Surface Layer Fluorination of TiO2 Electrodes for Electrode Protection LiBs: Fading the Reactivity of the Negative Electrode/Electrolyte Interface

Abstract

We demonstrate the possibility to master the negative electrode/electrolyte interface reactivity of a TiO2 based electrode with a protecting fluorinated layer, synthesized by a safe process applied for a first time on electrode materials. Pure XeF2 has been used as fluorinating agent with two different fluorination rates (LowF and HighF). This prospective study provides new insights of the surface fluorination benefits thanks to extreme surface spectroscopy analysis. Indeed, enhanced electrochemical performances have been correlated with the surface reactivity of fluorinated electrodes by the means of the Scanning Auger Mapping analysis (SAM) and to the X-ray Photoemission Spectroscopy (XPS). As shown by the XPS results, fluorinated electrodes exhibit thinner and stabilized solid electrolyte interphase (SEI) layer, providing more effective passivating properties. The SAM chemical mappings confirm this trend. The fluorination of the active material leads to fade the reactivity toward the electrolyte, consequently the products of the electrolyte degradation are deposited in lower quantities on the fluorinated electrode surface, especially the species originated from the salt degradation. Both of the fluorinated electrodes exhibit improved specific capacity by 10% after ten cycles. The polarization is reduced by 12.4% and 17% for LowF and HighF electrodes, respectively.