(Invited) Lithium Ion Transfer Kinetics of Size Regulated TiO2(B) Nanosheets with Vertical, Random, and Horizontal Alignment for High Rate Lithium Ion Capacitors and Batteries

Abstract

TiO2(B) has a high theoretical capacity of 335 mAh g-1 for Li+ intercalation and thus is a promising candidate as negative electrodes for lithium-ion capacitors and Li-ion batteries. For high rate lithium-ion transfer, it is important to shorten the Li+ diffusion path by using nanostructured TiO2(B). In this work, TiO2(B) nanosheets with different equivalent diameter D e of 300 nm and 30 nm were prepared. In addition, the orientation of the TiO2(B) nanosheets was manipulated by altering the deposition method and drying process. For large-sized TiO2(B)(300 nm) nanosheet electrodes, the orientation effect was not clear and the amount of lithiation (SOC) was almost the same. Even at a rather low loading of 0.35 mg/cm2, only 1/5 of the theoretical capacity could be intercalated (Li0.2TiO2, 20% SOC) at 0.2C rate. By downsizing TiO2(B) to D e=30 nm, a two time increase in rate performance (Li0.4TiO2 at 0.2C) was obtained for vertically-aligned electrodes. Although the orientation of small-sized TiO2(B) had a large influence on Li+ transfer kinetics, 100% SOC could not be achieved even at 0.2C, suggesting the lack of electronic conductivity as a governing factor.1 To better understand the Li+ transfer kinetics, the lack of electronic conductivity was circumvented by adopting a vertically aligned reduced graphene oxide (V-rGO) electrode2 as a porous current collector. Three different size TiO2(B) nanosheets (300, 150, and 70 nm) were deposited on V-rGO at a loading of 0.20 mg/cm2. CVs at 0.5 mV/s reveals that 334 and 332 mA/g (100% SOC) is obtained for TiO2(B)/V-rGO with D e=70 and 150 nm, respectively. On the other hand, for the 300 nm sized TiO2(B)/V-rGO, only 55% SOC was achieved (186 mAh/g). The trend in Li+ transfer kinetics becomes clearer at higher scan rate (10 mV/s) with the capacity scaling with decreasing equivalent diameter; 89, 194, and 233 mAh/g for D e=300, 150 and 70 nm TiO2(B)/V-rGO. Discussion on results of b-value analysis and Li+ penetration depth will be given in detail in the talk.This work was partially supported by an Advanced Low Carbon Technology Research Development Program (JST-ALCA, JPMJAL1008).[1] T. Yoshida, D. Takimoto, D. Mochizuki, W. Sugimoto, Electrochem., in press.[2] D. Mochizuki, R. Tanaka, S. Makino, Y. Ayato, W. Sugimoto, ACS Appl. Energy Mater.,2(2), 1033 (2019).