Sulfur–hydrazine hydrate-based chemical synthesis of sulfur@graphene composite for lithium–sulfur batteries

Jianmei Hanab, Baojuan Xi*a, Zhenyu Fenga, Xiaojian Maa, Junhao Zhangc, Shenglin Xionga and Yitai Qian*ad
aKey Laboratory of the Colloid and Interface Chemistry, Ministry of Education, and School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, P. R. China. E-mail: baojuanxi@sdu.edu.cn; Qianyt@sdu.edu.cn
bCollege of Chemistry and Chemical Engineering, Taishan University, Tai'an, 271021, P. R. China
cSchool of Environmental and Chemical Engineering and Marine Equipment and Technology Institute, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, PR China
dHefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, PR China

Received 19th November 2017 , Accepted 30th December 2017

First published on 2nd January 2018


Although the melt-diffusion method was applied to fabricate sulfur-based cathode for lithium–sulfur batteries, more efforts should be devoted to the development of synthetic methodology of sulfur-based hybrids. Herein, we report a sulfur–hydrazine hydrate chemistry-based method to prepare the composite of sulfur and N-doped reduced graphene oxide (S@N-rGO) with 76% sulfur content. Relying on the reaction of sulfur and hydrazine hydrate, cyclo-sulfur was broken to form soluble polysulfides. The subsequent refluxing with GO suspension rendered the transformation of soluble polysulfides to sulfur crystal homogeneously deposited on N-rGO due to direct nucleation from solution. Simultaneously, the nitrogen doping of GO was realized with hydrazine hydrate as doping agent in the one-pot route. The as-obtained S@N-rGO composite displayed a good rate capability and excellent capacity stability up to 300 cycles. This study may provide a new and facile method to construct the composite of sulfur and N-doped carbonaceous matrix, which makes the hybrid a competitive cathode material for lithium–sulfur batteries (LSBs).


1. Introduction

With the ever-growing demand for advanced electric vehicles and portable electronics, it is of high necessity to develop energy storage systems with high energy density. Lithium–sulfur batteries (LSBs) featured with the high theoretical specific capacity (1675 mA h g−1) and energy density (2600 W h kg−1) have recently attracted significant attention.1,2 Sulfur is considered as a promising cathode material owing to its abundance, low cost, and nontoxicity. However, the practical applications of LSBs are still challenging and prevented by several key issues associated with sulfur cathode. One is the poor electronic conductivity of sulfur and the discharge products (Li2S, Li2S2), which results in low specific capacity and poor rate performance.3–6 Another is the dissolution of intermediate Li2Sx products (4 ≤ x ≤ 8) that are formed during repeated electrochemical reactions, which leads to notorious shuttle effect, low Coulombic efficiency, and rapid capacity-fading.7–10 In addition, significant volume expansion (∼80%) during charge/discharge process may destroy the scaffold of the electrode, which accelerates the cycling fading.11

To address the aforementioned issues, one effective measure is to encapsulate sulfur with various carbon or conductive polymer materials.12 In particular, the porous carbonaceous materials with super conductivity and larger specific surface area, including micro/mesoporous carbon,13–15 carbon nanotubes,16,17 graphene18,19 and hollow carbon spheres20,21 not only strengthen the electronic conductivity of the sulfur cathode, but also buffer the volume effect and hinder the dissolution of sulfur. Except the non-polar carbon, a polar matrix can effectively enhance the sequestration of polysulfides, such as heteroatom doped carbons,22–25 primarily relying on the chemical bonding between them. Moreover, the hierarchical structures composed of carbon or other polar materials are devised for dual chemical/physical confinement of polysulfides.26,27

Currently, the preparation of sulfur-based composite cathodes primarily relies on steps combining mechanical mixing and thermal annealing. However, thermal treatment exhibits many disadvantages, such as time-/energy-consumption, formation of large pieces of aggregates and non-uniformity of sulfur within the carbon matrix. In contrast, chemistry-based deposition is applied to immobilize sulfur in various carbon materials, which is not only cost-effective, but can also control the size and homogeneity of sulfur coverage.28 For example, Wang et al. reported the synthesis of sulfur–graphene nanoribbon composite, in which sulfur originated from the oxidation of thioacetamide.21 Zhang et al. used sodium polysulfide as the sulfur source under acidic environment to prepare graphene oxide–sulfur composite.29 A facile approach was used to prepare a precursor of nitrogenous carbon coated ZnS from a rubber vulcanization accelerator, followed by the in situ oxidization by iodine to obtain a sulfur/nitrogenous carbon composite.30 Chemistry-based methods can effectively engineer sulfur particularly in terms of sulfur morphology and size control because of the in situ reaction in solution. Chen et al. prepared cathode materials through sulfur/amine-based chemistry method, in which ethylenediamine (EDA) and sulfur formed S–EDA precursor solution.31 Even though some efforts have been devoted to design the chemistry-based strategies to tune sulfur cathodes, more studies should be carried out to develop the methodology with the aim to improve the electrochemical performance of final product.

Herein, we report a sulfur–hydrazine hydrate chemistry-based method to prepare the composite of sulfur and N-doped reduced graphene oxide (S@N-rGO) as cathode materials for Li–S batteries. The sulfur hybridization and nitrogen doping of GO were achieved in the one-pot route, in which the hydrazine hydrate played a dual role of solvent for sulfur and a reductive/doping agent for GO. It was found that the sulfur can be recovered from the solution under reflux at 95 °C without the help of hydrochloric acid.31 By virtue of sulfur nucleation directly from solution, sulfur can uniformly distribute within the matrix of N-rGO, contributing to high capacity and long cycle stability of S@N-rGO.

2. Experimental section

2.1 Preparation of GO

Graphene oxide (GO) was synthesized from natural graphite powder (325 mesh, 99.95%, Aladdin) using the modified Hummers method.29 First, 1.0 g of graphite powder and 0.5 g sodium nitrate (AR, Sinopharm chemical reagent Co. Ltd) were mixed with 23 mL sulfuric acid (98%, Laiyangkangde chemical Co. Ltd) in an ice bath under vigorous stirring. Then, 3.0 g potassium permanganate was slowly added to the suspension at 20 °C. Successively, the temperature was raised to and maintained at 35 °C for 2 h. Following this, 46 mL ultrapure water was added dropwise, and the system temperature was further raised and maintained at 98 °C for 30 min. Finally, 140 mL of water and 20 mL of 30% hydrogen peroxide (AR, Sinopharm chemical reagent Co. Ltd) were added under stirring. After filtration and washing, the obtained GO aqueous dispersion was put into a dialysis bag for 2 weeks to remove the residuals. Finally, the GO aqueous suspension with a concentration of 1 mg mL−1 was sonicated for 2 h at room temperature for the subsequent syntheses.

2.2 Synthesis of the composite of S@N-rGO

First, 200 mg sublimed sulfur (Aladdin, 99.95%) was added into 0.6 mL hydrazine hydrate (85%, Sinopharm chemical reagent Co. Ltd) to form a dark red sulfur–hydrazine hydrate precursor solution. Second, the above red solution was slowly introduced into 100 mL of the pre-fabricated GO aqueous suspension and magnetically stirred for 1 h. Third, the mixed solution was transformed into a 150 mL round-bottom flask and refluxed at 95 °C for 2 h. The S@N-rGO composite was collected after filtration, washed with ultrapure water and freeze-dried for 24 h.

2.3 Preparation of a composite of S/N-rGO

For comparison, a control sample of S/N-rGO was prepared by the common melting-diffusion method. Typically, 100 mL of GO aqueous suspension was mixed with 0.6 mL hydrazine hydrate and refluxed in a 150 mL round-bottom flask at 95 °C for 2 h. The obtained N-rGO sample was washed and freeze-dried to get power. The introduction of a certain amount of sublimed sulfur was followed by ball-milling and thermal treatment at 155 °C for 12 h.

2.4 Electrochemical measurements

The working electrode was prepared by mixing the related active materials with acetylene black and polyvinylidene difluoride in a weight ratio of 65[thin space (1/6-em)]:[thin space (1/6-em)]25[thin space (1/6-em)]:[thin space (1/6-em)]15 with N-methyl pyrrolidone as a dispersant to form a slurry by ball-milling. The slurry was then coated on aluminum foil and dried at 50 °C for 24 h, followed by cutting into discs with a diameter of 12 mm. The areal sulfur loading of each disc was about 1 mg cm−2. R2016-type coin cells were assembled in an Ar-filled glove box with the concentration of moisture and oxygen below 0.1 ppm. Lithium foil and Celgard 2400 were used as the anode and the separator, respectively. The electrolyte was 1 M Li-bis-trifluoromethanesulphonylimide (LiTFSI) salt dissolved in 1,3-dioxolane and 1,2-dimethoxyethane (DME) (volumetric ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1) with 2 wt% of LiNO3. The galvanostatic charge–discharge measurements were carried out on a coin-cell test system (CT2001A LAND) at 30 °C at a cut-off voltage of 1.7–2.8 V. The cyclic voltammograms (CV) were detected using a CHI760E (Chenhua, Shanghai) electrochemical workstation at a scan rate of 0.1 mV s−1. Electrochemical impedance spectra (EIS) were also obtained on a CHI760D electrochemical workstation over a frequency range of 100 kHz to 0.01 Hz.
Visualized polysulfide adsorption study. The Li2S4 solution was prepared by dissolving stoichiometric mixture of sulfur and Li2S in solvent of DME and vigorously stirring at 50 °C for about one week. Then, an appropriate amount of N-rGO was added into prefabricated 5 mL of 5 mmol L−1 Li2S4/DME solution. The mixture was stirred for about 2 h and aged to form precipitate.

2.5 Materials characterization

The morphology and phase of products were characterized using field-emission scanning electron microscopy (FESEM, Zeiss G-500), transmission electron microscopy (TEM, JEOL JEM-1011) and X-ray powder diffraction (XRD, Bruker-D8 Advance, Cu Kα radiation with λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS, ESCALAB 250 spectrometer, PerkinElmer) was applied to obtain the chemical composition information. UV-vis absorption spectra were harvested on UV-2450 (Shimadzu Corporation). Thermal gravimetric analysis (TGA, Mettler Toledo TGA/SDTA851) was used to detect sulfur content at a heating rate of 5 °C min−1 under argon atmosphere. Raman spectra were recorded at ambient temperature on a Leica DM2700 microscope with a YAG laser (50 mW at 532 nm).

3. Results and discussion

In this study, the composite of S@N-rGO was synthesized based on a chemistry-based method. As illustrated in Fig. 1, the precursor solution of sulfur–hydrazine hydrate was prepared by the reaction of sublimed sulfur and hydrazine hydrate. The corresponding digital graph of the precursor solution is shown in Fig. S1, exhibiting the remarkably deep-red color, while pure hydrazine hydrate is colorless. The solution of hydrazine hydrate and sulfur or selenium had been used for the synthesis of metal sulfide and selenide.32,33 The species of sulfur in hydrazine hydrate was also investigated. Similarly, in the amine-solvent, the direct nucleophilic attack of nitrogen opens the octatomic ring of S8 and forms open chain alkylammonium polysulfides instead of octatomic molecules.31,32,34 Fig. 1B shows the UV-vis absorption spectrum similar to the reported profile of sulfur in ethylenediamine.31 The two notable peaks at 318 nm as well as 398 nm and another weak peak at 614 nm (magnified in the inset in Fig. 1B) demonstrate the presence of polysulfide ions. Subsequently, the precursor solution was slowly poured into a GO aqueous suspension. During the subsequent process of refluxing, GO is reduced and doped by hydrazine hydrate. Moreover, sulfur molecules are gradually released and deposited on the reduced graphene. The hydrazine hydrate shows dual functions, acting as not only a solvent for sulfur, but also as a reductive and doping agent for graphene oxide.
image file: c7qi00726d-f1.tif
Fig. 1 (A) Schematic synthetic procedure for preparation S@N-rGO composite, (B) UV-vis adsorption spectrum of sulfur in hydrazine hydrate.

The microstructure of S@N-rGO was analyzed by the technique of FESEM. As shown in Fig. 2, no large aggregates of sulfur were observed on N-rGO matrix, demonstrating the effectiveness of the present method. The loose N-rGO shows the structure of sponge with a large amount of wrinkles, clearly observed in Fig. 2B–D. Further confirmation can be notably observed from TEM description shown in Fig. 2E–F, displaying the uniform distribution of sulfur synthesized by the present sulfur–hydrazine hydrate chemistry-based method. Fig. 2G shows the corresponding elemental mappings including C, N and S, demonstrating the homogeneous distribution. The control sample of S/N-rGO was also detected by FESEM and TEM to monitor the structure. As shown in Fig. S2A–B, the large bulk of sulfur and the discrete N-rGO sheets (marked by arrows) are separated from each other. In Fig. S2C–D, the area with high contrast exhibits large sulfur particles, which are notably different from N-rGO, in consistence with the FESEM results. The corresponding elemental mappings of S/N-rGO are presented in Fig. S3. The corresponding EDX spectrum of S@N-rGO is supplied in Fig. S4. The results clearly prove the separation of large bulk of sulfur and loose N-rGO matrix. The mechanical mixing of sulfur and matrix cannot guarantee the uniformity of sulfur on surface. In comparison, the method developed in our study exhibits excellent effectiveness.


image file: c7qi00726d-f2.tif
Fig. 2 Morphological and structural description of S@N-rGO composite: (A–D) FESEM images; (E, F) TEM images; (G) FESEM and the corresponding elemental mapping images of S@N-rGO composite. Scale bars: (A) 2 μm, (B) 1 μm, (C, D) 200 nm, (E) 50 nm, (F) 100 nm, and (G) 400 nm.

The XRD patterns of S@N-rGO and S/N-rGO are displayed in Fig. 3A. In comparison with sublimed sulfur, their patterns are similar and can be well indexed as orthorhombic cyclo-sulfur. The Raman spectrum was analyzed to further characterize S@N-rGO. As shown in Fig. 3B, the two well-defined peaks at 1320 and 1580 cm−1 are ascribed to the D and G bands of carbon, respectively, with a 2D band at 2640 cm−1. The former band is related to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice (in a graphene layer) and the latter band results from the disorders caused by the finite particle size effect or lattice distortion of the graphite crystals.35 The intensity ratio of D to G band, i.e., ID/IG, can provide determinable information on the degree of graphitization.36,37 High ID/IG implies a large amount of defects in the N-rGO sheets, originating from the high degree of reduction and doping of GO.38,39 TGA curves of S@N-rGO and S/N-rGO (Fig. 3C–D) under argon atmosphere can accurately provide the sulfur content in each sample, which is about 76% and 74%, respectively.


image file: c7qi00726d-f3.tif
Fig. 3 (A) XRD patterns S@N-rGO and S/N-rGO composites. (B) Raman spectrum of S@N-rGO. (C, D) TGA curves of S@N-rGO and S/N-rGO.

In order to confirm the surface profile of S@N-rGO hybrids, X-ray photoelectron spectroscopy (XPS) measurements were carried out. As shown in Fig. 4A, the existence of carbon, oxygen, sulfur and nitrogen elements is confirmed in the sample. Oxygen can be detected in the composite due to the presence of oxygen-containing groups. In the fitted C 1s profile (Fig. 4B), there are four peaks at 284.7, 285.3, 286.3 and 287.7 eV, corresponding to C–C, C–O/C–S, C–N–C and N–C[double bond, length as m-dash]O, respectively. The presence of C–N–C and N–C[double bond, length as m-dash]O bonds distinctly reveals that N atoms and ketone groups are embedded in the carbon lattice.40,41 The C–O/C–S species are believed to be formed during the reflux process and they effectively immobilize sulfur on the surface of carbon hosts.42 The S 2p photoelectron spectrum (Fig. 4C) is typical of various valence states of sulfur in the S@N-rGO hybrids. Two prominent peaks centered at 164 and 165.2 eV are observed, which are attributed to the binding energy of S 2p3/2 and S 2p1/2 of S8 molecules, respectively.43 Furthermore, two weaker peaks located at 163.7 and 164.7 eV prove the presence of S–O species, which prove helpful in anchoring the sulfur and polysulfides within N-rGO matrix during the electrochemical process and promote the cycle stability for LSBs.42,44 The singlet peak at 168.7 eV was assigned to the presence of sulfates, which were likely produced via the oxidation of sulfur during the reflux reaction.45–48 Curve-fitting of high-resolution N 1s spectrum (Fig. 4D) reveals the presence of pyridinic N (399.8 eV), pyrrolic N (400.9 eV), and graphitic N (401.8 eV); the nitrogen content is calculated to be about 2.86 at% based on the rGO mass. Among these N species, graphitic N atoms are fully bonded, which is favorable for conductivity improvement of carbon.49 N doping can provide additional adsorption sites to lithium polysulfides, improving the cycling performance.50


image file: c7qi00726d-f4.tif
Fig. 4 XPS analysis of S@N-rGO hybrids: (A) survey spectrum, (B) C 1s, (C) S 2p, and (D) N 1s.

Cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.1 mV s−1. Fig. 5A shows the CV curves of S@N-rGO-based cells in the potential range of 1.7–2.8 V versus Li+/Li for the first five cycles. Two cathodic peaks at approximately 2.22 and 1.9 V are observable in the first scan, which accord with the multistep reduction of S8 molecules into higher-order lithium polysulfides (Li2Sn, 4 ≤ n < 8) and the further reduction to short-chain Li2Sn (1 < n < 4) or sulfides (Li2S).51–53 In the subsequent anodic scan, the peak at 2.48 V is related to the oxidation of lithium sulfides to sulfur. It is worth noting that as the cycling proceeds, the two reduction peaks shift to higher potential (2.26 and 2.05 V) and oxidation peaks to lower potential, indicating a slight polarization with cycling. The CV peaks almost overlap in position and intensity after the first activation cycle, revealing good electrochemical reversibility and capacity retention.


image file: c7qi00726d-f5.tif
Fig. 5 (A) CV curves of the S@N-rGO electrodes at a scan rate of 0.1 mV s−1 in the potential range of 1.7–2.8 V versus Li+/Li for the first five cycles; (B) rate capability of S@N-rGO and S/N-rGO tested at different current densities; (C) cycling performance of S@N-rGO and S/N-rGO cathodes for 300 cycles at a current rate of 0.5C; (D) cycling performance of S@N-rGO cathode at a current rate of 1C.

The rate performance of S@N-rGO and S/N-rGO cathodes was evaluated at different current densities. As shown in Fig. 5B, the initial specific capacity of S@N-rGO was 1234 mA h g−1 at a rate of 0.1C. When the current rate was increased to 0.5C, 1C and 2C, the discharge capacity of S@N-rGO cathode reached ∼695, ∼628 and ∼568 mA h g−1, respectively. When the current rate was restored to 0.1C, the capacity was effectively restored to 760 mA h g−1. In comparison, the specific capacity of S/N-rGO cathode fades drastically from 1188 to 442 mA h g−1 as the rate increases from 0.1 to 2C.

Fig. S5 shows the galvanostatic charge/discharge voltage profiles of the S@N-rGO and S/N-rGO cathodes between 1.7 and 2.8 V versus Li+/Li at different rate densities. Both samples display a similar trend: as the current density increases, the voltage gap between charge and discharge plateau is magnified, implying that more severe polarization is caused by the sluggish kinetics of ions.54 However, the voltage plateau of S@N-rGO is much longer than that of S/N-rGO at each current density, originating from the superior ability of sulfur immobilization of the former as compared to the latter.

The cycling performance of the S@N-rGO and S/N-rGO was investigated at 0.5C. As shown in Fig. 5C, the S@N-rGO offers the initial discharge capacity of 1102 mA h g−1 and a reversible capacity as high as 626.7 mA h g−1 after 300 cycles, maintaining a capacity retention of 56.8% and a slow capacity decay rate of 0.14% per cycle. The initial Coulombic efficiency can reach 98.8% and after several cycles it exhibits almost 100%. At the initial reaction stage, the sulfur primarily exists in the form of a crystal within N-rGO matrix as proved by the XRD pattern. During the initial cycles, sulfur would redistribute throughout the whole cathode by virtue of the repeated electrochemical reactions. Moreover, partial sulfur became soluble polysulfides, which were not immobilized firmly in the carbon scaffolds and released into the electrolyte, resulting in the degradation of reversible capacity.

Moreover, the EIS measurements of S@N-rGO before and after cycling at 0.8 A g−1 for 15 cycles were recorded and the results are presented in Fig. S6. The semicircle in the range of high-to-medium frequency is pertaining to the interfacial charge transfer resistance (Rct) and the tilted line represents the Warburg impedence (Zw). It is clear that Rct reduces with cycling, demonstrating that the activation process benefits the improvement of charge transfer and thus reversible capacity. FESEM images of S@N-rGO tested at the current density of 0.8 A g−1 for 30 cycles are presented in Fig. S7. The graphene structure is clearly observed with no visible aggregation, demonstrating the structure stability. In comparison, the S/N-rGO exhibits an original reversible-capacity of 1010 mA h g−1, which decreases to 406.8 mA h g−1 after 300 cycles with a capacity decay of 0.2% per cycle. The lower specific capacity and higher decay rate was attributed to the uneven distribution of sulfur on carbon and the large bulk of sulfur as demonstrated by the above FESEM results.

Fig. 5D represents the cycling performance of the S@N-rGO at 1C with the initial 10 cycles of activation at a lower current density of 0.5 A g−1. Upon immediately increasing to 1.6 A g−1 (1C), the reversible capacity of S@N-rGO reaches 765.7 mA h g−1. At the 170th cycle, a capacity of 596 mA h g−1 remains, maintaining a capacity retention of 77.8%. From the 10th cycle, the Coulombic efficiency of the S@N-rGO is maintained at approximately 100% due to the good confinement of polysulfides by chemical adsorption of N-rGO matrix and the uniform scattering of sulfur. Accordingly, the visualized adsorption experiment of polysulfides was carried out with Li2S4/DME as a model. As demonstrated in Fig. S8, the characteristic color of polysulfides disappeared after stirring with N-rGO. The result clearly indicated the strong absorption of N-rGO to polysulfides via chemical bonding.

In order to investigate the effect of sulfur content on the cycling behavior, another sample of S@N-rGO with 63% S loading was prepared and tested at 1C for 170 cycles (Fig. S9). As shown in Fig. S9B, during the initial 60 cycles, samples of S@N-rGO with 63% S and 76% S offer similar reversible capacity. However, from 60th cycle onwards, S@N-rGO with 63% S retained capacity higher than that of 76% S. Till the 170th cycle, a reversible capacity of 669 mA h g−1 was reserved. From the 10th cycle, the Coulombic efficiency was about 100%, similar to that of S@N-rGO with 76% S. From the above data, it is clear that S@N-rGO exhibits better rate capability and long-term cycle stability, relying on the advantageous structural features, including stronger chemical absorption ability of N-doped rGO and uniform distribution of sulfur in the carbon framework, ascribable to the present sulfur–hydrazine hydrate chemistry-based method.

4. Conclusions

In conclusion, a sulfur–hydrazine hydrate solution was employed to chemically prepare the composite of sulfur and N-rGO matrix with 76% sulfur content. After refluxing at 95 °C, sulfur crystal was homogeneously deposited within N-rGO matrix from the sulfur–hydrazine hydrate solution, in which the rGO was reduced and N-doped by hydrazine hydrate. Hence, the hydrazine hydrate, in this study, played a dual role, acting as not only a solvent for sulfur, but also a reducing and doping agent for graphene oxide. The doped N and residual oxygen atoms served as active sites to ensure effective confinement of polysulfides and the intimate contact of sulfur with the conducting matrix. The huge volume change during charge/discharge reactions could be accommodated by the porous N-rGO network scaffold. As a result, the as-fabricated S@N-rGO offered good reversibility and high rate capability.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the National Natural Science Fund of China (21601108), Young Scholars Program of Shandong University (2017WLJH15), the Fundamental Research Funds of Shandong University (no. 2016JC033), and the Taishan Scholar Project of Shandong Province (no. ts201511004).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qi00726d

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