Article pubs.acs.org/JPCC
Size-Tunable Single-Crystalline Anatase TiO2 Cubes as Anode Materials for Lithium Ion Batteries Xuming Yang, Yingchang Yang, Hongshuai Hou, Yan Zhang, Laibing Fang, Jun Chen, and Xiaobo Ji* College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China ABSTRACT: Single-crystalline anatase TiO2 cubes enclosed by {100} and {001} facets with tunable size, as designed based on the special three zigzag lithium diffusion paths along [100], [010], and [001] directions, were obtained through hydrothermal crystallization of TiO2 in a quaternary solution consisting of tetrabutyl titanate, acetic acid, water, and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmin][BF4]). The crystalline type and shape of as-prepared samples were confirmed by X-ray diffraction and scanning and transmission electron microscopes (XRD, SEM, and TEM). Particularly, selected area electron diffraction (SAED) patterns, high-resolution TEM images, and fast Fourier transformation (FFT) patterns of the plane and side view are given to determine the exposed {100} and {001} facets. Significantly prolonged plateaus that are very practical for power management and constant voltage output were achieved by the anatase TiO2 cubes in comparison with TiO2 nanoparticles. Besides, excellent cycle stability was presented through a long loop of discharge/charge processes with both shape and crystallinity well preserved, as ex situ SEM and XRD characterization manifested.
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INTRODUCTION Energy consumption in the form of lithium ion batteries (LIBs) is becoming more and more prevalent in a diverse range of applications from powering portable devices to driving electric vehicles as LIBs have incomparable advantages of large specific capacities, excellent cycle stability, slow self-discharge, and good environmental benignity over other secondary batteries such as lead−acid and Ni−Cd cells.1,2 Building better LIBs with higher energy and power densities, longer working life, and stricter security has generated increasing research interest, especially in the search for alternative cathode or anode materials.3−5 Currently, natural and artificial graphite are the dominant commercial anodes due to their high capacity at low and flat potentials (372 mA h g−1 with the stoichiometry of LiC6 below 0.2 V versus Li+/Li).6,7 However, along with the merit comes serious safety issues including decomposition of organic electrolyte and formation of solid electrolyte interface (SEI) films.6,8 In addition, graphite anodes suffer from poor rate capability and volume expansion (9−13%) during lithiation.9 These drawbacks lead to limited applications of graphite in the next-generation LIBs and leave an urgent need to find suitable substitutes. Titanium dioxide (TiO2) is regarded as a promising candidate for anode materials due to its natural abundance, its ready availability, as well as its intrinsic superiority of hosting lithium ions, which is closely related to its open-crystal structure.10 Among the four common crystal forms of TiO2 studied as LIB anode materials (anatase, rutile, brookite, and TiO2 (B)), anatase is the most extensively investigated.11−13 Anatase TiO2 belongs to the tetragonal crystal system with the space group of I41/amd. An anatase unit cell consists of four distorted TiO6 octahedrons and four more distorted empty oxygen octahedrons (octahedral vacancies) where lithium ions © XXXX American Chemical Society
would be accommodated during lithiation (Figure 1a and b). Each oxygen octahedron shares vertices with two TiO6 and four oxygen octahedra and shares edges with four TiO6 and four oxygen octahedra (Figure 1c). The line connecting two adjacent octahedral vacancies through their sharing edge is the very well defined migration route of lithium ions.14−16 These lines in the ac-planes can be grouped into a onedimensional coplanar zigzag channel array along the a-direction traversing down all lithium ion sites (Figure 1d).17 And along the b-direction exists an identical array but at a different height, which is determined by the helical symmetry of anatase TiO2. The two arrays are just interconnected with each other, which constructs zigzag channels in the c-direction (not coplanar) and results in the same possible lithium ion current as in the a- or bdirection, provided that the subtle volume increase during lithium intercalation (
