The mechanism of HF formation in LiPF6 based organic carbonate electrolytes

Spectroscopic ellipsometry was used to study the time-dependent formation of HF upon the thermal degradation of LiPF6 at 50 °C in a lithium ion battery electrolyte containing ethylene carbonate and diethyl carbonate. The generated HF was monitored by following the etching rate of a 300 nm thick SiO2 layer, grown on both sides of a silicon wafer substrate, as a function of the immersion time in the electrolyte at 50 °C. It was found that the formation of HF starts after 70 h of exposure time and occurs following several different phases. The amount of generated HF was calculated using an empirical formula correlating the etching rate to the temperature. Combining the results of the HF formation with literature data, a simplified mechanism for the formation of the HF involving LiPF6 degradation, and a simplified catalytical reaction pathway of the formed HF and silicon dioxide are proposed to describe the kinetics of HF formation.

► Novel setup for the determination of the formation of HF in LiPF6 containing organic carbonate based battery electrolytes.
► Time-dependent investigation of the HF formation and quantification of the formed amount.
► Identification of different stages in the process of HF generation upon exposure to elevated temperature.
► A simplified catalytic mechanism of the formation/reaction of LiPF6 and HF in battery electrolytes in the presence of silicon dioxide is proposed.