A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries

A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries

Accepted Manuscript A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries Zichao Yan, Li Li...

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Accepted Manuscript A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries Zichao Yan, Li Liu, Hongbo Shu, Xiukang Yang, Hao Wang, Jinli Tan, Qian Zhou, Zhifeng Huang, Xianyou Wang PII:

S0378-7753(14)01654-1

DOI:

10.1016/j.jpowsour.2014.10.045

Reference:

POWER 19965

To appear in:

Journal of Power Sources

Received Date: 22 July 2014 Revised Date:

5 October 2014

Accepted Date: 6 October 2014

Please cite this article as: Z. Yan, L. Liu, H. Shu, X. Yang, H. Wang, J. Tan, Q. Zhou, Z. Huang, X. Wang, A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries, Journal of Power Sources (2014), doi: 10.1016/j.jpowsour.2014.10.045. 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

A tightly integrated sodium titanate-carbon composite as an anode

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material for rechargeable sodium ion batteries

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Zichao Yan, Li Liu*, Hongbo Shu, Xiukang Yang, Hao Wang, Jinli Tan, Qian Zhou,

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Zhifeng Huang, Xianyou Wang*

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(Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,

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School of Chemistry, Xiangtan University, Hunan, Xiangtan 411105, China)

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Abstract:

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A novel sodium titanate-carbon (Na2Ti3O7/C) composite has been successfully

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synthesized via a rheological phase method. The homogeneous-dispersed carbon not

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only sheathes the single Na2Ti3O7 particle but also combines all individual Na2Ti3O7

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particles to a stable union, as characterized by X-ray diffraction, scanning electron

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microscopy (SEM), and high-resolution transmission microscopy (HRTEM). The

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uniformly distributed carbon forms a good network of electrically conductive paths

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among the Na2Ti3O7 particles, which is closely interlinked with each other. So

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Na2Ti3O7 active material can get electrons from all directions and be fully utilized for

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sodium ion insertion and extraction reactions, which can improve sodium storage

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properties with enhanced rate capability and super cycling performance. The

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Na2Ti3O7/C composite exhibits much better electrochemical performance than bare

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Na2Ti3O7, which displays a stable discharge capacity of 111.8 mAh g-1 at 1 C after

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100 cycles, while only 48.6 mAh·g-1 for bare Na2Ti3O7 at the same conditions.

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Furthermore, the composite shows relatively stable storage capacities during long

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*

Corresponding author. Tel.: +86-731-58292206;Fax: +86-731-58292477

E-mail addresses: [email protected] (L. Liu) ; [email protected] (X.Y. Wang) 1

ACCEPTED MANUSCRIPT term cycling even at 5 C. The remarkably improved cycling performance and rate

2

capability of Na2Ti3O7 are attributed to the tight integration between carbon and

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Na2Ti3O7 which may enhance the electronic conductivity, decrease the charge transfer

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resistance and improve the electrochemical stability during cycling, thus making a

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compelling case for its development as an advanced anode material for sodium ion

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batteries.

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Keywords: Sodium ion batteries; Sodium titanate; Carbon; Anode; Composite

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1. Introduction

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Renewable energy storages as important energy storage devices for mobile,

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electric vehicle and portable applications have been attracted a great deal of

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awareness due to the environmental disruption and economic recession [1-3]. Cheaper,

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safer and more environmentally benign energy storage technologies have been

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considered as the most suitable candidate for the future renewable energy storages [4,

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5]. Compared with lithium ion batteries, sodium ion batteries (NIBs) with higher

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availability and potential for lower cost of raw materials have drawn scientists’

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attention [6-8]. However, the electrode material (especially the anode material) has

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been considered to be one of the main restrictions for the widely application of NIBs.

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Sodium ion insertion into carbon anode materials was extensively studied by a

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lot of scientists. The results show that unless high pressures are used, sodium ion

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insertion into graphitic carbons is minimal [9-11]. Moreover, sodium metal cannot be

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used as the anode as well due to the possibility of dendrite formation and the low

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melting temperature of sodium metal from safety concerns [12]. In addition, hard 2

ACCEPTED MANUSCRIPT carbon with high discharge capacity and good capacity retention also has been studied

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[13]. However, the low voltage discharge plateau between 0 and 0.1 V vs. Na/Na+

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could raise safety concerns that there may be sodium dendrite formation at such a low

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voltage range. So, it is of great urgency for us to find a perfect anode material for

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future NIBs. Various promising anode materials, such as NiCo2O4 [14], Na3V2(PO4)3

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[15], Ni3S2 [16] and NaFeF3 [17] have been fabricated to improve the applicability of

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NIBs. However, all of those reported anode materials have disadvantages, such as

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their high cost, environmentally unfriendly, complicated synthesis process or the

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charge/discharge voltage plateaus being unobvious.

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Ti-based anode material with its inherent chemical stability, minimal toxicity,

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low cost, and high safety has been considered as one of the most suitable candidates

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for NIBs anode materials [18-20]. As a newly used anode material, sodium titanate,

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Na2Ti3O7, with lower discharge plateau and abundant raw material resources has been

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studied as Na hosts. Senguttuvan et al. [21] were the first to report Na2Ti3O7 as the

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anode materials for NIBs, and illustrated the proper potential of reversibly reaction in

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a discharge voltage region from 0.01 to 2.5 V. After that, Wang et al. [22] synthesized

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Na2Ti3O7 rods via a reverse microemulsion method, and a reversible capacity of 72

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mAh g-1 was achieved at 0.5 C, but only 20 cycles were lasted. Pan et al. [23]

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synthesized layered Na2Ti3O7 by a solid-state method. The layered Na2Ti3O7 shows a

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reversible capacity of 85 mAh g-1 (0.5 C) after 100 cycles, which indicates a good

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cycling stability. However, the Na2Ti3O7 also suffers from structural distortion, low

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electronic

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conductivity

and

poor

electrochemical 3

stability

upon

sodium

ACCEPTED MANUSCRIPT insertion/extraction, which may account for the low coulombic efficiency and

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continuous capacity fading of Na2Ti3O7 electrodes [23]. Rudola et al. [24] prepared

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Na2Ti3O7 particles via a solid-state method. The working electrode for sodium cell

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was fabricated by mixing the as-synthesized sample and carbon black via high-energy

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ball-milling process. The mixture electrode show good cycling performance. A

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reversible discharge capacity of 80 mAh g-1 can be retained after 100 cycles at 2 C.

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However, it did not say more about the long-term cycling performance at high rates

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and the specific capability needed to be further improved. So it is not very

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encouraging just via high-energy ball-milling process reducing the grains size or

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mixing with conductive materials to solve the poor electrochemical stability of

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Na2Ti3O7.

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Forming a composite with carbon is an effective approach to increase the

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electronic conductivity, rate performance and cycling stability of the anode material

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for lithium ion batteries [25-27]. After the very few reports about the electrochemical

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activity of bare Na2Ti3O7 in NIBs, there is no literature report on fabricating a tight

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integration of carbon and Na2Ti3O7 to improve the electrochemical performance of

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Na2Ti3O7. The effectiveness of the tight structure in improving the electrochemical

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performance of Na2Ti3O7 is unknown. Based on the above points, it is of great

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significance to fabricate stable Na2Ti3O7/C composite with high electrochemical

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activity in the view of both practical application and scientific research.

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With the aim to synthesize the Na2Ti3O7/C composite with stable and high

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electrochemical performance, we present a rheological phase method using anatase 4

ACCEPTED MANUSCRIPT TiO2 and Na2CO3 as raw material, glucose as carbon source. In this work, we

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successfully synthesized Na2Ti3O7/C composite for the first time via this rheological

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phase method and illustrated its excellent electrochemical performance in NIBs.

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2. Experimental

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2.1 Synthesis of Na2Ti3O7/C composite.

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Na2Ti3O7/C composite was achieved via a rheological phase method (shown in

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Fig. 1). 0.5 g anatase TiO2 spheres prepared by the soft-template method [28] and

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0.23 g Na2CO3 (controlling the molar ratio of Na:Ti to 2:3) were milled in the alcohol.

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The obtained well blended suspension was then dried at 80 °C. A mixture of Na 2CO3

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and the anatase TiO2 was preheated at 500 °C for 5 h, and then calcined at 750 °C for

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8 h in air. The calcined product and glucose were blended homogeneously and then

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added into the alcohol to obtain a solid–liquid rheological mixture. After continuous

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magnetic stirring of the mixture for 2 h, it was dried at 80 °C for 4 h in a vacuum oven

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to eliminate ethanol adequately. Finally, the mixture was calcined at 600 °C for 3 h in

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a tubular furnace under argon gas protecting, with a heating rate of 5 °C min-1, to form

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the Na2Ti3O7/C composite. The mass ratio of Na2Ti3O7 and glucose were settled as

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2:1. According to the carbon residue of glucose after thermal treatment under argon

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gas protecting, the content of carbon in Na2Ti3O7/C composite was about 5 wt%. For

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comparison, bare Na2Ti3O7 was also prepared in the same way except for the addition

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of glucose.

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Fig. 1 2.2 Structure and morphology characterization 5

ACCEPTED MANUSCRIPT The structures of the as-synthesized samples were characterized by X-ray

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diffraction. X-ray powder diffraction data were obtained using a Rigaku

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D/MAX-2500 powder diffractometer with a graphite monochromatic and Cu Kα

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radiation (λ= 0.15418 nm) operated at a scan rate of 5° min-1. Scanning electron

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microscope (SEM) images of the samples were collected using a JEOL JSM-6610

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scanning electron microscope, which were used to observe the morphology of the

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samples. Besides, high-resolution transmission electron microscopy (HRTEM) and

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fast-Fourier transform (FFT) measurements were carried out using a JEOL

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JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV.

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2.3 Electrochemical characterization

The working electrodes for sodium cells were fabricated by mixing the

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as-synthesized samples, carbon black, and Polyvinylidene fluoride (PVDF) binder

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with a weight ratio of 70:20:10 in N-methyl pyrrolidinone, which were then pasted on

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copper foil followed by drying under vacuum at 110 °C for 10 h. The average mass

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loading of the active material in the electrode was about 1.8 mg cm-2. The testing cells

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were assembled with metallic sodium as the negative electrode, glass fiber separator

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(Whatman GF/F), and 1 M NaClO4 in propylene carbonate (PC) electrolyte. The

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assembly of the testing cells was carried out in an argon-filled glove box, where water

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and oxygen concentration were kept less than 5 ppm. The charge-discharge cycle tests

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of NIBs (178 mA g-1 was assumed to be 1 C rate) were run at different current

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densities between 0.01-2.5 V. All the cells were allowed to age for overnight before

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testing.

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ACCEPTED MANUSCRIPT Cyclic voltammetry (CV) tests and EIS experiments were performed on a Zahner

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Zennium electrochemical workstation. CV tests were carried out at various scan rates

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on the potential interval 0.01-2.5 V (vs. Na+/Na). The ac perturbation signal was ±5

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mV and the frequency range was from 10 mHz to 100 KHz. All the tests were

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performed at room temperature.

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3. Results and discussion

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3.1 Characterization of Na2Ti3O7/C composite.

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The X-ray diffraction (XRD) measurement was used to study the phase structure

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of the as-prepared samples. Fig. 2 presents the XRD patterns of anatase TiO2 spheres,

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Na2Ti3O7/C composite and Na2Ti3O7. As shown in Fig. 2a, anatase TiO2 spheres show

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good crystal structure, of which the diffraction peaks in the XRD pattern can be

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indexed well based on anatase phase of TiO2 (JCPDS No. 21-1272). The inset of SEM

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image of anatase TiO2 spheres shows a well distribution of homogeneous spheres with

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a diameter of 0.5 µm. In Fig. 2b, the as-prepared Na2Ti3O7 shows good crystal

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structure, of which the diffraction peaks in the XRD pattern could be indexed well

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based on the Na2Ti3O7 pattern corresponding to JCPDS No. 31-1329. For the

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Na2Ti3O7/C composite, typical peaks at 10.5°, 15.8°, 25.6°, 43.8°, 47.7°, etc. can be

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found for monoclinic Na2Ti3O7 structure. As we can see that both Na2Ti3O7/C

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composite and Na2Ti3O7 are in good agreement with the XRD pattern of monoclinic

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Na2Ti3O7 structure. However, the peak intensity of Na2Ti3O7/C composite is lower

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than that of Na2Ti3O7, which can be greatly attributed to the introduction of carbon.

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This phenomenon is similar to other reports [29-31].

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ACCEPTED MANUSCRIPT Fig. 2

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The morphology of Na2Ti3O7 and Na2Ti3O7/C composite are displayed in Fig. 3.

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As shown in Fig. 3a, the SEM image of Na2Ti3O7 reveals the well distribution of

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homogeneous particles with relative smooth surface and diameters ranging from 0.2

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to 0.8 µm. From the SEM image of Na2Ti3O7/C composite (Fig. 3b), it is very clear

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that carbon as functional component wrap around the Na2Ti3O7 particles, which may

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benefit for electrical conductivity and stability during cycling.

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In order to further investigate morphology and structure of the final products,

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TEM and HRTEM images of Na2Ti3O7 and Na2Ti3O7/C composite are shown in Fig.

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3c-f. TEM image in Fig. 3c presents the spherical particles of Na2Ti3O7 with a

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diameter about 0.8 µm. From Fig. 3d, it can be found that Na2Ti3O7 particle is well

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inserted in carbon. It is expected that such a combination may effectively avoids the

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structural distortion and enhances the capacity and stability of Na2Ti3O7 electrodes

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upon sodium insertion/extraction. HRTEM (see Fig. 3e and f) analysis is employed to

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determine the crystal facets. Fig. 3e shows that Na2Ti3O7 particles display clear

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crystal lattices, its fast-Fourier transform (FFT) image of the same region (inset)

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reveals that the diffraction spots have a lattice spacing related to the (011), (001),

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(200), (401) and (-112) planes, corresponding to monoclinic Na2Ti3O7, which is in

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good agreement with the XRD results shown in Fig. 2. HRTEM image (Fig. 3f) taken

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on Na2Ti3O7/C composite shows that the interplanar distance between adjacent lattice

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planes are 0.34 and 0.84 nm, corresponding to (001) and (011) planes of monoclinic

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Na2Ti3O7. What’s more, it also demonstrates that the homogeneous-dispersed carbon

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ACCEPTED MANUSCRIPT not only sheathes the single Na2Ti3O7 particle but also combines all individual

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Na2Ti3O7 particles to a stable union, which is well agree with the SEM image (Fig.

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3b). Besides, the uniformly distributed carbon formed a good network of electrically

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conductive paths among the Na2Ti3O7 particles, which are closely interlinked with

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each other. So active Na2Ti3O7 material can get electrons from all directions and be

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fully utilized for sodium ion insertion and extraction reactions. All these above expect

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that the introduction of carbon might enhance electron transport and increase the

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stability of Na2Ti3O7 particles.

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3.2 Electrochemical analysis of Na2Ti3O7/C composite.

The electrochemical properties of Na2Ti3O7/C composite as sodium insertion

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electrodes were studied in order to examine the effectiveness of the tight integration

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of carbon and Na2Ti3O7 in improving the electrochemical performance of Na2Ti3O7

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electrode. Fig. 4a and b show the cycle performance and charge/discharge profiles

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(insets) of Na2Ti3O7 and Na2Ti3O7/C composite at 1 C. As shown in Fig. 4a, the

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discharge capacity of Na2Ti3O7 drops from 279.6 to 48.6 mAh g-1 sharply after 100

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cycles, with a large capacity loss of 82%. It is apparent that Na2Ti3O7 shows a huge

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irreversible capacity loss on the first cycle and low capacity retention after 100 cycles.

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Interestingly, the huge capacity fading of Na2Ti3O7 is ascribed to the first 10 cycles,

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which might be attributed to the instability caused by structure deformation during

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cycling [23]. However, the introduction of carbon improves the electrochemical

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performance of Na2Ti3O7 remarkably. The carbon wrapping may avoid the direct

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ACCEPTED MANUSCRIPT contact of material and electrolyte, which effectively enhance the stability of

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Na2Ti3O7 particles during the charge and discharge processes. Na2Ti3O7/C composite

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can deliver discharge capacity of 276.2 mAh g-1 at the first discharge process, and the

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capacity of 111.8 mAh g-1 is finally remained after 100 cycles (see Fig. 4b). Moreover,

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with the increasing of the cycle numbers, the charge capacity of Na2Ti3O7/C

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composite is getting stable and the coulombic efficiency is nearly 100% after the first

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several cycles. Compared with Na2Ti3O7, the capacity of Na2Ti3O7/C composite

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decreases in the initial cycle and it turns to be stable in the following cycles, which

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indicates a better cycling performance in NIBs. In order to exclude the effect from the

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carbon black additives, the relationship plot between specific capacity and cycle

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number for pure carbon black electrode in the range of 0.01 V-2.5 V at different rates

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in Na half-cells was utilized to present the sodium-storage performance of pure

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carbon black (see Fig S1 in Supporting Information). The pure carbon black can

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deliver a high discharge capacity of 186.4 mAh g-1 at the first discharge process at 1 C,

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and the capacity of 72.3 mAh g-1 is obtained in second cycle. And then a capacity

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below 50 mAh g-1 is remained after 20 cycles. A low discharge capacity of 25 mAh

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g-1 is obtained after 20 cycles at 5 C. However, the Na2Ti3O7/C composite shows

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much higher capacity than that of pure carbon black no matter at 1 C (111.8 mAh g-1

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after 100 cycles) or 5 C (72.8 mAh g-1 after 100 cycles). Besides, the additive amount

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of carbon black that mixed in the electrode is only 20 wt.%. So, the capacity which

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provides by carbon black in Na half-cells can be ignored. Fig. 4a (insets) presents the

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voltage profiles of Na2Ti3O7 at 1 C. It exhibits very rapidly decreasing Ti redox

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ACCEPTED MANUSCRIPT plateaus (at ~ 0.3V during discharge and 0.5V during charging) after initial 10 cycles.

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However, the reversibility of Ti redox reactions has been well proved by Wang et al.

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[32]. Therefore, this phenomenon may account for the high rate cycling. The capacity

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retention data and voltage profiles of Na2Ti3O7 at 0.2 C were included for comparison.

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Fig. S2 illustrates that the Ti redox plateaus can be clearly observed at 0.2 C even

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after 50 cycles. Fig. 4b (insets) shows the charge/discharge voltage profiles of

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Na2Ti3O7/C composite for the 1st, 2nd, 10th, 30th, 40th, 50th, 80th and 100th cycles at 1 C

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in the voltage range of 0.01-2.5 V. The sample shows sloped reaction plateaus locate

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at 0.47 V (charge) and 0.17 V (discharge), which is in accordance with the CV curves

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and delegates the representative charge/discharge profiles of Na2Ti3O7 electrode in

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NIBs [22, 23, 24]. Besides, the carbon wrapping effectively prevents from the

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continuous capacity fading of Na2Ti3O7 electrodes (especially for 2-10 cycles) which

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may account for the structural distortion of Na2Ti3O7 electrodes upon sodium

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insertion/extraction

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insertion/extraction amount of Na during different cycles is shown in Fig. S3. The

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result reveals that its actual sodium content is relatively lower than the theoretical

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value (two sodium) except the first cycle. This might be ascribed to the reactions of

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SEI or other side reactions of the electrolyte deterioration in the initial cycle [23, 24].

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In order to further explore the effectiveness of the composited carbon in improving

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the electrochemical performance of Na2Ti3O7 electrode, sodium cells made using

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Na2Ti3O7 and Na2Ti3O7/C composite were run at 5 C for 100 cycles to test the long

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term cycling performance at high rate (Fig. 4c). Just like the electrochemical 11

ACCEPTED MANUSCRIPT performance of Na2Ti3O7 electrode at 1 C, the second intensive decay of capacity

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from 2 to 10 cycles is also observed at 5 C. The discharge capacity of Na2Ti3O7 drops

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from 185.1 to 110 mAh·g-1 after the first cycle, which can be connected with the

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irreversible capture of sodium ion, formation of the SEI (which is actually a

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heterogeneous multilayer) films and other side reactions during the first cycle [23, 33,

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34]. And the capacity quickly drops to 67.8 mAh·g-1 after 10 cycles due to the

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instability of Na2Ti3O7 during the charge and discharge processes at high rate. After

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that, a poor capacity of 37 mAh g-1 is finally obtained after 100 cycles. However, the

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Na2Ti3O7/C composite shows excellent stability during cycling. The capacity of

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Na2Ti3O7/C composite starts at 220.1 mAh g-1 and still maintains at 95.3 mAh g-1

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after 10 cycles. And then a high capacity of 72.8 mAh g-1 is finally obtained after 100

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cycles, which is about twice the bare Na2Ti3O7 electrode. The good cycling

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performance and high specific capacity at fast discharging/charging processes indicate

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the super structure stability of the Na2Ti3O7/C composite during sodium

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insertion-extraction. In conclusion, Na2Ti3O7/C composite performs much superior

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cycling stability and capacity retention than bare Na2Ti3O7. These results suggest that

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the introduction of carbon greatly increases the conductivity and electrochemical

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stability of Na2Ti3O7.

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Fig. 4

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The rate capability of Na2Ti3O7/C composite and Na2Ti3O7 for NIBs was further

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investigated at various rates ranging from 0.5 to 5 C as shown in Fig. 5. For each

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stage, the process was taken with 11 cycles. Na2Ti3O7 can deliver discharge capacity 12

ACCEPTED MANUSCRIPT of 97.6 mAh g-1 at 0.5 C. However, the specific discharge capacity sharply reduces to

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64.7, 49.4 and 31.7 mA h g-1 at rates of 1 C, 2 C and 5 C, respectively. After the high

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rate of 5 C, the specific discharge capacity of 72.8 mAh g-1 is finally remained when

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reducing the rate to 0.5 C. Clearly, Na2Ti3O7/C composite has the better rate

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performance in capacities deliverable at various rates: the Na2Ti3O7/C composite

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electrode is initially cycled at 0.5 C where the capacity stabilized to 133.4 mA g-1

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after 11 cycles. The rate is then increased to 1 C, 2 C and 5 C. Stable discharge

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capacities of 121.2, 104.3 and 79.5 mAh g-1 are sustainable for 11 charge-discharge

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cycles at each of these rates. After deep cycling at high rates, a discharge capacity of

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120.6 mAh g-1 is restored upon reducing the rate to 0.5 C. Besides, the composites

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have excellent charge capacity retention even at high rates. The Na2Ti3O7/C

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composite exhibits a remarkable rate capability, which is even superior to the

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performance of reported Na2Ti3O7 rods [22] and Na2Ti3O7 small platelets [24].

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Fig. 5

Since the higher capacity observed at high rates suggests a stability enhanced

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response, further investigations were performed using cyclic voltammetry. The CV

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curves of Na2Ti3O7 and Na2Ti3O7/C composite are shown in Fig. 6. It can be seen that

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the oxidation peak of bare Na2Ti3O7 electrode (Fig. 6a) alters largely from 0.49 V to

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0.59 V at different scan rates ranging from 0.2 mV s-1 to 0.6 mV s-1, while the value

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of Na2Ti3O7/C composite electrode (Fig. 6b) is 0.47 V at the scan rate of 0.2 mV s-1,

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and alters slightly to 0.50 V when increasing the scan rate to 1.2 mV s-1. This

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confirms the better reversibility and lower polarization of the Na2Ti3O7/C composite

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ACCEPTED MANUSCRIPT electrode. Besides, a pair of redox peaks observed at 0.17 V (reduction peak) and 0.47

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V (oxidation peak) remarkable appears at CV curves, which is in good agreement

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with the charge/discharge voltage profiles (see the insets in Fig. 4) and other reports

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[22, 23]. However, it is interesting to note that the intensity of oxidation peak is

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higher than that of the reduction peak and the intensity of reduction peak is weakened

6

with the increasing of the scan rates. This can be attributed to the more facile

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extraction process of the Na+ ion upon charging at high rates comparing to the Na+ ion

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insertion process during discharging, which has been clearly proved by other reports

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[23, 24, 35].

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Fig. 6

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In order to explain the improved electrochemical performance of the composite,

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EIS measurement was introduced. The three-dimensional Nyquist plots of Na2Ti3O7

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and Na2Ti3O7/C electrodes after different numbers of cycling at around 1.8 V are

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shown in Fig. 7. The EIS is recorded during 1st to 30th charge/discharge cycles at

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room temperature. The shapes of the Nyquist plots for each cycle are similar. The EIS

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pattern is mainly composed of one semicircle in the high frequency region and a

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sloping line in the low frequency region (see Fig. 7a and b). Nyquist plots are fitted

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with the equivalent circuit model (see Fig. 7c), and the fitted impedance data are listed

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in Table 1. The fitting patterns show that fitting data are in well agreement with

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experimental data as shown in Fig. 7a and b. The equivalent circuit model includes

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the resistance of electrolyte (Rs), a constant phase element (CPE) associated with the

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interfacial resistance, and the semi-circle is correlated with the sodium charge transfer

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14

ACCEPTED MANUSCRIPT resistance at the interface Rct. The linear portion is designated to Warburg impedance

2

(Zw), which is attributed to the diffusion of sodium ion into the bulk of the electrode

3

materials. Rs denotes the solution resistances. It is explicit from Table 1 that the Rs

4

values of Na2Ti3O7/C composite are smaller than those of Na2Ti3O7 at the same

5

number of cycles, implying that the Na2Ti3O7/C composite has lower structural

6

distortion and the suppression of side reaction of electrolyte deterioration. The Rct of

7

Na2Ti3O7 sample is 260 Ω after 1 cycle, and this value increases to 760 Ω after 10

8

cycles, which is consistent with the trend of huge capacity loss in the first 10 cycles.

9

However, the Rct of the Na2Ti3O7/C sample is 122 Ω after 1 cycle, while this value

10

only increases to 172 Ω after 30 cycles. It is well-known that the lower increase of

11

charge transfer resistance during cycling means better cycle performance. These

12

results are consistent with the excellent electrochemical performance of Na2Ti3O7/C

13

composite.

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Fig. 7

16 17

Table 1

4. Conclusions

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In summary, Na2Ti3O7/C composite has been successfully synthesized via a

18

rheological phase method. SEM and TEM measurements have confirmed that the

19

Na2Ti3O7 particle shows a pupa-like structure. The special structure combines all

20

individual Na2Ti3O7 particles together and forms a good network of electrically

21

conductive paths among the Na2Ti3O7 particles. Electrochemical tests show that the

22

discharge capacity of Na2Ti3O7/C composite still remain as high as 72.8 mAh·g-1 after 15

ACCEPTED MANUSCRIPT 100 cycles at the rate of 5 C, while only 37 mAh·g-1 for bare Na2Ti3O7 at the same

2

conditions. The excellent electrochemical performance of Na2Ti3O7/C composite can

3

be ascribed to tight integration of carbon and Na2Ti3O7, which could enhance

4

electronic conductivity, decrease the charge transfer resistance, and improve the

5

electrochemical stability during cycling,

6

charge-discharge tests. All these results suggest that the Na2Ti3O7/C composite can

7

offer promising future for sodium ion battery anode materials.

8

Acknowledgments

confirmed

in CV,

EIS

and

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as

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1

This work is supported financially by the National Natural Science Foundation

10

of China (Grant No. 51202209), Doctoral Fund of Ministry of Education of China

11

(Grant No. 20114301120007), and Hunan Provincial Natural Science Foundation of

12

China (Grant No. 14JJ6017).

13

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Chem. Commun. 49 (2013) 8973.

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[34] H. K. Han, T. Song, J. Y. Bae, L. F. Nazar, H. S. Kim, U. Paik, Energy Environ.

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Sci. 4 (2011) 1986.

2

[35] K. Tang, X. Yu, J. Sun, H. Li, X. Huang, Electrochim. Acta 56 (2011) 4869.

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ACCEPTED MANUSCRIPT Table captions:

2

Table 1 Rs and Rct values of Na2Ti3O7 and Na2Ti3O7/C composite after different

3

cycles in Na half-cells.

4

Figure captions:

5

Fig. 1 Schematic illustration for the formation of Na2Ti3O7/C composite’s

6

microstructure.

7

Fig. 2 XRD patterns of (a) anatase TiO2 spheres, (b) Na2Ti3O7/C composite and

8

Na2Ti3O7. The inset shows the corresponding SEM image of anatase TiO2 spheres.

9

Fig. 3 SEM (a), TEM (c) and HRTEM (e) images of Na2Ti3O7; SEM (b), TEM (d)

10

and HRTEM (f) images of Na2Ti3O7/C composite. The inset of HRTEM (e) shows the

11

corresponding FFT pattern of Na2Ti3O7.

12

Fig. 4 Variation of charge (hollow) and discharge (solid) capacity versus cycle

13

number and charge/discharge profiles (insets) for the (a) Na2Ti3O7 and (b) Na2Ti3O7/C

14

composite at 1 C (178 mA g-1); (c) cycling performance of Na2Ti3O7 and Na2Ti3O7/C

15

composite at 5 C in the range of 0.01 V-2.5 V in Na half-cells.

16

Fig. 5 Rate capability of Na2Ti3O7 and Na2Ti3O7/C composite at 0.5-5 C in the range

17

of 0.01 V-2.5 V in Na half-cells. Charge (hollow) and discharge (solid).

18

Fig. 6 CV plots for (a) Na2Ti3O7 and (b) Na2Ti3O7/C composite electrodes for the

19

second cycle at various scan rates in a voltage window of 0.01–2.5 V.

20

Fig. 7 Three-dimensional Nyquist plots measured for (a) Na2Ti3O7 and (b)

21

Na2Ti3O7/C composite around 1.8 V after different numbers of cycling at 1 C in Na

22

half-cells; (c) the equivalent circuit model.

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cycles in Na half-cells. Rs (Ω)

Rct (Ω)

1st

10th

20th

30th

1st

Na2Ti3O7

13.2

18.2

20.7

25.5

Na2Ti3O7/C

10.8

11.1

11.5

13.6

SC

Samples

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Table 1 Rs and Rct values of Na2Ti3O7 and Na2Ti3O7/C composite after different

20th

30th

260

760

870

1050

122

130

150

172

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10th

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Fig. 1 Schematic illustration for the formation of Na2Ti3O7/C composite’s

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microstructure.

1

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(b)

(a)

10

20

30

50

2θ / degree

70

SC 10

80

20

30

Na2 Ti 3O7

40

2 θ / degree

50

60

006 421

-511 -320 -215 511

-204 401 -313 020 -214 411

Na 2Ti3 O7 JCPDS NO. 31-1329 -310

011 111 300 -112 -203

001 -101 101

215

204

60

116 220

200

40

105 211

004

101

JCPDS NO. 21-1272

200 -102

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Intensity(a. u.)

Intensity(a. u.)

Na2 Ti3O7 /C composites

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Fig. 2 XRD patterns of (a) anatase TiO2 spheres, (b) Na2Ti3O7/C composite and

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Na2Ti3O7. The inset shows the corresponding SEM image of anatase TiO2 spheres.

2

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Fig. 3 SEM (a), TEM (c) and HRTEM (e) images of Na2Ti3O7; SEM (b), TEM (d) and HRTEM (f) images of Na2Ti3O7/C composite. The inset of HRTEM (e) shows the corresponding FFT pattern of Na2Ti3O7.

3

ACCEPTED MANUSCRIPT

400

400

(a)

(b)

2.5

350

2.5

350

0.5

0.0

th

th

th

nd

100 80 50 th 40t h 30th 10 2 1s t

150 0

50

1 00

1 50

2 00

2 50

Special Capacity / mAh g-1

100 50

300

300 250

0

20

0.5

0.0

150

th

50

th

100 th 80 th 50t h 40 30 10t h 2 nd 1 st

0

100

50

100

15 0

2 00

Spe cial Capacit y / mA h g -1

250

300

Na2Ti3O7 /C 40

60

80

0

100

Cycle Number

0

20

40

60

80

100

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(c)

Na2 Ti 3O7

Na2 Ti 3O7 /C

200

Specific Capacity/ mAh g-1

1.0

200

Na2Ti3 O7 0

1.5

SC

200

1.0

+ Votage / V Na /Na

250

1.5

Specific Capacity/ mAh g-1

Votage / V Na+ /Na

Specific Capacity/ mAh g-1

300

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2.0

2.0

150

100

0

0

20

40

TE D

50

60

80

100

Cycle Number

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Fig. 4 Variation of charge (hollow) and discharge (solid) capacity versus cycle number and charge/discharge profiles (insets) for the (a) Na2Ti3O7 and (b) Na2Ti3O7/C

AC C

composite at 1 C (178 mA g-1); (c) cycling performance of Na2Ti3O7 and Na2Ti3O7/C composite at 5 C in the range of 0.01 V-2.5 V in Na half-cells.

4

ACCEPTED MANUSCRIPT

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350

Na2 Ti3O7 Na2 Ti3O7 /C

200

0.5 C

150

2C

100

0.5 C

5C

50 0

1C

SC

250

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Specific Capacity/ mAh g -1

300

0

10

20

30

40

50

60

TE D

Cycle Number

Fig. 5 Rate capability of Na2Ti3O7 and Na2Ti3O7/C composite at 0.5-5 C in the range

AC C

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of 0.01 V-2.5 V in Na half-cells. Charge (hollow) and discharge (solid).

5

0.2 mV/s 0.4 mV/s 0.6 mV/s

1.0

0.5

Current/A g-1

0.5

(b)

SC

(a)

0.0

-0.5

0.0

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0.2 mV/s 0.4 mV/s 0.6 mV/s 0.8 mV/s 1.0 mV/s 1.2 mV/s

-0.5

-1.0

-1.0

0.0

0.5

1.0

1.5

Voltage/V vs. Na+/Na

2.0

2.5

0.0

0.5

1.0

1.5

Voltage/V vs. Na +/Na

2.0

2.5

TE D

Fig. 6 CV plots for (a) Na2Ti3O7 and (b) Na2Ti3O7/C composite electrodes for the

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second cycle at various scan rates in a voltage window of 0.01–2.5 V.

6

ACCEPTED MANUSCRIPT

2000

500

(a)

(b)

Ω) -Z''(Ω

Ω) -Z' '(Ω

1000

500

300 200 100

1 num 20 ber

30

0

500 400

0 1 Cy 10 cle num 20 ber

100

300 200 ' (Ω ) Z

30 0

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2000 1500 1000 ) 500 ' (Ω Z

SC

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400

1500

Fig. 7 Three-dimensional Nyquist plots measured for (a) Na2Ti3O7 and (b)

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Na2Ti3O7/C composite around 1.8 V after different numbers of cycling at 1 C in Na

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half-cells; (c) the equivalent circuit model.

7

ACCEPTED MANUSCRIPT  Na2Ti3O7/C composite was firstly investigated.  Na2Ti3O7/C composite showed much better electrochemical performance than bare Na2Ti3O7.

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 Na2Ti3O7/C composite is promising anode materials for sodium ion batteries.

ACCEPTED MANUSCRIPT

Supporting Information for A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries

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Zichao Yan, Li Liu*, Hongbo Shu, Xiukang Yang, Hao Wang, Jinli Tan, Qian Zhou,

SC

Zhifeng Huang, Xianyou Wang*

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Specific Capacity/ mAh g-1

150

100

0

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50

0

20

40

60

80

1C 5C

100

EP

Cycle Number

Fig. S1 Variation of charge (hollow) and discharge (solid) capacity versus cycle

AC C

number at 1 C and 5 C for pure carbon black in the range of 0.01 V-2.5 V in Na half-cells.

*

Corresponding author. Tel.: +86-731-58292206;Fax: +86-731-58292477

E-mail addresses: [email protected] (L. Liu) ; [email protected] (X.Y. Wang)

ACCEPTED MANUSCRIPT 250

(b)

2.5

200 2.0

Votage / V Na +/Na

150

100

1.5

1.0

0.5

50 Na2Ti3O7 0

0

10

0.0 20

30

40

50

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Specific Capacity/ mAh g-1

(a)

50th 30th 0

50

20th

100

10th

2nd

150

200

Special Capacity / mAh g-1

Cycle Number

SC

Fig. S2 (a) Variation of charge (hollow) and discharge (solid) capacity versus cycle number and (b) charge/discharge profiles for Na2Ti3O7 at 0.2 C in the range of 0.01

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V-2.5 V in Na half-cells.

1st 2nd 100th

on

1.0

Ex tra c ti

1.5

EP

Votage / V Na+/Na

2.0

TE D

2.5

Insertion

0.5

AC C

0.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

mols of Na reacted / mol Na2Ti3O7

Fig. S3 Voltage versus the insertion/extraction amount of Na in Na2Ti3O7/C composite during different cycles in the range of 0.01 V-2.5 V in Na half-cells.