C composite nanofibers with high-capacity and long-cycle life as anode materials for sodium ion batteries

Accepted Manuscript TiO2-Sn/C composite nanofibers with high-capacity and long-cycle life as anode materials for sodium ion batteries Su Nie, Li Liu, Junfang Liu, Jing Xia, Yue Zhang, Jianjun Xie, Min Li, Xianyou Wang PII:

S0925-8388(18)33273-0

DOI:

10.1016/j.jallcom.2018.09.044

Reference:

JALCOM 47463

To appear in:

Journal of Alloys and Compounds

Received Date: 12 May 2018 Revised Date:

31 August 2018

Accepted Date: 4 September 2018

Please cite this article as: S. Nie, L. Liu, J. Liu, J. Xia, Y. Zhang, J. Xie, M. Li, X. Wang, TiO2-Sn/C composite nanofibers with high-capacity and long-cycle life as anode materials for sodium ion batteries, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.09.044. 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.

ACCEPTED MANUSCRIPT TiO2-Sn/C composite nanofibers with high-capacity and long-cycle life as anode

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materials for sodium ion batteries

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Su Niea, Li Liua,b*, Junfang Liua, Jing Xiaa, Yue Zhanga, Jianjun Xiea,

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Min Lia, Xianyou Wanga

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a

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Joint Engineering Laboratory for Key materials of New Energy Storage Battery,

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Hunan Province Key Laboratory of Electrochemical Energy Storage and Conversion,

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School of Chemistry, Xiangtan University, Xiangtan 411105, China

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b

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National Base for International Science & Technology Cooperation, National Local

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

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Nankai University, Tianjin 300071, China

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Abstract:

The novel TiO2-Sn/C composite nanofibers have been successfully fabricated by

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a simple and facile electrospinning process. A small amount of metal tin interacts with

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TiO2 nanoparticles in a carbon matrix, which makes TiO2-Sn/C nanofibers have the

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stability of TiO2 and the high capacity of Sn. At the same time, the TiO2-Sn/C

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nanofibers reveal an improved diffusion coefficient of sodium ions due to a small

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amount of Sn nanoparticles incorporation. Compared with TiO2/C nanofibers, the

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TiO2-Sn/C nanofibers electrode shows significantly improved specific capacity,

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substantial cycling stability, and remarkable rate capability. It delivers high reversible

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capacity of 255 mA h g-1 at at current densities of 0.05 A g-1 in the range of 0.01-2.5V

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vs. Na/Na+, and has specific capacities of 214 and 147 mA h g-1 at current densities of

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*Corresponding author at: College of Xiangtan University. China.

E-mail addresses: [email protected] (L. Liu). 1

ACCEPTED MANUSCRIPT 0.5 and 4 A g-1, respectively. Furthermore, TiO2-Sn/C nanofibers electrode

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demonstrates a discharge capacity 190.8 mA h g-1 with 95.4 % retention after 1000

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cycles at 1 A g-1 (the initial and second discharge capacity is 483 and 201 mA h g-1,

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respectively). Even up to 5 A g-1, the discharge capacity of 134.3 mA h g-1 with

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83.9 % retention is obtained after 1000 cycles. Outstanding electrochemical

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performance makes TiO2-Sn/C nanofibers as a hopeful anode material for sodium-ion

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batteries and to be applied in the field of large-scale energy storage.

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Keywords:

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Nanofibers; Titanium dioxide; Metal tin; Electrospinning; Sodium-ion batteries;

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Introduction

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In recent years, lithium-ion batteries (LIBs) have been widely applied to portable

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equipment, electric vehicles and other fields as an important energy storage device

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due to their merits of high energy density, long cycle life[1–4]. Nevertheless, the high

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cost limits the application of LIBs in large-scale energy storage for the limited and

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uneven distribution of lithium resources. Compared with lithium resources, sodium

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resources have the advantages of low cost and abundant resources. Therefore,

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sodium-ion batteries (SIBs) have attracted extensive interest and been regarded as a

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promising source to satisfy the requirement of large-scale energy storage and smart

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grid applications in recent years.[5–7] However, due to sodium ions have a large

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radius compared to lithium ions (ionic radius, 1.02 Å versus 0.76 Å), so it’s hard to

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discover applicable materials for reversible and rapid ion insertion/extraction.

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Among a variety of reported anode materials, titanium dioxide (TiO2) has been

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ACCEPTED MANUSCRIPT regarded as a hopeful anode material for SIBs due to the merits of low cost,

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structurally stable and environmental friendliness[8-10]. At present, there are various

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polymorphs of TiO2, for instance anatase[11,12], rutile[13], TiO2(B)[14], TiO2(H)[15],

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have been researched as anode materials for NIBs. Nonetheless, these materials show

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large differences in the electrochemical performance of sodium storage. The

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electrochemical capabilities of various phases of TiO2 nanospheres have been reported

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by Wang et al. [16], and concluded that anatase TiO2 had a preferable Na-storage

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ability compared with mixed anatase/rutile or amorphous TiO2. However, the poor

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electrical conductivity (~10-12 S cm-1) and low ion diffusion coefficients of TiO2 lead

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to inferior rate performance, which hinders its practical application.[17] Moreover, it

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is well known that TiO2 anodes show unattractive specific capacity, which is usually

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below 200 mA h g-1. These disadvantages greatly restrict the research an attractive

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and practical application of TiO2 anodes.

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Another type of anodes, based on the alloying materials[18, 8] and conversion

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materials[19, 20] (e.g. red P, Sb2S3, Fe2O3, MoS2 and so on) have high specific

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capacities. However, large volume expansion result in the crushing of the electrode

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material and subsequently generate the irreversible capacity loss and inferior

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cycle performance during the charge and discharge processes. Particularly, metallic tin

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(Sn) has received widespread attention as a sodium ion battery anode matter for high

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theoretical capacity (847 mA h g-1 for Na15Sn4) as well as excellent electronic

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conductivity[21]. However, the pulverization of the electrode material caused by

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serious volume changes (~520 %) hinders the practical application of Sn.

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Nanocrystallization of tin particles is an effective strategy to prevent the pulverization

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of Sn, which could reduce the mechanical stress produced during the alloying and

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de-alloying reaction to sodium ions[22]. Electrospinning technology is widely used in the preparation of one-dimensional

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nanostructured lithium/sodium ion battery electrode material due to its simple and

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versatile characteristics.[23, 24] At present, there have been a lot of research reports

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on the preparation of high-performance electrode materials (such as Sn/C, Fe3O4/C,

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TiO2/C) by electrospinning technology.[25-27] The advantages of electrospinning are

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mainly reflected in the following two aspects. On one hand, the obtained

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one-dimensional nanofibers have large specific surface area and a high surface area to

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volume ratio, providing many active sites. On the other hand, the porous structure

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effectively alleviates the large volume change during the reaction of the battery, which

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is beneficial to the sufficient wetting of the electrolyte and the increase of the

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potential, thereby improving the energy storage efficiency.[28, 29]

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Herein, we first report the preparation of TiO2-Sn/C composite nanofibers

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(denoted as TiO2-Sn/C NFs) through simple electrospinning technique. The

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as-prepared TiO2-Sn/C NFs combine the advantages of TiO2 and Sn, that a small

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amount of Sn nanoparticles increases the specific capacity of the electrode material

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obviously while retaining the excellent cycling stability of TiO2. The one-dimensional

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structure of nanofibers can provide a large specific surface area for sufficient

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contacting with the electrolyte and short ions/electrons transmission paths[30, 31].

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Furthermore, anatase TiO2 and Sn nano-particles are uniformly embedded in the

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ACCEPTED MANUSCRIPT carbon matrix, which not only enhances stability but also improves the overall

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conductivity. As expected, TiO2-Sn/C NFs electrode exhibits high reversible capacity

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255 mA h g-1 at 0.05 A g-1, superior rate property (214 and 147 mA h g-1 at current

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densities of 0.5 and 4 A g-1, respectively), and outstanding long cycling performance

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(134.3 mA h g-1 with 83.9 % retention at 5 A g-1 over 1000 cycles) as anode materials,

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offering a promising application in constructing low cost and high capacity SIBs.

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2. Experimental Section

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2.1. Preparation of Materials

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The TiO2-Sn/C NFs were manufactured by electrospinning technology followed

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by post-calcination treatment. The precursor solution for electrospinning was made as

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follows: First, 8 ml N, N-dimethylformamide (DMF, Kermel, 99.5 %) and 1.5 ml

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acetic acid glacial (CH3COOH, Kermel, 99.5 %) were mixed, then adding with 0.1 g

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tin dichloride dihydrate (SnCl2·2H2O, Kermel, 99.5 %) and 1.5 ml tetra-n-butyl

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titanate (C16H36O4Ti, Kermel, 99 %). After that, 0.75 g polyvinylpyrrolidone (PVP,

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Mw=1,300,000, Alfa-Aesar) was added into the above mixed solution under magnetic

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stirring for 8 h to obtain a precursor solution. The precursor solution was loaded into a

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10 mL syringe connected with a blunt-tip stainless-steel needle and spun on an

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electrospinning unit with the applied voltage of 15 kV. The needle-to-collector

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distance was 15 cm and the flow rate was 0.38 ml h-1. The as-collected nanofibers

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were first dried at 70 °C for 8 h in a vacuum oven and then pre-calcined at 200 °C for

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2 h with a heating rate of 4 °C min-1. Finally, The TiO2-Sn/C composite nanofibers

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can be obtained by carbonization at 600 °C for 4 h in Ar/H2 atmosphere with a heating

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ACCEPTED MANUSCRIPT rate of 4°C min-1. For comparison, the pristine TiO2/C NFs and other two kinds of

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TiO2-Sn/C composite nanofibers were prepared with similar methods by adjusting the

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additive amounts of SnCl2·2H2O to 0.0, 0.05, and 0.2 g, respectively.

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2.2. Structure and Physical Characterization

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The structure and crystallinity of the sample were determined by X-ray powder

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diffraction (XRD) in a Rigaku D/Max-2500 powder diffractometer with Cu-Kα

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radiation (λ=1.5418 Å). The surface morphologies and detailed microstructures of the

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synthesized products were observed by using a scanning electron microscope (SEM,

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JEOL, SM-71480) and a transmission electron microscopy (TEM, JEOL,

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JEM-100CX). X-ray photoelectron spectroscopy (XPS, ThermoFisher, Escalab 250XI)

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was

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Thermogravimetry

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adsorption-desorption isotherms were measured with TriStar II3020. Inductively

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Coupled Plasma Optical Emission Spectrometer (ICP-OES, Optima 5300DV) is used

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to determine the content of elements in composite fibers.

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2.3. Electrochemical Measurements

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materials. Nitrogen

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The working electrode consisted of active materials (TiO2-Sn/C NFs or TiO2/C

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NFs), carbon black, polyvinylidene fluoride (PVDF) binder in a mass ratio of

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70:20:10, dissolved in an appropriate amount of N-Methyl-2-pyrrolidinone (NMP)

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and the copper foil was employed as the current collector. For SIBs, the half-cells

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were assembled with pure sodium foil as the counter electrode and a glass fiber

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(Whatman, GF/C) as separator in an argon-filled glove box. The electrolyte solution

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ACCEPTED MANUSCRIPT was composed of 1 M NaClO4 dissolved in propylene carbonate/ethylene carbonate

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(PC/EC, 1:1 in volume). The loading mass of the active material is about 1.0 ± 0.1 mg

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cm-2. The coin cells were cycled by galvanostatic discharge–charge measurements

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with a battery testing system (Neware, China) at room temperature in the voltage

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interval of 0.01 and 2.5 V (vs. Na+/Na). Both cyclic voltammetry (CV) tests and

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electrochemical impedance spectra (EIS) experiments were performed on a CHI604E

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electrochemistry workstation within a potential window of 0.01–2.5 V. The signal of

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alternating current (AC) perturbation was ±5 mV and the frequency range was 100

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mHz-105 Hz. For LIBs, the testing batteries were assembled with Celgard 2300 film

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as the separator, lithium metal as the counter electrode, and 1 M LiPF6 dissolved in

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dimethyl carbonate/ethylene carbonate (DMC/EC, 1:1 in volume) as the electrolyte.

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Similarly, the coin cells were tested by galvanostatic discharge–charge measurements

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with a battery testing system (Neware, China) at room temperature in the voltage

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interval of 0.01 and 3.0 V (vs. Li+/Li).

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3. Results and discussion

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3. 1. Structure and morphology

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In order to analyze the phase composition and structures of the TiO2/C NFs and

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TiO2-Sn/C NFs, X-ray diffraction (XRD) measurements were implemented. All the

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diffraction peaks of TiO2/C NFs are well indexed to anatase TiO2 (JCPDS No.

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21-1272) in Figure 1a. The weak intensity of the TiO2 peaks in XRD patterns may be

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ascribed to TiO2 nanoparticles embedded in the carbon matrix[32]. As shown in

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Figure 1b, the main diffraction peaks of TiO2-Sn/C NFs can be well indexed to the

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ACCEPTED MANUSCRIPT standard XRD patterns of anatase TiO2 (JCPDS No. 21-1272) and minor peaks are

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correspond to tetragonal Sn (JCPDS No. 04-0673), which prove that TiO2-Sn/C NFs

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are made up of a lot of anatase TiO2 and a little amount of metal tin. In addition, the

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XRD pattern of TiO2-Sn/C NFs with different amounts of SnCl2·2H2O is illustrated in

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Figure S1a. It can be observed that the diffraction peak of tetragonal Sn is becoming

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sharper with the amount of SnCl2·2H2O increases, signifying a rise of Sn content in

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TiO2-Sn/C NFs. The XRD patterns of precursors of TiO2-Sn/C NFs and PVP NFs are

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shown in Figure S1b. Neither the precursor nor the pre-calcined sample can find the

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diffraction peak of SnCl2.2H2O and its peaks is consistent with the peaks of PVP

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precursor. The reason for this phenomenon may be that the content of SnCl.2H2O is

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small or TiO2 is an amorphous state, so that it can be hidden and cannot be found.

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Raman spectra in Figure 1c reveal two prominent peaks at 1350 and 1600 cm-1,

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corresponding to a disorder-induced feature (D-band) and the E2g mode of graphite

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(G-band), respectively. The low intensity ratio of D versus G band (ID/IG<1.0) can be

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used to indicate a certain degree of graphitization in a carbon matrix[33]. The values

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of ID/IG for TiO2/C NFs and TiO2-Sn/C NFs are 0.84 and 0.82 respectively, which

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represent high-quality electrical conductivity[34, 35]. Based on the weight losses in

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the TG curve under air atmosphere(Figure 1d), the total TiO2 and TiO2-Sn content in

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the as-prepared samples were determined to be 69.6 wt % and 71.2 wt %. The weak

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content difference may be related to the addition of SnCl2.2H2O in the precursor

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solution. Where, the reaction occurring between predominant weight loss step

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between 400˚C to 500˚C is oxidized of carbon. The thermogravimetric curves of the

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precursor and the annealed sample under N2 atmosphere are shown in Figure S1c. It

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can be seen that the temperature at which PVP is carbonized is about 450 ℃. The scanning electron microscopy (SEM) images for TiO2/C NFs (Figure 2a, b)

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and TiO2-Sn/C NFs (Figure 2c, d) are exhibited in Figure 2. It can be seen that both

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TiO2/C NFs and TiO2-Sn/C NFs show long and continuous uniform diameter of about

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110 nm. Figure 2b, d shows high magnification SEM images of TiO2/C NFs and

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TiO2-Sn/C NFs, respectively. Smooth surface and interconnected networks are

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observed in the obtained fibers. There are no aggregated particles can be discovered

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on the surface of the fibers. It implies that all TiO2 and Sn particles are embedded in

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the carbon matrix, which is corresponding with the XRD results. Further, the N2

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adsorption–desorption

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Brunauer-Emmett-Teller (BET) surface area and pore volume of TiO2-Sn/C NFs and

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TiO2/C NFs. As shown in Figure S2. The Brunauer–Emmett–Teller (BET) surface

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area and the pore volume of TiO2-Sn/C NFs are 136.9 m2 g-1 and 0.107 cm3 g-1

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relative to the TiO2/C NFs, which showed BET surface area and pore volume of 117.9

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m2 g-1 and 0.094 cm3 g-1. The increased specific surface area and pore volume may be

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because of gas generation during the reduction of tin dioxide to generate micropores,

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shown in the following formula[36]: S nO 2 + C → Sn + CO 2 (1)

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The larger specific surface area ensures enough contact between the electrode material

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and the electrolyte, thereby shortening the transport path to accelerate the rapid

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transfer of Na+ ions.

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Transmission electron microscopy (TEM) was further implemented to explore

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ACCEPTED MANUSCRIPT the morphology and microstructure of obtained fibers. As shown in Figure 3a, TiO2

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nanoparticles are dispersed in the carbon matrix to form TiO2/C composite nanofibers.

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In addition, it can be clearly observed in Figures 3b-c that the particles are dispersed

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uniformly in the carbon matrix, which not only greatly increases the conductivity of

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TiO2 but also prevents the aggregation of Sn particles. Simultaneously, the TEM

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mapping images of TiO2-Sn/C NFs in Figure 3d also show that the homogeneous

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distribution of C, O, Sn and Ti elements in the nanofibers. The distinct lattice fringes

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with a d-spacing of 0.35 nm and 0.15 nm can be observed in HRTEM image (Figure

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3e) and interplanar crystal spacing statistical tables (Figure 3f), corresponding to the

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(1 0 1) and (2 1 3) faces of anatase TiO2, respectively. However, it is regrettable that

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the lattice fringes of Sn could not be found, which may be related to the fact that the

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content of Sn is too little, or the rather smaller size of Sn nanoparticles and their

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lattice fringes were covered by the lattice fringes of TiO2[37]. As shown in the EDS

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spectrum (Figure S3), the content of Sn is only about 6.87 wt.%. In order to further

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understand the composite ratio of Sn and TiO2, ICP-OES tests were conducted on

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different samples. As shown in Table 1, the content of Sn in the composite fiber

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increases with the increase of SnCl2.2H2O addition.

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To further gain the elemental composition and valence, X-ray photoelectron

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spectroscopy (XPS) was applied to survey the as-fabricated TiO2/C NFs and

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TiO2-Sn/C NFs. The various elemental composition of two samples is marked in the

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wide spectrum (Figure 4a) and the high-resolution spectra of C, Ti, Sn are further

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displayed in Figure 4b-d, respectively. As shown in Figure 4b, there is no change of C

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ACCEPTED MANUSCRIPT 1s peaks in TiO2/C NFs and TiO2-Sn/C NFs. The curves at 284.8 eV, 286.2 eV and

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289.1 eV can be assigned to the C–C, C–O, and O–C=O bonds, respectively[38, 39].

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XPS results of Ti 2p of TiO2/C NFs and TiO2-Sn/C NFs are emerged in Figure 4c.

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There are Ti 2p1/2 and Ti 2p3/2 peak at ca.458.9 eV and ca.464.7 eV respectively,

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suggesting that the predominant state of Ti element in TiO2/C NFs is present in the

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form of Ti4+[40]. For TiO2-Sn/C NFs, the corresponding peaks shift slightly to the

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lower binding energy of 458.7 and 464.5 eV, which is related to the formation of

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oxygen vacancies that lead to changes in the chemical environment[41].The Sn 3d

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peak of TiO2-Sn/C NFs can be deconvoluted into Sn 3d3/2 (494.5 eV) and Sn 3d5/2

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(486.1 eV) in Figure 4d. Among them, the binding energy of Sn 3d5/2 (486.1 eV) is

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higher than 484.9 eV (Sn0) but lower that 486.3 (Sn2+), which means that a part of Sn

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particles are covered with a tin suboxide layer[42, 43]. Similarly, Figure S4 shows the

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O1s trend of TiO2-Sn/C NFs and TiO2/C NFs. It can be seen that only the appearance

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of Ti-O and Ti-OH appears in the O 1s diagram, which means that Sn is mainly in Sn0.

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In addition, both the main peaks of Ti-O and Ti-OH have the same tendency of slight

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deviation towards low binding energy as Ti 2p, which may be attributed to changes in

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the chemical environment.

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3.2. Electrochemical Performance

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Galvanostatic discharge–charge measurements were applied to examine the

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electrochemical performance of TiO2-Sn/C NFs and TiO2/C NFs. As indicated in

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Figure 5a, the specific discharge capacity of TiO2/C NFs electrode is only 341.3 mA h

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g-1 in the first discharge process and 171.4 mA h g-1 in the second cycle at 1 A g-1.

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ACCEPTED MANUSCRIPT After 1000 cycles, the capacity of TiO2/C NFs is 100.2 mA h g-1 and the

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corresponding capacity retention rate is 58.5 % (based on the capacity of the second

244

cycle). Under the same condition, the TiO2-Sn/C NFs electrode displays a higher

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capacity of 483.1 mA h g-1 at the first discharge process and can be maintain

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reversible capacity of 190.8 mA h g-1 after 1000 cycles at a current density of 1 A g-1,

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corresponding to 95.4 % capacity retention (according to the capacity of the second

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cycle (200.1 mA h g-1)). Correspondingly, the overall profiles for TiO2-Sn/C NFs in

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the voltage range of 0.01–2.5 V (vs. Na/Na+) are exhibited in Figure 5b. There was no

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significant change in the charging and discharging platform with the increase of the

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number of cycle, corresponding to the glorious cycling stability. The large capacity

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loss of the premiere cycle is principally connected with the interfacial reaction of the

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electrode with the electrolyte leading to the formation of the solid-electrolyte

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interfacial (SEI) film[44].

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The rate performance of two samples is expressed in Figure 5c and the

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TiO2-Sn/C NFs electrode reveals more excellent rate performance than TiO2/C NFs.

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TiO2-Sn/C NFs provide a discharge capacity of 255 mA h g-1 at 0.05 A g-1. The

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discharge capacities keep at 244, 227, 214, 195, 165, 156, 147 and 139 mA h g-1 when

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the current density increases to 0.1, 0.2, 0.5, 1, 2, 3, 4, and 5 A g-1, respectively. The

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specific capacity can still be maintained at 245 mA h g-1 in the following cycles when

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the current densities recovered to 0.05 A g-1. Ulteriorly, the rate capability of

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TiO2-Sn/C NFs and TiO2/C NFs is directly compared through capacity remaining ratio

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(shown in Figure S5). It is clearly seen that the TiO2-Sn/C NFs electrode releases

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ACCEPTED MANUSCRIPT much higher specific capacity than the TiO2/C NFs electrode at large current densities;

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e.g., 139 and 41.6 mA h g-1 at 5 A g-1 are acquired for TiO2-Sn/C NFs and TiO2/C NFs,

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corresponding to capacity retention rates of 49.6 % and 20.6 % (compared to the

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capacity of the second cycle at 0.05 A g-1), respectively. The high rate (or fast) charge

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and discharge performance of the TiO2-Sn/C NFs can be highly enhanced by the

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improved kinetics provided for electronically conducting metal tin nanoparticles

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reside in TiO2-Sn/C NFs. The discharge/charge curves emerge into Figure 5d, the

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specific discharge capacity gradually decrease with the increasing current density.

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However, the TiO2-Sn/C NFs electrode shows a less polarization at high current

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densities than TiO2/C NFs (Figure S6a), which is consistent with excellent rate

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performance. This excellent rate performance may originate from the interaction of

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TiO2 nanoparticles and a small amount of Sn nanoparticles in the carbon matrix.

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To explore the detailed information of the outstanding electrochemical

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performance of TiO2-Sn/C NFs, the detailed charge and discharge diagrams from the

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second cycle to the fifth cycle at a small current of 0.05 A g-1 and cyclic voltammetry

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was carried out. Compared to TiO2/C NFs (Figure S6b), there is one unspectacular

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plateau in the discharge curves of TiO2-Sn/C NFs at 0.35 V appears on the Figure 5e,

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corresponding to the sodiation course of metal tin[21]. A small anodic peak near 0.35

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V is observed in the CV plot of TiO2-Sn/C NFs for the second cycle at 0.1 mV s-1

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relative to TiO2/C NFs (shown in Figure 5f). This peak is well consistent with the

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platform of charge and discharge curves. From the above studies, it can be easily

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inferred that the outstanding electrochemical performance of TiO2-Sn/C NFs is

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ACCEPTED MANUSCRIPT derived from the electrochemical reaction of Sn. Concretely, the CV curves of the first

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four cycles of TiO2-Sn/C NFs and TiO2/C NFs are presented in Figure S7. There is a

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broad and weak reduction peak of TiO2-Sn/C NFs appears at ca. 0.97 V,

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corresponding to the typical irreversible reaction of Sn in the first sodication scan[45].

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In addition, the CV curve of TiO2-Sn/C NFs (Figure S7a) has a good coincidence after

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the initial cycle, which is in consonance with glorious cycling stability. To better

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understand the different sodium-alloying, CV test was performed at a small scanning

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speed of 0.05 mV s-1. As shown in Figure S8, no more peaks were found to

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correspond to sodium-Sn alloying except 0.35 V, which is the main peak of the

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sodium alloying reaction of Sn.[46, 47] This may be related to the low content of Sn

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in the composite fiber.

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For the sake of evidence that TiO2-Sn/C NFs have excellent electrochemical

298

performance, the long cycle testing was implemented at high current density of 5 A

299

g-1 (Figure 5g). It should be emphasized that there is 83.9 % retention (based on the

300

capacity of the second cycle (150.9 mA h g-1)) with specific capacity remaining at

301

134.3 mA h g-1 even for 1000 cycles. Besides, to confirm the effect of Sn addition on

302

the TiO2-Sn/C NFs composite, the electrochemical properties of samples with

303

different amounts of SnCl2·2H2O (0.0 g, 0.05 g, 0.1 g and 0.2 g) are shown in Figure

304

S9. It can be seen the addition of tin increases could improve remarkably its capacity.

305

Among those samples, the composite added with 0.1 g SnCl2·2H2O shows the best

306

electrochemical performance (delivers high reversible capacity of 232 mA h g-1 after

307

100 cycles at 0.1 A g-1 ).

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ACCEPTED MANUSCRIPT In order to further highlight the meliority of TiO2-Sn/C NFs, some outstanding

309

and representative reports about anatase TiO2 based anodes compared with TiO2-Sn/C

310

NFs for sodium-ion storage performance are presented in Figure 6. It can be seen

311

intuitively that TiO2-Sn/C NFs electrode has higher discharge capacity than many

312

other researches at different current densities. Although nanoporous anatase TiO2

313

exhibits superior specific capacity at a large current density, its cycle performance is

314

dissatisfactory. It has a discharge specific capacity of only 163 mA h g-1 after 500

315

cycles at a current density of 1C (335 mA g-1)[48].

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Based on the outstanding sodium-storage performance, it is significant to try to

317

extend the lithium-storage properties of the TiO2-Sn/C NFs sample. As shown in

318

Figure S10a, the TiO2-Sn/C NFs electrode also displays more outstanding rate

319

performance than TiO2/C NFs. The TiO2-Sn/C NFs electrode provides a discharge

320

capacity of 472.1 mA h g-1 at a current density of 0.05 A g-1. When the current density

321

increases to 0.1, 0.2, 0.5, 1, 2, 3, 4, and 5 A g-1, the discharge capacity moderately

322

decreases to 407.7, 357.8, 294.4, 247.1, 204.1, 182.4, 164.4, and 148.6 mA h g-1,

323

respectively. When the current density decreases to 50 mA g-1, the reversible capacity

324

can recover to 450.4 mA h g-1. Besides, the TiO2-Sn/C NFs electrode shows a higher

325

capacity of 762.2 mA h g-1 at the first discharge process and can stabilize the

326

discharge capacity at 284.7 mA h g-1 after 10 cycles at a current density of 1 A g-1.

327

Even after 500 cycles, the reversible capacity of 247.6 mA h g-1 can be maintained

328

(Figure S10b). The charge/discharge voltage profiles of the TiO2-Sn/C NFs and

329

TiO2/C NFs at different current densities in the voltage range of 0.01-3.0 V (vs. Li/Li+)

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ACCEPTED MANUSCRIPT are presented in Figure S10c, d, respectively. Compared with TiO2/C NFs, the

331

TiO2-Sn/C NFs electrode exhibits a less polarization as the current density increases.

332

From discussed above, tin metal has a significant effect on the TiO2-Sn/C electrode

333

for lithium ion battery as well as sodium ion battery.

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EIS measurement was introduced to explore dynamical properties of the

335

TiO2-Sn/C NFs electrode. As shown in Figure 7a, the EIS patterns are mainly

336

composed of three parts: small intercept at the high frequency region (Rs), semicircle

337

in the high frequency region (Rct) and sloping line in the low frequency region

338

(Zw)[55]. The Rs, Rct and Zw represent the resistance of the electrolyte contacting with

339

particles, the charge-transfer resistance and Na+ ion diffusion in the anode active

340

material, respectively. According to the fitting experimental data (see Table 2),

341

TiO2-Sn/C NFs show a lower Rct value of 242.8 Ω compared to TiO2/C NFs with a Rct

342

value of 297.1 Ω, indicative of the introduction of a small amount of Sn can achieve a

343

kinetics increase in the electrochemical reaction. The diffusion coefficient of sodion

344

ions (DNa+) could be figured out according to the following formulas[56, 57]:

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D Na + =

R 2T 2 (2) Z ' = Rs + Rct + σ w ω -1/2 (3) 2 A2 n 4 F 4 c 2σ 2w

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where R is the gas constant, T is the Kelvin temperature, A is the surface area of the

347

electrode, n is the number of electrons per molecule during the electronic transfer

348

reaction, F is the Faraday constant, c is concentration of sodium ions, and σw is the

349

Warburg factor which could be gained by calculating the slope of the line Z'~ω-1/2 as

350

shown in Figure. 7b. By using Eq. (2) and (3), the sodium ion diffusion coefficients

351

(DNa+) of TiO2-Sn/C NFs after the 1st cycle increases to 6.9×10-13 cm2 s-1 compared to 16

ACCEPTED MANUSCRIPT the TiO2/C NFs value of 2.4×10-13 cm2 s-1. The increased diffusion coefficient of

353

sodium ions (DNa+) may be attributed to a small amount of Sn nanoparticles

354

incorporation, and bring about better electrochemical performance for TiO2-Sn/C NFs

355

electrode.

356

4. Conclusions

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In summary, electrospun TiO2-Sn/C nanofibers (TiO2-Sn/C NFs) show high

358

specific capacity, excellent cycle performance, and outstanding rate capability due to

359

the synergistic effect of anatase titanium dioxide and metal tin. Besides, compared to

360

pristine TiO2/C NFs, TiO2-Sn/C NFs show a larger specific surface area, which

361

promotes the contact with the electrolyte and shortens the diffusion route for both

362

electrons and ions. EIS results also reveal an increased diffusion coefficient of Na+

363

ions (DNa+) in TiO2-Sn/C NFs electrode owing to the introduction of metallic tin. As a

364

consequence, TiO2-Sn/C NFs electrode demonstrates a higher discharge capacity of

365

255 mA h g-1 at 0.05 A g-1 and can be kept a reversible capacity of 190.8 mA h g-1

366

with 95.4 % retention after 1000 cycles at 1 A g-1. Even the current density increases

367

to 5 A g-1, it still has the discharge capacity of 134.3 mA h g-1 with 83.9 % retention

368

after 1000 cycles, indicating prominent cycle stability and excellent rate capability. In

369

addition, the results show that metal tin also has a significant impact on the TiO2-Sn/C

370

electrode for lithium ion battery. The present work demonstrates that a small amount

371

of metal tin incorporation is an efficient way to improve the electrochemical

372

performance of TiO2/C nanofibers, and this strategy could promote the application of

373

TiO2 anodes for sodium ion batteries and lithium ion batteries.

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Acknowledgements This work was supported financially by the National Science Natural Foundation

376

of China (Grant No. 51672234), the Research Foundation for Hunan Youth

377

Outstanding People from Hunan Provincial Science and Technology Department

378

(2015RS4030), Hunan 2011 Collaborative Innovation Center of Chemical

379

Engineering & Technology with Environmental Benignity and Effective Resource

380

Utilization, Program for Innovative Research Cultivation Team in University of

381

Ministry of Education of China (1337304), and the 111 Project (B12015).

382

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ACCEPTED MANUSCRIPT Table 1. Analysis of the content of Sn and Ti in different samples by ICP-OES. Sn

Ti

TiO2-Sn/C NFs (0.05 g)

3.32 wt%

33.79 wt%

TiO2-Sn/C NFs (0.1 g)

6.08 wt%

32.97 wt%

TiO2-Sn/C NFs (0.2 g)

10.11 wt%

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34.98 wt%

TiO2-Sn/C NFs

6.7

TiO2/C NFs

7.4

Rct (Ω)

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Table 2. EIS parameters of TiO2-Sn/C NFs and TiO2/C NFs samples.

1

DNa+ (cm2 s-1)

242.8

6.9×10-13

297.1

2.4×10-13

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Figure 1. (a) XRD pattern of TiO2/C NFs; (b) XRD pattern of TiO2-Sn/C NFs; (c) Raman spectra of TiO2/C NFs and TiO2-Sn/C NFs; (d) TG curves of TiO2/C NFs and TiO2-Sn/C NFs under air atmosphere.

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Figure 2. (a)(b) SEM images of TiO2/C NFs; (c)(d) SEM images of TiO2-Sn/C NFs.

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Figure 3. (a) TEM image of TiO2/C NFs; (b)(c) TEM image of TiO2-Sn/C NFs; (d)

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TEM elemental mapping images of C, O, Sn and Ti elements of TiO2-Sn/C NFs; (e)

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HRTEM image of TiO2-Sn/C NFs with lattice spacing; (f) Interplanar crystal spacing statistical tables.

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Figure 4. (a) XPS wide spectrum of TiO2/C NFs and TiO2-Sn/C NFs; High-resolution (b) C 1s and (c) Ti 2p XPS spectra of TiO2/C NFs and

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Figure 5. (a) Cycling performance of TiO2/C NFs and TiO2-Sn/C NFs electrode under a current density of 1 A g-1; (b) Continuous discharge and charge curves of TiO2-Sn/C NFs electrode under a current density of 1A g-1; (c) Rate

capability of TiO2/C NFs and TiO2-Sn/C NFs; (d) Charge-discharge curves of TiO2-Sn/C NFs at 0.05-5 A g-1 in the range of 0.01-2.5 V; (e) Charge-discharge curves of TiO2-Sn/C NFs from the second cycle to the fifth

cycle at a current density of 0.05 A g-1; (f) The CV plots of TiO2-Sn/C NFs and

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of TiO2-Sn/C NFs under the high current density of 5 A g-1.

Figure 6. Comparison of the electrochemical performance between TiO2-Sn/C NFs

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with other reported TiO2 in studies as anode materials for NIBs.

Figure 7. (a) Nyquist plots measured for the TiO2-Sn/C NFs and TiO2/C NFs electrodes after first cycle at 0.05 A g-1; (b) The relationship plot between Z' and ω-1/2 of the low-frequency region.

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