Amorphous and Crystalline TiO 2 Nanotube Arrays for Enhanced Li-ion Intercalation Properties
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Amorphous and Crystalline TiO2 Nanotube Arrays for Enhanced Li-Ion Intercalation Properties Dongsheng Guan, Chuan Cai, Ying Wang* Mechanical Engineering Department, Louisiana State University, Baton Rouge, LA 70803, USA ABSTRACT We have employed anodic oxidation of Ti foils to prepare self-organized TiO2 nanotube arrays which show enhanced electrochemical properties for applications as Li-ion battery electrode materials. The lengths and pore diameters of TiO2 nanotubes can be finely tuned by varying voltage, electrolyte composition, or anodization time. The as-prepared nanotubes are amorphous and can be converted into anatase nanotubes with heat treatment at 480oC and nanotubes of mixed anatase/rutile phases by heating at 580oC. The morphological features of QDQRWXEHVUHPDLQXQFKDQJHGDIWHUDQQHDOLQJ$PRUSKRXVQDQRWXEHVZLWKDOHQJWKRIȝPDQG DQRXWHUGLDPHWHURIQPGHOLYHUVDFDSDFLW\RI ȝ$KFP-2 DWDFXUUHQWGHQVLW\RIȝ$ cm-2, while those with a length of 25 µm and an outer diameter of 158 nm display a capacity of 533 ȝ$ K FP-2. The 3-ȝP ORQJ anatase nanotubes and nanotubes of mixed phases show lower FDSDFLWLHVRIȝ$KFP-2 DQGȝ$KFP-2, respectively at the same current density. The amorphous TiO2 nanotubes with a length of 1.9 ȝm exhibit a capacity five times higher than that of TiO2 compact layer even when the nanotube array is cycled at a current density 80 times higher than that for the compact layer. The amorphous nanotubes show excellent capacity retention ability over 50 cycles. Cycled nanotubes show little change in morphology compared to the nanotubes before cycling, indicating the high structural stability of TiO2 nanotubes. INTRODUCTION Rechargeable Li-ion batteries have drawn extensive attention due to their high specific capacity, light weight and long lifespan [1]. Graphite is the most commonly used anode material in the current commercially available Li-ion batteries. However, the graphite has some limitations which hinder the development of new Li-ion batteries with higher operation voltage and larger power density. For example, lithium dendrites possibly form in graphite-based Li-ion batteries, which can induce an explosion if exposed to air [2]. Therefore, quite a few new materials have been developed to replace the graphite, such as Si [3], Sn [4], Co3O4 [5], SnO2 [6] and TiO2. Among these materials, low-cost and nontoxic TiO2 is a promising alternative to the graphite due to its high operation voltage (~1.75 V vs. Li+/Li redox couple), high safety and low self-discharge, and good capacity retention during cycling [7]. Various polymorphs of TiO2 display Li-ion insertion/deinsertion properties. For example, Reddy et al. found that a maximum of 0.95 Li/TiO2 (molar ratio) can be inserted into brookite crystallites with a size of 10 nm [ 8]. Noailles et al. discovered that hollandite-type TiO2 shows low Li-ion intercalation capacity [9], but TiO2 (B) shows a large capacity (§335 mA h g-1) and
excellent cycleability when it is used as anode materia
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