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Nitrogen and sulfur dual-doped graphene sheets as anode materials with superior cycling stability for lithium-ion batteries
Electrochimica Acta 184 (2015) 24–31
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Nitrogen and sulfur dual-doped graphene sheets as anode materials with superior cycling stability for lithium-ion batteries Yuqi Zhoua , Yan Zenga , Dandan Xua , Peihang Lia , Heng-guo Wanga,* , Xiang Lia , Yanhui Lia , Yinghui Wangb,* a
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, PR China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 May 2015 Received in revised form 30 September 2015 Accepted 7 October 2015 Available online xxx
Novel nitrogen and sulfur dual-doped graphene sheets are synthesized via carbonization of poly(2,5dimercapto-1,3,4-thiadiazole) (PDMcT) functionalized graphene oxides. Here, PDMcT, polymerized by the commercially available 2,5-dimercapto-1,3,4-thiadiazole (DMcT) monomer is chosen as the nitrogen and sulfur precursor, which plays a key role in the formation of nitrogen and sulfur dual-doped graphene. When evaluated as anode materials for lithium ion batteries, the nitrogen and sulfur dual-doped graphene sheets show high initial specific capacity of 1428.8 mAh g1, excellent cycling stability over 5000 cycles, and good rate capability (107 mAh g1 even at a current density of 10 A g1), which can be attributed to the unique structure and dual-heteroatom doping. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Codoped graphene heteroatom-containing polymer anode lithium ion batteries
1. Introduction The ever-increasing demand of portable electronic devices and full electric vehicles has required the rapid development of lithium ion batteries (LIBs) with high energy and power densities as well as good cyclic performance [1–3]. However, graphite, the most commonly used anode material in commercial LIBs, suffers from some disadvantages such as low theoretical capacity (372 mAh g1) and poor rate performance [4], which hinders its further applications in state-of-the-art energy technologies. Alternatively, graphene, the ultrathin two dimensional networks, has become the most applicable candidate as a high-power and high-energy electrode material owing to its excellent conductivity, large surface, pronounced chemical stability as well as the open and flexible porous structure [5–10]. Remarkably, chemical functionalization of graphene could potentially produce localized highly reactive regions and thus unexpected properties [11]. Recently, it has been proposed that the chemical doping of graphene with foreign atoms such as nitrogen [12–26], boron [27,28] and sulfur [29,30] can enhance the reactivity and electric conductivity, and hence lead to significant enhancement in the electrochemical performance. For instance, nitrogen-doped graphene can induce a large number of defects and exhibit more favorable binding sites
* Corresponding authors. Tel.: +86 431 85583176; fax: +86 431 85306769. E-mail address: [email protected] (H.-g. Wang). http://dx.doi.org/10.1016/j.electacta.2015.10.026 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
for Li+-ion, and hence enhance the LIB properties [19–21]. As a result, several methods including chemical vapor deposition (CVD), nitrogen plasma treatment, and the arc-discharge method, even using the toxic and hazardous precursors (NH3 or pyridine) have been developed to synthesize heteroatom-doped graphene [31–35]. However, during the above synthesis, using toxic precursors, sophisticated equipment or rigorous conditions are usually inevitable and the resulting heteroatom-doped graphene is commonly inhomogeneous and low nitrogen doping. Furthermore, the synthesis of other heteroatom-doped graphene, for example sulfur-doped graphene, remains a big challenge, let alone, using a facile method to prepare dual-heteroatom codoped graphene. To address these problems, using heteroatom-containing polymers as the doped source seems to be the most applicable strategy. It is well known that the molecular skeleton of polymer could contain different functional groups composed of multiheteroatom. Therefore, choosing suitable polymer could easily prepare heteroatom-codoped graphene using a facile one-step reaction. Interestingly, graphene oxide (GO), an important derivative of graphene, has abundant oxygen-containing groups including hydroxyls, epoxides, and carboxyls on the surface, which can act as interfacial linkers to link the polymer with functional group by mutual electrostatic interactions [36]. Furthermore, reduced graphene oxide (RGO) also has rich stacking p electrons, which can facilitate the chemical functionalization with the conjugated polymer via aromatic p–p interactions [37]. Most importantly, after GO is coated by the polymers, the decomposition of polymers
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on GO surface not only induces the crumpling of GO, but also enables the nitrogen doping reaction to proceed on both sides of GO, leading to uniform and high-concentration nitrogen doping in graphene. [21] Therefore, exploring and developing the suitable polymer for the preparation of multi-heteroatom codoped graphene is urgently required. Herein, for the first time, we report an efficient strategy for elaborately constructing nitrogen and sulfur dual-doped graphene sheets (NSGs). Briefly, the NSGs are simply prepared via carbonization of poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT) functionalized GO. PDMcT, polymerized by the commercially available DMcT monomer, plays a key role in the formation of such nitrogen and sulfur dual-doped structures. The obtained NSGs with the unique structures comprised of crumpled and dual-heteroatom codoped nanostructure exhibit excellent performance as anode materials for LIBs. 2. Experimental 2.1. Synthesis of NSGs GO was prepared by a modified Hummers method [38]. 0.3 g DMcT was dissolved in 20 mL CH3OH. Then, the mixed solution was added into the flask containing 10 g homogeneous GO solution (8 mg g1). 0.1 g cetrimonium bromide (CTAB) was subsequently added under stirring. Then, the mixed solvent CH3OH/H2O (10 mL, 1:1 by volume) containing 1.37 g ammonium persulfate (APS) was added dropwise to the above solution. After stirring at room temperature for 24 h, the light-yellow precipitate was collected, washed with CH3OH/H2O, and dried overnight at 80 C. Finally, the mixture was put in a tubular furnace at 800 C for 2 h (heating rate 5 C min1) under a N2 gas flow. As a control experiment, NSGs was prepared using the similar procedure with the different concentration (0.05, 0.3, 0.6 and 1.0 g) of the DMcT monomer in the precursor mixed solution, which were denoted as NSGs-0.5, NSGs3, NSGs-6 and NSGs-10. In addition, GO was directly annealed in a tubular furnace at 800 C for 2 h (heating rate 5 C min1) under a N2 gas flow to obtain graphene (GS). 2.2. Characterization Scanning electron microscopy (SEM) images were collected with a Hitachi S-4800 instrument. Transmission electron
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microscope (TEM) images were carried out with a Tecnai G2 using 200 kV. X-ray diffraction (XRD) patterns were carried out with a Rigaku-Dmax 2500 diffractometer using Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) analysis was conducted with ESCALAB MK II X-ray instrument. Raman measurements were performed at room temperature using an inVia Reflex Raman spectrometer with Raman shift from 100 to 1000 cm1. 2.3. Electrochemical characterization 80 wt% active material, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) were mixed in N-methyl-2pyrrolidone (NMP) and then uniformly pasted on copper foil. After dried in vacuum at 80 C for 12 h to remove the solvent, the work electrodes were pressed and cut into disks. Thin lithium foil was used as the counter electrode, Celgard 2400 membrane was used as separator and lithium hexafluorophosphate LiPF6 (1 M) in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 vol %) was employed as the electrolyte. Galvanostatic charge-discharge tests were carried out between 0.01–3.0 V vs. Li+/Li on a Land Battery Measurement System (Land, PR China). Cyclic voltammetry (CV) was performed by using a VMP3 Electrochemical Workstation (Bio-logic Inc.). 3. Results and discussion The nitrogen and sulfur dual-doped graphene is prepared via a three-step route (Fig. 1): first, DMcT monomer was absorbed on the surface of graphene oxide (GO) due to the hydrogen bond and electrostatic interactions. Here, DMcTs made up of carbon, nitrogen and sulfur elements are selected as the doped source and GO is prepared from natural graphite by using a modified Hummer’s method. Then, the initiator was added into the mixed solution to induce the polymerization of the DMcT monomer. During this process, the uniform coatings on the surface of GO resulted in the formation of the thin and sandwich-like polymer/ graphene oxide/polymer. Finally, the obtained composite was dried and annealed under inert gas atmosphere to obtain nitrogen and sulfur dual-doped graphene. Fig. 1 summarizes the methodology for synthesizing the N and S dual-doped graphene. In order to demonstrate the proposed formation mechanism of the nitrogen and sulfur dual-doped graphene, we carry out a series of concentration-dependent experiments which samples are
Fig. 1. Schematic illustration of the fabrication of nitrogen and sulfur dual-doped graphene: adsorption of the monomer, formation of polymer and thermal annealing.
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prepared by adjusting the concentration of the DMcT monomer in the precursor mixed solution. The detailed experimental procedure is described in the Experimental Section. The morphology of NSGs is first investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As exhibited in Fig. 2a-b, many randomly distributed dimensional (2D) nanosheets, which are similar to GO, can be distinctly observed. Interestingly, the NSGs-3 show distinct crumpled structures, indicating the growth of PDMcT along the surface of the graphene oxide sheets. As shown in Fig. 2c, the thin planar/lamellar structures with a large amount of wrinkles, are also confirmed by TEM. It is worth emphasizing that the crumpled structure has been found to be responsible for the improvement of the electrochemical activity of graphene nanosheets [13,39]. In addition, the scanning TEM (STEM) and energy dispersive X-ray spectroscopic (EDS) elemental mapping analysis are shown in Fig. 2d. From these images, one can see that this hybrid is composed of C, N, and S elements and these elements are homogeneously distributed, indicating the formation of nitrogen and sulfur dual-doped graphene sheets. Subsequently, the morphology of NSGs-6 and NSGs-10 is further investigated by SEM. As exhibited in Fig. 3, the NSGs-6 and NSGs-10 also maintain the 2D structure and show distinct crumpled structures. Interestingly, the surface morphology is hardly unchanged compared to NSGs-3, indicating that the further increase of DMcT monomer in the precursor solution has no effect on the sandwich-like polymer/ graphene oxide/polymer. Structural characterization of NSGs is investigated using powder X-ray diffraction (XRD). As shown in Fig. 4a, two characteristic diffraction peaks can be observed at around 26.1
and 43.2 , which can be assigned to the (002) diffraction of the graphitic layer-by-layer structure and the (101) diffraction of graphite, respectively. In addition, the broad (002) diffraction peak ranging from 17 to 30 shows the disordered or amorphous nature of the stacked layers of doped graphene. According to the (002) diffraction peak, the interlayer spacing could be calculated to be 0.37 nm. Previously reported studies have demonstrated that the occurrence of large free space between graphene layers should be beneficial for the reversible storage of lithium ion. The reduction process from GO to NSGs is confirmed by Raman spectroscopy (Fig. 4b). The typical Raman spectra exhibit two remarkable peaks, corresponding to the D and G band, respectively [40]. The presence of both D band and G band confirms the more disordered structure, in agreement with the XRD observation. It is found that the intensities of the D band for GO (ID/IG = 0.85) become lower than that for NSGs (ID/IG = 1.00), which probably resulted from the generation of smaller nanocrystalline graphene domains, [41] and the incorporation of heteroatoms [31]. In addition, the downshift (from 1608 to 1598 cm1) of the G peak from GO to NSGs can be attributed to the restoration of the conjugated structure during pyrolysis [42] and the electron-donating capability of heteroatoms [31,33,43]. The disordered structure and the numerous defects are expected to be propitious to the insertion/extraction and the diffusion of Li-ions. The chemical composition and bonding configurations of NSGs are further characterized using X-ray photoelectron spectroscopy (XPS) (Fig. 5). As shown in Fig. 5a, it is obvious that the presence of the S2p peak at 165 eV, C1s peak at 284 eV, and N1s peak at 400 eV can be observed in the XPS survey spectrum, suggesting that both nitrogen and sulfur have been successfully incorporated into
Fig. 2. (a) Low- and (b) high-resolution SEM images of NSGs-3. (c) TEM image of NSGs-3 and (d) Dark-field TEM image of NSGs-3 and corresponding EDS elemental mapping images of carbon (C), nitrogen (N) and sulfur (S).
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Fig. 3. (a) Low- and (b) high-resolution SEM images of NSGs-6 and (c) Low- and (d) high-resolution SEM images of NSGs-10.
Fig. 4. XRD pattern (a) and Raman spectra (b) of NSGs-3.
graphene. As shown in Fig. 5b, there is no peak at a high binding energy in C1s XPS spectrum, suggesting that the oxygen functional groups on GO are largely removed, which further confirms the reduction process of GO [33]. In order to probe the chemical state of nitrogen and sulfur in the heteroatom-doped graphene, the high-resolution N1s and S2p are deconvoluted into different peaks. The high-resolution N1s could be deconvoluted into three peaks at 398, 400, and 401 eV (Fig. 5c). The low binding energy peaks at 398 and 400 eV are commonly attributed to pyridine-like nitrogen (pyridinic-N) and pyrrole-like nitrogen (pyrrolic-N), respectively, which contribute electron density to the p -conjugated system
with a pair of p- electrons in the graphene layers [44]. The high energy peak at 401 eV is corresponded to graphitic nitrogen (graphitic-N). Based on the intensity of XPS, the N binding configuration includes 35.6% pyridinic-N, 31.4% pyrrolic-N, and 33.0% graphitic-N. In addition, the high-resolution S2p could be resolved into three peaks at 163.7, 165.0, and 168.2 eV (Fig. 5d). The former two peaks are attributed to the thiophene–S of C S C and C¼S , respectively [45,46]. The third peak should arise from some oxidized S (C-SOx) [45,46]. Therefore, it can be inferred that the heteroatom-doped graphene has more defects than those in perfect graphene, which is beneficial to the charge
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Fig. 5. XPS spectra of NSGs-3: survey spectrum (a) and high-resolution C1s (b), N1s (c) and S2p (d).
transfer. In addition, XPS spectra of NSGs-6 and NSGs-10 are also tested and their results show that the further increase of DMcT monomer in the precursor solution hardly increase the N and S elemental content, that is, NSGs-3 is enough to coat the surface of
GO without additional doped source. This result further verifies the proposed formation mechanism of the N and S dual-doped graphene derived from the sandwich-like polymer/graphene oxide/polymer.
Fig. 6. (a) CV curves of NSGs-3 between 0 and 3.0 V at a potential sweep rate of 0.1 mV s1. (b) Charge-discharge profiles of NSGs-3 at 50 mA g1, and cycle performances of NSGs-3 at 100 mA g1 (c) and 1000 mA g1 (d).
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The lithium storage properties of the as-obtained NSGs-3 are assessed by cyclic voltammetry (CV) and galvanostatic dischargecharge experiments. First, the CV curves of the NSGs-3 electrode are shown in Fig. 6a. In the first cycle, a pronounced reduction peak between 0.97 and 0.3 V, which is less pronounced in the following cycles, is likely due to electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) layer and other side reactions. In addition, two broad oxidation peaks are observed at near 0 V and 1.2 V, respectively, which is consistent with that of the reported graphene anode [13,19,27]. The peak at lower potential can be attributed to the Li extraction from the graphitic layers. And the peak at 1.2 V is due to the Li extraction from the defects in the NSGs-3, such as pores, vacancies, edges and corners of the graphitic layers [47]. As has been reported, the peaks at higher potential are likely related to the heteroatoms (residual H) on the surface of NSGs [48]. Interestingly, in the following cycles, the CV curves almost overlapped, implying an excellent cyclability of the NSGs3 electrode. Subsequently, the galvanostatic charge-discharge profiles of the NSGs-3 electrode at 50 mA g1 are shown in Fig. 6b. The correlative plateau regions can be observed in the charge-discharge profiles, and the initial charge and discharge capacities are 1428.8 and 846.5 mAh g1, respectively, corresponding to a Coulombic efficiency of 59.2%. The 40.8% capacity loss can be mainly ascribed to the formation of the SEI. When the current density is increased to 100 mA g1 (Fig. 6c), the initial charge and discharge capacities are 1024.3 and 593.6 mAh g1, respectively, corresponding to a Coulombic efficiency of 57.9%. After the first cycles, the Coulombic efficiency quickly improves above 95.4% and after 500 cycles, the NSGs-3 electrode still maintains a specific reversible capacity of 490 mAh g1, corresponding to capacity retention ratios of 82.5%, which reveals a good cyclic performance and reversibility. In order to further study the long-term stability of the NSGs-3 electrode at high current density, the electrode is tested at 1 A g1. As shown in Fig. 6d, the NSGs-3 electrode still exhibits good specific reversible capacity of 211 mAh g1 after 5000 cycles. Similarly, the Coulombic efficiency increased to nearly 100% after the first cycles. Finally, the rate capability of the NSGs3 electrode is also evaluated at various current rates. As shown in Fig. 7a, reversible capacities of 400 mAh g1 at the discharge/ charge rate of 0.2 A g1, 335 mAh g1 at 0.5 A g1, 264 mAh g1 at 1 A g1, 207 mAh g1 at 2 A g1, 142 mAh g1 at 5 A g1, and 107 mAh g1 at 10 A g1, are achieved respectively, demonstrating the excellent rate performance of the NSGs-3 electrode. Interestingly, even after a high rate of 10 A g1 and 70 cycles measurement (Fig. 7b), the charge capacity reverts to 450 mA h g1 when the current density returns to 0.1 A g1, implying the good rate performance of this materials. Remarkably, the NSGs-3 electrode
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still maintains the restoring reversible capacity of 450 mAh g1 after 500 cycles, indicating the superior cycling stability. The excellent cyclic performance and rate performance demonstrate that the NSGs are promising candidate as anode materials for high-performance LIBs. Such good electrochemical performance results from the combination of the unique structure and heteroatom doping. On the one hand, the unique structure includes large surface area, larger interlayer spacing and crumpled structure. First, the high surface area provides sufficient electrode/ electrolyte contact area and a large quantity of active sites to absorb Li+ ions and promote rapid charge-transfer reaction [13]. Second, the larger interlayer spacing facilitates the intercalation/ extraction of Li+ ions into the graphitic shells [48]. Finally, recent researches have demonstrated that the crumpled structure has been found to be responsible for the improvement of the electrochemical activity of graphene nanosheets [14]. On the other hand, heteroatom doping plays pivotal roles in increasing the conductivity and the electrochemical activity of NSGs, which could lead to good electrochemical performance. In order to demonstrate the results, we compare the electrochemical performance of the pure graphene (GS) with that of the N and S dual-doped graphene prepared by adjusting the different monomer concentration (NSGs-0.5, NSGs-3 and NSGs-6) (Fig. 8). Obviously, the N and S dual-doped graphene shows the improved capacity compared with the pure graphene Recently, it has been proposed that the nitrogen doping could generate a large number of extrinsic defects and active sites, which can attract more Li ions and adjust the electron properties of the adjoining carbon atoms with the cycles
Fig. 8. Cycling performance of GS, NSGs-0.5, NSGs-3 and NSGs-6 at 100 mA g1.
Fig. 7. (a) Rate performance of NSGs-3 at different current densities in the range of 3–0.01 V. (b) Cycling performance of NSGs-3 after the back and forth high rate and 70 cycles measurement at 100 mA g1.
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[12–26]. Herein, N and S dual-doped graphene is able to generate more extrinsic defects and active sites and simultaneously increase the conductivity and the electrochemical activity, which is more beneficial to improving the lithium storage performance. Moreover, with the increase of the monomer concentration, NSGs electrode shows further increased performance from NSGs-0.5 to NSGs-3, which further demonstrates the enhancement effect of the doping. Interestingly, by further increasing the monomer concentration, the NSGs-6 show similar performance with NSGs-3, which is attributed that the sandwich-like polymer/graphene oxide/ polymer has no enough surfaces to dope additional heteroatoms. These observations corroborate that N and S dual-doped graphene are very effective for generating more extrinsic defects and active sites and improving the electrochemical performance. 4. Conclusions A simple, general yet effective strategy is developed to prepare N and S dual-doped graphene via carbonization of heteroatomcontaining polymers functionalized GO. As potential anode materials for LIBs, the NSGs exhibit excellent properties in terms of cycling performance and rate capability, benefiting from the unique structure and heteroatom doping. Obviously, using heteroatom-containing polymers as the doped source seems to be the most applicable and very helpful strategy to prepare promising graphene-based electrode materials. The novel method would open up new opportunities in the development of dual or multi-doped graphene for applications in LIBs, supercapacitors, photoelectrochemical and water splitting. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21404014), Jilin Science & Technology Department (the development plan project of science and technology No. 20150520002JH) and Training Programs of Innovation and Entrepreneurship for Undergraduates (No. 2014S034). References [1] M. Winter, J.B. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (2004) 4245–4259. [2] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [3] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496–499. [4] B.K. Guo, X.Q. Wang, P.F. Fulvio, M.F. Chi, S.M. Mahurin, X.G. Sun, S. Dai, Softtemplated mesoporous carbon-carbon nanotube composites for high performance lithium-ion batteries, Adv. Mater. 23 (2011) 4661–4666. [5] Y.W. Zhu, S. Murali, W.W. Cai, X.S. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906–3924. [6] X.L. Huang, R.Z. Wang, D. Xu, Z.L. Wang, H.G. Wang, J.J. Xu, Z. Wu, Q.C. Liu, Y. Zhang, X.B. Zhang, Homogeneous CoO on graphene for binder-free and ultralong-life lithium ion batteries, Adv. Funct. Mater. 23 (2013) 4345–4353. [7] S.M. Paek, E. Yoo, I. Honma, Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with three-dimensionally delaminated flexible structure, Nano. Lett. 9 (2009) 72–75. [8] X.G. Wang, X.F. Zhu, K. Yao, J.G. Zhang, Z. Liu, A SnO2/graphene compositeas a high stability electrode for lithium ion batteries, Carbon 49 (2011) 133–139. [9] S.B. Yang, X.L. Feng, S. Ivanovici, K. Müllen, Fabrication of grapheneencapsulated oxide nanoparticles: towards high-performance anode materials for lithium storage, Angew. Chem. Int. Ed. 49 (2010) 8408–8411. [10] A.K. Geim, Graphene: status and prospects, Science 324 (2009) 1530–1534. [11] S.Y. Wang, L.P. Zhang, Z.H. Xia, A. Roy, D.W. Chang, J.B. Baek, L.M. Dai, BCN graphene as efficient metal-free electrocatalyst for the oxygen reduction reaction, Angew. Chem. Int. Ed. 51 (2012) 4209–4212. [12] Z.Y. Lin, G. Waller, Y. Liu, M.L. Liu, C.P. Wong, Facile synthesis of nitrogen-doped graphene via pyrolysis of graphene oxide and urea, and its electrocatalytic activity toward the oxygen-reduction reaction, Adv. Energy Mater. 2 (2012) 884–888.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nitrogen and sulfur dual-doped graphene sheets as anode materials with superior cycling stability for lithium-ion batteries Yuqi Zhoua , Yan Zenga , Dandan Xua , Peihang Lia , Heng-guo Wanga,* , Xiang Lia , Yanhui Lia , Yinghui Wangb,* a
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, PR China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 May 2015 Received in revised form 30 September 2015 Accepted 7 October 2015 Available online xxx
Novel nitrogen and sulfur dual-doped graphene sheets are synthesized via carbonization of poly(2,5dimercapto-1,3,4-thiadiazole) (PDMcT) functionalized graphene oxides. Here, PDMcT, polymerized by the commercially available 2,5-dimercapto-1,3,4-thiadiazole (DMcT) monomer is chosen as the nitrogen and sulfur precursor, which plays a key role in the formation of nitrogen and sulfur dual-doped graphene. When evaluated as anode materials for lithium ion batteries, the nitrogen and sulfur dual-doped graphene sheets show high initial specific capacity of 1428.8 mAh g1, excellent cycling stability over 5000 cycles, and good rate capability (107 mAh g1 even at a current density of 10 A g1), which can be attributed to the unique structure and dual-heteroatom doping. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Codoped graphene heteroatom-containing polymer anode lithium ion batteries
1. Introduction The ever-increasing demand of portable electronic devices and full electric vehicles has required the rapid development of lithium ion batteries (LIBs) with high energy and power densities as well as good cyclic performance [1–3]. However, graphite, the most commonly used anode material in commercial LIBs, suffers from some disadvantages such as low theoretical capacity (372 mAh g1) and poor rate performance [4], which hinders its further applications in state-of-the-art energy technologies. Alternatively, graphene, the ultrathin two dimensional networks, has become the most applicable candidate as a high-power and high-energy electrode material owing to its excellent conductivity, large surface, pronounced chemical stability as well as the open and flexible porous structure [5–10]. Remarkably, chemical functionalization of graphene could potentially produce localized highly reactive regions and thus unexpected properties [11]. Recently, it has been proposed that the chemical doping of graphene with foreign atoms such as nitrogen [12–26], boron [27,28] and sulfur [29,30] can enhance the reactivity and electric conductivity, and hence lead to significant enhancement in the electrochemical performance. For instance, nitrogen-doped graphene can induce a large number of defects and exhibit more favorable binding sites
* Corresponding authors. Tel.: +86 431 85583176; fax: +86 431 85306769. E-mail address: [email protected] (H.-g. Wang). http://dx.doi.org/10.1016/j.electacta.2015.10.026 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
for Li+-ion, and hence enhance the LIB properties [19–21]. As a result, several methods including chemical vapor deposition (CVD), nitrogen plasma treatment, and the arc-discharge method, even using the toxic and hazardous precursors (NH3 or pyridine) have been developed to synthesize heteroatom-doped graphene [31–35]. However, during the above synthesis, using toxic precursors, sophisticated equipment or rigorous conditions are usually inevitable and the resulting heteroatom-doped graphene is commonly inhomogeneous and low nitrogen doping. Furthermore, the synthesis of other heteroatom-doped graphene, for example sulfur-doped graphene, remains a big challenge, let alone, using a facile method to prepare dual-heteroatom codoped graphene. To address these problems, using heteroatom-containing polymers as the doped source seems to be the most applicable strategy. It is well known that the molecular skeleton of polymer could contain different functional groups composed of multiheteroatom. Therefore, choosing suitable polymer could easily prepare heteroatom-codoped graphene using a facile one-step reaction. Interestingly, graphene oxide (GO), an important derivative of graphene, has abundant oxygen-containing groups including hydroxyls, epoxides, and carboxyls on the surface, which can act as interfacial linkers to link the polymer with functional group by mutual electrostatic interactions [36]. Furthermore, reduced graphene oxide (RGO) also has rich stacking p electrons, which can facilitate the chemical functionalization with the conjugated polymer via aromatic p–p interactions [37]. Most importantly, after GO is coated by the polymers, the decomposition of polymers
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on GO surface not only induces the crumpling of GO, but also enables the nitrogen doping reaction to proceed on both sides of GO, leading to uniform and high-concentration nitrogen doping in graphene. [21] Therefore, exploring and developing the suitable polymer for the preparation of multi-heteroatom codoped graphene is urgently required. Herein, for the first time, we report an efficient strategy for elaborately constructing nitrogen and sulfur dual-doped graphene sheets (NSGs). Briefly, the NSGs are simply prepared via carbonization of poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT) functionalized GO. PDMcT, polymerized by the commercially available DMcT monomer, plays a key role in the formation of such nitrogen and sulfur dual-doped structures. The obtained NSGs with the unique structures comprised of crumpled and dual-heteroatom codoped nanostructure exhibit excellent performance as anode materials for LIBs. 2. Experimental 2.1. Synthesis of NSGs GO was prepared by a modified Hummers method [38]. 0.3 g DMcT was dissolved in 20 mL CH3OH. Then, the mixed solution was added into the flask containing 10 g homogeneous GO solution (8 mg g1). 0.1 g cetrimonium bromide (CTAB) was subsequently added under stirring. Then, the mixed solvent CH3OH/H2O (10 mL, 1:1 by volume) containing 1.37 g ammonium persulfate (APS) was added dropwise to the above solution. After stirring at room temperature for 24 h, the light-yellow precipitate was collected, washed with CH3OH/H2O, and dried overnight at 80 C. Finally, the mixture was put in a tubular furnace at 800 C for 2 h (heating rate 5 C min1) under a N2 gas flow. As a control experiment, NSGs was prepared using the similar procedure with the different concentration (0.05, 0.3, 0.6 and 1.0 g) of the DMcT monomer in the precursor mixed solution, which were denoted as NSGs-0.5, NSGs3, NSGs-6 and NSGs-10. In addition, GO was directly annealed in a tubular furnace at 800 C for 2 h (heating rate 5 C min1) under a N2 gas flow to obtain graphene (GS). 2.2. Characterization Scanning electron microscopy (SEM) images were collected with a Hitachi S-4800 instrument. Transmission electron
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microscope (TEM) images were carried out with a Tecnai G2 using 200 kV. X-ray diffraction (XRD) patterns were carried out with a Rigaku-Dmax 2500 diffractometer using Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) analysis was conducted with ESCALAB MK II X-ray instrument. Raman measurements were performed at room temperature using an inVia Reflex Raman spectrometer with Raman shift from 100 to 1000 cm1. 2.3. Electrochemical characterization 80 wt% active material, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) were mixed in N-methyl-2pyrrolidone (NMP) and then uniformly pasted on copper foil. After dried in vacuum at 80 C for 12 h to remove the solvent, the work electrodes were pressed and cut into disks. Thin lithium foil was used as the counter electrode, Celgard 2400 membrane was used as separator and lithium hexafluorophosphate LiPF6 (1 M) in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 vol %) was employed as the electrolyte. Galvanostatic charge-discharge tests were carried out between 0.01–3.0 V vs. Li+/Li on a Land Battery Measurement System (Land, PR China). Cyclic voltammetry (CV) was performed by using a VMP3 Electrochemical Workstation (Bio-logic Inc.). 3. Results and discussion The nitrogen and sulfur dual-doped graphene is prepared via a three-step route (Fig. 1): first, DMcT monomer was absorbed on the surface of graphene oxide (GO) due to the hydrogen bond and electrostatic interactions. Here, DMcTs made up of carbon, nitrogen and sulfur elements are selected as the doped source and GO is prepared from natural graphite by using a modified Hummer’s method. Then, the initiator was added into the mixed solution to induce the polymerization of the DMcT monomer. During this process, the uniform coatings on the surface of GO resulted in the formation of the thin and sandwich-like polymer/ graphene oxide/polymer. Finally, the obtained composite was dried and annealed under inert gas atmosphere to obtain nitrogen and sulfur dual-doped graphene. Fig. 1 summarizes the methodology for synthesizing the N and S dual-doped graphene. In order to demonstrate the proposed formation mechanism of the nitrogen and sulfur dual-doped graphene, we carry out a series of concentration-dependent experiments which samples are
Fig. 1. Schematic illustration of the fabrication of nitrogen and sulfur dual-doped graphene: adsorption of the monomer, formation of polymer and thermal annealing.
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prepared by adjusting the concentration of the DMcT monomer in the precursor mixed solution. The detailed experimental procedure is described in the Experimental Section. The morphology of NSGs is first investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As exhibited in Fig. 2a-b, many randomly distributed dimensional (2D) nanosheets, which are similar to GO, can be distinctly observed. Interestingly, the NSGs-3 show distinct crumpled structures, indicating the growth of PDMcT along the surface of the graphene oxide sheets. As shown in Fig. 2c, the thin planar/lamellar structures with a large amount of wrinkles, are also confirmed by TEM. It is worth emphasizing that the crumpled structure has been found to be responsible for the improvement of the electrochemical activity of graphene nanosheets [13,39]. In addition, the scanning TEM (STEM) and energy dispersive X-ray spectroscopic (EDS) elemental mapping analysis are shown in Fig. 2d. From these images, one can see that this hybrid is composed of C, N, and S elements and these elements are homogeneously distributed, indicating the formation of nitrogen and sulfur dual-doped graphene sheets. Subsequently, the morphology of NSGs-6 and NSGs-10 is further investigated by SEM. As exhibited in Fig. 3, the NSGs-6 and NSGs-10 also maintain the 2D structure and show distinct crumpled structures. Interestingly, the surface morphology is hardly unchanged compared to NSGs-3, indicating that the further increase of DMcT monomer in the precursor solution has no effect on the sandwich-like polymer/ graphene oxide/polymer. Structural characterization of NSGs is investigated using powder X-ray diffraction (XRD). As shown in Fig. 4a, two characteristic diffraction peaks can be observed at around 26.1
and 43.2 , which can be assigned to the (002) diffraction of the graphitic layer-by-layer structure and the (101) diffraction of graphite, respectively. In addition, the broad (002) diffraction peak ranging from 17 to 30 shows the disordered or amorphous nature of the stacked layers of doped graphene. According to the (002) diffraction peak, the interlayer spacing could be calculated to be 0.37 nm. Previously reported studies have demonstrated that the occurrence of large free space between graphene layers should be beneficial for the reversible storage of lithium ion. The reduction process from GO to NSGs is confirmed by Raman spectroscopy (Fig. 4b). The typical Raman spectra exhibit two remarkable peaks, corresponding to the D and G band, respectively [40]. The presence of both D band and G band confirms the more disordered structure, in agreement with the XRD observation. It is found that the intensities of the D band for GO (ID/IG = 0.85) become lower than that for NSGs (ID/IG = 1.00), which probably resulted from the generation of smaller nanocrystalline graphene domains, [41] and the incorporation of heteroatoms [31]. In addition, the downshift (from 1608 to 1598 cm1) of the G peak from GO to NSGs can be attributed to the restoration of the conjugated structure during pyrolysis [42] and the electron-donating capability of heteroatoms [31,33,43]. The disordered structure and the numerous defects are expected to be propitious to the insertion/extraction and the diffusion of Li-ions. The chemical composition and bonding configurations of NSGs are further characterized using X-ray photoelectron spectroscopy (XPS) (Fig. 5). As shown in Fig. 5a, it is obvious that the presence of the S2p peak at 165 eV, C1s peak at 284 eV, and N1s peak at 400 eV can be observed in the XPS survey spectrum, suggesting that both nitrogen and sulfur have been successfully incorporated into
Fig. 2. (a) Low- and (b) high-resolution SEM images of NSGs-3. (c) TEM image of NSGs-3 and (d) Dark-field TEM image of NSGs-3 and corresponding EDS elemental mapping images of carbon (C), nitrogen (N) and sulfur (S).
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Fig. 3. (a) Low- and (b) high-resolution SEM images of NSGs-6 and (c) Low- and (d) high-resolution SEM images of NSGs-10.
Fig. 4. XRD pattern (a) and Raman spectra (b) of NSGs-3.
graphene. As shown in Fig. 5b, there is no peak at a high binding energy in C1s XPS spectrum, suggesting that the oxygen functional groups on GO are largely removed, which further confirms the reduction process of GO [33]. In order to probe the chemical state of nitrogen and sulfur in the heteroatom-doped graphene, the high-resolution N1s and S2p are deconvoluted into different peaks. The high-resolution N1s could be deconvoluted into three peaks at 398, 400, and 401 eV (Fig. 5c). The low binding energy peaks at 398 and 400 eV are commonly attributed to pyridine-like nitrogen (pyridinic-N) and pyrrole-like nitrogen (pyrrolic-N), respectively, which contribute electron density to the p -conjugated system
with a pair of p- electrons in the graphene layers [44]. The high energy peak at 401 eV is corresponded to graphitic nitrogen (graphitic-N). Based on the intensity of XPS, the N binding configuration includes 35.6% pyridinic-N, 31.4% pyrrolic-N, and 33.0% graphitic-N. In addition, the high-resolution S2p could be resolved into three peaks at 163.7, 165.0, and 168.2 eV (Fig. 5d). The former two peaks are attributed to the thiophene–S of C S C and C¼S , respectively [45,46]. The third peak should arise from some oxidized S (C-SOx) [45,46]. Therefore, it can be inferred that the heteroatom-doped graphene has more defects than those in perfect graphene, which is beneficial to the charge
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Fig. 5. XPS spectra of NSGs-3: survey spectrum (a) and high-resolution C1s (b), N1s (c) and S2p (d).
transfer. In addition, XPS spectra of NSGs-6 and NSGs-10 are also tested and their results show that the further increase of DMcT monomer in the precursor solution hardly increase the N and S elemental content, that is, NSGs-3 is enough to coat the surface of
GO without additional doped source. This result further verifies the proposed formation mechanism of the N and S dual-doped graphene derived from the sandwich-like polymer/graphene oxide/polymer.
Fig. 6. (a) CV curves of NSGs-3 between 0 and 3.0 V at a potential sweep rate of 0.1 mV s1. (b) Charge-discharge profiles of NSGs-3 at 50 mA g1, and cycle performances of NSGs-3 at 100 mA g1 (c) and 1000 mA g1 (d).
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The lithium storage properties of the as-obtained NSGs-3 are assessed by cyclic voltammetry (CV) and galvanostatic dischargecharge experiments. First, the CV curves of the NSGs-3 electrode are shown in Fig. 6a. In the first cycle, a pronounced reduction peak between 0.97 and 0.3 V, which is less pronounced in the following cycles, is likely due to electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) layer and other side reactions. In addition, two broad oxidation peaks are observed at near 0 V and 1.2 V, respectively, which is consistent with that of the reported graphene anode [13,19,27]. The peak at lower potential can be attributed to the Li extraction from the graphitic layers. And the peak at 1.2 V is due to the Li extraction from the defects in the NSGs-3, such as pores, vacancies, edges and corners of the graphitic layers [47]. As has been reported, the peaks at higher potential are likely related to the heteroatoms (residual H) on the surface of NSGs [48]. Interestingly, in the following cycles, the CV curves almost overlapped, implying an excellent cyclability of the NSGs3 electrode. Subsequently, the galvanostatic charge-discharge profiles of the NSGs-3 electrode at 50 mA g1 are shown in Fig. 6b. The correlative plateau regions can be observed in the charge-discharge profiles, and the initial charge and discharge capacities are 1428.8 and 846.5 mAh g1, respectively, corresponding to a Coulombic efficiency of 59.2%. The 40.8% capacity loss can be mainly ascribed to the formation of the SEI. When the current density is increased to 100 mA g1 (Fig. 6c), the initial charge and discharge capacities are 1024.3 and 593.6 mAh g1, respectively, corresponding to a Coulombic efficiency of 57.9%. After the first cycles, the Coulombic efficiency quickly improves above 95.4% and after 500 cycles, the NSGs-3 electrode still maintains a specific reversible capacity of 490 mAh g1, corresponding to capacity retention ratios of 82.5%, which reveals a good cyclic performance and reversibility. In order to further study the long-term stability of the NSGs-3 electrode at high current density, the electrode is tested at 1 A g1. As shown in Fig. 6d, the NSGs-3 electrode still exhibits good specific reversible capacity of 211 mAh g1 after 5000 cycles. Similarly, the Coulombic efficiency increased to nearly 100% after the first cycles. Finally, the rate capability of the NSGs3 electrode is also evaluated at various current rates. As shown in Fig. 7a, reversible capacities of 400 mAh g1 at the discharge/ charge rate of 0.2 A g1, 335 mAh g1 at 0.5 A g1, 264 mAh g1 at 1 A g1, 207 mAh g1 at 2 A g1, 142 mAh g1 at 5 A g1, and 107 mAh g1 at 10 A g1, are achieved respectively, demonstrating the excellent rate performance of the NSGs-3 electrode. Interestingly, even after a high rate of 10 A g1 and 70 cycles measurement (Fig. 7b), the charge capacity reverts to 450 mA h g1 when the current density returns to 0.1 A g1, implying the good rate performance of this materials. Remarkably, the NSGs-3 electrode
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still maintains the restoring reversible capacity of 450 mAh g1 after 500 cycles, indicating the superior cycling stability. The excellent cyclic performance and rate performance demonstrate that the NSGs are promising candidate as anode materials for high-performance LIBs. Such good electrochemical performance results from the combination of the unique structure and heteroatom doping. On the one hand, the unique structure includes large surface area, larger interlayer spacing and crumpled structure. First, the high surface area provides sufficient electrode/ electrolyte contact area and a large quantity of active sites to absorb Li+ ions and promote rapid charge-transfer reaction [13]. Second, the larger interlayer spacing facilitates the intercalation/ extraction of Li+ ions into the graphitic shells [48]. Finally, recent researches have demonstrated that the crumpled structure has been found to be responsible for the improvement of the electrochemical activity of graphene nanosheets [14]. On the other hand, heteroatom doping plays pivotal roles in increasing the conductivity and the electrochemical activity of NSGs, which could lead to good electrochemical performance. In order to demonstrate the results, we compare the electrochemical performance of the pure graphene (GS) with that of the N and S dual-doped graphene prepared by adjusting the different monomer concentration (NSGs-0.5, NSGs-3 and NSGs-6) (Fig. 8). Obviously, the N and S dual-doped graphene shows the improved capacity compared with the pure graphene Recently, it has been proposed that the nitrogen doping could generate a large number of extrinsic defects and active sites, which can attract more Li ions and adjust the electron properties of the adjoining carbon atoms with the cycles
Fig. 8. Cycling performance of GS, NSGs-0.5, NSGs-3 and NSGs-6 at 100 mA g1.
Fig. 7. (a) Rate performance of NSGs-3 at different current densities in the range of 3–0.01 V. (b) Cycling performance of NSGs-3 after the back and forth high rate and 70 cycles measurement at 100 mA g1.
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[12–26]. Herein, N and S dual-doped graphene is able to generate more extrinsic defects and active sites and simultaneously increase the conductivity and the electrochemical activity, which is more beneficial to improving the lithium storage performance. Moreover, with the increase of the monomer concentration, NSGs electrode shows further increased performance from NSGs-0.5 to NSGs-3, which further demonstrates the enhancement effect of the doping. Interestingly, by further increasing the monomer concentration, the NSGs-6 show similar performance with NSGs-3, which is attributed that the sandwich-like polymer/graphene oxide/ polymer has no enough surfaces to dope additional heteroatoms. These observations corroborate that N and S dual-doped graphene are very effective for generating more extrinsic defects and active sites and improving the electrochemical performance. 4. Conclusions A simple, general yet effective strategy is developed to prepare N and S dual-doped graphene via carbonization of heteroatomcontaining polymers functionalized GO. As potential anode materials for LIBs, the NSGs exhibit excellent properties in terms of cycling performance and rate capability, benefiting from the unique structure and heteroatom doping. Obviously, using heteroatom-containing polymers as the doped source seems to be the most applicable and very helpful strategy to prepare promising graphene-based electrode materials. The novel method would open up new opportunities in the development of dual or multi-doped graphene for applications in LIBs, supercapacitors, photoelectrochemical and water splitting. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21404014), Jilin Science & Technology Department (the development plan project of science and technology No. 20150520002JH) and Training Programs of Innovation and Entrepreneurship for Undergraduates (No. 2014S034). References [1] M. Winter, J.B. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (2004) 4245–4259. [2] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [3] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496–499. [4] B.K. Guo, X.Q. Wang, P.F. Fulvio, M.F. Chi, S.M. Mahurin, X.G. Sun, S. Dai, Softtemplated mesoporous carbon-carbon nanotube composites for high performance lithium-ion batteries, Adv. Mater. 23 (2011) 4661–4666. [5] Y.W. Zhu, S. Murali, W.W. Cai, X.S. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906–3924. [6] X.L. Huang, R.Z. Wang, D. Xu, Z.L. Wang, H.G. Wang, J.J. Xu, Z. Wu, Q.C. Liu, Y. Zhang, X.B. Zhang, Homogeneous CoO on graphene for binder-free and ultralong-life lithium ion batteries, Adv. Funct. Mater. 23 (2013) 4345–4353. [7] S.M. Paek, E. Yoo, I. Honma, Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with three-dimensionally delaminated flexible structure, Nano. Lett. 9 (2009) 72–75. [8] X.G. Wang, X.F. Zhu, K. Yao, J.G. Zhang, Z. Liu, A SnO2/graphene compositeas a high stability electrode for lithium ion batteries, Carbon 49 (2011) 133–139. [9] S.B. Yang, X.L. Feng, S. Ivanovici, K. Müllen, Fabrication of grapheneencapsulated oxide nanoparticles: towards high-performance anode materials for lithium storage, Angew. Chem. Int. Ed. 49 (2010) 8408–8411. [10] A.K. Geim, Graphene: status and prospects, Science 324 (2009) 1530–1534. [11] S.Y. Wang, L.P. Zhang, Z.H. Xia, A. Roy, D.W. Chang, J.B. Baek, L.M. Dai, BCN graphene as efficient metal-free electrocatalyst for the oxygen reduction reaction, Angew. Chem. Int. Ed. 51 (2012) 4209–4212. [12] Z.Y. Lin, G. Waller, Y. Liu, M.L. Liu, C.P. Wong, Facile synthesis of nitrogen-doped graphene via pyrolysis of graphene oxide and urea, and its electrocatalytic activity toward the oxygen-reduction reaction, Adv. Energy Mater. 2 (2012) 884–888.
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