Beyond Composition: Surface Reactivity and Structural Arrangement of the Cathode–Electrolyte Interphase

The role of the cathode–electrolyte interphase (CEI) on battery performance has been historically overlooked due to the anodic stability of carbonate-based electrolytes used in Li-ion batteries. Yet, over the past few decades, degradation in device lifetime has been attributed to cathode surface reactivity, ion transport at the cathode/electrolyte interface, and structural transformations that occur at the cathode surface. In this review, we highlight recent progress in analytical techniques that have facilitated these insights and elucidated not only the CEI composition but also the spatial distribution of electrolyte decomposition products in the CEI as well as cathode-driven reactions that occur during battery operation. With a deeper understanding of the CEI and the processes that lead to its formation, these advanced characterization tools can unlock routes to mitigate impedance rise, particle cracking, transition metal dissolution, and electrolyte consumption, ultimately enabling longer lasting, safer batteries.