Cathode–Electrolyte Interphase of Ni-Rich LayeredOxides: Evolving Structure and Implication on Stability

The cathode–electrolyte interphase (CEI) is recognized as a crucial component in battery systems; however, knowledge regarding its structure and function remains limited and often controversial. This study demonstrates the feasibility of revealing atomic-resolution CEI structures via cryogenic transmission electron microscopy (cryo-TEM). Using Ni-rich oxide (LiNi_0.8Co_0.1Mn_0.1O₂, NCM811) microparticles as the model cathode, ultrafine images of CEI were obtained, revealing its dynamic evolution over cycling and its impact on battery performance. In ethylene carbonate (EC)-based electrolytes, the CEI forms an amorphous organic-rich layer, while, in fluoroethylene carbonate (FEC)-based electrolytes, the CEI contains abundant LiF grains. Both CEIs thicken with cycling but exhibit different structural evolutions: the former adopts a mosaic structure, while the latter forms a dual-layer structure with a compact LiF inner layer. These unprecedented high-resolution images and comprehensive analysis advance the understanding of CEI dynamics, addressing existing controversies and providing insights for improving battery performance.