ARTICLE pubs.acs.org/JPCC
Facile Synthesis of Nanocrystalline TiO2 Mesoporous Microspheres for Lithium-Ion Batteries Jie Wang,† Yingke Zhou,*,† Yuanyuan Hu,† Ryan O’Hayre,‡ and Zongping Shao*,† †
State Key Laboratory of Materials Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, P. R. China ‡ Department of Metallurgical & Materials Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, United States ABSTRACT: TiO2 mesoporous nanocrystalline microspheres assembled from uniform nanoparticles were synthesized by a facile and template-free hydrolytic precipitation route in normal solvent media. The phase structure, morphology, and pore nature were analyzed by X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscopy, and BET measurements. The electrochemical properties were investigated by cyclic voltammetry, constant current discharge-charge tests, and electrochemical impedance techniques. Microspheres with diameters ranging from 0.2 to 1.0 μm were assembled by aggregation of nanosized TiO2 crystallites (∼8-15 nm) and yielded a typical type-IV BET isotherm curve with a surface area of ∼116.9 m2 g-1 and a pore size of ∼5.4 nm. A simplified model was proposed to demonstrate the nanoparticle packing modes to form the mesoporous structure. The initial discharge capacity reached 265 mAh g-1 at a rate of 0.06 C and 234 mAh g-1 at a rate of 0.12 C. The samples demonstrated high rate capacity of 175 mAh g-1 at 0.6 C and 151 mAh g-1 at 1.2 C even after 50 cycles, and the Coulombic efficiency was approximately 99%, indicating excellent cycling stability and reversibility. Details of the kinetic process of the nanocrystalline mesoporous microspheres electrode reaction from electrochemical impedance spectra provided further insights into the possible mechanisms responsible for the good reversibility and stability. These investigations indicate that TiO2 nanocrystalline mesoporous microspheres might be a promising anode material for high-energy density lithium-ion batteries.
1. INTRODUCTION Titanium dioxide (TiO2) has been successfully demonstrated as a prospective material for a variety of applications in catalysis,1 photovoltaics,2 and sensing.3 Recently, TiO2 and TiO2-based materials have also been demonstrated as promising anodes in rechargeable lithium-ion batteries.4-7 In comparison to the normally used carbonaceous materials, the titanium dioxide anode operates at ∼1.7 V vs Liþ/Li (carbon-based anodes at ∼0.1 V vs Liþ/Li), leading to a trade-off in terms of lower overall operating cell voltage but improved capacity retention, lower self-discharge, and enhanced safety/stability.8,9 TiO2 is also chemically stable, inexpensive, nontoxic, and environmentally benign.10 These properties make TiO2 particularly attractive for potential largescale energy storage applications, for example, in the emerging field of electric vehicles. TiO2 anode performance depends strongly on the morphology, porosity, and crystalline phase of the electrode structure. Various kinds of TiO2 anodes have been studied for Li-ion batteries, including electrodes fabricated from anatase, rutile, and TiO2(B), using a variety of structural titania forms including nanotubes, nanowires, nanorods, and nanoparticles. Bulk rutile titania is thermodynamically stable but accommodates a negligible amount of Li at room temperature (
