Electrochemical and electrical characteristics of ball milled Cs2Ti6O13 modified by the surface-to-bulk migration of hydroxyl groups

Abstract

Ball milling of solids under benign conditions leads to surface functionalization without altering the crystal structure and morphology. However, these additional surface functional groups are rarely fixed but instead mobilized across such ball milled solids. This phenomenon, including its effects on electrochemical and electrical properties, has received limited attention. We report herein that dry vibratory ball milling of lepidocrocite-type Cs₂Ti₆O₁₃ generated hydroxyl groups which subsequently migrated from surfaces to bulk. The increased number of bulk hydroxyl groups is deduced from Raman, IR, and solid state ¹H nuclear magnetic resonance spectroscopy, and thermogravimetric analysis. In contrast, the decrease in the relative proportion of surface hydroxyl groups/water and carbon–oxygen species was deduced from X-ray photoelectron spectroscopy. The inaccessible hydroxyl groups in ball milled Cs₂Ti₆O₁₃ lead to a smaller amount of stored charge and increased charge transfer resistance, according to galvanostatic charge–discharge experiments and electrochemical impedance spectroscopy studies in 1 M Na₂SO₄. The alternating current electrical properties were also measured, revealing fundamental insights such as the one-dimensional conduction pathway and the relaxation time in microseconds. A model has been proposed for this surface-to-bulk migration of the hydroxyl groups, which competes with surface dangling bonds leading to particle agglomeration.