Synthesis of Na2Ti6O13 nanorods as possible anode materials for rechargeable lithium ion batteries

Synthesis of Na2Ti6O13 nanorods as possible anode materials for rechargeable lithium ion batteries

Accepted Manuscript Title: Synthesis of Na2 Ti6 O13 nanorods as possible anode materials for rechargeable lithium ion batteries Author: Peng Li Pengfe...

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Accepted Manuscript Title: Synthesis of Na2 Ti6 O13 nanorods as possible anode materials for rechargeable lithium ion batteries Author: Peng Li Pengfei Wang Shangshu Qian Haoxiang Yu Xiaoting Lin Miao Shui Xi Zheng Nengbing Long Jie Shu PII: DOI: Reference:

S0013-4686(15)30827-6 http://dx.doi.org/doi:10.1016/j.electacta.2015.11.057 EA 26057

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

22-6-2015 9-11-2015 9-11-2015

Please cite this article as: Peng Li, Pengfei Wang, Shangshu Qian, Haoxiang Yu, Xiaoting Lin, Miao Shui, Xi Zheng, Nengbing Long, Jie Shu, Synthesis of Na2Ti6O13 nanorods as possible anode materials for rechargeable lithium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.11.057 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.

Synthesis of Na2Ti6O13 nanorods as possible anode materials for rechargeable lithium ion batteries

Peng Li, Pengfei Wang, Shangshu Qian, Haoxiang Yu, Xiaoting Lin, Miao Shui, Xi Zheng, Nengbing Long, Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang, China * Corresponding author: Jie Shu Tel.: +86-574-87600787 Fax: +86-574-87609987 E-mail: [email protected]

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Graphical abstract 600

potentiostatic charge

Intensity/ a.u.

charge

400 300 200

discharge

Time/ min

500

100 0

3

2

1

0

15

20

25

30

2/

+

Potential vs. (Li/Li )/ V

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35

40

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Highlights 

Na2Ti6O13 nanorods are prepared by a traditional solid state reaction.



Na2Ti6O13 nanorods show the potential as lithium storage materials.



Na2Ti6O13 nanorods reveal slight volume expansion during cycles.



In-situ XRD proves the structural and cyclic stability of Na2Ti6O13 nanorods.

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Abstract In this work, Na2Ti6O13 nanorods are prepared by a traditional solid state reaction and reported as anode materials for advanced lithium-ion batteries. The effect of calcining temperature on the size and electrochemical behavior of nanorodes is thoroughly described and compared within the temperature range of 800-1000 oC. It can be found that the size of nanorodes increases with the enhancing of calcining temperature. At 1000 oC, Na2Ti6O13 nanorodes melt into big bulks, which exhibit poor ionic conductivity and low lithium storage capacity. Although Na2Ti6O13 nanorodes with smaller size can be formed at 800 oC, its capacity retention is poor. In contrast, Na2Ti6O13 nanorodes obtained at 900 oC reveal high reversible capacity, rapid lithium ion diffusion behavior and outstanding rate property. In-situ and ex-situ analyses reveal that the structural evolution of Na2Ti6O13 during lithiation and delithiation process is quasi-reversible, which ensures the excellent electrochemical performance of Na2Ti6O13 nanorods for repeated lithium storage. Keywords: Na2Ti6O13; Anode material; Electrochemical behavior; In-situ X-ray diffraction; Lithium ion batteries.

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1. Introduction In the past few years, great attention has been paid on rechargeable lithium-ion batteries, mainly because of their highly efficient, flexible, portable and environmentally friendly technologies for energy storage [1, 2]. However, the traditional anode material graphite always suffers from serious safety problem when high charge/discharge rate are required. With the rapid development of electric vehicles and hybrid electric vehicles, it is of great demand to develop high power density, rechargeability, and safety anode materials to be used in high-power lithium-ion batteries [3-6]. Among previous reported materials, alkali and alkaline earth titanates have received increasingly interest due to their distinct structures, outstanding chemical stability and high ion conductivity [7-13]. The alkali titanates with a general formula A2O·nTiO2 (3≤n≤8, A = H, Li, Na, K) crystallize in a monoclinic structure. With a low alkali metal content (n=6-8), they show a tunnel structure and exhibit a good chemical stability [13]. Among the alkali titanates, Na2Ti6O13 has been widely investigated in the previous reports and presents excellent properties as photocatalytic material [14-18]. Several years ago, Na2Ti6O13 was used as lithium storage material for the first time [19-21]. The preliminary results reveal that the structure of Na2Ti6O13 can accommodate 3.0 Li per formula, corresponding to the electrochemical reaction mechanism of two solid-solution processes and a biphasic transition between 1.0 and 2.0 V [19]. Recently, J.C. Pérez-Flores et al also reported the structure and electrochemical behavior of Na2Ti6O13 [20, 21]. It is found that the 2c and 4i vacant sites within the hexatitanate

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unit channel allow for the insertion of 3.0 Li per formula in Na2Ti6O13. However, the actual observed insertion capacity is only 2.0 Li per formula in their experiment [22]. For comparison, Na2Ti6O13 nanotubes prepared by H. Zhang et al can deliver a lithium storage capacity of 3.0 Li per formula in the potential range of 1.0-2.5 V [21]. Actually, the maximum theoretical capacity of Na2Ti6O13 can reach 297 mAh g-1 based on the total reduction of six Ti4+ ions into six Ti3+ ions (6.0 Li per formula) in the structure, which may be similar with the electrochemical reaction between Li4Ti5O12 and Li4Ti9O12 in a broad working window (0.0-3.0 V) [24-26]. Therefore, it is interesting to present an investigation on the maximum lithium storage capacity and its reaction mechanism of Na2Ti6O13 between 0.0 and 3.0 V. In this work, Na2Ti6O13 nanorods are fabricated by a simple high temperature solid state reaction at 800, 900 and 1000 oC, and the best calcining temperature is verified according to the electrochemical measurements in the potential range of 1.0-3.0 V. Furthermore, we also make a thorough investigation on the structural evolution and lithium storage mechanism of Na2Ti6O13 nanorods in the potential range of 0.0-3.0 V by galvanostatic cycle, cyclic voltammetry, ex-situ high resolution transmission electron microscope (HRTEM) and in-situ X-ray diffraction (XRD) methods.

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2. Experimental The Na2Ti6O13 nanorods were prepared via a conventional high temperature solid state reaction [27]. All the chemical reagents were analytical grade in the experiment. C2H2O4·2H2O (Aladdin Chemical, 99 %), C2H3O2Na·3H2O (Aladdin Chemical, 99 %) and TiO2 (Aladdin Chemical, 99 %) in a molar ratio of 1:1:3 were mixed with ethanol and ground in the planetary ball mill for 12 h. The obtained mixture was dried at 80 o

C for 24 h, and the resultant specimen was re-ground and transferred into muffle

furnace. This precursor was progressively heated up to 600 oC to decompose the oxalate salts and then calcined at the temperatures varying from 800 to 1000 oC for 10 h in air atmosphere. After cooling down to room temperature naturally, the final product was formed. In the following section, the samples formed at 800, 900 and 1000 oC were named as Na2Ti6O13-800, Na2Ti6O13-900 and Na2Ti6O13-1000, respectively. The phase purity and crystal structure of the obtained samples were characterized by powder X-ray diffraction using a Bruker D8 diffractometer (Cu-K radiation, =1.5406 Å). The particle morphology was verified by Hitachi S4800 scanning electron microscopy (SEM) analysis. Crystal symmetry was in addition studied by means of high resolution transmission electron microscope and selected area electron diffraction (SAED) images recorded on a JEOL JEM-2010 electron microscope. Electrochemical lithium insertion and extraction experiments were performed by CR2032 coin-type lithium half-cells. The working electrode was prepared by pasting a homogeneous slurry containing 80 wt.% Na2Ti6O13-800 (Na2Ti6O13-900 or

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Na2Ti6O13-1000) active material, 10 wt.% carbon black conductive addictive, and 10 wt.% polyvinylidene difluoride binder dispersed in N-methyl-2-pyrrolidone on a Cu foil and subsequently dried in vacuum oven at 120 oC for 12 h. Then, the active material foil was cut into discs with a diameter of 15 mm. The coin cells were assembled in an argon-filled glove box with the as-prepared film as the working electrode, Li metal foil as the counter electrode, glass fiber as the separator and 1 mol L-1 LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 in volume) as the electrolyte. Galvanostatic discharge/charge (Li insertion and extraction) experiments were carried out in the potential range of 1.0-3.0 and 0-3.0 V with a multichannel Land CT2001A battery test system. Cyclic voltammetry was performed at a scan rate of 0.1 mV s-1 on a CHI 1000B electrochemical workstation. Electrochemical impedance spectroscopy (EIS) measurements were carried out using an impedance analyzer CHI 660D electrochemical workstation in the frequency range from 0.01 to 100000 Hz. All the above electrochemical tests were carried out in a constant temperature cabinet (25 o

C).

3. Results and discussion Fig. 1 shows the XRD patterns of Na2Ti6O13-800, Na2Ti6O13-900 and Na2Ti6O13-1000. It is obvious that the patterns of Na2Ti6O13 formed at 900 and 1000 o

C are extremely similar with that of Na2Ti6O13-800 (Fig. S1 in Supplementary

Materials). Seen from the XRD patterns, strong diffraction peaks at 11.84º, 14.09º,

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24.50º, 26.75º, 29.83º, 32.16º, 33.45º, 35.87º, 38.02º, 43.32º, 44.21º and 48.57º are in good agreement with the (200), (20-1), (110), (111), (310), (112), (402), (60-1), (11-3), (40-4), (602) and (020) planes in the JCDPS card No.14-0277 representing the compound of Na2Ti6O13. Furthermore, three refined XRD patterns as depicted in Fig. 1 also show no obvious impurity phases in the as-obtained samples. Refined lattice parameters for the three samples are listed in Table 1. As can be seen in Table 1, the lattice parameters of the three samples are practically the same regardless of the different synthetic temperatures. These lattice parameters are in agreement with the previous reports with only little deviation in  and V parameters [19]. Besides, the detailed refined data for atom positions are also shown in Tables S1-S3 (Supplementary Materials). The SEM images of the Na2Ti6O13 samples calcined at different temperatures are presented in Fig. 2. As depicted in Fig. 2a and 2b, the rod-like products formed at 800 o

C are well distributed in the image with the length of 200-500 nm and the width of

50-100 nm. In Fig. 2c and 2d, the sample calcined at 900 oC also consists of plenty of rods with the length of 200-700 nm and the diameter of 100 nm. While in Fig. 2e and 2f, the bulks of Na2Ti6O13-1000 reveal larger size than the previous two samples obtained at lower temperature, with the length of 0.5-2.0 μm and the width of 0.5-1.0 μm. It suggests that the size of Na2Ti6O13 increases with the enhancing of calcining temperature. Fig. 3a-3c depicts the discharge (Li insertion) and charge (Li extraction) profiles for Na2Ti6O13-800, Na2Ti6O13-900, and Na2Ti6O13-1000 samples in the potential range

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between 1.0 and 3.0 V at a constant current density of 50 mA g-1. The initial discharge and charge capacities for Na2Ti6O13-800, Na2Ti6O13-900, and Na2Ti6O13-1000 samples are 174/72.8 mAh g-1, 178.6/76.1 mAh g-1 and 107.1/28.7 mAh g-1 with the corresponding first coulombic efficiencies of 41.84 %, 42.61 % and 27.57 %, respectively. It can be seen that lithium insertion into Na2Ti6O13-800, Na2Ti6O13-900, and Na2Ti6O13-1000 samples produces a flat plateau at 1.35 V and a long slope between 1.0 and 1.2 V in Fig. 3a-3c. During the reverse Li extraction process, two platforms appear at 1.19 and 1.40 V. It indicates that the lithium storage behavior of Na2Ti6O13 is quasi-reversible. Fig. 3d shows the cycling performance for Na2Ti6O13-800, Na2Ti6O13-900, and Na2Ti6O13-1000 samples operated in the potential range between 1.0 and 3.0 V at a constant current density of 50 mA g-1. As can be seen, Na2Ti6O13-800 shows poor capacity retention (62.6 %) with a low reversible charge capacity of 45.6 mAh g-1 after 50 cycles. The large capacity loss of Na2Ti6O13-800 may be attributed to the formation of lattice imperfection during solid state crystallization at a low temperature of 800 oC, which induces the structural instability after plenty of lithium ions insertion between 1.0 and 1.2 V. Similar phenomenon (28.6 % capacity loss after 15 cycles) can be found in the previous reported by R. Dominko [19]. Although Na2Ti6O13-1000 delivers low initial charge capacity, it reveals superior capacity retention (92.3 %) after 50 cycles. In contrast, Na2Ti6O13-900 not only maintains a high charge capacity of 64.9 mAh g-1 after 50 cycles, but also presents good capacity retention of 85.3 %. The rate performance of Na2Ti6O13-800, Na2Ti6O13-900 and Na2Ti6O13-1000

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samples between 1.0 and 3.0 V is shown and compared in Fig. 3e. Corresponding discharge/charge curves are also presented in Fig. S2 (Supplementary Materials). It can be found that Na2Ti6O13-900 exhibits better high rate performance than that of Na2Ti6O13-800 and Na2Ti6O13-1000. For Na2Ti6O13-800, it can deliver the charge capacity of 54.8 mAh g-1 at 100 mA g-1, 44.1 mAh g-1 at 150 mA g-1, 36.9 mAh g-1 at 200 mA g-1, 32.4 mAh g-1 at 250 mA g-1 and 28.5 mAh g-1 at 300 mA g-1. Among the three samples, Na2Ti6O13-1000 shows the poorest rate performance with the charge capacity of 26.4 mAh g-1 at 100 mA g-1, 24.0 mAh g-1 at 150 mA g-1, 22.7 mAh g-1 at 200 mA g-1, 21.1 mAh g-1 at 250 mA g-1 and 19.6 mAh g-1 at 300 mA g-1. In contrast, Na2Ti6O13-900 can present higher charge capacity of 58.0 mAh g-1 at 100 mA g-1, 48.9 mAh g-1 at 150 mA g-1, 43.7 mAh g-1 at 200 mA g-1, 40.0 mAh g-1 at 250 mA g-1 and 37.2 mAh g-1 at 300 mA g-1. The superior rate performance of Na2Ti6O13-900 is consistent with the excellent cycling property as discussed above (Fig. 3d). Fig. 4a-4c displays the representative cyclic voltammograms of three Na2Ti6O13 samples recorded at a scan rate of 0.1 mV s-1 between 1.0 and 3.0 V. Two pairs of separated redox peaks (1.04/1.29 V and 1.33/1.41 V) can be clearly observed for Na2Ti6O13-800, Na2Ti6O13-900, and Na2Ti6O13-1000 samples, suggesting that lithium ions probably occupy at two different vacant sites in the potential range between 1.0 and 1.5 V. The appearance of these redox peaks is in good accordance with the discharge/charge platforms in Fig. 3 and characteristic electrochemical behavior of Na2Ti6O13 in the previous reports [9, 19]. In order to further investigate the electrode kinetics, the apparent electrochemical parameters of Na2Ti6O13 electrodes are

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calculated from EIS patterns. Fig. 4d depicts the Nyquist plots of Na2Ti6O13-800, Na2Ti6O13-900, and Na2Ti6O13-1000 samples before cycles. The corresponding equivalent circuit is presented as an inset graph. All the three EIS curves are composed of a depressed semicircle in the high frequency region and an inclined line in the low frequency region. The semicircle in the high frequency region represents the charge-transfer process, while the oblique line in the low frequency region reflects the lithium ion diffusion process. As illustrated by equivalent circuit in Fig. 4d, Rs is the resistance of the electrolyte, Rct refers the charge transfer resistance at the particle/electrolyte interface, and ZW means the Warburg impedance. According to the fitting results by ZSimpWin software, the charge transfer resistance (Rct) of Na2Ti6O13-900 is 6.63 Ω, which is lower than 6.98 and 7.67 Ω for Na2Ti6O13-800 and Na2Ti6O13-1000. This lower charge transfer resistance further proves that Na2Ti6O13-900 sample has a better kinetic behavior. Moreover, the lithium ion diffusion coefficient (DLi) of Na2Ti6O13 could be calculated from the low frequency plots according to the following equations [28, 29].

D=

R 2T 2 2 A2 n 4 F 4C 2 2

Z'=R s +R ct +

-

1 2

(1)

(2)

where R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of electrons transferred in the half-reaction for the redox couple, F is the Faraday constant, C is the concentration of lithium ion in solid, ω is the angular frequency and σ is the Warburg factor which has a relationship with Z’. The Z’ versus ω-1/2 plots and corresponding calculated values of DLi are presented in 12

Fig. 4e and 4f, respectively. It is obvious that Na2Ti6O13-900 shows the highest lithium diffusion coefficient (6.98×10-15 cm2 s-1) among all the three samples (3.35 ×10-15 cm2 s-1 for Na2Ti6O13-800 and 9.48×10-16 cm2 s-1 for Na2Ti6O13-1000), which is in consistent with its outstanding cycling performance as shown in Fig. 4. To check the high structural and cycling stabilities, Na2Ti6O13-900 nanorods are studied in a broad potential window (0.0-3.0 V). Fig. 5 gives the discharge/charge curves, cycling property curves, cyclic voltammograms and rate performance curves of Na2Ti6O13-900 nanorods. Seen from Fig. 5a, Na2Ti6O13-900 nanorods reveal another long slope below 1.0 V except for the lithiation platforms at high potentials during the discharge process. Furthermore, another long slope also appears in the reverse charge process between 0.0 and 1.2 V, which is in consistent with the appearance of oxidation peak between 0.0 and 0.5 V in the cyclic voltammograms as shown in Fig. 5b. With a careful observation in Fig. 5c, it is clear that Na2Ti6O13-900 nanorods exhibit the initial discharge/charge capacities of 556.2/285.1 mAh g-1 with its first columbic efficiency of 51.3 % in the potential range of 0.0-3.0 V at a constant current density of 50 mA g-1. After 50 cycles, Na2Ti6O13-900 nanorods can still keep the discharge/charge capacities of 166.6/158.7 mAh g-1 with the capacity retention of 55.7 %. The rate performance of Na2Ti6O13-900 nanorods in 0.0-3.0 V is also shown in Fig. 5d. It can be found that the rate performance of Na2Ti6O13-900 nanorods is evaluated at different current densities from 50 to 300 mA g-1. Viewed from Fig. 5d, Na2Ti6O13-900 nanorods can deliver the reversible charge capacities of 198.6 mAh g-1 at 100 mA g-1, 178.4 mAh g-1 at 150 mA g-1, 165.8 mAh g-1 at 200 mA g-1, 151.8 mAh

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g-1 at 250 mA g-1 and 149.3 mAh g-1 at 300 mA g-1. Therefore, Na2Ti6O13-900 nanorods not only display outstanding high-rate capability but also reveal excellent cyclic stability. For further exploring the insertion/extraction behavior of Na2Ti6O13-900 nanorods, in-situ XRD measurements are taken in the potential range of 0.0-3.0 V. As can be seen in Fig. 6 and Fig. S3-S5 (Supplementary Materials), the overall XRD patterns reveal clear diffraction peak changes and phase transitions in the potential range of 0.0-3.0 V, especially for the angle ranges in 11-20º, 23.5-25.5º, 28-34º and 42.5-45º. In addition, the in-situ XRD patterns with reduced number are also presented in Fig. S6 (Supplementary Materials). From the observations in Fig. 7, some of the diffraction peaks for the starting phase progressively vanish during the discharge process, and all of them can reappear in the reverse charge process. In Fig. 7a and 7b, the diffraction peaks in the angle range of 11-20º have no obvious changes in their relative intensity but the diffraction peak at 13.8º shows a slight shift to higher angles. Fig. 7c-7d gives a clear presentation that the diffraction peak at 24.3º diminishes gradually upon the discharge process and recovers to the previous Bragg position after a full charge process. Furthermore, a new diffraction peak appears at 23.5º during a discharge process to 0.0 V and then disappears in reverse charge process. It indicates that a new lithiated phase forms due to lithium ions occupation the vacant sites in Na2Ti6O13-900 nanorods. Similar evolutions of (310), (112) and (402) peaks can also be observed in Fig. 7e-7f. For comparison, the changes of (40-4) and (602) peaks are quite different from those mentioned above, which gradually shift

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towards lower angles during the discharge process and move back to their pristine Bragg positions after a full charge process (Fig. 7g and 7h). However, the relative intensities, shapes and positions of all the Bragg peaks cannot restore completely after the discharge/charge cycle, which suggests that the electrochemical reaction between Na2Ti6O13-900 nanorods and Li is quasi-reversible in 0.0-3.0 V. In the previous investigation, J.C. Pérez-Flores et al ever reported the lithium storage mechanism in Na2Ti6O13 between 1.0 and 3.0 V [22, 23], which is associated with the lithium ions insertion at the 2c and 4i vacant sites within a hexatitanate unit channel. In this work, it can be found that the structure of Na2Ti6O13 can still accommodate lithium ions below 1.0 V (Fig. 5). Therefore, additional vacant sites should be available for lithium storage. As shown in Fig. 8a, the basic structure of Na2Ti6O13 is the (Ti6O13)2- skeleton arrangement, which consists of corrugated chains of three edge- and corner-shared TiO6 octahedra assembled in such a way to form 3×1 tunnels along c-axis. After the 2c (0.5, 0.5, 0.5) and 4i (0.548, 0.5, 0.8088) sites fully occupied by lithium ions at high potentials (>1.0 V), the 2d (0.5, 0, 0.5) and another 4i (0.391, 1, 0.537) vacant sites are proposed to hold lithium ions at low potentials (<1.0 V) as shown in Fig. 8b. The lithiated crystal structure and corresponding Rietveld refinement result are also displayed in Fig. S7 and Table S1 (Supplementary Materials). This lithiation process at low potentials is similar with the lithium insertion behavior in Li4Ti5O12 at low potentials (<1.0 V) [24-26]. By using FullProf software, the lattice parameters of various lithiated and delithiated Na2Ti6O13-900 products are refined during the whole discharge/charge

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process and the calculated variations are presented in Fig. 9a-9d. Seen from Fig. 9a, we can find that the value of a parameter reveals no obvious changes and remains at about 1.5076 nm upon the whole charge/discharge process. In contrast, both b and c parameters show a pronounced increase during the discharge process and then gradually decrease during the reverse charge process (Fig. 9b-9c). As a result, the cell volume of Na2Ti6O13-900 nanorods also experiences an increase from 0.5085 to 0.5225 nm3 as depicted in Fig. 9d, which indicates that the volume expansion of Na2Ti6O13-900 nanorods is 2.75 % after a full lithiation. Upon a charge process to 3.0 V, the cell volume of delithiated Na2Ti6O13-900 nanorods is calculated to be 0.5122 nm3, which is close to that of the pristine material. It indicates that the structural change of Na2Ti6O13-900 is quasi-reversible. The expansion and shrinkage of Na2Ti6O13 lattice are also observed by ex-situ HRTEM technique as shown in Fig. S8-S10 (Supplementary Materials). For the (20-1) plane in ex-situ HRTEM observation, its pristine lattice spacing is 0.6345 nm, and this lattice spacing increases to 0.6372 nm after a full lithiation. Upon a reverse delithiation, it decreases to the value of 0.6313 nm. The quasi-reversible change of lattice spacing for (20-1) plane is in good accordance with the evolution of lattice parameters as revealed by in-situ XRD. This further demonstrates the high structural stability of Na2Ti6O13-900 nanorods. Thus, Na2Ti6O13-900 nanorods are structural stability materials for lithium storage.

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4. Conclusion In summary, we fabricate Na2Ti6O13 nanorods by a simple high temperature solid state reaction at 800, 900 and 1000 oC. Morphology analysis and electrochemical evaluation suggest that the best calcining temperature for Na2Ti6O13 nanorods is 900 o

C. Na2Ti6O13 nanorods formed at 900 oC not only can reveal a high lithium diffusion

coefficient (6.98×10-15 cm2 s-1), but also can deliver high charge capacities of 198.6 mAh g-1 at 100 mA g-1, 178.4 mAh g-1 at 150 mA g-1, 165.8 mAh g-1 at 200 mA g-1, 151.8 mAh g-1 at 250 mA g-1 and 149.3 mAh g-1 at 300 mA g-1 in the potential range of 0.0-3.0 V. In-situ XRD and ex-situ HRTEM results prove that Na2Ti6O13 nanorodes have high structural stability with only 2.75 % of volume expansion after a full lithiation. In addition, a probable lithium storage mechanism in Na2Ti6O13 is also proposed according to the Rietveld refinement results. All the evidences reveal that Na2Ti6O13 nanorodes can be used as anode candidates for lithium-ion batteries.

Acknowledgement This work is sponsored by Ningbo Key Innovation Team (2014B81005) and Ningbo Natural Science Foundation (2014A610042). The work is also supported by K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data associated with this article can be found at the end of this submission.

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flux at a relatively low temperature, European Journal of Inorganic Chemistry 2010 (2010) 2936-2940. 15 L. Zhen, C.Y. Xu, W.S. Wang, C.S. Lao, Q. Kuang, Electrical and photocatalytic properties of Na2Ti6O13 nanobelts prepared by molten salt synthesis, Applied Surface Science 255 (2009) 4149-4152. 16 K. Teshima, K. Yubuta, T. Shimodaira, T. Suzuki, M. Endo, T. Shishido, S. Oishi, Environmentally friendly growth of highly crystalline photocatalytic Na2Ti6O13 whiskers from a NaCl flux, Crystal Growth & Design 8 (2008) 465-469. 17 C.Y. Xu, J. Wu, P. Zhang, S.P. Hu, J.X. Cui, Z.Q. Wang, Y.D. Huang, L. Zhen, Molten salt synthesis of Na2Ti3O7 and Na2Ti6O13 one-dimensional nanostructures and their photocatalytic and humidity sensing properties, CrystEngComm 15 (2013) 3448-3454. 18 C. Liu, T. Sun, L. Wu, J. Liang, Q. Huang, J. Chen, W. Hou, N-doped [email protected] core-shell nanobelts with exposed {101} anatase facets and enhanced visible light photocatalytic performance, Applied Catalysis B: Environmental 170-171 (2015) 17-24. 19 R. Dominko, E. Baudrin, P. Umek, D. Arcon, M. Gaberscek, J. Jamnik, Reversible lithium insertion into Na2Ti6O13 structure, Electrochemistry Communications 8 (2006) 673-677. 20 R. Dominko, L. Dupont, M. Gaberscek, J. Jamnik, E. Baudrin, Alkali hexatitanates-A2Ti6O13 (A=Na, K) as host structure for reversible lithium insertion, Journal of Power Sources 174 (2007) 1172-1176.

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21 H. Zhang, X.P. Gao, G.R. Li, T.Y. Yan, H.Y. Zhu, Electrochemical lithium storage of sodium titanate nanotubes and nanorods, Electrochimica Acta 53 (2008) 7061-7068. 22 J.C. Pérez-Flores, F. García-Alvarado, M. Hoelzel, I. Sobrados, J. Sanz, A. Kuhn, Insight into the channel ion distribution and influence on the lithium insertion properties of hexatitanates A2Ti6O13 (A=Na, Li, H) as candidates for anode materials in lithium-ion batteries, Dalton Transactions 41 (2012) 14633-14642. 23 J.C. Pérez-Flores, M. Hoelzel, A. Kuhn, F. García-Alvarado, On the mechanism of lithium insertion into A2Ti6O13 (A=Na, Li), ECS Transactions 41 (2012) 195-206. 24 Z.Y. Zhong, C.Y. Ouyang, S.Q. Shi, M.S. Lei, Ab initio studies on Li4+xTi5O12 compounds as anode materials for lithium-ion batteries, ChemPhysChem 9 (2008) 2104-2108. 25 H. Ge, N, Li, D.Y. Li, C.S. Dai, D.L. Wang, Study on the theoretical capacity of spinel lithium titanate induced by low-potential intercalation, The Journal of Physical Chemistry C 113 (2009) 6324-6326. 26 T.F. Yi, Y. Xie, Y.R. Zhu, R.S. Zhu, H. Shen, Structural and thermodynamic stability of Li4Ti5O12 anode material for lithium-ion battery, Journal of Power Sources 222 (2013) 448-454. 27 J.C. Pérez-Flores, A. Kuhn, F. García-Alvarado, Synthesis, structure and electrochemical Li insertion behaviour of Li2Ti6O13 with the Na2Ti6O13 tunnel-structure, Journal of Power Sources 196 (2011) 1378-1385.

21

28 T.F. Yi, S.Y. Yang, M. Tao, Y. Xie, Y.R. Zhu, R.S. Zhu, Synthesis and application of a novel Li4Ti5O12 composite as anode material with enhanced fast charge/discharge performance for lithium-ion battery, Electrochimica Acta 134 (2014) 377-383. 29 T.F. Yi, S.Y. Yang, Y.R. Zhu, M.F. Ye, Y. Xie, R.S. Zhu, Enhanced rate performance of Li4Ti5O12 anode material by ethanol-assisted hydrothermal synthesis for lithium-ion battery, Ceramics International 40 (2014) 9853-9858.

22

Na2Ti6O13 (Sim)

(a) Na2Ti6O13-800

20

30

40

50

60

70

10

2 / 

30

40

50

60

70

2 / 

Na2Ti6O13 (Exp)

Na2Ti6O13-1000

Difference Observed Reflections

Intensity/ a.u.

20

20

Difference Observed Reflections

Na2Ti6O13 (Sim)

(c)

10

Na2Ti6O13 (Exp)

Na2Ti6O13-900

Intensity/ a.u.

Difference Observed Reflections

Intensity/ a.u. 10

Na2Ti6O13 (Sim)

(b)

Na2Ti6O13 (Exp)

30

40

50

60

70

2 / 

Fig. 1. XRD patterns of Na2Ti6O13 obtained at 800 oC (a), 900 oC (b) and 1000 oC (c).

23

Fig. 2. SEM images of Na2Ti6O13 nanorods obtained at 800 oC (a, b), 900 oC (c, d) and 1000 oC (e, f).

24

(a) Na2Ti6O13-800

2.5

1st 2nd 3rd 4th 5th

2.0 1.5 1.0 0

50

100

150

(b)

3.0

Potential vs. (Li/Li+)/ V

Potential vs. (Li/Li+)/ V

3.0

Na2Ti6O13-900

2.5

1st 2nd 3rd 4th 5th

2.0 1.5 1.0 0

200

50

Specific capacity/ mAh g-1

Potential vs. (Li/Li )/ V

+

Charge capacity/ mAh g-1

(c)

3.0

Na2Ti6O13-1000

2.5

1st 2nd 3rd 4th 5th

2.0 1.5 1.0 0

30

60

90

120

Charge capacity/ mAh g-1

100

150

200

Na2Ti6O13-800

(d)

Na2Ti6O13-900

80

Na2Ti6O13-1000 60 40 20 0

Specific capacity/ mAh g-1

10

20

30

40

50

Cycle number

(e)

80 60 40

100

Specific capacity/ mAh g-1

200 mA g-1 250 mA g-1 300 mA g-1

50 mA g-1 100 mA g-1 150 mA g-1

20

Na2Ti6O13-800

50 mA g-1

Na2Ti6O13-900

0

Na2Ti6O13-1000 0

10

20

30

40

50

60

70

Cycle number

Fig. 3. Galvanostatic discharge/charge curves (a-c), cycling performance (d) and rate performance (e) of Na2Ti6O13 nanorods obtained at different calcining temperatures.

25

0.05

(a) 0.0

first cycle second cycle third cycle

-0.1

-0.2 1.0

1.5

2.0

(b)

0.00

Na2Ti6O13-800

2.5

Current/ A

Current/ A

0.1

-0.05

Na2Ti6O13-900

-0.10

first cycle second cycle third cycle

-0.15 -0.20 -0.25

3.0

1.0

1.5

15

Rct

0.00

Na2Ti6O13-1000 first cycle second cycle third cycle

-0.15

1.0

1.5

2.0

2.5

-Z''/ ohm

Current/ A

12

-0.10

RS

CPE

9 o

800 C o 900 C o 1000 C

6 3 0

3.0

3

6

Potential vs. Li/Li+/ V

Z'/ ohm

(f) Na2Ti6O13-800 Na2Ti6O13-900 Na2Ti6O13-1000

1200 800 400 0 4

12

15

18

80

(e)

1600

9

Z'/ ohm

D-Li/10^-16)/ cm2S-1

2000

3.0

(d)

(c)

-0.05

2.5

Potential vs. (Li/Li )/ V

Potential vs. (Li/Li )/ V

0.05

2.0

+

+

5

6

7

8

9

60

40

33.502

20 9.477 0

10

69.803

800

-1/2/ s1/2

900

1000

Temperature/ oC

Fig. 4. Cyclic voltammograms (a-c), EIS patterns (d), the relationship between Z’ and ω-1/2 in low frequency region (e), and the lithium diffusion coefficient (f) of Na2Ti6O13 nanorods obtained at different calcining temperatures.

26

(a)

0.0

Na2Ti6O13-900

2

st

1 2nd 3rd 4th 5th

1

(b)

0.1

Current/ A

Potential vs. (Li/Li+)/ V

3

Na2Ti6O13-900

-0.1 -0.2

first cycle second cycle third cycle

-0.3 -0.4

0 0

100

200

300

400

500

600

0

1

Specific capacity/ mAh g-1

2

3

Potential vs. (Li/Li+)/ V

600

(c)

Capacity/ mAh g

-1

-1

Na2Ti6O13-900

400

charge capacity discharge capacity

300 200 100 0

0

10

20

30

40

(d)

600

Capacity/ mAh g

500

50mA g-1 100mA g-1 50mA g-1 150mA g-1 300mA g-1 200mA g-1 250mA g-1

400 300 200 100 0

50

Na2Ti6O13-900

500

0

10

20

30

40

50

60

70

Cycle number

Cycle number

Fig. 5. Galvanostatic discharge/charge curves (a), cyclic voltammograms (b), cycling performance (c) and rate property (d) of Na2Ti6O13-900 nanorods recorded at a current density of 50.0 mA g-1 in the potential range between 0.0 and 3.0 V.

27

600

potentiostatic charge

Intensity/ a.u.

charge

400 300 200

discharge

Time/ min

500

100 0

3

2

1

0

15

20

+

25

30

35

40

45

2/

Potential vs. (Li/Li )/ V

Fig. 6. 2D View of in-situ XRD patterns of Na2Ti6O13-900 nanorods recorded in the potential range of 0.0-3.0 V.

28

Fig. 7. Selected in-situ XRD patterns of Na2Ti6O13-900 nanorods in the 2θ range of 10-20º, 23.5-25.5º, 28.5-34º and 42.5-45º in the potential range of 0.0-3.0 V. 29

Fig. 8. The probable structural models for Na2Ti6O13 before (a) and after lithiation (b).

30

1.512

0.380

(a)

1.510

(b)

0.378

1.506

b/ nm

a/ nm

1.508

charge to 3 V

discharge to 0 V

1.504

0.376 0.374

discharge to 0 V

a

1.502

0.372

1.500

0.370

0

100

200

300

400

500

b 0

100

Time/ min

200

300

400

500

Time/ min

0.930

0.525

(c)

(d)

0.925

0.520

V/ nm

3

c/ nm

charge to 3 V

0.920

discharge to 0 V

0.915

0.910

charge to 3 V

0.515

discharge to 0 V charge to 3 V

0.510

V 0

100

200

300

400

0.505

500

Time/ min

0

100

200

300

400

500

Time/ min

Fig. 9. The evolution of lattice parameters (a, b, c and V) for Na2Ti6O13-900 nanorods between 0.0 and 3.0 V.

31

Table 1. Refined lattice parameters for Na2Ti6O13 nanorodes obtained at 800 oC, 900 o

C and 1000 oC.  (º)

V (nm3)

Compound

a (nm)

b (nm)

c (nm)

Na2Ti6O13-800

1.50569

0.37366

0.914664 99.0383

0.50821

Na2Ti6O13-900

1.50593

0.37374

0.914827 99.0282

0.50851

Na2Ti6O13-1000

1.50551

0.37358

0.914976 99.0042

0.50811

32