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Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries
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Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries Xuefeng Wang, Yejing Li, Yurui Gao, Zhaoxiang Wangn, Liquan Chen Key Laboratory for Renewable Energy, Chinese Academy of Sciences Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condense Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China Received 25 January 2015; received in revised form 9 March 2015; accepted 16 March 2015
KEYWORDS
Abstract
Sodium titanate; Nanotube array; Nano-architecture; Additive-free; Sodium ion batteries
Low cycling stability and poor rate performance are two of the distinctive drawbacks of most electrode materials for sodium (Na) ion batteries due to the large size of the Na ions. The TiO6 octahedrons interconnect with each other by edges or corners to form both layer- and tunnel-structured sodium titanates (NTO). Such open structures make NTO promising anode materials for sodium (Na) ion batteries. Herein, (conduction and binder) additive-free NTO nanotube array chemically engraved on a Ti foil is used as an electrode and exhibits outstanding rate performance and long-term cycling stability. When a current density of 3200 mA g 1 (i.e., one complete cycle in one minute) is applied, the array delivers a reversible capacity of 42 mAh g 1. Even after 5000 galvanostatic (400 mA g 1) cycles, the electrode retains a capacity of 55 mAh g 1. These excellent performances are attributed to the open structure and the nano-architecture of the NTO nanotube arrays. & 2015 Elsevier Ltd. All rights reserved.
Introduction The rapid development of the renewable or sustainable energy requires energy storage systems with high capacity,
Abbreviations: NTO, sodium titanate n Corresponding author. Tel./fax: +86 10 82649050. E-mail address: [email protected] (Z. Wang).
long cycle life and high safety. When it comes to practical application, especially in large scale energy storage facilities, abundance and cost of the material become important criteria that must be taken into account. In these regards, sodium (Na)-ion batteries are attractive due to the high abundance of Na and the similarity of its intercalation chemistry to that of the lithium (Li)-ion batteries [1–4]. The challenges for the Na-ion batteries include search for superior materials and construction of advanced electrodes.
http://dx.doi.org/10.1016/j.nanoen.2015.03.029 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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Compounds that work well in lithium intercalation may not function at all for Na ion (0.97 Å) accommodation due to the different properties (ionic radius, polarity, interaction with the lattice of the host, etc.) of Na and Li [5], with graphite as a good example [6,7]. Meanwhile, the volume changes of electrode materials that store Na by alloying or conversion reaction, such as tin (Sn) [8], antimony (Sb) [9], molybdenum sulfides (MoS2) [10,11] and so on, are even more severe upon Na insertion and extraction. This will leads to the fast capacity decay of a cell. Therefore, alleviation of the volume change or search for materials with stable structure during cycling is essential to achieve long-term cycling performance of Na-ion batteries. Titanium (Ti)-based compounds have been extensively explored and regarded as promising anode materials for Naion batteries duo to their low toxicity, wide abundance and low cost [12–17]. Layered Na2Ti3O7 is known for its lowest operation potential (about 0.3 V vs. Na + /Na) as an oxide anode but suffers from poor cycling performance [18,19]. On the other hand, the tunnel-structured Na2Ti6O13 exhibits a lifespan over 5000 cycles, but its capacity is less than 20 mAh g 1 [20]. A combination of advantages of the layered and tunnel-like structures is expected to enhance the structural stability and rate performance of the titanates [21]. Furthermore, nano-architectural designs are beneficial for the fast Na-ion diffusion and alleviating the volume variation. In addition, one-dimensional nanotubes grown directly on a current collector can effectively improve the electronic conductivity of the electrode due to their intimate electric contact, resulting in superior rate and cycle performances [22,23]. This (conduction and binder) additive-free electrode increases the total energy density of the Na-ion batteries. Therefore, in these senses, flexible binder-free sodium titanate (NTO) nanotube arrays with such structural features are prepared on the Ti substrate in this work. It exhibits long cycle life (5000 cycles) with a high reversible capacity of 55 mAh g 1 for Na-ion batteries.
Experimental section Material preparation Sodium titanate (NTO) nanotube array was obtained by engraving a commercial titanium (Ti) foil in a concentrated NaOH solution by hydrothermal treatment. Its formation mechanism can be found in an early report [24]. In short, layered NTO nanosheets formed from alkali titanate hydrogel dissolution scroll into nanotubes, vertically aligned on the Ti foil. In a typical experiment, a piece of cleaned Ti foil was treated with 80 mL 10 mol L 1 NaOH in a sealed Teflon reactor (100 mL) at 140 1C for 12 h. Then the Ti foil with a layer of NTO film was washed repeatedly with NaOH solutions of decreasing pH values. By extending the hydrothermal reaction time to 48 h, NTO nanotube powder was obtained. Further heattreatment was conducted to remove the H2O in the interlayers of NTO in a tube furnace at 300 1C for 2 h with flowing O2. The weight of NTO on the Ti foil was determined by comparing the weights of the Ti foil before and after hydrothermal treatment and by the inductively coupled plasma (ICP) method for the NTO powder. The average loading of NTO on the Ti foil is about 0.3 mg cm 2.
Structural characterization The structure of the sample was characterized on powder Xray diffractometer (D8 Advance with a LynxEye_XE detector, Bruker) with Cu Kα1 radiation (λ =1.5405 Å). The morphology was observed on an FEI XL30 Sirion FEG digital scanning electron microscope (SEM) and transmission electron microscope (Tecnai G2 F20 U-TWIN). The Raman spectra were recorded on a Renishaw Via-Reflex spectrometer (532 nm radiation) with a resolution of 2 cm 1. X-ray photoelectron spectra (XPS) were recorded on an Escalab 250XPS spectrometer (Perkin-Elmer Co). The spectrum was calibrated with C1s of the adventitious carbon in the vacuum chamber at 284.8 eV. All the ex situ tests were conducted in vacuum and under protection of pure Ar upon sample transferring.
Electrochemical evaluation The NTO film was cut into 8 8 mm2 sheets and used directly as electrode for Na-ion cells. In an Ar-filled glove box, CR2032-type coin cells were assembled with Na foil as the counter electrode, glass fiber as the separator, and 1 mol L 1 NaClO4 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (4:6 in volume) as the electrolyte. The electrochemical performance evaluation was carried out on a Land BT2000 battery tester (Wuhan, China) at room temperature. The cyclic voltammetry (CV) tests were performed on a CHI600D electrochemical workstation (Shanghai, China) at a scan rate of 0.05 mV s 1. The electrode sheets for ex situ characterization were obtained by rinsing the cycled electrode sheets with DMC, drying them in the vacuum chamber of the glove-box, transferring into the vacuum chamber of the instrument or tested in a sealed container.
Results and discussions Figure 1 shows the morphology of the NTO nanotube array. Some residual NTO nanosheets can be seen in the scanning electron microscopic image (SEM; Figure 1a). The crosssection view of the image (Figure 1a) shows that the oriented NTO nanotubes, about 2 μm long, are wellaligned on the Ti foil. Transmission electron microscopy (TEM) imaging (Figure 1b) shows that the outer and inner diameters of the NTO nanotubes are about 11.0 nm and 5.0 nm, respectively. The lattice fringes along the nanotube indicate that only few layers of nanosheets are scrolled into nanotubes and their spacings are determined to be ca. 0.71 nm, close to that of the (001) plane of NTO (calculated from the following XRD pattern). This observation suggests that these nanotubes grow perpendicular to the [001] direction of the NTO (Figure 1b). The structure of the NTO is characterized with X-ray diffraction (XRD; Figure 2a). The diffraction peaks of the Ti substrate are labeled with black dashes. The XRD patterns of the NTO array are consistent with the Ref. [24] and can be assigned to sodium titanates, e.g. Na2Ti3O7 and Na2Ti6O13. The diffractions (2θ) at ca. 10.51 and 28.51 are indexed to the (001) and (111) planes of Na2Ti3O7, while the peaks at 24.21 and 35.11 are indexed to the (110) and (020) planes of Na2Ti6O13. In general, the structure of the NTO are more like that of the NaTi3O6(OH) 2H2O, a newly reported
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
Additive-free sodium titanate nanotube array
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Figure 1 The SEM (a) and TEM (b) images of the NTO nanotube arrays. The images show that the oriented NTO nanotubes perpendicular to the [001] direction are well-aligned on the Ti foil.
Figure 2 XRD patterns of the as-prepared NTO nanotube arrays (a) and the schematic structures of some sodium titanates (b). The XRD patterns indicate that the structure of the as-prepared NTO is more open with its layer and tunnel hybrid features.
corrugated layer-structured titanate [21]. After dehydration at 300 1C, the structure of NTO shrinks along the c axis, resulting in shifting of the (001) diffraction from 9.91 to 10.51. Both Na2Ti3O7 and Na2Ti6O13 consist of TiO6 octahedrons connected with edges or corners to form layered and tunnel-type structures (Figure 2b), wherein the Na ions stay between the layers and within the tunnels, respectively [25]. Most titanates, denoted as Na2O nTiO2, share the general structural features of Na2Ti3O7 and Na2Ti6O13. Titanates with low Na/Ti ratios incline to form tunnel structures while those with high Na/Ti ratios tend to form layered structures [13]. In terms of NaTi3O6(OH) 2H2O, the edge- and corner-sharing TiO6 octahedrons form corrugated layers of (Ti6O14)4 units, between which are the hydrated Na + and proton (H + ) located [21]. As the Na/Ti atomic ratio of the as-prepared NTO powder is 2:5.1 according to the ICP analysis, somewhere between those of Na2Ti3O7 and Na2Ti6O13, the structure of the as-prepared NTO is believed to be more open with its layer and tunnel hybrid features (Figure 2b). Such structural characteristics are beneficial for the rate and long-term cycling performances of NTO as an anode material for Na-ion batteries.
The Ti foil supported NTO nanotube array is directly used as electrode for Na-ion batteries without adding any conduction or binding additives. Its electrochemical performances are shown in Figure 3. The potential profiles of the first two cycles (Figure 3a) show slopes ranging from 1.5 to 0.0 V vs. Na + /Na, different from that of Na2Ti3O7 [18] or Na2Ti6O13 [20] whose discharge plateaus are at 0.3 and 0.8 V, respectively, but similar to that of NaTi3O6(OH) 2H2O [21]. This slope-type potential profile suggests that probably a solid solution reaction occurs when the Na ions are inserted into or extracted out of the NTO nanotubes successively. The cyclic voltammetry (CV) curves (Figure 3b) exhibit broad redox peaks between 1.5 and 0.0 V. Analysis of the CV curves at various scanning rates (Figure S1) indicates that the Na ion storage mechanism in the NTO nanotube array is a combination of capacitance (about 74%) and intercalation (about 26%) reactions. The reduction peak at 0.2 V is ascribed to electrolyte decomposition and formation of solid electrolyte interphase (SEI), responsible for the irreversible capacity and low coulombic efficiency in the first cycle (65%; Figure 3a). Figure 3c depicts the cycling performance of the NTO nanotube arrays at various current densities. Reversible capacities of 145, 110 and 80 mAh g 1 are obtained for 200
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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Figure 3 The potential profile at a current density of 50 mA g 1 (a), cyclic voltammetry (b), cycling performance of NTO nanotube array at different current densities (c), rate performance (d), long cycling performance at a current density of 400 mA g 1 (e) of NTO nanotube array and the compared cycling performance of most reported titanates (f). The slight fluctuation of specific capacity of NTO is due to the subtle changes of the test atmosphere, such as temperature. The NTO nanotube array exhibit outstanding rate performance (42 mAh g 1 at a current density of 3200 mAh g 1) and high cycling stability (55 mAh g 1 after 5000 cycles). To the best of our knowledge, the rate performance and long-term cycling stability of our NTO nanotube array are better than that of any previously reported titanates anode materials (f).
cycles at current densities of 100, 200 and 400 mA g 1, respectively. Even under a current density of 3200 mA g 1 (corresponding to one complete cycle in one minute), a reversible capacity of 42 mAh g 1 can still be delivered (Figure 3d). Once the current density is recovered to 400 mA g 1, the capacity increases to 82 mAh g 1. Considering that no conduction additive is used in the electrode, this excellent rate performance of the nanotube array should be mainly attributed to the open structure, the nano-architecture of
NTO and the good electric contact between NTO array and the Ti foil. This is further evidenced with the better rate performance of NTO array than that of NTO powder (Figure S2). We also notice that the white NTO film becomes black after cycling, indicating that Na insertion enhances the electronic conductivity of the NTO, a phenomenon often observed in Li-inserted Li4Ti5O12 [26]. The long-term cycling performance of the NTO arrays is evaluated at a current density of 400 mA g 1 (Figure 3e).
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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Figure 4 The Raman (a) and XPS spectra (b) of the NTO nanotube arrays at various discharge/charge states in the first cycle. The results indicate that the inserted Na ions prefer to stay within the layers rather than in the TiO6-constructed tunnels. Some inserted Na ions are trapped in the structure of NTO, responsible for the capacity dropping and low coulombic efficiency but contributing to enhancing its electronic conductivity.
The reversible capacity of NTO drops in the first 20 cycles but then becomes stable at 80 mAh g 1. A capacity of 55 mAh g 1 is still obtained after 5000 cycles, with a coulombic efficiency of about 99.8%. The rate of the capacity delay between the 20th and the 5000th cycles is only 60 ppm. The similarity of the CV curves (Figure 3b) of the 5002nd and the 2nd cycles demonstrate the reversible reaction and structural stability of the NTO nanotube array. To our knowledge, the rate performance and long-term cycling stability of our NTO nanotube array are better than that of any previously reported titanates anode materials (Figure 3f) [12,16,20,27,28]. Raman and X-ray photoelectron spectroscopies (XPS) are employed to explore the structural and chemical state of the NTO arrays at various Na insertion and extraction states. As Figure 4a shows, the intensity of the Na…O–Ti bending mode at 183 cm 1 [29] fades after extensive Na insertion and recovers when the cell is charged to 2.5 V. Insertion of the Na ions in NTO hardly affects the structure of the corner-shared TiO6 (Raman bands at about 272 and 444 cm 1) [28] but leads to blueshifting of about 59 cm 1 for the mode of the edge-shared TiO6 (band at 697 cm 1) [29]. Similar blue-shifting occurs for the stretching of the shorter Ti–O bond (at 920 cm 1) in distorted TiO6 with non-bridging oxygen atoms [30,31]. These observations indicate that the inserted Na ions prefer to stay within the layers rather than in the TiO6-constructed tunnels as the edgeshared TiO6 unit and terminal oxygen atoms are present in the structure of layered Na2Ti3O7 in most cases. The Raman spectra of the NTO electrode charged to 2.5 V after 1 and 1000 cycles are similar to that of the as-prepared NTO, suggesting the reversible Na-ion insertion and extraction and the stable TiO6 octahedron framework. However, some Na ions are supposed to be trapped around the distorted TiO6 because the Ti–O stretching mode cannot be fully recovered till 2.5 V (905 cm 1 at 2.5 V vs. 922 cm 1 in the as-prepared NTO). The XPS results (Figure 4b) confirm the above trapping effect. The binding energy of Ti 2p3/2 is 458.4 eV in the asprepared NTO, corresponding to that of about Ti4 + . It shifts to 457.4 eV for the discharged sample, referred to the about Ti3 + . It goes back to 457.7 eV when the cell is recharged to 2.5 V. The
trapping of some inserted Na ions might be responsible for the capacity dropping and low coulombic efficiency in the first 20 cycles. On the other hand, Na trapping in the structure of NTO enhances its electronic conductivity and is beneficial for highrate cycling. Four important characteristics of the NTO nanotube array grown on Ti foil are responsible for its superior electrochemical performances. First, the open structure of NTO combined with layered and tunnel structures not only provides more channels and room for easy insertion/extraction and diffusion of the Na ions but also enhances its structural stability. Secondly, the stable TiO6 octahedron framework alleviates the volume variation and enhances the cycling stability of the nanotube arrays. Thirdly, the nanostructure along with the interlayers or tunnels of the NTO structure facilitate the diffusion of Na ions in it. Fourth, the nano-architectures of NTO nanotubes vertically aligned on Ti foil improve the charge transfer between the NTO and Ti current collector due to their intimate electric contact, resulting in outstanding rate performance of NTO. These nano-architectural designs further contribute to alleviating volume changes of the NTO arrays and ensure their long lifespan cycling performance. Moreover, this (conduction and binder) additive-free electrode increases the total energy density of the Na-ion batteries and free-standing nanotube arrays supported on bendable substrates exhibit great potential in flexible electrodes and batteries.
Conclusions In summary, sodium titanate (NTO) nanotube array is prepared by chemically engraving the titanium (Ti) foil in a concentrated alkaline solution. Its structure combines the advantages of layered Na2Ti3O7 and tunnel-type Na2Ti6O13. The open structure and nano-architecture of the NTO nanotube array provide more channels and shorter paths for easy diffusion of the Na ions, ensuring its outstanding rate performance (42 mAh g 1 at a current density of 3200 mAh g 1). The low coulombic efficiency
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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in the first cycle is mainly due to the electrolyte decomposition and the trapping of some inserted Na ions. On the other hand, the structural stability of NTO arrays and the good electric contact between the array film and the Ti substrate permit its high capacity retention as an advanced electrode (55 mAh g 1 after 5000 cycles). These conduction additive- and binder-free sodium titanate nanotube arrays on bendable substrates exhibit great potential in flexible electrodes and batteries.
Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Funding sources No competing financial interests have been declared.
Acknowledgments This work was financially supported by the National 973 Program of China (Grant no. 2015CB251100) and the National Natural Science Foundation of China (NSFC Nos. 51372268 and 11234013).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.03.029.
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Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries Xuefeng Wang, Yejing Li, Yurui Gao, Zhaoxiang Wangn, Liquan Chen Key Laboratory for Renewable Energy, Chinese Academy of Sciences Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condense Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China Received 25 January 2015; received in revised form 9 March 2015; accepted 16 March 2015
KEYWORDS
Abstract
Sodium titanate; Nanotube array; Nano-architecture; Additive-free; Sodium ion batteries
Low cycling stability and poor rate performance are two of the distinctive drawbacks of most electrode materials for sodium (Na) ion batteries due to the large size of the Na ions. The TiO6 octahedrons interconnect with each other by edges or corners to form both layer- and tunnel-structured sodium titanates (NTO). Such open structures make NTO promising anode materials for sodium (Na) ion batteries. Herein, (conduction and binder) additive-free NTO nanotube array chemically engraved on a Ti foil is used as an electrode and exhibits outstanding rate performance and long-term cycling stability. When a current density of 3200 mA g 1 (i.e., one complete cycle in one minute) is applied, the array delivers a reversible capacity of 42 mAh g 1. Even after 5000 galvanostatic (400 mA g 1) cycles, the electrode retains a capacity of 55 mAh g 1. These excellent performances are attributed to the open structure and the nano-architecture of the NTO nanotube arrays. & 2015 Elsevier Ltd. All rights reserved.
Introduction The rapid development of the renewable or sustainable energy requires energy storage systems with high capacity,
Abbreviations: NTO, sodium titanate n Corresponding author. Tel./fax: +86 10 82649050. E-mail address: [email protected] (Z. Wang).
long cycle life and high safety. When it comes to practical application, especially in large scale energy storage facilities, abundance and cost of the material become important criteria that must be taken into account. In these regards, sodium (Na)-ion batteries are attractive due to the high abundance of Na and the similarity of its intercalation chemistry to that of the lithium (Li)-ion batteries [1–4]. The challenges for the Na-ion batteries include search for superior materials and construction of advanced electrodes.
http://dx.doi.org/10.1016/j.nanoen.2015.03.029 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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X. Wang et al.
Compounds that work well in lithium intercalation may not function at all for Na ion (0.97 Å) accommodation due to the different properties (ionic radius, polarity, interaction with the lattice of the host, etc.) of Na and Li [5], with graphite as a good example [6,7]. Meanwhile, the volume changes of electrode materials that store Na by alloying or conversion reaction, such as tin (Sn) [8], antimony (Sb) [9], molybdenum sulfides (MoS2) [10,11] and so on, are even more severe upon Na insertion and extraction. This will leads to the fast capacity decay of a cell. Therefore, alleviation of the volume change or search for materials with stable structure during cycling is essential to achieve long-term cycling performance of Na-ion batteries. Titanium (Ti)-based compounds have been extensively explored and regarded as promising anode materials for Naion batteries duo to their low toxicity, wide abundance and low cost [12–17]. Layered Na2Ti3O7 is known for its lowest operation potential (about 0.3 V vs. Na + /Na) as an oxide anode but suffers from poor cycling performance [18,19]. On the other hand, the tunnel-structured Na2Ti6O13 exhibits a lifespan over 5000 cycles, but its capacity is less than 20 mAh g 1 [20]. A combination of advantages of the layered and tunnel-like structures is expected to enhance the structural stability and rate performance of the titanates [21]. Furthermore, nano-architectural designs are beneficial for the fast Na-ion diffusion and alleviating the volume variation. In addition, one-dimensional nanotubes grown directly on a current collector can effectively improve the electronic conductivity of the electrode due to their intimate electric contact, resulting in superior rate and cycle performances [22,23]. This (conduction and binder) additive-free electrode increases the total energy density of the Na-ion batteries. Therefore, in these senses, flexible binder-free sodium titanate (NTO) nanotube arrays with such structural features are prepared on the Ti substrate in this work. It exhibits long cycle life (5000 cycles) with a high reversible capacity of 55 mAh g 1 for Na-ion batteries.
Experimental section Material preparation Sodium titanate (NTO) nanotube array was obtained by engraving a commercial titanium (Ti) foil in a concentrated NaOH solution by hydrothermal treatment. Its formation mechanism can be found in an early report [24]. In short, layered NTO nanosheets formed from alkali titanate hydrogel dissolution scroll into nanotubes, vertically aligned on the Ti foil. In a typical experiment, a piece of cleaned Ti foil was treated with 80 mL 10 mol L 1 NaOH in a sealed Teflon reactor (100 mL) at 140 1C for 12 h. Then the Ti foil with a layer of NTO film was washed repeatedly with NaOH solutions of decreasing pH values. By extending the hydrothermal reaction time to 48 h, NTO nanotube powder was obtained. Further heattreatment was conducted to remove the H2O in the interlayers of NTO in a tube furnace at 300 1C for 2 h with flowing O2. The weight of NTO on the Ti foil was determined by comparing the weights of the Ti foil before and after hydrothermal treatment and by the inductively coupled plasma (ICP) method for the NTO powder. The average loading of NTO on the Ti foil is about 0.3 mg cm 2.
Structural characterization The structure of the sample was characterized on powder Xray diffractometer (D8 Advance with a LynxEye_XE detector, Bruker) with Cu Kα1 radiation (λ =1.5405 Å). The morphology was observed on an FEI XL30 Sirion FEG digital scanning electron microscope (SEM) and transmission electron microscope (Tecnai G2 F20 U-TWIN). The Raman spectra were recorded on a Renishaw Via-Reflex spectrometer (532 nm radiation) with a resolution of 2 cm 1. X-ray photoelectron spectra (XPS) were recorded on an Escalab 250XPS spectrometer (Perkin-Elmer Co). The spectrum was calibrated with C1s of the adventitious carbon in the vacuum chamber at 284.8 eV. All the ex situ tests were conducted in vacuum and under protection of pure Ar upon sample transferring.
Electrochemical evaluation The NTO film was cut into 8 8 mm2 sheets and used directly as electrode for Na-ion cells. In an Ar-filled glove box, CR2032-type coin cells were assembled with Na foil as the counter electrode, glass fiber as the separator, and 1 mol L 1 NaClO4 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (4:6 in volume) as the electrolyte. The electrochemical performance evaluation was carried out on a Land BT2000 battery tester (Wuhan, China) at room temperature. The cyclic voltammetry (CV) tests were performed on a CHI600D electrochemical workstation (Shanghai, China) at a scan rate of 0.05 mV s 1. The electrode sheets for ex situ characterization were obtained by rinsing the cycled electrode sheets with DMC, drying them in the vacuum chamber of the glove-box, transferring into the vacuum chamber of the instrument or tested in a sealed container.
Results and discussions Figure 1 shows the morphology of the NTO nanotube array. Some residual NTO nanosheets can be seen in the scanning electron microscopic image (SEM; Figure 1a). The crosssection view of the image (Figure 1a) shows that the oriented NTO nanotubes, about 2 μm long, are wellaligned on the Ti foil. Transmission electron microscopy (TEM) imaging (Figure 1b) shows that the outer and inner diameters of the NTO nanotubes are about 11.0 nm and 5.0 nm, respectively. The lattice fringes along the nanotube indicate that only few layers of nanosheets are scrolled into nanotubes and their spacings are determined to be ca. 0.71 nm, close to that of the (001) plane of NTO (calculated from the following XRD pattern). This observation suggests that these nanotubes grow perpendicular to the [001] direction of the NTO (Figure 1b). The structure of the NTO is characterized with X-ray diffraction (XRD; Figure 2a). The diffraction peaks of the Ti substrate are labeled with black dashes. The XRD patterns of the NTO array are consistent with the Ref. [24] and can be assigned to sodium titanates, e.g. Na2Ti3O7 and Na2Ti6O13. The diffractions (2θ) at ca. 10.51 and 28.51 are indexed to the (001) and (111) planes of Na2Ti3O7, while the peaks at 24.21 and 35.11 are indexed to the (110) and (020) planes of Na2Ti6O13. In general, the structure of the NTO are more like that of the NaTi3O6(OH) 2H2O, a newly reported
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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Figure 1 The SEM (a) and TEM (b) images of the NTO nanotube arrays. The images show that the oriented NTO nanotubes perpendicular to the [001] direction are well-aligned on the Ti foil.
Figure 2 XRD patterns of the as-prepared NTO nanotube arrays (a) and the schematic structures of some sodium titanates (b). The XRD patterns indicate that the structure of the as-prepared NTO is more open with its layer and tunnel hybrid features.
corrugated layer-structured titanate [21]. After dehydration at 300 1C, the structure of NTO shrinks along the c axis, resulting in shifting of the (001) diffraction from 9.91 to 10.51. Both Na2Ti3O7 and Na2Ti6O13 consist of TiO6 octahedrons connected with edges or corners to form layered and tunnel-type structures (Figure 2b), wherein the Na ions stay between the layers and within the tunnels, respectively [25]. Most titanates, denoted as Na2O nTiO2, share the general structural features of Na2Ti3O7 and Na2Ti6O13. Titanates with low Na/Ti ratios incline to form tunnel structures while those with high Na/Ti ratios tend to form layered structures [13]. In terms of NaTi3O6(OH) 2H2O, the edge- and corner-sharing TiO6 octahedrons form corrugated layers of (Ti6O14)4 units, between which are the hydrated Na + and proton (H + ) located [21]. As the Na/Ti atomic ratio of the as-prepared NTO powder is 2:5.1 according to the ICP analysis, somewhere between those of Na2Ti3O7 and Na2Ti6O13, the structure of the as-prepared NTO is believed to be more open with its layer and tunnel hybrid features (Figure 2b). Such structural characteristics are beneficial for the rate and long-term cycling performances of NTO as an anode material for Na-ion batteries.
The Ti foil supported NTO nanotube array is directly used as electrode for Na-ion batteries without adding any conduction or binding additives. Its electrochemical performances are shown in Figure 3. The potential profiles of the first two cycles (Figure 3a) show slopes ranging from 1.5 to 0.0 V vs. Na + /Na, different from that of Na2Ti3O7 [18] or Na2Ti6O13 [20] whose discharge plateaus are at 0.3 and 0.8 V, respectively, but similar to that of NaTi3O6(OH) 2H2O [21]. This slope-type potential profile suggests that probably a solid solution reaction occurs when the Na ions are inserted into or extracted out of the NTO nanotubes successively. The cyclic voltammetry (CV) curves (Figure 3b) exhibit broad redox peaks between 1.5 and 0.0 V. Analysis of the CV curves at various scanning rates (Figure S1) indicates that the Na ion storage mechanism in the NTO nanotube array is a combination of capacitance (about 74%) and intercalation (about 26%) reactions. The reduction peak at 0.2 V is ascribed to electrolyte decomposition and formation of solid electrolyte interphase (SEI), responsible for the irreversible capacity and low coulombic efficiency in the first cycle (65%; Figure 3a). Figure 3c depicts the cycling performance of the NTO nanotube arrays at various current densities. Reversible capacities of 145, 110 and 80 mAh g 1 are obtained for 200
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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Figure 3 The potential profile at a current density of 50 mA g 1 (a), cyclic voltammetry (b), cycling performance of NTO nanotube array at different current densities (c), rate performance (d), long cycling performance at a current density of 400 mA g 1 (e) of NTO nanotube array and the compared cycling performance of most reported titanates (f). The slight fluctuation of specific capacity of NTO is due to the subtle changes of the test atmosphere, such as temperature. The NTO nanotube array exhibit outstanding rate performance (42 mAh g 1 at a current density of 3200 mAh g 1) and high cycling stability (55 mAh g 1 after 5000 cycles). To the best of our knowledge, the rate performance and long-term cycling stability of our NTO nanotube array are better than that of any previously reported titanates anode materials (f).
cycles at current densities of 100, 200 and 400 mA g 1, respectively. Even under a current density of 3200 mA g 1 (corresponding to one complete cycle in one minute), a reversible capacity of 42 mAh g 1 can still be delivered (Figure 3d). Once the current density is recovered to 400 mA g 1, the capacity increases to 82 mAh g 1. Considering that no conduction additive is used in the electrode, this excellent rate performance of the nanotube array should be mainly attributed to the open structure, the nano-architecture of
NTO and the good electric contact between NTO array and the Ti foil. This is further evidenced with the better rate performance of NTO array than that of NTO powder (Figure S2). We also notice that the white NTO film becomes black after cycling, indicating that Na insertion enhances the electronic conductivity of the NTO, a phenomenon often observed in Li-inserted Li4Ti5O12 [26]. The long-term cycling performance of the NTO arrays is evaluated at a current density of 400 mA g 1 (Figure 3e).
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Figure 4 The Raman (a) and XPS spectra (b) of the NTO nanotube arrays at various discharge/charge states in the first cycle. The results indicate that the inserted Na ions prefer to stay within the layers rather than in the TiO6-constructed tunnels. Some inserted Na ions are trapped in the structure of NTO, responsible for the capacity dropping and low coulombic efficiency but contributing to enhancing its electronic conductivity.
The reversible capacity of NTO drops in the first 20 cycles but then becomes stable at 80 mAh g 1. A capacity of 55 mAh g 1 is still obtained after 5000 cycles, with a coulombic efficiency of about 99.8%. The rate of the capacity delay between the 20th and the 5000th cycles is only 60 ppm. The similarity of the CV curves (Figure 3b) of the 5002nd and the 2nd cycles demonstrate the reversible reaction and structural stability of the NTO nanotube array. To our knowledge, the rate performance and long-term cycling stability of our NTO nanotube array are better than that of any previously reported titanates anode materials (Figure 3f) [12,16,20,27,28]. Raman and X-ray photoelectron spectroscopies (XPS) are employed to explore the structural and chemical state of the NTO arrays at various Na insertion and extraction states. As Figure 4a shows, the intensity of the Na…O–Ti bending mode at 183 cm 1 [29] fades after extensive Na insertion and recovers when the cell is charged to 2.5 V. Insertion of the Na ions in NTO hardly affects the structure of the corner-shared TiO6 (Raman bands at about 272 and 444 cm 1) [28] but leads to blueshifting of about 59 cm 1 for the mode of the edge-shared TiO6 (band at 697 cm 1) [29]. Similar blue-shifting occurs for the stretching of the shorter Ti–O bond (at 920 cm 1) in distorted TiO6 with non-bridging oxygen atoms [30,31]. These observations indicate that the inserted Na ions prefer to stay within the layers rather than in the TiO6-constructed tunnels as the edgeshared TiO6 unit and terminal oxygen atoms are present in the structure of layered Na2Ti3O7 in most cases. The Raman spectra of the NTO electrode charged to 2.5 V after 1 and 1000 cycles are similar to that of the as-prepared NTO, suggesting the reversible Na-ion insertion and extraction and the stable TiO6 octahedron framework. However, some Na ions are supposed to be trapped around the distorted TiO6 because the Ti–O stretching mode cannot be fully recovered till 2.5 V (905 cm 1 at 2.5 V vs. 922 cm 1 in the as-prepared NTO). The XPS results (Figure 4b) confirm the above trapping effect. The binding energy of Ti 2p3/2 is 458.4 eV in the asprepared NTO, corresponding to that of about Ti4 + . It shifts to 457.4 eV for the discharged sample, referred to the about Ti3 + . It goes back to 457.7 eV when the cell is recharged to 2.5 V. The
trapping of some inserted Na ions might be responsible for the capacity dropping and low coulombic efficiency in the first 20 cycles. On the other hand, Na trapping in the structure of NTO enhances its electronic conductivity and is beneficial for highrate cycling. Four important characteristics of the NTO nanotube array grown on Ti foil are responsible for its superior electrochemical performances. First, the open structure of NTO combined with layered and tunnel structures not only provides more channels and room for easy insertion/extraction and diffusion of the Na ions but also enhances its structural stability. Secondly, the stable TiO6 octahedron framework alleviates the volume variation and enhances the cycling stability of the nanotube arrays. Thirdly, the nanostructure along with the interlayers or tunnels of the NTO structure facilitate the diffusion of Na ions in it. Fourth, the nano-architectures of NTO nanotubes vertically aligned on Ti foil improve the charge transfer between the NTO and Ti current collector due to their intimate electric contact, resulting in outstanding rate performance of NTO. These nano-architectural designs further contribute to alleviating volume changes of the NTO arrays and ensure their long lifespan cycling performance. Moreover, this (conduction and binder) additive-free electrode increases the total energy density of the Na-ion batteries and free-standing nanotube arrays supported on bendable substrates exhibit great potential in flexible electrodes and batteries.
Conclusions In summary, sodium titanate (NTO) nanotube array is prepared by chemically engraving the titanium (Ti) foil in a concentrated alkaline solution. Its structure combines the advantages of layered Na2Ti3O7 and tunnel-type Na2Ti6O13. The open structure and nano-architecture of the NTO nanotube array provide more channels and shorter paths for easy diffusion of the Na ions, ensuring its outstanding rate performance (42 mAh g 1 at a current density of 3200 mAh g 1). The low coulombic efficiency
Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029
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in the first cycle is mainly due to the electrolyte decomposition and the trapping of some inserted Na ions. On the other hand, the structural stability of NTO arrays and the good electric contact between the array film and the Ti substrate permit its high capacity retention as an advanced electrode (55 mAh g 1 after 5000 cycles). These conduction additive- and binder-free sodium titanate nanotube arrays on bendable substrates exhibit great potential in flexible electrodes and batteries.
Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Funding sources No competing financial interests have been declared.
Acknowledgments This work was financially supported by the National 973 Program of China (Grant no. 2015CB251100) and the National Natural Science Foundation of China (NSFC Nos. 51372268 and 11234013).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.03.029.
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Please cite this article as: X. Wang, et al., Additive-free sodium titanate nanotube array as advanced electrode for sodium ion batteries, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.03.029