Carbon-coated Li4Ti5O12 nanowires showing high rate capability as an anode material for rechargeable sodium batteries

Carbon-coated Li4Ti5O12 nanowires showing high rate capability as an anode material for rechargeable sodium batteries

Nano Energy (2015) 12, 725–734 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/nanoenergy RAPID COMMUNICATION ...

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Nano Energy (2015) 12, 725–734

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

RAPID COMMUNICATION

Carbon-coated Li4Ti5O12 nanowires showing high rate capability as an anode material for rechargeable sodium batteries Ki-Tae Kima, Chan-Yeop Yua, Chong Seung Yoonb, Sun-Jae Kima, Yang-Kook Sunc,n, Seung-Taek Myunga,nn a

Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea Department of Material Science and Engineering, Hanyang University, Seoul 133-791, South Korea c Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea b

Received 18 September 2014; received in revised form 10 January 2015; accepted 21 January 2015 Available online 31 January 2015

KEYWORDS

Abstract

Lithium titanates; Nanowires; Carbon coating; Anode; Sodium; Battery

This is the first report where crystalline carbon-coated Li4Ti5O12 nanowires are employed as an anode material for sodium-ion batteries. The Li4Ti5O12 nanowires are synthesized via a two-step ionic exchange process from Na2Ti3O7 nanowires to form hydrous lithium titanate nanowires, where excessive lithium oxide is adhered on the surface of the nanowires. The nanowire products are consequently heated to form Li4Ti5O12, and the resultant nanowires are subsequently coated by pitch as the carbon source. X-ray diffraction (XRD) and electron microscopic studies reveal that the carboncoated Li4Ti5O12 nanowires are highly crystalline products and that their nanowire features have been modified with carbon nanolayers (o10 nm in thickness). As a result, the electronic conductivity is approximately 3  10 1 S cm 1. The delivered capacities are about 168 mAh g 1 at a rate of 0.2 C (35 mA g 1), 117 mAh g 1 at a rate of 10 C, 88 mAh g 1 at a rate of 30 C, 67 mAh g 1 at a rate of 50 C, and 38 mAh g 1 at a rate of 100 C; these conductivity values are superior to those achieved with bare Li4Ti5O12. Continuous cycling testing reveals outstanding cycling stability, showing 96.3% capacity retention after cycles. Ex-situ XRD and X-ray photoelectron spectroscopic studies indicate that the electrode reaction is followed by Na + insertion and extraction, accompanied by the Ti4 + /3 + redox couple. We believe that the excellent high rate capacity and rechargeability upon cycling

n Corresponding author.Tel.: +82 2 2220 0524; fax: +82 2 2282 7329. nn Corresponding author. Tel.; +82 2 3408 3454; fax: +82 2 3408 4342. E-mail addresses: [email protected] (Y.-K. Sun), [email protected] (S.-T. Myung).

http://dx.doi.org/10.1016/j.nanoen.2015.01.034 2211-2855/& 2015 Elsevier Ltd. All rights reserved.

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K.-T. Kim et al. result from the unique morphology of the highly crystalline Li4Ti5O12 nanowires assisted by conducting thin carbon layers. & 2015 Elsevier Ltd. All rights reserved.

Introduction There are ever-increasing interests in rechargeable batteries for application in hybrid electric vehicles and as renewable or dispersed energy storage systems, which typically require higher charge/discharge rates, larger battery sizes, and put a greater emphasis on the price, rate capacity, and safety of the batteries [1]. In addition to price concerns, lithium is an exhaustible resource if largescale lithium batteries are progressively used in near future. Conversely, sodium resources are inexhaustible in seawater; therefore, they are potentially low-cost. For this reason, room temperature type sodium-ion batteries have attracted great interest for application in large-scale energy storage applications. Such investigations have mainly focused on finding cathode materials with layer [2–5] and olivine structures [6–8]. Some materials have been suggested as the anode materials, including Na2Ti3O7 [9], Na2C8H4O4 [10], Sn [11–13], Sb/C [14], SnSb/C [15], TiO2/C [16,17], Fe3O4 [18,19], and hard carbon [20–22]. These electrodes can be classified into three categories depending on their reaction type: insertion [9,10,16,17,20–22], conversion [18,19], and alloy formation with Na [11–15]. However, the conversion reaction usually results in severe volume expansion of the active materials, and the alloy formation reaction causes Na metal plating which causes safety concerns. From this point of view, insertion chemistry would be favorable to ensure capacity retention, although the capacity is lower than the above-mentioned conversion and alloy formation chemistries. Among the various types of Na + insertion electrodes, hard carbon is advantageous because it shows the lowest operation voltage (near zero V) versus Na/Na + [20–22], which can increase the energy density of the Na cell. Also, the irreversible capacity is lower than that of Na2Ti3O7 [9] and TiO2/C [16,17]. Operation at high rates is another challenge in Na systems. Although hard carbon delivers a relatively high capacity (300 mAh g 1) at 0.1 C, the capacity drops to 120 mAh g 1 at a rate of 2 C [22]. Another insertion electrode, Na2Ti3O7, shows a flat voltage plateau at 0.3 V when the applied current was C/25. At the low current, the delivered reversible capacity was approximately 100 mAh g 1 with a large irreversible capacity; namely, uptaking 2 mol of Na + ions into Na2Ti3O7 to form Na4Ti3O7. Very recently, Yan et al. [23] reported improvement in electrochemical properties of Na2Ti3O7 by modifying the surface with carbon that delivers a charge capacity about 120 mAh g 1 but results in sloppy discharge–charge curves at 1 C (178 mA g 1), although the carbon coating was not effective in reducing the irreversible capacity on the first cycle. This capacity drop reflects the difficulty of highrate operation using Na + insertion electrodes. Recently, Zhao et al. [24] reported Na + insertion into spinel Li4Ti5O12, showing an average operation voltage of

1 V versus Na/Na + and a reversible capacity of about 145 mAh g 1 at 0.1 C. Additionally, their subsequent work revealed the reaction mechanism that occurs when Li4Ti5O12 reacted with Na + ions between 0.5 V and 3 V: Li4Ti5O12 + 6Na + + 6e 2Li7Ti5O12 + Na6LiTi5O12 [25]. Using X-ray absorption spectroscopy, Yu et al. [26] demonstrated variations in the oxidation state during the above reaction. Intrinsically, Li4Ti5O12 has poor electric conductivity due to the empty Ti 3d state with band energy of 2 eV; under these conditions, cycling at a high current seems to be not possible. Another hurdle for Na + insertion or extraction is that, because the ionic radius of Na + (1.02 Å) is larger than that of Li + (0.76 Å), one can anticipate that fast insertion or extraction of Na + does not occur readily. This necessitates downsizing of the particle size and surface modification with electroconducting substances. From the above review, we hypothesized that a nanostructured Li4Ti5O12 compound modified with conducting carbon would be a suitable candidate as the anode material in sodium-ion batteries. Using these materials, because larger Na + ions are introduced into a host Li4Ti5O12 spinel structure, structural stability is a critical concern to ensure long term cycling stability. Thus, highly crystalline Li4Ti5O12 nanowires are synthesized via a two-step ionic exchange process utilizing a hydrothermal reaction, as shown schematically in Figure 1a. The surface of the product is also modified by carbon to endow high electric conductivity, which is expected to yield high rate capabilities.

Experimental Preparation of carbon-coated Li4Ti5O12 TiO2 powders (P-25, Degusa) were first dispersed in a 10 M NaOH aqueous solution and then transferred to a Teflonlined autoclave to obtain Na2Ti3O7 nanowires [27]. After the hydrothermal reaction at 170 1C for 48 h, the produced Ionic exchange

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Carbon-coated Li4Ti5O12 nanowires showing high rate capability white powders were washed by distilled water until a pH of 7 was obtained. Then, they were dried at 80 1C in air. The dried powders, presumably Na2Ti3O7 nanowires, were subjected to ionic exchange in an HCl aqueous solution to produce H2Ti3O7 nanowires, of which the Na/H mole ratio was 0.125. After the ion exchange, the resultants were washed by distilled water until a neutral pH was reached and dried again at 80 1C in air. Next, the obtained H2Ti3O7 powders (0.5 g) were again hydrothermally treated in a LiOH aqueous solution (50 ml) at 100 1C for 24 h to form lithiated titanates via hydrothermal ionic exchange. After washing the precipitates with ethanol until a pH of 7 was reached, the precipitates were dried at 60 1C. The dried powders were heated to 800 1C for 6 h in air to obtain Li4Ti5O12. The produced Li4Ti5O12 was mixed with an appropriate amount of pitch as a carbon source and calcined at 750 1C for 6 h in Ar to produce carbon-coated Li4Ti5O12.

Structural properties The crystalline phases of the products were characterized by powder X-ray diffraction (XRD, Rint-2000, Rigaku) analysis using Cu-Kα radiation. The XRD data were obtained with a step size of 0.031 and a count time of 5 s. The collected intensity data from the XRD were analyzed by the Rietveld refinement program Fullprof 2002 [28]. The particle morphologies of the produced powders were observed using scanning electron microscopy (SEM, JXA-8100, JEOL) and transmission electron microscopy (H-800, Hitachi). Raman spectra (Renishow, inVia Raman Microscope) were obtained for the carbon-coated Li4Ti5O12. The d.c. electrical conductivity was measured by a direct volt-ampere method (CMT-SR1000, AIT), in which disc samples were contacted with a four-point probe. An elemental analyzer (EA110, CE Instrument, Italy) was employed to determine the amount of carbon in the final products.

Electrochemical measurement For electrode fabrication, the synthesized Li4Ti5O12 was mixed with carbon black and polyvinylidene fluoride (85:7.5:7.5, where 7.5 for carbon black denotes the sum of added carbon black (3.8 wt%) and coated carbon (3.7 wt%) for the carboncoated Li4Ti5O12 electrode) in N-methylpyrrolidinon. The obtained slurries were coated onto Al foil and roll-pressed. The obtained slurry was coated onto Cu foil and roll-pressed. The electrodes were vacuum dried overnight at 80 1C prior to use. Charge–discharge tests were done using R2032 type coin cells having Na metal as the counter electrode. The electrolyte solutions were 1 M NaClO4 in a mixture of polycarbonate and fluoroethylene carbonate (98:2 in volume). The cells were cycled between 0.3 V and 2.5 V from 35 mA g 1 (0.2 C) to 17.5 A g 1 (100 C) at 25 1C. We also suggest another technique to dramatically reduce the irreversible capacity, namely, presodiation. For presodiation, the carbon-coated Li4Ti5O12 nanowire electrode was directly contact with Na metal in the presence of electrolyte to eliminate the additional capacity resulting from the reductive decomposition of electrolyte and a solid electrolyte interface (SEI) formation, and all process was performed in the Ar-filled glove box. During the presodiation process, we continuously checked the open circuit

727 voltage (OCV) of the carbon-coated Li4Ti5O12 nanowire electrode. After 20 min later, the anode electrode reached approximately 0.82 V versus Na/Na + , which corresponds to the initiation voltage that accommodates Na + ions in the spinel structure. Then, we finished the presodiation process.

Electrochemically desodiated/sodiated electrodes The ex-situ XRD patterns were carefully examined using the Cu current collector as the internal standard for lattice parameter calculation by a least square method. The electrodes were wrapped with a Mylar film (ChemplexTM) for the XRD measurement to avoid contamination from air and water molecules. X-ray photoelectron spectroscopy (XPS, PHI 5600, Perkin-Elmer, USA) measurements were performed to obtain information of the oxidation state of titanium in a macro-mode (3 mm  3 mm). The samples were first transferred into a hermitically-sealed transfer chamber (ULVAC) in the glove box, and they were again transferred into the vacuum chamber of XPS machine, thus no exposure to air and water molecules for XPS measurement.

Results and discussion For the synthesis of carbon-coated Li4Ti5O12 nanowires, we employed two-step hydrothermal reactions, as shown in Figure 1a. Hydrothermal reaction of TiO2 in a high concentration NaOH solution at 170 1C for 48 h produces highly crystalline nanowires (Figure 2a-1), and the white-colored products are several micrometers in length (Figure 2b-1). We assign the crystal structure of Na2Ti3O7 as follow: space group P21/m, a=8.5933(1) Å, b=3.7905(1) Å, c=9.1399(2) Å, β=102.281. Ionic exchange of the as-synthesized Na2Ti3O7 nanowires with hydrogen results in a slight deviation in the crystal structure compared with the XRD of Na2Ti3O7; in particular, a shift in the strongest reflection from 2θ=111 to 2θ=121 after the ionic exchange is observed (Figure 2a-2). There are no signs of the Na2Ti3O7-related product in the XRD pattern. The shrinkage of the interlayer is decisive evidence that the larger Na + ions were exchanged for smaller H + ions, and the resulting XRD pattern is assigned to H2Ti3O7 (space group C2/m, a=16.0274 (2) Å, b=3.7377(2) Å, and c=9.2196(4) Å, β=102.071). Also, morphological and color changes are not perceived after the ionic exchange (Figure 2b-2), indicating that the diluted HCl simply acted as a medium for the ionic exchange and did not affect the deformation of the morphology. The ionic-exchanged H2Ti3O7 was again hydrothermally treated with a 0.63 M LiOH aqueous solution at 100 1C for 24 h to exchange H + ions for Li + ions (Figure 2a-3). The hydrothermal treatment of H2Ti3O7 produces a slight shift of the strongest diffraction peak from 2θ =121 to 2θ= 111, and the other diffraction peaks also become stronger than those in H2Ti3O7. The XRD pattern is indexed as hydrous lithium titanate (Li1.81H0.19Ti2O5  2.2H2O, space group C-base-centered orthorhombic, a= 16.5204(1), b= 3.7091(1), and c= 3.0017(1) Å [29,30]). In contrast with the smooth surfaces of Na2Ti3O7 and H2Ti3O7 nanowires (Figure 2b-1 and b-2), we observe that small particles, presumably lithium oxide (Li2O), have adhered on the surface of hydrous lithium titanate nanowires (Figure 2b-3), although the length of the nanowires remains unchanged. For the above two steps, the as-synthesized Na2Ti3O7 and H2Ti3O7 nanowires were washed

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Cu K α 2θ θ / Degree Figure 2 (a) XRD patterns of products, 1: hydrothermal product of Na2Ti3O7, 2: H2Ti3O7 produced by ionic exchange of Na2Ti3O7 in a diluted HCl aqueous solution, 3: hydrous lithium titanate prepared by hydrothermal ionic exchange of H2Ti3O7 in a 0.63 M LiOH aqueous solution at 100 1C, 4: Rietveld refinement results of XRD data for as-synthesized Li4Ti5O12, and 5: carbon-coated Li4Ti5O12 using pitch as the carbon coating source; (b) SEM images of 1: hydrothermal product of Na2Ti3O7, 2: H2Ti3O7 produced by ionic exchange of Na2Ti3O7 in a diluted HCl aqueous solution, 3: hydrous lithium titanate prepared by hydrothermal ionic exchange of H2Ti3O7 in a 0.63 M LiOH aqueous solution at 100 1C, 4: as-synthesized Li4Ti5O12, and 5: carbon-coated Li4Ti5O12; and (c) TEM images of 1: as-synthesized Li4Ti5O12 and 2: carbon-coated Li4Ti5O12 and 3: magnified image of carbon-coated Li4Ti5O12.

with distilled water until the pH reached 7 to completely remove any unreacted NaOH and/or HCl from the surface of the nanowires. However, for the ionic exchange to form hydrous lithium titanate, we intended to leave the residual lithium adhered on the surface of the nanowires to satisfy the chemical composition of Li4Ti5O12, as designated. For this reason, we rinsed the as-produced hydrous lithium titanate nanowires with ethanol to remove any excess LiOH aqueous solution from the surface. Due to the weak X-ray scattering factor of lithium element, the compound adhered on the surface of the nanowires may not appear in the XRD pattern. The nanowires were consequently heated at 800 1C for 6 h in air (Figure 2a-4). Rietveld refinement of the XRD data from the resultant nanowires indicates the formation of highly crystalline cubic spinel Li4Ti5O12 without impurities. The small deviation between the observed and calculated XRD patterns demonstrates that Li, Ti, and O are located in their fixed sites (see, Table S1). The calculated lattice parameter, a=8.3611(1) Å, is close to the value reported in the literature [24–26]. This feature confirms the validity of our strategy; the intentional adherence of excessive lithium oxide on the surface of hydrous lithium titanate nanowires through the hydrothermal ionic exchange process is a significantly important action to create single phase Li4Ti5O12 nanowires. Indeed, direct heat treatment of the dehydrated anatase TiO2 nanowire and Li2CO3 gives rise

to bulk Li4Ti5O12 particles through consolidation of nanowires (Figure S1c and d). Even after the heat treatment, nanowire morphology of the resultants is not altered (Figure 2b-4). Hence, the appropriate amount of adhered lithium oxide is likely to diffuse into the hydrous lithium titanate nanowires during the short calcination time, such that the nanowire morphology is maintained. Other conditions, such as changing the content of the adhered lithium oxide or extending the calcination time, do not bring about the single phase Li4Ti5O12 nanowires (Figure S3). Using TEM (Figure 2c-1), we observe that although the nanowire morphology was maintained throughout the high temperature calcination, slight deformation of the surface is observed. In particular, the smooth edge line of the surface is transformed to a zigzag-shaped edge. This is presumably due to the initiation of crystal growth, which produces polygonal-shaped bulk particles at high temperatures. Therefore, the adherence of the appropriate amount of lithium oxide and the optimized heat treatment conditions enable the formation of Li4Ti5O12 nanowires. Carbon coating was conducted on the as-synthesized Li4Ti5O12 nanowires using pitch as the carbon sources. Rietveld refinement of the XRD pattern for the pitch-treated Li4Ti5O12 does not exhibit extra peaks caused by impurities such as Li2CO3 and TiO2 (Figure 2a-5). The calculated lattice parameter, a=8.3615(1) Å (Table S1), is also retained (similar to that of the carbon-free Li4Ti5O12, a=8.3611(1) Å), implying no

Carbon-coated Li4Ti5O12 nanowires showing high rate capability variation in the oxidation state of Ti in the compound. Provided that reduction occurred during heating, the larger lattice parameter likely appears in the carbon-coated sample because the ionic radius of Ti3 + (0.67 Å) is larger than that of Ti4 + (0.605 Å). From these results, we believe that our carbon coating method does not affect the variation in the crystal structure. As observed in SEM and TEM images (Figure 2b-5, c-2, and c-3), the original morphology of the nanowires is preserved after the carbon coating. As designated, thin carbon layers, estimated to be below 5 nm in thickness, completely encapsulate the nanowires. The residual carbon content in the product is found to be 3.7 wt%, as designated, because a carbon content of about 3 wt% was usually found to exhibit good electrode performance even for different electrode materials such as LiFePO4, LiMn1 xFexPO4, and TiO2 as reported our prior reports [16,17,31–35]. The resulting electronic conductivity for the carbon-coated Li4Ti5O12 is approximately 3  10 1 S cm 1. This high conductivity is most likely ascribed to the presence of the uniformly distributed thin carbon layers on the surface of Li4Ti5O12 nanowires. Carbon-related peaks are not observed for the bare Li4Ti5O12 (Figure S5a), while it is evident that the appearance of D- and G-bands indicates carbonization of pitch at the given heat-treatment condition on the carbon-coated Li4Ti5O12 (Figure S5b). The graphitized carbon is further confirmed on the outermost surface of carbon-coated Li4Ti5O12 in XPS spectra (Figure S5c). The as-synthesized Li4Ti5O12 and carbon-coated Li4Ti5O12 nanowires (hereafter referred to as C-LTO NWs) cells were galvanostatically cycled at 0.2 C (35 mA g 1) between 0.3 and 2.5 V using Na metal as the counter electrode at room temperature (Figure 3). Both bare LTO and C-LTO NWs electrodes exhibit the same feature on discharge (reduction): a slow voltage decay from 1.5 to 0.8 V and a flat plateau from 0.8 to 0.3 V, delivering discharge capacities of 243 mAh g 1 for the bare LTO NWs and 266 mAh g 1 for the C-LTO NWs (Figure 3a). These values exceed the theoretical capacity of Li4Ti5O12 (175 mAh g 1). In order to understand how this overcapacity was obtained, the lower cut-off voltage was changed to 0.8 V, 0.65 V, and 0.3 V using the C-LTO NWs electrode. The delivered capacity is only 9 mAh g 1 upon charging (oxidation) when the lower cut-off was 0.8 V, while the capacity reaches approximately 115 mAh g 1 when the

729 cut-off voltage is further lowered to 0.65 V. These results indicate that the capacity that appears above 0.8 V can be attributed to side reactions with the electrolyte such as reductive decomposition of the electrolyte and formation of SEI layer in the voltage range. The irreversible reaction is also noticed in the derivative curve (Figure 3b). The capacity is maximized at 0.3 V, showing 155 mAh g 1 for the LTO NWs and 168 mAh g 1 for the C-LTO NWs, approaching 96% of the theoretical capacity. Presodiation of the C-LTO NWs electrode is effective in elimination of the irreversible reaction, so that the delivered capacities are 165 mAh g 1 for discharge and charge capacity of 163 mAh g 1 for the charge (Figure 3a). The derivative curve also confirms the presence of voltage plateaus at 0.75 V and 0.55 V on discharge, which are coupled at 0.8 V and 1.0 V upon charging (Figure 3b). In attempt to make comparisons of the produced LTO nanowires with nanoparticles, the as-synthesized bare and C-LTO NWs were ballmilled (see details in Figure S6a–c). Interestingly, the milling induces more irreversible capacity associated with reductive decomposition of the electrolyte and SEI formation in the voltage region of 1.5–0.8 V in the first discharge (Figure S6d), presumably because of the increased specific surfaces (132 m2 g 1) after the ballmill. Note that the distinct voltage plateaus observed for the nanowires are not evident for the ballmilled LTO nanoparticles. The size effect is further confirmed (Figure S6d–f); namely, the ballmilled nanoparticles exhibit smooth voltage decays at the beginning and ending parts of charge and discharge process, compared to the highly crystalline nanowires (Figure 4a and c). Continuous cycling tests reveal that both bare LTO and C-LTO NWs exhibit excellent cyclability during 50 cycles, retaining 92% capacity for the bare LTO and 97% for the C-LTO NWs (Figure 4a and b). Unfortunately, the ballmilled nanoparticles exhibit faster capacity fade than those of nanowires although the retention was better for the carboncoated LTO nanoparticles (Figure S6e). Adhesion of active materials on the current collector would be an important concern to retain the capacity. The electrode geometry would be different between the nanoparticles and nanowires. The formation of Na6LiTi5O7 phase, inducing lattice volume expansion of 12.5% [25], may lead to detachment of the active

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Figure 3 (a) First discharge (reduction) and charge (oxidation) curves in the voltage range of 0.3–2.5 V (0.2 C, 35 mA g 1): bare Li4Ti5O12 (black straight line), carbon-coated Li4Ti5O12, where lower cut-off voltage varies to 0.8 V (blue dash and dot), 0.65 V (blue dash), and 0.3 V (blue straight line), and presodiated carbon-coated Li4Ti5O12 (red dash); (b) the resulting derivative curves for carbon-coated Li4Ti5O12.

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Figure 4 (a) Continuous discharge and charge curves of bare Li4Ti5O12 (top) and carbon-coated Li4Ti5O12 (bottom) (0.2 C, 35 mA g 1) and (b) the resulting cycling data versus capacity; (c) rate capability data of bare Li4Ti5O12 (top) and carbon-coated Li4Ti5O12 (bottom) from 0.5 C to 100 C (17.5 A g 1); and (d) comparison of the resulting capacity versus C-rates shown in red letters.

materials from the current collector, causing fast capacity fade, when the used binder was PVDF for electrode fabrication. By contrast, the long nanowires are tangled each other (see Figure S8b), and this may enhance the contact with current collector. Therefore, the nanowires with or without carbon coating is likely to result in the good capacity retention. These data suggest that the spinel structured nanowires are substantially stable, allowing for the repetitive insertion/extraction of the larger Na + ions (relative to Li + ions) during cycling. If structural collapse occurred as cycling continued, the discharge-charge profiles and the resulting capacity would not be retained during cycling. Achieving good rate capabilities is challenging in Na systems because they utilize large Na + ions. The rate capability was measured under various charge currents with a fixed discharge current density of 0.2 C (Figure 4c and d). As anticipated, the specific charge capacity of bare LTO NWs decreases markedly with increasing charge rate and finally demonstrates a disappointing capacity of 22 mAh g 1 at 20 C (3.5 A g 1, Figure 4c top). Meanwhile, the C-LTO NWs show extraordinary performance with increasing current densities, maintaining exceptionally high capacity: 117 mAh g 1 at 10 C (1.75 A g 1), 88 mAh g 1 at 30 C, 67 mAh g 1 at 50 C, and 38 mAh g 1 at 100 C (17.5 A g 1, Figure 4c bottom). Additionally, these capacities are maintained (Figure 4d). The

rate properties of the C-LTO NWs are far better than that of nanoparticles (Figure S6f). To the best of our knowledge, the present C-LTO material produces the best properties in terms of capacity at high rates. In contrast, the delivered capacity for the bare LTO NWs is only about 22 mAh g 1 at 20 C. The produced materials have a large surface area of 84 m2 g 1 for the nanowires and of 132 m2 g 1 for the nanoparticles; this enlarged reaction area is helpful in reducing the total current applied to the electrodes. This situation enables the nanomaterials to work substantially at high rates. Also, the dispersion of the applied current across the electrode reduces the stress loaded to the nanowires during high rate reactions. Additionally, crystallinity of the C-LTO NWs is another important concern to achieve such a high rate capability. Nevertheless, Na + ions seem unable to be readily accommodated into the spinel LTO structure due to their large size when high currents are applied for both charge and discharge (Figure S7). Ceder et al. [36] reported that Na + extraction/re-insertion from/into Na-containing materials is experimentally and computationally faster than those in Li system due to the low Lewis acidity of Na + . By contrast, insertion of Na + into non Na-containing compounds with a small bottleneck size could be slower than that of Li + . With this in mind, it is worth noting the high rate storage of Na + ions in the highly crystalline C-LTO NWs, which is motivated

Carbon-coated Li4Ti5O12 nanowires showing high rate capability

731 lithiated Li7Ti5O12, where Ti3 + /Ti4 + should theoretically be 60/40. These data suggest Na + insertion into the host Li4Ti5O12 during discharge via the following reaction: 2Li4Ti5O12 +6Na + +6e -Li7Ti5O12 + Na6LiTi5O12 [26]. The calculated lattice parameters, using a least square method, are approximately 8.384(1) Å for the Li7Ti5O12 phase and 8.637(1) Å for the Na6LiTi5O12 phase. These values agree with the synchrotron results by Yu et al. [25]. Interestingly, the diffraction intensity of the Li7Ti5O12 phase becomes lower after the appearance of the Na6LiTi5O12 phase (Figure 3b-3), and the resulting intensity is further lowered at 0.3 V (point 4) as more Na6LiTi5O12 is produced, where the Na6LiTi5O12 phase is more developed than at point 3. Therefore, it appears that once Na + ions are inserted into the host spinel structure, the original Li4Ti5O12 phase separates into Li7Ti5O12 and Na6LiTi5O12 phases, and the newly formed Li7Ti5O12 phase exhibits a reduced relative intensity in the XRD pattern compared to that of the Li4Ti5O12. This tendency, showing the reduced diffraction intensity of Li7Ti5O12, is also observed in other recent reports [25,26]. The full sodiation of Li4Ti5O12 induces the phase separation of Na6LiTi5O12 and Li7Ti5O12. Li7Ti5O12, which is a decomposition product, belongs to the parent Li4Ti5O12 so that the product would have high crystallinity though lower than that of parent oxide. Meanwhile, the Na6LiTi5O12 is the electrochemically produced resultant, so that the ordering of atoms would be rather random compared to that of Li7Ti5O12, which may cause broadening of XRD peaks with low intensity. For the reason, TEM was used to observe the

by the presence of the thin and uniform carbon nanolayers that directly improve the electronic conductivity. At present, the Li4Ti5O12 undergoes electrochemical reaction with the larger Na + ions, which are not likely to occupy tetrahedral 8a sites but prefer to settle at 16c octahedral sites. This is analogous to the Li + insertion process. Ex-situ XRD and XPS measurements were carried out to follow the structural evolution and chemical state of Ti during the first cycle from points 1 to 5 (Figure 5a). The fresh electrode exhibits the cubic spinel structure, and the oxidation state falls to Ti4 + (Figure 5b-1 and c-1). And the resulting selected-area diffraction (SAD) pattern is typically observed in cubic spinel structure (Figure 6a). Compared to the fresh electrode (point 1), the electrode discharged to 0.8 V (point 2) does not show any difference in the structure or the chemical state (Figure 5a-2 and c-2). This confirms that the delivered capacity (81 mAh g 1) is not related to the reduction of Ti but is caused by reductive decomposition of the electrolyte and the formation of an SEI layer. Discharging to point 3, the parent spinel structure does not vary, whereas several shoulder peaks form at 2θ= 17.71, 34.31, 41.41, 56.11, and 61.31 (Figure 5b-3), suggesting the presence of Na6[LiTi5]O12 [24–26]. Additionally, the trivalent Ti3 + is visible in the XPS spectra, showing a Ti3 + /Ti4 + ratio of 37/63. In consideration of the capacity achieved between 0.8 V and 0.65 V, the formation of Ti3 + is reasonable. Further lowering the voltage to 0.3 V (Figure 5b-4), the development of the Na6[LiTi5] O12 phase is noticed, and the ratio of Ti3 + /Ti4 + is close to 54/46 (Figure 5c-4). This agrees with the results of a fully

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Figure 5 (a) First discharge–charge curve of carbon-coated Li4Ti5O12 indicating the places where ex-situ XRD and XPS measurements were carried out; (b) ex-situ XRD patterns and (c) XPS profiles of carbon-coated Li4Ti5O12s; and (d) electrochemical reaction that occurred in carbon-coated Li4Ti5O12 nanowires in the Na cell.

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Figure 6 TEM image of (a) as-synthesized carbon-coated Li4Ti5O12 and its SAD pattern in [001] zone; (b) fully sodiated (at the end of discharge) carbon-coated Li4Ti5O12 that captures a single nanoparticles and that magnifies the end part in which submicro-scale domains evidently present in the single crystal nanowire and the resulting SAD pattern in [001] zone; (c) fully desosdiated (at the end of charge) carbon-coated Li4Ti5O12 and its SAD pattern in [110] zone; and (d) after extensive cycled carbon-coated Li4Ti5O12 and its SAD pattern showing a strong [001] direction texture.

fully sodiated electrode (Figure 6b). The original nanowire morphology is remained even after the phase separation. Besides, the nanowire exhibits two contrasts in the whole particle: dark gray and black parts (marked by white dash line in the magnified image), representing the presence of nano-scale domains within the single crystal nanowire which would be responsible for the weak XRD intensity for the newly formed Na6LiTi5O12. The dark gray part is mainly observed in each end part of the crystal, while the black part appears in the center part of the nanowire. Accordingly, two phases are noticed in the SAD pattern while only one phase is perceived in the SAD pattern before charge (Figure 6a). Assuming these phases as Na6LiTi5O12 and Li7Ti5O12, the ratio of d400(Na6LiTi5O12)/d400(Li7Ti5O12) is approximately 1.1, of which the value coincides with the result obtained our lab XRD and synchrotron XRD data that the expanded lattice volume is about 12.5% [25]. Upon charging, removal of Na + ions from the structure results in the restoration of the original structure without any traces of Li7Ti5O12 and Na6LiTi5O12 phases in the XRD pattern (Figure 5b-5). At the same time, the low diffraction intensity at point 4 becomes higher at point 5. The resulting SAD pattern also represents that the two phases are restored into one phase, Li4Ti5O12, in which the black contrast almost disappears in the TEM image (Figure 6c). The oxidation state of Ti also returns to tetravalent in the XPS spectra (Figure 5c-5). The aforementioned data suggest that Na + insertion into the cubic spinel structure gives rise to the separation of the phase into two cubic phases (Nafree Li7Ti5O12 and Na-rich Na6LiTi5O12), accompanied by a Ti4 + /3 + redox reaction (Figure 5d). This reaction is highly reversible through the recovery of the original Li4Ti5O12 structure. During the process, Na + ions prefer to occupy octahedral sites rather than tetragonal sites because the tetragonal sites are too small to accommodate Na + ions. Recently, Sun et al. [25] calculated the Na + migration using

DFT and suggested that Na + migration through octahedral sites occurs rather slowly relative to Li + migration. Na + movement at high rates is dramatically increased in our Li4Ti5O12 nanowires, which have been modified with a conducting carbon layer. The extensively cycled carbon-coated Li4Ti5O12 nanowires were examined by XRD (Figure S8a). Compared to the assynthesized, the widths of the diffractions peaks become somehow broadened. In comparison with the as-synthesized carbon-coated Li4Ti5O12 nanowires (Figure 2b-5 and Figure S2e), the original morphology of nanowire seems to be maintained after the cycling test in the SEM image (Figure S8b). However, TEM image presents split of the nanowires into several long nanowires along the longitudinal direction after the cycling test (Figure 6d). The repetitive insertion/extraction of the larger Na + ions into the small vacant 16c octahedral site probably induces the continuous phase separation into Li7Ti5O12 and Na6LiTi5O12 on discharge and convergence to Li4Ti5O12 on charge. With cycling, the phase transition would cause accumulation of more stress to the crystal structure of the nanowires, and this, in turn, splits the nanowires along the longitudinal direction, resulting in the appearance of polycrystalline SAD pattern with a strong [001] direction texture (Figure 6d). Therefore, the broadening of the XRD peaks is understood after the cycling (Figure S8a). Since we employed the highly crystalline single crystal nanowires lengthened to several micrometers along the longitudinal direction, we were able to find the morphological evolution during the repetitive Na + insertion/extraction into the cubic spinel Li4Ti5O12.

Conclusions To improve the rate performance of Li4Ti5O12 as the anode material for Na-ion batteries, we synthesize Li4Ti5O12 nanowires via a two-step ionic exchange process. These nanowires are

Carbon-coated Li4Ti5O12 nanowires showing high rate capability modified with conducting carbon to yield carbon-coated Li4Ti5O12 nanowires in which the carbon content are found to be 3.7 wt%. As a result, the electric conductivity of the product could reach approximately 3  10 1 S cm 1. Electrochemical tests demonstrate that the carbon-coated Li4Ti5O12 nanowires electrode exhibits satisfactory high capacity retention and excellent high rate performances. The reaction is based on Na + insertion and extraction through a three-phase reaction (Li4Ti5O12, Li7Ti5O12, and Na6LiTi5O12) followed by the formation of a Ti4 + /3 + redox couple, which is highly reversible. In addition, relatively high operation voltage for Li4Ti5O12 compared to hard carbon has the advantage of preventing the dendrite formation of Na metal, while lower operating voltage of hard carbon closed to 0 V could not have. These extraordinary electrode performances and stable phase transition during repetitive Na + insertion/extraction demonstrate the feasibility of using carbon-coated Li4Ti5O12 nanowires as a high rate anode material for rechargeable Na-ion batteries, particularly in low-cost energy storage applications.

Acknowledgments This research was partly supported by a grant from the National Research Foundation of Korea by the Korean government (MEST) (NRF-2009-C1AAA001-0093307). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology of Korea (NRF-2014R1A2A1A11051197).

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.2015.01.034.

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Chan-Yeop Yu is presently a graduate student under the supervision of Prof. Seung-Taek Myung at Sejong University. His research focuses on materials development in the fields of energy conversion and storage for Na-ion batteries.

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K.-T. Kim et al. Chong Seung Yoon received his Ph.D. degree from Massachusetts Institute of Technology, MA, USA in 1995. Since 2003, he has been a professor with the Department of Materials Science and Engineering at Hanyang University in Korea. His diverse research interests include microstructure characterization using transmission electron microscopy, electrode materials of Li-ion battery, thin film battery, and energy conversion using magnetic and biomimetic materials.

Yang-Kook Sun obtained his PhD degree from Seoul National University, South Korea, and is a Professor of Energy Engineering at Hanyang University, South Korea. His research interests include metal fluoride-coated cathodes, lithium transition metal oxide, olivine-related cathodes and core-shell concentration-gradient materials for advanced lithium ion batteries, and lithium metal-free, lithium sulfur and lithium air batteries.

Sun-Jae Kim is a full Professor in the Department Nanotechnology and Advanced Materials Engineering at Sejong University in Seoul, Korea. He received his PhD in Materials Science and Engineering from Korea Advanced Institute of Science & Technology, Korea in 1992. He studies on the fabrication of various oxide nanopowders such as TiO2, ZnO, LiMn2O4, LiCoO2, and titanate nanotubes for photocatalyst, secondary battery, and hydrogen storage materials. He is now working as a director of Nano-Materials Technology, National Research Foundation of Korea from February 2014.

Seung-Taek Myung is an Associate Professor of Nano Engineering at Sejong University, South Korea. He received his Ph.D. degree in Chemical Engineering from Iwate University, Japan, in 2003. His research interests include development of electro-active materials and corrosion of current collectors of rechargeable lithium and sodium batteries.