Nitrogen-Doped Carbon-Encapsulated SnO2@Sn Nanoparticles

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Nitrogen-doped Carbon-encapsulated SnO2@Sn Nanoparticles Uniformly Grafted on Three-dimensional Graphene-like Networks as Anode for High-performance Lithium Ion Batteries Yunyong Li, Haiyan Zhang, Yiming Chen, Zhicong Shi, Xiaoguo Cao, Zaiping Guo, and Pei Kang Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08340 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 14, 2015

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ACS Applied Materials & Interfaces

Nitrogen-doped Carbon-encapsulated SnO2@Sn Nanoparticles Uniformly Grafted on Three-dimensional Graphene-like Networks as Anode for High-performance Lithium Ion Batteries

Yunyong Lia, Haiyan Zhanga, *, Yiming Chena, Zhicong Shia, Xiaoguo Caoa, Zaiping Guoa, c,*, and Pei Kang Shenb, *

a

Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of

Materials and Energy, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, China b

Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University,

Nanning, Guangxi, 530004, PR China c

Institute for Superconducting and Electronic Materials, School of Mechanical, Materials and

Mechatronics Engineering, University of Wollongong, North Wollongong, New South Wales 2500, Australia

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ABSTRACT A peculiar nanostructure consisting of nitrogen-doped, carbon-encapsulated (N-C) SnO2@Sn nanoparticles grafted on three-dimensional (3D) graphene-like networks (designated as N-C@SnO2@Sn/3D-GNs) has been fabricated via a low-cost and scalable method, namely an in situ hydrolysis of Sn salts and immobilization of SnO2 nanoparticles on the surface of 3D-GNs, followed by an in situ polymerization of dopamine on the surface of the SnO2/3D-GNs, and finally a carbonization. In the composites, three-layer core-shell NC@SnO2@Sn nanoparticles were uniformly grafted onto the surfaces of 3D-GNs, which promotes highly efficient insertion/extraction of Li+. In addition, the outermost N-C layer with graphene-like structure of the N-C@SnO2@Sn nanoparticles can effectively buffer the large volume changes, enhance electronic conductivity, and prevent SnO2/Sn aggregation and pulverization during discharge/charge. The middle SnO2 layer can be changed into active Sn and nano-Li2O during discharge, as described by SnO2 + Li+ → Sn + Li2O, while the thusformed nano-Li2O can provide a facile environment for the alloying process and facilitate good cycling behavior, so as to further improve the cycling performance of the composite. The inner Sn layer with large theoretical capacity can guarantee high lithium storage in the composite. The 3D-GNs, with high electrical conductivity (1.50 × 103 S m-1), large surface area (1143 m2 g-1), and high mechanical flexibility, tightly pin the core-shell structure of the N-C@SnO2@Sn nanoparticles and thus lead to remarkably enhanced electrical conductivity and structural integrity of the overall electrode. Consequently, this novel hybrid anode exhibits highly stable capacity of up to 901 mAh g-1, with ~89.3% capacity retention after 200 cycles at 0.1 A g-1 and superior high rate performance, as well as a long lifetime of 500 cycles with 84.0% retention at 1.0 A g-1. Importantly, this unique hybrid design is expected to be extended to other alloy-type anode materials such as silicon, germanium, etc.


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KEYWORDS: nitrogen-doped carbon, SnO2, Sn, three-dimensional graphene, lithium ion battery



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INTRODUCTION Lithium-ion batteries (LIBs) have attracted great attention during the past several decades owing to their high energy density, light weight, and long cycle life. Nevertheless, current commercial graphite anodes with a low theoretical capacity of 372 mAh g-1 display limited energy capacity and rate performance when utilized in high energy consuming applications such as electric vehicles (EVs), plug-in hybrid electric vehicles (PHEVs), and smart grids1-3. Therefore, numerous efforts have been devoted to developing new electrode materials to meet the ever-growing performance demands on LIBs4-6. In this context, metallic Sn has been extensively explored as a promising alternative anode material for high-performance LIBs, due to its high theoretical capacity (992 mAh g-1 or 7262 mAh cm-3 for Li4.4Sn), good electronic conductivity, and moderate operating voltage that can improve the safety of LIBs during rapid charge/discharge processes7-10. Unfortunately, large volume expansion (~300%) commonly occurs in Sn during lithium ion insertion/extraction, which causes not only severe pulverization and subsequent electrical disconnection from the current collector, but also aggregation of Sn nanoparticles and continual formation of a very thick solid electrolyte interphase (SEI) on the Sn surfaces during cycling, thereby leading to rapid capacity decay and poor cyclability11-14. Therefore, it is still a critical challenge to further improve or optimize the electrochemical performance of Sn electrode. To overcome these obstacles, various strategies have been developed to improve the structural integrity of Sn-based materials, such as optimizing the particle size or morphology9, 15

, and fabricating carbon hybrids11, 16-18. Among these Sn-based materials, Sn-carbon hybrids,

such as Sn nanoparticles embedded in porous carbon16,

18-20

, and carbon-encapsulated Sn

nanostructures21-24 have been demonstrated as effective structures. These carbon matrices can provide spaces to buffer the mechanical stress induced by the volume changes in the Sn nanostructures, resulting in significantly improved cycling performance. 4 ACS Paragon Plus Environment

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Recently, three-dimensional (3D) graphene, as a special structure of graphene, has drawn special attention to its possibilities as a support for metal or metal oxides, in order to further enhance the electrochemical performances of LIB electrodes. This is due not only to the intriguing properties of this type of graphene, but to additional characteristics of its selfsupported structure, its high specific surface area and porosity25-27. Hence, Sn-3D graphene hybrids with special structures, such as directly decorated Sn-3D graphene28-30, sandwich-like graphene-supported hybrids31-32, and 3D porous graphene networks anchored with Sn@Graphene (Sn@G-PGNWs) hybrids7, have been gradually developed25-26, 30. Although these hybrid anodes can significantly improve the electrochemical performances of LIBs, they still have the following problems: (i) in some Sn-3D graphene hybrids28-30, the exposed active Sn nanostructures, which are simply decorated on the surfaces of 3D graphene, would come into direct contact with the electrolyte and thus result in side reactions at the interface between Sn and the electrolyte; meanwhile, the active materials can easily fall off of the graphene nanosheets or electrodes due to their volume expansion during the long cycling processes, thus resulting in limited cycling performance29, 33; (ii) delamination at multiple-interfaces of the sandwich-like graphene-supported hybrids can be introduced by the repeated chargedischarge processes in the multilayer structure, which leads to decreased stability of the batteries31-32; (iii) relatively complex and strictly controlled conditions (resulting in high cost) are necessary for the fabrication of some Sn-3D graphene hybrids, make them relatively unsuitable for large-scale application7. In addition, it is well known that nitrogen-doped carbon (N-C) materials have attracted great attention for improving the cycling performance and rate capacity of Sn-based composites because they can improve their electronic conductivity, the ionic permeability of the carbon layer, charge transfer at the interface, and the stability of SEI films11,

23, 34-38

.

Therefore, it will be interesting to design and fabricate a novel hybrid structure of Sn-3D graphene with encapsulation by N-C by a low-cost and scalable method. 5 ACS Paragon Plus Environment


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In this study, we have designed a unique nanostructure of N-C encapsulated SnO2@Sn nanoparticles grafted on 3D graphene-like networks (3D-GNs) (designated as NC@SnO2@Sn/3D-GNs composite), which is fabricated via a low-cost and scalable method, namely, in situ hydrolysis of Sn salts and immobilization of SnO2 nanoparticles on the surface of the 3D-GNs, followed by an in situ polymerization of dopamine (DA) on the surface of the SnO2/3D-GNs, and finally a carbonization, as shown in Scheme 1. In the composite, the threelayer core-shell N-C@SnO2@Sn nanoparticles were uniformly grafted on the surfaces of 3DGNs, which is favorable for highly efficient insertion/extraction of Li+. In addition, the thusformed N-C layer with graphene-like structure in the outermost layer of the N-C@SnO2@Sn nanoparticles can effectively buffer the large volume changes, enhance electronic conductivity,

and

prevent

SnO2/Sn

aggregation

and

pulverization

over

repeated

discharge/charge processes. The middle SnO2 layer can be changed into active Sn and nanoLi2O during discharge, as described by SnO2 + Li+ → Sn + Li2O, while the thus-formed nanoLi2O can provide a facile environment for the alloying process and facilitate good cycling behavior39-41, so as to further improve the cycling performance of the composite. The inner Sn layer possesses a large theoretical capacity, thereby guaranteeing high lithium storage in the composite. Moreover, the interconnected 3D porous graphene networks, with high electrical conductivity (1.50 × 103 S m-1), large surface area (1143 m2 g-1), and high mechanical flexibility, tightly pin the core-shell structure of the N-C@SnO2@Sn nanoparticles and thus lead to remarkably enhanced electrical conductivity and structural integrity of the overall electrode. Consequently, this novel hybrid anode exhibits a highly stable capacity of up to 901 mAh g-1, with ~89.3% capacity retention after 200 cycles at a current density of 100 mA g-1 and superior high rate performance in LIBs. RESULTS AND DISCUSSION


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As shown in Scheme 1, the synthesis of N-C@SnO2@Sn/3D-GNs involves an in situ hydrolysis and immobilization of SnO2 nanoparticles on the porous surfaces of 3D-GNs to obtain SnO2/3D-GNs composite, followed by an in situ polymerization of DA on the surface of SnO2/3D-GNs composites to yield a thin layer of polydopamine (PDA) that is coated on the surfaces of the SnO2/3D-GNs composite, and finally, a carbonization and simultaneous reduction of part of the SnO2 nanoparticles to Sn by the 3D-GNs. In the first step, SnO2/3DGNs composite was synthesized by an in situ hydrolysis of SnCl2 and immobilization of SnO2 nanoparticles on the porous surfaces of 3D-GNs. The 3D-GNs was first synthesized by a onestep ion-exchange/activation combination method using a cheap metal ion-exchange resin as the carbon precursor (see experimental section for details). The 3D-GNs shows a unique interconnected macroporous 3D network in scanning electron microscope (SEM) images (Figure 1A), which is similar to those previously reported42. After immobilization of SnO2 nanoparticles on the porous surfaces of the 3D-GNs, the SnO2/3D-GNs composite exhibits similar morphology in transmission electron microscope (TEM) images (Figure 1B) of the 3D macroporous structures to the pristine 3D-GNs sample (Figure 1A), suggesting that that the anchoring of SnO2 nanoparticles on 3D-GNs does not change the overall morphology of 3DGNs. The magnified TEM image (Figure 1C) exhibits a uniform distribution of SnO2 nanoparticles in highly dense regions. The ring-like mode in the selected-area electron diffraction (SAED) pattern (inset of Figure 1C) confirms the presence of polycrystalline SnO2 corresponding to the (110), (101), (221), and (301) crystalline planes of SnO2, which can be identified as tetragonal rutile-like SnO2 (PDF No. 41-1445, space group P42/mnm, a0 = b0 = 4.738 Å, c0 = 3.187 Å)43-45. The magnified high-resolution (HR) TEM image in Figure 1D shows highly crystalline SnO2 nanoparticles with the size of 3-4 nm anchored on the surfaces of the 3D-GNs, and well-defined lattice fringe of (110) plane of SnO2 with an interplanar spacing of 0.33 nm (PDF No. 41-1445) (Inset). Then the as-prepared SnO2/3D-GNs composite was coated with a thin layer of PDA by an in situ polymerization of DA in Tris7 ACS Paragon Plus Environment


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buffer solution (pH 8.5). As shown in Figure 1E, a similar 3D morphology was observed for the PDA@SnO2/3D-GNs composite were observed, suggesting the coating of PDA did not change the overall morphology of SnO2/3D-GNs. The magnified TEM image (Figure 1F) still displays uniformly dispersed SnO2 nanoparticles on the surfaces of the 3D-GNs, and a similar SAED pattern of SnO2 nanoparticles (inset of Figure 1F) was obtained, indicating that the coating of PDA did not change the dispersion or chemical structure of the SnO2 nanoparticles. The HR-TEM image (Figure 1G) of the PDA@SnO2/3D-GNs composite shows a smooth surface with an additional thin layer revealed in the edge (marked by arrows), indicating that the PDA layer had been successfully coated on the surfaces of the SnO2/3D-GNs composites. The further magnified HR-TEM (Figure 1H) image displays clear lattice fringes of (110) plane of SnO2 (PDF No. 41-1445) (Inset) and an obviously stacked layer structure for the PDA layer, which is mainly due to the planar conformation of the oligomers in melanin-like materials34, 46. In the final step, the N-C@SnO2@Sn/3D-GNs composites were synthesized by the carbonization of PDA@SnO2/3D-GNs composites at 700 oC for 1 h in N2. As shown in Figure 2(A and B), a large amount of uniformly dispersed N-C@SnO2@Sn nanoparticles were observed to be grafted onto the surfaces of 3D-GNs (more pictures in the Figure S1 in the Supporting Information). To clearly characterize the structures of the N-C@SnO2@Sn nanoparticles, three typical N-C@SnO2@Sn nanoparticles (C1, C2, and C3 in Figure 2) were analyzed by HR-TEM. The C1 nanoparticle of Figure 2, which was grafted on the surface of the 3D-GNs, clearly exhibits a three-layer core-shell structure and has a size of ~35 nm. In the outermost layer, a uniform thin N-C layer with a graphene-like structure (layered structure) was observed, as shown in image C1 of Figure 2. The N-C layer with a graphene-like structure was derived from the planar conformation of the oligomer in the melanin-like PDA, which accords with the reports of PDA as a precursor for preparing carbon with a layered structure (graphene-like materials)34, 46. For the second layer, this nanoparticle also shows a thin layer, 8 ACS Paragon Plus Environment

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which displays the well-defined lattice fringes of SnO2 (C1-1 of Figure 2) with an interplanar spacing of 0.33 nm, corresponding to the (110) planes of SnO2 (PDF No. 41-1445). The results demonstrate that the second layer is a SnO2 layer. The innermost layer (C1-2 of Figure 2) clearly presents a crystalline structure with an inter-planar spacing of 0.27 nm, corresponding to the (101) planes of Sn (PDF No. 04-0673). The formation of Sn in the innermost layer is mainly due to the reduction of part of the SnO2 nanoparticles by 3D-GNs at 700 oC in N2, corresponding to the reaction equation described by SnO2 + C → Sn + CO2, which is in accord with reports that SnO2 can be reduced to metallic Sn particles by carbon when the temperature increases to 700 oC47-48. In addition, the HR-TEM images of the other two N-C@SnO2@Sn nanoparticles (C2 and C3 in Figure 2) both clearly display a three-layer core-shell structure, and the lattice fringes of SnO2 in the middle layer and the lattice fringes of Sn in the inner layer were both clearly observed (C2-1 and C3-1 in Figure 2), respectively. According to the above results, we conclude that the three-layer core-shell structure of the NC@SnO2@Sn nanoparticles consists of a carbon layer in the outermost layer, SnO2 layer in the second layer, and Sn layer in the innermost layer, as illustrated in Figure 2D. The uniform distribution of N-C@SnO2@Sn nanoparticles was also confirmed by elemental mapping by TEM, as shown in Figure 3. Figure 3A shows the macroporous 3DGNs walls of N-C@SnO2@Sn/3D-GNs composite, which clearly display well-distributed NC@SnO2@Sn nanoparticles with uniform size. The TEM elemental mapping (Figure 3(B1B4)) of a N-C@SnO2@Sn nanoparticle, corresponding to the area outlined by the orange square in (B), demonstrated the uniform distribution of carbon (B1), nitrogen (B2), oxygen (B3), and tin (B4), respectively. The results demonstrated that the N-C and SnO2 were uniformly coated to form the outermost layer and the middle layer, respectively. To further evidence the structure and the composition of the composite, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the composite. Figure 4 shows the XRD patterns of 3D-GNs, and the SnO2/3D-GNs, PDA@SnO2/3D-GNs, 9 ACS Paragon Plus Environment


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and N-C@SnO2@Sn/3D-GNs composites. The XRD pattern of 3D-GNs shows a relatively sharp peak at 2θ = 26.2o, indicating a high degree of graphitization of 3D-GNs. After the immobilization of SnO2 on the surfaces of 3D-GNs, the SnO2/3D-GNs composite shows four prominent diffraction peaks at 2θ = 26.6o, 33.9o, 51.8o, and 65.9o, corresponding to the (110), (101), (211), and (301) peaks of tetragonal rutile-like SnO2 (PDF No. 41-1445, space group P42/mnm, a0 = b0 = 4.738 Å, c0 = 3.187 Å)43-45. The main (110) peak of the tetragonal SnO2 phase at 2θ = 26.6o almost overlaps with the main C (002) peak of the 3D-GNs matrix at 2θ = 26.2o. For the PDA@SnO2/3D-GNs composite, four prominent diffraction peaks at 2θ = 26.6o, 33.9o, 51.8o, and 65.9o were also observed, indicating that the chemical structure of SnO2 in the composite is not destroyed after the coating of PDA is applied on the surface of SnO2/3D-GNs composite, which is consistent with the TEM results. For the NC@SnO2@Sn/3D-GNs composite, the sharp diffraction peaks at 2θ = 26.6o, 33.9o, 37.9 o, 51.8o, and 65.9o correspond to the (110), (101), (211), and (301) peaks of tetragonal rutile-like SnO2 (PDF No. 41-1445, space group P42/mnm, a0 = b0 = 4.738 Å, c0 = 3.187 Å), respectively, while the sharp diffraction peaks at 2θ = 30.6o, 32.0o, 43.9 o, 44.9o, 55.3o, 62.5 o, 64.5 o, and 79.5o correspond to the (200), (101), (200), (211), (301), (112), (321), and (312) peaks of Sn (PDF No. 04-0673), respectively. These results further demonstrate the presence of SnO2 and Sn in the N-C@SnO2@Sn/3D-GNs composite. The composition and the valence states of elements in the above-mentioned composites were also determined by XPS, and the results are given in Figure 5. The peaks at ~284, ~400, ~487, ~495 and ~531 eV corresponded to the peaks of C 1s, N 1s, Sn 3d5/2, Sn 3d3/2, and O 1s, respectively. For the 3D-GNs, strong peaks in the C 1s spectrum and faint peaks in the O 1s spectrum were observed (Figure 5A), indicating that the 3D-GNs contains a small amount of oxygen-containing groups (~4.0 at%), which is mainly caused by the activation of KOH. For the SnO2/3D-GNs composite, the spectrum consists of peaks for Sn, O, and C (Figure 5A). 10 ACS Paragon Plus Environment

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The further high-resolution Sn 3d spectra (Figure 5B) of the SnO2/3D-GNs composite shows that the spin energy separation of Sn4+ 3d3/2 and Sn4+ 3d5/2, located at 495.8 and 487.3 eV, respectively, was 8.5 eV, which is in agreement with that reported for SnO2, indicating the 4+ oxidation state for Sn in the composite49-50. The results further confirmed the presence of SnO2 nanoparticles anchored on the 3D-GNs. For the PDA@SnO2/3D-GNs composite, the spectrum contains peaks for the four elements Sn, O, C, and N (Figure 5A). The N content is ~5.6 at.% in the composite, which is mainly derived from the C-N functional groups in the PDA structure34,

46, 51

. In addition, the high-resolution Sn 3d spectrum (Figure 5B) of

PDA@SnO2/3D-GNs composite shows two peaks with the same location as those of SnO2/3D-GNs composite, corresponding to their locations at 495.8 and 487.3 eV, respectively. This result demonstrates the 4+ oxidation state for Sn in the PDA@SnO2/3D-GNs composite, indicating that the oxidation state for Sn in the composite is not changed after the coating of PDA is applied on the surfaces of SnO2/3D-GNs composite, which is consistent with the TEM and XRD results. For the N-C@SnO2@Sn/3D-GNs composite, the spectrum consists of peaks for Sn, O, C, and N. The N content is ~3.9 at.% in the composite, which shows a little decrease compared with that in the PDA@SnO2/3D-GNs composite. This result may be caused by the decomposition of PDA in the process of carbonization. In addition, the highresolution N 1s spectrum (Figure 5C) of N-C@SnO2@Sn/3D-GNs composite shows that the nitrogen in composite consists of pyridinic, pyrrolic, and graphitic nitrogen species with binding energies at 398.7, 400.5, and 401.3 eV, respectively52-53. The area percentages of pyridinic, pyrrolic and graphitic nitrogen peaks are in the ratio of 15.8%:53.5%:30.7%, indicating that the nitrogen atoms doped into the composite are mainly in the form of pyrrolic and graphitic nitrogen. Furthermore, in the high-resolution Sn 3d spectrum (Figure 5B) of NC@SnO2@Sn/3D-GNs composite, two strong peaks, located at 495.8 and 487.3 eV, correspond to the peaks of Sn 3d3/2 and Sn 3d5/2 for Sn4+ respectively40, 54, while two faint peaks, located at 493.7 and 485.3 eV, correspond to the peaks of Sn 3d3/2 and Sn 3d5/2 for 11 ACS Paragon Plus Environment


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Sn(0) respectively40, 54, indicating the presence of SnO2 and Sn in the N-C@SnO2@Sn/3DGNs composite, which is in agreement with the TEM and XRD results. In addition, the weight percentages of Sn, O, N and C in the N-C@SnO2@Sn/3D-GNs composite were also determinated by TGA and elemental analyzer. The detailed processes were given in Supporting Information. The results show that the weight percentages of Sn, O, N and C in the composite is 66.01%, 2.92%, 1.21% and 29.86%, respectively (The content of carbon (containing 3D HPG and N-C) is 31.07%, the weight ratio of Sn to SnO2 is 55.18:13.75=4.01:1.). The porosity of the N-C@SnO2@Sn/3D-GNs composite was studied by nitrogen adsorption and desorption isotherms, and these isotherms and the pore-size distributions for 3D-GNs, and the SnO2/3D-GNs, PDA@SnO2/3D-GNs, and N-C@SnO2@Sn/3D-GNs composites are shown in Figure 6. The isotherm for the 3D-GNs (Figure 6A, open circles) exhibited the combined characteristics of type II/IV55, with a Brunauer-Emmett-Teller (BET) surface area of 1143 m2 g-1, and a total pore volume of 0.81 cm3 g-1. The corresponding density functional theory (DFT) pore size distribution of 3D-GNs is a hierarchical pore distribution with abundant micro-, meso-, and macropores (Figure 6B, open circles), which is consistent with our previous report42. After the SnO2 nanoparticles were anchored on 3D-GNs, the specific surface area and the total pore volume of the SnO2/3D-GNs composites were decreased (Figure 6 and Table S1), and the DFT pore size distribution mainly exhibits mesoand macropores (> 7 nm) but also contains a few micropores (Figure 6B). The results demonstrated that the small SnO2 nanoparticles may be mainly embedded in the micro- and mesopores (< 7 nm) of the 3D-GNs. The PDA@SnO2/3D-GNs composite exhibits a low specific surface area and total pore volume (Figure 6 and Table S1), while the DFT pore size distribution of PDA@SnO2/3D-GNs composite (Figure 6B) only exhibits large pores (> 7 nm). These results may mainly exist because the smooth PDA layer covered the surfaces of the SnO2/3D-GNs, leading to the complete filling or covering of the micro- and mesopores (< 12 ACS Paragon Plus Environment

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7 nm) of SnO2/3D-GNs. The N-C@SnO2@Sn/3D-GNs composite exhibits a specific surface area of ~105 m2 g-1, total pore volume of 0.36 cm3 g-1, and a broad pore size distribution form 7 to 100 nm (Figure 6, pink curve), which can provide sufficient void volume for the transport of lithium ions. In addition, compared with the PDA@SnO2/3D-GNs composite, the NC@SnO2@Sn/3D-GNs composite shows an obvious increase in the specific surface area and total pore volume, which may be mainly due to the carbonization of PDA and the formation of N-C@SnO2@Sn nanoparticles, leading to the formation or exposure of many large mesoporoes. Moreover, the powder conductivities of 3D-GNs and N-C@SnO2@Sn/3D-GNs composite were also examined by a four-probe method. The powder conductivity of the 3DGNs was about 1.50 × 103 S m-1, which is about three times higher than that of the KOHactivated graphene material56, and is also superior to those of most typically self-assembled 3D graphene (Table S2). The N-C@SnO2@Sn/3D-GNs composite had powder conductivity of about 1.52 × 103 S m-1, whcih is close to that of pristine 3D-GNs, indicating that the grafting of N-C@SnO2@Sn/3D-GNs nanoparticles on 3D-GNs did not block the electronic conducting pathways. For comparison, a Sn/3D-GNs sample was made by the same processes as the NC@SnO2@Sn/3D-GNs composite, but without the addition of dopamine hydrochloride. The XRD pattern of the Sn/3D-GNs sample (Figure S2) shows eleven sharp diffraction peaks, which exactly match the peaks of the standard PDF card of Sn (PDF No. 04-0673), indicating only the formation of Sn in this sample. These results further demonstrated that the SnO2 anchored on 3D-GNs was fully reduced to Sn without the coating of PDA on the surfaces of SnO2/3D-GNs. In addition, the TEM images of the Sn/3D-GNs sample (Figure S4A and S4B) show many irregular Sn nanoparticles of various sizes immobilized on the surfaces of 3DGNs, and the SAED pattern (Figure S4C) confirms the presence of polycrystalline Sn,



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corresponding to the (101), (211), (301), (321), and (312) crystalline planes of Sn (PDF No. 04-0673). From the above results, we know that three-layer core-shell N-C@SnO2@Sn nanoparticles with uniform sizes were obtained when there was a coating of PDA on the surfaces of SnO2/3D-GNs before heating at 700 oC in N2, while irregular nanoparticles of Sn with various sizes were formed when there was no coating of PDA on the surfaces of SnO2/3D-GNs before heating at 700 oC in N2, as shown in Figure S5. The results demonstrate that the PDA coating plays an important role in controlling the morphology of the nanoparticles anchored on the surface of 3D-GNs, which may be mainly because the PDA coated on surface of SnO2/3DGNs could protect the liquid Sn droplets (reduced by the 3D-GNs matrix) from arbitrary diffusion, evaporation and further agglomeration when the PDA@SnO2/3D-GNs sample was heated at 700 oC in N2, thereby forming three-layer core-shell N-C@SnO2@Sn nanoparticles with uniform size grafted on the surfaces of 3D-GNs. The electrochemical performance of the N-C@SnO2@Sn/3D-GNs composite as anode in LIBs was studied in comparison with Sn/3D-GNs composite and 3D-GNs. Figure 7A shows typical cyclic voltammograms (CV) of N-C@SnO2@Sn/3D-GNs composite electrode in the voltage range of 0.01-3.0 V with a constant scan rate of 0.1 mV s-1. In the first cathodic sweep process, the cathodic peak in the potential range of 0.45-1.55 V corresponds to the formation of a solid electrolyte interphase (SEI) and the reduction reaction of SnO2 and Li+, which is described as SnO2 + 4Li+ + 4e- → Sn(0) + 2 Li2O9, 49. This cathodic peak shifts to higher voltages (about 0.95 V) in the subsequent cycles, indicating the partially reversible reaction of SnO2 and Li+ 49. The peak at 0-0.43 V should be related to the formation of LixSn alloys and the insertion of Li+ into 3D-GNs57. In the anodic process, the anodic peak at 0.59 V represents the de-alloying process of LixSn57-58. This alloying/de-alloying Li-storage described by Sn + xLi+ + xe- → LixSn (0 ≤ x ≤ 4.4) is considered to be highly reversible and predominantly contributes to the reversible lithium storage capacity. The anodic peak at 1.31 V is related to 14 ACS Paragon Plus Environment

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the partially reversible reaction of SnO2 and Li+49, 59-60. All the reduction and oxidation peaks in the next 4 cycles are the same as those in the second cycle, indicating the good electrochemical reversibility and structural stability of the N-C@SnO2@Sn/3D-GNs composite electrode. The galvanostatic charge/discharge measurements were performed at the current density of 100 mA g-1 in the potential window of 0.01-3.0 V vs. Li+/Li. The specific capacity was calculated based on the total mass of the N-C@SnO2@Sn/3D-GNs composite. The typical charge/discharge curves in the 1st, 2nd, 5th, 10th, 20th, 50th, 100th, and 200th cycles are shown in Figure 7B. It is important to note that both charge and discharge profiles show little change from the second to the 200th cycle, further demonstrating that the N-C@SnO2@Sn/3D-GNs composite electrodes are stable during cycling. The cycling performance of the N-C@SnO2@Sn/3D-GNs composite, Sn/3D-GNs composite, and 3D-GNs samples at a current density of 100 mA g-1 are presented in Figure 7C. It can be seen that the initial coulombic efficiency (CE) of N-C@SnO2@Sn/3D-GNs, Sn/3D-GNs, and 3D-GNs is 56%, 46%, and 48%, respectively. Obviously, the initial CE of N-C@SnO2@Sn/3D-GNs is higher than another two. The results demonstrated that the N-C encapsulation layer in the N-C@SnO2@Sn/3D-GNs composite can efficiently improve the initial CE, which may be due to that the N-C encapsulation layer in the composite can effectively avoid Sn or SnO2 directly contacting with the electrolyte, thus reducing the decomposition of electrolyte and suppressing the SEI formation. As for the Sn/3D-GNs composite, the directly exposed active Sn nanoparticles would come into direct contact with the electrolyte and thus result in side reactions at the interface between Sn and the electrolyte, thereby accelerating the decomposition of electrolyte and forming a relatively thick SEI film78, 15, 38

. In addition, the N-C@SnO2@Sn/3D-GNs composite electrode shows the best

electrochemical performance, delivering a high reversible capacity of 901 mAh g-1 after 200 cycles, which is about 89.3% capacity retention compared with that in the second cycle (1009 15 ACS Paragon Plus Environment


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mAh/g). Here, the specific capacity in the second cycle for the N-C@SnO2@Sn/3D-GNs electrode is higher than the theoretical capacity of Sn (994 mAh g-1), which may be due to that the N-C@SnO2@Sn/3D-GNs composite owns high specific surface area (~105 m2 g-1) and great amount of large pores (> 7 nm) (the results was confirmed by N2 adsorption experiments, as shown in Figure 6 (pink curve)), thus existing numerous amount of defects in its surface, which can store extra lithium ions61-62. For comparison, the capacity of the Sn/3DGNs composite electrode decreased significantly with increasing cycle number, and the discharge capacity of the Sn/3D-GNs composites electrode was only 379 mAh g-1 after 200 cycles (~43.5% capacity retention). The severe capacity fading of the Sn/3D-GNs composites electrode should be attributed to its pulverization and exfoliation from the electrode during electrochemical processes63. As for the 3D-GNs, the material delivered a specific reversible capacity of 320 mAh g-1 after 200 cycles, indicating good cycling performance, but the value is much lower than that of N-C@SnO2@Sn/3D-GNs composite. Moreover, the capacity contribution of each component in the N-C@SnO2@Sn/3D-GNs electrode (such as that at the 200th cycle) was also illustrated in the Supporting Information. The results show that the contribution on the capacity of the carbon, Sn and SnO2, and interfacial lithium ion storage in the N-C@SnO2@Sn/3D-GNs electrode is 6.8% (60.9 mAh g-1), 73.0% (655.9 mAh g-1), and 20.3% (182.2 mAh g-1) at the 200th cycles, respectively. Figure 7D shows the rate performance of N-C@SnO2@Sn/3D-GNs composite and Sn/3DGNs composite. The N-C@SnO2@Sn/3D-GNs composite exhibited an initial discharge capacity of 958 mAh g-1 at current density of 0.1 A g-1. With increasing current density from 0.5 to 4.0 A g-1, the N-C@SnO2@Sn/3D-GNs composite exhibited an obvious advantage in rate capability. The N-C@SnO2@Sn/3D-GNs composite delivered discharge capacities of 790, 649, 491, and 417 mAh g-1 (~43.5% capacity retention) at current densities of 0.5, 1.0, 2.0, and 4.0 A g-1, respectively. Even under such rigorous testing conditions, the discharge capacity still recovered to 905 mAh g-1 when the cycling current was reduced back to 0.1 A g16 ACS Paragon Plus Environment

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1

, indicating that the N-C@SnO2@Sn/3D-GNs composite remained stable during the

extended rate cycling process. For comparison, the Sn/3D-GNs composite showed an initial discharge capacity of 824 mAh g-1 at current density of 0.1 A g-1 and only retained the discharge capacity of 50 mAh g-1 (~6.1% capacity retention) at a current density of 4.0 A g-1. Moreover, when the cycling current was reduced back to 0.1 A g-1, the discharge capacity only recovered to 414 mAh g-1. The results demonstrated that the N-C@SnO2@Sn/3D-GNs composite displayed excellent rate performance. In order to gain insight into the remarkable rate performance of N-C@SnO2@Sn/3D-GNs composite compared with Sn/3D-GNs composite, electrochemical impedance spectra (EIS) measurements were carried out and the Nyquist plots were shown in Figure S7. In the Nyquist plots, the high frequency semicircle (Rf) is associated with lithium-ion migration through the SEI film covered on the N-C@SnO2@Sn/3D-GNs or Sn/3D-GNs, the middle frequency semicircle is linked to charge transfer through the electrode-electrolyte interface and the steep sloping line represents solid-state diffusion of the lithium-ions in the electrode. According to Figure S7, it can be found clearly that the diameter of the semicircle for the NC@SnO2@Sn/3D-GNs electrode in the high-medium-frequency region is significantly smaller than that of the Sn/3D HPG composite, which suggested that N-C@SnO2@Sn/3DGNs possessed lower contact and charge-transfer resistances. The exact kinetic differences between N-C@SnO2@Sn/3D-GNs and Sn/3D-GNs were inspected by modeling AC impedance spectra based on the modified Randles equivalent circuit (Inset in Figure S7) and are summarized in Table S364. It can be seen that the values of SEI film resistance Rf and charge-transfer resistance Rct of the N-C@SnO2@Sn/3D-GNs electrode are 13.8 and 36.0 Ω, respectively, which were observed to be significantly lower than those of Sn/3D-GNs (27.7 and 48.3 Ω, respectively). This result validated that the N-C encapsulation layer in NC@SnO2@Sn/3D-GNs composite can significantly reduce the SEI resistance and charge transfer resistance of the electrode materials. 17 ACS Paragon Plus Environment


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The N-C@SnO2@Sn/3D-GNs composite electrode was also tested by long-term cycling at the higher current density of 1.0 A g-1 (Figure 8). The electrode exhibited a highly stable capacity of 550 mAh g-1, with 84% capacity retention after 500 cycles at 1.0 A g-1 compared with the value for the 2nd cycle (Figure 8). In addition, such high stability and large capacity of the composite electrodes are superior to those reported Sn-3D graphene composite electrodes shown in Table 1. In addition, the morphology of the N-C@SnO2@Sn/3D-GNs composite electrode after 500 cycles at 1.0 A g-1 was also given in Figure S8. Although the sample surface is covered with a thick and gel-like SEI layer, it can be seen clearly that the individual nanoparticles with diameter in the range 30-50 nm have not aggregated at all and still anchor homogeneously and firmly to the surface of the graphene-like walls (Figure S8 (A)), which is very similar to the morphology of the pristine product (see Figure 2B). The magnified TEM image (Figure S8 (B)) still clearly exhibited an encapsulated structure with a thin N-C shell and an extra SEI film in the outer surface of the composite, but the Sn core was pulverized into small particles and still encapsuled in the N-C shell. The same phenomenon was also been reported on some Sn@C or Sn@graphene12. In addition, the HRTEM image (Figure S8 (C)) clearly dispalyed the well-defined lattice fringes of Sn with an interplanar spacing of 0.27 nm, corresponding to the (101) planes of Sn (PDF No. 04-0673), which further confirmed that the Sn particles still encapsuled in the N-C shell after 500 cycles. This evidence implies that the N-C shell do not been destroyed under such rigorous testing conditions and can effectively buffer the mechanical stress resulting from the severe volume change of Sn nanoparticles during lithium ion insertion/extraction, which is also very beneficial for improving the cycling performance of the N-C@SnO2@Sn/3D-GNs electrode. The excellent overall electrochemical behavior of the as-prepared N-C@SnO2@Sn/3DGNs composite can be attributed to the multiple and possibly synergistic effects that stem from their design. Firstly, the N-C layer with graphene-like structure in the outermost layer of 18 ACS Paragon Plus Environment

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the N-C@SnO2@Sn nanoparticles can effectively buffer the large volume changes, enhance electronic conductivity, and prevent SnO2/Sn aggregation and pulverization over repeated discharge/charge processes. Secondly, the SnO2 layer in the middle layer of N-C@SnO2@Sn nanoparticles can be changed into active Sn and Li2O during discharge, as described by SnO2 + Li+ → Sn + Li2O, while the thus-formed Li2O can provide a facile environment for the alloying process and facilitate good cycling behavior, so as to further improve the cycling performance. The innermost Sn layer possesses a large theoretical capacity, thereby guaranteeing high lithium storage in the composite. Thirdly, the highly robust and elastic 3D graphene network acts as a buffer that reinforces the structural integrity of the overall electrode via tightly pinning the N-C@SnO2@Sn nanoparticles, thereby leading to excellent cycling stability. Finally, the overall N-C@SnO2@Sn/3D-GNs composite with superior electrical conductivity (1.52 × 103 S m-1), a 3D porous nature, and large surface area (105 m2 g-1) can significantly facilitate the diffusion and transport of electrons and ions, contributing to the remarkable improvements in the reversible capacity and rate capability. On the basis of the analysis presented above, we believe that the outstanding synergetic effects between the 3D porous graphene network and the N-C@SnO2@Sn nanostructures are responsible for the superior lithium storage performance of the overall electrode7, 65. CONCLUSIONS In summary, a unique nanostructure in the form of three-layer core-shell NC@SnO2@Sn/3D-GNs composite has been successfully fabricated via a low-cost and scalable method, that is, an in situ hydrolysis of Sn salts and immobilization of SnO2 nanoparticles on the surfaces of 3D-GNs, followed by an in situ polymerization of DA on the surfaces of SnO2/3D-GNs, and finally a carbonization. The as-obtained N-C@SnO2@Sn/3DGNs composite showed highly stable capacity of up to 901 mAh g-1 after 200 cycles at a current density of 100 mA g-1, reversible high rate charge-discharge performance, and a long 19 ACS Paragon Plus Environment


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lifetime of 500 cycles with 84 % capacity retention at 1.0 A g-1. Importantly, this unique hybrid design is expected to be extended to other alloy-type anode materials such as silicon, germanium, etc. EXPERIMENTAL SECTION Synthesis of the 3D-GNs, N-C@SnO2@Sn/3D-GNs composite, and Sn/3D-GNs. The 3D-GNs was synthesized by an improved procedure that was previously reported42, 66. In brief, the pre-treated macroporous acrylic type cation-exchange resin was firstly impregnated with 0.10 mol L-1 of nickel acetate solution (100 mL). The nickel ion-exchange resin was washed and dried. Then, the nickel ion-exchange resin (10 g) was added into 400 mL KOH/ethanol solution containing 20 g KOH under stirring and dried to form a nickel ion-exchange resin/KOH mixture. Finally, the mixture was heated at 850 oC for 2 h in N2 atmosphere with a heating rate of 2 oC min-1. After cooling down to room temperature, the resulting sample was treated in 3 mol L-1 HCl solution to remove the nickel nanoparticles and other impurities. The sample was finally washed and dried. The 3D highly pure graphene (HPG) powders were vacuum dried at 120 oC for 5 h. The N-C@SnO2@Sn/3D-GNs were prepared as follows (Scheme 1): (1) 100 mg asprepared 3D-GNs were firstly dispersed in a mixture of 200 mL ethylene glycol and 200 mL water by ultrasonication, followed by the addition of concentrated HCl (1.5 mL). Under vigorous stirring, 1.2 g SnCl2⋅2H2O was added into the dispersion, and the hydrolysis reaction was left to proceed at 90 oC under stirring for 1 h. The resulting precipitates were collected by centrifugation and washed three times with deionised (DI) water, then dried at 90 °C in a vacuum box overnight, and finally the SnO2/3D-GNs were obtained. (2) The as-obtained SnO2/3D-GNs were mixed with dopamine hydrochloride (150 mg, Aldrich) in Tris-buffer (75 mL, 10 mmol L-1; pH 8.5) and stirred for 12 h at 80 oC. The thus-obtained PDA@SnO2/3DGNs composites were collected by centrifugation and washed three times using water. (3) The 20 ACS Paragon Plus Environment

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as-prepared PDA@SnO2/3D-GNs powders were placed in a tube furnace and heated under N2 at 700 oC for 1 h with a heating rate of 5 oC min-1. Finally, the N-C@SnO2@Sn/3D-GNs composite was obtained. For comparsion, the Sn/3D-GNs sample was made by the same processes as the N-C@SnO2@Sn/3D-GNs composite but without the addition of dopamine hydrochloride. Characterizations. The X-ray diffraction (XRD) measurements were carried out on a D/Max-III (Rigaku Co., Japan) using Cu Kα radiation with a scan rate of 10o min-1, operating at 40 kV and 30 mA. The samples were ground with an agate mortar until they could pass a 325 mesh standard sieve. The transmission electron microscope (TEM) investigations and TEM mapping were carried out on a JEOL JEM-2010 (HR) at 200 kV and a FEI Tecnai G2 F30 at 300 kV, respectively. Scanning electron microscope (SEM) micrograms were collected on a JEM-6700F field emission scanning electron microscope. X-ray photoelectron spectroscopy (XPS) was conducted with two separate systems equipped with monochromatic Al Kα sources (ESCALab250, USA) to analyze the chemical composition of the samples. The N2 adsorption experiments using an ASAP 2420 Surface Area Analyzer (Micrometeritics Co., USA) were conducted to investigate the porosity of the samples. All the samples were outgassed at 150 oC in a nitrogen flow for 6 h prior to the measurement. Nitrogen adsorption data were recorded at liquid nitrogen temperature (77 K). The specific surface areas of the samples were calculated according to the Brunauer-Emmett-Teller (BET) equation from the adsorption data in the relative pressure range from 0.05 to 0.2. The pore volumes were estimated to be the liquid volume indicated by the adsorption (N2) data at a relative pressure of 0.98. The content of C and N elements in the as-obtained composite was determinated by elemental analyzer (Vario EL, Germany). TGA was conducted on a thermo-gravimetrydifferential scanning calorimetry instrument (NETZSCHSTA 409 PC) under air at a heating rate of 10 oC min-1 from room temperature to 900 oC. Electrode fabrication and electrochemical measurements. The anode slurry were 21 ACS Paragon Plus Environment


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prepared by mixing 80 wt% N-C@SnO2@Sn/3D-GNs composite, 3D-GNs, or Sn/3D-GNs sample, 10 wt% acetylene black, and 10 wt% polyvinylidene difluoride (PVDF) in Nmethylpyrrolidone (NMP) solvent dispersant, respectively. Negative electrodes were produced by coating the slurry on copper foil to form ~100 µm thick films and dried in vacuum oven at 120 °C for 12 h. After that, the dried material was pressed and then cut into 1.54 cm2 (diameter = 1.4 cm) disks. Here, the mass loading of active material in the electrodes is around 1.5 ± 0.3 mg cm-2. Preliminary cell tests were conducted with 2032 coin-type cells, which were fabricated in an argon-filled glove box using lithium metal as the counter electrode and a microporous polyethylene separator. The electrolyte solution was 1.0 mol L-1 LiPF6 in ethylene carbonate and diethyl carbonate (EC:DEC, 1:1 in volume). The chargedischarge performances of the cells were tested on a program-controlled test system (Shenzhen Neware Battery Co., China), and the potential window was kept between 0.01 and 3.0 V at room temperature. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy measurements (Amplitude: 5 mV, Frequency: 10 mHz~100 kHz) was conducted on a ZAHNER IM6e (Germany) electrochemical workstation. ASSOCIATED CONTENT Corresponding Author Haiyan Zhang* Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, China; Fax: (+8620)39322570; Tel: (+8620)-39322570; E-mail: [email protected]. Pei Kang Shen* Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University, Nanning, Guangxi, 530004, PR China; E-mail: [email protected].


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Zaiping Guo* Institute for Superconducting and Electronic Materials, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, North Wollongong, New South Wales 2500, Australia; E-mail: [email protected]. Notes. The authors declare no competing financial interest. Acknowledgements. This work was supported by the National Natural Science Foundation of China (Grant nos. 51502043, 21073241, 51276044, 51302042, 21176045), the link project of the National Natural Science Foundation of China and Guangdong Province (U1401246), the Natural Science Foundation of Guangdong Province of China (2014A030310382), the Science and Technology Program of Guangdong Province of China (Grant nos. 2014B010106005, 2015B010135011, 2015A050502047), the Science and Technology Program of Guangzhou City of China (201508030018), the research project of Guangdong Province Science & Technology Bureau (No. 2014A010106029), the Major International (Regional) Joint Research Project (51210002), the National Basic Research Program of China (2015CB932304), and the Pearl River New Star Plan of Science and Technology of Guangzhou City of China (2013J2200038). Supporting Information Available: Additional results for SEM images, TEM images, XRD, TGA, EIS, schematic illustration, physical characteristics, and comparison of the electrical conductivity of various samples. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1)

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Dong, Y.; Zhao, Z.; Wang, Z.; Liu, Y.; Wang, X.; Qiu, J., Dually Fixed SnO2 Nanoparticles on Graphene Nanosheets by Polyaniline Coating for Superior Lithium Storage. ACS Appl. Mater. Inter. 2015, 7, 2444-2451.

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Liu, H.; Hu, R.; Sun, W.; Zeng, M.; Liu, J.; Yang, L.; Zhu, M., Sn@SnOx/C 27 ACS Paragon Plus Environment


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Scheme 1. Schematic illustration of the synthesis processes for N-C@SnO2@Sn/3D-GNs composite: First, there was in situ hydrolysis of Sn salts and immobilization of 3-4 nm SnO2 nanoparticles on the surfaces of 3D-GNs to obtain SnO2/3D-GNs composite; second, a thin layer of polydopamine (PDA) was coated on the surfaces of the SnO2/3D-GNs composite via an in situ polymerization process to obtain PDA@SnO2/3D-GNs composite; Finally, the PDA@SnO2/3D-GNs composite was carbonized in N2, and simultaneously, part of the SnO2 nanoparticles were reduced to Sn by the 3D-GNs, forming the N-C@SnO2@Sn/3D-GNs composite.


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Figure 1. TEM image of 3D-GNs (A), TEM images of SnO2/3D-GNs (B-D) and PDA@SnO2/3D-GNs (E-H) at different magnifications. Insets to (C) and (F) are the corresponding SAED patterns of SnO2/3D-GNs and PDA@SnO2/3D-GNs, respectively. Inset to (D) is the enlarged view corresponding to the outlined area by the white square in (D).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (A-B) TEM images with different magnifications of N-C@SnO2@Sn/3D-GNs; high-resolution TEM images (C1, C2, and C3) and their further magnified TEM images (C1-1, C1-2, C2-1, and C3-1) of three N-C@SnO2@Sn nanoparticles; (D) schematic illustration of NC@SnO2@Sn nanoparticle. Inset in (C3-1) is the fast Fourier transform (FFT) diffraction pattern of the Sn layer in N-C@SnO2@Sn/3D-GNs.


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Figure 3. TEM images with different magnifications (A and B), and elemental mapping of carbon (B1), nitrogen (B2), oxygen (B3) and tin (B4), corresponding to the area outlined by the orange square in (B) for the N-C@SnO2@Sn/3D-GNs composite.



SnO2 (PDF #41-1445)

3D-GNs SnO2/3D-GNs

Sn (PDF #04-0673)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PDA@SnO2/3D-GNs N-C@SnO2@Sn/3D-GNs

20

30

40

50

60

2θ (degree) ) (

Figure 4. XRD patterns of final composite and precursors.


70

80

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Binding energy (ev) 500 A

3D-GNs

SnO2/3D-GNs

492

488

484 Sn3d

8.5 eV

Intensity (a.u.)

N1s

496

B

C1s

O1s Sn3d

SnO2/3D-GNs PDA@SnO2/3D-GNs

N-C@SnO2@Sn/3D-GNs

Sn4+3d3/2 Sn03d3/2 Sn4+3d5/2 Sn03d5/2

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PDA@SnO2/3D-GNs

N-C@SnO2@Sn/3D-GNs

550 500 450 400 350 300 250

C Pyrrolic/pyridone N

Pyridinic N

Graphitic N

408

404

400

396

Binding energy (ev)

Binding energy (ev)

Figure 5. XPS spectra of N-C@SnO2@Sn/3D-GNs and precursors (A) and their Sn 3d spectra (B); magnified fitting of N 1s spectrum (C) for N-C@SnO2@Sn/3D-GNs.


1000 800

A

3D-GNs SnO2/3D-GNs

Pore Volume (cm3 g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Adsorbed Volume (cm3 g-1)


PDA@SnO2/3D-GNs N-C@SnO2@Sn/3D-GNs

600 400 200 0 0.0

.2

.4

.6

.8

1.4

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3D-GNs SnO2/3D-GNs

B

1.2

PDA@SnO2/3D-GNs

1.0

N-C@SnO2@Sn/3D-GNs

.8 .6 .4 .2 0.0 1

1.0

10

100

Pore Diameter (nm)

P/P0

Figure 6. (A) Nitrogen adsorption/desorption isotherms and (B) DFT pore-size distribution curves of various materials.


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Figure 7. Electrochemical performances of N-C@SnO2@Sn/3D-GNs, Sn/3D-GNs, and3DGNs as anode materials. (A) Typical CV curves of N-C@SnO2@Sn/3D-GNs composite anode at a scan rate of 0.1 mV s-1, (B) galvanostatic discharge/charge profiles of NC@SnO2@Sn/3D-GNs composite anode for selected cycles between 0.01 and 3 V at a current density of 100 mA g-1, (C) cycling performance at a current density of 100 mA g-1 for the three anode materials, and (D) specific capacity versus current density for NC@SnO2@Sn/3D-GNs composite (1) and Sn/3D-GNs composite (2) electrodes, respectively.


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800

100

600 1.0 A/g 400

50

Charge

Discharge

200

0 0

100

200

300

400

Cycle number

500

Coulombic efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Specific capacity (mAh/g)


Figure 8. Cycling performance and its corresponding Coulombic efficiency at a current density of 1.0 A g-1 for N-C@SnO2@Sn/3D-GNs composite anode.


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Table 1. Comparison of the electrochemical performance of our as-prepared electrodes with reported Sn-graphene electrodes.

Classification

Cycle performance

Rate capability

Ref. (year)

Sn-rGO-650

0.05 A g-1/50th/611 mAh g-1

2.0 A g-1/470 mAh g-1

67

2015

G/Sn/G

0.1 A g-1/100th/838.4 mAh g-1

1.0 A g-1/639.7 mAh g-1

68

2014

G/Sn-S

0.1 A g-1/100th/650 mAh g-1

2.0 A g-1/440 mAh g-1

69

2014

3D Sn-GNs

0.5 A g-1/60th/ 552 mAh g-1

30

2014

Sn/Graphene

0.1 A g-1/100th/~500 mAh g-1

33

2014

Sn@N-RGO

0.1 A g-1/100th/481 mAh g-1

2.0 A g-1/307 mAh g-1

23

2013

3D Sn-graphene

0.293 A g-1/400th/794 mAh g-1

1.76 A g-1/300 mAh g-1

28

2013

Sn@C-graphene

0.05 A g-1/50th/630 mAh g-1

1.6 A g-1/

Nitrogen-Doped Carbon-Encapsulated SnO2@Sn Nanoparticles

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