High-Temperature Stable Anatase Titanium Oxide Nanofibers for

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High Temperature Stable Anatase Titanium Oxide Nanofibers for Lithium Ion Battery Anodes Sangkyu Lee, Wonsik Eom, Hun Park, and Tae Hee Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06631 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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ACS Applied Materials & Interfaces

High-Temperature, Stable Anatase Titanium Oxide Nanofibers for Lithium Ion Battery Anodes Sangkyu Lee†*, Wonsik Eom‡, Hun Park‡, and Tae Hee Han‡*

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Department of Materials Science and Engineering, Hanyang University, Seoul 133- 791, Korea

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Department of Organic and Nano Engineering, Hanyang University, Seoul, 133-791, Korea

*Corresponding Authors Prof. Tae Hee Han (T. H. Han) E-mail: [email protected] Dr. Sangkyu Lee (S. Lee) E-mail: [email protected]

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Abstract Control of the crystal structure of electrochemically active materials is an important approach to fabricating high performance electrodes for lithium ion batteries (LIBs). Here, we report a methodology for controlling the crystal structure of TiO2 nanofibers by adding aluminum isopropoxide to a common sol-gel precursor solution utilized to create TiO2 nanofibers. The introduction of aluminum cations impedes the phase transformation of electrospun TiO2 nanofibers from the anatase to the rutile phase, which inevitably occurs in the typical annealing process utilized for the formation of TiO2 crystals. As a result, high temperature stable anatase TiO2 nanofibers were created in which the crystal structure was well maintained even at high annealing temperatures of up to 700 °C. Finally, the resulting anatase TiO2 nanofibers were utilized to prepare LIB anodes, and their electrochemical performance was compared to pristine TiO2 nanofibers that contain both anatase and rutile phases. Compared to the electrode prepared with pristine TiO2 nanofibers, the electrode prepared with anatase TiO2 nanofibers exhibited excellent electrochemical performances, such as an initial Coulombic efficiency of 83.9%, a capacity retention of 89.5% after 100 cycles, and a rate capability of 48.5% at a current density of 10 C (1 C = 200 mA g-1).

KEYWORDS: Phase transformation; Titanium oxide; Anatase; Rutile; Nanofibers; Lithium ion battery

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Introduction TiO2 has been widely investigated as a potential anode material for lithium ion batteries (LIBs) due to its low cost and good structural stability during Li insertion/extraction.1-6 Unlike conventional anode materials, such as graphite, silicon or their composites, the high operating potential (1–3 V vs. Li/Li+) of TiO2 materials addresses safety issues of LIBs related to the formation of a solid electrolyte interface (SEI) layer and the growth of Li dendrites, leading to excellent electrochemical stability.1-5 Based on these advantageous characteristics of TiO2 materials, many studies for TiO2 anode materials have been reported until now.7-14 Additionally, the applications of TiO2 electrodes are further expanding into other energy storage devices such as lithium ion capacitors,15 sodium ion batteries,16 and lithium-sulfur batteries.17 TiO2 has well-known brookite, anatase, and rutile polymorphs. Among them, anatase TiO2 has remarkable electrochemical properties including a relatively high capacity of 167.5 mAh g-1, a long flat voltage region (plateau) at a high reaction voltage (~1.78 V vs. Li/Li+) and negligible volume expansion induced by lithium insertion (~4%).18 Direct comparison of the electrochemical performances of electrodes prepared with anatase and rutile TiO2 reveals clear differences.19-21 For example, an anatase TiO2 electrode presented a long voltage plateau at ~1.75 V, indicating the coexistence of a Li-poor phase (Li0.05TiO2) and Li-rich phase (Li0.5TiO2).22 In contrast, there was a sloped region without a plateau19 or plateau regions at lower potentials of ~1.4 and 1.1 V20-21 in the case of rutile TiO2 electrodes. Additionally, lithium diffusion in rutile TiO2 is limited due to its unique crystal structure,23 resulting in the poor electrochemical performance of rutile TiO2 materials.24 To overcome this drawback, the size of rutile TiO2 must be reduced to tens of nanometers.24 3



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Beyond the strategy to reduce the crystallite size of TiO2, however, the crystal phase transition of TiO2 from metastable anatase to thermodynamically stable rutile can easily occur at a temperature of ~500 °C.25 This should cause processing issues during the preparation of TiO2 materials. This issue is extremely critical in the electrospinning process. The electrospinning process has been widely utilized to fabricate TiO2-based LIB anodes26-33 because the resulting nanofibers have a large electrochemically active surface area and short diffusion length for Li ions. In general, the polymeric matrix in electrospun TiO2 nanofibers should be removed by an annealing process to form inorganic nanofibers for LIB applications.34-36 For example, the poly(vinylpyrrolidone) phase in electrospun nanofibers can be completely removed at a temperature as high as 500 °C.35 However, at such temperatures, the crystal structure of a small part of TiO2 can be transformed from anatase to rutile so that a trace of the rutile phase is included in the TiO2 nanofibers.27 Here, we report a methodology for preparing TiO2 nanofibers with a high temperature stable anatase phase for their application as LIB anodes. The addition of aluminum cations37 to the TiO2 precursor solution yields TiO2 particles with a high-temperature, stable anatase phase due to the retarded crystal transition effect of the aluminum dopant. In this work, aluminum isopropoxide was selected as the source of aluminum cations for preparing TiO2 nanofibers with a controlled crystal structure. The morphology and crystal structure of TiO2 nanofibers were investigated by increasing the annealing temperature from 500 to 700 °C and by varying the concentration of the aluminum component. Finally, the electrochemical performance of TiO2 nanofiber-based electrodes was evaluated to demonstrate the application of the LIB anodes.

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Experimental Details Fabrication of TiO2 nanofibers Titanium (IV) isopropoxide (97+%, Alfa Aesar, U.S.A.), aluminum isopropoxide (98%, Aldrich, U.S.A.), N,N-dimethyl formamide (DMF, 99.5%, Samchun Chemical, Korea), poly(vinyl acetate) (PVAc, Mw ~140,000, Aldrich), and acetic acid (99.5%, Samchum Chemical, Korea) were used to produce pristine TiO2 and aluminum component-containing TiO2 nanofibers (AlTiO2 nanofibers). For the preparation of the pristine TiO2 nanofibers, 3 g of PVAc was dissolved in 10 mL of DMF solvent. Then, 1.5 g of titanium (IV) isopropoxide and 0.6 g of acetic acid were subsequently added to the solution. After stirring for 6 hours, the homogeneous solution was transferred into a syringe equipped with a metal nozzle (18 G, NanoNC Co., Ltd, Korea). The electrical field between the nozzle and foil current collector was controlled to be 1 kV/cm. The feed rate of the precursor solution was 1.5 mL/h. To control the crystal structure of the TiO2 nanofibers, an adequate amount of aluminum isopropoxide was added to the precursor solution. The mixing molar ratio of Ti4+ to Al3+ was varied from 99:1 to 80:20. Electrospun mats were detached from the current collector and annealed at various temperatures ranging from 300 to 700 °C for 1 h under ambient conditions.

Assembly of LIBs TiO2 nanofibers, poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP, KynarFlex 2801, Arkema Inc., France), and carbon black (Super P, Alfa Aesar, U.S.A.) were homogenized using a planetary mixer (Thinky Mixer, Thinky Corporation, Japan) to prepare slurries for the TiO2 nanofiber anodes. The resulting slurries were coated on a Cu foil by doctor blading. The 5



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mixing ratio of active materials, binder, and conducting additive was 70:20:10, respectively. The average loading level of the active materials on the electrode was approximately 0.68 mg/cm2. The processing steps, including annealing, slurry preparation, and slurry casting, can affect the morphology of inorganic nanofibers. The morphological changes are described in Figure S1 in the Supporting Information. After drying in a vacuum oven at 80 °C for 12 h, 2032 coin cells were assembled by inserting a polypropylene separator between a lithium metal foil and the TiO2 nanofiber electrode. A solution of 1 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/diethylene carbonate (DEC) was utilized as the electrolyte (Panax E-Tec Co., Ltd, Korea). The volume ratio of EC to DEC was 1:1. The assembly was conducted in an argonfilled glove box.

Characterizations A field emission scanning electron microscope (FE-SEM, JSM 4700F, JEOL, Japan) and the high-resolution transmission electron microscope (HR-TEM, JEM-2100F, JEOL, Japan) were utilized to observe the morphologies of the TiO2 nanofibers. Atomic mapping was conducted via energy-dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) analysis was conducted using a Bruker Miller diffractometer with Cu-Kα radiation. X-ray photoelectron spectroscopy (XPS) was utilized to interpret the suppressed phase transformation of the TiO2 nanofibers in the presence of the aluminum component using a Thermo Scientific Theta probe with monochromatic Al Kα radiation. A microbalance (Sartorius SE2, resolution of 0.1 µg, Sartorius, Germany) was utilized to precisely measure the mass of the TiO2 nanofiber electrodes. Galvanostatic charge-discharge tests were performed in the voltage range of 1 to 3 V vs. Li/Li+ at 6


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a current density of 0.2 C to assess the cycle stability and at different current densities ranging from 0.2 to 10 C to evaluate the rate capability. In this study, 1 C corresponds to 200 mAh g-1. A battery tester (TOSCAT 3100, Toyo Systems, Japan) was utilized for the tests. Electrochemical impedance spectroscopy (EIS) analysis was performed using a potentiostat/galvanostat (SP-200, Bio-Logic SAS instrument, France). The measurements were conducted in the frequency range of 250 kHz to 10 mHz at an amplitude of 5 mV.

Results & Discussion Pristine TiO2 and Al-TiO2 nanofibers were produced by electrospinning and subsequent annealing at different temperatures (500, 600, and 700 °C). Their crystallographic information was obtained using XRD analysis, and the results are shown in Figure 1. The pristine TiO2 nanofibers that were annealed at 500 °C clearly contained both anatase and rutile phases (Figure 1a). The lowest peak observed at 25.3° corresponds to the characteristic peak of anatase TiO2 (JCPDS, No. 21-1272). The higher peak at 27.5° belongs to the rutile phase in TiO2 nanofibers (JCPDS, No. 76-1940). As the annealing temperature was increased, the majority phase of the TiO2 nanofibers became rutile. In the case of Al-TiO2 nanofibers, the anatase phase was maintained even at the annealing temperature of 700 °C, although a tiny peak corresponding to the rutile phase was observed at 27.5° (Figure 1b). This trend is similar to the results of a previous study involving TiO2 particles.37 According to that study, Al2O3 clusters on the surface of TiO2 particles interfered with the nucleation and growth of the rutile phase, which consequently impeded the phase transformation from the anatase to rutile phase. To verify this mechanism, the crystallite size of both pristine TiO2 and Al-TiO2 nanofibers was calculated 7



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using the Scherrer equation (Figure S2 in the Supporting Information). The crystallite size of the Al-TiO2 nanofibers was smaller than that of pristine TiO2 nanofibers, which signifies that the addition of aluminum content inhibits the growth of anatase phases and delaying the transformation into rutile phases. To analyze the chemical interactions occurring in the TiO2 nanofibers, pristine TiO2 nanofibers were annealed at 400 and 500 °C, whereas Al-TiO2 nanofibers were annealed at 300, 400, and 500 °C. As shown in Figure S3 in the Supporting Information, the color of the pristine TiO2 nanofibers annealed at 400 °C is light gray, but it changes to white when annealed at 500 °C. The Al-TiO2 nanofibers annealed at 400 °C are black colored, and they turn white when annealed at 500 °C. The difference in the powder color is related to the residual carbon present at lower temperatures. Therefore, the presence of aluminum components in the nanofibers seems to delay the decomposition of carbon-based components in TiO2 nanofibers. The XPS analysis results supported the delayed transformation of the anatase phase in the Al-TiO2 nanofibers. Figure 2 compares the O 1s and Ti 2p spectra of pristine TiO2 and Al-TiO2 nanofibers annealed at different temperatures. The pristine TiO2 nanofibers annealed at 400 and 500 °C show almost symmetric O 1s spectra with respect to 529.5 eV with a slight tail in the range of 530 to 543 eV. The clear peak at 529.5 eV corresponds to Ti-O bonding in the TiO2 nanofibers, whereas the tail regions are caused by the presence of H2O, C-O, and OH.38 The Al-TiO2 nanofibers annealed at 300 °C exhibit a broad peak centered at 531 eV. The peak is significantly reduced at higher annealing temperatures. After annealing at 500 °C, a nearly symmetric spectrum was obtained at 529.5 eV, which is similar to the results of pristine TiO2 nanofibers annealed at 400 and 500 °C. Consequently, the O 1s spectrum indicates that the 8


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presence of the aluminum component retards the formation of Ti-O bonding at lower temperatures. As shown in Figure 2b, pristine TiO2 nanofibers annealed at 400 and 500 °C exhibit clear peaks at 485.1 and 463.7 eV for Ti 2p3/2 and Ti 2p1/2, respectively. The Al-TiO2 nanofibers annealed at lower annealing temperatures of 300 and 400 °C show broad spectra. All peaks related to Ti-O bonding are shifted to a higher bonding energy level compared to the pristine TiO2 nanofibers. The Al-TiO2 nanofibers annealed at 500 °C show a similar spectrum as the pristine TiO2 nanofibers. The spectra of O 1s and Ti 2p signify that the presence of aluminum components delays the formation of stable Ti-O bonding in the TiO2 nanofibers. In addition, the Al 2p spectra of Al-TiO2 nanofibers were obtained by increasing the annealing temperature from 300 to 700 °C to examine the chemical state of the aluminum component in the Al-TiO2 nanofibers (Figure S4 in the Supporting Information). A broad peak was observed in the Al-TiO2 nanofibers annealed at 300 °C, and it sharpened as the annealing temperature increased. The Al-TiO2 nanofibers annealed at 500 °C exhibited a sharper peak with a maximum intensity at 73.3 eV at the binding energies for metallic Al (72.6 eV) and the Al-O bond (74.6 eV).39,40 Additionally, a further increase in the annealing temperature (> 500 °C) caused a peak shift toward a higher binding energy corresponding to the Al-O bond. It has been reported that the formation of Al2O3 clusters on the TiO2 surface prevents the nucleation and growth of the rutile phase in TiO2 particles.37 However, our XRD analysis (Figure 1) showed that the anatase phase of Al-TiO2 nanofibers is well maintained even at the annealing temperature of 500 °C. Therefore, such retarded phase transformation at 500 °C seems to be related not only to the formation of the Al-O bond but also to the presence of metallic Al. 9



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The morphological evolution of TiO2 nanofibers was evaluated as a function of the annealing temperature. At the annealing temperature of 500 °C, continuous nanofibers of a diameter of ~200 nm were formed (Figure 3a and d, and Figure S5 in the Supporting Information), irrespective of the presence of the aluminum component. We observed that pristine TiO2 nanofibers consist of a large amount of nanoparticles (~20 nm in diameter). This result is identical to the high resolution TEM image (Figure S6 in the Supporting Information). In contrast, the addition of aluminum components led to nanofibers with a smooth surface (Figure 3d); however, the high resolution TEM (Figure S6 in the Supporting Information) revealed that the resulting nanofibers consist of smaller nanoparticles (~ 10 nm in diameter). EDX analysis also revealed a homogeneous distribution of aluminum elements in the Al-TiO2 nanofibers (Figure S7 and S8 in the Supporting Information). With increasing annealing temperature, the grain size of the pristine TiO2 nanofibers increased (Figure 3b and c). In the case of Al-TiO2 nanofibers, higher annealing temperatures caused a relatively rougher surface (Figure 3e). A further increase in the annealing temperature up to 700 °C resulted in a porous-structured surface morphology that consisted of nanometer-sized bumps on the surface of the Al-TiO2 nanofibers (Figure 3f). To evaluate the influence of the annealing temperature on the electrochemical performance of the TiO2 nanofibers, galvanostatic charge-discharge tests were performed with the nanofibers prepared at different annealing temperatures (500, 600, and 700 °C). All TiO2 nanofiber electrodes were charged and discharged in the voltage range of 1 to 3 V vs. Li/Li+ at a constant density of 0.2 C (1C = 200 mAh g-1) for up to 100 cycles. Figure 4a shows the first charge and discharge curves of the pristine TiO2 nanofibers obtained at different annealing 10


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temperatures. The electrode consisting of pristine TiO2 nanofibers that were annealed at 500 °C exhibited charge and discharge capacities of 220.7 and 181.1 mAh g-1, respectively. The electrode also showed clear voltage plateaus at 1.90 and 1.75 V during the charge and discharge processes, respectively. However, the high temperature annealing of pristine TiO2 nanofibers led to significant decreases in both the charge and discharge capacities. With increasing annealing temperature for the pristine TiO2 nanofibers, multiple voltage plateaus were formed during the discharge process, as is often observed in rutile TiO2 electrodes.27 As the annealing temperature increased, the rutile content in the TiO2 nanofibers increased, resulting in rutile TiO2-like electrochemical properties. In addition, annealing at higher temperatures led to grain growth of the pristine TiO2 nanofibers (Figure 3a-c), decreasing the capacity of the TiO2 electrode due to the slow transport rate of Li ions in larger-sized rutile particles.27 Figure 4b shows the first charge and discharge curves of the Al-TiO2 nanofiber electrodes. The electrode prepared with Al-TiO2 nanofibers that were annealed at 500 °C showed slightly lower charge and discharge capacities than those of the electrode prepared with pristine TiO2 nanofibers. The nanofibers contain a large amount of the unreactive aluminum component (10 mol%). With increasing annealing temperature, the charge and discharge capacities during the first cycle slightly decreased. However, all Al-TiO2 nanofiber electrodes exhibited stable plateau regions at 1.90 and 1.75 V, irrespective of the annealing temperature. Based on the voltage profiles of both nanofibers (Figure 4a and b), it is likely that the large rutile content in TiO2 nanofibers results in an irregular voltage profile that exhibits multiple voltage plateaus.27 However, the addition of the aluminum component to the TiO2 nanofibers retards the phase

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transformation, resulting in a long plateau region, which is a typical electrochemical characteristic of an anatase TiO2 electrode. Additionally, the cycle performance of the pristine TiO2 and Al-TiO2 nanofiber electrodes was evaluated, and the results are shown in Figure 4c and d. The charge and discharge capacities of pristine TiO2 nanofibers annealed at 500 °C gradually decreased as cycling progressed (Figure 4c). After 100 galvanostatic charge-discharge cycles, the electrode consisting of pristine TiO2 nanofibers that were annealed at 500 °C only delivered 79.5% of its initial charge capacity. Meanwhile, the electrode consisting of the anatase TiO2 nanofibers prepared by adding the aluminum component showed superior cycle performance compared with the pristine TiO2 nanofiber electrode (Figure 4d) that had a capacity retention of 89.5%. However, slight capacity fading was observed in the Al-TiO2 nanofibers annealed at 700 °C. The cycle performance results indicate that the presence of a larger amount of the rutile phase in the TiO2 nanofibers decreases the charge and discharge capacities and causes significant capacity fading during the charge-discharge cycles. The addition of the aluminum component to TiO2 nanofibers also improved the Coulombic efficiency of the TiO2 nanofiber electrodes, as shown in Figure S9 in the Supporting Information. At the first charge and discharge, the electrode with Al-TiO2, which is the anatase TiO2, exhibited a Coulombic efficiency of 83.9%, whereas the electrode with pristine TiO2, which is mainly rutile TiO2, exhibited only a Coulombic efficiency of 65.4%. After 10 cycles, the Coulombic efficiencies of all TiO2 nanofiber electrodes exceeded 99.9%. Consequently, the low efficiency obtained at early cycle numbers of the pristine TiO2 nanofiber electrode is related to the rutile phase in TiO2 nanofibers. 12


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To assess the kinetics of the TiO2 nanofiber electrodes, the rate capability was evaluated by gradually increasing the current density from 0.2 to 10 C. After the test, the current density was subsequently decreased to the initial current density. Figure 5 compares the rate capabilities of the pristine TiO2 and Al-TiO2 nanofiber electrodes. At a low current density, there is little difference between the rate capabilities of both electrodes. In general, at higher current densities, the capacities of the electrodes gradually decrease due to the slow ion transport rate at higher current densities. Therefore, the difference in the rate capability between the two different electrodes becomes apparent at higher current densities. At the current density of 10 C, the pristine TiO2 nanofiber electrode only delivered 36.8% of the initial capacity. In contrast, 48.5% of the initial charge capacity of the Al-TiO2 TiO2 nanofiber electrode was maintained at the same current density. Long-term testing of the rate capability at 10 C was also performed for 100 cycles, and the results are shown in Figure S10 in the Supporting Information. To interpret the different electrochemical performances of the TiO2 nanofibers, electrochemical impedance spectroscopy (EIS) was utilized. First, the two electrodes were discharged to 1 V, and the measurement was conducted in the frequency range of 250 kHz to 10 mHz with an excitation amplitude of 5 mV. The typical shape of Nyquist plots consisting of a semicircle followed by a linear line was observed in both electrodes (Figure S11 in the Supporting Information). The semicircle is related to the charge transport in the SEI layer and charge transfer resistance at the electrode-electrolyte interface. However, the formation of the SEI layer barely occurred because TiO2 electrodes generally have a high reaction voltage. Therefore, the semicircle observed in the TiO2 electrodes is mainly related to the charge transfer resistance at the TiO2 electrode-electrolyte interface. The lower charge transfer resistance 13



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indicates the faster kinetics of the Faradaic reaction. However, as shown in Figure S11 in the Supporting Information, both electrodes showed similar charge transfer resistances, irrespective of the presence of the aluminum component in the TiO2 nanofibers. In contrast, in the charged state (3 V), the Al-TiO2 nanofiber electrode showed a lower charge transfer resistance compared with the pristine TiO2 nanofiber electrode (Figure S11c in the Supporting Information). This signifies that the Al-TiO2 electrode is capable of rapid desertion of Li ions from the lithiated TiO2 nanofibers. Consequently, such a lower charge transfer resistance in the Al-TiO2 electrode could be attributed to its excellent cycle performance and rate capability. To investigate the influence of aluminum components on the electrochemical performances of the TiO2 nanofiber electrodes, TiO2 nanofibers with different concentrations of the aluminum component were prepared. The mixing molar ratio of Ti4+ to Al3+ was varied from 99:1 to 80:20. As shown in Figure 6a, both the charge and discharge capacities of the TiO2 nanofibers were significantly affected by the aluminum content. Higher aluminum contents led to a decreased plateau region. This trend is natural because the aluminum component does not participate in the reaction with Li ions. However, an appreciable capacity fading was observed in the TiO2 nanofiber electrode, which contained a low concentration of the aluminum component (1 mol%) (Figure 6b). This result implies that the rutile contents in the Al-TiO2 nanofibers with 1 mol% of aluminum content are similar to those in the pristine TiO2 nanofibers. (Figure S12 in the Supporting Information). While the addition of 5 mol% aluminum content to the TiO2 nanofibers slightly improved the cycle performance of the TiO2 nanofiber electrodes, the electrode still exhibited slight capacity fading. However, the addition of 10 mol% aluminum content to the TiO2 nanofibers improved the cycle performance of the TiO2 nanofiber electrode 14


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in which no significant capacity fading was observed. The presence of a greater amount of the aluminum component (20 mol%) in the Al-TiO2 nanofibers improved the cycle performance of the electrode; however, it resulted in a decreased capacity. Al-TiO2 nanofibers containing more than 10 mol% of aluminum contents exhibited complete anatase (Figure S12 in the Supporting Information). Additionally, the electrodes prepared with the Al-TiO2 nanofibers exhibited excellent cycle performance without capacity fading. Thus, it is likely that the presence of an anatase phase in the Al-TiO2 nanofibers contributes to stable cycle performance.

Conclusion In summary, the phase transformation from the anatase to rutile phase in TiO2 nanofibers was controlled by adding an aluminum component to the precursor solution of the TiO2 nanofibers. The addition of aluminum components results in the incomplete burnout of the polymeric phase in the aluminum component-containing TiO2 nanofibers, which retards the formation of Ti-O bonds and consequently delays the phase transformation from anatase to rutile. The phasecontrolled TiO2 nanofibers were utilized to fabricate LIB anodes, and their electrochemical performances were evaluated. When the annealing temperature was increased, the capacity and cycle performance of the pristine TiO2 nanofibers degraded, whereas those of the anatase TiO2 nanofibers were not significantly impacted. The electrochemical performance of Al-TiO2 nanofibers was also evaluated as a function of the aluminum component concentration. With a low aluminum component concentration, the electrode demonstrated electrochemical properties similar to those of the rutile TiO2 nanofiber electrode. However, the addition of a high concentration of the aluminum component degraded the electrochemical properties of the 15



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electrode. By optimization of the annealing temperature and aluminum component concentration, TiO2 nanofiber anodes with excellent electrochemical properties were finally obtained. This methodology can also be extended to the implementation of comb-structured LIBs prepared with electrochemically active materials, which must be annealed at high temperatures. Associated content Supporting Information: SEM of TiO2 nanofibers at each fabrication step, graph of crystallite size and ratio of XRD peak intensity, pictures of The TiO2 nanofiber powder, XPS spectra (Al 2p) of the Al-TiO2 nanofibers, high resolution SEM images of the TiO2 nanofibers, high resolution TEM images and results of the EDX analysis, Coulombic efficiencies, rate capabilities, and Nyquist plots of the TiO2 nanofiber lithium ion battery, rate capabilities of the TiO2 nanofiber lithium ion battery, XRD patterns of Al-TiO2 with different aluminum contents

Acknowledgements This research was financially supported by the program for fostering next-generation researchers in engineering (2017H1D8A2032495), the Nano·Material Technology Development Program (2016M3A7B4905609), and Basic Science Research Program (2013R1A1A2013126) of the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning.

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(37) Ding, X.; Liu, L.; Ma, X.; Qi, Z.; He, Y. The Influence of Alumina Dopant on the Structural Transformation of Gel-Derived Nanometre Titania Powders. J. Mater. Sci. Lett. 1994, 13, 462464. (38) Luciu, I.; Bartali, R.; Laidani, N. Influence of Hydrogen Addition to an Ar Plasma on the Structural Properties of TiO2− x Thin Films Deposited by RF Sputtering. J. Phys. D: Appl. Phys. 2012, 45 (34), 345302. (39) Sarapatka, T. J. Palladium-Induced Charge Transports with Palladium/Alumina/Aluminum Interface Formation. J. Phys. Chem. 1993, 97(43), 11274-11277. (40) Hess A.; Kemnitz E.; Lippitz A.; Unger W.E.S.; Menz D.-H. ESCA, XRD, and IR Characterization of Aluminum Oxide, Hydroxyfluoride, and Fluoride Surfaces in Correlation with Their Catalytic Activity in Heterogeneous Halogen Exchange Reactions, J. Catal. 1994, 148, 270-280. Legends Figure 1. XRD patterns of (a) pristine TiO2 and (b) Al-TiO2 nanofibers annealed at 500, 600, and 700 °C.

Figure 2. (a) O 1s and (b) Ti 2p XPS spectra of pristine TiO2 (lower part) and Al-TiO2 nanofibers (upper part) annealed at different temperatures.

Figure 3. SEM images of (a-c) pristine TiO2 and (d-f) Al-TiO2 nanofibers prepared at different annealing temperatures of 500, 600, and 700 °C.

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Figure 4. Electrochemical performances of pristine TiO2 and Al-TiO2 nanofibers. First charge and discharge cycles of (a) pristine TiO2 and (b) Al-TiO2 nanofibers. Cycle performances of (c) pristine TiO2 and (d) Al-TiO2 nanofibers. The open and filled symbols correspond to discharge and charge, respectively.

Figure 5. Rate capabilities of pristine TiO2 and Al-TiO2 nanofibers. The numbers correspond to the current rate (1 C = 200 mA g-1).

Figure 6. (a) First charge-discharge curves and (b) cycle performance of Al-TiO2 nanofibers prepared with different aluminum contents. The results for TiO2 nanofibers with a 10 mol% aluminum content are the same as those shown in Figure 4b and d. The open and filled symbols represent discharge and charge, respectively.

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Figure 1. XRD patterns of (a) pristine TiO2 and (b) Al-TiO2 nanofibers annealed at 500, 600, and 700 °C.

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Figure 2. (a) O 1s and (b) Ti 2p XPS spectra of pristine TiO2 (lower part) and Al-TiO2 nanofibers (upper part) annealed at different temperatures.

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Figure 3. SEM images of (a-c) pristine TiO2 and (d-f) Al-TiO2 nanofibers prepared at different annealing temperatures of 500, 600, and 700 °C. 26


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Figure 4. Electrochemical performances of pristine TiO2 and Al-TiO2 nanofibers. First charge and discharge cycles of (a) pristine TiO2 and (b) Al-TiO2 nanofibers. Cycle performances of (c) pristine TiO2 and (d) Al-TiO2 nanofibers. The open and filled symbols correspond to discharge and charge, respectively.

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Rate Capabiltiy (%)


100

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0.2

0.2 0.5

80

1.0 2.0

60

5.0

40

10 C

TiO2 NF

20

Al-TiO2 NF 0

5

10

15

20

25

30

35

Cycle (#) Figure 5. Rate capabilities of pristine TiO2 and Al-TiO2 nanofibers. The numbers correspond to the current rate (1 C = 200 mA g-1).

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Figure 6. (a) First charge-discharge curves and (b) cycle performance of Al-TiO2 nanofibers prepared with different aluminum contents. The results for TiO2 nanofibers with 10 mol% aluminum content are the same as those shown in Figure 4b and d. The open and filled symbols represent discharge and charge, respectively.

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Table Of Contents (TOC) graphic

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High-Temperature Stable Anatase Titanium Oxide Nanofibers for

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