Solid electrolyte interphase (SEI) at TiO₂ electrodes in li-ion batteries: Defining apparent and effective SEI based on evidence from X-ay photoemission spectroscopy and scanning electrochemical microscopy

The high (de)lithiation potential of TiO₂ (ca. 1.7 V vs Li/ Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO₂ electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO₂ electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO₂ electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO₂ electrodes to differentiate it from the protecting and ef fective SEI formed at graphite electrodes.