Ultrafast Nano-Crystalline and Highly-Dispersed-TiO2 (B) /CNF Composites for Negative Electrode of ‘Nanohybrid Capacitor’

Abstract

We have been applying our original in-situ material processing technology called ‘Ultra-Centrifuging (UC) Treatment’ to prepare a novel ultrafast nano-crystalline Li4Ti5O12 (nc-LTO) /carbon nano fiber (CNF) composite electrode for the generation-II capacitive energy storage device of ‘Nanohybrid Capacitor’ producing more than triple energy density of a conventional electrochemical capacitor.[1] For the further improvement in the energy density of ‘Nanohybrid Capacitor’, LTO alternatives are needed. Among different candidates, TiO2 (B) possesses several advantages such as its 2-folds theoretical capacity (335 mAh g-1) of LTO (175 mAh g-1) and the comparable redox potential (1.6 V vs. Li/Li+). In addition, TiO2 (B) is an industrially attractive material because of its inexpensive raw materials and its environmentally benign synthesis using only aqueous solution (e.g., water-soluble complexes [2]) without any Li-sources.[2-4] However, TiO2 (B) has intrinsic problems; its low electronic conductivity and Li+ diffusivity due to the b-axis oriented growth of TiO2 (B) [3]. In this report, we successfully synthesized b-axis shortened and hyper-dispersed nc-TiO2 (B)/CNF composites, which has a potential to substitute nc-LTO/CNF as a negative electrode of ‘Nanohybrid capacitor’. Inhibition of b-axis growth and hyper dispersion of TiO2 (B) into the carbon matrix were simultaneously achieved by the UC treatment and the subsequent hydrothermal method [4]. The prepared nc-TiO2/CNF exhibited 236 mAh g-1 (active material) at high rate of 300 C, which even exceeds that of nc-LTO/CNF composites. A significant decrease of peak intensity corresponding to the C-site intercalation around 2 V vs. Li/Li+ indicates that the Li+ was directly accommodated into A1 and A2 site of TiO2 (B) owing to the limitation of b-axis growth. Furthermore, the combined analysis of the N2 adsorption experiment and electochemical characterization suggests that the "dispersibility" of nc-TiO2 (B) on CNF matrix correlates closely with its electrochemical performance.