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

Understanding the formation and evolution of the cathode-electrolyte interphase (CEI) is essential for elucidating degradation mechanisms in high-voltage lithium-ion batteries. Here, liquid electrochemical transmission electron microscopy (ec-TEM), gas chromatography-mass spectrometry (GC-MS), and correlated 4D-scanning transmission electron microscopy (STEM) automated crystal orientation mapping (ACOM) are combined to investigate CEI evolution under high-voltage conditions. Cycling LP30 electrolyte between 4 and 6 V vs Li produces dispersed 1.0-1.5 μm particles composed of crystalline LiF embedded in an amorphous phase that limits dissolution. In contrast, cycling between 2.5 and 5.5 V forms an approximately 36 nm amorphous interphase without detectable crystalline LiF. GC-MS reveals that ethylene carbonate oxidation generates HF, promoting LiF formation at sufficiently high potentials. LiF dissolution proceeds through a two-step mechanism involving oxidative formation of soluble intermediates followed by reduction into species capable of dissolving LiF. These results provide direct mechanistic insight into voltage-dependent CEI formation and degradation, guiding the design of more stable electrolytes and interphases.