Solid Electrolyte Interphase (SEI) Formation on the Graphite Anode in Electrolytes Containing the Anion Receptor Tris(hexafluoroisopropyl)borate (THFIPB)

Addition of small amounts of tris(hexafluoroisopropyl)borate (THFIPB) has previously been shown to improve the capacity of graphite anodes in a wide temperature window during long-term cycling in electrolytes based on LiPF6 in ethylene carbonate (EC) and diethylene carbonate (DMC) solvents. Here, we demonstrate that the addition of THFIPB accelerates the LiPF6 hydrolysis, and consumes residual water. The SEI formation and composition was studied by operando electrochemical mass spectrometry (OEMS), X-ray photoelectron spectroscopy (XPS), and ex situ diffuse reflectance spectroscopy (DRIFT). Differences in the surface products are observed for the THFIPB containing electrolyte already upon exposure of graphite anodes to the electrolyte. Both the mechanism and the kinetics of the EC reduction reaction are affected, as evidenced by the shift in the C2H4 evolution peak to higher potentials as compared to the reference electrolyte. Addition of THFIPB leads to formation of a SEI layer enriched in inorganic components; salt reduction products at high potentials, and inorganic carbonate at lower potentials. The SEI formed is more conductive but slightly less passivating. The SEI formed in the reference electrolyte is dominated by organic compounds, also at high potentials, facilitated by the trace amounts of water inevitably found in the electrolyte.