A CTAB-modified S/C nanocomposite cathode for high performance Li–S batteries

Abstract

Two unique S/C/Ni foam nanostructure electrodes are fabricated by a facile cetyltrimethyl ammonium bromide (CTAB) modified electrodeposition method. With this strategy, S nanodots and carbon nanocomposites (carbon nanotube (CNT) or reduced graphene oxide (RGO)) are directly deposited onto the Ni foams, which obviously increase the S loading and electron/ion conductivity of the whole electrodes. Ascribed to the unique structure, the hybrid electrodes exhibit balanced high performance, including: large reversible capacity, stable cyclability and high coulombic efficiency. Among them, the S/RGO/Ni foam nanostructure electrode exhibits better performance, with a high reversible capacity of 1421 mA h g⁻¹ initially and over 790 mA h g⁻¹ after 500 cycles with a very low capacity degradation rate of less than 0.09% per cycle and high coulombic efficiency of 98% at a high-rate of 0.5C.

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1. Introduction

Due to the rapidly growing global energy consumption and worsening environmental pollution, the development of sustainable green power sources has become an urgent demand in various fields such as electric vehicles, hybrid electric vehicles, and many other energy storage applications.^1–5 The Li–S battery as one of the most promising next generation energy storage devices, has exceptional advantages, such as a high theoretical capacity of the S cathode (1675 mA h g⁻¹) and Li anode (3860 mA h g⁻¹), abundance, low-cost and eco-friendliness of S,^6–13 thus attracting intensive attention in recent years. However, the wide-spread commercialization of Li–S batteries are restricted by some fundamental issues.^14,15 Their inferior cycle performance, low S utilization, and low coulombic efficiency are the major technical obstacles, mainly due to the insulating nature of S (5 × 10⁻³⁰ S cm⁻¹)¹⁶ and the high solubility of intermediate long-chain lithium polysulfides (Li₂S_n, n ≥ 4) in liquid electrolyte, thereby resulting in poor mechanical stability and severe capacity fading.^17–20

In order to solve the aforementioned problems with Li–S batteries, the strategies of impregnating S in a porous carbon or polymers,^21,22 and fabricating yolk–shell nanostructures with an internal void space for S-storage have been used to enhance the cathode conductivity and improve the electrochemical performance of S-based batteries.²³ However, low S content of composites still limits the practical application of Li–S battery. Therefore, it is important to prepare a cathode with high S content, meanwhile maintaining high electrode integrity during the cycling without electrode collapse. The previous studies have demonstrated that 3-dimensional Ni foams are attractive bifunctional current collector for Li–S batteries.^24,25 Ni foam substrates as current collectors have porosity and high conductivity, which can accommodate sulfur and carbon material for a practical cell. The substantial agglomeration of S may exist in Ni foam by the paste-absorption method.²⁵

It has been reported that electrodeposition is an effective technique to fabricate nanostructured material.^26–28 The electrodeposition method offers many advantages such as simplicity, low cost and controllable deposition, which can allow the well incorporation of sulfur into the 3-dimensional Ni foam and carbon structure due to the penetration of the electrolyte into the material interior. However, pure nanostructured S is prone to aggregate forming large particles in the Ni foam and exposed in the electrolyte completely, resulting in the dissolved polysulfide species shuttling between the cathode and anode, which limits the efficient utilization and stabilization of active material. In this paper, self-supporting nanostructured sulfur carbon composites were directly deposited on the collector by a facile one-step electrodeposition method, which can exclude the influence of PVDF. A novel kind of S/C/Ni foam nanostructure electrode was synthesized by a modified method that reduced graphene oxide (RGO) or carbon nanotube (CNT) and reactive S nanodots were aligned simultaneously onto the Ni foams as a high-performance cathode for Li–S batteries. During this process, we further used cetyltrimethyl ammonium bromide (CTAB) to modify the structure of nanocomposite to obtain a significantly improved cycle life. In this structure, S nanodots were coated and immobilized by CTAB-modified carbon materials which can protect S from dissolution, facilitate the easy access of electrolyte and provide effective accommodation of the volume change of S during the cycling processes. Among them, the CTAB-modified S/RGO/Ni foam nanocomposites electrode exhibits a high reversible capacity of 1421 mA h g⁻¹ initially and over 790 mA h g⁻¹ after 500 cycles at 0.5C with excellent cyclability and high-rate performance, which shows great potential as the advanced cathode for Li–S battery.

2. Experimental details

2.1. Material synthesis

Ni foam was immersed in a 1 M HCl solution for 20 min and cleaned with ultrasonic for 10 min to get rid of the possible surface oxide layer. Then conductive substrate was purified with acetone, deionized water and dried at 50 °C in a vacuum oven before the electrodeposition processes.

Synthesis of pure S cathode materials. the electrodeposition experiment was carried out in a two-electrode system at room temperature. Both working electrode and counter electrode were identical Ni foams. Na₂S·9H₂O was dissolved in deionized water to form a 0.1 M aqueous solution as working-solution. S nanodots on Ni foam were prepared through a constant current electrodeposition method and the current density was about 3 mA cm⁻². Cathodes with different S contents were easily obtained by different deposition time. The deposition time of 2.5 h was adopted.

Synthesis of CTAB-modified S/RGO nanocomposites. Graphene oxide (GO) was synthesized from natural graphite by a modified Hummers' method.²⁹ The prepared GO was dispersed in deionized water with ultrasonic for 0.5 h. Moderate CTAB was added into the mixture of 0.1 mg ml⁻¹ GO dispersion and 0.1 M Na₂S·9H₂O and then reacted for 2 h under magnetic stirring. And then the electrodeposition processes were similar to the above method conducted in a two electrode system. Meanwhile, the GO nanosheets covered on the surface of Ni foam transformed into RGO in the electrodeposition processes,³⁰ which would facilitate charge transfer during the charge/discharge reaction process of cathode.

Synthesis of CTAB-modified S/CNT nanocomposites. 0.5 g CNT and 0.5 g CTAB were added to 100 ml deionized water and then formed suspension through magnetic stirring for 0.5 h and ultrasonic for 0.5 h. The as-prepared suspension was dispersed into 500 ml 0.1 M Na₂S·9H₂O aqueous solution and sonicated for 0.5 h to form a uniform solution. The electrodeposition was carried out in a two-electrode system, as described above.

Then, the prepared cathodes were collected, washed for several times by deionized water, and then dried at 50 °C in vacuum for 24 h.

2.2. Structural characterizations

The structure of synthesized materials was characterized by X-ray diffraction (XRD, Rigaku Smartlab3) with Cu Kα radiation. The morphologies were observed by a scanning electron microscope (SEM, FEI Inspect F50), equipped with an energy-dispersive X-ray spectroscope (EDX). The valence states of samples were studied by X-ray photoelectron spectroscopy (XPS, ThermoFisher SCIENTIFIC) with Al Kα X-ray source (hν = 1486.6 eV).

2.3. Electrochemical characterizations

The area density of Ni is 0.03 g cm⁻² and the thickness of Ni foam is 0.5 mm. The mass of sulfur carbon composites can be obtained by the difference of mass value before and after the deposition. The total mass loading of active sulfur materials in electrode was approximately 0.7 mg cm⁻² calculated by the ratio of sulfur carbon according to thermogravimetric analysis (TG, SDT-Q600, Fig. 1b). The electrodes were prepared by cutting the Ni foams with S/RGO or S/CNT nanocomposites into regular squares, and then pressed under a fixed pressure. The sulfur content in electrode is about 0.5 mg with a certain electrodes area, and the mass of the conductive Ni foam substrates as bifunctional current collectors is about 0.02 g in each electrode. The coin cells were fabricated in an argon-filled glove box. Pure lithium foil was applied as the counter electrode and a Celgard 2400 separator was used to separate the cathode and anode with 1 mol L⁻¹ LiTFSI in DOL/DME (1 : 1 by volume) containing 1 wt% LiNO₃ addition as the electrolyte (80 µL). Galvanostatic charge and discharge measurements were taken on an NEWARE battery testing system with a voltage ranging from 1.5–2.8 V. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were conducted by RST 5000 and CHI 660 Electrochemical Workstation.

Fig. 1 (a) XRD patterns of pure S, S/RGO and S/CNT nanocomposites, with the standard data of S (PDF# 08-0247). (b) TG analyses of S/RGO and S/CNT nanocomposites.

3. Results and discussion

The XRD patterns of pure S, S/RGO and S/CNT nanocomposites are shown in Fig. 1a. The strong diffraction peaks at 44.5°, 51.8° and 76.4° correspond to the (111), (200) and (220) peaks of Ni (PDF# 70-1849), respectively. All the rest of diffraction peaks of these three samples can be indexed to the monoclinic S phase (PDF# 08-0247) and no other impurities are detected, indicating the high purity of as-synthesized products and the successful reduction of sulfide and the formation of highly crystalline phase during the electrodeposition processes. We further measured the XRD pattern of the cathode after cycling, and the formation of NiS has been clearly excluded (Fig. S1 in ESI†). The TG reveals that the S contents of S/RGO and S/CNT nanocomposites are 77.3% and 62.7%, respectively (Fig. 1b), by the volatilization of S at around 250 °C.³¹

The morphologies of the as-prepared nanocomposites were investigated by SEM. Fig. 2a shows the porous structure of Ni foam before electrodepositon at low magnification, which can be used as the backbone of the electrodes. Fig. 2b shows the SEM image of the pure S cathode. As can be seen, S tends to aggregate and form particles of size around 1 µm which randomly distribute on the surface of Ni foam by electrodeposition. Fig. 2c shows the morphologies of the S/RGO nanocomposites. It can be clearly seen that the RGO nanosheets are densely adsorbed on the surface of Ni foam, showing well-organized and macroporous structure. Such kind of structure can increase the total surface area and pore volume, which will be essential to enhance the S loading and ensure the good electrolyte infiltration and fast Li-ion transfer to achieve an outstanding high-rate cell performance. In addition, the RGO layer would retard polysulfide diffusion by coating S particles to improve the electrochemical reversibility. Furthermore, EDX mapping of C and S verify the uniformed distribution without substantial agglomeration of S (Fig. 3a). Similarly, the structure of S/CNT nanocomposite was observed. As can be seen in Fig. 2d, CNT are evenly anchored on the surface of Ni foam, forming a network. S nanoparticles are uniformly distributed on the CNT network, which was further confirmed by the EDX mapping of S (Fig. 3b). This CNT network is a conducting matrix for both electrons and Li-ions, which can make S nanoparticles more accessible to the electrolyte during the charge–discharge process. The uniform distribution of RGO and CNT was due to surfactant CTAB improved solution dispersibility.

Fig. 2 The morphologies of (a) pure Ni foam, (b) S nanodots, (c) S/RGO and (d) S/CNT nanocomposites.

Fig. 3 SEM image, elemental mapping and EDX spectra for the selected areas in (a) S/RGO and (b) S/CNT, showing the uniform distribution of element S, C, and Ni.

Raman spectroscopy is a powerful analytical tool to characterize the microstructure and vibrational properties. The Raman spectra of the obtained electrode materials are shown in Fig. 4. The strong peak at 1124 cm⁻¹ derives from Ni foam. There are two peaks at 430 and 460 cm⁻¹ observed for pure S and S/C nanocomposite cathodes, which can be attributed to the S–S band. Two strong peaks can be observed at about 1346 and 1599 cm⁻¹ for S/C nanocomposites, which can be assigned to the first-order D and G bands, respectively. It is known that the D band is related to the C–C stretching vibrations of the disordered sp³ carbon structure, while the G band corresponds to the C–C stretching vibrations of the planar sp² carbon structure as in graphite structures.³² The D/G intensity ratio of these bands signifies the disorder degree of carbon materials.³³ The intensity ratio in CTAB-modified RGO material (I_D/I_G = 0.75 < 0.85 of before reduction) indicates that the number of defects decreased, the order degree improved and GO reduced, which is helpful for enhancing the cathode conductivity and adsorption.³⁴ A new small peak was formed at about 630 cm⁻¹, which could be assigned to a –S–O–C bond (600–700 cm⁻¹).^35,36 Furthermore, the interactions between sulfur and graphene, and the presence of C–O, C–S and S–O bonds can be identified by XPS measurements (Fig. S2 in ESI†). This analysis confirms that there are strong physical and chemical interaction between RGO and S for anchoring S and intermediate polysulfide products. The D/G ratio of CTAB-modified CNT is 1.24, while the ratio of pure single-walled carbon nanotubes is very small, indicating a certain content of disordered sp² carbon structures, some amorphous carbon impurities and most of multi-walled carbon nanotubes.^37,38 Most elemental S can be adsorbed by the multi-walled CNT, therefore physical absorption plays an important role to anchor S and polysulfide in S/CNT nanocomposite. Strong physical and chemical interaction between carbon material and S contribute to increase the stability of cathode and enhance the cycling performance of Li–S batteries.

Fig. 4 Raman spectra of Ni foam, pure S, S/RGO and S/CNT nanocomposites.

In order to investigate the electrochemical performance of the composites as the cathode of Li–S battery, a series of electrochemical measurements were carried out. Fig. 5a–c display CV curves of the nanocomposites cathodes for the initial three cycles between 1.0 and 3.0 V at a scan rate of 0.5 mV s⁻¹. Two major reduction peeks and one oxidation peak are observed, which agrees well with previous reports about the multistep reduction mechanism of element S.^39,40 The reduction peak at high voltage (about 2.2 V) is the reduction process from elemental S to long chain polysulfides, while the other reduction peak (about 2.0 V) is associated with the reduction process from long chain of lithium polysulfides to insoluble short-chain lithium sulfides. The overlapping oxidation peak refers to the reverse process. The redox peaks of the pure S cathodes showed significant decrease in the sequence of charge–discharge cycle, owing to S particles exposed in the electrolyte completely and the serious shuttle of dissolved polysulfide species. Compared with the cell of pure S (Fig. 5a), the cells of S/RGO and S/CNT nanocomposites showed small polysulfide loss during cycling, based on nearly the same height of redox peaks (Fig. 5b and c), revealing that polysulfide species shuttle was effectively suppressed by the RGO layer and CNT-network and the reversibility and stability of electrodes were both improved.

Fig. 5 CV plots of (a) pure S, (b) S/RGO and (c) S/CNT nanocomposites, at a scan rate of 0.5 mV s⁻¹ with 1–3 V voltage windows. (d) The charge–discharge profiles of three composites at 0.1C.

The plateaus in the voltage profiles after the cyclic stability (Fig. 5d) match the peak voltages of CV curves for the reduction and oxidation cycles (Fig. 5a–c). Meantime, Fig. 5d shows that the S/RGO and S/CNT nanocomposites cathodes display excellent discharge capacity of 1408 and 1246 mA h g⁻¹ at 0.1C rate, respectively, indicating highly efficient S utilization due to the holistic rational design of the cathode structure.

Galvanostatic discharge–charge experiments were conducted to evaluate the capacity and cycling performance of S/C composites electrodes at a rate of 0.1C between 1.5 V and 2.8 V (Fig. 6). Notably, a high capacity of 1450 mA h g⁻¹ was achieved for S/RGO nanocomposites after the capacity nearly stabilizes. This value is much higher than that of pure S nanomaterial cathode. Remarkably, a high capacity of 1071 mA h g⁻¹ was obtained after 145 cycles. In the beginning, the capacity of the battery shows the tendency of growth, which is likely due to the activation of the battery and the gradual enhancement of the S utilization on the following several circles. The battery of S/CNT nanocomposites delivered an initial discharge capacity of 1299 mA h g⁻¹ and stabilized at about 1000 mA h g⁻¹ after cycling for several times. In contrast, the cycle performance of pure S cathode clearly decreased with increasing the times of cycling. When the surfactant and conductive carbon materials (RGO and CNT) were employed, the cell cyclability was significantly improved. After 145 cycles, the capacity of S/RGO and S/CNT are still 1071 and 671 mA h g⁻¹, while pure S is only 300 mA h g⁻¹, representing the superior ability for immobilizing polysulfides within the cathode region of the cell. Moreover, a high Coulomb efficiency of more than 96% is kept during the cycling processes, which leads to the good capacity retaining, indicating the excellent ability to suppress the shuttle effect. The oxygen functional groups in RGO can improve the cycle performance of the battery, as some papers already pointed out that oxygen can trap the polysulfide.³⁹

Fig. 6 (a) The cycling performance of pure S, S/RGO and S/CNT composites at 0.1C and the Coulombic efficiencies of CTAB-S/RGO and CTAB-S/CNT composites. (b) The corresponding rate performance at various current rates.

Rate capabilities of S/RGO and S/CNT were also measured to examine the tolerance at high current rates. The cathodes were cycled at various current rates from 0.2C to 2C (Fig. 6b), S/RGO and S/CNT nanocomposites delivered high reversible capacities. The S/RGO and S/CNT can exhibit 1425 and 1300 mA h g⁻¹ at 0.2C during the first 10 cycles. Even at a high current rate of 1C, S/RGO and S/CNT can still exhibit 738.1 and 717.4 mA h g⁻¹, respectively, showing extraordinary high rate performances. More importantly, the capacities of S/RGO and S/CNT nanocomposites are recovered when the rate is turned back to 0.2C, which is about 95% capacity retention compared with the initial values. However, pure S exhibits worse C-rate discharge performances at high rates. The capacity of pure S is only 556 mA h g⁻¹ at a high current rate of 1C.

When the current density increased to 0.5C, the cathodes were further tested for long-term cycling. As shown in Fig. 7, the S/RGO and S/CNT nanocomposites cathodes still exhibit excellent cycling performance after 500 cycles. Therefore, the results of cells show significant progress, not only on the stabilized coulombic efficiency (high efficiency above 98%), but also on the capacity retention. It should be noted that the electrolyte with LiNO₃ can improve the coulombic efficiency rate and the electrochemical performances. The unstable coulombic efficiency rates were found in the beginning of cycles, which is only about 95% with LiNO₃-free electrolyte (Fig. S3 in ESI†). Another interesting phenomenon is that at the beginning of 0.1C cycling processes, the capacity increases with increasing cycle number. This is possibly attributed to the initial wetting process between electrode and electrolyte and a consequent maximization of the cathode utilization.³¹ There is a activation process in the initial state of charge and discharge processes, and the high current rate charge/discharge might accelerate the activation process thus higher initial capacity at 0.5C.^31,41 The cells with S/RGO and S/CNT nanocomposite cathodes show very low capacity fading rate of less than 0.09% and 0.12% per cycle at 0.5C, and high discharge capacity of 794 and 582 mA h g⁻¹ after 500 cycles, respectively. This unique structure of S/C nanocomposites can not only minimize the volume change of S but also provide the high electrical conductivity of electrodes owing to the homogeneous distribution of carbon materials.

Fig. 7 Long-term cycling performance and coulombic efficiency of pure S, S/RGO and S/CNT nanocomposite cathodes at 0.5C.

To further explore the mechanism of enhanced electrochemical performances of S/RGO and S/CNT nanocomposites, we performed electrochemical impedance spectroscopy (EIS), as shown in Fig. 8. The spectra were analyzed and fitted with an equivalent circuit model. R_e represents the impedance of the electrolyte and C_PE represents the capacitance of the electrical double layer. R_ct is the charge-transfer resistance, reflecting the resistance of the electrochemical reaction at the boundary of electrode–electrolyte.⁴² From the fitting results of Zview by equivalent circuit, we can find that the charge-transfer resistance of S/RGO (48.7 Ω) and S/CNT (41.5 Ω) are lower than pure S (64.7 Ω), owing to the high electrolyte absorption capability of carbon materials and its homogeneous distribution, which enhanced not only the Li⁺ ion but also the electron transportation. As a result, the rate capabilities and discharge capacity were significantly increased.

Fig. 8 EIS spectra of cells with pure S, S/RGO, and S/CNT cathodes. The inset is the equivalent circuit used to fit the impedance spectra.

The solution with CTAB has good dispersion (Fig. S4a in ESI†). Moreover, CTAB is one kind of cationic surfactant used to modify the surface functionality of carbon nanomaterials and sulfur can be immobilized by the functional groups on the nanomaterials. In a controlled experiment, S/RGO nanocomposites were synthesized without CTAB and many of the RGO sediments were found at the bottom of the beaker during the electrochemical assembly processes (Fig. S4b in ESI†). The nanocomposites on the surface of Ni foam were unstable, so actives S should be naturally inclined to dissolve in liquid electrolytes, thereby resulting in poor mechanical stability and severe capacity fading (Fig. S5a and b in ESI†). CTAB can significantly affect the adsorption capability of sulfur when it is attached on the surface of carbon nanomaterials.^43,44 The structure of CTAB-S/C nanocomposites can be improved, with a high specific surface area and pore volumes. So, this could also improve the uniformity of the sulfur distributing on the RGO surfaces and increase the sulfur utilization.

In our work, 3-dimensional structure Ni foam served as 3-dimensional interlinked current collector for long range electron transfer, and CNT or rGO served as the short-range electron transfer pathway. The well combination of 3-dimensional Ni foam with hollow CNTs and RGO is an effective route toward a high utilization efficiency of the active materials. The 3-dimensional hierarchical structure not only ensures good conductivity between the sulfur and current collector but also can accommodate ultrahigh amounts of active materials for the applications in Li–S batteries.^39,45,46 The residual oxygen functional groups on the surface of rGO and CNTs provide active sites to anchor the sulfur particles and trap the soluble polysulfide intermediates within the electrode.³⁹ Therefore, a multifunctional, hierarchical, hybrid CTAB-S/C/Ni-based structure was achieved to meet both the demand for flexible devices and practical high-performance Li–S batteries.

4. Conclusions

In summary, S/RGO and S/CNT were simultaneously deposited onto Ni foams by a novel constant current electrodeposition strategy without binder. Excellent electrochemical performance has been achieved in these S/C nanocomposites cathodes. The RGO nanosheet structure increases the total surface area and pore volume. The hollow CNTs provide natural electron pathways and construct a 3-dimensional highly conductive porous network that largely improves the conductivity of cathodes. Strong physical and chemical interaction between carbon material and S are effective in immobilizing S and suppressing the dissolution of polysulfide. Moreover, RGO nanosheets and mesh CNT network can accommodate the volume change of the electrode during the Li–S electrochemical reactions. As a result, the charge transfer impedance of the cathodes is significantly decreased, leading to more remarkable S utilization, C-rate performance and cycle stability compared with the cathodes of pure S. Due to the advantages of RGO, such as good dispersion, high specific surface area, and functional group effect, the electrode with RGO displayed better performance than that with CNTs. The S/RGO composites exhibit a high reversible capacity of 1421 mA h g⁻¹ initially and over 790 mA h g⁻¹ after 500 cycles at high-rate of 0.5C.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51172044, 51471085, 51407029), the Natural Science Foundation of Jiangsu Province of China (BK20151400), China Postdoctoral Science Foundation (2012M520968), the International Postdoctoral Exchange Fellowship Program 2014 by the Office of China Post-doctoral Council, and the open research fund of Key Laboratory of MEMS of Ministry of Education, Southeast University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16361k

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