Letter pubs.acs.org/NanoLett
Anatase Titania Nanorods as an Intercalation Anode Material for Rechargeable Sodium Batteries Ki-Tae Kim,† Ghulam Ali,‡ Kyung Yoon Chung,‡ Chong Seung Yoon,§ Hitoshi Yashiro,∥ Yang-Kook Sun,*,⊥,¶ Jun Lu,□ Khalil Amine,*,¶,□ and Seung-Taek Myung*,† †
Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea Center for Energy Convergence, Korea Institute of Science and Technology, Seoul 136-791, South Korea § Department of Material Science and Engineering, Hanyang University, Seoul 133-791, South Korea ∥ Dapartment of Chemical Engineering, Iwate University, Morioka, Iwate 020-8551, Japan ⊥ Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea ¶ Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia □ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ‡
S Supporting Information *
ABSTRACT: For the first time, we report the electrochemical activity of anatase TiO2 nanorods in a Na cell. The anatase TiO2 nanorods were synthesized by a hydrothermal method, and their surfaces were coated by carbon to improve the electric conductivity through carbonization of pitch at 700 °C for 2 h in Ar flow. The resulting structure does not change before and after the carbon coating, as confirmed by X-ray diffraction (XRD). Transmission electron microscopic images confirm the presence of a carbon coating on the anatase TiO2 nanorods. In cell tests, anodes of bare and carboncoated anatase TiO2 nanorods exhibit stable cycling performance and attain a capacity of about 172 and 193 mAh g−1 on the first charge, respectively, in the voltage range of 3−0 V. With the help of the conductive carbon layers, the carbon-coated anatase TiO2 delivers more capacity at high rates, 104 mAh g−1 at the 10 C-rate (3.3 A g−1), 82 mAh g−1 at the 30 C-rate (10 A g−1), and 53 mAh g−1 at the 100 C-rate (33 A g−1). By contrast, the anode of bare anatase TiO2 nanorods delivers only about 38 mAh g−1 at the 10 C-rate (3.3 A g−1). The excellent cyclability and high-rate capability are the result of a Na+ insertion and extraction reaction into the host structure coupled with Ti4+/3+ redox reaction, as revealed by Xray absorption spectroscopy. KEYWORDS: Natase TiO2, nanorods, carbon coating, intercalation, anode, sodium battery the delivered capacities differ from synthetic methods.5 The random structure of hard carbon appears to be the major obstacle to fast Na+ insertion. Several types of TiO2 have been introduced as alternative anode materials to graphite in rechargeable lithium batteries.6−11 These TiO2 materials have many advantages such as low operation voltage (1.6−1.9 V vs Li/Li+), high capacity, nontoxicity, and low production cost. The TiO2 forms various polymorphs such as anatase,6−8 rutile,9 brookite,10 and TiO2B.11 Among them, the anatase structure is believed to be the most feasible material for Li+ insertion because its threedimensional open structure allows Li+ insertion.
Demands for rechargeable batteries are increasing for applications in mobile devices, electric-powered transportation, and stationary energy storage. For these applications, lithiumion batteries have been the most promising system because of their long cycle life, high energy density, safety, and so on. However, the limited world’s lithium resources could be rapidly depleted if large lithium batteries are mass produced in the near future. This shortcoming has motivated us to study rechargeable sodium batteries because sodium is the most abundant natural resource on earth. Also different from the lithium system, inexpensive Fe is available for use in the cathode material, namely, as NaFeO2.1,2 The large interslab distances of the Na layers delay sliding of Fe from the transition metal layer to the Na layer. Recently, several groups suggested use of hard carbon as a Na+ insertion anode material.3−5 However, the hard carbon exhibits limited capacity at high rate because of the physical properties of this material. Even worse, © 2014 American Chemical Society
Received: July 24, 2013 Revised: December 24, 2013 Published: January 8, 2014 416
dx.doi.org/10.1021/nl402747x | Nano Lett. 2014, 14, 416−422
Nano Letters
Letter
Figure 1. Power XRD patterns of (a) a bare anatase TiO2 nanorods; carbon-coated anatase nanorod TiO2 carbonized from pitch at 700 °C for 2 h in Ar, (b) 1.4 wt %, (c) 2.9 wt %, and (d) 6.7 wt % carbon-coated nanorods; the resulting TEM images of (e) the bare, (f) 1.4 wt %, (g) 2.9 wt %, and (h) 6.7 wt % carbon-coated nanorods; (i) XPS spectra of bare and 2.9 wt % carbon-coated anatase nanorod TiO2.
could cause side reactions, particularly with electrolyte. Also, absorption and/or adhesion of water molecules is possible, which would be significantly detrimental for operation of the Na cell. Results and Discussion. To overcome the abovementioned drawbacks, we modified as-synthesized anatase TiO2 by carbon coating using pitch as a carbon source (see Experimental Section). The carbon-coated powders are black irrespective of the amount of added pitch (see the insets of Figure 1a−d). The added pitch (3, 5, and 10 wt % versus anatase TiO2) was carbonized, and the detected carbons were found to be approximately 1.4, 2.9, and 6.7 wt %, respectively, as confirmed by a CHN analyzer. The produced anatase nanorod TiO2 exhibits the typical Raman spectrum of anatase TiO2 (Supporting Information S-Figure 1a), and it is evident that the appearance of D- and G-bands indicates carbonization in the given heat-treatment condition (Supporting Information S-Figure 1b−d). From the XRD patterns (Figure 1a−d), the carbon coating does not alter the original anatase TiO2 structure during the heat treatment in argon atmosphere, as evidenced by the typical XRD peaks of anatase TiO2 (2θ = 25.3, 37.8, 48, 53.8, 55, 62.7, 68.7, and 70°) being observed after the pitch treatment. Also, the calculated lattice parameters
There are only two reports on TiO2 operated in a Na cell. Xiong et al.12 investigated amorphous nanowire TiO2 grown on a Ti substrate. Interestingly, capacity increased gradually with charge−discharge cycling, although the delivered capacity reached a maximum of only 120 mAh g−1 (50 mA g−1), and the charge−discharge curves for voltage versus capacity were not consistent with cycling, presumably owing to the amorphous structure. The X-ray absorption study by Xiong et al. indicated that the electrochemical reaction is based on the Ti4+/3+ redox couple, even though they concluded that anatase TiO2 is inactive in their Na cell. Huang et al.13 also tested low crystalline TiO2(B) in a Na cell. The delivered capacity reached approximately 80 mAh g−1 at the first charge (oxidation) based on the Ti4+/3+ redox confirmed by X-ray photoelectron spectroscopy, though the capacity gradually decreased to 40 mAh g−1 upon cycling. In addition, the obtained capacity was quite small at 400 mA g−1 (1.3 C, about 30 mAh g−1) and the sodium storage has been kinetically sluggish due to large Na+ (1.02 Å) ion relative to Li+ (0.76 Å). Nanorod-type anatase TiO2 with one-dimensional nanostructures is worth investigation because the diffusion path of sodium ions can be shortened, and this may result in improved electrode performance. However, the large specific surface area 417
dx.doi.org/10.1021/nl402747x | Nano Lett. 2014, 14, 416−422
Nano Letters
Letter
Figure 2. (a) First discharge curve of bare and 2.9 wt % carbon-coated anatase nanorod TiO2 measured at 10 mA g−1. The carbon-coated anatase nanorod TiO2 electrodes were discharged to different cutoff voltages to 0.8, 0.3, and 0.01 V and subsequently charged to 3 V. Presodiated carboncoated anatase nanorod TiO2 electrode was also added to compare the initial irreversibility; continuous discharge and charge curves of (b) bare and (c) 2.9 wt % carbon-coated anatase nanorod TiO2 measured at 10 mA g−1; rate capability of (d) bare and (e) 2.9 wt % carbon-coated anatase nanorod TiO2 measured from 0.5 C-rate (165 mA g−1) to 100 C-rates (33 A g−1). Those cells were discharged at 10 mA g−1 prior to charge at high rates; (f) resulting cycling and rate capability data; high rate cycling test fixing both discharge and charge rates to (g) 5 C-rates (1.65 A g−1) and (h) resulting high rate cycling data.
of the bare and the carbon-coated TiO2, a = 3.790(1) Å and c = 9.506(1) Å, are close to the values reported in the literature (JCPDS 84-1285). As evident in Figure 1e, the produced TiO2 particles are tens of nanometers in length. Apparently, the nanorods are agglomerated in the image. The presence of carbon coating layers is confirmed in Figure 1f−h, showing thin coating layers of about 1−2 nm thickness. X-ray photoelectron spectroscopic (XPS) data further confirm the presence of carbon on the outermost surface. For the bare nanorod TiO2 (Figure 1i), an air-formed carbon layer (C with H, N, and O) is mainly observed in the photon energy range of 284−288 eV. Another peak observed at 288−291 eV is ascribed to the carbonate layer formed in air. For the carboncoated anatase nanorod TiO2 (Figure 1i), carbon (like graphite) is evident at 284.6 eV. In addition, a carbide-related peak is not found in the range of 280.5−283 eV. These results
indicate that the carbon is not doped but coated on the surface of the anatase nanorod TiO2. The bare and carbon-coated-nanorod TiO2 materials were tested as an anode in a Na cell using Na metal as the counter electrode. Irrespective of carbon contents (0−6.7 wt %), the TiO2 electrodes show a large capacity on first discharge, exceeding 700 mAh (g-TiO2)−1 (Figure 2a). In addition, the voltage profiles exhibit three distinct plateaus: 1.5−1.0, 1.0−0.3, and 0.3−0.01 V. A similar tendency is also observed in the anatase nanowire TiO2 and nanowire TiO2(B) (Supporting Information S-Figure 2). Hence, three voltage plateaus appear to be a typical feature of nanostructured TiO2 in the Na cell. Note that TiO2 does not show similar behavior in the Li cell.10 The first charge capacities delivered are 172 mAh g−1 for the bare (Figure 2b) and 193 mAh g−1 for the anatase nanorod TiO2 coated with 2.9 wt % carbon (Figure 2c), indicating a large irreversible capacity greater than 500 mAh g−1 with low 418
dx.doi.org/10.1021/nl402747x | Nano Lett. 2014, 14, 416−422
Nano Letters
Letter
Figure 3. (a) Ex situ XRD patterns of 2.9 wt % carbon-coated anatase nanorod TiO2; (b) the first charge and discharge curve indicating that the XRD measurements were carried out and the resulting lattice parameters were plotted as a function of capacity; (c) HR-TEM images and SAD pattern of the as-synthesized (left, point 1), SAD pattern with TEM image of point 4, after the first discharge (center), and SAD pattern with TEM image of extensively cycled electrode (right). Wide views of HR-TEM images are shown in Supporting Information S-Figure 6; (d) XANES k-edge spectra of 2.9 wt % carbon-coated anatase nanorod TiO2 at the first cycle; (e) schemating drawing of Na+ insertion and extraction based on the data of ex situ XRD and XANES.
Coulombic efficiency (
