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

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.     

References

[1] K. Naoi, et al., Energy & Environ. Sci., RSC, 5, (2012) 9363.

[2] a) M. Kobayashi, et al., Chem. Mater., 19, (2007) 5373.

[3] A. R. Armstrong et al., Chem. Mater., 22, (2010), 6426.

[4] Y. Ren, et al., Angewandte. Chem. Int. Ed, 51, (2012) 2164.