Evolution of Solid Electrolyte Interface on TiO2 Electrodes in an Aqueous Li-Ion Battery Studied Using Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) was applied for in situ visualization of solid electrolyte interface (SEI) evolution on the TiO2 anode in a concentrated aqueous electrolyte during cycling. In nonaqueous electrolytes, the SEI is an electronic insulative layer composed of organic and inorganic components covering the electrode surface. However, in concentrated aqueous electrolytes, the SEI is mostly composed of inorganic compounds formed by electrolyte decomposition, such as LiF and Li2CO3, which are randomly distributed over the TiO2 surface. In addition, the Ti3+ and Li residuals accumulated through multiple cycling lead to an increase in the overall electronic conductivity of the TiO2 anode. During the resting period after cycling, an inverse process is observed with partial dissolution of the formed SEI and the diffusion of Li residuals back into the electrolyte.