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Hydrogenation Driven Conductive Na2Ti3O7 Nanoarrays as Robust Binder-Free Anodes for Sodium-Ion Batteries Shidong Fu, Jiangfeng Ni, Yong Xu, Qiao Zhang, and Liang Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01805 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016
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Hydrogenation
Driven
Conductive
Na2Ti3O7
Nanoarrays as Robust Binder-Free Anodes for Sodium-Ion Batteries Shidong Fu, †,§ Jiangfeng Ni,†,§ Yong Xu,‡ ,§ Qiao Zhang*,‡ and Liang Li*,† † College of Physics, Optoelectronics and Energy, Center for Energy Conversion Materials & Physics (CECMP), the Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, P. R. China ‡ Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano and Soft Materials (FUNSOM), the Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China
Keywords: Sodium-ion battery, Hydrogenation, Nanoarray, Rate capability
Abstract We present a general and rational approach to fabricate highly accessible and affordable sodium-ion battery anodes by engineering three-dimensional hydrogenated Na2Ti3O7 nanoarrays supported on flexible Ti substrates. The hydrogenated Na2Ti3O7 nanoarrays exhibit desirable properties for sodium storage, such as high surface area, high electrical conductivity and Na+ diffusivity. The as-obtained nanoarrays demonstrate remarkably stable and robust Na-storage performance when tested as binder-free anodes for sodium-ion battery. They can afford a high reversible (desodiation) capacity of 227 mAh g–1 and retain a capacity of 65 mAh g–1 over 10,000 continuous cycles at a high rate of 35 C. Therefore, through this synergy of array architecture and hydrogenation, it is possible to engineer numerous anodes that can reversibly store Na+ ions in a fast and stable manner.
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Electrochemical energy storages have been playing a critical role in our daily life and energy-intensive civilization.1-3 As a major solution towards efficient energy storage, lithium-ion batteries take the center stage for electronics and automobiles. The utilization of sulfur cathode and silicon anode has boosted the energy density and reduced the cost.4-7 Recently, sodium-ion batteries (SIBs) are capturing increasing attention as a alternative energy solution, owing to their similar electrochemical properties but more abundant resources and lower cost of sodium compared to lithium.8-10 One critical challenge at present is to develop affordable and efficient SIB anodes that can store large amounts of Na+ ions durably and robustly.11-13 Ti-based materials have attracted immense interest due to their high activity, low cost and environmental benignity.14-17 With the open layered framework, Na2Ti3O7 allows for storage of ~3.5 Na+ ions per formula unit in the interlayer space, resulting in a high theoretical capacity of 310 mA h g–1.18-25 As the large Na+ ions destabilize the host considerably, the intercalation of Na+ ions into Na2Ti3O7 lattice occurs at a low potential of 0.3 V (vs. Na+/Na, unless otherwise denoted),25 which is highly desirable because it lies beyond the potential of Na dendrite growth, whereas does not sacrifice the operating voltage and energy density in practices. Implementation of Na2Ti3O7 in SIBs is challenging, since it suffers from sluggish Na reaction kinetics caused by the insulating nature of lattice associated with a large bandgap of 3.7 eV.19 Na2Ti3O7 materials also exhibit insufficient Na cycling, as the insertion of Na+ ions causes substantial lattice expansion and strain.24-26 Although one promising solution is to fabricate nanostructured Na2Ti3O7, which can distinctly reduce the travel distance of the electron and Na+ ion, the controllable fabrication of nanostructures is equipment-intensive and procedure-complicated, and thus less manipulable.27 Three-dimensional (3D) Na2Ti3O7 nanoarrays on metal substrates have drawn significant attention.28,
29
The ordered
configuration enables a direct electron transport from/to conductive substrate and the free 2 ACS Paragon Plus Environment
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diffusion of ions, making it a highly accessible architecture for energy storage.30-32 These Na2Ti3O7 nanoarrays preferentially grow along the longitudinal axis, limiting the transport of electrons along this direction. This limitation imposes a striking challenge in the achievement of high-rate Na2Ti3O7 nanoarray electrodes. It has been demonstrated recently that hydrogenation of oxide electrodes can drastically enhance their electrical conductivity and rate capability.33-36 Such an idea has not yet been attempted on the Na2Ti3O7 yet, possibly due to the lack of clear understanding on its electronic structure. Herein, we demonstrate that hydrogenated Na2Ti3O7 (denoted as H-Na2Ti3O7) nanoarrays directly grown on flexible Ti substrate can serve as binder-free anode for SIBs. The H-Na2Ti3O7 nanoarrays are composed of ordered nanotubes, ~3 µm in length and 25~35 nm in external diameter. When tested as the anode, these nanoarrays exhibit excellent electrochemical activities. They afford a high reversible capacity of 227 mAh g−1 in the potential range of 0.1–2.5 V, and a robust capability of 65 mAh g–1 at 35 C (6.2 A g–1, 1 C is equal to 2 Na+ insertions into Na2Ti3O7 in 1 h, ca 177 mA g−1),18 outstripping any other Na2Ti3O7 based materials. More importantly, the H-Na2Ti3O7 nanoarrays can sustain 10,000 continuous cycles at 35 C without appreciable capacity fading. To the best of knowledge, this is the longest stability reported for Na2Ti3O7 and even possible for Na anodes up to date.29 H-Na2Ti3O7 nanoarrays have been prepared by hydrothermally treating metallic Ti foil in an alkaline solution and a subsequent hydrogenation reaction in an Ar/H2 mixture atmosphere, as schematically illustrated in Figure 1a (see Experimental Section for more details, Supporting Information). Briefly, ordered Na2Ti3O7 nanotubes are firstly hydrothermally grown in 1 M NaOH solution at 220 °C.35 The morphological features of the Na2Ti3O7 products characterized by scanning electron microscopy (SEM) are present in Figure S1, Supporting Information. The growth of Na2Ti3O7 nanoarrays undergoes an intermediate of nanowalls, which splits to nanotube arrays upon reaction extension to 3 hours or more. 3 ACS Paragon Plus Environment
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Structure evolution of the Na2Ti3O7 products over hydrothermal duration is monitored using X-ray diffraction (XRD). The 3-hour product exhibits a high crystalline feature (Figure S2a, Supporting Information), and is further annealed in a mixed Ar/H2 atmosphere (H-Na2Ti3O7) or air (denoted as A-Na2Ti3O7) at 450 °C for 2 hours. The XRD patterns of both samples can be resolved into monoclinic Na2Ti3O7 (PDF #31-1329), with the (002), (201), and (020) peaks being clearly visible (Figure S2b, supporting information). SEM images reveal that the H-Na2Ti3O7 well retains the morphology of the hydrothermal product, consisting of periodic nanotubes vertically aligned on the Ti foil with an average longitudinal length of ~3 µm (Figure 1b-c). Nevertheless, the H-Na2Ti3O7 evidences a darker color versus the A-Na2Ti3O7 (Figure S3a, supporting information), indicating a strong absorption in the visible region due to hydrogen diffusion into Na2Ti3O7 lattice.33
In
addition, hydrogenation results in a decreased band gap of 3.49 eV compared to 3.85 eV for pristine Na2Ti3O7, as confirmed by diffuse reflectance spectroscopy (Figure S3b-c, supporting information). The nanotubular characteristic of the H-Na2Ti3O7 products (Figure 1d-e) has also been confirmed by the transmission electron microscopy (TEM) analysis. The inner and the outer diameters of the nanotubes are about 5 nm and 25~35 nm, respectively. The H-Na2Ti3O7 nanotubes exhibit a core-shell structure composed of a crystalline core and disordered shell (Figure 1f).35 The crystalline core shows well-resolved (001) facet with typical monoclinic Na2Ti3O7 spacing of 0.84 nm. The thickness of the disordered surface layer in the end of tubes is about 5 nm, which is thicker than that of the side part. The different thicknesses can be attributed to the facile diffusion of hydrogen along the tube axial direction. The presence of Na, Ti, and O elements has been confirmed by the energy dispersive X-ray spectroscopy (Figure S4, Supporting Information). There is a decrease in the intensity of O signal after hydrogenation, suggesting the formation of oxygen vacancies.
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Raman spectroscopy was used to examine structural variation in the Na2Ti3O7 nanotubes after hydrogenation (Figure 2a). The A-Na2Ti3O7 exhibits several scattering bands at 192, 275, 435, 671, and 702 cm‒1, in good agreement with the characteristics of Na2Ti3O7 framework.28 Compared with A-Na2Ti3O7, the H-Na2Ti3O7 sample shows clear band broadening, suggesting a structural variation related to oxygen vacancies.33 The chemical states of the Na2Ti3O7 nanoarray products are probed by X-ray photoelectron spectroscopy (XPS). Figure 2b shows the normalized Ti 2p core level spectra of H-Na2Ti3O7 and A-Na2Ti3O7. The Ti 2p doublets (2p1/2 and Ti 2p3/2) at 465.1 and 459.0 eV are observed clearly, confirming the main species are Ti4+. Nonetheless, the H-Na2Ti3O7 exhibits a shift of 0.3 eV towards lower energy, suggesting that its Ti element is partially reduced upon hydrogenation. The difference between the two Ti 2p spectra shows additional peaks centered at 463.6 and 457.9 eV, correlating with the characteristic Ti 2p doublets of Ti3+. This result confirms both the presence of Ti3+ ions and the generation of oxygen vacancies in the H-Na2Ti3O7 product during hydrogenation. Figure 2c compares high-resolution XPS spectra of O 1s of the H-Na2Ti3O7 and the A-Na2Ti3O7. The prominent peak centered at 530.2 eV is attributed to O-Ti-O, while the broad peak centered at 532.5 eV can be ascribed to Ti−OH, which has a higher binding energy compared with the former one. The H-Na2Ti3O7 exhibits a higher intensity in the Ti−OH peak compared with the as-obtained A-Na2Ti3O7, indicating that more hydroxyl groups are generated through hydrogenation.36 The electrochemical impedance spectroscopy (EIS) has been carried out to probe the effect of hydrogenation on the electrical properties of Na2Ti3O7. The EIS spectra are recorded on the H-Na2Ti3O7 and A-Na2Ti3O7 samples with 1 kHz frequency in the dark. Accordingly, Mott-Schottky plots are generated using the capacitances derived from EIS, as shown in Figure 2d. The carrier densities are then calculated to be 1.4 × 1020 and 7.2 × 1020 cm−3 for A-Na2Ti3O7 and H-Na2Ti3O7, respectively. The hydrogenated sample manifests a five-fold 5 ACS Paragon Plus Environment
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higher carrier density, due to the increased oxygen vacancy induced by hydrogenation. In addition, the I-V measurement shows that the H-Na2Ti3O7 features a higher electrical conductivity of 1.2×10−4 S cm−1 than that of A-Na2Ti3O7 (1.7×10−7 S cm−1) (Figure S5, Supporting Information). Owing to the high conductivity, the 3D H-Na2Ti3O7 nanoarrays can be considered promising binder-free electrodes for Na storage. The electrochemical behavior of the H-Na2Ti3O7 nanoarrays was firstly investigated by cyclic voltammetry (CV), as presented in Figure 3a. The CV profile shows a marked redox peaks at 0.25/0.65 V, characteristic of Na+ insertion/extraction in the monoclinic Na2Ti3O7 phase.18 A weak pair of redox peaks at 1.15/1.25 V might be attributed to the reduced Ti3+ species by hydrogen.28 The highly reproducible CV profiles suggest a reversible Na storage process in the H-Na2Ti3O7. The Na-storage behavior was furtehr probed by galvanostatic intermittent titration technique (GITT). The GITT curves shown in Figure 3b reveal a relaxed potential plateau at 0.4/0.5 V with negligible polarizations, implying a high Na-storage reversibility in the H-Na2Ti3O7. Therefore, the observed hysteresis in the CV profiles of the H-Na2Ti3O7 may be attributed to a kinetic issue rather than a thermodynamic one.19 Galvanostatic results show that the H-Na2Ti3O7 delivers a charge (desodiation) capacity of 227 mAh g−1 at 0.2 C, corresponds to reversible storage of ~2.5 Na in per unit formula of Na2Ti3O7 (Figure 3c). The initial charge and discharge profiles reveal a low Coulombic efficiency of 45%, which can be enhanced to 84% using a simple pre-sodiation (Figure S6, Supporting Information).37 By contrast, the A-Na2Ti3O7 only affords a reversible desodiation capacity of 153 mAh g−1. Thus, the H-Na2Ti3O7 exhibits markedly improved capacity and efficiency, due to higher conductivity enabled by hydrogenation. This hydrogenation-induced improvement in the Na-storage activity is more pronounced at higher rate. As shown in Figure 3d, the H-Na2Ti3O7 affords a capacity of 195 mAh g−1 at 1 C, which is almost twice 6 ACS Paragon Plus Environment
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that of A-Na2Ti3O7. Furthermore, the H-Na2Ti3O7 retains 94% of the initial capacity, compared with 88% for the A-Na2Ti3O7 over 200 cycles. The improved kinetics and stability of the H-Na2Ti3O7 electrode correlates with the reduced charge transfer resistance and the stable electrode/electrolyte interphase, as confirmed by EIS (Figure S7, Supporting Information) and SEM observation (Figure S8, Supporting Information). To investigate the effect of hydrogenation on the high rate behaviors, the Na2Ti3O7 anodes were galvanostatically tested at various rates ranging from 0.2 to 35 C. Figure 4a-b present the rate curves for H-Na2Ti3O7 and A-Na2Ti3O7, respectively. Superior rate characteristic for the hydrogenated Na2Ti3O7 nanoarray is manifest. Notably, the H-Na2Ti3O7 nanoarrays afford reversible (desodiation) capacities of 227, 210, 190, 164, 130, and 99 mAh g−1, respectively, at rates of 0.2, 0.5, 1, 2, 5, 10 C. At very high rates of 20 and 35 C, the capacities are 77 and 71 mAh g−1 (or 40% of the available capacity), respectively. At the rate of 35 C, the H-Na2Ti3O7 fulfills a full cycle of charge and discharge within 80 seconds. This rate capability is superior to the A-Na2Ti3O7 counterpart and other Na2Ti3O7 based electrodes reported recently (Figure 4c), such as Na2Ti3O7 nanocrystals,20 Na2Ti3O7 nanospheres,21 and Na2Ti3O7 nanotubes.28 In another rate test, the H-Na2Ti3O7 electrodes were cycled five times at each current rate (Figure 4d). The capacity level is rather stable at each rate, regardless the rate cycling history. More significantly, when the applied rates decrease, the capacity gradually restores to 225 mAh g−1 at 0.2 C, signaling a remarkable reversibility. Furthermore, the H-Na2Ti3O7 was cycled at a rate of 35 C (6.2 A g−1) to acquire its durability under extreme condition. It exhibits an outstanding capability, retaining 65 mAh g−1 over 10,000 cycles without appreciable decay (Figure 4e). To our knowledge, such an ultrastable and robust capability has not been realized for Na2Ti3O7 electrodes yet. This result strongly implies the effectiveness of our material design.
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To unravel the superior Na storage behaviors, the kinetics of the H-Na2Ti3O7 were analyzed with CV techniques. Figure 5a shows highly similar CV curves at various sweep rates ranging from 0.1 to 1.5 mV s−1. When the redox peak currents related to Na+ insertion/extraction in Na2Ti3O7 are plotted versus the sweep rates, a linear relation is observed for both anodic and cathodic processes (Figure 5b), which is indicative of a surface charging or pseudocapacitive mechanism.38 At higher sweep rates, the CV profiles show distortion from the basic shape (Figure S9, Supporting Information). This distortion can arise from several sources such as increased Ohmic contribution and/or diffusion constrains.39-41 Furthermore, the contribution of the total charge (Q) stored can be analyzed using Trasatti’s method.42 The Q involves charge from the surface capacitive process (Qs) and from the bulk insertion of Na+ ion (Qb). The latter part is a diffusion-controlled process, i.e., a function of the diffusion rate. Thus, ܳሺ ݒሻ = ܳ௦ + ܳ = ܳ௦ + ݇ሺݒ
ିଵൗ ଶሻ
where k is a constant and v is the potential sweep rate. Figure 5c shows the normalized capacity versus the reciprocal square root of the sweep rates. The surface charge (Qs) can be determined from the linear relationship part (v ≤ 1.5 mV s−1); the extrapolated y-intercept gives the contribution of Qs. Also, the relative contributions related to both Na+ insertion and capacitive processes can be identified, as illustrated in Figure 5d. The relative ratio of Qs gradually increases with increasing sweep rate. The ratio is 58% at a scan rate of 0.1 mV s−1, and reaches 85% at 1.5 mV s−1. Such a robust and durable Na cycling in the H-Na2Ti3O7 can be attributable to the unique 3D nanoarray architecture and hydrogenation. Conventional electrodes adopt a flat structure and contain inactive components such as polymer binder, which usually suffers from insufficient ion permeation and breaking up of electron transport caused by insulating binder. By contrast, 3D binder-free nanoarray electrodes can efficiently resolve these issues by 8 ACS Paragon Plus Environment
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enabling direct electrical contact with substrates and facile ion diffusion through nanopores.32 In addition, the 3D configuration ensures good mechanical adhesion of nanoarrays to the flexible metal substrate, avoiding the destruction of structural integrity upon Na cycling. Hydrogenation of Na2Ti3O7 nanotubes renders the product a high electrical conductivity,35 which drastically reduces the charge transfer resistance and boosts the Na reaction kinetics. Full cells using H-Na2Ti3O7 anodes and Na2/3(Ni1/3Mn2/3)O2 cathodes were assembled to evaluate the feasibility of the binder-free anodes in practice.43 Pre-sodiated H-Na2Ti3O7 electrodes were directly used as anodes, and composite electrodes consisting of Na2/3(Ni1/3Mn2/3)O2, conductive carbon and binder as cathodes. The mass ratio of anode against cathode was adjusted to be 1:3.5. The Na2/3(Ni1/3Mn2/3)O2 material was synthesized via a sol-gel assisted solid state approach.44 The material features a P2-type structure with particle size of 2~3 µm and exhibits a reversible capacity of ~80 mAh g−1 (Figure S10, Supporting Information). Electrochemical results shown in Figure 6 reveal that the H-Na2Ti3O7//Na2/3(Ni1/3Mn2/3)O2 full cells afford a high reversible capacity of 240 mAh g−1 (based on the anode mass) with an average cell voltage of 2.7 V. The energy density reaches a high value of 125 mAh g−1 (based on the mass of both electrodes). Additionally, the full cells exhibit an impressively high rate capacity and high cycling stability, sustaining 100 cycles at various current rates with Coulombic efficiency above 97%. More importantly, two charged full cells in-series can work well to light up LED arrays in a wrist strap, thereby demonstrating the potential in flexible and wearable electronics.45 In conclusion, hydrogenated Na2Ti3O7 nanoarrays supported on flexible Ti substrates were directly fabricated for the first time, through an alkaline hydrothermal reaction and a subsequent
hydrogenation
process.
The
well-engineered
H-Na2Ti3O7
demonstrates
remarkable anode activity for SIBs with high capacity and high stability. The as-obtained H-Na2Ti3O7 is capable of sustaining rapid potential sweeping, displaying a pseudocapacitive 9 ACS Paragon Plus Environment
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behavior. This is a clear indication that the Na reaction kinetics and durability in the Na2Ti3O7 materials can be elaborately tailored by architecture design and structural hydrogenation. These joint material designs significantly promote transport of both electron and ion, leading to remarkably enhanced electrochemical activity. Furthermore, this material engineering is applicable to modulating other electrodes for improved electrochemical energy storage.
ASSOCIATED CONTENT Supporting Information Experimental details, more SEM images, XRD patterns, UV spectra, CV and charge/discharge curves, and conductivity results. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] (Q. Z);
[email protected] (L. L) Author Contributions §
S.F. and J.N. contributed equally to this work.
NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge the financial support of the National Natural Science Foundation of China (51302181, 51372159, 51422206, 21401135), the Thousand Youth Talents Plan, the Jiangsu Shuangchuang Plan, the Jiangsu Natural Science Foundation (BK20151219, BK20140009, 10 ACS Paragon Plus Environment
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BK20140304), the China Postdoctoral Science Foundation (2015T80580), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Collaborative Innovation Center of Suzhou Nano Science and Technology.
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(35) Shen, L.; Uchaker, E.; Zhang, X.; Cao, G. Adv. Mater. 2012, 24, 6502-6506. (36) Lu, X.; Wang, G.; Zhai, T.; Yu, M.; Gan, J.; Tong, Y.; Li, Y. Nano Lett. 2012, 12, 1690-1696. (37) Ni, J.; Fu, S.; Wu, C.; Zhao, Y.; Maier, J.; Yu, Y.; Li, L. Adv. Energy Mater. 2016, 6, 1502568. (38) Li, S.; Qiu, J.; Lai, C.; Ling, M.; Zhao, H.; Zhang, S. Nano Energy 2015, 12, 224-230. (39) Ni, J.; Zhao, Y.; Liu, T.; Zheng, H.; Gao, L.; Yan, C.; Li, L. Adv. Energy Mater. 2014, 4, 1400798. (40) Ni, J.; Zhao, Y.; Li, L.; Mai, L. Nano Energy 2015, 11, 129–135. (41) Ni, J.; Li, Y. Adv. Energy Mater. 2016, 6, 1600278. (42) Ardizzone, S.; Fregonara, G.; Trasatti, S. Electrochim. Acta 1990, 35, 263-267. (43) Yu, D. Y.; Prikhodchenko, P. V.; Mason, C. W.; Batabyal, S. K.; Gun, J.; Sladkevich, S.; Medvedev, A. G.; Lev, O. Nat. Commun. 2013, 4, 2922. (44) Wen, Y.; Wang, B.; Zeng, G.; Nogita, K.; Ye, D.; Wang, L. Chem. Asian J. 2015, 10, 661-666. (45) Li, L.; Wu, Z.; Yuan, S.; Zhang, X.-B. Energy Environ. Sci. 2014, 7, 2101-2122.
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Nano Letters
Figures
Figure 1. Fabrication and microscopy observations of hydrogenated Na2Ti3O7 (H-Na2Ti3O7) nanoarrays grown on Ti foil. (a) Schematic of fabrication process. SEM images of (b) top view and (c) side view of H-Na2Ti3O7. (d, e) TEM images of H-Na2Ti3O7 show nanotubular structure with diameter of 25–35 nm. (f) High-resolution TEM image of H-Na2Ti3O7 shows clear lattice spacing of 0.84 nm, corresponding to the (001) facet of the monoclinic Na2Ti3O7. The surface amorphous phase is separated from the crystalline bulk by white line. The arrows indicate the amorphous area near surface.
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Nano Letters
(b) 275 192
386
435
671
702 915
A-Na2Ti3O7
H-Na2Ti3O7
Ti 2p3/2
Intensity (a. u.)
Intensity (a. u.)
(a)
A-Na2Ti3O7 Ti 2p1/2
3+
Ti (457.9 eV) 3+
Ti (463.6 eV) 4+
Ti (465.1 eV) 4+
H-Na2Ti3O7
200
Ti (459.0 eV)
400
600
800
1000
454
−1
456
458
460
462
464
466
468
Binding energy (eV)
Wavenumber (cm )
(d) 3.0
(c)
2.5
4
1/C (10 F cm )
O-Ti A-Na2Ti3O7
2.0
7
−2
Ti-OH H-Na Ti O 2 3 7
1.5 1.0
2
Intensity (a. u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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A-Na2Ti3O7
527 528 529 530 531 532 533 534 535
H-Na2Ti3O7
0.5 0.0 -0.1
0.0
0.1
0.2
0.3
Potential vs. Ag/AgCl (V)
Binding energy (eV)
Figure 2. Structure characterization of H-Na2Ti3O7 and A-Na2Ti3O7 nanoarrays grown on Ti foil. (a) Raman spectra. High resolution XPS spectra of (b) Ti 2p and (c) O 1s of H-Na2Ti3O7 and A-Na2Ti3O7 nanoarrays. The blue dash line in (b) is the difference in their spectra (H-Na2Ti3O7 minus A-Na2Ti3O7). (d) Mott-Schottky plots of H-Na2Ti3O7 and A-Na2Ti3O7.
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2.0
+
Current (mA)
0.1
Potential (V vs. Na /Na )
(b) 2.5
(a) 0.2
0.5 mV s
0.0
−1
-0.1
1st 2nd 3rd
-0.2 0.0
0.5
1.0
1.5
2.0
2.5
Current pulse: 120 s Relaxation: 3600 s
1.5 1.0 0.5 0.0
Measured curve Relaxed curve
0
300
400
500
(d) 250 −1
Capacity (mAh g )
+
200
Capacity (mAh g )
(c) 2.5 2.0 1.5 1.0 −1
35.4 mA g / 0.2 C
0.5 H-Na2Ti3O7
0.0
100
−1
+
Potential vs. Na /Na (V)
Potential vs. Na /Na (V)
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Nano Letters
A-Na2Ti3O7
0
50
100
150
200
250
300
350
−1
H-Na2Ti3O7
200 150
−1
177 mA g / 1 C A-Na2Ti3O7
100 50 0
0
50
100
150
200
Cycle number
Capacity (mAh g )
Figure 3. Electrochemical Na-storage properties of H-Na2Ti3O7 nanoarrays. (a) CV and (b) galvanostatic intermittent titration technique (GITT) curves of H-Na2Ti3O7. The GITT was operated by introducing current pulse of 1 C for 120 s followed by relaxation for 3600 s. Comparison of (c) galvanostatic curves and (d) cycling performance of H-Na2Ti3O7 and A-Na2Ti3O7. The electrodes were activated at 0.2 C for 5 cycles prior to cycling tests.
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H-Na2Ti3O7
(b) 2.5 Potential vs. Na /Na (V)
+
+
2.0 1.5 0.2C 0.5C 1C 2C 5C 10C 20C 35C
1.0 0.5 0.0
0
50
100
150
200
250
300
A-Na2Ti3O7
2.0 1.5 0.2C 0.5C 1C 2C 5C 10C 20C 35C
1.0 0.5 0.0
0
50
−1
−1
Capacity (mAh g )
Na2Ti3O7 array Microsized Na2Ti3O7 Na2Ti3O7 sphere Nanosized Na2Ti3O7
100
0
0.1
1
250 0.2
250
300
200 150
1C
1 2
2 5
5 10
100
20
50 0
10
0.2 0.5
0.5 1
Discharge Charge
0
20
40
60
80
100
Cycle number
Rate (C)
(e)
200
(d) 300
H-Na2Ti3O7
200
150
Capacity (mAh g )
A-Na2Ti3O7
−1
Capacity (mAh g )
(c) 300
100
−1
Capacity (mAh g )
200
−1
150
150 100 100
−1
6.2 A g / 35 C
50 50
0
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0 10000
Coulombic effciency (%)
Potential vs. Na /Na (V)
(a) 2.5
Capacity (mAh g )
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Cycle number
Figure 4. Rate capability of H-Na2Ti3O7 nanoarrays. Galvanostatic profiles of (a) H-Na2Ti3O7 and (b) A-Na2Ti3O7 at various current rates. (c) Comparison of the rate capability of the H-Na2Ti3O7 with reported high-performance Na2Ti3O7 materials. Note that the reversible (desodiation) capacity is selected for comparison. (d) Rate cycling performance of H-Na2Ti3O7. The H-Na2Ti3O7 electrodes were firstly tested at each rate for 5 cycles then cycled at a fixed rate of 1 C. (e) Long-term cycling of H-Na2Ti3O7 at a robust rate of 35 C for 10,000 cycles. Activation at 0.2 C for 5 cycles is necessary prior to high rate tests.
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Current (mA)
0.2
0.0
-0.2
−1
−1
0.1 mV s −1 0.5 mV s −1 1.0 mV s
-0.4 0.0
0.5
1.0
1.5
0.2 mV s −1 0.8 mV s −1 1.5 mV s
2.0
Peak current (mA)
(b) 0.8
(a) 0.4
Anodic
0.4
0.0
2
R > 0.99
-0.4
Cathodic
-0.8 0.0
2.5
0.5
1.0
1.5
2.0
−1
+
Sweep rate (mV s )
Potential vs. Na /Na (V)
(c)
(d)100 1.0
Ratio of Qs (%)
Normalized capacity
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0.8 0.6 0.4 0.2 −1
>1.5 mV s
