g-C3N4 composite as enhanced performance anode material for lithium ion batteries

g-C3N4 composite as enhanced performance anode material for lithium ion batteries

Chemical Physics Letters 715 (2019) 284–292 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/lo...

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Chemical Physics Letters 715 (2019) 284–292

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

SnO2 nanosheets/graphite oxide/g-C3N4 composite as enhanced performance anode material for lithium ion batteries

T

Huu Ha Trana, Phi Hung Nguyena, Van Hoang Caoa, Le Tuan Nguyena, Van Man Tranb, ⁎ My Loan Phung Leb, Sung-Jin Kimc, Vien Voa, a

Department of Chemistry, Quy Nhon University, 170 An Duong Vuong, Quy Nhon, Binh Dinh, Viet Nam Applied Physical Chemistry Laboratory, Faculty of Chemistry, VNUHCM – University of Science, 227 Nguyen Van Cu Street, District 5, Ho Chi Minh City, Viet Nam c Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea b

H I GH L IG H T S :

of SnO nanosheets on GO, g-C N and GO/g-C N supports were prepared. • Composites storage performance for the composites was investigated. • Lithium • The GO/g-C N support causes a better cycling performance for lithium storage. 2

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A R T I C LE I N FO

A B S T R A C T

Keywords: Graphite oxide g-C3N4 Lithium ion battery Nanosheets SnO2

SnO2/graphite oxide/g-C3N4 composite was synthesized by hydrothermally growing SnO2 nanosheets on graphite oxide/g-C3N4 using tin dichloride and mercapto acetic acid as tin source and structure-directing agent for SnO2 nanosheets, respectively. For comparison, SnO2/graphite oxide and SnO2/g-C3N4 composites were prepared with the same procedure except different supports. The obtained composites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric-differential thermal analysis and electrochemical properties. The results show that the SnO2/graphite oxide/g-C3N4 composite with the graphite oxide/ g-C3N4 support enhanced reversible capacity and cycling performance for lithium storage.

1. Introduction The environmental pollution has become a serious issue in recent years. Especially, the emission of carbon dioxide, a greenhouse gas derived mainly from the use of fossil fuels, has increased, while the fossil fuels have been depleted. Therefore, the demand for electric transports will be increased significantly in the near future. However, the recent rechargeable batteries technology does not meet this requirement because of their low energy density. Among the current rechargeable batteries, Lithium ion batteries (LIBs) are most potential to improve the capacity for this requirement [1]. Therefore, the investigation on materials with high lithium storage capacity is posed worldwide. Tin dioxide (SnO2) has been widely studied as gas sensors [2,3], supercapacitors [4,5]. Besides, SnO2 has attracted as a potential material for LIBs anode materials because of its high theoretical capacity of 790 mAh/g, low toxicity, high natural abundance, and safe working



[6–13]. As shown in the publications, alone SnO2 possesses some drawbacks, such as large volume variation during the lithium insertion/ extraction process, poor electronic conductivity, and poor capacity retention over extended charge–discharge cycling [14,15]. These limitations impede its practical application in LIBs technology. To overcome these drawbacks, some strategies including preparation of SnO2 in nano scale such as nanoparticles [12,13,16], nanowires [17], porous nanostructures [18], hollow [19], nanosheets [20], nanotubes [21]; and dispersion of SnO2 on supports have been applied [12,13,22,23]. By these ways, an improvement of capacity and stability for LIB anodes was demonstrated, which can be explained by that the void space of nanostructured materials could buffer the volume change, leading to equilibrium of charge – discharge structural state. For the composites of SnO2 on supports, the supports can not only cushion the internal stress induced during the volume change, but also make the composite more conductive. Regarding effect of morphology, nanosheets are of interest for LIBs anode materials. Two-dimensional structure of SnO2

Corresponding author.

https://doi.org/10.1016/j.cplett.2018.11.052 Received 5 September 2018; Received in revised form 23 November 2018; Accepted 26 November 2018 Available online 27 November 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.

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nanopores as a result of its combination of sp2- and sp-hybridized carbon, which has attracted much attention in materials for Lithium-ion battery anodes [29–31]. Besides, graphitic carbon nitride (g-C3N4), a type of organic polymer with graphene-like structure investigated widely in photocatalysis [32], has recently allured attention in investigation on LIB anode materials [33–39]. In these works, g-C3N4 was used as a support, which enhanced electrochemical properties for the LIB anodes. This phenomenon can be ascribed to some factors such as; (i) g-C3N4 with N-rich carbon framework can generate high electronic conductivity by creating more surface defects and active sites for lithium ion movement; (ii) The porous structure with high surface area of this material used as a matrix can reduce the strain of electrodes during charge/discharge process and provide a shorter diffusion pathway for lithium ions and electrons [34]. Furthermore, with the graphite-like structure, g-C3N4 can form hydrogen bonding with other appropriate species, such as GO into a single hybrid material for harnessing their mutual 2D–2D molecular interactions, which can improve cycling performance of the obtained LIB anode materials [36–39]. However, rare publications have been reported on this material as supports for LIB anodes and more investigation on g-C3N4 for LIB anodes materials should be carried out. In this paper, we reported the preparation of SnO2 nanosheets/GO/ g-C3N4 composite, in which, the SnO2 nanosheets were grown on the GO/g-C3N4 support. This material was investigated on its lithium storage ability.

Fig. 1. XRD patterns of SO (a), SGO (b), SCN (c), SGC (d), g-C3N4 (e) and GO (f).

nanosheets is dynamic, which can reduce the volume changes [24,25]. However, few works on SnO2 nanosheets have been published [20,24,25]. The supports for dispersion of the active materials normally require the following features: (i) large surface area, (ii) no hindrance of storing lithium ions, and (iii) good conductivity. The using of supports reduces the internal stress induced during the volume change as well as enhances the ionic conductivity of composite. Thus, carbon materials are usually used in order to satisfy these conditions. Among the carbon materials, graphite oxide (GO) has been widely investigated as a support for LIBs anode materials [26–28]. Recently, graphdiyne, an artificial carbon allotrope, possesses 2D framework with in-plane

2. Experimental 2.1. Synthesis of materials All the chemicals were purchased from Sigma-Aldrich and used as received. Graphite oxide (GO) was synthesized by a modified Hummers

Fig. 2. SEM images of SO (a), SGO (b), SCN (c) and SGC (d). 285

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Fig. 3. SEM image (a); and carbon (b), nitrogen (c), oxygen (d) and tin (e) element mapping images of SGC.

2.2. Characterization of material structure

method [40]. In a typical synthesis of the SnO2 nanosheets/GO/g-C3N4 composite, firstly, GO/g-C3N4 was prepared by the following procedure. GO (30 mg) and urea (5 g) were well ground, which was then transferred to a furnace and heated up to 550 °C for 2 h under flowing N2 at a rate of 10 °C/min. After washing two times with water, once with ethanol and drying at 80 °C overnight, the resulting dark yellow powder was referred as GO/g-C3N4. In the next step, GO/g-C3N4 (3 mg) was dispersed in 40 mL of 10 mM mercapto acetic acid solution as the morphology directing agent of SnO2 by ultrasonication for 30 min. HCl solution (37 wt%, 0.3 mL) was then added to the mixture with stirring for 15 min. 100 mg of tin(II) chloride dehydrate (SnCl2·2H2O) and 0.5 g of urea were added to the resulting mixture. After stirring for 1 h without cover, the reaction mixture was then transferred to an autoclave and heated at 150 °C for 10 h. The autoclave was then naturally cooled down to room temperature. The light brown solid was separated by centrifugation, washed with ethanol three times, dried at 80 °C for 12 h and referred as SGC. For comparison, SnO2 nanosheets, SnO2 nanosheets/GO and SnO2 nanosheets/g-C3N4 were prepared with the same procedure above except absence of GO/g-C3N4, replacing GO/gC3N4 by GO and replacing GO/g-C3N4 by g-C3N4, respectively, and denoted as SO, SGO and SCN. g-C3N4 was synthesized by the same procedure for the preparation of GO/g-C3N4 except absence of GO.

X-ray diffraction (XRD) of the samples was measured on a Bruker D8 Advance. Scanning electron microscopy (SEM) and element mapping images were recorded on the JEOL JSM-600F. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a JEOL JEM-2100F. Infrared spectra were conducted on IRAffinity-1S, Raman scattering spectroscopy measurements were carried out with 780 nm excitation using a FT-IR spectrometer Bruker Vertex 70 with Raman module Bruker RAM II. X-ray photoelectron spectroscopy (XPS) was conducted using Theta Probe AR-XPS spectrometer (Thermo Fisher Scientific). Thermogravimetricdifferential thermal analysis (TG-DTA) was characterized on Labsys TG SETARAM under air flow. 2.3. Characterization of electrochemical properties For electrochemical characterization of the materials, CR 2032 coin cells were used, in which the working electrode, lithium foil counter electrode, electrolyte (80 μL) and separator (Celgard 2325) were assembled in an argon-filled glove box with contain of water and oxygen below 1 ppm. The working electrodes were prepared by casting the 286

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Fig. 4. TEM image and SAED pattern (the upper part of inset) of SGC.

slurry method. Typically, 10 mg of g-C3N4 was mixed well with 2 droplet of Tripton-X to form homogeneous suspension. The final slurry was casted on the fluorine-doped tin oxide (FTO) glass with the area of 1 cm × 1 cm and dried on the hot plate at 300 °C for 15 min, then stored at 80 °C. The EIS tests were performed in a 0.1 M KCl solution with a frequency range from 0.01 Hz to 100 kHz at 0.3 V, and the amplitude of the applied sine wave potential in each case was 5 mV.

3. Results and discussion As mentioned earlier, in order to compare various supports, three composites based on SnO2, SnO2 nanosheets/GO (SGO), SnO2 nanosheets/g-C3N4 (SCN) and SnO2 nanosheets/GO/g-C3N4 (SGC) were prepared. The crystal structure and phase purity of the materials were characterized by XRD (Fig. 1). All diffractions of Fig. 1a are corresponding to the (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 2 0), (3 1 0) and (3 0 1) planes of the tetragonal rutile structure of SnO2, (JCPDS no. 411445). However, these peaks are not sharp, which may be attributed to that this material possesses low crystallinity or nanostructure. For GO (Fig. 1f), a characteristic peak at 10.7° corresponding to (0 0 2) plane with interlayer separation of 0.826 nm confirms graphite oxide structure [27]. Fig. 1e shows a characteristic peak at 27.3° corresponding to (0 0 2) plane of g-C3N4 [32]. The XRD patterns of the SGO, SCN and SGC composites are also shown in Fig. 1. It can be observed that all the diffraction peaks corresponding SnO2 in Fig. 1a can be seen in the patterns for the composites, which demonstrates that SnO2 is a main component of the materials. Besides, Fig. 1b shows a week peak at 10.7°, indicating the presence of a small content of GO in SGO. However, this peak cannot be detected for SGC, which can be explained by presence of a smaller GO amount in this material compared to that in SGO. The characteristic peak for g-C3N4 at 27.4° cannot be observed in SCN and SGC, which can be ascribed to that this peak is overlapped by the peak corresponding (1 1 0) plane of SnO2. These results demonstrate the presence of a relatively large content of SnO2 in the composites. Morphology of the samples was characterized by SEM (Fig. 2). It is worth to note that for all the samples, nanosheets agglomerated to form cauliflower-like morphology were clearly obtained. Without using mercapto acetic acid in the reaction mixture, no nanosheets were observed (not shown), indicating that mercapto acetic acid plays a

Fig. 5. IR spectrum of SO (a), SGO (b), SCN (c), SGC (d), g-C3N4 (e) and GO (f).

slurry containing of 75 wt% active material, 15 wt% black carbon, and 10 wt% polyacrylic acid (PAA, MW 450,000) as a binder onto a copper foil, which was dried in a vacuum oven at 110 °C for 12 h and then punched into 16 mm-diameter disks. The mixture of 1.0 M LiPF6 in ethylene carbonate, ethyl diethyl carbonate, and dimethyl carbonate corresponding volume ratio of 3:3:4, containing 10 wt% of fluoroehtylene carbonate was used as electrolyte. The cyclic voltammetry was conducted at a scan rate of 1 mV/s between 0.01 and 1.5 V (vs. Li/ Li + ) using SGC electrode as a representative working one, and Li metal as counter and reference electrodes. For evaluation of electrochemical cycling performance, the cells were cycled between 0.01 and 2.0 V at a rate of C/10 for the first cycle and then charge/discharge at 1C for the additional cycles on a Biologic potentiostat at 25 °C. The specific capacity was calculated on the weight of SnO2. Electrochemical impedance spectroscopy (EIS) tests were performed with a voltage of 5 mV amplitude over a frequency range of 100 kHz–0.01 Hz. The EIS for g-C3N4 was carried out on a Autolab PGSTAT302N electrochemical workstation using a standard three-electrode cell with a Pt sheet and an Ag/AgCl electrode as the counter electrode and the reference electrode, respectively. The electrode was prepared following 287

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Fig. 6. XPS spectra of SGC.

For more information on morphology, a representative sample, SGC, was characterized by TEM (Fig. 4). SnO2 nanosheets with thickness of about 12 nm and the presence of GO matrix can be elucidated. The inset also shows the SAED pattern of SGC, which is in agreement with the cassiterite structure (JCPDS Card No. 41-1445). There are two diffraction rings corresponding to (1 1 0), (1 0 1) planes from the center to the outside [26]. Fig. 5 shows IR spectra of the samples. For GO, the peaks at 1400 cm−1 and 1622 cm−1 are attributed to carbonyl groups. The presence of carboxyl groups can be detected at 1730 cm−1, 1233 cm−1 and 1058 cm−1. The peak at 1233 cm−1 is also attributable to epoxy groups [42]. Fig. 5e shows a sharp peak at 810 cm−1 due to the characteristic breathing mode of the triazine units and peaks in the range 1247–1642 cm−1 attributable to the vibration of the CN heterocycles were obtained. This observation is in accordance with the IR spectrum of g-C3N4 reported in literatures [34,35]. In Fig. 5a, it can be seen clearly that only one strong peak at 640 cm−1 is appeared, which may come from Sn-O bond in SnO2 [28]. For the composites, the shape of IR spectra is very close to that of SO, indicating that a greater content of SnO2 in these samples. XPS technique was used to detect chemical states of elements on surface of the materials and the results of a representative sample, SGC, are presented in Fig. 6. For the C 1s (Fig. 6a), three peaks at 284.6, 286.0, and 287.8 eV were observed, which can be attributed to sp2 CeC, CeN and sp2 C atoms from the aromatic rings NeC]N, respectively [43]. The earlier one may comes from GO and the latter peaks may correspond to bonds in g-C3N4. Deconvolution of the N 1s peaks (Fig. 6b) shows three peaks at 398.3, 399.0, and 400.8 eV, which

Fig. 7. TG-DTA curves of SO (a), SGO (b), SCN (c) and SGC (d).

significant role in control of shape for SnO2, which is accordant with the previous publications [33,41]. In principle, for composites, homogeneity is very important. In order to clarify distribution of components in the composites, a representative material, SGC was characterized by element mapping technique (Fig. 3). The carbon, nitrogen, oxygen and tin element mapping images of SGC indicate a highly homogeneous distribution of the elements in SGC. A high signal density of Sn and O with a low one of N can be observed, reflecting a larger content of SnO2 compared to GO and g-C3N4. 288

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Cyclic voltammetry (CV) measurement was performed to characterize the electrochemical properties of the samples using electrode of a representative sample, SGC in the range 0.01–1.5 V with a scanning rate of 1 mV/s. The CV curves of the first 3 cycles are shown in Fig. 8. The Li+ insertion/deinsertion reactions in LIBs using SnO2 as an anode material were previously reported as following equations [45]: SnO2 + 4Li+ + 4e → Sn + 2Li2O

(1)

Sn + xLi + xe ⇄ LixSn (4,4 ≥ x ≥ 0)

(2)

+

The step (1) is irreversible and (2) is reversible. In the first cathode sweep, irreversible peak at 0.89 V is attributed to decomposition of SnO2 to Sn (step 1) and formation of a solid electrolyte interface (SEI) [12], which disappeared in the next cycles. Also in this sweep, a peak at about 0.13 V related to alloying between Li and Sn (step 2) is observed [12]. In the first anode sweep, a strong peak appeared at about 0.57 V may be due to de-alloying LixSn. For the second and third cathode sweeps, the peak at 0.13 V shifted to higher potential and became clearer. In these anode sweeps, a strong peak at 0.53 V and weak peaks at 0.64, 0.75, and 0.81 V, may be attributed to subsequent regeneration of Li3.5Sn, Li2.33Sn, LiSn, and Sn, respectively, from delithiation of Li4.4Sn [46]. This result indicates complete conversion of SnO2 into Sn. Fig. 9 shows the charge-discharge profiles of the samples in the range of 0.01–1.5 V at a current density of 60 mA/g for the first cycle and 600 mA/g for the next cycles. In the first cycle, two plateaus at ∼0.84 V and around 0.14 V can be observed from the discharge curve. The first one is clear and can be attributed to the formation of Sn from SnO2, which becomes indiscernible in the next cycles. The second one is poorly defined, which may be ascribed to formation of LixSn alloy. This plateau becomes clearer in the following cycles. For the charge curves, one main plateau at ∼0.5 V is observed, which may correspond to the de-alloying LixSn. This one still remains in the latter cycles. These phenomenons are in good agreement with the CV results, and nearly the same for the samples, SO, SGO, SCN and SGC. A high initial discharge capacity in the range of 1200–1300 mAh/g can be obtained for the samples, which is normally observed in SnO2-based anode materials [12]. This meets the fact that SnO2 is irreversibly reduced to metallic Sn during the first discharge as described by Eq. (1). The cycling performance of the anodes using the samples as active

Fig. 8. CV curves of the first 3 cycles for anode prepared from SGC with a scan rate of 1 mV/s in the range of 0.01–1.5 V.

demonstrates the presence of pyridinic, pyrrolic, and graphitic N, respectively [44]. For Sn element (Fig. 6c), two peaks at 495.0 and 486.5 eV corresponding to Sn 3d5/2 and Sn 3d3/2 can be observed. The two peaks are symmetric and possess an energy separation of 8.5 eV and a spin-orbital branching ratio of 1.67, which agrees well with the reports on SnO2 [12,26]. The O 1 s XPS spectrum is fitted into three peaks at 530.3, 531.5 and 532.3 eV corresponding to SneO, C]O and OeH, respectively [26]. These results further demonstrate SGC consisting of three components, SnO2, GO and g-C3N4. Thermal properties of these materials were also investigated by TGDTA. Fig. 7 shows the two steps of weight loss for all the samples. The first one is from room temperature to 150 °C, which may be caused by physically adsorbed water. The second, main loss occurring in the temperature range from 400 to 600 °C can be assigned to combustion of GO and g-C3N4. The residual weight percentages are 99%, 95%, 98% and 97% corresponding to SnO2 in SO, SGO, SCN, SGC, respectively, which is consistent with the above characteristics that the samples contain a significant amount of SnO2.

Fig. 9. Charge–discharge profiles of SO (a), SGO (b), SCN (c) and SGC (d). 289

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Fig. 10. Cycling performance (a–c) and rate performance (d) of the samples.

first ten cycles for all the materials, while from the 10th to 100th cycle, a weak decline was observed except for SO. After 100 cycles, the discharge capacity retentions of 41.4, 53.7, 52.0 and 65.2% for SO, SGO, SCN and SGC, respectively, were observed. The better capacity retention of SGC compared to that of SO, SGO and SCN can be elucidated in Fig. 10c, which indicates a more stable cycling performance. After the first cycle, the coulombic efficiencies are improved and closed to 98% for all the samples. Fig. 10d shows the cyclic stability of the electrodes under various current rates within 24 cycles. When the current density increased from 0.1C to 10C, the reversible capacity shows a slight decrease for the SGO, SCN and SGC composites, while a significant decrease in the capacity for alone SnO2 (SO) was observed. This phenomenon can be explained by that the use of supports including GO, g-C3N4 and GO/gC3N4 seems to maintain the homogeneous distribution of SnO2 against agglomeration and/or structural collapse of SnO2 nanosheets during repeated charge/discharge cycling, resulting in an improved cycling performance, especially at high-rates. Among the supports, GO/g-C3N4 can improve cycling performance for SGC. When the current density reversed to 0.1C, the capacity recovery of SGC is also better that of SGO, SCN. The superior capacity retention and good rate capability of SGC can be explained by the structural characteristics of layered GO/g-C3N4 hybrid support. The layered GO can provide a electron- and ion-conductive medium, leading to good reaction kinetics. Besides, integration of mesoporous g-C3N4 nanosheets into the GO to form composite can

Fig. 11. Nyquist plots of the SGO, SCN and SGC electrodes after 5 cycles.

materials are shown in Fig. 10(a–c). In the first cycle (0.1C), the discharge capacities of 1456.1, 1228.3, 1323.4 and 1230.6 mAh/g and initial coulombic efficiencies of 47.2, 55.4, 53.6 and 51.0% for SO, SGO, SCN and SGC, respectively, were obtained. The low initial coulombic efficiencies are ascribed to the irreversible reaction of SnO2 at the first cycle (Eq. (1)) and the formation of SEI by electrolyte decomposition [47]. It can be seen a strong reduction of capacities in the 290

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to the previous report [50]. This phenomenal can be attribute to the partial conversion from π-bonds to σ-bonds in tri-s-triazine structure of g-C3N4, caused by structural distortion [50]. 4. Conclusion The composites of SnO2 nanosheets agglomerated to form cauliflower-like morphology on GO, g-C3N4 and GO/g-C3N4 supports were successfully synthesized using mercapto acetic acid as a morphologycontrolled agent for SnO2 nanosheets in hydrochloric acid medium. The composites with the supports exhibited an improved cycling performance compared to those of the stand-alone SnO2 nanosheets. Among the supports, GO/g-C3N4 with a unique porous layered structure provides an improved cycling performance. This result presents an advantage of combination of GO and g-C3N4 to prepare potential support for anode materials of lithium ion batteries. However, more investigation on usage of g-C3N4 as a support for other active materials in lithium ion batteries electrodes should be conducted.

Fig. 12. Raman spectrum of g-C3N4.

Declaration of interests We, all of the authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research is funded by Vietnamese Ministry of Education and Training under the grant B2016-DQN-01. The authors also thank the Vietnam National Foundation for Science and Technology Development (No. 104.06–2015.94) for partial financial support to this work. References [1] C.M. Hayner, X. Zhao, H.H. Kung, Materials for rechargeable lithium-ion batteries, Annu. Rev. Chem. Biomol. Eng. 3 (2012) 445–471. [2] X. Han, M. Jin, S. Xie, Q. Kuang, Z. Jiang, Y. Jiang, Z. Xie, L. Zheng, Synthesis of tin dioxide octahedral nanoparticles with exposed high-energy 221 facets and enhanced gas-sensing properties, Angew. Chem. Int. Ed. 48 (2009) 9180–9183. [3] G. Xi, J. Ye, Ultrathin SnO2 nanorods: template- and surfactant-free solution phase synthesis, growth mechanism, optical, gas-sensing, and surface adsorption properties, Inorg. Chem. 49 (2010) 2302–2309. [4] K.C. Ng, S.W. Zhang, C. Peng, G.Z. Chen, Individual and bipolarly stacked asymmetrical aqueous supercapacitors of CNTs/SnO2 and CNTs/MnO2 nanocomposites, J. Electrochem. Soc. 156 (2009) A846–853. [5] R.K. Selvan, I. Perelshtein, N. Perkas, A. Gedanken, Synthesis of hexagonal-shaped SnO2 nanocrystals and [email protected] nanocomposites for electrochemical redox supercapacitors, J. Phys. Chem. C 112 (2008) 1825–1830. [6] J.F. Ye, H.J. Zhang, R. Yang, X.G. Li, L.M. Qi, Morphology-controlled synthesis of SnO2 nanotubes by using 1D silica mesostructures as sacrificial templates and their applications in lithium-ion batteries, Small 6 (2010) 296–306. [7] J.S. Chen, C.M. Li, W.W. Zhou, Q.Y. Yan, L.A. Archer, X.W. Lou, One-pot formation of SnO2 hollow nanospheres and α[email protected] nanorattles with large void space and their lithium storage properties, Nanoscale 1 (2009) 280–285. [8] Y. Deng, C. Fang, G. Chen, The developments of SnO2/graphene nanocomposites as anode materials for high performance lithium ion batteries: a review, J. Power Sources 304 (2016) 81–101. [9] J.S. Chen, X.W. Lou, SnO2-based nanomaterials: synthesis and application in lithium-ion batteries, Small 9 (2013) 1877–1893. [10] Y. Zhao, J. Li, N. Wang, C.X. Wu, G.F. Dong, L.H. Guan, Fully reversible conversion between SnO2 and Sn in [email protected]@PPy coaxial nanocable as high performance anode material for lithium ion batteries, J. Phys. Chem. C 116 (2012) 18612–18617. [11] C. Wang, Y. Zhou, M.Y. Ge, X.B. Xu, Z.L. Zhang, J.Z. Jiang, Large-scale synthesis of SnO2 nanosheets with high lithium storage capacity, J. Am. Chem. Soc. 132 (2010) 46–47. [12] Md.S.A.S. Shah, J. Lee, A.R. Park, Y. Choi, W.J. Kim, J. Park, C.H. Chung, J. Kim, B. Lim, P.J. Yoo, Ultra-fine SnO2 nanoparticles doubly embedded in amorphous carbon and reduced graphene oxide (rGO) for superior lithium storage, Electrochim. Acta 224 (2017) 201–210. [13] V.M.H. Ng, S. Wu, P. Liu, B. Zhu, L. Yu, C. Wang, H. Huang, Z.J. Xu, Z. Yao, J. Zhou, W. Que, L.B. Kong, Hierarchical SnO2-graphite nanocomposite anode for lithiumion batteries through high energy mechanical activation, Electrochim. Acta 248 (2017) 440–448. [14] Q. Tian, Z. Zhang, J. Chen, L. Yang, S. Hirano, Carbon [email protected] SnO2

Fig. 13. Nyquist plot of g-C3N4.

support forming a stable SEI layer and accommodating the volume change during the lithiation/delithiation process, which significantly improves the cycle performance of the electrode. Additionally, the mesoporous g-C3N4 nanosheets can be beneficial for mass transport of ions and electrolyte, resulting to a significantly enhanced usage efficiency of active materials [33–39]. To support this assumption, the EIS measurements for the electrodes prepared from the materials were carried out. The Nyquist plots of the SGO, SCN and SGC electrodes after 5 cycles in Fig. 11 show that the charge transfer resistance of SGC electrode possesses a smaller value than that of SGO, SCN electrodes, indicating that the presence of GO/g-C3N4 as a support can improve electron and lithium ion transport for SGC. As mentioned earlier, in this paper, we would like to demonstrate that g-C3N4 can be used as a good support for LIBs anodes. To further investigate physicochemical properties of g-C3N4, the Raman spectroscopy and EIS were measured. For the Raman spectroscopy, to overcome the fluorescence interference, we employed a laser of 780 nm for excitation, and obtained the Raman spectrum in Fig. 12. Two intensive peaks at 708.8 and 1231.2 cm−1 corresponding to heptazine ring breathing mode and stretching vibration modes of C]N and CeN heterocycles, respectively, in g-C3N4 can be observed [48]. Besides, the peaks with lower intensity at 790.0, 980.0, 1151.5 and 1312.1 cm−1 can be assigned to vibration modes of melem, an intermediate product of the process to form g-C3N4 from urea precursor [49]. Fig. 13 shows the Nyquist plot of g-C3N4. This result is appropriate 291

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