Solid Electrolyte Interphase (SEI) at TiO₂ Electrodes in Li-Ion Batteries: Defining Apparent and Effective SEI Based on Evidence from X-ray Photoemission Spectroscopy and Scanning Electrochemical Microscopy

Abstract

The high (de)lithiation potential of TiO (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 TiO electrodes at potentials above 1.0 V vs Li/Li. We investigate the formation of a SEI at anatase TiO 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 TiO 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 TiO electrodes to differentiate it from the protecting and effective SEI formed at graphite electrodes.

Understanding the Conductive Carbon Additive on Electrode/Electrolyte Interface Formation in Lithium-Ion Batteries via in situ Scanning Electrochemical Microscopy

“…Ferrocene was chosen as the free-diffusion redox mediator and the corresponding cyclic voltammetry between 2.8 and 3.8 V is shown in Figure 1A. The redox potential of the ferrocene/ferrocenium (Fc/Fc + ) combination is 3.26 V vs. Li + /Li, which is suitable for solid electrolyte interface characterization in LIBs (Zampardi et al, 2013;Ventosa et al, 2017). During the feedback measurement ( Figure 1B), the microprobe has an applied potential of 3.6 V vs. Li + /Li as it moves toward the substrate, and the current at the microprobe increases when the substrate is conductive due to the increase of Fc regeneration rate (positive feedback).…”

Robust electrochemistry of black TiO2 as stable and high-rate negative electrode for lithium-ion batteries

“…Harutyunyana et al showed that high concentration of defects and edges in the electrode matrix is responsible for excess capacity in first few cycles [62]. Similarly, Schuhmann et al demonstrated that SEI formed in case of TiO 2 has high Liion transport ability and thereby it enhances the cycle stability of TiO 2 [39].…”

“…3 Raman spectrum of the B-TiO 2 Fig. 4 SEM images (a, b) and EDAX (c) of the B-TiO 2 NPs to 0.7-0.8 V vs Li [37][38][39]. The couple of peaks at 0.65 and 1.67 V in the cathodic sweep are assigned to insertion of lithium into TiO 2 matrix and to the structure transformation from tetragonal TiO 2 to orthorhombic Li x TiO 2 , where x is insertion coefficient.…”

“…Overall electrochemical reaction taking place may be illustrated by the following equation: Figure 8 shows the galvanostatic charge-discharge profile of B-TiO 2 at 1 C rate (1 C = 335 mA h g −1 ), in the potential range 0.1-3.0 V. The first lithiation and delithiation capacities were found to be 575 and 345 mA h g −1 , respectively. The excess capacity in the first few cycles is attributed to widening of potential range, defective TiO 2 and formation of SEI [39,57,[61][62][63][64][65][66]. Vereecken et al demonstrated an increase in the capacity by ~ 30% when the lower potential barrier was dragged from 1.0 to 0.1 V for TiO 2 .…”

Electrochemical Characteristics of a Triphenylamine Derivative by Microelectrode Voltammetry

“…Based on these advantages, microelectrodes have been recently applied in the biomedical and analytical fields. Microelectrodes enable micromanipulation in the biomedical field and functionalization and characterization of solid surfaces in the analytical field when used in scanning electrochemical microscopy. Microelectrodes are fabricated in various forms, such as disk‐cylinders , spheres , and line‐arrays .…”