Investigation of Cathode Electrolyte Interphase Formation via Coupling Liquid Electrochemical TEM and GC/MS

Abstract

A deeper understanding of the cathode electrolyte interphase (CEI) formation mechanism is essential to elucidate battery degradation. Here, we combine Liquid Electrochemical Transmission Electron Microscopy (ec-TEM) with Gas Chromatography/Mass Spectrometry (GC/MS) to monitor CEI evolution in a realistic electrochemical environment, focusing on electrolyte behavior under high voltages. The correlation between the electrochemical response, gas and liquid analysis after cycling, and the observation of deposited species on the working electrode (WE) reveals the processes governing CEI formation, stability, and composition. Cycling between 4 and 6 V vs Li leads to dispersed particles instead of a continuous film. These are partly composed of LiF and an amorphous phase that prevents dissolution at high potential. When cycled between 2.5 and 5.5 V, an anodic current peak indicates the formation of a 36 nm amorphous thin film without crystalline LiF, attributed to EC oxidation producing HF and subsequent LiF at a higher potential. LiF dissolution appears to follow a two-step pathway: electrolyte oxidation forms soluble intermediates, which are later reduced at lower potential to yield species capable of dissolving LiF. These results provide new insights into CEI formation and dissolution mechanisms, underscoring the need for further studies across different potential windows and with non carbonate electrolytes to validate these findings.