Hexagonal Hollow Mesoporous TiO2 Nanoparticles As an Advanced Anode Material for Lithium-Ion Batteries
Authors: Ma Chunrong , Yuan Tao , Zhang Weimin , Ma Zi-Feng 
Published: February 2020

Recently, TiO2 received tremendous attention due to its low cost, chemical stability, excellent rate performance, proper Li-intercalated potential and environmental friendliness1-3. However, the poor electronic conductivity and relatively low specific capacity lead to the major obstacles for large-scale application in the LIBs. To overcome these deficiencies, a series of efforts have been made to enhance the performance of TiO2 anode. It has been demonstrated that the morphology and nanostructure show significant impacts on the performance of electrochemical performance. However, most of the reported hollow structures have a spherical shape so far, non-spherical hollow structure might exhibit unusual propertioes when used as the anode materials.Hence, we explore the synthesis of the non-spherical hollow structures with favourable shell architectures, and used as the anode materials of LIBs.
Here, we introduced an easy scale-up strategy for the fabrication of well-organized hexagonal hollow mesoporous TiO2(HHM-TiO2)materials by a facile, cost-effective template-engaged method at room temperature. We further introduce a conformal carbon coating layer on the hexagonal hollow mesoporous TiO2, and investigate the electrochemical performance.
The hollow TiO2 is designed on the basis of the interplay and synergy of controlled hydtolysisi of Ti4+ and accompaning gradual etching of Cd(OH)2. During the process, interfacial reaction between (NH4)2TiF6 and Cd(OH)2 occurred simultaneously,TiO2 layer is deposited on the scaffold of Cd(OH)2 template through accelerated hydrolysis of [TiF6]2
- by consumption of H+.To evaluate the promising use of the TiO2 as an anode material for LIBs, a conformal carbon coating layer was introduced to coat on the porous TiO2 particles by hydrothermal method using glucose as the carbon source.
Fig. 1 The morphology characterizations of HHM-TiO2 and HHM-TiO2/C. (a) SEM, (b) TEM and (c) HRTEM of HHM-TiO2, (d) TEM and (e)HRTEM of HHM-TiO2/C, (f) the SEAD pattern of HHM-TiO2

The morphology of the as-prepared HHM-TiO2 and HHM-TiO2/C were characterized by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM.). Fig. 1a indicated that the HHM-TiO2 is consisted of hexagonal particles with the particle-size range from 250 to 350 nm. Meanwhile, the hollow interior can be directly observed from the broken nanoparticle (inset in Fig. 1a) and TEM images (Fig. 1b), while the thickness of TiO2 shell is about 30 nm. A close examination (HRTEM images in Fig. 1c) of the sample reveals a highly porous shell that is consisted of small TiO2 nanoparticles with a size of several nanometers.
After introducing the carbon coating layer, the feature of HHM-TiO2/C was analyzed in Fig. 1d-f. The TEM images of HHM-TiO2/C matching well with the pure HHM-TiO2 confirmed that the carbon coating process does not alter the structure of the HHM-TiO2. The selective area electron diffraction (SAED) pattern (Fig. 1f) showed bright diffraction rings indexed to (101), (004), (200), (211), and (204) planes of anatase TiO2, indicating its polycrystalline nature
In summary, a new structure and well-organizedhexagonal hollow mesoporous TiO2 was fabricated by a simple cost-effective method. After wrapping with an elastic carbon matrix, The HHM-TiO2/C electrode showed better rate, with a good rate capability of 175 mA h g-1 at 5 C, 152 mA h g-1 at 10 C and 126 mAh g-1 at 20 C. The higher capacity retention at higher current rates demonstrates a superior rate capability for the HHM-TiO2/C composite.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21336003, 21333007).
References
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Figure 1