High Rate TiO2(B) Electrodes Derived from TiO2 Nanosheets with Different Size
Authors: Yoshida Tomohiro , Makino Sho , Ayato Yusuke , Mochizuki Dai , Sugimoto Wataru 
Published: February 2020

TiO2(B), with a theoretical capacity of 335 mAh g-1, is expected to possess high rate characteristics and may be a promising electrode material for lithium-ion batteries and lithium-ion capacitors. TiO2(B) prepared from TiO2 nanosheets shows high capacity of 200 mAh g-1 and excellent rate performance 1). We anticipated that TiO2(B) prepared from small size nanosheets may provide higher power performance in comparison with large size TiO2(B) due to the shorter diffusion length of Li+. In this study, we focused on the size effect of the precursor nanosheets.
Different sized TiO2(B) were prepared from different size TiO2 nanosheets. Colloidal TiO2 nanosheet was synthesized according to a previous report 2). Small size TiO2 nanosheets were prepared by extended ultrasonication. Figure (a) shows the synthesis scheme of the thin film TiO2(B) electrode. First, colloidal TiO2 nanosheet was dropped on the Ti substate and then 10 M HCl was added to the nanosheet colloid droplet to induce flocculation. After drying, TiO2 nanosheet/Ti was washed with ultra pure water, then heat treated at 350°C for 2 hours to convert to TiO2(B)/Ti. The TiO2(B) electrode was tested at 25°C by galvanostatic charge discharge tests and cyclic voltammetry using a three-electrode system consisting of a Li metal foil as the counter electrode, a Li metal chip as the reference electrode and 1 M LiPF6in ethylene carbonate and diethyl carbonate of 1:1 volume ratio.
Figure (b), (c) shows FE-SEM images of the surface of the TiO2(B)/Ti electrodes prepared from large and small size TiO2 nanosheets. Compared with TiO2(B) prepared from large size TiO2 nanosheets, the TiO2(B)/Ti electrode prepared from small size TiO2 nanosheet has a more porous texture. Figure (d), (e) shows the discharge curves of TiO2(B)/Ti from large and small size TiO2 nanosheets. TiO2(B) electrode prepared from large size TiO2 nanosheets shows 170 mAh g-1 at 0.2 C and 13 mAh g-1 at 25 C. On the contrary, TiO2(B) electrode prepared with small sized TiO2 nanosheets shows enhanced kinetics with 197 mAh g-1 at 0.2 C and 99 mAh g-1 at 25 C. The results suggest that the insertion/extraction of Li+ is faster for TiO2(B) prepared from small size TiO2 nanosheet which may be due to the more porous micro structure and shorter Li+diffusion length.
Figure (f), (g) shows cyclic voltammograms for TiO2(B)/Ti from large and small size TiO2 nanosheets at a scan rate of 0.5 mV s-1. Both sample exhibited a cathodic peak at 1.5 V and an anodic current peak at 1.6 V, which are characteristic for insertion/extraction of Li+ to/from TiO2(B) phase. In addition, both TiO2(B)/Ti prepared from large and small size TiO2 nanosheet exhibited a cathodic current peak at 2.0 V and an anodic current peak at around and 1.7 V, which is characteristic for insertion/extraction of Li+ of anatase phase. The presence of the anatase phase is one of the reasons that the capacity is lower than theoretical value for TiO2(B), and thus suggesting that high capacity may be achieved by decreasing the amount of the anatase phase.
This work was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).

References

1) H. Jang, S. Suzuki and M. Miyayama, J. Power Sources, 203, 97 (2012).
2) W. Sugimoto, O. Terabayashi, Y. Murakami, and Y. Takasu, J. Mater. Chem. 12, 3814 (2002).

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