Effect of Size of TiO2(B) Nanosheets on Electrochemical Properties for Fast Li Ion Intercalation
Authors: Hideshima Sho , Nagatani Itsuki , Takimoto Daisuke , Sugimoto Wataru 
Published: December 2020

Bronze-phase TiO2, TiO2(B), has a theoretical Li+ storage capacity comparable to graphite and thus has been studied as an anode material for lithium-ion capacitors and lithium-ion batteries. Since Li+ diffusion proceeds along the b axis in the open tunnel structure of TiO2(B), its power density is expected to improve by shortening the b-axis dimension. Although miniaturized TiO2(B), such as TiO2(B) nanotubes1 or nanoparticles2, have been proposed as high-rate Li+ storage materials, difficulty in size-controlled synthesis of such materials impedes sufficient investigation to verify the effect of shortened b-axis dimension of TiO2(B) on the performance. TiO2(B) nanosheets can be downsized in b-axis dimension by controlling the sonication condition for TiO2 nanosheets used as a precursor material. In this study, by preparing size-regulated TiO2(B) nanosheets, the charge storage mechanism of TiO2(B) related to fast Li+ intercalation was analyzed.
TiO2 nanosheets, prepared by exfoliating H2Ti4O9 nanosheets, were downsized by ultrasonic treatment, resulting in the fabrication of size-regulated TiO2 nanosheets. TiO2 nanosheets were vacuum impregnated into a porous current collector, i.e. vertically-aligned rGO nanosheet film,3 and thermally treated at 350°C for 2 hours under N2 flow conditions to convert to TiO2(B). The TiO2(B) nanosheet electrodes were used as the working electrode and lithium foil was used for the counter and the reference electrodes. A three-electrode flat cell was used for electrochemical characterization. The electrolyte was 1 M LiPF6 in a 1:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC).
Based on atomic force microscopy images, small, medium, and large size TiO2 nanosheets with average diameter of 70, 150, and 300 nm were successfully obtained by changing the sonication time. Cyclic voltammograms revealed that the capacities of TiO2(B) nanosheet electrodes at a scan rate of 0.5 mV s−1 and 20 mV s−1 were 186 mAh (g-TiO2)−1(=Li0.56TiO2) and 76 mAh (g-TiO2)−1 (=Li0.23TiO2) for the large-sized nanosheets, 334 mAh (g-TiO2)−1 (=Li1.0TiO2) and 156 mAh (g-TiO2)−1 (=Li0.4TiO2) for the medium-sized nanosheets, and 332 mAh (g-TiO2)−1 (=Li1.0TiO2) and 211 mAh (g-TiO2)−1 (=Li0.63TiO2) for the small-sized nanosheets, respectively. The medium- and small-sized TiO2(B) nanosheets exhibited Li+ storage capability comparable to the theoretical capacity of TiO2(B) (335 mAh g−
1), while the large-sized TiO2(B) nanosheets with longer Li+ transport path could only be charged to ~50% SOC under the same conditions. The charge storage mechanism of TiO2(B) nanosheets based on b-value analysis4 suggested that shortening the b-axis length of TiO2(B) was effective for increasing the capacitive contribution related to near-surface Faradic reaction.
Acknowledgments
This work was partly supported by Advanced Low Carbon Technology Research and Development Program (ALCA) (JPMJAL1008) from Japan Science and Technology Agency (JST).
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