Carbon coated sodium-titanate nanotube as an advanced intercalation anode material for sodium-ion batteries

Accepted Manuscript Carbon coated sodium-titanate nanotube as an advanced intercalation anode material for sodium-ion batteries Meng Li, Xuezhang Xiao, Xiulin Fan, Xu Huang, Yujie Liu, Lixin Chen PII:

S0925-8388(17)31298-7

DOI:

10.1016/j.jallcom.2017.04.098

Reference:

JALCOM 41502

To appear in:

Journal of Alloys and Compounds

Received Date: 10 February 2017 Revised Date:

5 April 2017

Accepted Date: 11 April 2017

Please cite this article as: M. Li, X. Xiao, X. Fan, X. Huang, Y. Liu, L. Chen, Carbon coated sodiumtitanate nanotube as an advanced intercalation anode material for sodium-ion batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.098. 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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Graphical abstract

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Carbon coated sodium-titanate nanotube as an advanced intercalation anode material for sodium-ion batteries Meng Li1, Xuezhang Xiao1*, Xiulin Fan2*, Xu Huang1, Yujie Liu1, Lixin Chen1 1

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State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. E-mail: [email protected]

Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, USA. E-mail: [email protected]

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Abstract

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Na2Ti3O7 is a promising intercalation anode material for sodium ion batteries. However, low electronic conductivity and structural instability restrict its practical applications. Herein, a novel carbon coated sodium-titanate nanotube was successfully synthesized via a facile solvothermal method. The carbon combines all

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individual Na2Ti3O7 nanotubes into a stable union, which is characterized and confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The carbon encapsulation together with the unique nanotube structure of the Na2Ti3O7

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endows the favorable high conductivity and improves the structural stability during

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cycling. Therefore, Na2Ti3O7@C electrode shows a much higher specific capacity and good cycling stability (142.2 mAh g-1 at 311 mA g-1 after 100 cycles), whereas the bare Na2Ti3O7 only shows a capacity of 84.9 mAh g-1 at 311 mA g-1 after 100 cycles). Furthermore, Na2Ti3O7@C composite electrode has relatively stable capacities at high rate current (84 mAh g-1 at 3.11 A g-1). The present study provides a facile and scalable method to escalate the electrochemical performance of the intercalation anode materials for sodium ion batteries. 1

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Keywords: Na2Ti3O7; Nanotubes; Carbon coated; Electrochemical; Sodium-ion

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batteries

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1. Introduction Lithium-ion batteries (LIBs) have become the main electrochemical energy storage

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and conversion systems for a large scale of environmental applications, such as electrical vehicles and smart grid [1-6]. However, the scarcity and uneven distributions of lithium resources greatly restrict the extensive applications on the

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grid-scale energy storage systems. Therefore, sodium-ion batteries (SIBs) are revived in the field of large-scale energy storage systems because of its similar

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electrochemical properties as lithium, and more importantly the natural abundance and fairly low cost [7, 8].

The performance of the cathodes in SIBs has been achieved large progress in the

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past five years and is found to be comparable with their counterparts of LIBs [9-12]. However, the anode performance is still the major scientific challenge restricting the immediate practical applications, especially for the graphite which is the highly

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efficient and commercial intercalation anode for LIBs, cannot be directly utilized as

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anode for SIBs in the carbonate electrolytes [13, 14]. Extensive efforts have been devoted to developing high-power and high-capacity anode materials for the SIBs [15-18]. Among these anode candidates, the anodes with intercalation mechanism are the

most

promising

one

because

of

the

low

volume

change,

good

intercalation/deintercalation kinetics, and thereby the potential of long cycling stability. Recently, Na2Ti3O7 received intensive investigations due to its proper low voltage (0.3V vs Na/Na+) [16], and low cost with earth-abundant elements. However, 3

ACCEPTED MANUSCRIPT Na2Ti3O7 electrode shows poor cyclic stability and limited rate capability [6], because of these two vital issues, which are needed to be addressed before the application: (i) low electronic conductivity, (ii) structural instability of the intercalated phase. In order

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to improve the electrochemical performance of Na2Ti3O7, various approaches such as forming a composite with carbon [8, 17], synthesizing special morphological Na2Ti3O7 [18], structuring a novel architecture [19], have been developed. However,

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Na2Ti3O7 anode still exhibits poor cycle performance as reported in the previous

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papers [18].

Herein, we firstly synthesized Na2Ti3O7 nanotubes (NTs) by a hydrothermal method, and then coated Na2Ti3O7 NTs with carbon to obtain Na2Ti3O7@C composite through a solvothermal method. The structural and electrochemical properties of the Na2Ti3O7

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and Na2Ti3O7@C were systematically investigated. The coated carbon greatly enhances the electron conductivity, while the specially designed nanotube with the carbon layer facilitates the transport of the Na ions and improves the structural

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stability during sodium intercalation. Therefore, our Na2Ti3O7@C composite shows

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greatly enhanced electrochemical performance with long cycling stability and good rate capability.

2. Material and methods 2.1 Synthesis of Na2Ti3O7 NTs 4

ACCEPTED MANUSCRIPT First, 500 mg TiO2 in 10 mL NaOH aqueous solution (10 M) was stirred vigorously at room temperature for 3 h. Then, the solution was transferred into a Teflon-lined stainless-steel autoclave with a volume of 100 mL, and subsequently

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sealed and heated at 130 ºC for 3 d in an oven. The resulting white sample was

120 ºC for 18 h to produce the Na2Ti3O7 NTs. 2.2 Synthesis of carbon coated Na2Ti3O7 NTs

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centrifuged and washed with deionized water several times and dried in an oven at

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300 mg Na2Ti3O7 NTs was dispersed in 5 mL NaOH aqueous solution (10 M) by ultrasonication to form a suspension. 600 mg glucose (C6H12O6) was dissolved in 15 mL deionized water. The former suspension together with 10 mL ethanol was added into the solution under stirring. The resulting suspension was transferred into a

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Teflon-lined stainless-steel autoclave with a volume of 100 mL, and subsequently sealed and heated at 190 ºC for 15 h in an oven. The carbon precursor carbon coated Na2Ti3O7 NTs were washed with deionized water and dried at 90 ºC in an oven. The

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resulting sample was heated in a quartz tube at 400 ºC in an Ar atmosphere for 4 h.

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2.3 Characterizations

X-ray diffraction (XRD) experiments of the samples were performed on an X’Pert

Pro X-ray diffractometer (PANalytical, the Netherlands) with Cu Kα radiation at 40 kV and 40 mA. Carbon structure was analyzed by using Raman spectroscopy (JDbin-yvon, LabRamHRUV). Transmission electron microscopy (TEM, Tecnai G2 F30) were performed to examine the morphology and microstructure. The electrochemical properties of the synthesized Na2Ti3O7@C and Na2Ti3O7 5

ACCEPTED MANUSCRIPT samples were evaluated by assembling CR2025 coin type cells in an argon-filled glove box. The working electrode was prepared by forming slurry of Na2Ti3O7 or Na2Ti3O7 @C, carbon black and sodium alginate (NaAlg) in a weight ratio of 70 : 15 :

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15. The slurry was coated on copper foil disks and dried overnight in a vacuum oven at 80 °C. Coin cells were fabricated using Na foil as the counter electrode, a glass fiber (GF/D) from Whatman was used as the separator and NaClO4 (1 M) in ethylene

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carbonate/dimethyl carbonate (EC/DMC, 1:1 vol%) with 10 w% fluoroethylene

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carbonate (FEC) additive as the electrolyte. The charge–discharge curves were measured at a constant current density between 0.01 and 2.5 V using a LAND CT2001A Battery Cycler at room temperature. Cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) experiments were performance on an

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electrochemical workstation (CHI600E, CH Instruments, Inc.). CV tests were carried at different rates between 0.01 and 2.5 V (vs. Na/Na+). The EIS measurements were performed under a frequency range between 100 kHz and 10 mHz with an applied

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voltage of 5 mV.

3. Results and discussions The overall synthesis process of Na2Ti3O7@C NTs is illustrated in Fig. 1a.

Na2Ti3O7 NTs were first prepared via a facile hydrothermal method in NaOH aqueous solution (10 M). After synthesis of Na2Ti3O7 NTs, a carbon layer with a thickness of several nanometers was coated on the surface of Na2Ti3O7 using glucose solution as 6

ACCEPTED MANUSCRIPT the carbon precursor [20]. Detail processes are presented in the Experimental section. To confirm integral morphology of the as-prepared sample, SEM image of Na2Ti3O7 NTs is shown in Fig. 1b. The tube structure is Na2Ti3O7 nanotube and non-tube

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structure is carbon. Thus, we can clearly see Na2Ti3O7 is coated by the carbon

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Fig. 1 (a) Schematic representation for the preparation of Na2Ti3O7@C NTs, (b) SEM image of Na2Ti3O7@C NTs.

TEM images (Fig. 2a and 2b) show that after hydrothermal reaction, the TiO2

nanoparticles are transformed to the uniform Na2Ti3O7 NTs with size of ~10 nm in outside diameter and several hundred nanometers in length. The high resolution TEM (HRTEM) image is shown in Fig. 2c, which confirmed that the as-synthesized Na2Ti3O7 is polycrystalline and no other layers on the surface of Na2Ti3O7. Lattice 7

ACCEPTED MANUSCRIPT fringes with a spacing of 0.19 and 0.30 nm can be seen from the HRTEM image, consistent with the (020) and (300) planes of Na2Ti3O7, respectively. These results indicate that large-scale production of uniform-sized Na2Ti3O7 NTs can be

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self-assembled through a simple hydrothermal method using TiO2 nanoparticles as precursors in NaOH aqueous solution. This is the first report for the Na2Ti3O7 NTs synthesized by the simple hydrothermal method to the best of our knowledge. TEM

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images (Fig. 2d and 2e) clearly demonstrate that Na2Ti3O7 NTs are tightly

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encapsulated by carbon layers. Na2Ti3O7 nanotubes are uniformly dispersed and entangled in the carbon matrix. The structure can be further confirmed from the HRTEM image (Fig. 2f). Crystalline Na2Ti3O7 NTs are well encapsulated by amorphous carbon. Lattice fringes with a spacing of 0.19 nm can be seen from the

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HRTEM image, in good agreement with the (020) planes of Na2Ti3O7.

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Fig. 2 (a,b) and (c) are TEM and HRTEM of Na2Ti3O7 NTs; (d,e) and (f) are TEM and HRTEM of Na2Ti3O7@C NTs.

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The phase structure and composition of samples were further identified by XRD

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and Raman spectroscopy. XRD patterns of the Na2Ti3O7 and Na2Ti3O7@C are shown in Fig. 3a. It is found that all the diffraction peaks can be well indexed to layer structure of Na2Ti3O7 (JCPDS31-1329). After coating carbon, no other diffraction peaks show up, indicating that carbon is present as amorphous structure, which is in

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line with the HRTEM results shown in Fig. 2. To analyze the structure of carbon-layers on the nanotubes, Raman spectroscopy was performed. As shown in Fig. 3b, there are two intense broad peaks at 1375 and 1587 cm-1 that can be assigned to

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the D (disordered) and G (graphene) bands of the deposited carbon, respectively [21].

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The D and G bands are related to the sp3-type carbon and sp2 graphite-like structure, respectively. The IG/ID ratio approximately correlates to the amount of graphene clusters in the disordered carbon, with higher ratios corresponding to higher electronic conductivity [22]. These carbon layers with evident graphene structures greatly improve the electron conductivity of the Na2Ti3O7@C composite. To determine the chemical composition of Na2Ti3O7@C NTs, TGA was performed as shown in Fig.3c. The sample was heated to 800 ºC in air so that carbon is oxidized to CO2. According 9

ACCEPTED MANUSCRIPT to the remaining weight of Na2Ti3O7, the original fraction of Na2Ti3O7 is calculated to be 89 wt% and the carbon content is 11 wt%. (b) Intensity (a.u.)

Na2Ti3O7@C ♣ ♣

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♣ Na2Ti3O7 (JCPDS31-1329)

(001)

Intensity (a.u.)

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95

11%

90 85 80 75 70

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400

600

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Temperature (°C)

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Fig. 3 (a) XRD patterns of Na2Ti3O7 and Na2Ti3O7@C NTs; (b) Raman spectra of Na2Ti3O7@C NTs; (c) TGA profile of Na2Ti3O7@C NTs.

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The electrochemical performance was evaluated by preparing coin-type half cells that employ Na2Ti3O7 or Na2Ti3O7@C as the working electrode and metal Na as the counter/reference electrode. In this study, the capacity is calculated based on the total mass of the composite including Na2Ti3O7 and carbon layers. The charge–discharge curves of the bare Na2Ti3O7 and Na2Ti3O7@C composite electrodes at a rate of 1C in the voltage range of 0.01–2.5V (vs. Na+/Na) for the 1st , 2nd , 50th and 100th cycles are illustrated in Fig. 4a and 4b. Upon cycling, the first discharge and charge capacities of

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discharge and charge capacities of 405.1 and 133.3 mAh g-1, giving a Coulombic efficiency of ~32.9%. It is noted that the first discharge capacity of Na2Ti3O7@C composite is much higher than that of the theoretical values for Na2Ti3O7 and carbon.

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The reason for the larger capacity can be ascribed to the formation of a solid

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electrolyte interphase (SEI) layer resulting from electrolyte decomposition at low voltage and additional interfacial/active site Na storage mechanism due to nano-scale [23]. From the second cycle, this plateau becomes much shorter, indicating that some differences take place between the first and the following cycles and the existence of

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irreversible reaction. Upon further cycling, no obvious changes to either charge or discharge process are observed for Na2Ti3O7 or Na2Ti3O7@C composite electrode, suggesting the highly reversible electrode reaction process and a stable electrode

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structure characteristic for the samples after the first cycle. The capacity of

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Na2Ti3O7@C composite electrode is higher than that of Na2Ti3O7 electrode, which indicates the carbon coating can enhance the conductivity of composite obviously and thereby dramatically improves the utilization efficiency of the Na2Ti3O7. Fig. 4c shows the cycling performances of Na2Ti3O7, Na2Ti3O7@C and the

corresponding Coulombic efficiency of Na2Ti3O7@C. The Na2Ti3O7@C composite electrode exhibits higher capacity and good cyclic retention compared to bare Na2Ti3O7. The reversible capacities of the second cycle are 274.2 and 149.4 mAh g-1 11

ACCEPTED MANUSCRIPT for Na2Ti3O7@C and Na2Ti3O7, respectively. After 100 cycles, the Na2Ti3O7@C still delivers a reversible capacity of 142.2 mAh g-1, while the capacity of Na2Ti3O7 decreases to 84.6 mAh g-1. Furthermore, the Coulombic efficiency of Na2Ti3O7@C

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increased to 98% after 10 cycles. This excellent property could be attributed to the specially designed nanotube structure of Na2Ti3O7 and the conducting carbon coating layers. For comparison, the cycling performance of carbon at 1C is shown in the inset

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of (c). These amorphous carbon exhibits a quite low capacity of ~10 mAh g-1 at 1C,

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confirming that the carbon make little contribution to the total capacity of the hybrid electrode. The rate performance of the Na2Ti3O7 and Na2Ti3O7 @C composite electrodes are shown in Fig. 4d. Compared with Na2Ti3O7, Na2Ti3O7@C can deliver higher reversible specific capacities of 220, 180, 170, 150, 137, 108, 84 mAh g-1 at

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the current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10 C (1C=311 mA g-1). More interestingly, when the current rate was reduced to 0.1C after 35 cycles, a stable high capacity of 220 mAh g-1 for Na2Ti3O7@C was resumed, which indicates that Na2Ti3O7@C

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electrode has the excellent tolerance for the rapid sodium ion insertion/extraction

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cycles. Table 1 compares the electrochemical performance of the reported Na2Ti3O7 electrodes, demonstrating that our specially designed Na2Ti3O7@C composite electrode exhibits superior electrochemical properties. Table 1. Electrochemical performance of different Na2Ti3O7

Type of material Single crystalline Na2Ti3O7 rods [24]

Specific capacity (mAh g-1) 85 mAh g-1 at 85 mA g-1

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Cycle performance 55 mAh g-1 after 20 cycles at 85 mA g-1

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105 mAh g-1 after 50 cycles at 17 mA g-1

Na2Ti3O7 nanotubes [18]

80 mAh g-1 at 3.54 A g-1

150 mAh g-1 after 100 cycles at 354 mA g-1

Na2Ti3O7/C composites (include C) [17]

79.5 mAh g-1 at 890 mA g-1

72.8 mAh g-1 after 100 cycles at 890 mA g-1

Na2Ti3O7@CNT (exclude CNT) [8]

100 mAh g-1 at 3.4 A g-1

100 mAh g-1 after 100 cycles at 1.7 A g-1

This work

83.75 mAh g-1 at 3.11 A g-1 142 mAh g-1 after 100 cycles at 311 mA g-1

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Na2Ti3O7 nanotubes [25]

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Fig. 4e and f present the CV curves of bare Na2Ti3O7 and Na2Ti3O7@C composite electrodes for the first three cycles in the potential range of 0.01-2.5 V (vs. Na/Na+) at a scan rate of 0.1 mV s-1. In the first scan, some irreversible reductive peaks were present. The cathodic peak appearing at about 1 V is attributed to the

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decomposition of FEC forming the SEI layer. 21 In the second and third cycle, the peak of bare Na2Ti3O7 electrode at 0.50 V is attributed to the extraction of Na+ from

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Na2Ti3O7, while the value of Na2Ti3O7@C composite electrode is 0.48 V, which confirms lower polarization of Na2Ti3O7@C composite electrode. It is noted that the

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current density of the Na2Ti3O7@C are higher than those of the bare Na2Ti3O7, which can be attributed to the fast Na+ and electron transport and a higher utilization efficiency for Na2Ti3O7 in the Na2Ti3O7@C material.

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2.5

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100 200 300 -1 400 Capacity (mAh g )

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Fig. 4 Electrochemical performance of Na2Ti3O7 and Na2Ti3O7@C: (a and b) charge/discharge curves of Na2Ti3O7 and Na2Ti3O7@C electrodes at 1C between 0.01 and 2.5V (vs. Na/Na+); (c) cycling performances of Na2Ti3O7, Na2Ti3O7@C electrodes at 1C and cycling performance of carbon at 1C (glucose after heated at 400 ºC) is shown in the inset of (c); (d) rate performance of Na2Ti3O7 and Na2Ti3O7@C electrodes at different current. (1C=311 mA g-1); CV curves of (e) bare Na2Ti3O7, (f) Na2Ti3O7@C for the first three cycles at a scan rate of 0.1 mV s-1 between 0.01 and 2.5 V (vs. Na/Na+).

To further investigate the effect of carbon layers on the Na+ diffusivity in Na2Ti3O7, cyclic voltammetry measurements were carried out at various scan rates in the range of 0.1– 0.8 mV s-1 (as shown in Fig.5). The Na+ diffusion coefficient was 14

ACCEPTED MANUSCRIPT obtained using the Randlese-Sevcik equation at room temperature [26]. Ip=26900n3/2AD1/2Cv1/2

(1)

In the above equation, n is the number of electrons per molecule during the

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intercalation, A is attributed to the surface area of the anode, C is related to the concentration of Na+ , D presents the diffusion coefficient of Na+ and v is the scan rate. A linear relationship between Ip and v1/2 was shown in Fig. 6c and 6d. In this electrode

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reaction, n= 3.5 mol, A=1.77 cm2, C=1 mol L-1.The diffusion coefficient of Na+ are

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7.86×10-14 and 3.14×10-13 cm2 s-1 for Na2Ti3O7 and Na2Ti3O7@C, respectively, proving that the porous Na2Ti3O7 NTs with carbon layers possess a higher sodium ion conductivity. Therefore, the improved capacity could be ascribed to the porous structure of Na2Ti3O7 nanotube, the conductivity of carbon layers and shorten Na+

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To verify the superior electrochemical performance of hierarchical Na2Ti3O7@C,

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electrochemical impedance measurements were performed. The Nyquist plots for the samples with a frequency range of 100 kHz to 0.01 Hz are performed and shown in Fig. 6. The shapes of the Nyquist plots of both electrodes are quite similar, which

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consist of a depressed semicircle with a medium-frequency semicircle overlap each

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other and a long low-frequency line. The inclined line in the low-frequency region represents the Warburg impedance (Zw), which is related to solid-state diffusion of Na+ in the electrode materials. The semicircle in the middle frequency range indicates the charge-transfer resistance (Rct), relating to charge transfer through the

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electrode/electrolyte interface. In the AC impedance spectra, the diameter of the semicircle for the hybrid hierarchical Na2Ti3O7@C composite electrode in the high-medium frequency region is much smaller than that of Na2Ti3O7. The value of

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Rct is ~90 Ω for the Na2Ti3O7@C, much lower than that of the corresponding

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Na2Ti3O7 (~150 Ω). It can be attributed to the increased contact area at the electrode–electrolyte interface as well as the enhanced electrical conductivity for the Na2Ti3O7@C electrode [26]. In the low frequency region, a more vertical straight line of Na2Ti3O7@C compared to Na2Ti3O7 is an evidence for the faster Na+ diffusion behavior of Na2Ti3O7@C electrode [28]. Moreover, EIS under different cycles (three or five cycles) have the similar results, which reflects the stability of the charge transfer process in cycling. Based on above analysis, it can be concluded that the 16

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Na2Ti3O7@C increases the transport and diffusion process of Na+ and electron.

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Fig. 6 Nyquist plots (100 kHz–10 mHz) of Na2Ti3O7 and Na2Ti3O7@C composite electrode after (a) three and (b) five cycles..

To understand the improved electrochemical performance of Na2Ti3O7@C, TEM

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analyses were performed for the electrode after 5 cycles, as shown in Fig. 7. It can be seen from Fig. 7a that the Na2Ti3O7@C composite still remains the original nanotube

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structure with carbon layers outside. HRTEM image (Fig. 7b) shows that small parts of encapsulated nanotubes of Na2Ti3O7 were pulverized into smaller nanoparticles due

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to intercalate or deintercalate of Na+ from the composite [29, 30], while the carbon layers can prevent the exfoliation and disconnection of active materials and effectively cage these pulverized particles. Based on above analysis, the overall structural stability and durability of the Na2Ti3O7@C hybrid electrode are improved.

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Fig. 7 (a) and (b) are TEM and HRTEM images of Na2Ti3O7@C after 5 cycles.

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4. Conclusions

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In summary, the Na2Ti3O7@C composite consisting of Na2Ti3O7 NTs (~10 nm in

outside diameter and several hundred nanometers in length) with carbon layers (~5 nm) was designed and fabricated. The unique hierarchical structure for the Na2Ti3O7@C composite electrode ensures favorable transport kinetics for electrons and Na+, and therefore exhibits excellent electrochemical performance. The as-synthesized Na2Ti3O7@C composite delivers a high reversible capacity of 142.2 mA h g-1 for over 100 cycles at 1C (311 mA g-1) and shows an improved rate 18

ACCEPTED MANUSCRIPT capability (83.75 mA h g-1 at 10 C). The better electrochemical performance of the Na2Ti3O7@C composite electrode can be ascribed to the synergistic effect of unique structure of Na2Ti3O7 and outside carbon layers: (1) the outer layer of carbon can

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improve the electron conductivity of Na2Ti3O7 and enhance the stability of Na2Ti3O7 NTs structure during the electrochemical cycling; (2) the carbon layers in association with nanotubes structure provide fast transport and diffusion of Na+. In addition,

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Na2Ti3O7 were pulverized into smaller nanoparticles during cycling due to the

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intercalate or deintercalate of Na+ from the composite, while the carbon layers can prevent the exfoliation and disconnection of active materials and effectively cage these pulverized particles to ensure the better performance of Na2Ti3O7. Such a simple and scalable route to construct a carbon encapsulated core-shell structure could be

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further extended to other intercalation anode materials for sodium-ion battery

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applications.

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Acknowledgements

The authors gratefully acknowledge the financial supports for this research from

the National Natural Science Foundation of China (51571179 and 51671173), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), the Zhejiang Provincial Science & Technology Program of China (2015C31035).

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References

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[1] J. M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359-367. [2] K. Kang, Y. S. Meng, J. Breger, C. P. Grey, G. Ceder, Electrodes with high power and high capacity for rechargeable lithium batteries, Science 311 (2006) 977-980. [3] L. M. Suo, Y. S. Hu, H. Li, M. Armand, L. Q. Chen, L. Q. Chen, A new class of solvent-in-salt electrolyte for high-energy rechargeable metallic lithium batteries, Nat. Commun. 4 (2013) 1481-1489. [4] M. Li, X, Z. Xiao, Y. J. Liu, W. Zhang, Y. W. Zhang, L. X. Chen, Ternary perovskite cobalt titanate/graphene composite material as long-term cyclic anode for lithium-ion battery, J. Alloys Compd. 700 (2017) 54-60. [5] K. Chen, K. Cao, C. Xing, Y. Hu, J. Liu, Y. He, J. Wang, A. Li, H. Qin, In-situ TEM study of the lithiation and delithiation of FeS nanosheets, J. Alloys Compd. 688 (2016) 946-952. [6] H. Kang, Y. Liu, K. Cao, Y. Zhao, L. Jiao, Y. Wang, H. Yuan, Update on anode materials for Na-ion batteries, J. Mater. Chem. A 3 (2015) 17899-17913. [7] H. L. Pan, X. Lu, X. Q. Yu, Y. S. Hu, H. Li, X. Q. Yang, L. Q. Chen, Sodium storage and transport properties in layered Na2Ti3O7 for room-temperature sodium-ion batteries, Adv. Energy Mater. 3 (2013) 1186-1194. [8] S. Y. Dong, L. F. Shen, H. S. Li, P. Nie, Y. Y. Zhu, Q. Sheng, X. G. Zhang, Pseudocapacitive behaviours of Na2Ti3O7@CNT coaxial nanocables for high-performance sodiumion capacitors, J. Mater. Chem. A 3 (2015) 21277-21283. [9] L. Mu, S. Xu, Y. Li, Y. S. Hu, H. Li, L. Chen, X. Huang, Prototype sodium-ion batteries using an air-stable and Co/Ni-free O3-layered metal oxide cathode, Adv. Mater. 27 (2015) 6928-6933. [10] S. Y. Wei, X. Y. Wang, M. L. Jiang, R. Zhang, Y. Q. Shen, H. Hu, The FeF3 center dot 0.33H(2)O/C nanocomposite with open mesoporous structure as high-capacity cathode material for lithium/sodium ion batteries, J. Alloys Compd. 689 (2016) 945-951. [11] P. Barpanda, G. Oyama, S. i. Nishimura, S.C. Chung, A. Yamada, A 3.8 V earth-abundant sodium battery electrode, Nat. Commun. 5 (2014) 4358-4365. [12] X. Xiang, K. Zhang, J. Chen, Recent advances and prospects of cathode materials for sodium-ion batteries, Adv. Mater. 27 (2015) 5343-5364. [13] Z. Y. Lan, M. G. Chen, X. Y. Xu, C. C. Xiao, F. Wang, Y. Wang, Y. H. Lu, Y. Z. Jiang, J. Z. Jiang, Investigations on molybdenum dinitride as a promising anode material for Na-ion batteries from first-principle calculations, J. Alloys Compd. 701 (2017) 875-881. [14] Y. Wen, K. He, Y. Zhu, F. Han, Y. Xu, I. Matsuda, Y. Ishii, J. Cumings, C. Wang, Expanded graphite as superior anode for sodium-ion batteries, Nat. Commun. 5 (2014) 4033-4042. [15] Y. Zhang, L. Guo, S. Yang, Novel sodium/lithium-ion anode material based on ultrathin Na2Ti2O4(OH)2 nanosheet, Nanoscale 7 (2015) 14618-14626. 20

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AC C

EP

TE D

M AN U

SC

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[16] P. Senguttuvan, G. Rousse, V. Seznec, J. M. Tarascon, M. R. Palacin, Na2Ti3O7: lowest voltage ever reported oxide insertion electrode for sodium ion batteries, Chem. Mater. 23 (2011) 4109-4111. [17] Z. C. Yan, L. Li, H. B. Shu, X. K. Yang, H. Wang, J. L. Tan, Q. Zhou, Z. F. Huang, X. Y. Wang, A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries, J. Power Sources 274 (2015) 8-14. [18] W. Wang, C. J. Yu, Z. S. Lin, J. G. Hou, H. M. Zhu, S. Q. Jiao, Microspheric Na2Ti3O7 consisting of tiny nanotubes: an anode material for sodium-ion batteries with ultrafast charge-discharge rates, Nanoscale 5 (2013) 594-599. [19] Y. P. Zhang, L. Guo, S. H. Yang, Three-dimensional spider-web architecture assembled from Na2Ti3O7 nanotubes as a high performance anode for a sodium-ion battery, Chem. Commun. 50 (2014) 14029-14032. [20] W. M. Zhang, X. L. Wu, J. S. Hu, Y. G. Guo, L. J. Wan, Carbon coated Fe3O4 nanospindles as a superior anode material for lithium-ion batteries, Adv. Funct. Mater. 18 (2008) 3941-3946. [21] F. Tuinstra, J. L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (1970) 1126-1128. [22] Nakamura T, Miwa Y, Tabuchi M, Yamada Y, Structural and surface modifications of LiFePO4 olivine particles and their electrochemical properties, J. Electrochem. Soc. 153 (2006) A1108-A1114. [23] S. Xu, C. M. Hessel, H. Ren, R. Yu, Q. Jin, M. Yang, H. Zhao, D. Wang, Alpha-Fe2O3 multi-shelled hollow microspheres for lithium ion battery anodes with superior capacity and charge retention, Energy Environ. Sci. 7 (2014) 632-637. [24] W. Wang, C. Yu, Y. Liu, J. Hou, H. Zhu, S. Jiao, Single crystalline Na2Ti3O7 rods as an anode material for sodium-ion batteries, RSC Advances 3 (2013) 1041-1044. [25] J. Yin, L. Qi, H. Wang, Sodium titanate nanotubes as negative electrode materials for sodium-ion capacitors, ACS Appl. Mater. Interfaces 4 (2012) 2762-2768. [26] Z.H. Bi, M. P. Paranthaman, B. K. Guo, R. Unocic, H. M. Meyer III, C.A. Bridges, X.G. Suna, S. Daia, High performance Cr, N-codoped mesoporous TiO2 microspheres for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 1818-1824. [27] B. G. Choi, M. Yang, W. H. Hong, J. W. Choi, Y. S. Huh, 3D macroporous graphene frameworks for supercapacitors with high energy and power densities. ACS Nano 6 (2012) 4020-4028. [28] X. Fan, J. Shao, X. Xiao, L. Chen, X. Wang, S. Li, H. Ge, Carbon encapsulated 3D hierarchical Fe3O4 spheres as advanced anode materials with long cycle lifetimes for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 14641-14648. [29] S. Ni, X. Lv, J. Ma, X. Yang, L. Zhang, Electrochemical characteristics of lithium vanadate, Li3VO4 as a new sort of anode material for Li-ion batteries, J. Power Sources 248 (2014) 122-129. [30] S. Ni, J. Zhang, J. Ma, X. Yang, L. Zhang, X. Li, H. Zeng, Approaching the Theoretical Capacity of Li3VO4 via Electrochemical Reconstruction, Adv. Mater. Interfaces 3 (2016) 1500340. 21

ACCEPTED MANUSCRIPT Highlights:

Carbon coated Na2Ti3O7 nanotubes (NTs) were synthesized for Na-ion battery.

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Na2Ti3O7@C show better electrochemical performance than bare Na2Ti3O7 NTs.

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Na2Ti3O7@C NTs deliver high reversible capacity of 142.2 mA h g-1 for over 100

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cycles.

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Na2Ti3O7@C NTs show relatively stable capacities at high rate current.

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