Evaluation and Characterization of SEI Composition in Lithium Metal and Anode‐Free Lithium Batteries

Abstract Interfaces, particularly the solid electrolyte interface (SEI), play a crucial role in the performance and durability of batteries. Peled first proposed the inception of the SEI. The SEI, which is formed by electrolyte decomposition on the electrode surface, affects battery stability, electrochemistry, and cycle life. The structural properties of the SEI are related to lithium stripping and plating efficiency, as well as to the overall battery lifespan. In lithium metal batteries, the SEI must manage the significant volume changes of the electrode and prevent dendrite growth that can lead to short circuits and capacity losses. This challenge is exacerbated in anode‐free lithium batteries, where uncontrolled SEI growth can cause rapid capacity degradation. Improving SEI stability is vital for enhancing battery performance, and researchers are exploring various strategies, such as the use of electrolyte additives and synthetic SEI films. Advanced in situ characterization methods, such as atomic force microscopy and X‐ray photoelectron spectrometry, provide insights into the evolution of SEIs under operating conditions. This review covers recent research on SEI formation in lithium‐metal and anode‐free lithium batteries, emphasizes stabilization strategies, and examines new real‐time characterization methods.