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Highly Ordered TiO2 Microcones with High Rate Performance for Enhanced Lithium-Ion Storage Oonhee Rhee, Gibaek Lee,* and Jinsub Choi* Nano & Energy Materials Lab, Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, South Korea S Supporting Information *
ABSTRACT: The perpendicularly oriented anatase TiO2 microcones for Li-ion battery application were synthesized via anodization of a Ti foil in aqueous HF + H3PO4 solution. The TiO2 microcones exhibited a high active surface area with a hollow core depending on applied voltage and reaction time, confirmed by SEM, XRD and TEM with EDS mapping. Li insertion/desertion into TiO2 microcones was evaluated for the first time in half-cell configuration in terms of various current density and long-term cyclability. The electrochemical experiments demonstrated that the as-prepared TiO2 microcones as anode material exhibited 3 times higher capacity as compared with TiO2 nanotubular structures, excellent rate performance (0.054 mAhcm−2 even at 50 C) and reliable capacity retention during 500 cycles, which was attributed to facile diffusion of Li-ions induced in hollow anatase TiO2 microcones structure with multilayered nanofragment. KEYWORDS: titanium dioxide, anodization, microcone, anode, lithium ion batteries
1. INTRODUCTION Although rechargeable Li-ion batteries (LIB) are a popular energy storage system in daily use devices, improvements are needed in terms of safety, power/energy density, cycle life, and cost.1−5 In particular, decomposition of the organic electrolyte, Li-ion dendrite formation during Li insertion, and expansion of electrodes during charging/discharging should be avoided to improve safety. In this respect, TiO2 has attracted attention as an alternative to graphite due to its suitable discharge potential (∼1.7 V vs Li/Li+), in which organic electrolyte decomposition and Li metal dendrite formation upon overcharging do not occur.6,7 The TiO2 anode was calculated at a theoretical capacity of 336 mAh/g when 1.0-Li was fully inserted per unit cell of TiO2 (LiTiO2). The maximum inclusion amount of Li into TiO2 was reported at nearly 0.8 for each unit of TiO2 (Li0.8TiO2) and a practical capacity of 168 mAh/g (Li0.5TiO2) was reported in most cases due to structural changes from tetragonal to orthorhombic when more than 0.5 Li was inserted per unit cell of TiO2.8−13 However, Li0.5TiO2 still shows a higher capacity when compared to that of other Ti-based anodes such as Li4Ti5O12 (∼175 mAh/g),14 LiCrTiO4 (∼157 mAh/g), and TiP2O7 (∼121 mAh/g).15 In addition, abundance, nontoxicity, simple synthetic routes, and low cost are regarded as merits of the TiO2 anode.6,16−18 Because bulk TiO2 has sluggish Li ion diffusion and poor electronic conductivity, many attempts to overcome this drawback through doping and nanostructuring, such as nanowires, nanobelts, and nanotubes, have been reported recently.6,12,17,19−26 Reddy et al. reported that the electrochemical performance in LIBs was improved using single-phase anatase TiO2 nanofibers and nanoparticles by electrospinning and molten salt method.27,28 In addition, © XXXX American Chemical Society
binder-free TiO2 electrodes, in which TiO2 are directly connected with the current collector of Ti substrate without active components and binders, have been reported to improve the capacity per original weight.6 Anodization is one of several well-defined methods to produce TiO2 nanotubes over a large Ti substrate in a costeffective process.29 Several groups have already reported that the use of anodic TiO2 nanotubes in LIB results in good life cycle retention and small volume expansion, indicating the possibility of producing binder-free ultrathin-film batteries.6,27 However, because nanotubes are in the amorphous phase, postthermal treatment to convert them to the anatase phase is essential.30 In addition, because TiO2 nanotubes are not fully available for the insertion/desertion of Li because of Li-ion diffusion problems in high-aspect-ratio geometry, the experimentally realized capacity did not achieve initial expectations. In this study, anodic hollow crystalline microcones with many multilayered nanofragments were prepared for use in LIB anodes, showing more than 3 times higher capacity, fairly good life cycle retention, and no structure distortion. High-surface, hollow TiO2 anatase microcones prepared via single anodization without any post-treatment mitigated the Li-ion mass transfer problem that is typically observed in high-aspect-ratio nanotubular TiO2. Received: March 13, 2016 Accepted: May 24, 2016
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DOI: 10.1021/acsami.6b03099 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2 Microcones. Titanium foils 0.127 mm in thickness with 99.7% purity (Sigma-Aldrich) were cut and cleaned with acetone, ethanol, and distilled water (DI) sequentially via ultrasonification, followed by drying in the air. The anodization was performed in a mixture of 1 M H3PO4 (Aldrich) and 0.5 wt % HF (Aldrich) under a constant voltage using a DC power supply (N8761A, Agilent Technologies). All experiments for anodization were carried out in two-electrode electrochemical cells composed of titanium foil as the working electrode and platinum mesh as the counter electrode at room temperature. After anodizing, anodized TiO2 specimens were immediately rinsed with DI water and then dried in the air. For comparison, the TiO2 nanotube electrodes were prepared through a well-defined anodization procedure,31 in which anodization is carried out at 20 V in 1 M H3PO4, 1 M NaOH (Aldrich) and 0.5 wt % HF solution at room temperature for 4 h. Afterward, TiO2 nanotube samples were rinsed in DI water and dried in the air. 2.2. Physical Characterization. The morphology and structure of as-prepared TiO2 samples were observed via a field-emission scanning electron microscope (FE-SEM, 4300S, Hitachi) with energy-dispersive X-ray spectroscopy (EDS) mapping and a field-emission transmission electron microscope (FE-TEM, JEM-2100F, JEOL) equipped with selected-area electron diffraction (SAED) analysis. The crystallinity of the samples was analyzed via X-ray diffraction (XRD, Rigaku D/maxRB) with Cu Kα radiation (1.54056 Å). Detailed structural information on the samples was conducted via X-ray photoelectron spectroscopy (XPS, VGESCALAB 220i-XL spectrometer, Fisons) analysis using an Al Kα X-ray source. 2.3. Electrochemical Measurements. The electrochemical properties of the TiO2 microcone (or nanotube) sample with a diameter of approximately 10 mm as the working electrode were studied in coin-type cells (CR2032, Wellcos corporation), which were assembled and dissembled in an argon-filled glovebox (
