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Large-scale carbon framework microbelts anchoring ultrafine SnO2 nanoparticles with enhanced lithium storage properties
Accepted Manuscript Large-scale carbon framework microbelts anchoring ultrafine SnO2 nanoparticles with enhanced lithium storage properties Qishang Wang, Junqi Xu, Guangyue Shen, Yaqing Guo, Xun Zhao, Yanjie Xia, Haibin Sun, Peiyou Hou, Wenhe Xie, Xijin Xu PII:
S0013-4686(18)32659-8
DOI:
https://doi.org/10.1016/j.electacta.2018.11.175
Reference:
EA 33176
To appear in:
Electrochimica Acta
Received Date: 25 September 2018 Revised Date:
16 November 2018
Accepted Date: 25 November 2018
Please cite this article as: Q. Wang, J. Xu, G. Shen, Y. Guo, X. Zhao, Y. Xia, H. Sun, P. Hou, W. Xie, X. Xu, Large-scale carbon framework microbelts anchoring ultrafine SnO2 nanoparticles with enhanced lithium storage properties, Electrochimica Acta (2018), doi: https://doi.org/10.1016/ j.electacta.2018.11.175. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Large-scale carbon framework microbelts anchoring ultrafine SnO2 nanoparticles with enhanced lithium
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storage properties Qishang Wang, a,b Junqi Xu,a,b* Guangyue Shen,a,b Yaqing Guo,a,b Xun Zhao,a,b Yanjie Xia,a,b Haibin Sun,a,b Peiyou Hou,c Wenhe Xie,a,b* and Xijin Xu c*
Key Laboratory of Microelectronics and Energy of Henan Province, Department of Physics
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a
and Electronic Engineering, Xinyang Normal University, Xinyang 464000, PR China Energy-Saving Building Materials Innovative Collaboration Center of Henan Province,
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b
Xinyang Normal University, Xinyang 464000, PR China c
School of Physics and Technology, University of Jinan, PR. China
* Corresponding Author. E-mail adress: [email protected] or [email protected] or
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[email protected]
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ACCEPTED MANUSCRIPT Abstract Varieties of nanostructured SnO2 have been widely investigated as promising anode material for next generation lithium-ion batteries (LIBs). However, traditional nanostructures suffer from re-agglomeration and excessive side reactions, which lead to low coulombic efficiency,
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poor rate performance and dramatic capacity decay. Here we develop an easy and robust strategy to fabricate carbon framework microbelts anchoring ultrafine SnO2 nanoparticles (USnO2 NPs@ CF-MBs), which takes advantage of the synergistic effect between high
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conductivity of large-size carbon framework and high activity of ultrafine SnO2 nanoparticles. The as-fabricated U-SnO2 NPs@C-BsF composite deliver high capacity of 925 mAh g-1 after
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250 cycles at current density of 200 mA g-1, high rate capacity of 464 mAh g-1 at a high current density of 5000 mA g-1 and long cycle performance of 788 mAh g-1 after 1000 cycles at current density of 1500 mA g-1 in half cells. When applied in a full cell by coupling with a LiCoO2 cathode, the fabricated U-SnO2 NPs@ CF-MBs composite full cells keep a high
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capacity of 510 mAh g-1 after 80 cycles. Notably, the electrode exhibit two platforms located at 3.3 and 2.6 V, which indicate that the conversion between SnO2 and Sn is also highly reversible in full cells. The excellent lithium storage of large-scale U-SnO2 NPs@ CF-MBs
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ensures its great promise for commercial ultilization in the future LIBs.
Keywords: SnO2, Carbon microbelts, Reversible conversion, Lithium ion batteries, Full cells.
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ACCEPTED MANUSCRIPT 1. Introduction Up to now, the most popular energy storage devices are lithium ion batteries (LIBs) [1-4], which have wide-range application in many fields including smart phone, tablet, and electric vehicle etc. The key evaluating indicators of these LIBs are the energy density determined by
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specific capacity and operating voltage, which are highly related to the used cathode and anode electrode materials. Unfortunately, graphite, the broadly commercial anode material, has a low theoretical specific capacity of 372 mAh g-1, which cannot fully satisfy the growing
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energy density requirements. Thus it is urgent to explore novel advanced anode materials with higher specific capacity [2, 5-17].
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Tin dioxide (SnO2) has been widely researched as an anode electrode material due to its low cost, friendly to environment, high specific capacity and high nature abundance[11, 1821]. Compared with graphite, SnO2 possesses much higher capacity because of its unique two step lithium storage mechanism[22, 23]. Specifically, in the charge/discharge process, SnO2
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firstly undergo a typical conversion reaction to form Li2O and Sn, and then from the Sn alloy with Li. Assuming that the two reaction processes are completely reversible, the theoretical specific capacity of SnO2 is as high as 1489 mAh g-1. Additionally, comparing with most of
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the anode materials, the corresponding working potential of SnO2 is relatively low. These advantages make SnO2 an ideal alternative to graphite as anode materials in LIBs applications.
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Nevertheless, the practical application of SnO2 anode materials is largely impeded by volume expansion challenges during redox reactions. On the one hand, the huge volume expanding (more than 360 %) may shake the electrode, leading to electrode structural pulverization and active material exfoliation from the current collector[24, 25]. On the other hand, the huge volume changes crack solid electrolyte interphase (SEI), which result in the expose of the anode electrode surface in the electrolyte and repeat formation fresh SEI. Accordingly, the unstable SEI growth continuously consumes large amounts of limited electrolyte, leading to a high resistance for charge transfer. Therefore, the electrodes show dramatic capacity 3
ACCEPTED MANUSCRIPT attenuation, low coulombic efficiency and poor rate performance. To overcome the volume changes, various complex SnO2 nanostructures and their composites including SnO2 nanowire [26-28], SnO2 nanotube[29, 30], SnO2 nanosheet[31], SnO2 nanosphere[32, 33], SnO2/graphene[34] etc have been designed as anode materials. These attempts are based on
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the fact that these nano-based materials not only shorten the lithium ion diffusion distance but also offer rich void space to buffer the volume changes, which greatly improve the electrochemical lithium storage of batteries[35].
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Although, the above mentioned nano-design strategy cause new problems. For instance, the nanoscale SnO2 may re-agglomerate and destroy the original nanostructure, which causes
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poor actual capacity because of the irreversible reaction between SnO2 and Sn [36, 37]. In addition, the large specific surface and porosity endow these nanomaterials with more contact interface between electrode and electrolyte, which inevitably bring about more side reactions or irreversible reactions, thus leading to the significant capacity fade phenomenon in the long-
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term electrochemical cycles. To overcome the re-agglomeration problems, researchers have tried different methods, such as graphene combination with nanoparticle[38], carbon coating on the surface of nanostructures [39], and yolk (nanopraticle)-shell (carbon) designing [40]
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etc. However, graphene combination method bring about the exfoliation of the active material from the graphene due to the weak van der Waals force interaction; carbon coating cannot
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completely solve such problem because the inner active material is big that they may break the carbon shell; yolk-shell designing method is an effective way to tackle the reagglomeration problem, but the synthesis procedure is very complicated and is difficult to control[41]. Based on the above analysis, developing an easy and effective strategy to tackle the volume expansion and the re-agglomeration of high-capacity SnO2 anode remains a huge challenge. Various synthesis methods have been used to fabricate the bare SnO2 (including nanopartciles, nanowires, nanobelts etc) and SnO2/C composite (including SnO2/graphene[42], 4
ACCEPTED MANUSCRIPT SnO2/CNT[43], SnO2/polymer[44], SnO2/ graphene oxide[22], etc ), however, there are rare reports about a facile electrospinning method for synthesis of SnO2-carbon ultrafine nanosheets. The controllable and uniform morphology can be conveniently generated by adjusting the electrospinning ambient temperature. Herein, we firstly report a simple and
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robust strategy to fabricate carbon framework microbelts anchoring ultrafine SnO2 nanoparticles (U-SnO2 NPs@CF-MBs), in which the ultrafine SnO2 nanoparticles (~6.5 nm) are uniformly separated by the large-size carbon micro belts (~1.6 µm) framework. The as-
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synthesized U-SnO2 NPs@CF-MBs integrates with the quick electron conduction and fast ion diffusion for the electrochemical performance. Firstly, compared with the bare SnO2
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nanstructures (including nanopartciles, nanowires, nanobelts with thick thiness etc), the characteristics of thin thickness, the micro-size width and large length of carbon belts, endow the electron conduction passageway for electrochemical properties. Secondly, compare with the SnO2/C composite (including SnO2/graphene[42], SnO2/CNT[43], SnO2/polymer[44],
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SnO2/graphene oxide[22], etc ), the tight coupling and uniform anchoring of SnO2 ultrafine nanoparticle in the carbon framework benefit for the complete reaction between SnO2 and Li. This novel design effectively increases electron and ion conductivity, avoids the re-
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agglomeration, and reduces side effects, which cause both the conversion of SnO2 to Sn and Sn alloying processes are reversible and thus significantly enhance the electrochemical
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performance.The unique U-SnO2 NPs@C-BsF shows a much enhanced SnO2 anode electrode performance with high specific capacity (925 mAh g-1 after 250 cycles at a current density of 200 mA g-1), superior rate performance (464 mAh g-1 at a current density of 5000 mA g-1) and long cycle life performance (788 mAh g-1 after 1000 cycles at current density of 1500 mA g-1). Besides, the full cell using this U-SnO2 NPs@ CF-MBs anode and LiCoO2 cathode also delivers high reversible capacity of 510 mAh g-1 after 80 cycles at current density of 200 mA g-1. Notably, the oxidation peaks located at 1.3 V in half cell and 2.6 V in full cell demonstrate the high reversibility of conversion reaction between Sn and SnO2, which is 5
ACCEPTED MANUSCRIPT rarely reported in other literatures[45-47]. These excellent lithium storage properties indicate the potential application of as-synthesized U-SnO2 NPs@ CF-MBs in the advanced LIBs. 2. Experimental
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2.1. Fabrication of carbon belts framework anchoring ultrafine SnO2 nanoparticles
The large scale carbon framework belts anchoring ultrafine SnO2 nanoparticles (U-SnO2 NPs@CF-MBs) composite were synthesized by a facile process, which include an
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electrospinning method and the following anneal process, as depicted in Fig. 1. The raw
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materials N,N-dimethylformamide (DMF), Polyvinylpyrrolidone (PVP, Mw = 1300000) and SnCl2·2H2O were purchased from Sigma-Aldrich without further purification. First of all, 1.8 g PVP is first dissolved in 10 mL DMF with magnetic stirring to form a uniform viscous solution. Then, 1.0 g SnCl2·2H2O was added into the solution with magnetic stirring for another 8 hours to form a uniform liquid gel (Fig. 1a). The prepared tin-based liquid gel was
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transferred into a syringe and applied on electrospinning equipment (WL-2, Beijing Aibo Zhiye Ion Technology Limited Company, China). The setting voltage, solution feed rate, collected distance and electrospinning environment temperature were 13.5 kV, 2.5 mL h-1,15
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cm and 50 oC, respectively. An aluminum foil was used to collect the electrospinning
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products (Fig. 1b). After the collection, the products were transferred to a vacuum oven to evaporate residual solvent. In order to obtain U-SnO2 NPs@CF-MBs composite, the dried sample was heated at 260 oC for 4 hours to convert tin salt into SnO2 under air atmospheric conditions, and then carbonized at 500 oC for 4 hours under Ar atmospheric conditions (Fig. 1c). Finally, the resulted samples were heated at 260 oC for 10 hours under air atmospheric condition to form the U-SnO2 NPs@CF-MBs (Fig. 1d).
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ACCEPTED MANUSCRIPT 2.2. Characterization of the U-SnO2 NPs@CF-MBs composite The structure and morphology of the samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron
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microscopy (TEM, FEI Tecnai G2 F20 S-TWIN). The crystal structure and compositions were identified by X-ray diffraction (XRD, Rigaku SmartLab 9kW), micro Raman spectroscopy (Jobin-Yvon LabRAM HR800 UV, YAG 532 nm) and X-ray photoelectron spectroscopy
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(XPS, K-Alpha, Thermo Fisher Scientific).
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2.3 Electrochemical measurements
The working electrode were prepared by mixing U-SnO2 NPs@CF-MBs (80 wt.%), super P conductive agent (10 wt.%) and sodium alginate binder (10 wt.%) with distill water as the solvent. The obtained slurry was coated onto a copper foil and then dried in a vacuum oven at 90 oC for 12 hours. The active load of the U-SnO2 NPs@CF-MBs is about 0.8-2.0 mg cm-2.
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For the half-cells fabrication, lithium foils were used as counter and reference electrode. For the full cell fabrication, the LiCoO2 was used as cathode electrode. Both the half cells and full
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cells were assembled in an argon-filled glove box (H2O, O2<0.1 ppm) and glass fiber was served as separator membrane. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate
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(EC) and dimethyl carbonate (DEC) mixture in a volume ratio of 1:1. Electrochemical performance tests were performed on Neware multi-channel battery testers (BTS-760) and an electrochemical workstation (CHI-660E).
3. Results and discussion 3.1. The Morphology and Structure of the U-SnO2 NPs@CF-MBs XRD pattern of U-SnO2 NPs@CF-MBs is shown in Fig. 2a, in which all the sharp diffraction peaks are well indexed to the tetragonal structure of SnO2 (JCPDS No#41-1445) 7
ACCEPTED MANUSCRIPT while a broad peak (dotted red line) is attributed to the amorphous carbon content. Raman spectrum Fig. 2b with two characteristic D band at 1368 cm-1 and G band at 1590 cm-1 are attributed to the crystal defect or disordered graphite, and graphitic lattice vibration mode with E2g symmetry [48], respectively. No obvious peak of SnO2 can be observed from the
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Raman analysis, which can be attributed to the fact that the response factor of carbon is much higher than that of SnO2 and most SnO2 particles were covered by carbon. The chemical composition and the oxidation state of U-SnO2 NPs@CF-MBs were characterized by X-ray
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photoelectron spectroscopy (XPS) as shown in Fig. 2(c, d). A representative full scan XPS spectrum Fig. 2c reveals the existence of the tin, oxygen, nitrogen and carbon element. The
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precise scan spectrum of Sn 3d3/2 located at 495.1 eV and Sn 3d5/2 located at 486.7 eV (Fig. 2d) accord well with the early reports[49]. The C 1s XPS peaks (Supporting information, Fig. S1a) can be deconvoluted into the main components for Sp2-C at 284.5 eV and two weaker bands associated with C-N at 285.2 eV and C-O at 285.9 eV. The high-resolution XPS N 1s
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peaks (Supporting information, Fig. S1b) can be deconvoluted into the pyridinic nitrogen at 398.2 eV and pyrrolic nitrogen at 400.6 eV. The existence of N 1s and C 1s species in the hybrid is mainly due to the raw materials of PVP and the solvent DMF. It should be noted that
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the nitrogen doping in the carbon frameworks is beneficial to increase the electrode conductivity and create additional active sites for Li+ storage[50]. The pore structure of the U-
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SnO2 NPs@CF-MBs is explored by the nitrogen adsorption-desorption isotherms (Supporting information, Fig. S2). The specific surface area of the U-SnO2 NPs@CF-MBs is about 37.1 m2 g-1 according to the Brunauer-Emmentt-Teller (BET) theory (Fig. S2a). The BarrettJoyner-Halenda (BJH) analysis (Fig. S2b) presents a pore size distribution ranging from 2 to 100 nm, corresponding to a total pore volume of 0.016 m3 g-1. The TGA analysis in Fig. S3 (Supporting information) shows that the mass percentage of SnO2 is about 60 wt % in the USnO2 NPs@CF-MBs sample.
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ACCEPTED MANUSCRIPT The typical structures and morphologies of the as-fabricated U-SnO2 NPs@CF-MBs were characterized by field-emission scanning electron microscope (FESEM). As shown in Fig. 3ab, the hybrid is composed by large-quantity “belts-like” structures with length of several hundred microns, in which each microbelt has a width of ~1.6 µm (Fig. 3c) and a thickness of
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~123 nm (Supporting information, Fig. S4). Such structural construction endows the hybrid highway for the electrons rapid transporting, as well as ions diffusion through the belt surfaces. TEM (Fig. 3e), high resolution TEM (Fig. 2d, 2f) and selected-area diffraction
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pattern (SAED) (Fig. 2f) images further reveals that ultrafine SnO2 nanoparticles with a mean diameter of ~6.5 nm (Supporting information, Fig. S5) are uniformly dispersed through the
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carbon belts. The corresponding TEM elemental mappings (Fig. 3g-j) also clearly confirm that the elements of Sn, O, C and N uniformly distributed through the whole structure. We investigate the morphologies of the as-prepared sample at the different ambient temperature. Fig. 4 shows the SEM images spun at a ambient temperature 18 oC, 35 oC and 50 o
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C, respectively. Fig. 4a and Fig. 4c demonstrate the almost wire-like and belts-like
morphology, respectively, while the sample exhibits the mixtures of wires and belts (Fig. 4b). Based on the above results, a possible growth mechanism of microbelts [51, 52] can be
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proposed as shown in Fig. 5. Initially, fibers spun out from the nozzle uniformly compose of SnCl2, PVP and solvent DMF (Fig. 5a). Rapidly, the spinning ambient brings about the
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evaporation of the solvent at the exterior of the fibers and thus results in phase separation [53, 54] between the core (solution) and shell (skin) (Fig. 5b). If the spinning temperature is higher (50 oC), the solvent evaporates quickly and thus leads to anisotropy shrinkage among different part of the same fiber because that great difference exist in the local parameters, such as imperfect circular cross section of the fiber, defects, voids, local temperature etc. The circular cross section of the fibers become elliptical (Fig. 5c1) and then flat (Fig. 5d1) by a critical pressure differential. After annealing, the precursors will finally become the composite microbelts uniformly containing SnO2 and carbon particles (Fig. 5e1). On the other hand, if 9
ACCEPTED MANUSCRIPT the spinning temperature is lower (room temperature, 18 oC), the solvent evaporates relatively slow and thus lead to isotropy shrinking among different parts of the same fiber. The circular cross section of the fiber uniformly shrinks (Fig. 5c2) and finally shrinks to a solid fiber (Fig. 5d2) as the material (chemical molecules) [55] further migrates from the core to the shell.
SnO2 and carbon particles (Fig. 5e2).
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3.2. Electrochemical performance of the U-SnO2 NPs@CF-MBs
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After annealing, the precursors will finally become a compact solid fiber uniformly containing
A series of electrochemical tests were carried out to evaluate the lithium storage properties
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of U-SnO2 NPs@CF-MBs as anode material of half cells. Fig. 6a show the initial 5 cyclic voltammograms (CVs) curves measured at a scan rate of 0.2 mV s-1 in the voltage window range from 0.02-3.0 V (vs. Li/Li+). During the initial cathodic scan, a broad peak at about 0.8 V is observed, which is caused by the conversion of SnO2 to Sn and the irreversible formation
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of SEI layer. Another cathodic peak located at about 0.25 V attributed the Sn alloying process is also found. In the following anodic scan, two broad oxidation peaks at 0.5 V and 1.3 V corresponding to the dealloy of IixSn and conversion of Sn back to SnO2 are detected[56].
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The CV curves after 150 galvanostatic density charge/discharge cycles were displayed in Fig. 6b, and it can also be observed two pronounced oxidation peaks located at about 0.5 V
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and 1.3 V in anodic scan process, corresponding to the formation of amorphous Sn and SnO2, respectively[22, 57, 58]. The relatively strong oxidation peak intensity of SnO2 demonstrates that the conversion of SnO2 to Sn is highly reversible. This highly reversible property of SnO2 materials is rarely reported in literatures[59, 60]. Hence, the whole electrochemical reaction mechanism can be described as follow[2, 61, 62]. SnO2 + 4 Li + + 4e− ↔ Sn + 2 Li2O
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Sn + xLi + + xe− ↔ Lix Sn(0 ≤ x ≤ 4.4)
(2)
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ACCEPTED MANUSCRIPT The galvanostatic discharge/charge profiles of the initial 5 cycles were shown in Fig. 6c, and it can be observed the first discharge profile and capacity is quite different from the following profiles, which is attributed to the formation of SEI layer and some related irreversible reactions. Two obvious plateaus located at about 0.5 and 1.3 V can be observed in
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the charge profile, which reflects the two-step formation mechanism of SnO2. The fact that the curves from 3rd to 5th are almost overlap each other indicates the anode materials is very stable (i.e. no side reaction and noticeable capacity decay). In the following cycles, the
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representative discharge/charge profiles of 50th, 100th, 150th, 200th and 250th (Fig. 6d) are highly reversible, in accordance with the conclusion from Fig. 6b. These data from the
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electrochemical workstation and the multi-channel battery testers confirm that the conversion from SnO2 to Sn is highly reversible[63].
Fig. 7a shows the cyclic performance and coulomb efficiency at a current density of 200 mA g-1, and the initial charge and discharge capacities are 1124 and 1541 mAh g-1,
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respectively, corresponding to an initial coulomb efficiency (CE) of 72.9%. The high initial CE is higher than most of pure and complex SnO2 materials[64-67], which is a result of the highly reversible from SnO2 to Sn. The half cells deliver high reversible capacity of 925 mAh
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g-1 after 250 cycles, which is only a slightly fade comparable to the initial cycle. As a contrast, the cyclic performance of pure SnO2 (the morphology is microbelts (Fig. S6a) and the crystal
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phase is determined by XRD (Fig. S6b); the corresponding synthesis procedure please refer to the experiment S1; supporting information) and the C nanostructures (the morphology is microbelts (Fig. S7a) and the crystal phase is determined by Raman spectra (Fig. S7b); the corresponding synthesis procedure please refer to the experiment S2; supporting information) are also displayed in Fig. 7a. The capacity of the bare SnO2 microbelts decays very fast and delivers a low capacity of 478 mAh g-1 after 150 cycles at a current density of 200 mAh g-1. The capacity of carbon microbelts is very stable and delivers a low capacity of 295 mAh g-1 after 100 cycles at current density of 200 mAh g-1. Thus the cyclic performance of U-SnO2 11
ACCEPTED MANUSCRIPT NPs@CF-MBs is much better than that of pure SnO2 and C electrode, which may be due to the synergistic effects originated from good conduction channel of carbon framework microbelts and higher theoretical capacity of SnO2. In addition, the coulombic efficiency (upper lines) is more than 98 % after the initial 3
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cycles, which further verifies the highly reversible lithium storage ability of the novel U-SnO2 NPs@CF-MBs anode materials. The rate performance was evaluated from 0.2 A g-1 to 5 A g-1, as shown in Fig 5b, the half cells deliver high reversible capacities of 937, 850, 746, 641, 543
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and 464 mAh g-1 at current densities of 0.2, 0.4, 0.8, 1.6, 3.2 and 5 A g-1, respectively. Noticeably, when the current density was reverted back to a low current density of 0.2 A g-1,
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the cell capacity restore to a high value of 889 mAh g-1, equivalent of 94.9 % of initial cycle capacity. As a result, both the capacity retention at large current density (5A g-1) and recovery property are superior to nanofiber[68-71], nanotube[72-78] based anode materials. The long cyclic performance was tested at a high current density of 1.5 A g-1 as shown in Fig. 7c, and it
1000 cycles.
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can be seen that the reversible capacity are very stable and maintain 788 mAh g-1 even after
All these results demonstrate that the as-synthesized U-SnO2 NPs@CF-MBs based half
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cells possess the good performance evidenced by high capacity, superior rate and long cycle life [78-80]. The red curve of Fig. S8a shows the electrochemical impedance spectra (EIS) for
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the half cell of the U-SnO2 NPs@CF-MBs after 1000 cycles. The Nyquist plot shows a semicircle and the kenetic parameters obtained from the equivalent circuit fitting are listed in the Fig. S8b. The charge-transfer resistance (Rct) calculated by using the equivalent circuit model for the sample is only 106.7 Ω after 1000 cycles, indicating the high ionic conductivity for the U-SnO2 NPs@CF-MBs electrode. To investigate the excellent cycling stability mechanism and structure change of the USnO2 NPs@CF-MBs, the lithium half cell in Fig. S9 is disassembled after cycling tests. The morphology of the U-SnO2 NPs@CF-MBs sample is studied using SEM and techniques. 12
ACCEPTED MANUSCRIPT Before washing out the electrolyte, it seems that that the electrode’s surface is covered by a layer of electrolyte from the SEM of Fig. S9a and TEM image of Fig. S9b. However, the beltlike characteristics of the U-SnO2 NPs@CF-MBs electrode can still be clearly observed even after cycles. In order to further study the detailed microstructure of the U-SnO2 NPs@CF-
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MBs electrode, the after-cycled sample is dispersed in ethanol under intense ultrasound. The low-magnification TEM images for the after-cycled electrode are illustrated in Fig. S9c, in which the electrode is uniform and similar to that of uncycled U-SnO2 NPs@CF-MBs. An
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HRTEM images for the after-cycled electrode are showed in Fig. S9d, in which no obvious SnO2 particles can be observed, suggesting that the electrode convert to amorphous structure
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in the long cycling process. From these SEM and TEM images displayed in Fig. S9, even after 1000 cycles, the integrity of the whole electrode is well sustained, which might be the reason accounting for the excellent cycling stability of the electrode. The outstanding lithium storage capability of as-synthesized U-SnO2 NPs@CF-MBs
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electrode is associated with the unique structure, in which the ultrafine SnO2 nanoparticle is separated by the carbon network. Firstly, the micron-size carbon framework supports long and wide highway for convenient electronic transport. Secondly, the ultrafine SnO2 nanoparticles
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were uniform anchored in the large-size carbon structure, which greatly reduces the lithium ions diffusion distance. The U-SnO2 NPs@CF-MBs still remain the belt-structure after
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cycling test (supporting information). Therefore, these designs take the advantages of synergistic effects of both micrometer size and nanometer size. Consequently, the electrode structure has a good stability and the electrochemical reaction of SnO2 to Sn is highly reversible. Finally, to prove the practical application of as synthesized U-SnO2 NPs@CF-MBs, we further assemble full cells to evaluate the energy storage performance when coupled with commercial LiCoO2 cathode. Prelithiation of anode electrode can effectively offset the lithium consumption and improve the full cell performance. Specifically, the U-SnO2 13
ACCEPTED MANUSCRIPT NPs@CF-MBs anode directly contact with metal lithium for five minutes to conduct the prelithiation process. Then the full cells of the as-synthesized U-SnO2 NPs@CF-MBs anode coupled by a LiCoO2 cathode were tested at a current density of 200 mA g-1 (based on the active mass of U-SnO2 NPs@CF-MBs) in the voltage range of 1.5-3.9 V. Fig. 8a presents the
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representative discharge/charge profiles of 2nd and 12th cycle. Apparently, there exists two discharge plateaus located at 3.3 and 2.6 V, which is further confirmed by the two corresponding discharge peaks at CV measurement in Fig. 8b. These results demonstrate that
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the conversion of Sn to SnO2 is also reversible when the as synthesized U-SnO2 NPs@CFMBs electrode is applied in the full cell. Fig. 8c shows that the reversible capacity of full cell
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is 510 mAh g-1 after 80 cycles and the coulombic efficiency is about 99.5 %. The black curve of Fig. S8a shows the electrochemical impedance spectra (EIS) for the full cell of the U-SnO2 NPs@CF-MBs after cycling. The The Nyquist plot shows a large semicircle and the kenetic parameters obtained from the equivalent circuit fitting are listed in the Fig. S6b. As can be
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seen from the Fig. S6b, the charge-transfer resistance (Rct) calculated by using the equivalent circuit model for the sample is only 300.4 Ω after cycling, which is larger than the Rct of the half cell. To prove the application of the full cell, we design a Sn logo (Fig. 8d), which
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consists of 15 yellow LEDs and 17 red LEDs. It can be observed that a fully charged full cell
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(with an absolute capacity about 1.2 mAh) can light these 32 LEDs for at least for 40 minutes.
4. Conclusions
In summary, we have successfully synthesized the U-SnO2 NPs@CF-MBs anode material, i.e. massive of ultrafine SnO2 particles were anchored uniformly in the carbon network, which renders lithium ions and electrons with convenient and fast electrochemical reaction channels. It was found that the conversion between Sn and SnO2 is highly reversible in both half and 14
ACCEPTED MANUSCRIPT full cells. The half cells deliver high capacity of 925 mAh g-1 at current density of 200 mA g-1, high rate capacity of 464 mAh g-1 at current density of 5000 mA g-1 and long cycle performance of 788 mAh g-1 after 1000 cycles at current density of 1500 mA g-1. The full cell also delivers high reversible capacity of 510 mAh g-1 after 80 cycles at current density of 200
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mA g-1. It is worth noting that that the full cells of as synthesized anode materials coupled with LiCoO2 can light 32 LEDs for more than 40 minutes. These results demonstrate that the novel design of combining the micron and nano advantages can promote the electrochemical
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performance of SnO2 in the next-generation LIBs and this facile technique can be easily
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expanded in the fabrication of other related anode materials.
Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 61675175), Program Projects of Science and Technology Innovative Research Team of Henan Province
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(18IRTSTHN017), Key Research Projects of Henan Provincial Department of Education (17A140011), Science and Technology Project of Henan Province (182300410218) and and
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Nanhu Scholars Program for Young Scholars of XYNU.
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Figure Caption Fig. 1. Sketch illustration for fabricating of carbon framework microbelts anchoring ultrafine
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SnO2 nanoparticles: (a) uniform tin-based gel for electronspinning; (b) electronspinning process for nanobelts; (c) anneal in Ar atmosphere and 500 oC; (d) final products; (e) TEM observation results.
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analysis and (d) XPS precise scan spectra of Sn 3d.
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Fig. 2. (a)XRD patterns of the U-SnO2 NPs@CF-MBs. (b)Raman spectra. (c) Full scan XPS
Fig. 3. SEM characterization: (a) low-magnification of the nanobelts; (b)high-magnification. (c) The width distribution of the U-SnO2 NPs@CF-MBs. TEM characterization: (e) a typical low-magnification image; (d) and (f) HRTEM image of the U-SnO2 NPs@CF-MBs and the
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corresponding SAED pattern (inset of Fig. 3f); (h-j) the element mapping.
Fig. 4. SEM images of the sample for electrospinning ambient of (a) 18 oC, (b) 35 oC and (c) 50 oC, respectively.
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Fig. 5. The schematic of the growth mechanism for SnO2 composite. (a) Fibers spun out from
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the nozzle at the beginning. (b) Phase separation between the core (solution) and shell (skin). (c1) Anisotropy shrinkage among different part of the same fiber at higher ambient temperature. (d1) Skin-core structures shrink into belt structure. (e1) SnO2 composite nanobelt after annealing. (c2) Isotropy shrinkage among different part of the same fiber at lower ambient temperature. (d2) Skin-core structures shrink into wire structure. (e2) SnO2 composite nanowire after annealing.
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ACCEPTED MANUSCRIPT Fig. 6. Electrochemical performances of U-SnO2 NPs@CF-MBs electrode based half cell: (a) CV curves for the initial 5 cycles; (b) CV curves galvanostatic charge/discharge test after 150 cycles; (c) Galvanostatic discharge-charge profiles of the initial 5 cycles; (d) representative
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galvanostatic discharge/charge profiles of 50th, 100th , 150th, 200th and 300th.
Fig. 7. Electrochemical tests of U-SnO2 NPs@CF-MBs electrode based half cell: (a) cycling performance
and coulombic efficiency of U-SnO2 NPs@CF-MBs, and the cycling
performance of bare SnO2 and C electrode; (b) rate performance; (c) long cycling
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performance tested at 1500 mA g-1.
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Fig. 8. Electrochemical test of U-SnO2 NPs@CF-MBs electrode based full cell: (a) representative galvanostatic discharge-charge profiles of 2nd and 12th; (b) representative CV curves; (c) cycling performance tested at 200 mA g-1 and coulombic efficiency; (d) a typical
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digital image of 32 LEDs powered by the integrated full cell.
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S0013-4686(18)32659-8
DOI:
https://doi.org/10.1016/j.electacta.2018.11.175
Reference:
EA 33176
To appear in:
Electrochimica Acta
Received Date: 25 September 2018 Revised Date:
16 November 2018
Accepted Date: 25 November 2018
Please cite this article as: Q. Wang, J. Xu, G. Shen, Y. Guo, X. Zhao, Y. Xia, H. Sun, P. Hou, W. Xie, X. Xu, Large-scale carbon framework microbelts anchoring ultrafine SnO2 nanoparticles with enhanced lithium storage properties, Electrochimica Acta (2018), doi: https://doi.org/10.1016/ j.electacta.2018.11.175. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Large-scale carbon framework microbelts anchoring ultrafine SnO2 nanoparticles with enhanced lithium
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storage properties Qishang Wang, a,b Junqi Xu,a,b* Guangyue Shen,a,b Yaqing Guo,a,b Xun Zhao,a,b Yanjie Xia,a,b Haibin Sun,a,b Peiyou Hou,c Wenhe Xie,a,b* and Xijin Xu c*
Key Laboratory of Microelectronics and Energy of Henan Province, Department of Physics
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a
and Electronic Engineering, Xinyang Normal University, Xinyang 464000, PR China Energy-Saving Building Materials Innovative Collaboration Center of Henan Province,
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b
Xinyang Normal University, Xinyang 464000, PR China c
School of Physics and Technology, University of Jinan, PR. China
* Corresponding Author. E-mail adress: [email protected] or [email protected] or
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[email protected]
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ACCEPTED MANUSCRIPT Abstract Varieties of nanostructured SnO2 have been widely investigated as promising anode material for next generation lithium-ion batteries (LIBs). However, traditional nanostructures suffer from re-agglomeration and excessive side reactions, which lead to low coulombic efficiency,
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poor rate performance and dramatic capacity decay. Here we develop an easy and robust strategy to fabricate carbon framework microbelts anchoring ultrafine SnO2 nanoparticles (USnO2 NPs@ CF-MBs), which takes advantage of the synergistic effect between high
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conductivity of large-size carbon framework and high activity of ultrafine SnO2 nanoparticles. The as-fabricated U-SnO2 NPs@C-BsF composite deliver high capacity of 925 mAh g-1 after
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250 cycles at current density of 200 mA g-1, high rate capacity of 464 mAh g-1 at a high current density of 5000 mA g-1 and long cycle performance of 788 mAh g-1 after 1000 cycles at current density of 1500 mA g-1 in half cells. When applied in a full cell by coupling with a LiCoO2 cathode, the fabricated U-SnO2 NPs@ CF-MBs composite full cells keep a high
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capacity of 510 mAh g-1 after 80 cycles. Notably, the electrode exhibit two platforms located at 3.3 and 2.6 V, which indicate that the conversion between SnO2 and Sn is also highly reversible in full cells. The excellent lithium storage of large-scale U-SnO2 NPs@ CF-MBs
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ensures its great promise for commercial ultilization in the future LIBs.
Keywords: SnO2, Carbon microbelts, Reversible conversion, Lithium ion batteries, Full cells.
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ACCEPTED MANUSCRIPT 1. Introduction Up to now, the most popular energy storage devices are lithium ion batteries (LIBs) [1-4], which have wide-range application in many fields including smart phone, tablet, and electric vehicle etc. The key evaluating indicators of these LIBs are the energy density determined by
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specific capacity and operating voltage, which are highly related to the used cathode and anode electrode materials. Unfortunately, graphite, the broadly commercial anode material, has a low theoretical specific capacity of 372 mAh g-1, which cannot fully satisfy the growing
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energy density requirements. Thus it is urgent to explore novel advanced anode materials with higher specific capacity [2, 5-17].
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Tin dioxide (SnO2) has been widely researched as an anode electrode material due to its low cost, friendly to environment, high specific capacity and high nature abundance[11, 1821]. Compared with graphite, SnO2 possesses much higher capacity because of its unique two step lithium storage mechanism[22, 23]. Specifically, in the charge/discharge process, SnO2
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firstly undergo a typical conversion reaction to form Li2O and Sn, and then from the Sn alloy with Li. Assuming that the two reaction processes are completely reversible, the theoretical specific capacity of SnO2 is as high as 1489 mAh g-1. Additionally, comparing with most of
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the anode materials, the corresponding working potential of SnO2 is relatively low. These advantages make SnO2 an ideal alternative to graphite as anode materials in LIBs applications.
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Nevertheless, the practical application of SnO2 anode materials is largely impeded by volume expansion challenges during redox reactions. On the one hand, the huge volume expanding (more than 360 %) may shake the electrode, leading to electrode structural pulverization and active material exfoliation from the current collector[24, 25]. On the other hand, the huge volume changes crack solid electrolyte interphase (SEI), which result in the expose of the anode electrode surface in the electrolyte and repeat formation fresh SEI. Accordingly, the unstable SEI growth continuously consumes large amounts of limited electrolyte, leading to a high resistance for charge transfer. Therefore, the electrodes show dramatic capacity 3
ACCEPTED MANUSCRIPT attenuation, low coulombic efficiency and poor rate performance. To overcome the volume changes, various complex SnO2 nanostructures and their composites including SnO2 nanowire [26-28], SnO2 nanotube[29, 30], SnO2 nanosheet[31], SnO2 nanosphere[32, 33], SnO2/graphene[34] etc have been designed as anode materials. These attempts are based on
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the fact that these nano-based materials not only shorten the lithium ion diffusion distance but also offer rich void space to buffer the volume changes, which greatly improve the electrochemical lithium storage of batteries[35].
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Although, the above mentioned nano-design strategy cause new problems. For instance, the nanoscale SnO2 may re-agglomerate and destroy the original nanostructure, which causes
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poor actual capacity because of the irreversible reaction between SnO2 and Sn [36, 37]. In addition, the large specific surface and porosity endow these nanomaterials with more contact interface between electrode and electrolyte, which inevitably bring about more side reactions or irreversible reactions, thus leading to the significant capacity fade phenomenon in the long-
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term electrochemical cycles. To overcome the re-agglomeration problems, researchers have tried different methods, such as graphene combination with nanoparticle[38], carbon coating on the surface of nanostructures [39], and yolk (nanopraticle)-shell (carbon) designing [40]
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etc. However, graphene combination method bring about the exfoliation of the active material from the graphene due to the weak van der Waals force interaction; carbon coating cannot
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completely solve such problem because the inner active material is big that they may break the carbon shell; yolk-shell designing method is an effective way to tackle the reagglomeration problem, but the synthesis procedure is very complicated and is difficult to control[41]. Based on the above analysis, developing an easy and effective strategy to tackle the volume expansion and the re-agglomeration of high-capacity SnO2 anode remains a huge challenge. Various synthesis methods have been used to fabricate the bare SnO2 (including nanopartciles, nanowires, nanobelts etc) and SnO2/C composite (including SnO2/graphene[42], 4
ACCEPTED MANUSCRIPT SnO2/CNT[43], SnO2/polymer[44], SnO2/ graphene oxide[22], etc ), however, there are rare reports about a facile electrospinning method for synthesis of SnO2-carbon ultrafine nanosheets. The controllable and uniform morphology can be conveniently generated by adjusting the electrospinning ambient temperature. Herein, we firstly report a simple and
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robust strategy to fabricate carbon framework microbelts anchoring ultrafine SnO2 nanoparticles (U-SnO2 NPs@CF-MBs), in which the ultrafine SnO2 nanoparticles (~6.5 nm) are uniformly separated by the large-size carbon micro belts (~1.6 µm) framework. The as-
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synthesized U-SnO2 NPs@CF-MBs integrates with the quick electron conduction and fast ion diffusion for the electrochemical performance. Firstly, compared with the bare SnO2
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nanstructures (including nanopartciles, nanowires, nanobelts with thick thiness etc), the characteristics of thin thickness, the micro-size width and large length of carbon belts, endow the electron conduction passageway for electrochemical properties. Secondly, compare with the SnO2/C composite (including SnO2/graphene[42], SnO2/CNT[43], SnO2/polymer[44],
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SnO2/graphene oxide[22], etc ), the tight coupling and uniform anchoring of SnO2 ultrafine nanoparticle in the carbon framework benefit for the complete reaction between SnO2 and Li. This novel design effectively increases electron and ion conductivity, avoids the re-
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agglomeration, and reduces side effects, which cause both the conversion of SnO2 to Sn and Sn alloying processes are reversible and thus significantly enhance the electrochemical
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performance.The unique U-SnO2 NPs@C-BsF shows a much enhanced SnO2 anode electrode performance with high specific capacity (925 mAh g-1 after 250 cycles at a current density of 200 mA g-1), superior rate performance (464 mAh g-1 at a current density of 5000 mA g-1) and long cycle life performance (788 mAh g-1 after 1000 cycles at current density of 1500 mA g-1). Besides, the full cell using this U-SnO2 NPs@ CF-MBs anode and LiCoO2 cathode also delivers high reversible capacity of 510 mAh g-1 after 80 cycles at current density of 200 mA g-1. Notably, the oxidation peaks located at 1.3 V in half cell and 2.6 V in full cell demonstrate the high reversibility of conversion reaction between Sn and SnO2, which is 5
ACCEPTED MANUSCRIPT rarely reported in other literatures[45-47]. These excellent lithium storage properties indicate the potential application of as-synthesized U-SnO2 NPs@ CF-MBs in the advanced LIBs. 2. Experimental
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2.1. Fabrication of carbon belts framework anchoring ultrafine SnO2 nanoparticles
The large scale carbon framework belts anchoring ultrafine SnO2 nanoparticles (U-SnO2 NPs@CF-MBs) composite were synthesized by a facile process, which include an
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electrospinning method and the following anneal process, as depicted in Fig. 1. The raw
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materials N,N-dimethylformamide (DMF), Polyvinylpyrrolidone (PVP, Mw = 1300000) and SnCl2·2H2O were purchased from Sigma-Aldrich without further purification. First of all, 1.8 g PVP is first dissolved in 10 mL DMF with magnetic stirring to form a uniform viscous solution. Then, 1.0 g SnCl2·2H2O was added into the solution with magnetic stirring for another 8 hours to form a uniform liquid gel (Fig. 1a). The prepared tin-based liquid gel was
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transferred into a syringe and applied on electrospinning equipment (WL-2, Beijing Aibo Zhiye Ion Technology Limited Company, China). The setting voltage, solution feed rate, collected distance and electrospinning environment temperature were 13.5 kV, 2.5 mL h-1,15
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cm and 50 oC, respectively. An aluminum foil was used to collect the electrospinning
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products (Fig. 1b). After the collection, the products were transferred to a vacuum oven to evaporate residual solvent. In order to obtain U-SnO2 NPs@CF-MBs composite, the dried sample was heated at 260 oC for 4 hours to convert tin salt into SnO2 under air atmospheric conditions, and then carbonized at 500 oC for 4 hours under Ar atmospheric conditions (Fig. 1c). Finally, the resulted samples were heated at 260 oC for 10 hours under air atmospheric condition to form the U-SnO2 NPs@CF-MBs (Fig. 1d).
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microscopy (TEM, FEI Tecnai G2 F20 S-TWIN). The crystal structure and compositions were identified by X-ray diffraction (XRD, Rigaku SmartLab 9kW), micro Raman spectroscopy (Jobin-Yvon LabRAM HR800 UV, YAG 532 nm) and X-ray photoelectron spectroscopy
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(XPS, K-Alpha, Thermo Fisher Scientific).
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2.3 Electrochemical measurements
The working electrode were prepared by mixing U-SnO2 NPs@CF-MBs (80 wt.%), super P conductive agent (10 wt.%) and sodium alginate binder (10 wt.%) with distill water as the solvent. The obtained slurry was coated onto a copper foil and then dried in a vacuum oven at 90 oC for 12 hours. The active load of the U-SnO2 NPs@CF-MBs is about 0.8-2.0 mg cm-2.
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For the half-cells fabrication, lithium foils were used as counter and reference electrode. For the full cell fabrication, the LiCoO2 was used as cathode electrode. Both the half cells and full
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cells were assembled in an argon-filled glove box (H2O, O2<0.1 ppm) and glass fiber was served as separator membrane. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate
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(EC) and dimethyl carbonate (DEC) mixture in a volume ratio of 1:1. Electrochemical performance tests were performed on Neware multi-channel battery testers (BTS-760) and an electrochemical workstation (CHI-660E).
3. Results and discussion 3.1. The Morphology and Structure of the U-SnO2 NPs@CF-MBs XRD pattern of U-SnO2 NPs@CF-MBs is shown in Fig. 2a, in which all the sharp diffraction peaks are well indexed to the tetragonal structure of SnO2 (JCPDS No#41-1445) 7
ACCEPTED MANUSCRIPT while a broad peak (dotted red line) is attributed to the amorphous carbon content. Raman spectrum Fig. 2b with two characteristic D band at 1368 cm-1 and G band at 1590 cm-1 are attributed to the crystal defect or disordered graphite, and graphitic lattice vibration mode with E2g symmetry [48], respectively. No obvious peak of SnO2 can be observed from the
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Raman analysis, which can be attributed to the fact that the response factor of carbon is much higher than that of SnO2 and most SnO2 particles were covered by carbon. The chemical composition and the oxidation state of U-SnO2 NPs@CF-MBs were characterized by X-ray
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photoelectron spectroscopy (XPS) as shown in Fig. 2(c, d). A representative full scan XPS spectrum Fig. 2c reveals the existence of the tin, oxygen, nitrogen and carbon element. The
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precise scan spectrum of Sn 3d3/2 located at 495.1 eV and Sn 3d5/2 located at 486.7 eV (Fig. 2d) accord well with the early reports[49]. The C 1s XPS peaks (Supporting information, Fig. S1a) can be deconvoluted into the main components for Sp2-C at 284.5 eV and two weaker bands associated with C-N at 285.2 eV and C-O at 285.9 eV. The high-resolution XPS N 1s
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peaks (Supporting information, Fig. S1b) can be deconvoluted into the pyridinic nitrogen at 398.2 eV and pyrrolic nitrogen at 400.6 eV. The existence of N 1s and C 1s species in the hybrid is mainly due to the raw materials of PVP and the solvent DMF. It should be noted that
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the nitrogen doping in the carbon frameworks is beneficial to increase the electrode conductivity and create additional active sites for Li+ storage[50]. The pore structure of the U-
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SnO2 NPs@CF-MBs is explored by the nitrogen adsorption-desorption isotherms (Supporting information, Fig. S2). The specific surface area of the U-SnO2 NPs@CF-MBs is about 37.1 m2 g-1 according to the Brunauer-Emmentt-Teller (BET) theory (Fig. S2a). The BarrettJoyner-Halenda (BJH) analysis (Fig. S2b) presents a pore size distribution ranging from 2 to 100 nm, corresponding to a total pore volume of 0.016 m3 g-1. The TGA analysis in Fig. S3 (Supporting information) shows that the mass percentage of SnO2 is about 60 wt % in the USnO2 NPs@CF-MBs sample.
8
ACCEPTED MANUSCRIPT The typical structures and morphologies of the as-fabricated U-SnO2 NPs@CF-MBs were characterized by field-emission scanning electron microscope (FESEM). As shown in Fig. 3ab, the hybrid is composed by large-quantity “belts-like” structures with length of several hundred microns, in which each microbelt has a width of ~1.6 µm (Fig. 3c) and a thickness of
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~123 nm (Supporting information, Fig. S4). Such structural construction endows the hybrid highway for the electrons rapid transporting, as well as ions diffusion through the belt surfaces. TEM (Fig. 3e), high resolution TEM (Fig. 2d, 2f) and selected-area diffraction
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pattern (SAED) (Fig. 2f) images further reveals that ultrafine SnO2 nanoparticles with a mean diameter of ~6.5 nm (Supporting information, Fig. S5) are uniformly dispersed through the
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carbon belts. The corresponding TEM elemental mappings (Fig. 3g-j) also clearly confirm that the elements of Sn, O, C and N uniformly distributed through the whole structure. We investigate the morphologies of the as-prepared sample at the different ambient temperature. Fig. 4 shows the SEM images spun at a ambient temperature 18 oC, 35 oC and 50 o
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C, respectively. Fig. 4a and Fig. 4c demonstrate the almost wire-like and belts-like
morphology, respectively, while the sample exhibits the mixtures of wires and belts (Fig. 4b). Based on the above results, a possible growth mechanism of microbelts [51, 52] can be
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proposed as shown in Fig. 5. Initially, fibers spun out from the nozzle uniformly compose of SnCl2, PVP and solvent DMF (Fig. 5a). Rapidly, the spinning ambient brings about the
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evaporation of the solvent at the exterior of the fibers and thus results in phase separation [53, 54] between the core (solution) and shell (skin) (Fig. 5b). If the spinning temperature is higher (50 oC), the solvent evaporates quickly and thus leads to anisotropy shrinkage among different part of the same fiber because that great difference exist in the local parameters, such as imperfect circular cross section of the fiber, defects, voids, local temperature etc. The circular cross section of the fibers become elliptical (Fig. 5c1) and then flat (Fig. 5d1) by a critical pressure differential. After annealing, the precursors will finally become the composite microbelts uniformly containing SnO2 and carbon particles (Fig. 5e1). On the other hand, if 9
ACCEPTED MANUSCRIPT the spinning temperature is lower (room temperature, 18 oC), the solvent evaporates relatively slow and thus lead to isotropy shrinking among different parts of the same fiber. The circular cross section of the fiber uniformly shrinks (Fig. 5c2) and finally shrinks to a solid fiber (Fig. 5d2) as the material (chemical molecules) [55] further migrates from the core to the shell.
SnO2 and carbon particles (Fig. 5e2).
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3.2. Electrochemical performance of the U-SnO2 NPs@CF-MBs
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After annealing, the precursors will finally become a compact solid fiber uniformly containing
A series of electrochemical tests were carried out to evaluate the lithium storage properties
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of U-SnO2 NPs@CF-MBs as anode material of half cells. Fig. 6a show the initial 5 cyclic voltammograms (CVs) curves measured at a scan rate of 0.2 mV s-1 in the voltage window range from 0.02-3.0 V (vs. Li/Li+). During the initial cathodic scan, a broad peak at about 0.8 V is observed, which is caused by the conversion of SnO2 to Sn and the irreversible formation
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of SEI layer. Another cathodic peak located at about 0.25 V attributed the Sn alloying process is also found. In the following anodic scan, two broad oxidation peaks at 0.5 V and 1.3 V corresponding to the dealloy of IixSn and conversion of Sn back to SnO2 are detected[56].
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The CV curves after 150 galvanostatic density charge/discharge cycles were displayed in Fig. 6b, and it can also be observed two pronounced oxidation peaks located at about 0.5 V
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and 1.3 V in anodic scan process, corresponding to the formation of amorphous Sn and SnO2, respectively[22, 57, 58]. The relatively strong oxidation peak intensity of SnO2 demonstrates that the conversion of SnO2 to Sn is highly reversible. This highly reversible property of SnO2 materials is rarely reported in literatures[59, 60]. Hence, the whole electrochemical reaction mechanism can be described as follow[2, 61, 62]. SnO2 + 4 Li + + 4e− ↔ Sn + 2 Li2O
(1)
Sn + xLi + + xe− ↔ Lix Sn(0 ≤ x ≤ 4.4)
(2)
10
ACCEPTED MANUSCRIPT The galvanostatic discharge/charge profiles of the initial 5 cycles were shown in Fig. 6c, and it can be observed the first discharge profile and capacity is quite different from the following profiles, which is attributed to the formation of SEI layer and some related irreversible reactions. Two obvious plateaus located at about 0.5 and 1.3 V can be observed in
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the charge profile, which reflects the two-step formation mechanism of SnO2. The fact that the curves from 3rd to 5th are almost overlap each other indicates the anode materials is very stable (i.e. no side reaction and noticeable capacity decay). In the following cycles, the
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representative discharge/charge profiles of 50th, 100th, 150th, 200th and 250th (Fig. 6d) are highly reversible, in accordance with the conclusion from Fig. 6b. These data from the
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electrochemical workstation and the multi-channel battery testers confirm that the conversion from SnO2 to Sn is highly reversible[63].
Fig. 7a shows the cyclic performance and coulomb efficiency at a current density of 200 mA g-1, and the initial charge and discharge capacities are 1124 and 1541 mAh g-1,
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respectively, corresponding to an initial coulomb efficiency (CE) of 72.9%. The high initial CE is higher than most of pure and complex SnO2 materials[64-67], which is a result of the highly reversible from SnO2 to Sn. The half cells deliver high reversible capacity of 925 mAh
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g-1 after 250 cycles, which is only a slightly fade comparable to the initial cycle. As a contrast, the cyclic performance of pure SnO2 (the morphology is microbelts (Fig. S6a) and the crystal
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phase is determined by XRD (Fig. S6b); the corresponding synthesis procedure please refer to the experiment S1; supporting information) and the C nanostructures (the morphology is microbelts (Fig. S7a) and the crystal phase is determined by Raman spectra (Fig. S7b); the corresponding synthesis procedure please refer to the experiment S2; supporting information) are also displayed in Fig. 7a. The capacity of the bare SnO2 microbelts decays very fast and delivers a low capacity of 478 mAh g-1 after 150 cycles at a current density of 200 mAh g-1. The capacity of carbon microbelts is very stable and delivers a low capacity of 295 mAh g-1 after 100 cycles at current density of 200 mAh g-1. Thus the cyclic performance of U-SnO2 11
ACCEPTED MANUSCRIPT NPs@CF-MBs is much better than that of pure SnO2 and C electrode, which may be due to the synergistic effects originated from good conduction channel of carbon framework microbelts and higher theoretical capacity of SnO2. In addition, the coulombic efficiency (upper lines) is more than 98 % after the initial 3
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cycles, which further verifies the highly reversible lithium storage ability of the novel U-SnO2 NPs@CF-MBs anode materials. The rate performance was evaluated from 0.2 A g-1 to 5 A g-1, as shown in Fig 5b, the half cells deliver high reversible capacities of 937, 850, 746, 641, 543
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and 464 mAh g-1 at current densities of 0.2, 0.4, 0.8, 1.6, 3.2 and 5 A g-1, respectively. Noticeably, when the current density was reverted back to a low current density of 0.2 A g-1,
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the cell capacity restore to a high value of 889 mAh g-1, equivalent of 94.9 % of initial cycle capacity. As a result, both the capacity retention at large current density (5A g-1) and recovery property are superior to nanofiber[68-71], nanotube[72-78] based anode materials. The long cyclic performance was tested at a high current density of 1.5 A g-1 as shown in Fig. 7c, and it
1000 cycles.
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can be seen that the reversible capacity are very stable and maintain 788 mAh g-1 even after
All these results demonstrate that the as-synthesized U-SnO2 NPs@CF-MBs based half
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cells possess the good performance evidenced by high capacity, superior rate and long cycle life [78-80]. The red curve of Fig. S8a shows the electrochemical impedance spectra (EIS) for
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the half cell of the U-SnO2 NPs@CF-MBs after 1000 cycles. The Nyquist plot shows a semicircle and the kenetic parameters obtained from the equivalent circuit fitting are listed in the Fig. S8b. The charge-transfer resistance (Rct) calculated by using the equivalent circuit model for the sample is only 106.7 Ω after 1000 cycles, indicating the high ionic conductivity for the U-SnO2 NPs@CF-MBs electrode. To investigate the excellent cycling stability mechanism and structure change of the USnO2 NPs@CF-MBs, the lithium half cell in Fig. S9 is disassembled after cycling tests. The morphology of the U-SnO2 NPs@CF-MBs sample is studied using SEM and techniques. 12
ACCEPTED MANUSCRIPT Before washing out the electrolyte, it seems that that the electrode’s surface is covered by a layer of electrolyte from the SEM of Fig. S9a and TEM image of Fig. S9b. However, the beltlike characteristics of the U-SnO2 NPs@CF-MBs electrode can still be clearly observed even after cycles. In order to further study the detailed microstructure of the U-SnO2 NPs@CF-
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MBs electrode, the after-cycled sample is dispersed in ethanol under intense ultrasound. The low-magnification TEM images for the after-cycled electrode are illustrated in Fig. S9c, in which the electrode is uniform and similar to that of uncycled U-SnO2 NPs@CF-MBs. An
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HRTEM images for the after-cycled electrode are showed in Fig. S9d, in which no obvious SnO2 particles can be observed, suggesting that the electrode convert to amorphous structure
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in the long cycling process. From these SEM and TEM images displayed in Fig. S9, even after 1000 cycles, the integrity of the whole electrode is well sustained, which might be the reason accounting for the excellent cycling stability of the electrode. The outstanding lithium storage capability of as-synthesized U-SnO2 NPs@CF-MBs
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electrode is associated with the unique structure, in which the ultrafine SnO2 nanoparticle is separated by the carbon network. Firstly, the micron-size carbon framework supports long and wide highway for convenient electronic transport. Secondly, the ultrafine SnO2 nanoparticles
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were uniform anchored in the large-size carbon structure, which greatly reduces the lithium ions diffusion distance. The U-SnO2 NPs@CF-MBs still remain the belt-structure after
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cycling test (supporting information). Therefore, these designs take the advantages of synergistic effects of both micrometer size and nanometer size. Consequently, the electrode structure has a good stability and the electrochemical reaction of SnO2 to Sn is highly reversible. Finally, to prove the practical application of as synthesized U-SnO2 NPs@CF-MBs, we further assemble full cells to evaluate the energy storage performance when coupled with commercial LiCoO2 cathode. Prelithiation of anode electrode can effectively offset the lithium consumption and improve the full cell performance. Specifically, the U-SnO2 13
ACCEPTED MANUSCRIPT NPs@CF-MBs anode directly contact with metal lithium for five minutes to conduct the prelithiation process. Then the full cells of the as-synthesized U-SnO2 NPs@CF-MBs anode coupled by a LiCoO2 cathode were tested at a current density of 200 mA g-1 (based on the active mass of U-SnO2 NPs@CF-MBs) in the voltage range of 1.5-3.9 V. Fig. 8a presents the
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representative discharge/charge profiles of 2nd and 12th cycle. Apparently, there exists two discharge plateaus located at 3.3 and 2.6 V, which is further confirmed by the two corresponding discharge peaks at CV measurement in Fig. 8b. These results demonstrate that
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the conversion of Sn to SnO2 is also reversible when the as synthesized U-SnO2 NPs@CFMBs electrode is applied in the full cell. Fig. 8c shows that the reversible capacity of full cell
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is 510 mAh g-1 after 80 cycles and the coulombic efficiency is about 99.5 %. The black curve of Fig. S8a shows the electrochemical impedance spectra (EIS) for the full cell of the U-SnO2 NPs@CF-MBs after cycling. The The Nyquist plot shows a large semicircle and the kenetic parameters obtained from the equivalent circuit fitting are listed in the Fig. S6b. As can be
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seen from the Fig. S6b, the charge-transfer resistance (Rct) calculated by using the equivalent circuit model for the sample is only 300.4 Ω after cycling, which is larger than the Rct of the half cell. To prove the application of the full cell, we design a Sn logo (Fig. 8d), which
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consists of 15 yellow LEDs and 17 red LEDs. It can be observed that a fully charged full cell
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(with an absolute capacity about 1.2 mAh) can light these 32 LEDs for at least for 40 minutes.
4. Conclusions
In summary, we have successfully synthesized the U-SnO2 NPs@CF-MBs anode material, i.e. massive of ultrafine SnO2 particles were anchored uniformly in the carbon network, which renders lithium ions and electrons with convenient and fast electrochemical reaction channels. It was found that the conversion between Sn and SnO2 is highly reversible in both half and 14
ACCEPTED MANUSCRIPT full cells. The half cells deliver high capacity of 925 mAh g-1 at current density of 200 mA g-1, high rate capacity of 464 mAh g-1 at current density of 5000 mA g-1 and long cycle performance of 788 mAh g-1 after 1000 cycles at current density of 1500 mA g-1. The full cell also delivers high reversible capacity of 510 mAh g-1 after 80 cycles at current density of 200
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mA g-1. It is worth noting that that the full cells of as synthesized anode materials coupled with LiCoO2 can light 32 LEDs for more than 40 minutes. These results demonstrate that the novel design of combining the micron and nano advantages can promote the electrochemical
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performance of SnO2 in the next-generation LIBs and this facile technique can be easily
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expanded in the fabrication of other related anode materials.
Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 61675175), Program Projects of Science and Technology Innovative Research Team of Henan Province
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(18IRTSTHN017), Key Research Projects of Henan Provincial Department of Education (17A140011), Science and Technology Project of Henan Province (182300410218) and and
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Nanhu Scholars Program for Young Scholars of XYNU.
15
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ACCEPTED MANUSCRIPT [80] F. Zhang, C. Yang, X. Gao, S. Chen, Y. Hu, H. Guan, Y. Ma, J. Zhang, H. Zhou, L. Qi,
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ACS Appl. Mater. Interfaces 9 (2017) 9620-9629.
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Figure Caption Fig. 1. Sketch illustration for fabricating of carbon framework microbelts anchoring ultrafine
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SnO2 nanoparticles: (a) uniform tin-based gel for electronspinning; (b) electronspinning process for nanobelts; (c) anneal in Ar atmosphere and 500 oC; (d) final products; (e) TEM observation results.
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analysis and (d) XPS precise scan spectra of Sn 3d.
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Fig. 2. (a)XRD patterns of the U-SnO2 NPs@CF-MBs. (b)Raman spectra. (c) Full scan XPS
Fig. 3. SEM characterization: (a) low-magnification of the nanobelts; (b)high-magnification. (c) The width distribution of the U-SnO2 NPs@CF-MBs. TEM characterization: (e) a typical low-magnification image; (d) and (f) HRTEM image of the U-SnO2 NPs@CF-MBs and the
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corresponding SAED pattern (inset of Fig. 3f); (h-j) the element mapping.
Fig. 4. SEM images of the sample for electrospinning ambient of (a) 18 oC, (b) 35 oC and (c) 50 oC, respectively.
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Fig. 5. The schematic of the growth mechanism for SnO2 composite. (a) Fibers spun out from
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the nozzle at the beginning. (b) Phase separation between the core (solution) and shell (skin). (c1) Anisotropy shrinkage among different part of the same fiber at higher ambient temperature. (d1) Skin-core structures shrink into belt structure. (e1) SnO2 composite nanobelt after annealing. (c2) Isotropy shrinkage among different part of the same fiber at lower ambient temperature. (d2) Skin-core structures shrink into wire structure. (e2) SnO2 composite nanowire after annealing.
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ACCEPTED MANUSCRIPT Fig. 6. Electrochemical performances of U-SnO2 NPs@CF-MBs electrode based half cell: (a) CV curves for the initial 5 cycles; (b) CV curves galvanostatic charge/discharge test after 150 cycles; (c) Galvanostatic discharge-charge profiles of the initial 5 cycles; (d) representative
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galvanostatic discharge/charge profiles of 50th, 100th , 150th, 200th and 300th.
Fig. 7. Electrochemical tests of U-SnO2 NPs@CF-MBs electrode based half cell: (a) cycling performance
and coulombic efficiency of U-SnO2 NPs@CF-MBs, and the cycling
performance of bare SnO2 and C electrode; (b) rate performance; (c) long cycling
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performance tested at 1500 mA g-1.
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Fig. 8. Electrochemical test of U-SnO2 NPs@CF-MBs electrode based full cell: (a) representative galvanostatic discharge-charge profiles of 2nd and 12th; (b) representative CV curves; (c) cycling performance tested at 200 mA g-1 and coulombic efficiency; (d) a typical
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digital image of 32 LEDs powered by the integrated full cell.
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