Depth-Resolved Probing of Native Solid Electrolyte Interphase Formation and Dynamics in Li Metal Batteries by Cryogenic X-Ray Photoelectron Spectroscopy.

Solid electrolyte interphase (SEI) critically governs lithium (Li) battery performance. Yet, understanding the native SEI remains challenging due to the lack of techniques capable of depth profiling of the interphase layer under electrolyte conditions (wet-SEI). In this work, cryogenic X-ray photoelectron spectroscopy (cryo-XPS) coupled with argon gas cluster ion beam (GCIB) sputtering was developed to extensively investigate the vitrified wet-SEI of Li metal batteries without chemical damage. First, the combined cryo-XPS and GCIB platform captures the full composition of the native SEI in the presence of electrolyte, which comprises organic polymeric hydrocarbons and inorganic species like LiC, LiF, LiO, and LiCO. These results are significantly distinct from conventional XPS characterizations of dry-SEI (i.e., SEI without electrolyte) showing a depletion of inorganic species and thus highlight the strength of this hybrid approach in revealing the real motif of the native SEI. Second, a graded SEI architecture has been revealed with electrochemical decomposition products (LiF and LiCO) dominating the electrolyte-facing region, and chemically derived species (LiO and LiC) accumulating at the electrode-facing region. Lastly, this approach is capable of scrutinizing the dynamic evolution of SEI during Li deposition, unravelling a compositional shift from electrochemical SEI to a graded complex SEI architecture, with a thickness increase from the nanometer- to micrometer-scale. Therefore, depth-resolved cryo-XPS serves as a promising methodology for elucidating the dynamic heterogeneous chemical signatures across evolving solid-liquid interfaces in electrocatalysis and energy storage processes.