Tailoring high-surface-area nanocrystalline TiO2 polymorphs for high-power Li ion battery electrodes
Introduction
In recent years, considerable efforts have been made to improve the Li ion battery technologies, leading to a 100–150% increase in the storage capability of energy per unit weight and volume [1], [2], [3]. The general consensus is that nanotechnology plays a key role for future battery applications and its market adoption because of the beneficial aspects of the nanostructured electrode over its comparable bulk counterpart, including a higher electrode/electrolyte interfacial area, shorter Li-ion transport path lengths, and a facile strain relaxation from the Li uptake/removal [1], [2], [3], [4], [5]. Some effective approaches for nanostructured anode materials have attracted intensive interest for the high-power, high-energy Li ion batteries. Titania-based nanomaterials, including various TiO2 polymorphs as well as Li4Ti5O12, are considered alternatives to the conventional graphite anodes because they are inherently safe and chemically compatible with the electrolyte.
To date, TiO2 polymorphs, including rutile (tetragonal, P42/mnm), anatase (tetragonal, I41/amd), and brookite (orthorhombic, Pbca), TiO2-B (bronze), TiO2-R (ramsdellite), TiO2-H (hollandite), TiO2-II (columbite) and TiO2-III (baddeleyite) have been reported [6], [7]. Nano-sized rutile and anatase TiO2, which are common in nature, have attracted much attention for their numerous applications as key materials for photocatalysts [8], dye-sensitized solar cells [9], gas sensors [10], electrochromic devices [11], and biological applications [12]. Very few reports have examined the synthesis of pure brookite TiO2, but more recently, nanostructured brookite TiO2 has also been investigated as a good material for photocatalysts and anode materials for Li ion batteries [13], [14], [15]. Nanocrystalline brookite TiO2 has been synthesized with a crystal size of ∼50 nm in an organic media [16]. Nanocrystalline brookite TiO2 particles were also synthesized through the hydrolysis of a TiCl3 or TiCl4 solution as the precursor [17], [18], [19]. Recently, TiO2 nanorods, nanotubes, and nanoflowers with a brookite structure have been synthesized via a hydrothermal route at temperatures over 180 °C [14], [20], [21], [22]. The formation of brookite nanoparticles through the urea [(NH2)2CO]-mediated hydrolysis/precipitation process in an aqueous solution has also been reported [14], [23].
In this work, a simple, high yield synthetic route was proposed for the preparation of the brookite nanorods and the anatase nanoflowers with extraordinary large surface areas. The phase evolution from brookite to anatase TiO2 was systematically demonstrated during the urea forced hydrolysis process. Furthermore, the electrochemical performance of the anatase nanoflowers was investigated, and these nanoflowers exhibited a high capacity and an excellent rate capability compared to the brookite nanorods.
Section snippets
Preparation of TiO2 nanoparticles
A series of TiO2 nanoparticles were prepared using the modified urea-mediated precipitation method that was explained elsewhere [14], [23]. Urea (Alfa Aesar, 99.3%) at a concentration of 0.3–3.0 M was dissolved in an aqueous solution containing 0.015 M of titanium trichloride (TiCl3, Alfa Aesar, 20% in 3% hydrochloric acid) at room temperature. The solution was heated and maintained at 90–100 °C for 4 h with stirring. The product was centrifuged and repeatedly washed (five times with deionized
Results and discussion
The structures of all of the TiO2 samples that were prepared below 100 °C by adjusting the urea concentration were identified using powder X-ray diffraction (XRD) at room temperature, in Fig. 1. Little precipitation was observed below a urea concentration of 0.3 M. At a urea content of 0.3 M, almost pure brookite TiO2 was synthesized. Noteworthy, anatase TiO2 started to form and gradually increased with increasing urea concentration, which was characterized by the decrease in a diffraction peak
Conclusions
In summary, the successful phase and morphology evolution of nano-sized TiO2 were demonstrated using a urea forced hydrolysis/precipitation process at a low temperature below 100 °C. The crystalline brookite nanorods were transformed into the anatase nanoflowers through the proper adjustment of the urea concentration. The anatase nanoflowers that were assembled with relatively thin nanorods exhibited a large surface area and were well suited for improving the reversible capacity and the rate
Acknowledgments
This research was supported by Future-based Technology Development Program (Nano Fields) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0019116).
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