Formation mechanisms and mass transfer characteristics of solid–electrolyte interphase in lithium batteries

The solid–electrolyte interphase (SEI) remains the most central yet enigmatic factor determining the performance of lithium (Li) batteries. As a nanoscale layer composed of insoluble electrolyte decomposition products between the anode and electrolyte, the SEI serves two key functions in governing battery performance: passivating the anode against further electrolyte reactions and enabling Li-ion transport across the interface. The simultaneous fulfillment of these two functions, in which reactions and mass transfer are intrinsically coupled, considerably complicates the understanding of the role of the SEI. Furthermore, the sensitivity of the SEI to air and electron beams poses additional challenges for its characterization and understanding. Consequently, a thorough understanding of SEI formation reactions and associated mass transport characteristics is imperative for the continued advancement of Li batteries. Herein, we first elucidate the formation mechanism of the SEI, covering the generation of reduction products, nucleation and growth, and evolution, as well as the resulting composition and structural models. The mass transfer characteristics of the SEI are then systematically summarized, with emphasis on grain-boundary-mediated and nanopore-confined transport pathways. Subsequently, representative strategies for regulating the SEI to achieve improvement in passivation capability and ion transport are introduced. Finally, we offer insights into future research directions for deepening the fundamental understanding of SEI-related scientific issues and enabling rational SEI design.