Cathode-Electrolyte Interphase of Ni-Rich Layered Oxides: 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 (LiNiCoMnO, 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.