Effect of Surface Fluorination on the Charge/Discharge Properties of High Potential Negative Electrode TiO<sub>2</sub>(B) for LIBs

Abstract

Surface fluorination of TiO2(B) powder was conducted by pure F2 gas at room temperature for 1 h and the effect on the charge/discharge properties was examined as a negative electrode of Li-ion batteries (LIBs). X-ray diffraction (XRD) pattern was not changed before and after the surface fluorination though the peak intensities became weaker than that of the pristine sample, indicating the etching of the surface of SF-TiO2(B) power. This was supported by scanning electron microscopy (SEM) observation. However, X-ray photoelectron spectroscopy (XPS) analysis clearly revealed that F atoms exist on the surface of TiO2(B) particles and probably were covalently bonded with Ti atoms near the surface. From the charge/discharge tests at a C/6 rate, the SF-TiO2(B) exhibited a higher 1st discharge (203 mAh g-1) than the pristine sample (181 mAh g-1) with a good cycleability. Impedance analysis revealed that both resistances of solid electrolyte interphase (SEI) film and charge transfer at the SEI /active material interface were reduced by surface fluorination, implying the improvement of SEI film and permeability of the electrolyte solution to the interphase. The rate capability was improved by the surface fluorination up to 1C rate, at which the SF-TiO2(B) exhibited a high discharge capacity of around 150 mAh g-1.