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Ultrafast Lithium Storage using Antimony-Doped Tin Oxide Nanoparticles Sandwiched between Carbon Nanofibers and a Carbon skin Geon-Hyoung An, Do-Young Lee, Yu-Jin Lee, and Hyo-Jin Ahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10868 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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Ultrafast Lithium Storage using Antimony-Doped Tin Oxide Nanoparticles Sandwiched between Carbon Nanofibers and a Carbon skin Geon-Hyoung An,† Do-Young Lee,‡ Yu-Jin Lee,‡ and Hyo-Jin Ahn*†, ‡ †

Program of Materials Science & Engineering, Convergence Institute of Biomedical Engineering and

Biomaterials, Seoul National University of Science and Technology, Seoul 139-743, Korea ‡

Department of Materials Science and Engineering, Seoul National University of Science and

Technology, Seoul 139-743, Korea KEYWORDS: Li-ion battery, Anode, Antimony-doped tin oxide, Sandwich structure, Carbon skin

ABSTRACT: The metal oxides as anode materials for Li-ion batteries (LIBs) are of significant interest to many potential technologies, because of their high theoretical capacity value, low-price, and environmentally friendly features. In spite of these considerable benefits, and ongoing progress in the field, momentous challenges exist, related with structural disintegration due to volume expansion of electrode materials. This leads to rapid capacity decline, and must be resolved in order to progress for the realistic utilize of LIBs with ultrafast cycling stability. This paper proposes a novel architecture of Sb-doped SnO2 nanoparticles sandwiched between carbon nanofiber and carbon skin (CNF/ATO/C), using electrospinning and hydrothermal methods. The CNF/ATO/C exhibits superb electrochemical behavior such as high specific capacity and outstanding cycling stability (705 mA h g–1 after 100

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cycles), outstanding high-rate performance (411 mA h g–1 at 2000 mA g–1), and ultrafast cycling stability (347 mA h g–1 at 2000 mA g–1 after 100 cycles), which is high compared to any reported value using SnO2-based anode materials. Thus, this unique architecture furnishes profitable effects, including electroactive sites, structural stability, and electrical conductivity, which can potentially realize for ultrafast LIBs.

INTRODUCTION Development of energy storage devices is key to realizing the global aim of reducing fossil fuel consumption and moving to environmentally friendly renewable energy resources. LIBs have been extensively employed in electronic devices as well as new generations of energy systems. They have generated substantial interest owing to their attractive advantages, including high energy density, outstanding cycling stability, low toxicity, and lack of memory effects.1-3 To move LIB technologies forward, towards practical applications as electric bikes, the improved anode materials are essential for the meeting of industrial requirements, which perform crucial roles in proving the battery performance, volume, weight, and price.4-9 Consequently, numerous strategies, using the reasonable design of anode, have been intensively studied to enhance the specific capacity in LIBs through the introduction of various composite materials using metals, metal oxides, metal dichalcogenides, and Si as substitutes to the graphite having low theoretical specific capacity of 372 mA h g–1.10-16 Of these alternatives, metal oxides have been extensively used as hopeful candidates for LIB.17-19 Metal oxides suffer from two main problems: (I) severe volume expansion and aggregation during the lithiation/delithiation process causes rapid decay of the specific capacity; ( Ⅱ ) low electrical conductivity with a slow conversion reaction leads to poor high-rate performance.20-22 To conquer these troubles, composite materials composed of metal oxides and carbon materials (i.e., metal oxides loaded on the carbon) have been used to enhance structural stability and electrical conductivity, leading to the improved capacity and the enhanced cycling stability.20-24 Relatively poor cycling stability was observed

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in these composites, which was attributed to a reduction of electroactive sites due to aggregation of metal oxides on the carbon. In addition, ultrafast cycling stability of anode materials at high current densities are also highly important for practical utilize of LIBs. The low electrical conductivity of metal oxides remains an issue, and is the reason for poor high-rate capacity and low ultrafast cycling stability.25-36 To overcome these issues, the introduction of sandwich structure with a physical buffer (i.e., metal oxides sandwiched between carbon and a physical buffer) is a persuasive solution for improving the cycling stability. In addition, the doping technology is a powerful candidate for increasing the ultrafast cycling stability. Functional doped metal oxides, with high electrical conductivity and high optical transmission, are often used in transparent conducting electrodes, electrochromic windows, and heated mirrors.37,38 However, doped metal oxides, consisting of a sandwich structure with carbon materials and physical buffers, have not yet been studied for ultrafast cycling stability of anode materials at high current densities. In this paper, we report a novel architecture of Sb-doped SnO2 (ATO) sandwiched between the carbon nanofiber (CNF) and the carbon skin as a physical buffer, fabricated using a simple approach based on electrospinning and hydrothermal methods. The electrochemical performance of these materials in LIBs is investigated. These novel architectures may prevent the issues associated with volume change and agglomeration of metal oxides, because of the excellent physical and electrochemical stability of the carbon skin on ATO, which leads to improved cycling stability. ATO is sandwiched between the CNF and the carbon skin, which has the high theoretical capacity (SnO2: 782 mAh g–1), low price, as well as increased electrical conductivity, resulting from Sb doping in the SnO2 lattice.25-36 In the core region, CNFs having unique the one dimensional (1-D) nanostructures are used, owing to their high surface area (443 m2 g–1), high electric conductivity (102 S cm–1), and excellent physical / chemical property.39-43

RESULTS AND DISCUSSION Figure 1 schematically illustrates the synthetic process for preparing ATO nanoparticles sandwiched

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between the CNF and the carbon skin (CNF/ATO/C) with novel architecture. The CNF was made using electrospinning and carbonization process, as shown in Figure 1a. To obtain ATO nanoparticles loaded on CNFs (CNF/ATO), the ATO nanoparticles were loaded on CNFs using a hydrothermal method, using the precursor mixture consisting of SnCl4 and SbCl3, as show in Figure 1b. Finally, the carbon skin was uniformly formed on the ATO nanoparticles using a hydrothermal method by glucose as the carbon source (Figure 1c). The morphologies were examined by field emission scanning electron microscopy (FESEM) measurements. Figure S1 shows images of ATO particles produced using a hydrothermal method without the CNF. The X-ray diffraction (XRD) patterns of ATO particles showed the distinct diffraction peaks, indicating that the hydrothermal method offered the excellent crystallinities of ATO particles, leading to the improved electrochemical performance relative to capacity and high-rate performance.7,18 On the other hand, the ATO particles, consisting of nano-sized grains (5-11 nm), exhibited irregular morphologies with diameter of 0.2 - 1.3 mm, indicating that these nanoparticles easily aggregate due to high surface energy, causing rapid decay of the specific capacity in LIBs. ATO nanoparticles therefore require complexing with carbon materials to obtain good dispersion to increase electroactive sites. This leads to improved specific capacity and stability in LIBs. For this reason, CNFs were used to overcome the existing challenges with ATO nanoparticles. Figure 2 displays low-magnification (Figure 2a–c) and high-magnification (Figure 2d–f) FESEM images of CNF, CNF/ATO, and CNF/ATO/C. All nanofibers demonstrate that network structures can supply effective electron transfer and swift ion diffusion of the electrolyte during cycling, leading to high-performance LIBs. Figure 2a shows FESEM images of CNFs with a diameter of 217 – 231 nm, exhibiting smooth surfaces (Figure 2d). After hydrothermal processing, the CNF/ATO (Figure 2b and e), with diameter of 245–266 nm, presented uneven surfaces composed of nano-sized grains, and displayed no agglomeration. The FESEM results indicate that ATO nanoparticles are evenly loaded on CNF surface using optimized conditions of the 3.3 mM precursor mixture consisting of SnCl4 and SbCl3. It was observed that increasing the amount of precursor mixture

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led to agglomeration of the ATO nanoparticles (Figure S2). As shown in Figure 2c and f, CNF/ATO/C, with diameter of 265–289 nm, displayed even surfaces and consistent morphology without agglomerated ATO on the surface, showing that ATO nanoparticles were fully sandwiched between CNF and carbon skin. This novel architecture of CNF/ATO/C as anode materials for LIBs can efficiently reconcile the volume expansion, resulting in excellent cycling stability. Figure 3 shows low-magnification (Figure 3a–c) and high-magnification (Figure 3d–f) transmission electron microscopy (TEM) images of CNF, CNF/ATO, and CNF/ATO/C. CNF (Figure 3a and d) showed a uniform contrast because only a single carbon phase exists. CNF/ATO (Figure 3b and e) was relatively dark, implying the presence of ATO nanoparticles on the CNF surface. In addition, the size range of ATO nanoparticles was 6 to 9 nm. The high-magnification TEM image of CNF/ATO (Figure 3e) shows the smaller lattice spacing (3.29 Å) compared to pure SnO2 (3.35 Å), which corresponds to the (110) plane of ATO.27,29,44 The XRD results indicated that Sb was successfully doped in the SnO2 lattice. Interestingly, CNF/ATO/C (Figure 3c and f) was totally encapsulated using the carbon skin without morphological change of the ATO nanoparticles (Figure 3b). The uniform carbon skin consisting of a thickness of ~ 11.3 nm, on the ATO nanoparticles was obviously observed using highmagnification TEM (Figure 3f). This could contribute to the formation of a physically strong buffer skin. Also, CNF/ATO/C had a lattice spacing of 3.29 Å, which corresponding to the (110) plane of ATO.27,29,44 To prove the distribution of carbon, tin, antimony, and oxygen of CNF/ATO/C, TEM– Energy dispersive spectrometer (EDS) mapping analysis was performed, and the results are shown in Figure 4. The EDS consequence indicates that all atoms are consistently dispersed, indicating that ATO nanoparticles are uniformly synthesized and sandwiched between a CNF and the carbon skin. Figure 5a shows the XRD patterns arising from the investigation of the crystalline phases and crystallinities of the samples. The CNF indicates wide peak at around 25°, corresponding to the (002) layers of graphite. In addition, the main diffraction peaks of CNF/ATO and CNF/ATO/C are observed

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at 26.8°, 34.1°, 38.1°, 52.0° and 55.0°, corresponds to the (110), (101), (200), (211) and (220) planes, respectively, which are slightly shifted to higher angles when compared to those of the SnO2 phases with a tetragonal structure (space group P42/mnm[136]). These results indicate that the lattice parameters of ATO become smaller than SnO2 because Sb5+ ions (radius: 6.2 Å) substitute the positions of Sn4+ ions (radius: 7.1 Å) in the SnO2 lattice, indicating successful doping of Sb ions into the SnO2 lattice.45,46 The XRD results for the Sb doping in SnO2 show good agreement with the X-ray photoelectron spectrometer (XPS) results (Figure S3). In addition, there are no XRD peaks from impurities such as Sb2O3 or Sb2O5, indicating that Sb ions are totally doped into the SnO2 lattice, forming an ATO lattice. From the diffraction patterns of CNF/ATO and CNF/ATO/C, the grain sizes of ATO nanoparticles can be determined using the Scherrer equation (which λ is the X-ray wavelength, β is the full width at half maximum (FWHM), and θ is the Bragg angle).47 D = 0.9λ/(β cos θ) The sizes of ATO nanoparticles are calculated to be 7.7 nm for CNF/ATO and 7.8 nm for the CNF/ATO/C, based on the (110), (101), and (211) planes, which are in good accordance with the TEM results. To further investigate the content of CNF, CNF/ATO, and CNF/ATO/C, thermal gravimetric analysis (TGA) analysis (Figure 5b) was conducted. CNF exhibited a weight loss of 100 %, signifying the existence of pure carbon without impurities. CNF/ATO exhibited a weight loss of 38%, because of the presence of ATO nanoparticles on the CNF. CNF/ATO/C showed higher weight losses of 68% than CNF/ATO, which implies that the increased weight loss is ascribed to the existence of carbon skins. The TGA consequences are therefore in well concurrence with the TEM results. To examine the effect on electrical conductivity of Sb doping in the SnO2 lattice, samples of undoped SnO2 nanoparticles sandwiched between the CNF and carbon skin were prepared (referred to as “CNF/SnO2/C”). The morphologies of CNF/SnO2/C were similar to CNF/ATO/C as shown in Figure S4a. The electrical conductivities of CNF/SnO2/C and CNF/ATO/C were 0.6 and 1.7 S cm–1,

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respectively, as shown in Figure S4b, i.e., CNF/ATO/C exhibited higher electrical conductivity than CNF/SnO2/C. This indicated that Sb doping in SnO2 lattice contributes to the increase of electrical conductivity, which leads to enhanced high-rate performance. To evaluate the cycling stability in LIBs, charge-discharge evaluations were preformed using a cycler system were carried out with CNF, ATO particles, CNF/ATO, and CNF/ATO/C electrodes. Tests were conducted for up to 100 cycles at a current density of 100 mA g-1 in a voltage range of 0.05 – 3.00 V (vs. Li/Li+). The first specific charge and discharge capacities were 362 and 570 mAh g–1 for CNF, 811 and 1511 mAh g–1 for ATO particles, 990 and 1518 mAh g–1 for CNF/ATO, and 1161 and 1521 mAh g– 1

for CNF/ATO/C, respectively. In addition, the voltage profiles and Cyclic voltammetry (CV) curves of

the electrodes are presented in Figure S5 and S6, respectively. The first discharge capacities of ATO particles, CNF/ATO, and CNF/ATO/C are high compared to the theoretical capacity for SnO2 (i.e., 782 mAh g-1), owing to the direct growth of solid electrolyte interface (SEI) layers on the electrode surface, which have the ability to store charge via interfacial charging at the metal/Li2O interface.48,49 In general, the SEI layers usually appear during the first cycles, owing to reductive decomposition of electrolyte element on the electrode surface.48,49 The high irreversible capacity losses in the first cycle result from SEI layers on the electrode surface. As shown in Figure S5, the CNF/ATO/C electrode indicated a high Coulombic efficiency of 76.3% compared with the CNF (63.6%), ATO particles (53.7%), and CNF/ATO (65.2%) electrodes. The high Coulombic efficiency of the CNF/ATO/C electrode suggests that sandwiched ATO nanoparticles between a CNF and a carbon skin can perform a significant role for the improved LIB performance during the first cycle. In addition, all electrodes reached a Coulombic efficiency of nearly 100% after 5 cycles, which means high reversible performance was attained. Figure 6a displays the cycling stability of electrodes during 100 cycles. For the CNF electrode, a low specific discharge capacity of 361 mAh g–1 remained nearly constant between 5 and 100 cycles. The ATO particles presented a swift drop in the specific discharge capacity to 97 mA h g–1 after 100 cycles.

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These results indicate that ATO particles undergo from a large volume change during the cycling. Complexing with carbon materials is necessary to reconcile the volume change. To optimize the amount of ATO nanoparticles on the CNF/ATO, samples were prepared using 1.6, 3.3, and 4.9 mM precursor mixture (composed of SnCl4 and SbCl3). The optimum mixture was identified as the 3.3 mM precursor mixture as shown in Figure S7. The CNF/ATO electrode exhibited a low cycling stability (469 mA h g–1 after 100 cycles). These results are attributed to the reduction of electroactive sites, arising from the agglomeration of ATO nanoparticles on the CNF during cycling (Figure S8). For this reason, we synthesized the sandwich structure with a carbon skin to overcome the existing challenges with ATO nanoparticles. CNF/ATO/C with a sandwich structure showed impressive cycling stability with a specific discharge capacity of 705 mAh g–1 after 100 cycles. To disclose the superior cycling stability of CNF/ATO/C, the cycled electrode materials were investigated using FESEM and TEM measurements (Figure S9). The structures and the morphologies of CNF/ATO/C were well retained after 100 cycles, implying that sandwich structure with the carbon skin is helpful in easing the volume expansion during the cycling. In addition, this capacity is higher than any reported value for SnO2-based anode materials with carbon materials, as organized in Table S1.25,26,27,30,33,35,50-58 We believe that the improved specific capacity with outstanding cycling stability is mainly because of the novel sandwich structure. Figure 6b shows the high-rate performance obtained at current densities of 100, 300, 700, 1000, 1500, 2000 and 100 mA g–1. The CNF/ATO/C electrode showed excellent high-rate performance of 411 mA h g–1 at 2000 mA g–1, which recovered to 707 mA h g–1, 87 % of the specific discharge capacity at 100 mA g–1. The outstanding high-rate performance of the CNF/ATO/C electrode is the highest more than previously reported studies for ATO and SnO2-based anode materials with a carbon material (Figure 6c).25-36 Additionally, the CNF/ATO/C electrode exhibited good ultrafast cycling stability with a high specific discharge capacity of 347 mA h g–1 after 100 cycles at an ultrafast current density of 2000 mA g–1 (Figure 6d), which means that the optimized CNF/ATO/C electrode can be efficiently utilized for

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ultrafast LIBs. The enhanced high-rate performance and enhanced ultrafast cycling stability are ascribed to the increased electrical conductivity arising from Sb doping in the SnO2 lattice, owing to fast electron transport during the cycling process. It can also be ascribed to the existence of the carbon skin as a physical buffer layer. The cycling stability and high-rate performance test was carried out using CNF/SnO2/C under equivalent conditions in order to verify the doping effect of Sb in the SnO2 lattice in LIBs. The CNF/SnO2/C showed a relatively low specific discharge capacity (Figure S10a) and low high-rate performance (Figure S10b) compared to CNF/ATO/C, indicating that Sb doping in the SnO2 lattice could provide the improved specific discharge capacity and the enhanced high-rate performance, based on increased electrical conductivity relative to fast electron transport. In particular, the doping effect of Sb in the SnO2 lattice has a more significant impact on the ultrafast current density of 2000 mA g–1 (Figure S10b), manifesting that consideration of electrical conductivity is a key design factor for ultrafast LIBs. The novel architecture of the CNF/ATO/C electrode exhibits improved electrochemical performance in LIBs due to three main effects (Figure 7). Firstly, the good dispersion of ATO nanoparticles can provide an increased number of electroactive sites, thus improving the specific discharge capacity. Secondly, the sandwich structure with the carbon skin greatly helps to prevent volume change and agglomeration of ATO nanoparticles during cycling, leading to enhanced cycling stability. Thirdly, the doping effect of Sb in the SnO2 lattice can increase electrical conductivity relative to fast electron transport, lead to outstanding ultrafast cycling stability.

CONCLUSIONS CNF/ATO/C was synthesized by a sequential process of electrospinning and hydrothermal techniques. The optimized CNF/ATO/C exhibited a novel architecture consisting of well-dispersed ATO nanoparticles, which were sandwiched between CNFs and the carbon skin. The optimized CNF/ATO/C ACS Paragon Plus Environment

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electrode displayed enhanced lithium-storage properties, including outstanding cycling stability, with the highest specific discharge capacity (705 mA h g–1 after 100 cycles), and outstanding high-rate performance (411 mA h g–1 at 2000 mA g–1) compared with CNF, ATO particles and CNF/ATO electrodes. In addition, the optimized CNF/ATO/C electrode exhibited impressive ultrafast cycling stability (347 mA h g–1 at 2000 mA g–1 after 100 cycles). The enhanced electrochemical achievement can be defined by three factors: (I) the improved specific discharge capacity is related to the good dispersion of ATO nanoparticles; (II) the excellent cycling stability arises from the unique sandwich structure with the carbon skin; (Ⅲ) the outstanding high-rate performance and the superb ultrafast cycling stability are attributed to the doping effect of Sb in the SnO2 lattice. The authors believe that this novel architecture has a prodigious potentiality for utilize in high-performance LIBs.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Supporting figures relative to the ATO particles and the table relative to the cycling stability comparison of previously reported SnO2 and carbon composites as anode materials for LIBs.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (H.-J. Ahn) Notes The authors declare no competing financial interest.

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2015, 3, 19445-19454. 16 Yu, S.; Jung, J.-W.; Kim, I.-D. Single Layers of WS2 Nanoplates Embedded in Nitrogen-Doped Carbon Nanofibers as Anode Materials for Lithium-Ion Batteries. Nanoscale, 2015, 7, 1194511950. 17 Qiu, J.; Yang, Z.; Li, Y. N-Doped Carbon Encapsulated Ultrathin MoO3 Nanosheets as Superior Anodes with High Capacity and Excellent Rate Capability for Li-Ion Batteries. J. Mater. Chem. A 2015, 3, 24245-24253. 18 Etacheri, Y.; Marom, R.; Elazai, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: a Review. Energy Environ. Sci. 2011, 4, 3243-3262. 19 Wang, H.; Zhuo, S.; Liang,Y.; Han, X.; Zhang, B.; General Self-Template Synthesis of Transition-Metal Oxide and Chalcogenide Mesoporous Nanotubes with Enhanced Electrochemical Performances. Angew. Chem., Int. Ed. 2016, 55, 9055 -9059. 20 Zhang, H. X.; Feng, C.l; Zhai, Y. C.; Jiang, K. L.; Li, Q. Q.; Fan, S. S. Cross-Stacked Carbon Nanotube Sheets Uniformly Loaded with SnO2 Nanoparticles: A Novel Binder-Free and HighCapacity Anode Material for Lithium-Ion Batteries. Adv. Mater. 2009, 21, 2299-2304. 21 Zhang, L. S.; Jiang, L. Y.; Yan, H. J.; Wang, W. D. Wang, W.; Song, W. G.; Guo, Y. G.; Wan, L. J. Mono Dispersed SnO2 Nanoparticles on Both Sides of Single Layer Graphene Sheets as Anode Materials in Li-Ion Batteries. J. Mater. Chem. 2010, 20, 5462-5467. 22 Wu, Z. S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H. M. Graphene Anchored with Co3O4 Nanoparticles as Anode of Lithium Ion Batteries with Enhanced Reversible Capacity and Cyclic Performance. ACS Nano 2010, 4, 3187-3194. 23 Gohier, A.; Laik, B.; Kim, K. H.; Maurice, J. L.; Ramos, J. P. P.; Cojocaru, C. S.; Van, P. T. High-Rate Capability Silicon Decorated Vertically Aligned Carbon Nanotubes for Li-Ion Batteries. Adv. Mater. 2012, 24, 2592-2597. 24 An, G. H.; Ahn, H. J. Carbon Nanofiber/Cobalt Oxide Nanopyramid Core-shell Nanowires for High-Performance Lithium-Ion Batteries. J. Power Sources 2014, 272, 828-836. 25 Zhang, J.; Ma, Z.; Jiang, W.; Zou, Y.; Wang, Y.; Lu, C. Sandwich-like CNTs@SnO2/SnO/Sn Anodes on Three-Dimensional Ni Foam Substrate for Lithium Ion Batteries. J. Electroanal. Chem. 2016, 767, 49-55. 26 Dirican, M.; Yanilmaz, M.; Fu, K.l; Lu, Y.; Kizil, H.; Zhang, X. Carbon-Enhanced Electrodeposited SnO2/Carbon Nanofiber Composites as Anode for Lithium-Ion Batteries. J. Power Sources 2014, 264, 240-247. 27 Zhang, L. S.; Jiang, L. Y.; Yan, H. J.; Wang, W. D.; Wang, W.; Song, W. G.; Guo, Y. G.; Wan, L. J. Mono Dispersed SnO2 Nanoparticles on Both Sides of Single Layer Graphene Sheets as Anode Materials in Li-Ion Batteries. J. Mater. Chem. 2010, 20, 5462-5467. 28 Tian, Q.; Zhang, Z.; Yang, L.; Hirano, S. I. Synthesis of SnO2/Sn@carbon Nanospheres Dispersed in the Interspaces of a Three-Dimensional SnO2/Sn@Carbon Nanowires Network, and Their Application as an Anode Material for Lithium-Ion Batteries. J. Mater. Chem. A 2014, 2, 12881-12887. 29 Bhaskar, A.; Deepa, M.; Ramakrishna, M.; Rao, T. N. Poly(3,4-Ethylenedioxythiophene) Sheath 12 ACS Paragon Plus Environment

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Over a SnO2 Hollow Spheres/Graphene Oxide Hybrid for a Durable Anode in Li-Ion Batteries. J. Phys. Chem. C 2014, 118, 7296-7306. 30 Yang, Y.; Ji, X.; Lu, F.; Chen, Q.; Banks, C. E. The Mechanistic Exploration of Porous Activated Graphene Sheets-Anchored SnO2 Nanocrystals for Application in High-Performance Li-Ion Battery Anodes. Phys. Chem. Chem. Phys. 2013, 15, 15098-15105. 31 Wang, Y.; Chen, T. Nonaqueous and Template-Free Synthesis of Sb Doped SnO2 Microspheres and Their Application to Lithium-Ion Battery Anode. Electrochim. Acta. 2009, 54, 3510-3515. 32 Xia, G.; Li, N.; Li, D.; Liu, R.; Wang, C.; Li, Q.; Lu, X.; Spendelow, J. S.; Zhang, J.; Wu, G. Graphene/Fe2O3/SnO2 Ternary Nanocomposites as a High-Performance Anode for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2013, 5, 8607-8614. 33 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. Interfaces 2015, 7, 2444-2451. 34 Shen, Z.; Hu, Y.; Chen, Y.; Chen, R.; He, X.; Zhang, X.; Shao, H.; Zhang, Y. Controllable Synthesis of Carbon-Coated Sn–SnO2–Carbon-Nanofiber Membrane as Advanced Binder-Free Anode for Lithium-Ion Batteries. Electrochim. Acta. 2016, 188, 661-670. 35 Wang, L.; Wang, D.; Dong, Z.; Zhang, F.; Jin, J. Interface Chemistry Engineering for Stable Cycling of Reduced GO/SnO2 Nanocomposites for Lithium Ion Battery. Nano Lett. 2013, 13, 1711-1716. 36 Wang, Y.; Djerdi, I.; Smarsly, B.; Antonietti, M. Antimony-Doped SnO2 Nanopowders with High Crystallinity for Lithium-Ion Battery Electrode. Chem. Mater. 2009, 21, 3202-3209. 37 Ouyang, P.; Zhang, H.; Wang, Y.; Chen, W.; Li, Z. Electrochemical & Microstructural Investigations of Magnetron sputtered Nanostructured ATO Thin Films for Application in LiIon Battery. Electrochim. Acta. 2014, 130, 232-238. 38 Wu, F. D.; Wu, M.; Wang, Y. Antimony-Doped Tin Oxide Nanotubes for High Capacity Lithium Storage. Electrochem. Commun. 2011, 13, 433-436. 39 An, G. H.; Ahn, H. J. Activated Porous Carbon Nanofibers Using Sn Segregation for HighPerformance Electrochemical Capacitors. Carbon 2013, 65, 87-96. 40 Pneg, S.; Li, L.; Lee, J. K. Y.; Tian, L.; Srinivasan, M.; Adams, S.; Ramakrishna, S. Electrospun Carbon Nanofibers and Their Hybrid Composites as Advanced Materials for Energy Conversion and Storage. Nano Energy 2016, 22, 361-395. 41 An, G. H.; Koo, B. R.; Ahn, H. J. Activated Mesoporous Carbon Nanofibers Fabricated Using Water Etching-Assisted Templating for High-Performance Electrochemical Capacitors. Phys. Chem. Chem. Phys. 2016, 18, 6587-6594. 42 Zhang, B.; Kang, F.; Tarascon, J. M.; Kim, J. K. Recent Advances in Electrospun Carbon Nanofibers and Their Application in Electrochemical Energy Storage. Prog. Mater. Sci. 2016, 76, 319-380. 43 An, G. H.; Ahn, H. J. Hong, W. K. Electrochemical Properties for High Surface Area and Improved Electrical Conductivity of Platinum-Embedded Porous Carbon Nanofibers. J. Power ACS Paragon Plus Environment

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Figure caption Figure 1 Schematic illustration of synthetic routes for CNF/ATO/C. (a) CNFs using electrospinning. (b) CNF/ATO obtained by a hydrothermal method using SnCl4 and SbCl3. (c) CNF/ATO/C fabricated using a hydrothermal method using glucose. Figure 2 Low-magnification (a–c) and high-magnification (d–f) FESEM images of CNF, CNF/ATO, and CNF/ATO/C. Figure 3 Low-magnification (a–c) and high-magnification (d–f) TEM images of CNF, CNF/ATO, and CNF/ATO/C. Figure 4 TEM-EDS mapping data of CNF/ATO/C. Figure 5 (a) XRD patterns of CNF, CNF/ATO, and CNF/ATO/C. (b) TGA curves of the CNF, CNF/ATO, and CNF/ATO/C from 200 to 800 °C at a heating rate of 10 °C min−1 in air. Figure 6 (a) The cycling stability of CNF, ATO particles, CNF/ATO, and CNF/ATO/C at current densities of 100 mA g –1 up to 100 cycles. (b) The high-rate performance at current densities of 100, 300, 700, 1000, 1500, 2000 and 100 mA g–1. (c) Comparison of high-rate performance with previously reported studies of ATO and SnO2-based anode materials with the carbon material in LIBs. (d) The ultrafast cycling stability of CNF, ATO particles, CNF/ATO, and CNF/ATO/C at current densities of 2000 mA g –1 up to 100 cycles.

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Figure 7 Schematic of the charge transport and the Li-ion diffusion in CNF/ATO/C with three main effects for high-performance LIBs.

Figure 1

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Table of Contents Graphic

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