrode delivered a slightly higher initial discharge capacity of 933 mA h g⁻¹, however, it decreased to 893 mA h g⁻¹ at the 2nd cycle and 809 mA h g⁻¹ at the 5th cycle, indicating a rapid capacity loss for the initial cycles. A discharge capacity of 670 mA h g⁻¹ can be obtained after 240 cycles, with 71.8% capacity retention, much lower than that of the S@GHCS composite electrode. It is noted that the coulombic efficiency of the two electrodes both are approximately above 98% during 240 cycles. The highly graphitized GHCS has more stable mechanical and electrochemical properties compared to the amorphous HCS, moreover, it contains fewer oxygen species, which is thought to lead unwanted side reactions and deteriorate the cycling performance.⁵¹

For the S@HCS composite, though it delivers favorable rate and cycling performance due to confinement of sulfur shuttling by the conductive amorphous porous carbon shells. However, after highly crystalline of the carbon shells, expected high electrical conductivity, excellent mechanical and electrochemical stability, more compact structure and fewer oxygen species could be achieved for the GHCS. These characteristics are favorable to lower the polarization and lead to better confinement of sulfur dissolution/shuttling, less unwanted side reactions and stable mechanical structure during cycling. The unique structure characteristics of the GHCS result in the high rate capability and stable cycling performance of the S@GHCS electrode.

4. Conclusions

In summary, we proposed a highly graphitized GHCS by simply annealing the amorphous HCS at 2600 °C as sulfur host for Li–S batteries. The obtained S@GHCS composite shows better rate capability with ∼800 mA h g⁻¹ at 4C rate and more stable cycle life with high capacity retention of 93.7% after 240 cycles at 1C rate compared to the S@HCS composite. The excellent electrochemical performance should be ascribed to the expected high electrical conductivity, excellent mechanical and electrochemical stability, more compact structure and extremely low oxygen species of the graphitized carbon shells. Our results indicate the hollow structure with highly graphitized carbon shells is a promising sulfur cathode for Li–S batteries.

Acknowledgements

This work was supported by the Research Project of National University of Defense Technology (ZDYYjcYj20140701).

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Footnote

† Electronic supplementary information (ESI) available: XPS, BET, XRD, TGA results and additional electrochemical properties such as EIS, charge–discharge curves and electrical conductivity measurement. See DOI: 10.1039/c6ra22652c

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